Failure Mode of Micron-thickness Hard Coatings on Soft Substrate in Scratch Test

Hard coatings of 1-3 μm thickness,like DLC,TiC,TiN and Ti-Si-N,were prepared by magnetron sputtering on soft stainless steel substrates.They failed in the same way during the scratch test,i.e.,along the brim of scratch,cracking and spalling took place throughout the test,but at the center,cracking appeared first,then partial spalling and finally massive spalling occurred.In contrast,for submicron-thickness(0.5μm) DLC coatings,cracking and spalling occurred at the center appeared from the very beginning of the scratch.The intensity and frequency of acoustic emission peaks correlated to the scale of cracking and spalling in the thick coatings,but this was not the case for DLC coatings of sub-micron thickness.