Material Removal Characteristics of Single-Crystal 4H-SiC Based on Varied-Load Nanoscratch Tests

As third-generation semiconductor materials with a wide bandgap, single-crystal silicon carbides (SiC), such as 4H-, 6H-, 15R-, and 3C-SiC, are hexagonal, rhombohedral, or cubic structures that have a high hardness, strong stability, low thermal conductivity, favorable temperature resistance, and other excellent physical and chemical properties [1, 2]. Single-crystal SiC is advantageous for producing electronic and sensor devices under extreme temperature, pressure, frequency, power, and radiation conditions and is widely used in the aerospace, telecommunication, automotive, and semiconductor production fields. Recently, researchers have demonstrated increased interest in the performance characterization, applications, and growth of high-quality and large-sized single-crystal SiC [3, 4].

To obtain various high-quality components with low surface damage, grinding, polishing, and other precision and ultraprecision machining methods are used to process single-crystal SiC and ensure high surface integrity in the finished components [5‐8]. However, single-crystal SiC is a typical hard-to-machine material due to its high hardness, brittleness, and low fracture toughness [9]. These properties result in surface defects and subsurface damage such as cracks, scratches, chipping, and spalling during the machining process, which seriously affect the accuracy and service life of the components [10]. Therefore, it is important to study the material removal characteristics and ductile removal mechanism of single-crystal SiC to achieve machining with high efficiency and precision, low emissions, and carbon reduction.

The use of the nanoscratch test with a diamond indenter is an effective method for investigating the material removal mechanism, crack propagation, surface deformation, and damage characteristics in machining and has been widely applied in property tests for various types of workpieces and films.

Huang et al. [11] studied the surface and subsurface deformation characteristics during the nanoscratching of cemented carbide and showed that smooth surfaces were produced at loads of less than 3 mN. When large loads were applied, debris, fractures, and crushed particles were generated. Li et al. [12] systematically studied the effect of strain rate on the surface deformation characteristics and surface damage by conducting nanoscratch tests at different scratch speeds on Lu₂O₃ single crystals. The results showed that a higher scratch speed results in a shallower scratch depth and more continuous chips. Marshall et al. [13] studied transverse crack propagation in the contact field of sharp indenters and discussed the prospect of predicting the transverse fracture degree of other ceramics. Huang et al. [14] performed nanoscratch tests on dense bulk alumina ceramic samples with three grain sizes, and the results showed that the plastic deformation ability of ultra-fine-grained alumina ceramic samples increases at the nano-and microscales. Kovalchenko et al. [15] used abrasive particles of different shapes to characterize monocrystalline silicon. The results showed that spherical WC particles induce a uniform cutting pressure and limit the degree of brittle fracture. Moreover, the sharp edges and corners of the particles produce stress concentration areas, leading to cracking and spalling. Wu et al. [16] studied the lattice bending and dislocations, twins, and stacking faults during the process of carving GaAs. The results showed that the residual stress may be the cause of local lattice bending. Wasmer et al. [17] studied the mechanical deformation of GaAs nanoindentation and scratching and found that twins were the main deformation processes during the indentation process, and only slip bands and ideal dislocations were observed during the scratch process. Qiu et al. [18] conducted double-scratch tests on glass ceramics to study the mechanisms of crack propagation and material removal. The results showed that the interaction between adjacent scratches depends on the scratch depth and separation distance. Cai et al. [19] performed a scratching study on AIN ceramics and found that when the double scratch interval was between 0.5 μm and 1 μm, the second scratch merged with the first scratch to form a single scratch, resulting in an increase in the depth of the second scratch.

In this study, varied-load nanoscratch tests were performed on single-crystal 4H-SiC using a nanoindenter system with a Berkovich indenter to investigate the material removal, crack propagation, surface deformation, and damage characteristics of single-crystal SiC in ultraprecision machining. The material removal characteristics under different planes, indenter directions, loading rates, and scratch intervals are discussed, and the nanoscratch groove surface morphology, distribution law of cracks, material deformations and fractures, and cross-sectional characteristics are analyzed using SEM, FIB, and 3D profilometry. Finally, the mechanisms of material removal and crack propagation in single-crystal 4H-SiC are studied. The proposed research provides a guideline for the high-efficiency and precision machining of single-crystal SiC to ensure high accuracy and a long service life.

The workpiece material was 4H-SiC (Hefei Yuanjing Technology Material Co., Ltd., China), where H and 4 denote the hexagonal structure and periodicity of the tetrahedrons along the [0001] c-axis of the crystal, respectively. The differences between the [0001] Si- and [000-1] C-plane of 4H-SiC in nanoscratch tests are discussed herein. Figure 1 shows the microstructure diagram [20]. The surface of the 5×5×0.41 mm workpiece was polished, and the roughness was less than 15 nm. Additionally, the workpiece was fixed to a copper substrate using a special solid adhesive. The size of the substrate was 20 ×10 ×1 mm, and its surface was polished before adhesive bonding. To ensure cleanliness, it was necessary to clean the workpiece before the tests. First, the workpiece was placed in an acetone solution and rinsed twice using an ultrasonic cleaning machine for 10 min. The samples were then cleaned using distilled water for 10 min before being dried in a dust-free environment.

As shown in Figure 2, varied-load nanoscratch tests were carried out using a Nano Indenter G200 (Keysight Technologies, USA) to observe the stage and characteristics of the brittle, ductile, and brittle-ductile removal of the materials and to explore the crack propagation and surface deformation of the single-crystal 4H-SiC during machining. The diamond triangular-pyramidal Berkovich indenter was used because it has the same surface and projection area as the Vickers indenter and avoids the influence of the tip cross-edge [21]. The geometric shape of the Berkovich indenter used in this study is shown in Figure 2(b). Its tip radius is 90 nm, the included angle between the centerline and each edge is 77.05°, and the included angle between the centerline and each surface is 65.3° [22].

Figure 3 shows the experimental scheme for the varied-load nanoscratches used in this study. The material removal characteristics under different planes (Si- and C-plane), indenter directions (edge, face, and side face forward), normal loading rate v_p (5, 7, 10 mN/s), and scratch intervals d (3, 5, 8, 11, 14 μm) were discussed, and the mechanism of material removal and crack propagation was also analyzed. A field-emission scanning electron microscope (MIRA3 TESCAN, TESCAN ORSAY HOLDING, a. s., Czech Republic) and a focused ion beam scanning electron microscope (Crossbeam 350, Carl Zeiss Microscopy GmbH, Germany) were employed to observe the microcracks, fractures, and deformations of the surface and subsurface, and an optical surface profilometer was used to determine the 3D morphology and cross-sectional characteristics of the double nanoscratch.

In this study, single- and double-varied-load nanoscratch tests were performed on single-crystal 4H-SiC using Berkovich indenter. The main conclusions drawn are as follows: