Stability of borosilicide coatings on high-heat metals at high temperatures

The possibility of improving the heat resistance of silicide coatings by causing the dissolving of disilicides on a metal substrate to slow down has been investigated. The dissolving was decelerated by the method of borosilification, i.e. by the successive saturation of metals with boron and silicon. In order to clarify the question of the effectiveness of employing the boride substrate, the kinetics of formation and the structure of the intermediate phases of borosilicide coatings upon columbium in the temperature range between 1200 and 1400 deg C and upon molybdenum and tungsten in the temperature range from 1400 to 1600 deg C have been studied. The heating time was 1,3,5 and 10 hours, the residual pressure being 1x10-4 mm Hg. The study was carried out by the methods of metallography, microdurometry, X-ray phase and microscopic X-ray spectrum analyses. It has been found that a preliminary saturation with boron makes it possible to prolong the service life of heat-resistant silicide coatings. The greatest stability at high temperatures is displayed by the columbium-based borosilicide coating, the least, by that coated upon molybdenum, the coating on the tungsten substrate occupying the intermediate position. The recommended thicknesses of the barrier layers for silicide coatings on refractory metals are as follows: NbB from 8 to 10μ, ternary phases: Mo5(B,Si)3 from 10 to 15μ and W5(B,Si)3 from 5 to 8μ