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

Interest in electroless plating of nickel-based ternary alloys has increased because of their excellent corrosion, wear, thermal and electrical resistance. They also possess good magnetic properties. In the present investigation, the effect of copper and tungsten in alkaline electroless nickel baths has been studied in depositing Ni–Cu–P and Ni–W–P alloys and also the synergistic effect of ions in depositing Ni–W–Cu–P alloys. Deposits were characterized using XRD, scanning electron microscopy (SEM), energy-dispersive analysis of X-ray (EDX) and atomic force microscopy (AFM) techniques. XRD results revealed that not much variation in structure and grain size has been found in Ni–Cu–P deposit. A decrease in phosphorus content and a marginal increase in grain size have been observed due to the tungsten addition in the Ni–P deposit. Addition of copper in Ni–W–P baths has resulted in a quaternary deposit, Ni–W–Cu–P, with increased crystallinity. SEM studies showed that presence of coarse nodules in ternary Ni–Cu–P and Ni–W–P deposits. Addition of copper in Ni–W–P baths has resulted in a very smooth deposit. Studies by AFM on deposits have proved that the copper has suppressed coarse nodules by inhibiting their growth in quaternary deposit. No considerable change in hardness has been noticed in both as-plated and heat-treated deposits due to the inclusion of copper in Ni–W–P deposit. A marginal improvement in corrosion resistance has been observed in quaternary alloy compared to ternary (Ni–Cu–P or Ni–W–P) alloys.

Electroless deposition of Ni–Cu–P, Ni–W–P and Ni–W–Cu–P alloys

Electrochemical studies on electroless ternary and quaternary Ni-P based alloys

Four samples of electroless nickel–phosphorus (EN) deposits coated on mild steel substrate have been analysed for their hardness changes in relation to the deposit phosphorus contents as well as different heating temperatures at isothermal (100–500 °C for 1 h) and linear heating (to 300–600 °C at 20 °C/min) conditions. It was found that the hardness of the EN samples increased with decreasing phosphorus content at as-deposited condition, and could be enhanced by appropriate heating. The results of Vickers microindentation testing showed that the peak hardness of the EN samples could be achieved after heat-treating at 400–450 °C. This is caused by the formation of intermetallic Ni 3 P stable phase at this temperature range, acting as a function of precipitation hardening. The Knoop microindentation testing on the cross sections of the samples indicated variations of the hardness across the depth (distance from the sample surface towards the sample/substrate interface), but for most samples the tendencies of change were not clear. Scanning electron microscopy analysis has shown that the lamellar structure present in the cross sections of the as-deposited EN samples tends to disappear, and agglomeration occurs when the heat-treating temperature is increased. The concept of kinetic strength ( K s ) is adopted to interpret the kinetic energy ( Q ) of increased hardening effects using the Vickers hardness data after the isothermal experiments, but the result has not been satisfactory.

Hardness evolution of electroless nickel-phosphorus deposits with thermal processing

The crystallization behavior of electroless NiP and NiCuP was studied comparatively by using differential scanning calorimetry and X-ray diffractometry. It is apparent that low-P NiP deposits transform to the stable phase Ni3P directly, but low-P (high-Cu) NiCuP deposits transform to the metastable phase Ni5P2 first, and then to the stable Ni3P. Both the hypereutectic amorphous NiP deposits and amorphous NiCuP deposits with high phosphorus content transform to the metastable phases Ni5P2 and Ni12P5 first, then to the stable phase Ni3P. For the amorphous NiP and NiCuP deposits with P content of approximately 10 wt.%, the crystallization temperature of the latter is markedly higher than that of the former. In addition, the crystallization temperature of the hypereutectic NiP deposit is nearly the same as that of the amorphous NiCuP deposit with a similar P content. For the crystalline NiP and NiCuP deposits with low P content, the temperature at which the stable Ni3P forms in the latter is obviously higher than that of the former.

Q.X. Mai

Papers

Structural changes induced by heating in electroless nickel-phosphorus alloys