Structure, Mechanical Properties and Tribology of W–N and W–O Coatings

Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals & Hard Materials

journal homepage: www.elsevier.com/locate/IJRMHM

Structure, mechanical properties and tribology of W–N and W–O coatings

T. Polcar a,*, A. Cavaleiro b a Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, Prague 6, Czech Republic b SEG-CEMUC – Department of Mechanical Engineering, University of Coimbra, Rua Luís Reis Santos, P-3030 788 Coimbra, Portugal article info abstract

Article history: The tribological properties of nitrides and oxides of transition metal thin films deposited by reactive mag- Received 21 February 2009 netron sputtering have been thoroughly studied for three decades. Nevertheless, there are still several Accepted 13 July 2009 gaps in knowledge. The majority of studies are focused on a limited number of metals, namely Ti, Al and Cr, while other potentially attractive compounds are aside the main attention. Even in case of TiN, probably the most studied hard thin film, the frictional and wear behaviour brings many controversies. Keywords: Despite significant progress of analytical and computational methods, the analysis of the wear behaviour High temperature tribology is still a great challenge. Tungsten nitride We are presenting here a summary of our recent work on tungsten nitride (nitrogen content 0–58 at.%) Tungsten oxide Protective coatings and tungsten oxide (oxygen content 0–75 at.%) coatings deposited by reactive magnetron sputtering. Our aim has been the analysis of the connection of fundamental properties of these films, such as chemical composition, structure, hardness, Young’s modulus and residual stress, with their tribological properties – friction coefficient and wear rate. We have been focused mainly on the description of the dominant wear mechanisms influencing the tribological properties. The tribological tests have been carried out both at room and elevated temperature; the temperature was increased in steps until immediate coatings failure. The tungsten nitride coatings with the ‘‘worst” parameters generally considered as vital for high wear resistance, such as hardness, were considered to have the best tribological performance. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction W–N coatings were studied for mechanical applications [2]; nevertheless, the main attention was paid to ternary systems pre- Coatings of transition metals nitrides are known for their high pared by the addition of different elements (e.g., W–Ti–N [3] and hardness and excellent wear resistance, being titanium nitride W–Si–N [4]), with the final scope of improving the mechanical the best known member of this family. However, its tribological properties of the coatings. Tribological properties of tungsten ni- properties are significantly diminished when the coating is ex- tride coatings at room [2,5] and high [6] temperature were thor- posed to elevated temperatures, particularly due to rapid oxida- oughly studied by present authors. It has been shown that the tion. In this respect, chromium nitride possesses significant tribological properties of tungsten nitride coatings are strongly advantages compared to TiN, mainly higher oxidation resistance, influenced by the formation of a tungsten oxide tribolayer even thermal stability and, particularly, corrosion resistance. On the at room temperature. Further increase of environment tempera- other hand, as main drawback CrN exhibits a significantly higher ture led to the oxidation of the coatings, particularly in the area friction coefficient, which is not desired in many applications [1]. of the contact. A great amount of research work was also carried Other nitrides, such as MoN, VN or AlN, could outperform TiN or out by the authors in the W–O system [7,8]. Besides a complete CrN in many parameters; however, their commercial use is very and detailed basic characterization of the coatings, particular inter- limited. Tungsten nitride stays still aside of attention despite some est was paid to their thermal stability in both protective and significant advantages, such as extremely high hardness and excel- oxidant atmospheres [9]. In some specific cases, the overall tribo- lent adhesion on typical steel substrates. logical behaviour of the coatings was assessed by pin-on-disk against different counterbody materials [5–7]. This paper summarizes our work dealing with the tribological properties of tungsten nitrides deposited by magnetron sputtering. Additional results concerned with the W–O system will be also * Corresponding author. Tel./fax: +420 224 312 439. presented as a tool for interpreting and understanding the global E-mail address: [email protected] (T. Polcar). tribological behaviour of these systems.

0263-4368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.07.013 16 T. Polcar, A. Cavaleiro / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22

2. Experimental details + + Tungsten oxide and nitride coatings were deposited by DC reac- W25O75 + * tive magnetron sputtering from a tungsten target in a Ar + O2 or N2 # atmosphere, respectively, onto high speed steel AISI M2 substrates W45N55 # * heat treated to have a hardness close to 9 GPa. The deposition runs º W88N12 º º for both tungsten compounds were performed keeping constant º * the following parameters: total working pressure (0.3 Pa), target W81O13 º º 2 º current density (10 mA cm ), substrate at floating potential, no & W90N10 substrate rotation, inter-electrode distance (65 mm) and substrate Intensity (arb. units) & temperature (<350 °C, with no external heating). In order to W100 achieve different N and O contents in the films, the coatings were deposited with different flow ratios of the reactive gas, N2/Ar from 0 to 3 and O2/Ar from 0 to 1, respectively. Before any deposition, an 10 30 50 70 ultimate vacuum pressure better than 5 104 Pa was reached 2 Theta (deg) and the substrates surface was ion cleaned with an ion gun.

