THERMAL RESIDUAL STRESSES AND THEIR TOUGHENING EFFECT IN Al 2 O 3 PLATELET REINFORCED GLASS R. I. TODD{ 1 {, A. R. BOCCACCINI 2 , R. SINCLAIR 1 , R. B. YALLEE 1 and R. J. YOUNG 1 1 Manchester Materials Science Centre, University of Manchester and UMIST, Grosvenor Street, Manchester M1 7HS, U.K. and 2 FG Werkstotechnik, Technische Universitaet Ilmenau, 98684 Ilmenau, Germany (Received 28 July 1998; accepted 26 May 1999) AbstractÐFluorescence spectroscopy has been used to measure the thermal residual stresses in Al 2 O 3 -plate- let/borosilicate glass composites. Tensile residual stresses were found in the platelets, implying the presence of compressive residual stresses in the glass matrix. Measurements of stresses in the bulk of the composite could be obtained using ¯uorescence from platelets below the specimen surface. The measured stresses lay between the predictions of models for spherical particles and thin platelets, but were closer to the former for the range of platelet contents investigated (5±30 vol.%). Estimates of the increase in toughness associ- ated with the compressive residual stresses in the matrix suggest that this mechanism makes a signi®cant contribution to the toughening eect of the Al 2 O 3 platelets. # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Raman spectroscopy; Toughness; Structural ceramics; Composites; Residual stress 1. INTRODUCTION One eective way to improve the mechanical prop- erties of glass and glass±ceramics is to form a com- posite material. Such a composite can be fabricated by reinforcing the low-modulus, low-strength matrix with a high-modulus, high-strength and/or high- ductility second constituent in the form of ®bres, whiskers, platelets or particles [1, 2]. The resulting properties of the composites formed vary strongly, depending on the characteristics and geometry of the dispersed phase used. Although the improvement of the fracture beha- viour, i.e. fracture toughness and strength, of dis- persion reinforced glass and glass±ceramic composites may not be as pronounced as that achieved using continuous ®bre reinforcement [3, 4], the use of less expensive powder technology prep- aration techniques makes dispersion reinforcement an attractive alternative [3, 5]. In particular, ceramic platelets are increasingly being considered as the reinforcing component in ceramic matrices because they have improved environmental safety, pro- duction economics, and thermal stability compared with whiskers [6]. Although ceramic platelets have been used successfully to reinforce many engineer- ing ceramics [6±9], there has been only limited work on their use as a reinforcing constituent in glass and glass±ceramics. To date, only SiC platelets have been investigated extensively in such matrices [2, 10±12]. In recent studies, the incorpor- ation of Al 2 O 3 platelets in glass±ceramics and glass has also been considered [5, 13]. In particular, Al 2 O 3 platelets were shown to be eective in rein- forcing a commercial borosilicate glass: the mechan- ical behaviour of the composites in terms of fracture strength, Young's modulus, hardness and fracture toughness was improved over that of the unreinforced glass matrix [5, 14]. The development of subsurface damage induced by indentation in these composites has also been investigated [15]. It was found that the extent and depth of subsurface damage decrease with the addition of platelets. Qualitatively, the mechanical property improve- ments were ascribed to the increase in the Young's modulus of the composite over the unreinforced glass, the introduction of compressive residual stres- ses in the matrix due to the thermal expansion mis- match between the phases, and also crack de¯ection processes occurring around the platelets. The pre- sence of internal residual stresses and their positive eect on composite toughening was postulated on the basis of the considerable thermal expansion mis- match existing in these composites: the thermal expansion coecient of the glass matrix (a m  3:3  10 6 =8C) [14] is much lower than that of the alumina platelets ( a p  8:9  10 6 =8C) [16]. This should result in a net compressive stress in the matrix during cooling from the processing tempera- Acta mater. Vol. 47, No. 11, pp. 3233±3240, 1999 # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/99 $20.00 + 0.00 PII: S1359-6454(99)00177-9 {Present address: Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K. {To whom all correspondence should be addressed. 3233