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Analysis of wear behaviour of titanium carbonitride coatings
WEAR ELSEVIER
Wear 210 (1997) 245-254
Analysis of wear behaviour of titanium carbonitride comings Yeong Yan Guu, Jen Fin Lin
*
Department of Mechanical Engineering. National Cheng Sung Universio'. Tainan 70101, Taiwan
Received 5 December 1996: accepted I 1 March 1997
Abstract The kinetics of chemical reactions occurring during the coating process and the wear behaviour of Ti(C,N) coatings, including variations in wear displacement with the temperature of the lower specimen, were -:.nvestigated in this work. The variations in the wear displacement and the temperature of the lower specimen are regressed by an eight-order polynomial function. The lower specimens were coated by a titanium film as undedayer and three kinds of coating material including TiN, Ti (C,N) or TiC were deposited as the top layer. The Ti(C,N) coatings were prepared by varying the gas flow rates of nitrogen and acetylene to form eight kinds of specimen. The tribological behaviour demonstrated by these eight specimens is di~ussed. The experimental data for the atomic ratios of [C] and [N] can be well expressed using the theory of diffusion rate and the theory of reaction rate for the deposition of ceramic coatings. The variations in the wear displacement gradient with the temperature of the lower specimen can give information on the adhesive behaviour arising before and after three-body wear. The wear rates of the upper and lower specimens due to adhesive wear are dependent on the operating conditions. The specimen with a higher final wear displacement was likely to produce on the upper specimen a higher wear rate when operating at 0.705 m s- ~. The thicker the adhesive layer, the lower the wear rate of the lower specimen produced. When the sliding speed was elevated to 1.41 m s- n the specimen with a higher final wear displacement often produced a lower wear rate on the upper specimen, and also caused higher wear rates on the lower specimens. © 1997 Elsevier Science S.A. Kowords: Titaniumcm~onitride: Wear displacemenu Temperature: Diffusion
1. I n t r o d u c t i o n
The tribological behaviour of Ti(C,N) coating film is varied by changing the coating factors including the substrate, deposition parameters, stoichiometry, wear mechanism, etc. The composition and the microstructure ofTi (C,N) films has been reported in several recent studies [ I - 9 ] . Damond et al. [2] studied the deposition of Ti(C,N) film using arc-evaporated titanium and the reactive mixture of CH4 and N~ gases. The films were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray microanalysis for chemical composition and crystal structures. The chemical composition of the Ti(C,N) films was found to depend strongly on the mass flow rate ratio CH4"N2 and on the total pressure during deposition, but was weakly dependent on the bias voltage and not affected by variation of the arc current [2]. lvanchenko et at. [5] investigated the structure and carbon content in a series of samples of Ti(C,N) film deposited by the ion-plasma spraying method using the results of X-ray phase, micro-X-ray-spectrum analyses and Auger spectroscopy. They found that the microhardness of * Corresponding author. 0043-1648197/$17.00 © 1997 Elsevier Science S.A. All rights reserved PII S 0 0 4 3 - 1 6 4 8 ( 97 )00056-2
the samples and lattice constant increased while the growth in carbon content growth is changed substantially. Cheng and Hong [ !0] studied the thin films of titanium carbonitride prepared by chemical vapour deposition on a cemented tungsten carbide substrate. They presented two different reaction mechanisms, gas diffusion control and surface kinetic control. The deposition rate of Ti(C,N) was a function of deposition temperature and the total flow rate of reactive gas. The dry-sliding wear mechanisms of silicon nitride, partially stabilized zirconia and alumina were studied by Gautier and Kato [ I l ] using a ball-on-disk wear apparatus. A wide range of sliding velocities and normal loads were tested in order to draw experimental load-velocity friction and wear maps. in addition, the wear rate of each component was shown to be related to the 'frictional power' which was provided to the tribological syst ~m. in this study, the lower specimens (flat washer) were coated with a titanium film underlayer and three kinds of coating material including TiN, Ti(C,N) or TiC were deposited as the top layer. The Ti(C,N) coatings were prepared by varying the mass flow rates of nitrogen and acetylene gases to form eight kinds of spec,.'men. The kinetics of chemical reactions occurring during the coating process was investi-
246
Y. ¥, Guu. J.F. Lin / Wear 210 (1997) 245-2~4
gated and it fits the theory of diffusion rate. The diffusion rates of N and C ions were expressed as an exponential function of the mass flow rate ratio of nitrogen and acetylene gases. The tribological behaviour demonstrated in these eight kinds of specimen was investigated at two sliding speeds. The variations of wear displacement with the temperature of the lower specimen were also investigated in this study to evaluate the thermal effect on wear.
~ ~e
"~
ROTARYSPINDLE(WITIIDELVE PINS& BALLSEAT)
~:~..,~
ROTARYSPECIMENHOLDER
U'f,PF~SPECIMEN m'JtUS'l"WASHER)
, ~ ~ ~
L~ x , ~ j . ~ j ~ ' TORQUE
~ ~ ~" LOAD
LOWER SPECLME~
(FLATWASHER] STATIONARYSP£CIME:NHOLDEP,
Fig. !. Schematic diagram of the thrust-wu~her adapter.
2. Exper/mental details
By varying the mass flow rates of N2 a n d C2H2 gases during the coating process in reactive depositions, ceramic coatings of the lower specimens were prepared by the cathodic arc ion plating process (CA). The lower specimens were heated to about 300 °C in a vacuum deposition chamber at the pressure 10-3.. ! 0 - 4 Pa. The steered CA plasma source with a titanium arc target was then operated with an arc current of i 00 A. After the titanium ion bombardment at - 1000 V bias voltage, the Ti film was deposited as underlayer at a substrate bias voltage of - 2 0 0 V. Finally, the reactant gases nitrogen and acetylene with a total pressure of 2.7 × 10- ~Pa were fed into the vacuum deposition chamber. Several mass flow rate ratios N2:C2H2 were "ased in the coating process, as shown in Table 1. The ceramic layer was deposited onto the lower specimens as the. top layer at the temperature of 400 °(2. The end-process surface roughness of the deposited layer was about 0.3--0.4 ttm R,. The thickness of the top layer was about 3 ttm, and the thickness of the underlayer was 0.1 ttm. The composition of the ceramic coating film was determined using a JEOL !~;M-35 wavelength-dispersive X-ray analyser. Reference san~ples with known titanium, carbon and nitrogen contents were used for calibration. The friction and wear tests were carried out on a multispecimen wear testing machine using a thrust-washer adapter. The upper specimen (thrust washer), which was rotating at a constant sliding speed during the wear process, was pres~d against a stationar) lower specimen (fiat washer) to simulate the tribological behaviour of dry surface contacts. Fig. I Table 1 The stoichiometries of eight types of coating films produced by varying the mass flow rate of N 2 and C2H2 gases during the coating process; the underlayer of the coating is O. 1 p,m thick and the top layer is 3 p.m thick
shows a schematic diagram of the thrust-washer adapter assembly. The ECR (electrical contact resistance )-equipped test machine was designed to be insulated from electric current between the driver of the upper specimen and the support of the lower specimen as long as the two specimens had no direct contact. The variation in voltage can be detected when wear debris is entrapped in the interface between the upper and lower specimens during wear testing [ 12]. A long levered bar connected to the specimen support was utilized to measure the relative displacement of the upper and lower specimens during the wear test. A wear gauge was positioned vertically at the end of the lever bar, with the ratio of the reading of specimen displacement to the wear gauge of 1:6. The wear gauge was used to record the wear displacement of the lower specimen. The experimental conditions including the speed of rotation, the specimen temperature, the applied load, the driving torque, the electrical contact resistance and the specimen displacement were monitored electrically by a digital instrumentation system. The accuracy of temperature measurements using the thermocouple is :!: 0.5 °C, while the accuracy of rotating speed measurements is + 3 rev rain- '. The substrates of the upper and lower specimens were both made of the same material JIS (34404 SKD61 steel. Before depositing the two coating layers, the lower specimens were finely polished to a surface roughness of 0.3 Ixm R,,. The dimensions of the upper and lower specimens were prepared according to the requirement of the ASTM D3702-78 standard test method [ 13]. These specimens were tested under the constant normal pressure of ! .69 MPa, but at two different sliding speeds, 0.705 m s - ' and 1.41 m s - ~. All sliding tests ended at a distance of 2032 m. in order to take the repeatability into account, the test results for friction coefficient and wear rate were obtained from the average of three readings.
Specimen Mass flow rate ratio of N., and C:H:.,nr~::mc:i~:
Mass flow rate ratio ofC_-H. ~*:H-
Stoichiometry
A B C D E F O H
0.00 0.09 0.17 0.23 0.29 0.33 0.41 1.00
TiNo.s2 TiQ,..~o 4 TiQi.sNo.~ TiQ, sN,.,
3. Results
TiC,..~,~Io..~H
3. I. Chemical composition of the coating fihn
100:0 I ~J0: ! 0 100:20 100:30 100:40 100:50 100:70 0:100
Note: ~bc,x~= mc~_w.I(raN:+ inc.,x:).
TiC~I.~4No.~ TiC,,7.,No.22 TiCp.2~
The chemical composition of the coating film can be determined by wavelength-dispersive X-ray analysis. Table l demonstrates the stoichiometries of eight types of Ti(C,N)
E Y. Gut+. J.F. Lin / Wear 2 I0 ~1997~ 245-254 . . . . . . . .