The chemical composition and the structure were determined Fig. 1a. Typical X-ray diffraction patterns of W–N and W–O sputtered coatings. by a Cameca SX-50 electron probe microanalysis apparatus (EPMA) (* – Substrate; + – WO3;o–b-W, # – W2N, and – a-W). and an X-Pert Philips X-ray diffractometer (XRD), respectively. The oxides microstructure was analyzed by high resolution transmis- sion electron microscope (HR-TEM – JEOL 2010-FEG, operated at 200 kV). The mechanical properties of the coatings were evaluated by depth-sensing indentation technique using a Fischer Instru- ments-Fischerscope, with a maximum load of 50 mN and following the testing procedure described elsewhere [10]. The adhesion/ cohesion of the coatings was evaluated by scratch-testing technique using a Revetest, CSM Instruments. The load was increased linearly from 0 to 50 N, using a Rockwell C 200 lm radius indenter tip, loading rate of 100 N/min, and scratching speed of 10 mm/min. Wear testing was done using a high temperature pin-on-disc tribometer (CSEM Instruments). Al2O3 balls with a diameter of 6 mm were used as counter-parts. All measurements were pro- vided with a load of 5 N and a linear speed of 0.05 m/s; the relative humidity of the air was kept constant at (25 ± 5)%. The maximum static Hertzian pressure for the elastic contact between the ball and the coating was about 1.5 GPa. The worn volume of the balls was evaluated by optical microscopy. The morphology of the coating surface, ball scars, wear tracks and wear debris were examined by scanning electron microscopy (SEM); the chemical analysis of the wear tracks and the wear debris was obtained by energy-dis- persive X-ray analysis (EDX). The profiles of the wear tracks were measured by mechanical profilometer. The wear rates of the ball Fig. 1b. HR-TEM image of W26O74 coating. and the coating were calculated according to [6] as the worn material volume per sliding distance and normal load. The average value of five profiles measured on each wear track was used to calculate Typical XRD diffractograms for W–N and W–O are shown in the coating wear rate. Fig. 1a [5,8]. The structure of the W–N and W–O coatings can be correlated with their chemical composition: (i) for low-nitrogen or oxygen contents (N,O < 9 at.%), the coatings exhibited the equi- librium bcc a-W phase with a (1 1 0) preferential orientation; (ii) 3. Results and discussion for coatings with intermediate nitrogen and oxygen contents up

to 30 at.% (e.g., W88N12,W85N15 or W81O19) the b-W phase is de- 3.1. Coating characterization at room temperature tected mixed with the a-W; in the N-containing ﬁlms for at.%

N > 15at.% fcc. NaCl-type b-W2N is also occurring (iii) for coatings 3.1.1. Chemical composition and structure with higher reactive element contents (N,O > 30 at.%) the structure The increase of the nitrogen partial pressure ratio p =p from is either formed by the NaCl-type nitride compound, b-W N, with ð N2 ArÞ 2 0 to 3 led to a linear increase of the nitrogen content in the films (2 2 0) preferential orientation, in W–N coatings or by a low crys- from 0 to 55 at.%. The oxygen content originating from the residual tallinity order phase. It should be pointed out that an increase of atmosphere and the chamber leaks was lower than 5 at.%; when nitrogen content over 55 at.% was followed by the formation of higher nitrogen flows were used, the oxygen content decreased. hexagonal d-WN phase [6] whereas the structure of the W–O coat- Similar results were obtained for the W–O system, but being pos- ings with high oxygen content can be described as nanocrystalline sible to reach much higher O contents (75 at.%) even if the max- or quasi-amorphous. Fig. 1b shows a high resolution TEM image of imum used partial pressure ratio was much lower p =p 1 . the microstructure of this film. The presence of very small crystal- ð O2 Ar ¼ Þ This is due to the much higher affinity of W for O than for N. It lites embedded in a low order crystallinity matrix is clearly obser- should be pointed out that the low reactivity of tungsten allows vable. SEM micrographs of all W–N coating cross-sections showed sputtering of nitrides and oxides in all ranges of chemical typical columnar morphology contrasting with the featureless compositions. W–O films (Fig. 2). T. Polcar, A. Cavaleiro / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22 17

40 W-N W-O 35

10 Critical load Lc2 (N) 5

0 1020304050607080 N or O content (at.%)

