Tribological properties of structural steel modified by fine particle bombardment (FPB) and diamond-like carbon hybrid surface treatment

To improve the poor adhesion of diamond-like carbon (DLC) coatings to steel substrates, we have proposed a hybrid surface treatment, which is a combination of fine particle bombardment (FPB) and DLC coating. Specimens were treated by FPB using Cr shot particles prior to coating with DLC by the ionizing deposition method. Specimens with only the DLC coating without the FPB treatment (single-treated) were also prepared. The FPB treatment increased substrate hardness and roughness. In addition, a Cr-rich layer was formed on the surface because of the mechanical mixing of Cr shot particles into steel substrate. Reciprocating sliding wear tests were conducted to investigate the coefficient of friction. While the single-treated specimens showed a sudden increase in the friction coefficient resulting from delamination of the DLC coating, the hybrid-treated specimens maintained a low friction coefficient during wear tests indicating strong adhesion of the DLC coating. The reasons for good adhesion of the hybrid-treated specimens were investigated in terms of surface hardness and Cr enrichment. The relationship between substrate hardness and friction coefficient was investigated by preparing substrates with different hardness values. The results indicated that an increase in hardness is effective in suppressing of DLC coatings delamination, whereas a significant increase in hardness was not achieved by the FPB treatment. A roughened substrate without the Cr-rich layer was also prepared to examine the effects of substrate roughness and the presence of Cr. The results imply that strong adhesion of the DLC coating of the hybrid-treated specimens was due to the presence of Cr, which suppressed the delamination of the DLC coatings.