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Lubricated sliding wear of physically vapour deposited titanium nitride
Surface and Coatings Technology, 50 (1992) 117-126
117
Lubricated sliding wear of physically vapour deposited titanium nitride* S. J. B u l l t a n d P. R. C h a l k e r AEA Industrial Technology, Harwell Laboratory, Oxfordshire, 0 X l l ORA (UK)
(Received March 17, 1991; accepted August 17, 1991)
Abstract Coated components are used in conjunction with uncoated materials in many tribological environments that contain lubricants developed for the uncoated substrates. There is no reason to believe that a lubricant optimized for a steel substrate will have a similar effect on the coated surface and thus there is a need to assess the effects of lubricants on the sliding wear of coated components and identify any deleterious lubricant--coating interactions. The lubricated sliding wear performance of titanium nitride films sliding against 52 100 steel and polycrystalline sapphire counterfaces was investigated as a function of coating microstructure for a range of different physical vapour deposition (PVD) processes. Improvements in both coating wear rate and lifetime have been found, even using unformulated base oils, though variations in behaviour between lubricants have been observed which can be detrimental. When the microstructure of titanium nitride coatings on steel substrates is open (zone 1), the use of an unformulated oil can improve wear performance until it is comparable with that of a fully dense film with the zone T microstructure. Furthermore, the use of lubricants led to a substantial improvement in wear lifetime over similar unlubricated coating contacts owing to reductions in the amount of coating detachment during wear. The presence of the lubricant reduces the amount of transferred material from the sliding counterface (particularly for the steel sphere), minimizing coating-counterface adhesion and hence friction which reduces the incidence of premature failure by coating detachment. The implications of these observations are discussed for the use of TiN coated components in real tribological applications.
I. Introduction Ceramic coatings are finding an increasing range of engineering applications because of their combination of properties, including, for example, high hardness and good wear and corrosion resistance. One such example is titanium nitride coatings produced by physical vapour deposition (PVD) [1] or chemical vapour deposition (CVD) [2] which have b e e n used in a wide range of tribological applications from high p e r f o r m a n c e engines to cutting tools. Ideally, each c o m p o n e n t used in a particular application would be designed to take into consideration the presence of coated parts, but a combination of surface coating and a change in operating lubricant may well be cheaper than redesigning the component. In m a n y cases the coating is applied to an existing c o m p o n e n t to solve a specific wear problem and is required to function in the existing operating environment. In m a n y cases this compromise m e a n s *Paper presented at the 18th International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, April 22-26, 1991. tAuthor to whom correspondence should be addressed.
that the coated c o m p o n e n t must operate in lubricants developed for the uncoated material and it is therefore essential to identify any deleterious lubricant--coating interactions if the performance of the coated components is to be adequate. Several workers have studied the effects of lubricants on the sliding wear of bulk ceramic materials (see for example refs. 3-5), but relatively little work has b e e n reported for ceramic coatings [6-8]. In general, low friction and wear is only achieved for like-on-like ceramic contacts in unlubricated sliding, but the friction and wear of c e r a m i c - m e t a l contacts can be improved by the use of a lubricant. It has been observed that the lowest friction coefficients for titanium nitride coatings are recorded for like-on-like contacts in unlubricated sliding [8] and improvements have been recorded for the lubricated sliding of steel against the coating [2]. A recent study has shown that considerable improvements in wear performance can be achieved by the use of a hard coating when there is a deleterious reaction between the lubricant and the substrate [8]. In this study a systematic investigation of the lubricated sliding wear behaviour of P V D titanium nitride coatings in the sphere-on-disc test was made. Friction
Elsevier Sequoia
118
s.J. Bull, P. R. Chalker / Sliding wear of PVD titanium nitride
and wear performance was determined for films deposited by several deposition technologies using a range of processing conditions to identify any effects of microstructure on lubricated sliding wear. Studies using a range of lubricants and nominally identical coatings were carried out to identify deleterious lubricant--coating interactions.
2. Experimental details
2.1. Sample preparation Stainless steel (18 wt.% Cr, 9 wt.% Ni, 1 wt.% Ti) and M2 tool steel substrates were polished to a 3 /xm diamond finish using standard metallographic techniques. The samples were ultrasonically cleaned and vapour degreased and then coated with titanium nitride by sputter ion plating [10, 11]. After ion cleaning the coatings were deposited using a substrate bias in the range 0 V to - 1 3 0 V to a thickness of 2-5 /zm as determined by metallographic cross-sections. To investigate the effects of changing the deposition technology coatings of similar thickness were deposited onto stainless steel substrates by magnetron sputtering [12] over an applied bias range of 0 to - 2 0 0 V and by arc evaporation [13] at - 1 0 0 V applied bias. 2.2. Wear testing Wear tests were performed on a sphere-on-disc tester using 3 mm uncoated spheres sliding against the coated disc. A test load of 2 N and a sliding speed of 0.1 m s-~ was used in all cases and the wear rates of disc and sphere were determined by the methods detailed in ref. 8. The coated sample is clamped to a rotating platen which is surrounded by a sealed container connected to a lubricant recirculation system which enables a continuous supply of lubricant to be fed to the contact. Debris is filtered out of the lubricant during the recycling process. The contact is kept submerged by an excess supply of the lubricant. Three wear tests were performed with each lubricant--coating combination and the results averaged to give the data presented in the figures. Though there was considerable variation between tests, the wear rates for each combination were consistent and the errors are comparable with the size of the plot symbols. A range of lubricants was investigated in this study which is detailed in Table 1. For the microstructural and deposition technology experiments, the rotary pump oil coded Vac 15 was used in isolation whereas all the lubricants were used to test each substrate-sphere combination in the investigation of lubricant effects. The spheres used in the test were either uncoated 52 100 bearing steel or uncoated sapphire. Steel spheres were used for the microstructural studies since it was
known that they lead to considerable transfer of material to the coating in dry sliding [9]. Both steel and sapphire spheres were used to look at the effect of various lubricants on the interactions at metal-ceramic and ceramic-ceramic contacts. Prior to testing with each lubricant, the variation of friction coefficient with sliding velocity for a 52 100 sphere sliding against the uncoated substrate was determined to check if the oils had significantly different behaviour. No major variations were discovered though it is expected that lubricant film thickness will vary between them. The ability of the oils to form films depends on their pressure-viscosity coefficients which were not available in this study. In all cases boundary lubrication was maintained for all lubricants.
