Toughening nitride ceramic coatings by nanotwin design

Because of high hardness, good electrical and thermal conductivities, and excellent wear resistance, transition metal nitride (TMN) ceramic coatings are widely used in protective coatings of cutting tools and structural components operating in extreme environments. However, they are also very brittle due to a mixture of ionic, covalent, and metallic bonding, and they easily suffer from brittle fractures. To avoid brittle failure, in addition to providing high strength, TMN hard coatings should also be tough. For several decades, toughening TMN ceramic coatings has been one of the central goals in materials science and the coating community in order to improve brittleness and avoid catastrophic failure during applications. In this project, we aim to develop a new strategy to toughen TMN coatings significantly, subsequently increasing the damage tolerance. The approach we proposed is, for the first time, to introduce a high density of nanotwins into the nitride ceramic coatings by selecting relative lower stacking fault energy nitrides as starting materials and optimizing the deposition conditions, and eventually achieve designing a high-density nanotwins-mediated ceramic coatings with superior mechanical properties. Based on the acquired receipt, we then turn to high-stacking fault energy nitride coatings to incorporate a high-density nanotwin. Why could dense nanotwins toughen the ceramic significantly? When incorporating a high density of nanotwins in ceramic coatings, substantial crack deflection by nanotwins and massive nanocrack growth (thus nanocracks toughening) under loads will occur. This provides many routes to dissipate the fracture energy, dramatically increasing damage tolerance. We envisage that highly dense nanotwin-mediated ceramic coatings can realize high strength and super toughening simultaneously and exhibit excellent damage tolerance compared to ordinary un-twinned TMN ceramic coatings. Moreover, we use advanced transmission electron microscopy, theoretical simulations, and modeling to explore the nanotwins-toughening mechanism in ceramic coatings through real-time in-situ and ex-situ multiscale structure characterization. We aim to understand the mechanism from the atomic scale. If successful, we will provide a novel and practical approach to constructing and designing new hard-yet- tough materials with potential applications in the manufacturing industry. We then will open a new field in TMN ceramic coatings.