Inhibitory effect of Ni-P coating on thermal expansion of carbon steel

The thermal expansion of carbon steel can be inhibited by electroless deposited Ni-P coating. The diameter of sample increases by only 3% after plated. The linear coefficient of thermal expansion (CTE) of the substrate is decreased from 16 ppm/K to as low as 12.68 ppm/K with a reduction rate of 20.72%. At 613 K, Ni-P coating crystallizes with a volume shrinkage which partly counteracts the thermal expansion of the substrate and then induces a small CTE, which is named crystallization compensation mechanism. After crystallization, a diffusion layer composed mainly of FeNi3, one of the primary phases of NiFe Invar alloys, generates in-situ at the interface of coating and substrate. This diffusion layer plays a key role in inhibiting the thermal expansion of iron substrate, which is named diffusion-layer inhibitory mechanism. These findings are, in principle, applicable to all the ferrous alloys so that a potential easy way can be promoted to decrease the CTEs of ferrous alloys. (C) 2017 Elsevier B.V. All rights reserved.