o19 Modelling of Combined reinforcement of Ceramic Composites by Whisker and Transformation Toughening G. M. Song, Y Zhou, Y Sun &T C. Lei School of Materials Science and Engineering, PO Box 433, Harbin Institute of Technology, Harbin 150001 People's Republic of China ( Received 10 April 1997; accepted 30 May 1997) Abstract: An appropriate model of combine toughening in ceramic composites toughened with whiskers and transformation particles is presented to identify the effects of whisker toughening and transformation toughening. whisker tougher ing includes whisker bridging and crack deflection. For transformation toughen- ng, the shear effect is considered by using a shear factor, based on the dilatation contribution. Calculated results show that there are interactions among whisker bridging, crack deflection and transformation toughening. Crack brid- ging and crack deflection give rise to the contribution of transformation tough- ening, but transformation toughening produces a reduce to the contributions of whisker bridging and crack deflection. Predictions of the toughness of ZrO2(2mol%Y203)/Al2O3 based on the present model are showed to be in good agreement with the experimental results. c 1998 Elsevier Science Limited and Techna S.r.I 1 INTRODUCTION Several theoretical analyses of fibre bridging in fibre-reinforced ceramics has been proposed.9,0A Whisker is found to be effective in strengthening continuum model is introduced by Budiansky et and toughening ceramic materials, - and the al. for an elastic solid which contains particles toughening is realised by main three toughening that undergo an irreversible stress-induce dilation mechanisms, whisker bridging, whisker pullout and transformation. A model which includes both crack deflection. On the other hand toughness has shear and dilation effects of transformation tough been improved remarkably by ZrO2 1-m transfor- ening, has also been established, 2 which is perhaps mation in many ceramics --b Toughness values can more realistic than the early model which considers further be increased when both whiskers and parti- dilatation only. Amazigo et al. 3 also presented a cles are incorporated into ceramic matrix. In some analysis model of synergistic toughening including cases, synergistic toughening has been observed and crack bridging by ductile particles and dilation lized.7.In the combine toughened ceramics with transformation of ZrO2 particles both Sic whiskers and Zro transformation parti Whisker is a kind of short fibre and its orienta- cles, there are various toughening mechanisms that tion in ceramic matrix is usually random, which operate simultaneously, including whisker bridging, different to unidirectional aligned long fibre. Crack crack deflection, whisker pullout, microcracking deflecting along whisker/matrix interfaces is very and transformation, etc. du uring crack propagating. frequently observed. Addition of ZrOz transfo It is very difficult to identify experimentally th mation particles in whisker-toughened ceramics relative contribution and the role of each toughen- makes toughening analysis more complicated To ing mechanism. So theoretical modelling of com- date, there is not a suitable model to discuss bine toughening in ceramics toughened by multiple this problem of combine toughening with both toughening methods is a long standing subject whiskers and Zro2 transformation particlesModelling of Combined Reinforcement of Ceramic Composites by Whisker and Transformation Toughening G. M. Song, Y. Zhou, Y. Sun & T. C. Lei School of Materials Science and Engineering, PO Box 433, Harbin Institute of Technology, Harbin 150001, People's Republic of China (Received 10 April 1997; accepted 30 May 1997) Abstract: An appropriate model of combine toughening in ceramic composites toughened with whiskers and transformation particles is presented to identify the e€ects of whisker toughening and transformation toughening. Whisker toughen￾ing includes whisker bridging and crack de¯ection. For transformation toughen￾ing, the shear e€ect is considered by using a shear factor, f, based on the dilatation contribution. Calculated results show that there are interactions among whisker bridging, crack de¯ection and transformation toughening. Crack brid￾ging and crack de¯ection give rise to the contribution of transformation tough￾ening, but transformation toughening produces a reduce to the contributions of whisker bridging and crack de¯ection. Predictions of the toughness of SiCw/ ZrO2(2mol%Y2O3)/Al2O3 based on the present model are showed to be in good agreement with the experimental results. # 1998 Elsevier Science Limited and Techna S.r.l. 1 INTRODUCTION Whisker is found to be e€ective in strengthening and toughening ceramic materials,1±3 and the toughening is realised by main three toughening mechanisms, whisker bridging, whisker pullout and crack de¯ection.3 On the other hand, toughness has been improved remarkably by ZrO2 t±m transfor￾mation in many ceramics.4±6 Toughness values can further be increased when both whiskers and parti￾cles are incorporated into ceramic matrix. In some cases, synergistic toughening has been observed and utilized.7,8 In the combine toughened ceramics with both SiC whiskers and ZrO2 transformation parti￾cles, there are various toughening mechanisms that operate simultaneously, including whisker bridging, crack de¯ection, whisker pullout, microcracking and transformation, etc., during crack propagating. It is very dicult to identify experimentally the relative contribution and the role of each toughen￾ing mechanism. So theoretical modelling of com￾bine toughening in ceramics toughened by multiple toughening methods is a long standing subject. Several theoretical analyses of ®bre bridging in ®bre-reinforced ceramics has been proposed.9,10 A continuum model is introduced by Budiansky et al.11 for an elastic solid which contains particles that undergo an irreversible stress-induce dilation transformation. A model which includes both shear and dilation e€ects of transformation tough￾ening, has also been established,12 which is perhaps more realistic than the early model which considers dilatation only. Amazigo et al.13 also presented a analysis model of synergistic toughening including crack bridging by ductile particles and dilation transformation of ZrO2 particles. Whisker is a kind of short ®bre and its orienta￾tion in ceramic matrix is usually random, which is di€erent to unidirectional aligned long ®bre. Crack de¯ecting along whisker/matrix interfaces is very frequently observed. Addition of ZrO2 transfor￾mation particles in whisker-toughened ceramics makes toughening analysis more complicated. To date, there is not a suitable model to discuss this problem of combine toughening with both whiskers and ZrO2 transformation particles. Ceramics International 24 (1998) 521±525 # 1998 Elsevier Science Limited and Techna S.r.l. Printed in Great Britain. 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