Crystallisation kinetics and phase transformation behaviour of electroless nickel–phosphorus deposits with high phosphorus content

Abstract

The effects of continuous heating on the crystallisation kinetics and the phase transformation behaviour of electroless nickel–phosphorus deposits plated on mild steel substrates, with high phosphorus contents of 12 and 16 wt.%, were studied. Both the deposits exhibited an amorphous X-ray profile in the as-deposited condition. The crystallisation temperatures of the deposits increased with decreasing phosphorus content, and increasing heating rate. The activation energies during the crystallisation processes were calculated from the differential scanning calorimetry (DSC) curves of the deposits at heating rates ranging from 5 to 50°C/min. It was found that the activation energy is slightly higher in the deposit with lower phosphorus content. X-ray diffraction (XRD) analyses were also conducted on the deposits after heating processes in the DSC apparatus to 300–800°C at 20°C/min. The sequence of phase transformations was found to be: amorphous phase→intermediate metastable phases+stable Ni 3P phase (+f.c.c. nickel)→stable Ni 3P phase (+f.c.c. nickel). Electron microprobe analysis was carried out on the 16 wt.% P deposit and showed that the iron content in the deposit was affected by heating processes. Film growth in the 16 wt.% P deposit has been shown by scanning electron microscopy (SEM) and electron microprobe analysis on the cross-sections of the deposit after heating to 400 and 800°C (at 20°C/min).