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 -- >

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. XRD Examinations

The XRD data are shown in Table 2. The Ti,

TiO(mon), TiO(cub), Ti02 (anatase), Ti02 (rutile) phases were identified both in the scale at temperatures up to 900°C and in the inner layers of the scale at 1000-1200°C after oxidation during 60 min. In the Time, minutes later stages (300 min), Ti203 oxide appeared, and at the Fig. 10: The squares of parabolic rate constants vs same time TiO monoxide was not recorded. This points

time (k] ,k2,1200°C). to possible oxidation of TiO monoxide to Ti203-oxide.

247 Vol. 16, No. 4, 1997 Mechanism of the High Temperature Oxidation of Titanium Carbide

Table 1 Parabolic Kp (kg^m4 h χ 104) and linear K, (kg/m2 h χ 102) rate constants for the oxidation of TiC

Steps of oxidation * Temperature, °C parabolic 1 II III linear „ „„+O.05 0.98 ... 900 — -0.06 — 2.0-4.0

_ +0.05 2 +1 05 , ,+0-74 0.96 5.4^ 15.1 · 7.7 1000 -0.08 -0.15 -1.29 -0.81 0.083 - 0.83 0.83-1.58 1.58-4.0 4.0-6.0

102 2 61 + 20 7.,a+8 29.0 74.0Λ 3.4^· 1100 -L21 -0.82 -2.90 -0.12 0.083 - 0.75 0.75-1.58 1.58-3.0 3.0-5.0

,, Λ+3·55 44 2 45 76.0 128.2 · 250.0+ 1200 -2.80 -3.5 -4.8 -0.35 0.083-0.50 0.50-1.0 1.0-3.0 3.0-5.0

* - above bar is the rat· constant; under bar is the duration of the stage (from start of oxidation).

TEMPERATURE, °C An outer layer of scale at high temperatures consists of Ti0 (rutile). 1200 1100 1000 900 2 The scale structure at high temperatures is shown in Fig. 13. Starting with 1000°C, the scale is two-layered. The inner layer is porous and fine-grained, the outer layer is more compact and coarse-grained. The scale has a complex composition which varies with temperature and time (Table 2).

DISCUSSION

4.1. Parabolic Oxidation

4.1.1. First Parabolic Stage The XRD examination (Table 2) and the metallo- graphic investigation identified titanium (600-800°C, 4 min), the dark-blue colored thin film consisting of

oxycarbide TiC0.42O0.58 and titanium dioxide Ti02 in both the anatase and rutile modifications (800°C, 15 RECIPROCAL TEMPERATURE, ϊΓ1 min) on oxidized surfaces. This fact is explained by the Fig. 12: Logarithm of parabolic (A-C) and linear (D) features of oxidation of titanium carbide and the oxidation constants (Ink) as a function of formation of oxycarbides /18,19/ in the initial stages of reciprocal temperature (1/T), A - first step, Β oxidation. Dissolution of oxygen in the titanium - second step, C - third step, D - linear carbide lattice is accompanied by substitution of part of oxidation in the temperature range of 900 - the carbon atoms with oxygen and subsequent 1200°C vs time. formation of oxycarbides with vacancies in both

248 V.B. Voitovich High Temperature Materials and Processes

Table 2 XRD investigation of the scale

Temperature, °C Identified phases Time, min Starting sample TiC 600 TiO·; Γ1Ο2 (rutile),TKtyanatase) 300 700 Ti, TiO*,TiO**; TChirutile), TiOj (anatase) 300 TiCo>uOo.58; TiOi(rutile), Γ1Ο2 (anatase) 15

800 TiO*; Ti02(rutile) 120 Ti, TiO*,TiO·*; Ti02(rutile), T1O2 (anatase) 300

900 Ti, TiO*,TiO**; Ti02(rutile) 300

TiO*,TiO**; Ti02(rutile) 20 1000, inner layer Γι, TiO*,TiO**; Ti02(rutile) 60 Γι, TiO*,TiO**; TiOa(rutile), 120 Ti, TiO*,TiO**; TiiOa, TiOi(rutüe) 300 1000, outer layer T1O2 (rutile) 300 1100, inner layer Γι, ΓιΟ**; T12O3, TiOiirutile) 300 1100, outer layer Γ1Ο2 (rutile) 300 1200, inner layer Ti, TiO**; T12O3, TiOi(nitile) 300 1200, outer layer TiO: (rutile) 300

Note: * -monoclinic modification of TiO , type A2/m ** - cubic modification of TiO, type Fm3m (NaCl -type). Note: A/2m structure is similar to Fm/3m structure, but differ by presence of ordered the vacant lattice sites.

Fig. 13: SEM micrographs of scale for oxidized sample TiC (1000°C, 6 hours); a - general view, b - inner scale and matrix/scale interface; c - inner scale; d - outer scale.

