Oxidation behavior matrix cracks

The main advantage of CMCs over monolithic ceramics is their superior toughness, tolerance to the presence of cracks and defects, and non-catastrophic mode of failure. It is widely accepted that in order to avoid brittle fiacture behavior in CMCs and improve the damage toloance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subsequent fiber pullout . Historically, following the development of SiC fibers, fiber coatings such as C or BN have been employed to promote the desired composite behavior. However, the non-oxide fiber/non-oxide matrix composites generally show poor oxidation resistance , particularly at intermediate... 

The first CMCs to be developed consisted of three major components a ceramic matrix, fibers embedded in the matrix, and a tailored interface between the fiber and the matrix. Although these materials show damage tolerance and non-brittle behavior, the non-oxide materials that compose the CMCs are prone to oxidation, especially when matrix cracks are present. Lately, the development of an all-oxide CMC has captured the researchers attention. In these oxide/oxide composites, fracture toughness is achieved through crack deflection inside the matrix. A controlled level of matrix porosity will provide suitable conditions for crack deflection while inherently impeding oxidation during high temperature service. ... 

Tests on tin oxide fiber coatings in model composite systems indicated some crack deflection at the coating-fiber interface (Siadati et al., 1991 Venkatesh and Chawla, 1992). However, tensile tests of tin oxide coated alumina fiber-reinforced alumina matrix composites demonstrated a decrease in the extent of fiber pullout as the density of the matrix phase was increased. This led to increasingly brittle fracture behavior in these composites (Goettler, 1993). Tin oxide also has thermal stability problems at elevated temperatures (Norkitis and Hellmann, 1991). For example, in the presence of air at temperatures above 1300°C (2,372°F), tin oxide (solid) decomposes into SnO (gas) and Oj (gas). This decomposition occurs at even lower temperatures when the partial pressure of oxygen in the test environment is reduced. 

Monticelli, C. Zucchi, F. Brunoro, G. Trabanelli, G. (1997). Stress corrosion cracking behaviour of some aluminium-based metal matrix composites. Corrosion Science, Vol. 39, No. 10-11, pp. 1949-1963, ISSN 0010938X Muhamed Ashraf, P. Shibli, S. M. A. (2007). Reinforcing aluminium with cerium oxide A new and effective technique to prevent corrosion in marine environments. Electrochemistry Communications, Vol. 9, No. 3, pp. 443-448, ISSN 13882481 Niino, M. Maeda, S. (1990). Recent Development Status of Fxmctionally Gradient Materials. ISIJ International, Vol. 30, No. 9, pp. 699-703, ISSN 09151559 Nunes, P. C. R. Ramanathan, L. V. (1995). Corrosion behavior of alumina-aluminium and silicon carbide-aluminium metal-matiix composites. Corrosion, Vol. 51, No. 8, pp. 610-617, ISSN 00109312... 

On the Figure 9 it appears that the cracks seem to be healed inside the matrix since they cannot be observed after 13h under air at 850 C. Moreover, the oxidation of TiB2 leads to the formation of Ti02 grains (reaction 5) which are covering the matrix surface (due to an increase of 1.64 in the molar volume ) and can act as an environmental barrier. Further characterizations on the self-healing behavior of this material are under progress. 

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