Reactions of Molybdenum and Tungsten Carbides with Oxygen at High Temperatures
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THE oxidation of carbides is a rather complex process since gaseous reaction products like CO, CO 2 and volatile metal oxides can be formed in addition to oxides in condensed state. Therefore, most papers on this subject give only qualitative descriptions of the phenomena, e.g. for Mo and W-carbides. j r Frequently drastic changes of the reaction mechanism are observed in various temperature and pressure ranges. A systematic description of the processes occurring is found in the paper of W. W. Webb, T. T. Norton, and C. Wagner, ~but in this treatment the formation of volatile oxides is not considered. This reaction is, however, most important for the Mo and W metal oxidation at temperatures higher than 1300 K 7-19and thermochemical data indicate a strong CO-reaction for Mo and W carbides. 2~ Based on these facts the following relatively simple reaction mechanism is expected to be responsible for the high temperature low pressure oxidation of Mo and W carbides MexC + 1/202 ~ x M e
+ CO
MexC + 02 ~ xMe + CO2
[1] [2]
No oxide scales are formed on the surface and the CO and CO 2 reactions govern the initial stage of the process. Thus, a metal scale should grow on the surface and the carbide decompose at the phase boundary carbide/metal inside the specimen in a metal-carbon solid solution. The carbon atoms diffuse through the metal layer to the surface and there react with impinging 0 2 molecules to form CO and CO 2. In addition to this reaction volatile oxides evaporate from molybdenum and tungsten in oxygen-containing atmospheres, e.g. according to the reaction equation. Mo + 0 2 ~ MOOy(g)
[3]
This process gives rise to a large metal loss in addition to the pure metal evaporation 7-~9and causes a reduction S. G. KANG, formerly Visiting Scientist at the Max-PlanckInstitut is now Professor in the Dept. of Material Engineering, Han Yang University, Seoul 133, Korea. E. FROMM is Research Scientist at the Max-Planck-lnstitut fiat Metallforschung, Institut ffir Werkstoffwissenschaften, D-7000 Stuttgart-l, Germany. Manuscript submitted November 19, 1980.
of the thickness of the metallic surface layer on carbide specimens. The interrelation between the CO reaction (Eq. [1]) and the volatile oxide formation (Eq. [3]) finally ends up in a steady state reaction with a constant metal layer thickness and a constant weight loss according to an overall reaction, e.g. for M02C when only CO and MoO2(g) is formed M02C + 5/202 --~ CO + 2MoO2(g )
[4]
This reaction mechanism should hold only for high temperatures and low 0 2 pressures. At temperatures below 1300 K and 02 pressures higher than 10 -3 bar oxide layers can be formed on the surface of the metal layer 2~ and, thus, the reaction mechanism is changed drastically. In order to obtain quantitative data on the high temperature oxidation behavior of Mo and W carbides and to check the reaction mechanism gravimetric experiments were performed between 1300 and 2100 K with Mo2C, W2C and WC samples in an 0 2 atmosphere ranging from 10 -7 to 10 -2 bar. They were complemented by par
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