“Ridge effect” in oxidation kinetics of hetero-modulus ceramics based on titanium carbide

Abstract

Recent studies on oxidation kinetics of the TiC-7 vol.% C (graphite) hetero-modulus ceramics at temperatures of 400-1000°C and oxygen pressures of 0.13-65 kPa have led to the discovery of a temperature-pressure-dependent phenomenon called "ridge effect". The oxidation rate of the composite rises rapidly to a maximum at ridge values of oxygen pressure (p O 2 ) or temperature (T), but then it declines with subsequent growth of oxidation parameters. The ridge values mark a change in the prevailing mechanism, as while the ridge parameter traversing the values of apparent activation energy Q or order of reaction m change its sign. The oxidation mechanisms, essentially different within the ranges of parameters, are identified according to the developed ridge-effect model, which can be applied probably to a variety of systems with inorganic compounds.

Initial stages of oxidation of near-stoichiometric titanium carbide at low oxygen pressures

“…Previously, Stewart and Cutler [11] have reported a maximum in the oxidation rates of TiC 1−x . Voitovich and Pugach [12] found out that quasi-stoichiometric TiC 1−x possessed the highest oxidation rate at 800 • C. These observations have recently been explained by the present author, Shabalin [13], in terms of a 'ridge effect'.…”

“…3) was calculated by taking into account only the formation of TiO 2 , as the small contribution of oxycarbide phases to the overall weight change of the TiC 1−x samples could be reasonably neglected, and no other products were identified in the oxidized samples. The residual carbon, which is dispersed throughout the scale, as has been well documented previously [12,13], is a result of non-stoichiometric oxidation of the carbide and inhibited carbon burn-off at earlier stages of the process. An illustration of this is provided by comparison of the number of titanium atoms reacting with oxygen at 700 • C and 1.3 kPa over an hour with those of carbon; the former is about two times more than the latter, showing a clear dominance of the metal oxide scale producing mechanism.…”

“…4), at 600 • C and 13 kPa oxygen pressure, enabled the determination of the O/Ti ratio as 0.28 ± 0.08, which corresponds to the incubation of the oxide on the solid-gas interface. On the basis of data from XRD, SEM/EDX and EPMA, the initial linear step appears to be connected with the dissolution of oxygen in the carbide [13][14][15] via formation of an oxycarbide (solid solution) phase:…”

Physicomechanical properties of ultrahigh temperature heteromodulus ceramics based on group 4 transition metal carbides

“…The discovery of ridge effect [1][2][3][4] in the oxidation of carbide-carbon heteromodulus ceramics 5,6 (HMC) put forward this type of ultrahigh temperature materials in the leading position for potential applications in the extreme environments, which are faced with exploitation of various thermal protection systems in aerospace technologies and nuclear engineering. Most modern advances in rocket motors and propulsion assemblies are connected with increase of intensive thermochemical erosion and thermomechanical loading effects on the surface of materials in the area of critical cross-section of nozzles.…”

Prospects of nanotechnology and materials design on the basis of refractory compounds

“…формально-кинетические константы (энергия активации, экспонента давления) меняют свой знак на противоположный. На диаграмме «температура-давление» интервалы параметров окисления, в которых наблюдается подобное поведение материалов, получили название областей отрицательной кинетики [28,29]. Используя именно такие параметры, возможно получение бланкет-преоксидированных кар-бидно-углеродных материалов (рис.…”