Fig. 4. Critical load Lc2 for W–N and W–O coatings.

diffraction angles. Therefore, the nitrogen is probably being placed in interstitial positions in the lattice increasing the dilatation of the lattice parameter and inducing increasing compressive stresses from 3.7 to 4.8 GPa. Consequently, the hardness also increases from 22 to 40 GPa. In the case of W–O coatings the effect of the interstitial position of oxygen in the W lattice does not give rise to any hardening effect. Firstly, the distortion caused in the W lattice is much lower for O than for N (e.g., for 10 at.% N(O), 0.9% for O against 2.5% of N [8,12]) and, secondly, the higher electronega- tivity of O in relation to N decreases the covalent character of the

Fig. 2. SEM cross-sections of W65N35 (a) and W65O35 (b) coatings (Si substrates). bond, giving rise to lower materials strength. b-W phase, which is dominant for W88N12 and W84N16 coatings, is known to have low compressive residual stress [13], or even tensile stress [14]. 5 The residual stress value of these two coatings dropped to 3.3 44 W-N - Hardness 4 and 3.4 GPa, respectively, leading to a decrease in the hardness 40 W-O - Hardness W-N - Res. stress 3 down to values close to 29 GPa. On the other hand, the occurrence 36 W-O - Res. stress of this phase in the W–O coating gives rise to a small increase in 2 32 the hardness (W87O13 ﬁlm). 1 28 For high nitrogen content (N P 34 at.%), the coatings display the 0 b-W2N phase, without or with (2 2 0) preferential orientation. The 24 -1 hardness of these coatings decreases proportionally from 41 to 20 -2 30 GPa. It can be interpreted by the synergetic effect of the com- 16 -3 pressive residual stress decrease with the change in the preferen-

Hardness (GPa) Hardness tial orientation of the films. In the W–O films the hardness 12 -4 linearly decreases from 25 (W87O13) down to 7.7 GPa (W25O75). 8 -5 Residual stress (GPa) The amorphisation of the coatings associated with the increasing 4 -6 importance of the W–O ionic bonding type justifies the drop in 0 -7 the coatings hardness. 0 10203040506070 The adhesion/cohesion values of the coatings were measured by N or O content (at.%) scratch-test apparatus to calculate the critical loads Lc1 and Lc2 (see Fig. 4). These critical loads are defined as the first cohesive Fig. 3. Hardness and residual stress of W–N and W–O coatings with different nitrogen and oxygen content, respectively. and adhesive failures, respectively. With increasing nitrogen content Lc1 and Lc2 values decreased linearly from 18 to 9 N and from 50 to 13 N, respectively. Pure tungsten coating exhibited Lc2 value 3.1.2. Hardness, residual stress, and adhesion higher than the maximum applied load (50 N). The scratch-est The evolution of the residual stress and the hardness as a func- behaviour is even worse for W–O films, Lc2 decreases abruptly tion of the nitrogen content in W–N coatings, depicted in Fig. 3, fol- down to 10 N for O content up to 30 at.% and is kept approxi- lows similar trends. A linear relationship can be established mately constant for higher contents (W25O75 film 9 N). between the hardness and the residual stress, showing that the lattice distortions related to the structural defects that induce 3.1.3. Friction and wear of the low-Nitrogen W–N coatings increasing intrinsic stresses are determinant effects influencing Fig. 5 shows the evolution of the friction coefficient and the the measured hardness of the coating. coating wear rate with increasing nitrogen content when the coat-

All coatings display a compressive residual stress state, result- ings are sliding against Al2O3 balls. The main feature which seems ing from the backscattered energetic neutral impinging on the to determine the inversion in the tribological behaviour of W–N growing ﬁlm surface [11]. The increasing nitrogen content from 0 coatings is the presence of the NaCl-type W2N phase. In fact, the to 9 at.% promotes a shift in the a-W (1 1 0) peak position to lower drop in the friction and wear coefﬁcients was registered for the 18 T. Polcar, A. Cavaleiro / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22

0.7 4.0 β-W N α-W α-W + β-W N β-W N 2 2 2 δ-WN 3.5 0.6 3.0 /Nm) 3 2.5 0.5 mm -6 2.0

0.4 1.5 Friction coefficient 1.0

0.3 Wear rate (10 0.5

0.2 0.0 0 102030405060 Fig. 7. SEM micrograph of the wear track, coating W88N12 (back-scattered mode). The defects are clearly visible (see black arrows). Nitrogen content (at.%)

Fig. 5. Tribological properties of W–N ﬁlms at room temperature. The dominant structure zones are indicated.

