Molecular dynamics simulations of nanoindentation followed by nanoscratching were conducted on single crystal aluminum (with the crystal set up in the (001) [100] orientation and scratching performed in the [100] direction) at extremely fine scratch depths (from 0.8 nm to almost zero) to investigate the atomic-scale friction. The friction coefficients at these depths were found to be rather high (\ensuremath{\sim}0.6), nearly constant, and independent of scratch depth except for zero depth when the magnitudes of the forces were extremely small. The high values of the friction coefficient even at these fine scratch depths are attributed to the finite value of the scratch force involved in breaking and reforming of the atomic bonds, the high negative rake angle generally presented by the indenter (in the present case -45\ifmmode^\circ\else\textdegree\fi{}) at fine scratch depths, which results in higher normal force (about twice the scratch force), and the absence of any lubricating film or contaminant between the sliding surfaces. The friction coefficient was also found to be close to the mean grinding coefficient, which is the ratio of the cutting to the thrust force with a high negative rake tool. Consequently, it appears that whenever material removal is involved in atomic-scale friction even at extremely fine scratch depths, the magnitude of the friction coefficient can be high, dependent on the rake angle presented by the tool, and independent of the normal force. This is because the magnitude of both normal and scratch forces increases with an increase in scratch depth and negative rake angle. Both the scratch hardness and indentation hardness were found to increase with decreasing scratch/indentation depth, strongly suggesting a size effect at fine scratch depths.

Molecular dynamics simulation of atomic-scale friction

Diamond is an exciting material with many outstanding properties; see, for example Field J E (ed) 1979 The Properties of Diamond (London: Academic) and Field J E (ed) 1992 The Properties of Natural and Synthetic Diamond (London: Academic). It is pre-eminent as a gemstone, an industrial tool and as a material for solid state research. Since natural diamonds grew deep below the Earth's surface before their ejection to mineable levels, they also contain valuable information for geologists. The key to many of diamond's properties is the rigidity of its structure which explains, for example, its exceptional hardness and its high thermal conductivity. Since 1953, it has been possible to grow synthetic diamond. Before then, it was effectively only possible to have natural diamond, with a small number of these found in the vicinity of meteorite impacts. Techniques are now available to grow gem quality synthetic diamonds greater than 1 carat (0.2 g) using high temperatures and pressures (HTHP) similar to those found in nature. However, the costs are high, and the largest commercially available industrial diamonds are about 0.01 carat in weight or about 1 mm in linear dimension. The bulk of synthetic diamonds used industrially are 600 µm or less. Over 75% of diamond used for industrial purposes today is synthetic material. In recent years, there have been two significant developments. The first is the production of composites based on diamond; these materials have a significantly greater toughness than diamond while still maintaining very high hardness and reasonable thermal conductivity. The second is the production at low pressures by metastable growth using chemical vapour deposition techniques. Deposition onto non-diamond substrates was first demonstrated by Spitsyn et al 1981 J. Cryst. Growth 52 219–26 and confirmed by Matsumoto et al 1982 Japan J. Appl. Phys. 21 L183–5. These developments have added further to the versatility of diamond. Two other groups of materials based on carbon, namely the fullerenes and graphines have been identified in recent years and are now the subject of intense research.

https://iopscience.iop.org/article/10.1088/0034-4885/75/12/126505/pdf

The mechanical and strength properties of diamond

Strength, fracture and friction properties of diamond

An anisotropic friction and wear model

Diamond turning of steel in carbon-saturated atmospheres

The friction has been measured as a function of the applied load, the direction of sliding and the degree of lubrication of the surface. It is found that the direction in which the cube surface has been polished affects both the magnitude and the symmetry of the friction. Our results imply, in contrast to earlier work, that the friction arises from a ratchet-type mechanism, and that polished diamond surfaces have unusual properties.

http://iopscience.iop.org/article/10.1088/0022-3727/6/15/303/pdf

J Wilks

Papers

The friction of diamond sliding on polished cube faces of diamond