北京化工大学：《材料导论》课程阅读材料（无极非金属）The_influence_of_the_molar_ratio_of_Al2O3_to_Y2O3_on_sintering_behavior_and_the_mechanical_properties_of_a_SiC2O3_ceramic_composite

A ELSEVIER Materials Science and EngineeringA4(2008)62-66 www.elsevier.com/locate/mse The influence of the molar ratio of Al2O3 to Y2O3 on sintering behavior and the mechanical properties of a SiC-Al2O3-Y203 ceramic composite N.Zhang2.*,H.Q.Rub,Q.K.Cai,X.D.Sun b Abstract The influe tio of AlO,to Y2O(i.e was3,not 53.which iscustomarily lar rati for the formation of YAG (Y3AlsO)phase.When 3.ma enals existed in wo pha Keywonds:Ceramic composite:Malo/Myo:Microstructure:Mechanical properties:Sintering temperature 1.Introduction sinteringadditives[15-17].but the componentsarealllocatedin the two-phase area of YAG and Al2O3 in the equilibrium phase uid phase sinterin conditions.the of the surface finish ceramic material and sintering densification requires high declined,and machining costs increased.The result of changing By usingasuitable k de the sintering temperature by adding AlO and Y2O3 to form was based on earlier work with sintering technology at 1850C a YAG liquid phase.Using pressureless sintering technology. for 30min [19].The influence of the molar ratio of Al2Os to Alo of a 2.Experimental procedure d at nhas of YAG and YAM.The 0.5 of the low melting point eutectic liquid phase (YAG+YAM) values of the total mass +862462266018 Weighed amounts of SiC.Al2O3 and Y203 powders were put into a nylon ball-milling kettle with high-purity Al2O3 balls and

Materials Science and Engineering A 486 (2008) 262–266 The influence of the molar ratio of Al2O3 to Y2O3 on sintering behavior and the mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite N. Zhang a,∗, H.Q. Ru b, Q.K. Cai a, X.D. Sun b a Scientific Research Center, Shenyang University, Shenyang 110044, China b School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China Received 16 May 2007; received in revised form 31 August 2007; accepted 25 September 2007 Abstract The influence of the molar ratio of Al2O3 to Y2O3 (i.e. MAl2O3 /MY2O3 ) on sintering densification, microstructure and the mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite were studied. It was shown that the optimal value of MAl2O3 /MY2O3 was 3/2, not 5/3, which is customarily considered the optimal molar ratio for the formation of YAG (Y3Al5O12) phase. When MAl2O3 /MY2O3 is 5/3, materials existed in two phases of YAG and very little YAM phases. The sintering mechanism of the solid phase occurred at 1850 ◦C. When MAl2O3 /MY2O3 was 3/2, materials existed in the two phases YAG (Y3Al5O12) and YAM (Y4Al2O9). The formation of the low melting point eutectic liquid phase (YAG + YAM) increased sintering densification. Flexure strength, hardness and relative density were all higher. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramic composite; MAl2O3 /MY2O3 ; Microstructure; Mechanical properties; Sintering temperature 1. Introduction SiC is very hard, very strong, very durable, resistant to high temperature and chemical attack, and has the potential for exten￾sive application in modern industry. However, pure SiC is a ceramic material [1] and sintering densification requires high temperature and pressure. By using a suitable sintering additive, the sintering temperature can be reduced and a dense ceramic material is produced. Since 1990, some scientists have decreased the sintering temperature by adding Al2O3 and Y2O3 to form a YAG liquid phase. Using pressureless sintering technology, they obtained a dense SiC ceramic composite [2–4]. It was gen￾erally thought that the optimal molar ratio of Al2O3 to Y2O3 (i.e. MAl2O3 /MY2O3 ) was 5/3 for formation of the YAG liquid phase by the reaction 5Al2O3 + 3Y2O3 →2Y3Al5O12 [5–14]. However, we found that the optimal value of MAl2O3 /MY2O3 for sintering densification is not 5/3 but 3/2. As a result, the mate￾rial entered a two-phase area of YAG and YAM. The formation of the low melting point eutectic liquid phase (YAG + YAM) increased sintering densification. There has been some research into the effects of different proportions of Al2O3and Y2O3 as ∗ Corresponding author. Tel.: +86 24 62266946; fax: +86 24 62266018. E-mail address: zhangning5832@163.com (N. Zhang). sintering additives[15–17], but the components are all located in the two-phase area of YAG and Al2O3 in the equilibrium phase diagram of Al2O3–Y2O3 [18]. Because more Al2O3 volatilized easily, the bury-sintering method was adopted. Under the liq￾uid phase sintering conditions, the quality of the surface finish declined, and machining costs increased. The result of changing the proportions of sintering additives Al2O3 and Y2O3 should be investigated in the two-phase area of YAG and YAM in order to avoid the Al2O3 phase volatilizing. The work described here was based on earlier work with sintering technology at 1850 ◦C for 30 min [19]. The influence of the molar ratio of Al2O3 to Y2O3 on sintering behavior and the mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite were investigated. 2. Experimental procedure The raw materials were SiC powder (average particle size 0.5m and mass percent 99.8 wt.%), Y2O3 powder (average particle size 1 m and mass percent 99 wt.%) and Al2O3 pow￾der (average particle size 1 m and mass percent 99 wt.%). The values of MAl2O3 /MY2O3 tested were 3/2, 5/3, 7/4 and 2/1. The quantity of Al2O3 and Y2O3 powder accounted for 10 wt.% of the total mass. Weighed amounts of SiC, Al2O3 and Y2O3 powders were put into a nylon ball-milling kettle with high-purity Al2O3 balls and 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.09.052

