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Sliding wear of thermal-sprayed chromia coatings
Wear, 138 (1990)
93 - 110
93
SLIDING WEAR OF THERMAL-SPRAYED BU-QIAN
WANG
and ZHENG
Mechanical Engineering ALAN
RONG
Department,
CHROMIA
COATINGS*
SHUI
Xian Institute of Highways, Xian, Shaanxi (China)
V. LEVY
Lawrence Berkeley Laboratory,
University of California, Berkeley,
CA 94720
(U.S.A.)
Summary
Several thermal-sprayed Cr,Os coatings were applied to disc-shaped test specimens and tested at 25 “C and 400 “C in unlubricated sliding wear against a hard chromium-plated washer at low contact pressures of 0.69 and 6.9 MPa. The behavior of the coatings was compared with that of a slurryapplied, mixed oxide coating, SCA 1000, which had been evaluated in experimental diesel engines. The wear rates of the chromia coatings at the 400 “C test temperature were orders of magnitude lower than those tested at room temperature and that of the SCA 1000 coating at both temperatures. Also, the dynamic coefficient of friction of the best coating at 400 “C was 0.2. The promising performance of the chromia coating at 400 “C appears to be due to a super polishing surface effect that could be the result of the wear debris acting as a fine polishing agent.
1. Introduction
The possible use of ceramic thermal barrier layers [l - 41 on the combustion zone surfaces of diesel engines, i.e. piston crowns, exhaust valve faces and cylinder head fire decks may require that a thermal protective layer of material be placed on the upper portion of the cylinder wall liner. The sliding motion between the top piston ring and such a protective layer at higher surface temperatures than occur in today’s engines will require the development of new types of lubricants. If no satisfactory lubricants of either organic or inorganic base can be developed, the surfaces will have to operate in an unlubricated condition, which would be problematic [5]. In an earlier effort to gain some insight into such a requirement [6], a number of plasma-sprayed hard material coatings, including WC-MO, Cr,C,MO, Tic-Mo and Al,Os-TiO, were tested in unlubricated sliding wear against a slurry applied, mixed chromium, aluminum, silicon oxide coating that had appeared promising as an oxidation barrier coating in diesel engine tests. The *Paper presented at the International U.S.A., April 8 - 14, 1989.
Conference
0043-1648/90/$3.50
0 Elsevier
on Wear of Materials,
Sequoia/Printed
Denver,
CO,
in The Netherlands
94
tests were carried out at 25, 425 and 725 “C. It was determined that both the wear rates and the coefficients of friction were much too high to be practical, even at the low contact pressures that were used, 0.69 to 14.7 MPa, in the laboratory thrust washer tests. The possible use of a Cr,O, base coating that would have an acceptable level of wear and coefficient of friction was indicated in some work on the wear behavior of chromia former alloys [7]. In this work, it was observed that the sliding action produced a hard, very smooth, glazed layer of Cr,O, on the metal surface at the elevated temperatures that were used. To investigate whether Cr,O, would form an acceptable sliding wear surface because of the way its surface is modified from the as-applied surface by the sliding action, a series of thermal-sprayed Cr,O, coatings were tested. Relatively low contact pressures were used compared with those which occur in the diesel engine application because of the limitations of the thrust washer test machine that was used. The coatings were applied by both plasma- and oxyacetylene flame-spray methods using different types of starting powders and rod. The behavior of the Cr,O, coatings was compared with that of the slurry-applied, mixed oxide coating which had shown some promise in experimental diesel engine tests.
2. Test conditions The sliding wear tests were carried out on a Falex 6 thrust washer test device using oscillatory and rotating relative motion between the surfaces. The operational characteristics of this type of tester are described in ASTM standard D-3702-78. The test area and the typical washer and disc test specimen pair are shown in Fig. 1. The test coatings were applied on 3.2 cm (1.25 in) OD 1018 steel discs and were rough surface ground prior to testing. The mating washers were 1.5 mm (0.060 in) wide by 2.5 cm (1 in) OD hard chromium-plated 1018 steel. The test coatings are described in Table 1. Their proprietary nature and the restricted scope of the project prevented us from obtaining a full characterization of the coatings. The test conditions are noted in Tables 2 - 4. Tests were carried out at two contact pressures, 0.69 MPa (100 PSI) and 6.9 MPa (1000 PSI). These pressures are low compared with those that occur in a diesel engine and thus they provide only a first indication of the potential of the coatings for use in actual sliding wear service.
3. Results and discussion 3.1. Wear rates and coefficient of friction Most of the tests used an oscillatory, 90” reversal wear motion. The wear rates measured in these tests are listed in Table 2. As a result of there being a limited number of specimens prepared by the supplier, only
95
Fig. 1. Falex
TABLE
6 wear tester
and washer
and disc test specimens.
1
Disc materialsa
Designation
Kind of Cr203 coating on substrate
R S C F K
Rokide C flame-sprayed CrzOs Spray-dried plasma-sprayed CrzOs Coarse fused plasma-sprayed CrzOs Fine fused plasma-sprayed CrzOs SCA 1000, slurry-applied Si02-Cr*O-Al203
aThe test coatings, for the commercially plated 1018 steel.
1018 steel
Thickness 0.28 0.24 0.32 0.40 0.05
(mm)
- 0.42 - 0.08
all applied by hand, were proprietary, experimental coatings except available Rokide Coating. The washer was made of hard chromium-
single tests were performed at each condition. The flame-sprayed Cr,O, coating, R, had the lowest steady state wear rate under all conditions. All of the Cr,O, thermal-sprayed coatings greatly exceeded the performance of the slurry-applied, SCA 1000 coating which had been successfully tested in diesel engines on the cylinder wall liner by others [8]. While none of the thermal-sprayed coatings failed during the test times used, the very much thinner SCA 1000 coating wore through in all but the low contact pressure test at 25 “C. At both test temperatures the wear was significantly greater for
TABLE
2
0.2 0.2 0.52 2.7 2.74
aHard chromium plating on substrate 1018 steel. bP = 0.69 MPa. cP = 6.9 MPa.
