Preparation and characterization of silicon nitride codeposited electroless nickel composite coatings

Ferrous based bearings can be suitably used for water lubricated applications by reducing the corrosion with the application of electroless nickel coating. Enhancement of wear resistance can be achieved by codepositing silicon nitride (Si3N4) which possesses high hardness and chemical inertness. This paper reports the characteristic of a typical medium phosphorous (6.5 wt.%) electroless nickel coating. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies showed the amorphous to crystalline transition with precipitation of Ni3P and Ni around 340 °C even when 2.9 wt.% Si3N4 was present in the composite. Results showed increasing co-deposition of second phase upon increasing temperature, pH and bath loading; highest incorporation (4 wt.%) was at 10 g/l of suspension. Scanning electron microscopic (SEM) photographs of the cross-section and surface of coated specimen revealed most desired uniformity at 2.9 wt.% from 5 g/l bath loading. Electrochemical measurements showed that the electroless nickel composite coating with 2.9 wt.% Si3N4 (ENC) has almost similar corrosion rate as that of carbon steel in demineralised water adjusted to pH 10 by addition of LiOH. Tribological tests of AISI 52100 ball-bearings and races coated with 4 μm thick ENC in demineralised water of pH 10 using four-ball tester showed satisfactory performances in terms of weight loss, pitting levels and coefficient of friction and life time of about 9 years under the actual application environment.