Electroless Deposition and Characterization of High Phosphorus Ni-P-Si3N4 Composite Coatings

Composite coatings were prepared using hypophosphite reduced electroless nickel bath containing 1g/L submicron silicon nitride particles at pH 4.6 ± 0.2 and temperature 85 ± 2°C. Deposition rate was 6-8 μm/hour for both plain Ni-P and composite coatings. The amount of silicon nitride particles codeposited in the Ni-P matrix was around 3.5 wt.%. As-deposited coating surface composition analysis, carried out by Energy Dispersive Analysis of X-ray (EDX), results showed that plain Ni-P and Ni-P-Si3N4 deposits were having around 10 wt.% phosphorus. The X-ray diffraction (XRD) pattern of Ni-P-Si3N4 coating was very similar to that of plain electroless Ni-P coating in as-deposited condition. Presence of a single, broad peak around 45° 2θ which corresponds to Ni (111) peak was seen in both deposits. The calculated grain size by Debye-Scherrer method for both deposits was around 1.2 nm. Optical micrograph of the deposit cross-section revealed that the particles incorporation was uniform throughout the thickness of the coating. Phase transformation behavior studied by Differential Scanning Calorimeter (DSC) indicates that the particle incorporation had not influenced the crystallization temperature of the composite coatings. Presence of metastable phases like NiP2 and Ni5P4 were observed for both coatings annealed at crystallization temperature. Increase in grain size of the deposits from 1.2 nm to 21 nm was also observed due to the annealing at crystallization temperature. Microhardness measurements made on the as-deposited and annealed (400°C) cross-sectional coatings showed that there was about 10% and 22% increase in hardness values respectively with the codeposition of silicon nitride particles.