Fig. 6. Typical wear debris taken from the ball, tests with low-nitrogen coatings. The large particles are clearly visible. Fig. 8. SEM micrograph of W85N15 wear track.

coating in which this phase started to be dominant. Al2O3 ball scars cal increase of temperature, which enhances the tribo-oxidation of were very small and transfer of the coating material to the ball sur- the coating. The tungsten oxides are much softer than the nitrides face has not been observed abundantly; the only exception was the and, thus, local plastic deformation can occur. Oxidation of mate- test with W88N12 film, where a measurable ball scar was detected. rial in the defect was confirmed by EDX. This coating exhibits as SEM analysis of the wear particles has revealed that there are well the lowest value of the critical load Lc1, which can enhance two different types of wear particles: small round particles with the formation of large delaminated particles. Moreover, the ball an average dimension of less than 50 nm and large particles with wear scar is covered with deep scratches and, thus, the delaminat- sharp edges with a grit size of 1–3 lm(Fig. 6). The highest concen- ed particles are abrasive enough even to damage the hard Al2O3 tration of the large particles occurs in the wear debris of the coat- ball surface. No major defects of plastic deformations are visible ing with 12 at.% N; however, even in this case, the amount of the in the wear track of the W85N15 coating (Fig. 8). As a consequence, small particles is clearly dominant. The chemical composition of compared to W88N12 sample, the wear rate and the friction coeffi- the small particles measured by EDX is almost stoichiometric tung- cient decrease significantly. This different behaviour can not be sten trioxide with no vestiges of nitrogen, while the large particles attributed to any change in the mechanical properties, since hard- are not oxidized. ness, residual stress and critical load of both coatings are almost

The wear track of W95N5 coating shows only shallow scratches identical. However, as referred to above, the XRD analysis has re- parallel to the relative sliding movement. Defects in the wear vealed that from one coating to the other, the presence of the b- tracks could be observed by SEM in the films with further increase W2N phase was detected which seems to affect the tribological of nitrogen content. The W90N10 wear track is very similar to the behaviour. In fact, the coatings formed predominantly by the b- previous one; however, cohesive failures of the coating could be W2N phase present similar wear mechanisms as those of W85N15 detected. These failures contributed to the increase of the wear one. rate, since the wear debris containing large tungsten nitride parti- In general, for low-nitrogen content films (N content < 15 at.%), cles can cause abrasive wear due to their sharp edges and high the wear is driven by a combination of abrasion, delamination and hardness. The W88N12 sample exhibits the highest wear rate and tribo-oxidation. The influence of the interlayer of wear debris, friction coefficient. The number of delaminations in the wear track which is formed between the surfaces in the contact zone, is neg- rapidly increases as well as the number of large W–N particles, ligible, since it does not adhere to any surface and is rapidly driven causing very deep scratches in the coating surface. The largest out from the contact area. Tribo-oxidation occurs during the entire scratches are finished by the accumulation of coating material test, since the majority of the wear debris particles are oxidized. revealing plastic deformation (Fig. 7). It can be assumed that a The frictional heat generated by dry sliding is almost exclusively large particle may get jammed in the coating surface causing a lo- dissipated away through the asperities in contact causing a signif- T. Polcar, A. Cavaleiro / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22 19 icant local increase of temperature leading to the oxidation of each for the W–Nitride calculated from XRD patterns. In case of asperity [15]. Thus, during the contact, as tungsten oxides have W45N55 the wear debris covers almost all the wear track. lower density than the nitrides, the asperities will oxidize and ex- The sliding of the coatings with high nitrogen content is typical pand lifting apart the remaining asperities. This effect results in the of a three-body abrasive wear. In this case, the wear particles are concentration of both the frictional energy dissipation and the free to slide over the surface, since they are not held rigidly. Con- mechanical load on a few asperities only. The tungsten oxides sequently, the wear rate is much lower than for a two-body wear, are soft and the oxidized asperities can be plastically deformed characteristic of the low-N content films. The three-body wear is and easily removed from the coating surface. This mechanism is allowed by a rapid production of the wear debris in the first stages typical for W95N5 coating and can be described as a polishing of the sliding. The thick interlayer between opposing surfaces is and mild wear mode. responsible for the increase of the contact area resulting in a de- The delamination of large particles leads to tungsten-rich (i.e., crease of the contact pressure. Consequently, the wear rate and not oxidized) particles in the wear debris for W90N10 coatings the friction coefficient decrease. Moreover, the wear debris adheres revealing that other mechanism of material removal has to be con- on the sides of the wear track, where the contact pressure reaches sidered. This coating exhibits the highest hardness, Young’s modu- minimal values. The surface of the central groove of the wear track, lus and, particularly, residual stress values. It is possible that the which is neither covered by adhered oxide layer nor oxidized, is expansion of the oxidized asperities can not be accommodated the only area of the coating material exposed to direct wear, since by the coating and, together with the stresses induced by the fric- adhered oxide layers totally protect the coating surface. tion, creates cracks and enhances their propagation leading to coating chipping. The presence of hard sharp-edged particles in 3.1.5. Sliding of selected W–O coatings the contact is typical of abrasive wear, as it is confirmed by the When the tribological behaviour of the W–O coatings is ana- presence of scratches in the wear track. The wear and friction coef- lyzed and compared to W–N films it can be immediately concluded ficient increases as a result of the presence of the non-oxidized W– that their performance is inferior concerning both the friction coef- N particles in the contact. Therefore, the abrasive process and ficient and the wear rates. For example, for low interstitial content delamination are the main wear mechanisms for W90N10 film. films W88N12 and W87O13 the values are l = 0.6 and 0.73 and the These mechanisms are complemented by local plastic deformation wear rate 3.3 and 4.4 106 mm3/Nm, respectively, whereas for in the case of W88N12. In this case, in spite of having a global lower compound films W45N55 and W25O75 the values are l = 0.33 and stress state due to the presence of b-W phase, the delamination can 0.49 and the wear rate 0.15 and 2.9 106 mm3/Nm, respectively still occur due to either the high local stresses in the a-W phase [5,7]. In the low O content film, also containing the b-W phase, in (see Fig. 3) or due to dimensional changes in b-W during sliding. spite of its moderate hardness (25 GPa) no formation of big wear As the hardness is lower, the large particles can be embedded in particles takes place due to the low local stresses in the a-W phase. the coating surface and be plastically deformed. Thus, the wear track is rather smooth with only shallow scratches.

