Al2O3-ZrO2 composites were prepared in two compositional ranges, 15 wt% ZrO2 and 29 wt% ZrO2 with or without yttria or magnesia stabilizers. While 1.5 wt% Y2O3 produced tetragonal ZrO2 and fine grain microstructure, the 4.5 wt% Y2O3 developed cubic and tetragonal ZrO2 with similar microstructure. Al2O3 with 29.5 wt% ZrO2-1.5 wt% Y2O3 composition had the highest strength (3,300 kg/cm2). The bending strength remained more or less the same after the first thermal shock, and then it decreased gradually, but retained some strength after 20 cycles of quench. The load vs displacement curve became nonlinear after thermal shock possibly because of formation of microcracks which could be seen by microstructural studies

Aggarwal, P S

Biswas, Narayan Chandra

Mitra, B L

IR@CGCRI - Central Glass and Ceramic Research Institute (CSIR)

BuIl. Mater, Sei., Vo[. 15, No. 2. April 1992, pp. 131-141. ~ Printed in India. Thermomechanical properties and microstracture of alumina-zirconia B L MITRA, N C BISWAS and P S AGGARWAL Central Glass and Ceramic Research institute, Calcutt2_ 700032, I~dia MS received 28 August gq0, revised 30 April 1991 Abstract, AI,Os~ZrO= composites were prepared in two compositional r nges, 15 ~t/o' o/ ZrO2 and 29 wt,~ ZrO~ with or without yttria or magnesia stabilizers. While 1.5 wt% Y2Oa . produced tetragonal ZrO 2 and fine grain mierostrueture, th  4.5 wt~ Y20 s developed cubic and tetragonaI Zr02 with similar minrostrueture. AI=O~ with 29.5 wt~ ZrO_,-1.5 wt~ Y2Os composition had the highest strength (3,300 kg./cm 2 I. The beading sf:rength remained more nr less ~he same after tllc 6rs~ thermal shock, and then it decreased gradually, but retained some strength after 20 cycles of quench. The toad vs displacement curve became nonlinear alter thermal shock possibly because of formation of microcracks which could be seen by microstructural Stt d[es. Ke).words~ Alumina-zlreonia composites; thermal shock resistance; microcracks; inelasticity. 1. Introduction It is well-known that dispersion of zirconia in alumina substantially improves its strength (Hori e~ al I986; Marshall 1986; Lange and Miller 1987~ Wang and Stevens 1988; Srinivasa Rue and Cannon 19891 and thermomechanieal properties. The grain growth of alumina is also hindered by the addition of zirconia (Lunge and Hirlinger 1984; Lin and Lu 1988), but the densification rate of alumina during sintering is reduced linearly with ZrO2 addition (Majumdar et al 1986). The strengthening ofalumir~a by zirconia depends on severn factors. During stress-induced martensitic phase transformation of tetragonal zirconia particles to the monoclinic form, considerable stress is absorbed from the stress field of propagating cracks (Rfihle et al 1986). The residual stresses around which already transformed monoclinic zirconia can cause rnicrocracking and give further strengthening (Hori et al 1986). Cubic zirconia is also a toughening agent of alumina ceramics intered in air (Kibbel and Heuer 1984). Crack deflection also plays an important role (Wang and Stevens 1988). Annealing at higher temperature (1200~C or above)gives rise to grain coarsening effect (Kibbel and Heuer 19861. The intergranular ZrO: particles appear to coarsen by coalescence as particles are dragged by migrating A12Os grain boundaries (Nettleship and Stevens 1987). This is controlled by ZrO2-A120 ~ diffusion kinetics. The intragranular particles coarsen at a much slower rate (KibbeI and Heuer 1986). tn general, AI203-ZrO ~ composites would often cope with appreciable thermal stresses. In the present work, the bending strength of as-fired, annealed and thermal~shocked samples of ZrO2--AI= Os composites containing 13'5 to 29 wt% ZrO2 composites is compared. Microstructure and phase analysis of