《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_microstuctural design of A-Z

Scripta Materialia, Vol. 38. No -5.1998 Lhe PIIS13596462(97)00443-0 MICROSTRUCTURAL DESIGN IN ALUMINA-ALUMINA/ZIRCONIA LAYERED COMPOSITES A Javier Sanchez-Herencia, Jose S Moya*and Antoni P. Tomsia *Instituto de Ceramica y Vidrio(CSIC), Arganda del Rey, 28500 Madrid, Spain Materials Sciences Division, Lawrence Berkeley Laboratory 1 Cyclotron Road, Berkeley, ( Received April 28, 1995) (Accepted October 7, 1997) Introduction Very recently several authors(1-4) have pointed out the extremely important role of microstructural design in developing structural ceramic materials for long term high temperature applications. In this sense Raj [4] has identified several boundary conditions: (i) Resistance to oxidation, (ii)Resistance to grain boundary sliding and cavitation. (ifi) Good strength and toughness at room temperature The aspiration is to eliminate grain boundaries which can act as cavitation sites, without using single crystals which typically exhibit low toughness. In this regard ceramics with single crystal-like mor- phologies, e.g., large elongated grains, with good fracture toughness and high bending strength ha One route to find these apparently contradictory characteristic is by building up layered microa chitectures where layers with high toughness and high bending strength coexist with layers with high creep resistance. These conditions can be met in the case of Al2O3/Al2O3-Zro2 laminates The present work was directed to the study of the microstructural features and properties of AL2O3/Al,O3+ unstabilized ZrO2 and Al2O/Al2O3+ t-ZrO2(3 mol%Y2O3layered composites Experimental Procedure The following starting materials have been used: Al2O3( Condea HPA 0.5, Germany) with dso -0.5 um, specific surface area of 9.5 m/g and 99.99% purity; monoclinic ZrO2 (Dinamit Nobel, Germany) with dso -1 um and specific surface area of 66.7 m-g and tetragonal ZrO2(3 mol%Y2O3)(TZ-3Ys Tosoh, Japan)with dso 0.4 um and specific surface area of 6.7 m-7 Stable aqueous suspensions of Al2O3, A,O3+15 vol m-zrO2 and Al2O3+15 vol %t-ZrO2(Y2O3) ith 70 wt solid load were prepared by adjusting the pH with HCl to a value (5)of 4. The rheological properties of the different slips were studied using a rotational viscosimeter(Haake Rotovisco HV20) at a constant temperature of 25C. Monolithic and laminated composites were obtained following the flow chart of Fig. 1. The thickness of the different layers in the laminate compacts were calculated using the Al2O3 wall thickness ratio(Fig 2) Flexural strength test sample bars with dimensions of 40 x 4 X 2 mm were cut from fired plates with tensile surfaces oriented parallel to the layers. Surfaces were polished successively with 9, 6 and I um

LAYERED COMPOSITES Vol. 38. No. 1 Slurry of Al, O, Slumy ol A2O3+15wo%mz。2 Slurry of Al,o,+15 vol. "%t-zro, Slip Casting of Paris moid to build up the layered Drying at room temperature for Presintering at 1.C/1 Machining to reach flatness Firing at 1700C/2h under load(P=3 MPa) nder 5x 10 torr vacuum ructural analysis Figure 1. Processing flow chart describing the procedure for making monolithic and laminated composites 2 02004006008001000120014001600 Figure 2. Wall thickness versus time for slip cast alumina slurry