W85N15 coating stands for the transition in the wear mechanism However, local well adherent plates of tungsten oxide cover par- from low-N to high-N content films. In fact, in spite of having tially the wear track. For the W25O75 coating a high amount of wear approximately the same mechanical properties (H, E, Lc, rr)as debris is formed promoting a very compact and homogeneous W88N12 film, its tribological behaviour starts to be different. Com- layer covering entirely the wear track. Moreover, also on the ball pared to W88N12 films, the structure of the W85N15 coating is differ- surface tungsten oxide can be detected. In this case, an almost ent: (i) the amount of b-W phase decreases, (ii) the a-W phase exclusive third-body wear mechanism prevails with self-mated recovers, with a significant decrease in its lattice distortion and lo- tungsten oxide. cal stresses, and (iii) W2N phase appears. The combine effect of At a first sight it seems that in both W–N and W–O systems the these structural changes results in similar mechanical but different sliding mechanisms are quite similar, i.e., for low interstitial con- tribological properties. Neither plastic deformation defects in the tents the contact between the ball and the coating is direct, with- wear track nor W–N particles in the wear debris were detected out or with low amount of third-body in-between, whereas for after the running test. The aspect of the wear track is only of pol- high N/O contents a high amount of wear debris are present in ishing wear, resulting in a significant decrease of the friction coef- the sliding contact. Therefore, the differences in the friction coeffi- ficient and the wear rate. Moreover, for this film no adhered plates cients for both systems are surprising, particularly when the fric- could be observed in the wear track. tion of oxides is higher than that of nitrides – quite unusual under similar conditions. Particularly the case with low-N/O con- 3.1.4. Sliding of W–N coatings with high nitrogen content tent is difficult to explain, since the typical abrasive wear for

