Thermal stability and microstructure characterization of sputtered Ni–P and Ni–P–Cr coatings

Ni–P and Ni–P–Cr alloy coatings were deposited onto silicon substrates by the RF magnetron sputtering technique with dual targets of electroless nickel–phosphorus alloy and chromium. To evaluate the effect of doping with a third element on the enhancement of thermal stability in Ni–P coatings, differential scanning calorimetry (DSC) was used to identify the onset temperature of phase transformation. The crystallization behavior at different stages of phase transition in the DSC plot was further evidenced by X-ray diffraction (XRD). Microstructural evolution of the Ni–P based coating in the as-deposited and heat-treated states were investigated by transmission electron microscopy (TEM). The broad peak in the DSC plot of both Ni–P and Ni–P–Cr deposits is attributed to the grain coarsening of nickel on the basis of XRD and TEM analyses. The addition of Cr atoms into Ni–P based coating not only suppresses the formation of Ni3P but also reduces the grain size of precipitates.