Structural behavior and mechanical properties of high-entropy coatings synthesized by HVOF

This paper discusses the technology, structure and functional–mechanical properties of surface layers made of high-entropy materials with a high-temperature shape memory effect, nickel (Ni)–cobalt (Co)–titanium (Ti)–zirconium (Zr)–hafnium (Hf), nickel–copper (Cu)–titanium–zirconium–hafnium and titanium–nickel–zirconium–hafnium–cobalt–copper. The developed technology and equipment for the formation of high-entropy coatings on the surface of steel products included mechanical activation of powders, high-velocity oxygen fuel (HVOF) spraying in a protective atmosphere and subsequent thermal and thermomechanical treatment. This allowed carrying out this process in a single technological cycle. The authors have developed statistical models of the formation process of highly entropic HVOF coatings in a protective atmosphere according to the criteria of adhesive strength and porosity with optimization of parameters. Based on complex X-ray diffraction and electron microscopy studies, the authors determined the structural parameters of high-entropy alloy coatings obtained by HVOF, followed by thermal and thermomechanical treatment. It was shown that all three alloys, nickel–cobalt–titanium–zirconium–hafnium, nickel–copper–titanium–zirconium–hafnium and titanium–nickel–zirconium–hafnium–cobalt–copper, which make up the surface compositions, are in an austenitic–martensitic state with a grain size of 32–120 nm. The authors performed calorimetric studies of the functional properties of the coatings. The studies showed the manifestation of austenitic–martensitic transformations. The authors also carried out tests for ‘friction wear’ and mechanical multicycle fatigue of the obtained high-entropy compositions on a steel surface.