c01052e5 lence ELSEVIER Composites: Part A 34(2003)163-170 Fabrication and testing of all-oxide cfcc tubes M.G. Holmquist& b, T C. Radsick2, O.H. Sudre F.F. Lange, k Department of Materials, University of Califomia at Sa ara. Santa Barbara. CA 93106 USA vOlvo Aero Corporation, 4618 intan, Sweden Rockwell Scientific Company, 1049 Camino Dos housand Oaks. CA 91360. USA Received 15 August 2001: revised 15 September 2002: accepted 23 October 2002 Abstract A novel, low-cost processing method was used to manufacture all-oxide ceramic composite tubes. Fibre cloths were infiltrated with a pre consolidated slurry composed of a mixture of mullite and alumina powders. The pre-consolidated slurry was first formulated at a low volume fraction of powder to produce a short-range repulsive interparticle pair potential that allowed consolidation via pressure filtration, yet produced a consolidated body that could be fluidised by vibration. Efficient infiltration of fibre tows and a homogeneous microstructure were demonstrated. The infiltrated cloths, which could be frozen for latter use, were rolled to tubular shapes. After drying, the powder matrix strengthened by cyclic infiltration and pyrolysis of an alumina precursor. When the tubes were pressurised, they delaminated at an average hoop stress of 47 MPa Delamination initiated within the porous matrix where the outer most layer terminated. A failure analysis is presented using a strain energy release rate function for this specimen/crack configuration. Values of fracture energy inferred from the analysis were in close agreement with literature data for porous ceramic Published by Elsevier Science Ltd. Keywords: E. Powder processing: E Prepreg: B. Fracture: Oxide composites 1. Introduction on the fibres. which must be isolated from matrix cracks to take advantage of their high strength. Two different All-oxide continuous fibre reinforced ceramic compo- strategies for isolating fibres from matrix cracks can be sites(CFCCs) have attracted growing interest for use distinguished; one is to develop a crack deflecting interface high temperature applications that include gas turbine between the fibres and the surrounding matrix [13-18] ngines[1-6], hot gas filters [7, 8]and thermal protection second is to produce a porous matrix [5,18,19,35].The systems [9]. These composites are stable in oxidising failure mechanisms of composites fabricated with a porous environments. However, applications are limited to 1000- matrix have been studied and described in some detail 6, 20 1200C, due to the currently available fibres [10]. More 21]. Upon loading the stress/strain behaviour is nearly conventional, SiC based, CFCCs may find applications at linear, yet the matrix continuously microcracks before the higher temperatures(=1200C), although their life may onset of fibre failure [6, 20]. The fibres are isolated from the be shorter because of their susceptibility to oxidative matrix cracks because a continuous crack front cannot exist embrittlement at stresses that exceed the matrix strength within a porous matrix. Namely, crack extension within the [11,12」 matrix occurs by the breaking of grain pairs at grain Apart from fibre development, major efforts are boundaries within the matrix. when the matrix becomes focused on the issue of making oxide CFCCs damage ufficiently dense to support a continuous crack front, tolerant and insensitive to notches. These properties depend matrix cracks extend through the fibres [36]; when this occurs, the fibres are no longer isolated from the matrix Corresponding author. Tel : +1-805-893-8248; fax: + 1-805-893-8486. crac E-mail address: flange @engineering. ucsb. edu(FF. Lange) I Present address: Advanced Engineering. SAAB. 461 80 Trollhattan Mullite is an ideal matrix material because of its high creep resistance, low modulus, low CTE and sluggish 2 Present address: USAFA/DFEM. US Air Force Academy, Colorado densification behaviour at temperatures below -1300C [21]. In the current study, mullite is used as the major matrix Published by Elsevier Science Ltd. PI:S1359-835X(02)00208-7Fabrication and testing of all-oxide CFCC tubes M.G. Holmquista,b,1, T.C. Radsicka,2, O.H. Sudrec , F.F. Langea,* a Department of Materials, University of California at Santa Barbara, Santa Barbara, CA 93106, USA b Volvo Aero Corporation, 461 81 Trollha¨ttan, Sweden c Rockwell Scientific Company, 1049 Camino Dos Rios, Thousand Oaks, CA 91360, USA Received 15 August 2001; revised 15 September 2002; accepted 23 October 2002 Abstract A novel, low-cost processing method was used to manufacture all-oxide ceramic composite tubes. Fibre cloths were infiltrated with a pre￾consolidated slurry composed of a mixture of mullite and alumina powders. The pre-consolidated slurry was first formulated at a low volume fraction of powder to produce a short-range repulsive interparticle pair potential that allowed consolidation via pressure filtration, yet produced a consolidated body that could be fluidised by vibration. Efficient infiltration of fibre tows and a homogeneous microstructure were demonstrated. The infiltrated cloths, which could be frozen for latter use, were rolled to tubular shapes. After drying, the powder matrix was strengthened by cyclic infiltration and pyrolysis of an alumina precursor. When the tubes were pressurised, they delaminated at an average hoop stress of 47 MPa. Delamination initiated within the porous matrix where the outer most layer terminated. A failure analysis is presented using a strain energy release rate function for this specimen/crack configuration. Values of fracture energy inferred from the analysis were in close agreement with literature data for porous ceramics. Published by Elsevier Science Ltd. Keywords: E. Powder processing; E. Prepreg; B. Fracture; Oxide composites 1. Introduction All-oxide continuous fibre reinforced ceramic compo￾sites (CFCCs) have attracted growing interest for use in high temperature applications that include gas turbine engines [1–6], hot gas filters [7,8] and thermal protection systems [9]. These composites are stable in oxidising environments. However, applications are limited to 1000– 1200 8C, due to the currently available fibres [10]. More conventional, SiC based, CFCCs may find applications at higher temperatures ($1200 8C), although their life may be shorter because of their susceptibility to oxidative embrittlement at stresses that exceed the matrix strength [11,12]. Apart from fibre development, major efforts are focused on the issue of making oxide CFCCs damage tolerant and insensitive to notches. These properties depend on the fibres, which must be isolated from matrix cracks to take advantage of their high strength. Two different strategies for isolating fibres from matrix cracks can be distinguished; one is to develop a crack deflecting interface between the fibres and the surrounding matrix [13–18], a second is to produce a porous matrix [5,18,19,35]. The failure mechanisms of composites fabricated with a porous matrix have been studied and described in some detail [6,20, 21]. Upon loading the stress/strain behaviour is nearly linear, yet the matrix continuously microcracks before the onset of fibre failure [6,20]. The fibres are isolated from the matrix cracks because a continuous crack front cannot exist within a porous matrix. Namely, crack extension within the matrix occurs by the breaking of grain pairs at grain boundaries within the matrix. When the matrix becomes sufficiently dense to support a continuous crack front, matrix cracks extend through the fibres [36]; when this occurs, the fibres are no longer isolated from the matrix cracks. Mullite is an ideal matrix material because of its high creep resistance, low modulus, low CTE and sluggish densification behaviour at temperatures below ,1300 8C [21]. In the current study, mullite is used as the major matrix 1359-835X/03/$ - see front matter Published by Elsevier Science Ltd. PII: S1 35 9 -8 35 X( 02 )0 0 20 8 -7 Composites: Part A 34 (2003) 163–170 www.elsevier.com/locate/compositesa 1 Present address: Advanced Engineering, SAAB, 461 80 Trollha¨ttan, Sweden. 2 Present address: USAFA/DFEM, US Air Force Academy, Colorado Springs, Co.,USA. * Corresponding author. Tel.: þ1-805-893-8248; fax: þ1-805-893-8486. E-mail address: flange@engineering.ucsb.edu (F.F. Lange)