Strength of free-standing chemically vapour-deposited diamond measured by a range of techniques

Abstract

The strength of free-standing chemically vapour-deposited diamond was determined using a bursting-disc test, a three-point bend test and a four-point bend test. The bursting-disc technique has advantages for determining the strength of ceramic materials since the maximum tensile stress is at the centre of the disc. The much smaller stresses near the sample edges are rarely responsible for failure and so edge effects are negligible. However, the bursting-disc test is often impractical for ascertaining the strength of chemically vapour-deposited diamond since the cost of samples is high. A three-point bend test is often employed instead, since it may be performed on smaller samples. However, the area stressed in the three-point bend test is smaller, and so there is less chance of stressing a large flaw, resulting in higher strength values. A further disadvantage is that specimen edges are stressed and, unless the specimen sides are polished, as well as its faces, misleading results may be obtained (although this is not the case for chemically vapour-deposited diamond). A four-point bend test is often used because it stresses a larger volume of material than the three-point bend test and so the results are more reliable. Furthermore, the diamond used in a four-point bend test can be of similar size to those used in a three-point bend test and so the expenditure is comparable. However, ensuring that the four-point bend test is applied correctly is technically more difficult. Strength data are reported for both the growth and the nucleation sides, for difference nucleation densities and for as-received and polished samples.

Acknowledgements

This research has been supported by grants from Element Six Ltd. R. Marrah (Cavendish) is thanked for his technical support.