Role of nickel undercoat and reduction-flame heating on the mechanical properties of Cr-C deposit electroplated from a trivalent chromium based bath

In this study, a surface hardening method for Cr-C deposits using flame heating for a short period is proposed. The hardness and wear resistance behavior of as-plated and flame-heated Cr-C deposits were investigated. The Cr-C deposits were electroplated on high carbon tool steel in a bath with trivalent chromium ions. Experimental results show that Cr-C deposits with an extremely high hardness of ca. 1700 Hv can be achieved after reduction-flame heating for 1 s. Meanwhile, the wear resistance of Cr-C deposited steel was improved after flame heating. To increase the wear resistance and bonding strength of the Cr-C deposited specimen, pre-electrodeposition of a thin Ni deposit between the Cr-C deposit and the steel substrate is recommended. A Ni undercoat with a thickness of few micrometers could reduce the crack density in the Cr-C deposit and increase the bonding strength between the Cr-C deposit and the steel substrate. As determined by a microstructure study, the as-plated Cr-C deposit has an amorphous structure, but transforms to a crystalline structure after flame heating. The main hardening mechanism is a result of the precipitation of nanograined diamond membranes, which can be extracted by immersing the flame-hardened Cr-C deposit in an etching solution comprised of 33 vol.% HNO₃ and 67 vol.% HCl.