Composites of corundum/mullite have been developed from mixtures of fly ash and alumina by reaction sintering at 16000C. The physico-mechanical properties of the composites have been evaluated. The microstructural evolution of the composites was investigated by scanning electron microscope(SEM). The spherical corundum

crystals were found reinforced with randomly oriented acicular mullite matrix leading to dense microstructure and consequently improved mechanical, wear resistant and

refractory properties. The reasons of the improved properties have been discussed in the paper

Kumar, Sanjay

Singh, K K

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198DEVELOPMENT OF CORUNDUM/MULLITE COMPOSITES FROM FLYASH AND ALUMINA MIX BY REACTION SINTERINGSANJAY KUMAR & K.K.SINGHNational Metallurgical Laboratory Jamshedpur - 831 007AbstractComposites of corundum/mullite have been developed from mixtures of fly ashand alumina by reaction sintering at 16000C. The physico-mechanical properties ofthe composites have been evaluated. The microstructural evolution of the compositeswas investigated by scanning electron microscope(SEM). The spherical corundumcrystals were found reinforced with randomly oriented acicular mullite matrix leadingto dense microstructure and consequently improved mechanical, wear resistant andrefractory properties. The reasons of the improved properties have been discussed inthe paper.Key Words: Mullite, Corundum, Composites, Microstructure, PropertiesIntroductionMullite has become a very important material for advanced structural and func-tional ceramics. The reasons are some of its outstanding properties like low thermalexpansion, low thermal conductivity and excellent creep resistance but exhibits lowstrength and fracture toughness [ 1-3 ]. To enhance the mechanical properties of mullitesceramics, development of mullite-matrix composites has taken place. The addition ofvarious alumina grains in mullite matrix composites to form ceramic composites withbetter strength and toughness has become widely recognised as a method for produc-ing materials for structural applications.Commercial mullite is prepared from the mixture of kaolin and alumina byreaction sintering method[4]. Mullite from various starting materials has been inten-sively investigated by many workers[5,61. Synthesis of mullite from mixture of flyash and alumina was reported by several authors[7-9]. They observed that these mulliteshave comparable properties with that of pure commercial mullite. Use of fly ash forthe development of hard sintered wear resistant products was also reported by someworkers[ 10,11 ].The approach selected to synthesize corundum/mullite composites was reac-tion sintering. Mullite(3A1,03.2SiO,) is a single crystal phase composed of 72 wt% ofalumina and 28 wt% of silica[ 12]. The composition of fly ash falls in the range of 12COMPOSITES FROM FLY ASH AND ALUMINA MIX 199to 27 wt% alumina and 31 to 63 wt% of silica[ 13]. The additional alumina added tofly ash to make up the deficiency and under the appropriate sintering conditions, thefly ash and alumina mixture can react to form corundum/mullite composites.ExperimentalThe starting materials used to provide necessary A1203 and SiQ., for the desiredphase were fly ash and calcined alumina. The raw materials were wet milled inti-mately in a pot mill for specific duration to a fineness of around 5µm. The slurryobtained was then dried, powdered and homogenised. The prepared powder was com-pacted using 300 to 350 kg/cm2 pressure. The compacted samples were again driedand sintered at 160011C with 2 hours soaking. The rate of heating was kept constant at6°C/min.Laboratory Process Flow-SheetRaw MaterialFly Ash + a-A1,0 34,Wet Milling4,Sieving4,Drying4,Homogenisation4PowderPreparationyCompactionyDrying14,Sintering4,CharacterizationThe chemical