Comparing both surface images, the worn surface with abnormal wear is more clearly roughened, while in the normally worn surface in which the initial break-in condition is completed, many smooth regions are distributed. As for the quantitative expression of the advancement of the break-in condition by the statistical processing, Sugimura et al. tried the geometrical expression of the break-in process by the "break-in equation"4).
In this expression, the advancement of truncation, i.e., the process that the wear is preferentially advanced at the tip of projections of the worn surface as mentioned above, can be expressed by the simulation using the equation.
When the surface profile is changed by the wear, the peak gradually approaches the prescribed value irrespective of the initial value, and this phenomenon is verified by the agreement of both of the calculation and the test results.
In addition, in the simulation to obtain the surface roughness (the density function) in the stationary condition with the advanced break-in condition, the results are obtained that the peak gradually approaches the prescribed value irrespective of the initial value of the calculated value, and the gradually approached value is determined by the granularity distribution and the shape of the wear debris. As a result by Finkin in which similar trend was obtained experimentally, the theoretical support is given to the results5) that the surface roughness of the worn surface reaches the prescribed balanced value, and the value corresponds to the mean grain size of the wear debris.
The roughness distribution curve in Fig. 9 clarifies that projected parts are reduced in number in the normally worn surface where the initial break-in condition is completed, the peak of the curve becomes sharp, projected parts become dull in the peak though they become smaller in number when the surface is roughened, and recessed parts are increased in number. In the consideration together with these findings, the advancement of the break-in condition in the spray coated ceramics is considered to be the process in which tips of projections are preferentially cut off similar to that of the metal.
The condition of the advancement is obtained by the statistically processed results in which the differential curve is obtained from the Abbot's load curve to be obtained by accumulating the data on the surface roughness of the worn surface by each small threshold in the height direction, and the quantitative interpretation is considered to be possible from the results. That means, when the break-in process in the test is viewed from the surface roughness, it is presumed that the break-in condition of the composite material of ceramics and metal is advanced in the same process as that of metal.
In addition, from the results of the partial contacting wear test in the present test, it seems effective to share the load under the partially high bearing pressure through the partial contacting and to reduce the degree of the load on the hard film by the coating which is relatively lower in hardness such as Ni-P + PTFE plating in order to improve the initial break-in condition of the hard film such as the spray coated ceramics.
4. Conclusion
The newly devised partial contacting wear test was carried out using the spray coated ceramics and the spray coated ceramics was compared with the Ni-P + PTFE plating selected to improve the initial break-in condition. In addition, to quantitatively interpret the break-in surface, the sliding surface after the partial contacting wear test was observed, and examination was made employing various kinds of observation methods. The obtained results are summarized as follows.
(1) The partial contacting wear test method seems to be suitable for the test method to reappear the film separation or abnormal wear by the partially high bearing pressure contact of the actual piston ring.
(2) The initial break-in condition of the spray coated ceramics is largely improved by coating the Ni-P + PTFE plating.
(3) From the results of the observation of the sliding surface after the partial contacting wear test, it is proved the following characteristic differences are generated in the judgment of the surface profile dependent on only the observation by the optical microscope, possibly due to the peculiarity of the spray coated ceramics. That means, recessed pans such as cracks small in the light reflection look fairly deep in the observation by the optical microscope, but the compositions of sulfur and phosphor are left behind, and the recessed parts are fairly shallow actually. The molybdenum phase with metallic lustar looks projected in the observation by the optical microscope, but in reality, slightly lower than the ceramics (chromium oxide) phase.
(4) From the results of the EPMA analysis, sulfur and phosphor derived from the extreme-pressure additive in the lubricating oil is less in the sliding surface, and it is considered difficult to form the extreme-pressure film on chromium oxide or molybdenum.
