*Article* **Modeling and Experiment of the Critical Depth of Cut at the Ductile–Brittle Transition for a 4H-SiC Single Crystal**

#### **Peng Chai, Shujuan Li \* and Yan Li \***

School of Mechanical and Precision Instrument Engineering, Xi an University of Technology, No. 5 South Jinhua Road, Xi'an 710048, Shaanxi, China; chaipeng@stu.xaut.edu.cn **\*** Correspondence: shujuanli@xaut.edu.cn (S.L.); jyxy-ly@xaut.edu.cn (Y.L.)

Received: 19 April 2019; Accepted: 5 June 2019; Published: 7 June 2019

**Abstract:** In this paper, a theoretical model of the critical depth of cut of nanoscratching on a 4H-SiC single crystal with a Berkovich indenter is proposed, and a series of scratch tests in a nanomechanical test system was performed. Through nanoindentation experimentation on fused quartz, the Berkovich indenter nose radius was indirectly confirmed using least squares. The range of critical depths of cut at the ductile–brittle transition was obtained by SEM observation, and the size of cracks was amplified with increasing scratching depth. The theoretical result of the critical depth of cut at the ductile–brittle transition for a 4H-SiC single crystal is 91.7 nm, which is close to the first obvious pop-in point of the relation curve between tangential force and lateral displacement. Repeated experimental results show good consistency and good agreemen<sup>t</sup> with other references.

**Keywords:** 4H-SiC; critical depth of cut; Berkovich indenter; cleavage strength; nanoscratching
