Nanoindentation, Scratching and Coating Wear

Molecular dynamics study of twisted bilayer graphene-coated silicon carbide under nanoindentation

The effect of twisted bilayer graphene (TBG) with different torsion angles (θ) as a barrier coating for 4H silicon carbide (4H-SiC) on the enhancement of hardness-toughness synergy is investigated under nanoindentation using molecular dynamics methods. We find that (i) the average elastic modulus and intrinsic hardness of TBG-coated 4H-SiC are increased by 25.1% and 13.5%, respectively, compared to bare 4H-SiC. (ii) In synergy with 4H-SiC substrate, the large-angle TBG coatings show superior resistance to rigid indenter penetration than the small- angle coatings due to the enhanced slipping effect by the weakened interlayer coupling, where 21.78◦-TBG coating is the most effective. (iii) At θ = 21.78◦, the diamond-like structure formed by the localized sp2-to-sp3 phase transition in the slip zone of TBG coating improves the hardness and significantly delays the nucleation and propagation of the dominant full dislocations with a Burgers vector of 1/3 in 4H-SiC substrate. (iv) Despite the perforation of 21.78◦-TBG coating, the sudden increase of other dislocations competing with the full dislocations further prevents damage and failure of 4H-SiC. (v) Nanoindentation behavior shows temperature insensitivity in the elastic stage but temperature sensitivity in the plastic stage, along with sensitivity to instantaneous indentation depth. The findings reveal the key role and mechanism of twisted TBG coatings in protecting 4H-SiC and other similar materials underneath from extreme high-pressure conditions.