Hard Transition-Metal Diboride Coatings

Transition-metal diborides (TMB2, TM = Ti, Zr, Hf, V, Nb, Ta, Co, Mo, and W) form extremely temperature-stable single-phase compositions belonging to ultra-high temperature ceramics. The strong ionic and covalent bonds between atoms of transition metal and boron atoms in the hexagonal a-AlB2 or w-W2B5 type of TMB2 structure led to the high hardness, excellent wear resistance, and chemical inertness of these materials. Physical deposition methods enhance TMB2’s intrinsic character by forming hard coatings. Hard TMB2 coatings typically have a nanocomposite structure consisting of nanocolumns with a diameter of a few nanometers and an amorphous, so-called B-tissue phase. These coatings are very hard, often more than 40 GPa. However, TMB2 coatings still do not fulfill their application potential in demanding thermal–mechanical conditions due to their low oxidation resistance compared to the bulk equivalent, inherent brittleness, and poor crack propagation resistance. This chapter demonstrates a variety of approaches to tuning the mechanical properties and improving the weaknesses of these promising materials using both deposition methods (changing deposition parameters and ionizing sputtered species) and material design (changing stoichiometry, alloying, and multilayer architecture).