Mechanical Properties of Hard W-C Coating on Steel Substrate Deduced from Nanoindentation and Finite Element Modeling

Mechanical properties of a hard and stiff W-C coating on steel substrate have been investigated using nanoindentation combined with finite element modeling (FEM) and extended FEM (XFEM). The significant pile-up observed around the indents in steel substrate caused an overestimation of hardness and indentation modulus. A simple geometrical model, considering the additional contact surfaces due to pile-up, has been proposed to reduce this overestimation. The presence of W-C coating suppressed the pile-up in the steel substrate and a transition to sink-in behavior occurred. The FEM simulations adequately reproduced the surface topography of the indents in the substrate and coating/substrate systems as well. The maximum principal stresses of the indented W-C/steel coated system were tensile; they were always located in the coating and evolved in 3 stages. Cohesive cracking occurred during loading in the sink-in zone (stage III) when the ultimate tensile strength (sigma(max)) of the coating was reached. The obtained hardness (H-c), indentation modulus (E-c), yield stress (Y) and strength (sigma(max)) of the W-C coating were H-c = 20 GPa, E-c = 250 GPa, Y = 9.0 GPa and sigma(max) = 9.35 GPa, respectively. XFEM resulted in fracture energy of the W-C coating of G = 38.1 J.m(-2) and fracture toughness of K-IC = 3.5 MPa.m(0.5).