CHEM ELSEVIER Materials Chemistry and Physics 75(2002)284-290 www.elsevier.com/locate/matchem In situ synthesis of Al2O3-ZrO2-SiCw ceramic matrix composites by carbothermal reduction of natural silicates L. Mariappana, T.S. Kannan, A M. Umarji National Aerospace Laboratories, Material Science Division, Bangalore 560017, India Indian Institute of Science, Material Research Center, Bangalore 560012, India Abstract In situ formations of Al2O3 ZrO2 SiCw termary composite powders have been obtained by carbothermal reduction of a mixture of Sillimanite, Kaolin and Zircon using two different carbon sources. Products formed were mixtures of alumina and zirconia along with silicon carbide in the form of whiskers. The effects of composition of the reactants, the role of fineness of the starting precursors and the nature of the carbon source on the final product powder obtained are presented. XRD and SEM analyses indicate complete reaction of the precursors to yield Al2O3 +ZrO2 +SiCw as product powders, with the Sic having whisker morphology. It is also seen that zirconia could be stabilised to some extent in the tetragonal form without any stabilising agent by tailoring the starting materials and their composition Keywords: Carbothermal reactions; In situ synthesis; Alumina zirconia silicon carbide whisker ceramic composites 1. Introduction Carbothermic reactions as the name indicates, are high temperature reactions, where carbon is used as a reducing Ceramic matrix composites are finding increasing appli- agent as well as a reactant. The reaction has been found cations as structural components where high temperature quite effective in reducing several oxides to lower oxides, strength, wear and corrosion resistance are required and are metallic oxides to metals and even to metallic carbides not satisfied by metals and super alloys. Exceptional im- and oxycarbides [1]. The carbothermic reaction is normally provement can be obtained in high temperature properties carried out in an inert gas atmosphere of argon or nitrogen such as creep, fracture toughness, hardness and strength to prevent oxidation of carbon. Reduction of silicates with ith decreasing variability in properties and performance carbon has been under active consideration for the past 2 y using these composites. Among the various compos- decades. Bechtold and Cutler [2] studied the separation of ites whisker reinforced composites are one particular class. clay minerals for obtaining alumina by carbothermic reac- These whisker reinforced composites are prepared in two tions and in the process they were able to obtain silicon ways viz, physical mixing and in situ synthesis. In situ carbide whiskers( SiCw)as well synthesis of alumina(Al2O3)+ zirconia(ZrO2)+ silicon Based on these early trials, which were not then known carbide whisker(SiCw)ceramic composites was attempted as a route to the production of ceramic matrix composite o as to overcome some of the difficulties associated with precursor powders by carbothermic reaction, Chaklader et al physical mixing and avoid direct handling of very expensive [3] exploited this reaction for production of Al2O3-SiCw as well as hazardous sic whiskers composite powders. Simultaneously we were able at NAL to Much of the earlier work reported on the ceramic com- successfully synthesise ceramic composite powder mixtures posite systems Al2O3+SiCw, ZrO SiCw or Al2O3+ of Al2 O3+SiCw and ZrO2+SiCw by carbothermic reaction ZrO2+SiCw was via ceramic processing route, i. e, physical [4-6] using appropriate mineral silicates, acting as a cheap mixing of the constitutional oxides and the costly whiskers. and inexpensive method for generating both the ceramic Thus a chemical route to generate these ceramic composite oxide matrix and Sicw in situ precursor powders via carbothermal reactions was consid In the present work, an attempt is made to synth ered to be an effective as well as a much cheaper route three phase mixture of alumina +zirconia silicon carbide whisker ceramic composite using a combination of alu- monosilicate and Zircon minerals as cheap and abundantly E-mailaddress:kannan@lycos.com(T.S.Kannan) available sources. It is also suggested [7, 8 that additional 0254-0584/02/S-see front matter o 2002 Published by Elsevier Science B.V. 0584(02)00077-9Materials Chemistry and Physics 75 (2002) 284–290 In situ synthesis of Al2O3–ZrO2–SiCw ceramic matrix composites by carbothermal reduction of natural silicates L. Mariappan a, T.S. Kannan a,∗, A.M. Umarji b a National Aerospace Laboratories, Material Science Division, Bangalore 560017, India b Indian Institute of Science, Material Research Center, Bangalore 560012, India Abstract In situ formations of Al2O3 + ZrO2 + SiCW ternary composite powders have been obtained by carbothermal reduction of a mixture of Sillimanite, Kaolin and Zircon using two different carbon sources. Products formed were mixtures of alumina and zirconia along with silicon carbide in the form of whiskers. The effects of composition of the reactants, the role of fineness of the starting precursors and the nature of the carbon source on the final product powder obtained are presented. XRD and SEM analyses indicate complete reaction of the precursors to yield Al2O3 + ZrO2 + SiCW as product powders, with the SiC having whisker morphology. It is also seen that zirconia could be stabilised to some extent in the tetragonal form without any stabilising agent by tailoring the starting materials and their composition. © 2002 Published by Elsevier Science B.V. Keywords: Carbothermal reactions; In situ synthesis; Alumina + zirconia + silicon carbide whisker ceramic composites 1. Introduction Ceramic matrix composites are finding increasing appli￾cations as structural components where high temperature strength, wear and corrosion resistance are required and are not satisfied by metals and super alloys. Exceptional im￾provement can be obtained in high temperature properties such as creep, fracture toughness, hardness and strength with decreasing variability in properties and performance by using these composites. Among the various compos￾ites whisker reinforced composites are one particular class. These whisker reinforced composites are prepared in two ways viz, physical mixing and in situ synthesis. In situ synthesis of alumina (Al2O3) + zirconia (ZrO2) + silicon carbide whisker (SiCw) ceramic composites was attempted so as to overcome some of the difficulties associated with physical mixing and avoid direct handling of very expensive as well as hazardous SiC whiskers. Much of the earlier work reported on the ceramic com￾posite systems Al2O3 + SiCw, ZrO2 + SiCw or Al2O3 + ZrO2+SiCw was via ceramic processing route, i.e., physical mixing of the constitutional oxides and the costly whiskers. Thus a chemical route to generate these ceramic composite precursor powders via carbothermal reactions was consid￾ered to be an effective as well as a much cheaper route. ∗ Corresponding author. E-mail address: kannan@lycos.com (T.S. Kannan). Carbothermic reactions as the name indicates, are high temperature reactions, where carbon is used as a reducing agent as well as a reactant. The reaction has been found quite effective in reducing several oxides to lower oxides, metallic oxides to metals and even to metallic carbides and oxycarbides [1]. The carbothermic reaction is normally carried out in an inert gas atmosphere of argon or nitrogen to prevent oxidation of carbon. Reduction of silicates with carbon has been under active consideration for the past 2 decades. Bechtold and Cutler [2] studied the separation of clay minerals for obtaining alumina by carbothermic reac￾tions and in the process they were able to obtain silicon carbide whiskers (SiCw) as well. Based on these early trials, which were not then known as a route to the production of ceramic matrix composite precursor powders by carbothermic reaction, Chaklader et al. [3] exploited this reaction for production of Al2O3–SiCw composite powders. Simultaneously we were able at NAL to successfully synthesise ceramic composite powder mixtures of Al2O3+SiCw and ZrO2+SiCw by carbothermic reaction [4–6] using appropriate mineral silicates, acting as a cheap and inexpensive method for generating both the ceramic oxide matrix and SiCw in situ. In the present work, an attempt is made to synthesise a three phase mixture of alumina + zirconia + silicon carbide whisker ceramic composite using a combination of alu￾minosilicate and Zircon minerals as cheap and abundantly available sources. It is also suggested [7,8] that additional 0254-0584/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S0254-0584(02)00077-9