Friction and Wear Mechanisms of Ceramic Surfaces, As Well As on Acquiring Knowledge About the Properties of the New Surfaces Created During Wear

Till Pappa, Mamma, Jens och Benny

List of Papers

I On the role of tribofilm formation on the alumina drive components of an ultrasonic motor J. Olofsson, F. Lindberg, S. Johansson, S. Jacobson, Wear, 267 (2009) 1295-1300

II The influence of grain size and surface treatment of the tribofilm formation on alumina components J. Olofsson, S. Jacobson, submitted to: Journal of American Ceramic Society

III Influence from humidity on the alumina friction drive system of an ultrasonic motor J. Olofsson, S. Johansson, S. Jacobson, Tribology International, 42 (2009) 1467-1477

IV Tribofilm formation of lightly loaded self mated alumina contacts J. Olofsson, U. Bexell, S. Jacobson, submitted to Wear

V On the influence from micro topography of PVD coatings on friction behaviour, material transfer and tribofilm formation J. Olofsson, J. Gerth, H. Nyberg, U. Wiklund, S. Jacobson, Wear 271 (2011) 2046-1057

VI Evaluation of silicon nitride as a wear resistant and resorbable alternative for total hip joint replacement J. Olofsson, T. M. Grehk, T. Berlind, C. Persson, S. Jacobson, H. Engqvist, submitted to: Journal of Biomaterials Research part B

VII Fabrication and evaluation of SixNy coatings for total joint replacements J. Olofsson, M. Pettersson, N. Teuscher, A. Heilmann, K. Larsson, K. Grandfield, C. Persson, S. Jacobson, H. Engqvist, submitted to: Journal of Materials Science – Materials in Medicine

Reprints were made with permission from the publishers.

Author’s Contribution to the Publications

Paper I Major part of planning, major part of experimental work excluding TEM analyses, major part of evaluation and writing.

Paper II Major part of planning, experimental work, evaluation and writing.

Paper III Major part of planning, experimental work, evaluation and writing.

Paper IV Major part of planning, major part of experimental work excluding XPS and SIMS analyses, major part of evaluation and writing.

Paper V Part of planning, part of experimental work excluding coating deposition and XPS analyses, part of evaluation and major part of writing.

Paper VI Major part of planning, major part of experimental work excluding blood plasma incubations XPS, XRD, and ICP-MS analyses, major part of evaluation and writing.

Paper VII Major part of planning, part of experimental work excluding DFT calculations, XRD and TEM analyses, major part of evaluation and writing.

Parts of this thesis have been previously published (Papers I, III and V). These papers are reprinted with the kind permission from Elsevier.

Contents

Introduction...... 9 Aim of the Thesis ...... 10 Overview of Tribofilms ...... 11 Friction and Wear of Ceramics ...... 13 Wear Mechanisms ...... 13 Influence of Atmosphere ...... 14 Ceramics ...... 15 Ceramic Coatings ...... 16 Coatings ...... 17 PVD – Sputter Coating ...... 17 Coating Deposition ...... 18 Friction Drive System of an Ultrasonic Motor ...... 20 Tribology in Hip Joint Replacements ...... 22 Hip Joint Replacements ...... 22 Biotribology...... 23 Tribological and Mechanical Testing ...... 25 Bench Testing ...... 25 Ball-on-Disc Test...... 26 Nanoindentaion...... 27 Solubility Tests of Silicon Nitride ...... 27 Surface Analysis ...... 28 Surface Characterisation...... 28 Chemical Surface Analysis...... 29 Tribofilm Formation, Friction and Wear of Alumina against Alumina...... 31 Tribofilm Formation on Alumina Surfaces ...... 31 Tribofilm Formation in Water and Different Humidity...... 37 Hardness of the Tribofilm...... 38 Chemical Composition of the Tribofilm...... 39 Friction...... 40

Friction Behaviour and Tribofilm Formation of TaC/a-C Coating...... 43 Friction, Wear and Solubility of Silicon Nitride for Total Hip Joint Replacements ...... 46 Evaluation of Bulk Silicon Nitride ...... 46 Evaluation of SixNy Coatings...... 49 Conclusions...... 53 Sammanfattning på svenska (Summary in Swedish)...... 55 Acknowledgements...... 58 References...... 60

Introduction

Friction, wear and lubrication have to be considered in most everyday situations where physical movement is of importance. The science and technology of interacting surfaces in relative motion, which encompasses friction, wear and lubrication, is called Tribology [1]. This means that knowledge of tribology can also be applied for development of sports and sports equipment. For example, football shoes have studs which control the grip, i.e. the friction between the shoes and the grass. However, if it rains, water will act as a lubricant and the friction between the grass and the shoes decreases. If the grass has dried but not the soil, the soil under the grass is the most easily sheared material, and then the shearing takes place in the soil and yet another friction level will apply. In addition, the surface properties could change over time. Grass and soil could adhere to the shoes, smoothen the surfaces and thus change the friction behaviour. Further, it should be considered that the wear of the football field increases when the soil is softer. Contrastingly, if the field is hard and contains a lot of sand and stones, the studs will be worn and hence the friction decreases. In conclusion, in order to understand the friction and wear behaviour it is of high importance to consider the surface properties as they will develop over time due to the tribological contact. The initial surfaces will always change. Friction arises as a resistance to motion when a solid surface moves over another surface. The tangential friction force (FF) is proportional to the normal load (FN) by the coefficient of friction (µ), therefore the friction equation can be expressed as:

F µ F (1) FN

This proportional law is called the First Law of Friction by Amonton 1699 [1]. The coefficient of friction depends on the materials in contact, and will change with time as the surfaces change. At each point of contact, the softest material will deform and the material with lowest shear strength ( ) will be sheared. However, when the surrounding conditions change, e.g. a lubricant is added (water on grass), the temperature changes (different shear strength of

9 the soil), and so on, the coefficient of friction will change. The friction is not only a parameter for the original materials in contact, it is a parameter for the whole, dynamic system! The materials in contact have to be well adapted to the system and therefore materials science is essential for tribology. In terms of football, different types of shoes and materials are used for different grounds to achieve the optimal friction between the ground and the shoe. The optimal friction is different for different systems. To reduce the energy losses in machine elements, the friction should be low. A low friction is also desired in hip joints to facilitate body movement. However, the optimal friction is high in an ultrasonic motor’s friction drive system in order to transfer movement.

Aim of the Thesis The aim of this thesis is to improve the function of ultrasonic motors, hip joint replacements and low-friction applications using ceramic materials and coatings. The investigations focus on understanding the friction and wear mechanisms of ceramic surfaces, as well as on acquiring knowledge about the properties of the new surfaces created during wear. Ultimately, this understanding can be used to develop ceramic systems offering both high and low friction, while minimising material losses.

10 Overview of Tribofilms

The surfaces of two bodies that slide against each other are subjected to high local stresses and pressures. This results in local shear deformation and fracture of the surfaces and locally high temperatures can arise. The high temperatures can accelerate chemical reactions on the surfaces or even melt the surfaces locally. These conditions are not only destructive to the surface, they are also necessary for the formation of new surface compounds, i.e. tribofilms [2]. These compound layers, or tribofilms, give the surfaces new tribological properties by changing the surface topography, chemistry and mechanical properties. Jacobson and Hogmark [2] divide the tribofilm formation into two groups: Transformation Type Tribofilms and Deposition Type Tribofilms. Transformation Type films includes transformation of the original surface by plastic deformation, phase transformations, diffusion, etc., without any material transfer, see Figure 1a. In contrast, Deposition Type films are formed by molecules fed from the counter surface, the environment, or by wear debris, exemplified in Figure 1b,c and Figure 2.

Figure 1. Schematic image of tribofilms. The arrows indicate the sliding direction of the bodies; (a) Transformation Type, the lower surface have transformed, chemically or mechanically; (b) Deposition Type, wear particles from both surfaces has formed a tribofilm on the lower surface; (c) Deposition Type, the surface has reacted with the environment during contact and formed a tribofilm of chemical reaction products.

Figure 2. Tribofilm on a steel ball surface, formed by agglomeration and tribo- sintering of wear debris from the ceramic coating on the counter surface.

Tribofilms often act as protective layers on a surface [3-5]. Therefore, it is common to design surfaces or lubricants to achieve a certain tribofilm. The deposition of low-friction coatings (such as diamond like carbon (DLC) coatings) is one way to control the friction and wear properties by tribofilm formation. Often one surface is coated and slides against a counter steel surface. During running in, coating material adheres to the steel surface, i.e. forms a tribofilm. Thereafter, the sliding takes place in the easily sheared layers between the coated surface and the tribofilm. This decreases the friction and protects the steel surface from wear, while the wear rate of the coating reaches a low steady state level [6]. Most of the lubricants used in engines contain additives. Some of these additives are used in order to form a tribofilm on the steel surface. A tribofilm can be chemically formed by reactions between the lubricant and the solid surfaces. The most frequently used additive to preserve the steel surfaces from wear is zinc dialkyldithiophosphate (ZDDP) [7,8]. At high pressures, phosphates are formed and adsorb to the steel surface which in turn protects the surface from wear. There are also unwanted tribofilms, for example galling on forming tools. Galling occurs when lumps of work material adhere to the forming tool and cause damage to the next work material [2,9]. This kind of tribofilm formation is often prevented by decreasing the surface roughness of the forming tool [10].

12 Friction and Wear of Ceramics

Ceramics are used in a wide range of applications, both in lubricated and unlubricated contacts. Due to their hardness and high wear resistance, ceramics are often used in applications such as water pump bearings, ball bearings, cutting tools, etc. [11,12]. Ceramics are relatively inert and are therefore suitable in harsh chemical environments as well as in medical applications. Alumina (Al2O3), zirconia (ZrO2), silicon carbide (SiC) and silicon nitride (Si3N4) are some of the most common sintered bulk ceramics used for engineering applications. To overcome the low ductility of bulk ceramics, ceramics are commonly deposited as thin coatings on more ductile materials. The ductile bulk withstands high mechanical stresses while the coatings improve the hardness and wear resistance of the surface. One of the main advantages of using ceramics against ceramics in unlubricated contacts is that the chemical bonds between the surfaces are easier to break than the metallic bonds that form between clean metal surfaces. When metal slides against metal, oxides normally cover the surfaces, which often decreases the coefficient of friction. However, if the oxides become worn off, strong metallic bonds form between the exposed metal surfaces. Further sliding will require shearing of the metallic interface, which increases the coefficient of friction [1]. Typically, in tribological contacts between ceramics and metals, the metal adheres to the ceramic surface and then the sliding takes place in the created metal-metal interface [13].

Wear Mechanisms Wear of ceramics are dominated by cracking and chemically induced surface transformations. Plastic deformation does occur but more rarely [14]. There are typically two wear regimes for ceramics, mild and severe wear. In sliding under low contact pressures the wear is mild, the surfaces become polished, the roughness decreases and the wear rate is low [12,15]. Smooth surfaces and low pressures give lower coefficients of friction [16,17]. Conversely, severe wear takes place under higher contact pressures where stresses exceed the tensile strength of the material and grain fracture lead to formation of wear debris and an increased wear rate [12,15]. At very high pressures, whole grains will chip out from the ceramic matrix and the surface roughness increases drastically [18]. The size of wear debris typically increases with contact pressure. Severe wear is typical during running and as the

13 surfaces become smoother, the wear regime transforms to mild wear. Further, the sliding speed has an influence on the wear of ceramics. The wear normally increases with increasing sliding velocity [17]. It has also been demonstrated that an increased grain size increases the wear rate in sliding contacts [19]. In repeated contact, the wear debris that is formed in the contact will be ground to smaller sizes. In case of ceramic sliding against ceramic, the wear debris is usually generated from both surfaces [20]. In dry contacts (i.e. without any lubricant) the wear debris formed may fill up cavities and create agglomerates on the surfaces. The local pressures and temperatures in a tribological contact are high enough to sinter the agglomerated particles to a solid tribofilm. This phenomenon is called tribo-sintering [21]. This type of tribofilm has been shown to smoothen surface topography and may have a thickness up to 6 µm [5].

