Interaction between SiC and Ti powder
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The interaction between SiC and Ti powder at 1073-1523 K was investigated employing a combination of x-ray diffraction, scanning electron microscopy with EDS, Auger spectroscopy, and transmission electron microscopy. As a result of the interaction, a triple-layer reaction zone was formed. The most important part of the reaction zone was a mixed TiC-Ti 5 Si 3 (C) layer. Thin TiC sublayers were formed on both the inner and the outer sides of the mixed reaction layer. The reaction zone was found to grow by a parabolic law with the kinetic constant, it = 1.3 X 10~3 e x p ( - 2 1 8 0 0 / r ) cm 2 /s. The growth process of the SiC/Ti reaction zone was assumed to be controlled by diffusion of all three components of the system: Ti, Si, and C. Thin reaction layers (5—6 fxva) spalled off the SiC surface and remained on the Ti powder side. Examples of reaction films obtained after different exposures at various temperatures are shown in Fig. 1. It can be seen that the coatings remaining on the SiC surface are covered with cracks normal to the SiC/reaction zone interface. These cracks must have appeared during cooling down to room temperature, because of the thermal mismatch stresses acting at the interface. In contrast, there are far fewer cracks in the thicker reaction films that spalled off the SiC surface. Apparently, at reaction layer thicknesses of > 5 /Ltm, the SiC/reaction zone interface becomes weaker than the reaction layer,9 and the thermal mismatch strains are more easily accommodated by cracking along the interface. SEM micrographs of the fracture surface of the reaction films, Figs. l(b)-l(d), do not reveal the presence of morphologically or compositionally different layers in the reaction zone. On the contrary, the reaction films look like a uniform layer with a grain size of ~ 0.5-1 /jura. EDS analysis (including x-ray mapping) also 2726
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suggests a very uniform distribution of Si and Ti in the reaction zone. Reaction layers obtained at reasonable exposures (up to 4 h) at 1073-1123 K are so thin that their phase composition can hardly be analyzed employing XRD. Diffraction spectra taken from thicker reaction films obtained at different exposures at 1173-1523 K, e.g., 1273 K X 1 h, Fig. 2(a), show that the reaction zone contains a Ti silicide, Ti5Si3, and a Ti carbide, TiC. No additional peaks that can be indicative of the presence of the ternary compound, Ti 3 SiC 2 , are observed. The lattice parameter of TiC measured from the XRD spectrum (ao ~ 0.430 nm) is significantly lower than that of the stoichiometric TiC (a0 = 0.433 nm 10 ) and even of the hypostoichiometric compound TiC0.6i (a0 = 0.431 nm 11 ). This implies that the composition of TiC in the reaction film is strongly hypostoichiometric and corresponds to TiCo.5-o.6- Another factor leading to the decrease of the lattice parameter of TiC may be oxygen contamination.12 The lattice parameters of T15S13 measured from the XRD spectrum (a0 =» 0.742 nm and Co ~ 0.513 nm) are also lower than those of a pure Ti 5 Si 3 (a0 ~ 0.74
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