**1. Introduction**

Single crystal SiC is a third-generation semiconductor which performs well in terms of high breakdown electric field, high thermal conductivity, large band gap, high saturated electron drift velocity, low dielectric constant and good resistance to radiation damage, all of which open up completely new classes of commercial and military applications which are currently impossible or una ffordable with silicon or gallium arsenide [1,2]. The most widely used examples include their use as sources of white light for optical storage, as displays, and in radar and communication technology, and in automotive and oil exploration industries and in high-radiation environments [3,4]. Of the many di fferent silicon carbide polytypes, 3C-SiC, 4H-SiC and 6H-SiC are the most commonly applied. Depending on the particular application, the requirements for the resulting surface of SiC substrates are stringent and often stipulate that the roughness (Ra) must be less than 0.3 nm, the wafers must have flat, smooth, and damage free surfaces; this means the machined quality directly determines the applied value and performance of the device [5,6]. However, the high hardness, sti ffness, and strength of single crystal SiC make it very di fficult to obtain a high-quality surface finish by mechanical methods, even with a diamond cutting tool [7].

In recent years some researchers have become interested in analyzing the mechanical properties of single crystal SiC; Yin et al. used Vickers indention and nanoindentation to carry out indentation experiments on the 6H-SiC substrates, and used grinding and polishing tests to investigate microfracture, residual damage, and surface roughness associated with material removal and surface generation [8]. Meng et al. performed a nanoscratching test on 6H–SiC with a Berkovich diamond indenter and found that the deformation and removal of 6H-SiC while nanoscratching with a sharp indenter was completely different from the single-point diamond turning (SPDT) method [9]. Goel et al. carried out a diamond turning test on single crystal 6H-SiC at a cutting speed of 1 m/s on a precision diamond turning machine to elucidate the microscopic origin of ductile-regime machining, and obtained a surface finish of Ra = 9.2 nm [10]. Yan et al. used a Berkovich nanoindenter to investigate the subsurface damage of SiC in nanoindentation tests, and found that the depth of subsurface damage was much larger than that in indentation tests, and the damaging mechanism of SiC was completely different from single crystalline silicon [11]. Xiao et al., carried out molecular dynamics (MD) simulations to investigate the atomic scale details of ductile deformation while machining of 6H SiC, and found that a taper cutting experiment on a single crystal 6H SiC wafer produced a ductile-cut surface. Moreover, a micro Raman spectroscopy of the machined surface revealed no peaks for amorphous SiC, which agreed with the MD result [12]. Lee et al. proposed a hybrid polishing technique using a mixed abrasive slurry (MAS) with colloidal silica and nanodiamonds to investigate the hybrid removal mechanism of MAS on SiC [13]. Goel et al. carried out diamond turning of single crystal 6H-SiC at a cutting speed of 1 m/s on a precision diamond turning machine to elucidate the microscopic origin of ductile-regime machining; a surface finish of Ra = 9.2 nm was obtained and a brittle–ductile transition was observed [14]. Li et al. carried out the nanoindentation test for 6H-SiC with a Berkovich indenter and set up the three-dimensional finite element simulation, the plastic deformation and cracks morphology and mechanical properties were analyzed with the maximum load P (max) [15]. Nawaz et al. investigated the nanoscale elastic-plastic deformation behavior of single crystal 6H-SiC systematically by using nanoindentation with a Berkovich indenter and observed the effect of loading rates on the critical pop-in load, pop-in displacement and maximum shear stress [16]. Pang et al. presented an experimental and numerical analysis of the deformation behavior of single-crystal 6H-SiC in nanoindentation, the results showed that classical crystal plasticity theory can be reliably applied in predicting plastic deformation of ceramic at small scales [17]. Lu et al. described the mechanical planarization machining of SiC substrates involving the Si face and C face of N-type 4H-SiC, N-type 6H-SiC, and V-type 6H-SiC with a sol-gel polishing pad, the removal mechanism of SiC substrates was investigated by nanoindentation and nanoscratching [18].

While these studies did not present concrete nanomechanical data and generally used the mechanical data in the literature to analyze material removal mechanism, nor did they analyze the material removal mechanism of single crystal SiC from a mechanical perspective whilst using abrasives in different fixed methods. In this paper, a nanomechanical test system was used to test the nanoindentation of single crystal 6H-SiC materials to obtain the hardness, modulus and loads for elastic-plastic and plastic-brittle transitions of this material. The single crystal 6H-SiC materials produced by the same factory were machined by rotational grinding with fixed abrasives, lapping with free abrasives and magnetorheological (MR) finishing of semi-fixed abrasives, respectively. Then, the mechanical properties of the materials in brittle-plastic transition under the three fixed methods of abrasives with optimal experimental conditions were calculated. On this basis, this study revealed the mechanical behavior and material removal mechanisms of single crystal 6H-SiC under the effects of abrasives by combining the morphologies of the machined surfaces and the results of nanoindentation experiments.
