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)00077-9
Materials 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 applications as structural components where high temperature strength, wear and corrosion resistance are required and are not satisfied by metals and super alloys. Exceptional improvement 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 composites 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 composite 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 considered 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 reactions 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 aluminosilicate 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
toughening might be obtained by the generation of ZrO2 in tends to be more pure in carbon content. Hence these two addition to Al2 O3 and SiCw in this process sources were used to study its effect on the overall product Kaolin(M/s English India Clays, India), Sillimanite and 2. Experimental Zircon(M/s Indian Rare Earths, India) were taken as the 2.. Ranw materials given in Table 1 whilst particle size distributions of the various precursor powders(measured using a sedigraph The basic ingredients used for the carbothermic reac- 5100-Ms Micromeritcs, USA) are presented in Fig. 1. AC tion to produce the composite precursor powder mixtures (M/s Emerck, India)and CB (M/s Degussa, Germany)with are the silicate minerals(containing alumina and zirconia) a BET surface area of 1000 and 300 m-gm- were used and finely divided carbon Silicate minerals were chosen for SEM photographs of these carbon sources are presented in their low cost and their availability. The action of carbon Fig. 2 is twofold viz, as a reducer of the silicate and also a part goes into the product being formed. Carbon was taken in 2.2. Experimental procedt two forms, activated charcoal (AC) and carbon black(CB) AC being obtained from natural sources usually does have a The required amounts of AC or CB, Kaolin, Silliman- small percentage of impurities On the other hand, CB which ite and Zircon were taken to contain C and Sioz con- is obtained from the distillation of crude oil or lamp black tent(in silicates) in the molar ratio of 5.5: 1. The starting Table 1 Chemical composition of the silicate starting materials used for carbothermic synthesis Compounds Mineral Zircon (%, M/s Sillimanite(%, M/s Kaolin(%, M/ Indian Rare Earths) Indian Rare Earths) glish India Clays) 39.00 36.50 TIO, 00.03 Loss on ignition 14.30 —c b60 Fig. 1. Particle size distribution of the starting precursors used for the reaction:(a) Kaolin,(b) Zircon;(c) Sillimanite
L. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 285 toughening might be obtained by the generation of ZrO2 in addition to Al2O3 and SiCw in this process. 2. Experimental 2.1. Raw materials The basic ingredients used for the carbothermic reaction to produce the composite precursor powder mixtures are the silicate minerals (containing alumina and zirconia) and finely divided carbon. Silicate minerals were chosen for their low cost and their availability. The action of carbon is twofold viz, as a reducer of the silicate and also a part goes into the product being formed. Carbon was taken in two forms, activated charcoal (AC) and carbon black (CB). AC being obtained from natural sources usually does have a small percentage of impurities. On the other hand, CB which is obtained from the distillation of crude oil or lamp black Table 1 Chemical composition of the silicate starting materials used for carbothermic synthesis Compounds present Mineral Zircon (%, M/s Indian Rare Earths) Sillimanite (%, M/s Indian Rare Earths) Kaolin (%, M/s English India Clays) Al2O3 – 60.00 39.00 ZrO2 65.60 – – SiO2 31.10 36.50 46.00 TiO2 00.60 00.20 00.50 Fe2O3 00.08 00.50 00.52 CaO – – 00.06 MgO – – 00.06 K2O – – 00.03 Na2O – – 00.08 Loss on ignition – – 14.30 Fig. 1. Particle size distribution of the starting precursors used for the reaction: (a) Kaolin; (b) Zircon; (c) Sillimanite. tends to be more pure in carbon content. Hence these two sources were used to study its effect on the overall product formation. Kaolin (M/s English India Clays, India), Sillimanite and Zircon (M/s Indian Rare Earths, India) were taken as the silicate precursors. Chemical analysis of the precursors is given in Table 1 whilst particle size distributions of the various precursor powders (measured using a sedigraph 5100-M/s Micromeritcs, USA) are presented in Fig. 1. AC (M/s Emerck, India) and CB (M/s Degussa, Germany) with a BET surface area of 1000 and 300 m2 gm−1 were used. SEM photographs of these carbon sources are presented in Fig. 2. 