Preparation and characterization of autocatalytic low phosphorus nickel coatings containing submicron silicon nitride particles

This paper addresses the structural characteristics and phase transformation behavior of plain electroless Ni–P and Ni–P–Si3N4 composite coatings. Composite coatings were prepared using a low phosphorus electroless nickel bath containing 1 and 5 g/L submicron silicon nitride particles at pH 6.5 and temperature 85 ± 2 °C. Deposition rate was around 6 μm/h for both plain and composite coatings. The amount of silicon nitride particles codeposited in the Ni–P matrix is 3.5 wt.% (N1) and 4.5 wt.% (N2) with the addition of 1 and 5 g/L particles in the bath. Deposit surface composition analysis carried out by energy-dispersive analysis of X-ray (EDX) results shows that plain Ni–P and Ni–P–Si3N4 deposits are having around 3.5 wt.% phosphorus. The X-ray diffraction (XRD) patterns of Ni–P–Si3N4 coatings are very similar to that of plain electroless Ni–P coating in as-deposited condition. Presence of a single high intensity peak at 44.8° 2θ which corresponds to Ni(1 1 1) peak is present in both deposits. Presence of Ni(2 0 0) and (2 2 0) peaks is also observed in composite coatings apart from Ni(1 1 1) peak. The calculated grain size using Debye–Scherrer method for both deposits is between 11 and 14 nm. It shows that inclusion of silicon nitride particles has a marginal influence on the grain size of the composite coatings. Optical micrographs of deposit cross-sections reveal that particles incorporation is uniform throughout the thickness of the coating. Anodically etched metallographic cross-sections of both deposits revealed no banded/lamellar structure. It shows that the phosphorous content in the deposit is uniform. This was confirmed with the cross-sectional scanning electron microscope (SEM)–EDX carried out on the deposits. A marginal variation in phosphorus concentration is observed. Phase transformation behavior studied at different scanning rates by differential scanning calorimeter (DSC) indicates that there is a slight increase in crystallization temperature around 20 °C with the incorporation of silicon nitride particles compared to plain Ni–P coatings. Microhardness measurements made on the cross-section of as-deposited coatings showed that there is about 15% improvement in microhardness with the codeposition of silicon nitride particles in Ni–P matrix.