2.3
2.12
0.42
0.059
0.0040
Steady state (4 h x5,@
0.11
Steady state (12 min X4)c
Initial stage (2.5 min XS)b
(I min X5,F
Initial stage
Steady state (4 h ~5)~
~.0005 0.0032 0.0032 0.0022 0.13 (wore off)
Initial stage (2.5 min x5Jb
low5 crne2 s-l)
0.88 1.55 1.55 0.88 5.8
A uerage wear rate at T = 400 “C
x
1.03 1.67 4.97 3.97
I.81 1.68 4.91 4.13 31.78 (wore off)
state
Auerugt? wear rate at T = 25 “C
Average oscillatory wear rate of washera (g
TABLE 3
0.025 0.043 0.052 0.073 0.12
Steady (4 wa
Znitial stage (2.5 minja
Steady state (12 min)b
Initial stage (1 min,Ib
Znitial stage (2.5 rnirQa
Steady state (4 hla
Wear rate at T = 400 “C!
Wear rate at T = 25 “C
aP = 0.69 MPa. bP = 6.9 MPa.
R S c F K
Sample
Crz03 coating oscillatory wear rate g x lO-$ cmd2 s~‘l
0.55
Initial stage (5 min X41c
0.388 0.10 1.14 0.93 (1 min) 48.40 (wore off)
Initial stage (5 minp
0.56
Steady state (20 min X4je
0.095 0.138 0.37 0.15
Steady-state test time (20 min)b
97 TABLE
4
Wear rate (g
X
10e5 cme2 sC~)~
Sample
R
S
c
F
K
Initial stage (2.5 min) Steady state (4 h)
0.91
0.35
0.98
0.83
6.10
0.0014
0.0026
0.0032
0.0028
0.40 (2 h wore off)
aT = 400 “C, P = 0.69 MPa, rate of revolution 46% - 52%.
is 200 rev min-’
and relative
humidity
is
all of the coatings at the high contact pressure of 6.9 MPa than at the low contact pressure of 0.69 MPa. A marked reduction occurred in the steady state wear rates of the Cr,O, coatings at 400 “C compared with their wear rates at 25 “C at both contact pressures. The dynamic coefficient of friction and surface morphology were correspondingly improved, as will be shown later. The relative wear rates of the S, C and F plasma-sprayed coatings indicate that the effect of the starting powder, as indicated in Table 1, on the wear behavior of the plasma-sprayed coatings is a factor. The hard chromium plating on the washer, which was a commercial coating used for wear resistance, behaved well at both temperatures, showing no sign of softening at the 400 “C test temperature, see Table 3. The same chromium-plated washer was used for several tests, hence the designation X5 and X4 in Table 3 to indicate the number of tests on each specimen. It is interesting to note that all the Cr203 coatings out-performed the chromium plating at some test condition with the R and S coatings doing better under all the test conditions. Figure 2 is a bar graph of the test results. The effects of increasing the temperature and the contact pressure on the resultant wear can readily be seen. The SCA 1000 coating, K, can only be applied to a thickness of 0.08 mm because of the nature of its application process. It therefore wore through during the low contact pressure test at 400 “C even though it had a wear rate that was near its 25 “C wear rate. The relative performances of the Cr,Os coatings and the SCA 1000 coating can easily be seen in Fig. 2. The incremental wear rates of the coatings are plotted in Fig. 3. This type of curve differs from the usual cumulative wear curve in that each point on the curve represents the amount of wear that occurred in an increment of time. It can be seen that all of the materials reached a steady state wear rate where the same amount of wear occurred in each succeeding increment of time after only l/2 to 2 h of testing. All of the coatings had higher initial wear rates because their high asperities were being removed, as occurs in the sliding [9] and erosive [lo] wear of plasma-sprayed coatings. As will be seen in the micrographs, the coating “wear-in” occurs until a super polished condition is reached after which losses are low and uniform. Coatings S and R
98
1
1 03
i& 86906969 20°C: 4KPc
Coating
ZO’C
3 4wc
F
K
6969
869
06
0696s 2O’C 4W’C
4wc
&$I
0696906969 MPa MT
400°C
R
S
C
Fig. 2. Bar graph of coating wear rates.
.-------em
F
A
c
..........................
.---s . -.-,-.-,-
1
2
R
3
4
Time (Hour)
Fig. 3. Incremental wear rate curves of coatings for unlubricated 25 “C and 0.69 MPa.
wear at 200 rev min-‘,
99
reached steady state conditions in less than 0.5 h of testing while coating C reached steady state after 1.5 h of testing. The reason for this difference is not known. Figure 4 shows the coefficient of friction for 4 h of testing in one direction, rotating wear on one specimen of each type of coating at 400 “C. The super polishing of the Crz03 coating that will be shown and discussed in the next section resulted in an unlubricated coefficient of friction as low as 0.2 at steady state conditions. It is interesting to note that the two Cr,03 coatings with the highest wear rates, F and C, had the lowest coefficients of friction. The reason for this is not known.
0.6
O_----_-_R .. .. .. . . .. . .. .. . .. .
4
0.7
$ 0.6 ‘G z2 z z :t =8 0
0.5 0.4 0.3 0.2
I
1 l/2
I
I
2 l/2
3 l/2
I
I
4 l/2
Time (Hour)
Fig. 4. Sustained coefficient of friction test at 400 “C. Test conditions: 0.69 MPa (100 lbf in-2); 200 rev min-‘; relative humidity 46% - 52%; 400 ‘C; all washer, hard chromium plating on 1018 steel substrate.
