CERAMICS INTERNATIONAL ELSEⅤIER Ceramics International 28(2002)565-573 Interphase effects on the bend strength and toughness of an oxide fibre/oxide matrix composite Ramanan venkatesh School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai Setatan Pulau Pinang, Malaysia Received 30 April 2001; received in revised form 12 October 2001; accepted 3 December 2001 Abstract The effect of alumina-20 wt ZrO2(PRD-166) fibre and Sno2 coating properties on the bending strength and toughness of alumina fibre/SnO2/glass matrix composites have been investigated. The mean strength of as-received alumina-20 wt ZrO2 fibres was 1380 MPa for a gage length of 17 mm and decreased with increase in heat treatment temperatures. It was also observed that as the Sno, coating thickness increased, roughness of the coating increased and this decreased the strength of the fibres. This rough ness effect had serious implications on the fracture characteristics of PRD-166/SnO2/glass and Saphikon/SnO2/glass matrix com- posites. PRD-166/SnO,glass matrix composites exhibited non-planar failure with fiber bridging and fibre debonding as major toughening mechanisms. Saphikon/SnO2/glass matrix composites failed in a tough manner with extensive fibre pullout. The differ- nce the failure mode between PRD-166/ SnO2/glass and Saphikon/SnO2/glass matrix composites was attributed to the clamping stress associated with fiber roughness at the PrD-166/SnO, interface as compared to the smoother Saphikon/ Sno, interface C 2002 Elsevier Science Ltd and Techna S.r. l. All rights reserved Keywords: Interface effects; Bend strength; Toughness; Oxide fibre composite Introduction irregularities at the fibre /matrix interface. In composites with strong bonding at the interface, cracks originating Fibre reinfor great potential for in the brittle matrix tensile cut through the fibres. improving strength and toughness of ceramic materials resulting in a planar brittle failure of the composite In [1-5]. Parameters that influence the properties of fibres composite with a weak bonding at the interface, when in ceramic matrix composites(CMCs) include: tensile matrix tensile strength is exceeded, multiple cracking of strength, strain to failure, Weibull modulus, aspect ratio the matrix takes place with the fibres having enough and surface roughness. Alumina, mullite and zirconia strength to bridge the cracks. Further increase in stress are the principal polycrystalline oxide fibres developed causes fibre debonding due to interfacial stress and [6-21]. Oxide fibre/oxide matrix composites are con- Poisson's effect. Continued stressing of the composite sidered for potential use at extremely high temperatures beyond fibre debonding causes the failure of the fibre (1400-1600C)and in severe environments [22-31]. along its length and then depending on residual stress, Failure strength and toughness of CMCs depend on a Poissons ratio of fibre and matrix and interfacial fric multitude of mechanisms involving matrix microcrack- tional stress, fibre pullout occurs. Fibre pullout is the ing, matrix prestressing, fibre debonding and fibre pull- main toughening mechanism in CMCs. Hence for a out. Strength and toughness of CMCs are greatly tough composite, the interface bonding should be influenced by the interfacial bonding at the fibre/matrix strong enough to allow load transfer but weak enough interface. Interfacial strength is a strong function of the to aid crack deflection, fibre debonding and fibre pull- degree of bonding(chemical or mechanical) between out Interfacial strength can be controlled by modifying fibre and matrix and the thermal mismatch between fibre the fibre/matrix reactions at the processing and service and matrix. Mechanical bonding is primarily due to temperatures either through proper selection of materi als or by means of interface gs. For a operate successfully the following conditions should be 0272-8842/02/S22.00C 2002 Elsevier Science Ltd and Techna S.r. l. All rights reserved. PII:S0272-8842(02)00011-1Interphase effects on the bend strength and toughness of an oxide fibre/oxide matrix composite Ramanan Venkatesh School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai Setatan, Pulau Pinang, Malaysia Received 30 April 2001; received in revised form 12 October 2001; accepted 3 December 2001 Abstract The effect of alumina–20 wt.% ZrO2 (PRD-166) fibre and SnO2 coating properties on the bending strength and toughness of alumina fibre/SnO2/glass matrix composites have been investigated. The mean strength of as-received alumina–20 wt.% ZrO2 fibres was 1380 MPa for a gage length of 17 mm and decreased with increase in heat treatment temperatures. It was also observed that as the SnO2 coating thickness increased, roughness of the coating increased and this decreased the strength of the fibres. This rough￾ness effect had serious implications on the fracture characteristics of PRD-166/SnO2/glass and Saphikon/SnO2/glass matrix com￾posites. PRD-166/SnO2/glass matrix composites exhibited non-planar failure with fiber bridging and fibre debonding as major toughening mechanisms. Saphikon/SnO2/glass matrix composites failed in a tough manner with extensive fibre pullout. The differ￾ence in the failure mode between PRD-166/SnO2/glass and Saphikon/SnO2/glass matrix composites was attributed to the clamping stress associated with fiber roughness at the PRD-166/SnO2 interface as compared to the smoother Saphikon/SnO2 interface. # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: Interface effects; Bend strength; Toughness; Oxide fibre composite Introduction Fibre reinforcement offers a great potential for improving strength and toughness of ceramic materials [1–5]. Parameters that influence the properties of fibres in ceramic matrix composites (CMCs) include: tensile strength, strain to failure, Weibull modulus, aspect ratio and surface roughness. Alumina, mullite and zirconia are the principal polycrystalline oxide fibres developed [6–21]. Oxide fibre/oxide matrix composites are con￾sidered for potential use at extremely high temperatures (1400–1600
C) and in severe environments [22–31]. Failure strength and toughness of CMCs depend on a multitude of mechanisms involving matrix microcrack￾ing, matrix prestressing, fibre debonding and fibre pull￾out. Strength and toughness of CMCs are greatly influenced by the interfacial bonding at the fibre/matrix interface. Interfacial strength is a strong function of the degree of bonding (chemical or mechanical) between fibre and matrix and the thermal mismatch between fibre and matrix. Mechanical bonding is primarily due to irregularities at the fibre/matrix interface. In composites with strong bonding at the interface, cracks originating in the brittle matrix tensile cut through the fibres, resulting in a planar brittle failure of the composite. In a composite with a weak bonding at the interface, when matrix tensile strength is exceeded, multiple cracking of the matrix takes place with the fibres having enough strength to bridge the cracks. Further increase in stress causes fibre debonding due to interfacial stress and Poisson’s effect. Continued stressing of the composite beyond fibre debonding causes the failure of the fibre along its length and then depending on residual stress, Poisson’s ratio of fibre and matrix and interfacial fric￾tional stress, fibre pullout occurs. Fibre pullout is the main toughening mechanism in CMCs. Hence for a tough composite, the interface bonding should be strong enough to allow load transfer but weak enough to aid crack deflection, fibre debonding and fibre pull￾out. Interfacial strength can be controlled by modifying the fibre/matrix reactions at the processing and service temperatures either through proper selection of materi￾als or by means of interfacial coatings. For a coating to operate successfully the following conditions should be met. 0272-8842/02/$22.00 # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(02)00011-1 Ceramics International 28 (2002) 565–573 www.elsevier.com/locate/ceramint E-mail address: ram5nan@tm.net.mu (R. Venkatesh)