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

摘要

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 gamma-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 L-C = 37 N have been measured in scratch adhesion tests. Impact tests brought further evidence for strong bonding. For coated gamma-TiAl alloys, thermogravimetric quasi-isothermal oxidation tests in air at 750 degrees 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 H-2/H2S/H2O atmosphere the CrAlYN/CrN protected gamma-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 degrees C.