Wear resistance of ceramics can be improved by suppressing fracture, which can be accomplished either by decreasing the grain size or by reducing the size of the deformation zone. We have combined these two strategies with the goal of understanding the atomistic mechanisms underlying the plasticity-controlled friction and wear in nanocrystalline (nc) silicon carbide (SiC). We have performed molecular dynamics simulations of nanoscale wear on nc-SiC with 5 nm grain diameter with a nanoscale cutting tool. We find that grain-boundary (GB) sliding is the primary deformation mechanism during wear and that it is accommodated by heterogeneous nucleation of partial dislocations, formation of voids at the triple junctions, and grain pull-out. We estimate the stresses required for heterogeneous nucleation of partial dislocations at triple junctions and shear strength of GBs. Pile up in nc-SiC consists of grains that were pulled out during deformation. We compare the wear response of nc-SiC to single-crystal (sc) SiC and show that scratch hardness of nc-SiC is lower than that of sc-SiC. Our results demonstrate that the higher scratch hardness in sc-SiC originates from nucleation and motion of dislocations, whereas nc-SiC is more pliable due to additional mechanism of deformation via GB sliding.
Physical vapor deposition (PVD) with a slow deposition rate and substrate temperature near the glass transition temperature can produce glasses with exceptional properties that include higher density, lower enthalpy, and better kinetic stability than the corresponding properties in ordinary liquid-cooled glass. Despite intensive investigations into thermodynamic and kinetic properties of PVD glasses, little is known about their mechanical properties, such as hardness and creep. In this study, we use nanoindentation to show that the mechanical properties of N,N′bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) glass can be manipulated by varying the deposition temperature. The elastic modulus of TPD glass was found to be correlated with molecular orientation, whereas the hardness was found to be correlated with density. In addition, our studies reveal that time-dependent deformation plays a key role in the mechanical response of these glasses.
Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC) and nanocrystalline silicon carbide (nc-SiC) is investigated using single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic force microscopy (AFM) tips under dry conditions, and the wear behavior is compared to that of single-crystal silicon with both thin and thick native oxide layers. We discovered a transition in the relative wear resistance of the SiC samples compared to that of Si as a function of contact size. With larger nanoindenter tips (tip radius ≈ 370 nm), the wear resistances of both sc-SiC and nc-SiC are higher than that of Si. This result is expected from the Archard's equation because SiC is harder than Si. However, with the smaller AFM tips (tip radius ≈ 20 nm), the wear resistances of sc-SiC and nc-SiC are lower than that of Si, despite the fact that the contact pressures are comparable to those applied with the nanoindenter tips, and the plastic zones are well-developed in both sets of wear experiments. We attribute the decrease in the relative wear resistance of SiC compared to that of Si to a transition from a wear regime dominated by the materials' resistance to plastic deformation (i.e., hardness) to a regime dominated by the materials' resistance to interfacial shear. This conclusion is supported by our AFM studies of wearless friction, which reveal that the interfacial shear strength of SiC is higher than that of Si. The contributions of surface roughness and surface chemistry to differences in interfacial shear strength are also discussed.
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