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(between 380 and 550°C) was reported by Shimada and Ishii [5.157]. They reported that oxidation commenced at 300°C at all partial pressures of oxygen between 0.66 and 39.5 kPa, and that complete oxidation occurred at different temperatures depending on the oxygen pressure. Shimada and Ishii suggested that rapid initial oxidation occurred through the formation of an oxycarbide, Zr(CxO1-x). Following this initial oxidation, the mechanism changed around 470°C to one that formed cubic zirconia as the product of oxidation along with the generation of microcracks. Selected area electron diffraction around the edges of the ZrC grains oxidized below 470°C, exhibited the presence of cubic ZrO2 nuclei that were not observable by XRD. At low temperatures, when the reaction was about 75% complete, carbon was found to be present as hexagonal diamond. Hexagonal diamond was also produced initially (reaction 40% complete) at higher temperatures.

Arun et al. [5.158] reported the following order of TiC>HfC>ZrC for the oxidation resistance for these three carbides at 1273 K. The oxidation of these materials is much greater when they are incorporated into hot pressed compositions of TiC-ZrO2, ZrC-ZrO2, and HfC-HfO2. Arun et al. also reported a greater oxidation of TiC when incorporated into ZrO2 as opposed to Al2O3.

Boron carbide is chemically very stable. It will dissociate in a vacuum above 2600°C into boron gas and solid carbon [5.159]. The oxidation of B4C starts at about 600°C, forming a B2O3 film. Moisture in the air will lower this temperature to 250°C. Chlorine reacts with B4C at 1000°C forming BCl3 and graphite.

5.2.3 Borides

Several of the diborides are of considerable interest because of their high melting points and high strengths at elevated temperatures. Probably the one that has received the most attention is TiB2; however, ZrB2, HfB2, NbB2, and TaB2 are also of interest. These are the most attractive because of their high stability compared to the other diborides. Like the carbides

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Chapter 5

and nitrides, the diborides possess the undesirable characteristic of oxidation. The oxidation of the diborides generally forms B2O3 and a metallic oxide according to:

(5.49)

B2O3 readily vaporizes above 1100°C, and therefore applications at high temperatures result in porous reaction layers. At lower temperatures, where the B2O3 is molten (Tm=490°C), a surface layer of glassy material is formed over an inner layer of metallic oxide [5.160].

When these diborides are used as particulate reinforcement for oxide matrices, various reactions may take place depending upon the oxide matrix. In a study of the oxidation of hot-pressed composites in air at 1650, 1850, and 2050°C, Vedula et al. [5.160] reported that in a zirconia matrix, the titania that formed by oxidation of the diboride went into solution into the zirconia and the B2O3 vaporized. In an yttria matrix tested in vacuum at 1600°C, they found significant reaction, but were unable to determine its exact nature because of the lack of published phase equilibria data. During hot pressing of a composite with an alumina matrix, the B2O3 that formed by oxidation of the diboride during heat-up reacted with the alumina forming a low melting liquid. Subsequent heating to 1600°C in vacuum caused reaction between the alumina and titania to form an intermediate aluminum titanate.

During sintering studies of titanium diboride, Walker and Saha [5.161] reported the following reactions:

(5.50)

(5.51)

In addition to these reactions, excess CO2 or CO will oxidize the TiC formed to TiO2. Davies and Phennah [5.162] have shown that TiB2 reacts with CO2 forming titanium borate, in addition to the TiO2 and B2O3 formed.

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Silicon hexaboride exhibits an oxidation resistance better than the above diborides as a result of the formation of a wellattached borosilicate film [5.159].

5.2.4 Silicides

The oxidation of MoSi2 has been reported to occur by several mechanisms by Fitzer [5.163] depending upon the temperature and the oxygen partial pressure. Initially, only MoO3 formed, but volatilized, allowing the formation of SiO2. The partial pressure of oxygen at the interface MoSi2/SiO2 then decreased, allowing the oxidation of only silicon to continue. At very high temperatures (>1200°C) and low oxygen pressures (<10-6 atm), active oxidation occurred with the formation of volatile silicon monoxide, as long as the silicon content on the surface was sufficient. At low pressures of oxygen, selective oxidation of silicon occurred because of its greater affinity for oxygen than molybdenum. The selective oxidation of silicon led to the formation of a sublayer of Mo5Si3. At moderately high temperatures (around 1000°C) and high oxygen pressures (≈10- 2 atm), the evaporation of the molybdenum oxides formed led to a protective SiO2 layer. During the volatilization of the molybdenum oxides, the SiO2 layer was very porous allowing rapid oxidation with temperature increase. At lower temperatures, where the molybdenum oxides did not volatilize but remained as solid oxide reaction products, a continuous silica layer could not form. This occurred at temperatures below 600°C, and is called pesting, which can lead to total destruction of the material. The actual amount of pesting that occurred, however, was dependent upon the microstructure [5.164]. Lin et al. [5.165] found that the oxidation of MoSi2 in a combustion gas environment (i.e., natural gas) was very similar to that described above. Initially, SiO2 and volatile MoO3 formed until the silica layer was of sufficient thickness to diminish the H2O/ CO2 activity (equivalent to a low partial pressure of oxygen) to a level where Mo5Si3 formed. The change in mechanisms took only minutes at temperatures of 1370 and 1600°C.

Copyright © 2004 by Marcel Dekker, Inc.