the composites are also presented, 132 B L Mitra, N C Biswas and P S Aggarwa! Table 1. Batch composition, water absorpt ion al~d bulk density. Batch M ~ terial wL% A B C D E A[zO~ 85,90 84.0 7@00 6%00 66.99 ZrO,  15.00 t4,50 29"00 29.50 28.64 YaO~ - -  / '50 - -  1.50 z*.37 MgO - -  - -  1"0 - -  - -  (MgCO3) I2'1) Water absorptiott(%) 1-53 0"85 2.35 0.53 1.78 Bulk density (g/eel 3.920 3,976 3"920 4.165 3.950 Thee retical devasity(%) 92.9 95-8 88"3 94.0 91'9 2. Experimeatal Five compositions marked A, B, C,D, E (table 1) with stabilizing agents Y2 03 as well as MgO (batch C) in some cases were prepared by wet ball milling for 30 h. Bars of approximate dimensions 78 x 18 x ~t-5 mm were made by dry-pressing at 650 kg/cm 2 followed by isostatic pressing at 3, 165kg/cm 2 (45,000psi). Two-step sintering combining lh  isothermal heating at 1400~ and followed by 90rain soaking at 1560-1580~ was adopted. The fired sample surfaces were ground by 220 and 400 mesh SiC powders. The samples were annealed by heat treating the sample at [ 140~C for 30min before quenching. Thermal shock test was performed by thermal cycling the samples between I200~ and drought-free air, giving at least 10min soaking at t200~ every time. Three-point bending strength and Young's modulus were determined by Instron. Fractured surfaces were etched by 15% HF-20~ HCI mixed etehant and observed in scanning electron microscope (SEM). X-ray diffraction analysis (XDA) was carried out using Cu target. Bulk density was determined by the Archemedes method. 3. Results and discussion The firing shrinkage of the samples averaged at 20-23~, a value, which was accounted for in making precision technical ceramics. Bulk density and water absorption values were recorded as shown in tame 1. The use of yttria improved the density with respect to that obtained by the use of MgO additive (batch C). Small addition of MgO had a beneficial effect on sintering of A1203-ZrO e (Kosmac et al 1982) and acted as a stabilizer for ZrO 2 (BansaI and Heuer 1975; Zoz et al 1980). One MgO-composition of 1 wt~ MgO was studied to get a good product. By X-ray diffraction it was found that addition of Y20 3 increased tetragonal phase retet~tion. The diffraetogram traces for (111)m, (111) t, e and (111) m reflections were as shown in figure 1. The high angle diffraction for (400) t, (400) c, (004) t is as shown in figure 2. Composition B was almost free from monoclinic phase since ( i l l )  m reflection at 28.4 ~ and (111)m reflection at 315 ~ were found negligible. High aogle Thermomechanical . . . .  alumina-zh-conia 133 taJ z A 30.3 Figm'e 1, z.93"A SO.5" B 28,~"  c AI2 03 3, 43"A 26.0" 2 28,14  ~ O 30.45' i I 28.5 ~01~RACrION ANGLE Ze'~ Low angle X-ray diffraction (peaks) diagrams, 2~.1r., ~ ) i U5 _z @ A 74.4" 74.0* c D . 7435. = E ~ 7~.8"] . DIFFRACTION ANGLE 2~ Figure 2. High artgle X-ray diffraction (peaks) diagrams. t34 B L Mitra. N C BL~wa.r and P S Aggarwal 'fable 2. Bending ~tre~gth {ist kg/cm~') after different hermal cycling. Batch 1 cycle ] c~,cles 5 cycles 10 cycles 15 cycles 20 cyelex Samples thermal ef therm,~l of thermal of therrna[ of thermal of thermal as such shock shock shock ~hock shock shock A 2.250 2, 060 560 570 520 - -  404 + 200 + 20~) B :2, 960 2, 640 712 600 • 90 660 483 438 ::L 200 4- 200 C 1,8513 1, 89(I 646 604 542 463 270 _+ 300 4- 300 D 3,000 3, 290 780 740 660 538 471 + 300 + 300 E 2,430 2, 35(1 608 650 710 446 243 • 200 4- 200 diffraction of sample B gave (400) reflection at 74-4: which according to Benedetti et al (1989) was for tetrago~lal ZrO~. Other compositions (A,C,D,E) contained monoctinie zirconia, c~-Alz 03 and another ZrO2 phase, It was difficult to distinguish between tetr~qganal and cubic ZrO~ from (i1 l) reflection at 30,4~ But