Mechanism of the High Temperature Oxidation of Titanium Carbide
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Mechanism of the High Temperature Oxidation of Titanium Carbide V.B. Voitovich Institute for Problems of Materials Science, Kiev, Krzhyzhanovsky Str. 3, 252680, GSP. Ukraine ABSTRACT should resist high-temperature corrosion better than the simple Co-binder in the WC-based cermets 111. These High-temperature oxidation of titanium carbide in features have led to the intensive studies of high- the temperature range of 600-1200°C in air has been temperature oxidation of titanium carbide. studied by TGA, XRD, SEM, and AES methods. High-temperature oxidation of titanium carbide was Oxidation kinetics is multistage. The time intervals and studied by many investigators /3-17/. Nevertheless, temperature dependence of the rate constants of three oxidation mechanism has not been clearly studied up to parabolic and linear stages for oxidation at tempera- the present day. The oxidation mechanism was inter- tures 900-1200°C were determined. According to SEM preted as diffusion of oxygen through rutile (Ti02.x) and XRD data, a two-layer scale was formed at scale /7,9-ll/. The effect of titanium diffusion on temperatures exceeding 1000°C. The outer scale titanium carbide oxidation was not essentially consisted mainly of Ti02 (rutile). The inner scale considered. consisted of various titanium oxides. The effect of Oxidation of titanium carbide at temperatures above diffusion of both titanium and oxygen on the oxidation 900°C follows a parabolic kinetics /6-12/. It indicates processes was studied. The mechanism of the oxidation that diffusion processes control the oxidation rate. process is discussed. Parabolic oxidation was interpreted as a one-step process with an activation energy from 181.4 to 204.8 kJ/mole/9,11-13/. KEY WORDS At temperatures below 500°C scale consists of anatase (Ti02.x) /8/ and at higher temperatures scale Titanium carbide, oxidation, kinetics, scale, oxide, consists of rutile /6-11/. It was supposed /ll/ that diffusion deviation of the oxidation kinetics from parabolic behavior with time is caused by cracking of thick oxide layers. The disturbance of continuity of an outer layer 1. INTRODUCTION at temperatures above 1000°C was observed by Zhilyaev et al. /13/ as well. Scale consists of two layers Titanium carbide has the Bl-type (NaCl) crystal of rutile; the inner layer is black and porous, and the structure and exists over a wide substoichiometric com- outer layer is more compact, light-grey colored and has position range while maintaining the same crystal a more coarse-grained structure /7,9,11-13/. structure. Titanium carbide and cermets on its base III The carbon content in the outer layer varies from are finding expanding application in the manufacture 0.06% to 0.16 wt% /6,7,9,13/. The outer layer consists of wear-resistance materials, nozzles, forming fingers, of rutile, anatase and free carbon with a maximum hot rod mill roller guides, coatings, etc. /1,2/. The content of 0.38 wt% (900°C, 1500 m), and, at oxidation resistance of TiC is much greater than that of temperatures above 1000°C, the scale consists of rutile WC and the Ni-Co-Mo binder in the TiC-based cermets only/12/. 243 Vol. 16, No. 4, 1997 Mechanism of the High Temperature Oxidation of Titanium Carbide It is widely supposed that oxygen diffusion via 3 with a Cu K«- source was used. Chemical anionic vacancies is the sole process that causes the heterogeneities in the thin surface layers were studied formation of scale. However, this mechanism does not by Auger spectroscopy (AES) with an Auger explain the features of formation of a two-layer scale at microprobe JAMP-10S. high temperatures. The existence of both an inner layer /7,9,11-13/ and an outer layer has cast doubt on the 2.2. Materials models assuming the diffusion of only one type of point Titanium carbide samples were prepared by powder defects (anions or cations) completely controls the metallurgy routes. Samples of 5 χ 5 χ 5 mm in size with oxidation rate. previously ground faces were used for TGA measure- On the basis of analysis of a two-layer scale in ments. Specimens with the stoichiometric composition oxidation of a titanium carbide single crystal in the TiCo. 5 with the following purity data: 0.12 %N , 0.30 temperature range of 800-1200°C, Reichle and Nickl 9 2 % 0 , 0.015 % Mo, 0.004 % Al, 0.010 % Si, 0.005 % /12/ concluded that, up to 800°C, scale is formed at the 2 Cr, 0.003 % Μη, 0.12 % Fe, 0.014 % Ni contain up to TiC/Ti0 interface due to oxygen diffusion. The 2 0.2 wt% of free carbon. significance of diffusion of titanium cations in the formation of scale at Ti02/gas interface is substantially increased at higher temperatures. 