Influence of Atmosphere The friction and wear properties of ceramics strongly depend on the surrounding atmosphere [22]. The ability to form a sintered tribofilm on alumina decreases with increasing humidity. The hydrophilic alumina wear particles are less prone to agglomerate since they adsorb water which leads to lower adhesive forces [23]. It is claimed that aluminium hydroxides (AlOOH, Al(OH)3) are formed on the alumina surfaces in water and humid air [4,24,25]. This hydroxide is said to have lower shear strength than alumina and therefore contributes to a lower friction. Silicon nitride sliding against silicon nitride demonstrates a similar behaviour of tribochemical wear. Silicon nitride reacts with water and forms silica (SiO2) [26]. This silica layer contributes to very low coefficients of friction and low wear rates [27]. However, Xu et al. [27] have also shown that when the silica layer has reached a critical thickness, it delaminates from the silicon nitride surface and a new layer of silica builds up on the surface. The chemical reactions with water will continue to dissolve the silica, creating a reaction product consisting of silicon hydroxide (Si(OH)4). The majority of crystalline wear particles are produced at an early stage of the test, whereas with increased sliding distance the wear particles produced are amorphous or consist of silica or silicon hydroxide. However, Tomizawa et al. [26] claim that no solid wear particles are generated from the contact of silicon nitride against silicon nitride in water.

14 Ceramics

Ceramics are a large group of materials exhibiting a wide range of properties. Ceramic materials can be defined as compounds of metallic and non- metallic elements as well as non-metallic, non-organic materials [28]. The characteristic properties of ceramics include high melting point, low density, high corrosion resistance, high hardness and low thermal expansion. Further, they are much stronger in compression than in tension and their main disadvantage is brittleness. The plastic deformation of crystalline ceramics and metals takes place by dislocation motions. The atomic bonding in ceramics is of a covalent or ionic character. With these bonding types there are a limited number of slip planes, along which dislocations can move [29]. For ionically bonded atoms, the repulsive forces of the electrically charged atoms restrict the slip of the atoms. For covalently bonded atoms, the strength of the covalent bonds pre- vents the slip. The ions in metallic bonding share electrons and therefore no electrostatic forces prevent slip of atoms. Due to the restricted slip and therefore restricted dislocation motion, plastic deformation is limited for ceramics and they often fracture before plastic deformation can occur. Also due to the restricted number of slip systems, the strength of ceramics in tension is limited by the defects within the material. A defect in a ceramic material weak- ens the structure by creating a stress concentration from which the crack propagation starts. Therefore, bulk ceramics should not be subjected to high tensile stresses. For non-crystalline ceramics, there is no regular atomic structure and consequently plastic deformation does not occur by dislocation motion. These materials deform by viscous flow [29]. Applied shear forces cause the atoms to slide past each other, by breaking and reformation of interacting bonds, without a favourable sliding direction. Bulk ceramics are produced by the compaction of powder and a subsequent densification of the same through sintering/firing at high temperatures. Due to high hardness and brittleness, any following treatment such as shap- ing is complicated and often requires diamond cutting tools and abrasives. Sharp edges and corners should be avoided due to concentrations of tensile stresses. Ceramics are often used in applications that are too hot for metals, such as ovens and heat engines. They are also used as electrical insulators and heat barriers. Moreover, ceramics can have semiconducting character (for example silicon carbide) and can also be used for heat transfer [28].

15 The most common bulk ceramics in tribological use are based on alumina (Al2O3), silicon carbide (SiC), silicon nitride (Si3N4), zirconia (ZrO2) or boron nitride (B4N). To exemplify, silicon nitride may be used in bearings and metal cutting tools, alumina in cutting tools and hip joints, silicon carbide in mechanical seals, zirconia in dies and hip joints and boron nitride is mainly used in cutting and abrasive applications.

Ceramic Coatings Ceramic coatings are deposited to give surfaces the beneficial properties of a ceramic material while maintaining the strength and ductility of the underlying bulk material. Ceramic coatings, only a few micrometres thick, are enough to improve the life of cutting tools, which need to withstand extreme stresses and temperatures.

16 Coatings

Many engineering components can be improved with a coating. The coating can act as a protective layer on top of a bulk material or a brittle, wear resistant material can be improved by the support of a more ductile bulk material. This makes it possible to focus specific properties where they are most needed. It is also possible to combine several material properties within a coating, for example combining a hard phase with a lubricious phase. The hard phase is usually a carbide or nitride and the lubricious phase is often amorphous carbon (a-C) that can reduce adhesion and sticking to the counter surface. These low-friction wear resistant coatings are frequently used in various machine elements to reduce energy losses. There are several deposition techniques for the production of coatings, where PVD (physical vapour deposition) is a common method for applying thin coatings (1-10 µm) aimed for low friction and/or wear resistance. PVD is often chosen due to the possibility to produce high quality coatings of variable composition at relatively low substrate temperatures. Further, the technique allows combination of several materials to fabricate multilayer structures. PVD can be achieved by either evaporation or sputtering of a solid source in vacuum. In both cases, the vapour can form a coating either with the same composition as the evaporated material or in an altered form after reaction with a gas introduced into the deposition chamber (such as TiN formed from sputtered Ti reacted with N2 gas).

PVD – Sputter Coating The purpose of sputter coating is to generate free atoms from a solid target such that these emitted atoms collide with a substrate surface, condense and form a coating. The process begins with evacuation of the chamber, followed by introduction of a working gas (usually Ar) and application of an electric potential between the target and a shield around the target. The electric field accelerates ions towards the target, which in turn creates more ions and electrons upon bombardment with gas molecules, thereby a plasma is formed. The positively charged ions in the plasma are accelerated towards the target and upon impact eject atoms which are finally deposited as a coating on the substrate. To increase the ionization and hence the deposition rate, magnetron sputtering is an alternative [30]. In magnetron sputtering, a magnetic field per-

17 pendicular to the electron current is utilised to obtain a helical path for the electrons. The secondary electrons released from the target by the ionic bombardment are trapped by the magnetic field and move in the same pattern as the other electrons. This concentration in electron density leads to a locally increased ionisation and thus a higher sputter rate of the target in this area, forming the so-called racetrack. In the simplest sputter methods, the substrates are earthed. A negative potential (bias) can be applied to the substrates to accelerate positive ions also towards the substrate, to generate a mild sputtering. This controls the coating growth and could also improve adhesion of the coating. A negative bias can also be used for sputter cleaning of the substrate prior to deposition, in order to get rid of oxides and surface contaminations. However, it has been revealed that sputter cleaning may also roughen the surfaces. One important example of this is tool steels, where the sputter rate of the carbides is lower than that of the matrix [31,32]. If the potential applied between the shield and target is constant over time, it is termed as direct current (DC) sputtering. However, this potential can be pulsed and if the frequency is sufficiently high, it is referred to as radio frequency (RF) sputtering. RF sputtering is preferable to avoid charging if the target material does not have sufficient conductivity. To deposit more complex coating compositions, sputtering can be made from two targets simultaneously, so called co-sputtering.

Coating Deposition In this thesis, two types of coatings have been deposited by PVD-sputtering. The coating of TaC/a-C* was produced because it has been shown to give a low coefficient of friction, about 0.05 in a ball-on-disc apparatus against a ball of steel [33]. The other coating of SixNy was produced in order to achieve wear resistant bearing surfaces for hip joint replacements, which case a minimum amount of wear particles that are resorbable in vivo†.

TaC/a-C Coating The TaC/a-C coating was deposited in the BAI640R PVD coating system by sputtering from a planar DC magnetron source. In order to simultaneously sputter tantalum and carbon, the carbon target was partly covered by a tantalum foil, a procedure earlier used by Nilsson et al. [34]. Metallurgical powder high speed steel ASP 2053‡ substrates and silicon wafers were simultaneously coated. After cleaning and mounting, the samples were heated to 400 °C for 30 min and sputter cleaned for 1 minute. The sputter

* Tantalum carbides (TaC) in an amorphous phase of carbon (a-C) † In vivo: Within the living body ‡ Material composition (wt.): 2.48% C, 5.2% Cr, 3.1% Mo, 4.2% W, 8.0% V, balance Fe

18 cleaning/etching process was performed at an Ar-pressure of 1.5 x 10-6 bar and a substrate bias of -200 V. The 240 minute sputtering process was performed using a magnetron power of 1.5 kW at a total chamber pressure of 3.5 x 10-6 bar. The substrate bias was set to 0 V (floating potential).

SixNy Coatings * The SixNy coatings were deposited on CoCr, ASTM F1537 substrates as well as on silicon wafers. The coatings were deposited by reactive RF sputtering (13.56 Hz), utilising a 4 inch silicon target and an Ar/N2 gas mixture. The coating system (Hochvakuum, Dresden, Germany) was not equipped with a bias control of the substrates and hence no sputter cleaning was performed prior to deposition. In order to optimise the coating composition, microstructure etc., several coating parameters were varied. The total chamber pressure during coating was 1±0.5 bar, the target power was in most cases set to 300 W, the substrate temperature was in most cases set to 280 °C and the gas flow rate was about 40 sccm with various gas compositions. To avoid overheating of the target, the coating process alternated between 10 min sputtering and 3 min breaks. The total deposition time was 2 hours, except for the coating sputtered with a target power of 150 W, which had a deposition time of 3 hours. In two of the coating processes, ethylene (C2H4) gas was let into the chamber as a reactive agent in order to dope the coating with carbon.

* Material Composition (wt.): 28% Cr, 6 %, Mo, C 0.35%, balance Co

19 Friction Drive System of an Ultrasonic Motor

The demands for portable products with several functions utilising mechanical movements are steadily increasing. Ultrasonic motors are well suited in systems where miniature size, high speed, good precision and low power consumption are essential features. The drive mechanisms of ultrasonic motors are based on the converse piezoelectric effect, where the piezoelectric material elongates when an electric field is applied [35,36]. Traditional electromagnetic motors are typically based on a rotating system which needs gear-boxes, lead screws and nuts to accomplish a linear movement. Miniaturisation of such systems is complicated due to difficulties in manufacturing and assembling. However, the movement of piezoelectric elements can give a direct linear motion, requiring only a friction drive system which is well adapted to miniaturization. With fewer components, the size can be considerably reduced. A commercial such ultrasonic motor is shown in Figure 3 and the main parts of the friction drive system are sche- matically illustrated in Figure 4. The main function of this motor is to operate a drive rail back and forth.

Figure 3. The ultrasonic, piezoelectric motor PiezoWaveTM from PiezoMotor AB. The orange tail is a part of the flexible printed circuit board and has contact pads for electricity supply.

The piezoelectric elements are soldered to the flexible printed circuit board and the drive pads are glued to the piezoelectric elements. The flexible printed circuit board is folded and assembled into a plastic housing, which places the elements in their right positions, according to Figure 4. The drive rail is inserted into the plastic housing and integrated bearings keep the rail in place. A spring attached to the flexible printed circuit board presses the elements towards the drive rail and creates a normal force between the drive

20 pads and the drive rail. When electric signals are applied to the elements they will oscillate. Due to a shift of the electric phase, the middle of the elements, i.e. the assembled drive pads, will describe a rotating motion. The drive pads transfer the movement to the drive rail by pushing it in the desired driving direction during half the cycle of the rotating motion. The rail is pushed approximately 1-3 µm per cycle, at a frequency of 96 kHz. Due to the high frequency, the continuous motion of the drive rail has a speed of approximately 100 mm/s. The total stroke of the investigated motor is approximately 6 mm. This implies that the drive pads push the drive rail approximately 5000 times per stroke and always with the same area in contact. The average contact pressure between the drive pads and the drive rail is approximately 10 MPa.