249 Vol. 16, No. 4, 1997 Mechanism of the High Temperature Oxidation of Titanium Carbide sublattices /13,18,20/. As oxygen content in oxycarbide increases, the dN/dE 0 complement of titanium sublattice decreases /18,20/. At substitution of oxygen atoms for carbon the additional amount of both carbon and titanium atoms has to be Ti additionally removed from the carbide lattice. The preceding is in accordance with the metallo- graphic and XRD examinations of oxidized samples. At 600°C and during 300 min at 700°C for the initial stage (up to 15 min), dark films of oxycarbide solid solutions, inclusions of carbon and some of the Ti02 oxide were observed and identified in the surface layers. Formation of both modifications of titanium dioxide, viz. both anatase and rutile, during the formation of oxycarbides with a low oxygen content and with defects in both sublattices 1201, may be caused 418 by the oxidation of titanium, evolving dining Π oxycarbide formation. In time, both in the scale formed at 700-900°C and in the inner scale at 1000-1200°C, 507 titanium was recorded, but it was always present simultaneously with TiO (cub) titanium monoxide. Thin gold-colored layers of TiO (Table 2) are 387 formed as oxygen is dissolved in the oxycarbides, and 10I 0 20I0 30ι0 ENERG Y OOI O 501 0 eV with time they form a continuous layer on the surface of samples. Under oxidation of the {100} face of the Fig. 14: Auger spectrum from surface of oxidized titanium carbide single crystal at 900°C /21/, the scale grains of titanium carbide (800°C, 60 min). at low oxygen pressures consisted of TiO, and at higher oxygen pressures of T1O2. grains of titanium carbide. The shape of the low-energy The Auger-spectrum from the surface of oxidized part of the C-KW-lines in Fig. 16 c,d is intermediate carbide grains (800°C, 60 min) with the following between Fig. 16a and Fig. 16b. In this manner, the chemical composition: 44.9 at.% Ti, 47.6 at.% 02, and surface of oxidized grains is enriched with carbon. 7.5 at.% C, is shown in Fig. 14. It corresponds to a titanium monoxide TiO and some carbon (graphite). The first steps of oxidation are characterized by the The Auger spectrum derived from the surface of highest values of an activation energy - 334.4 kJ/mole. titanium carbide in subscale layers is shown in Fig. 15. Dissolution of oxygen and formation of, respectively, The chemical composition (49.1 at.%Ti, 48.3 at.%C oxycarbides and titanium monoxide take place in this and 2.6 at.%02) corresponds to a titanium carbide. It step. This value is slightly lower than the should be noted that, since for different modifications corresponding value for diffusion of oxygen in the of carbon the electron density of the collectivization lattice of a titanium carbide single crystal (TiCo.97) - valence band changes with energy 272 eV 1221, the 381.6 kJ/mole/24/. shape of the carbon KW-Auger line with energy 272 eV differs for different carbon compounds /23/. Some 4.1.2. Second and Third Parabolic Stages Auger spectra of carbon are shown in Fig. 16. The The second stage of parabolic oxidation starts at spectra in Fig. 16a and Fig. 16b are C-KW lines of 900°C (Fig. 2-11, Table 1). The formation of the inner carbon and titanium carbide, respectively. The spectra scale takes place at this stage. The formation of the in Fig. 16 c, d are derived from partially oxidized outer layer in a two-layer scale starts at 1000°C during

250 V.B. Voitovich High Temperature Materials and Processes

. dN/dE • dN/dE

• Λ f AV r Ό

Jf "

• eV eV

. dN/dE . dN/dE : Λ Λ / Γ c) d)

eV eV

ENERGY Fig. 15: Auger spectrum of titanium carbide. Fig. 16: Auger spectra of carbon: a - graphite, b - titanium carbide, c-d - inclusions on the the third stage of parabolic oxidation. surface of oxidized grains (800°C, 60 min). Such features of inner scale structure (Fig. 13) as porosity, stratified structure, inclusions of carbon in cracks and in pores point to the fact that this layer is formed by oxygen diffusion into carbide and initial 46.8 and 49.8 at% at a temperature of 350°C for 10 formation of oxycarbide. It determines the complex min 1261. phase composition of the scale (Table 2). The scale The transition to the second stage of parabolic oxi- consists of titanium, two modifications of monoxide dation is accompanied by a sharp increase of the TiO, oxide Ti203 and dioxide Ti02. It should be noted oxidation rate (Fig. 1-11, Table 1) due to changes in that the formation of phase TiO(mon) may be explained the oxidation mechanism. At this stage, alongside with by oxidation of titanium, and the formation of TiO(cub) lower titanium oxides, titanium dioxide Ti02.x (rutile) by further oxidation of oxycarbide (TiCxOy). The is formed. Oxygen difiusion in rutile is a rate- presence of titanium in a scale is due to formation of controlling step. titanium monoxide since it has a wide region of The value of the activation energy decreases up to homogeneity and defects in both the metallic and the 234.1 kJ/mole. The activation energies for oxygen oxygen sublattices /25/. With the increase in oxygen difiusion in rutile single crystals fell in the range of 276 content during oxidation, the amount of vacancies in kJ/mole 727/ - 282.6 kJ/mole /28/. the metallic sublattices increases. It occurs by titanium Transition to the third parabolic stage takes place, release from the TiO lattice. Titanium was found on the with time, at temperatures exceeding 1000°C. The surface of oxidized samples during oxidation of difiusion of titanium cations through dense compact titanium monoxide (TiO) with an oxygen content of rutile layers is a rate-controlling step. It brings about

