Fiber Coatings Derived from Molecular Precursors
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ber Coatings Derived from Molecular Precursors
Factors Involved in Coating Fibers
H. Eric Fischer, David J. Larkin, and Léonard V. Interrante Introduction Interphases in Fiber-Reinforced Composites It is generally agreed that the mechanical performance of continuous fiberreinforced composites will dépend to a large extent on the nature of the interface between the fiber and the matrix.1 In the case of many inorganic composites, compositional and/or morphological gradients can arise over an extended région between the matrix and reinforcing fiber as a resuit of processing conditions, mechanical forces, or chemical interactions. The term "interphase" has been applied to such a région.2-3 There hâve been numerous theoretical and expérimental treatments of this subject in an effort to quantify the stress and strain in the interphase région, and to correlate the présence of such forces with the mechanical performance of the attendant composites.4 Much of the expérimental work has emphasized controlling the nature of the interphase through careful control of the processing conditions5 or by the introduction of spécial interface layers, ofteh by applying a coating to the reinforcing fiber prior to incorporating it into the matrix. ,5c'6 Coatings as divergent as metals,7,8 ceramics, and pyrocarbon or silicon8 hâve been applied to fibers incorporated into reinforced composites. Fibers hâve been coated to aid in the processing of composites, for example, by improving the ability of the matrix to wet the reinforcing fiber8,9 or as a barrier layer providing protection from chemical or mechanical damage during incorporation into the matrix.10 Coatings hâve also been emMRS BULLETIN/APRIL1991
ployed to effect the performance of composites under "in-use" conditions such as modification of the fiber to matrix bond11 and as a reaction barrier to permit the use of composites in corrosive environments.12 Coatings that hâve received particular attention in the literature are metals (e.g., Ni and Cu),7 oxides (e.g., A1203, Y203, and Zr0 2 ), 13 and non-oxide ceramics 812 " 14 (e.g., nitrides,15" carbides,10b16 and borides15b). A diverse array of methods hâve been employed to produce fiber coatings ranging from merely oxidizing the fiber surface9 to the application of multilayer coatings using rf sputtering.16 This article will highlight precursors that permit application of coatings by thermal processing using simple apparatus and low pyrolysis températures. This includes both low-weight, volatile molécules suitable for MOCVD (metalorganic chemical vapor déposition) application and polymers which may be applied by dipping or spray coating their solutions. In some situations, plasmas or lasers hâve been used to facilitate déposition from thèse organometallics at low tem-
TiCl3 + 2 BC13 + 2 m2 BC13 + xs NH 3 SiH4 (or SiCl4) + K n H x
Cranmer has briefly reviewed fiber coating and characterization techniques.17 As noted in the Cranmer review, most of the work leading to carbide, boride, and nitride coatings employs conventional atmosphe
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