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Effect of temperature and sliding velocity on TiN coating wear
Surface and Coatings
Technology
91-95
(1997)
53-59
Effect of temperature and sliding velocity on TIN coating wear
Abstract Dry shding wear experiments at specific temperatures ranging between ‘25 and 500°C were conducted on a PVD (reactive ion plated) TiN coating deposited onto an austenitic stainless steel substrate. The coating was worn against high-speed steel pins using a pin-on-disc sliding configuration at a low contact load (10 N) and various sliding speeds. Three wear regimes were identified: (1) at low temperatures and sliding speeds up to -1 .O m s-‘, there was minimal damage to the ion-plated TIN and the formation of a protective iron oxide transfer layer on the TiN surface: (2) at temperatures in the -ZOO-450°C range, polishing of the TIN occurred, accompanied by increased wear rates as speed was increased; (3) above -45O’C ductile deformation and failure of the coating occurred. Steel pin wear rates dropped significantly and the wear track was coated with a titanium/iron oxide transfer layer. The wear rate data are summarised in map form on temperature vs. sliding speed axes and discussed together with corresponding SEM, EDS and optical metallographic evidence. 0 1997 Elsevier Science S.A. Kqwords:
Sliding velocity; TiN coating; Wear; Wear map
1. Introduction
coating above -300°C. The transition was attributed mostly to thermal softening of the steel substrateand lowering of
When engineering components are subjectedto dry sliding contact conditions with other materials, the generation of friction heat in the contact zone can induce significant changesin wear rates and mechanismsin either one or both materials[1,2]. Increasingthe severity of wear conditions by applying higher contact loads, speedsor external heating generally raises the energy input to the contact region. The attainment of a critical temperature or temperature range in the bulk material microstructure at the contact surface is thus conducive to initiation of wear transitions arising from thermal softening effects [I]. Recent studieson dry sliding of a monolithic Al alloy [3] againsttool steelshowed order of magnitude transitions from mild to severe wear in the alloy on attainment of a critical transition temperature (123 + 5°C) in the bulk microstructure below the sliding surface. Wear transitions arising from thermal softening effects in ultra-hard TiN-coated steels have also been observed; an investigation involving dry sliding contact between Si3N4 balls and a TiN-coated 316 stainlesssteel [4] revealed a general transition from tribochemical polishing of TIN at low temperatures,to ductile deformation and fracture of the * Corresponding author.
0257~897297/$17.00 0 1997 Else&x PIZ SO257-8972(97)00475-l
Science S.A. All rights reserved
coating hardness,
where reduced
compressive
thermal
mis-
match stressesin the TiN arise as the coating is heated [S]. This has beenillustrated in investigations on the hot-indentation resistance of TiN and Tic-coated steels, where-a general degradation in coating hardness occurs above -300°C [5,6]. Wear data from investigations
where
a broad
range of
load, speedand/or temperature test conditions are used are increasingly being presented in the literature in wear map form. In general, the data are plotted on normal load or contact pressureversus sliding speed,or number of sliding cycles, map axes, offering a practical meansby which transitions and wear regimes can be identified and delineated, according to test condition ranges[2]. Somerecent examples include mapsfor sliding wear comparisonsof nitrided and non-nitrided steels against tool steel pins [7], TiN and AlzOj-coated steels [8]. A1203 vs. A120j, Sic vs. Sic and Si3N?vs. SisN4 ceramic sliding pairs [9]: and Al alloy wear [31.
In this paper, the effect of increased temperature and sliding speedon the behaviour of a TiN-coated stainless steel, in dry sliding contact against high-speed steel pins is reported. The wear data are presentedin wear map form on temperature versus sliding speed axes with dominant
S. Wihn,
I 2 s
p
A. Z Alprts
/ Sqtitc~
rind
Coniings
500 0
1.~.1'~~J~'*@*~1" 0 10
Subsurface
20 Depth
30 (pm)
Fig. I. (a) Optical micrograph showing the subsurface microstructure of the TiN-coated 316 stain& steel. The micrograph w;1s obtained from ;i taper section inclined ut 3S” to Ihe TiN coating surfucc. Variations in N concentration during the depocition process produced interlayers of different Ti and N stoichiometry, visible as layers in the micrograph. (b) Mensurements of bulk coating/substrate Vi&r& microhardness profile along the hubsurface taper section shown in a.
mechanismsof coating wear damagebeing identified and marked on the maps. These results are then discussedin terms of the thermomechanicalbehaviour of TiN coatings.
