In-situ SEM study of crack initiation, propagation and interfacial debonding of Ni-P coating during tensile tests: Heat treatment effect

This study focuses on the effect of annealing temperature onto the mechanisms of crack initiation, propagation and interfacial debonding of the electroless nickel/phosphorus (Ni-P) coatings deposited on steel. An in-situ tensile test has been carried out in a SEM chamber for coated samples annealed at 400 and 600 degrees C, and at a strain rate of 0.1 mu m/s. It is observed that when an as deposited Ni-P on steel system is subjected to a uniaxial tensile test, a bended crack is formed on the surface of coating and leads to a debonding of coating at a small strain level. Incontrast, in case of an annealed sample, the N-iP coating shows a severe multiple parallel crackings due to the localized plastic deformation of the substrate. The crack density firstly increases with the rising tensile strain, and then reaches saturation. The spacing between the resulting saturated parallel cracks in the coating surface is defined in the shear lag model to quantify the interfacial shear strength. Despite the large plastic deformation of the substrate, there is no spallation observed at the interface between the two materials. The hardness and the young's modulus of the coating before and after heat treatment are determined by nano-indentation tests. The stiffness of the coatings increases owing to the Ni3P phase precipitation during heat teatment. (C) 2016 Elsevier Inc. All rights reserved.