ering A46(2008)262-26d 6 the ra 30 atC.sieved,molded at an isotonic cold pressure of 200MPa and sintered in a graphite vacuum tube fumace.Argon gas was nfrared optica pyrom rate was 15C/min,with pressureless sintering conditions of 1850C for 30 min. madzu EPM-8100 scanning electronic microse was used 1.75 tothe microscure of the material.Vickerhard Fig.2.Relatic ship between weight loss and nals,respectively 3.Results and discussion According to the lever principle.the materialo 3.1.The influence of MAho,/My0 on sintering ingadditive.which contributed to the glide and rearrangeme of grain organization.Under the action of capillary force.sin on rose rapidly:as a re the re Fig.1 shows the relationship between MAh/Myo and rel- ative density under sintering conditions of 1850C for 30min. (a)show Fig. which indicate that the material was composed of three kinds of phases with SiC.YAG and YAM,confirming the experimental was 32 and the m orial wos in the sin lowest when MAlO/MY0 was 5/3.When MAL0/MY0 ratio increasedto74,relative density increased:but began to deerease gle phase of YAG (in Fig.3).where the sintering temperatur was ra 1850C)was lower than the melting point of YAG(1930C) n or the T 53 to 7/4.but inereased when was raised from h 714to2/1. 口88ee9 ao0。￥O7 YAM) in the two-pl area c 200 AP 94.5 L*AI:0 AM+YAO a 15 1.75 Al2O3.wt% Fig.1.Relationship between,/My,o,and relative density. ()15.(7