Figure 4. Principle of the friction drive system on an ultrasonic motor (Piezo- WaveTM); (a) Schematic cross section, the black springs indicate the normal force (1 N) applied between the drive pads and the drive rail; (b) Bending of the piezoelectric bimorph elements due to elongation of selected layers by an applied electric field. The drive pads on the elements give the drive rail a linear motion. The arrows indicate the motion of the drive pads.

The drive pads and the drive rail are made of alumina, due to its relatively high wear resistance and high coefficient of friction. For some applications it is of importance that the friction drive system produces a minimal amount of wear particles so as not to disturb the function of adjacent components. The driving force of the motor equals the friction force between the drive pads and the drive rail, thus the driving force of the motor can never exceed the friction in the friction drive system. Therefore, it is important that the friction force is relatively stable and on a relatively high level. With higher friction forces, the motor become stronger and can be suitable for several applications.

21 Tribology in Hip Joint Replacements

The human body is a complex system with several sensors and actuators. Whenever something happens to it, e.g. an injury or another kind of compli- cation, it senses and reacts to it, in purpose to defend itself and stay intact. When a foreign material is inserted in vivo, i.e. in the body, the involved tissue will be damaged regardless of implantation method. The reactions the body sets in motion will attempt to heal the wound or remove the foreign material. When optimising an implant, the aim is to minimise the foreign body reactions. The variation in intensity and duration of the inflammatory response or wound-healing process depends on the size, shape, chemical and physical properties of the biomaterial. It is important that the biomaterial is biocompatible, i.e. it is able to perform with an appropriate host response for its intended function [37]. When it comes to total hip joint replacements, it is the whole system, rather than the individual materials, that have to be biocompatible.

Hip Joint Replacements Total joint replacements constitute one of the most common and successful procedures in orthopaedics [38]. In the United States alone, approximately 202 500 primary total hip joint replacements were performed in 2003. By 2030 the number of primary procedures is estimated to grow to 572 000 [39]. “Primary procedure” refer to the first time the total joint replacement is performed, while any following procedures are referred to as revisions. To occupy in the space of the previous replacement, the dimension of each revision has to be larger than the previous. For example the stem of a first revision hip is longer than the primary hip stem (Figure 5a). A total hip joint replacement has an average life span of approximately 15 years before it has to be replaced by a revision [40]. An increasing number of younger patients are receiving implants and an aging population [41]. It is therefore highly desirable to increase the implant longevity, in order to reduce the number of revisions. A total hip joint replacement consists of several parts, see Figure 5. The stem placed in the thighbone (i.e. femoral bone) is often composed of titanium. The ball (i.e. the femoral head) is often composed of cobalt chromium (CoCr), alumina (Al2O3) or zirconia (ZrO2). The acetabular cup is positioned in the pelvis bone and is often composed of CoCr, steel or alumina. If poly-

22 ethylene is used for the cup, it is often used as a liner between the femoral head and the acetabular cup, as shown in Figure 5a. The diameter of the femoral heads is typically around 28 mm, with a diameter clearance to the cup of typically 100-200 µm.

Figure 5. Total hip joint replacement. (a) This implant is for the third revision of a replacement. The stem is of titanium and coated with hydroxyapatite for improved bone integration, the femoral head is of alumina, the liner of polyethylene and the acetabular cup is of titanium; (b) X-ray image, the patient’s left hip has been replaced.

Biotribology Failures of total joint replacements can be due to several factors including insufficient fixation and positioning, mechanical loosening and infections. One of the most critical factors is related to tribology, i.e. wear of the cup, head and liner. It is the produced wear particles rather than the material loss from the joint surfaces that become the limiting factor. The wear particles that are accumulated close to the implant may cause inflammation and osteolysis (degeneration of bone tissue) which eventually leads to implant loosening [40,42,43]. The most important demands on the material combination in the implant are high wear resistance and biocompatibility. In addition, smooth bearing surfaces are important to reduce the generation of wear particles. Common material combinations for hip joint bearings include cups of ultra-high molecular weight polyethylene (UHMWPE) that slide against a head of CoCr, alumina or zirconia. These material combinations exhibit a

23 low coefficient of friction, but the UHMWPE often wears to produce a relatively large amount of wear particles [44,45]. In order to reduce the generation of wear particles from the UHMWPE, alternative material combinations are used, for example CoCr against CoCr, alumina against alumina or zirconia against zirconia. Other metals than CoCr have also been used, including titanium alloys and stainless steels [40]. Although wear particles from metals and ceramics have also been shown to cause osteolysis [46,47], wear particles of UHMWPE are considered to be more biologically detrimental [47]. However, it has been demonstrated that corrosion of metallic wear particles increases the metal ion content in the body, which is a cause of concern since Co and Cr ions are known carcinogens [48]. Bulk ceramics (mainly alumina and zirconia) against themselves have been used for hip joint bearings due to their high corrosion resistance. They have also been shown to produce less and smaller wear particles than other material combinations [43,45,49]. However, bulk alumina and zirconia are brittle in comparison to metals, which can lead to catastrophic failure in vivo [50]. To improve the ductility of the hip joint implant as well as minimising the production of metallic wear particles, ceramic coatings of ZrO2, TiN, CrN, CrCN, DLC have been deposited on CoCr alloys and tested in vitro (in an artificial environment) [51,52]. It has also been reported that TiN, CrN and hydrogenated amorphous carbon (a-C:H) coatings have been deposited on titanium alloys, and tested in vitro [53]. Alternative ceramics that have shown potential in in vitro tests include silicon nitride (Si3N4) and silicon carbide (SiC) [54,55]. Silicon nitride as a bulk material has been shown to be bioinert and exhibit a good biocompatibility [56,57]. When exposed to water, the surface of Si3N4 becomes hydro- lysed and oxidised, and the silica layer is then leached in the water [27,58]. This in combination with low wear rates suggests that the use of silicon nitride bearings would produce less wear particles and those produced would be biocompatible and degradable. The less body response to the wear particles, the longer is the life span of the implant. Even though silicon nitride is a relatively tough ceramic material, the bearing component would be more ductile if designed as a silicon nitride coating on a metallic substrate. CoCr is a good alternative as a substrate since it has already proven its function as a bearing material for hip joints.

24 Tribological and Mechanical Testing

The ultimate goal of tribological and mechanical testing is to find the best possible material solution for the specific application with respect to friction properties and wear resistance. When field tests are complicated and expen- sive, an alternative test method that mimics the tribological conditions has to be employed. Fundamental knowledge about tribology is required to choose the best suited test method. Model tests in the lab are often performed to test the wear of single components, while other tests measure the tribological function of a whole system.

Bench Testing An individual bench test was set up to measure the actual friction force between the drive pads and the drive rail (Figure 4) of the ultrasonic motor PiezoWaveTM, see Figure 6. The drive rail consists of 99% -alumina and the drive pads consist of 96% - alumina. The contacting surfaces are tested as sintered, i.e. not further treated after sintering.

Figure 6. Schematic image of the friction force test apparatus for the ultrasonic motor PiezoWave TM. The friction force between the drive pads and the drive rail in the motor are measured while the motor is turned off.

The friction force measured here was the actual “dynamic holding force” of the motor. The normal force of 1 N, applied by the spring, was always present. The friction force between the drive pads and the drive rail was continuously monitored while the drive rail was pushed through the motor at a constant speed of 3 mm/s, using an external operating motor. A position gauge continuously monitored the position of the drive rail.

25 Ball-on-Disc Test The ball-on-disc test [59] is a model test that includes a stationary ball sliding against a rotating disc while the friction force is continuously measured, see Figure 7. The ball-on-disc is used to simulate various sliding, high pressure contact situations, but is best suited for contacts where one surface is constantly in contact.

Figure 7. Schematic image of the ball-on-disc set up, N is the applied normal force and FF is the measured friction force.

The tribological properties of an alumina drive pad in contact with the alumina drive rail were simulated with a ball-on-disc apparatus. A polished -alumina ball of 6 mm was slid against a disc of -alumina. The load and speed were selected to represent the contact between the drive pads and the dive rail. The normal load was 1 N and the sliding speed 0.1 mm/s. By measuring the wear marks on the drive pad and ball after 200 000 passages, the contact pressure was estimated to approximately 10 MPa. Also the TaC/a-C coating was tested in the ball-on-disc apparatus and slid against a ball bearing steel ball of 6 mm in diameter. The setup involving one uncoated part sliding against a carbon based low-friction coating was selected since it has become a common technical solution with many advantages [60]. The normal force was 5 N and the sliding speed was 0.005 m/s. All the tests were performed unlubricated in ambient room conditions with a relatively humidity of 50 ± 10%. For surface studies of the running-in procedure, tests of three different sliding durations were performed: 5, 20 and 1000 revolutions. The ball-on-disc has also been utilised for pre-scanning of potential materials for hip joint replacements. Various material combinations were tested in this work. Discs of bulk CoCr, silicon nitride, as well as discs coated with SixNy were slid against balls of silicon nitride, and some tests were also exe- cuted with balls of alumina against CoCr. The balls had a diameter of 6 mm, the track radius was 2.5 mm and the sliding speed 0.04-0.05 m/s. The tests have been carried out in a synthetic body fluid PBS (Phosphate Buffered Saline), which has ion concentrations close to human blood plasma [61]. Tests have also been performed in a solution of 25% bovine serum diluted in

26 deionised water. The sliding speed, track radius and bovine solution was in accordance with the ASTM standard test [62]. In the long tests, the deionised water had to be refilled once or twice a day, to make up for evaporation losses. The continuous sliding in the ball-on-disc test is quite far from the complex motion of a hip joint. In addition, the contact pressure is very much higher than the average pressure in a hip joint. Due to the small ball radius and the limitation of lowest load for the ball-on-disc apparatus, the maximum Hertzian pressure [63] of the contact between the silicon nitride ball * and the SixNy coated CoCr disc can be estimated to 0.76 GPa , neglecting the influence from the thin coating. This would correspond to a load of 545 kN on a hip joint with a diameter of 28 mm and a radial clearance to the cup of 100 µm, assuming both head and cup are made of CoCr coated SixNy, again neglecting the small influence from the thin coating on the contact pressure.

Nanoindentaion For hardness measurements of a surface layer, such as a coating or a tribofilm, the indentation depth should not exceed 1/10 of the layer thickness to avoid influence from the underlying material [64]. In this thesis, the superficial hardness measurements were performed with an ultra nano hardness tester (CSM Instruments UNHT) equipped with a Berkovich tip†. The typical indent depths range from 30 nm to 250 nm. One advantage with this equipment is that the exact location of the indents can be chosen using an optical microscope. The indents were analysed in accordance with the Oliver-Pharr method [65].

Solubility Tests of Silicon Nitride To estimate the solubility and possible dissolution of silicon nitride wear particles in the body, a commercial silicon nitride powder (P95H, Akzo Nobel) was mixed with PBS using three different pH values: 4.8, 6.5 and 7.4. These pH levels were selected to match the pH around an implant, which has been shown to vary between 4.4-7.7 [66]. PBS has a natural pH of 7.4 so hydrochloric acid was added to decrease the value. 100 mg of Si3N4 powder was mixed with 15 ml PBS. The average grain size of the powder was approximately 1 µm. The PBS/silicon nitride mixtures were stored in plastic tubes with plastic lids. The tubes were placed on a rocking platform shaker and stored at 37°C during 35 and 75 days respectively. After the stor- age time, the mixture was filtered through a 0.2 µm PTFE membrane using a syringe. Subsequently to filtration, ICP-MS (Inductively Coupled Plasma Mass Spectrometry) was used to determine the silicon ion content of the solutions.

* Youngs Modulus for Si3N4 320 GPa and for CoCr 241 GPa. Poisson’s ratio 0.27 for Si3N4 and 0.3 for CoCr. † Three-sided pyramidal diamond tip

27 Surface Analysis

Besides designing proper tests, surface analysis has been the most important step in the tribological investigations of this thesis. The ultimate goal of the surface analysis is to reveal the set of tribological mechanisms that have modified the interacting surfaces in the tribological contact. This was done by studying the appearance, topography and chemical composition of the surfaces.

