Adhesion of Ti(C,N) PECVD coating on plasma nitrided L-PBF 18Ni300 maraging steel

This study aims to investigate the effect of varying plasma nitriding intensities, followed by coating and the light grinding step (between the nitriding and coating), which is designed to enhance the adhesion properties of L-PBF 18Ni300 maraging steel. Adhesion performance was evaluated using Rockwell indentation and scratch testing. More intense plasma nitriding significantly improves surface hardening and thickness of the diffusion layer, thereby increasing load-bearing capacity and reducing coating delamination. The surface hardening provided by the presence of Fe4N and Mo2N precipitates was observed by detailed TEM observation. The sample subjected to soft grinding exhibited acceptable adhesion in indentation tests but demonstrated lower adhesion in scratch tests. This discrepancy is due to the different loading modes and sensitivity to interfacial features between the indentation and scratch tests. The presence of interfacial oxides, steep residual stress gradient, and relatively low surface roughness compromised adhesion under scratch testing. Anisotropic behaviour was also observed. In intense nitrided and ground samples, scratch testing along the parallel direction resulted in better adhesion compared to the perpendicular one. This work highlights that coating adhesion in duplex-treated L-PBF maraging steels is not solely controlled by hardness and diffusion layer thickness, but is also strongly influenced by microstructure of the interface, surface roughness, residual stresses and build direction. Overall, the intensely nitrided and coated sample has demonstrated the best adhesion performance, considering both testing methods.