N. Zhang et al. / Materials Science and Engineering A 486 (2008) 262–266 263 alcohol, and milled for 48 h. The mass ratio of the Al2O3 balls to the raw material was 5/1. The milled mixed powder was air-dried at 90 ◦C, sieved, molded at an isotonic cold pressure of 200 MPa and sintered in a graphite vacuum tube furnace. Argon gas was used as a protective atmosphere. An infrared optical pyrome￾ter was used to measure the sintering temperature; the heating rate was 15 ◦C/min, with pressureless sintering conditions of 1850 ◦C for 30 min. A D/max-RB X-ray diffraction instrument was used to analyze the phase composition of the material. A Japanese Shi￾madzu EPM-810Q scanning electronic microscope was used to analyze the microstructure of the material. A Vickers hard￾ness tester and electronic universal testing machine were used to analyze the hardness and flexure of materials, respectively. 3. Results and discussion 3.1. The influence of MAl2O3 /MY2O3 on sintering densification Fig. 1 shows the relationship between MAl2O3 /MY2O3 and rel￾ative density under sintering conditions of 1850 ◦C for 30 min. Fig. 1 shows that the influence of the value of MAl2O3 /MY2O3 on relative density was very complex. Relative density and densification were highest when MAl2O3 /MY2O3 was 3/2, and lowest when MAl2O3 /MY2O3 was 5/3. When MAl2O3 /MY2O3 ratio increased to 7/4, relative density increased; but began to decrease again when MAl2O3 /MY2O3 was raised to 2/1. Fig. 2 shows the complex relationship between MAl2O3 /MY2O3 and sintering weight loss at 1850 ◦C for 30 min. The sintering weight loss was lowest when MAl2O3 /MY2O3 was 3/2, and highest when MAl2O3 /MY2O3 was 5/3. The sintering weight loss decreased when MAl2O3 /MY2O3 was raised from 5/3 to 7/4, but increased when MAl2O3 /MY2O3 was raised from 7/4 to 2/1. Fig. 3 shows the equilibrium phase diagram of Al2O3–Y2O3 [18], which included five compounds: Al2O3, Y2O3, YAG (Y3Al5O12) with 5/3 of MAl2O3 /MY2O3 , YAM (Y4Al2O9) with 1/2 of MAl2O3 /MY2O3 and YAP (YAlO3) with 1/1 of MAl2O3 /MY2O3 . When MAl2O3 /MY2O3 was 3/2, the material was in the two-phase area of YAG and YAM. A low melting point eutectic liquid phase (YAG + YAM) was formed at 1850 ◦C. Fig. 1. Relationship between MAl2O3 /MY2O3 and relative density. Fig. 2. Relationship between weight loss and MAl2O3 /MY2O3 . According to the lever principle, the material contained lower melting point eutectic liquid phase with 57% of the total sinter￾ing additive, which contributed to the glide and rearrangement of grain organization. Under the action of capillary force, sin￾tering densification rose rapidly; as a result, the relative density was very high, and weight loss was low. Fig. 4(a) shows the X-ray diffraction patterns with a MAl2O3 /MY2O3 ratio of 3/2, which indicate that the material was composed of three kinds of phases with SiC, YAG and YAM, confirming the experimental result described above. When MAl2O3 /MY2O3 was 5/3, the material was in the sin￾gle phase of YAG (in Fig. 3), where the sintering temperature (1850 ◦C) was lower than the melting point of YAG (1930 ◦C). The capillary force prevented sintering densification of the material. The solid sintering additive did not contribute to the glide and rearrangement of grain organization. Therefore, den￾sification was very slow, and the sintering weight loss was high, appearing as diffraction peaks of SiC, YAG and YAM in Fig. 4(b). Sintering densification was slow, and the Al2O3 phase volatilized easily (Al2O3 + SiC→Al2O(g) + SiO(g) + CO Fig. 3. Equilibrium phase diagram of Al2O3–Y2O3 [18]: (a) 1.5, (b) 1.67 and (c) 1.75

264 N.Zhang et al.Materials Science and Engineering A 486 (2008)262-266 00 a 500 300 200 15 1.6 175 2 成小 MA2o3/M203 with different value decreased simultaneously.The AlO:diffraction peak did no d appear,only the SiC and YAG diffraction peaks were found (Fig.4(c)). 0 50 70 20/dea. 3.2.The influence of Mo,/Myo on mechanical raterials with different values of properties ceramic composite material.When MAlo,/MY2o,was 32. The c liquid phase area and yag two-phase area hut the content of the yamphase was very low.Densification was not easy because the material was rapidly before Al2O3 phase was volatilized.Weight loss was sel dom in the liquid phase. the owest,and relat s high as 7/4 or 2/1.the ial as in the YAG and liquid ph 50d YAG solid P (Fig.3).Sintering densification roseagain because of the lig fication process go along slowly.The Al phase had been volatilize before m tenal was in sinterng den tive.However.the liquid phase began to appear at 1760C rose to 7/4 weight loss was less hut relative Volatilization of Al2O had occurred before sintering densifica- tion at 1850C.Weight loss rose and the sintering densification 5um Fig.6.Microstructure of the rials with di values of MAho/My-o:(a)1.67 and (b)1.5

