Anti-adhesion performance of various nitride and DLC films against high strength steel in metal forming operation

High strength steel (HSS) is widely used for automobile reinforcement parts and the quantity required is rapidly grown. However, the strength and hardness of the steel are relatively high, its formability is very low and adhesion to tool material can be easily found under forming operation. This paper aimed to evaluate the anti-adhesion performance of commercial nitride and DLC films coated on cold work tool steel against HSS in forming operation. The friction coefficient and wear rate of the non-coated ball (SKD11; hardness 60 ± 2 HRC), balls coated with TiN-PVD, TiCN-PVD, AlTiN-PVD, Nitride + CrN and DLC have been evaluated in sliding contact against SPFH 590 (JIS) disk. The scratch and nano-indentation tests were done on each type of coated tools to characterize the adhesive strength between the film and the substrate, and the hardness and the elastic modulus, respectively. The anti-adhesion performance of various films coated tool in metal stamping process was also investigated by performing U-bending experiment. The cold roll carbon steel; SPCC (JIS) was also used to compare a material transfer problem to the case of using HSS (JIS: SPFH590). As the results, for HSS sheet, the adhesion of workpiece material on a non-coated die surface was detected after 49 strokes whereas adhesion could not be found in case of stamping SPCC sheet up to 500 strokes. The TiCN, AlTiN, and Nitride + CrN films showed good anti-adhesion performance when forming HSS, while the TiN and DLC films did not provide the satisfied results.