CERAMICS INTERNATIONAL ELSEⅤIER Ceramics International 28(2002)223-226 Mechanical properties and microstructure of a Si3 N4/Ti3SiC2 multilayer composite Luo Yong-Ming, Pan Wei, Li ShuQin, Chen Jian, Wang Rui Gang, Li Jian Qiang State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering Tsinghua University, Beijing 100084, China Received 30 July 2001; received in revised form 9 August 2001; accepted 27 September 2001 Abstract A new Si3 N4/Ti3SiC, laminated composite which was manufactured by the powder metallurgy method is presented. The work of fracture was measured in four-point bending at a crosshead speed of 0.05 mm/min. The results show that the mechanical properties of multilayered composites consisting of Ti3SiC2 layers in Si3 N4 were significantly enhanced. Three-step toughening behaviors were found in the material. The crack deflection is regarded as a dominant toughening mechanism, meanwhile the unique microstructure d properties of Ti3SiC, contributed to the improvement, thus preventing catastrophic failure. C 2002 Elsevier Science Ltd and Techna S.r. 1. All rights reserved Keywords: B Microstructure; C. Mechanical properties; Si3N4/Ti3 SiC2; Multilayer composites 1. Introduction shock, high strength, high toughness, high melting points low density and thermal stability [8-15]. Thus Ti3 SiC2 is Silicon nitride is considered a potentially useful struc- considered to be effective to overcome the lack of relia- tural ceramic for its combination of toughness, strength, bility. In present research we selected Ti3 SiC2 as the weak hardness, and chemical and thermal durability. Unfortu- interface to improve the Si, n4 toughness without sacrifice nately, the lack of its reliability in mechanical properties the silicon nitride ceramic properties. The microstructure has been a critical problem preventing widespread struc- and mechanical properties of the Si3N4/ Ti3SiC2 multi tural applications. Cook et al. [1] suggested that this pro layer ceramic have been evaluated blem can be overcome by introducing weak interfaces to deflect a growing crack. Following Cooks idea Si3N4 ceramic composites with a layered structure have been 2. Experimental procedure evaluated [2-7. These layered composites were reported to present several improved properties(e. g. increase in 2. 1. Raw materials and fabrication processing fracture toughness, crack arrest, multiple cracking, and so forth). To overcome the lack of reliability, the fea Si3N4 powder(E-10, UBE Industries, Tokyo, Japan) tures of layered composites are especially attractive. with MgO( 8 mass%, from Shin-etsu Chemical Co A In this paper we have successfully fabricated Si, N4/ Tokyo, Japan, 99.99% pure)and CeO2(2 mass%,from 13SiC2 multilayer ceramic by powder metallurgy and Shin-etsu Chemical Co., Tokyo, Japan, 99.99% pure) hot-pressing. It is well known that Ti3SiC2 is a novel were ball-milled in ethanol for 48h. The slurry was stirred structural/functional material. Hexagonal Ti3 SiC2 has a drying with rotary evaporator at 60C, the mixture was layer structure on the atomic scale. Recent studies show reground in a mortar and pestle and screened through a that Ti3 SiC2 combines the merits of both ceramics and 100-mesh sieve. Ti3 SiC2 powder was synthesized directly metals, such as being electrically and thermally conductive, from elemental titanium, silicon, and graphite powders as well as easy to machine and resistant to thermal with solid-liquid method at 1270oC. Pure Ti3? pow ders were obtained [16 by first removing TiSi2 using HF solution, and then heating the powders in air at 500C for 5 h, finally washing the powders with 0272-8842/02/S22.00C 2002 Elsevier Science Ltd and Techna S.r. l. All rights reserved. PII:S0272-8842(01)00082-7
