Part A: applied scienc and manufacturing ELSEVIER Composites: Part A 30(1999)445-4 Ceramic composites: roles of fiber and interface J. P Singh", D. Singh, M. Sutaria nergy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA Abstract Results are presented that elucidate: (a) the effects of fiber coating on retained fiber strength and mechanical properties of Nicalon-fiber reinforced SiC matrix composites; and(b)the role of residual stresses in the interfacial bond strength of SiC-fiber-reinforced reaction-bonded Si3 N4 matrix composites. For Nicalon-fiber-reinforced SiC matrix composites that were fractured in a flexural mode, retained in-situ fiber strength, ultimate strength and work-of-fracture(wOF) of the composites increased with increasing thickness of the fiber coating and cached maximum values at a coating thickness of 0. 3 um. a direct correlation between the variation of in-situ fiber strength and the variation of ultimate strength and woF of the composites clearly indicates the critical role of the retained in-situ strength of reinforcing fibers in composites. Fiber pushout tests performed on SiC-fiber-reinforced reaction-bonded Si3N4 matrix composites indicate that both debonding and frictional shear stresses decreased with increasing fiber content. These variations are consistent with the variation of residual radial stress on fibers, as measured by neutron diffraction, i. e. residual stresses decreased with increasing fiber content. Because fracture behavior is strongly controlled by interfacial bond strength, which is proportional to the residual radial stress, appropriate control of residual stresses is critical in the design of composites with desired fracture properties. o 1999 Elsevier Science Ltd. All rights reserved Keywords: Composites; In-situ fiber strength, B. Residual/internal stress; B. Interface/interphase 1. Introduction The strength of the fiber/matrix interface bond should be optimized to facilitate desired fiber pullout during compo Continuous-fiber ceramic composites(CFCCs)are candi- site fracture, this in turn will lead to substantial energy date materials for structural applications in various indus- dissipation and improved fracture toughness. Bond strength tries, including automotive, aerospace and utilities, is controlled by both interfacial characteristics(such as fiber primarily because of their improved flaw tolerance, large coating [5])and residual stresses induced by thermal expan- ork of fracture(WOF)and noncatastrophic mode of failure sion mismatch between fibers and matrix and can be tailored [1, 2]. The mechanical behavior of these composites is by appropriately coating the fiber surface Fiber coating not greatly influenced by the strength of the reinforcing fibers, only controls bond strength but also protects the fiber from the characteristics of fiber/matrix interface and the residual being damaged by flaws generated during processing and stresses caused by thermal expansion mismatch between service. Therefore, an improved understanding of the role of fibers and matrix. Fiber strength is an important parameter fiber coating and residual stresses in fracture behavior will that controls the fracture behavior of CFCCs. High strength lead to the design and processing of CFCCs with reliable of reinforcing fibers is critical because once a matrix crack performance. In this paper, we present results that will eluc is initiated and extended, load is transferred from the matrix date: (1)the effects of fiber coating on flaw generation, to the fibers in the wake of the crack. Weak fibers fracture retained fiber strength and mechanical properties of Nica and lead to catastrophic failure of the composite, whereas lon-fiber-reinforced SiC matrix composites; and (2)the role strong fibers accommodate the stresses. Theoretical analysis of residual stresses in the interfacial bond strength of Sic and experimental observations have shown that the amount fiber-reinforced reaction-bonded Si, N4 matrix composites of fiber pullout(which contributes to the toughening of omposite)is strongly infuenced by the mean strength and the variability in strength of the reinforcing fibers [3]. Also, 2 Specimens and experimental procedure the ultimate load-bearing capacity of the composite is deter mined by fiber strength characteristics [4] To evaluate the effects of fiber coating, specimens of Nicalon-fiber-reinforced SiC matrix composites with Corresponding author. Tel: +1-630-252-5 123; fax: pproximately 42 vol. fiber reinforcements and carbon 3604 interlayer coating of varying thickness(0-1. 