1.0
~
]
~.m,-.c,~n
C, ~ m , i , ~ M ...........
i
+, i
Let [C] and [ N ] represent the atomic stoichiometric number ratios C:Ti and N:Ti respectively, then the rates of reaction for nitrogen and carbon ions are written as follows [ 14]:
i .
I¢1
/
diN] =_Kn4,N,( ! -- [N]) dt
d[C]
dt = Kc~c~n:( ! -- [C] )
0.0
~---,.~
0.0
,-T~---:-r-:
O. 1
.......
0.2
~ .........
0.3
r';
,
0.4
247
0.5
mass f l ~ re.~oflhe C2H2gss, ,q,,~ Fig. 2. Variationsof the atomic stoichiometricratioC:Ti and N:Tiwithmass flow rate ratio C_,H.,4k-.,.,. film produced by varying the mass flow rates of nitrogen and acetylene gases during the coating process. Fig. 2 depicts the variations of the atomic stoichiometric ratios C:Ti and N:Ti with the mass flow rate ratio 4~c_,H_- which is defined as tbc,..,_=mc,.H,_l(mc,,,+mm,), where m represents the gas mass flow rate. These data are presented for the coating films only considering cases B to 13. Let [CI and IN] represent the atomic stoichiometric number ratios of C:Ti and N:Ti respectively. A linear relationship can be established between the atomic ratio and the mass flow rate ratio (~C2H:, tbr both [C! and [NI. Increasing the mass flow rate ratio OC.-H:would increase the atomic ratio [ C ], but would decrease the atomic ratio [ NI. These two straight lines regressed well by the data of [C] and [NI intersect at a point having mass flow rate ratio ~C'.H.,-------0.12. If the mass flow rate ratio tbc,., in the chamber is lower than 0. ! 2, the atomic ratio [ C 1 in the chemical composition ofTi (C,N) is less than the atomic ratio [ N ]; conversely, if the ratio q~c,,., is higher than O. 12. the atomic ratio [ C l in the chemical composition would surpass I N 1.
3.2. Kinetics of surface reactions in the coating process For the purpose of studying the variations in the atomic stoichiometric ratios [C] and [N] in the chamber with the mass flow rates of the C2H2 and N_, gases, the kinetics of the reactions occurring during the coating process was investigated using both the diffusion rate theory and the concentration theory. If it is assumed that the rate of reaction between the titanium surface and the N_, and C,H, gases is high, the reaction between these two ions and the titanium surface is generally dependent on the mass flow rate ratios 4~C_,H_,and 4~.,. The reaction rates for nitrogen and carbon ions can be expressed as the atomic stoichiometric ratios N:Ti and C:Ti respectively.
3.2. I. The diffusion rate theor), In a higher temperature region, with a relatively lower activation energy, the growth mechanism of Ti(C,N) films is primarily gas diffusion controlled. The two atomic stoichiometric number ratio,,;, using the theory of diffusion rate, can be expressed as a function of 4~c:., and ~ : respectively.
(in)
(Ib)
where d~,-mN,/(mc:n,_+mm,.), (~:H:~'ff/C2H.,/(//IC_~H_, + ms_,), and t is the reaction time of particles. The constants Ks and Kc are functions of the diffusion coefficients of nitrogen and carbon ions respectively. The general solutions of Eqs. ( l a ) and ( l b ) are thus written as 1 - [ N ] = exp( - KNd~,_t) I -
[ C ] -- exp( - Kc~::,H,t)
(2a) (2b)
The time t for the chemical reaction may be expressed as a function of the ion speed and the surface characteristic. Assuming that the mean distance L between two reactant particles and the average ion speed V are always maintained steady during the coating process, the reaction time is thus expressed as a function of LIV. Eqs. (2a) and (2b) can be rewritten as 1 - [ N ] =/3N exp(aN~,~,)
(3a)
l - [C] =/3c exp(Otcd~c,,u2)
(3b)
where/3N is a function of the diffusion coefficient of nitrogen ions, a s is the available reaction (diffusion) time for nitrogen ions,/3c is a function of the diffusion coefficient of carbon ions, and ac is the available reaction (diffusion) time for carbon ions. The experimental data in Fig. 3 marked by circles and triangles can be regressed by F_.qs. (3a) and (3b) respectively. Fig. 3 shows the good matches of the experimental results with the theoretical expressions. They are given as 1 - [ C ] -- 0.874 exp( - 2.86ti~:n:)
(4a)
and I - [N] = 1.283 exp( -0.8954h, j.,)
(4b)
Increasing the mass flow rate ratio would always enhance the reactions arising between the titanium surface and either the nitrogen or the acetylene gas. However, the reaction rate associated with [ N ] is relatively higher for the same increase in the mass flow rate.
3.2.2. The concentration theory Cheng and Hon [ 10] have proposed another reaction model of kinetics suitable for the CVD coating process. According to their study on the crystal growth of titanium carbonitride by CVD, the deposition rate has a relatively strong dependence, and can be expressed as an Arrhenius equation. The reaction rate of formation of the Ti(C,N) coatings is dependent on the reactive gas flow rate and deposition
248
EE Guu. J.F. Linl Wear210 (1997) 245-254 '[.~ --
t.0-
.m
I
]
•A
,,-,-ilq ] -.-~-ilSl •
•
•
mq.(t,m~
1.0 m
l 0+
!
A
I~u.(u)
0~s &
0.0
0.1
02
i
0,3
0.4
o.,
0.5
~ w m l rUlo ofC21~ I m
I .........
cLs
0,5
0.6
0.7 0.8 m m flow r m mJo ofN2 i ~
0.9
11.0
Fig. 3. Plo~ of I - [C] and i - [ N I vs. mass flow race rauoofC,H2 f,as and N2 gas respectively.
temperature. The growth rate of Ti(C,N) coatings can be expressed by an empirical formula: Ga
~"
exp(
- AEIRT)
(5)
where G is the growth rate of the Ti(C,N) coating, ~bis the reactive gas flow rate, ~ is a slowly varying function of deposition temperature, AE (J mol- ') is the apparent activation energy (not a function of gas flow rate), R (Jmol - t K -I ) is the gas constant, and T (K) is the deposition temperature. Thus, the atomic ratios [C] and [N] can be expressed by the following equations: [c] =ac(~,m) ~
(6a)
[N] = B N ( ~ , ) ''~
(6b)
where Bc and BN represent the proportional constants which depend on the deposition temperature. Fig. 4 demonstrates a good agreement of the experimental results with the models demonstrated in Eqs. (6a) and (6b). The experimental data marked by the circles can be regressed well by the following expression: I t ] = 1.114ckc,.x2 o,,~,,
(7a)
and the data marked by the triangles can be expressed as
[N] = , ,n. . ,~w, ~, e, .N4 ,, 9
(7b)
The reaction behaviour arising in the cathode..,,'- ion plating process associated with atomic stoichiometric ratios C:Ti and N:Ti was observed from the physical viewpoint in terms of the diffusion theory or the chemical viewpoint in terms of the concentration theory. As the expressions of exponential form Eqs. (4a) and (4b) show, both atomic stoichiometric
lmm
b .........
~s
of I .........
ih Ca
8m
I .........
O.7 oJ n,mm flow me ~lhelqT. ~
o, ~ ........
o,o
oo "-I
'~.o
Fig. 4. Plot of atomic ratio [C] and I N I vs. mass flow rate ratio of C~H, gas and N: gas respectively.
ratios, [C] and [N], are increased by increasing the mass flow rate ratios 0t:2x., and ~ 2 respectively. The same effect is also demonstrated in Eqs. (7a) and (Tb) on the basis of the concentration theory Athough t'te w-xiations of [ C ] and IN] are expressed in a po¢+t:~,'ban.
3.3. Wear rate of lower specimen affected by coating material The wear rate of the lower specimen is strongly dependent on the material composition. As the wear rate data of the eight types of specimen in Table 2 show, the wear rates of the specimens with TiN coating as the top layer are always the lowest for both rotational speeds. The lower specimen with TiC coating, operating at the sliding speed of 0.705 m s - ' , showed a much larger wear rate than the specimens with the TiN coating. As the sliding speed was increased to 1.41 m s - ', the gap in wear rate between the TiC and TiN coatings shrunk. The chemical composition of the Ti(C,N) coating can be altered by adjusting the mass flow rates of the nitrogen and acetylene gases during the coating process. It is interesting that the specimens prepared with ~bc.~x2of 0.09 and 0.29 have lower wear rates at both sliding speeds (0.705 and 1.41 m s - ' ). Although the material hardness associated with the latter is higher than that of the former, the wear rate of the latter is slightly higher than that of the former.
3.4. Variations of wear displacement with specimen temperature The wear displacement presents the relative displacement of the upper and lower specimens during the wear test. A
E E Guu, ZF, Lin / Wear 210 0997) 245-254
249
Table 2 Friction coefficientsand wear rates of the lower specimens and the upper specimen: the underlayerof the coating is 0.1 gm thick and the top layer is 3 ~m thick; the operating conditions include applied load producing the normal pressure of 1,69 MPa, and sliding distance of 2032 m Case
4~,:.,H..