Mechanical properties and microstructure of a Si3N4/Ti3SiC2 multilayer composite Luo Yong-Ming*,Pan Wei,Li ShuQin,Chen Jian,Wang RuiGang,Li JianQiang State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering Tsinghua University, Beijing 100084, China Received 30 July 2001; received in revised form 9 August 2001; accepted 27 September 2001 Abstract A new Si3N4/Ti3SiC2 laminated composite which was manufactured by the powder metallurgy method is presented. The work of fracture was measured in four-point bending at a crosshead speed of 0.05 mm/min. The results show that the mechanical properties of multilayered composites consisting of Ti3SiC2 layers in Si3N4 were significantly enhanced. Three-step toughening behaviors were found in the material. The crack deflection is regarded as a dominant toughening mechanism,meanwhile the unique microstructure and properties of Ti3SiC2 contributed to the improvement,thus preventing catastrophic failure. # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: B. Microstructure; C. Mechanical properties; Si3N4/Ti3SiC2; Multilayer composites 1. Introduction Silicon nitride is considered a potentially useful structural ceramic for its combination of toughness,strength, hardness,and chemical and thermal durability. Unfortunately,the lack of its reliability in mechanical properties has been a critical problem preventing widespread structural applications. Cook et al. [1] suggested that this problem can be overcome by introducing weak interfaces to deflect a growing crack. Following Cook’s idea Si3N4 ceramic composites with a layered structure have been evaluated [2–7]. These layered composites were reported to present several improved properties (e.g. increase in fracture toughness,crack arrest,multiple cracking,and so forth). To overcome the lack of reliability,the features of layered composites are especially attractive. In this paper we have successfully fabricated Si3N4/ Ti3SiC2 multilayer ceramic by powder metallurgy and hot-pressing. It is well known that Ti3SiC2 is a novel structural/functional material. Hexagonal Ti3SiC2 has a layer structure on the atomic scale. Recent studies show that Ti3SiC2 combines the merits of both ceramics and metals,such as being electrically and thermally conductive, as well as easy to machine and resistant to thermal shock,high strength,high toughness,high melting points, low density and thermal stability [8–15]. Thus Ti3SiC2 is considered to be effective to overcome the lack of reliability. In present research we selected Ti3SiC2 as the weak interface to improve the Si3N4 toughness without sacrifice the silicon nitride ceramic properties. The microstructure and mechanical properties of the Si3N4/ Ti3SiC2 multilayer ceramic have been evaluated. 2. Experimental procedure 2.1. Raw materials and fabrication processing Si3N4 powder (E-10,UBE Industries,Tokyo,Japan) with MgO (8 mass%,from Shin-etsu Chemical Co., Tokyo,Japan,99.99% pure) and CeO2 (2 mass%,from Shin-etsu Chemical Co.,Tokyo,Japan,99.99% pure) were ball-milled in ethanol for 48h. The slurry was stirreddrying with rotary evaporator at 60 C,the mixture was reground in a mortar and pestle and screened through a 100-mesh sieve. Ti3SiC2 powder was synthesized directly from elemental titanium,silicon,and graphite powders with solid-liquid method at 1270 C. Pure Ti3SiC2 powders were obtained [16] by first removing TiSi2 using HF solution,and then heating the powders in air at 500 C for 5 h,finally washing the powders with hot 0272-8842/02/$22.00 # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(01)00082-7 Ceramics International 28 (2002) 223–226 www.elsevier.com/locate/ceramint * Corresponding author. E-mail address: lym98@mails.tsinghua.edu.cn (L. YongMing)