2. Experimental 2. I. Muterids The substrateused was a 316L austenitic stainlesssteel which had been burnished in an alumina and porcelain media, prior to washing. After drying, the steel plates were sputter cleaned followed by reactive ion plating of TiN onto their surfacesat Liburdi EngineeringLtd., Hamilton, Ontario, Canada.The deposition process comprisedan electron beam evaporation processby which titanium was deposited under vacuum onto cathodically charged substrate surfaces [IO]. Nitrogen was added into the plasma and a TiN film produced on the surface by reacting with the titanium ions. A coating temperatureof 420°C and high Ti deposition rate were usedto produce a nitrogen-deficient Ti layer between the substrateand a nitrogen-rich TiN surface. The use of a relatively soft 3 16stainless-steelsubstrate was consideredto be useful for examining TiN wear at low temperatures and substratesoftening effects at higher temperatures. Fig. la is an optical micrograph of a low-angle taper section of the coating. The taper section was prepared by mounting the coating in resin at an inclined angle of 3.5” followed by metallographic polishing to give a magnified view of its subsurfacemicrostructure. The microstructural features aligned vertically at - 10 pm, 18 pm and 20 pm
Technology
94-95
(1997)
53-59
show some evidence of microporosity which presumably arise from fluctuations in nitrogen gas pressure during Ti deposition. Fig. lb showsthe change in the microhardness (Hv soa)as a function of subsurfacedepth measuredacross the taper section. Since the size of indentations madeacross the taper range between-6 pm at the coating surface to -20 pm in the substrate,the hardnessis thus a bulk or composite measurementof the coating, its substructure and the substratesresistanceto indentation. The hardest region (H, 5og 2546 i 315kg mm-‘) is comprisedof a TiN outer layer that is approximately 10 pm thick which subsequentlydecreases in hardnessto the substrate which is -20 pm beneath the surEace.Hardnessvariations in this region have arisen as a result of fluctuations in nitrogen and titanium concentrations during deposition [ lo]. The pre-testsurfaceroughnessof the TiN coating wasmeasuredasR, = 0.34 510.09 pm. The 316 stainless-steel substrate is significantly softer (Hv 5~ 279.2 + 6.2 kg mm-‘) than the coating. High-speed steel pins (AISI M4, nominal composition 1.3 C, 5.1 Cr, 3.0 V, 1.7 Ni, 0.1 Si, 0.3 S, bal.Fe, in wt.%) 2.3 mm in diameter and hardnessHv 5og242.2 1 8.7 kg mm-’ (normalisedcondition) were used as the slider material for wear tests. 2.2. Weuv tesrs The dry sliding tests were done using ion-plated TiN coupon specimens on an elevated temperature sliding wear apparatus,Details of the operating principles of this machineare given in Ref. [4]. Experiments were performed at set temperaturesbetween 25°C and 500°C and sliding speedsbetween 0.05 and 1.0 m s-‘. The apparatusis comprised of a variable speed rotating shaft arrangement to which is attached the TIN specimen holder. A vertical shaft, to which is attached the pin specimen, is lowered onto the rotating TiN coupons to produce a circular wear track 11.0 mm in average diameter. Heat is provided by enclosing the specimensin furnace controlled by a thermostat and temperaturesmonitored through the useof a K type ungrounded thermocouple affixed to the pin specimen holder. Temperature readingswere monitored by software interface with a personalcomputer. A constantsliding distanceof 1000m and contact load of 10 N was used in all experiments. The nominal contact pressurebetween pin and coating was 2.41 MPa for each test, and the pin contact geometry with the coating usedwas a flat-on-flat configuration. In the elevated temperaturetests the specimenswere heated to the required temperature and allowed to equilibrate for at least 30 min before testing. Wear rates of both TiN and steel pins were obtained by measuring massloss/gain after each test and dividing by the total sliding distance. Scanning electron microscopy (SEM) under low vacuum conditions (-15 Pa) and energy dispersion spectroscopy (EDS) X-ray techniques were employed to study the various wear mechanismsand debris compositions.
S. Wilsm,
A.T. Alps
/S!q%ce
rind Courings
3. Results 3.1. Wear rates of ion-plated TiN coating and M4 steel pins Fig. 2 shows wear rates of both the ion-plated TiN coating and steel pins for tests conducted at 0.12 m s-r and temperatures between 25°C and 500°C. All wear rates were determined from mass loss measurements accurate to iO.0002 g and variations in wear rates are expected to be i0.2 x IO-’ g m-l. The TiN has a negative wear rate (i.e. mass gain) at 25°C and zero wear at 200°C followed by an increase to a maximum wear rate of 8.4 x lo-’ g m-’ at 450°C. Wear of the TIN at 5OO’C is characterised by a slight decrease to 3.4 x 10” g mei by comparison to the peak at 450°C. The steel pin shows a significant drop in wear rate from 39.7 to 51.8 x lo-‘g m-r in the below 45O’C temperature range, to 0.0 x lo-’ g m-l at 450°C and 6.0 x lo-’ g m-’ at 500°C. Wear rates at other sliding speeds and temperatures for the TIN and steel pins are summarised in map form in Figs. 3 and 4, respectively. The maps have linear temperature versus sliding speed axes, and each plotted point represents the conditions for a particular wear test. Wear rate data have been marked adjacent to each point. 3.2. Oxidation wear regime The TiN experiences negative wear rates below -0.16 m s-l at 25’C (Fig. 3) and wear tracks were covered with a characteristic red/brown transfer layer. Fig. 5a shows a scanning electron micrograph of the TiN wear track after a test at 25°C and 0.12 m s-r showing the powder-like transfer layer compacted onto the TiN surface. There has been little damage to the TiN surface. An EDS spectrum of the transfer layer (Fig. 5b) reveals the presence of iron, oxygen and titanium, suggesting that it is comprised of mostly, iron and titanium oxides. Wear debris produced at these test conditions were separated using a magnet into a speed= 5e-6 -
4e-6
C &?I* L
‘c 9
se-6
c
-$ tti
2e-6
i?
91-93
500
-
3.6 0 ductile
-
4.8.
TiN Wear Rates for Dry Sliding vs. AISI M4 (x1O”g.m )
deformation deformation and fa!iure
polishing -
6.0 0 12.13.m
1570
100 0
0.0
0.6 0.8 0.2 0.4 Sliding Speed (m.s-I)
1.0
1.2
Fig. 3. Wear map showing TIN wear rates at different sliding speeds and temperature conditions. The wear rates have been multiplied by IO’.
non-magnetic red/brown powder and a smaller fraction of a magnetic black one. This is an indication that the red/brown powder is probably rhombohedral hematite (FelO;) which is a non-magnetic oxide, whilst the magnetic black one is most probably small iron particles generated by the abrasion of the steel pin against the hard TiN coating. Wear tracks of TiN specimens for 25°C teats at the higher sliding speeds also had the red/brown transfer layer present, although it was observed to be significantly diminished at 1.0 m s-r. The transfer layer was also marginally present on the TiN wear track at 0.12 m s-’ and 200°C. An ‘oxidation’ regime has been demarcated on the TIN wear map (Fig. 3) denoting test conditions where the red/brown oxide transfer layer was visible. 600 ,.,..,.,,,a,,....,nn
2
i
k
AISI M4 Pin Wear Rates for Dry Sliding vs. TiN (x1U’g.m.‘)
6.0.