from the high angle diffraction of figure 2, compositions A and C were found to contain cubic ZrO2 (along with m onocIinic ZrO;). Composition D contained tetragonal long with mc~noclinic ZrOz. Composition E developed tetragonal ZrO z (74.6~ 0), cubic ZrOz (73.7~2 ~9) and monocIinic ZrOz (28-15~ 8) together. The formation of cubic phase in A was probably due to the impurities from raw materials as well as from grinding the batches. The three-point bending strength of unannealed samples was higher in the case of I5 wt~ ZrO: batches and decreased eonsiderably at29 wt}g ZrO~ addition in batch C (table 2). But when 1.5wt% Y:O3 was added to 29wt% ZrO-~ batch~ the highest strength was obtained. However whe• Yz 03 was increased to about 4.5~, the strength decreased. This eoMd have been due to the formation of non-transformable cubic ZrO2 in p/ace oftetragonal ZrO~ (Nett!eship and Stevens 1987). The bending strength remained more or less the same after the first thermal shock, increased on first cycle in case of batches C and D, after that lhe strength decreased abruptly with increasing number of thermal shock cycling. This was evident from the results of third cycIes, But even after 20 cycles of thermal shocks, the.~e samples retained ~ppreciaNe strength (240 to 470 kg/cm2). Yaar~ et al (1986) reported lower strength of zirconia-mullite oa thermal cyelirlg (after 3 thermal shocks 460-1000 kg/cm2). However, in our case, the load vs dispiaeement curve was found to be nonlinear, especially after thermal shock cycling {figures 3a-e). The plastic deformation and the nature of the ~mrves after thermal shock treatment suggested that microcracks were formed in the samples due to thermal shock and these microeracks were absorbing some amount of stress giving rise to plasticity. Nonlinear stress-strain beh.aviour and plsstic deformation in fransformation- toughened materials have also been discussed by several workers (Marshall I986; Matsai et a11986; Heuer eta11986; Swain 1990; Wu and Chen 1990). The pseudoplastic: behaviour in these materials possibly arises both from stress-induced martensitic transformation as well as microcracking. Marshall (1986) observed linear stress-strain Thermomechanical . . . .  alumina-zirconia 135 "I O OQ 800 C -s 6oo  Z 400 0 o 2QO ta l  A E Displacement C 0 4-. g o, Af fer  5 cyc les  200 Ib) O 150 C ~ :///J [Displacement .~ lc150 .-7100 O O5O Af ter  10 cyc les  [c )  E Displacement 200 C o 150 Z tO0 "0 0 o - -  50  After 19 cycles (d) D C /yl/ Displacement 7 -O ,O 200 _ . . . . .  After 20 cycles (el i 50  B / "'00/i o 50  Displacemenf Figure 3, Load-dhplacement curves on t]cxural bending, behaviour in averaged material, whereas nonlinear behaviour was seen m the toughened material. The increasing nonlinear component Was cvidcnt at higher strains. Young's modulus (E) was determined from the load displacement curve of 3-point bcnding test, from the following equations, E = wP/481,~, (1) 136 B L Mitra, N C Biswas and P S A q,qarwal Table 3, Young's modulus (in • 10' kg/cra 2) after different hermal cycling. Batch 3 cycles 5 cycles I0 oyeles 15 cycles 20 uycles Sample A~nealed of tl~ermal of thermal of thermal of thermal of therrtml as such 1 cycle ~hock shock shock shock shock A 1.64 1.45 0.8 0"7 0-5 - -  0.80 B 2.00 1.95 1"2 1.2 0"7 0.80 0.83 C 1.33 007 0,7 0-65 0,4 ff43 0.50 D 1.50 0.98 0-75 0.6 0'5 0.81 0.52 E 1.30 0"95 0.65 0.6 0'5 0,80 0-47 where the moment of inertia, I -- bt3/12, (2) and deflection ax2mm 6 = Chart drive x cross head speed, (3) Here W is load, ! the span, b the breadth, t the thickness and a---chart reading corresponding to W. Batches A and B had higher Yotmg's modulus than batches C, D and E, although the strength of the composition D was the highest. This was probably