Table 4 lists the wear rates of the coatings in one-directional rotational wear at 400 “C at steady state conditions after 4 h of testing. The wear rates of the Cr,O, coatings were the same as for oscillatory testing while the wear rate of the SCA 1000 coating was considerably higher. The one-directional rotational tests were done to determine whether the super polishing was an effect of the oscillatory movement of the wearing surface. 3.2. Metallographic analysis 3.2.1. Hard chromium-plated washer The surface of the single hard chromium-plated washer used wore at a slow, even rate during the course of several tests. Figure 5 shows the surface before and after testing. The speckled appearance in the micrographs on the right is probably due to some very small wear debris particles of chromium oxide. The morphology of the surfaces before and after testing appears to be the same with the asperities removed. This indicates that reusing the
(b)
(a) Fig. 5. Surface of hard at 25 “C and 0.69 MPa.
chromium-plated the test series.
chromium
plating
(a)
before
washer did not adversely
testing
and
(b) after
testing
affect the comparability
for
factor
20 h
in
3.2.2. Coating R The flame-sprayed Cr,O,, coating R, had the lowest wear rate of the Cr,O, thermal-sprayed coatings. Figure 6 shows the surface and crosssectional microstructures of the coating. The micrographs indicate that the coating was quite dense, but did have a number of rough areas of varying sizes and depths on the ground surface that were probably pores from splat boundaries (Fig. 6(a)). After the 4 h test at 25 “C the worn surface had many areas that were smoother than the unworn surface as well as areas that were much coarser (Fig. 6(b)). The cross-section of the wear surface (Fig. 6(d)) shows how shallow the affected wear surface was and reveals the density of the coating. No evidence of large splat boundaries was observed in any of the cross-sectional micrographs, although there was evidence of their presence in the form of pores in the surface micrographs (see also Figs. 6(c) and 6(e)). The very low wear rate of coating R at 400 “C and 0.69 MPa contact pressure relates to the very smooth surface of the tested coating shown in Fig. 6(c). Some of the small voids in the untested surface have been increased in size, but several have been either filled in with wear debris or worn down to the level of the surrounding solid oxide material. The crosssection in Fig. 6(e) shows how smooth the wear surface was. The role of wear debris as a polishing agent at the elevated temperature requires further investigation. The wear behavior of coating R at 25 “C and a contact pressure of 6.9 MPa resulted in a much more textured wear surface, as can be seen in Fig. 7.
(4
(b)
(d)
(e) Fig. 6. Surfaces of coating R (a) before testing and after testing for 4 h at (b) 25 “C and (c) 400 “C. Also shown are cross-sections of the specimens tested at (d) 25 “C and (e) 400 “C. In (b), (c), (d), (e) P = 0.69MPa.
This morphology correlates with the large increase in the wear rate that occurred (see Table 2). The degree of damage done, including the severe localized plastic flow and the cracking of the coating, is shown in Fig. 7(b).
(a)
(cl Fig. 7. (a), (b) Surface and 6.9 MPa.
and (c) cross-section
of coating
R after testing
for 12 min at 25 “C
The cross-section of the wear surface in Fig. 7(c) shows the depth of the damage that occurred. It also shows what appears to be a splat boundary crack, deep in the cross-section of the coating. The depth of surface damaged material compared with the depth of the splat boundary is an important indication of why the flame-sprayed Cr203, coating R, behaved so well. It presents dense, pore-free, chromia splats at the wear surface which are highly resistant to wear attack. Plasma-sprayed coatings, on the other hand, have a small amount of fine porosity within each splat that results in a greater susceptibility to wear, even though they have finer splat boundaries and an overall lower level of porosity. 3.2.3. Coating S The spray-dried powder, plasma-sprayed coating had the second lowest wear rate of the Cr,03 thermal-sprayed coatings. The macrographs of the wear tracks at the lower contact pressure for the 25 and 400 “C tests are shown in Fig. 8. There appears to be the beginning of a crack across the wear face of the 400 “C test specimen. The more polished surface of the 400 “C
(a) Fig. 8. Macrographs and (b) T = 400 “C.
(b) of wear
tracks
on coating
S tested
for 4 h at 0.69
MPa:
(a) T = 25 “C
test disc resulted in its lower wear rate and lower coefficient of friction compared with the 25 “C test specimen (see Table 2 and Figure 4). The unworn and worn surfaces are shown in Fig. 9. The worn surfaces at 25 “C for the two contact pressures (Figs. 9(b) and 9(c)) appear very similar even though there was a major difference in their wear rates. The reason for this is not known. The 400 “C test specimens (Figs. 9(d) and 9(e)) had different degrees of a very smooth surface that were markedly different from the 25 “C specimen surfaces. Figure 9(e) shows that the whole surface was polished to a significantly finer finish than that of the untested surface (Fig. 9(a)). Whether the polished surface is smeared wear debris or a grounddown surface using wear debris as the grinding media, or both, is not known. 3.2.4. Coating C The coarse fused powder plasma-sprayed coating had a much higher wear rate than the R or S coatings at all but the 400 “C, 0.69 MPa test condition where all four Cr,Os coatings had similar wear rates (see Table 2). The wear tracks shown in Fig. 10 were much smoother at 400 “C than at 25 “C at the low contact pressure, but cracks developed in the coating within the wear track at 400 “C. The highly polished surface that developed at 400 “C (see Fig. 10(b)) can be compared with the coarser surface from the 25 “C test. The increased number of cracks in the wear track, compared with coating S at 400 “C, did not result in an increased wear rate (see Table 2). It appears that the wear rate at 400 “C is primarily a function of the surface smoothness and not the amount of cracking that occurs. This is not so at the higher contact pressure where cracking in the wear track contributes to the higher wear rates. The unworn and worn wear surfaces of coating C, as seen in Fig. 11, do not vary as much as those of coating S. The texture of the worn surfaces is considerably rougher than that of the unworn surface. At 6.9 MPa contact pressure, the surfaces appear to be rougher at 25 “C than at 400 “C (see Figs. 11(d) and 11(e)), and their wear rates are considerably different. The
(a)
(b)
(d)
(e) Fig. 9. Surface of (a) unworn coating S and surfaces tested (b) for 4 h at 25 “C and 0.69 MPa, (c) for 12 min at 25 “C and 6.9 MPa, (d) for 4 h at 400 “C and 0.69 MPa, and (e) for
20 min at 400 “C and 6.9 MPa.
smooth, polished surface which occurred at 400 “C and 0.69 MPa contact pressure can be seen in Fig. 11(c) in the region where one of the cracks in the wear track was located. There appears to have been some preferential
(a)
(b)
Fig. 10. Macrographs of wear tracks on coating C tested for 4 h at 0.69 MPa: (a) T = 25 “C and (b) T = 400 “C.
wear occurring in the crack zone, but it did not increase the steady state wear rate (see Table 2) at least for the relatively short duration of the test.
