40TH ANNIVERSARY J MATER SCI41(006)951-962 Damage characterisation of thermally shocked cross-ply ceramic composite laminates C. KASTRITseas. P A. SMITHJ.A. YEOMANS' School of Engineering(H6, University of Surrey, Guildford, GU2 7XH, Surrey U E-mail: j- yeomans@surrey. ac uk amage due to thermal shock in cross-ply Nicalon/calcium aluminosilicate ceramic matrix composites has been investigated. Heated specimens of two simple [(0/90 )s and (90/0@)s] and two multi-layer [(0%/90)3s and(90/0)3s] materials were quenched into water at room temperature Crack morphologies were assessed by reflected light microscopy and scanning electron microscopy. The use of image assembling software allowed the generation of reflected light microscopy images of all of the thermally-shocked surfaces onto which the crack patterns were then superimposed. This allowed clear identification of damage mechanisms and accurate quantification of damage accumulation with increasing severity of thermal shock. Damage was first detected in the central plies of each composite Composites with 0ocentr plies exhibited slightly higher resistance to thermal shock than their counterparts with 90 central plies. Although damage extended to the outer plies as the severity of the shock increased, crack density was found to vary with position at every shock: it was highest in the central plies and gradually reduced towards the outer plies. Multiple matrix cracking erpendicular to the fibre direction was the damage mode identified in 0 plies while 90 plie contained cracks that ran along the ply length At more severe shocks the morphology of these crack patterns was affected in significantly different ways. In addition, the thinner, simple cross-ply composites exhibited much higher resistance to thermal shock than their multi-layer counterparts. 2006 Springer Science Business Media, Inc 1. Introduction deflecting fibre-matrix interfaces ensures that the fibres As fibre-reinforced ceramic matrix composites(CMCs) remain largely unaffected, thus preserving the integrity of are being considered increasingly for high-performance the material. However, damage due to thermal shock still engines and other applications, it is becoming apparent has an adverse affect on mechanical and thermal proper- that there is a need to understand better their behaviour ties. In addition microstructural changes due to high tem under conditions of"thermal shock. This term describes perature exposure have a detrimental effect on the perfor n event in which a sudden(usually downward)tempera- mance of these materials under thermal shock conditions ture change generates stresses in the material that can lead Recent studies have concentrated mostly on materials to cracking and long-term property degradation [1]. Such with continuous unidirectional (UD) fibres or on various events are common in high-temperature machinery, e.g. types of porous 2-D SiC/SiC prepared by chemical vapour in the case of an emergency shut-down of a gas turbine. infiltration. It was found that multiple matrix cracking Fibre-reinforced CMCs have been shown to exhibit bet- perpendicular to the fibres was the main damage mode ter behaviour under conditions of thermal shock than their on the faces of UD CMCs that contained longitudinal monolithic or particulate-reinforced counterparts [2]. fibres, accompanied by the appearance of ' thumb-nail With optimum selection of fibres and matrices, favourable or 'thermal debond matrix cracks on their end faces [3- residual stress conditions can be established in the matrix, 10]. Large-scale porosity affected the behaviour of 2-D which lead to increased resistance to crack initiation due to SiC/SiC, as the pores acted as crack initiation sites at thermal shock. After cracks appear, the presence of crack- shocks of moderate severity [11-13]. Information on the * Author to whom all correspondence should be addressed. 0022-2461◎2006S Science Business Media, Inc. DOI:10.1007/s10853-006-6594-840TH ANNIVERSARY J MATER SCI 4 1 (2 0 0 6 ) 9 5 1 –9 6 2 Damage characterisation of thermally shocked cross-ply ceramic composite laminates C. KASTRITSEAS, P. A. SMITH, J. A. YEOMANS∗ School of Engineering (H6), University of Surrey, Guildford, GU2 7XH, Surrey, UK E-mail: j.yeomans@surrey.ac.uk Damage due to thermal shock in cross-ply Nicalon/calcium aluminosilicate ceramic matrix composites has been investigated. Heated specimens of two simple [(0◦/90◦)s and (90◦/0◦)s] and two multi-layer [(0◦/90◦)3s and (90◦/0◦)3s] materials were quenched into water at room temperature. Crack morphologies were assessed by reflected light microscopy and scanning electron microscopy. The use of image assembling software allowed the generation of reflected light microscopy images of all of the thermally-shocked surfaces onto which the crack patterns were then superimposed. This allowed clear identification of damage mechanisms and accurate quantification of damage accumulation with increasing severity of thermal shock. Damage was first detected in the central plies of each composite. Composites with 0◦ central plies exhibited slightly higher resistance to thermal shock than their counterparts with 90◦ central plies. Although damage extended to the outer plies as the severity of the shock increased, crack density was found to vary with position at every shock: it was highest in the central plies and gradually reduced towards the outer plies. Multiple matrix cracking perpendicular to the fibre direction was the damage mode identified in 0◦ plies, while 90◦ plies contained cracks that ran along the ply length. At more severe shocks the morphology of these crack patterns was affected in significantly different ways. In addition, the thinner, simple cross-ply composites exhibited much higher resistance to thermal shock than their multi-layer counterparts. C 2006 Springer Science + Business Media, Inc. 1. Introduction As fibre-reinforced ceramic matrix composites (CMCs) are being considered increasingly for high-performance engines and other applications, it is becoming apparent that there is a need to understand better their behaviour under conditions of ‘thermal shock’. This term describes an event in which a sudden (usually downward) tempera￾ture change generates stresses in the material that can lead to cracking and long-term property degradation [1]. Such events are common in high-temperature machinery, e.g. in the case of an emergency shut-down of a gas turbine. Fibre-reinforced CMCs have been shown to exhibit bet￾ter behaviour under conditions of thermal shock than their monolithic or particulate-reinforced counterparts [2]. With optimum selection of fibres and matrices, favourable residual stress conditions can be established in the matrix, which lead to increased resistance to crack initiation due to thermal shock. After cracks appear, the presence of crack- ∗Author to whom all correspondence should be addressed. deflecting fibre-matrix interfaces ensures that the fibres remain largely unaffected, thus preserving the integrity of the material. However, damage due to thermal shock still has an adverse affect on mechanical and thermal proper￾ties. In addition, microstructural changes due to high tem￾perature exposure have a detrimental effect on the perfor￾mance of these materials under thermal shock conditions. Recent studies have concentrated mostly on materials with continuous unidirectional (UD) fibres or on various types of porous 2-D SiC/SiC prepared by chemical vapour infiltration. It was found that multiple matrix cracking perpendicular to the fibres was the main damage mode on the faces of UD CMCs that contained longitudinal fibres, accompanied by the appearance of ‘thumb-nail’ or ‘thermal debond’ matrix cracks on their end faces [3– 10]. Large-scale porosity affected the behaviour of 2-D SiC/SiC, as the pores acted as crack initiation sites at shocks of moderate severity [11–13]. Information on the 0022-2461 C 2006 Springer Science + Business Media, Inc. DOI: 10.1007/s10853-006-6594-8 951