As for the deformation by the coefficient of thermal expansion, no difference in deformation of 0.2 μm can be generated unless the contact interface is of the order of several thousands K. Thus, it may be concluded that the molybdenum phase low in hardness was selectively worn as a result that the attacking effect to the counter surface by fine asperity such as the ceramics phase high in hardness which is the main composition and the abrasive effect by foreign matters such as free wear debris are multiplied.
3.2.4 Results of Statistical Processing of Surface Roughness
Two kinds of the replicas of the sliding surface of the actual piston ring were sampled from the normal worn surface on which the initial break-in condition was completed, and the worn surface in which abnormal wear was generated, the surface roughness was statistically processed, and the quantitative interpretation on the phenomenon of the break-in condition was examined.
Fig. 9 Comparison of normal and abnormal wear surface of actual piston ring
Upper side： three dimensional SEM images
Lower side： results of statistic treatment of surface profile
The surface profile of the sliding surface is accumulated for each small threshold in the height direction and the so-called Abbot's load curve is obtained. The differential curve of this curve is the roughness distribution curve. This indicates the probability that the sample surface is present in the height direction, and the probability for the ideally machined surface is formally distributed. Fig. 9 shows the roughness distribution curve obtained by the above-mentioned method. (a) is a normally worn surface while (b) is an abnormally worn surface. What is noted here is that the projection and recess of the actual surface is reversed by the replica.
Firstly, both roughness distribution curves show the uneven distribution in which one side of the normal distribution is cut off, and the left side to the peak, i.e., the recessed part is small. This means that the projected area is small in the actual surface. It can be interpreted that tips of the projections of the worn surface were preferentially cut off by the wear. In the roughness distribution curve of the normally worn surface in (a), the peak is sharp, and the probability of presence of the height is concentrated while, in the worn surface having the abnormal wear in (b), the peak is dull, and the probability of presence of the height is relatively distributed.
