Mechanical properties of DLC coating sputter deposited on surface nanocrystallized 304 stainless steel

Surface nanocrystallization (SNC) can markedly improve surface mechanical properties of metallic materials and accelerate thermal diffusion of elemental atoms. In this work SNC is used as a pretreatment method to improve mechanical properties of the DLC coating on the 304 stainless steel. Surface mechanical attrition treatment (SMAT) was used to generate a nanocrystalline surface layer on the steel substrate. Then the DLC coating with Cr adhesive interlayer was sputter deposited on the coarse-grained and the surface nanocrystallized steel substrates (noted as CG-DLC, SNC-DLC samples, respectively). X-ray diffraction confirms an amorphous nature of the carbon coating. Elemental analysis by energy dispersive X-ray reveals the enhancement of Cr diffusion into the steel substrate for the SNC-DLC sample. The microhardness, scratch, Rockwell-C indentation, cyclic impact and wear tests show that the SNC-DLC sample has higher surface hardness and load-bearing capacity than the CG-DLC sample. The mechanical tests also evidence that the SNC-DLC sample possesses higher film cohesion strength and better wear resistance as compared to the CG-DLC sample. Mechanisms for the enhancement of cohesion and adhesion strength of the DLC coating on SNC stainless steel substrate are discussed. This work may provide an advantageous hybrid processing for improving surface mechanical properties of austenitic stainless steel and other relatively soft structural steels compared with the conventional duplex treatment consisting of thermal–chemical processes (nitriding, carburizing, etc.) and hard coatings.