The wear debris produced during sliding of high-N content films W88N12 coating should exhibit higher friction. The possible reason is very homogenous containing only small round particles with a for higher friction of W87O13 can be the adhesion of the tungsten diameter of 20–30 nm. Oxidation studies performed in W–N coat- oxide tribolayer to the transferred oxidized coating material on ings showed that, in coatings with high N content, the oxide scale the alumina ball. Similar process could take place in sliding of spalled off as it was being formed whereas it stayed attached to the W25O75. Alumina balls sliding against nitrides, on the other hand, base coating in the case of low-N coatings. This can explain the showed not vestiges of adhered coating material. In other words, easy formation of the very small wear particles in these coatings the steady-state sliding of W45N55 film is represented mainly by compared to the low-N content ones. Microanalysis of the wear Al2O3 ball – W–O tribolayer contact, whereas self-mating of tung- debris has shown that their chemical composition is stoichiometric sten oxides become dominant after running in with W25O75 coat- tungsten trioxide. The volume of adherent wear debris is much ing, which can lead to strong adhesive forces. higher compared to that of low N-content coatings. The wear tracks are very smooth, neither defects nor cracks have been ob- 3.2. Characterization of WN films at elevated temperature served. The large volume of adhered wear debris, which consider- ably exceeds the worn volume of the coating, can be easily 3.2.1. Structure and hardness explained by the difference in the density of the coatings and the As referred to above, the tribological behaviour of W–N coatings adhered wear debris. The wear debris is exclusively stoichiometric at room temperature could be divided into two groups with differ- tungsten trioxide, which is much less dense than bulk WO3 with a ent dominant wear mechanisms. The coatings with low-nitrogen density of only 7.16 g cm3 [16], when compared to 16.2 g cm3 content (up to 15 at.%) exhibited higher values of friction coeffi- 20 T. Polcar, A. Cavaleiro / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22 cient and wear rate, since their sliding properties were dominated a 45 by abrasive wear and delamination. High nitrogen contents (33– 55 at.%) led to a typical three-body wear with a thick layer of tung- 40 sten trioxide consisting of small round particles. Considering the 35 hardness and the wear rate of tungsten nitrides, three chemical compositions from the later group were selected for the tribologi- 30 cal tests at elevated temperatures due to the following reasons: (i) 25 the coatings with high nitrogen content exhibited lower wear, (ii) the high wear resistance of these coatings was caused by the for- 20 mation of a third-body of tungsten trioxide; this phenomenon

Hardness (GPa) 15 should be enhanced at higher temperatures, and (iii) the selected W N chemical compositions cover coatings with either the highest 10 70 30 W N hardness or the lowest friction and wear. 53 47 5 All coatings showed similar diffraction patterns from room tem- W42N58 perature up to 400 °C, see Fig. 9. Only a small shift of the XRD peaks 0 positions to higher diffraction angles can be observed being attrib- 0 100 200 300 400 500 600 uted to stress relaxation. The presence of small oxide peaks close to Annealing temperature (ºC) 2h 25° revealed that at 500 °C surface oxidation already occurred while, at 600 °C, XRD patterns of the coatings after annealing b 450 exhibited only the m-WO3 structure (WO3 monoclinic phase, ICDD card nr. 83-0950) with (0 0 2) preferential orientation. The absence 400 of any signs of nitride phases clearly demonstrated the entire oxidation of the coating. It should be pointed out that the XRD pat- 350 terns obtained after annealing at 600 °C were identical for all the tested coatings. Finally, the coating series deposited with the highest nitrogen 300 content, W42N58, clearly exhibited at room temperature a mixture of d-WN and b-W2N phases (it is not clear from XRD spectra of 250 W45N55 coating [5] whether d-WN is present or not). W N The variation of the hardness and the Young’s modulus with the modulus (GPa) Young's 30 70 200 W N annealing temperature is shown in Fig. 10. At room temperature, 53 47 W42N58 150 0 100 200 300 400 500 600 Annealing temperature (ºC)

Fig. 10. Hardness (a) and Young’s modulus (b) of annealed W–N coatings.

W70N30 was the hardest coating (41 GPa), while the coatings with higher nitrogen content, W53N47 and W42N58, exhibited similar hardness values close to 30 GPa (see more details above). With increasing annealing temperature up to 400 °C a slow decrease of the hardness could be observed. The difference in H values between the room temperature and 400 °C was approximately 8 GPa in all the three coatings. Since neither variation of the structure nor oxidation was observed, the hardness decrease with temperature can be attributed to a decrease of the compressive stress, which should be closely related to the decrease of defect density induced by the heating and the consequent stress relaxation. These effects complemented with the start of the oxidation process jus- tify the steeper drop in the hardness observed for higher temperatures. As shown in previous section, the coatings are fully oxidized at 600 °C. As a consequence, at this temperature the hardness is constant for all the coatings, 6 GPa, value typical for the tungsten trioxide [7].

The coatings W70N30 and W53N47 exhibited similar E values up to 400 °C, close to 380 GPa. At 500 °C, the Young’s modulus slightly decreased to 310 and 340 GPa, respectively, and at 600 °C dropped

down to 180 GPa, again a value corresponding to WO3 [8]. The Young’s modulus of the coating with the highest nitrogen content,

W42N58, was lower than that of the other coatings.

