Development of ceramic particle reinforced nickel based composite coatings with electroless coating method

Studies focused on the electroless nickel coatings onto aluminum substrates has increased due to their excellent properties for many applications to enhance the surface properties of the aluminum. In the present thesis, development of electroless nickel polyalloy deposits is considered as the most effective method to alter the chemical and physical properties of binary NiP and NiB alloy system. Although many researchs on polyalloys of electroless nickel deposition were reported in literature, there are few works on electroless coatings with combined boron and phosphorus. In this thesis, two different reducing agents are used simultaneously to reduce nickel and impregnate the boron and phosphorous into reduced nickel matrix. The evolution mechanism of electroless NiBP deposition and NiX (X=B,P) systems were examined comparatively. Electroless composite coatings have been widely used in many applications to provide protection against corrosion as well as wear. In order to analyze the effect of the secondary ceramic particles in NiBP matrix, Si3N4 and SiC particulates were incorporated into NiBP coatings. The surface and cross-section microstructures of the coatings were observed with Scanning Electron Microscopy (SEM). The corrosion properties were characterized by Tafel extrapolation method in NaCl solution. Moreover, nanoindentation tests were performed on the cross-section of deposits before and after the heat treatment to measure the hardness. Tribological tests were performed on a reciprocating ball-on-disk against an ø 6 mm Al2O3 ball according to DIN 50 324 and ASTM G 99-95a standards in a ball-on-disk configuration. The results showed that the best corrosion and tribological performance was obtained in the composite coatings prepared with 20 g/L Si3N4 ceramic particles in the NiBP coating bath.