Strength, fracture and friction properties of diamond

Data on the strength, fracture and frictional properties of diamond in its various forms are reviewed. Most of the data for the strength of natural diamond have been obtained from indentation testing. It must be remembered that such testing subjects only a small volume of material to high stress and only a limited part of the flaw distribution is activated. With chemical vapour deposited (CVD) diamond, larger specimens are available and a wider range of test geometries are being used. In some of these, quite large volumes are stressed, and a much broader range of flaw sizes can be activated. It is essential not only to be aware of this, but also to quote, with any strength value, the precise geometry of the test. This is particularly important since there is at present considerable expenditure of time and money in optimizing the strength of CVD diamond. It is additionally important, in using a strength value, to have the essential application in mind. For example, if the strength in an erosion situation is important, the value obtained by indentation is probably more realistic. However, if it is for a ''window'' in an aerospace application, the strength value from a burst test, which stresses a larger volume, is probably more appropriate. The friction of diamond is low, which makes it attractive. However, natural diamond shows anisotropic effects and also coefficients of friction which depend on the environment. Termination of dangling bonds by hydrogen or other adsorbates gives diamond its low friction. Moisture can reduce the friction even further under certain conditions. Provided that CVD diamond surfaces are smooth, the friction coefficient can be as low as for natural diamond, and moisture has a similar effect. The factors affecting the mechanisms of energy loss in the friction of diamond sliding on diamond are reviewed. The application of molecular dynamics and atomic force microscopy to diamond surfaces offers exciting prospects.