2.2. Experimental procedure The required amounts of AC or CB, Kaolin, Sillimanite and Zircon were taken to contain C and SiO2 content (in silicates) in the molar ratio of 5.5:1. The starting
Mariappan et al. /Materials Chemistry and Physics 75(2002)284-290 Table 2 ZrO2)+35 vol. SiCw from starting composition Starting materials Weight Product phases in weight AlO zrO, SiC 44.07 17.28 16.8 (a Sample code ratIo 15 Zircon Zircon Carbon 1540 Fig 2 SEM pictures of the two types of carbon sources: (a)AC(Activated 2040 Charcoal) and(b)CB(Carbon Black). using an agate pestle and mortar. The alcohol evaporated Sillimanite after good physical blending and the resulting lumps were Zircon again crushed in the same pestle and mortar. The powder mix was sieved through a 100 mesh sieve to obtain -100 ZAC/ZCB Zircon mesh-free flowing powders. This powder was finally Carbon 36.25 in an air oven at 110C over night. These mixed powders were loaded into graphite crucibles. The furnace was heated in a programmed manner from room temperature to 1250C 1500.C to final temperature(1550, 1650 and 1700C)at expected product weight percentage are given in labe oy at 15. from 1250 to 1500.C at 10.C min-l and is given in Table 2. The quantities of starting materials 5Cmin. Soaking at the selected final temperature for Their eventual conversion to volume percentage is shown in I h, after which the furnace with the samples was cooled to Table 4 room temperature. The reacted powders were decarburised by heating in air at 700oC for 2 h to eliminate the excess carbon Table 4 Seven compositions were made to form various ratios Typical conversion table from weight percentage to volume percentage of silicon carbide, zirconia and alumina in the final prod- for the products ucts of the carbothermal reaction. The starting composi- Product Volume(cm) Volume (% tions were constituted according to the formula ((1-x) 52.59a Al2O3+(r)ZrO2)(l-y)+ SiC(y) to generate the product ZrOz phases as desired. As an example, the sample coded 2035 24.54 orresponds to ((0. 8)Al2O3 +(.2)ZrO2 J0.65+(0.35)SiCw a volume ratio of Al O3: ZrO2=80.39: 19.61
286 L. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 Fig. 2. SEM pictures of the two types of carbon sources: (a) AC (Activated Charcoal) and (b) CB (Carbon Black). materials and the carbon source were mixed in ethyl alcohol using an agate pestle and mortar. The alcohol evaporated after good physical blending and the resulting lumps were again crushed in the same pestle and mortar. The powder mix was sieved through a 100 mesh sieve to obtain −100 mesh-free flowing powders. This powder was finally dried in an air oven at 110 ◦C over night. These mixed powders were loaded into graphite crucibles. The furnace was heated in a programmed manner from room temperature to 1250 ◦C at 15 ◦C min−1, from 1250 to 1500 ◦C at 10 ◦C min−1 and 1500 ◦C to final temperature (1550, 1650 and 1700 ◦C) at 5 ◦C min−1. Soaking at the selected final temperature was for 1 h, after which the furnace with the samples was cooled to room temperature. The reacted powders were decarburised by heating in air at 700 ◦C for 2 h to eliminate the excess carbon. Seven compositions were made to form various ratios of silicon carbide, zirconia and alumina in the final products of the carbothermal reaction. The starting compositions were constituted according to the formula {(1 − x) Al2O3 + (x)ZrO2}(1−y) + SiC(y) to generate the product phases as desired. As an example, the sample coded 2035 corresponds to {(0.8)Al2O3 + (0.2)ZrO2}0.65 + (0.35)SiCw Table 2 Typical calculation to arrive at the product 65 vol.% (80:20, Al2O3: ZrO2) + 35 vol.