Thermal reliability of electroless Ni–P–W coating during the aging treatment

Abstract

The Ni–P–W coating was fabricated by electroless technique in the alkaline solution. The phase transition temperature of Ni–P compound analyzed by differential scanning calorimetry (DSC) was 406 °C. To evaluate the thermal stability of Ni–P–W coating under long-term aging treatment, the operation temperature below and above the crystallization temperature of Ni–P compound was chosen at 375 and 450 °C, respectively. The phase transformation during thermal evolution was studied by X-ray diffraction (XRD). The microstructure and grain size at different stages of aging time were identified by transmission electron microscope (TEM). The surface roughness at as-deposited and final heat treatment states were explored by atomic force microscope (AFM). During the long-term heat treatment at 375 °C, the microhardness firstly increased owing to the crystallization of nickel as well as Ni 3P and then maintained an elevated hardness value. The Ni–P–W coating at 450 °C also exhibited a high elevated-temperature reliability after aging treatment. The hardness of the Ni–P–W coating was substantially increased to 1350 HK for 30 min and reached a maximum hardness of 1460 HK, with grain size of 13 nm for 4 h. However, the strength of the coating was gradually degraded after excessive aging and reached a steady state value around 1200 HK over 20 h, with the grain size pinned around 30 nm.