Solving Problems in Surface Engineering and Tribology by Means of Analytical Electron Microscopy

Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 12

ERNESTO CORONEL

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List of Papers

I The effect of carbon content on the microstructure of hydrogen- free titanium carbide films grown on high speed steel by physical vapour deposition E. Coronel, U. Wiklund and E. Olsson, In manuscript

II Microstructure of d.c. magnetron sputtered TiB2 coatings M. Berger, E. Coronel and E. Olsson, Surface & Coatings Technol- ogy, 180, (2004), 240-244

III An analytical TEM study of (Ta,Al)C:C coatings in as-deposited and oxidised states E. Coronel, D. Nilsson, S. Csillag and U. Wiklund, In manuscript

IV TEM studies of tribofilm formation - compatibility between two metal doped DLC coatings and lubricant E. Coronel, N. Stavlid and U. Wiklund, In manuscript

V On the wear mechanisms of CVD diamond when sliding in nitrogen and argon atmosphere E. Coronel and J. Andersson and U. Wiklund, In manuscript

VI Surface analysis of laser cladded Stellite exposed to self-mated high load dry sliding D. Persson, E. Coronel, S. Jacobson and S. Hogmark, Submitted to Wear

VII Wear induced material modification of cemented carbide rock drill buttons U. Beste, E. Coronel and S. Jacobson, Accepted for publication in International journal of refractory metals and hard materials

The author of this thesis has performed all high resolution analytical electron microscopy of all papers.

Contents

Part I Background 9 I.1 Introduction 11 I.1.1 Tribology 11 I.1.2 Surface Modification 12 I.1.3 The Active Surface 13 I.1.4 Tribological Testing 13 General 13 Tests used in this thesis 14 I.1.5 Tribofilm Analysis 15 I.2 Surface Engineering 17 I.2.1 Deposition of Coatings 17 Magnetron Sputtering 17 Electron beam evaporation 18 CVD 19 Laser Powder Cladding 20 I.3. Analytical Microscopy 21 I.3.1 Electron Scattering 21 Elastic Scattering 22 Inelastic Scattering 25 I.3.2 Microscopical Techniques 26 Transmission ElectronMicroscopy 27 Scanning Electron Microscopy 28 Focused Ion Beam 28 Conventional TEM sample preparation 30 I.3.3 Energy Dispersive X-ray Spectroscopy 31 I.3.4 Electron Energy Loss Spectroscopy 32 Thickness measurements 34 Relative Quantification 35 I.3.5 Energy Filtered TEM 36 Three Windows Technique 37

Part II Contributions 39 II.1 Introduction 41 II.2 Structure and composition of as-deposited and heat treated thin coatings (Papers I, II, III) 41 II.2.1 Effect of carbon content on the microstructure of TiC coatings (Paper I) 41 Conclusions 45 II.2.2 Effect of substrate bias on the microstructure of TiB2 coatings (Paper II) 45 Conclusions 47 II.2.3 The role of Al on structure and stability of (Ta,Al)C:C coatings (Paper III) 47 As-deposited coating 48 Heat treated coatings 50 Conclusions 51 II.3 Revealing mechanisms of friction and wear (Papers IV, V, VI) 53 II.3.1 Mechanisms of tribofilm formation for lubricated Me-DLC coatings (Paper IV) 53 Surface morphology of tribofilms 54 Structure of tribofilms 54 Composition of tribofilms 55 Conclusions 58 II.3.2 Wear mechanisms of diamond surfaces in different atmospheres (Paper V) 59 Sliding tests 59 Analysis of wear debris 60 Internal film characterizations 61 Composition of diamond surface 61 Conclusions 62 II.3.3 Friction mechanisms of Stellite 21 (Paper VI) 62 Sample preparation 62 Microstructure 63 Composition 63 Conclusions 65 II.4 Solution of a practical problem-Wear of cemented carbide in rock drilling (PaperVII) 67 II.4.1 Rock drilling- A tough application of cemented carbide 67 II.4.2 Sample Preparation 68 II.4.3 Wear mechanisms of drill buttons 68 Drilling quartzitic granite 68 Drilling magnetite 70 II.4.4 Conclusions 70 II.5 Summary of Contributions 71 II.6 Future challenges of HR-TEM 72 Sammanfattning på svenska 73 Bakgrund 73 Bidrag och resultat 74 Acknowledgements 77 References 79 Part I Background

I.1 Introduction

The development of new materials and surface layers is one of the most sig- nificant motors for the technical progress of our society. New materials may be developed by curiosity or even by chance, but much more often as a respond to pronounced demands. The demands are of several categories; economical (longer life, lower price, easier to machine, etc.) technical (higher performance, lighter, stronger, less prone to wear and corrosion, etc.) and – today of increasing importance – environmental and health related (non- toxic, renewable, etc.). Successful development of new and improved materials is a complicated activity, requiring the collected efforts and skills from people of several fields of expertise. The development is always iterative, including several loops of material synthesis, testing, and modifications based on the evaluation. The research behind the present thesis encompasses work with coating synthesis and tribological testing and evaluation based on a solid knowledge about the inner structure and composition of the materials. By employing advanced surface analysis and high-resolution microscopy, the materials development and tribological evaluation processes are made much more efficient. Moreover, this evaluation can be performed both on the as-synthesised material and after tribological testing or practical use. Knowl- edge on how the material interacts on the atomic scale with the counter surfaces, provides even more information and gives hints on how to make the materials even better suited to performing their intended tasks.

I.1.1 Tribology Tribology covers the science and technology of surfaces in contact and relative motion. This broad definition makes it extremely common and hence an important field of study. It includes friction, wear and lubrication and associated surface layers on the contacting bodies1. Tribology is also of great economic significance in all industrial sectors. In late years this has been especially evident in the development and use of tools, machine components and vehicles to meet today’s increasing economical and environmental restric- tions.

11 Surfaces in tribological contact are subject to very harsh environments characterized by extreme local pressures, temperatures and deformation. Under these conditions, surface films are formed through reactions between the contacting materials and the surrounding atmosphere or lubricant, but also through phase changes resulting in a surface layer with different properties compared to the original surface. These surface films, often referred to as tribofilms, may be very thin i.e. from a couple of nanometers to a few micrometers. Despite their insignificant thickness they will govern the behaviour of critical tribological components. Obviously, the study of tribofilms or related material properties requires high resolving power which is provided by techniques such as Auger electron spectroscopy (AES), transmission electron microscopy (TEM), scanning electron microscopy (SEM) or atom force microscopy (AFM), among others.

I.1.2 Surface Modification Low friction films have been used by humans for a very long time and yet there are areas of applications which just recently started to benefit from the advantages of surface modification or coating technology. The ancient Egyp- tians, for instance, used wooden sledges to drag the large stones used to build the pyramids. The sledge was dragged on a wooden track and recov- ered wall paintings reveal that a lubricant, probably water, was used to alter the surface properties and reduce the friction. This early documented use of friction-reduction technology shows that man has addressed this problem for a long time, and that the use of lubricants and oils today bear a close resemblance with the use then. However, in vacuum technology, space applications or food industry, where there is a need for low friction components, one cannot make use of liquid lubricants. In the case of vacuum technology and space applications it is difficult to keep the lubricants in position due to the low pressure2-4, and in the food industry a lubricant might contaminate the food5. To solve this problem, solid low-friction materials have been developed and used successfully. Today, thin layers of solid lubricants are used to reduce friction and wear in many applications with sliding contacts such as on video tapes, computer hard disks and reader heads6. The great economic significance of tribological development can be easily understood when considering the shortened work time in metal cutting. In the beginning of the 20:th century it took more than one hour of turning to remove the surface layer of a 0.5 m long steel rod with a diameter of 0.1 m using a carbon steel tool material7. When high speed steel (HSS) was introduced the working time was reduced to 30 minutes and with the first cemented carbide (CC) materials it could be done within only 6 minutes. In order to further increase the life time and quality of cutting tools as well as to decrease the working time, Sandvik introduced the Gamma Coating (GC)

12 process in 1969. This process made it possible to deposit thin (some µm thick) titanium carbide (TiC) coatings on CC tools. These new cutting tools reduced the working time so that the work that took more than one hour in the beginning of the 20:th century now could be performed in less than 1 minute. The success story of materials science and surface engineering in tooling application is still unfolding and today it is followed by a similar development in the tribological field of machine components.

I.1.3 The Active Surface Immediately after a tribological surface has been set to work, reactions may build up a surface layer with significantly different properties compared to the original surface. Very often these layers determine the tribological behaviour of the component. A pin on disc experiment with a Teflon pin and an aluminium disc could serve as an illustrative example. In this experiment it was somewhat surprisingly observed that the Teflon pin produced signifi- cant wear of the disc, although Teflon is a very soft polymer material. It was found that very hard alumina particles from the aluminium surface were formed and embedded in the Teflon. Teflon thus was more capable of retain- ing alumina than aluminium itself and hence hard alumina particles abraded the soft aluminium disc. This shows the importance of studying the active surfaces of a tribological system in order to understand the friction and wear mechanisms encountered. Here, the counter surface sliding against aluminium was no longer the original soft Teflon surface but an alumina reinforced abrasive Teflon pin.

I.1.4 Tribological Testing General The most reliable tribotest to simulate a real component in its authentic application is of course, a field test. However, this is often neither practically nor economically feasible. Instead, simplified laboratory tests mimicking the real application have to be used. The compatibility between the test and the real application has to be ensured, however. This is usually done by verify- ing that the wear mechanisms are the same for the test and the authentic application. A vast number of tribotests have been developed during the years to assess different wear, friction, seizure and lubrication mechanisms8-10. Here follows a description of the tests performed in this work.

13 Tests used in this thesis A pin-on-disc or ball-on-disc experimental set-up was used to measure friction during sliding as a function of sliding distance, see Fig. 1, either with a rotating disc or a disc moving back and forth. It is a versatile set-up because different materials can be chosen for the ball and disc or plane in addition to the possibility of controlling temperature, atmosphere and lubrication.

Figure 1. Schematic image of the ball-on-disc rotating set-up and ball-on-plane fretting experimental set-up.

In this test friction is calculated by measuring the normal load and lateral forces as the disc or plane moves. After finishing the friction measurement it is possible to asses the wear from the weight reduction or by making a depth profile along the wear track. Further, wear debris accumulated along the rim of the wear track can be analysed with microscopic and spectroscopic techniques to establish its nature. This analysis may be of interest for coatings on both components and tools to examine any tribofilm formed on the surface, and hence identifying friction and wear mechanisms. The tribological load scanner was used to study friction, wear, seizure and surface damage as a function of load for a single stroke or after reciprocating strokes11, 12, see Fig. 2.

Figure 2. Schematic image illustrating the motion of the rod during a single stroke tribological load scanner experiment.