% SiCw from starting composition Starting materials Weight Product phases in weight Al2O3 ZrO2 SiC Sillimanite 70 44.07 – 17.28 Kaolin 5 2.297 – 1.80 Zircon 25 – 16.8 5.46 Table 3 Composition/formulation for obtaining the product {(1 − x)Al2O3 + (x)ZrO2}(1−y) + SiC(y) Sample code Precursor Weight ratio x y 1535 Sillimanite 77.00 15 35 Kaolin 05.00 Zircon 18.00 Carbon 40.41 2035 Sillimanite 70.00 20 35 Kaolin 05.00 Zircon 25.00 Carbon 40.09 2535 Sillimanite 55.00 25 35 Zircon 45.00 Carbon 38.65 1540 Sillimanite 52.00 15 40 Kaolin 30.00 Zircon 18.00 Carbon 43.02 2040 Sillimanite 46.00 20 40 Kaolin 30.00 Zircon 24.00 Carbon 42.74 2540 Sillimanite 40.00 25 40 Kaolin 30.00 Zircon 30.00 Carbon 42.46 ZAC/ZCB Zircon 100.00 100 38 Carbon 36.25 is given in Table 2. The quantities of starting materials and expected product weight percentage are given in Table 3. Their eventual conversion to volume percentage is shown in Table 4. Table 4 Typical conversion table from weight percentage to volume percentage for the products Product Weight Volume (cm3) Volume (%) Al2O3 46.36 11.8 52.59a ZrO2 16.8 2.8 12.38a SiC 24.54 7.9 34.94 a Volume ratio of Al2O3:ZrO2 = 80.39:19.61
L. Mariappan et al. / Materials Chemistry and Physics 75(2002)284-29 3. Results and discussion being natural minerals after the reaction, the sic content will be 33 vol. for Sillimanite. 49% for Kaolin and 38% The results were obtained by the carbothermal reduc- for Zircon. Since it had been decided to make a ZTA-SiC tion of mixtures of Kaolin. Sillimanite and Zircon taken composite, a minimum of 15% ZrO2 had to be taken with various proportions. The formation of the product phases 85% Al2O3 to form the matrix. When this ratio was taken in with respect to variations in temperature and variations composition has been analysed with the help of XRd while percentage works out to be 35%. For comparison by varying their morphology was analysed using SEM. The conditions SiC,40% compositions were also made. This also explains favouring the formation of tetragonal zirconia without the the reason why two sources of aluminosilicates were used addition of stabilisers is also enumerated for the experiments 3.1. Role of variation in composition 3.2. Formation of tetragonal zirconia As given in Table 2, seven compositions were made Three ratios of alumina zirconia was taken for these ex get various ratios of Al2O3+ZrO2+SiCw. The precursors periments. It is now known by the work of Lange [9], that ZAC 00090000 2540 2040 00 1540 0000 1535 35404550 O→a-Al2O3;V→β-SiC;◇→mZrO2寥→t-zrO2 Fig 3. Product phases obtained at 1650C for different compositions with AC. (O)a-Al2O3, (V) B-SiC, (o)m-ZrO and(4)t-zrO2
L. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 287 3. Results and discussion The results were obtained by the carbothermal reduction of mixtures of Kaolin, Sillimanite and Zircon taken in various proportions. The formation of the product phases with respect to variations in temperature and variations in composition has been analysed with the help of XRD while their morphology was analysed using SEM. The conditions favouring the formation of tetragonal zirconia without the addition of stabilisers is also enumerated. 3.1. Role of variation in composition As given in Table 2, seven compositions were made to get various ratios of Al2O3 + ZrO2 + SiCW. The precursors Fig. 3. Product phases obtained at 1650 ◦C for different compositions with AC. () -Al2O3, () -SiC, () m-ZrO2 and ( ) t-ZrO2. being natural minerals after the reaction, the SiC content will be 33 vol.% for Sillimanite, 49% for Kaolin and 38% for Zircon. Since it had been decided to make a ZTA–SiC composite, a minimum of 15% ZrO2 had to be taken with 85% Al2O3 to form the matrix. When this ratio was taken in natural mineral form the minimum content of SiC by volume percentage works out to be 35%. For comparison by varying SiC, 40% compositions were also made. This also explains the reason why two sources of aluminosilicates were used for the experiments. 3.2. Formation of tetragonal zirconia Three ratios of alumina:zirconia was taken for these experiments. It is now known by the work of Lange [9], that
