Titanium-base coating on plane silica substrates

Abstract

Plane silica substrates were coated with a titanium-based deposit by gaseous cementation at temperatures ranging from 600 to 800 ° C, over 1 h to several tens of hours. The cement consisted of hydrogen chloride and titanium. The bilayer structure of the coating was established by X-ray diffraction, X-ray electron microprobe analysis, scanning electron microscopy, low-energy electron-induced X-ray spectroscopy. X-ray photoelectron spectroscopy, optical microscopy and X-ray fluorescence spectroscopy. Thin coatings were amorphous and the composition of the outer zone was close to TiO. Thicker coatings ranging from 2 to 30 μm were crystalline. In this case, the outer layer, designated α-Ti(O) or TiO x with 0.44 ⩽ × ⩾ 0.49, corresponded to an ordered solid solution of oxygen in a close-packed hexagonal titanium. The inner layer in contact with the substrate was Ti5Si3. At fixed temperatures (600, 650, 700, and 800 ° C), the thickness of the coating increased according to a parabolic law. Activation energy and diffusion coefficient were calculated. Thermodynamic considerations concerning the interactions between the gaseous phase of cementation and the silica substrate, are presented from theoretical calculations and from Ti-Si-O phase diagrams experimentally constructed at 800, 1000 and 1300 ° C.