chapter  7
42 Pages

Inorganic Fibers

I. Introduction 162

II. Glass Fibers 165

III. Fibers on a Substrate 167 A. Boron fibers 167 B. Silicon carbide fibers 170

IV. Carbon Fibers 170 A. Carbon fibers from PAN precursors 170 B. Carbon fibers from pitch-based precursors 171

V. Alumina-Based Fibers 174 A. Alumina silica fibers 174 B. a-Alumina fibers 179

VI. SiC-Based Fibers 189 A. NL-200 Series of Nicalon fibers 189 B. Tyranno LOX-M 190 C. Hi-Nicalon and Tyranno LOX-E fibers 191 D. Near-stoichiometric fibers 194

VII. Si-C-N-Based Fibers 196 A. Si-C-N-O fibers 196 B. Silicon nitride fibers 197 C. Si-B-(N,C) fibers 197

VIII. Continuous Monocrystalline Filaments 197

IX. Whiskers 199

X. Conclusion 199

References 201

I. INTRODUCTION Ceramics as fibers seems at first view to be a strange concept. We are used to thinking of ceramics as being brittle, inflexible bulk materials with highly crystal­ line structures, not as flexible filaments which can be woven and turned into complex shapes. Nevertheless, natural ceramic fibers, such as asbestos fibers and rock wool, have been used for many decades. Man-made glass fibers have been available since the mid-1930s and interest in advanced fibers for reinforcement was stimulated by the development of carbon fibers made from organic precursors in the mid-1960s. The surfaces of glass fibers have to be protected by a size so as to prevent their deterioration by abrasion and by the environment and this size also acts as a lubricant and coupling agent to the matrix. The surfaces of carbon fibers are treated so as to improve interfacial bonding with the resin matrix and a coupling agent can also be added to the fiber surface. Synthetic ceramic fibers were developed in the 1960s, although they had large diameters and could not be woven. Today, the main interest in this type of fiber is for the reinforcement of titanium, and for this, the surface of the fiber has to be protected by a complex coating to avoid degradation during composite manufacture. Some continuous ceramic fibers with small diameters of around 20 |im were developed in the 1970s, and since 1980, a number of small-diameter ceramic fibers have been developed, some of which can be woven and which offer the possibility of rein­ forcing bulk ceramic matrices for use at very high temperatures. The inclusion of ceramic fibers in a brittle ceramic or vitroceramic matrix implies a control of the fibers’ surfaces, as the bonding of the fibers with the matrix has to be carefully controlled so as to maximize the control of crack propagation.