3.2.5. Coating F The fine fused powder plasma-sprayed coating had lower wear rates than the coarse fused powder coating (see Table 2). However, differences in the surface morphology of the wear tracks of the two coatings are not evident. The wear tracks of coating F in Fig. 12 are similar to those of coating C in Fig. 10. The surfaces of the wear tracks at all four test conditions are shown in Fig. 12. The unworn and worn surfaces of coating F are shown in Fig. 13. The same pattern of surface morphology as a function of test conditions as occurred on coating C can be seen in Fig. 13. More polished areas occurred in the 400 “C tests than in the 25 “C tests. The morphologies of all of the test surfaces are much more alike than their different wear rates would indicate. More indepth analysis is required to improve understanding of the lack of correlation between surface morphologies and wear rates for this coating. The higher magnification views of the unworn surfaces and worn surfaces at 6.9 MPa at 25 “C are shown in Fig. 14. The similarity between the two surfaces is striking when the high wear rate of the worn surface is considered.
3.2.6. Coating K The surfaces of the slurry-applied, SCA 1000, coating are shown in Fig. 15. The morphology of the tested coating in Fig. 15(b) is very similar to that of the untested surface. The pits in the untested surface (Fig. 15(a)) are interesting. The three tested surfaces (Figs. 15(c), 15(d) and 15(e)) all show the underlying steel substrate with a few pieces of coating remaining in Fig. 15(d).
(a)
(d)
Fig. 11. Surfaces of (a) unworn coating C and surfaces tested (b) for 4 h at 25 “C and 0.69 MPa, (c) for 4 h at 400 “C and 0.69 MPa, (d) for 4 b at 25 “C and 6.9 MPa, and (e) for 20 min at 400 % and 6.9 MPa.
4. Conclusions (1) All of the thermal-sprayed Cr,03 coatings were superior to the slurry-applied, mixed oxide coating by nearly an order of magnitude at 25 “C
(b)
(cl
(d)
Fig. 12. ~acrographs of wear tracks on coating F: (a) 4 h at 25 “C and 0.69 MPa; (b) 4 h at 400 “C and 0.69 MPa; (c) 12 min at 25 “C and 6.9 MPa; (d) 20 min at 400 “C and 6.9 MPa.
and two orders of magnitude at 400 “C at contact pressures of 0.69 and 6.9 MPa. (2) The 400 “C test temperature coefficient of friction of the Cr,O, coatings at unlubricated steady state conditions was 0.2 - 0.25, which correlates with the low wear rates of these materials. (3) The Cr,O, coatings had wear rates one or more orders of magnitude lower at 400 “C than at 25 “C at both contact pressures. (4) The flame-sprayed Cr,O, coating had the lowest wear rate of the Cr,03 coatings because of the high density of the individual splats. (5) At the contact pressure of 6.9 MPa the wear rates of Cr,O, coatings R, S, C and F were two orders of magnitude higher than at the low contact pressure, 0.69 MPa. (6) The R and S coatings had lower wear rates than the hard chromium plate at 25 “C. At 400 “C all of the Cr,O, coatings had lower wear rates than the chromium plate.
(a)
Cd)
(e) Fig. 13. Surfaces of (a) unworn coating F and surfaces tested (b) for 4 h at 25 “C and 0.69 MPa, (c) for 4 h at 400 “C!and 0.69 MPa, (d) for 12 min at 25 “C and 6.9 MPa, and (e) for 20 min at 400 “C and 6.9 MPa.
(4
(b)
Fig. 14. Surfaces of coating F: (a) unworn and (b) worn at 25 “C and 6.9 MPa for 12 min.
(4
(b)
(cl
(d)
Fig. 15. (continued).
Fig. 15. Surfaces of (a) unworn coating K and surfaces tested (b) for 4 h at 25 “C and 0.69 MPa, (c) for 4 h at 400 “C and 0.69 MPa, (d) for 1 min at 25 “C and 6.9 MPa, and (e) for 1 min at 400 “C and 6.9 MPa.
References 1 R. Kamo and W. Bryzik, Ceramics in heat engines, SAE Tech. Paper 790645, June, 1979, Dearborn, MI. on low heat rejection diesel engine development. Proc. 2 K. L. Hoag, A perspective DOE 1987 Coatings for Advanced Heat Engines Workshop, Paper H-9, DOE Conf. 870762, Castine, Maine, July 1987, U.S. Department of Energy, Washington, DC, 1987. et al., Thick thermal barrier coatings for diesel engines. Proc. DOE 3 T. M. Yonoshonis Coatings for Advanced Heat Engines Workshop, Paper III-II, DOE Conf. 870762, Castine, Maine, Jury 1987, U.S. Department of Energy, Washington, DC, 1987. Proc. DOE 4 D. I. Biehler, Thick thermal barrier coatings for diesel engine components. 1987 Coatings for Advanced Heat Engines Workshop, Paper III-1 9, DOE Conf. 870762, Castine, Maine, July 1987, U.S. Department of Energy, Washington, DC, 1987. 5 K. F. Dufrane and W. A. Glaeser, Wear of ceramics in advanced heat engine applications. In K. C. Ludema (ed.), Proc. Int. Conf. on Wear of Materials, Houston, TX, April 5 - 9, 1987, American Society of Mechanical Engineers, New York, 1987, pp. 285 - 292. sliding wear of ceramic materials. In K. C. Ludema 6 A. Levy and N. Jee, Unlubricated (ed.), Proc. Int. Conf. on Wear of Materials, Houston, TX, April 5 - 9, 1987, American Society of Mechanical Engineers, New York, 1987, pp. 459 - 476. 7 F. .H. Stott, J. Glascott and G. C. Wood, The sliding wear of in-situ formed oxides on iron and nickel-base alloys. Proc. NACE Conf. on Corrosion-Erosion-Wear of Materials at Elevated Temperatures, Berkeley, CA, January, 1986, pp. 267 - 277, NACE, Houston, TX, 1986. 8 R. B. Coers, L. D. Fox and D. J. Jones, Cummins uncooled 250 engine. Proc. SAE Int. Conf. and Exposition, Detroit, MI, February, 1984. coatings for 9 A. Levy, D. Boone, E. January and A. Davis, Sliding wear of protective diesel engine components, Wear, 101 (2) (1985) 127 _ 140. 10 A. G. Davis, D. H. Boone and A. Levy, Erosion of ceramic thermal barrier coatings, Wear, 110 (2) (1986) 101 - 116.
