Influence of the composition of a boron-containing active medium in the form of a coating on the structure and properties of a diffusion layer on titanium parts

Abstract

A comparative study of VT-1.0 titanium boriding, carboboronizing and borosiliconizing methods was carried out in order to increase wear resistance in aggressive environments at elevated temperatures. The microstructure of diffusion coatings was investigated, their thickness and microhardness were determined. Diffusion saturation of 10×10×25 mm VT-1.0 titanium samples was carried out from saturating coatings based on boron carbide. Process temperature of 950 °C, and saturation time of 1.5 h were used as saturation conditions. At the end of high-temperature exposure, samples were removed from the furnace and cooled in air to room temperature, cleaned from saturating coatings with wooden spatulas, and boiled in the soap and soda solution for 1 h. A continuous diffusion layer 80–100 μm thick forms on the titanium surface. The borosiliconized diffusion layer obtained by titanium saturation from the mixture of 45%B 4 C–5%Na 2 B 4 O 7 –22%Si–5%NaF–3%NaCl– 20%CrB 2 has a higher microhardness: 1520 HV 0.1 versus 1280 HV 0.1 for carboboride one and 1120 HV 0.1 for boride one. In this case, boride and carboboride coatings, obtained, respectively, by saturation from 45%B 4 C–5%Na 2 B 4 O 7 –5%NaF–25%Al 2 O 3 –20%CrB 2 and 70%B 4 C– 5%Na 2 B 4 O 7 –5%NaF–20%CrB 2 coatings have a pronounced zonal structure. The upper zone of these coatings having high microhardness also features high brittleness indicators, which makes it impossible to accurately measure microhardness distribution due to chipping and cracking at microhardness measurement points. The qualitative composition of coatings on titanium was studied by X-ray diffraction using the DRON-6 X-ray diffractometer in filtered CuK α radiation (λ = 1.5418 Å) in the angle range of 2θ = 20÷80°. The diffusion coating exhibits reflections of titanium carbide, chromium and titanium borides, and a certain amount of the Cr 2 Ti intermetallic compound. Boride phases of chromium and titanium refer to high boron phases with high specific boron content: TiB, CrB, Ti 2 B 5 , Ti 3 B 4 и Cr 2 B 3 .