Wear, 121 (1988)
363 - 380
SLIDING WEAR OF CERAMIC MATERIALS*
ALAN V. LEVY and NANCY JEEt Lawrence
Summary The potential use of thermal insulating ceramic coatings on the cylinder walls of diesel engines at elevated temperatures requires that an understanding of the operative friction and wear mechanisms be gained. An investigation was carried out to determine the unlubricated sliding wear behavior of a thermal insulating oxide coating in combination with a number of carbide, boride, and mixed oxide coatings at room and elevated temperatures to 730 “C. The coatings were applied up to several hundred micrometers thick by plasma spray and chemical vapor deposition (CVD) processes on flat washers and discs and tested at contact pressures up to 6.9 MPa to steady state wear conditions in a rotational reversing motion. Wear rates, coefficients of friction and wear surface compositions and morphologies were determined. It was found that the binder metal in sprayed metalcarbide composites had a dominant influence on the wear, more so than the composition of the carbide. Different wear surface morphologies and attendant wear rates occurred at each of the test temperatures. The shape of the deposited grains in the CVD coatings had a major effect on the wear rates; rounded grains being better than angular grains of the same material. A film formed on the T&-mixed oxide wear pairs that could have a significant effect on their wear behavior.
1. Introduction The use of ceramic thermal barrier coatings on the surfaces of diesel engine components to enhance performance will result in higher gas temperatures in the combustion zone area that could adversely affect the upper portion of the cylinder wall. The use of protective coatings on the cylinder wall will require that it and the mating surface of the piston ring exhibit acceptable reciprocating sliding wear behavior. The purpose of this investigation was to determine the nature of the behavior of several promising coating materials in unlubricated sliding wear at room and elevated temperatures. *Paper presented at the International Conference on Wear of Materials, Houston, TX, U.S.A., April 5 - 9, 1987. +Present address: Smith International Inc., Irvine, CA, U.S.A. 0043-1648/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
The study was exploratory and tested a comparatively wide range of materials in a manner where some understanding of the operative wear mechanisms could be determined as well as the measurement of their wear rates and coefficients of friction. The work concentrated on relating the morphology of the worn surface to the amount of material that was removed, the removal mechanism and the dynamic coefficient of friction, rather than primarily on a ranking of their performance. The materials discussed herein were among those studied in a larger program that determined the behavior of 10 different material pairs [ 11. At least one of each type of coating tested is discussed in this paper. The type of wear test used wore flat annular surfaces against each other in an oscillating or continuous rotational mode. The contact pressure and surface area stayed constant through the life of the test. The sliding wear literature, particularly studies made at elevated temperatures, primarily reported on pin-on-disc specimen geometries. Because the pin undergoes a considerable change in contact surface geometry and pressure distribution during a test it was felt that pin-on-disc test results did not relate to the steady state wear behavior obtained in the washer-on-disc tests reported herein. The test results are not intended to be used to rank coatings or predict wear behavior of components in diesel engine service. They generally represent only two tests at a given test condition which is not considered a sufficient number to predict wear rates. Rather they describe regimes of behavior in terms of active wear mechanisms. Since the tests were run in an unlubricated state, the behavior of the materials tested may be quite different when lubrication is used.
2. Test conditions The tests were carried out using a Falex 6 wear tester which slides a narrow surface washer, 1.5 mm X 2.5 cm outside diameter, against a wider surface disc, 8 mm X 3.2 cm outside diameter, in either a rotational or reversing oscillatory mode. The test is described in ASTM standard D-3702-78. The rotational mode was used for the coefficient of friction tests and the oscillatory mode for the wear rate tests, The test area on the Falex 6 tester and the two test specimens in each pair are shown in Fig. 1. The protective oxide coating on the disc for all of the tests was a proprietary slurry applied and subsequently impregnated chromia-silicaalumina coating designated SCA 1000. This material is a candidate for use on the cylinder walls of diesel engines. The coating is basically a porous mixed oxide that is painted, sprayed, or dipped on to the surface to be coated in a water slurry. The water is then driven off by baking in a furnace at 525 “C. The resulting porous coating is repeatedly impregnated with chromic acid which is thermally transformed to chromia, resulting in the dense mixed oxide coating that is approximately 75 @rn (0.003 in) thick.
