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Diamond Physics

Research into the friction and polishing/wear rate of diamond has been an active field of research for more than sixty years in universities around the world yet there is still no comprehensive picture of the detailed mechanisms involved (diamond on diamond) nor a fundamental explanation of the observed anisotropy. Recent work conducted in this laboratory has highlighted the importance of adhesion and suggested that a possible surface shear-induced phase transition (sp3-sp2) may be responsible for the easy removal of material in certain directions. This idea was put forward after analysis of polishing debris using electron energy loss spectroscopy (EELS). How this process might compete with a micro-fracture mechanism still remains unclear and new research is concentrating on the influence of surface chemistry, the detection (or not) of a phase transition and spectrographic analysis of light emissions at the diamond-diamond interface.

Some properties of diamond exhibit marked anisotropy e.g. friction coefficient, compressive and tensile strengths, polishing/ wear rate, the existence of a preferred fracture plane. The fundamental aims of our research on this topic are to describe and explain the precise nature of these processes, including the anisotropy. The research is largely experimental but there is a long history of collaboration with theoretical groups. Links with industry are very strong and a great deal of applied research is conducted which tackles important industrial problems.

Our current research in this area is on the strength-fracture, shock, erosion and frictional properties of natural, synthetic and CVD diamond.

 

High-speed impact sequence (5µs inter-frame time) of a 1mm sapphire ball impacting onto the (100) surface of a single crystal diamond at 110m/s.

Cone crack viewed in plane polarised light.

 

Our research into the fracture of diamond has concentrated on the role of the intrinsic anisotropy in crystal strength in causing a preference for fracture of one plane/direction over another. The elastic constants, in particular the Poisson ratio(s) vary considerably with direction in the crystal and we are beginning to examine how this affects an applied stress-field on the scale of a crack-tip. It has been well known for centuries that diamond exhibits a symmetry-related set of preferred fracture planes- the easy cleavage planes or {111}. The dominance of this type of plane over all others, in the brittle failure of diamond, is very great and has hitherto been explained using very basic surface energy arguments.

When this type of calculation is performed for a number of crystallographic planes it becomes clear that the {111} surface energy is indeed the lowest. However, the energies for many of the other planes calculated in this way are apparently too close to explain the predominance of {111} fracture in diamond, particularly since the energies that are supplied to a crystal in order to break it are often much larger than the necessary minimum. Ab initio quantum-mechanical calculations of a stretched diamond crystal are currently being made in order to quantify differences (with direction) in the intrinsic crystal strength and take account of the energy for bond-bending as well as just the stretch. This extra contribution to the energy required to stretch the diamond crystal normal to planes other than {111} is thought to be quite considerable as the diamond lattice is known to strongly resist this type of distortion.

Research is also being conducted on the erosion, fracture and polishing of chemical-vapour-deposited (CVD) polycrystalline diamond. This material is rapidly becoming an important commercial product and recent work in the group has been to characterise its mechanical properties/fracture behaviour. One such commercial application is as a window both for high power lasers and also for infrared imaging.

Further reading:

2009 Zaayman E., Morrison G. and Field J.E. "Edge flaking in diamond" Int. J. Refract. Metals Hard Mater. 27 409-416

2006 Willmott G.R. and Field J.E. "A high-speed photographic study of fast cracks in shocked diamond", Philos. Mag. 86 4305-4318

2005 Davies A.R., Field J.E., Takahashi K., Hada K. "Tensile and fatigue strength of free-standing CVD diamond" Diamond Related Mater. 14 6-10

2004 Davies A.R., Field J.E. "The strength of free-standing CVD diamond" Wear 256 153-158

2004 Davies A.R., Field J.E., Takahashi K., Hada K. "The toughness of free-standing CVD diamond" J. Mater. Sci. 39 1571-1574

2003 Willmott G.R., Proud W.G. and Field J.E. "Shock properties of diamond and kimberlite" J. Phys. IV France 110833-838

2003 Davies A.R., Field J.E. and Pickles C.S.J. "Strength of free-standing chemically vapour-deposited diamond measured by a range of techniques" Philos. Mag. 834059-4070

2000 Telling R.H., Pickard C.J., Payne M.C. & Field J.E. "Theoretical strength and cleavage of diamond" Phys. Rev. Letts 84 5160-516

1999 Telling R.H. & Field J.E. "Fracture and erosion of diamond", Diamond Related Mater. 8850-854

1998 Telling R.H. & Field J.E. "Fracture in CVD diamond", Int. J. Refract. Metals Hard Mater. 16269-276

1996 Field J.E. & Pickles C.S.J. "Strength, fracture and friction properties of diamond." Diamond Related Mater. 5625-634

1995 Nicholson E.D., Partridge P.G. & Ashfold M.N.R. "The mechanical properties of CVD diamond films, and diamond-coated fibres and wires." Mater. Res. Soc. Symp. Proc. 383 101-113

1995 van Bouwelen F.M., Bleloch A. & Field J.E. "Scanning reflection electron microscopy study of a cleaved diamond surface." Inst. Phys. Conf. Ser. 147221-224

1994 Nicholson E.D. & Field J.E. "The mechanical and thermal properties of thin films." J Hard Mater. 5 89-132

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