J.Am. Cera.Soc.85I2864-6602002 urna Side-Surface Structure of a Commercial B-Silicon Carbide Whisker Lin Geng, Jie Zhang, Qing-Chang Meng, and Cong-Kai Yao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People's Republic of China The side surfaces of a reial B-sic whisker were ana- displayed f zigzag structure and were composed of Ized by calculating ce energy and observing the nes. In the present research, the side-surface microstructure of the The results indicated that the structure of iC whiskers was studied from the point of side surfaces displayed of zigzag strueture and were crystal str energy analysis and was examined using composed of (111],(110), and(100) crystal planes. high-resolution transmission electron microscopy (TEM) L. Introduction Il. Experimental Procedure ILICON CARBIDE(SiC) whiskers have been produced using numer of the Tws-100 B-SiC whiskers used ous methods. The commercial B-siC whiskers used in this report in the 0. 1-1.0 um and 30-100 um, respec- (Product TWS-100, Tokai Carbon Co. Ltd, Tokyo, Japan)were Preparation of asic whisker specimen that was suitable for prepared by a vapor-iquid-solid (VLS)technique. These whiskers have been widely used as an effective reinforcement and toughening hase in aluminum-matrix composites" and ceramic-matrix compo (a) tes. SiC-whisker-reinforced aluminum alloy(SICwAl) composites have shown high mechanical and physical properties' and have (112 ential use in various structural applications I composites are usually fabricated by wder-metallurgy techniques. 6,7 The nature of the Sic/alur minum terfaces in the SiCwAl composites that have been produced by the different techniques is obviously different, which, in turn, affects the mechanical and physical properties of the composites. Previous Al Sic research results have shown that the bonding strength of the sic/ aluminum interface in the SICwAl composites made using the ueeze-casting method is very high(higher than the matrix shear no interfacial reaction and mutual diffusion of elements has occurred at the interface. To explain the high bonding strength of the sicaluminum interface. a semicoherent bondin mechanism has been highlighted ,u In this mechanism. the melt aluminum-matrix alloy is considered to have crystallized on the urfaces of the Sic whiskers during squeeze casting, so that some _02Bm crystal-orientation relationships might exist between the Sic whisker nd the aluminum-alloy matrix According to this mechanism, the side-surface structure of the SiC whisker is important in the nature and erties of the sicaluminum surface in the sic Ye et al4 also found that the side surfaces of the sic whiskers were very important, in regard to the properties of the alumina(Al,O3 matrix composites The results of the studies on the crystal structure of the B-sic agonal and triangular) existed for the p-Sic whiskers and both were single-crystal structures with a face-centered cubic (fcc structure and grew along the [1lllfcs direction( that of the whisk SiC axis).-3 However, almost no research reports exist in regard to the side-surface structure of the B-sic whiskers. Liu et al. reported that the side surfaces of the B-SiC whiskers are(112ifce however. the results of nutt 2, 3 indicated that the side surfaces 0.2um Fig. 1. TEM images of the B-SiC whiskers (a)
Side-Surface Structure of a Commercial -Silicon Carbide Whisker Lin Geng,† Jie Zhang, Qing-Chang Meng, and Cong-Kai Yao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China The side surfaces of a commercial -SiC whisker were analyzed by calculating the surface energy and observing the microstructure of the whiskers. The results indicated that the side surfaces displayed a type of zigzag structure and were composed of {111}, {110}, and {100} crystal planes. I. Introduction SILICON CARBIDE (SiC) whiskers have been produced using numerous methods.1 The commercial -SiC whiskers used in this report (Product TWS-100, Tokai Carbon Co. Ltd., Tokyo, Japan) were prepared by a vapor–liquid–solid (VLS) technique. These whiskers have been widely used as an effective reinforcement and toughening phase in aluminum-matrix composites2,3 and ceramic-matrix composites.4 SiC-whisker-reinforced aluminum alloy (SiCw/Al) composites have shown high mechanical and physical properties5 and have potential use in various structural applications. SiCw/Al composites are usually fabricated by squeeze-casting and powder-metallurgy techniques.6,7 The nature of the SiC/aluminum interfaces in the SiCw/Al composites that have been produced by the different techniques is obviously different, which, in turn, affects the mechanical and physical properties of the composites. Previous research results have shown that the bonding strength of the SiC/ aluminum interface in the SiCw/Al composites made using the