Parameters such as speed, load range and number of strokes can be inde- pendently varied in a load scanner. Experiments can also be performed with addition of lubricants. Samples are typically 100 mm long rods with a diameter of 10 mm.

14 I.1.5 Tribofilm Analysis The small dimensions encountered in tribology make microscopy indis- pensable when assessing results of tribological contacts. There are many different microscopy techniques available but one of the most versatile instruments is the scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) detector. This combined instrument has the ability to image a surface with nanometre resolution and simul- taneously perform elemental analysis at the sub-micrometer level. Its imaging resolution is surpassed only by Scanning Probe Microscopy (SPM) techniques1. A focused ion beam (FIB) instrument combined with an SEM- function provides the capability of producing a metallographical cross- section or transmission electron microscopy (TEM) sample at a precise location. The TEM can provide chemical as well as crystallographic information with atomic resolution and its spatial resolution is unsurpassed by any other technique. However, the actual volume that is studied is extremely small, and care has to be taken when generalising the results to the whole sample. In fact a rough calculation gives that the combined volume of all samples studied in all TEM´s since they became commercially available, in the 1960’ies, is only13 0.6 mm3. Spectroscopy techniques often used to study tribofilms are AES, Electron Spectroscopy for Chemical Analysis (ESCA), Glow Discharge Optical Emission Spectroscopy (GDOES) and Secondary Ion Mass Spectroscopy (SIMS). These techniques provide good statistics, i.e. relatively larger areas are analysed with a high depth- and spectral resolution.

I.2 Surface Engineering

There are many means of producing surface layers suitable for tribological applications. In some cases the original surface is modified by heating14 or ion bombardment15 and in other cases a new surface is produced through deposition of new material. The coatings studied in this work were deposited using different deposition techniques, such as Physical Vapour Deposition (PVD), Hot Filament Chemical Vapour Deposition HF-CVD and Laser Powder Cladding (LPC).

I.2.1 Deposition of Coatings Many different processes are used to deposit coatings on components or tools. Among these are the Chemical Vapour Deposition (CVD) and PVD processes. In CVD, different gaseous precursors are brought to react on a heated surface, i.e. the substrate, which might be a tool or a mechanical component. The surface is often heated to temperatures exceeding 1000°C16. The high temperature restricts the number of materials which may be considered as substrates. For instance, any tool steel would soften at these elevated temperatures. From this reason the hard, stiff and heat resistant CC materials are the most commonly used tribological substrates for CVD deposition. During the years there has of course been a desire to put coatings also on steels and other more temperature sensitive materials resulting in an evolu- tion of deposition processes performed at lower process temperatures17. Sev- eral different deposition techniques are available in the PVD family, such as sputtering, electron beam evaporation, cathodic arc and laser ablation. Of these magnetron sputtering and electron beam evaporation have been used in this work and are described below.

Magnetron Sputtering The magnetron sputtering device consists of a water cooled cathode mounted on an array of magnets or electromagnets. The poles of the magnet are mounted so that the centre of the cathode surface is one magnetic pole and the outer rim is the other. The magnetic field lines extend with near para- bolic shape from the central pole to the outer one, as depicted in Fig. 3. By

17 orienting the magnetic field and the electric field in this particular way there will be a closed EuB drift path on the magnetron surface.

Figure 3. Schematic image of a rectangular magnetron with the magnetic field lines and the ExB drift path indicated.

The coating chamber is first evacuated and then filled with argon, where after a plasma is ignited. The free electrons will tend to be partially constrained to, and travel along the drift path, and as a result, ionisation will be most intense in the vicinity of these tracks, producing a locally much denser plasma. Ionised atoms are accelerated towards the cathode by the negative potential. There they will erode the target material by sputtering and hence provide released target atoms which will then condense on the surroundings, atom by atom. The process of sputtering can be compared to billiard ball collisions where the atoms in the target material are knocked out from their initial position by incoming energetic ions as shown in Fig. 4.

a) b) Figure 4. When an argon ion impinges onto the target material a) it will scatter out target-material atoms b). The expelled atoms are then deposited onto any exposed surface.

Electron beam evaporation Electron beam evaporation performed in an evacuated chamber, can be used to deposit a wide range of materials (considering it being a heat evaporation technique) including compounds, alloys, metals and ceramics18. The materi-

18 als to be deposited are put into a water cooled crucible, often made of cop- per. The water cooling allows the melt to remain separate from the crucible by a barrier of solid material and hence prevents any reaction between the melt and the crucible. A typical electron beam evaporation unit bends the beam into a 270q arc by a coil positioned as depicted in Fig. 5, i.e. opposite to the electron filament. This construction allows protection of the filament as well as the possibility to scan the beam to vary the extent of the melt. The material in the crucible is heated, melted and brought to evaporation by the high energy electron beam. As with sputtering, atoms condensate every- where on the surrounding surfaces.

Figure 5. Schematic image of a typical 270° electron beam evaporation unit

CVD Synthetic diamond can be produced by several different processes such as combustion synthesis, microwave plasma assisted diamond deposition, plasma torch and HF-CVD deposition. The diamond films studied in this work were produced by means of HF-CVD. In this process, tungsten fila- ments are heated to temperatures ranging 2200í2800°C, see Fig. 6. A working pressure of 30í40 mBar was used and the gas was a mixture of H2 and CH4. Deposition of diamond is achieved through reactions occurring on the filament surface, in the gas and on the growing diamond surface19.

19 Figure 6. The HF-CVD sample holder shown together with the W filament wires in front.

Laser Powder Cladding The laser powder cladding (LPC) process uses gas-atomized powder that is melted onto the substrate by a laser beam, see Fig. 7. The rapid solidification of the clad layer, due to efficient cooling from the substrate material, results in a metallically bonded fine structured coating.

Figure 7. Principles of the laser cladding process. The laser beam is focused to melt the added metal powder onto the substrate. Inert gas protects the clad layer from detrimental oxidation.

20 I.3. Analytical Microscopy

As the requirements on the quality of tribological components continue to rise, the need for understanding the factors that govern their properties also raise. Many of today’s coating have structures so small that characterization on a nanometre level is required in order to reveal the microstructure or nanostructure of the coating. Often, it is necessary to combine several characterization techniques in order to fully uncover the overalll as well as the local chemistry and structure of such coatings. In this work, electron microscopy techniques have been extensively used and the reason for using electrons instead of light is that the acquired resolution, G, is directly proportional to the wavelength, O, of the light or electro- magnetical wave as shown in formula (1)13.

0.61O G (1) P sin E

This means that when using visible light the highest resolution we may achieve is around 0.3 µm13 for green light. Electrons accelerated with 300 kV, however, have a significantly shorter wavelength and hence makes it possible to achieve a much higher resolution. The theoretically calculated resolution is though not attainable in an electron microscope. This is due to lens aberrations such as spherical aberration, astigmatism and chromatic aberration.

I.3.1 Electron Scattering Electron scattering or diffraction is essential for any kind of electron microscopy. Scattering denotes a deflection by collision while diffraction denotes a deviation of a wave direction by an obstacle. As the electron can attain both wave and particle properties, both scattering and diffraction are used to de- scribe electron and matter interaction. A negatively charged electron passing through a material interacts strongly with the atoms and gives rise to different signals, see Fig. 8. A primary electron, i.e. incident electron, may interact with the atomic nucleus, bounded electrons or free electrons but it may also be transmitted through a material unaffected. If the material does not scatter

21 or in other way respond to electron irradiation, it is invisible in electron microscopes. Especially in SEM analysis, when studying bulk materials, the incident electrons often loose all their energy and get absorbed by the specimen.

Figure 8. A schematic image showing the different emerging signals as a specimen is irradiated with an electron beam. Transmitted electrons are only observed if the sample is thin enough.

Electron scattering is often divided into elastic scattering and inelastic scattering. Both types of scattering give structural and phase information.

Elastic Scattering Elastically scattered electrons are responsible for most of the contrast observed in TEM images and they build up the intensity distribution in diffraction patterns. In the SEM they are responsible for the Z contrast observed from back-scattered electrons. To better understand the mechanisms behind the contrast observed both in the TEM and the SEM it is necessary to look closer into the interaction processes between electrons and matter. Elastic scattering may be divided into scattering from single atoms or scattering from several atoms together. Interaction with a single atom may occur in two different ways, both of which involve interaction with Coulomb forces. Firstly, it may interact with the electron cloud resulting in a small angular deviation or secondly, it may penetrate the electron cloud, interact with the nucleus and deviate through a larger angle, see Fig.9. The deviation can exceed 90° resulting in backscattering of electrons.

22 Figure 9. Schematic figure of the interaction between high energy electrons and an atom. Coulomb interaction in the electron cloud, i.e. with a large impact parameter b, results in low angle scattering while Coulomb interaction with the nucleus, i.e. with a small impact parameter b, results in high angle scattering or even backscattering.

Scattering at larger angles can be explained by Rutherford scattering, in which the screening by the atomic electrons is neglected. Electron scattering V from atoms per unit solid angle : can be described by the differential cross section

dV 2 f (2) d: where f is the complex scattering factor, i.e. a function of the scattering vector q depicted in Fig. 10 or the scattering angle T. The differential cross section may also be described as a function of the elastic form factor F(q):

2 dV 4 2 4J 2 2 4 F q 2 4 Z f x q (3) d: a0 q a0 q where a0 is the first Bohr radius, q is the magnitude of the scattering vector, 2 2 -1/2 J=(1-v /c ) is a relativistic factor, Z is the atomic number and ƒx(q), the Fourier transform of the electron density inside the atom, is the atomic scattering factor or form factor for an incident x-ray. Since Z occurs in equation 3, it is obvious that an incident electron is scattered by the total electrostatic field in an atom. Applying classical mechanics and neglecting the screening influence of the atomic electrons gives the following expression for the differential cross section:

23 dV 4J 2 Z 2 2 4 (4) d: a0 q

This equation gives a sensible approximation for large scattering angles, i.e. a small impact parameter. There are several different approximations available to include electron screening such as the Wentzel expression in which the nuclear potential is attenuated exponentially. Cross sections with higher accuracy are calculated using Hartree-Fock and Hartree-Slater methods20.

