Availableonlineatwww.sciencedirect.com SCIENCE DIRECTO composites Part A: applied science ELSEVIER Composites: Part A 34(2003)613-622 www.elsevier.com/locate/composite Mesostructure of whipoX all oxide cmcs M. SchmuckerA. Grafmuller h. schneide German Aerospace Center(DLR), Institute of Materials Research, D51147 KOln, Germany Received 29 October 2002; revised 26 February 2003: accepted 14 March 2003 Abstract Wound highly porous oxide matrix(WHIPOX) ceramic matrix composites consist of oxide fibers(mullite- or alumina-type) which are embedded in mullite- or alumina-rich matrices, respectively. In the ideal case the fiber distribution is homogeneous; in reality, however, fabrication(winding)-induced matrix agglomerations do occur. As knowledge on the homogeneity of the material is crucial for the prediction f the mechanical behavior a technique to describe the mesostructure of WhiPOX quantitatively has been developed by means of optical microscopy(transmitted light). The technique makes use of light conductivity and opacity of fibers and matrix, respectively. Three dimensional plots of the matrix agglomerations were obtained by tomographic methods using 25 individual slices of 1.5 mm thickness for mesostructurally. The study showed that delamination- induced failure of WhIPOX is essentially controlled by localized interlaminate matrix agglomerations. Compression of WhiPOX plates in the pre-sintering moist stage helps to achieve a better homogeneity and thus improved near strength of WHIPOX components C 2003 Elsevier science ltd. all rights reserved Keywords: A Ceramic-matrix composites(CMCs); C. Statistical properties/methods; D. Optical microscopy 1. Introduction matrix concept. WHIPOX CMCs exhibit excellent mech anical, thermomechanical, and thermal properties [4-71 Oxide ceramics have a high potential WHIPOX CMCs are made up of oxide fibers(ce protection systems(TPS)in combustion chambers of ga Nextel, 3M, fibers of type 610, 650 or 720)which are turbine engines and for re-entry space vehicles. A promising embedded in a porous mullite or alumina matrix. Adjacent way to achieve the required tough and damage tolerant matrix infiltrated fiber bundles typically combine to form ceramics is the reinforcement of the ceramic substrate bodies laminates with the occurrence of relatively large fiber free matrix agglomerations(Fig. 1). The fractography of by continuous ceramic fibers [1]. The non-brittle quasI- WHIPOX components and structures has shown that plastic behavior of fiber-reinforced ceramics is due to a relatively weak bonding between fibers and the matrix thus delamination is the predominant failure mechanism allowing crack deflection and fiber pull-out [2]. To achieve a ggesting that interlaminate matrix agglomerations have weak fiber/matrix bonding either suitable fiber coatings have a controlling influence on the material's mechanical behavior. An improved design of the meso- and micro- phases)or, in an alternative approach, a highly porous matrix structure of WHlPoX therefore requires reliable and may be used. Materials making use of the porous matrix statistically robust information on the amount and the concept exhibit only few local fiber/matrix contacts and spatial orientation of fiber free areas which, however, is not hence the matrix acts like a weak fiber/matrix interphase [3] able ye WHIPOX (Wound highly porous oxide matrix) Different methods have been presented for the quanti- ites have been designed according to this porou tative microstructural analysis of fiber-reinforced materials such as metal or polymer matrix composites. These Corresponding author. Tel: +49-22-03-601-2462: fax: +49-22-03 procedures all are focused on analyzes of the fiber 696-480 ns using Dirichlet tessalation [8, 9], fiber cell E-mail address: martin. schmuecker@dlr. de(M. Schmucker) analysis [10], or radial fiber-fiber distance distributions [9] 1359-835X/03/S- see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/s1359-835X03)00100-3Mesostructure of WHIPOX all oxide CMCs M. Schmu¨cker*, A. Grafmu¨ller, H. Schneider German Aerospace Center (DLR), Institute of Materials Research, D-51147 Ko¨ln, Germany Received 29 October 2002; revised 26 February 2003; accepted 14 March 2003 Abstract Wound highly porous oxide matrix (WHIPOX) ceramic matrix composites consist of oxide fibers (mullite- or alumina-type) which are embedded in mullite- or alumina-rich matrices, respectively. In the ideal case the fiber distribution is homogeneous; in reality, however, fabrication (winding)-induced matrix agglomerations do occur. As knowledge on the homogeneity of the material is crucial for the prediction of the mechanical behavior a technique to describe the mesostructure of WHIPOX quantitatively has been developed by means of optical microscopy (transmitted light). The technique makes use of light conductivity and opacity of fibers and matrix, respectively. Three￾dimensional plots of the matrix agglomerations were obtained by tomographic methods using <25 individual slices of 1.5 mm thickness for each sample. Samples from different sites of a WHIPOX plate, and samples which have been differently pressed prior to sintering were examined mesostructurally. The study showed that delamination-induced failure of WHIPOX is essentially controlled by localized interlaminate matrix agglomerations. Compression of WHIPOX plates in the pre-sintering moist stage helps to achieve a better homogeneity and thus improved shear strength of WHIPOX components. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites (CMCs); C. Statistical properties/methods; D. Optical microscopy 1. Introduction Oxide ceramics have a high potential for thermal protection systems (TPS) in combustion chambers of gas turbine engines and for re-entry space vehicles. A promising way to achieve the required tough and damage tolerant ceramics is the reinforcement of the ceramic substrate bodies by continuous ceramic fibers [1]. The non-brittle quasi￾plastic behavior of fiber-reinforced ceramics is due to a relatively weak bonding between fibers and the matrix thus allowing crack deflection and fiber pull-out [2]. To achieve a weak fiber/matrix bonding either suitable fiber coatings have to be employed (cleavable, porous, low toughness inter￾phases) or, in an alternative approach, a highly porous matrix may be used. Materials making use of the porous matrix concept exhibit only few local fiber/matrix contacts and hence the matrix acts like a weak fiber/matrix interphase [3]. WHIPOXe (Wound highly porous oxide matrix) composites have been designed according to this porous matrix concept. WHIPOX CMCs exhibit excellent mech￾anical, thermomechanical, and thermal properties [4–7]. WHIPOX CMCs are made up of oxide fibers (commercial Nextel, 3M, fibers of type 610, 650 or 720) which are embedded in a porous mullite or alumina matrix. Adjacent matrix infiltrated fiber bundles typically combine to form laminates with the occurrence of relatively large fiber free matrix agglomerations (Fig. 1). The fractography of WHIPOX components and structures has shown that delamination is the predominant failure mechanism suggesting that interlaminate matrix agglomerations have a controlling influence on the material’s mechanical behavior. An improved design of the meso- and micro￾structure of WHIPOX therefore requires reliable and statistically robust information on the amount and the spatial orientation of fiber free areas which, however, is not available yet. Different methods have been presented for the quanti￾tative microstructural analysis of fiber-reinforced materials such as metal or polymer matrix composites. These procedures all are focused on analyzes of the fiber distributions using Dirichlet tessalation [8,9], fiber cell analysis [10], or radial fiber–fiber distance distributions [9]. 1359-835X/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-835X(03)00100-3 Composites: Part A 34 (2003) 613–622 www.elsevier.com/locate/compositesa * Corresponding author. Tel.: þ49-22-03-601-2462; fax: þ49-22-03- 696-480. E-mail address: martin.schmuecker@dlr.de (M. Schmu¨cker)