-
O’
\
55
53-59
-I’,““,““,““,
0.21 m.s-’
-y
(1997)
600
500
, , , , , , I , , , , , , , , , ,
6e-6
Technology
. po-*. I -1.. --.. 47.0* I-._ -1.. 602.
51.8.6g~g.422.
-~~~~~~~.~ : 95,2.44J J47.1 l
&!3.3
48.2@
,.1.1.~‘.‘,.‘.,‘.‘,,,,‘,,,,‘,,”
-2e-6=“““““““““““““‘0
100
200
300
Temperature
400
500
("C)
Fig. 2. Graph showing variation in TiN and steel pin wear rates with te,t temperature at a constant sliding speed of 0.12 m s-‘.
0.0
0.2 0.4 0.6 0.8 Sliding Speed (m.s-')
Fig. 4. Wear map showing and iemperature conditions.
1.0
1.2
steel pin wear rates at different sliding speeds The wear rates have been multiplied by IO’.
S. Wilson, A.T. Alpns / Sq‘kr
56
mci Comings
Technology
94-9.5
(19973 53-59
and the rest of the coating is undamaged. Surface roughness measurements taken on the wear track in the direction of sliding, revealed an average R, = 0.05 ?. 0.01 (.cm. A ‘polishing’ wear regime has been demarcated on the TiN wear map (Fig. 3) for conditions where this mechanism was observed. 3.4. Ductile deformation and failure regime
Wooo
Above -450°C the worn TIN surfaces still showed evidence of ‘polishing’ type wear, although darker patches of transfer material were visible to the naked eye in the wear track. Fig. 9a is an SEM micrograph of the TiN surface after a wear test at 500°C and 0.12 m s-‘. The edge of the wear track shows distinct signs of polishing type wear adjacent to which is a region of powdery wear debris. The region closer towards the centre of the wear track is comprised of a compacted transfer layer which corresponds to the black region visible to the naked eye. The layer has also cracked and delaminated in some regions. EDS analysis of the transfer material (Fig. 9b) reveals it is comprised of mostly titanium and iron oxides. Part of the black layer was scraped from the wear track and was found to be magnetic, indicating that
-r-r , , / r----
I
0
2
I
I
I
I
I
I
I
4
11,
I’,
6
8
”
1
10
“,I
’
J
12
Energy (eV) Fig. 5. (a) Scanning electron micrograph of at 25°C and 0.12 m s-’ showing a powdery the TiN surface. There is little damage to spectrum of the transfer layer reveals the titanium.
the TIN wear track after a test transfer layer compacted onto the TIN surface. (b) An EDS prrscnce of iron, oxygen and
Above -2OO’C, the oxide transfer iayer becomes less visible, and the TiN wear track surfaces are more polished in appearance. The SEM micrograph in Fig. 6a shows the surface of the TiN after wear at 300°C and 0.12 m s-l where the wear track shows signs of slight smoothing of surface asperities, and there are patches of compacted powder transfer material. EDS analysis of the transfer material (Fig. 6b) shows it is comprised of mostly oxygen, titanium and iron, i.e. oxides of these two metals. 3.3. ‘Polishing ’ wear regime
The SEM micrograph in Fig. 7 shows the TiN surface after wear at 300°C and 1.2 m s-‘. Here the surface has undergone significantly more polishing and there is less evidence of oxide transfer in the wear track. The increase in polishing effect with raised sliding speed at 300°C also corresponds to a parallel increase in the TiN wear rate (Fig. 3). Fig. 8 shows the depth to which polishing wear damage extends beneath the TiN wear track (3OO”C, 0.6 m s-l). Only the first 5-8 pm of the coating surface has been worn away
4
Fig. and and the
6 Energy
8
10
12
(eV)
6. (a) Scanning electron micrograph of the TiN after wear at 300°C 0.12 m s-‘. The wear track shows slight smoothing of surface asperities there are patches of compacted transfer material. (b) EDS spectrum of transfer material in a.
5’. Wilson, A.T. Alps
/ Swf~ce
arwi Comings
Fig. 7. TIN surface after wear at 300°C and 1.2 m s-‘. Here the ht$“ace has undergone significa?tly more polishing and there is less evidence of oxide transfer in the wear track.
any iron oxides present were most likely magnetite (FesOJ; a black, magnetic, high-temperature oxide. It is unlikely that debris would be‘metallic in this regime due to the highly oxidising conditions. The optical micrograph in Fig. 10 shows the extent of subsurface damage beneath the TiN wear track at 5OO’C. The TiN has plastically deformed at the interface with the stainless steel substrate beneath the centre of the wear track, and there is also evidence for coating failure. Wear rates of the M4 steel pins (Fig. 4) appear to show little correlation with concurrent changes in sliding speed or temperature below -45O’C. However at -450°C and above, wear rates drop dramatically indicating a significant change in wear mechanism.
4. Discussion The wear behaviour of ion-plated TIN against high-speed steel shows analogous features to recent work on a similar
-
.
*
15jxm Fig. 8. Shows the depth to which polishing wear damage extends beneath the Tick wear track for wear at 300°C and 0.6 m s-‘. Only the first 5-8 pm of the coating surface has been worn away, and the rest of the coating
substructureis undamaged.
Technology
94-95
(b) luoo // 1
57
(1997J 53-59
/
/
,
Fe
/
b
800
,
/ TiN worn surface 500%, 0.12 m.s”
1 4
II
0
2
4
8
6 Energy
3
IO
12
(W
Fig. 9. (a) TiN surface after a svear test at 500°C and 0.12 m s-‘. The edge of the wear track shows distinct signs of polishing wear, adjacent to which is a region of transferred oxide wear debris compacted in the centre of the wear track. (b) EDS spectrum of the transfer material in a.