251 Vol. 16, No. 4, 1997 Mechanism of the High Temperature Oxidation of Titanium Carbide formation of an outer scale over the initially formed 4.3. Linear Oxidation inner scale. The oxidation mechanism changes. The Transition to linear oxidation takes place after 4 h values of the rate constants increase (Fig. 2-11, Table at 1000°C and after 3 h at 1100°C and 1200°C. The 1). value of an apparent activation energy is 124.7 kJ/mole. Participation of titanium cations in the process of The continuity of the scale is disturbed with time. formation of the outer layer is caused by the presence of These phenomena were also observed by Stewart and the TiO (cub) phase in the inner scale (Table 2). The Cutler /ll/ and by Zhilyaev et al. IUI. Scale of a coefficient of cationic diffusion increases with an critical thickness begins to crack and peel due to increase in oxygen content, i.e., with the rise in the stresses; this decreases the protective properties of the amount of cationic vacancies /29/. scale, and oxidation rate increases. Linear kinetics During oxidation of titanium monoxide (TiO) indicates that the reaction rate is controlled by reactions titanium cations diffuse through the inner scale (Ti0 .x) 2 at the carbide/ inner scale interface and becomes the into the inner scale/outer scale interface. Diffusion of rate-controlling step of high-temperature oxidation of cations, evolving from oxycarbides while their oxygen titanium carbide with time. content is increased, may also play some role in this process. These diffusion processes control the formation of 5. CONCLUSIONS outer layers of scale at later stages at temperatures in excess of 1000°C. The coefficient of titanium cation 1. High-temperature oxidation of titanium carbide has diffusion in the rutile lattice exceeds that for the oxygen 2 been studied in the temperature range of 600- diffusion by a factor of 10 111,30-32/. It promotes 1200°C using TGA, XRD, SEM and AES methods. more intensive formation of an outer layer in It is shown for the first time that oxidation of comparison with an inner layer at high temperatures. compact titanium carbide at high temperatures The value of the activation energy in stage ΙΠ (219 (1000-1200°C) has four stages. Oxidation kinetics kJ/mole) is slightly lower than stage Π (234.1 kJ/mole). follows the three-stage parabolic law, parabolic This is also consistent with data for titanium diffusion oxidation is displaced by linear behavior with time. in single crystal of rutile /33,34/. 2. The activation energy for the first parabolic step is Diffusion of titanium cations dining the third stage the highest (333.4 kJ/mole), and depends on the causes the formation of a dense and sintered outer scale dissolution of oxygen in the titanium carbide lattice. (Fig. 13). Contrary to a multiphase inner scale, an outer The activation energies for step Π (234.1 kJ/mole) scale consists of TiO^x (rutile) only. Both the uni- and step ΙΠ (219.1 kJ/mole) are lower than for the directional diffusion of oxygen, forming an inner scale first step. They are controlled by the diffusion of during stages I and Π of parabolic oxidation, and the oxygen in an inner scale and of titanium cations in two-way diffusion of both titanium and oxygen an outer scale, respectively. The activation energy simultaneously, take place during stage ΠΙ of high- for linear oxidation is 124.7 kJ/ mole. Linear temperature oxidation of titanium carbide. oxidation depends on the processes at the This study makes it possible for the first time to "matrix/inner scale" interface. explain the complex and multistage mechanism of the 3. A one-layer scale is formed at 900°C. A two-layer interaction of titanium carbide and oxygen, based on scale is formed in the temperature range of 1000- the dissolution of oxygen in the carbide lattice, the 1200°C. The structural features of the scale were formation of a multiphase inner scale consisting of studied. Phase composition of the scale was identi- various titanium oxides, and the formation of an outer fied for different temperatures and time ranges. An layer due to the diffusion of titanium cations during inner layer has a multiphase composition and is high-temperature oxidation. composed of various titanium oxides (TiO, Ti 0 , This study indicates that the diffusion of titanium 2 3 Ti0 ). An outer layer is composed of Ti0 . (rutile) cations is of primary importance in the high- 2 2 x only. temperature oxidation of titanium carbide.

