Strength degradation of SiC fiber during manufacture of titanium matrix composites by plasma spraying and hot pressing
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INTRODUCTION
FIBER strength is one of the most important factors determining mechanical properties in continuous fiber reinforced composites.[1,2,3] Ceramic fibers, such as SiC or Al2O3, possess a wide spread of strengths because of inherent internal/surface flaws, which are usually assumed to be randomly distributed. A Weibull distribution function is commonly used to describe the spread of fiber strengths about a mean value.[3,4] A wide distribution of fiber strengths can result in a large scatter in composite mechanical properties,[2,3] and failure behavior and longitudinal strength of a composite containing fibers of varying strengths is dominated by the lower strength fiber, irrespective of the fraction of low strength fiber.[2] Fiber strength degradation by either chemical or mechanical damage has been found during composite manufacture by a variety of routes including foilfiber-foil (FFF), powder cloth, matrix coated fiber (MCF), and thermal spraying.[3–9] Low pressure plasma spraying (LPPS) has been explored as an economic method of manufacturing continuous fiberreinforced metal matrix composites.[10–15] The matrix, in the form of powder feedstock, is injected into a high-temperature plasma flame where it is melted to form a spray of liquid droplets. The droplets are accelerated toward a rotating cylinder on which a continuous fiber or monofilament is wound with a precise spacing. The sprayed matrix infiltrates into K.H. BAIK, Research Fellow, and P.S. GRANT, Director, Oxford Centre for Advanced Materials and Composites, are with the Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom. Manuscript submitted January 17, 2001.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the interfiber region, solidifies, and binds the fibers into a single layer composite or monotape. A number of monotapes are then stacked together and consolidated by hot isostatic pressing or unidirectional hot pressing. LPPS allows a variety of matrices to be sprayed including Al, Ti, and MoSi2 based alloys.[6,12–15] This technique has been further developed by the authors to manufacture a multilayer, circumferentially fiber-reinforced composite ring, rather than monotape, in a single spraying operation by combining LPPS with simultaneous fiber winding, termed spray/wind.[6,12–15] This process allows the production of composite preforms of up to 98 pct density, with uniformly spaced fibers. A major disadvantage of the LPPS route for the manufacture of fiber-reinforced composites is fiber damage or breakage (oxidation and other contamination can also be problematical in some matrices such as Ti alloys[15]). The impact of sprayed droplets, typically with velocities of several hundred meters per second, can cause severe thermal shock and mechanical damage.[5] Fiber damage in subsequent consolidation of monotapes, or foil/fiber preforms obtained from other routes, has been studied extensively, and stress localization at any asperity contacts in the preform between matrix and fiber leads to local fiber bending or microcr
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