of the glass fibers did not appear to play a role in the degradation of the mechanical properties. Even after 2 years, it was minimal. The degree of hydration product growth and its density was directly related to the degree of embrittlement. This embrittlement was attributed to an increase in pull-out bond strength due to the growth of CaO·H2O. In a composite where no hydration products formed, ductile fracture occurred as evidenced by fiber pull-out.

Ready [7.59] suggested that the water pressures developed during the reaction of hydrogen with particulate NiTiO3 in a matrix of titania at temperatures between 700°C and 1000°C were sufficient to cause microstructural degradation (i.e., grain boundary cracks). Thermodynamic data indicated that pressures as high as 6 MPa could be developed. The proposed mechanism involved the diffusion of hydrogen through TiO2 grains, reduction of NiTiO3 producing Ni metal and H2O gas at the TiO2/NiTiO3 interface, and subsequent grain boundary separation. The separation of the grain boundaries allowed additional hydrogen ingress. According to Ready, the location of pores (i.e., water vapor bubbles) at only the matrix/ particulate interface suggested that the reduction of the NiTiO3 was controlled by oxygen diffusion out of the TiO2 grains toward the interface.

Arun et al. [7.60] reported the following order of TiC>HfC>ZrC for the oxidation resistance of these three carbides at 1273 K. The oxidation of these materials was much greater when they were 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.

7.3.2 Nonoxide-Matrix Composites

Si3N4 Matrix Composites

A Si3N4 composite containing 30 wt.% ZrO2 (also containing 3 mol% Y2O3), when oxidized at 1200°C, exhibited

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decomposition of the zirconia grains as reported by Falk and Rundgren [7.61]. The oxidation proceeded by first forming faceted cavities close to the zirconia grain boundaries due to release of nitrogen dissolved in the zirconia. Prolonged oxidation formed silica-rich films upon the pore walls. Hot pressing at 1800°C apparently formed zirconia containing a variation in the amount of yttria that led to the formation of some monoclinic zirconia after oxidation for 20 min at 1200°C. At shorter times, only cubic and tetragonal zirconia were detected. Cristobalite formed in the oxide scale after 2 hr of oxidation. Short-term oxidation was suggested as a means to enhance mechanical properties; however, long-term oxidation resulted in disintegration of the composite.

SiC-Matrix Composites

Oxidation. As discussed in Sec. 7.2.2 on fiber coatings, an interphase material is either deposited onto the fibers before composite fabrication or the interphase is formed in situ. If the interphase is carbon, the composite must receive an exterior surface protective coating. This is the case for SiC fiber/SiC matrix composites. Once the carbon interphase has been oxidized leaving behind an annular cavity surrounding the fibers, continued oxidation fills the cavity with silica. The amount of silica present is dependent upon the proximity of the reaction site to the location of oxygen ingress. The time/ temperature schedule required for complete filling of the cavities with silica is also dependent upon the interphase layer thickness. Filipuzzi et al. [7.16] reported that a time of 10 hr in flowing oxygen at temperatures between 900°C and 1300°C was required to consume completely a 1-µm-thick carbon interphase in a 13×3×3 mm sample. Filipuzzi et al. reported that composites with thin interphase layers (on the order of 0.1 µm) resulted in microcracking due to the volume increase associated with the SiC to SiO2 conversion. Microcracking was not observed at high temperatures (i.e., 1200°C) presumably due to stress release through the lower viscosity silica glass nor was it observed in composites with

Copyright © 2004 by Marcel Dekker, Inc.

a thick interphase presumably due to the cavity not being filled with silica.

In a graphite-coated Nicalon™ fiber/SiC composite tested at 600 and 950°C in air, Lin and Becher [7.62] found that lifetimes were more dependent upon open porosity (15–25%) than upon parameters such as graphite coating thickness or fiber layout design. Increases in performance were obtained by the use of boron-containing oxidation inhibitors. This was attributed to the oxidation of the boron forming a glass that sealed cracks in the matrix thus minimizing the ingress of oxidation. The oxidation of graphite was the predominant mode of deterioration at low temperatures, but oxidation of SiC occurred at temperatures of 425°C. Verrilli et al. [7.63] found similar results in their investigations of graphite-coated Nicalon™ fiber/SiC composites tested at 500–1300°C. Oxidation of the interfacial graphite occurred first and then oxidation of the SiC fibers occurred evidenced by the formation of surface pits and radius reduction (most severe between 700 and 800°C). Other investigators have reported the degradation of carbon-coated Nicalon™ fiber/SiC [7.29] resulting from the oxidation of the fibers at intermediate temperatures (600–800°C).

The graphite oxidizes to CO and CO2 [reactions (7.7) and

(7.8) below]. Then additional oxygen reacts with the SiC forming free Si, which then continues to react to form SiO2, filling the space originally occupied by the graphite [reactions (7.9) and (7.10) below]. These reactions are all temperatureand oxygen partial pressure-dependent as discussed in Chaps. 2 and 5. This causes embrittlement and loss of toughness.

(7.7)

(7.8)

(7.9)

(7.10)

The latest preference for SiC/SiC composite is one with fibers of improved microstructure and chemistry called Sylramic™*

Copyright © 2004 by Marcel Dekker, Inc.

incorporated into a matrix of melt-infiltrated SiC matrix. In addition, the interphase material of choice has become BN, although its oxidation is essentially the same as carbon. Ogbuji [7.25] attributed the problems with BN interphase to one involving a thin film of carbon that formed under the BN either from carbon-rich fibers or sizing char that oxidized first exposing the BN interphase. PVA sizing and carbon-free fibers, although not completely removing this problem, at least tremendously decreased the severe pesting† that did occur.

Moisture Attack. In the study of alumina composites reinforced with SiC whiskers, Kim and Moorhead [7.64] found that the room-temperature flexural strength after exposure to H2/H2O at 1300°C and 1400°C was significantly affected by pH2O. Reductions in strength were observed when active oxidation of the SiC occurred at pH2O<2×10-5 MPa. Kim and Moorhead also reported that long-term exposures greater than 10 hr resulted in no additional loss in strength. At higher water vapor pressures, reductions in strength were less severe due to the formation of an aluminosilicate glass and mullite upon the surface of the sample. For exposures at 1400°C for 10 hr above pH2O=5×10-4 MPa, strength increases were observed due to the healing of cracks caused by glass formation at the sample surface.

Even at high temperatures, moisture may attack silica containing materials in the same fashion it does silicate glasses at ambient conditions. This is of concern for those materials like silicon nitride and carbide that form a protective layer of silica on their surfaces at high temperatures. Once the protective layer is broken, oxidation of the underlying material may take place. The protective layer does not even need to be broken for continued oxidation in moist environments. According to Williams [7.65], the diffusion of oxygen through silica is an

* Sylramic™, Dow Corning Corp., Midland, MI.

† Pesting was originally used to describe the formation of a powder-like deposit on the surface of metallic silicides during oxidation; however, it is now used to describe a similar phenomenon on any material.

Copyright © 2004 by Marcel Dekker, Inc.