Heat treatment effects on EN-PTFE-SiC composite coatings

Electroless nickel (EN) composite coatings embedded with SiC and PTFE particles were deposited on mild steel substrates. Postdeposition heat treatment effects on coating properties were studied in a N2 atmosphere. Differential scanning calorimetry (DSC), X-ray diffractometry (XRD), scanning electron microscopy (SEM), EDX and EIS were used to analyse the temperature dependence of the microstructure, composition, mechanical and corrosion properties of the coatings. It was found that the as-deposited EN-PTFE-SiC coatings had a homogeneous structure with microsized SiC and PTFE particles embedded in an amorphous Ni–P matrix. Phase transition from an amorphous Ni–P matrix to a mixture of polycrystalline Ni and Ni3P alloy occurred at around 340 °C. The highest microhardness and best adhesion properties were observed for the samples annealed at 400–450 °C for 1 h. It is interesting to note that the embedded PTFE particles were still stable even when heat treated at 450 °C, probably owing to the Ni–P metal matrix package effect. EIS results showed that proper heat treatments also enhanced the corrosion resistance in both acidic (1.0 N H2SO4) and salty (3% NaCl) media, attributing to the coating density and structure improvement.