Fracture toughness evaluation of nanocrystalline, bilayer, gradient and gradient-multilayer diamond films via nanoindentation

Abstract

Diamond films are widely used as tool coatings for machining hard materials, yet balancing hardness and fracture toughness remains challenging. Here, nanocrystalline (NCD), bilayered multilayer (2L-M), gradient (G), and gradient- multilayer (G-M) diamond films were deposited by hot filament chemical vapor deposition (HFCVD). Fracture toughness of these four diamond films was evaluated via nanoindentation following Anstis's equation. Results showed that 2L-M films had the lowest fracture toughness suffering from its sharp interfaces with high sp² phase content and stress concentrations, whereas G films obtained improved toughness through gradual grain size reduction. Notably, the fracture toughness of G-M film reached 9.62 MPa·m^1/2 with highest hardness of 71.8 GPa, even surpassing traditional NCD films. This enhancement was attributed to the novel gradient-multilayer architecture, which improved interfacial integrity, reduced residual stress at interfaces through moderate transitions, and suppressed crack propagation by combining gradually decreasing grain sizes in the MCD layer with multiple interfaces. The work demonstrates that gradient-multilayer designs effectively reconcile high hardness and fracture toughness in diamond films, offering a viable strategy for advanced tool-coating applications.