A; B1 C! D! E! FI GI HI A2 B2 C2 D2 E2 F2 G2
0.00 0.09 0,17 0.23 0.29 0.33 0.41 1.00 O.00 0.09 O. 17 0.23 0.29 0.33 0.41
H2
!.00
Sliding speed (m s - i)
0.705
1.41
Average friction coefficient
Wear rate × I0 - ~ (mm ~ s - *) Lower specimen
Upper specimen
Final wear displacement (p.m)
0.547 0,504 0.543 0.522 0,554 0,513 0,508 0.522 0.442 0.440 0.430 0.412 0.438 0.411 0.442
0,322 0.414 0.739 0.633 0,446 0.450 1,845 1.467 0.045 0.088 O. 140 0.102 0.095 0.135 0.163
2.532 2.063 2.139 2.161 2.152 2,183 1.317 ! .250 8.223 8.971 5..~80 3.791 4.557 3.337 3.142
200 158 199 184 199 200 153 186 223 192 243 273 249 238 278
0,444
0.070
6.675
249
TM (°C)
245 250 250 250 235 238 240 245
T; ( °C )
25O
245 245
233 230 25O 260 235 270 27O 240
TM is the specimen temperature corresponding to the maximum wear displacement.T~ is the specimen temperature az which Ihtee-body wear was absent,
charactedstc for a C 1 specimen at the sliding speed of 0.705 m s - ' is exhibited in Fig. 5(a). The curve shows that the wear displacement increased continuously on raising the specimen temperature until a peak value; beyond this point there is a small drop. The specimen temperatures of all eight types of specimen corresponding to the maximum wear displacement are given in Table 2; they lie in the range between 235 °C and 250 °C. An attempt to exploit the specimen temperature corresponding to the maximum wear displacement as the critical borderline was made to separate two different wear mechanisms. In the range below the maximum wear displacement, the ceramic coating in the lower specimen was not fully worn out; the material of the upper specimen was continuously adhesive to the lower specimen, thus resulting in a continuous increase in wear displacement However, when the specimen temperature rose above this critical value, the ceramic coating film deposited onto the lower specimen was mostly removed out of the wear track; adhesive wear was greatly impeded, and the rubbing surfaces were operating under the circumstance of metal-to-metal contact. The wear displacement thus showed a small decline in this region. Changing the operating conditions by varying the sliding speed brought about noticeable distinctions in the fluctuation s of specimen temperature and wear displacement curves. Fig, 5(b) shows the variation of wear displacement of the C2 specimen at the sliding speed of 1.41 m s- t. The curves of all eight cases exhibit similar characteristics: they start with a small smooth region, then are connected by a curve of zig-zag form in the three-body wear region, and finally are restored to a smooth curve until the designated sliding distance is completed. The smooth curve in the first region was caused by the mechanism of abrasive wear, as shown in Fig. 6(a). When the specimen temperature was continuously
elevated, the three-body wear eventually replaced abrasive wear as the prevailing wear mechanism; numerous fluctuations appear oil the curves of wear displacement. As the specimen temperature was further elevated, the three-body wear was impeded gradually, and the adhesive wear mechanism became prevailing (Fig. 6 ( b ) ) . In this region, the ceramic coating film still remained partly on the surface of the wear track although the designated sliding distance had finished. This phenomenon illustrates that the restoration of a smooth curve in this region resulted mainly from the significant change in wear mechanism due to the temperature rise. At the sliding speed of IAI m s - ' , the three-body wear of all eight types of specimen was effectively impeded when the specimen temperature rose above about 250 °C. Actually, being dependent on ~he chemical composition of the ceramic coating, this critical temperature changes somewhat in a small range. The critical temperature is slightly lower for some types of specimen. Table 2 demonstrates the temperature of the specimen at which three-body wear was absent. Variations of the wear displacement and specimen temperature are regressed by an eighth-order polynomial function for the convenience of analysis. Wear displacements varying with specimen temperature are demonstrated in Fig. 7(a); they were obtained from eight types of specimen operating at the sliding speed of 0.705 m s- =. The relationship of wear displacement increasing with increasing temperature of the lower specimen holds for all eight specimens. The genuine starting points of these eight curves in Fig. 7(a) ate actually different. However, a constant temperature ( 70 °(2) was chosen as the common starting point for the purpose of fitting the wear displacements well to the polynomial. There exists one or more inflection points on each wear displacement curve. The first inflection point reflects the behaviour arising
E E Guu. J.F. Lin / P/ear 210 (1997) 245-254
250
~,~,:
,.,. •
-..
. . . .
,il
-..,~:c,
~:0-.
~-,o0 i~o 80
0.8 -
:_
J0o
~-- 200
o..i
2O0
--
lo.._:: 0.2
,....
~. ~...
:, ~
•
•
_
_
~,~
--
OO
~ o (b~"
O
!
,mmN~:~
300
oo
EO0
900
I ~0
- .soo
~ riO0
t,o.
Fig. 5. Plo~s of friction ceeflicient, wear displacement, electrical voltage and specimen temperature vs. sliding time at the sliding speed of (a) 0.705 m s- ~and (b) !.41 m s - ~.The operating conditions include applied normal pressure of 1.69 MPa, and sliding distance of 2032 m.
near the end of the three-body wear. The wear displacement ahead of the first inflection point is obviously less in magnitude than that behind this point. This feature indicates that the material adhesion occurring during three-body wear is quantitatively lower than that occurring during the process for which adhesive wear was dominant. The genuine starting points of these eight curves are found to have negative displacements. Negative displacements were commonly produced at the time before the presence of three-body wear; they imply that abrasive wear, instead of material adhesion, governed the wear mechanism in this interval. The experi-
Fig. 6. Micrographs of the worn surface generated by the specimen of case C2 at the sliding speed of IAI m s -~. The operating conditions include applied normal pressure of 1.69 MPa, and sliding distance of 2032 m: (a) first region (abrasive wear); (b) last region (adhesive wear).
mental results reveal that the F1 specimens have the largest wear displacement for the same lower specimen tt'.mperature. The final wear displacements of these eight curves, which are considered as the approximate increases in thickness of the adhesive layer, are shown in Table 2. The data for these final wear displacements, according to their magnitude, give the array order AI,Fl >C1,EI > H I > D l > B I > G I . The wear rate data give the array AI > F I > DI,EI,C! > B I > G I , H i for the upper specimens and the array GI > H I > C I > D! > FI,EI > B! > A! for the lower specimens. The following conclusion can thus be drawn from the investigatio,I of the above two array orders: the specimens with a higher final wear displacement were likely to produce a higher wear rate of the upper specimen when operating at 0.705 m s- *. The evidence reveals that the upper and lower specimens operating at 0.705 m s- * were normally in possession of a thick adhesive layer on the wear tracks: this layer, in general, can protect the lower specimen from severe wear. The thicker the adhesive layer, the lower the wear rate of the lower specimen produced. As the sliding speed was increased to 1.41 m s-*, the curves of wear displacement vs. specimen temperature, as shown in Fig. 7(b), become similar in appearance to the curves demonstrated at the sliding speed of 0.705 m s - t . However, the final wear displacements are relatively higher. The data for the final wear displacements of the eight curves give the array order G2 > D2 > E2,H2 > C2 > F2 > A2 > B2.
Y.Y. (;uu. J.F. Lin
200-:
/ Wear 210 t 1997) 245.-2~4
~~=o.~nn
(i)
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-'~
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,~/lj/
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]
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~.o
-*-
,
i
.~,
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~Ill/l~
iiq]i ........ iT,',,iiVii i i ..... I00 150 200 IOWI~ StXl;:~:I~ ~ (igl~R:¢ C
l'i
IAl I
0
i,i
i 250
=o_]
0"0/
................ 50
I ......... 100
I ......
150
200
1 250
+' ~., i+4nucs ~ i d i ~ d l m ~ # ZOOm
,'-m, NO,
.....o
~" --i-
-
#
,o,, , -~- .., I.
~
L _+_
I
I
./t'Zl
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,
!
~"'
o
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,
r:
j,i
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?'~i b , l
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i I
'-........... ~ ~/'/_ /,
!t
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"i~
j*
'', ,~: il
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k~x~,rr, m~d¢~=C
Fig. 7. Plots of wear displacement vs. specimen temperature at the sliding speed of (a) 0.705 m s-~ and (b) 1,41 m s - i . The operating conditions include applied normal pressure of 1.69 MPa, and sliding distance of 2032 m,
The wear rate data at the sliding speed of 1.4 i m s - ' give the array B2 > A2 > H2 > C2 > E2 > D2 > F2,G2 for the upper specimens, and G2 > C2 > F2 > D2 > E2 > B2 > H2 > A2 for the lower specimens. It is interesting that the specimens with higher final wear displacements often produced lower wear rates of the upper specimens, but brought out higher wear rates for the lower specimens. Consequently, the behaviour of wear loss is strongly dependent on the operating conditions; the characteristic associated with the specimens operating at higher sliding speeds perhaps demonstrates behaviour exactly opposite to that exhibited at lower sliding speeds.
o,o 50
100
150
200
250
300
350
Fig. 8. Plots of the displacement-temperature gradient vs. specimen temperalure at the sliding speed of (a) 0.705 m s - ' and (b) 1.41 m s I '. The operating conditions include applied normal pressure of 1.69 MPa. and sliding distance of 2032 m.