Surface Characterisation

Scanning Electron Microscopy – SEM [67] The surface appearances before and after tribological tests were analysed using SEM. A LEO 1550 SEM with a FEG (field emission gun) and a LEO 440 SEM with a LaB6 filament were utilised. To prevent charging that would disturb the imaging process, insulating samples were sputter coated with a gold palladium film.

Transmission Electron Microscopy – TEM [68] When the resolution offered by the SEM is not enough, TEM is applicable to resolve atomic contrasts. TEM is practical when it comes to analysis of the interface between a material and its tribofilm. It is also functional for the analysis of the crystal structure and microstructure. A TEM sample has to be electron transparent, i.e. having a thickness of not more than 100 nm. The particular instruments used in this work are a FEI Tecnai F30 ST TEM and a JEOL 2000 FX for the electron diffraction patterns.

Focused Ion Beam – FIB [69] A FIB (FEI DB235) equipped with a SEM column has been used to manu- facture superficial cross-sections as well as TEM-samples. The ion beam can be used for imaging and deposition of materials but mainly for cutting/- milling a well-defined section. The area to be cross sectioned was first covered with a layer of platinum, to protect the original surface from damage during the subsequent milling.

28 Optical Profilometry [70] The surface roughness, as well as worn cross-section areas of the various wear tests, was measured using light interference optical microscopy utilising a Wyko NT-110. The method has a vertical resolution in the sub nano- metre range. However, the lateral resolution is limited to around a microme- tre. The main advantage with the method is that it measures the surface profile over an area without physical contact, i.e. it is a non destructive method.

Atomic Force Microscopy – AFM [71] The AFM, XE-150, PISA has been used for measurements of the surface topography of tribofilms as well as for surface imaging. In AFM a sharp tip on a cantilever is scanned over the surface in a very precise manner. The surface topography results in a microscopic vertical movement of the cantilever which is recorded. The AFM is best suited for relatively smooth surfaces, since the height of the tip and the bending of the cantilever limit the analysis for rough surfaces. The AFM has a better lateral resolution than the optical profilometer, however the maximum analysis area for each image is limited to 90 x 90 µm for the equipment used here.

Chemical Surface Analysis

Energy Dispersive X-ray Spectroscopy – EDS [67] The elemental chemical composition of tribofilms and coatings has been analysed with EDS. The analyses were carried out with the SEM/EDS EDAX LEO 440 with a solid state detector equipped with a thin polymer window facilitating permeability of light elements. In EDS, the impinging electrons excite atoms which emit X-ray photons from a depth of 1-6 µm, which defines the analysis depth [67]. The energy of the photons is a function of the energy levels of the atoms and is element specific. The quantita- tive analysis of light elements in a matrix of heavy elements is a limitation of the EDS analysis because x-rays from the light elements can be absorbed by the heavier elements. However, this is often to some extent compensated by the software.

X-ray Diffraction – XRD [72]

The crystalline phases of bulk Si3N4, powder Si3N4 as well as SixNy coatings were examined with XRD. For the analysis of the coatings, a constant incidence angle of 1°, called grazing incidence (GI), was used to enhance the signal from the coating. The present XRD analyses were performed on a Siemens D5000 using Cu K radiation.

29 X-ray Photoelectron Spectroscopy – XPS [73] XPS analysis is suitable for chemical analysis of tribofilms and coatings due to a shallow analysis depth of less than 5 nm [73]. The tribofilms on Al2O3 surfaces, as well as tribofilms on CoCr and Si3N4 surfaces that have been in contact with PBS or bovine serum were analysed with XPS. Also the TaC/ a-C coatings were analysed with XPS. The XPS analyses were performed with a Physical Electronics Quantum 2000 spectrometer using monochro- matic Al K radiation. The technique is based on the photoelectron effect. Photons from the incoming x-ray radiation excite electrons of the studied surface. The kinetic energies of the ejected photoelectrons are analysed and the binding energies are calculated. The binding energy of the photoelectrons is specific for each chemical bond. Depth profiling is possible by alternating sputtering (using Ar-ions) and analysis.

Inductively Coupled Plasma Mass Spectrometry – ICP-MS [74] Inductively coupled plasma mass spectrometry, was used in this thesis to determine the silicon content of the PBS/silicon nitride solutions. The analysis was performed externally by SP Technical Research Institute of Sweden in Borås, Sweden. ICP-MS is preferable for the detection of low concentrations of positive metal ions. The aqueous solution is introduced into a plasma, the solution is turned into gas and then the atoms are ionised. Subse- quently, the ions travel through vacuum and are separated by their mass-to- charge ratio.

30 Tribofilm Formation, Friction and Wear of Alumina against Alumina

In the alumina friction drive system of an ultrasonic motor, the friction between the contacting, force transferring surfaces has to be relatively high and stable. The aim for this study was to characterise the friction force, friction mechanisms and surface transformation of the contacting alumina surfaces, in order to achieve a high and stable driving force of the motor.

Tribofilm Formation on Alumina Surfaces The as-sintered (without further treatment after sintering) alumina surfaces of the drive rail has round shaped grains and a surface roughness (Ra) of 0.8 µm, see Figure 8a. The drive pad has a cylindrical shape and round shaped grains about the same size as the dive rail. However, the surface is smoother with an Ra value of 0.4 µm, see Figure 8b.

Figure 8. Unworn alumina surfaces. (a) Drive rail; (b) Drive pad.

When the alumina disc or drive rail slides against the alumina drive pad or alumina ball, grains become scratched, plastically deformed and fractured. Wear debris is scattered all over the contact area, see Figure 9. The wear is initially rapid, but soon levels out.

Figure 9. Surface appearance of initial wear; (a) Drive rail after about 1000 strokes in the motor; (b) Drive pad after 10 strokes of the drive rail corresponding to approximately 18 000 steps of the drive pad.

The wear debris is further ground by the counter surface. This debris agglomerates and sinters due to the tribological contact, in this way forming a tribofilm on the surface. This film fills up cavities, thereby smoothing out the original roughness of the drive rail from Ra 0.8 µm to 0.6 µm. The wear volume is very small, after 350 000 strokes the wear is still within the outermost layers of grains, see Figure 10. In addition, contacting area of the drive pad is worn flat and the tribofilm on top of the flat grains is much thin- ner than that on the drive rail.

Figure 10. Surface appearance after about 350 000 strokes of the drive rail, corresponding to 2.1 × 109 steps of the drive pad; (a) Drive rail, patches of tribofilm covering the surface; (b) Drive pad, the contacting area has been worn flat.

The width of the contact area of the drive pad is more than 100 µm (Figure 10b) and the step of the drive pad is at least 1 µm, which means that every spot on the dive pad meets the drive rail about 100 times during each stroke. During that time the tribofilm on the drive rail is kneaded several times so that edges and superficial parts of the film will easily become worn off. These wear particles may again be ground into smaller pieces, leading to circulation of material forming the films. Therefore, the outer part of the

32 tribofilm is denser while coarser wear particles are embedded in the bottom of the tribofilm, see Figure 11.

Figure 11. Cross-section produced with FIB of a drive rail after about 200 000 strokes. The tribofilm has several cracks and a coarser structure in the lower parts of the film.

Selected area electron diffraction (SAED) in the TEM as well as fast fou- rier transformation (FFT) of the TEM image revealed that the tribofilm on the drive rail is amorphous or nano-crystalline and has a dense structure without any large pores, see Figure 12 and Figure 13. The tribofilm can be relatively thick when filling deep cavities but also thin on top of flat alumina grains. The thickness typically varies from tens of nanometres up to 4 µm, see Figure 11 and Figure 12.

Figure 12. TEM image of drive rail surface showing the tribofilm filling a recession but also covering the flat top of the grain. The inserted electron diffraction patterns reveal the amorphous structure of the tribofilm and the crystalline alumina grain. The black part in the top of the image is the protective platinum layer, deposited prior to the FIB cross-section milling.

High resolution TEM revealed that the mainly amorphous tribofilm bonds intimately to the alumina grains, see Figure 13.

Figure 13. High resolution TEM image showing the intimate interface between the tribofilm and the alumina grain. The corresponding FFT patterns reveal (top insert): mainly amorphous tribofilm with some crystallinity and (lower insert) a crystalline alumina grain.

Characteristic Wear of Different Test Methods The wear tracks on alumina discs slid against alumina balls showed smoother (Ra=0.4 µm) and more substantial tribofilms than the corresponding wear tracks on the drive rail, see Figure 14-Figure 16. The worn drive rail has an Ra of 0.6 µm and only patches of tribofilm.

Influence of Grain size on the Tribofilm Formation In a study of the influence of grain size on the tribological behaviour, as- sintered alumina discs of three different grain size ranges (1-5 µm, 1-10 µm and 5-10µm) were slid against polished alumina balls. (The standard grain size range of alumina discs and alumina drive rails in this work is 5-10 µm.) It was found that the tribofilm formation was less extensive on surfaces with the smallest grain size (1-5 µm), Figure 14. Whereas, almost the entire wear track on the surfaces of grain sizes 1-10 µm and 5-10 µm were covered with a tribofilm, Figure 15 and Figure 16. The approximately 250 µm wear track on the surface with the smallest grains was also narrower than the others, which measured 350 µm and 450 µm for grain sizes 1-10 µm and 5- 10 µm, respectively.

Figure 14. As-sintered alumina surface of grain size 1-5 µm; (a) Unworn surface; (b) After 100 000 revolutions ball-on-disc test. Large parts of the surface are covered with tribofilm but somewhat less than surfaces with larger grain sizes.

Figure 15. As-sintered alumina surface of grain size 1-10 µm; (a) Unworn surface; (b) After 100 000 revolutions ball-on-disc test. The tribofilm includes some cracks and covers almost the entire wear track.

The surface roughness (Ra = 0.8 µm) of the as-sintered alumina surface of grain size 5-10 µm was coarser, without smaller grains filling up the larg- est cavities, Figure 16a. The as-sintered surfaces with smaller grains both showed Ra 0.4 µm.

Figure 16. As-sintered alumina surface of grain size 5-10 µm; (a) Unworn surface; (b) After 100 000 revolutions ball-on-disc test. The tribofilm includes some cracks and covers almost the entire wear track.

35 The Influence of Surface Treatment on the Tribofilm Formation With the objective to better understand the influence of surface pre-treatment on the tribological behaviour, as-sintered alumina surfaces of grain sizes 5-10 µm were treated in different ways prior to ball-on-disc tests. One surface was ground with 800 grit SiC grinding paper and the other was coarsely grit blasted with 200 µm SiC particles. The surface became rougher from the grit blasting, exhibiting many sharp protrusions and a roughness increase from Ra 0.8 to Ra = 1.4 µm, see Figure 17a. The grinding treatment reduced the Ra value to 0.1 µm, see Figure 17b.

Figure 17. Treated alumina surfaces of grain size 5-10 µm before wear; (a) Grit- blasted surface; (b) Ground surface. Compare with the as-sintered surface in Figure 16.

The many sharp protrusions on the grit blasted surface were crushed leading to an extensive formation of wear particles during initial sliding. This also resulted in formation of more and thicker tribofilms, see Figure 18a and c. This tribofilm was cracked and did not cover the entire wear track. Many loose wear particles were in vicinity of the tribofilm. The ground surface was further ground by the wear. Fewer wear particles were generated and no tribofilm was formed on top of the flat grains see Figure 18a and c. The plateaus in the wear track of the fine ground surface are extremely smooth, with an Ra of approximately 3 nm.

Figure 18. Wear track from the ball-on-disc test after 100 000 revolutions; (a) Grit blasted surface, the main part of the surface is covered with a thick tribofilm with cracks; (b) Ground surface, fine polished grains without visible tribofilm coverage; (c) Grit blasted surface, superficial cross-section using FIB over an area with two levels of a thick tribofilm; (d) Ground surface, superficial cross-section using FIB. Flat worn surface without visible tribofilm.

