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 degrees C. Deposition rate was around 6 mu m/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 I 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 degrees 20 which corresponds to Ni(111) 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(I 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 degrees 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. (c) 2007 Elsevier B.V. All rights reserved.