because batches A and B contained higher quantities of alumina which was more elastic than ZrO:. Young's modulus decreased on thermal shock treatment but started increasing after 15 cycles. Intergranular pores and cracks relaxed residual stresses (Buresh i984). Ttae reduction i  E was indicative of considerable mierocrack development (Swain 1990) on thermal shock. The rise of E after 15 cycles could have been due to the resistance to crack-growth by network of microeracks developed. The scanning electron micrographs (SEM) of fractured surfaces are shown in figure 4(a) to 4(0. Figures 4a and 4b are fraetographs of batches A and B prepared from ball-milled alumina and hence the grains are of bigger sizes. The remaining figures are those of samples made from vibro-energy-mflled alumina which is much finer than bali-milled alumina. Grain size and pore sizes were much smaller in these cases. It was difficult to distinguish between alumina nd zirconia grains in scanning electron micrographs without back scattering. But from XRD anaIysis and by comparison with the reported microstrueture of other workers (Heuer et al 1982; Kibbel and Herder 1986h sm',dl round and irregular-shaped grains were considered as zirconia. Intergranular zireonia crystallites as well as few intragranular Z Oa phase were seen enmeshed in A1203 crystals. The surrounding A1203 grains exert matrix constraint upon ZrO2 grains thereby reducing the tendency of Zr02 grains to transform. At higher magnification (figure 2d), intragranutar p ecipitates are revealed as engraved witNn the AI: Oa crystallites. In the miorographs of batches B and D, strong grain-to-grain bonding took place as a result of good sintering. The grain size of alumina was comparatively smaller in Y2 03 containing batches B (average 3.45 #m, pot milled batch) and D (average 1.76/~m, vibro energy-milled batch). The average grain size of AlzO 3 in batch A was 7-3 ~ml (pot-milled batch)and in batch C, 2-7/2m (Vibro energy-milled batch), yttria had more pronounced grain growth retarding effect Thermomechan ica l  . . . .  a lumina-z i rconia 137 a 9 ,~  I~  ,~  ,e  i ' "  ' , o Figure 4(a---e). For caption, see p, 138, f8 d Figure 1. SEM fractagraphs, a. Composition A f rom ball-milled alumina, b. Composi- tion B from ball-rn111ed alumina, e. Composilion C rmm vibro-energy mi]led A]; 9 3, d. Composilion C w{th higher magnification, e. Composition D from vibro-energy milled AI,O 3 and 1". Composition E from vJbra-energy milled AI203 . Thermomechan ica l  . . . .  a lumina-z i rcon ia  139 a x Figure 5, SEM fractographs after thermal ~hock test, a. S-anaple R after 5 thermal shocks. b. Sample C after 5 thermal shocks. r Sample E after 10 thermal shocks. 140 B L Mitra, N C Biswas and P S Aggacwal than magnesia in AI203-ZrO~ composites. Some pores were seen as black phases in the micrographs. Figures 3a and 3b show the microstructures of compositions B and C respectively after t'lvc thcrlnal shock treatments, l=igure 3C is for composition E (Lhermal-cycIed l0 times}, tn all these cases, grain-to-grain bonding had weakened ue to thermal cycling giving rise to separation of grains or formation ofmicrocracks with dimensions similar to the grain size. Marshall (1986) observed formation ofmicrocraeks equivalent to the grain size under load. These microcracks absorb some energy during their subcritical growth undcr stress (microc.rack toughening}. Some plasticity was developed due to this effect. Examinatio,~ in back-scattered mode (Green 1982) would have clarified the mierocracks better. Although open porosity was negligible in these materials as understood from water absorption values, a lot of pores were still seen in the microstructure. This also agreed witt~ theoretical density values showl~ in table 1. Because of the presencc of pores, the strength values were much less. However, t?ne presence of pores was beneficial in several applications; for example, ii~ bioimplan~, the presence of porosity was sometimes beneficial for bone growth. Thesc materiaIs were used as refractory materials by virtue of their good thermal shock resistance and strength. 