Tribofilm also formed on the balls that slid against the discs. The film covered about two thirds of the wear mark and it was thickest in the leading end. A thicker tribofilm formed on the ball that slid against the grit blasted surface than the one slid against the ground surface.

Tribofilm Formation in Water and Different Humidity The influence from presence of water or different levels of humidity was tested with as-sintered alumina discs (grain size 5-10 µm) that slid against polished alumina balls. It was revealed that water and the relative humidity strongly affected both wear rate and appearance of the tribofilm. The width of the wear tracks increased with increasing humidity (RH) and the widest track was formed on the disc tested in water. The tribofilm formed in dry air (RH 3%) only covered patches while that formed in humid air (RH 90%) covered almost the entire wear track, see Figure 19. The tribofilm formed in dry air was smooth within the individual patches, showed much less surface cracks and there was free debris along its edges. Further, debris agglomerated into cylinder shaped particles. The tribofilm formed in humid air exhibited a flake-like shape, some of which were more protruding and about to flake off. However, from a cross sectional view (not shown here), it seems like the tribofilms formed in dry and humid air have a similar structure.

Figure 19. Appearance of alumina surfaces after 100 000 revolutions in a ball-on- disc tests; (a) Tribofilm formed in dry air (RH 3%); (b) Tribofilm formed in humid air (RH 90%).

In the water lubricated tests, it initially seemed like there was a thin tribofilm formed on top of the grains but after further testing the grains were polished flat and only wear particles remained packed in cavities between grains, see Figure 20. The worn water lubricated surface was very similar to the worn ground surface, Figure 18b. In both cases, there was no tribofilm formation, due to lack of wear particles. The wear particles that initially formed in the water lubricated test were transported away by the water and in the absence of a protective film the surfaces became polished flat.

Figure 20. Appearance of alumina surface after 200 000 revolution in a water lubricated ball-on-disc test.

Hardness of the Tribofilm The hardness of the tribofilm (indent depth of 30 nm) was approximately 10 GPa, which is softer than that of the original alumina grains, with a hardness of approximately 35 GPa. The superficial hardness of the tribofilms showed a wide distribution, from 0.5 to approximately 50 GPa, especially for the shallowest indents of 30 and 50 nm. For those small indents the

38 probability to mainly hit single embedded wear particles within the tribofilm is relatively high. Such measurements will give values close to the hardness level of the original grains. Furthermore, the tribofilm formed in humid air (RH 90%) was softer than the tribofilm formed in dry air (RH 3%), see Figure 21.

Figure 21. Hardness of tribofilm formed in ball-on-disc tests. Distribution of individual nano hardness indents with 30 nm indentation depth and their representative mean values.

Chemical Composition of the Tribofilm The XPS analysis revealed no clear differences in the chemical state or composition between tribofilms formed in RH 60%, RH 3% and of unworn alumina surface. The Al2p peaks on the tribofilms formed in RH 3%, RH 60% and on the unworn surface were found at the same energy, 73.8 eV, after sputtering of the surface, see Figure 22, which is the expected energy for -alumina [75]. The peaks for the tribofilm formed in RH 60% as well as for the unworn surface from the most superficial measurements without sputtering, were somewhat broadened and shifted towards lower energies. If aluminium hydroxide would have been detected, the Al2p would be shifted towards higher energies than that of -alumina [75,76]. Sputtering 2 keV argon ions of 1 and 10 minutes corresponds to a etch depth of approximately 2.5 nm and 25 nm, respectively [77-79]. All of the surfaces (after 1 and 10 minutes of sputtering in the XPS) had an atomic concentration with an O/Al ratio of approximately 1.7, which is slightly higher than that of alumina at 1.5. However, the surfaces also contained carbon and the tribofilm formed in RH 60% had about twice as high carbon content as the other surfaces.

Figure 22. XPS spectra of tribofilms showing Al2p measured after three different times of sputtering; (a) Formed in dry air (RH 3%); (b)Formed in RH 60%; (c) Unworn alumina reference surface.

Friction All of the friction force measurements of alumina slid against alumina, irre- spective of test apparatus, surface treatment and humidity, showed a common initial value of the coefficient of friction. It was always approximately 0.2. Subsequently, most of the tests had a relatively rapid friction increase before stabilisation. From the friction measurements of the motor, it was found that the initial friction force, when the motors only had been running through a single stroke, was relatively low, about 0.2. After longer use, the friction level as well as the friction variation increased. The friction started to stabilise after about 2000 strokes, see Figure 23. By comparing the development of the coefficient of friction with the surface appearance of the drive rails, it was found that the tribofilm build-up correlates to the increase in friction.

Figure 23. Coefficient of friction vs. number of strokes of the motor. Each dot represents a mean value of the measurements along the whole drive rail.

Despite the different appearances of the worn surfaces with different grain sizes and surface treatments, only the grit blasted surface revealed a friction behaviour deviating from the others. The grit blasted surface had a mean friction of approximately 0.7, but with a scattering range of about 0.5-1. The mean friction level for the others was just below 0.4, however, the smoother surfaces showed less friction scattering, see Figure 24. The grit blasted surface also formed the thickest tribofilm while the ground surface did not form a tribofilm.

Figure 24. Friction force during 100 000 revolutions in a ball-on-disc test of various treatments of alumina surfaces.

The ambient humidity did have an effect on the friction between alumina and alumina. Tests in dryer atmospheres achieved higher friction. The average friction level was very similar for the medium humidity levels (20%, 40% and 60%), while in dry air (RH < 0.7%) it was distinctively higher. The lowest friction was found in water lubricated contacts, Figure 25. However, in many of the dryer tests high friction peaks appeared. The peaks were not just instant phenomena, the high values lasted for approximately 40 000- 50 000 revolutions.

Figure 25. Coefficient of friction from ball-on-disc tests at different test conditions. The bars indicate the mean value of the whole 200 000 revolutions tests while the lines indicate typical peak levels.

42 Friction Behaviour and Tribofilm Formation of TaC/a-C Coating

The tribofilm formation involved in the use of ceramic, low-friction coatings are of high importance for the friction as well as the wear of the uncoated counter surface. To gain a deeper understanding, the phenomena behind the influence of initial surface topography of a low-friction coating was investigated. In order to produce different substrate topographies, the high speed steel substrates were plasma etched prior to deposition of the TaC/a-C coating. The slower etching rate of the carbides results in an increasing roughness with increasing etching time. Here, four different etching times were employed. The protruding carbides on the substrate surface did affect the resulting coating topography. However, the protrusions on the coating were considerably lower than the protruding substrate carbides before coating, see Figure 26.

Figure 26. Surface appearance of sample before and after coating deposition; (a) 10 min etched high speed steel sample with protruding carbides. The height of the carbides is 100-180 nm; (b) TaC/a-C coating on the same steel substrate. The height of the protrusions formed on top of the carbides is 25-65 nm.

The friction measurements revealed an interesting relation between friction behaviour and coating topography, Figure 27. The coated surfaces with highest protrusions showed a higher initial friction level as well as a slower friction decrease. The rougher samples required longer time for the friction to stabilise.

Figure 27. Schematic illustration of the friction behaviour found for the coated surfaces against a steel ball. The duration of each stage was strongly dependent on the initial roughness.

Immediately when the ball starts sliding over the coated samples it gener- ates and picks up coating wear fragments. These accumulate primarily along the leading edge of the circular wear mark and thus constitute the first stage of tribofilm formation, see Figure 28a. Subsequently, the protrusions on the coating scratch and wear this tribofilm. The fragments then accumulate along the leading edges of the protrusions on the coating, see Figure 28b. For the smoothest coatings, the protrusions were quickly worn flat and after the test the entire wear track was smooth.

Figure 28. Surface appearance of surfaces sliding against each other after 5 revolutions; (a) Worn ball, sliding direction of the counter surface is indicated with an arrow, the insert is a close-up of the accumulated fragments along the leading edge; (b) Wear track of the coating on the high speed steel substrate etched for 20 min. A considerable amount of wear debris has been collected in front of the protrusions. Sliding direction of the counter surface was from top to bottom.

44 After further sliding, the amount of collected fragments increased and flattened the areas between the protrusions. During that time the coefficient of friction was still on a relatively high level and there was a substantial amount of debris in circulation. However, eventually the wear debris in circulation was ground down to finer sizes, the tribofilm on the ball became very smooth and consisted of fine ground particles, and the coefficient of friction started to decrease. The coefficient of friction reached its minimum and stable level (approximately at 0.1-0.15) when the protrusions of the coating surface had become smooth, the tribofilm of fine sized particles was mechanically stable and covering a large part area of the wear scar on the ball and very little debris was in circulation, see Figure 29. This means that the shearing only took place between the outermost layer of fine ground particles on the ball and the smooth coating surface.

Figure 29. Surface appearance of surfaces sliding against each other after 1000 revolutions; (a) Worn ball, sliding direction of the counter surface is indicated with an arrow. The insert is a close-up of the tribofilm where outermost layer consists of fine sized wear particles; (b) Wear track of the coating on the high speed steel substrate etched for 20 min. The surface is smoothened, the protrusions are flattened and wear fragments have filled up the area between the protrusions.

The surfaces with lower protrusions smoothened faster and did not scratch the transferred layer of the ball to the same extent as those with higher protrusions. Hence, less and finer wear particles were in circulation and the stable tribofilm of small particles built up faster. This contributed to a faster friction decrease and friction stabilisation.

45 Friction, Wear and Solubility of Silicon Nitride for Total Hip Joint Replacements

Evaluation of Bulk Silicon Nitride

In this work, bulk silicon nitride (Si3N4) has been evaluated as a material in total hip joint replacements. Silicon nitride is of interest because it is pre- dicted to show high wear resistance in water as well as produce soluble wear particles in vivo. This combination would minimise the risk of osteolysis and implant loosening and consequently prolong the implant life span.

Friction and Wear

The friction and wear properties of bulk Si3N4 against bulk Si3N4 for total hip joint replacements were tested with a ball-on-disc apparatus in PBS and bovine serum solutions. This material combination was compared to a CoCr disc slid against balls of Al2O3 and Si3N4. The friction levels for various materials and lubricant combinations were dramatically different, see Figure 30. There was also a difference in friction fluctuation. All the PBS lubricated tests started with a coefficient of friction around 0.4, but in the time that followed, the material combinations were divided into three distinct friction levels. For Si3N4 slid against Si3N4 in PBS, the coefficient of friction was at 0.01 except for the running-in period and some peaks which probably were connected to the refilling of deionised water (to make up for evaporation losses). In contrast to the PBS lubricated tests, all serum lubricated tests demonstrated the same friction behaviour with respect to both friction level and fluctuations, independent of material combination.

Figure 30. Coefficient of friction as a function of number of revolutions in the ball- on-disc test, in either PBS or a serum solution.

The amount of wear of the CoCr and Si3N4 discs depended on counter surface and lubricant. The CoCr discs exhibited a larger amount of wear than the Si3N4 discs, see Figure 31. The Si3N4 disc that slid against a Si3N4 ball in PBS had a significantly lower wear than the others, with a wear track width 2 of 150 µm, depth of 30 nm and a cross-sectional area of 3 µm . The Si3N4 disc that slid against a Si3N4 ball in the serum solutions had a larger wear track, with a width of 600 µm, a depth of about 1.4 µm and a cross sectional area of 670 µm2.

Figure 31. Optical profile images of worn discs and the calculated cross-sectional areas of the wear tracks. All the images have the same magnification except for the enlarged area of the Si3N4 disc that slid against a Si3N4 ball in PBS. The z-scale is magnified 8 times more than the x and y scales.

