Incipient plasticity in 4H-SiC during quasistatic nanoindentation

Silicon carbide (SiC) is an important orthopedic material due to its inert nature and superior mechanical and tribological properties. Some of the potential applications of silicon carbide include coating for stents to enhance hemocompatibility, coating for prosthetic-bearing surfaces and uncemented joint prosthetics. This study is the first to explore nanomechanical response of single crystal 4H-SiC through quasistatic nanoindentation. Displacement controlled quasistatic nanoindentation experiments were performed on a single crystal 4H-SiC specimen using a blunt Berkovich indenter (300nm tip radius) at extremely fine indentation depths of 5nm, 10nm, 12nm, 25nm, 30nm and 50nm. Load-displacement curve obtained from the indentation experiments showed yielding or incipient plasticity in 4H-SiC typically at a shear stress of about 21GPa (~an indentation depth of 33.8nm) through a pop-in event. An interesting observation was that the residual depth of indent showed three distinct patterns: (i) positive depth hysteresis above 33nm, (ii) no depth hysteresis at 12nm, and (iii) negative depth hysteresis below 12nm. This contrasting depth hysteresis phenomenon is hypothesized to originate due to the existence of compressive residual stresses (upto 143MPa) induced in the specimen by the polishing process prior to the nanoindentation.