3. Results
3.1. Effect of coating microstructure The coatings produced by sputter ion plating were deposited using a range of substrate bias voltages to change the microstructure of the film; as the bias voltage is increased the density of the film increases owing to an "ion polishing" effect and the structure changes from open zone 1 to dense zone T according to Thornton's structure model [14, 15] (see Fig. 1). The precise changes in surface roughness which are produced by the applied substrate bias depend on the thickness and texture of the coating but typically the Ra value for an unbiased film would be around 80 nm and for a smoother - 6 0 V bias film around 40 nm (which is the roughness of the uncoated substrate). Such structure changes are expected to affect the friction and wear behaviour of the films in dry sliding, which is indeed observed in the wear tests against steel spheres (Fig. 2). In unlubricated sliding the friction coefficient is about 0.65 and does not vary significantly with bias voltage except at -130 V bias (Fig. 2(a)). However, there is a minimum in disc wear rate at - 6 0 V bias and a marked change in coating wear rate on either side of this minimum (Fig. 2(b)). When tested under lubricated sliding conditions using a vacuum pump oil for the lubricant (see Table 1), the friction coefficient was dramatically reduced for all the bias voltages used, to around 0.09. The wear rates for the coated discs in lubricated conditions did not show as much variation compared with unlubricated sliding and were lower than all but the minimum unlubricated wear rate. The use of lubricants apparently improves the performance of the poorer TiN coatings to such an extent that it is comparable with that of the best coating in unlubricated contact (deposited at - 6 0 V bias). The reasons for this improvement become apparent on microscopic analysis of the worn surfaces. In un-
119
S. J. Bull, P. R. Chalker / Sliding wear o f PVD titanium nitride
TABLE 1. Designation and properties of lubricants Name
Grade
Viscosity (cSt) 25 °C
Density (g cm -3)
40 °C
Additives
Comments
None
Base oil
100 °C
150SN RL142
15W50
17.3
Not known
Bad reference oil
RL138
10W40
14.6
Not known
Good reference oil
Vac 15
SAE20
-
Rotary pump oil (straight mineral oil)
68
Medos 100
5.6 100
11.1
0.883
General purpose oil (straight mineral oil)
AWS32
32
5.26
0.873
Anti-oxidant Anti-rust Anti-foam Anti-wear
Hydraulic fluid and high speed bearing oil
BD68
68
8.8
0.888
EP Oiliness "Tackiness"
Slideway oil
PD68
68
8.85
0.879
Not known
Compressor oil
Anti-corrosion Anti-wear Anti-oxidant
Engine oil
RX Super
Water DC200 Silicone
15W40
14.2
0.89
0.65
0.28
200
1.0
Tap water
0.97
Precision equipment lubricant, poor for steels in sliding wear
Inl
Fig. 1. Scanning electron fraetographs of (a) unbiased (zone 1) and (b) biased (zone T) titanium nitride coatings produced by sputter ion plating. Note the pyramidal end caps on the unbiased coating (Ra=0.1 /zm) and the much smoother surface topography produced by the application of a substrate bias during deposition (Ra=0.04 p.m). l u b r i c a t e d sliding t h e r e is c o n s i d e r a b l e t r a n s f e r o f i r o n f r o m t h e s t e e l s p h e r e to t h e c o a t i n g s u r f a c e d u r i n g t h e test (Fig. 3) w h i c h c a n b e i d e n t i f i e d b y e n e r g y d i s p e r s i v e X - r a y m i c r o a n a l y s i s ( E D X ) in t h e s c a n n i n g e l e c t r o n m i c r o s c o p e . I n t h e first few p a s s e s t h e f r i c t i o n coefficient
b e t w e e n T i N a n d s t e e l is a r o u n d 0.2, b u t as this t r a n s f e r film f o r m s t h e friction rises to a r o u n d 0.65. T h e transf e r r e d d e b r i s is o x i d i z e d d u r i n g t h e w e a r p r o c e s s a n d the formation of various iron oxide phases has been reported depending on the test conditions and the slider
120
S.J. Bull, P. R. Chalker / Sliding wear of PVD titanium nitride 0.8
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100
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10 -14 4,
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,
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Fig. 2. Variation of (a) coefficient of sliding friction, (b) coated disc wear rate and (c) uncoated sphere wear rate for a steel sphere sliding against sputter ion plated TiN on stainless steel in lubricated and unlubricated sliding.
material [9, 16]. In this study the debris collected was identified by X-ray diffraction as a mixture of TiO2 and F e 3 0 4. The high friction coefficients associated with transfer move the maximum shear stress component from within the substrate to the coating surface near to the rear of the sliding contact [17]. Consequently, the probability of coating removal or failure within the coating during the test increases. The amount of transfer can be measured using a metallographic etch for stainless steel to remove the oxidized transfer layer from the coating surface after the test. The area of transferred material is then determined by taking surface profilometer traces along
the same track before and after this process. Figure 4 shows the variation in the amount of transfer as a function of applied substrate bias for the films tested here. Considerable transfer is observed for the rougher low bias films which have the more open zone I structure, suggesting that the intercolumnar gaps may promote the trapping of transferred material. For the dense zone T structures, the amount of transfer is small and this leads to the best wear performance. The film deposited at - 130 V bias is an exception to this trend; although this film is dense and very smooth it does show some transfer. Microscopic observation shows the presence of elongated needle-like debris [9] which has been suggested to be titanium oxide "rollers" formed by stripping and rolling up the native oxide from the TiN surface [16]. These rollers are responsible for the reduction in friction in unlubricated sliding for this coating. The oxygen content of the nitride is very low (less than 0.5% by Rutherford backscattering) and it is unlikely that any significant oxide formation would occur during deposition. On removal from the coating system some oxidation of the coating surface occurs on exposure to the atmosphere. The rollers could be formed by the removal of this native oxide from the TiN surface. This may lead to increased reactivity with the slider material as it passes over the stripped region, leading to transfer of material. However, the mechanism for the increased transfer for high bias coatings cannot be positively identified from the data in this study. No evidence for transfer was found in any of the lubricated sliding wear tests performed here, though it may be that some limited transfer is taking place at asperity contacts at a reduced level. The tests were all performed under conditions of boundary lubrication where a thin film of lubricant separates the sliding contacts and only a few direct contacts will occur between the sliding materials at asperity tips. Though no gross transfer occurs, some transfer may still be occurring at asperity tips and this is beyond the resolution of EDX. Such transfer can be observed by more sensitive techniques such as X-ray photoelectron spectroscopy on some samples. The large reduction in transfer leads to low friction coefficients and consequently no coating was detached from the substrate at any time during the lubricated sliding wear tests performed here.
3.2. Effect of deposition technology The importance of metal transfer in determining the wear performance of sputter ion plated TiN was shown to be dependent on coating microstructure in the previous section. Since the structure of a TiN coating is dependent on both the choice of deposition technology and the processing parameters used [17], similar measurements were made on magnetron sputtered and arc
S. J. Bull, P. R. Chalker / Sliding wear o f PVD titanium nitride
121
r
Fig. 3. Scanning electron fractographs of worn surfaces of an uncoated sphere sliding on a - 6 0 V bias TiN coated disc; (a) general view of the track showing areas of transferred material (unlubricated), (b) fiat on the sphere showing debris (unlubricated), (c) general view of the track showing no transfer (lubricated), and (d) flat on the sphere showing little or no debris (lubricated).
