Deformation and removal characteristics in nanoscratching of 6H-SiC with Berkovich indenter
Introduction
Silicon carbide (SiC) is an ultra-hard structural ceramic material and an important wide-band-gap semi-conductor as well. It shows excellent resistance to thermal shock, oxidation, and corrosion under the conditions of high temperature and pressure [1], for which SiC boasts of its widespread applicable prospect in such fields as structural composites and electronic materials [2]. As a typical brittle material, however, in classical or Griffith sense with high stiffness (E/r) and extreme micro-hardness, it is really difficult to obtain high surface quality by mechanical methods, even by the diamond cutting tool [3], [4], [5].
In recent years, some researchers show great interest in the ductile machining of single crystal silicon carbide. Initially, ductile regime machining of single-crystal silicon carbide (6H) was achieved by using single-point diamond turning (SPDT) at penetration depth less than 500 nm, and the nose radius of cutting tools is about 2 mm [6]. This material is believed to have high pressure phase (HPP) like other types of semi-conductor and ceramic in ductile machining [7], [8], [9]. Jacob et al. performed a single pass cut experiment on 6H-SiC by using a round nose tool with nose radius about 1 mm. The critical depth of cut hc for 6H-SiC measured by a Raman microscope is about 70 nm [10]. Another report shows that a surface finish of Ra=9.2 nm was obtained with an SPDT method [11]. They found that the diamond fly cutting tool with negative rake angle caused a significant increase in the thrust force [11], which can facilitate the realization of ductile-regime machining of hard and brittle materials [12].
The nanoscratch technique was used as a method to analyze the mechanical properties of materials [13]. This approach provides with an insight into the deformation and removal mechanism of grinding. The removal mechanism of ultra-precision grinding is different from that of fly-cutting method. Nose radius and rake angle of the grains are so much smaller than those of fly-cutting tools that will generate greater hydrostatic pressure and more micro-cracks. Hardly any report about this can be found until today. Obviously, it requires more work to explain the mechanism of deformation for single crystal silicon carbide during the process of grinding. This paper aims at performing an intensive analysis of ductile and brittle characteristics of 6H-SiC during a nanoscratching process with a Berkovich diamond indenter and providing with core information for further study of damage-free grinding of this material.
Section snippets
Experimental details
The experiment was conducted on the (0001) surface of N-type 6H-SiC substrate (Hefei Kejing Materials Technology Co., Ltd) precisely polished and the roughness parameter Ra is less than 1 nm. The sample size is 10×5×0.33 mm3. As shown in Fig. 1, the nanoscratching experiment was carried out with the Nano Indenter XP (MTS Systems Corp.) by using a diamond Berkovich indenter (triangular-based pyramid) under a progressive load at fixed loading rate. Nose radius of the indenter is about 940 nm, as
Characteristics of the 6H-SiC (0001) after nanoscratching test
The optical image of the scratch groove on the surface of 6H-SiC is shown in Fig. 3. The whole process was divided into five regimes: the elastic regime, the plastic regime, the subsurface cracking regime, the surface and subsurface cracking regimes and the mico-abrasive regime [14], [15]. In the last regime, some debris which is linked to a non-reproducible process appears on the surface.
As shown in Fig. 4, the penetration depth h increases smoothly in the previous section. Since the applied
Conclusion
The nanoscratching test with Berkovich indenter was conducted on 6H-SiC (0001) over the progressive load (0.1–100 mN) to analyze the deformation features on the nanoscale and removal mechanism for this material. Based on the test results and theoretical analysis, the following conclusions are reached:
- 1.
Detailed analysis for the typical characteristics of regimes from the micro-ductile regime to the micro-abrasive regime during nanoscratching was performed. The critical depth of cut measured in
Acknowledgment
This work was supported by the National Key Basic Research and Development Program of China (973 program, Grant no. 2011CB 013202) and the National Natural Science Foundation of China (Grant no. 51175126). We would like to express our gratitude to Professor Liangchi Zhang who has offered us valuable suggestions during our studies.
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