Abstract
The effect of spherical indenter stiffness on Hertzian-contact-induced fracture initiation was examined in hot-pressed silicon carbides (SiCs). Hertzian ring crack initiation forces were measured using zirconia, steel, silicon nitride, alumina, or tungsten carbide spherical indenters (elastic moduli ranging between 213 and 630 GPa). The two (flat target) SiCs were fully dense, and had equivalent elastic moduli (~450 GPa) and fracture toughnesses; however, about 20% of the grains in one SiC were larger than the largest grains in the other. Decreasing the indenter elastic modulus consistently resulted in lower ring crack initiation forces and those differences were statistically significant. Such a decrease in Hertzian ring crack initiation force with decreased indenter elastic modulus indicates the presence of a non-zero friction coefficient. Additionally, independent of the indenter material, ring crack initiation occurred at lower Hertzian indentation forces in the SiC containing larger grains suggesting that the grains in that tail of the grain-size-distribution acted as Griffith-type flaws. Lastly, selecting a spherical indenter material that has the same or similar elastic modulus as the target material provides simpler interpretation, and estimates of ring crack initiation stresses with greater usefulness and fidelity. Such a "matched" condition serves to circumvent the complexities that a ubiquitously unknown coefficient of friction introduces in the estimation of Hertzian ring crack initiation stress.