10 -13 10 "14t e~
10 -15
1016 10 -17
10"18 10-19
I0"20 0
i
I
f
100 50 Negative applied substrate bias (V)
150
Fig. 4. Variation of the amount of transfer with substrate bias for sputter ion plated titanium nitride in unlubricated sliding.
evaporated titanium nitride coatings. The sliding friction of the magnetron sputtered material is reduced by lubrication during the wear test by an amount similar to that of the sputter ion plating films reported in the previous section (Fig. 5(a)). Similarly the coefficient of friction is reduced for arc evaporated TiN films deposited at a range of thicknesses (Fig. 6(a)). The wear behaviour of the magnetron sputtered TiN as a function of applied substrate bias is very complicated for the unlubricated tests (Fig. 5(b)) but this can be related to the effect of coating microstructure on the amount of transfer during the wear test. The smoothness and density of the coating increase with bias up to - 8 0 V as for the sputter ion plated coating, and throughout this bias range the amount of transfer (and hence coating wear) is reduced. However, at - 1 0 0 V bias the coating
S. J. Bull, P. R. Chalker / Sliding wear o f P V D titanium nitride
122
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0.8
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2e-14 n •
0.0
250
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i
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3
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4
5
C o a t i n g T h i c k n e s s (~tm) 1 0 -11
,~
Unlubricated Lubricated
10 -1~
[
t_
~
-le-14
.~
-2e-14
~
10 -12
-~
IO -I~
O I
-3e-14 0
(b)
50 Negative
,
I
100
i
I
,
150
I
,
200
applied substrate bias
I
~
10-15
I
250
2
(b)
(V)
i
I
i
3
1
*
4
Thickness (I.tm) 10 -12
10 -13
"g
Z ~
1 0 -14
10 -14 10 -15 10-1~
10 -15 10 -17 10.16
t_
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t_ [-
10 -19 1 0 -17 0
(c)
10 -18
t 50
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i 100
'
i
150
applied substrate bias
I 200
,
250
(v)
Fig. 5. Variation of (a) coefficient of sliding friction, (b) coated disc w e a r rate, and (c) transfer rate as a function o f bias voltage for 5 /zm thick m a g n e t r o n s p u t t e r e d TiN films in lubricated and unlubricated sliding.
undergoes a change in texture from {200} to {422} [17] and this is accompanied by an opening out of the coating structure (Fig. 7), allowing the trapping of debris and increased transfer (Fig. 5(c)). At greater than - 100 V bias voltage the coating density increases and the amount of transfer is reduced but not to the same extent as at low bias voltages. In lubricated sliding there is again no evidence of gross transfer in scanning electron microscopy (SEM) investigations and the wear rates for coatings deposited at all bias voltages are broadly similar. It is interesting to note that the coating wear rates for low rate sputter ion plated TiN and high rate magnetron sputtered TiN
i
I
2
I
(c)
3
4
5
T h i c k n e s s (~tm)
Fig. 6. Variation o f (a) coefficient o f sliding friction, (b) coated disc wear rate, and (c) transfer rate as a function of coating thickness for - 100 V bias arc e v a p o r a t e d titanium nitride coatings in lubricated and unlubricated sliding.
are similar at low bias voltages in lubricated sliding, though the magnetron sputtered material shows better performance at greater than - 8 0 V. The wear performance of the arc evaporated TiN coatings is also considerably improved by the use of a lubricant (Fig. 6(b)). For the arc evaporated coatings, transfer of titanium-rich material from the coating to the sphere was also observed owing to the reactivity of the titanium-rich macrodroplets within the coating (Fig. 8). These particles have been shown to influence the abrasive wear performance of arc evaporated TiN [19]. Even with the use of a lubricant, the wear rate of the evaporated coatings is greater than that of the sputtered films and thus it is essential to take steps
S. J. Bull, P. R. Chalker / Sliding wear of PVD titanium nitride
123
(b)
Fig. 7. Scanning electron fractographs of magnetron sputtered TiN on austenitic stainless steel, (a) -80 V bias and (b) -100 V bias, showing the reduction in coating density at the higher bias voltage.
Fig. 8. Scanning electron micrographs of (a) the wear track and (b) the flat on the sphere for a 1.7 p.m arc evaporated TiN coating on stainless steel tested in unlubricated sliding. to reduce the incidence of macrodroplet formation if the best coating performance is to be achieved for arc evaporated TiN. 3.3. Effect o f lubricant
In order to assess the sliding wear performance of TiN coatings in a range of lubricants, a number of identical titanium nitride coatings were produced on M2 tool steel. For each lubricant used three tests were performed: (1) 52 100 steel sphere against uncoated M2 disc; (2) 52 100 steel sphere against TiN coated M2 disc; (3) alumina sphere against TiN coated M2 disc. Though the effects o f any lubricant additive packages on the steel components in the sliding couples will be important in tests (1) and (2), the final test represents
ceramic on ceramic sliding and none of the lubricants used were originally intended for this contact situation. In all the tests described in this section, a total sliding distance of around 1 km was used; in the lubricated tests there was no evidence for gross transfer from the steel to the TiN substrates in test (2) and there was no evidence for coating spallation in either test (2) or (3). In all the lubricated sliding experiments performed here, the coefficient of sliding friction was around 0.05 and the variations between lubricants were within the experimental scatter of tests performed using a single lubricant. Figure 9 shows plots of sphere wear against disc wear for all the contact situations investigated and individual lubricant data are presented in Fig. 10. It
124
S.J. Bull, P. R. Chalker / Sliding wear o f PVD titanium nitride 10 -13]
10-13 None
10 -14
Walet m
10-15
z
[]
10-16 10-17 ¢o
[]
10.18
~_. =
10 -19
[]
10 -20 ........ I0-18
(a)
t_
ts
m ........ I0-17
, ........ 10"16
~ ........ 10"15 2
, 10-14
Uncoated M2 disc wear (m /N)
o
~
,~
z
"~
S
~
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~
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10-13 None 10 -14
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n
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10-15 1
z
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.=_. _= ,< 10-181 10 -17 (c)
Z t,',l 10-16
10 -15
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10 -13
2
TiN/M2 Disc wear (m IN)
Fig. 9. Variation of sphere wear rate against disc wear rate in lubricated sliding for (a) a steel sphere against an uncoated M2 disc, (b) a steel sphere against a TiN coated M2 disc, and (c) an alumina sphere against a TiN coated M2 steel disc. The straight line is drawn through the results for the hydrocarbonbased mineral oil lubricants.
is readily apparent that the highest wear rates were measured in unlubricated sliding, but that high wear rates were also observed when water was used as the lubricant. Considerable corrosion of the steel substrate was observed (by SEM and X-ray diffraction) in the tests on uncoated M2 and the steel sphere also showed some signs of rusting. However, even in the alumina-titanium-nitride contacts the wear rate was high when water was used as a lubricant so it appears that some chemical interaction between the water and the ceramic surfaces has occurred to promote wear. This effect could be related to the chemomechanical softening
Av
(c)
a
Fig. 10. Bar charts of sphere and disc wear rates for the lubricants used here: (a) steel sphere against uncoated M2 disc, (b) steel sphere against TiN coated M2, and (c) alumina sphere against TiN coated M2.
