Effect of microstructure on the adhesion strength of TiBx coating on Ti6Al4V substrate

TiBx coatings were deposited on Ti6Al4V and Si (100) wafer substrates by D.C. magnetron sputtering with various target-to-substrate distances (T.S. distances) from 50 mm to 200 mm. The influence of T.S. distance on the microstructure, hardness and adhesion strength of TiBx coatings and Ti6Al4V substrate system was investigated. Results showed that the microstructure of TiBx coatings transformed from dense to fibre columnar grain with the increase in T.S. distance, whilst the hardness decreased from 20.9 GPa to 9.4 GPa. The Rockwell-C indentation adhesion strength grade was also improved from HF6 to HF1. An adhesion evaluation factor G, which is related to the mechanical properties and the microstructure of TiBx coating, is proposed based on the test results. The adhesion strength increased with G, which corresponded well with the results of indentation test. The high-speed rubbing test with a sliding speed of 300 m/s was performed to check the Al-adhesion resistance of the TiBx coating against Al-hBN seal coating.