STRUCTURE AND THERMAL PROPERTIES OF COATINGS, OBTAINED BY DETONATION GAS SPRAYING
Authors: Kharchenko S.D. 
Published: January 2021

The thermophysical properties and wear resistance of materials are influenced, firstly, by the structure and phase composition of their surface layer and, secondly, by the secondary structures formed. The evolution of the processes of mechanochemical adaptation determines the formation of secondary structures of the first and second types on the friction surfaces, and their formation is carried out under the cooperative influence of deformation, heating and diffusion. It can be considered reliable that the process is accompanied by the dispersion of the surface layer structure and the formation, as a result of compaction and sintering, of protective films that reduce the shear resistance.

It is shown that when studying the relationship between the fracture resistance of composite detonation coatings, their structure, phase composition, thermophysical properties, the influence of external factors that determine the operational stability of friction units, the leading value determines the choice of research methods. The capabilities of the methods and equipment used largely determine the depth and likelihood of ideas about the processes occurring during contact interaction.

It was confirmed that the microstructure of the surface films has a finely dispersed structure and consists of a mixture of phases of the composite coating and products of interaction with air oxygen. According to the stoichiometric composition, the difficultly activated complex has the form of a finely dispersed mixture of oxides Cr2O3, Al2O3, NiO and complex phases such as chromoxides NiCr2O4.The presence of texture maxima testifies to the directional orientation of the elements of the ultradispersed structure, while the structure consists of crystals oriented in the direction of the velocity vector with sizes of the order of several interatomic ones.

Electron diffraction studies of highly dispersed heterogeneous thin-film structures formed on the friction surfaces of the test coatings showed that they do not correspond to both supersaturated solid solutions of oxygen in metals and chemical compounds of non-stoichiometric composition. And in their structure they are close to the structure of a dispersion-hardened composite material. Dispersed inclusions, which are intermetallic compounds, borides, aluminides, metal oxides included in the composition of the coatings, have sizes from 10 to 25 nm, the nature of their distribution is opposite with a pronounced orientation in the direction of the sliding velocity vector. This circumstance confirms that the processes of structural activation play a decisive role in the formation of secondary structures. The dependence of the friction coefficients on the sliding speed of the tested detonation coatings changes significantly during heat treatment of the coatings.

Heat treatment of the coatings was carried out at a temperature of 1250°C for 24 hours. Thermal and thermophysical properties of coatings after heat treatment increased more than 2.5 times. The cracks around the indenter track became significantly less, which indicates an increase in the crack resistance of the coating. The thermal conductivity of the samples after spraying is practically independent of the spraying modes and the initial state of the powder material. This fact is associated with the disorder of the crystal structure of the substrate material and the microstructure of the coating itself. In the coating samples after heat treatment, the thermal conductivity significantly increases due to a decrease in the grain size of the coating phases and an increase in the ordering of the coating structure.