Luo Yong M ing et al./ Ceramics International 28(2002)223-226 (NH4)2SO4+ H2SO4 solution. The resulting powder had 2. 2. Characterization average particle size 1.0 um(purity higher 98%). The dried Ti3 SiC2 and Si3 N4 starting mixtures were carefully Specimens for flexural tests were cut and weighed and alternately layered in a steel die(p50 mm the billets to nominal dimensions of inner diameter) in a sequence of Si3 N4/ Ti3 SiC?/ Si3 N4/ Flexure strength was measured in a three-poi Ti,SiC2// Si3N4, then were compressed at 100 MPa. test with spans of 30 mm at a crosshead speed of 0.5 The layer thickness of Si3 N4 and Ti3 SiC2 were 0.30 and mm/min. The prospective tensile surfaces were ground 0. 15 mm, respectively. The green compact was hot- and polished to achieve a mirrorlike surface finish prior pressed in graphite dies at 25 MPa under flowing argon to mechanical testing. The work of fracture was mea at 1600C for 2 h. The heating and cooling rates were sured in four-point bending test with outer and inner 10 and 15C/min, respectively spans of 40 and 20 mm, respectively. The bending tests 12971020KuQ394Nn 124822K《829*2N Fig. 1. Macrograph of the laminated composite Si N/Ti3Sic 1284320KU636*2N 12070820Kv0474HN Fig. 3. SEM of the fracture surface of Si3 Na/ SiC2 laminated com- Fig. 2. Crack propagation in the laminated composite Si3 N4/Ti]SiC posites: (a)Si3N4 layer; (b)Ti3SiC, layer
(NH4)2SO4+H2SO4 solution. The resulting powder had average particle size 1.0 mm (purity higher 98%). The dried Ti3SiC2 and Si3N4 starting mixtures were carefully weighed and alternately layered in a steel die (50 mm inner diameter) in a sequence of Si3N4/ Ti3SiC2/ Si3N4/ Ti3SiC2/.../Si3N4,then were compressed at 100 MPa. The layer thickness of Si3N4 and Ti3SiC2 were 0.30 and 0.15 mm,respectively. The green compact was hotpressed in graphite dies at 25 MPa under flowing argon at 1600 C for 2 h. The heating and cooling rates were 10 and 15 C/min,respectively. 2.2. Characterization Specimens for flexural tests were cut and ground from the billets to nominal dimensions of 4336 mm. Flexure strength was measured in a three-point bending test with spans of 30 mm at a crosshead speed of 0.5 mm/min. The prospective tensile surfaces were ground and polished to achieve a mirrorlike surface finish prior to mechanical testing. The work of fracture was measured in four-point bending test with outer and inner spans of 40 and 20 mm,respectively. The bending tests Fig. 1. Macrograph of the laminated composite Si3N4/Ti3SiC2. Fig. 2. Crack propagation in the laminated composite Si3N4/Ti3SiC2. Fig. 3. SEM of the fracture surface of Si3N4/Ti3SiC2 laminated composites: (a) Si3N4 layer; (b) Ti3SiC2 layer. 224 Luo YongMing et al. / Ceramics International 28 (2002) 223–226
Luo Yong Ming et al./ Ceramics International 28(2002)223-226 were performed at a crosshead speed of 0.05 mm/min. where p is the load, a is the distance between the inner The work of fracture of each specimen was calculated by and outer loading points, b is width, and h is the thick determining the area under the load-crosshead deflection ness of the bar. The relationship between the maximum curve and dividing it by twice the cross-sectional area of shear stress and the maximum tensile stress for the four the sample. The fracture surface of the laminated com- point load condition can be written as [17] posites were examined by scanning electron microscope (SEM). Energy-dispersive X-ray analysis was used to qualitatively display the compositional distribution in Ot,max ne laminated composites Shear fracture will occur in these specimens if the naximum shear stress exceeds the shear strength of the 3. Results and discussion interface layer. Based on these considerations, the frac ture behavior of a multiplayer ceramic in a bending test The mechanical properties were measured using the depends not only on the microstructure but also on the hot-pressed laminated sample. The work of fracture of specimen geometry. The shear cracking would make the Si3N4/Ti3SiC2 laminated composites was 4760 Jm, and specimen more compliant in bending, therefore relieving the flexure strength was 746.3+10.2 MPa, while work some of the tensile stresses of the fracture of Si3 N4 ceramic was only 100 J/m. The Fig. I shows their structure of Si3 N4/Ti3SiC, laminated work of the fracture of Si3 N4/Ti3SiC2 laminated compo- composites. The silicon nitride layers appear gray and the sites was higher than that of Si3 N4 monolithic