analysis and physico-mechanical properties such as hardness(Mohsscale ), linear shrinkage , bulk density(Archimedes method), flexural strength (threepoint load), fracture toughness , % apparent porosity(ASTM C373) and compressive20 0strength(ASTM C377) were evaluated by the standard procedures. The particle sizeafter milling was measured with the help of sedigraph. The erosion rate was deter-mined by the volume loss of material in cin3lkg of crodent at 45° impact angle. Abra-sion rate was measured on Morgan Marshal abradability index. X-Ray Diffraction(XRD) data were obtained with a diffractometer using Ni filtered Cu(K.) radiation.Scanning Electron Microscopy(JEOL JSM 840) has been carried out on the fracturedsurface of sintered specimen to study the crystal morphology.Results and DiscussionThe bulk chemical composition of the rawv materials used are given in Table-ITable - 1Chemical Compositions of the Raw MaterialsConstituentsRack M aterials (Wt %)Fly ash AluminaSio, 59.92 -AI O 27.79 99.5Fc_C7 4 .46 <0.05CaO 1.44 -MgO 0.0 -TiO. 0.02 -11.0.1 3.21 0.2The particle size distribution of the fly ash and clay mixture after 12 hours ofball nulling is given in Fig. 1. It can be observed from the figure that more than 95% ofparticles are smaller than 5 microns. The majority of the particle belongs to the sizerange of 1 to 4 microns . This very fine particle distribution has contributed towardsattaining the bulk density of the composites near the theoretical density values.Fig.2 shows the X.RD pattern ofcorundum/mullite composites. The major phasesidentified are mullite and corundum The mullitization started at the interface betweenpressed pellets of alumina and silica present in the raw mix. The mullite crystal growsas a result of movement of SiO- into AI,O1 zone and a glassy phase was also formeddue to diffusion of A1,03 into SiO,. The excess of alumina present in the compositionhas given rise to the corundum phase. Both primary and secondary mullite have beenformed. The primary mullite has conic from the host fly ash while the secondarymullite has formed by reaction sintering of fee SiO, present in fly ash and A1.,O.COMPOSITES FROM FLY ASH AND ALUMINA MIX 201The mullite formation started at the lower temperatures but major mullitisationhas taken place at 1600°C. The reaction sintering at 1600°C is the result of presenceof impurities like Fe,03. TiO,. CaO etc. in fly ash. Further, additives played a signifi-cant role in promoting the sintering at lower temperatures. Johnson and Pask(14] alsoobserved that addition of small amount of Fe_03. TiO,, and CaO to A1„O3-SiO,, mix-tures enhanced the crystal growth.The microphotographs taken on the fractured surface is given in Fig .3. Thespherical corundum grains are uniformly dispersed throughout the mullite matrix.The needle shaped mullite crystals are randomly oriented and interlocked togethergiving rise to very compact microstructure. Dekevsen[ 151 also reported that whenmullite is formed in the presence of small amount of intergranular glassy phase, italways exhibits an acicular or elongated morphology.The mechanical and refractory properties of the presently developed compos-ites are presented in Table 2.Table - 2Physicu-mechanical & refractory properties of the carundum/mullite composites.Physico- Mechanical PropertiesBulk Density 2.89 g/ccApparent Porosity 0.56%Hardness (Moll, Scale) 9Compressive Strength 990 MPaFlexural Strength 197 MPaFracture Toughness : 3.58 MPaintErosion Rate 0.015Abradability Index 22.21Refractory PropertiesP.C.E.