squeeze-casting method is very high (higher than the matrix shear strength), although no interfacial reaction and mutual diffusion of elements has occurred at the interface.8 To explain the high bonding strength of the SiC/aluminum interface, a semicoherent bonding mechanism has been highlighted.9,10 In this mechanism, the melt aluminum-matrix alloy is considered to have crystallized on the surfaces of the SiC whiskers during squeeze casting, so that some crystal-orientation relationships might exist between the SiC whisker and the aluminum-alloy matrix. According to this mechanism, the side-surface structure of the SiC whisker is important in the nature and properties of the SiC/aluminum surface in the SiCw/Al composites. Ye et al.4 also found that the side surfaces of the SiC whiskers were very important, in regard to the properties of the alumina (Al2O3) matrix composites. The results of the studies on the crystal structure of the -SiC whiskers have shown that two types of transverse sections (hexagonal and triangular) existed for the -SiC whiskers and both were single-crystal structures with a face-centered cubic (fcc) structure and grew along the [111]fcc direction (that of the whisker axis).11–13 However, almost no research reports exist in regard to the side-surface structure of the -SiC whiskers. Liu et al.11 reported that the side surfaces of the -SiC whiskers are {112}fcc; however, the results of Nutt12,13 indicated that the side surfaces displayed a form of zigzag structure and were composed of {111}fcc crystal planes. In the present research, the side-surface structure of the -SiC whiskers was studied from the point of crystal structure and energy analysis and was examined using high-resolution transmission electron microscopy (TEM). II. Experimental Procedure The diameter and length of the TWS-100 -SiC whiskers used in the present report were 0.1–1.0 m and 30–100 m, respectively. Preparation of a SiC whisker specimen that was suitable for J. J. Petrovic—contributing editor Manuscript No. 187309. Received January 23, 2002; approved April 2, 2002. † Author to whom correspondence should be addressed. Fig. 1. TEM images of the -SiC whiskers in transverse section ((a) triangular whisker and (b) hexagonal whisker). 2864 journal J. Am. Ceram. Soc., 85 [11] 2864–66 (2002)
Communications of the American Ceramic Sociery 2865 Fig. 2. Cells of some crystal planes in fcc structure(a)1111, (b)(110), (c)(100), (d)(112))of the B-Sic whisker(() first layer atom, () second layer atom,(X) third layer atom,(A) fourth layer atom, (v) fifth layer atom, and(+)sixth layer atom). [lll]WHisker [l1llTWhisker axIs axIs T一 (a)Model I (b)Model ll Fig. 3. Side-surface crystal models of the B-SiC whiskers("T" denotes twins). observation via TEM was difficult; therefore, a SicaL composite The total surface energy (o can be calculated using the was fabricated using a squeeze-casting route. The thin lamellae of following equation the composite were cut by spark machining and then thinned via on milling. In the composite specimen, many Sic whiskers, (2) riented in various directions, were present and were suitable for TEM observation. Therefore, the microstructure and crystal de where ohk, is the surface energy of the (hk, plane and Shk is the B-SiC whiskers could be examined via TEM urface area of the (hkl) plane, The calculated results are shown in Table ll Table ll shows that Model I has the highest total surface energy II. Results and Diseussion and Model Ill has the lowest total surface energy. Note that Model Il is suitable only for whiskers with a high density of microtwins Figure I is a collection of TEM images showing the morphol- in the(111) plane perpendicular to the whisker axis. High gy of the transverse section of the B-SiC whiskers. The side resolution TEM observation results have shown that a high density urfaces of both types of Sic whiskers have been proved to be of microtwins exists in the hexagonal p-SiC whisker, as shown in parallel to the(112) fee crystal planes, as shown in Fig. 1. Fig. 4(b).Therefore, Model Ill is a possible side-surface structure However, in the fcc crystal structure, the (112)fcc crystal planes for the hexagonal B-SiC whiskers. However, for the triangular eph贴hcm图s盒saat0tMha,吗0mFg4mps lave a higher surface energy, in comparison with that of other lower-index crystal planes, langular B-SiC whisker to form its side the whiskers, should be dependent on the total surface en of that in Model Ill. Therefore, one can conclude that Model Il is a the whiskers possible side-surface structure for the triangular B-SiC whiskers 2 shows the cells of the (111ifce,(110 fcc,(100) ce, and From the above-given analysis, the side surface of the p-siC crystal planes of the SiC whisker. The surface energy per whisker should be a type of zigzag