Coating-matrix interface

Oxidation of the fiber-coating-matrix interface is one of the major problems that have prevented the widespread use of non-oxide ceramic composites (Luthra, 1997b). This interface can be exposed to oxidizing environments when the ends of coated fibers are exposed to the surrounding atmosphere or when matrix cracks are present, allowing atmospheric oxygen to reach the fiber coatings. 

FIGURE 6-2 Schematic representations of the progression of oxidation along the fiber/coating/matrix interface of uniaxial SiC/C/SiC composites with fiber ends exposed. Only one section of the annular region along the fiber/ coating/matrix interface is shown. Source Luthra, 1994. 

A prerequisite for an effective fiber coating is chemical stability with the fiber and matrix phases of a composite. If reactions occur, the fiber-coating-matrix interface would become strongly bonded, and the composite would exhibit brittle fracture behavior. Based on currently available phase diagrams, oxide compounds have been identified that do not show compound formation with commercially available oxide fibers. Particular attention has been paid to fibers that show the highest temperature capability (alumina fibers, such as Fiber FP, PRD-166, and Nextel 610). Oxide compounds identified as stable with alumina included tin oxide, zirconia, titania (at temperatures below 1,150°C [2,102°F]), zirconia titanate (ZrXi04, also at temperatures below 1,150°C [2,102°F]), and possibly zirconia tin titanate (Zr(Sn,Ti)04). 

Increase in the area of hysteresis loops with the number of cycles leading to a decrease in ability to carry interfacial shear stress (t) resulting from a local sliding at the fiber/coat or coat/matrix interface. 

Figure 17 Raman spectra of a glass fiber/matrix interfaces. (A) styrene monomer (B) untreated E-glass fiber coated with polystyrene, (C) E-glass fiber treated with y-methacryloxy propyl trimethoxy silane.

For the successful development of fiber-reinforced ceramics the design of the fiber/matrix interface plays a key role. The coating of the fibers should meet the following demands ... 

E, V and a are the Young s modulus, Poisson ratio and CTE, respectively, and the subscript i refers to the interlayer or coating. The residual stress at the fiber/matrix interface, (Tai, for the composite without an interlayer can be obtained for , =... 

Three underlying mechanisms are responsible for the nonlinearity.17,18 (1) Frictional dissipation occurs at the fiber/matrix interfaces, whereupon the sliding resistance of debonded interfaces, r, becomes a key parameter. Control of t is critical. This behavior is dominated by the fiber coating, as well as the fiber morphology.19,20 By varying r, the prevalent damage mechanism and the resultant non-linearity can be dramatically modified. (2) The matrix cracks... 

The thermomechanical properties of coatings at fiber-matrix interfaces are critically important. A consistent characterization approach is necessary and the most commonly adopted hypothesis is that there are two parameters (Fig. 1.8). One is associated with fracture and the other with slip.1 33,136... 

For preforms of fiber reinforcements, a thin coating is applied to the fibers using chemical vapor infiltration (CVI). This coating step is essential both to protect the fiber from chemical attack by the strongly reducing aluminum alloy and to provide for a weak fiber/matrix interface in the composite. Because the coating is thin, the CVI step requires only a few hours, unlike CVI matrix formation processes, where long times are necessary to achieve sufficient densification. 

Wider use of fiber-reinforced ceramic matrix composites for high temperature structural applications is hindered by several factors including (1) absence of a low cost, thermally stable fiber, (2) decrease in toughness caused by oxidation of the commonly used carbon and boron nitride fiber-matrix interface coatings, and (3) composite fabrication (consolidation) processes that are expensive or degrade the fiber. This chapter addresses how these shortcomings may be overcome by CVD and chemical vapor infiltration (CVI). Much of this chapter is based on recent experimental research at Georgia Tech. 

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