materials. Ti3 SiC2 interfaces appear as thin, bright layers separating Accordingly Ti3SiC2 increased reliability of si3 N4 cera he Si3N4 layers. Fig. 2 shows a typical crack propagation mic in mechanical properties when it was interface layer, manner of the hot-pressing laminated composite. As thus preventing catastrophic failure shown in Fig. 2, a major tensile crack propagates On a four-point bending test fixture, conventional through the specimen although it is deflected by Ti3Sic2 ceramics fail by tensile cracking. In contrast, these mul- layers. One remarkable feature observed in Fig. 2 is the tilayer ceramics with low interface shear strength could interlocking of toothlike, debonded layers. This indi- Tail by a combination of shear and tensile cracking. The cates that the maximum shear stress must exceed the maximum tensile stress in a four-point-bend bar can be shear strength of the Ti3Sic2 layer expressed as[17] Fig. 3 shows the micrographs of Si3 N4/Ti3 SiC2 lami- nated composites specimen fractured at room tempera ture. Whisker like B-Si3N4 is clearly seen in the Si3N4 Ot, max bh () layer, which indicates a-Sis N4 has transformed to the B-Si3N4 phase, but sintered temperature was rather aver Ti3 SiC? 12848220KU742HN Fig. 4. SEM of Ti]SiC2 grains in different positions
were performed at a crosshead speed of 0.05 mm/min. The work of fracture of each specimen was calculated by determining the area under the load-crosshead deflection curve and dividing it by twice the cross-sectional area of the sample. The fracture surface of the laminated composites were examined by scanning electron microscope (SEM). Energy-dispersive X-ray analysis was used to qualitatively display the compositional distribution in the laminated composites. 3. Results and discussion The mechanical properties were measured using the hot-pressed laminated sample. The work of fracture of Si3N4/Ti3SiC2 laminated composites was 4760 J/m2 ,and the flexure strength was 746.310.2 MPa,while work of the fracture of Si3N4 ceramic was only 100 J/m2 . The work of the fracture of Si3N4/Ti3SiC2 laminated composites was higher than that of Si3N4 monolithic materials. Accordingly Ti3SiC2 increased reliability of Si3N4 ceramic in mechanical properties when it was interface layer, thus preventing catastrophic failure. On a four-point bending test fixture,conventional ceramics fail by tensile cracking. In contrast,these multilayer ceramics with low interface shear strength could fail by a combination of shear and tensile cracking. The maximum tensile stress in a four-point-bend bar can be expressed as [17] t;max ¼ 3ap bh2 ð1Þ where p is the load, a is the distance between the inner and outer loading points, b is width,and h is the thickness of the bar. The relationship between the maximum shear stress and the maximum tensile stress for the fourpoint load condition can be written as [17] max t;max ¼ h 4a ð2Þ Shear fracture will occur in these specimens if the maximum shear stress exceeds the shear strength of the interface layer. Based on these considerations,the fracture behavior of a multiplayer ceramic in a bending test depends not only on the microstructure but also on the specimen geometry. The shear cracking would make the specimen more compliant in bending,therefore relieving some of the tensile stresses. Fig. 1 shows their structure of Si3N4/Ti3SiC2 laminated composites. The silicon nitride layers appear gray and the Ti3SiC2 interfaces appear as thin,bright layers separating the Si3N4 layers. Fig. 2 shows a typical crack propagation manner of the hot-pressing laminated composite. As shown in Fig. 2,a major tensile crack propagates through the specimen although it is deflected by Ti3SiC2 layers. One remarkable feature observed in Fig. 2 is the interlocking of toothlike,debonded layers. This indicates that the maximum shear stress must exceed the shear strength of the Ti3SiC2 layer. Fig. 3 shows the micrographs of Si3N4/Ti3SiC2 laminated composites specimen fractured at room temperature. Whisker like b-Si3N4 is clearly seen in the Si3N4 layer,which indicates a-Si3N4 has transformed to the b-Si3N4 phase,but sintered temperature was rather Fig. 4. SEM of Ti3SiC2 grains in different positions. Luo YongMing et al. / Ceramics International 28 (2002) 223–226 225