Fig. 1. Falex 6 wear tester and specimens.
The coatings on the wear surface, 1.5 mm wide, of the washer were an attempt to simulate the contact surface of a piston ring. They were applied by plasma spray or chemical vapor deposition (CVD) processes. The specific composition and process details of the plasma spray coatings, which were sprayed on 1018 steel, were considered too proprietary by the coating supplier to be disclosed. The CVD TiB2 coating was applied on a WC-Co substrate at 900 “C using a gas mixture of HP, BCIs, and TiCI in an 80 min decomposition cycle. The specimens were wear tested in a dry unlubricated condition using 90” reversal oscillatory motion at 250 cycles min-’ at contact pressures from 0.17 to 6.9 MPa (25 to 1000 lbf ine2). Test time periods were up to 8 h at three test temperatures; 25, 425, and 730 “C. The weight losses were determined on the 25 and 425 “C test specimens using a balance that measured to 0.1 * 0.4 mg. The friction coefficient tests were run on single specimens, periodically changing the test temperature and stabilizing it before making each successive measurement using the load cell system on the Falex 6 machine. The friction tests were run using the constant rotational mode of the machine at 200 rev min-‘.
After testing and final weighing, the specimens were prepared for detailed metallographic examination of the wear surface and cross-sections using optical and scanning electron microscopes with energy-dispersive X-ray (EDX) attachment, to determine the surface chemistry changes which occurred.
3. Results and discussion 3.1. Carbide coatings The wear test results for the materials selected for analysis in this paper are listed in Table 1. All of the plasma-sprayed coatings on the washers had nearly the same wear losses after 15 min of testing at 25 “C. There was only a minor effect on the wear rate of a lo-fold increase in contact pressure for the Cr,C$-Mo coating. The other carbides and the mixed oxide coatings exhibited similar behavior. The common coating on the disc, SCA 1000 mixed oxide, wore at a higher rate than its washer pairing for all of the materials tested. The discs sliding on all of the carbide coatings at the 0.69 MPa contact pressure had essentially the same wear rate at 25 “C. In the 425 “C tests, the washer coatings wore at a higher rate than they did at 25 “C, wearing through in 2 min in the case of the Cr&+Mo material. The TiBz rounded grain coating had the lowest wear rate of all of the materials tested at 25 “C. The low wear rate of this material resulted in a low wear rate on its mating SCA 1000 disc. The wear losses shown in Table 1 related to the morphologies of the worn surfaces, as will be presented later. Longer time tests, up to 8 h, indicated that the wear reached a steady state rate after approximately 1 h of testing, see Fig. 2. Initially, a higher rate occurred which approached the steady state rate after 10 - 15 min of testing. The steady state rate remained constant until incipient wear through of either the washer or disc material occurred.
t % 2 -_u
* b x
tl s 5 -c 12345678
Fig. 2. Longer term wear test of WC-MO washer on SCA disc at 0.69 MPa and at 25 “c.
TiB2 - nodular grains
TiB2 - rounded grains 0.03
Material wear rates
Test temperature (g=)
Contact pressure (MPa(lbf iK2))
Exposure time (min)
Wear rates (g cm-’ 25 x10-‘)
Washer and disc wear tracks polished Washer and disc wear tracks polished Wore through washer Washer and disc tracks polished Washer surface speared molybdenum on disc surface Washer and disc wear tracks polished Disc wear track polished Washer and disc wear tracks polished Disc wore through at end of test Washer locally polished
3.2. Coefficient of friction The measured coefficients of friction in the unlubricated condition are listed in Table 2. Two of the values obtained were obviously incorrect and were left off the table as there were no additional specimens to rerun. Overall, the measured values of the unlubricated friction coefficient were high and fluctuating, indicating that these materials, in the forms tested and in the unhrbricated condition, would not be suitabIe for service on the cylinder wall liners and piston rings of diesel engines. The measured values of friction coefficient followed some patterns, but with discrepancies. For example, both the WC-MO and the Tic-Mo coatings had lower friction coefficients as the test temperature increased to 245 “C after which the values increased considerably. The reduction in friction coefficient corresponds to the super