Experimental investigation on the surface and subsurface damages characteristics and formation mechanisms in ultra-precision grinding of SiC

Surface and subsurface damages appear inevitably in the grinding process, which will influence the performance and lifetime of the machined components. In this paper, ultra-precision grinding experiments were performed on Reaction-bonded Silicon Carbide (RB-SiC) ceramics to investigate surface and subsurface damages characteristics and formation mechanisms in atomic scale. The surface and subsurface damages were measured by a combination of scanning electron microscopy (SEM), atomic force microscopy (AFM), raman spectroscopy and transmission electron microscope (TEM) techniques. Ductile-regime removal mode is achieved below critical cutting depth, exhibiting with obvious plough stripes and pile-up. The brittle fracture behavior is noticeably influenced by the microstructures of RB-SiC such as impurities, phase boundary and grain boundary. It was found that subsurface damages in plastic zone mainly consist of stacking faults (SFs), twins and limited dislocations. No amorphous structure can be observed in both 6H-SiC and Si particles in RB-SiC ceramics. Additionally, with the aid of high resolution TEM analysis, SFs and twins were found within the 6H-SiC closed packed plane i.e. (0001). At last, based on the SiC structure characteristic, the formation mechanisms of SFs and twins was discussed, and a schematic model was proposed to clarify the relationship between plastic deformation induced defects and brittle fractures

Subsurface damages beneath fracture pits of reaction-bonded silicon carbide after ultra-precision grinding

This paper investigated the structural defects beneath the fracture area of 6H-SiC in reaction-bonded silicon carbide (RB-SiC) ceramics after ultra-precision grinding. The nano-indentation technique was used to evaluate the evolution of deformation behavior and find the critical transition condition among elastic, plastic and fracture. It was found that beneath the fracture pits, dislocations accompanied with micro-cracks (lateral and median) were the two types of subsurface damage. However, no amorphous phase was detected. In addition, a two-beam analysis confirmed that the dislocations were activated on basal and dissociated into the Shockley partial dislocations in 6H-SiC particle. The following indentation experiments revealed that the existence dislocations in the ground subsurface should be occurred earlier than cleavage. These dislocations were the predominant yielding mechanism in 6H-SiC, which initiated at a shear stress of about 23.4-28.4 Gpa through a pop-in event on load-displacement curve. Afterwards, cracks emerged when the maximum tensile stress beneath the indenter increased to 31.6 Gpa. It was identified that cracks could be activated under the intersection effect of non-uniform high density dislocations. Meanwhile, the blocking effect on sliding motion of dislocations caused by cross propagating dislocations, phase boundary and sintering agents play an important role in the evolution of fracture during grinding process. At last, the deformation behavior was further elaborated to discuss the slippage on the basal plane determined by Schmidt factor and structure characteristic of 6H-SiC