Figure 10. Diagram of elastic scattering; k0 and k1 are the wave vectors of the incident and scattered electron wave while q is the scattering vector. The magnitude of the scattering vector is q=2k0sin(ș/2)

Bragg’s Law The following treatment of electron diffraction is not physically valid but provides a good mathematical model. Bragg’s law applies to reflecting waves at glance angles and not to transmitted waves as is the case in TEM analysis. However, the atomic planes seams to behave as mirrors for the incident electron beam. A crystal can be thought of as being an array of atoms or slits. When an electron wave impinges on the crystal it undergoes diffraction. To understand diffraction we need to use the wave nature of electrons. The waves passes the crystal and when these waves exit the sample, interference effects will occur resulting in constructive and destructive interference. This yields a periodic oscillation of diffraction intensity. For certain directions, which may be calculated from Bragg's law:

nO 2d sin T B (5)

§ d · O 2¨ ¸sin T B (6) © n ¹

24 constructive interference is obtained, where O is the wavelength of the electron wave, d is the distance between the atomic planes, TB the Bragg angle and n is an integer. From this equation we can observe that the shorter the distance between the lattice planes the larger the scattering angle since O is constant. The integer number n can be thought of as introducing additional atomic planes, for instance n=2 can be visualized as introducing an additional atomic plane in the centre between the original ones. In electron diffraction, reflections of specific sets of atomic planes are observed. These are shown in diffraction patterns, which can be used to iden- tify the atomic structure of observed crystals since only specific sets of planes meet the diffraction conditions, i.e. are present in the diffraction pattern. Structure factor analysis selection rules, i.e. rules for which reflections might be observed for a specific atomic structure. From the diffraction pattern it is also possible to deduce the planar spacing dhkl for specific (hkl) planes, where hkl are the Miller indices of that specific set of atomic planes.

Inelastic Scattering Inelastic scattering which, involves energy transfer provides the possibility to extract chemical information, such as the constituent elements in the sample. An electron that passes through the sample may undergo phonon scattering or plasmon scattering or excite an inner or outer shell electron in an atom. These inelastic processes and upcoming signals reveal information about the sample. Even without being irradiated by electrons, the atoms in the sample vi- brates about their lattice sites because of their thermal energy. An electron passing through a sample may generate more vibration in the form of phonons. Phonons are quanta of atomic vibration and they are of the order of kT in energy where k is Boltzmann's constant and T is the absolute temperature. This means that the energy loss for an electron upon generating a phonon is below 0.1 eV20. Phonon scattering is, in contrast to other scattering processes, temperature dependent and increases with factor of 2-4 between 10 K and room temperature. It also increases with the atomic number, Z, of the scattering atom. Plasmon scattering is due to excitation of electron waves in the sample. The valence electrons in the sample may be seen as oscillators which reacts with each other through electrostatic forces. The incoming electron produces an electric field, which through electrostatic forces starts oscillations of the valance electrons. This oscillation occurs at a characteristic angular fre- quency, Zp. The energy loss suffered by an primary electron differs between different materials but is in the range of approximately 2 to 40 eV An incoming electron may interact with an atomic electron, transfer energy to it and then continue through the sample. If the transferred energy exceeds a certain limit the atomic electron is ejected and hence escapes the

25 attractive force of the nucleus. When an inner shell electron is ejected the atom is left in an excited state with a higher energy than its ground state. It may return to a lower energy state if an outer shell electron fills the hole. In this process the atom may release the excess energy by emitting either radiation in the form of an X-ray photon or an Auger electron, i.e. a low energy electron. The fluorescence yield, Z, is the possibility ratio of X-ray emission to inner shell ionization and can be calculated using the following equa- tion13:

Z 4 Z (7) a Z 4 where a is approximately 106 for K shell. X-ray analysis is thus well suited for heavier elements but not so well for light elements. Also other factors, like absorption in the sample, further reduces the sensitivity to detect light elements. Secondary electrons (SE) are emitted from the sample as a result of electron irradiation. There are different types of secondary electrons and these might be divided into different groups. The fast SE are those that are ejected from an inner-shell and these electrons may have up to 50% of the incoming electron energy. So called slow SE are ejected from the valence or conduc- tion band and have energies typically below 50 eV. The third kind of SE are the Auger electrons, which also have low energies. These latter electrons are ejected by a characteristic X-ray giving its energy to an outer shell electron. Detecting these signals all have different pros and cons. The slow SE have low energy and are hence strongly absorbed. This results in that the slow SE that may escape from the sample are those ejected from near the surface, i.e. only the surface is probed upon using these electrons for analysis. The same is true for Auger electrons that have an energy of only some hundreds of eV. The difference between the two is that Auger electrons have characteristic quantized energies that can be traced to a specific element. This is not the case with either fast or slow SE since the process behind the creation of these electrons can be quite complex.

I.3.2 Microscopical Techniques There are two types of electron microscopes namely SEM and TEM. The surface technique SEM gives good topographic information together with, chemical information, although the spatial resolution is limited and not always enough for the required characterization. Advantages of the SEM are though that it can analyze relatively large areas, it is easy to use and the sample preparation is quite straight forward meaning that little or nothing

26 has to be done to the sample before it is put into the microscope. On the other hand the contrast in the image can be difficult to interpret correctly since many effects may alter the secondary electron signal from the sample. In the case of chemical analysis using the EDS detector it is important to realize that the x-ray signal is not emerging from the surface alone but also from an interaction volume that may reach 1 µm into the sample. In a homogenous sample it might not be a problem but whenever dealing with in- homogeneous samples, such as coatings there is uncertainty about what is included in the probed volume.

Transmission ElectronMicroscopy A TEM is used for studying materials at high magnifications. It is a very powerful tool in materials science and its uniqueness lies in the possibility to extract for instance crystallographic, elemental and chemical information with extremely high spatial resolution. The TEM has an electron source, focusing lenses, i.e. condenser lenses, an objective lens, several magnifying lenses and electron intensity detectors, such as fluorescent screens and cameras. As men- tioned earlier, electrons are diffracted by crystals in the specimen and hence a diffraction pattern is formed at the back focal plane of the objective lens. At the back focal plane of a lens parallel rays coming into the lens will be imaged to a point. Since Bragg diffraction diffracts electron beams into specific angles, the back focal plane thus exhibit bright spots separated by an angle of TB for the specific diffracting planes. This presents several possibilities when imaging the specimen. If a diaphragm, in this case the objective aperture is introduced in the back focal plane of the objective lens this enables imaging either with electrons scattered by specific planes or by “unscattered” electrons. The former imaging mode is called dark field (DF) while the latter is called bright field (BF). There is also a selected area diffraction aperture (SAD), positioned at an image plane, with which it is possible to choose areas in the sample that contributes to the diffraction pattern. More information is available in books by Williams & Carter and Joy13, 21. The sample preparation prior to TEM analysis is highly dependent on the nature of the sample. A sample needs to be thinned to at least 100 nm over as large area as possible without the sample becoming too brittle to handle. The classical approach is to grind a dimple in a thinned disc with a grinding wheel. The final thinning is done in vacuum using a beam of argon ions with a small angle of incidence. During this stage some argon implantation will take place as well as amorphization of the surface layer13. A low angle of incidence of Ar ions reduces its thickness and hence the effect of these artefacts. It is though important to keep these effects in mind as they can distort the results of the analysis. Other artefacts can occur during polishing, for instance the use of SiC polishing laps and diamond laps will damage the sample within a depth of approximately 2 times the polishing particle size.

27 The main contribution to contrast in the TEM is elastic scattering so when studying inelastic signals from the TEM to get chemical information, the elastic scattering might redistribute the inelastic scattering from the sample. In EDS analysis strong diffracting conditions should be avoided since the diffraction contrast might modulate the EDS signal. Furthermore, thickness effects will also change the signal. The same applies to the TEM techniques of electron energy loss spectroscopy (EELS) and energy filtered TEM.

Scanning Electron Microscopy The SEM is considered to be one of the most important instruments when studying tribological wear mechanisms and is the most widely used electron microscope. It is used for characterizing all kind of materials, including bio- logical samples. In an SEM an electron beam is scanned over the sample and the emissions generated by the beam are detected. The intensity of the signal controls the contrast in the image. An SE image shows surface topography due to the placement of the detector at the side of the sample. This leads to shadow and thus topographic contrast. Intensity of BSE images is dependent on the atomic number because backscattering increases with atomic number. BSE images also exhibit shadowing effects due to BSE trajectories being straight lines.

Focused Ion Beam In the FIB station, a beam of ions is scanned over the studied surface and the out coming electrons or ions are collected by a detector. The amount of out coming electrons or ions is directly proportional to the intensity of the acquired image. I.e. an FIB instrument works essentially like an SEM with the main difference that the scanning beam is composed of ions instead of electrons. The impinging ions sputter the material and even at low currents mass removal can be observed. The possibility to sputter or etch a material is used to dig into materials to expose their internal structure. It has been proven by many scientists that the FIB instrument is a very versatile and valuable tool when studying materials exposed to tribological contact or materials in general. The main reason is that the area of study and subsequent etching can be selected with nanometre precision. Also, the crystal-orientation modulated signal is beneficial when studying polycrystalline samples. The heavy ion penetrates deep into the material under investigation and the penetration depth depends on the crystallographic orientation. For certain crystal orientations, ions may tunnel along atom rows and hence end up very deep into the material. Ions interact with matter and SEs are produced. How- ever, the SE signal strongly depends on the depth where they are produced since they have a short escape depth. This means that the contrast in the im-

28 age will depend on the penetration depth of the ions which in turn depends on the crystallographic orientation. The instrument used in this work has also an additional SEM column, providing a possibility to acquire SEM images of the area being etched. The two columns are 52° apart but focus on the same point, i.e. the working point, see Fig. 11. This provide a possibility to study the etched wall and measure features along the ion beam direction at the same time as material is being etched. It is hence possible to acquire SEM images during etching without having to tilt or rotate the sample, see Fig.12. In many FIB applications it is essential to protect the surface of the material that will be investigated from the intense ion beam, especially in tribology were the main interest is devoted to the very surface region. This is ac- complished by depositing a thin metal film by ion or electron beam induced CVD inside the FIB instrument.

Figure 11. Schematic image showing the orientation of the electron and ion columns, the sample and the working point.

a) b) Figure 12. Image taken from the same are with a) the ion column and b) with the electron column.

29 TEM samples are rapidly produced using the FIB instrument by etching trenches on either side of the area of interest, see Fig. 12. The ion current is reduced during the thinning process to accomplish a smoother cut and less radiation damage of the final sample.

Conventional TEM sample preparation TEM specimens can be prepared in a variety of ways but in this work they have been prepared by classical TEM sample preparation techniques and by FIB sample preparation. The goal with the specimen preparation is to achieve an electron transparent area at the site of interest. In classical preparation, the sample has to be cut into small pieces first, see Fig.13. The pieces are glued together and the resin let to cure. After this, the "sandwich" and resin is introduced into a brass tube with an outer diameter of 3 mm and the resin let to cure. Now the brass tube is cut into thin 0.7 mm discs which are polished from both sides in several steps first with sili- con carbide paper grade 800 and finally with 1 µm diamond lap. At this stage the disc should be about 100 µm thick and it is time to focus on the interesting area. Using a dimpel grinding machine, see Fig. 14a, the disc is ground in the centre to a thickness of 20 µm or less. The dimpled area is also finally polished with 1 µm diamond to achieve a smooth surface.

