The effects of continuous heating on the crystallisation kinetics and the phase transformation behaviour of electroless nickelphosphorus 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 508C / 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 300-8008C at 208C / min. The sequence of phase transformations was found to be: amorphous phase→intermediate metastable phases1stable Ni P phase (1f.c.c. nickel)→stable Ni P phase (1f.c.c. nickel). 3 3 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 8008C (at 208C / min).

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Electroless nickel-phosphorus deposits with 5-8 wt% P and 3-5 wt% P were analysed for the effects of continuous heating on the crystallization kinetics and phase transformation behaviour of the deposits. The as-deposited coatings consist of a mixture of amorphous and microcrystalline nickel phases, featuring in their X-ray diffraction patterns. Continuous heating processes to 300• C-800• C at 20• C/min were carried out on the deposits in a differential scanning calorimetric apparatus. The subsequent X-ray diffraction analyses show that the sequence of phase transformation process was: amorphous phase + microcrystalline nickel → f.c.c. nickel + Ni 3 P stable phases. Preferred orientation of nickel {200} plane developed in the deposits after the heating processes. Differential scanning calorimetry of the deposits indicates that the crystallization temperatures increased with decreasing phosphorus content, and increasing heating rate. Crystallization activation energies of the deposits (230 and 322 kJ/mol, respectively) were calculated using the peak temperatures of crystallization process, from the differential scanning calorimetric curves at the heating rates ranging from 5 to 50• C/min. It was found that the deposit with lower phosphorus content has higher activation energy. C 2002 Kluwer Academic Publishers

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The crystallisation and phase transformation behaviour of electroless Ni-P deposits during thermal processing have a vital role in determining their material properties. Conflicting results have been reported for the correlation of crystallisation and phase transformation behaviour with deposit composition and thermal processing, and hence microstructure and material properties. These variations are believed to be caused by variations in factors including plating conditions, substrate composition and surface preparation, and testing and measuring techniques. The mechanism of electroless Ni-P plating is discussed briefly and the literature on the effect of crystallisation and phase transformation behaviour on the microstructure and material properties of electroless Ni-P deposits is critically reviewed. A comparison is made with other commonly used engineering deposits such as electroplated nickel and hard chromium. The advantages, limitations, and development of electroless Ni-P are outlined and the use of heat treatment to tailor properties for specific industrial applications are described. Finally, the challenges this technology faces in the future are considered.