Novel CrAlYN/CrN nanoscale multilayer PVD coatings produced by the combined High Power Impulse Magnetron Sputtering/Unbalanced Magnetron Sputtering technique for environmental protection of γ-TiAl alloys

CrAlYN/CrN coatings utilising nanoscale multilayer structure with a typical bi-layer thickness of 4.2 nm have been deposited on light-weight Ti–45Al–8Nb alloys known as γ-TiAl. The surface pre-treatment was carried out by bombardment with Cr1+ ions generated by High Power Impulse Magnetron Sputtering, (HIPIMS) discharge, whereas the nanoscale multilayer CrAlYN/CrN coating was deposited by Unbalanced Magnetron Sputtering (UBM). Scanning Transmission Electron Microscopy (STEM), revealed that the coating/substrate interface was extremely clean and sharp. Large areas of coating growing epitaxially were observed. STEM–Energy Dispersive Spectroscopy (EDS) profile analyses further showed that, during the HIPIMS ion bombardment, Cr was implanted into the substrate to a depth of 5 nm. Adhesion critical load values as high as LC = 37 N have been measured in scratch adhesion tests. Impact tests brought further evidence for strong bonding. For coated γ-TiAl alloys, thermogravimetric quasi-isothermal oxidation tests in air at 750 °C after 1000 h exposure showed four times smaller weight gain compared to the uncoated material. In sulphidation tests after 1000 h exposure to aggressive H2/H2S/H2O atmosphere the CrAlYN/CrN protected γ-TiAl alloys showed a weight gain reduced by factor of four as compared to the uncoated substrate. High temperature pin-on-disc tests revealed that the friction coefficient reduces from 0.56 at room temperature to 0.4 at 630 °C.