Oxidation of porous nanocrystalline titanium nitride. II. Scale structure and composition

Abstract

We have used x-ray phase analysis to study the composition of the products of reaction between oxygen and nanocrystalline powders with particle sizes 15, 40, 55, and 80 nm, and also specimens pressed (and sintered) from them. The powders were oxidized in air at 100°C (400 h) to 500°C (5 min), while the sintered specimens were oxidized at 600-900°C for 15, 120, and 240 min. In all cases, in the initial oxidation step the oxynitride Ti(O x N y ) is formed, which over time is oxidized to TiO, Ti 2 O 3 , Ti 3 O 5 , TiO 2 (anatase) and TiO 2 (rutile). In the range 600-800°C, formation of a continuous oxide layer and conversion of anatase to rutile slows down diffusion of oxygen in the scale. We have established that at 900°C, the growth rate of the scale thickness increases and so the reflections from the oxynitride are barely noticeable on the diffraction patterns taken from the surface of the oxidized specimen. In these diffraction patterns, along with strong reflections from the rutile, we also observed weak reflections from lower oxides and anatase, which may be due to reaction between oxygen and the titanium ions diffused to the scale surface. We have concluded that at T > 850°C, the mechanism for oxidation of TiN changes. This is due to superposition of counterdiffusion of titanium ions on the diffusion of oxygen.Earlier [1] we studied the kinetics of oxidation in air of specimens sintered from nanocrystalline titanium nitride powders. The kinetic isotherms were evidence for the uniqueness of the oxidation mechanism for porous specimens at temperatures above 600 and 850°C. This work is a continuation of these studies and is devoted to a study of the composition of the products of reaction between TiN and oxygen under various oxidation conditions.We carried out x-ray phase analysis (DRON, copper and molybdenum radiation) of the original and oxidized powders and also specimens sintered from those powders. The reflections were indexed using data on the crystal structure of the phases we were looking for [2-6]. The microhardness and microstructure of the original and oxidized specimens were studied on a MMT-3 hardness tester. The results of the x-ray studies are presented in Tables 1-3. Table 3 gives the calculated values of sin 2 θ (θ is the Bragg reflection angle) for the reflections found and the corresponding relative intensities I/I 0 , determined by eye on a ten-point scale; the phases and indices (hkl) of the reflections matching those found are also indicated.We studied TiN powders with particle sizes 15 and 40 nm, obtained by vapor deposition, and powders with particle size 55 and 80 nm obtained by the plasmochemical method. Some powders were coated with a protective film of an organic material, which was removed by annealing in hydrogen for 6 h at 600°C. All the TiN powders had a facecentered crystal lattice (NaCl type) with lattice parameter within the range 0.4228-0.4231 nm (Table 1), which is consistent with the data in [4,7,8]. According to the x-ray phase analysis results, within the sensitivity of...

Oxidation of porous nanocrystalline titanium nitride. III. Oxidation mechanism

“…Moreover, starting powders had micron particles in all cases. Therefore, this study focuses on analyzing the results obtained earlier in [1,5]. We can employ a comparative method here since the basic requirements [12] are met: the samples were produced by the same technology, had the same weight, shape, dimensions, and porosity [1].…”

“…The weight balance calculation permits finding the weight of the formed titanium dioxide ΔM 1 and the weight of the reacted titanium nitride ΔM 2 (material consumption), which characterize TiN oxidation resistance along with q, and Δm. The x-ray phase analysis [5] has revealed that the scale consists mainly of anatase after 2 to 4-hour oxidation at t ≤ 600°C and of rutile at t > 600°C. Therefore, the calculations covered oxidation for 2 and 4 h. Preliminary weighed samples were placed into a furnace heated to a certain temperature, held for 2 or 4 h, cooled in air, and weighed; then the change in their weight ΔP (mg) was determined and the actual weight increment Δm (mg) was calculated.…”

“…The calculations have covered conditions where the scale, according to [5], mainly consists of TiO 2 (2 and 4-h oxidation). Note that the total reaction surface area includes both the external (geometrical) surface area of the sample and the internal surface area of the pore oxidized.…”

“…After active initial oxidation (∼30 min) at 600°C, the oxidation q increases during a period of 2 h because of oxidation of a large reaction surface area ( Table 2). Anatase is the main oxidation product, a protective film not being formed [5]. In 2 h, anatase transforms into rutile, which is denser [13].…”

“…At 700°C, oxygen interacts more actively with the reaction surface area, and q in the first minutes is greater than at 600°C [1]. A solid oxide film consisting of rutile forms in 15 min; there are TiO and Ti 2 O 3 under the film [5].…”