3.2.2. Friction and wear The average friction coefﬁcient and the wear rate as a function of the testing temperature of the coatings are presented in Fig. 11. Fig. 9. XRD of W–N coatings selected for high temperature tribology tests. Both the friction and the wear exhibited similar trends for all the T. Polcar, A. Cavaleiro / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22 21

1.0 7 W N W N W N 70 30 53 47 42 58 W N W N W N 70 30 53 47 42 58 6 0.8 ]

5 -1 m -1 N 0.6 4 3 mm 3 -6 0.4 2 Friction coefficient 0.2 1 Wear rate [10 Wear

0.0 Fig. 13. Cracks in the wear track, W70N30 coating, tested at temperature 500 °C. 0 100 200 300 400 500 Temperature [ oC] the wear track. The cracks covered the entire wear track at Fig. 11. Friction and wear rate of W–N coatings as a function of the testing temperature. Open symbols represent friction coefﬁcient, full ones wear rate. 500 °C(Fig. 13), and the iron signal detected by EDX revealed the local destruction of the coating. Therefore, the wear rate for 500 °C is higher than that indicated in Fig. 11. The wear tracks of coatings. The friction increased from the lowest level at room tem- W53N47 and W42N58 coatings were almost identical in all the temperature to a maximum in the temperature range from 200 to perature range. At 300 and 400 °C, the ﬁrst damages appeared in 300 °C. A further rise in the temperature was followed by a small the wear tracks. The partial destruction of the coating always oc- decrease of the friction. Considering the error of the measurement, curred under the adhered tungsten oxide layer. A further rise in the friction value was almost identical at 300 and 400 °C for all the the temperature to 500 °C brought a radical change, since the nitrogen contents, while the lowest friction was exhibited by the adhesive tungsten oxide vanished and long straps showing a se-

W42N58 coating at temperatures up to 200 °C and by W70N30 coat- vere damage were observed inside the otherwise smooth wear ing at the temperature of 500 °C. The worn volume was almost not track. measurable up to 200 °C and then increased for higher tempera- The structural and the hardness analysis shows that the evolu- tures. All the coatings tested at 600 °C were worn out after the test; tion of the wear rate with increasing temperature can not be con- thus, the friction and the wear data are not presented. nected exclusively with either the oxidation or the considerable As referred to above, the wear tracks produced at room temper- softening of the coatings. The increase of the wear rate in the ature showed a thick layer of tungsten oxide adhered on their bor- range 300–400 °C for W70N30 is a typical example of such behav- ders; only on the central groove, the direct contact between the ball iour, since both the hardness and the structure are almost iden- and the tungsten nitride took place. The adhered layer on the side tical in this temperature range. The identification of the started to disappear with increasing temperature and vanished at tungsten nitride wear mechanisms at elevated temperature is a 300 °C. The central part of the wear tracks, uncovered at room tem- very difficult task. Therefore, the discussion about the possible perature, became covered by isolated isles of tungsten oxide at wear mechanisms at elevated temperature is, in many points, 200 °C(Fig. 12). The wear tracks produced on the different tungsten speculative. nitrides were very similar in the range of 20–200 °C, while signifi- The observation of the wear track shows the moving of the po- cant differences were observed at higher temperatures. sition of the adhered oxide layer with increasing temperature. The

The wear track produced on the coating W70N30 when sliding at thick layer localised in the wear track sides at room temperature 300 °C exhibits small lateral cracks originated from the centre of disappears and a thin layer of tungsten oxide is formed in the centre of the wear tracks at 300 °C. Moreover, the first cracks appear in all coatings at this temperature and they are located in the zone with the oxide layer. Based on the results presented above, three main wear mechanisms occur when the temperature is increased. The first dominant wear mechanism was the three-body wear with a thick tungsten trioxide tribolayer, typical for room temperature and to some extent up to 300 °C. A further increase of the temperature (300–400 °C) leads to the vanishing of the adhered oxide layer and the consequent crack formation. Finally, at 500 °C, the cracks propagate reaching the substrate and the rapid coating destruction is then observed, particularly in the centre of the wear track. The evolution of the friction coefficient supports these pro- posed three stages. The initial value is the lowest, since the contact is represented mainly by the adhered tungsten oxide. As the adhered layer disappears with increasing temperature, the friction rises, as it is observed. The friction stabilises in the range 300– 400 °C, since there is no significant change observed in the wear tracks of the individual coatings. The value of the friction coefficient measured at 500 °C cannot be strictly considered as repre- Fig. 12. Wear track of W70N30 coating produced by sliding at 200 °C (SEM). Note the adhered tribolayer in the centre of the track. Other two tested WN coatings senting the tungsten nitride (or oxide), since the coatings are exhibited similar appearance at this temperature. partially worn out. 22 T. Polcar, A. Cavaleiro / Int. Journal of Refractory Metals & Hard Materials 28 (2010) 15–22