(Pvroiuetric Cone : 36Equivalent) ValueLinear Shrinkage 12.8%Data of the above table shows that--the bulk density of the composites (2.89 v/cc)has achieved almost 95 `%, of the theoretical density. The low apparent porosity at1600°C has resulted both from the crystalline structure of mullite corundum crystalsand glass phase matrix kept rigid by a lattice of secondary mullite in which the crys-202tals of primary mullite grows as closely knitted long needles. This dense net of mullitecrystals and the viscous glassy phase occupied the pore spaces and reduced the poros-ity.The hardness of the composite is 9 on Mohs scale which is equal to corundumand next to diamond. Comparing with mullite, there is not much improvement incompressive and flexural strength. This may be due to presence of impurities whichleads to glass formation and consequently low strength properties. However, an irzrprovement in fracture toughness value(3.58 MPa m112) was obtained as compared tomullite which is 2.73 M Pa m112 as reported by Ismail et al. [16]. The erosion andabrasion resistant results confirm the suitability of these composites for wear resistantapplications.The pyrometric cone equivalent of 36 and %linear shrinkage of 12.8 indicatethat the composites can be used for high temperature refractory applications.ConclusionsOn the basis of the present study. the following conclusions can be drawn:1. Fly ash can be used as one of the raw materials for the developmentof corundumlmullite composites.2. The presently developed composites consist of spherical shaped corundumcrystals and needle shaped mullite which are reinforced together.3. These composites show improved mechanical and refractory properties.4. These composites can be used for structural, wear resistant and hightemperature applications.AcknowledgementThe authors are grateful to Prof . P.Ramachandra Rao, Director , National Met-allurgical Laboratory, Jamshedpur , India for his permission to present and publishthis paper.References1 Engineered Materials Handbook, 4, Ceramic & Glasses, ASM Inter-national, 1991.2 Harbison-Walker Refractories Co. : Modem Refractory Practice, The WilliamFeather Co., Cleveland, 1961.3 Zhong,Xian-Chong ;Sun,Geng-Chong, Lirr's Sel.Pap.. Refract., 2 (1990) 23.COMPOSITES FROM FLY ASH AND ALUMINA MIX 2034 Hawkes,WH, Trans.Brit.Cer.Soc., 61 (1962) 689.5 Okada,K ;Otsuka,N ;Somiya,S, Am.Cer. Soc.Bull ., 70 (1991) 1633.6 Aksay,I.A ;Dabbs,D.M ;Sarikaya,M, J.Am.Cer.Soc., 74 (1991) 2343.7 Ohtake,Takeshi; Uchida,Kunio; Ikazaki,Fumikazu; Kawamura,M; Ohkubo,T; Kamiya, K: Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi,99 (3) (1991) 239.8 Hwang,J.Y ;Huang,X ;Hein,A.M, JOM, 46 (5) (1994) 36,9 Huang,X ; Hwang,J.Y ;Mutsuddy,B.C, Interceram, 44 (2) (1995) 69.10 Das,S.K ;Roy,K.C ;Mittra,B.K ;Gupta,K.N, Ind.Cer., Techna Publishers,Italy,13, N 3/4, 1993, 155.11 Kumar,Sanjay ;Singh,K.K, Intl.Conf.Fly Ash Disposal & Utilisation,India, January, 1998, 10, 23.12 Rodrigo,PD.D ;Boch,P, Int.J.High Techn.Cer., 1 (1985) 3.13 Hwang,J.Y ;Hein,A.M et al., Application of characterization of fly ashbeneficiation to produce quality controlled products : Process MineralogyXI - Characterization of metallurgical and recyclable products, Hansen,D.Met al.(Eds.), The Minerals, Metals & Materials SQciety, 199114 Johnson,S ;Pask,J.A, Ain, Cer.Soc. Bull., 61 (1982) 838.15 Dekeysen,W.L, Science of Ceramics II, Stewart, G.H(Ed),Academic Press NY11963, 24316 Ismail,M.G.M.U ; Nakai, Z ;Somiya,S, J.Am.Cer.Soc., 73 (1990) 537.2040.5 1.0 1.5 2 .0 2.5 3 .0 3.5 4 .0 4.5 5 .0 5.5 6.0Particle size in micronsFig.1 - Particle size distribution of fly ash & clay mixture after ball milling.7O•J,M &I i7a, r P's PA i!+9i fi .v YrU i PAITWO - THETA / D - SPACINGFig.2 - XRD pattern of corundum/mullite compositionCOMPOSITES FROM FLY ASH AND ALUMINA MIX 205Fig.3 - SEM of fractured surface of compositese

Development of Corundum/Mullite Composites from fly Ash & Alumina Mix by Reaction Sintering

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Development of Corundum/Mullite Composites from fly Ash & Alumina Mix by Reaction Sintering

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