Luo Yong Ming et al./ Ceramics International 28(2002)223-226 Mgk 13? and Mg elements have diffused in fringe of Ti3Sic, layers This indicates that during the sintered process produced liquid in Si3N4 layers diffused to Ti3SiC2 layers and accelerated growth of the size of Ti3 SiC, grains CeLal, 129 4. Conclusions New Si3 N4/Ti3 SIC laminated composites have been manufactured by powder metallurgy method. Multilayer TiKa. 970 materials have shown noncatastrophic failure in four point bend tests. The three-step toughening behavior observed is a fundamental appearance in Si3N4/Ti3SiC2 laminated composites. Crack deflection along the weak interfaces was thought to be the major mechanism for improving fracture resistance. The secondary toughening behavior seems to be the layer structure of Ti3SiC2 on the 然伤 atomic scale, pull-out and micro-plastic deformation of Ti3 SiC2 grains. The difference size of Ti3 SiC2 grains was tertiary toughening behavior. Multiple toughening mechanisms obviously enhanced fracture resistance. The average apparent work of fracture of the laminated composites was about 4760 J/m- References J. Cook, J E. Gordon, Proc. Roy. Soc. London A282(1964) 2.-y. Liu, S.M. Hsu, J. Am. Ceram Soc. 79(1996)2452. 4 D. Popovic, S. Baskaran, G. Zywicki, C. Arens, J.w. Halloran, in: B W. Sheldon, S.C. Danforth(Eds ) Silicon-Based Structural Ceramics, vol. 42, American Ceramic Society, Westerville, OH Fig. 5. Element distribution in the laminated composites material. 5 P. Sajgalik. Z. Lences, J. Dusza, in: D.S. Yan, R. Fu.s.x. Shi lowly, so size of the B-si3N Is was small. The char- (Eds ) Proceedings of the 5th International Symposium on Ceramic acteristic of the laminated Ti3SiC2 grains can easily be Materials and Components for Engines(Shanghai, China May identified in these micrographs. Upon analysis of the 994), World Scientitic Publishing, Singapore, 1995, p. 198. fracture surface, the failure in the Ti3 SiC2 layers appears [6 Yasuhiro Shigegaki, J Am Ceram Soc. 79(1996)2197 7 P. Wei, et al., Mater. Lett., in press to be intergranular in nature while exhibiting a highly [8YP. Zeng. D L. Jiang, T. Watanable, J. Am. Ceram. Soc. 83 layered structure. This feature along with the rough 2000)2999 nature of the fracture surface has led to the suggestion 9 J.J. Nickl, K.K. Schweitzer, P. Luxenberg, J. Less-Common that this ceramic possesses a weak interface that leads to Metals26(1972)335 a behavior of a nanolaminate[18, 19]. It is seen that pull [0R. Pampuch, J. Lis, L. Stobierski, M. Tymkiewicz, J. Eur Ceran.Soc.5(1989)283 out of Ti3? grains is evidence. Micro-plastic defor- [11 T. El-Raghy, M W. Barsoum, A. Zavaliangos, S. Kalidindi, mation can be seen in such region, which may act more Am. Ceran.Soc.82(1999)2855 effectively as an energy-absorbing mechanism and [12]M.W. Barsoum, T El-Raghy, J Am Ceram Soc. 79(1996)1953 toughen the material further [ T. El-Raghy, A. Zavaliangos, M.W. Barsoum, S. Kalidindi, J In the microstructure of laminated composites we [14) L.M. Low, S. K Lee, B Lawn, M.W. Barsoum, I Am Ceram Soc.8l(1998)225 layer was bigger than for those removed from the Si3 N4 [15] TGoto, T Harai, Mater Res Bull. 22(1987)119 layer(see Fig. 4). Qualitative energy-dispersive X-ray [16] C. Racault, F. Langlais, R. Naslain, J Mater. Sci. 29(1994)3384 microanalysis was used to detect the element distribu- [7 J.V. Mullin, A C. Knoell, Basic concepts in composite beam tion in the laminated composites material(see Fig. 5) sting, Mater Res Stand. (1970)16-20. [18 M.W. Barsoum, T El-Raghy, J Am Ceram Soc. 79(1996)1953. Fig 5 shows the Mg, Ce and Ti element line map of the [19] M.w. Barsoum, T. El-Raghy, J Mater. Synth. Proc. 5(1997) region of the laminated material. We found that the Ce 197