25 um)were 835X/99/- see front matter 1999 Elsevier Science Ltd. All rights reserved 1359-835X(98)00133-XCeramic composites: roles of fiber and interface J.P. Singh*, D. Singh, M. Sutaria Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA Abstract Results are presented that elucidate: (a) the effects of fiber coating on retained fiber strength and mechanical properties of Nicalon-fiber￾reinforced SiC matrix composites; and (b) the role of residual stresses in the interfacial bond strength of SiC-fiber-reinforced reaction-bonded Si3N4 matrix composites. For Nicalon-fiber-reinforced SiC matrix composites that were fractured in a flexural mode, retained in-situ fiber strength, ultimate strength and work-of-fracture (WOF) of the composites increased with increasing thickness of the fiber coating and reached maximum values at a coating thickness of <0.3 mm. A direct correlation between the variation of in-situ fiber strength and the variation of ultimate strength and WOF of the composites clearly indicates the critical role of the retained in-situ strength of reinforcing fibers in composites. Fiber pushout tests performed on SiC-fiber-reinforced reaction-bonded Si3N4 matrix composites indicate that both debonding and frictional shear stresses decreased with increasing fiber content. These variations are consistent with the variation of residual radial stress on fibers, as measured by neutron diffraction, i.e. residual stresses decreased with increasing fiber content. Because fracture behavior is strongly controlled by interfacial bond strength, which is proportional to the residual radial stress, appropriate control of residual stresses is critical in the design of composites with desired fracture properties. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Composites; In-situ fiber strength; B. Residual/internal stress; B. Interface/interphase 1. Introduction Continuous-fiber ceramic composites (CFCCs) are candi￾date materials for structural applications in various indus￾tries, including automotive, aerospace and utilities, primarily because of their improved flaw tolerance, large work of fracture (WOF) and noncatastrophic mode of failure [1,2]. The mechanical behavior of these composites is greatly influenced by the strength of the reinforcing fibers, the characteristics of fiber/matrix interface and the residual stresses caused by thermal expansion mismatch between fibers and matrix. Fiber strength is an important parameter that controls the fracture behavior of CFCCs. High strength of reinforcing fibers is critical because once a matrix crack is initiated and extended, load is transferred from the matrix to the fibers in the wake of the crack. Weak fibers fracture and lead to catastrophic failure of the composite, whereas strong fibers accommodate the stresses. Theoretical analysis and experimental observations have shown that the amount of fiber pullout (which contributes to the toughening of a composite) is strongly influenced by the mean strength and the variability in strength of the reinforcing fibers [3]. Also, the ultimate load-bearing capacity of the composite is deter￾mined by fiber strength characteristics [4]. The strength of the fiber/matrix interface bond should be optimized to facilitate desired fiber pullout during compo￾site fracture, this in turn will lead to substantial energy dissipation and improved fracture toughness. Bond strength is controlled by both interfacial characteristics (such as fiber coating [5]) and residual stresses induced by thermal expan￾sion mismatch between fibers and matrix and can be tailored by appropriately coating the fiber surface. Fiber coating not only controls bond strength but also protects the fiber from being damaged by flaws generated during processing and service. Therefore, an improved understanding of the role of fiber coating and residual stresses in fracture behavior will lead to the design and processing of CFCCs with reliable performance. In this paper, we present results that will eluci￾date: (1) the effects of fiber coating on flaw generation, retained fiber strength and mechanical properties of Nica￾lon-fiber-reinforced SiC matrix composites; and (2) the role of residual stresses in the interfacial bond strength of SiC- fiber-reinforced reaction-bonded Si3N4 matrix composites. 2. Specimens and experimental procedure To evaluate the effects of fiber coating, specimens of Nicalon-fiber-reinforced SiC matrix composites with approximately 42 vol.% fiber reinforcements and carbon interlayer coating of varying thickness (0–1.25 mm) were Composites: Part A 30 (1999) 445–450 1359-835X/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S1359-835X(98)00133-X * Corresponding author. Tel: 1 1-630-252-5123; fax: 1 1-630-252- 3604