The effect of nitriding on adhesion and mechanical properties of electroless Ni–P coating on AISI 4140 steel

In this work, electroless nickel has been deposited onto steel substrates by a selfcatalytic process, which involves temperature and time to create a Ni–P layer with a uniform thickness. The major drawback of electroless nickel is the interdiffusion post heat treatment (PHT) that is necessary to create a metallurgical bond between the steel substrate and the Ni–P deposit. The PHT results in softening of the steel. The aim of this ongoing research is to investigate the effect of introducing a nitrided layer prior to deposition of Ni–P layer, as an attempt to lessen the softening of the steel. To this end, Ni–P coatings were obtained over nitrided and non-nitrided steel substrates. The PHT was done at 400°C or 610°C. The nitrided layer, indeed, avoided the softening of the steel substrate. In addition, the nitrided layer improves the adhesion of the Ni–P coating even when the PHT was done at 400°C.