TiN coating sliding against Si3NJ balls (Hv =,og2549.9 F 286.3 kg mm-‘) [4], where polishing wear modes were observed at low temperatures, sliding speeds and contact loads, accompanied by the formation of a thin tribochemitally formed SiO/Si(OH)x transfer layer on the TiN surface. As temperatures were raised, Si3Nq tribochemical decomposition was diminished resulting in greater ball contact stresses and cracking damage to the TIN coating. At temperatures approaching 600°C softening and plastic relaxation of the TiN coating and substrate resulted in a ductile deformation wear mechanism, accompanied by a significant lowering of Si3N4 wear rates. In the present investigation, the AISI M4 steel pin specimens are an order of magnitude softer (242.2 F 8.7 kg mm-‘) than the TiN coating (2546 + 315 kg mm-‘), and are thus expected to wear preferentially and inflict less damage to the coating than what is seen for TiN wear against a correspondingly harder SijNJ ball [4]. Any softening of the TIN coating and substrate arising from relaxation in elastic mismatch stresses [6,7] and/or reduced shear stress of the substrate at elevated temperatures would have minimal influence on the steel pin wear rates, given the large hardness differential. A low
58
S. Wilson,
: plastic deformation/-. * . _’
A.T. Alpas / Sqike
3 16stainlesssteel
and Coatings
Technology
94-95
(1997)
53-59
ishing wear and lowering of oxide transfer to the TIN surface. As sliding speed is increased, there will be an expected increase in friction heating effects at the sliding contact surface [2j and subsequent softening of the TiN and substrate in the localised contact region. Increased wear of the TiN coating therefore arises as a result of thermal softening. Here again, reduced oxide transfer and lubrication effects at the higher speeds would also contribute to increased wear. 15&m
5. Conclusions Fig. 10. Opticai micrograph bhowing subsurface damage beneath the TiN wear track at 500°C and 0.12 m s-‘. The TiN has plastically deformed into the stainless-steel substrate beneath the centre of thr track, and there is evidence for coating failure.
pin hardness is conducive to accommodation of contact stresses at the sliding interface by plastic deformation of the pin material, thereby reducing the chances of indentation cracking damage in the TiN which generally arise when stresses generated by sliding contact attain a critical level [ 11,121. However, once the coating hardness is sufficiently diminished by thermal effects and plastic relaxation of its substrate to levels close to or below those of the steel pin, there will be an expected drop in pin wear. This effect is observed in Figs. 2 and 4, where steel pin wear rates drop markedly at 45O’C and above, whilst remaining relatively unaffected by wear conditions below this temperature. The TiN and its 3 16 stainiess steel substrate have softened to an extent whereby minimal wear is inflicted on the opposing steel pin. The softening effect is also apparent on inspection of the damage beneath the wear track (Fig. lo), where the TiN has plastically deformed into the stainless steel substrate. Oxidation of the steel pin material and the TiN surface appear to provide protection of both contact surfaces from wear by the formation of a protective oxide transfer layer. This is seen especially for room temperature wear at low speeds, where oxide transfer to the TiN results in mass gains and negative wear rates (Fig. 3). Increases in temperature and sliding speed appear to reduce the stability of the FepOj transfer layer and increase polishing wear damage. Whilst increasing the sliding speed would reduce the FelOa transfer layer stability on the TiN, it is unclear as to why temperature increases into the polishing regime, above -200°C and below -45O”C, have a similar effect. A possible reason may be due to the increased formation of TiO, species at elevated temperatures which are known lubricants and may therefore inhibit Fe203 transfer from the steel [ 131. The TiN polishing wear modes seen at temperatures above -200°C are infiuenced by sliding speed, as is evidenced by the increase in TiN wear rates with speed at 3OO’C (Fig. 3). Here, there is a concurrent increase in pol-
Dry sliding wear experiments at specific temperatures ranging between 25” and 500°C were conducted on a PVD TiN coating deposited onto an austenitic stainlesssteel substrate. The wear rate data were summarised in map form on temperature vs. sliding speed axes, and three wear regimes were identified: 1. At low temperatures and sliding speeds up to -1 .Om s-j, there was minimal damage to the TiN and the formation of a protective Fez03 transfer layer on the TiN surface. 2. At temperatures in the -2OO-45O’C range, polishing of the coating occurred. Wear rates of the TiN increased with higher sliding speeds in this regime. This effect was attributed to further softening of the coating arising from additional friction heating effects at higher speeds. 3. Above -45O”C, ductile deformation and failure of the coating occurred. Steel pin wear rates dropped signiiicantly, and the wear track was coated with a lubricating titanium oxide and iron oxide transfer layer.
Acknowledgements
Financial support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a strategic grant program is gratefully acknowledged. The authors would like to thank Mr. D. Nagy of Liburdi Engineering Ltd., Hamilton, ON, for support and interest in this work and provision of test materials.
References [I] J.K. Lancaster, Proc. Phys. Sm. B, 70 (1957) 112. 121 SC. Lim and M.F. Ashby, Am Met& 35 (1987) 1. [3] J. Zhang and A.T.Alpas, Actu iWater., 45 (1997) 513. [4] S. Wilson and A.T. Alpas, Surf: Coat. Technol., 86-87 (1996) 75. [S] S.J. Bull, D.S. Rickerby, J.C.Knight and T.F.Page, Sud Eng., 8 (1992) 193. 161 J.C. Knight, T.F. Page and I.M. Hutchings, SUI$ Eng., 5 (1989) 213. [7] H. Kato, T&Eyre and B. Ralph, Acta Mefull. Mater., 42 (1994) 1703. IS] K. Kate, S& Cont. Technol., 76-77 (1995) 469.
S. Wilson,
A.T. Alpas / Swface
and Coatings
[9] K. Kato, in H.M. Hawthorne and T. Trccynski (eds.), Advanced Ceramics far Structural nnd Tribological Appl., Proc. 34 ConJ CM, Vancouver, 1995, p. 23. [IO] V.R. Parameswaran, J.P. Immarigeon and D. Nagy, Su$ Cont. Technol., 52 (1992) 251.