93 - 110
93
SLIDING WEAR OF THERMAL-SPRAYED BU-QIAN
WANG
and ZHENG
Mechanical Engineering ALAN
RONG
Department,
CHROMIA
COATINGS*
SHUI
Xian Institute of Highways, Xian, Shaanxi (China)
V. LEVY
Lawrence Berkeley Laboratory,
University of California, Berkeley,
CA 94720
(U.S.A.)
Summary
Several thermal-sprayed Cr,Os coatings were applied to disc-shaped test specimens and tested at 25 “C and 400 “C in unlubricated sliding wear against a hard chromium-plated washer at low contact pressures of 0.69 and 6.9 MPa. The behavior of the coatings was compared with that of a slurryapplied, mixed oxide coating, SCA 1000, which had been evaluated in experimental diesel engines. The wear rates of the chromia coatings at the 400 “C test temperature were orders of magnitude lower than those tested at room temperature and that of the SCA 1000 coating at both temperatures. Also, the dynamic coefficient of friction of the best coating at 400 “C was 0.2. The promising performance of the chromia coating at 400 “C appears to be due to a super polishing surface effect that could be the result of the wear debris acting as a fine polishing agent.
1. Introduction
The possible use of ceramic thermal barrier layers [l - 41 on the combustion zone surfaces of diesel engines, i.e. piston crowns, exhaust valve faces and cylinder head fire decks may require that a thermal protective layer of material be placed on the upper portion of the cylinder wall liner. The sliding motion between the top piston ring and such a protective layer at higher surface temperatures than occur in today’s engines will require the development of new types of lubricants. If no satisfactory lubricants of either organic or inorganic base can be developed, the surfaces will have to operate in an unlubricated condition, which would be problematic [5]. In an earlier effort to gain some insight into such a requirement [6], a number of plasma-sprayed hard material coatings, including WC-MO, Cr,C,MO, Tic-Mo and Al,Os-TiO, were tested in unlubricated sliding wear against a slurry applied, mixed chromium, aluminum, silicon oxide coating that had appeared promising as an oxidation barrier coating in diesel engine tests. The *Paper presented at the International U.S.A., April 8 - 14, 1989.
Conference
0043-1648/90/$3.50
0 Elsevier
on Wear of Materials,
Sequoia/Printed
Denver,
CO,
in The Netherlands
94
tests were carried out at 25, 425 and 725 “C. It was determined that both the wear rates and the coefficients of friction were much too high to be practical, even at the low contact pressures that were used, 0.69 to 14.7 MPa, in the laboratory thrust washer tests. The possible use of a Cr,O, base coating that would have an acceptable level of wear and coefficient of friction was indicated in some work on the wear behavior of chromia former alloys [7]. In this work, it was observed that the sliding action produced a hard, very smooth, glazed layer of Cr,O, on the metal surface at the elevated temperatures that were used. To investigate whether Cr,O, would form an acceptable sliding wear surface because of the way its surface is modified from the as-applied surface by the sliding action, a series of thermal-sprayed Cr,O, coatings were tested. Relatively low contact pressures were used compared with those which occur in the diesel engine application because of the limitations of the thrust washer test machine that was used. The coatings were applied by both plasma- and oxyacetylene flame-spray methods using different types of starting powders and rod. The behavior of the Cr,O, coatings was compared with that of the slurry-applied, mixed oxide coating which had shown some promise in experimental diesel engine tests.
2. Test conditions The sliding wear tests were carried out on a Falex 6 thrust washer test device using oscillatory and rotating relative motion between the surfaces. The operational characteristics of this type of tester are described in ASTM standard D-3702-78. The test area and the typical washer and disc test specimen pair are shown in Fig. 1. The test coatings were applied on 3.2 cm (1.25 in) OD 1018 steel discs and were rough surface ground prior to testing. The mating washers were 1.5 mm (0.060 in) wide by 2.5 cm (1 in) OD hard chromium-plated 1018 steel. The test coatings are described in Table 1. Their proprietary nature and the restricted scope of the project prevented us from obtaining a full characterization of the coatings. The test conditions are noted in Tables 2 - 4. Tests were carried out at two contact pressures, 0.69 MPa (100 PSI) and 6.9 MPa (1000 PSI). These pressures are low compared with those that occur in a diesel engine and thus they provide only a first indication of the potential of the coatings for use in actual sliding wear service.
3. Results and discussion 3.1. Wear rates and coefficient of friction Most of the tests used an oscillatory, 90” reversal wear motion. The wear rates measured in these tests are listed in Table 2. As a result of there being a limited number of specimens prepared by the supplier, only
95
Fig. 1. Falex
TABLE
6 wear tester
and washer
and disc test specimens.
1
Disc materialsa
Designation
Kind of Cr203 coating on substrate
R S C F K
Rokide C flame-sprayed CrzOs Spray-dried plasma-sprayed CrzOs Coarse fused plasma-sprayed CrzOs Fine fused plasma-sprayed CrzOs SCA 1000, slurry-applied Si02-Cr*O-Al203
aThe test coatings, for the commercially plated 1018 steel.
1018 steel
Thickness 0.28 0.24 0.32 0.40 0.05
(mm)
- 0.42 - 0.08
all applied by hand, were proprietary, experimental coatings except available Rokide Coating. The washer was made of hard chromium-
single tests were performed at each condition. The flame-sprayed Cr,O, coating, R, had the lowest steady state wear rate under all conditions. All of the Cr,O, thermal-sprayed coatings greatly exceeded the performance of the slurry-applied, SCA 1000 coating which had been successfully tested in diesel engines on the cylinder wall liner by others [8]. While none of the thermal-sprayed coatings failed during the test times used, the very much thinner SCA 1000 coating wore through in all but the low contact pressure test at 25 “C. At both test temperatures the wear was significantly greater for
TABLE
2
0.2 0.2 0.52 2.7 2.74
aHard chromium plating on substrate 1018 steel. bP = 0.69 MPa. cP = 6.9 MPa.