Figure 13. The sample is cut into smaller pieces. b) Glue is applied and c) the pieces of sample are glued together under heat and pressure. d) The “club sandwich” is encased with epoxy into a brass tube and then cut into discs.

The last step is to further thin the area until it is suitable for TEM analysis, i.e. around 100 nm or thinner. This is done by ion etching, see figure 14b. The sample is bombarded with argon ions at low incident angle until the disc has a small hole in the middle. At this stage the material in the vicinity of the hole should be thin enough to allow electrons to be transmitted through the sample.

a) b) Figure 14. a) : Schematic figure of the dimple grinder setup. A dimple is produced by the simultaneous rotation of the sample and the grinding wheel. b) A simplified view showing how the sample is rotated while Ar ions are accelerated towards the sample at low angles, typically between 1ºand 10º.

The easiest way to produce samples is suitable if the studied material is in powder form. Then the powder is simply mixed with ethanol to a slurry and then a thin carbon grid is dipped into the slurry and dried. The carbon grid will support the sample.

I.3.3 Energy Dispersive X-ray Spectroscopy EDS analysis is often performed inside the electron microscope in order to retrieve the chemical composition of the analyzed material. A typical spectrum is depicted in Fig. 15.

Figure 15. A typical spectrum from an EDS analysis inside the TEM.

31 Primary electrons may interact with atoms by transferring enough energy to eject a bounded inner-shell electron, thus leaving the atom in an excited state. The excited atom returns to its ground state by replacing the missing electron with an electron from an outer shell. In this transition, an Auger electron or a characteristic X-ray is emitted. The probability of X-ray emission versus Auger electron emission increases with atomic number and hence making EDS a suitable technique for heavier elements. The energy of the emitted X-ray is equal to the difference in energy between the two electron shells involved in the relaxation process and hence unique to the atom of origin. If now the intensity of the out coming X-rays is plotted as a function of their respective energy a spectrum is acquired. The energy-position of the peaks in the spectrum reveals the elements present in the analyzed material, i.e a qualitative elemental analysis is performed. However, quantitative analysis is often wanted and in order to perform good quantitative analysis effects such as absorption, fluorescence, bremstrahlung, escape peaks, sum peaks and k factors have to be considered. For sufficiently thin TEM samples it is possible to use the thin film approximation, in which absorption effects and fluorescence effects can be neglected. The mass concentration ratio of two elements, A and B, can be written as22

C A I A k AB (8) CB I B where CA is the weight concentration of element A, IA the measured X-ray intensity and kAB the Cliff-Lorimer factor, accounting for differences in ionization cross sections, fluorescence yield and detector efficiency between the two elements. The kAB values can be measured using standard samples but they are also often calculated when performing standardless quantification.

I.3.4 Electron Energy Loss Spectroscopy Electron Energy-Loss Spectroscopy (EELS) can be performed in a TEM where it will benefit from the high spatial resolution of the microscope. Due to its high energy resolution, it is used to study chemical states of elements, measure sample thickness and to perform qualitative as well as quantitative elemental analysis. The nature of the intensity distribution in the spectrum makes it however best suited for light element analysis. As described earlier, there are many processes in which an electron might loose energy inside the specimen and hence the energy distribution of the transmitted electrons will extend over a large energy interval. For instance, when an incoming electron loses energy by exciting one single atom in the

32 specimen by knocking out a core electron it looses a characteristic amount of energy. The electron energy loss spectrum, i.e. the intensity of transmitted electrons as a function of energy loss, actually depicts the probability of an electron loosing a specific amount of energy as a function of energy loss. The differential cross section can be written as follows20:

2 ª 4 º½ dV i 4J Z 1 ° T 0 ° ®1 « »¾ (9) d: a 2 k 4 2 2 2 2 2 2 2 0 0 T T E ¯° ¬« T T E T 0 ¼»¿°

2 where T is the scattering angle, TE=E/(Jm0v ) is the characteristic scattering angle for inelastic scattering with E being the energy loss,

2 șE E/(Ȗ m 0 v ) (10)

-1 where E is the mean energy loss, and T0 = (k0r0) where r0 is the screening -1/3 radius defined for a Wentzel potential and equal to a0Z using the Thomas- Fermi model. Calculation of cross sections is required for quantitative EELS analysis. Often an angle limiting aperture, the objective aperture, is used to select the central beam. In that case the differential cross section should be recalculated to take also the limiting angle aperture into account. The EEL spectrometer bends the electron beam and disperses the electron as a function of their energy. A typical EEL spectrum as the one shown in figure 16, is composed by the intense zero loss peak, the low loss region, and the characteristic energy loss region.

Figure 16. A typical EELS spectrum showing the different energy-loss regions.

The phonon scattering is not resolved since the resolution of the EEL spectrometer is seldom better than 0.5 eV and phonon scattering is of the order of 0.1 eV. Next to the zero loss peak the plasmon peak is observed, both surface and volume plasmons may exist. Depending on the thickness of the sample we see one or several equidistant plasmon peaks. Since the probability of an electron exciting a plasmon can be expressed in terms of a mean

33 free path, obviously an electron might excite two plasmons and hence have lost twice the energy of one plasmon and so on. Beyond the plasmon peaks the intensity decays exponentially until the first characteristic ionizing edge. Depending on the probability of a core electron being either excited to an empty state or just excited to a free level, the energy loss near edge structure (ELNES) will look quite different as can be seen in the case of graphite and diamond, see figure 17. From this it is possible to extract chemical information with the high spatial resolution provided by the TEM.

a) b) Figure 17. EELS spectra showing the differences in energy-loss near edge structure (ELNES) between graphite (a) and diamond (b). Actually diamond should not exhibit the observed ʌ* peak at 284 eV energy-loss, However in case it is present due to amorphous carbon originating from the sample preparation.

Thickness measurements From the EELS spectrum it is possible to retrieve the thickness of the observed sample. This is achievable since the inelastic scattering will increase with increasing thickness. The intensity ratio between the zero loss intensity and the total intensity of the spectrum is governed by the total mean free path, O, for all inelastic scattering

§ I t · t O ln¨ ¸ (11) © I 0 ¹

If O is known for the studied sample the local thickness can be retrieved. O varies between approximately 50 nm to 200 nm for different materials. If an angle limiting aperture is introduced then O(E) should be used.

34 Relative Quantification Relative quantification can be performed if the background is subtracted from the edge (see figure 18). The following equation can be used for such quantification of two materials A and B:

N I (E ,')V (E ,') A A A (11) N B I B (E ,')V B (E ,') where N is the number of atoms per unit area contributing to the edge, I the edge intensity above background (see figure 18) and V the cross section. Note that the angle limiting aperture or collection aperture with semi angle E and the integration windows ' affect both the intensity I and the cross section V. Prior to a quantitative analysis, the background contribution has to be subtracted. The background under the edge is often calculated using the power law model23:

I AE r (12) where I is the intensity at energy E and A and r are fitting parameters. The fitting parameters are calculated using the intensity in the window or windows, depending on the employed technique, i.e. two windows or three windows technique20.

Figure 18. An EELS spectrum of the carbon K-edge of TiC with the background calculated beyond the edge. IK , IB and ǻ are also shown.

35 If the cross sections are not known, changes in the ratio will reflect changes in composition, though they are not quantitative but qualitative.

I.3.5 Energy Filtered TEM Energy Filtered TEM (EFTEM) offers the possibility to image large sample areas using electrons with well defined energy hence making it possible to acquire elemental distribution maps within minutes. An energy filter is basically a spectrometer with additional lenses making it possible to reconstruct the image of a sample area from the EEL spectrum. At the energy plane, the plane where an energy loss spectrum is formed, a slit is inserted thus selecting an energy range for the transmitted electrons contributing to the energy filtered image. This is possible due to a number of post slit lenses correcting aberrations in the image due to the fact that the electrons have already been dispersed as a function of energy. The attainable resolution of energy filtered TEM images, roughly 1 nm, is mostly limited by the delocalization error, the chromatic aberration and the diffraction limit24. Delocalization error arises because a fast electron can pass at a certain distance from an atom and still ionize it. Chromatic aberration in the objective lens, i.e. electrons possessing different amounts of ki- netic energy are focused with different strength affects the resolution. Dif- fraction limit is due to the size of the objective aperture and the fact that a lens will not image a point object to a point image but to a disc. All these errors can be summed in quadrature, e.g.

x 2 y 2 z 2 (13)

The effect on the resolution of each of these parameters is depicted in Fig. 19 together with their sum in quadrature.

36 Figure 19. Attainable resolution at the carbon K energy (284 eV) for 300 kV electrons and using a energy selecting slit of 20 eV width.

Chromatic aberration can be reduced by using a small energy selecting slit and a small objective aperture. The objective aperture should be chosen considering the resolution that is required. However, larger objective aperture gives more intensity in the image, hence better signal to noise ratio.

Three Windows Technique To produce an elemental map, the background intensity in the image has to be subtracted. This is performed by acquiring images with energy interval positioned at lower energies than the edge of interest, see Fig. 20. The two pre edge images are then used to calculate the background intensity at the post-edge image, i.e. the image with energy interval under the edge.

37 Figure 20. An EELS spectrum of the carbon edge in TiC showing possible position of the pre edge windows and the post edge window.

The background is calculated for each pixel in the post-edge image using the pre-edge images and then subtracted from the initial value of the post-edge image. In this way an image with the background subtracted, i.e. an elemental map, is produced. If two elemental maps are acquired then the same methods used in EELS quantitative analysis can be used here. Using equation 11 and hence dividing two maps and multiplying them with the corresponding cross sections gives an elemental ratio image with quantitative values23.

Artefacts such as thickness effects due to varying thickness and diffraction effects, often observed in crystalline TEM samples, affects the intensity distribution also of an elemental map. A way to cancel these effects is by producing a jump ratio image. This image is calculated by simply dividing the post edge image with a pre edge image. Jump ratio images have low noise levels but are not quantitative even though they often follow the intensity observed in elemental maps.

38 Part II Contributions

II.1 Introduction

The primary aim of this work was to forward the knowledge of tribological surfaces by investigating the internal microstructure of thin coatings and surface layers used in tribological applications, and study their modifications due to tribological contact. Knowledge of the tribological response of the surface material to mechanical, thermal and chemical loads down to the atomic level will aid in the design of new and better coatings and materials. It will also be useful when selecting the best tribological materials for given applications. In all papers included in the thesis this has been achieved through the use of state of the art analytical electron microscopy.