4. Concluding remarks Acknowledgements

The tribological behaviour of a hard coating tested by pin-on- This work was supported by the Grant Agency of the Academy disc is often explained by the dominant influence of one of the of Sciences of the Czech Republic through the project mechanical properties of the coatings. The improvement in the KJB201240701 and by the Ministry of Education of the Czech wear resistance is attributed to an increase of the hardness, the Republic (project MSM 6840770038). scratch-test resistance or the plasticity parameter H3/E2 among many others. Nevertheless, W–N coatings show a different References behaviour. Although the hardness, adhesion/cohesion or morphology may influence the friction coefficient or the wear rate [1] Polcar T, Kubart T, Novák R, Kopecky´ L, Široky´ P. Comparison of tribological behaviour of TiN, TiCN and CrN at elevated temperatures. Surf Coat Technol of the coating, none of these properties can be considered as 2005;193:192. dominant. Moreover, coatings exhibiting similar mechanical [2] Castanho J, Cavaleiro A, Vieira MT. Study of tungsten sputtered films with low properties can show very different tribological behaviour. In this nitrogen content. Vacuum 1994;45:1051. [3] Cavaleiro A, Trindade B, Vieira MT. Influence of Ti addition on the properties of case, the type of the wear mechanism, together with the pres- W–Ti–C/N sputtered films. Surf Coat Technol 2003;174–175:68. ence of a wear debris interlayer forming a third-body, plays the [4] Louro C, Cavaleiro A. Hardness versus structure in W–Si–N sputtered coatings. dominant role in the tribological testing. In fact, the best wear Surf Coat Technol 1999;116–119:74. resistance was achieved for the coatings with high N-content [5] Polcar T, Parreira NMG, Cavaleiro A. Tribological characterization of tungsten nitride coatings deposited by reactive magnetron sputtering. Wear exhibiting low hardness and low values of the scratch-test critical 2007;262:655. loads. [6] Polcar T, Parreira NMG, Cavaleiro A. Structural and tribological With the exception of tribo-oxidiation, present in all tested characterization of tungsten nitride coatings at elevated temperature. Wear 2008;265:319. coated samples, the main wear mechanisms of low N-content [7] Polcar T, Parreira NMG. Tungsten oxide with different oxygen content: sliding coatings was abrasive wear, delamination and, in the particular properties. Vacuum 2007;81:1426. case of W N sample, plastic deformation. On the contrary, the [8] Parreira NMG, Carvalho NJM, Cavaleiro A. Synthesis, structural and mechanical 88 12 characterization of sputtered tungsten oxide coatings. Thin Solid Films sliding of the coatings with high nitrogen content was predomi- 2006;510:191. nantly influenced by the formation of a third-body consisting of [9] Parreira NMG, Polcar T, Cavaleiro A. Thermal stability of reactive sputtered very small tungsten trioxide particles. Consequently, the main tungsten oxide coatings. Surf Coat Technol 2007;201:7076. [10] Antunes JM, Cavaleiro A, Menezes LF, Simões MI, Fernandes JV. Ultra- wear mechanism was three-body abrasive wear with combination microhardness testing procedure with Vickers indenter. Surf Coat Technol with polishing and mild wear. The improvement of the wear resis- 2002;149:27. tance in case of high N-content films was explained by the pres- [11] Parreira NMG, Carvalho NJM, Vaz F, Cavaleiro A. Mechanical evaluation of unbiased W–O–N coatings deposited by d.c. reactive magnetron sputtering. ence of the protecting interlayer between the opposing surfaces Surf Coat Technol 2006;200:6511. in contact. [12] Parreira NMG, Carvalho NJM, Cavaleiro A. On the structural evaluation of The selected W–N coating works well up to 200 °C due to for- unbiased W–O–N sputtered coatings. Mater Sci Forum 2006;514–516:825. mation of tungsten oxide tribolayer; further increase of the tem- [13] Villain P, Goudeau P, Ligot J, Benayoun S, Badawi KF, Hantzpergue J-J. X-ray diffraction study of residual stresses and microstructure in tungsten thin films perature prevents tribolayer formation leading to higher wear sputter deposited on polyimide. J Vac Sci Technol A 2003;21:967. rates and coating damage. [14] Petroff P, Sheng TT, Sinha AK, Rozgonyi GA, Alevander FB. Microstructure, Despite the existence of similar adhered tribolayer (coating growth, resistivity, and stresses in thin tungsten films deposited by r.f. sputtering. J Appl Phys 1973;44:2545. with low W content) or similar dominant phase (high W content), [15] Stachowiak GW, Andrew WB. Engineering tribology. Butterworth– the friction coefficient of oxides is higher than that of nitrides. This Heinemann; 2001. behaviour has been attributed to the transfer of tungsten oxide [16] Mitsugi F, Hiraiwa E, Ikegami T, Ebihara K, Thareja RK. WO3 thin films prepared by pulsed laser deposition. Jpn J Appl Phys 2002;41:5372. material to the ball surface.