2.3
2.12
0.42
0.059
0.0040
Steady state (4 h x5,@
0.11
Steady state (12 min X4)c
Initial stage (2.5 min XS)b
(I min X5,F
Initial stage
Steady state (4 h ~5)~
~.0005 0.0032 0.0032 0.0022 0.13 (wore off)
Initial stage (2.5 min x5Jb
low5 crne2 s-l)
0.88 1.55 1.55 0.88 5.8
A uerage wear rate at T = 400 “C
x
1.03 1.67 4.97 3.97
I.81 1.68 4.91 4.13 31.78 (wore off)
state
Auerugt? wear rate at T = 25 “C
Average oscillatory wear rate of washera (g
TABLE 3
0.025 0.043 0.052 0.073 0.12
Steady (4 wa
Znitial stage (2.5 minja
Steady state (12 min)b
Initial stage (1 min,Ib
Znitial stage (2.5 rnirQa
Steady state (4 hla
Wear rate at T = 400 “C!
Wear rate at T = 25 “C
aP = 0.69 MPa. bP = 6.9 MPa.
R S c F K
Sample
Crz03 coating oscillatory wear rate g x lO-$ cmd2 s~‘l
0.55
Initial stage (5 min X41c
0.388 0.10 1.14 0.93 (1 min) 48.40 (wore off)
Initial stage (5 minp
0.56
Steady state (20 min X4je
0.095 0.138 0.37 0.15
Steady-state test time (20 min)b
97 TABLE
4
Wear rate (g
X
10e5 cme2 sC~)~
Sample
R
S
c
F
K
Initial stage (2.5 min) Steady state (4 h)
0.91
0.35
0.98
0.83
6.10
0.0014
0.0026
0.0032
0.0028
0.40 (2 h wore off)
aT = 400 “C, P = 0.69 MPa, rate of revolution 46% - 52%.
is 200 rev min-’
and relative
humidity
is
all of the coatings at the high contact pressure of 6.9 MPa than at the low contact pressure of 0.69 MPa. A marked reduction occurred in the steady state wear rates of the Cr,O, coatings at 400 “C compared with their wear rates at 25 “C at both contact pressures. The dynamic coefficient of friction and surface morphology were correspondingly improved, as will be shown later. The relative wear rates of the S, C and F plasma-sprayed coatings indicate that the effect of the starting powder, as indicated in Table 1, on the wear behavior of the plasma-sprayed coatings is a factor. The hard chromium plating on the washer, which was a commercial coating used for wear resistance, behaved well at both temperatures, showing no sign of softening at the 400 “C test temperature, see Table 3. The same chromium-plated washer was used for several tests, hence the designation X5 and X4 in Table 3 to indicate the number of tests on each specimen. It is interesting to note that all the Cr203 coatings out-performed the chromium plating at some test condition with the R and S coatings doing better under all the test conditions. Figure 2 is a bar graph of the test results. The effects of increasing the temperature and the contact pressure on the resultant wear can readily be seen. The SCA 1000 coating, K, can only be applied to a thickness of 0.08 mm because of the nature of its application process. It therefore wore through during the low contact pressure test at 400 “C even though it had a wear rate that was near its 25 “C wear rate. The relative performances of the Cr,Os coatings and the SCA 1000 coating can easily be seen in Fig. 2. The incremental wear rates of the coatings are plotted in Fig. 3. This type of curve differs from the usual cumulative wear curve in that each point on the curve represents the amount of wear that occurred in an increment of time. It can be seen that all of the materials reached a steady state wear rate where the same amount of wear occurred in each succeeding increment of time after only l/2 to 2 h of testing. All of the coatings had higher initial wear rates because their high asperities were being removed, as occurs in the sliding [9] and erosive [lo] wear of plasma-sprayed coatings. As will be seen in the micrographs, the coating “wear-in” occurs until a super polished condition is reached after which losses are low and uniform. Coatings S and R
98
1
1 03
i& 86906969 20°C: 4KPc
Coating
ZO’C
3 4wc
F
K
6969
869
06
0696s 2O’C 4W’C
4wc
&$I
0696906969 MPa MT
400°C
R
S
C
Fig. 2. Bar graph of coating wear rates.
.-------em
F
A
c
..........................
.---s . -.-,-.-,-
1
2
R
3
4
Time (Hour)
Fig. 3. Incremental wear rate curves of coatings for unlubricated 25 “C and 0.69 MPa.
wear at 200 rev min-‘,
99
reached steady state conditions in less than 0.5 h of testing while coating C reached steady state after 1.5 h of testing. The reason for this difference is not known. Figure 4 shows the coefficient of friction for 4 h of testing in one direction, rotating wear on one specimen of each type of coating at 400 “C. The super polishing of the Crz03 coating that will be shown and discussed in the next section resulted in an unlubricated coefficient of friction as low as 0.2 at steady state conditions. It is interesting to note that the two Cr,03 coatings with the highest wear rates, F and C, had the lowest coefficients of friction. The reason for this is not known.
0.6
O_----_-_R .. .. .. . . .. . .. .. . .. .
4
0.7
$ 0.6 ‘G z2 z z :t =8 0
0.5 0.4 0.3 0.2
I
1 l/2
I
I
2 l/2
3 l/2
I
I
4 l/2
Time (Hour)
Fig. 4. Sustained coefficient of friction test at 400 “C. Test conditions: 0.69 MPa (100 lbf in-2); 200 rev min-‘; relative humidity 46% - 52%; 400 ‘C; all washer, hard chromium plating on 1018 steel substrate.