Technology
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(1997)
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59
[ll] S. Wilson and A.T. Alpas, li’enr; 196 (1996) 270. [12] S.J. Bull, Sur$ Cont. Techno!., 50 (1991) 25. 1131 MN. Gardos, H. Hong and W.O. Winer, Tribology Trnns., 22 (1990) 209.
Technology
91-95
(1997)
53-59
Effect of temperature and sliding velocity on TIN coating wear
Abstract Dry shding wear experiments at specific temperatures ranging between ‘25 and 500°C were conducted on a PVD (reactive ion plated) TiN coating deposited onto an austenitic stainless steel substrate. The coating was worn against high-speed steel pins using a pin-on-disc sliding configuration at a low contact load (10 N) and various sliding speeds. Three wear regimes were identified: (1) at low temperatures and sliding speeds up to -1 .O m s-‘, there was minimal damage to the ion-plated TIN and the formation of a protective iron oxide transfer layer on the TiN surface: (2) at temperatures in the -ZOO-450°C range, polishing of the TIN occurred, accompanied by increased wear rates as speed was increased; (3) above -45O’C ductile deformation and failure of the coating occurred. Steel pin wear rates dropped significantly and the wear track was coated with a titanium/iron oxide transfer layer. The wear rate data are summarised in map form on temperature vs. sliding speed axes and discussed together with corresponding SEM, EDS and optical metallographic evidence. 0 1997 Elsevier Science S.A. Kqwords:
Sliding velocity; TiN coating; Wear; Wear map
1. Introduction
coating above -300°C. The transition was attributed mostly to thermal softening of the steel substrateand lowering of
When engineering components are subjectedto dry sliding contact conditions with other materials, the generation of friction heat in the contact zone can induce significant changesin wear rates and mechanismsin either one or both materials[1,2]. Increasingthe severity of wear conditions by applying higher contact loads, speedsor external heating generally raises the energy input to the contact region. The attainment of a critical temperature or temperature range in the bulk material microstructure at the contact surface is thus conducive to initiation of wear transitions arising from thermal softening effects [I]. Recent studieson dry sliding of a monolithic Al alloy [3] againsttool steelshowed order of magnitude transitions from mild to severe wear in the alloy on attainment of a critical transition temperature (123 + 5°C) in the bulk microstructure below the sliding surface. Wear transitions arising from thermal softening effects in ultra-hard TiN-coated steels have also been observed; an investigation involving dry sliding contact between Si3N4 balls and a TiN-coated 316 stainlesssteel [4] revealed a general transition from tribochemical polishing of TIN at low temperatures,to ductile deformation and fracture of the * Corresponding author.
0257~897297/$17.00 0 1997 Else&x PIZ SO257-8972(97)00475-l
Science S.A. All rights reserved
coating hardness,
where reduced
compressive
thermal
mis-
match stressesin the TiN arise as the coating is heated [S]. This has beenillustrated in investigations on the hot-indentation resistance of TiN and Tic-coated steels, where-a general degradation in coating hardness occurs above -300°C [5,6]. Wear data from investigations
where
a broad
range of
load, speedand/or temperature test conditions are used are increasingly being presented in the literature in wear map form. In general, the data are plotted on normal load or contact pressureversus sliding speed,or number of sliding cycles, map axes, offering a practical meansby which transitions and wear regimes can be identified and delineated, according to test condition ranges[2]. Somerecent examples include mapsfor sliding wear comparisonsof nitrided and non-nitrided steels against tool steel pins [7], TiN and AlzOj-coated steels [8]. A1203 vs. A120j, Sic vs. Sic and Si3N?vs. SisN4 ceramic sliding pairs [9]: and Al alloy wear [31.
In this paper, the effect of increased temperature and sliding speedon the behaviour of a TiN-coated stainless steel, in dry sliding contact against high-speed steel pins is reported. The wear data are presentedin wear map form on temperature versus sliding speed axes with dominant
S. Wihn,
I 2 s
p
A. Z Alprts
/ Sqtitc~
rind
Coniings
500 0
1.~.1'~~J~'*@*~1" 0 10
Subsurface
20 Depth
30 (pm)
Fig. I. (a) Optical micrograph showing the subsurface microstructure of the TiN-coated 316 stain& steel. The micrograph w;1s obtained from ;i taper section inclined ut 3S” to Ihe TiN coating surfucc. Variations in N concentration during the depocition process produced interlayers of different Ti and N stoichiometry, visible as layers in the micrograph. (b) Mensurements of bulk coating/substrate Vi&r& microhardness profile along the hubsurface taper section shown in a.
mechanismsof coating wear damagebeing identified and marked on the maps. These results are then discussedin terms of the thermomechanicalbehaviour of TiN coatings.
