Effect of Different Pretreatments and Heat Treatment on Wear Properties of Electroless Ni-B Coatings on 7075-T6 Aluminum Alloy

AbstractThe effect of various pretreatments on wear behavior of electroless nickel - boron (ENi-B) deposits obtained from a thallium free bath of nickel chloride as source of nickel is presented in this paper. For any quality coating pretreatment is required. The adhesion of coating significantly depends on the pretreatment. Hence in the present investigations five different pretreatments such as zincate (Zn), Ni strike 1 (Ni 1) and Ni strike 2 (Ni 2), absorbed hypophosphite layer (AHL) and Palladium strike (PD) were given to the substrate prior to the ENi-B coating and their corresponding wear properties were examined. The quality of coatings with and without heat treatments was estimated by pin on disc wear test method as per ASTM G99 standard. The surface morphology of the coated samples were examined by scanning electron microscope (SEM). The structural characterization of the coated samples was examined by X- ray diffraction (XRD) technique. The results obtained indicated that the wear resistance of the ENi-B deposits were significantly influenced by the pretreatment conditions. Among the five different pretreatments, AHL provided better adhesion of coatings over the aluminum alloy. The quality of pretreatments with respect to wear and coefficient of friction are rated as follows: AHL > PD > Ni 2 > Zn > Ni 1. AHL pretreatment significantly improved the wear resistance and decreased the average coefficient of friction. Further to know the influence of heat treatment on wear behaviour of coated samples, the samples were heated at 400° C. It was found that heat treatment improved the wear resistance from ∼ 30μm to ∼ -70μm. After heat treatment the average coefficient of friction of AHL samples decreased from 0.079 to 0.009. The complete experimental results and analysis are reported in main paper