Invited Paper SE-TuA4
Deposition of M_n+1AX_n(n=1-3)Phase Coatings by Magnetron Sputtering from Compound Targets and High Velocity Oxy-Fuel Spraying

Tuesday, October 16, 2007, 2:40 pm, Room 617

Magnetron sputtering has proven to be a favorable route for synthesis of films of the ternary carbides or nitrides referred to as the M_n+1AX_n(n=1-3) phases. Sputtering from elemental targets has enabled the growth of several materials systems; predominately carbidic phases with Ti as the carbide-forming transition metal (M) and with either Al, Si, Ge or Sn as the A-element, but with extensions to other metals as in the V-Ge-C system or to nitride based systems as in the Ti-Al-N system. The majority of the films have been deposited on Al₂O₃(0001) substrates to promote epitaxial growth at temperatures typically above 700 °C, but for V₂GeC as low as 450 °C. In addition to deposition of the known phases in each of the studied systems this type of process also enables the growth of new phases such as Ti₃SnC₂ and so-called intergrown structures in the Ti-Si-C and Ti-Ge-C systems. Using sputtering from compound targets of Ti₃SiC₂ and Ti₂AlC in either Ar or Ar/N₂ plasmas we are currently investigating the growth conditions for the respective phases as well as potentially quaternary Ti-Si-CN and Ti-Al-CN phases. The studies show that sputtering of the compound targets in pure Ar plasmas favors epitaxial growth conditions for Ti₃SiC₂ and Ti₂AlC when the sources are co-sputtered with Ti. The as-deposited coatings consist predominately of MAX phase, but with intergrown layers of TiC. This behavior is attributed to higher carbon content in the deposited films compared to the target composition. Addition of N₂ during growth of both systems results in films of lower crystalline quality, and with preferential nucleation of TiC or TiCN for higher N₂ flows. XPS shows that up to 30 at. % N₂ is incorporated in the Ti-Al-CN films, and that the content of nitrogen in the deposited films scales with the amount of nitrogen available in the plasma. For deposition of thick (appr. 200 µm) Ti₂AlC coatings, we have applied high velocity oxy-fuel (HVOF) spraying. Characterization shows that dense and well-adherent coatings can be deposited on steel. Structural analysis shows that the HVOF coatings consist of Ti₂AlC (the powder phase) together with Ti-Al melting phases and residual TiC and Ti₃AlC₂. Vickers indentation experiments and scanning electron microscopy imaging of the indented area showed a hardness of appr. 6 GPa, with concentric cracks at the indent.