2. Experimental 2. I. Muterids The substrateused was a 316L austenitic stainlesssteel which had been burnished in an alumina and porcelain media, prior to washing. After drying, the steel plates were sputter cleaned followed by reactive ion plating of TiN onto their surfacesat Liburdi EngineeringLtd., Hamilton, Ontario, Canada.The deposition process comprisedan electron beam evaporation processby which titanium was deposited under vacuum onto cathodically charged substrate surfaces [IO]. Nitrogen was added into the plasma and a TiN film produced on the surface by reacting with the titanium ions. A coating temperatureof 420°C and high Ti deposition rate were usedto produce a nitrogen-deficient Ti layer between the substrateand a nitrogen-rich TiN surface. The use of a relatively soft 3 16stainless-steelsubstrate was consideredto be useful for examining TiN wear at low temperatures and substratesoftening effects at higher temperatures. Fig. la is an optical micrograph of a low-angle taper section of the coating. The taper section was prepared by mounting the coating in resin at an inclined angle of 3.5” followed by metallographic polishing to give a magnified view of its subsurfacemicrostructure. The microstructural features aligned vertically at - 10 pm, 18 pm and 20 pm
Technology
94-95
(1997)
53-59
show some evidence of microporosity which presumably arise from fluctuations in nitrogen gas pressure during Ti deposition. Fig. lb showsthe change in the microhardness (Hv soa)as a function of subsurfacedepth measuredacross the taper section. Since the size of indentations madeacross the taper range between-6 pm at the coating surface to -20 pm in the substrate,the hardnessis thus a bulk or composite measurementof the coating, its substructure and the substratesresistanceto indentation. The hardest region (H, 5og 2546 i 315kg mm-‘) is comprisedof a TiN outer layer that is approximately 10 pm thick which subsequentlydecreases in hardnessto the substrate which is -20 pm beneath the surEace.Hardnessvariations in this region have arisen as a result of fluctuations in nitrogen and titanium concentrations during deposition [ lo]. The pre-testsurfaceroughnessof the TiN coating wasmeasuredasR, = 0.34 510.09 pm. The 316 stainless-steel substrate is significantly softer (Hv 5~ 279.2 + 6.2 kg mm-‘) than the coating. High-speed steel pins (AISI M4, nominal composition 1.3 C, 5.1 Cr, 3.0 V, 1.7 Ni, 0.1 Si, 0.3 S, bal.Fe, in wt.%) 2.3 mm in diameter and hardnessHv 5og242.2 1 8.7 kg mm-’ (normalisedcondition) were used as the slider material for wear tests. 2.2. Weuv tesrs The dry sliding tests were done using ion-plated TiN coupon specimens on an elevated temperature sliding wear apparatus,Details of the operating principles of this machineare given in Ref. [4]. Experiments were performed at set temperaturesbetween 25°C and 500°C and sliding speedsbetween 0.05 and 1.0 m s-‘. The apparatusis comprised of a variable speed rotating shaft arrangement to which is attached the TIN specimen holder. A vertical shaft, to which is attached the pin specimen, is lowered onto the rotating TiN coupons to produce a circular wear track 11.0 mm in average diameter. Heat is provided by enclosing the specimensin furnace controlled by a thermostat and temperaturesmonitored through the useof a K type ungrounded thermocouple affixed to the pin specimen holder. Temperature readingswere monitored by software interface with a personalcomputer. A constantsliding distanceof 1000m and contact load of 10 N was used in all experiments. The nominal contact pressurebetween pin and coating was 2.41 MPa for each test, and the pin contact geometry with the coating usedwas a flat-on-flat configuration. In the elevated temperaturetests the specimenswere heated to the required temperature and allowed to equilibrate for at least 30 min before testing. Wear rates of both TiN and steel pins were obtained by measuring massloss/gain after each test and dividing by the total sliding distance. Scanning electron microscopy (SEM) under low vacuum conditions (-15 Pa) and energy dispersion spectroscopy (EDS) X-ray techniques were employed to study the various wear mechanismsand debris compositions.
S. Wilsm,
A.T. Alps
/S!q%ce
rind Courings
3. Results 3.1. Wear rates of ion-plated TiN coating and M4 steel pins Fig. 2 shows wear rates of both the ion-plated TiN coating and steel pins for tests conducted at 0.12 m s-r and temperatures between 25°C and 500°C. All wear rates were determined from mass loss measurements accurate to iO.0002 g and variations in wear rates are expected to be i0.2 x IO-’ g m-l. The TiN has a negative wear rate (i.e. mass gain) at 25°C and zero wear at 200°C followed by an increase to a maximum wear rate of 8.4 x lo-’ g m-’ at 450°C. Wear of the TIN at 5OO’C is characterised by a slight decrease to 3.4 x 10” g mei by comparison to the peak at 450°C. The steel pin shows a significant drop in wear rate from 39.7 to 51.8 x lo-‘g m-r in the below 45O’C temperature range, to 0.0 x lo-’ g m-l at 450°C and 6.0 x lo-’ g m-’ at 500°C. Wear rates at other sliding speeds and temperatures for the TIN and steel pins are summarised in map form in Figs. 3 and 4, respectively. The maps have linear temperature versus sliding speed axes, and each plotted point represents the conditions for a particular wear test. Wear rate data have been marked adjacent to each point. 3.2. Oxidation wear regime The TiN experiences negative wear rates below -0.16 m s-l at 25’C (Fig. 3) and wear tracks were covered with a characteristic red/brown transfer layer. Fig. 5a shows a scanning electron micrograph of the TiN wear track after a test at 25°C and 0.12 m s-r showing the powder-like transfer layer compacted onto the TiN surface. There has been little damage to the TiN surface. An EDS spectrum of the transfer layer (Fig. 5b) reveals the presence of iron, oxygen and titanium, suggesting that it is comprised of mostly, iron and titanium oxides. Wear debris produced at these test conditions were separated using a magnet into a speed= 5e-6 -
4e-6
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-
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TiN Wear Rates for Dry Sliding vs. AISI M4 (x1O”g.m )
deformation deformation and fa!iure
polishing -
6.0 0 12.13.m
1570
100 0
0.0
0.6 0.8 0.2 0.4 Sliding Speed (m.s-I)
1.0
1.2
Fig. 3. Wear map showing TIN wear rates at different sliding speeds and temperature conditions. The wear rates have been multiplied by IO’.
non-magnetic red/brown powder and a smaller fraction of a magnetic black one. This is an indication that the red/brown powder is probably rhombohedral hematite (FelO;) which is a non-magnetic oxide, whilst the magnetic black one is most probably small iron particles generated by the abrasion of the steel pin against the hard TiN coating. Wear tracks of TiN specimens for 25°C teats at the higher sliding speeds also had the red/brown transfer layer present, although it was observed to be significantly diminished at 1.0 m s-r. The transfer layer was also marginally present on the TiN wear track at 0.12 m s-’ and 200°C. An ‘oxidation’ regime has been demarcated on the TIN wear map (Fig. 3) denoting test conditions where the red/brown oxide transfer layer was visible. 600 ,.,..,.,,,a,,....,nn
2
i
k
AISI M4 Pin Wear Rates for Dry Sliding vs. TiN (x1U’g.m.‘)
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Technology
. po-*. I -1.. --.. 47.0* I-._ -1.. 602.
