Impact Testing of Pure
and Doped Alumina
The NanoTest 600 low load impact testing option has been used to mechanically differentiate two alumina-based ceramic materials. This technique utilises sample oscillation to achieve pendulum recoil from the surface followed by diamond impact. Both the frequency and static normal force are variable, although in the present work fixed conditions were employed for both materials since the objective was to demonstrate a basic difference in behaviour.
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:
| Pure Alumina | Doped Alumina | |
| Mean depth increase due to impact/nm | 2442 | 376
|
| Mean depth increase due to erosive wear/nm | 42 | 9.6
|
