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 b
e remembered that such testing subjects only a small volume of materia
l to high stress and only a limited part of the flaw distribution is a
ctivated. With chemical vapour deposited (CVD) diamond, larger specime
ns 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 a
t present considerable expenditure of time and money in optimizing the
strength of CVD diamond. It is additionally important, in using a str
ength value, to have the essential application in mind. For example, i
f the strength in an erosion situation is important, the value obtaine
d 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. T
he friction of diamond is low, which makes it attractive. However, nat
ural diamond shows anisotropic effects and also coefficients of fricti
on which depend on the environment. Termination of dangling bonds by h
ydrogen or other adsorbates gives diamond its low friction. Moisture c
an 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 di
amond sliding on diamond are reviewed. The application of molecular dy
namics and atomic force microscopy to diamond surfaces offers exciting
prospects.