of the surface of (mainly oxide) ceramics which has been attributed to adsorbed water [20, 21]. Clearly, this will have an important bearing on the selection of cutting fluids for use with TiN coated drills; the inference is that oil based cutting fluids are preferred to water based fluids if wear of the coating is to be
S. J. Bull, P. R. Chalker / Sliding wear o f PVD titanium nitride
minimized. As well as water, the use of the DC200 silicone lubricant in cases where one of the sliding counterfaces is steel leads to high wear rates in both sphere and disc. This lubricant is known to give poor performance for steels in sliding owing to its low loadbearing capacity. However, it appears to be perfectly adequate for the ceramic on ceramic contacts. For the hydrocarbon mineral oil based lubricants, general trends can be identified for each contact situation. For the steel sphere sliding on the uncoated M2 substrate the sphere wear is approximately constant with a wide range of disc wear rates measured. The use of PD68 compressor oil seems to give a much lower sphere wear rate, and the rotary pump oil (Vac 15) gives a higher wear rate than this mean level but it is difficult to tell from the experiments reported here if this scatter is significant. Another exception is the RXsuper engine oil. The data for this lubricant are not plotted in Fig. 9(a) because there was no detectable disc wear owing to the formation of a thick reaction layer over the wear track as a result of the anti-wear additives in the oil. For the steel sphere sliding on the TiN coated M2 (Fig. 9(b)) the sphere wear rate is again approximately constant for the mineral oil lubricants, but is higher than for the metal-metal contact owing to the harder sliding counterface. In this case the only real exception is the rotary pump oil which leads to a lower sphere wear rate. There is a considerable variation in the TiN coated disc wear rate. Those lubricants which showed the best performance in the steel-steel contacts do not show the best performance in the TiN-steel case and the lubricant ranking order has changed (Fig. 10). No attempt has yet been made to identify the factors in the oils which lead to the low TiN wear rates - the magnitude of the variation between the oils may not be significant in many applications. For the alumina sphere sliding on'the TiN coated M2 (Fig. 9(c)), the wear of the TiN coated disc is approximately constant but there is a large variation in the wear of the sphere. The function of the lubricants in this case cannot simply be to separate the sliding surfaces because the observed changes in wear rate do not correlate with the viscosity data presented in Table 1 and the sliding speeds used here are too low for hydrodynamic lubrication to be occurring. The thickness of the lubricant film will also depend on the variation of lubricant viscosity with pressure. It is expected that a wide range of lubricant film thicknesses have been investigated in this study but in the absence of calculated thicknesses of experimental data it is not possible to comment on the variation of wear with film thickness. Some chemical reactions take place between some constituents of the lubricant (apart form the additive package) and one or other of the sliding surfaces. It
125
appears that such reactions occur more readily on titanium nitride than alumina and help to protect the nitride surface from wear during sliding. However, the reactions occur even more readily on 52 100 steel, reducing the amount of sphere wear in the other experiments. This is expected since, as 52 100 is a bearing steel, the lubricants will be more likely to have been developed to work with it than tool steel or titanium nitride. The wear rates of individual lubricants are shown in Fig. 10. For the uncoated M2 disc (Fig. 10(a)), best wear performance is obtained for RX-super, an engine oil, Medos 100, a general purpose oil and the rotary pump oil, Vac 15. In the case of the engine oil a well defined reaction layer due to the anti-wear additives in the oil was found at the end of the test which was visible by reflected light microscopy. The precise composition of this layer could not be ascertained by Xray diffraction and EDX, but it was found that the reaction layers formed for other oils had different compositions which implies that the reaction layer chemistry is a strong function of the oil formulation. For the other lubricants which performed well, such anti-wear additives were not present. However, other oils with additives such as AWS32 did not perform well for the M2 substrate. In the case of the steel sphere sliding against the TiN coated M2 disc (Fig. 10(b)) these other oils performed better than the unformulated oils and the general purpose oil, Medos 100. In this case the good reference oil (RL142) led to best performance. Both the good reference oil and the engine oil also gave good results for the alumina sphere sliding against TiN coated M2 (Fig. 10(c)). Clearly, the additive packages present in commercial oils do react to differing extents with both ceramic coatings and bulk ceramics. In this study, no attempt was made to identify these reactions, but work is underway to identify the constituents of the lubricants which lead to good and bad coating performance. In most cases these reactions result in a reduction in wear, but this reduction may not be as pronounced as the reduction in wear for a steel on steel contact. In cases where the lubricant package is very effective in reducing wear for the steel on steel contact the performance of the steel sphere against TiN coated flat is relatively poor. However, for all other contact situations the wear performance is at least as good as the uncoated case.
4. Conclusions
The use of a lubricant in applications where steel slides against TiN can result in considerable reductions in wear because it limits the amount of transfer of the steel to the coated surface. As the smoothness of the
126
S.J. Bull, P. R. Chalker / Sliding wear of PI,'D titanium nitride
coating increases, this transfer is reduced without the need for the lubricant, but the performance for rough coated surfaces is improved to that of the best smooth coating if one is used. The use of a lubricant leads to performance benefits for titanium nitride coatings produced by sputtering and arc evaporation; in the case of the arc evaporated TiN the lubricant reduces the adhesion between unreacted titanium in the coating and the steel counterface. The precise nature of the additive package in the oil affects the wear of the coating, as does the nature of the sliding contact. The wear of both a TiN coating and bulk alumina has been shown to be sensitive to the choice of lubricant used, with additive package reactions occurring to a varying extent during wear. In general the performance of the titanium nitride coating is no poorer than that of uncoated tool steel in lubricated sliding wear. However, where the additive package performance is very good with the steel, the TiN performance is relatively poor. Clearly, to obtain the best performance from coated components, lubricants will need to be developed specifically for coated contacts with additive packages designed for them. In the meantime, current lubricants can be used but care should be taken to select a lubricant which does not compromise performance if anti-wear additives have been particularly effective for the uncoated component.
were developed for the CARE programme) were provided by Castrol Ltd. Bill Sproul of BIRL is thanked for the provision of magnetron sputtered samples and Robert Barrell of JJ Castings for the arc evaporated material.
References 1 2 3 4
5 6
7 8 9 10 11 12 13 14 15 16 17
Acknowledgments 18
This work was performed as part of the Corporate Research Programme of AEA Technology. The authors would like to thank Peter Fernback for carrying out some of the wear testing. The reference oils (which
19 20 21
A. Matthews, Surf. Eng., 1 (1985) 93. H. E. Hintermann, Wear, 100 (1984) 381. P. Studt, Wear, 115 (1987) 185. J. J. Habeeb, A. G. Blakey and W. N. Rogers, Proc. Conf. on Tribology, 1987, Institute of Mechanical Engineers, London, 1987, pp. 555-564. K.-H. Zum Gahr, Wear, 133 (1989) 1. J.F. Braza, in T. S. Sudarshan, D. G. Bhat and H. Hintermann (eds.), Proc. 3rd Int. Conf. on Surface Modification Technologies, TMS, Warrendale, PA, 1990, pp. 115-131. W. D. Sproul and R. Rothstein, Thin Solid Films, 126 (1985) 257. D. J. Carr6, Surf. Coat. Technol, 43--44 (1990) 609. S. J. Bull, D. S. Rickerby and A. Jain, Surf. Coat. Techno~ 41 (1990) 269. R. A. Dugdale, Thin Solid Films, 45 (1977) 541. D. S. Rickerby and R. Newbery, Vacuum, 38 (1988) 161. W. D. Sproul, Surf. Coat. Technol., 33 (1987) 73. H. Randhawa, Surf. Coat. Technol., 33 (1987) 53. J. A. Thornton, Ann. Rev. Mater Sci., 7 (1977) 239. D. S. Rickerby and S. J. Bull, Surf. Coat. Technol., 39--40 (1989) 315. I. L. Singer, S. Fayeulle and P. D. Ehni, Proc. Conf. on Wear of Materials, April 1991, in the press. G. M. Hamilton and L. E. Goodman, J. App. Mech., 33 (1966) 371. D. S. Riekerby, S. J. Bull, T. Robertson and A. Hendry, Surf. Coat. Technol., 41 (1990) 63. A. R. C. Westwood, J. S. Ahern and J. J. Mills, Colloids Surf., 2 (1981) 1. J. T. Czernuska, Ph.D. Thesis, University of Cambridge, 1985. S. Jahanir and T. E. Fischer, Tribol. Trans., 31 (1988) 32.