polishing of the coating which occurred in the 25 “C tests [l]. The subsequent increase may be related to the roughening of the surface observed in the 425 “C tests fl]. However, the Cr3Cz-Mo continued to have lower friction coefficients at test temperatures to 540 “C even though its surface morphology was similar to those of the other two carbide coatings tested [l]. The two mixed oxide coatings, Al,Os-TiO, and SCA 1000, had similar patterns of friction coefficient us. temperature wherein the values increased as the temperature increased to 540 “C!. The friction coefficient of the SCA 1000 coating did not vary much with test temperature, compared with the other materials. This behavior correlated with the surface mo~hology of the SCA 1000 coating [I] which remained similar in appearance at 25 and 425 “C. Thus, of the plasma spray coatings tested, the carbide-metal materials had
Dynamic Temperature (“C) 19 132 245 354 466 540 460 351 243 130 LLcy
Dynamic coefficient Cr3C2-MO 1.20 1.16 1.16 1.03 0.95
0.86 0.69 0.69 0.73 0.82 +0.05 -
WC -MO 1.26 0.53 0.42 0.58 0.74 1.26 1.05 0.84 0.95 1.10 0.74 kO.05
_0.55 _0.79 - 0.95 - 1.05
Tic-Mo 1.04 0.30 0.45 0.86 0.98 1.05 1.05
0.98 0.90 0.90 0.90 kO.07
0.82 0.88 0.85 0.94 1.00 1.06 1.03 0.79 0.68 0.76 0.62 rto.03
0.77 0.89 1.11 1.24 1.32 1.08 1.08 0.93 0.77 0.85 to.08
0.59 0.85 0.85 0.81-0.87 0.88 0.90 - 0.81 0.92 0.88 -0.79 0.8i -0.76 0.81 0.63 co.04
of reading aTest conditions:
tion; all discs, SCA coating.
25 lbf in-2 ; revolving
at 200 rev min-‘;
30 - 60 s dura-
decreasing friction coefficients with increasing temperature while the mixed oxides had increasing friction coefficients with increasing test temperature. The CVD TiBz followed the pattern of the mixed oxides in that its friction coefficient increased with temperature. On the way down from the maximum test temperature, the friction coefficients generally decreased, compared with their values at 540 “C. The materials did not have as large a spread in values of friction coefficients as occurred during the increasing temperature part of each test. The reason for this behavior may be related to the surface morphology of the materials as they are worn smoother at successively lower temperatures. The friction coefficients of the carbide and boride materials had lower values, in coming down to the final test temperature of 36 “C, than their starting coefficients at 19 “C. The friction coefficients of the mixed oxide materials behaved in the opposite manner, being lower at the initial 19 “C than at the final 36 “C! test.
3.3. Metallographic analysis The worn surfaces of the materials exhibited characteristics which helped to explain the likenesses and differences between their wear rates. For example, Fig. 3 shows the as-honed and post-tested surfaces of three carbide plasma spray coatings. The 25 “C tests appeared to super-polish the wear surfaces, removing their honing marks and sharply distributing the metallic and carbide phases. All three materials had similar distributions of carbide particles and metal binder as can be seen in Fig. 4. The metallic binder, particularly the molybdenum, predominated on the washer’s wear surface with relatively few carbide particles present. It appears to be primarily the wear behavior of the molybdenum, and to a lesser extent the wear behavior of the Ni-Cr binder metals, that determine the wear behavior of the coating system. Thus all three carbides had very similar wear rates, as can be seen in Table 1. The higher wear rates that occurred at 425 “C were due to a change in the wear mechanism. Figure 5 shows that the surface of the WC-MO washer was entirely different from its 25 “C test morphology. The surface was torn and smeared with essentially all of the WC particles embedded below the wear surface. Molybdenum had smeared over the whole surface and there was essentially no distribution of separate phases as was observed in the 25 “C tests. The wear surface of the SCA 1000 mixed oxide coating on the mating disc, shown in Fig. 6, had a considerable amount of molybdenum transferred from the washer smeared on its surface as well. While no wear rates were determined at a test temperature of 730 ‘C, because oxidation of the sides of the 1018 steel washer negated weight loss measurements, the worn surfaces were microscopically analyzed. Figure 7 shows the washer and disc surfaces from a 730 “C test. Molybdenum oxide formed on the washer’s surface while a majority of the disc’s surface was covered with smears of molybdenum transferred from the washer.
