Nakonechnyi S. Formation of the structure and properties of protective coatings from a mixture of powders «high-entropy alloy - refractory compound» by cold gas-dynamic spraying

The PhD thesis is devoted to the establishing of principles of the formation of the structure, phase composition and a complex of properties of protective composite coatings based on a high-entropy alloy (HEA) with the addition of refractory compounds (RC) obtained by cold gas-dynamic spraying (CGS) for their potential use in oxidizing and corrosive aggressive environments, high temperatures, and wear. The paper provides a literature review of recent research on the topic of the thesis. An analysis of the main features and properties of HEA, as well as composite materials based on them, is presented. Literature data on different types of coatings from HEAs and composites based on them, as well as the advantages and disadvantages of various methods of obtaining them, are widely reviewed and analysed. The second chapter of the thesis provides information on the composition of the HEA, the explanation of the selection of the initial metal components for the HEA synthesis and the RC for reinforcement. The technique of cold spraying of coatings and all relevant methods for studying the structure, phase composition, mechanical properties and thermal stability of powder materials and composite coatings, as well as the resistance of coatings to corrosion, oxidation, and wear are presented. The third chapter of the thesis presents the results of studying the structure and phase composition of the initial HEA powders and shows that AlNiCoFeCrTi HEA has a composite structure based on BCC solid solution with inclusions of σ-phase and TiC, and AlNiCoFeCr HEA consists of BCC solid solution and inclusions of FCC solid solution. In the fourth chapter of the thesis the influence of technological parameters of CS (pressure and temperature of the compressed air flow) on the characteristics (thickness, relative density, structure, phase composition, and mechanical properties) of coatings obtained of HEA powders and their mixtures with RC was investigated. It was found that, at different parameters of pressure (from 0.7 MPa to 0.9 MPa) and temperature (from 200 °C to 550 °C) of the compressed air flow, the phase composition and structure of the initial powders are preserved, while an increase in compressed air pressure and/or temperature increases the thickness of the coatings several times, and also leads to the grinding of their structure. The addition of refractory compounds to the AlNiCoFeCr HEA significantly increases the kinetic energy of particles and increases the thickness of composite coatings. At the same time, obtained composite coatings have an excellent combination of microhardness and fracture toughness compared to composite coatings based on pure metals and their alloys and other HEAs obtained by high-temperature spraying methods. In the fifth chapter of the thesis, the thermal stability of the structure, phase composition, and mechanical properties of the obtained composite coatings based on AlNiCoFeCr HEA with the addition of RC was investigated. It has been established that the stability of the phase composition and nanostructural state of the coatings obtained by CGS is maintained up to a temperature of 1000 °C and exceeds the thermal stability of composites with a matrix of traditional alloys due to the multi-element composition of the HEA matrix and the influence of high mixing entropy that increases the stability of solid solutions at high temperatures compared to intermetallic and other ordered phases. In addition, the possibility of increasing the hardness, fracture toughness, and wear resistance of composite CGS coatings based on AlNiCoFeCr HEA with the addition of RC by annealing at a temperature of 1000 °C (in the zone of FCC⟶BCC phase transformation temperatures) was established for the first time. In the sixth chapter of the thesis, the stability of the obtained composite coatings based on AlNiCoFeCr HEA with the addition of RC under conditions of electrocorrosion and high-temperature oxidation was investigated. It has been established that the coatings reinforced with titanium diboride have higher corrosion resistance than 316L stainless steel in a 3.5 % NaCl solution due to the formation of a continuous and dense passivating chromium oxide film on the surface, while the coatings reinforced with composite ceramics do not have anticorrosive properties due to the absence of a continuous passivating film on the surface and the formation of complex oxides of different composition. The annealing of composite coatings at 800 °C and 1000 °C increases their resistance to oxidation due to both an increase in the size of grains/subgrains and, accordingly, a decrease in the number of grain/subgrain boundaries (pathways of accelerated diffusion) and a lower oxygen diffusion rate, and the formation of a continuous and dense layer of aluminium oxide, which prevents the diffusion of oxygen atoms and protects the coating and substrate from internal oxidation for 100 hours at a temperature of 900 °C.