Investigation on interfacial adhesion of Ti-6Al-4V/nitride coatings

Both 10-meter ball-on-disk tests and 0.01-meter single pass sliding tests were performed under the same test conditions to compare the adhesion mechanism of titanium alloy Ti-6Al-4V to AISI 52100 steel and two nitride coatings deposited by physical vapor deposition technique (PVD), namely, AlTiN and CrN. The sliding surfaces and the interface region between the transfer layer and the counterface surfaces were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and energy dispersive spectroscopy (EDS). The results revealed that the adhesion of titanium alloy to the counterface materials initiated during the initial stage of sliding in unlubricated sliding contact and resulted in the transfer of titanium alloy to the counterface surfaces. It was found that the transferred Ti-6Al-4V had a layered amorphous structure in which nanocrystalline and polycrystalline oxides were embedded. Severe oxidation of titanium was observed in all the tests. However, Ti-6Al-4V showed less severe oxidation behavior when sliding against CrN. High resolution TEM (HRTEM), electron diffraction, and fast Fourier transform (FFT) investigations carried out at the interface region between the transfer layer and the counterface surfaces revealed the presence of nanocrystalline TiO (C2/m) at the transfer layer interface with steel and AlTiN. However only α-titanium (P63/mmc) was detected at the CrN/transfer layer interface.