252 V.B. Voitovich High Temperature Materials and Processes

4. The effect of the diffusion of titanium cations on the 15. D.K. Chatteijee and H.A. Lipsitt, Met. Trans., high-temperature oxidation of titanium carbide was A13, 1837 (1982). studied for the first time. It was assumed that an 16. V.B. Voitovich and V.A. Lavrenko, High Temp. & outer scale is formed over the inner scale via Mater. Sei., 34, 249 (1995). diffusion of titanium cations, whereas the inner 17. S. Shimada and M. Kozeka, J. Mater. Sei., 27, layer is formed by the diffusion of oxygen. 1869 (1992). 5. A multistage mechanism of high-temperature 18. S.I. Alyamovskiy, Yu.G. Zainulin and P.V. Geld, oxidation of compact titanium carbide was proposed in: Proceedings of the Institute for Chemistry, and discussed. Ural Science Centre, 25, 15 (1973) [in Russian], 19. Yu.G. Zainulin, S.I. Alyamovskiy and G.P. Shveikin, Dokl. Akad. Nauk SSSR, 223, 904 REFERENCES (1975) [in Russian], 20. D.T. Bogomolov, S.I. Alyamovskiy, G.P. 1. S.S. Kiparisov, Yu. V. Levinsky and A.P. Petrov, Schveikin and V.D. Lubimov, Izv. Akad. Nauk Titanium Carbide: Manufacture, Properties, SSSR, Neorg. Mater., 6, 1405 (1970) [in Russian], Application, Metallurgiya, Moscow, 1987; 216 pp. 21. G.E. Mollox and RE. Smallman, Phil. Mag., 13, [in Russian], 1 (1966). 2. B.B. Clark and B. Roebuck, J. Refractory Metals 22. P.E. Pahrsson and D. Ramaker, J. Mater. Res., 8, & Hard Materials, 11, 23 (1992). 2716 (1993). 3. F.M. Pollard and P. Woodward, Trans. Faraday 23. M. Guttman, Μέίανχ (Corros. Ind.), 52, 373 Soc., 46, 190 (1950). (1977). 4. R Kieffer and F. Kölbl, Planseeber. Pulvermetal, 24. Μ. Shuhmacher and P. Ereno, Solid State Ionics, 1, 17 (1952). 12, 263 (1984). 5. W. Kinna and O. Rüdiger, Arch. Eisenhiittenw., 25. S. Andersson, Acta Chem. Scand., 13, 415 (1959). 24, 535 (1953). 26. RF. Voitovich, E.I. Golovko, E.T. Kachkovskaya 6. G.V. Samsonov and N.K. Golubeva, Zh. Fiz. and V.P. Smirnov, Zh. Fiz. Khimii, 58, 2606 Khimii, 30, 1258 (1956) [in Russian], (1984) [in Russian], 7. W. Webb, J.T. Norton and C. Wagner, J. 27. D.J. Derry, D.G. Lees and J.M. Calvert, Proc. Electrochem. Soc., 103, 112 (1956). Brit. Ceram. Soc., 19, 77 (1971). 8. N.F. McDonald and C.E. Ransley, Powder Met., 28. D.J. Deny, D.G. Lees and J.M. Calvert, J. Phys. 3, 172 (1959). Chem. Solids, 42, 57 (1981). 9. A. Münster, Ζ. Elektrochem., 63, 806 (1959). 29. T.S. Lundy, RA. Padgett and M.D. Banus, Met. 10. Ε. Nikolaiski, Ζ. Phys. Chem., 24, 406 (1960). Trans., 4, 1179 (1973). 11. R.W. Stewart and J. Cutler, J. Amer. Ceram. Soc., 30. T.S. Lundy and W.A. Coghean, J. Phys. (Paris) 50, 176 (1967). Colloq., 34, 299 (1973). 12. M. Reichle and J.J. Nicki, J. Less-Common 31. D.A. Vencatu and L.E. Poteat, Mater. Sei. Eng., Metals, 27, 213 (1972). 5, 258 (1970). 13. V.A. Zhilyaev, V.D. Lubimov and G.P. Shvekin, 32. R. Haul and G. Dumbgen, J. Phys. Chem. Solids, Izv. Alcad. Nauk SSSR, Neorg. Mater., 10, 47 26, 1 (1965). (1974) [in Russian], 33. K. Hoshino, N.L. Peterson and C.L. Wiley, J. 14. RF. Voitovich, in: Oxidation of Carbides and Phys. Chem. Solids, 46, 1397 (1985). Nitrides, Naukova Dumka, Kiev, 1981, p. 37 [in 34. J.R Akse and H.B. Whitehurst, J. Phys. Chem. Russian], Solids, 39, 457 91965).

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