117
Lubricated sliding wear of physically vapour deposited titanium nitride* S. J. B u l l t a n d P. R. C h a l k e r AEA Industrial Technology, Harwell Laboratory, Oxfordshire, 0 X l l ORA (UK)
(Received March 17, 1991; accepted August 17, 1991)
Abstract Coated components are used in conjunction with uncoated materials in many tribological environments that contain lubricants developed for the uncoated substrates. There is no reason to believe that a lubricant optimized for a steel substrate will have a similar effect on the coated surface and thus there is a need to assess the effects of lubricants on the sliding wear of coated components and identify any deleterious lubricant--coating interactions. The lubricated sliding wear performance of titanium nitride films sliding against 52 100 steel and polycrystalline sapphire counterfaces was investigated as a function of coating microstructure for a range of different physical vapour deposition (PVD) processes. Improvements in both coating wear rate and lifetime have been found, even using unformulated base oils, though variations in behaviour between lubricants have been observed which can be detrimental. When the microstructure of titanium nitride coatings on steel substrates is open (zone 1), the use of an unformulated oil can improve wear performance until it is comparable with that of a fully dense film with the zone T microstructure. Furthermore, the use of lubricants led to a substantial improvement in wear lifetime over similar unlubricated coating contacts owing to reductions in the amount of coating detachment during wear. The presence of the lubricant reduces the amount of transferred material from the sliding counterface (particularly for the steel sphere), minimizing coating-counterface adhesion and hence friction which reduces the incidence of premature failure by coating detachment. The implications of these observations are discussed for the use of TiN coated components in real tribological applications.
I. Introduction Ceramic coatings are finding an increasing range of engineering applications because of their combination of properties, including, for example, high hardness and good wear and corrosion resistance. One such example is titanium nitride coatings produced by physical vapour deposition (PVD) [1] or chemical vapour deposition (CVD) [2] which have b e e n used in a wide range of tribological applications from high p e r f o r m a n c e engines to cutting tools. Ideally, each c o m p o n e n t used in a particular application would be designed to take into consideration the presence of coated parts, but a combination of surface coating and a change in operating lubricant may well be cheaper than redesigning the component. In m a n y cases the coating is applied to an existing c o m p o n e n t to solve a specific wear problem and is required to function in the existing operating environment. In m a n y cases this compromise m e a n s *Paper presented at the 18th International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, April 22-26, 1991. tAuthor to whom correspondence should be addressed.
that the coated c o m p o n e n t must operate in lubricants developed for the uncoated material and it is therefore essential to identify any deleterious lubricant--coating interactions if the performance of the coated components is to be adequate. Several workers have studied the effects of lubricants on the sliding wear of bulk ceramic materials (see for example refs. 3-5), but relatively little work has b e e n reported for ceramic coatings [6-8]. In general, low friction and wear is only achieved for like-on-like ceramic contacts in unlubricated sliding, but the friction and wear of c e r a m i c - m e t a l contacts can be improved by the use of a lubricant. It has been observed that the lowest friction coefficients for titanium nitride coatings are recorded for like-on-like contacts in unlubricated sliding [8] and improvements have been recorded for the lubricated sliding of steel against the coating [2]. A recent study has shown that considerable improvements in wear performance can be achieved by the use of a hard coating when there is a deleterious reaction between the lubricant and the substrate [8]. In this study a systematic investigation of the lubricated sliding wear behaviour of P V D titanium nitride coatings in the sphere-on-disc test was made. Friction
Elsevier Sequoia
118
s.J. Bull, P. R. Chalker / Sliding wear of PVD titanium nitride
and wear performance was determined for films deposited by several deposition technologies using a range of processing conditions to identify any effects of microstructure on lubricated sliding wear. Studies using a range of lubricants and nominally identical coatings were carried out to identify deleterious lubricant--coating interactions.
2. Experimental details
2.1. Sample preparation Stainless steel (18 wt.% Cr, 9 wt.% Ni, 1 wt.% Ti) and M2 tool steel substrates were polished to a 3 /xm diamond finish using standard metallographic techniques. The samples were ultrasonically cleaned and vapour degreased and then coated with titanium nitride by sputter ion plating [10, 11]. After ion cleaning the coatings were deposited using a substrate bias in the range 0 V to - 1 3 0 V to a thickness of 2-5 /zm as determined by metallographic cross-sections. To investigate the effects of changing the deposition technology coatings of similar thickness were deposited onto stainless steel substrates by magnetron sputtering [12] over an applied bias range of 0 to - 2 0 0 V and by arc evaporation [13] at - 1 0 0 V applied bias. 2.2. Wear testing Wear tests were performed on a sphere-on-disc tester using 3 mm uncoated spheres sliding against the coated disc. A test load of 2 N and a sliding speed of 0.1 m s-~ was used in all cases and the wear rates of disc and sphere were determined by the methods detailed in ref. 8. The coated sample is clamped to a rotating platen which is surrounded by a sealed container connected to a lubricant recirculation system which enables a continuous supply of lubricant to be fed to the contact. Debris is filtered out of the lubricant during the recycling process. The contact is kept submerged by an excess supply of the lubricant. Three wear tests were performed with each lubricant--coating combination and the results averaged to give the data presented in the figures. Though there was considerable variation between tests, the wear rates for each combination were consistent and the errors are comparable with the size of the plot symbols. A range of lubricants was investigated in this study which is detailed in Table 1. For the microstructural and deposition technology experiments, the rotary pump oil coded Vac 15 was used in isolation whereas all the lubricants were used to test each substrate-sphere combination in the investigation of lubricant effects. The spheres used in the test were either uncoated 52 100 bearing steel or uncoated sapphire. Steel spheres were used for the microstructural studies since it was
known that they lead to considerable transfer of material to the coating in dry sliding [9]. Both steel and sapphire spheres were used to look at the effect of various lubricants on the interactions at metal-ceramic and ceramic-ceramic contacts. Prior to testing with each lubricant, the variation of friction coefficient with sliding velocity for a 52 100 sphere sliding against the uncoated substrate was determined to check if the oils had significantly different behaviour. No major variations were discovered though it is expected that lubricant film thickness will vary between them. The ability of the oils to form films depends on their pressure-viscosity coefficients which were not available in this study. In all cases boundary lubrication was maintained for all lubricants.
