Journal of the European Ceramic Society zyxwvutsrqponmlkjihgf 15 (1995) 297-305 0 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0955.2219/95/$9.50 Toughening of X-Sialon with A1,0, Platelets Y. Zhou, J. Vleugels, T. Laoui & 0. Van der Biest Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, De Croylaan 2, B-3001 Leuven, Belgium (Received 28 January 1994.; revised version received 15 July 1994; accepted 11 August 1994) Abstract zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Toughening of an X-sialon matrix with up to 53 vol- ume percent A&O3 platelets was investigated. The eflect of platelet content on both matrix phase compo- sition and mechanical properties of the composites was evaluated The extent of cracks introduced by Vickers hardness indentations showed a reduced length with increasing platelet content, indicating an increase in fracture toughness. The toughness has been shown to increase from 1.77 MPa ’ m1’2 for pure X-sialon up to 4.16 A4Pa . m’” for the composite containing 28 volume percent platelets. For higher platelet additions, toughness increases further but the composition of the matrix shafts to p ‘-sialon. The E-modulus of the composites increases with the amount of platelets. The improvement in fracture toughness was attributed to debonding, crack bridging and crack deflection. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1 Introduction Sialon ceramics have been investigated extensively due to their remarkable properties which make them suitable for high temperature structural applications. I-3 They are also being considered for cutting tools; p’-sialons for instance are already being used for machining of cast irons, but they are not suitable for most steels. Our recent research has shown that at the high cutting speeds at which ceramic cutting tools need to be used to be competitive, chemical compatibility with the workpiece material may be a more important selection criterion than other material properties such as hardness.4T5 A recent study showed that X-sialon ceramics exhibit a much better chemical compatibility with steel at high temperatures com- pared to p-sialon based ceramics.6 The high chemical wear resistance of X-sialon may indicate that cutting tools could be an interesting potential application. However, actual turning trials7 indi- cated that the poor toughness of the material caused frequent chipping of the cutting edge. An indentation fracture toughness value of about 1.77 MPa m1’2 was measured.6 The low fracture toughness of X-sialon will thus limit the use of this ceramic in the field of cutting tools. To overcome this common problem encountered in the application of monolithic ceramic materials, many attempts have been made to improve their fracture toughness. For instance, the incorporation of a second phase into the monolithic ceramic matrix can result in an improved fracture toughness of the composite. Toughening of ceramics has usually focused on the introduction of whiskers into the matrix phase. A recent concern regarding the reinforcing components has shown less interest in whiskers because of several unfavourable aspects, including the high price, toxicity and difficulty in achieving a uniform dispersion during processing. Platelets could be a good alternative to whiskers as a reinforcement in monolithic ceramics because the toughening mechanisms contributing to whisker composites such as crack bridging, debonding, pullout and crack deflection could also operate in platelet ceramic composites.8y9 Furthermore, the use of platelets in ceramic composites has some ad- vantages over whisker reinforced ceramics, including the ease of dispersion into ceramic matrix at high loading without agglomerates, safer handling and lower cost. In this study, Al203 platelets are se- lected to toughen X-sialon ceramic matrix. According to the Si-Al-O-N phase diagram’ both matrix and reinforcement should be in chemical equilibrium. A limited number of studies have been published concerning platelet reinforced ceramics. To our knowledge, toughening of X-sialon with Al203 platelets has never been reported in the literature. In a Si,N,-SiC platelet system, Hanninen reported that there was a dependence of fracture toughness on platelet content and direction.” A maximum value of 9.9 MPa . m1’2was obtained at 25 vol.% platelets in the plane parallel to the hot press axis and perpendicular to the platelet basal faces, whereas a toughness of 68 MPa . m1’2 was obtained for the unreinforced Si3N4 matrix.” Claussen achieved a toughness of 8 MPa . m1’2 from approxi- mately 4.8 MPa . m1’2 for the matrix, in RBSN with 20 vol.% Sic platelets. 9 In both cases toughness was enhanced, but strength dropped with the 297