Fig. 8(a) demonstrates the displacement-temperaturegradients, d(wear displacement)/d(specimen temperature), of the eight types of specimen that were tested at the sliding speed of 0.705 m s - '. They are actually the slopes of the curves shown in Fig. 7(a). Each of these eight curves shows at least one peak value in the displacement-temperature gra-dient. The first peak lies within the temperature range between about 130 °C and ! 80 °(2. These extreme values are actually
El'. Guu, J.F. 1.in/Wear210 (1997) 245-254
252
t h slopes of the wear displacement curves at the inflection point. Beyond this point, metal-to-metal frictional contacts in terms of adhesive wear and microcutting gradually replaced the three-body wear and became the dominant wear mechanisms. The second peak on each curve is located at the point where the maximum wear displacement occurs. It should be mentioned here that at the sliding speed of 0.705 m s- t, the eight curves of wear displacement all have the maximum value near the end of the sliding time. The curves in Fig. 8(b) depict the displaceraent-temperature gradients, d(wear displacement) /d( specimen temperature), for the specimens operating at the sliding speed of 1.41 m s - n; these curves have one to three peaks. The first peak on each curve corresponds to the inflection points of the curves shown in Fig. 7(b). These inflection points occurred at the temperature within the range 155-250 °C. The wear displacements of the eight types of specimen always increased with sliding time, the maximum wear displacements generally being presented at the end of the sliding motion. The second and third peaks shown in Fig. 8(b) ate simply related to the variations of wear displacement with the lower specimen temperature: they are generated by the wear displacement curves with more than one inflection point. Eh'astic rises in the displacementtemperature gradient in seve:al of these eight curves appear near the end of each curve; the sharp increase is attributable to the combined effect of continuous increases in wear dis-
In order to understand the wear mechanisms that occurred ahead and behind the maximum wear displacement, two results relevant to the specimens of case D ! were obtained at the sliding speed of 0.705 m s - t . Fig. 9(a) shows the wear profile before the maximum wear displacement occurred (at the sliding time of 1500 s), whereas Fig. 9(b) exhibits the profile after the maximum wear displacement (at the sliding time of 2900 s). Since the layer of ceramic coating still existed on the wear track before reaching the maximum wear displacement, adhesive wear was then the dominant wear mechanism. The wear track of the lower specimen was filled with the material delaminated from the upper specimen, thus the wear profile does not exhibit a deep and broad concave shape. Fig. 9(b) demonstrates the wear profile obtained after the maximum wear displacement. This wear profile was obtained when the ceramic coating layer was damaged and was removed out of the wear track of the lower specimen. Adhesive wear at this moment was impeded and metal-to-metal friction contacts caused the wear track of the lower specimen to be broad and deep.
Co) 10 -.3
'
a l t ~ t e . , - 2~0 mu~t~
¢xmkp
0
3.5. Wear characteristics ahead and behind the maximum wear displacement
11~'11; |'~.l
o.o_
p~vr
placement but nearly constant specimen temperature in this region.
''
}o. -10
.... I ......... I' .... ' H ' I
1
2
3
memt~lle~th, mm 1.0
I,,,,,,,,,I,,,,,,,,,I,,,,,,,~] 0 I 2 3 r a ~ , ~ l o n ~ turn
(c_)
~'0,0 dld~ ~ o t400tomb 1 ~-].4t -1,0 0
1
2
n',~n~l knob, mm
3
Fig. 9. Wear profiles of the specimen for case D: (a) before and (b) after the maximum wear displacement at the sliding speed of 0.705 m s- *; (c) the wear track under a sliding speed of 1.41 m s - ,. The operating conditions include applied normal pressure of 1.69 MPa. 1"he top layer is 3 ttm thick, and the underlayer is O. i lxm thick.
Y.E Guu, ZF. 1.in/Wear210(1997) 245-254 0.4
ill
mNL
0.1
~h
IFI
m~ -o.1
app~
. . . . . . . . . . . . . . . . .
lOO
- ~ ~l~,
=~2.4N I . . . . . . . . .
15o
I.
. . . . . . . .
20o
] 250
Fig. 10. The variation of the rate of wear displacement with the specimen temperature. The operating conditions include sliding speed of 0.705 m s- '. applied normal pressure of 1,69 MPa, and sliding distance of 2032 m.
If the sliding speed was increased to 1.41 m s - ' , the ceramic coating film still remained on part of the wear track, even after the sliding distance of 2032 m. The adhesive layer cumulated continuously on the wear track until the sliding motion ended, the concave shape never being exhibited in the wear track (Fig. 9(c) ).
3.6. Wear displacement rate va~b~g with specimen temperature The curves in Fig. 10 are presented to illustrate the variations of the rate of wear displacement with the rise in specimen temperature. These results were obtained at the sliding speed of 0.705 m s - ~. Negative slopes of all these eight curves indicate that the increase in thickness of the adhesive layer before reaching the point of maxiwum displacement was lowered by the continuously rising specimen temperatare. Of the eight kinds of specimen, the relatively noticeable decline in the rate of wear displacement due to per unit temperature rise is present for specimens in cases El and FI.
4. Discussion For the purpose of investigating the variation s of the atomic stoichiometric ratios C:Ti and N:Ti with the mass flow rates o f t h e C2H2 a n d N 2 gases, the atomic stoichiometric number ratios C:Ti and N:Ti have been proved to agree well with the kinetics of surface reactions in terms of either the diffusion rate theory or the concentration theory. The growth rate of the Ti(C,N) films arising in the cathodic arc ion plating process can be investigated frem the physical viewpoint based on the diffusion theory or from the chemical viewpoint
253
based on the concentration theory. Actually, it is well known that the diffusion rate of reaction particles in the deposition chamber is effectively increased by increasing the concentration of particles. Consequently, the same effect on the atomic stoichiometric ratios could be obtained although they are evaluated on the basis of different theoretical models. The adhesion of the material from the upper specimen (SKD61 steel) onto the lower specimen is dependent quantitatively on whether tic ceramic coating remained or not during the wear process. If ~ ceramic coating film was retained, a significant difference in material hardness between the upper and lower specimens caused conditions favourable for material adhesion: this adhesive behaviour terminated gradually when the ceramic coating film was worn out of the lower specimen. The rate of adhesion in the period of threebody wear was decreased by increasing the temperature of the specimen. The wear performances for all eight types of specimen started with a short time of abrasive wear, followed by the three-body wear, and finally the mechanism of plastic deformation in the case of metal-to-metal contact. The wear behaviour demonstrated in the lower specimen is strongly dependent on the operating conditions. That is, the wear mechanism of the ceramic coating specimen is governed by both thermal and mechanical effects. The performance shown in the specimens at lower sliding speeds might be exactly opposite to that operating at higher sliding speeds.
5. C o n c l u s i o n s
i. The experimental data for die atomic ratie~[C] and [N] can be fitted well by both the expressions derived from the theory of diffusion rate and the theory of reaction rate of forming ceramic coating. 2. Adhesive wear is apt to occur at the specimens undergoing three-body wear or tnetal.to-metal frictional contact. The extent of material adhesion is dependent on the operating conditions and the condition of the ceramic film after three-body wear. The remainder of the ceramic coating film on the wear track encourages the increase in wear displacement. 3. The variations of the wear displacement gradient with the temperature of the lower specimen can offer information on the adhesive behavionr arising before and after threebody wear. The first peak of the wear displacement gradient curves is always present at the point where the three-body wear was about to end. 4. The wear rates of the upper and lower specimens due to adhesive wear are dependent on the operating conditions; they demonstrated exactly opposite behaviour at two sliding speeds. The specimens with a higher final wear displacement were likely to produce a higher wear rate of the upper specimen when operating at 0.705 m s - ' . The thicker the adhesive layer, the lower the wear rate of the
254
E Y. Guu. J.F. Lin / Wear 210 (1997) 245-254
lower specimen produced. When the sliding speed was elevated to 1.41 m s -~, the specimen with a higher final w e a r d i s p l a c e m e n t often p r o d u c e d a l o w e r w e a r rate o f the u p p e r specimen, and caused a h i g h e r w e a r rate o f the l o w e r specimen.
l 1l H. RmIMwa, Cathodic arc plasma depmition of TiC and Ti(C,N) films, Thin 3olid Films 153 (1987) 209-218. [2] E. Damond, P. Jacquor. J. Pagny, Ti(C,N) coatings by using the arc evaporation technique, Mater. Sci. Eng. AI40 ( 1991) 838-841. [3] K.T. Rie, A. Gebauer, Plasma-assisted chemical vaponr deposition of hard coatings with metallo-organic compounds, Mater. Sci. Eng. A139 (i-2) ( 1991) 61-66. [41 Y. C'hen, Y. Sun, F. Zhang, H. Mou, Preferential growth in ion beam enlmnced deposition of Ti(C,N) films, Vacuum42 (16) ( 1991 ) 1059. [5] LA. Ivanchenko. V.V. Paskal, N.A. Limvchenko, O.F. Gusarova, S.Y. Pilipovskij, G.A. Fmlov, Study of sm~ture and physico-chemical lXepertks of titanium cmbonilride with different compositions. Fiz. Khim. Obr. Mater. (4) (1992) 83-87. i6l J.M. Schneider, A. Voevodin,C. Reblmlz. A. Matthews, J.H.C. Hogg, D.B. Lewis, M. Ives, X-ray diffraction investigations of magnetron spottered Ti(C,N) coatings, Surf. Coat. Technol. 74-75 (!-3) (1995) 312-319.
