Hard and Superhard TiAlBN Coatings Deposited by Twin Electron-Beam Evaporation

Superhard nanostructured coatings, prepared by plasma-assisted chemical vapour deposition (CVD) and physical vapour deposition (PVD) techniques, such as vacuum arc evaporation and magnetron sputtering, are receiving increasing attention due to their potential applications for wear protection. In this study we report the synthesis and characterisation of PVD nanocomposite Ti-Al-B-N coatings deposited by electron-beam (EB) evaporation. Coatings were deposited onto Si (100), AISI316 and M2 substrates by co-evaporating Ti and hot isostatically pressed Ti-Al-B-N material, consisting of a mixture of 50 wt.% TiB2, 30 wt.% BN and 20 wt.% AlN, from a thermionically enhanced twin crucible EB evaporation source in an Ar plasma at 450°C. A combination of optical emission spectroscopy and partial pressure control was utilised to control the evaporation rates and hence the composition in the coating. The coating stoichiometry, relative phase composition, nanostructure and mechanical properties were determined using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), in combination with transmission electron spectroscopy (TEM) and nanoindentation measurements. Al (app. 10 at.% in coatings) was found to substitute for Ti in the cubic TiN structure. The formation of nanocrystalline (Ti,Al)N grains separated by an intergranular amorphous BN phase was observed. (Ti,Al)B0.14N1.12 coatings, consisting of app. 90 mol% (Ti,Al)N and app. 10 mol% BN with an average (Ti,Al)N grain size of 26 nm, showed hardness and elastic modulus values of 40 and 360 GPa, respectively. These coatings retained their mechanical properties for more than 90 months at room temperature in air, comparing results gathered from eight different nanoindentation systems and one laser acoustic surface wave method. Vacuum annealing experiments revealed a moderate, but significant, increase in hardness to 45 GPa for (Ti,Al)B0.14N1.12 coatings. Independent of the composition, all coatings examined exhibited structural stability in vacuum to temperatures in excess of 900°C.JRC.I.4-Nanotechnology and Molecular Imagin