4, Conclusions The foIIowing conclusions arc drawn from the above observations. (i) 3-1203- 29.5wt7/~ ZrO z--- 1.5% Y20 3 composition has the highest strength and AI20 3 - -  15w/~ ZrO 2 has a somewhat higher strength compared to A I ;O a .... 29 ~l,t~/,, ., , _ l wt}• MgO composition. (ii) Addition of 1"5 wt% Yz 03 has the beneficial effect of retaining tetragonal ZrOz and producing fine-grained microstructure and good grain-to-grain bonding. . O /  Off) The h~crease ofY:O.~ up to 4 5~o causes formation ofctlbic-tetragonal-monoctinic zireonia and reduction in strength. The MgO-stabilized batch and the batch without any deliberate addition of stabilizer also had significant amount of cubic phase ir~ addition to monodinic ZrO 2 probably because of ~he impurities. (iv) The bending s:rength remained the same or slightly increased in some cases after the first thermal cycling; thereafter the strength decreased abruptly on repeated thermal shock cycling: but even after 20 cycles, the samples possessed some strength. (v) Young's modulus also decreases on thermal shock treatment up to 15th cycles, thereafter it increases. The nonlinear behaviour of elasticity or load-disptacernent diagram after thermal shock treatment is due to microcracks. Acknowledgements The authors thank Drs K K Phani, Suchitra Sen and Sri Dilip Ohosh for their kind technical assistance and Dr J Mukherji for suggestions. References Banzai O K and Honer A H I975 J. Am. Ceram, Noc. 58 235 Benedet~i A, Fagherazzi O and Pinna F 1989 d. AJn. Col'am. Soc, 72 467 Thermomechan ica l  . . . .  a lumina-z i rcon ia  14t Buresh F E 1984 in Adz, antes in ceramics. Science and technoloOy of airconia II (eds) N Claussen, M R(ihle and A H Heuer (Cotumbus, Ohio; Am, Ceram. Soc.) 12 p 306 Green D J 1982 J, Am. Cerarn, $oc, 65 610 Heuer A I-I, Claassen N, Kriven W M and Riihle M 1982 J, Am. Ceram, Soc. 65 642 Heuer A I--I, Lange F F. Swain M V and Evans A G t986 J, Am. Ceram. Soc. 69 i-iv Horl S, Yoshimuta M and Somiya S 1986 J. Am. Ceram, 8oc. 69 169 Kibbe] B W anct Heuer A It 1984 in Advances in ceramics. Science and technology of zireonia 11 (eds) N Claussen, M R~ihle ~nd A I-I Heuer (Columbus. Ohio; Am. Ceram. Soc.] 12 p 415 Kibbel B and Heuer A H t986 J, Am Ceram. Soc. 69 231 Kosmae T, Wallace J S and Clanssen N 1982 J. Am. Ceram. See. 65 C66 Lange F F slid Hirlinger M M 1984 .I. Am, Ceram. Soc. 67 164 l,ange F F and Miller K T 1987 ,L AJn, Cera~. Soc. 70 896 Lin J T and Lu Hy 1988 Ceram. Int. 14 251 Majumder R. Gitbert E and Brook R J I986 Br. Ceram, Trans. J. 85 156 Marshall D B 1986 J. Am. Ceram. Soc. 69 173 Matsui M, Soma T and Oda I 1986 .I, Am. Ceram. See. 69 198 Nettlesllip I and Stevens R 1987 Int..I, ~ri~h Tech, Ceram. 3 ] R(ihJe M. Claussen N and Heuer A H 1986 3-. Am. C~ram. Soc, 69 I95 Srinivasa Rao A and Cannon W R 1989 Ceram. Intern. 15 179 Swain M V 1990 d. Am. Ce:ara, Sor 73 621 Wang J and Stevens R 1988 J. Mater, Sci, 23 804 W~.J Xin and Chert I W 1990 J, Am. Ceram. Soc. 73 746 Yuan Q M, Tan J Q and j'in Z G 1986 .L Am. Ce~am. Soc, 69 265 Zoz E 1, Karaalov A G, Rtldyak [ N, Gul'ko IN V and Karyakina E L 1980 haor#. Mater. 16 728 

Thermomechanical properties and microstructure of alumina-zirconia

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Thermomechanical properties and microstructure of alumina-zirconia

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