lowly,so size of the b-Si3N4 grains was small. The characteristic of the laminated Ti3SiC2 grains can easily be identified in these micrographs. Upon analysis of the fracture surface,the failure in the Ti3SiC2 layers appears to be intergranular in nature while exhibiting a highly layered structure. This feature along with the rough nature of the fracture surface has led to the suggestion that this ceramic possesses a weak interface that leads to a behavior of a nanolaminate [18,19]. It is seen that pullout of Ti3SiC2 grains is evidence. Micro-plastic deformation can be seen in such region,which may act more effectively as an energy-absorbing mechanism and toughen the material further. In the microstructure of laminated composites we found that the size of the Ti3SiC2 grains near the Si3N4 layer was bigger than for those removed from the Si3N4 layer (see Fig. 4). Qualitative energy-dispersive X-ray microanalysis was used to detect the element distribution in the laminated composites material (see Fig. 5). Fig. 5 shows the Mg,Ce and Ti element line map of the region of the laminated material. We found that the Ce and Mg elements have diffused in fringe of Ti3SiC2 layers. This indicates that during the sintered process produced liquid in Si3N4 layers diffused to Ti3SiC2 layers and accelerated growth of the size of Ti3SiC2 grains. 4. Conclusions New Si3N4/Ti3SiC2 laminated composites have been manufactured by powder metallurgy method. Multilayer materials have shown noncatastrophic failure in fourpoint bend tests. The three-step toughening behavior observed is a fundamental appearance in Si3N4/Ti3SiC2 laminated composites. Crack deflection along the weak interfaces was thought to be the major mechanism for improving fracture resistance. The secondary toughening behavior seems to be the layer structure of Ti3SiC2 on the atomic scale,pull-out and micro-plastic deformation of Ti3SiC2 grains. The difference size of Ti3SiC2 grains was tertiary toughening behavior. Multiple toughening mechanisms obviously enhanced fracture resistance. The average apparent work of fracture of the laminated composites was about 4760 J/m2 . References [1] J. Cook,J.E. Gordon,Proc. Roy. Soc. London A282 (1964) 508. [2] H.-y. Liu,S.M. Hsu,J. Am. Ceram. Soc. 79 (1996) 2452. [3] S. Baskaran,S. Nunn,D. Popovic,J.W. Halloran,J. Am. Ceram. Soc. 76 (1993) 2209. [4] D. Popovic,S. Baskaran,G. Zywicki,C. Arens,J.W. Halloran, in: B.W. Sheldon,S.C. Danforth (Eds.),Silicon-Based Structural Ceramics,vol. 42,American Ceramic Society,Westerville,OH, 1994,p. 173. [5] P. Sajgalik. Z. Lences,J. Dusza,in: D.S. Yan,R. Fu. S.X. Shi (Eds.),Proceedings of the 5th International Symposium on Ceramic Materials and Components for Engines (Shanghai,China May 1994),World Scientitic Publishing,Singapore,1995,p. 198. [6] Yasuhiro Shigegaki,J. Am. Ceram. Soc. 79 (1996) 2197. [7] P. Wei,et al.,Mater. Lett.,in press. [8] Y.P. Zeng,D.l. Jiang,T. Watanable,J. Am. Ceram. Soc. 83 (2000) 2999. [9] J.J. Nickl,K.K. Schweitzer,P. Luxenberg,J. Less-Common Metals 26 (1972) 335. [10] R. Pampuch,J. Lis,L. Stobierski,M. Tymkiewicz,J. Eur. Ceram. Soc. 5 (1989) 283. [11] T. El-Raghy,M.W. Barsoum,A. Zavaliangos,S. Kalidindi,J. Am. Ceram. Soc. 82 (1999) 2855. [12] M.W. Barsoum,T. El-Raghy,J. Am. Ceram. Soc. 79 (1996) 1953. [13] T. El-Raghy,A. Zavaliangos,M.W. Barsoum,S. Kalidindi,J. Am. Ceram. Soc. 80 (1997) 513. [14] I.M. Low,S.K. Lee,B. Lawn,M.W. Barsoum,J. Am. Ceram. Soc. 81 (1998) 225. [15] T. Goto,T. Harai,Mater. Res. Bull. 22 (1987) 1195. [16] C. Racault,F. Langlais,R. Naslain,J. Mater. Sci. 29 (1994) 3384. [17] J.V. Mullin,A.C. Knoell,Basic concepts in composite beam testing,Mater. Res. Stand. (1970) 16–20. [18] M.W. Barsoum,T. EI-Raghy,J. Am. Ceram. Soc. 79 (1996) 1953. [19] M.W. Barsoum,T. EI-Raghy,J. Mater. Synth. Proc. 5 (1997) 197. Fig. 5. Element distribution in the laminated composites material. 226 Luo YongMing et al. / Ceramics International 28 (2002) 223–226