Anisotropic mechanical amorphization drives wear in diamond

Abstract

Diamond is the hardest material on Earth. Nevertheless, polishing diamond is possible with a process that has remained unaltered for centuries and is still used for jewellery and coatings: the diamond is pressed against a rotating disc with embedded diamond grit. When polishing polycrystalline diamond, surface topographies become non-uniform because wear rates depend on crystal orientations. This anisotropy is not fully understood and impedes diamond's widespread use in applications that require planar polycrystalline films, ranging from cutting tools to confinement fusion. Here, we use molecular dynamics to show that polished diamond undergoes an sp(3)-sp(2) order-disorder transition resulting in an amorphous adlayer with a growth rate that strongly depends on surface orientation and sliding direction, in excellent correlation with experimental wear rates. This anisotropy originates in mechanically steered dissociation of individual crystal bonds. Similarly to other planarization processes, the diamond surface is chemically activated by mechanical means. Final removal of the amorphous interlayer proceeds either mechanically or through etching by ambient oxygen.

Tribological properties of nanocrystalline diamond films deposited by hot filament chemical vapor deposition

“…Furthermore, low friction is directly related to the amorphization/graphitization of diamond near the sliding contact point. 13,16 In one of the study it is seen that when diamond is polished, it undergoes sp 3 to sp 2 transition resulting in an amorphous adlayer that depends on surface orientation and sliding direction. The structural phase transformation and amorphization influence the change in friction and wear.…”

“…The structural phase transformation and amorphization influence the change in friction and wear. 13 To proof amorphization/graphitization of diamond films, Raman spectra were recorded on the film surface and in the wear track formed at 3 N load for each sample, as shown in Figure 11. All spectra have the following peaks in common: i) substrate-related silicon peaks at 520 cm −1 and 944-948 cm −1 ; ii) the fundamental diamond line (D*) at about 1332 cm −1 stemming from crystalline diamond; iii) the known graphite-related G and D bands at 1573-1598 cm −1 and 1350 cm −1 and (iv) peaks at 1168 cm −1 and 1470 cm −1 corresponding to the formation of transpolyacetylene (TPA) segment, which are only observed for nano-and ultrananocrystalline diamond.…”

“…9,10 Nevertheless, the friction and wear of nanocrystalline diamond are quite anisotropic and depends on various factors, such as the chemical reactivity of the surface, crystallite size, crystallographic orientation, existence of dangling covalent bonds, sliding direction of oriented planes, surface roughness, properties of transfer layers, test environment and finally test parameters which influence the wear behavior, such as load and sliding velocity. 8,[11][12][13][14][15] In addition to all known important factors and tribological test parameters which may influence the wear behavior of the NCD films, the amount of hydrogen and sp 3 /sp 2 bonding ratio play a dominant role in the friction and wear performances of these materials. 16 There are several physical and chemical methods to synthesize NCD films.…”

Minimal graphene thickness for wear protection of diamond

“…The polishing rate has been shown to depend on the surface orientation and sliding direction. 9 The amorphous layer which develops at the sliding interface is easily removed leading to wear of the surface. This amorphous layer also leads to a high friction coefficient because it has many bonds at the interface.…”

“…For the case of bare diamond slabs, following Ref. 9 we randomly place a few carbon atoms between them to prevent cold welding, that is the joining of the two slabs. When the slabs are separated by one or two graphene layers, we use these atoms to mimic possible imperfections of the surfaces or the presence of reactive adsorbates.…”