Effect of high-temperature annealing on the structure and micromechanical properties of NiCrBSi coatings formed by high-velocity gas-thermal spraying

Problem. Alloys of the NiCrBSi system are widely used for gas-thermal spraying of protective coatings due to their low melting point. However, coatings formed by high-velocity gas-thermal spraying are characterised by residual porosity and a layered structure, which limits their strength properties. A literature review revealed a lack of systematic data on the influence of subsequent high-temperature annealing on the transformation of the structural-phase composition, continuity, and the complex of micromechanical characteristics of such coatings. Aim. To evaluate the effect of vacuum annealing at 1050 °C on the structural-phase composition, continuity, and micromechanical properties (microhardness, Martens hardness, contact elastic modulus) of NiCrBSi coatings produced by high-velocity gas-thermal spraying. Methods. NiCrBSi alloy coatings were deposited by high-velocity gas-thermal spraying. The samples were subjected to vacuum annealing at 1050 °C with an isothermal hold of 2 h and subsequent furnace cooling. The study of structural-phase changes was carried out using scanning electron microscopy, X-ray phase analysis, and energy-dispersive X-ray microanalysis. Micromechanical properties were evaluated by measuring microhardness (recovered indentation method) and instrumented microindentation. Results. It was experimentally confirmed that annealing at 1050 °C leads to the formation of a dense homogeneous structure without layering. The formation of large strengthening phases – carbides (Cr7C3 and Cr23C6) and CrB chromium borides – was recorded, which increases the strength characteristics of the coating during indentation by 25–30 %. It was found that the contact elastic modulus increases from 130 to 228 GPa due to the elimination of discontinuities. Additional heating to 900 °C does not cause changes in the structure and hardness, which confirms the high thermal stability of the coating subjected to high-temperature (at 1050 °C) annealing. Conclusions. Hightemperature vacuum annealing at 1050 °C is an effective post-treatment method for sprayed NiCrBSi coatings, providing a 1.3-fold increase in hardness and a 1.7-fold increase in the elastic modulus due to the formation of larger strengthening phases and a reduction in the number of discontinuities.