II.2 Structure and composition of as-deposited and heat treated thin coatings (Papers I, II, III) A number of different PVD coatings for tribological applications were studied. Two of the coatings i.e. TiC and TiB2 are commonly used for tool applications while TaC:C is a low friction coatings intended for use on mechanical components. The fine scale microstructure was analyzed with respect to sensitivity to changes in substrate bias, composition and oxidation stability. Techniques used were analytical electron microscopy together with spectroscopy techniques such as EDS, EELS and EFTEM. This combination is well suited for uncovering the atomic and chemical structure of coatings.

II.2.1 Effect of carbon content on the microstructure of TiC coatings (Paper I)

TiCx coatings were deposited by sputtering carbon and electron beam evapo- rating titanium. During deposition the substrates were cyclically exposed to both sources by means of a carrousel mechanism. Different carbon to titanium ratios were achieved by varying the power of the magnetron25.

41 From SEM images it was concluded that the carbon content has a signifi- cant effect on the coating morphology. High carbon content leads to the formation of large loosely attached columns of material while low carbon content results in a fibrous structure, see Fig. 21.

a) b) Figure 21. SEM micrographs of fractured cross-sections of thin TiC coatings with 59 at% of carbon (a) and 43 at% of carbon (b). The columnar morphology of the coating in (a) emanates from protruding carbides in the substrate surface.

The microstructure of TiC coatings was studied with TEM in order to uncover atomic scale structure.

a) b) Figure 22. Bright field TEM micrograph of the 43 at% of carbon TiC coating (a) and a corresponding selected area diffraction pattern (b) showing the 200 texture of the thin coating.

A low carbon content led to the formation of columnar grains with a strong texture with the <001> direction being parallel to the substrate normal, see Fig. 22. An increase of carbon content, however led to the formation of ran-

42 domly ordered nanocrystalline TiC grains dispersed in an amorphous carbon matrix. According to earlier studies this element composition is suggested to correspond to TiC+C26. The high amount of carbon in the 59 at% of carbon TiC coating led to the formation of randomly ordered nanocrystalline TiC grains with agglomeration of larger grains at the edge of the growth columns, see Fig. 23.

a) b) Figure 23. Dark field TEM micrograph of a TiC film with 59 at% of carbon (a) and corresponding SAED pattern (b). Bright areas in (a) correspond to Bragg diffraction conditions. The diffraction pattern in (b) reveals a structure of randomly oriented TiC nanocrystals.

Elemental analysis was performed by EFTEM. Both types of coatings were analyzed at thin parts, see Fig. 24. It was found that the Ti/C ratio increased as a function of distance from the substrate/coating interface until a steady state was achieved, see Fig. 25. It was concluded that this was a result of the temperature of the titanium evaporation source. The amount of available titanium increases with the temperature of the evaporation source which had not saturated when carbon sputtering begun. The initially deposited titanium layer, clearly observed in Fig. 25, has the purpose to enhance the adhesion of the coating to the substrate. Its Ti/C ratio is not comparable with the Ti/C ratio for the coating since there is no carbon in the interlayer.

43 a) b) Figure 24. TEM micrograph of thin areas of the 43 at% (a) and 59 at% (b) carbon coatings (b). Note that much of the coatings has been milled away during specimen preparation.

b) Figure 25. Mapping of the Ti/C ratio and corresponding intensity profiles along the marked areas of the coatings with 43 at% (a) and 59 at% (b) carbon content.

44 Conclusions x Low carbon content resulted in coatings with columnar grains tex- tured with the <001> direction parallel to the substrate normal. x High carbon content resulted in randomly ordered TiC crystals in an amorphous carbon matrix. x These observations indicate that the morphology of carbon based coatings can be tailored by varying the carbon content.

II.2.2 Effect of substrate bias on the microstructure of TiB2 coatings (Paper II)

Thin coatings of TiB2 were deposited by sputtering from a TiB2 target. Though TiB2 offers excellent properties such as high hardness good chemical stability and good thermal and electrical conductivity its use for tribological applications has been limited. This is largely because TiB2 is prone to develop excessively high residual stresses during deposition27. However, it was found that by applying a positive bias to the substrate during deposition instead of the normal negative, it was possible to deposit TiB2 with low compressive residual stress maintaining its high hardness28. A positive substrate bias results in high mobility of adatoms on the growing film surface during deposition through thermal activation by predomi- nantly electron bombardment. Negative bias cause bombardment solely by ions which more likely introduces defects and high compressive stress. Three coatings deposited on cemented carbide at three different biases; Vs = -110 V, floating potential Vf § -6 V and Vs = +50 V were studied. TEM cross section specimens were prepared by classical TEM specimen preparation. All three coatings exhibited a columnar structure with the grain axes being parallel to the substrate normal, see Fig. 26.

45 a) b) c)

Figure 26. TEM micrographs of the three TiB2 coatings deposited with different bias voltage. The images are montages of several individual micrographs. a) -110 V, b) Vf and c) +50V.

Near the substrate, however, a layer with more random orientation of the grains was observed. The texture developed gradually during growth, see Fig. 27. This same behaviour was also observed for the TiC coatings studied in Paper I. The thickness of this layer was approximately 50 nm and similar for all coatings.

Figure 27. SAED patterns acquired at different levels in the TiB2 coating deposited with Vf. Each SAED enclosed several grains.

46 From Fig. 26 it was observed that the columnar grain size decreased as the bias increased from negative to positive voltage. For the Vf and Vs= +50 V the grain diameter was approximately 5 nm and observed to be constant from 200 nm from the substrate interface and up to the surface. A model for the development of film texture through evolutionary selection during growth has been proposed by van der Drift29. The texture results from differences in growth rate for different crystallographic direction. In this work the (0001) planes parallel to the substrate surface should be fa- vored. This results in a texture development where finally the [0001] direction grows on the expense of other crystallographic orientations. At the beginning of the growth, however, [0001] grains will grow in the lateral direction also, but just until they meet each other thus hindering further lateral grain growth. This happens after 200 nm of film growth for the Vf and Vs = +50 V TiB2 films. In the case of the Vs = -110 V, the surface should have experienced higher intensity of ion bombardment than the other two films. This could have rear- ranged the atoms on the surface and led to the reduction of nucleation sites in the nucleation zone. Eventually, this would result in larger grain size in the film made with negative bias.

Conclusions x By increasing the bias from negative to positive voltage it is possible to reduce the grain size and at the same time reduce the internal stresses while maintaining a high hardness. x The latter means that the thickness of TiB2 coatings can be dramati- cally increased without the risk of spontaneous coating detachment. x All coatings were found to be dense with no voids or pores. x All coatings exhibited a texture with the <0001> direction parallel to the substrate normal. x The same advantage should be achievable in other coating systems that suffer from excessively high residual stresses such as TiN or (AlCr)N.

II.2.3 The role of Al on structure and stability of (Ta,Al)C:C coatings (Paper III) Carbon coatings have attracted much interest due to their tribological properties such as low friction and good chemical stability. However, metal addi- tions have been observed to reduce friction with maintained wear rate30. The

47 addition of metals may also serve to alter the wear rate by changing the oxidation properties of the coating. Aluminium added to a TaC:C coating was shown to improve the oxidation resistance with almost unaltered friction properties for a certain concentration of Al31. However, the details of the role of aluminium was unknown. Tests of oxidation resistance were performed at different temperatures and Al contents. This work concerned a coating with about 11 at% Al, 15 at% of Ta,70 at% of carbon, 1.4 at% of O and 1.5 at% of Ar. The coatings were deposited by magnetron sputtering from a single carbon target partially covered with foils of tantalum and aluminium. Cross section TEM samples were produced by classical TEM specimen preparation techniques on one as- deposited coating and one heat treated at 500º C for about two hours.

As-deposited coating The coating exhibited the same microstructure throughout the whole thickness with an unclear layered structure, see Fig.28a.

a) b) Figure 28. TEM micrographs of the as deposited (Ta,Al)C:C coating. a) A layered structure of the coating seen in low magnification. Observe that much of the coating has been milled away during sample preparation. b) TEM lattice image showing small crystallites.

Electron diffraction suggested that the coating was composed of randomly ordered carbide grains. They could be revealed at high magnification, see Fig. 28b. It was also observed that the addition of Al changed the morphology of the TaC coating by removing the columnar structure usually develop- ing in TaC coatings32. The layered structure was studied in detail using STEM EDS. The intensity in HAADF STEM images follows Rutherford scattering, i.e. as a function of Z2. Further, the HAADF signal is incoherent in nature meaning that

48 its intensity follows the sample thickness and atomic number Z. In this work it was noted that the HAADF intensity followed the Ta signal intensity in the acquired EDS spectra. The explanation for this variation was deduced from a previously unnoticed effect during the deposition process. The metal foils covering the magnetron were thermally expanded and somewhat buckled. This would have led to an uneven distribution of Ta and Al above the magnetron. The complex rotation of the substrate makes the substrate pass over a specific part of the magnetron every three revolutions. During the three revolutions the coating grows approximately 4 nm which corresponds very well to the periodicity observed in the HAADF STEM image. EELS analysis was performed on these coatings to verify the chemical state of Ta and Al before and after the heat treatment. EELS spectra were also acquired on standard samples of bulk Ta, TaC and Al4C3. The acquired spectra were corrected for plural scattering by Fourier-ratio deconvolution using the built in function in the Digital Micrograph software33. When comparing the spectra it was found that the both the Al and Ta was in a carbide phase, see Figs. 29 and 30. This finding is in agreement with ESCA analysis31.

a) b) Figure 29. EELS spectra from as-deposited (Ta,Al)C:C coatings. a) The Al L edge. b) EELS spectra of different aluminum compounds shown for comparison. The top 33 Al and the Al2O3 specta are from the EELS atlas .

49 a) b) Figure 30. EELS spectra from as-deposited (Ta,Al)C:C coatings. a) The Ta O edge. 33 b) EELS pectra of TaOx, Ta and TaC, shown here for comparison.

Heat treated coatings After heat treatment in air two distinct layers were observed: one oxygen rich layer, i.e. the top layer and one oxygen deficient layer, i.e. the bottom layer. The interface between the layers is only 8 nm thick as measured in STEM, see Fig. 31. This suggests an oxidation process where the chemical reaction is the rate controlling mechanism rather than the diffusion of oxygen.

Figure 31. A STEM image of a cross-section of the heat treated (Ta,Al)C:C coating with a top oxygen rich layer and the lower oxygen deficient layer. Observe the very sharp transition region between both layers.

No crystallinity was found in the oxygen rich layer though this was suggested by X-ray analysis at glancing angle31. However small randomly ordered TaC grains were observed in the oxygen deficient layer. EELS analysis of both regions showed that no change in chemical state had occurred in the oxygen deficient layer compared to the as-deposited coating meaning that heat alone does not alter the structure.