L Mariappan er al. / Materials Chemistry and Physics 75(2002) 284-290 y controlling the particle size to less than I um, complete case of only zirconia as the matrix, though the absolute stabilisation in tetragonal form could be obtained up to content of tetragonal form increases, the ratio of tetragonal its content of 15 vol. in the sample without the addition to monoclinic is reduced. XRD figures presented reveal the of stabilisers. Further increase in the volume percent re- fine changes occurring in various formation reactions of sults in a part of the zirconia remaining in the monoclinic tetragonal zirconia with respect to changes in composition form. This was attributed to the higher elastic modulus of Typical XRD patterns obtained at 1650oC with AC for alumina acting as a constraining matrix on the tetragonal various compositions and at various temperatures for the zirconia. Above 15 vol. the effect of constraint is reduced composition 1535 are presented in Figs. 3 and 4. Typical and therefore a part reverts to monoclinic. Since the possi- SEM pictures of product powders obtained at 1650C with bility of generating tetragonal zirconia from a carbothermal AC and CB are presented in Figs. 5 and 6 reaction was reported to be difficult by Chaklader et al. It is seen from the SEM micrographs from Fig. 5 that [10], the initial compositions were formulated to generate the whiskers are thicker in the case of AC even when all a ZTA matrix wherein the toughening is mainly due to the the other parameters are constant. This could be due to the effect of microcracking. Hence 20 and 25 vol. of zirconia high reactivity of AC at these temperatures, and the gener was also taken in the compositions for this study. In the ation of Sio vapours would be far faster resulting in fewer 1650C/hr 60 m→)ax-Al2O3;t→β-SiC;A→m-ZrO2G→+t-zrO2 Fig. 4. Product phases obtained at different temperatures for the composition 1535 with AC.(m)a-Al2O3,(t)B-SiC,(A)m-ZrO2,(G)t-ZrO2,(B) mullite, ()zircon and (u) aluminium oxy carbide
288 L. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 by controlling the particle size to less than 1 m, complete stabilisation in tetragonal form could be obtained up to its content of 15 vol.% in the sample without the addition of stabilisers. Further increase in the volume percent results in a part of the zirconia remaining in the monoclinic form. This was attributed to the higher elastic modulus of alumina acting as a constraining matrix on the tetragonal zirconia. Above 15 vol.% the effect of constraint is reduced and therefore a part reverts to monoclinic. Since the possibility of generating tetragonal zirconia from a carbothermal reaction was reported to be difficult by Chaklader et al. [10], the initial compositions were formulated to generate a ZTA matrix wherein the toughening is mainly due to the effect of microcracking. Hence 20 and 25 vol.% of zirconia was also taken in the compositions for this study. In the Fig. 4. Product phases obtained at different temperatures for the composition 1535 with AC. (m) -Al2O3, (t) -SiC, (A) m-ZrO2, (G) t-ZrO2, (B) mullite, (I) zircon and (u) aluminium oxy carbide. case of only zirconia as the matrix, though the absolute content of tetragonal form increases, the ratio of tetragonal to monoclinic is reduced. XRD figures presented reveal the fine changes occurring in various formation reactions of tetragonal zirconia with respect to changes in composition. Typical XRD patterns obtained at 1650 ◦C with AC for various compositions and at various temperatures for the composition 1535 are presented in Figs. 3 and 4. Typical SEM pictures of product powders obtained at 1650 ◦C with AC and CB are presented in Figs. 5 and 6. It is seen from the SEM micrographs from Fig. 5 that the whiskers are thicker in the case of AC even when all the other parameters are constant. This could be due to the high reactivity of AC at these temperatures, and the generation of SiO vapours would be far faster resulting in fewer