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-~~~~~~~.~ : 95,2.44J J47.1 l
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400
500
("C)
Fig. 2. Graph showing variation in TiN and steel pin wear rates with te,t temperature at a constant sliding speed of 0.12 m s-‘.
0.0
0.2 0.4 0.6 0.8 Sliding Speed (m.s-')
Fig. 4. Wear map showing and iemperature conditions.
1.0
1.2
steel pin wear rates at different sliding speeds The wear rates have been multiplied by IO’.
S. Wilson, A.T. Alpns / Sq‘kr
56
mci Comings
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and the rest of the coating is undamaged. Surface roughness measurements taken on the wear track in the direction of sliding, revealed an average R, = 0.05 ?. 0.01 (.cm. A ‘polishing’ wear regime has been demarcated on the TiN wear map (Fig. 3) for conditions where this mechanism was observed. 3.4. Ductile deformation and failure regime
Wooo
Above -450°C the worn TIN surfaces still showed evidence of ‘polishing’ type wear, although darker patches of transfer material were visible to the naked eye in the wear track. Fig. 9a is an SEM micrograph of the TiN surface after a wear test at 500°C and 0.12 m s-‘. The edge of the wear track shows distinct signs of polishing type wear adjacent to which is a region of powdery wear debris. The region closer towards the centre of the wear track is comprised of a compacted transfer layer which corresponds to the black region visible to the naked eye. The layer has also cracked and delaminated in some regions. EDS analysis of the transfer material (Fig. 9b) reveals it is comprised of mostly titanium and iron oxides. Part of the black layer was scraped from the wear track and was found to be magnetic, indicating that
-r-r , , / r----
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Energy (eV) Fig. 5. (a) Scanning electron micrograph of at 25°C and 0.12 m s-’ showing a powdery the TiN surface. There is little damage to spectrum of the transfer layer reveals the titanium.
the TIN wear track after a test transfer layer compacted onto the TIN surface. (b) An EDS prrscnce of iron, oxygen and
Above -2OO’C, the oxide transfer iayer becomes less visible, and the TiN wear track surfaces are more polished in appearance. The SEM micrograph in Fig. 6a shows the surface of the TiN after wear at 300°C and 0.12 m s-l where the wear track shows signs of slight smoothing of surface asperities, and there are patches of compacted powder transfer material. EDS analysis of the transfer material (Fig. 6b) shows it is comprised of mostly oxygen, titanium and iron, i.e. oxides of these two metals. 3.3. ‘Polishing ’ wear regime
The SEM micrograph in Fig. 7 shows the TiN surface after wear at 300°C and 1.2 m s-‘. Here the surface has undergone significantly more polishing and there is less evidence of oxide transfer in the wear track. The increase in polishing effect with raised sliding speed at 300°C also corresponds to a parallel increase in the TiN wear rate (Fig. 3). Fig. 8 shows the depth to which polishing wear damage extends beneath the TiN wear track (3OO”C, 0.6 m s-l). Only the first 5-8 pm of the coating surface has been worn away
4
Fig. and and the
6 Energy
8
10
12
(eV)
6. (a) Scanning electron micrograph of the TiN after wear at 300°C 0.12 m s-‘. The wear track shows slight smoothing of surface asperities there are patches of compacted transfer material. (b) EDS spectrum of transfer material in a.
5’. Wilson, A.T. Alps
/ Swf~ce
arwi Comings
Fig. 7. TIN surface after wear at 300°C and 1.2 m s-‘. Here the ht$“ace has undergone significa?tly more polishing and there is less evidence of oxide transfer in the wear track.
any iron oxides present were most likely magnetite (FesOJ; a black, magnetic, high-temperature oxide. It is unlikely that debris would be‘metallic in this regime due to the highly oxidising conditions. The optical micrograph in Fig. 10 shows the extent of subsurface damage beneath the TiN wear track at 5OO’C. The TiN has plastically deformed at the interface with the stainless steel substrate beneath the centre of the wear track, and there is also evidence for coating failure. Wear rates of the M4 steel pins (Fig. 4) appear to show little correlation with concurrent changes in sliding speed or temperature below -45O’C. However at -450°C and above, wear rates drop dramatically indicating a significant change in wear mechanism.
4. Discussion The wear behaviour of ion-plated TIN against high-speed steel shows analogous features to recent work on a similar
-
.
*
15jxm Fig. 8. Shows the depth to which polishing wear damage extends beneath the Tick wear track for wear at 300°C and 0.6 m s-‘. Only the first 5-8 pm of the coating surface has been worn away, and the rest of the coating
substructureis undamaged.
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/
/
,
Fe
/
b
800
,
/ TiN worn surface 500%, 0.12 m.s”
1 4
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2
4
8
6 Energy
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Fig. 9. (a) TiN surface after a svear test at 500°C and 0.12 m s-‘. The edge of the wear track shows distinct signs of polishing wear, adjacent to which is a region of transferred oxide wear debris compacted in the centre of the wear track. (b) EDS spectrum of the transfer material in a.