3. Results
3.1. Effect of coating microstructure The coatings produced by sputter ion plating were deposited using a range of substrate bias voltages to change the microstructure of the film; as the bias voltage is increased the density of the film increases owing to an "ion polishing" effect and the structure changes from open zone 1 to dense zone T according to Thornton's structure model [14, 15] (see Fig. 1). The precise changes in surface roughness which are produced by the applied substrate bias depend on the thickness and texture of the coating but typically the Ra value for an unbiased film would be around 80 nm and for a smoother - 6 0 V bias film around 40 nm (which is the roughness of the uncoated substrate). Such structure changes are expected to affect the friction and wear behaviour of the films in dry sliding, which is indeed observed in the wear tests against steel spheres (Fig. 2). In unlubricated sliding the friction coefficient is about 0.65 and does not vary significantly with bias voltage except at -130 V bias (Fig. 2(a)). However, there is a minimum in disc wear rate at - 6 0 V bias and a marked change in coating wear rate on either side of this minimum (Fig. 2(b)). When tested under lubricated sliding conditions using a vacuum pump oil for the lubricant (see Table 1), the friction coefficient was dramatically reduced for all the bias voltages used, to around 0.09. The wear rates for the coated discs in lubricated conditions did not show as much variation compared with unlubricated sliding and were lower than all but the minimum unlubricated wear rate. The use of lubricants apparently improves the performance of the poorer TiN coatings to such an extent that it is comparable with that of the best coating in unlubricated contact (deposited at - 6 0 V bias). The reasons for this improvement become apparent on microscopic analysis of the worn surfaces. In un-
119
S. J. Bull, P. R. Chalker / Sliding wear o f PVD titanium nitride
TABLE 1. Designation and properties of lubricants Name
Grade
Viscosity (cSt) 25 °C
Density (g cm -3)
40 °C
Additives
Comments
None
Base oil
100 °C
150SN RL142
15W50
17.3
Not known
Bad reference oil
RL138
10W40
14.6
Not known
Good reference oil
Vac 15
SAE20
-
Rotary pump oil (straight mineral oil)
68
Medos 100
5.6 100
11.1
0.883
General purpose oil (straight mineral oil)
AWS32
32
5.26
0.873
Anti-oxidant Anti-rust Anti-foam Anti-wear
Hydraulic fluid and high speed bearing oil
BD68
68
8.8
0.888
EP Oiliness "Tackiness"
Slideway oil
PD68
68
8.85
0.879
Not known
Compressor oil
Anti-corrosion Anti-wear Anti-oxidant
Engine oil
RX Super
Water DC200 Silicone
15W40
14.2
0.89
0.65
0.28
200
1.0
Tap water
0.97
Precision equipment lubricant, poor for steels in sliding wear
Inl
Fig. 1. Scanning electron fraetographs of (a) unbiased (zone 1) and (b) biased (zone T) titanium nitride coatings produced by sputter ion plating. Note the pyramidal end caps on the unbiased coating (Ra=0.1 /zm) and the much smoother surface topography produced by the application of a substrate bias during deposition (Ra=0.04 p.m). l u b r i c a t e d sliding t h e r e is c o n s i d e r a b l e t r a n s f e r o f i r o n f r o m t h e s t e e l s p h e r e to t h e c o a t i n g s u r f a c e d u r i n g t h e test (Fig. 3) w h i c h c a n b e i d e n t i f i e d b y e n e r g y d i s p e r s i v e X - r a y m i c r o a n a l y s i s ( E D X ) in t h e s c a n n i n g e l e c t r o n m i c r o s c o p e . I n t h e first few p a s s e s t h e f r i c t i o n coefficient
b e t w e e n T i N a n d s t e e l is a r o u n d 0.2, b u t as this t r a n s f e r film f o r m s t h e friction rises to a r o u n d 0.65. T h e transf e r r e d d e b r i s is o x i d i z e d d u r i n g t h e w e a r p r o c e s s a n d the formation of various iron oxide phases has been reported depending on the test conditions and the slider
120
S.J. Bull, P. R. Chalker / Sliding wear of PVD titanium nitride 0.8
e..
.o_
0.6
L.
0.4
I:]
.
Lubricated
]
0.2' •
0
0.0 [ 0
I
• ,
50
(a)
z
• i
---,O, I
l
,
150
100
Negative applied substrate bias (V) 10-]1 Unlubricated Lubricated
10 -12
1 0 -13
10 -14 4,
10 -15 O
,
1 0 -16
I
0
,
50
I
i
100
I
150
Negative applied substrate bias (V)
(b) 10-1"
z
~
"g
n
Unlubricated Lubricated ]
l0-1,
~:
1 0 -1:
e,
10-16 0
(c)
i
I
50
i
I
100
I
I
150
Negative applied substrate bias (V)
Fig. 2. Variation of (a) coefficient of sliding friction, (b) coated disc wear rate and (c) uncoated sphere wear rate for a steel sphere sliding against sputter ion plated TiN on stainless steel in lubricated and unlubricated sliding.
material [9, 16]. In this study the debris collected was identified by X-ray diffraction as a mixture of TiO2 and F e 3 0 4. The high friction coefficients associated with transfer move the maximum shear stress component from within the substrate to the coating surface near to the rear of the sliding contact [17]. Consequently, the probability of coating removal or failure within the coating during the test increases. The amount of transfer can be measured using a metallographic etch for stainless steel to remove the oxidized transfer layer from the coating surface after the test. The area of transferred material is then determined by taking surface profilometer traces along
the same track before and after this process. Figure 4 shows the variation in the amount of transfer as a function of applied substrate bias for the films tested here. Considerable transfer is observed for the rougher low bias films which have the more open zone I structure, suggesting that the intercolumnar gaps may promote the trapping of transferred material. For the dense zone T structures, the amount of transfer is small and this leads to the best wear performance. The film deposited at - 130 V bias is an exception to this trend; although this film is dense and very smooth it does show some transfer. Microscopic observation shows the presence of elongated needle-like debris [9] which has been suggested to be titanium oxide "rollers" formed by stripping and rolling up the native oxide from the TiN surface [16]. These rollers are responsible for the reduction in friction in unlubricated sliding for this coating. The oxygen content of the nitride is very low (less than 0.5% by Rutherford backscattering) and it is unlikely that any significant oxide formation would occur during deposition. On removal from the coating system some oxidation of the coating surface occurs on exposure to the atmosphere. The rollers could be formed by the removal of this native oxide from the TiN surface. This may lead to increased reactivity with the slider material as it passes over the stripped region, leading to transfer of material. However, the mechanism for the increased transfer for high bias coatings cannot be positively identified from the data in this study. No evidence for transfer was found in any of the lubricated sliding wear tests performed here, though it may be that some limited transfer is taking place at asperity contacts at a reduced level. The tests were all performed under conditions of boundary lubrication where a thin film of lubricant separates the sliding contacts and only a few direct contacts will occur between the sliding materials at asperity tips. Though no gross transfer occurs, some transfer may still be occurring at asperity tips and this is beyond the resolution of EDX. Such transfer can be observed by more sensitive techniques such as X-ray photoelectron spectroscopy on some samples. The large reduction in transfer leads to low friction coefficients and consequently no coating was detached from the substrate at any time during the lubricated sliding wear tests performed here.
3.2. Effect of deposition technology The importance of metal transfer in determining the wear performance of sputter ion plated TiN was shown to be dependent on coating microstructure in the previous section. Since the structure of a TiN coating is dependent on both the choice of deposition technology and the processing parameters used [17], similar measurements were made on magnetron sputtered and arc
S. J. Bull, P. R. Chalker / Sliding wear o f PVD titanium nitride
121
r
Fig. 3. Scanning electron fractographs of worn surfaces of an uncoated sphere sliding on a - 6 0 V bias TiN coated disc; (a) general view of the track showing areas of transferred material (unlubricated), (b) fiat on the sphere showing debris (unlubricated), (c) general view of the track showing no transfer (lubricated), and (d) flat on the sphere showing little or no debris (lubricated).
