Lubricated Sliding

The monolithic ceramics were run self-mated with SAE-10W mineral oil, API Service SF/CC, lubrication at room temperature in an air environment to evaluate their sliding performance with a conventional lubricant. The speeds varied from 500 to 1000 rpm with progressive increases in ring loading from 9 N/mm to 25 N/mm. The lubricant was supplied to the sliding interfaces by drip feeding at a rate of approximately one drop per second (0.05 cm3/s) on each stationary specimen.

Both the silicon nitride and silicon carbide showed excellent performance in the room-temperature lubricated tests. With ring loads to 25 N/mm and speeds to 1000 rpm, the friction coefficient was in the range of

Figure 2. Edge chipping and surface cracks on YPSZ after dry sliding.

0.04 to 0.09. Wear on the ring specimens after 1.5 hours of operation was confined to 5 ^m over narrow contact bands, which became highly polished. Wear on the cylinder specimens was confined to local polishing of the protruding areas. The low wear and general insensi-tivity of friction forces to load were indicative of operation with hydrodynamic lubrication. Examination of the worn ring surfces showed them to be highly polished. Resolving details of the wear mechanism required examination by scanning electron microscopy at high magnifications, Figure 3. The wear process was a very fine abrasive polishing mechanism. No deep scratching or evidence of transfer was observed.

In contrast to the performance of the silicon nitride and silicon carbide, all three zirconias experienced problems in the room-temperature lubricated tests. The friction coefficient remained at levels of 0.13 to 0.16,

Figure 3. Polished aSiC ring wear surface run with mineral oil lubrication.

which was lower than experienced with dry sliding, but significantly higher than measured with the silicon nitride and silicon carbide. Visible hot spots were observed on the surfaces of the zirconia specimens, as shown in Figure 4. The hot spots moved across the contact surfaces and varied from distinct spots to streaks. This phenomenon, known as thermoelastic instability, begins with the local thermal expansion of frictionally heated areas on the contact surfaces (10,11). As the areas grow, they carry higher portions of the applied load and thereby are heated more intensely. The formation of such hot spots on a particular material depends upon the material properties, sliding speed, and friction coefficient. The low thermal conductivity and high thermal expansion coefficient of zirconia make it particularly susceptible to the formation of hot spots. The hot spots were also associated with more severe wear. Ring wear depths of 0.27 mm were measured after 1 hour of operation (compared with 5 fiim on the silicon carbide and silicon nitride).

Figure 4. Hot spots on YPSZ cylinder specimen at room temperature with mineral oil lubrication.

Consistent with the higher wear rates, the surfaces of the zirconias exhibited much more severe wear processes than those of silicon carbide and silicon nitride. Figure 5 shows the typical network of thermal-shock cracks formed on the contact surfaces of both the ring and cylinder specimens. Worn grooves were also present, which tended to broaden and eventually cover the width of the contact surfaces. At higher magnification, Figure 6, the edges of the cracks appear to have released small chips of debris. Some of these apparently were trapped between the sliding surfaces and caused the observed short grooves.

Experiments were also run at 260 C using a polyal-phaolefin (SDL-1) lubricant. Baseline experiments with chromium-plated rings arid cast iron cylinder specimens at 100 C showed a good correlation with actual field service. However, at 260 C the wear rates of the chromium plated rings were significantly higher. Therefore, evaluating the various ceramic coatings at 260 C provided an opportunity to compare their performance

Figure 5. Cracking, spalling, and wear grooves on YPSZ resulting from hot spots.

under marginal lubricating conditions. An operating temperature of 260 C was found to be the upper practical limit for the SDL-1 lubricant.

Wear Rate Calculations

The ring wear rates were calculated in terms of a wear coefficient to permit a direct comparison for.the various material combinations obtained at different load and speed combinations. The average wear volume for the two ring specimens was used in the calculation. Since the primary wear was found to occur on the ring specimens, the wear of the ring specimens was used as the measure of performance. The wear coefficients were calculated from the Archard wear equation (12):

where:

k = wear coefficient, p = hardness of wearing member, V = wear volume, L = applied load, and x = sliding distance.

A summary of the friction and wear results Is presented in Table 3. For a basis of comparison, the wear coefficient of the top piston ring in a conventional diesel truck engine was calculated. A wear depth of 0.25 mm was assumed for 6,000 hrs of service at an average ring loading of 18 N/mm. The resulting wear coefficient of 5 x 10 -9 is very low and would be associated with hydrodynamic lubrication. It provides the overall goal for the rings in advanced engines if the longevity of current engines is to be approached. An experiment at 100 C using a chromium plated ring and cast iron cylinder specimens resulted in a wear coefficient of 8 x 10-8. While slightly higher than the 5 x 10-9 from actual service, the result indicated that the apparatus was capable of reproducing the actual conditions sufficiently well to permit comparisons.

The best results at 260 C were obtained with ring coatings of.Jet-Kote sprayed cobalt-bonded tungsten carbide (WC) and plasma sprayed Cr203. Wear coefficients of 10-7, while higher than the baseline tests with chromium plated rings at 10-s, indicated wear rates that could be considered in actual engine service. The improvement obtained with these materials under the deteriorated lubrication conditions at 260 C can be seen by comparing the 10 ~5 wear coefficient of chromium plated rings and cast iron cylinder specimens at 260 C. Cylinder specimens of SCA-1000 (Cr203) and silicon carbide whisker-reinforced alumina (SiC/AI203) performed well with the WC and Cr203 ring specimens. Good performance was also obtained with a monolithic Si3N4 ring specimen operating against a Si3N4 cylinder specimen having a Ceraprep surface modification treatment. The treatment resulted in a reduction in ring wear coefficient from 3 x 10-6 to 1 x 10-6.

For comparison purposes, Table 3 also includes selected results with YPSZ and chromium plated rings operating against Si3N4. Wear coefficients of 10-4 obtained in these experiments would be unsatisfactory in practical engine applications.

Figure 6. Local spalling along cracks on MPSZ.
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