47 Tribofilm Formation

The XPS spectra of the wear track on the Si3N4 surface that slid against Si3N4 in PBS showed the presence of a thin SiO2/SiOx-OHy deposit, Figure 32. This tribofilm contributed to the extremely low coefficient of friction in the ball-on-disc test. The XPS analysis also revealed that the similarity in friction for all serum lubricated tests was due to the presence of the same type of tribofilm. The tribofilms on both surfaces consisted of metal oxide/hydroxide mixed with fragments of serum proteins. The amount of protein fragments was highest in the wear track, even if the areas outside the wear track also demonstrated traces of these fragments. The components C1, C2 and C3 in Figure 32 on the Si3N4 surface are the same as for those observed on the CoCr surface.

Figure 32. XPS spectra recorded in un-sputtered condition obtained from bovine and PBS lubricated Si3N4 surfaces that slid against Si3N4; (a) Si 2p peak, SiB is the bulk component and SiS is the component associated with SiO2/SiOx-OHy [80,81]; (b) N 1s peak; N1 is the bulk component and N2 is the component associated with C-NH2 groups [82]; (c) C 1s peak, C1 is C-C and C-H, C2 is C-O and C3 N-C=O bonds [82,83].

Solubility of Si3N4-powder Due to the small amount of wear and lack of an adequate method to collect wear particles of Si3N4, powder of Si3N4 was mixed with PBS in order to study the in vitro solubility of wear particles or small particles of Si3N4. The concentration of Si in the filtered PBS solutions was about 75 ml/l for all pH variations and also approximately the same for the different incuba- tion periods. This indicates that some of the Si3N4 powder was dissolved in PBS.

48 Evaluation of SixNy Coatings

Bulk Si3N4 has shown its potential as a wear resistant, resorbable alternative for total hip joint replacements. In order to improve the ductility of the implant construction, it would be favorable to deposit Si3N4 as a coating on a ductile material. CoCr is a material approved for hip joint replacements and it also has a sufficient hardness to support the coating and is therefore a good candidate to be used as substrate material under SixN4 coating.

Coating Design

Si3N4 has been shown to be soluble. However, the stability and reactivity of the wear particles can be affected by doping the Si3N4 structure. DFT (Density Functional Theory) calculations revealed that fragments of Si3N4 will be largely destabilised and more reactive to H and O with higher concentrations of dopant C. This means that the amount of C in the Si3N4 structure will control the resorbtion of wear particles from that surface. Therefore, C2H4-gas was introduced into the chamber during some of the coating processes, in order to dope the Si3N4 structure with C.

Coating Characterisation The coatings that were deposited had an N/Si ratio of about 1.5 (±0.3), which is higher than that of Si3N4 (1.33). The variation in N/Si ratio could not be correlated to the argon and/or nitrogen flow during the coating process. Furthermore, the coatings deposited with C2H4 introduced into the chamber had about the same carbon content as the rest of the coatings (3- 5%). Neither was there much difference in the nanostructure of the coatings, as is exemplified in Figure 33.

Figure 33. Surface appearance and microstructure of SixNy-coating; (a) Coating surface at two magnifications; (b) Cross-section of the coating showing the resulting columnar microstructure. The SEAD pattern in TEM produced ring-like patterns with sharp dark speckles, representing a nanocrystalline structure, Figure 34a. Via indexing, it was evident that all rings represented Si3N4. Furthermore, the high resolution TEM images exhibit lattice fringes consistent with a polycrystalline material, Figure 34b.

Figure 34. TEM diffraction pattern and image of SixNy-coating; (a) SAED pattern indicating Si3N4; (b) High resolution TEM image of SixNy-coating.

The crystallinity that was revealed in TEM could not be detected in XRD, probably due to the deficiency of detecting very small crystals with XRD [84]. The hardness of the coatings as measured with an indentation depth of 50 nm, ranged between 12 and 32 GPa with a mean value of approximately 22 GPa. This is similar to the superficial hardness of bulk Si3N4, approximately 23 GPa (with the same indentation depth). The CoCr substrates on which the coatings were deposited have a superficial hardness of approximately 8 GPa.

Friction and Wear of Coatings

The coatings were tested in a serum solution, slid against balls of bulk Si3N4. The friction and wear results were compared to bulk Si3N4 and bulk CoCr, also slid against balls of bulk Si3N4 in serum solution. There was no distinct difference in friction between the different types of coatings, bulk Si3N4 and bulk CoCr, Figure 35. However, for coatings that flaked off during the test, the coefficient of friction increased up to 0.45 and then eventually decreased to the same level as the intact coatings (not displayed in the figure).

Figure 35. Coefficient of friction during 10 000 revolutions for Si3N4-balls slid against the different SixNy-coatings, bulk Si3N4 and bulk CoCr in serum solution. The grey area represents the region containing all the coating curves, except for coatings that flaked-off.

The fine coating structure was gradually polished during sliding and the wear track became smooth after 10 000 revolutions, as exemplified in Figure 36. During some of the tests, the coatings cracked and flaked off in the wear track, which is exemplified in Figure 37. The flaking off was probably initi- ated by randomly distributed defects noted on several coatings. However, it was demonstrated that the coating adhered better to the Si substrate than to the CoCr substrate.

Figure 36. Surface appearance of the wear track on SixNy-coating (no. 8) after 10 000 revolutions at two different magnifications. The coating is smoothed out in the wear track.

Figure 37. Surface appearance of wear track where the coating has flaked off and the CoCr substrate is exposed and worn.

The coatings that did not flake off showed good wear resistance, almost matching that of bulk Si3N4, see Figure 38. The uncoated bulk CoCr showed a wear rate much higher than SixNy-coating on CoCr and bulk Si3N4. The superior wear resistance of the coating in comparison to bulk CoCr demonstrated the potential of these coatings for use on bearing surfaces of joint replacements.

Figure 38. Optical profile image of worn surfaces and the calculated cross-sectional areas of the wear tracks after 10 000 revolutions. The z-scale is magnified 23 times more than the x and y scales.

52 Conclusions

This thesis has provided insights and suggested technical solutions to achieve the desired friction and wear properties for ceramic surfaces. The work has generated a broad applicability to various applications but has here been exemplified by use in micro motors and hip joint replacements. Control of the tribofilm formation is fundamental for the friction and wear characteristics. This holds for tribofilms formed from wear particles as well as for tribofilms formed by deposition of products from reactions between the surface and the environment. Three different tribological systems with ceramic surfaces have been investigated. Most of the friction and wear tests were performed in a ball-on- disc apparatus and the subsequent surface analyses were essential for acquiring the knowledge about the tribofilm formation.

Alumina against Alumina for the Friction Drive System in an Ultrasonic Motor

The tribofilm that is built up on the as-sintered alumina (Al2O3) surfaces fills up cavities, smoothens the surface and has shown to bond intimately to the underlying alumina grains. There is a strong correlation between the build- up of tribofilm and the increase in friction. The tribofilm is softer than the original surface and consequently a thicker tribofilm gives higher friction due to a larger area of real contact and more deformation. The tribofilm formation depends on the amount and size of the wear particles generated. Polished surfaces or water lubricated surfaces exhibit no or very thin tribofilms. Hence these surfaces exhibit lower friction. In addition, increased surrounding humidity decreases the coefficient of friction. The appearance of the tribofilms differs as a consequence of different humidity. However, there was no obvious difference in their chemical composition. The strongest ultrasonic motors with a friction drive system of alumina will be found in dry conditions, where the alumina surfaces initially were made rough to promote the formation of a thick tribofilm.

TaC/a-C against Steel for Various Machine Elements It was revealed that even if the coating is relatively thick compared to the microscopic protrusions of the substrates, the final topography of the coating surface will be affected. The friction depends on the surface topography, with the smoother surfaces causing a faster tribofilm build-up on the counter

53 surface (of steel) and subsequently a faster friction decrease. The protrusions of the rougher surfaces scraped off the tribofilm from the steel ball, which resulted in more material circulation, longer time for stabilisation of the tribofilm and consequently longer time for friction decrease and stabilisation.

Silicon Nitride against Silicon Nitride in Hip Joint Replacements

In PBS, silicon nitride (Si3N4) against silicon nitride showed both lower wear rate and lower friction than cobalt chromium (CoCr) against silicon nitride and CoCr against alumina, due to the build-up of a tribofilm of SiO2/SiOx- OHy. In serum solution the friction was independent of material combination, due to build-up of similar tribofilms of proteins originating from the serum solution. However, the wear of silicon nitride was still much lower than that of CoCr. SixNy-coatings were deposited on CoCr substrates in order to provide the total hip joint replacement with a more ductile construction. The coatings were amorphous or nanocrystalline and exhibited the same friction level as silicon nitride and CoCr in a bovine solution. The coatings had a relatively poor adhesion to the substrates but demonstrated almost as good wear resistance as bulk silicon nitride. Furthermore, Si3N4 powder was shown to be soluble in PBS, which implies that the wear particles from a nano-crystalline Si3N4 coating also are resorbable. The small amount of wear particles that would be released from the joint contact in the body will thus eventually be resorbed. This could reduce the problems with inflammation and osteolysis connected to wear particles.

Tribological systems inevitably change with wear. Rather than trying to avoid these changes, a better strategy can be to design systems that, after the expected changes, achieve the desired friction and wear properties.

54 Sammanfattning på svenska (Summary in Swedish)

Friktions- och nötningsmekanismer hos keramiska ytor - För användning i mikromotorer och höftledsimplantat När två ytor glider mot varandra uppstår det friktion och det kan hända att någon eller båda ytorna nöts. Ytornas översta lager deformeras och deras egenskaper förändras ofta efter bara en kort stunds nötning. Den här avhand- lingen beskriver hur keramiska ytor kan utnyttja innötningsprocessen för att skapa de önskade egenskaperna. Tribologi kallas den vetenskap som innefattar friktion, nötning och smörj- ning för ytor i relativ rörelse. Friktionskraften (FF) verkar som bromsande då en yta rör sig i förhållande till en annan. Den är beroende av normallasten (FN) och förhållandet där emellan kallas friktionskoefficienten, den beteck- nas med µ och beräknas enligt:

F µ F (1) FN

Friktionskoefficienten och nötningen för två ytor som möts varierar beroende på materialkombination och dess omgivning. Friktionskoefficienten är därför inte en materialparameter, den är en systemparameter! Exempelvis så är en fotbollsplan hal när det regnar och den nöts mycket när vattnet gjort planen alltför mjuk. Medan när planen är hård och saknar gräs så är det dobbarna på fotbollsskorna som nöts. Optimalt grepp/friktion - uppstår när planen är mjuk nog för att dobbarna ska sjunka in något men fast nog för att inte trasas sönder vid snabba stopp eller starter. Den får heller inte vara så blöt så att friktionen mot gräset blir alltför låg. När ytor växelverkar med varandra uppstår det lokalt höga spänningar och temperaturer, viket gör att ytorna lokalt går sönder samt att kemiska reaktioner accelereras. Dessa händelser gör att nya produkter bildas på ytorna, så kallade tribofilmer. Dessa filmer ger ytan nya egenskaper och fungerar ofta som ett skyddande lager. Tribofilmerna kan vara uppbyggda av nötningspar- tiklar från de interagerande ytorna eller av reaktionsprodukter, som skapats via reaktioner mellan ytorna och omgivningen.

