Impact of metastable graphene-diamond coatings on the fracture toughness of silicon carbide

Abstract

Silicon carbide has excellent mechanical properties such as high hardness and strength, but its applications for body armor and protective coating solutions are limited by its poor toughness. It has been demonstrated that epitaxial graphene-coated SiC can enhance SiC mechanical properties due to the pressure-activated phase transition into a sp(3) diamond structure. Here, we show that atomically thin graphene coatings increase the hardness of SiC even for indentation depths of similar to 10 µm. Very importantly, the graphene coating also causes an increase of the fracture toughness by 11% compared to bare SiC, which is in contradiction with the general indirect variation of hardness and fracture toughness. This is explained in terms of the presence of a diamond phase under the indenter while the rest of the coating remains in the ultra-tough graphene phase. This study opens new venues for understanding hardness and toughness in metastable systems and for the applications of graphene-coatings. The phase-transition of atomically thin graphene coating into a diamond phase increases the hardness and the fracture toughness of SiC even for indentation depths of 10 µm compared to bare SiC, which is against the general indirect relation.