[7] K. Aigner, W. Lengauer. D. Rafaja, P. Ettmayer, Lattice parameters and thermal expansion of TiC~NI _,, Zr(C,NI _,), Hf(C,NI_ ,) and TiN~ _, from 298 g to 1473 g as investigated by high-temperature Xray diffraction. J. Alloys Comp. 215 (i-2) (1994) 121-126. [8] R. Bertoncello, A. Ca.~grande, M. Casatin, A. Glisenti, E. Lanzoni, L. Mirenghi. E. TondellooTiN. TiC and Ti (C,N) film characterization and its relationship to tribological F~ehavior,Surf. Interface Anal. 18 (7) (1992) 525-531. [9] D.J. Chang, W.P, Sun, M.H, lion, Morphology and stmcgu:e of chemically vapour-depositedTi(C.N) coatings.Thin SolidFilms 146 ( I ) (1987) 45-53. [ 10] DJ. Cheng, M.H. Hon, Crystal growth of titanium carbonitride by chemical vapor deposition, MRL Bull. Res. Devel. 2 (2) (1988) 2733. [ I 1] P. Gautier. K. Kato, Wear mechanisms of silicon n~tride, partially stabilized zirconia and alumina in unlubricated sliding against steel. Wear 162-164 (1993) 305-313. [ 121 J.F. Lin, C.C. Chou, S.T. Chen, The effect oi"surface chemistry in lubrication on the tribological behavionr of steel milers under roilingsliding contacts. Tribotest J. 2-3 (1996) 205-234. 1131 ASTM Standard D3702, Standard test method for wear rate of materials in self-lubricated rubbing contact using a thrust washer testing machine, Vol. 5-3, ASTM, Philadelphia. PA. 1983. pp. 417423. [ 14] I. lliuc, Tribology of Thin Layers, Tribology Series° Vol. 4, Elsevier, Oxford, 1980, pp. I I I-112.
Wear 210 (1997) 245-254
Analysis of wear behaviour of titanium carbonitride comings Yeong Yan Guu, Jen Fin Lin
*
Department of Mechanical Engineering. National Cheng Sung Universio'. Tainan 70101, Taiwan
Received 5 December 1996: accepted I 1 March 1997
Abstract The kinetics of chemical reactions occurring during the coating process and the wear behaviour of Ti(C,N) coatings, including variations in wear displacement with the temperature of the lower specimen, were -:.nvestigated in this work. The variations in the wear displacement and the temperature of the lower specimen are regressed by an eight-order polynomial function. The lower specimens were coated by a titanium film as undedayer and three kinds of coating material including TiN, Ti (C,N) or TiC were deposited as the top layer. The Ti(C,N) coatings were prepared by varying the gas flow rates of nitrogen and acetylene to form eight kinds of specimen. The tribological behaviour demonstrated by these eight specimens is di~ussed. The experimental data for the atomic ratios of [C] and [N] can be well expressed using the theory of diffusion rate and the theory of reaction rate for the deposition of ceramic coatings. The variations in the wear displacement gradient with the temperature of the lower specimen can give information on the adhesive behaviour arising before and after three-body wear. The wear rates of the upper and lower specimens due to adhesive wear are dependent on the operating conditions. The specimen with a higher final wear displacement was likely to produce on the upper specimen a higher wear rate when operating at 0.705 m s- ~. The thicker the adhesive layer, the lower the wear rate of the lower specimen produced. When the sliding speed was elevated to 1.41 m s- n the specimen with a higher final wear displacement often produced a lower wear rate on the upper specimen, and also caused higher wear rates on the lower specimens. © 1997 Elsevier Science S.A. Kowords: Titaniumcm~onitride: Wear displacemenu Temperature: Diffusion
1. I n t r o d u c t i o n
The tribological behaviour of Ti(C,N) coating film is varied by changing the coating factors including the substrate, deposition parameters, stoichiometry, wear mechanism, etc. The composition and the microstructure ofTi (C,N) films has been reported in several recent studies [ I - 9 ] . Damond et al. [2] studied the deposition of Ti(C,N) film using arc-evaporated titanium and the reactive mixture of CH4 and N~ gases. The films were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray microanalysis for chemical composition and crystal structures. The chemical composition of the Ti(C,N) films was found to depend strongly on the mass flow rate ratio CH4"N2 and on the total pressure during deposition, but was weakly dependent on the bias voltage and not affected by variation of the arc current [2]. lvanchenko et at. [5] investigated the structure and carbon content in a series of samples of Ti(C,N) film deposited by the ion-plasma spraying method using the results of X-ray phase, micro-X-ray-spectrum analyses and Auger spectroscopy. They found that the microhardness of * Corresponding author. 0043-1648197/$17.00 © 1997 Elsevier Science S.A. All rights reserved PII S 0 0 4 3 - 1 6 4 8 ( 97 )00056-2
the samples and lattice constant increased while the growth in carbon content growth is changed substantially. Cheng and Hong [ !0] studied the thin films of titanium carbonitride prepared by chemical vapour deposition on a cemented tungsten carbide substrate. They presented two different reaction mechanisms, gas diffusion control and surface kinetic control. The deposition rate of Ti(C,N) was a function of deposition temperature and the total flow rate of reactive gas. The dry-sliding wear mechanisms of silicon nitride, partially stabilized zirconia and alumina were studied by Gautier and Kato [ I l ] using a ball-on-disk wear apparatus. A wide range of sliding velocities and normal loads were tested in order to draw experimental load-velocity friction and wear maps. in addition, the wear rate of each component was shown to be related to the 'frictional power' which was provided to the tribological syst ~m. in this study, the lower specimens (flat washer) were coated with a titanium film underlayer and three kinds of coating material including TiN, Ti(C,N) or TiC were deposited as the top layer. The Ti(C,N) coatings were prepared by varying the mass flow rates of nitrogen and acetylene gases to form eight kinds of spec,.'men. The kinetics of chemical reactions occurring during the coating process was investi-
246
Y. ¥, Guu. J.F. Lin / Wear 210 (1997) 245-2~4
gated and it fits the theory of diffusion rate. The diffusion rates of N and C ions were expressed as an exponential function of the mass flow rate ratio of nitrogen and acetylene gases. The tribological behaviour demonstrated in these eight kinds of specimen was investigated at two sliding speeds. The variations of wear displacement with the temperature of the lower specimen were also investigated in this study to evaluate the thermal effect on wear.
~ ~e
"~
ROTARYSPINDLE(WITIIDELVE PINS& BALLSEAT)
~:~..,~
ROTARYSPECIMENHOLDER
U'f,PF~SPECIMEN m'JtUS'l"WASHER)
, ~ ~ ~
L~ x , ~ j . ~ j ~ ' TORQUE
~ ~ ~" LOAD
LOWER SPECLME~
(FLATWASHER] STATIONARYSP£CIME:NHOLDEP,
Fig. !. Schematic diagram of the thrust-wu~her adapter.
2. Exper/mental details
By varying the mass flow rates of N2 a n d C2H2 gases during the coating process in reactive depositions, ceramic coatings of the lower specimens were prepared by the cathodic arc ion plating process (CA). The lower specimens were heated to about 300 °C in a vacuum deposition chamber at the pressure 10-3.. ! 0 - 4 Pa. The steered CA plasma source with a titanium arc target was then operated with an arc current of i 00 A. After the titanium ion bombardment at - 1000 V bias voltage, the Ti film was deposited as underlayer at a substrate bias voltage of - 2 0 0 V. Finally, the reactant gases nitrogen and acetylene with a total pressure of 2.7 × 10- ~Pa were fed into the vacuum deposition chamber. Several mass flow rate ratios N2:C2H2 were "ased in the coating process, as shown in Table 1. The ceramic layer was deposited onto the lower specimens as the. top layer at the temperature of 400 °(2. The end-process surface roughness of the deposited layer was about 0.3--0.4 ttm R,. The thickness of the top layer was about 3 ttm, and the thickness of the underlayer was 0.1 ttm. The composition of the ceramic coating film was determined using a JEOL !~;M-35 wavelength-dispersive X-ray analyser. Reference san~ples with known titanium, carbon and nitrogen contents were used for calibration. The friction and wear tests were carried out on a multispecimen wear testing machine using a thrust-washer adapter. The upper specimen (thrust washer), which was rotating at a constant sliding speed during the wear process, was pres~d against a stationar) lower specimen (fiat washer) to simulate the tribological behaviour of dry surface contacts. Fig. I Table 1 The stoichiometries of eight types of coating films produced by varying the mass flow rate of N 2 and C2H2 gases during the coating process; the underlayer of the coating is O. 1 p,m thick and the top layer is 3 p.m thick
shows a schematic diagram of the thrust-washer adapter assembly. The ECR (electrical contact resistance )-equipped test machine was designed to be insulated from electric current between the driver of the upper specimen and the support of the lower specimen as long as the two specimens had no direct contact. The variation in voltage can be detected when wear debris is entrapped in the interface between the upper and lower specimens during wear testing [ 12]. A long levered bar connected to the specimen support was utilized to measure the relative displacement of the upper and lower specimens during the wear test. A wear gauge was positioned vertically at the end of the lever bar, with the ratio of the reading of specimen displacement to the wear gauge of 1:6. The wear gauge was used to record the wear displacement of the lower specimen. The experimental conditions including the speed of rotation, the specimen temperature, the applied load, the driving torque, the electrical contact resistance and the specimen displacement were monitored electrically by a digital instrumentation system. The accuracy of temperature measurements using the thermocouple is :!: 0.5 °C, while the accuracy of rotating speed measurements is + 3 rev rain- '. The substrates of the upper and lower specimens were both made of the same material JIS (34404 SKD61 steel. Before depositing the two coating layers, the lower specimens were finely polished to a surface roughness of 0.3 Ixm R,,. The dimensions of the upper and lower specimens were prepared according to the requirement of the ASTM D3702-78 standard test method [ 13]. These specimens were tested under the constant normal pressure of ! .69 MPa, but at two different sliding speeds, 0.705 m s - ' and 1.41 m s - ~. All sliding tests ended at a distance of 2032 m. in order to take the repeatability into account, the test results for friction coefficient and wear rate were obtained from the average of three readings.