Table 4 lists the wear rates of the coatings in one-directional rotational wear at 400 “C at steady state conditions after 4 h of testing. The wear rates of the Cr,O, coatings were the same as for oscillatory testing while the wear rate of the SCA 1000 coating was considerably higher. The one-directional rotational tests were done to determine whether the super polishing was an effect of the oscillatory movement of the wearing surface. 3.2. Metallographic analysis 3.2.1. Hard chromium-plated washer The surface of the single hard chromium-plated washer used wore at a slow, even rate during the course of several tests. Figure 5 shows the surface before and after testing. The speckled appearance in the micrographs on the right is probably due to some very small wear debris particles of chromium oxide. The morphology of the surfaces before and after testing appears to be the same with the asperities removed. This indicates that reusing the
(b)
(a) Fig. 5. Surface of hard at 25 “C and 0.69 MPa.
chromium-plated the test series.
chromium
plating
(a)
before
washer did not adversely
testing
and
(b) after
testing
affect the comparability
for
factor
20 h
in
3.2.2. Coating R The flame-sprayed Cr,O,, coating R, had the lowest wear rate of the Cr,O, thermal-sprayed coatings. Figure 6 shows the surface and crosssectional microstructures of the coating. The micrographs indicate that the coating was quite dense, but did have a number of rough areas of varying sizes and depths on the ground surface that were probably pores from splat boundaries (Fig. 6(a)). After the 4 h test at 25 “C the worn surface had many areas that were smoother than the unworn surface as well as areas that were much coarser (Fig. 6(b)). The cross-section of the wear surface (Fig. 6(d)) shows how shallow the affected wear surface was and reveals the density of the coating. No evidence of large splat boundaries was observed in any of the cross-sectional micrographs, although there was evidence of their presence in the form of pores in the surface micrographs (see also Figs. 6(c) and 6(e)). The very low wear rate of coating R at 400 “C and 0.69 MPa contact pressure relates to the very smooth surface of the tested coating shown in Fig. 6(c). Some of the small voids in the untested surface have been increased in size, but several have been either filled in with wear debris or worn down to the level of the surrounding solid oxide material. The crosssection in Fig. 6(e) shows how smooth the wear surface was. The role of wear debris as a polishing agent at the elevated temperature requires further investigation. The wear behavior of coating R at 25 “C and a contact pressure of 6.9 MPa resulted in a much more textured wear surface, as can be seen in Fig. 7.
(4
(b)
(d)
(e) Fig. 6. Surfaces of coating R (a) before testing and after testing for 4 h at (b) 25 “C and (c) 400 “C. Also shown are cross-sections of the specimens tested at (d) 25 “C and (e) 400 “C. In (b), (c), (d), (e) P = 0.69MPa.
This morphology correlates with the large increase in the wear rate that occurred (see Table 2). The degree of damage done, including the severe localized plastic flow and the cracking of the coating, is shown in Fig. 7(b).
(a)
(cl Fig. 7. (a), (b) Surface and 6.9 MPa.
and (c) cross-section
of coating
R after testing
for 12 min at 25 “C
The cross-section of the wear surface in Fig. 7(c) shows the depth of the damage that occurred. It also shows what appears to be a splat boundary crack, deep in the cross-section of the coating. The depth of surface damaged material compared with the depth of the splat boundary is an important indication of why the flame-sprayed Cr203, coating R, behaved so well. It presents dense, pore-free, chromia splats at the wear surface which are highly resistant to wear attack. Plasma-sprayed coatings, on the other hand, have a small amount of fine porosity within each splat that results in a greater susceptibility to wear, even though they have finer splat boundaries and an overall lower level of porosity. 3.2.3. Coating S The spray-dried powder, plasma-sprayed coating had the second lowest wear rate of the Cr,03 thermal-sprayed coatings. The macrographs of the wear tracks at the lower contact pressure for the 25 and 400 “C tests are shown in Fig. 8. There appears to be the beginning of a crack across the wear face of the 400 “C test specimen. The more polished surface of the 400 “C
(a) Fig. 8. Macrographs and (b) T = 400 “C.
(b) of wear
tracks
on coating
S tested
for 4 h at 0.69
MPa:
(a) T = 25 “C
test disc resulted in its lower wear rate and lower coefficient of friction compared with the 25 “C test specimen (see Table 2 and Figure 4). The unworn and worn surfaces are shown in Fig. 9. The worn surfaces at 25 “C for the two contact pressures (Figs. 9(b) and 9(c)) appear very similar even though there was a major difference in their wear rates. The reason for this is not known. The 400 “C test specimens (Figs. 9(d) and 9(e)) had different degrees of a very smooth surface that were markedly different from the 25 “C specimen surfaces. Figure 9(e) shows that the whole surface was polished to a significantly finer finish than that of the untested surface (Fig. 9(a)). Whether the polished surface is smeared wear debris or a grounddown surface using wear debris as the grinding media, or both, is not known. 3.2.4. Coating C The coarse fused powder plasma-sprayed coating had a much higher wear rate than the R or S coatings at all but the 400 “C, 0.69 MPa test condition where all four Cr,Os coatings had similar wear rates (see Table 2). The wear tracks shown in Fig. 10 were much smoother at 400 “C than at 25 “C at the low contact pressure, but cracks developed in the coating within the wear track at 400 “C. The highly polished surface that developed at 400 “C (see Fig. 10(b)) can be compared with the coarser surface from the 25 “C test. The increased number of cracks in the wear track, compared with coating S at 400 “C, did not result in an increased wear rate (see Table 2). It appears that the wear rate at 400 “C is primarily a function of the surface smoothness and not the amount of cracking that occurs. This is not so at the higher contact pressure where cracking in the wear track contributes to the higher wear rates. The unworn and worn wear surfaces of coating C, as seen in Fig. 11, do not vary as much as those of coating S. The texture of the worn surfaces is considerably rougher than that of the unworn surface. At 6.9 MPa contact pressure, the surfaces appear to be rougher at 25 “C than at 400 “C (see Figs. 11(d) and 11(e)), and their wear rates are considerably different. The
(a)
(b)
(d)
(e) Fig. 9. Surface of (a) unworn coating S and surfaces tested (b) for 4 h at 25 “C and 0.69 MPa, (c) for 12 min at 25 “C and 6.9 MPa, (d) for 4 h at 400 “C and 0.69 MPa, and (e) for
20 min at 400 “C and 6.9 MPa.
smooth, polished surface which occurred at 400 “C and 0.69 MPa contact pressure can be seen in Fig. 11(c) in the region where one of the cracks in the wear track was located. There appears to have been some preferential
(a)
(b)
Fig. 10. Macrographs of wear tracks on coating C tested for 4 h at 0.69 MPa: (a) T = 25 “C and (b) T = 400 “C.
wear occurring in the crack zone, but it did not increase the steady state wear rate (see Table 2) at least for the relatively short duration of the test.