50 The Ta O-edge in the oxygen rich layer is similar to the TaOx edge from the EELS atlas although it has a small shift suggesting a mix of TaO and another element, compare Figs. 30b and 32a. Also the Al L edge in the oxygen rich layer did not resemble the reference Al2O3 signature thus implying that pure Al2O3 had not been formed but rather some other compound such as a mixed tantalum aluminium oxide, compare Figs. 29b and 32b.

a) b) Figure 32. EELS spectra from the heat treated (Ta,Al)C:C coating. a) The Ta O- edge. The Al L edge in the oxygen rich as well as in the oxygen deficient layer.

Conclusions x Addition of aluminium results in non-columnar structure. x Heat treatment of an aluminium alloyed TaC:C coating resulted in a mixed tantalum aluminium oxide superficial layer with a virtually constant oxygen content through the layer, covering a layer without oxygen. x An extremely thin interface region formed between these layers. x These observations suggest that the oxidation process is controlled by the oxidation rate rather than the diffusion rate of oxygen.

II.3 Revealing mechanisms of friction and wear (Papers IV, V, VI)

The tests described in § I.1.5 are often used to evaluate friction, wear, seizure and surface damage. Coatings of the type presented in § II.1 are rou- tinely tested to assess their properties with the purpose to improve them and to find new applications for them. To explain the wear mechanisms, analytical electron microscopy was applied to study surfaces subjected to tribological testing. The FIB is the ultimate tool to extract samples both for cross- section studies in SEM and for TEM analysis.

II.3.1 Mechanisms of tribofilm formation for lubricated Me-DLC coatings (Paper IV) This work concerns the compatibility of metal doped DLC coatings with additivated lubricants. Two commercially available Me-DLC coatings doped with W and Cr, respectively, see Fig. 33, were deposited on ball bearing steel (BBS) rods. The coated rods were then tested in boundary lubricated contact against ball bearing steel rods using the tribological load scanning device, described in § I.1.4. The experiment was run for 5000 cycles with a base oil containing the commonly used Extreme Pressure (EP) additive Sul- furized Isobutylene (SIB). TEM samples were then made using the FIB instrument at a location corresponding to a load of 1200 N. Areas of interest on the BBS rods were found using the SEM in the FIB and a protective platinum layer was first deposited using the electron beam and then the ion beam for faster platinum deposition. The cut-out samples were welded to a special TEM grid inside the FIB and further thinned to electron transpar- ency.

53 a) b) Figure 33. ESCA elemental depth profiles of the Me-DLC coatings. a) A 2.0 µm W- doped coating. b) A 2.2 µm Cr-doped coating.

Surface morphology of tribofilms The surface morphology of the tribofilms formed on the BBS surfaces was significantly different depending on the counter surface, i.e. W-doped or Cr-doped DLC coating, see Fig. 34. The tribofilm formed on BBS when slid against W-doped DLC had a higher degree of coverage than the tribofilm formed against Cr-doped DLC.

a) b) Figure 34. SEM micrographs of the tribofilms formed on BBS surfaces worn against W-doped (a) and Cr-doped (b) DLC coatings, respectively. Arrows indicating sliding direction.

Structure of tribofilms Bright field TEM imaging showed that the tribofilms adhered well to the BBS surface and no pores or cracks were observed in the interface, see Fig. 35. The internal microstructure of the BBS material nest to the interface was observed to bear close resemblance with the surface morphology.

54 a) b) Figure 35. TEM micrographs of and the tribofilms formed on the BBS surfaces when tested against W-doped DLC (a) and Cr-doped DLC (b), respectively.

At higher magnifications, nano crystallites were observed in both tribofilms.

Composition of tribofilms It is evident from the STEM EDS analysis that the W-doped DLC coating was totally worn out since Cr was found in the tribofilm coming from the Cr interlayer, see Figs. 33a and 36c. The essentially constant Cr content in the tribofilm means that tribofilm formed before worn out has been removed. The high wear rate of the W-doped DLC coating is attributed to chemical wear through reaction between the WC phase and the lubricant. The Cr-doped DLC showed very low wear rate as evidenced by only mi- nor traces of Cr in the tribofilm, see Figs. 36b and d. The wear mechanism of the Cr-doped DLC coating is not tribochemical in the same manner as that of W-doped coating, due to the lack of Cr in the centre part of the Cr-doped DLC coating. Owing to the milder conditions during formation of tribofilm against Cr-doped DLC, its morphology is different compared to the tribofilm formed in the W-doped DLC.

55 a) b)

c) d) Figure 36. STEM images of the tribofilm formed on BBS when worn against W- doped (a) and Cr-doped (b) DLC coating. The EDS line profiles were obtained along the indicated lines in a) and b), respectively. Corresponding to W-doped (c) and Cr- doped DLC coatings.

EFTEM was performed on the tribofilms to examine the elemental distribution inside the tribofilms. Jump ratio images were calculated and are shown in Figs. 37 and 38.

56 a) b)

c) d) Figure 37. Cross-section images of the tribofilm on the BBS after testing against the W-doped DLC. A) Zero energy loss filtered image. b)-d) Sulphur, iron and chro- mium jump ratio images.

The distribution of S, Fe and Cr is essentially homogenous in the tribofilm formed on the BBS surface when worn against a W-doped DLC coating, see Fig. 38.

57 a) b)

c) d) Figure 38. Cross-section images of the tribofilm formed on the BBS during testing against the Cr-doped coating. a) Zero energy loss filtered image. b)-d) Sulphur, iron and carbon jump ratio images.

The S jump ratio image shows large intensity variations that should be free of thickness effects. However, jump ratio images may suffer from differences in background slope showing up when dividing two pre edge im- ages23. Some variations show up but only between the agglomerates suggesting that background slope is responsible for some of the variations encountered but not all. When comparing the S and Fe images of Figs. 38b and d, the intensity was observed to be anticorrelated, which was also observed in EDS line profiles.

Conclusions x This work clearly highlights the importance of careful selection of lubricants when using coated components. The EP additivated lubri-

58 cant in this study displayed very different reaction tendencies towards the two coatings and the steel surfaces. x Against the Cr doped coating reactions were constrained to the steel surface, giving a contact with a classical EP tribofilm. The function of the coating would in this case be to provide a mechanical protection in case of lubrication shortage. x In the case of a W doped coating the additive reacted also with the coating, causing accelerated wear of the coating. The tribofilm formed also incorporates coating constituents. It is concluded that this tribofilm is exposed to wear, presumably by abrasive action of the WC phase in the coating itself. x Both tribofilms formed on the BBS surface showed a high degree of crystallinity, with randomly oriented sulphide crystallites.

II.3.2 Wear mechanisms of diamond surfaces in different atmospheres (Paper V) Diamond sliding against diamond is known to exhibit high friction34 in vacuum, on the order of 0.5 to 1. This is in contrast to the low friction measured in normal atmosphere, on the order of 0.01 to 0.135. This phenomenon is attributed to the lack of saturation of carbon bonds at the surface in inert atmospheres leading to strong adhesive forces between the sliding surfaces. Though a number of investigations have been done on diamond friction in different environments the theory is jet not consensus mainly due to the complex nature of the tribological contact. This work deals with analytical TEM studies of wear debris and worn surfaces resulting from self mated diamond sliding in nitrogen and argon environments. The FIB was used to prepare cross section TEM samples of the surface region in the wear track and on an unworn surface.

Sliding tests The wear experiments were performed in a ball-on-flat setup for several passes in argon and nitrogen environments respectively. The friction coefficient was around 0.05 in ambient atmosphere starting from a friction value of 0.1. The friction coefficient in argon increases rapidly from an initial value of 0.1 to a steady-state level of 0.8. A similar behaviour was observed for nitrogen though the steady-state value is 0.7.

59 Analysis of wear debris The wear debris produced in sliding contact in nitrogen environment was found to be amorphous, see Fig. 39a, with highly varying thickness. Wear debris found in the experiment run in argon environment possessed graphitic like structure with bent planes, see Fig. 39b. The distance between planes as measured in the image was 3.4 Å thus in accordance with the planar distance for graphitic planes in graphite.

a) b) Figure 39. TEM micrographs of collected wear debris showing an amorphous structure of debris from a wear test run in nitrogen environment (a) and graphitic planes in the debris from a test run in argon environment (b).

EELS analysis of the wear debris produced in the experiment run in nitrogen showed a small content of nitrogen, see Fig. 40.

Figure 40. EELS spectrum of the carbon K-edge and the Nitrogen K-edge.

60 The EELS spectra were acquired with a convergence angle of 3.1 mrad and a collection angle of 1.1 mrad as to have low momentum transfer. However, this condition affects plural scattering removal algorithms20. Performing quantitative analysis on thin areas, i.e. t/Ȝ 0.4 and integrating the signal close to the edge onset should minimize the contribution of plural scattering. With these settings and a Fourier Ratio deconvolution, quantification yields a nitrogen content of around 10 at%.

Internal film characterizations Cross section TEM samples were produced from the wear track run in nitrogen environment. CVD diamond had similar appearance showing the polycrystalline structure, regardless of whether it had been subject to friction test or not, see Fig. 41. However, the surface of the worn diamond exhibited small protrusions as can be observed in Fig. 41b.

a) b) Figure 41. Bright field TEM micrographs showing the original diamond surface (a) and diamond surface worn against diamond in nitrogen (b). The dark layer on top of both surfaces is the deposited platinum. Two protrusions are observed in (b).

Composition of diamond surface STEM EELS line profiles were acquired on both samples, i.e. worn in nitrogen and unworn diamond. In both samples a thin carbon rich layer, resulting from the initial platinum deposition was observed. It originates from the incomplete decomposition of the organometallic gas used to deposit platinum thus leaving high amounts of carbon in the initial platinum layer. This is observed when using the electron beam to deposit platinum since the 5 keV energy of the electrons is low compared to the energy of the 30 keV Ga ions. An EELS line scan was also acquired over one of the protrusions observed on the worn surface. This line scan showed a trace signal of nitrogen

61 thus suggesting that the protrusion is wear debris resulting from the sliding experiment. In accordance with earlier investigations, there is no indication of graphitization of the diamond surface due to wear. However, nitrogen was incor- porated in the wear debris although being known for its inertness. Probably, nitrogen reduces the high shear-force by partly terminating the interfacial bonds.

Conclusions x No evidence of surface graphitization of the diamond surface tested in nitrogen was found. x Wear debris produced in nitrogen atmosphere was amorphous x Wear debris from experiments in argon atmosphere showed a graphitic like structure, i.e. curved and layered graphite planes x Formation of graphite planes do not automatically give low friction.