Vol. 38. No. I LAYERED COMPOSITES Figure 3. SEM micrograph of the polished cross-section of Al, O/Al,O,(m-ZrO2) laminated composite diamond paste and the edges were rounded during polishing operation. Four-point bend strength tests were conducted with upper and lower spans of 0.95 and 1.90 pectively, and a crosshead speed of 0.05 cm/min Fracture toughness(K,c) was determined by SENB method in monolithic compacts For microstructural analysis a light microscope and a scanning electron microscope(SEM) were used. Analyses were performed on polished or fracture surfaces Results and discussion The apparent viscosity of the studied slurries was found to be very low($12 MPas). The thickness of e zirconia-containing layers and alumina layers were kept at -100 um and -150 um respectively in all the laminates. No bubbles were observed in any of the fired plates The SEM micrograph of the AL2O3/AL2O3(ZrO2)are shown in Fig 3. As can be clearly observed a Al, O,+m-Zro Zro igure 4. Thermal expansion curve corresponding to the monolithic compact indicated in the figure

LAYERED COMPOSITES Vol. 38. No. I Figure 5. SEM micrograph of the polished cross-section of alumina/alumina(Y-TZP)laminated composite large number of perpendicularly oriented cracks are located in the alumina layers. This fact can be explained as follows on (a) The fraction of the zirconia which is monoclinic in the zirconia containing layer was determined polished specimens using Garvie's equation [61 and was found to be -80%6, During cooling dilatation takes place at 700-650oC range(Fig. 4)as a consequence of the t-m martensitic transfor mation of the coarse zirconia grain (b) Because of the strain mismatch(Ae) due to tetragonal to monoclinic zirconia trar esidual stress developed in the alumina layer according to equation Figure 6. SEM micrograph of the fracture surface alumina/alumina (Y-TZP)laminated composite

Vol. 38. No. 1 LAYERED COMPOSITES TABLE I Sample Fr(MPa) Kc(MPa×m2) 50± 5.0±0.5 lumina/ 600±50 The bending test was made keeping the alumina-t-ZrO, layer in tension E 21 here E is Youngs modulus of the alumina layer and v is Poisson coefficient. Considering that Ae is 2.5X10>(Fig. 4), this residual stress was calculated to be 0.6 GPa. This residual stress value justifies the cracking observed in the alumina layers Fig 5 shows the SEM micrograph of the polished cross-section of alumina/alumina(Y-TZP) laminates. In this particular case, the thermal expansion of zirconia-containing layer is only slightly higher( Fig. 4)than the alumina layer, consequently a crack-free dense(99.8% th) laminated compact is obtained. a remaining porosity formed by small pores(- l um)located at alumina grain boundaries is observed at the alumina layers. The average grain size of the Y-TZP containing layers is 3 um Conversely in the alumina layer the average grain size is 40 um(Fig. 6). This laminate composite has both high strength(or"600 MPa) and a duplex grain size structure It is known that coarse ceramic microstructure ensures a good high temperature creep behavior ccording to Nabarro-Herring equation the creep rate, E-1/d- and according to Raj(4)the crack propagation rate da/dt-Idd being the main grain size of the ceramic compacts. That is, the alumina layers with coarse grain size may contribute to keep a high creep resistance in this laminate. Conversely the Y-TZP containing layers could provide high toughness and high bending strength to the layered The results obtained in the present investigation show that the sequential slip casting is a promising processing route to design microarchitectures with duplex grain size structure(3/40 um) and high bending strength (-600MPa)which can be considered as a potential material for high temperature structural applications. Tensile fatique testing and high temperature creep testing is in progress Acknowlegments Work supported by CICYT Spain under contract No. MAT91-0878. APT Research supported by irector, Office of Energy Research of U.S. Dept. of Energy under Contract No. DE-AC03-76SF00098 References 1. K Ni 2.M.P er. H. M. Chan and G. A Miller, J. Am. Ceram. Soc. 75, 1715(1992 A. J. Sanchez-Herencia R. Moreno, P. Pena and J. Requena, in Layered Ceramics, Proceedings of Third Euro-Ceramics, Vol. 3. Pp. 289-300, ed. P. Duran, Faenza, Edetrice 4. R. Raj, J Am Ceram Soc. 76, 2147(1993) 5. J Requena, R. M and J S. Moya, J. Am. Ceram. Soc. 72, 1511(198 6. R.C. Garvie and P. S, Nicholson, 55, 303(1972