55 Keramiska material anses generellt ha låg kemisk reaktivitet, vara hårda, nötningsbeständiga och de används därför i många tribologiska kontakter. Nötningen av keramer domineras av sprickbildning, men kemiska reaktioner och plastisk deformation förekommer också. Nackdelen med keramer är att de är spröda och svaga när de utsätts för dragkrafter. För att minska sprödhe- ten hos komponenten men samtidigt behålla en hård och nötningsbeständig yta, kan lösningen vara att deponera en keramisk beläggning på ett metalliskt substrat. Kraften hos en mikromotor som använder sig av ett friktionsdrivsystem av aluminiumoxid (Al2O3) för att skapa en linjär rörelse, är begränsad av friktionskraften i systemet. Motorn är inte starkare än den lägsta friktionskraften som uppstår mellan de greppande ytorna i friktionsdrivsystemet. De aktiva ytorna i friktionsdrivsystemet är av aluminiumoxid, som är obehand- lad efter sintring. Detta material är valt för att det har låg kemisk reaktivitet, ger en relativt hög och stabil friktionskraft samt är nötningsbeständigt. När dessa ytor nöts mot varandra bildas det ett nytt skikt. Skiktet består av nöt- ningspartiklar från båda ytorna som sintrats samman till en så kallad tribofilm. Filmen fyller upp gropar och på så vis slätas ytan ut. Denna film bildas relativt tidigt under nötningsförloppet och bidrar till att friktionen ökar, dvs. motorn blir starkare. Genom att använda en grövre yta med många spetsiga toppar genereras initialt fler nötningspartiklar. Mer material finns då i omlopp och en tjockare tribofilm bildas, vilket också ökar friktionskraften. I system där låg friktion eftersträvas, är det vanligt att ena ytan beläggs med en keramisk lågfriktionsbeläggning innehållande amorft kol. Vid nöt- ning av beläggningen verkar det amorfa kolet som ett fast smörjmedel och sänker friktionen. För att studera ytjämnheternas betydelse för bildandet av tribofilm, deponerades TaC/a-C-beläggningar* på snabbstål av olika ytfinhet. Friktionstester och efterföljande ytanalys visar att material från beläggningen överförs till den motstående stålytan. I fallet med slätare beläggningar påbör- jas en finfördelning och kompaktering av den överförda filmen, vilket bidrar till en friktionssänkning. För beläggningar med grövre topografi skrapar beläggningens yttoppar av delar av den överförda filmen, vilket skapar mycket nötningsmaterial i omlopp som bidrar till en friktionshöjning. Varef- ter beläggningen slätas ut finfördelas och kompakteras tribofilmen på den motstående stålytan och friktionen sjunker så småningom till samma nivå som för de slätare beläggningsytorna. Keramiska ytor har även potential att förbättra friktions- och nötnings- egenskaper hos höftledsimplantat. Dagens höftledsimplantat har en livslängd i kroppen på ca 15 år innan de behöver bytas ut. En av de viktigaste orsaker- na till utbyten är ansamlingen av nötningspartiklar runt leden. Dessa nöt- ningspartiklar orsakar inflammationer som leder till osteolys (tillbakabild-

* TaC/a-C-beläggning: Beläggning som innehåller tantalkarbider (TaC) i en matris av amorft kol (a-C).

56 ning eller upplösning av benvävnad), vilket gör att implantatet lossnar och behöver bytas ut. Kiselnitrid (Si3N4) har visat sig ha högt nötningsmotstånd och ger låg friktion. Dessutom visar arbetet att de få och små nötnings- partiklar som trots allt bildas, kan förväntas lösas upp i kroppen. Detta skulle minimera risken för osteolys, vilket skulle öka livslängden för höftledsim- plantatet i kroppen. I nötnings - och friktionstester gav Si3N4 mot Si3N4 en betydligt lägre friktion och nötning i PBS* (syntetisk kroppsvätska), än andra tänkbara material- kombinationer som kobolt krom (CoCr) mot Si3N4 eller mot Al2O3. Detta orsakas av att det bildas en lättskjuvad tribofilm på Si3N4-ytorna. Filmen består av reaktionsprodukter av Si3N4-ytan och PBS-lösningen. För tester utförda in en lösning av serum† var friktionen däremot oberoende av materialkombination, då samma sorts tribofilm bildades på ytorna i alla fall. Däremot uppvisade materialkombinationen Si3N4 mot Si3N4 klart mindre nötning än CoCr mot Si3N4. Trots att Si3N4 är segt för att vara en keram, är det säkrare att använda en beläggning av Si3N4 på ett metalliskt substrat för att undvika ett katastrofalt sprödbrott. I detta fall deponerades amorfa/nanokristallina Si3N4-belägg- ningar på skivor av CoCr, vilket är ett väl beprövat material för höftledsim- plantat. Nötnings- och friktionstester av beläggningarna samt bulk Si3N4 och CoCr utfördes mot bulk Si3N4 i en lösning av serum. Det visade sig att friktionen var på samma nivå för alla tre materialkombinationerna. Däremot var bulk Si3N4 och beläggningarna betydligt mer nötningsbeständiga än CoCr. Beläggningarna av amorf/nanokristallin Si3N4 har potential som material för höftledsimplantat, då de ger ifrån sig få nötningspartiklar som dessutom kan lösas upp i kroppen.

Denna avhandling visar tydligt att nötningen av keramiska system ändrar de aktiva ytornas egenskaper. Den påvisar också potentialen i att skapa system som utnyttjar nötningens ytomvandling, i stället för att försöka förhindra förändringen. Med rätt förutsättningar kan man låta nötningen skapa de öns- kade egenskaperna för varje applikation, så som hög friktion, låg friktion eller små och ofarliga nötningspartiklar.

* PBS: Förkortning för Phosphate Buffered Saline som är en syntetisk kroppsvätska med samma salthalt som kroppen, saknar proteiner och har en viskositet liknande vatten. † Serum: I detta fall är det en animalisk produkt som innehåller proteiner. Lösningen som användes är en blandning av serum och avjonat vatten.

57 Acknowledgements

Det här hade varit mycket svårare och inte lika roligt utan all hjälp och stöd från Er andra, ett extra varmt tack till min handledare Staffan, för allt du har lärt mig om allt från tribologi till nya ord som pepplig och för att jag fått sitta på ditt rum och diskuterat nötnings- mekanismer min andra handledare Håkan, för inspiration och för att du pushat mig i mitt arbete min f.d. handledare och chef Stefan, för att du gav mig chansen att få börja på ett intressant projekt

Cecilia, för att du alltid tar dig tid, är snabb på att korrekturläsa och kommer med bra kommentarer

Thank you Nico Teuscher, for helping me with PVD sputtering, taking care of me in the lab, as well as inviting me to your home and your family

Thank you Andreas Heilmann, for letting me work in your lab at Fraunhofer IWM

Mårten Ohlsson, för all hjälp på PiezoMotor bl.a. med friktionstest- utrustningen

Maria, för ett bra samarbete och trevligt sällskap i labbet

Urban, Julia och Harald, för bra jobb och intressanta diskussioner om tribo- filmens uppbyggnad under sena kvällar

Åsa och Sture, för att ni alltid tar er tid och bidrar med bra diskussioner

Nils, som hjälpt mig med ESCA-undersökningar

Frida, för hjälp med mycket som rört ESCA, för trevligt ressällskap, för många, alltför långa tepauser men inte minst för att du är en bra vän

Jonatan, för att du har fixat alla mina små och stora dataproblem

Janne, för all praktiskt hjälp och stöd i labbet, både innan och efter att något har gått sönder

Fredric, för hjälp med diverse analysutrustning

Wei, for always having time to help me with lab work, answering questions, and with XRD

Kathryn, för att du tog dig tid att korrekturläsa kappan

Karin och Anja, för all administrativ hjälp

Peter, Julia, Fredrik, Martina, Harald, Maria, Janna och Petra, för att ni har varit som en extrafamilj i Ångströmvärlden och för alla roliga resor vi har varit på

MiM:are, MikMek:are, ÅSTC:are och JonDeTech:are, för att ni har varit med och bidragit till att jag trivts så bra på jobbet, speciellt i fikasoffan

PiezoMotorkollegorna, för en bra tid på PiezoMotor vännerna utanför Ångström, för att det är så roligt att umgås med er

Sala-tjejerna, som har stöttat mig i 25 år

Jens och Kristina, för allt stöd och skratt både under plugg och fritid

Mamma och Pappa, för all kärlek och för att ni alltid ställer upp för mig min älskade Benny, för din kärlek och för att ditt tålamod alltid tar vid där mitt slutar – du är bäst!

Uppsala, augusti 2011

59 References

1. Hutchings IM. Tribology, Friction and Wear of Engineering Materials. London: Edward Arnold, a division of Hodder Headline PLC; 1992. 2. Jacobson S, Hogmark S. Tribofilms - on the Crucial Importance of Tri- bologically Induced Surface Modifications. In: Nikas G, editor. Recent De- velopments in Wear Prevention, Friction and Lubrication. Trivandrum, In- dia: Research Signpost; 2010. p 197-225. 3. Biswas SK. Some mechanisms of tribofilm formation in metal/metal and ceramic/metal sliding interactions. Wear 2000;245(1-2):178-189. 4. Fischer TE, Zhu Z, Kim H, Shin DS. Genesis and role of wear debris in sliding wear of ceramics. Wear 2000;245(1-2):53-60. 5. Blomberg A, Hogmark S, Lu J. An electron microscopy study of worn ceramic surfaces. Tribology International 1993;26(6):369-381. 6. Liu Y, Erdemir A, Meletis EI. A study of the wear mechanism of diamond- like carbon films. Surface and Coatings Technology 1996;82(1-2):48-56. 7. Minfray C, Martin JM, Esnouf C, Le Mogne T, Kersting R, Hagenhoff B. A multi-technique approach of tribofilm characterisation. Thin Solid Films 2004;447-448:272-277. 8. Willermet PA, Dailey DP, Carter RO, Schmitz PJ, Zhu W. Mechanism of formation of antiwear films from zinc dialkyldithiophosphates. Tribology International 1995;28(3):177-187. 9. Wiklund U, Hutchings IM. Investigation of surface treatments for galling protection of titanium alloys. Wear 2001;251(1-12):1034-1041. 10. Podgornik B, Hogmark S, Sandberg O. Influence of surface roughness and coating type on the galling properties of coated forming tool steel. Surface and Coatings Technology 2004;184(2-3):338-348. 11. Chen M, Kato K, Adachi K. Friction and wear of self-mated SiC and Si3N4 sliding in water. Wear 2001;250(1-12):246-255. 12. Andersson P, Blomberg A. Alumina in unlubricated sliding point, line and plane contacts. Wear 1993;170(2):191-198. 13. Buckley DH. Friction Characteristics in Vacuum of Single Polycrystalline Aluminum Oxide in Contact with Themselves and with Various Metals. In: Jahanmir S, editor. Tribology of Ceramics, Fundamental. Park Ridge: Soci- ety of Tribologists and Lubriction Engineers; 1987. 14. Andersson P, Holmberg K. Limitations on the use of ceramics in unlubricated sliding applications due to transfer layer formation. Wear 1994;175(1-2):1-8. 15. Kato K, Adachi K. Wear of advanced ceramics. Wear 2002;253(11- 12):1097-1104.

