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The overall goal of this project is to discover fundamental mechanisms responsible for deformation and wear of ultrananocrystalline diamond (UNCD). Ultra-nanocrystalline diamond (UNCD) is an excellent candidate for drill coatings because its tribological, chemical, and mechanical properties are very similar to that of single crystal diamond, which is the strongest material known. Recently UNCD has been successfully deposited on micro end mills, which resulted in a considerable reduction in cutting forces and burr formation.

In the previous report we reported a study of friction between diamond surfaces and diamond-like-carbon atomic force microscope tips. The study led to a discovery of friction laws in dry nanoscale contacts and the results have been published in Nature. In a related study we have put forth a physical interpretation of an experimentally observed isotope effect on solid friction. The results have also been described in the previous report. This year we continued the investigation of mechanical response of nanoscale friction and determined that roughness theories developed for macroscale contacts apply to dry nanoscale contacts with atomic roughness.

The highlight of this year activities is our discovery of atomistic mechanisms underlying deformation of UNCD. We have performed massively parallel molecular dynamics simulations to nanoindentation and of uniaxial compression and we found that the dominant deformation mechanism in UNCD is grain boundary sliding. No dislocation activity has been observed during our simulations, in agreement with experimental reports. We discovered that hardness and yield strength of UNCD scale linearly with the shear strength (friction) of grain boundaries. Grain boundary friction can be controlled by the presence of dopants, e.g., hydrogen atoms. We also found that phenomenological theories of nanoindentation that were developed for materials that exhibit dislocation-based plasticity apply in the case where plasticity is controlled entirely by grain boundary sliding. The manuscript is now being prepared.

This grant enabled research of a graduate student Yifei Mo, who has obtained his PhD degree last summer and is currently a postdoctoral researcher at MIT. The funding also helped the PI establish a research program in nanotribology and mechanical properties of nanocrystalline ceramics.