264 N. Zhang et al. / Materials Science and Engineering A 486 (2008) 262–266 Fig. 4. X-ray diffraction patterns of the materials with different values of MAl2O3 /MY2O3 : (a)1.5, (b) 1.67 and (c) 1.75. [20,21]). Therefore, the total amount of sintering additive was decreased. The YAM diffraction peak appeared because of the volatilization of Al2O3. The material went through the YAM and YAG two-phase area, but the content of the YAM phase was very low. Densification was not easy because the material was seldom in the liquid phase. When MAl2O3 /MY2O3 was as high as 7/4 or 2/1, the mate￾rial was in the YAG and liquid phase two-phase area at 1850 ◦C (Fig. 3). Sintering densification rose again because of the liq￾uid phase and capillary force. According to the lever principle, with MAl2O3 /MY2O3 as high as 7/4 to 2/1, the quantity of Al2O3 increased from 43% to 47.5% of the total sintering addi￾tive. However, the liquid phase began to appear at 1760 ◦C. Volatilization of Al2O3 had occurred before sintering densifica￾tion at 1850 ◦C. Weight loss rose and the sintering densification Fig. 5. Relationship between flexure strength and hardness with different values of MAl2O3 /MY2O3 . decreased simultaneously. The Al2O3 diffraction peak did not appear, only the SiC and YAG diffraction peaks were found (Fig. 4(c)). 3.2. The influence of MAl2O3 /MY2O3 on mechanical properties Fig. 5 shows the influence of the value of MAl2O3 /MY2O3 on flexure strength and Vickers hardness of the SiC–Al2O3–Y2O3 ceramic composite material. When MAl2O3 /MY2O3 was 3/2, material existed in low melting point eutectic liquid phase area of (YAG + YAM), which contributed to the guild and rearrange￾ment of grain particles. Therefore, material had been compacted rapidly before Al2O3 phase was volatilized. Weight loss was the lowest, and relative density, flexure strength and Vickers hardness were all highest. When MAl2O3 /MY2O3 rose to 5/3, material main was in YAG solid phase area, sintering densi- fication process go along slowly. The Al2O3 phase had been generated to volatilize before material was in sintering den￾sification. Weight loss was the highest, and relative density, flexure strength and Vickers hardness were the lowest. When MAl2O3 /MY2O3 rose to 7/4, weight loss was less but relative density and flexure strength increased. When MAl2O3 /MY2O3 rose from 7/4 to 2/1, flexure strength and Vickers hardness Fig. 6. Microstructure of the materials with different values of MAl2O3 /MY2O3 : (a) 1.67 and (b) 1.5

N.Zhang etal /Materials Scie imgA46200)262-26 10u四 10μm 10um 10μm Fig.7.frac als with diffar nt values of/MYo:(a)1.5.(b)1.67,(e)1.75 and (d)2.0 declined gradually.because AbO:volatilized at the high tem- 4.Conclusions perature. 3.3.The influence of the molar ratio of Aboi to y2or on ing bel microstructure are summarized as follows: of the SiC-Al2O3 was 312 at 1850C for porosity were very larger because of Al hat no micro-crack existed.As a resul,flexure strength some mic co-crack in（al were all highes AlO phase was volatilized,so that the microstructure was very very little YAM,with very little liquid phase and no cap A vell eurendiiyr Fig.7 shows the fracture texture of the SiC-AlO3-Y2O Cfor 30 mi ponding to 5/3 and 3/2 of MA mentione grain structure and fracture pattem were not affected by differ- above.The optimal molar ratio of towas ences in the value of MAlO,/My0 not 5/3

