The Structure, Thermal Stability, Heat Resistance, and Diffusion-Barrier Properties of Coatings in the Mo-Y-Si-B System

The Mo-Si-B and Mo-Y-Si-B coatings were obtained by magnetron sputtering in the direct-current mode using a 90% MoSi2 + 10% MoB target equipped with yttrium segments. The composition and structure of the coatings were studied by scanning electron microscopy, X-ray phase analysis, and optical emission spectroscopy of a glow discharge. The diffusion-barrier properties and heat resistance were evaluated by annealing at temperatures from 700 to 1000 degrees C. The thermal stability was determined by heating the coating lamellae in a column of a transmission electron microscope in the in situ mode. According to the chemical analysis, the main elements were evenly distributed over the thickness of the coatings. The coating growth rate did not change upon the introduction of yttrium and was in the range of 325-350 nm/min. The Mo-Si-B coating was characterized by a columnar structure with grains of the hexagonal phase h-MoSi2 about 50 nm in size, as well as an insignificant volume fraction of the boron-containing amorphous phase (alpha-MoB). An increase in the concentration of Y up to 4 at % led to a decrease in the size of h-MoSi2 crystallites of up to 10 nm. A coating with a maximum concentration of yttrium (7 at %) contained predominantly an amorphous phase alpha-MoB with phase crystallites dispersed in it h-MoSi2 and t-MoB. The introduction of the optimal amount of yttrium (4 at %) led to a decrease in the thickness of the oxide film by a factor of 6 after annealing at a temperature of 800 degrees C and by three times at 900-1000 degrees C. The coating with the maximum concentration of yttrium (7 at %) had the best diffusion-barrier properties of all the studied samples at 700-1000 degrees C, and also surpassed the Mo-Si-B sample in terms of thermal stability. Positive effects from the introduction of yttrium into the composition of the coatings were connected with (a) structure modification, including suppression of columnar grain growth; (b) transition to a structure with a higher volume fraction of the amorphous phase; (c) isolation of grain boundaries and lengthening of the diffusion path of metal atoms from the substrate; and d) formation of protective surface layers on based on yttrium oxide when heated in air.