10 -13 10 "14t e~
10 -15
1016 10 -17
10"18 10-19
I0"20 0
i
I
f
100 50 Negative applied substrate bias (V)
150
Fig. 4. Variation of the amount of transfer with substrate bias for sputter ion plated titanium nitride in unlubricated sliding.
evaporated titanium nitride coatings. The sliding friction of the magnetron sputtered material is reduced by lubrication during the wear test by an amount similar to that of the sputter ion plating films reported in the previous section (Fig. 5(a)). Similarly the coefficient of friction is reduced for arc evaporated TiN films deposited at a range of thicknesses (Fig. 6(a)). The wear behaviour of the magnetron sputtered TiN as a function of applied substrate bias is very complicated for the unlubricated tests (Fig. 5(b)) but this can be related to the effect of coating microstructure on the amount of transfer during the wear test. The smoothness and density of the coating increase with bias up to - 8 0 V as for the sputter ion plated coating, and throughout this bias range the amount of transfer (and hence coating wear) is reduced. However, at - 1 0 0 V bias the coating
S. J. Bull, P. R. Chalker / Sliding wear o f P V D titanium nitride
122
=L
0.8
0.8
=-
C
O
•,~
0.6
0
0.4
I
u
Unlubricated Lubricated
0.6
&
I
•*~
0.2
E
0.4
n
Unlubricated
4
Lubricated
0.2
,$
0
0.0 0 (a)
!
i
50
100
i 150
200
Negative applied substrate bias (V)
le-14
I
Unlubricated Lubricated
I
i
2 (a)
2e-14 n •
0.0
250
I
i
I
3
i
I
4
5
C o a t i n g T h i c k n e s s (~tm) 1 0 -11
,~
Unlubricated Lubricated
10 -1~
[
t_
~
-le-14
.~
-2e-14
~
10 -12
-~
IO -I~
O I
-3e-14 0
(b)
50 Negative
,
I
100
i
I
,
150
I
,
200
applied substrate bias
I
~
10-15
I
250
2
(b)
(V)
i
I
i
3
1
*
4
Thickness (I.tm) 10 -12
10 -13
"g
Z ~
1 0 -14
10 -14 10 -15 10-1~
10 -15 10 -17 10.16
t_
[.,
t_ [-
10 -19 1 0 -17 0
(c)
10 -18
t 50
Negative
i 100
'
i
150
applied substrate bias
I 200
,
250
(v)
Fig. 5. Variation of (a) coefficient of sliding friction, (b) coated disc w e a r rate, and (c) transfer rate as a function o f bias voltage for 5 /zm thick m a g n e t r o n s p u t t e r e d TiN films in lubricated and unlubricated sliding.
undergoes a change in texture from {200} to {422} [17] and this is accompanied by an opening out of the coating structure (Fig. 7), allowing the trapping of debris and increased transfer (Fig. 5(c)). At greater than - 100 V bias voltage the coating density increases and the amount of transfer is reduced but not to the same extent as at low bias voltages. In lubricated sliding there is again no evidence of gross transfer in scanning electron microscopy (SEM) investigations and the wear rates for coatings deposited at all bias voltages are broadly similar. It is interesting to note that the coating wear rates for low rate sputter ion plated TiN and high rate magnetron sputtered TiN
i
I
2
I
(c)
3
4
5
T h i c k n e s s (~tm)
Fig. 6. Variation o f (a) coefficient o f sliding friction, (b) coated disc wear rate, and (c) transfer rate as a function of coating thickness for - 100 V bias arc e v a p o r a t e d titanium nitride coatings in lubricated and unlubricated sliding.
are similar at low bias voltages in lubricated sliding, though the magnetron sputtered material shows better performance at greater than - 8 0 V. The wear performance of the arc evaporated TiN coatings is also considerably improved by the use of a lubricant (Fig. 6(b)). For the arc evaporated coatings, transfer of titanium-rich material from the coating to the sphere was also observed owing to the reactivity of the titanium-rich macrodroplets within the coating (Fig. 8). These particles have been shown to influence the abrasive wear performance of arc evaporated TiN [19]. Even with the use of a lubricant, the wear rate of the evaporated coatings is greater than that of the sputtered films and thus it is essential to take steps
S. J. Bull, P. R. Chalker / Sliding wear of PVD titanium nitride
123
(b)
Fig. 7. Scanning electron fractographs of magnetron sputtered TiN on austenitic stainless steel, (a) -80 V bias and (b) -100 V bias, showing the reduction in coating density at the higher bias voltage.
Fig. 8. Scanning electron micrographs of (a) the wear track and (b) the flat on the sphere for a 1.7 p.m arc evaporated TiN coating on stainless steel tested in unlubricated sliding. to reduce the incidence of macrodroplet formation if the best coating performance is to be achieved for arc evaporated TiN. 3.3. Effect o f lubricant
In order to assess the sliding wear performance of TiN coatings in a range of lubricants, a number of identical titanium nitride coatings were produced on M2 tool steel. For each lubricant used three tests were performed: (1) 52 100 steel sphere against uncoated M2 disc; (2) 52 100 steel sphere against TiN coated M2 disc; (3) alumina sphere against TiN coated M2 disc. Though the effects o f any lubricant additive packages on the steel components in the sliding couples will be important in tests (1) and (2), the final test represents
ceramic on ceramic sliding and none of the lubricants used were originally intended for this contact situation. In all the tests described in this section, a total sliding distance of around 1 km was used; in the lubricated tests there was no evidence for gross transfer from the steel to the TiN substrates in test (2) and there was no evidence for coating spallation in either test (2) or (3). In all the lubricated sliding experiments performed here, the coefficient of sliding friction was around 0.05 and the variations between lubricants were within the experimental scatter of tests performed using a single lubricant. Figure 9 shows plots of sphere wear against disc wear for all the contact situations investigated and individual lubricant data are presented in Fig. 10. It
124
S.J. Bull, P. R. Chalker / Sliding wear o f PVD titanium nitride 10 -13]
10-13 None
10 -14
Walet m
10-15
z
[]
10-16 10-17 ¢o
[]
10.18
~_. =
10 -19
[]
10 -20 ........ I0-18
(a)
t_
ts
m ........ I0-17
, ........ 10"16
~ ........ 10"15 2
, 10-14
Uncoated M2 disc wear (m /N)
o
~
,~
z
"~
S
~
0
~
~
~
o.
~
~
m
(a)
10-13 None 10 -14
[]
10 -15
DC200 [] z
10 -16
[]
[]
m []
[]
l0 -17 10-18 10 -18
(b)
[]
[] L . . . . . . . .
n
. . . . . . . .
10 -17
TiN/M2
n
. . . . . . . .
n
10 -16
. . . . . . . .