Specimen Mass flow rate ratio of N., and C:H:.,nr~::mc:i~:
Mass flow rate ratio ofC_-H. ~*:H-
Stoichiometry
A B C D E F O H
0.00 0.09 0.17 0.23 0.29 0.33 0.41 1.00
TiNo.s2 TiQ,..~o 4 TiQi.sNo.~ TiQ, sN,.,
3. Results
TiC,..~,~Io..~H
3. I. Chemical composition of the coating fihn
100:0 I ~J0: ! 0 100:20 100:30 100:40 100:50 100:70 0:100
Note: ~bc,x~= mc~_w.I(raN:+ inc.,x:).
TiC~I.~4No.~ TiC,,7.,No.22 TiCp.2~
The chemical composition of the coating film can be determined by wavelength-dispersive X-ray analysis. Table l demonstrates the stoichiometries of eight types of Ti(C,N)
E Y. Gut+. J.F. Lin / Wear 2 I0 ~1997~ 245-254 . . . . . . . .
1.0
~
]
~.m,-.c,~n
C, ~ m , i , ~ M ...........
i
+, i
Let [C] and [ N ] represent the atomic stoichiometric number ratios C:Ti and N:Ti respectively, then the rates of reaction for nitrogen and carbon ions are written as follows [ 14]:
i .
I¢1
/
diN] =_Kn4,N,( ! -- [N]) dt
d[C]
dt = Kc~c~n:( ! -- [C] )
0.0
~---,.~
0.0
,-T~---:-r-:
O. 1
.......
0.2
~ .........
0.3
r';
,
0.4
247
0.5
mass f l ~ re.~oflhe C2H2gss, ,q,,~ Fig. 2. Variationsof the atomic stoichiometricratioC:Ti and N:Tiwithmass flow rate ratio C_,H.,4k-.,.,. film produced by varying the mass flow rates of nitrogen and acetylene gases during the coating process. Fig. 2 depicts the variations of the atomic stoichiometric ratios C:Ti and N:Ti with the mass flow rate ratio 4~c_,H_- which is defined as tbc,..,_=mc,.H,_l(mc,,,+mm,), where m represents the gas mass flow rate. These data are presented for the coating films only considering cases B to 13. Let [CI and IN] represent the atomic stoichiometric number ratios of C:Ti and N:Ti respectively. A linear relationship can be established between the atomic ratio and the mass flow rate ratio (~C2H:, tbr both [C! and [NI. Increasing the mass flow rate ratio OC.-H:would increase the atomic ratio [ C ], but would decrease the atomic ratio [ NI. These two straight lines regressed well by the data of [C] and [NI intersect at a point having mass flow rate ratio ~C'.H.,-------0.12. If the mass flow rate ratio tbc,., in the chamber is lower than 0. ! 2, the atomic ratio [ C 1 in the chemical composition ofTi (C,N) is less than the atomic ratio [ N ]; conversely, if the ratio q~c,,., is higher than O. 12. the atomic ratio [ C l in the chemical composition would surpass I N 1.
3.2. Kinetics of surface reactions in the coating process For the purpose of studying the variations in the atomic stoichiometric ratios [C] and [N] in the chamber with the mass flow rates of the C2H2 and N_, gases, the kinetics of the reactions occurring during the coating process was investigated using both the diffusion rate theory and the concentration theory. If it is assumed that the rate of reaction between the titanium surface and the N_, and C,H, gases is high, the reaction between these two ions and the titanium surface is generally dependent on the mass flow rate ratios 4~C_,H_,and 4~.,. The reaction rates for nitrogen and carbon ions can be expressed as the atomic stoichiometric ratios N:Ti and C:Ti respectively.
3.2. I. The diffusion rate theor), In a higher temperature region, with a relatively lower activation energy, the growth mechanism of Ti(C,N) films is primarily gas diffusion controlled. The two atomic stoichiometric number ratio,,;, using the theory of diffusion rate, can be expressed as a function of 4~c:., and ~ : respectively.
(in)
(Ib)
where d~,-mN,/(mc:n,_+mm,.), (~:H:~'ff/C2H.,/(//IC_~H_, + ms_,), and t is the reaction time of particles. The constants Ks and Kc are functions of the diffusion coefficients of nitrogen and carbon ions respectively. The general solutions of Eqs. ( l a ) and ( l b ) are thus written as 1 - [ N ] = exp( - KNd~,_t) I -
[ C ] -- exp( - Kc~::,H,t)
(2a) (2b)
The time t for the chemical reaction may be expressed as a function of the ion speed and the surface characteristic. Assuming that the mean distance L between two reactant particles and the average ion speed V are always maintained steady during the coating process, the reaction time is thus expressed as a function of LIV. Eqs. (2a) and (2b) can be rewritten as 1 - [ N ] =/3N exp(aN~,~,)
(3a)
l - [C] =/3c exp(Otcd~c,,u2)
(3b)
where/3N is a function of the diffusion coefficient of nitrogen ions, a s is the available reaction (diffusion) time for nitrogen ions,/3c is a function of the diffusion coefficient of carbon ions, and ac is the available reaction (diffusion) time for carbon ions. The experimental data in Fig. 3 marked by circles and triangles can be regressed by F_.qs. (3a) and (3b) respectively. Fig. 3 shows the good matches of the experimental results with the theoretical expressions. They are given as 1 - [ C ] -- 0.874 exp( - 2.86ti~:n:)
(4a)
and I - [N] = 1.283 exp( -0.8954h, j.,)
(4b)
Increasing the mass flow rate ratio would always enhance the reactions arising between the titanium surface and either the nitrogen or the acetylene gas. However, the reaction rate associated with [ N ] is relatively higher for the same increase in the mass flow rate.
3.2.2. The concentration theory Cheng and Hon [ 10] have proposed another reaction model of kinetics suitable for the CVD coating process. According to their study on the crystal growth of titanium carbonitride by CVD, the deposition rate has a relatively strong dependence, and can be expressed as an Arrhenius equation. The reaction rate of formation of the Ti(C,N) coatings is dependent on the reactive gas flow rate and deposition
248
EE Guu. J.F. Linl Wear210 (1997) 245-254 '[.~ --
t.0-
.m
I
]
•A
,,-,-ilq ] -.-~-ilSl •
•
•
mq.(t,m~
1.0 m
l 0+
!
A
I~u.(u)
0~s &
0.0
0.1
02
i
0,3
0.4
o.,
0.5
~ w m l rUlo ofC21~ I m
I .........
cLs
0,5
0.6
0.7 0.8 m m flow r m mJo ofN2 i ~
0.9
11.0
Fig. 3. Plo~ of I - [C] and i - [ N I vs. mass flow race rauoofC,H2 f,as and N2 gas respectively.
temperature. The growth rate of Ti(C,N) coatings can be expressed by an empirical formula: Ga
~"
exp(
- AEIRT)
(5)
where G is the growth rate of the Ti(C,N) coating, ~bis the reactive gas flow rate, ~ is a slowly varying function of deposition temperature, AE (J mol- ') is the apparent activation energy (not a function of gas flow rate), R (Jmol - t K -I ) is the gas constant, and T (K) is the deposition temperature. Thus, the atomic ratios [C] and [N] can be expressed by the following equations: [c] =ac(~,m) ~
(6a)
[N] = B N ( ~ , ) ''~
(6b)
where Bc and BN represent the proportional constants which depend on the deposition temperature. Fig. 4 demonstrates a good agreement of the experimental results with the models demonstrated in Eqs. (6a) and (6b). The experimental data marked by the circles can be regressed well by the following expression: I t ] = 1.114ckc,.x2 o,,~,,
(7a)
and the data marked by the triangles can be expressed as
[N] = , ,n. . ,~w, ~, e, .N4 ,, 9
(7b)
The reaction behaviour arising in the cathode..,,'- ion plating process associated with atomic stoichiometric ratios C:Ti and N:Ti was observed from the physical viewpoint in terms of the diffusion theory or the chemical viewpoint in terms of the concentration theory. As the expressions of exponential form Eqs. (4a) and (4b) show, both atomic stoichiometric
lmm
b .........
~s
of I .........
ih Ca
8m
I .........
O.7 oJ n,mm flow me ~lhelqT. ~
o, ~ ........
o,o
oo "-I
'~.o
Fig. 4. Plot of atomic ratio [C] and I N I vs. mass flow rate ratio of C~H, gas and N: gas respectively.
ratios, [C] and [N], are increased by increasing the mass flow rate ratios 0t:2x., and ~ 2 respectively. The same effect is also demonstrated in Eqs. (7a) and (Tb) on the basis of the concentration theory Athough t'te w-xiations of [ C ] and IN] are expressed in a po¢+t:~,'ban.
