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Experimental
The samples selected for investigation were:
Sample 1, Pure Alumina, wear rate 42 nm/s
Sample 2 Alumina Doped, with MgO and SiO2 wear rate 9.6 nm/s
The samples was mounted together on the
same metal stub by means of a cyanoacrylate adhesive. Prior to
testing, the surfaces were cleaned with acetone followed by alcohol,
and finally blown dry with "Air Duster".
A 25 µm radius spherical diamond and an oscillation frequency of 80 Hz
were used for all tests. The instrument was programmed to apply a
constant normal load of 25 mN.
Before beginning the sample oscillation, the diamond was left in
contact for a short period with the 25 mN load applied in order to
provide an initial reference position (range 1 in the first figure).
The oscillation was then started with a relatively low amplitude
(range 2) before being increased to a higher amplitude (range 3).
Finally, the oscillation was removed to provide a final, static depth
measurement (range 4).
Results
Figures 4 and 5 show the diamond displacement data before, during and
after impacting for the pure alumina sample. Figures 6 and 7 show the
corresponding data for the doped alumina. The total time for each
experiment was approximately 10 min.
Conclusions
1. The pure alumina showed significant depth increases during both
impact periods.
2. Figure 5 is particularly interesting. Here, in both the low and
high amplitude ranges the depth remained constant for an initial
period until it abruptly began to increase. This is similar behaviour
to that observed for fused quartz, where it appeared that a certain
level of microscopic damage was necessary before significant material
removal occurred.
3. The occurrence of an initial plateau in Figure 5 confirms that
impact damage rather than plastic deformation was the mechanism
responsible for the depth increase. Note also that the depth increase
during the 25 mN hold period at the beginning was negligible.
4. From the data density in Figures 4 and 5, it can be seen that the
pendulum recoil behaviour changed after the depth increase.
Essentially, at larger depths the average recoil amplitude was
smaller, indicating that the damaged contact point made energy
transfer to the pendulum more difficult. This has also been observed
with glass.
5. For the doped alumina (Sample 2), the pendulum recoil behaviour was
uniform during the initial impact period, and indicated only a very
small depth increase during the subsequent, higher amplitude impact
period.
6. The static depth increases (region 4 - region 1) after impact were: |
