Influence of the Composition of a Boron-Containing Active Medium in the Form of a Coating on the Structure and Properties of the Diffusion Layer of Titanium Parts

Abstract

In this work, a comparative study of the methods of boriding, carboboronizing, and borosiliconizing of VT-1.0 titanium is carried out in order to increase the wear resistance in aggressive environments at elevated temperatures. The microstructure of diffusion coatings is investigated, and their thickness and microhardness are determined. Diffusion saturation of VT-1.0 titanium specimens with dimensions of 10 × 10 × 25 mm was carried out from saturating mixtures based on boron carbide. Saturation mode: process temperature of 950°C, saturation time of 1.5 h. At the end of the high-temperature exposure, the samples were removed from the furnace and cooled in air to room temperature, the saturating coating was cleaned from the samples with wooden spatulas, and the samples were boiled in soap and soda solution for 1 h. A continuous diffusion layer 80–100 µm thick forms on the titanium surface. The borosilicized diffusion layer obtained by saturation of titanium from a mixture of 45% B₄C–5% Na₂B₄O₇–22% Si–5% NaF–3% NaCl–20% CrB₂ has a higher microhardness: 1520 HV_0.1 versus 1280 HV_0.1 for carboboronizing one and 1120 HV_0.1 for boriding. In this case, boriding and carboboronizing coatings obtained, respectively, by saturation from daubs of the composition 45% B₄C–5% Na₂B₄O₇–5% NaF–25% Al₂O₃–20% CrB₂ and 70% B₄C–5% Na₂B₄O₇–5% NaF–20% CrB₂ have a pronounced zonal structure. The upper zone of these coatings, having a high microhardness, also has high brittleness indicators, which does not allow accurately measuring the microhardness distribution indicators because of chipping and cracking in the places where the microhardness is measured. X‑ray diffraction studies of the qualitative composition of coatings on titanium were carried out on a DRON-6 X-ray diffractometer in filtered CuK_α radiation (λ = 1.5418 Å) in the angle range 2θ = 20°–80°. The diffusion coating exhibits reflections of titanium carbide, chromium and titanium borides, and a certain amount of Cr₂Ti intermetallic compound. Boride phases of chromium and titanium refer to high boron phases with a high specific boron content: TiB, CrB, Ti₂B₅, Ti₃B₄, and Cr₂B₃.