N. Zhang et al. / Materials Science and Engineering A 486 (2008) 262–266 265 Fig. 7. fracture structure of materials with different values of MAl2O3 /MY2O3 : (a) 1.5, (b) 1.67, (c) 1.75 and (d) 2.0. declined gradually, because Al2O3 volatilized at the high tem￾perature. 3.3. The influence of the molar ratio of Al2O3 to Y2O3 on microstructure Fig. 6 shows the SEM microstructure of the SiC–Al2O3– Y2O3 ceramic composite material produced by sintering at 1850 ◦C for 30 min with different values of MAl2O3 /MY2O3 . Fig. 6(a) and (b) shows that pore size and porosity were affected greatly by different values of MAl2O3 /MY2O3 . When MAl2O3 /MY2O3 was 5/3, sintering densification occurred very slowly, pore size and porosity were very larger because of Al2O3 phase volatilization, and the microstructure was so loose that it included some micro-crack in (a). Thus, flexure strength, hard￾ness and relative density were all lower. When MAl2O3 /MY2O3 was 3/2, sintering densification had occurred rapidly before Al2O3 phase was volatilized, so that the microstructure was very dense and no micro-crack existed in (b). The porosity and pore size were all lower, and material densification was higher. Thus, flexure strength, hardness and relative density were all higher as well. Fig. 7 shows the fracture texture of the SiC–Al2O3–Y2O3 ceramic composite material after sintering at 1850 ◦C for 30 min with different values of MAl2O3 /MY2O3 . No variation of grain size was visible. All samples had inter-crystalline cracks. The grain structure and fracture pattern were not affected by differ￾ences in the value of MAl2O3 /MY2O3 . 4. Conclusions The influence of the value of MAl2O3 /MY2O3 on the sinter￾ing behavior and mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite were investigated. The main results obtained are summarized as follows: (i) When the value of MAl2O3 /MY2O3 was 3/2 at 1850 ◦C for 30 min, material just existed in low melting point eutec￾tic liquid phase area of (YAG + YAM), which contributed to the glide and rearrangement of grain particle. Sinter￾ing densification had occurred rapidly before Al2O3 phase volatilized. The microstructures of materials were so dense that no micro-crack existed. As a result, flexure strength, hardness, and relative density were all highest. (ii) When the value of MAl2O3 /MY2O3 was 5/3 at 1850 ◦C for 30 min, material entered into two-phase area of YAG and very little YAM, with very little liquid phase and no cap￾illary force. Thus, sintering densification occurred slowly. Due to volatilization of Al2O3 phase, the microstructure was so loose that it included some micro-crack. As a result, flexure strength, hardness and relative density were all low￾est. (iii) When the value of MAl2O3 /MY2O3 was 7/4 or 2/1, flexure strength, hardness and relative density were between those corresponding to 5/3 and 3/2 of MAl2O3 /MY2O3 mentioned above. The optimal molar ratio of Al2O3 to Y2O3 was 3/2, not 5/3

N.Zhang etal /Materials Science and(2008)62-66 Acknowledgements Zhao.S.H.Tan.J.I 726-73 ng.H Yang.Co M This work was supported by the National Natural Science 45-949 a(a0s.50372041 Lioinin Nature Science Fund (no 20052002h and 5 20B [13]L Gao, HZ W H.Kawaoka.T.Sckino.K.Nihara.Eur.Ceram References ('s sie. I E 0e23200311115-1122 .A.W.Erickson.D.C.Kunerth.J.Mater.Sci.3(001) M.CI n,D.L Ji 0c.26(1998)192-197. 99139-142 Huang.H.Gu.J Zhang.D.L Jang.Acta Matcr.53 (2005)2521- 443 7o012213 T.Bhatia.A.Swarmima.J.Am.Ceram.Soc.8 (2001) [8P.Nitin Padture,J.Am.Ceram.Soc.7()519-525. 1211 H.O.Ra.XD.Sm Rare Eanth 23 (2005)1-136

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