3.3. Wear rate of lower specimen affected by coating material The wear rate of the lower specimen is strongly dependent on the material composition. As the wear rate data of the eight types of specimen in Table 2 show, the wear rates of the specimens with TiN coating as the top layer are always the lowest for both rotational speeds. The lower specimen with TiC coating, operating at the sliding speed of 0.705 m s - ' , showed a much larger wear rate than the specimens with the TiN coating. As the sliding speed was increased to 1.41 m s - ', the gap in wear rate between the TiC and TiN coatings shrunk. The chemical composition of the Ti(C,N) coating can be altered by adjusting the mass flow rates of the nitrogen and acetylene gases during the coating process. It is interesting that the specimens prepared with ~bc.~x2of 0.09 and 0.29 have lower wear rates at both sliding speeds (0.705 and 1.41 m s - ' ). Although the material hardness associated with the latter is higher than that of the former, the wear rate of the latter is slightly higher than that of the former.
3.4. Variations of wear displacement with specimen temperature The wear displacement presents the relative displacement of the upper and lower specimens during the wear test. A
E E Guu, ZF, Lin / Wear 210 0997) 245-254
249
Table 2 Friction coefficientsand wear rates of the lower specimens and the upper specimen: the underlayerof the coating is 0.1 gm thick and the top layer is 3 ~m thick; the operating conditions include applied load producing the normal pressure of 1,69 MPa, and sliding distance of 2032 m Case
4~,:.,H..
A; B1 C! D! E! FI GI HI A2 B2 C2 D2 E2 F2 G2
0.00 0.09 0,17 0.23 0.29 0.33 0.41 1.00 O.00 0.09 O. 17 0.23 0.29 0.33 0.41
H2
!.00
Sliding speed (m s - i)
0.705
1.41
Average friction coefficient
Wear rate × I0 - ~ (mm ~ s - *) Lower specimen
Upper specimen
Final wear displacement (p.m)
0.547 0,504 0.543 0.522 0,554 0,513 0,508 0.522 0.442 0.440 0.430 0.412 0.438 0.411 0.442
0,322 0.414 0.739 0.633 0,446 0.450 1,845 1.467 0.045 0.088 O. 140 0.102 0.095 0.135 0.163
2.532 2.063 2.139 2.161 2.152 2,183 1.317 ! .250 8.223 8.971 5..~80 3.791 4.557 3.337 3.142
200 158 199 184 199 200 153 186 223 192 243 273 249 238 278
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TM (°C)
245 250 250 250 235 238 240 245
T; ( °C )
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245 245
233 230 25O 260 235 270 27O 240
TM is the specimen temperature corresponding to the maximum wear displacement.T~ is the specimen temperature az which Ihtee-body wear was absent,
charactedstc for a C 1 specimen at the sliding speed of 0.705 m s - ' is exhibited in Fig. 5(a). The curve shows that the wear displacement increased continuously on raising the specimen temperature until a peak value; beyond this point there is a small drop. The specimen temperatures of all eight types of specimen corresponding to the maximum wear displacement are given in Table 2; they lie in the range between 235 °C and 250 °C. An attempt to exploit the specimen temperature corresponding to the maximum wear displacement as the critical borderline was made to separate two different wear mechanisms. In the range below the maximum wear displacement, the ceramic coating in the lower specimen was not fully worn out; the material of the upper specimen was continuously adhesive to the lower specimen, thus resulting in a continuous increase in wear displacement However, when the specimen temperature rose above this critical value, the ceramic coating film deposited onto the lower specimen was mostly removed out of the wear track; adhesive wear was greatly impeded, and the rubbing surfaces were operating under the circumstance of metal-to-metal contact. The wear displacement thus showed a small decline in this region. Changing the operating conditions by varying the sliding speed brought about noticeable distinctions in the fluctuation s of specimen temperature and wear displacement curves. Fig, 5(b) shows the variation of wear displacement of the C2 specimen at the sliding speed of 1.41 m s- t. The curves of all eight cases exhibit similar characteristics: they start with a small smooth region, then are connected by a curve of zig-zag form in the three-body wear region, and finally are restored to a smooth curve until the designated sliding distance is completed. The smooth curve in the first region was caused by the mechanism of abrasive wear, as shown in Fig. 6(a). When the specimen temperature was continuously
elevated, the three-body wear eventually replaced abrasive wear as the prevailing wear mechanism; numerous fluctuations appear oil the curves of wear displacement. As the specimen temperature was further elevated, the three-body wear was impeded gradually, and the adhesive wear mechanism became prevailing (Fig. 6 ( b ) ) . In this region, the ceramic coating film still remained partly on the surface of the wear track although the designated sliding distance had finished. This phenomenon illustrates that the restoration of a smooth curve in this region resulted mainly from the significant change in wear mechanism due to the temperature rise. At the sliding speed of IAI m s - ' , the three-body wear of all eight types of specimen was effectively impeded when the specimen temperature rose above about 250 °C. Actually, being dependent on ~he chemical composition of the ceramic coating, this critical temperature changes somewhat in a small range. The critical temperature is slightly lower for some types of specimen. Table 2 demonstrates the temperature of the specimen at which three-body wear was absent. Variations of the wear displacement and specimen temperature are regressed by an eighth-order polynomial function for the convenience of analysis. Wear displacements varying with specimen temperature are demonstrated in Fig. 7(a); they were obtained from eight types of specimen operating at the sliding speed of 0.705 m s- =. The relationship of wear displacement increasing with increasing temperature of the lower specimen holds for all eight specimens. The genuine starting points of these eight curves in Fig. 7(a) ate actually different. However, a constant temperature ( 70 °(2) was chosen as the common starting point for the purpose of fitting the wear displacements well to the polynomial. There exists one or more inflection points on each wear displacement curve. The first inflection point reflects the behaviour arising
E E Guu. J.F. Lin / P/ear 210 (1997) 245-254
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Fig. 5. Plo~s of friction ceeflicient, wear displacement, electrical voltage and specimen temperature vs. sliding time at the sliding speed of (a) 0.705 m s- ~and (b) !.41 m s - ~.The operating conditions include applied normal pressure of 1.69 MPa, and sliding distance of 2032 m.
near the end of the three-body wear. The wear displacement ahead of the first inflection point is obviously less in magnitude than that behind this point. This feature indicates that the material adhesion occurring during three-body wear is quantitatively lower than that occurring during the process for which adhesive wear was dominant. The genuine starting points of these eight curves are found to have negative displacements. Negative displacements were commonly produced at the time before the presence of three-body wear; they imply that abrasive wear, instead of material adhesion, governed the wear mechanism in this interval. The experi-
Fig. 6. Micrographs of the worn surface generated by the specimen of case C2 at the sliding speed of IAI m s -~. The operating conditions include applied normal pressure of 1.69 MPa, and sliding distance of 2032 m: (a) first region (abrasive wear); (b) last region (adhesive wear).
mental results reveal that the F1 specimens have the largest wear displacement for the same lower specimen tt'.mperature. The final wear displacements of these eight curves, which are considered as the approximate increases in thickness of the adhesive layer, are shown in Table 2. The data for these final wear displacements, according to their magnitude, give the array order AI,Fl >C1,EI > H I > D l > B I > G I . The wear rate data give the array AI > F I > DI,EI,C! > B I > G I , H i for the upper specimens and the array GI > H I > C I > D! > FI,EI > B! > A! for the lower specimens. The following conclusion can thus be drawn from the investigatio,I of the above two array orders: the specimens with a higher final wear displacement were likely to produce a higher wear rate of the upper specimen when operating at 0.705 m s- *. The evidence reveals that the upper and lower specimens operating at 0.705 m s- * were normally in possession of a thick adhesive layer on the wear tracks: this layer, in general, can protect the lower specimen from severe wear. The thicker the adhesive layer, the lower the wear rate of the lower specimen produced. As the sliding speed was increased to 1.41 m s-*, the curves of wear displacement vs. specimen temperature, as shown in Fig. 7(b), become similar in appearance to the curves demonstrated at the sliding speed of 0.705 m s - t . However, the final wear displacements are relatively higher. The data for the final wear displacements of the eight curves give the array order G2 > D2 > E2,H2 > C2 > F2 > A2 > B2.
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The wear rate data at the sliding speed of 1.4 i m s - ' give the array B2 > A2 > H2 > C2 > E2 > D2 > F2,G2 for the upper specimens, and G2 > C2 > F2 > D2 > E2 > B2 > H2 > A2 for the lower specimens. It is interesting that the specimens with higher final wear displacements often produced lower wear rates of the upper specimens, but brought out higher wear rates for the lower specimens. Consequently, the behaviour of wear loss is strongly dependent on the operating conditions; the characteristic associated with the specimens operating at higher sliding speeds perhaps demonstrates behaviour exactly opposite to that exhibited at lower sliding speeds.
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Fig. 8. Plots of the displacement-temperature gradient vs. specimen temperalure at the sliding speed of (a) 0.705 m s - ' and (b) 1.41 m s I '. The operating conditions include applied normal pressure of 1.69 MPa. and sliding distance of 2032 m.
