Morphology and Properties of Chemical Coatings on Steels After Extreme Thermal Laser Radiation Effects

The results of metal-physical studies of the specifics of structure formation of the chemical Ni–P coatings under pulsed laser irradiation and rapid crystallization of the surface layers of coated steels are presented. As a result, Ni₃P type chemical compounds with a particle size of 0.05–0.25 μm and a given spatial orientation are formed on the surface of irradiated steels, which occupy 30–50% of the irradiated zone volume. This leads to an increase in hardness, wear resistance, and a decrease in the friction coefficients of the irradiated material surfaces. The problem of improving the properties, preventing cracking and grain growth in the “coating – steel substrate” composition is solved. In this case, by using certain modes of pulsed laser surface irradiation, it becomes possible to preserve the properties of the core metal. It is shown that an increase in adhesion between coatings and a metal substrate occurs due to the formation of a 2–3 μm thick transition layer. This is more than the thickness of the transition layer (0.16–0.2 μm) formed in the process of extended bulk heating of the coatings. According to the metal-physical studies, this ensures a smooth transition of properties from the coating to the steel substrate along with a decrease in the localization of stresses at the “coating – substrate” interface. The reason for this has to do with the diffusion processes during mixing of the coating with the surface layer of the steel substrate melted by laser irradiation. It has been established that the possible perturbation of the micro-geometry of the irradiated surface of the coatings should be corrected by choosing the coefficient of overlapping of the irradiation spots in the range of 0.7–0.8. Regression models for predicting the structure and properties of the irradiated “chemical coating – steel substrate” composition have been created, which allow obtaining the required mechanical, technological, and performance properties of the irradiated products with various functionalities by purposefully selecting the laser treatment process parameters. It was found that after the laser treatment of Ni–P coatings, the hardness of the surface layers reaches its maximum values at a radiation power density of 100 MW/m² (steels U8A, P6M5, and P18) and 90 MW/ m² (steel X12M), while the values of hardness and performance properties are 10–60% higher than those of the coatings without laser treatment.