L. Mariappan et al. / Materials Chemistry and Physics 75(2002)284-29 Fig. 5. Typical SEM micrographs of the composite precursor powders with AC:(a)1535 and (b)1540 the volume fractions present, which could in part be due to nucleation sites for the whiskers to grow. On the other particles of SiC other than whiskers. Reaction conditions hand, the whiskers grown with CB from Fig. 6 show leaner and longer whiskers, which indicates that the reaction had seem to play a role in this distribution Decomposition of Sic does not occur because SiC is gone on for sufficient time and was kinetically slower. This stable with respect to both Al2 O3 and ZrO2 at these temper- resulted in the whiskers nucleating at a more number of atures. Zircon gives the monoclinic form immediately after sites and followed by an orderly growth decomposition, but at these temperatures the thermodynam ically stable form is tetragonal zirconia. As temperature increases there is grain growth. When the reacted powders 4. Discussion are cooled to room temperature, any particles below a crit ical size remain in the tetragonal form and those above this The variations observed with different carbon sources critical size transform to monoclinic. The critical size for could be attributed due to the fine particle si of this transformation is determined by the Al2O3 and Sic ursors and the high reactivity associated with AC at high matrix. This explains the presence of only monoclinic zir temperatures. AC with its fine microporous structure and conia in some cases after the treatment at 1550C, largely higher surface area gives a larger area with which the Sio tetragonal at 1650C for AC and at 1700C for CB. The vapours can react. Also at pressures higher than ambient reduction in the volume fractions of t-ZrOz at 1700C for and carbon ratios greater than unity, aluminium carbide AC could be due to grain growth formation could take place since this occurs at 1867C in the system Al2O3-SiC-C at ambient pressure and a carbon content of unity [11]. Indeed, this is what is observed for 5. Conclusions compositions with AC at 1700C. Whisker morphology is en for SiC at all reaction temperatures. It is to be noted Al2O3 ZrO2 SiCw composite powders were syn that the Sic is not all in whisker form. The XRD data gives thesised by the carbothermal reduction of cheap natural
L. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 289 Fig. 5. Typical SEM micrographs of the composite precursor powders with AC: (a) 1535 and (b) 1540. nucleation sites for the whiskers to grow. On the other hand, the whiskers grown with CB from Fig. 6 show leaner and longer whiskers, which indicates that the reaction had gone on for sufficient time and was kinetically slower. This resulted in the whiskers nucleating at a more number of sites and followed by an orderly growth. 4. Discussion The variations observed with different carbon sources could be attributed due to the fine particle sizes of the precursors and the high reactivity associated with AC at high temperatures. AC with its fine microporous structure and higher surface area gives a larger area with which the SiO vapours can react. Also at pressures higher than ambient and carbon ratios greater than unity, aluminium carbide formation could take place since this occurs at 1867 ◦C in the system Al2O3–SiC–C at ambient pressure and a carbon content of unity [11]. Indeed, this is what is observed for compositions with AC at 1700 ◦C. Whisker morphology is seen for SiC at all reaction temperatures. It is to be noted that the SiC is not all in whisker form. The XRD data gives Fig. 6. Typical SEM micrographs of the composite precursor powders with CB: (a) 1535 and (b) 1540. the volume fractions present, which could in part be due to particles of SiC other than whiskers. Reaction conditions seem to play a role in this distribution. Decomposition of SiC does not occur because SiC is stable with respect to both Al2O3 and ZrO2 at these temperatures. Zircon gives the monoclinic form immediately after decomposition, but at these temperatures the thermodynamically stable form is tetragonal zirconia. As temperature increases there is grain growth. When the reacted powders are cooled to room temperature, any particles below a critical size remain in the tetragonal form and those above this critical size transform to monoclinic. The critical size for this transformation is determined by the Al2O3 and SiC matrix. This explains the presence of only monoclinic zirconia in some cases after the treatment at 1550 ◦C, largely tetragonal at 1650 ◦C for AC and at 1700 ◦C for CB. The reduction in the volume fractions of t-ZrO2 at 1700 ◦C for AC could be due to grain growth. 5. Conclusions Al2O3 + ZrO2 + SiCw composite powders were synthesised by the carbothermal reduction of cheap natural
