DEPENDENCE OF PHYSICAL AND MECHANICAL PROPERTIES OF NI‒P-COATING ON THE HEAT TREATMENT MODE AND THE FORMED PHASE COMPOSITION

Abstract

The appearance of defects is possible on the surface of parts coated in the process of chemical nickel plating after its crystallization annealing. The effects of heat treatment modes of steel samples with composite Ni‒P-coating on the phase composition, hardness and levels of elastic deformations of the substrate leading to the formation of cracks in the coating has been evaluated. By uniaxial tension at a low rate (1 mm/min) of load application on specially prepared samples, the cracking load was initially recorded and repeatedly confirmed. The fact of coating defects (cracks and pore chains) was established by means of wetting the controlled surface with indicator solution. The experimental data were recalculated by the method of sections into true values of stresses arising in the material of the sample and coating. The Vickers hardness at a load of 100 g was checked on each sample. The dependence of the surface hardness level of coated samples on the ratio of crystalline phases in its composition was revealed. A coating consisting of 70 % of crystalline nickel and 15 % Ni3P is characterized by high hardness (at the level of 1010 HV) and low cracking threshold (100‒170 MPa). If more than 65 % nickel phosphide is present in the structure, the hardness of the coating is slightly higher at 1050 HV. At the same time, the level of deformations contributing to the appearance of defects is higher by 100 % and it is within the range of 270‒340 MPa. The revealed patterns of formation of physical and mechanical characteristics of the Ni‒P-coating made it possible to determine and recommend the technological mode of heat treatment of parts with coating which creates the optimal properties of parts used in gas, oil and petroleum products transportation equipment.