TiN coating sliding against Si3NJ balls (Hv =,og2549.9 F 286.3 kg mm-‘) [4], where polishing wear modes were observed at low temperatures, sliding speeds and contact loads, accompanied by the formation of a thin tribochemitally formed SiO/Si(OH)x transfer layer on the TiN surface. As temperatures were raised, Si3Nq tribochemical decomposition was diminished resulting in greater ball contact stresses and cracking damage to the TIN coating. At temperatures approaching 600°C softening and plastic relaxation of the TiN coating and substrate resulted in a ductile deformation wear mechanism, accompanied by a significant lowering of Si3N4 wear rates. In the present investigation, the AISI M4 steel pin specimens are an order of magnitude softer (242.2 F 8.7 kg mm-‘) than the TiN coating (2546 + 315 kg mm-‘), and are thus expected to wear preferentially and inflict less damage to the coating than what is seen for TiN wear against a correspondingly harder SijNJ ball [4]. Any softening of the TIN coating and substrate arising from relaxation in elastic mismatch stresses [6,7] and/or reduced shear stress of the substrate at elevated temperatures would have minimal influence on the steel pin wear rates, given the large hardness differential. A low
58
S. Wilson,
: plastic deformation/-. * . _’
A.T. Alpas / Sqike
3 16stainlesssteel
and Coatings
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ishing wear and lowering of oxide transfer to the TIN surface. As sliding speed is increased, there will be an expected increase in friction heating effects at the sliding contact surface [2j and subsequent softening of the TiN and substrate in the localised contact region. Increased wear of the TiN coating therefore arises as a result of thermal softening. Here again, reduced oxide transfer and lubrication effects at the higher speeds would also contribute to increased wear. 15&m
5. Conclusions Fig. 10. Opticai micrograph bhowing subsurface damage beneath the TiN wear track at 500°C and 0.12 m s-‘. The TiN has plastically deformed into the stainless-steel substrate beneath the centre of thr track, and there is evidence for coating failure.
pin hardness is conducive to accommodation of contact stresses at the sliding interface by plastic deformation of the pin material, thereby reducing the chances of indentation cracking damage in the TiN which generally arise when stresses generated by sliding contact attain a critical level [ 11,121. However, once the coating hardness is sufficiently diminished by thermal effects and plastic relaxation of its substrate to levels close to or below those of the steel pin, there will be an expected drop in pin wear. This effect is observed in Figs. 2 and 4, where steel pin wear rates drop markedly at 45O’C and above, whilst remaining relatively unaffected by wear conditions below this temperature. The TiN and its 3 16 stainiess steel substrate have softened to an extent whereby minimal wear is inflicted on the opposing steel pin. The softening effect is also apparent on inspection of the damage beneath the wear track (Fig. lo), where the TiN has plastically deformed into the stainless steel substrate. Oxidation of the steel pin material and the TiN surface appear to provide protection of both contact surfaces from wear by the formation of a protective oxide transfer layer. This is seen especially for room temperature wear at low speeds, where oxide transfer to the TiN results in mass gains and negative wear rates (Fig. 3). Increases in temperature and sliding speed appear to reduce the stability of the FepOj transfer layer and increase polishing wear damage. Whilst increasing the sliding speed would reduce the FelOa transfer layer stability on the TiN, it is unclear as to why temperature increases into the polishing regime, above -200°C and below -45O”C, have a similar effect. A possible reason may be due to the increased formation of TiO, species at elevated temperatures which are known lubricants and may therefore inhibit Fe203 transfer from the steel [ 131. The TiN polishing wear modes seen at temperatures above -200°C are infiuenced by sliding speed, as is evidenced by the increase in TiN wear rates with speed at 3OO’C (Fig. 3). Here, there is a concurrent increase in pol-
Dry sliding wear experiments at specific temperatures ranging between 25” and 500°C were conducted on a PVD TiN coating deposited onto an austenitic stainlesssteel substrate. The wear rate data were summarised in map form on temperature vs. sliding speed axes, and three wear regimes were identified: 1. At low temperatures and sliding speeds up to -1 .Om s-j, there was minimal damage to the TiN and the formation of a protective Fez03 transfer layer on the TiN surface. 2. At temperatures in the -2OO-45O’C range, polishing of the coating occurred. Wear rates of the TiN increased with higher sliding speeds in this regime. This effect was attributed to further softening of the coating arising from additional friction heating effects at higher speeds. 3. Above -45O”C, ductile deformation and failure of the coating occurred. Steel pin wear rates dropped signiiicantly, and the wear track was coated with a lubricating titanium oxide and iron oxide transfer layer.
Acknowledgements
Financial support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a strategic grant program is gratefully acknowledged. The authors would like to thank Mr. D. Nagy of Liburdi Engineering Ltd., Hamilton, ON, for support and interest in this work and provision of test materials.
References [I] J.K. Lancaster, Proc. Phys. Sm. B, 70 (1957) 112. 121 SC. Lim and M.F. Ashby, Am Met& 35 (1987) 1. [3] J. Zhang and A.T.Alpas, Actu iWater., 45 (1997) 513. [4] S. Wilson and A.T. Alpas, Surf: Coat. Technol., 86-87 (1996) 75. [S] S.J. Bull, D.S. Rickerby, J.C.Knight and T.F.Page, Sud Eng., 8 (1992) 193. 161 J.C. Knight, T.F. Page and I.M. Hutchings, SUI$ Eng., 5 (1989) 213. [7] H. Kato, T&Eyre and B. Ralph, Acta Mefull. Mater., 42 (1994) 1703. IS] K. Kate, S& Cont. Technol., 76-77 (1995) 469.
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A.T. Alpas / Swface
and Coatings
[9] K. Kato, in H.M. Hawthorne and T. Trccynski (eds.), Advanced Ceramics far Structural nnd Tribological Appl., Proc. 34 ConJ CM, Vancouver, 1995, p. 23. [IO] V.R. Parameswaran, J.P. Immarigeon and D. Nagy, Su$ Cont. Technol., 52 (1992) 251.
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[ll] S. Wilson and A.T. Alpas, li’enr; 196 (1996) 270. [12] S.J. Bull, Sur$ Cont. Techno!., 50 (1991) 25. 1131 MN. Gardos, H. Hong and W.O. Winer, Tribology Trnns., 22 (1990) 209.