60 16. Zum Gahr KH, Schneider J. Surface modification of ceramics for improved tribological properties. Ceramics International 2000;26(4):363-370. 17. Blomberg A, Olsson M, Hogmark S. Wear mechanisms and tribo mapping of Al2O3 and SiC in dry sliding. Wear 1994;171(1-2):77-89. 18. Chaiwan S, Hoffman M, Munroe P, Stiefel U. Investigation of sub-surface damage during sliding wear of alumina using focused ion-beam milling. Wear 2002;252(7-8):531-539. 19. Zum Gahr KH, Bundschuh W, Zimmerlin B. Effect of grain size on friction and sliding wear of oxide ceramics. Wear 1993;162-164(Part 1):269-279. 20. Ajayi OO, Ludema KC. Mechanism of transfer film formation during re- peat pass sliding of ceramic materials. Wear 1990;140(2):191-206. 21. Adachi K, Kato K. Formation of smooth wear surfaces on alumina ceramics by embedding and tribo-sintering of fine wear particles. Wear 2000;245(1-2):84-91. 22. Sasaki S. The effects of the surrounding atmosphere on the friction and wear of alumina, zirconia, silicon carbide and silicon nitride. Wear 1989;134(1):185-200. 23. Singha Roy R, Basu D, Chanda A, Mitra M, K. Distinct Wear Characteris- tics of Submicrometer-Grained Alumina in Air and Distilled Water: A Brief Analysis on Experimental Observation. Journal of the American Ceramic Society 2007;90(9):2987-2991. 24. Chaiwan S, Hoffman M, Munroe P. Investigation of sliding wear surfaces in alumina using transmission electron microscopy. Science and Technol- ogy of Advanced Materials 2006;7(8):826-833. 25. Gee MG. The formation of aluminium hydroxide in the sliding wear of alumina. Wear 1992;153(1):201-227. 26. Tomizawa H, Fischer TE. Friction and Wear of Silicon Nitride and Silicon Carbide in Water: Hydrodynamic Lubrication at Low Sliding Speed Ob- tained by Tribochemical Wear. ASLE Transactions 1986;30(1):41-46. 27. Xu J, Kato K. Formation of tribochemical layer of ceramics sliding in water and its role for low friction. Wear 2000;245(1-2):61-75. 28. Vlack LHV. Physical Ceramics for Engineers: Addison-Wesely Publishing Company; 1964. 29. Callister WD. Materials Science and Engineering - An Introduction. New York: John Wiley & Sons, Inc.; 2000. 30. Teer DG. Evaporation and Sputter Techniques. In: E L, editor. Coatings for High Temperature Applications New York: Elsevier Science Publishing Co., Inc.; 1983. 31. Harlin P, Bexell U, Olsson M. Influence of surface topography of arc- deposited TiN and sputter-deposited WC/C coatings on the initial material transfer tendency and friction characteristics under dry sliding contact conditions. Surface and Coatings Technology 2009;203(13):1748-1755. 32. Nordin M, Ericson F. Growth characteristics of multilayered physical vapour deposited TiN/TaNx on high speed steel substrate. Thin Solid Films 2001;385(1-2):174-181.

61 33. Nilsson D, Svahn F, Wiklund U, Hogmark S. Low-friction carbon-rich carbide coatings deposited by co-sputtering. Wear 2003;254(11):1084- 1091. 34. Nilsson D, Wiklund U. A flexible laboratory-scale approach to alloying and tailoring of thin films by single-magnetron co-sputtering. Thin Solid Films 2004;467(1-2):10-15. 35. Taylor GW, Gagnepain JJ, Meeker TR, Nakamura T, Shubalov LA. Piezo- electricity. Berkshire: Gordon and Breach Science Publishers; 1985. 3-7 p. 36. Dorey AP, Moore JH. Advances in Actuators. Bristol and Philadelphia: Institute of Physics Publishing; 1995. 117-121 p. 37. Ratner BD, Hoffman AS, Schoen FJ, Lemons JE. Biomaterials Science - An Introduction to Materials in Medicine. San Diego: Elsevier Science 1996. 445-447 p. 38. Garrett WE, Jr., Swiontkowski MF, Weinstein JN, Callaghan J, Rosier RN, Berry DJ, Harrast J, Derosa GP, the Research Committee of The American Board of Orthopaedic S. American Board of Orthopaedic Surgery Practice of the Orthopaedic Surgeon: Part-II, Certification Examination Case Mix. J Bone Joint Surg Am 2006;88(3):660-667. 39. Kurz S, Ong K, Lau E, Mowat F, Halpern M. Projections of Primary and Revision Hip and Knee Arthroplasty in the United States from 2005 to 2030. The Journal of Bone & Joint Surgery 2007;89(4). 40. Sargeant A, Goswami T. Hip implants: Paper V. Physiological effects. Materials & Design 2006;27(4):287-307. 41. Dixon T, Shaw M, Ebrahim S, Dieppe P. Trends in hip and knee joint replacement: socioeconomic inequalities and projections of need. Annals of the Rheumatic Diseases 2004;63(7):825-830. 42. Landgraeber S, von Knoch M, Löer F, Wegner A, Tsokos M, Hußmann B, Totsch M. Extrinsic and intrinsic pathways of apoptosis in aseptic loosening after total hip replacement. Biomaterials 2008;29(24-25):3444-3450. 43. Bizot P, Sedel L. Alumina bearings in hip replacement: Theoretical and practical aspects. Operative Techniques in Orthopaedics 2001;11(4):263- 269. 44. Xiong D, Ge S. Friction and wear properties of UHMWPE/Al2O3 ceramic under different lubricating conditions. Wear 2001;250(1-12):242-245. 45. Derbyshire B, Fisher J, Dowson D, Hardaker C, Brummitt K. Comparative study of the wear of UHMWPE with zirconia ceramic and stainless steel femoral heads in artificial hip joints. Medical Engineering & Physics 1994;16(3):229-236. 46. Aspenberg P, Anttila A, Konttinen YT, Lappalainen R, Goodman SB, Nordsletten L, Santavirta S. Benign response to particles of diamond and SiC: bone chamber studies of new joint replacement coating materials in rabbits. Biomaterials 1996;17(8):807-812. 47. Bozic KJ, Ries MD. Wear and Osteolysis in Total Hip Arthroplasty. Semi- nars in Arthroplasty 2005;16(2):142-152. 48. Sargeant A, Goswami T. Hip implants - Paper VI - Ion concentrations. Materials & Design 2007;28(1):155-171.

62 49. Willmann G, Früh HJ, Pfaff HG. Wear characteristics of sliding pairs of zirconia (Y-TZP) for hip endoprostheses. Biomaterials 1996;17(22):2157- 2162. 50. Chevalier J, Gremillard L. Ceramics for medical applications: A picture for the next 20 years. Journal of the European Ceramic Society 2009;29(7):1245-1255. 51. Yen SK, Guo MJ, Zan HZ. Characterization of electrolytic ZrO2 coating on Co-Cr-Mo implant alloys of hip prosthesis. Biomaterials 2001;22(2):125-133. 52. Fisher J, Hu X, Tipper J, Stewart T, Williams S, Stone M, Davies C, Hatto P, Bolton J, Riley M and others. An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface-engineered femoral heads. Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 2002;216(4):219-230. 53. Österle W, Klaffke D, Griepentrog M, Gross U, Kranz I, Knabe C. Poten- tial of wear resistant coatings on Ti-6Al-4V for artificial hip joint bearing surfaces. Wear 2008;264(7-8):505-517. 54. Bal BS, Khandkar A, Lakshminarayanan R, Clarke I, Hoffman AA, Raha- man MN. Fabrication and Testing of Silicon Nitride Bearings in Total Hip Arthroplasty: Winner of the 2007 "HAP" PAUL Award. The Journal of Ar- throplasty 2009;24(1):110-116. 55. Cappi B, Neuss S, Salber J, Telle R, Knüchel R, Fischer H. Cytocompati- bility of high strength non-oxide ceramics. Journal of Biomedical Materials Research Part A 2010;93A(1):67-76. 56. Neumann A, Unkel C, Werry C, Herborn CU, Maier HR, Ragoß C, Jahnke K. Prototype of a silicon nitride ceramic-based miniplate osteofixation system for the midface. Otolaryngology - Head and Neck Surgery 2006;134(6):923-930. 57. Guedes e Silva CC, Higa OZ, Bressiani JC. Cytotoxic evaluation of silicon nitride-based ceramics. Materials Science and Engineering: C 2004;24(5):643-646. 58. Zhmud BV, Bergström L. Dissolution Kinetics of Silicon Nitride in Aque- ous Suspension. Journal of Colloid and Interface Science 1999;218(2):582- 584. 59. Czichos H, Becker S, Lexow J. International multilaboratory sliding wear tests with ceramics and steel. Wear 1989;135(1):171-191. 60. Podgornik B, Jacobson S, Hogmark S. DLC coating of boundary lubricated components--advantages of coating one of the contact surfaces rather than both or none. Tribology International 2003;36(11):843-849. 61. Dulbecco R, Vogt M. Plaque Formation and Isolation of Pure Lines with Poliomyelitis Viruses. The Journal of Experimental Medicine 1954;99(2):167-182. 62. ASTM, Standards. Standard test for Wear Testing of Polymeric Materials Used in Total Joint Prostheses. ASTM International 2003;F 732-00. 63. Johnson KL. Contact mechanics. Cambridge: The press syndicate of the university of Cambridge; 1985. 90-104 p.

63 64. Atar E, Çimenoglu H, KayalI ES. Hardness characterisation of thin Zr(Hf,N) coatings. Surface and Coatings Technology 2003;162(2-3):167- 173. 65. Oliver WC, Pharr GM. An improved technique for determing hardness and elastic modulus using load and displacement sensing indentation experi- ments. Journal of Materials Research 1992;7(6):1564-83. 66. Konttinen YT, Takagi M, Mandelin J, Lassus J, Salo J, Ainola M, Li T-F, Virtanen I, Liljeström M, Sakai H and others. Acid Attack and Cathepsin K in Bone Resorption Around Total Hip Replacement Prosthesis. Journal of Bone and Mineral Research 2001;16(10):1780-1786. 67. Goldstein JI, Newbury DE, Echlin P, Joy DC, Fiori C, Lifshin E. Scanning Electron Microscopy and X-Ray Microanalysis. New York: Plenum Press; 1981. 68. Williams DB, Carter CB. Transmission Electron Microscopy. New York: Plenum Press; 1996. 69. Giannuzzi LA, Stevie FA. Introduction to Focused Ion Beams. Boston, MA: Springer US; 2005. 70. Gwidon W. Stachowiak AWBaGBS. 6 Characterization of Test Specimens. Tribology and Interface Engineering Series: Elsevier; 2004. p 115-150. 71. Vickerman JC, Gilmore IS. Surface Analysis The principal Techniques. Chichester: John Wiley & Sons Ltd; 2009. 511-523 p. 72. Woodruff DP, Delchar TA. Modern Techniques of Surface Science. Cam- bridge: University Press; 1994. 73. Briggs D, Seah MP. Practical Surface Analysis West Sussex: John Wiley & Sons, Ltd; 1983. 143-145 p. 74. Hill SJ. Inductively Coupled Plasma Spectrometry and its Applications. Oxford: Blackwell Publishing Ltd; 2007. 75. Moulder JF, Sobol WF, Bomben KD. Handbook of X-ray Photoelectron Spectroscopy. Minnesota: Physical Electronics Inc; 1995. 170-171, 40-41 p. 76. Thomas S, Sherwood PMA. Valence Band Spectra of Aluminum Oxides, Hydroxides, and Oxyhydroxides Interpreted by X Calculations. Analytical Chemistry 1992;64(21):2488-2495. 77. Orlinov V, Mladenov G, Petrov I, Braun M, Emmoth B. Angular distribution and sputtering yield of Al and Al2O3 during 40 key argon ion bombardment. Vacuum 1982;32(12):747-752. 78. Persson A, Ericson F, Thornell G, Nguyen H. Etch-stop technique for pat- terning of tunnel junctions for a magnetic field sensor. Journal of Micro- mechanics and Microengineering 2011;21(4). 79. NPL. Ar Sputter Yields, Group 1, Li to Si, 45. NPL; 2005. 80. Ingo GM, Zacchetti N, della Sala D, Coluzza C. X-ray photoelectron spectroscopy investigation on the chemical structure of amorphous silicon nitride (a-SiN[sub x]). Journal of Vacuum Science & Technology A: Vac- uum, Surfaces, and Films 1989;7(5):3048-3055. 81. Taylor JA. Further examination of the Si KLL Auger line in silicon nitride thin films. Applications of Surface Science 1981;7(1-2):168-184.

64 82. Vanea E, Simon V. XPS study of protein adsorption onto nanocrystalline aluminosilicate microparticles. Applied Surface Science 2011;257(6):2346-2352. 83. Hodgson AWE, Kurz S, Virtanen S, Fervel V, Olsson COA, Mischler S. Passive and transpassive behaviour of CoCrMo in simulated biological solutions. Electrochimica Acta 2004;49(13):2167-2178. 84. Klug DHP, Alexander LE. X-Ray Diffraction Procedures For Polycrystal- line and Amorphous Materials. New York: John Wiley & Sons. Inc; 1954. 491 p.