ICMCTF2014 Session B4-5: Properties and Characterization of Hard Coatings and Surfaces

Nano-crystalline zirconium tungsten nitride (Zr-W-N) thin films have been deposited on Si (100) substrates by magnetron sputtering at 200^°C substrate temperature. The effect of substrate bias on structure and mechanical properties of deposited films has been studied in details using X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy and nano-indentation. All Zr-W-N films exhibit a crystalline fcc phase with (111) and (200) preferred crystallographic orientations of grains. Increasing substrate bias decreases the crystallite size and increases the roughness of the Zr-W-N films. The lattice parameter and thickness of Zr-W-N films varies non-monotonically with increasing substrate bias. The Zr-W-N films have columnar structure which becomes finer at high substrate bias. Nano-indentation hardness and reduced elastic modulus of the films increases as the substrate bias goes up. Maximum wear resistance (H/E_r~0.22) and maximum resistance to fatigue fracture (H³/E²~1.1 GPa) have been achieved for the film deposited at -80 V bias voltage.

Residual stress was measured on plasma sprayed Nickel-Aluminum (Ni-Al), Alumina (Al₂O₃) and Alumina-Titania (Al₂O₃-TiO₂) single splats, by using the incremental focused ion beam micron-scale ring-core method (IµRCM). Results showed a tensile residual stress for polycrystalline Ni-Al, where the quenching stress is only partially relaxed by edge curling, through-thickness yielding and interfacial sliding. Conversely, a significant compressive stress for the amorphous Al₂O₃-TiO₂ splats and a relatively lower compressive stress for the large micro-cracked crystalline Al₂O₃ splats were measured.

The observed differences between the Al₂O₃ and Al₂O₃-TiO₂ splats suggest that, for these materials, a complex interaction exists between the stress component due to different thermal contraction and the quenching stress one, which is supposed to be reduced by different relaxation mechanisms, also correlated to the microstructural evolution and during splat formation.

The experimental methodology developed provided a unique way for the study of the residual stress build-up mechanisms in amorphous and crystalline single splats obtained by plasma spraying, and gave further insights into the actual micro-scale phenomena that give rise to adhesion and nano-mechanical behavior of thermally sprayed coatings.