Fig. 3. Untested and tested surfaces of washer.
The wear surface of the carbide-MO plasma-sprayed coatings on the washers had a significantly different surface morphology at each test temperature. While only the WC-MO coating is shown in Figs. 4, 5, and 7, the
Fig. 4. Micrograph and X-ray maps of the surface of WC-MO washer from 25 “C!test.
Cr3C2-Mo and Tic-Mo coatings were very similar in appearance. The binder metal was the primary constituent on the worn surfaces and determined the amount of wear which occurred on both the washer and the disc surfaces.
Fig. 5. Micrograph and X-ray maps of the surface of WC-MO washer from 425 % test.
Whether it is even necessary to use a hard carbide in these types of coatings is questionable. Also, the potential of sulfur in the combustion gases, transforming the molybdenum into an MO& solid film lubricant, is an intriguing possibility. Without such a reaction, to reduce the comparatively high coef-
~em~erature=425*C Contact pressure=O.I7MPa
from WC-MO washer
Fig. 6. Micrograph and X-ray maps of the surface of the SCA 1000 coating from 425 “C test.
ficient of friction which occurred, the carbide-MO plasma spray coatings do not appear to be feasible for use on the sliding surfaces of piston rings in an unlubricated condition.
Fig. 7. Surface of washer and disc of WC-MO - SCA 1000 pair tested at 730 “C.
3.4. Ti& CVD coating Several TiBs coatings on WC-Co washers were tested that had a range of mo~holo~es from Roth-s~~~ nodules to smooth rounded grains to sharp faceted grains of different sizes. The nodule and rounded grain versions of the TiBz discussed herein had the best performance of those tested. The TiB2 coatings on the washer were tested against the SCA 1000 mixed oxide coating on the discs for 60 min. 3.5. Nodular TiBz The incremental wear rate of the TiBz coating, designated CR554 on the washer sliding on the SCA 1000 coating on the disc, is plotted in Fig. 8. It shows typical curves for hard brittle materials El] with a low steady state wear rate of 0.01 g cm -* s-’ for the TiBz washer and 0.035 g cmm2s-i for the SCA disc. The weight gain of the washer at the beginning of the test was due to the transfer of SCA disc material to the washer surface which formed a film over portions of the washer surface. The TiB2 coating had an asdeposited surface that consisted of small rough-surfaced nodules covered over in places by small flakes of a thin scale
Fig. 8. Incremental wear curves of TiBz - nodule test (upper washer, TiBz number CR 554; lower disc, SCA 1000). Test description: Falex 6 thrust washer tester; unlubricated oscillatory motion at 200 cycles min -‘; 90” reversals; temperature, 25 “C; contact pressure, 0.17 MPa; l, washer; 0, disc.
as shown in Fig. 9, top micrographs. After the test, the washer surface had the appearance shown in the lower two micrographs. A film of varying thickness had formed over a portion of the specimen’s wear surface while the remainder of the TiBz surface maintained the initial nodule morphology without the flakes of scale on it. The specific composition of the surface in areas where the film occurred could not be determined either by EDX or X-ray diffraction. The elements that could be identified included titanium and some silicon, chromium, and aluminum transferred from the SCA 1000 mixed oxide coated disc. The surface area of the washer shown on the lower right side that did not have the film on it is a cleaner nodule version of the as-deposited surface. Some TiB2 was transferred to the SCA 1000 disc. The film that formed on the disc surface was analyzed in more detail. Figure 10 shows that the film varied in levels of continuity, morphology, and thickness. The EDX peaks in the area of the disc’s surface where the film was located show that it contained titanium and the three constituents of the SCA disc material, chromium, silicon, and aluminum. The larger, thicker, smoother segments of the film, EDX peak number 1, had higher concentrations of the three major constituent elements in the washer and disc coatings, i.e. titanium, silicon, and chromium, while the thinner areas, EDX peak number 2, had a lower titanium content, as did the overall composition of the surface analyzed. The higher titanium content film areas were rather severely cracked. Whether the cracks were present during the wear process or whether they formed on cooling after the tests is not known. 3.6. Rounded grain TiBz The wear rate of the TiBz coating designated number 557 was a low 0.004 g cm-’ s- ‘. The SCA 1000 disc had a wear rate of 0.06 g cm-* s-l.
