Microstructure of Oxide Scale

P-SijAION JCPDS 48-1615 p-Si4Al2O2N0 JCPDS 48-1616 p-SijAljOjNj JCPDS36-1333). (Reproduced with permission from Ref. [23] 

For Z2-1000 as well, no obvious oxide crystals could be seen, but its surface looked more porous than the surface of Zl-1000. However, for Z3-1000, the microstructure looked very different from that of Zl-1000 and Z2-1000, and there were visible agglomerates of oxide particles on the surface, and these should be mullite according to the results of XRD. 

After 1200 °C oxidation (see Z-1200), there were large numbers ofwhite needlelike oxides of less than 1 pm in length on the surface of Zl-1200. Combining the EDS analysis and XRD result, we are convinced that the needle-like oxides should be mullite crystals which grew in amorphous Si02. Clearly, no oxide crystals were seen on the surface of Z2-1200, except for large numbers of white spots which should be mullite oxide as same as the needle-like crystals in Zl-1200. However, their size was smaller than that in Zl-1200. For Z3-1200, we can see that the surface of the specimen was oxidized considerably. Combining the element analysis with the result of XRD, we can conclude that the oxide is mullite. 

When the oxidation temperature was increased to 1400 ° C (Z-1400), the needlelike mullite crystals in Zl-1400 grew up to more than about 10 pm in length. In Z2-1400, the mullite showed two kinds of shapes, needle-like crystals and agglomerates. The needle-like crystals were same as those in Zl-1400, and the agglomerates were same as those in Z3-1400. The whole surface of Z3-1400 is covered with the scale consisting of agglomerates of mullite. 

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G. D. West, S. Birosca and R. L. Higginson, Phase determination and microstructure of oxide scales formed on steel at high temperature , J. Microscopy 217, 122-129... 

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