10 -15
10 -14
Disc wear (m z/N)
10-15 1
z
Water[] None
10 -16
,~
~
>
~
~
8
m
~
o
~
~
~
~
~"
o
~
~x
<
IX)
a:
(b)
10"17 I
.=_. _= ,< 10-181 10 -17 (c)
Z t,',l 10-16
10 -15
I0 -14
10 -13
2
TiN/M2 Disc wear (m IN)
Fig. 9. Variation of sphere wear rate against disc wear rate in lubricated sliding for (a) a steel sphere against an uncoated M2 disc, (b) a steel sphere against a TiN coated M2 disc, and (c) an alumina sphere against a TiN coated M2 steel disc. The straight line is drawn through the results for the hydrocarbonbased mineral oil lubricants.
is readily apparent that the highest wear rates were measured in unlubricated sliding, but that high wear rates were also observed when water was used as the lubricant. Considerable corrosion of the steel substrate was observed (by SEM and X-ray diffraction) in the tests on uncoated M2 and the steel sphere also showed some signs of rusting. However, even in the alumina-titanium-nitride contacts the wear rate was high when water was used as a lubricant so it appears that some chemical interaction between the water and the ceramic surfaces has occurred to promote wear. This effect could be related to the chemomechanical softening
Av
(c)
a
Fig. 10. Bar charts of sphere and disc wear rates for the lubricants used here: (a) steel sphere against uncoated M2 disc, (b) steel sphere against TiN coated M2, and (c) alumina sphere against TiN coated M2.
of the surface of (mainly oxide) ceramics which has been attributed to adsorbed water [20, 21]. Clearly, this will have an important bearing on the selection of cutting fluids for use with TiN coated drills; the inference is that oil based cutting fluids are preferred to water based fluids if wear of the coating is to be
S. J. Bull, P. R. Chalker / Sliding wear o f PVD titanium nitride
minimized. As well as water, the use of the DC200 silicone lubricant in cases where one of the sliding counterfaces is steel leads to high wear rates in both sphere and disc. This lubricant is known to give poor performance for steels in sliding owing to its low loadbearing capacity. However, it appears to be perfectly adequate for the ceramic on ceramic contacts. For the hydrocarbon mineral oil based lubricants, general trends can be identified for each contact situation. For the steel sphere sliding on the uncoated M2 substrate the sphere wear is approximately constant with a wide range of disc wear rates measured. The use of PD68 compressor oil seems to give a much lower sphere wear rate, and the rotary pump oil (Vac 15) gives a higher wear rate than this mean level but it is difficult to tell from the experiments reported here if this scatter is significant. Another exception is the RXsuper engine oil. The data for this lubricant are not plotted in Fig. 9(a) because there was no detectable disc wear owing to the formation of a thick reaction layer over the wear track as a result of the anti-wear additives in the oil. For the steel sphere sliding on the TiN coated M2 (Fig. 9(b)) the sphere wear rate is again approximately constant for the mineral oil lubricants, but is higher than for the metal-metal contact owing to the harder sliding counterface. In this case the only real exception is the rotary pump oil which leads to a lower sphere wear rate. There is a considerable variation in the TiN coated disc wear rate. Those lubricants which showed the best performance in the steel-steel contacts do not show the best performance in the TiN-steel case and the lubricant ranking order has changed (Fig. 10). No attempt has yet been made to identify the factors in the oils which lead to the low TiN wear rates - the magnitude of the variation between the oils may not be significant in many applications. For the alumina sphere sliding on'the TiN coated M2 (Fig. 9(c)), the wear of the TiN coated disc is approximately constant but there is a large variation in the wear of the sphere. The function of the lubricants in this case cannot simply be to separate the sliding surfaces because the observed changes in wear rate do not correlate with the viscosity data presented in Table 1 and the sliding speeds used here are too low for hydrodynamic lubrication to be occurring. The thickness of the lubricant film will also depend on the variation of lubricant viscosity with pressure. It is expected that a wide range of lubricant film thicknesses have been investigated in this study but in the absence of calculated thicknesses of experimental data it is not possible to comment on the variation of wear with film thickness. Some chemical reactions take place between some constituents of the lubricant (apart form the additive package) and one or other of the sliding surfaces. It
125
appears that such reactions occur more readily on titanium nitride than alumina and help to protect the nitride surface from wear during sliding. However, the reactions occur even more readily on 52 100 steel, reducing the amount of sphere wear in the other experiments. This is expected since, as 52 100 is a bearing steel, the lubricants will be more likely to have been developed to work with it than tool steel or titanium nitride. The wear rates of individual lubricants are shown in Fig. 10. For the uncoated M2 disc (Fig. 10(a)), best wear performance is obtained for RX-super, an engine oil, Medos 100, a general purpose oil and the rotary pump oil, Vac 15. In the case of the engine oil a well defined reaction layer due to the anti-wear additives in the oil was found at the end of the test which was visible by reflected light microscopy. The precise composition of this layer could not be ascertained by Xray diffraction and EDX, but it was found that the reaction layers formed for other oils had different compositions which implies that the reaction layer chemistry is a strong function of the oil formulation. For the other lubricants which performed well, such anti-wear additives were not present. However, other oils with additives such as AWS32 did not perform well for the M2 substrate. In the case of the steel sphere sliding against the TiN coated M2 disc (Fig. 10(b)) these other oils performed better than the unformulated oils and the general purpose oil, Medos 100. In this case the good reference oil (RL142) led to best performance. Both the good reference oil and the engine oil also gave good results for the alumina sphere sliding against TiN coated M2 (Fig. 10(c)). Clearly, the additive packages present in commercial oils do react to differing extents with both ceramic coatings and bulk ceramics. In this study, no attempt was made to identify these reactions, but work is underway to identify the constituents of the lubricants which lead to good and bad coating performance. In most cases these reactions result in a reduction in wear, but this reduction may not be as pronounced as the reduction in wear for a steel on steel contact. In cases where the lubricant package is very effective in reducing wear for the steel on steel contact the performance of the steel sphere against TiN coated flat is relatively poor. However, for all other contact situations the wear performance is at least as good as the uncoated case.
4. Conclusions
The use of a lubricant in applications where steel slides against TiN can result in considerable reductions in wear because it limits the amount of transfer of the steel to the coated surface. As the smoothness of the
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S.J. Bull, P. R. Chalker / Sliding wear of PI,'D titanium nitride
coating increases, this transfer is reduced without the need for the lubricant, but the performance for rough coated surfaces is improved to that of the best smooth coating if one is used. The use of a lubricant leads to performance benefits for titanium nitride coatings produced by sputtering and arc evaporation; in the case of the arc evaporated TiN the lubricant reduces the adhesion between unreacted titanium in the coating and the steel counterface. The precise nature of the additive package in the oil affects the wear of the coating, as does the nature of the sliding contact. The wear of both a TiN coating and bulk alumina has been shown to be sensitive to the choice of lubricant used, with additive package reactions occurring to a varying extent during wear. In general the performance of the titanium nitride coating is no poorer than that of uncoated tool steel in lubricated sliding wear. However, where the additive package performance is very good with the steel, the TiN performance is relatively poor. Clearly, to obtain the best performance from coated components, lubricants will need to be developed specifically for coated contacts with additive packages designed for them. In the meantime, current lubricants can be used but care should be taken to select a lubricant which does not compromise performance if anti-wear additives have been particularly effective for the uncoated component.
were developed for the CARE programme) were provided by Castrol Ltd. Bill Sproul of BIRL is thanked for the provision of magnetron sputtered samples and Robert Barrell of JJ Castings for the arc evaporated material.
References 1 2 3 4
5 6
7 8 9 10 11 12 13 14 15 16 17
Acknowledgments 18
This work was performed as part of the Corporate Research Programme of AEA Technology. The authors would like to thank Peter Fernback for carrying out some of the wear testing. The reference oils (which
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