II.3.3 Friction mechanisms of Stellite 21 (Paper VI) Stellite 21 belongs to the Stellite family of Co-based alloys and is used as low-friction, galling resistant material in high load dry sliding contact applications. Its properties of Stellite 21 were evaluated by using the tribological load scanning equipment described in § I.1.4. In previous studies it has been shown that laser cladded Stellite exhibits a face-centred-cubic (fcc) structure with a strong (100) texture with the (100) planes parallel to the substrate surface36. When exposed to dry sliding in the load scanner the surface material changes to a hexagonal-close-packed (hcp) phase with the (0002) basal planes parallel to the worn surface. Basal planes in hcp structures often exhibit relatively low friction values due to their low shear resistance.

Sample preparation

The studied Stellite coating was deposited by CO2-laser cladding of cobalt-rich powders on austenitic stainless steel. Friction tests were performed using a tribological load scanning device and exposing the samples for 100 double passes with a maximum normal load of 2400 N. A TEM sample was extracted from the wear track, at an area exposed to a load of 1800 N, by means of FIB sample preparation. At this load, the material was plastically deformed with a macroscopically flat surface. During sample preparation an initial platinum layer was deposited using the electron beam in order to protect the surface region, after which more

62 platinum was deposited using the ion beam to increase the deposition rate. The thin TEM lamellae was then lifted out using a micromanipulator and put on TEM carbon grid.

Microstructure TEM bright field images revealed a polycrystalline microstructure composed of many twin boundaries. Diffraction patterns revealed a complete transition to from fcc to hcp with the hcp planes oriented in different directions depending on the analysed area, see Fig. 42. This was observed for the entire foil, i.e. to a depth of 5 µm, apart from a 30 nm thick tribofilm discussed in the following section.

a) b) c) Figure 42. TEM bright field micrograph of the surface region of worn Stellite 21 (a). Diffraction patterns were acquired from the surface region (b) and at a distance of 1.5 µm from the surface (c). The areas of the acquired SAED patterns are marked in (a).

Composition STEM EDS mapping and line scans were performed on the part of the sample that contained the external surface, see Fig. 43.

63 Figure 43. STEM image of the surface region including the 30 nm superficial tribofilm (top left). A convergent beam electron diffraction pattern from the tribofilm is shown (top right). The elemental maps of Co, Cr and Mo were acquired by STEM EDS within the white rectangular of the STEM image (bottom left). The C and O Jump ratio images were acquired by EFTEM (bottom right).

In the top surface region, a 30 nm thick surface layer was observed. It was also observed that a Mo, Cr, C rich particle (carbide) was covered by the layer suggesting that this layer had been smeared out during sliding contact. It can be identified as a tribofilm. This tribofilm was also observed to have a slightly higher Co content compared to the rest of the bulk material, see Fig. 43. Further, convergent beam electron diffraction showed that major parts of the layer had the (0002) basal planes oriented parallel to the surface thus providing low friction due to the low shear resistance of basal planes. It was concluded that the formation of easily sheared basal planes parallel to the surface in the tribofilm explains the extremely good friction and wear properties of Stellite 21 in severely loaded sliding contacts. This easy to shear surface film is backed up by a deformation hardened layer of Stellite 21. Consequently, the prerequisite for low friction is self-generated. From EFTEM images it was learned that the top 10 nm of the layer had been oxi- dized.

64 Conclusions x It is suggested that the mechanisms behind the excellent low friction properties and galling resistance of the Co-based material are a formation of a thin tribofilm of easily sheared hcp basal planes, combined with the formation of a load carrying deformation hardened sub-surface layer. x The friction coefficient stabilises at a level of 0.2 after the initial plastic deformation during the first stroke. x The wear track shows no signs of galling. x The tribofilm is about 30 nm thick and enriched with Co, but con- tains less Cr and Mo than the bulk material. x The outermost 10 nm of the tribofilm is enriched with O, which in- dicates a mild oxidative wear mechanism.

II.4 Solution of a practical problem-Wear of cemented carbide in rock drilling (PaperVII)

II.4.1 Rock drilling- A tough application of cemented carbide Rock drilling requires high percussive energy and bit rotation, generated by a rock drill rig, see Fig. 44a. The active part of the rock drill, the drill crown (or drill bit) is equipped with WC/Co cemented carbide buttons (or inserts). The energy is generated by the machine and is transmitted through the drill rod to the drill buttons, which performs the crushing of the rock. Rock drilling offers a cruel environment to any exposed material, whether it is the WC/Co CC buttons or the crown steel as can be understood from Fig. 44b.

a) b) Figure 44. Drill rod with a fresh drill bit ready for rock drilling (a) and drill bit re- covered after drilling approximately 100m (b).

WC/Co is used because it combines high hardness and high fracture tough- ness, provided by hard tungsten carbides contained in a tough cobalt matrix. During drilling, the buttons are subject to impact, local heat, cooling and shear resulting in fatigue wear of the buttons. The wear rate, however, is slow and electron microscopy is required to assess the degree of wear and the mechanisms behind it. From earlier studies it was concluded that rock material covers and penetrates into the cemented carbide37. The wear can be divided into five classes of deterioration mechanisms and five classes of material removal mechanisms38. The deterioration mechanisms are: rock

67 covering, intermixing and penetration, embrittlement of the binder phase, composite scale crack formation, cracking of WC grains and oxidation and corrosion of WC grains. However, it is believed that the rock penetration into the surface area of the button plays an important role for the continuous wear rate. It is therefore necessary to analyze cross-sections of the worn surface region in order to understand the wear mechanisms of cemented carbide used in rock drilling.

II.4.2 Sample Preparation The drilling is an example of an application where field testing is fairly easily performed since it is quick and provides abundant of test material for microanalysis. In this work, CC buttons used to drill 18 m in a hard rock type (quartzitic granite) and 20 m in a medium hard rock type (magnetite), respectively, were analyzed by analytical electron microscopy. The areas of interest were regions where rock material had adhered to the surface of the CC button. Several such areas were located by SEM. It was concluded that the best way to prepare samples of the used drill buttons was by means of FIB, since the packed rock was unevenly dispersed over their surfaces. The rock drill button surface suffers from stresses from micro structural changes and penetrated rock material. Thus, the affected CC surface layer would probably not withstand the mechanical stress related to classical TEM sample preparation. To protect the external sample surface area against the ion beam in the FIB, a thin layer of Pt was deposited. The sample was then welded to a special TEM grid inside the FIB and subsequently mildly polished using low angle and low energy Ar ions.

II.4.3 Wear mechanisms of drill buttons Several analytical microscopy techniques, such as STEM EDS and EFTEM were used to uncover the mechanism behind material deterioration of WC/Co buttons.

Drilling quartzitic granite TEM images revealed the distribution of WC grains, Co phase and penetrated rock material, see Fig. 45.

68 a) b)

Figure 45. TEM cross section images on CC button surface drilled 18 m in quartzite. a) STEM image showing the WC grains, Co binder phase and the rock material. b) TEM bright field image showing pores formed in the interface region between WC grains and rock material (1) with small cobalt grains (2). The pores are surrounded by a carbon rich rim (3). Region (4) conatins intermixed rock, Co-particles and WC fragmens. c) TEM lattice image of area (5) in b) showing the interface region between WC and amorphous quartz. A nano sized Co grain is observed in the amorphous quartz.

Several areas between the WC grains were analyzed using EDS and convergent beam revealing a high degree of hcp-Co binder phase. The rock cover was found to exhibit several interesting features hence providing evidence of a totally different composition of the active surface than that of the original WC/Co material. Amorphous rock material is attached to the WC grains at the atomic level, see Fig. 45c. Further, small pores were observed in the interface between the WC and the rock cover, see Fig. 45b. EFTEM analysis verified that small Co crystallites were formed surrounding the observed pores and in certain penetrated areas of the sample. The pores observed in the penetrated rock material are suggested to be due to solidification shrink-

69 age and the lower melting temperature of Co provided the time needed to crystallize.

Drilling magnetite Drilling in magnetite, a softer rock material, resulted in a polished surface appearance significantly different compared to drilling in quartzitic granite. Also here a porous structure was observed in the rock cover, see Fig. 46.

Figure 46. FIB ion image showing a cross section through a button surface after drilling in magnetite. Pores were observed in the adhered magnetite rock.

EDS analysis in the TEM showed that the rock cover layer included amorphous magnetite, crystalline magnetite, fcc-Co and traces of WC fragments. It was also found that Co binder phase was intermixed with magnetite rock and WC fragments. Moreover, the amount of fcc-Co increased with the distance from the surface region of the WC/Co button.

II.4.4 Conclusions x It was found that FIB and TEM together presented a very powerful combination when studying the detailed mechanisms of WC/Co wear in rock drilling. x The surface region was strongly modified and intermixing of rock and Co phase takes place when drilling in both types of rock material. This information could only have been retrieved by analytical TEM analysis performed on FIB samples extracted from precise lo- cations due to the small dimensions encountered. x The fragile samples exclude classical TEM specimen preparation

70 II.5 Summary of Contributions Performance and lifetime of almost any type of tool or mechanical component is limited by its tribological contacts. Both friction and wear are associated to large changes in structure and composition of superficial material layers as a result of deformation and chemical reactions between constituents in the surface and surrounding medium or lubricant This is, in turn, stimu- lated by the temperature raise generated by friction. The relative sliding between two materials in a tribological contact involves breaking and re-establishing of atomic bonds. Thus, friction is a result of the resistance to shear across the interface between the contacting surfaces, a resistance that is ultimately determined by the inter-atomic bond strength. Wear defined as loss of materials from a tribological surface is associated to the generation of wear debris, which is associated to different fracture mechanisms that also can be characterised down to the atomic scale. Consequently, the understanding of the detailed mechanisms behind friction and wear, necessary for further development of tribological materials and systems, requires microscopical studies and materials analysis at the highest possible resolution. The results presented in this thesis demonstrate the power of modern analytical techniques and high-resolution electron microscopy by applying them to a number of selected tribological materials and tribological systems.

x Development of new low friction, carbon based coatings was aided by detailed descriptions of the structure and composition of experimental coatings of TiC, TiB2 and (TaAl)C:C; as deposited and heat treated. (Papers I-III) x One of the most important tribological subjects of today is to design systems that function without any toxic constituents in lubricants or lubricant additives. To aid in this field, the structure and composition of tribo films formed in the presence of additivated lubricant could be revealed for the sliding contact between uncoated and Cr- doped or W-doped DLC coated ball bearing steel by the modern analytical techniques associated to high resolution TEM. (Paper IV) x The friction properties and wear resistance of CVD diamond coatings is not yet fully elucidated. High resolution TEM revealed the structure of wear debris and the outmost 2-3 atomic planes of worn diamond surfaces. The fragments form tests in argon atmosphere were graphitic, whereas fragments from tests in nitrogen atmosphere were amorphous. No evidence of graphitisation was found in the worn surface. (Paper V)