Fig. 9. Surface of TiBz number 554 before and after testing: temperature, pressure, 0.17 MPa.
25 “C; contact
Figure 11 shows the morphology of the TiBz wear surface, along with EDX peak analyses. Micrograph 1 shows the rounded grains of this coating that were identical to those on the untested surface. Only TiBz was present. This coating was processed for twice the deposition time used for coating number
Fig. 10. Surface of tested SCA 1000 diic in area of film on the surface: temperature, 26 “C; contact pressure, 0.17 MPa.
554 as well as having considerably different gas compositions in the retort, which probably accounted for the difference in their surface morphologies. Micrograph 2 shows a mixture of a thin film and the initial TiBz rounded grains. The EDX peak count was not sufficient to pick up the small amount of silicon and chromium transferred from the disc that were contained in the film in this view. View 3 has a thin but more extensive film and its EDX peak shows the silicon and chromium in it. This film is much thinner than that shown in Fig. 10, as shown by its translucency, whereby the texture of the rounded facets of TiB2 can be seen through the film. The low silicon and chromium peak heights are evidence of their being only a thin film on the surface. Some titanium had transferred to the SCA disc. Figure 12 shows the surface of the disc where titanium was identified. As in the case of the washer, the presence of titanium, silicon, and chromium was indicated by
Fig. 11. Surface of TiB2 number 557 after testing: temperature, 25 “c; contact pressure, 0.17 MPa.
the occurrence of a cracked film on the surface of the disc. The high silicon and chromium contents indicated by the EDX peaks are due to the SCA disc itself and not the formation of a thick film as was shown in Fig. 10.
Fig. 12. Surface of SCA 1000 disc in area of surface film: temperature, pressure, 0.17 MPa.
25 ‘32; contact
The compound containing relatively high amounts of titanium, that also contained silicon and chromium, formed at various locations on both the T&-coated washer and SCA-coated disc surfaces. It had different morphologies at different test temperatures and covered various percentages of the wear test surfaces but it occurred at all test conditions on all surfaces. The compound could not be identified by X-ray diffraction. Other analytical techniques that could identify lower atomic number elements such as boron were not used. It was speculated that the compound was B20s with titanium, silicon, and chromium dissoived in it. BzOs melts at 450 “C, which could be achieved
on the wear surface, particularly if elements such as the identified titanium, silicon, and chromium, which are reported to be soluble in B203, lowered its melting temperature. The film-like appearance of the compound, particularly in the low temperature 25 “C tests, indicated that melting and resolidification could have occurred. If the reaction that formed the film is detrimental to the wear behavior, it could mean that TiBz as a hard wear-resistant coating is not chemically compatible with an oxide ceramic thermal barrier coating. If, alternatively, the formation of the compound improved the wear resistance by acting as a lubricant or as a super smooth surface, the formation of the compound would be desirable. 4. Conclusions (1) The coatings tested in an unlubricated condition had room and elevated temperature wear rates and coefficients of friction that were too high to be practical for use in a diesel engine. (2) The coatings reached a steady state wear rate after approximately 1 h of testing and the wear rate remained constant until incipient failure of one or both of the surfaces occurred. (3) The wear rates of the carbide and mixed oxide coatings increased as the test temperature increased and their surface morphology and wear mechanism changed. (4) The metallic binder, particularly molybdenum, dominated the surface of the carbide coatings, markedly reducing the effect of the hard carbide phases on the wear behavior. (5) The directions of the changes in the friction coefficients could be roughly related to the morphology changes which occurred on the worn surfaces of the carbide and mixed oxide coatings. (6) The rounded grain TiBz coating had the lowest wear rate, at 25 “C, of all of the materials tested. (7) A compound, which formed on both the TiB? and the mating SCA 1000 coating surfaces, probably played a major role in the wear behavior of the pair. Acknowledgment This work was supported by Martin Marietta Corporation, Oak Ridge National Laboratory, work order number 34667-5225 (AC423GAl), through an agreement with the U.S. Department of Energy under contract number AC03-76SF00098. Reference 1 A. V. Levy and N. Jee, Elevated temperature sliding wear of ceramic and hard metal coating, Corrosion ‘86, Houston, TX, March 1986, National Association of Corrosion Engineers, Houston, TX, 1986, Paper 112.