Fiber-matrix interface coatings

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. 

R. H. Richards, The Chemical Vapor Deposition of Hexagonal Aluminates as a Fiber-Matrix Interface Coating for Oxide-Oxide Composites, M.S. Thesis, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, February 1995. 

The generic process for fabrication of fiber-reinforced aluminum oxide matrix composites by directed metal oxidation includes preforming, fiber-matrix interface coating, matrix growth and removal of residual aluminum. A flow chart with the various processing steps is shown in Fig, 1. 

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 ... 

Composite system (fiber/ matrix or coating) a Required interface strength, ol (MPa) Calculated transverse strength, oj (MPa) ... 

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. 

TiAl/SiC composite by interface reactions has recently been studied (Ochiai et al., 1994). The kinetics of the phase reactions is determined by interdiffusion, which has been studied in the case of TiAl-Mo (Zhang etal., 1992a). It has been shown that both the creep resistance and the toughness of a TiAl matrix composite can be improved by using coated fibers with weak fiber/matrix interfaces (Weber et al., 1993). 

The fiber is only one of three constituents of a CMC system, which also includes an interface and a matrix. Matrix processing is clearly beyond the scope of this report, but fiber-matrix interfaces (or interphases) are not. The manufacturing cost of potential interphase materials and the processes used to apply them to fibers and preforms is not well defined or understood. Interphase coatings are not mature at this point, and the cost impact of the interphase application process should be an important consideration in future efforts to commercialize CMCs. Therefore, the committee concludes that efforts should be made to determine the impact of the cost of fiber interphases on the cost of the total CMC system. 

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