Fig. 8(a) demonstrates the displacement-temperaturegradients, d(wear displacement)/d(specimen temperature), of the eight types of specimen that were tested at the sliding speed of 0.705 m s - '. They are actually the slopes of the curves shown in Fig. 7(a). Each of these eight curves shows at least one peak value in the displacement-temperature gra-dient. The first peak lies within the temperature range between about 130 °C and ! 80 °(2. These extreme values are actually
El'. Guu, J.F. 1.in/Wear210 (1997) 245-254
252
t h slopes of the wear displacement curves at the inflection point. Beyond this point, metal-to-metal frictional contacts in terms of adhesive wear and microcutting gradually replaced the three-body wear and became the dominant wear mechanisms. The second peak on each curve is located at the point where the maximum wear displacement occurs. It should be mentioned here that at the sliding speed of 0.705 m s- t, the eight curves of wear displacement all have the maximum value near the end of the sliding time. The curves in Fig. 8(b) depict the displaceraent-temperature gradients, d(wear displacement) /d( specimen temperature), for the specimens operating at the sliding speed of 1.41 m s - n; these curves have one to three peaks. The first peak on each curve corresponds to the inflection points of the curves shown in Fig. 7(b). These inflection points occurred at the temperature within the range 155-250 °C. The wear displacements of the eight types of specimen always increased with sliding time, the maximum wear displacements generally being presented at the end of the sliding motion. The second and third peaks shown in Fig. 8(b) ate simply related to the variations of wear displacement with the lower specimen temperature: they are generated by the wear displacement curves with more than one inflection point. Eh'astic rises in the displacementtemperature gradient in seve:al of these eight curves appear near the end of each curve; the sharp increase is attributable to the combined effect of continuous increases in wear dis-
In order to understand the wear mechanisms that occurred ahead and behind the maximum wear displacement, two results relevant to the specimens of case D ! were obtained at the sliding speed of 0.705 m s - t . Fig. 9(a) shows the wear profile before the maximum wear displacement occurred (at the sliding time of 1500 s), whereas Fig. 9(b) exhibits the profile after the maximum wear displacement (at the sliding time of 2900 s). Since the layer of ceramic coating still existed on the wear track before reaching the maximum wear displacement, adhesive wear was then the dominant wear mechanism. The wear track of the lower specimen was filled with the material delaminated from the upper specimen, thus the wear profile does not exhibit a deep and broad concave shape. Fig. 9(b) demonstrates the wear profile obtained after the maximum wear displacement. This wear profile was obtained when the ceramic coating layer was damaged and was removed out of the wear track of the lower specimen. Adhesive wear at this moment was impeded and metal-to-metal friction contacts caused the wear track of the lower specimen to be broad and deep.
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Y.E Guu, ZF. 1.in/Wear210(1997) 245-254 0.4
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Fig. 10. The variation of the rate of wear displacement with the specimen temperature. The operating conditions include sliding speed of 0.705 m s- '. applied normal pressure of 1,69 MPa, and sliding distance of 2032 m.
If the sliding speed was increased to 1.41 m s - ' , the ceramic coating film still remained on part of the wear track, even after the sliding distance of 2032 m. The adhesive layer cumulated continuously on the wear track until the sliding motion ended, the concave shape never being exhibited in the wear track (Fig. 9(c) ).
3.6. Wear displacement rate va~b~g with specimen temperature The curves in Fig. 10 are presented to illustrate the variations of the rate of wear displacement with the rise in specimen temperature. These results were obtained at the sliding speed of 0.705 m s - ~. Negative slopes of all these eight curves indicate that the increase in thickness of the adhesive layer before reaching the point of maxiwum displacement was lowered by the continuously rising specimen temperatare. Of the eight kinds of specimen, the relatively noticeable decline in the rate of wear displacement due to per unit temperature rise is present for specimens in cases El and FI.
4. Discussion For the purpose of investigating the variation s of the atomic stoichiometric ratios C:Ti and N:Ti with the mass flow rates o f t h e C2H2 a n d N 2 gases, the atomic stoichiometric number ratios C:Ti and N:Ti have been proved to agree well with the kinetics of surface reactions in terms of either the diffusion rate theory or the concentration theory. The growth rate of the Ti(C,N) films arising in the cathodic arc ion plating process can be investigated frem the physical viewpoint based on the diffusion theory or from the chemical viewpoint
253
based on the concentration theory. Actually, it is well known that the diffusion rate of reaction particles in the deposition chamber is effectively increased by increasing the concentration of particles. Consequently, the same effect on the atomic stoichiometric ratios could be obtained although they are evaluated on the basis of different theoretical models. The adhesion of the material from the upper specimen (SKD61 steel) onto the lower specimen is dependent quantitatively on whether tic ceramic coating remained or not during the wear process. If ~ ceramic coating film was retained, a significant difference in material hardness between the upper and lower specimens caused conditions favourable for material adhesion: this adhesive behaviour terminated gradually when the ceramic coating film was worn out of the lower specimen. The rate of adhesion in the period of threebody wear was decreased by increasing the temperature of the specimen. The wear performances for all eight types of specimen started with a short time of abrasive wear, followed by the three-body wear, and finally the mechanism of plastic deformation in the case of metal-to-metal contact. The wear behaviour demonstrated in the lower specimen is strongly dependent on the operating conditions. That is, the wear mechanism of the ceramic coating specimen is governed by both thermal and mechanical effects. The performance shown in the specimens at lower sliding speeds might be exactly opposite to that operating at higher sliding speeds.
5. C o n c l u s i o n s
i. The experimental data for die atomic ratie~[C] and [N] can be fitted well by both the expressions derived from the theory of diffusion rate and the theory of reaction rate of forming ceramic coating. 2. Adhesive wear is apt to occur at the specimens undergoing three-body wear or tnetal.to-metal frictional contact. The extent of material adhesion is dependent on the operating conditions and the condition of the ceramic film after three-body wear. The remainder of the ceramic coating film on the wear track encourages the increase in wear displacement. 3. The variations of the wear displacement gradient with the temperature of the lower specimen can offer information on the adhesive behavionr arising before and after threebody wear. The first peak of the wear displacement gradient curves is always present at the point where the three-body wear was about to end. 4. The wear rates of the upper and lower specimens due to adhesive wear are dependent on the operating conditions; they demonstrated exactly opposite behaviour at two sliding speeds. The specimens with a higher final wear displacement were likely to produce a higher wear rate of the upper specimen when operating at 0.705 m s - ' . The thicker the adhesive layer, the lower the wear rate of the
254
E Y. Guu. J.F. Lin / Wear 210 (1997) 245-254
lower specimen produced. When the sliding speed was elevated to 1.41 m s -~, the specimen with a higher final w e a r d i s p l a c e m e n t often p r o d u c e d a l o w e r w e a r rate o f the u p p e r specimen, and caused a h i g h e r w e a r rate o f the l o w e r specimen.
l 1l H. RmIMwa, Cathodic arc plasma depmition of TiC and Ti(C,N) films, Thin 3olid Films 153 (1987) 209-218. [2] E. Damond, P. Jacquor. J. Pagny, Ti(C,N) coatings by using the arc evaporation technique, Mater. Sci. Eng. AI40 ( 1991) 838-841. [3] K.T. Rie, A. Gebauer, Plasma-assisted chemical vaponr deposition of hard coatings with metallo-organic compounds, Mater. Sci. Eng. A139 (i-2) ( 1991) 61-66. [41 Y. C'hen, Y. Sun, F. Zhang, H. Mou, Preferential growth in ion beam enlmnced deposition of Ti(C,N) films, Vacuum42 (16) ( 1991 ) 1059. [5] LA. Ivanchenko. V.V. Paskal, N.A. Limvchenko, O.F. Gusarova, S.Y. Pilipovskij, G.A. Fmlov, Study of sm~ture and physico-chemical lXepertks of titanium cmbonilride with different compositions. Fiz. Khim. Obr. Mater. (4) (1992) 83-87. i6l J.M. Schneider, A. Voevodin,C. Reblmlz. A. Matthews, J.H.C. Hogg, D.B. Lewis, M. Ives, X-ray diffraction investigations of magnetron spottered Ti(C,N) coatings, Surf. Coat. Technol. 74-75 (!-3) (1995) 312-319.
[7] K. Aigner, W. Lengauer. D. Rafaja, P. Ettmayer, Lattice parameters and thermal expansion of TiC~NI _,, Zr(C,NI _,), Hf(C,NI_ ,) and TiN~ _, from 298 g to 1473 g as investigated by high-temperature Xray diffraction. J. Alloys Comp. 215 (i-2) (1994) 121-126. [8] R. Bertoncello, A. Ca.~grande, M. Casatin, A. Glisenti, E. Lanzoni, L. Mirenghi. E. TondellooTiN. TiC and Ti (C,N) film characterization and its relationship to tribological F~ehavior,Surf. Interface Anal. 18 (7) (1992) 525-531. [9] D.J. Chang, W.P, Sun, M.H, lion, Morphology and stmcgu:e of chemically vapour-depositedTi(C.N) coatings.Thin SolidFilms 146 ( I ) (1987) 45-53. [ 10] DJ. Cheng, M.H. Hon, Crystal growth of titanium carbonitride by chemical vapor deposition, MRL Bull. Res. Devel. 2 (2) (1988) 2733. [ I 1] P. Gautier. K. Kato, Wear mechanisms of silicon n~tride, partially stabilized zirconia and alumina in unlubricated sliding against steel. Wear 162-164 (1993) 305-313. [ 121 J.F. Lin, C.C. Chou, S.T. Chen, The effect oi"surface chemistry in lubrication on the tribological behavionr of steel milers under roilingsliding contacts. Tribotest J. 2-3 (1996) 205-234. 1131 ASTM Standard D3702, Standard test method for wear rate of materials in self-lubricated rubbing contact using a thrust washer testing machine, Vol. 5-3, ASTM, Philadelphia. PA. 1983. pp. 417423. [ 14] I. lliuc, Tribology of Thin Layers, Tribology Series° Vol. 4, Elsevier, Oxford, 1980, pp. I I I-112.