Oxidation of porous nanocrystalline titanium nitride. III. Oxidation mechanism

Abstract

The air oxidation mechanism of nanocrystalline TiN at 500 to 900°C is examined. It is shown that at t ≤ 800°C the oxidation of titanium nitride is controlled by the diffusion of oxygen and at t > 800°C the interdiffusion of titanium ions is observed. The oxidation properties of porous TiN are determined by the chemical interaction of oxygen and the reaction surface, which includes the external surface of samples and the internal surface of the pores into which oxygen penetrates. The time and temperature dependence of the weight increment complies with the porous material oxidation model. Active initial oxidation is due to the interaction of oxygen and large internal surface. Short-term self-heating of porous samples is also possible. At t ≤ 800°C, the pores are obliterated with oxides with time, the internal reaction surface reduces, an external oxide film is formed, the oxygen diffusion and weight increment slow down, and the process stabilizes. With temperature increase, these processes are activated and lead to a smaller weight increment at the final stage (2 to 4 h) at 800°C as compared with 600°C. At t > 800°C the pore obliteration rate increases, but due to the interaction of oxygen and titanium ions that diffuse into the external scale surface, weight increment continuously increases with both time and oxidation temperature. The phase composition of the scale also affects the oxidation mechanism of porous TiN. Oxynitride of terminal composition plays a protective role; the transformation of anatase into rutile is accompanied by a decrease in the oxygen diffusion rate; Ti 2 O 3 formed in pores accelerates their obliteration.The kinetics of air oxidation (at 500 to 900°C) of compacted and sintered samples of TiN nanocrystalline powder (grain size 55 nm) was studied in [1]. The 200-mg cylindrical samples were 2 mm in height and 5 mm in diameter. Their porosity was approximately 12%, and the average pore diameter determined by statistical lowtemperature nitrogen desorption was 43 nm. The oxidability was characterized by q = ΔP/S, where ΔP (mg) is the weight increment of the sample determined by continuous weighing; S is the area of external (geometrical) surface, which was equal to 0.78 cm 2 for all samples. It is shown that at t ≤ 800°C the oxidability dependence on time q = f(τ) and temperature q = f(t) complies with the porous material oxidation model [2][3][4]. The behavior of this dependence changes at t > 800°C.The paper [5] deals with x-ray phase analysis of the starting and oxidized nanocrystalline TiN powders (grain size 15, 40, 55, and 80 nm) and also sintered samples based on them. Oxynitride TiN x O y forms at the initial oxidation stage and TiO, Ti 2 O 3 , Ti 3 O 5 , TiO 2 (anatase), and TiO 2 (rutile) grains at the subsequent stages.The objective of this paper is to describe air oxidation at 500 to 900°C for porous nanocrystalline titanium nitride. This is complicated since results of different investigations need to be compared. Hence, the papers [6, 7] deal with oxidation kinetics at 600 to 800...

Mechanical Properties of Titanium Nitride Nanocomposites Produced by Chemical Precursor Synthesis Followed by High-P,T Treatment

“…It will be interesting to see if this extends to carbonitride structures that can be prepared by a similar chemical precursor route [ 10 , 12 ], and also as the nanoscale crystallites evolve, with further annealing and potential N 2 loss, to approach the stochiometric TiN composition. Another interesting direction would involve controlled incorporation of oxygen in the crystalline or amorphous component of the nanostructures that also affects the hardness [ 52 , 53 , 54 ].…”