3.2.5. Coating F The fine fused powder plasma-sprayed coating had lower wear rates than the coarse fused powder coating (see Table 2). However, differences in the surface morphology of the wear tracks of the two coatings are not evident. The wear tracks of coating F in Fig. 12 are similar to those of coating C in Fig. 10. The surfaces of the wear tracks at all four test conditions are shown in Fig. 12. The unworn and worn surfaces of coating F are shown in Fig. 13. The same pattern of surface morphology as a function of test conditions as occurred on coating C can be seen in Fig. 13. More polished areas occurred in the 400 “C tests than in the 25 “C tests. The morphologies of all of the test surfaces are much more alike than their different wear rates would indicate. More indepth analysis is required to improve understanding of the lack of correlation between surface morphologies and wear rates for this coating. The higher magnification views of the unworn surfaces and worn surfaces at 6.9 MPa at 25 “C are shown in Fig. 14. The similarity between the two surfaces is striking when the high wear rate of the worn surface is considered.
3.2.6. Coating K The surfaces of the slurry-applied, SCA 1000, coating are shown in Fig. 15. The morphology of the tested coating in Fig. 15(b) is very similar to that of the untested surface. The pits in the untested surface (Fig. 15(a)) are interesting. The three tested surfaces (Figs. 15(c), 15(d) and 15(e)) all show the underlying steel substrate with a few pieces of coating remaining in Fig. 15(d).
(a)
(d)
Fig. 11. Surfaces of (a) unworn coating C and surfaces tested (b) for 4 h at 25 “C and 0.69 MPa, (c) for 4 h at 400 “C and 0.69 MPa, (d) for 4 b at 25 “C and 6.9 MPa, and (e) for 20 min at 400 % and 6.9 MPa.
4. Conclusions (1) All of the thermal-sprayed Cr,03 coatings were superior to the slurry-applied, mixed oxide coating by nearly an order of magnitude at 25 “C
(b)
(cl
(d)
Fig. 12. ~acrographs of wear tracks on coating F: (a) 4 h at 25 “C and 0.69 MPa; (b) 4 h at 400 “C and 0.69 MPa; (c) 12 min at 25 “C and 6.9 MPa; (d) 20 min at 400 “C and 6.9 MPa.
and two orders of magnitude at 400 “C at contact pressures of 0.69 and 6.9 MPa. (2) The 400 “C test temperature coefficient of friction of the Cr,O, coatings at unlubricated steady state conditions was 0.2 - 0.25, which correlates with the low wear rates of these materials. (3) The Cr,O, coatings had wear rates one or more orders of magnitude lower at 400 “C than at 25 “C at both contact pressures. (4) The flame-sprayed Cr,O, coating had the lowest wear rate of the Cr,03 coatings because of the high density of the individual splats. (5) At the contact pressure of 6.9 MPa the wear rates of Cr,O, coatings R, S, C and F were two orders of magnitude higher than at the low contact pressure, 0.69 MPa. (6) The R and S coatings had lower wear rates than the hard chromium plate at 25 “C. At 400 “C all of the Cr,O, coatings had lower wear rates than the chromium plate.
(a)
Cd)
(e) Fig. 13. Surfaces of (a) unworn coating F and surfaces tested (b) for 4 h at 25 “C and 0.69 MPa, (c) for 4 h at 400 “C!and 0.69 MPa, (d) for 12 min at 25 “C and 6.9 MPa, and (e) for 20 min at 400 “C and 6.9 MPa.
(4
(b)
Fig. 14. Surfaces of coating F: (a) unworn and (b) worn at 25 “C and 6.9 MPa for 12 min.
(4
(b)
(cl
(d)
Fig. 15. (continued).
Fig. 15. Surfaces of (a) unworn coating K and surfaces tested (b) for 4 h at 25 “C and 0.69 MPa, (c) for 4 h at 400 “C and 0.69 MPa, (d) for 1 min at 25 “C and 6.9 MPa, and (e) for 1 min at 400 “C and 6.9 MPa.
References 1 R. Kamo and W. Bryzik, Ceramics in heat engines, SAE Tech. Paper 790645, June, 1979, Dearborn, MI. on low heat rejection diesel engine development. Proc. 2 K. L. Hoag, A perspective DOE 1987 Coatings for Advanced Heat Engines Workshop, Paper H-9, DOE Conf. 870762, Castine, Maine, July 1987, U.S. Department of Energy, Washington, DC, 1987. et al., Thick thermal barrier coatings for diesel engines. Proc. DOE 3 T. M. Yonoshonis Coatings for Advanced Heat Engines Workshop, Paper III-II, DOE Conf. 870762, Castine, Maine, Jury 1987, U.S. Department of Energy, Washington, DC, 1987. Proc. DOE 4 D. I. Biehler, Thick thermal barrier coatings for diesel engine components. 1987 Coatings for Advanced Heat Engines Workshop, Paper III-1 9, DOE Conf. 870762, Castine, Maine, July 1987, U.S. Department of Energy, Washington, DC, 1987. 5 K. F. Dufrane and W. A. Glaeser, Wear of ceramics in advanced heat engine applications. In K. C. Ludema (ed.), Proc. Int. Conf. on Wear of Materials, Houston, TX, April 5 - 9, 1987, American Society of Mechanical Engineers, New York, 1987, pp. 285 - 292. sliding wear of ceramic materials. In K. C. Ludema 6 A. Levy and N. Jee, Unlubricated (ed.), Proc. Int. Conf. on Wear of Materials, Houston, TX, April 5 - 9, 1987, American Society of Mechanical Engineers, New York, 1987, pp. 459 - 476. 7 F. .H. Stott, J. Glascott and G. C. Wood, The sliding wear of in-situ formed oxides on iron and nickel-base alloys. Proc. NACE Conf. on Corrosion-Erosion-Wear of Materials at Elevated Temperatures, Berkeley, CA, January, 1986, pp. 267 - 277, NACE, Houston, TX, 1986. 8 R. B. Coers, L. D. Fox and D. J. Jones, Cummins uncooled 250 engine. Proc. SAE Int. Conf. and Exposition, Detroit, MI, February, 1984. coatings for 9 A. Levy, D. Boone, E. January and A. Davis, Sliding wear of protective diesel engine components, Wear, 101 (2) (1985) 127 _ 140. 10 A. G. Davis, D. H. Boone and A. Levy, Erosion of ceramic thermal barrier coatings, Wear, 110 (2) (1986) 101 - 116.