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 3 P 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. © 2004 Elsevier B.V. All rights reserved.