L Mariappan er al. /Materials Chemistry and Physics 75(2002) 284-290 inerals such as Kaolin, Sillimanite and Zircon. The reac- Mr. Krishna (lISc)for taking XRD patterns and Mr. M.A. tion starts at 1550C and is completed at 1650C Beyond Venkataswamy (NAL) for helping us in SEM studies 1650C the product phases start reacting among them- selves and form new phases [ll]. Carbon black gives a Ref better product both in terms of whisker formation as well as in generating a controlled reaction at the optimum tem- [1]SE Khalafalla, L.A. Haas, High Temp. Sci. 2(1970)95 perature of 1650C. The tetragonal form of zirconia could 22] Bechtold, Cutler, J. Am. Ceram Soc. 63(1980)271 be obtained without any addition of stabilisers by this pro [3]AC D. Chaklader, S Das Gupta, E.C.Y. Lin, J. Am. Ceram. Soc. cess if the starting precursor sizes were maintained around 75(1992)2283 I um. The other matrix materials, alumina and Sic having [4] P.K. Panda, L Mariappan, T.S. Kannan, Ceram. Int. 25(1999)467 5] P.K. Panda, L. Mariappan, V.A. Jaleel, T.S. Kannan, J. Dubois, G. a higher elastic modulus than zirconia, help in retaining the Fantozzi, J Mater. Chem. 6(1996)1395 tetragonal form of zirconia. [6] P.K. Panda, L. Mariappan, V.A. Jaleel, T.S. Kannan, J. Dubois, A mroune, G. Fantozzi, J. Mater. Sci. 31(1996)4277. [7 M. Bohmer, E.A. Almond, Mater Sci Eng. A 105-106(1988)105 [8E.L. Exner, C K. Jun, L L. Moravansky, Ceram. Eng Acknowledgements (1988)597 [9] F.F. Lange, J Mater. Sci. 17(1982)225 [10A.C D. Chaklader, S.M. B. Veiga, M.M. Veiga, J.C. Bressiani, Advan- The authors thank Miss K. Geetha (lISc)for the help ced Ceramic Structures for Tribological Applications, 1995, P. 107. rendered during the course of this work. We also thank [11]A K Mishra, J. Am. Ceram Soc. 74( 1991)345
290 L. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 minerals such as Kaolin, Sillimanite and Zircon. The reaction starts at 1550 ◦C and is completed at 1650 ◦C. Beyond 1650 ◦C the product phases start reacting among themselves and form new phases [11]. Carbon black gives a better product both in terms of whisker formation as well as in generating a controlled reaction at the optimum temperature of 1650 ◦C. The tetragonal form of zirconia could be obtained without any addition of stabilisers by this process if the starting precursor sizes were maintained around 1m. The other matrix materials, alumina and SiC having a higher elastic modulus than zirconia, help in retaining the tetragonal form of zirconia. Acknowledgements The authors thank Miss K. Geetha (IISc) for the help rendered during the course of this work. We also thank Mr. Krishna (IISc) for taking XRD patterns and Mr. M.A. Venkataswamy (NAL) for helping us in SEM studies. References [1] S.E. Khalafalla, L.A. Haas, High Temp. Sci. 2 (1970) 95. [2] Bechtold, Cutler, J. Am. Ceram. Soc. 63 (1980) 271. [3] A.C.D. Chaklader, S. Das Gupta, E.C.Y. Lin, J. Am. Ceram. Soc. 75 (1992) 2283. [4] P.K. Panda, L. Mariappan, T.S. Kannan, Ceram. Int. 25 (1999) 467. [5] P.K. Panda, L. Mariappan, V.A. Jaleel, T.S. Kannan, J. Dubois, G. Fantozzi, J. Mater. Chem. 6 (1996) 1395. [6] P.K. Panda, L. Mariappan, V.A. Jaleel, T.S. Kannan, J. Dubois, A. Amroune, G. Fantozzi, J. Mater. Sci. 31 (1996) 4277. [7] M. Bohmer, E.A. Almond, Mater. Sci. Eng. A 105–106 (1988) 105. [8] E.L. Exner, C.K. Jun, L.L. Moravansky, Ceram. Eng. Sci. Proc. 9 (1988) 597. [9] F.F. Lange, J. Mater. Sci. 17 (1982) 225. [10] A.C.D. Chaklader, S.M.B. Veiga, M.M. Veiga, J.C. Bressiani, Advanced Ceramic Structures for Tribological Applications, 1995, p. 107. [11] A.K. Mishra, J. Am. Ceram. Soc. 74 (1991) 345.