71 x The mechanisms behind the low friction and anti galling properties of Stellites (Co-based materials) at severe sliding conditions, often discussed in the literature, was revealed to be a superficial phase transformation from an fcc structure to a nanometre thin tribo-layer of easily sheared hcp structure. Consequently, a low friction layer was self-generated during the sliding contact. This mechanism had not been realised before. (Paper VI) x Finally, applying modern analytical techniques and high-resolution electron microscopy to an actual tribological component; rock drilling tool button of cemented carbide (CC), has proved very useful. Here, the power of the FIB technique was utilised to extract thin TEM specimens directly out of the worn CC surface of used drill buttons. A totally new mechanism of rock material penetration into the CC material during drilling was revealed. The Co matrix of the original WC/Co composite material is successively replaced by a mixture of rock material and Co metal in the surface of the drill button. This new CC matrix is brittle, and wear of the CC buttons occur primarily by matrix removal. (Paper VII)

II.6 Future challenges of HR-TEM HR-TEM and its associated analytical techniques will increasingly serve as a means to gain atomic level understanding of materials, coatings and surface layers in disciplines of materials science. Within the field of tribological materials, the following areas can be identified as the most urgent. x Low friction coatings for dry applications in ambient air or vacuum. x Coatings adopted for environmentally acceptable lubricants. x Wear resistant coatings possible to deposit down to 150º C, to avoid over-tempering of component steel. x Wear resistant, galling free surfaces for tooling applications. x Self cleaning, anti sticking surfaces for windows , solar panels, buildings, etc.

72 Sammanfattning på svenska

Bakgrund Tribologi handlar om teknik och vetenskap av ytor i kontakt och relativ rö- relse. Det gör att tribologi finns överallt och därmed är ett viktigt forsk- ningsområde. Egenskaper förenade med tribologi är friktion, nötning och smörjning samt bildandet av tunna lager, så kallade tribofilmer, som åter- finns på ytor utsatta för tribologisk kontakt. Tribologi är också av stor eko- nomisk betydelse för alla industrisektorer. Under senare tid har detta varit uppenbart i utvecklandet av verktyg, maskinkomponenter och fordon som skall möta framtidens allt strängare miljökrav. En tribologisk kontakt utsätts för oerhört stora påfrestningar lokalt, d.v.s. inte över hela ytan, med mycket höga temperaturer, nästan upp till smält- punkten, och extremt höga tryck. Det gör att nya material som kan utstå dessa ogästvänliga miljöer måste utvecklas. Sedan slutet av 1960-talet har det blivit allt vanligare att verktyg och komponenter får en beläggning som skall skydda det underliggande materialet, substratet, mot nötning eller ge det nya bättre egenskaper såsom låg friktion och bättre oxidationsbeständighet. Detta resulterar ofta i användandet av tunna keramiska beläggningar på stål eller hårdmetall. Då keramer är hårda men sköra och substratet ofta segt men ej så hårt bildar dessa två tillsammans ett material som är både hårt och segt. Denna komposit klarar påfrestningar som de ingående materialen inte skulle klara på var sitt håll. Vid användandet av material i tribologiska tillämpningar är det vanligt att tribofilmer bildas. De är resultatet av kemiska reaktioner med omgivningen, och är oftast väldigt tunna, i storlekordningen ett par miljondels millimeter till några tusendels millimeter. Trots tribofilmernas obetydliga tjocklek är det vanligt att de styr kritiska tribologiska komponenters egenskaper. Utifrån detta är det lätt att inse att studier av tribologiska komponenter erfordrar analysmetoder med en upplösning i paritet med de minsta beståndsdelarna i en tribologisk kontakt, det vill säga atomär upplösning. I detta arbete har det visats hur väl lämpad, och ofta helt avgörande, elek- tronmikroskopi är för att på detaljnivå karakterisera beläggningar och tribofilmer med avseende på deras atomstruktur och sammansättning.

73 Bidrag och resultat Det primära målet med arbetet var att öka förståelsen av tribologiska ytor genom att undersöka mikrostrukturen hos beläggningar och tribofilmer upp- komna som resultat av tribologisk kontakt. Med denna nyvunna förståelse vore det möjligt att tillverka skräddarsydda beläggningar för olika tillämp- ningar. Mikrostrukturen studerades hos tre olika beläggningar med avseende på beläggningsparametrar och oxidationsbeständighet. Titankarbid (TiC) som använts sedan 1969 som beläggning på skärverktyg är ett mönstermaterial när det gäller att utvekla nya beläggningsmetoder och studera beläggnings- parametrar. Detta kommer sig av att TiC har samma kubiska koksaltsstruktur för ett stort intervall av titan till kol förhållande. Vid högre kolinnehåll än titaninnehåll bildas TiC med ett till ett förhållande plus fritt kol. Studien av TiC visade att det var möjligt att påverka materialets struktur och därmed dess egenskaper genom att ändra innehållet av kol i beläggningen. Ett annat sätt att påverka en beläggnings struktur är genom ändring av den elektriska spänningen på materialet som beläggs, under själva beläggningsprocessen. Även här visade det sig att mikrostrukturen ändrades och gjorde det möjligt att tillverka titandiborid (TiB2) med låga inre spänningar och bebehållen hårdhet. Fördelen med att få bort de inre spänningarna är att tjockare belägg- ningar kan produceras utan att riskera att de flagar av. Syre är som bekant, ett mycket reaktivt grundämne inte minst vid höga temperaturer. Som sades tidigare kan mycket höga temperaturer förväntas lokalt vid tribologiska tillämpningar. Vid dessa höga temperaturer oxiderar många material och den yta som oxideras kan i regel lätt skrapas av vilket leder till att ny, opåverkad yta blottläggs och utsätts för oxidation. Denna process leder ofta till en snabb avverkningstakt av beläggningar. För att motverka detta gäller det att få ner oxidationstakten. Reduktion av oxida- tionstakt hade påvistas för tantalkarbid innehållandes aluminium men inte varför det förhöll sig så. Genom att studera ett tvärsnitt av ett oxiderat prov och jämföra detta med ett icke oxiderat prov framgick det att den oxiderade beläggningen inte var genomoxiderad samt att oxidationen hölls separarerad från resten av det opåverkade materialet av ett ytterst tunt gränsskikt. Dessa resultat kommer att vara behjälpliga vid utvecklandet av nya beläggningar. Studier rörande tribofilmer och deras påverkan på yttre egenskaper som friktion och nötning genomfördes på ett antal tribologiska system. En av studierna behandlade metalldopade kolbeläggningar i gränsskiktssmord kontakt med olja och additiv mot kullagerstål. Resultatet från dessa experiment visade att vissa metaller i kolbeläggningen reagerade med additivet vilket ledde till hög nötning av beläggningen. Detta gäller dock inte alla metaller vilket visade att det viktigt att studera kolbeläggningar kompatibilitet med additiv i oljor. Ett material som ofta nämns i materialsammanhang är diamant. Tidigare studier har visat på stora skillnader i friktion när diamant

74 glider mot diamant i vakuum. Däremot har diamant glidandes mot diamant låg friktion vid försök i luft. Orsakerna till denna skillnad är inte helt utfors- kade och försök gjordes för att ta reada på vad som händer på ytan vid förök i kvävgas och argongas. Resultaten tyder på att kväve reagerar med diamantytan under glidande kontakt och därmed ger lägre friktion än vad som är fallet med argongas. Vidare visade det sig att nötningsfragmenten bildade vid nötning i kvävgas innehöll kväve och var amorfa. Experimentet i argon däremot gav grafitliknande struktur hos nötningsfragmenten. Det fanns hel- ler inga spår efter grafitisering hos diamantytan utsatt för glidande kontakt i kvävgas. I fallet Stellit 21 gjorde en ytlig tribofilm på 30 miljondels millimeter att låg friktion erhölls vid mycket hög last. Slutligen applicerades dessa moderna analysmetoder på en äkta tribologisk komponent; hårdmetallstift för borrning i berg. Här användes en fokuse- rad jonstråle för att ta fram transmissionselektronmikroskopiprover från bergsborrstiftet. Vid närmare undersökning visade det sig att berget trängt in i hårdmetallen och därmed bildat en blandning av berg, och kobolt som er- satte den tidigare rena koboltfasen. Detta ledde till skörare hårdmetall där nötningen sker primärt genom borttagande av koboltmatris. Med användandet av den här typen av analysmetoder förväntas utveck- lingen gå framåt när det gäller framtagandet av nya miljöanpassade material för tribologiska tillämpningar, beläggningar av temperaturkänsliga material samt utvecklandet av lågfriktionsbeläggningar för torr glidande kontakt i såväl luft vakuum.

Acknowledgements

This work has been carried out at the Tribomaterials group, Engineering sciences, the Ångström Laboratory, Uppsala University. The financial support from the Swedish Research Council for Engineering Sciences (TFR) is gratefully acknowledged. I would also like to express my thanks and grati- tude to all of you involved in this work, former and present friends and col- leagues.

Especially I would like to thank:

Prof. Sture Hogmark, my supervisor, for giving me the opportunity to continue the work at the Materials Science Division and providing help in writing and analysing the results. Ass. Prof. Urban Wiklund, my co-supervisor, for support, advice, interesting discussions and late time reading and preparations of presentations anywhere in the world, at any time. Professor Staffan Jacobson for helping out with the thesis. Ass. Prof. Stefan Csillag for advice, support and discussions about practical and theoretical topics regarding microscopy in general and EELS in particular. Prof. Eva Olsson for guidance, advice, discussions and support. Thank you for introducing me to the fantastic area of electron microscopy. Dr. Martin Saunders for sharing your knowledge in electron microscopy and other areas of interest. Anders, Jesper and Mattias for all the fun during our time here and else- where, who can forget CORSICA? Ulrika for kindly guidance, support and for being the greatest room-mate. I will try to help you as much as you helped me during the writing. Mattias Carlsson for knowing what a thesis writing student needs, COFFE and CANDY. Daniel Persson for help with reading the best parts of the thesis. Ulrik Beste for helping out with the thesis and being nice company at con- ferences. Daniel Nilsson and Mattias Berger for providing interesting materials to analyze. Lars Hammerström for last minute reading. Joakim Andersson for helping with paper writing and food intake.

77 Nils Stavlid for late time help in correcting papers. Sima Valizadeh for reading parts of the thesis and providing valuable help. Fredric Ericson, Rein Kalm, Jan-Åke Gustafsson and Masanori Mori for keeping the instruments running. Finally I would like to thank my whole family, my parents, sisters and broth- ers for your love and for helping me out when I most need it. Thank You Märta for your love, support and encouragement.

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