3. RESULTS Many authors /3-12/ did not associate the formation of an inner layer during oxidation of titanium carbide 3.1. Oxidation Kinetics with the possible formation of oxycarbide phases. The Oxidation kinetics of the compact samples of features of formation of these phases are considered by titanium carbide was studied under isothermal heating Alyamovskiy et al. /18,19/. Zhilyaev et al. 1131, based in air in the temperature range of 600-1200°C for 5 h. on a study of oxidation of titanium carbide powders, Oxidation shows a complex behavior and changes with proposed the mechanism of multistep oxidation temperature and time (Fig. 1). Up to 800°C the processes in terms of oxycarbide solid solution formation. Nevertheless, this mechanism as applied to the compact samples was not considered.The activation energy for the wide temperature and time intervals was 209 kJ/mole /13/. 2. EXPERIMENTAL PROCEDURES 2.1. Procedures The titanium carbide composition was evaluated by wet chemical analysis. Oxidation kinetics in the tem- perature range of 600-1200°C in air were studied using thermogravimetry (TGA) with continuous sample weighing (Am = ±2 χ 10"4 g). The scale morphology and structure of the oxidized samples were studied using an MIM-7 metallograph and by scanning electron microscopy (SEM) with a microanalyzer JCXA-733. A high-purity 12-15 nm thick gold layer was prepared by cathode sputtering on the oxidized specimens for SEM examinations using a JFC-1100 unit. The phase com- position of the oxidation products was examined by X- ray diffraction (XRD). An X-ray diffractometer DRON- of 900-1200°C. 244 V.B. Voitovich High Temperature Materials and Processes titanium carbide samples are oxidized slightly and no weight gain was recorded. At 900°C some weight gain was recorded after oxidation for 120 min, and at a later 1.0 .. time oxidation follows a parabolic kinetics (Fig. 1). k — 100CTC With further temperature rise up to 1000°C oxidation rate increases sharply and oxidation follows a complex paralinear kinetics. Authors /9-13/ have interpreted the oxidation 1Γ=" kinetics in the temperature range of 900-1200°C X 4- ^ 0.5 exclusively by one value of an activation energy. These CM values were equal to 181.4 kJ/mole 191, 192.7 kJ/mole S /10/, 192.0 kJ/mole /ll/, 204.8 kJ/mole IUI and 209.0 kJ/mole /13/ respectively. >W The plots in the coordinates (Am/s)2 = f(x) (Fig. 2- <3 11) basically show that oxidation kinetics in the temperature range of 1000-1200°C for 300 min follows three parabolic steps (I, Π, ΙΠ) and a linear (IV) one at —I 1— a time. Parabolic oxidation is changed by a linear (IV) 20 30 in time. There are four steps of oxidation, viz. Ι-ΙΠ are Time , min ate s parabolic and IV is linear. At 1000°C transition to Fig. 3: The squares of parabolic rate constants vs linear oxidation takes place after 4 h, and at 1100°C time (ki, 1000°C). and 1200°C after 3 h. The parabolic oxidation is controlled by the diffiision processes in a scale, whereas linear oxidation is controlled by reactions at the scale/matrix interface. The values of rate constants and k1tkg - 1000°C k2 - 900°C 2 ΕΟ- κ 3 1 -- ><Si < 15 30 45 60 75 90 -Time , min ate θ The squares of parabolic rate constants vs 120 180 240 time (ki,k2), 1000°C). Time, minutes Fig. 2: The squares of parabolic rate constants vs corresponding time intervals are listed in Table 1. time (k2, 900°C). The temperature dependence of the rate constants 245 Vol. 16, No. 4, 1997 Mechanism of the High Temperature Oxidation of Titanium Carbide 1100°C Time, minutes 0 15 30 4-5 60 75 Time , min ute s Fig. 5: The squares of parabolic rate constants vs Fig. 7: The squares of parabolic rate constants vs time (k2,k3) 1000°C). time (ki,k2, 1100°C). k^ - "1100°C - 200 v- 150 •s 3 if s 100 w > 0 3 50 Time, minutes Time, minutes Fig. 6: The squares of parabolic rate constants vs Fig. 8: The squares of parabolic rate constants vs time (ki, 1100°C). time (k2>k3, 1000°C). 246 V.B. Voitovich High Temperature Materials and Processes Time, minutes Time, minutes Fig. 11: The squares of parabolic rate constants vs Fig. 9· The squares of parabolic rate constants vs time (k3, 1200°C). time (ki, 1200°C). Ink = f(T') for all stages of oxidation is represented by straight lines (Fig. 12). Calculated values of an apparent activation energy are as follows: Stage I - 334.1 kJ/mole, Stage Π - 234.1 kJ/mole, Stage ΠΙ, 219.0 kJ/mole, and Stage IV - 124.7 kJ/mole, respectively. Equations of the temperature dependence are as follows: kj = (5.01 ± 0.26) χ 1013exp-3344±4/RT 10 2341±4/RT kn - (2.21 ± 0.17) χ 10 exp' 10 219 to4/RT km = (1.48 ± 0.16) χ 10 exp" 6 124 7±4/RT kIV = (1.01 ± 0.18) χ 10 exp" 3.2.