Mardarevych R. Development of a two-layer electrochemical composition coatings of Ni-B-Cr systems for the increase of wear- and corrosion resistanсe of carbon steels wares

The process of electrochemical deposition and thermal treatment of two-layer Ni-B-Cr composite coating and its properties. Development of new composite coatings based on nickel, boron and chromium, to improve durability of carbon steels under conditions of stress, aggressive media and high temperatures. Optical and scanning electron microscopy, X-ray diffraction, EDS, differential thermal and chemical methods of analysis, DC polarization methods, mechanical, fatigue, wear and corrosion tests. The technology of forming two-layer composite electrochemical coatings of Ni-B-Cr system for tribotechnical application was developed. Their wear resistance under dry friction at specific loads of 0.5 - 2.0 MPa is in 1.5 - 5.3 times higher comparing with galvanic chrome plating or hardened carbon steel. It was shown that Ni- B-Cr coatings correspond to number 2 severity of corrosion (on a 10-point scale) and due to dissolution rate in 26% sulphuric acid can be related to the group of very resistant materials. Heat-resistant coatings in air at temperatures of 700-900°C is close to heat-proofness of Nickel-Chromium alloys. Coatings increase cyclic durability of carbon steels on 50-75% at elevated (500-700 ° C) temperatures. It was calculated and experimentally confirmed that the high-quality filled coatings of stable composition form at a suspension flow rate of 0.15-0.2 m/sec. Surfactants under optimal concentration were used for the deposition of Ni-B primer layer of hypereutectic composition. Minimal temperatures (420 and 490°C) of solid state reactions during heating of two-layer compositions, the influence of annealing parameters on the diffusion redistribution of coating components and peculiarities of boride phases formation in double-layer coatings with different content of boron were determined. Technological recommendations of composite coatings formation for surface hardening of metallizators, extruders, casting forms and other products used in machine-building enterprises were developed.