The wear rate was measured as a function of depth using pin-on-disk wear tests by first measuring the wear depth vs. the number of cycles. The results of these tests are shown in Fig. 2. There are two regions in this figure. Early in the test (until the wear depth was about 110 nm), the wear rate was relatively low and decreasing slightly with depth. Then there is a dramatic increase in the wear rate near the end of the test. In terms of the fraction of the life of the implanted layer, the first region consumes about 90%of the total number of cycles to failure (where failure is defined as the number of cycles required to produce a wear depth of 200 nm) and the second region consumes the remaining 10%. It should be noted that these results are for a variety of loads and spin rates, each of which provide a different wear rate. Appropriate normalization allows us to put all these cases on a universal curve, allowing general conclusions to be made independent of the actual wear rate.
In order to estimate the wear rate, the data in the first (low wear
rate) region was fit to a polynomial. The wear rate at an
appropriate depth was then determined to be the slope of this curve.
This provides a wear rate with units of depth, because the number of
cycles was normalized. That is, the wear rate is taken to be
where is the depth of the wear track,
is the number of cycles
required to produce that depth, and
is the number of cycles
required to produce a wear depth of 200 nm. These wear rates will be
compared to hardness and nitrogen concentration profiles in a later
section.
The dramatic increase in the wear rate at depths greater than 110 nm is expected, because this depth is beyond the range of the implanted ions and one would expect wear rates on the order of the wear rate of unimplanted Ti-6Al-4V. The explanation of the behavior at small depths is less obvious. One would expect a low wear rate throughout the implanted layer, but Fig. 2 shows a decreasing wear rate with increasing depth. This may be due to break-in effects, as have been seen in other studies [17].