surface that is composed of unit area(o) for each crystal plane is given as anes such as i11 110}c,and{100}e It is difficult to confirm this fact directly via TEM observation n(UJ2) High-resolution TEM observation results have show that the side surface of the B-sic whisker is really a form of zigzag surface that 1g where U is the ener uired to break one atom bond(a is the lattice constant of the Sic whisker), S the area of the crystal cel d n the number of the broken atomic bonds in each crystal cel Table L. Calculation Results of the Surface Energy for The calculation results are shown in Table I. Table I shows that Four Types of Crystal Planes in the fee Structure of for the same area of the four types of crystal planes, fill) has β- SiC Whisker the lowest surface energy and the i fce has the highest surface Number of broken Figure I shows that the side surfaces of the B-siC whiskers Crystal plane Surface area, S crystal cell,n both hexagonal and triangular) should be parallel to the (112 rystal planes. In this case, there are three types of side-surface 111} (V3/2a2 6 3(Ua2) rystal models, labeled as Models I, Il, and Il, respectively(see 1 Fig 3: the length and width of the side surface are noted as / and 3v2(UJa) (v6/2a respectively)
observation via TEM was difficult; therefore, a SiCw/Al composite was fabricated using a squeeze-casting route. The thin lamellae of the composite were cut by spark machining and then thinned via ion milling. In the composite specimen, many SiC whiskers, oriented in various directions, were present and were suitable for TEM observation. Therefore, the microstructure and crystal defects of the -SiC whiskers could be examined via TEM. III. Results and Discussion Figure 1 is a collection of TEM images showing the morphology of the transverse section of the -SiC whiskers. The side surfaces of both types of SiC whiskers have been proved to be parallel to the {112}fcc crystal planes, as shown in Fig. 1. However, in the fcc crystal structure, the {112}fcc crystal planes have a higher surface energy, in comparison with that of other lower-index crystal planes, such as {110}fcc, {100}fcc, and {111}fcc. The crystal planes, which compose the side surfaces of the whiskers, should be dependent on the total surface energy of the whiskers. Figure 2 shows the cells of the {111}fcc, {110}fcc, {100}fcc, and {112}fcc crystal planes of the SiC whisker. The surface energy per unit area () for each crystal plane is given as nUa/ 2 S (1) where Ua is the energy required to break one atom bond (a is the lattice constant of the SiC whisker), S the area of the crystal cell, and n the number of the broken atomic bonds in each crystal cell. The calculation results are shown in Table I. Table I shows that, for the same area of the four types of crystal planes, {111}fcc has the lowest surface energy and the {112}fcc has the highest surface energy. Figure 1 shows that the side surfaces of the -SiC whiskers (both hexagonal and triangular) should be parallel to the {112} crystal planes. In this case, there are three types of side-surface crystal models, labeled as Models I, II, and II, respectively (see Fig. 3; the length and width of the side surface are noted as l and w, respectively). The total surface energy (t ) can be calculated using the following equation: t hklShkl (2) where hkl is the surface energy of the {hkl} plane and Shkl is the surface area of the {hkl} plane. The calculated results are shown in Table II. Table II shows that Model I has the highest total surface energy and Model III has the lowest total surface energy. Note that Model III is suitable only for whiskers with a high density of microtwins in the (111) plane perpendicular to the whisker axis. Highresolution TEM observation results have shown that a high density of microtwins exists in the hexagonal -SiC whisker, as shown in Fig. 4(b). Therefore, Model III is a possible side-surface structure for the hexagonal -SiC whiskers. However, for the triangular -SiC whiskers, no microtwins exist in the (111) plane perpendicular to the whisker axis, as shown in Fig. 4(a), so it is impossible for the triangular -SiC whisker to form its side surface similar to that in Model III. Therefore, one can conclude that Model II is a possible side-surface structure for the triangular -SiC whiskers. From the above-given analysis, the side surface of the -SiC whisker should be a type of zigzag surface that is composed of low-energy crystal planes such as {111}fcc, {110}fcc, and {100}fcc. It is difficult to confirm this fact directly via TEM observation. High-resolution TEM observation results have show that the side surface of the -SiC whisker is really a form of zigzag surface that is similar to that shown in Fig. 5. Fig. 2. Cells of some crystal planes in fcc structure ((a){111}, (b) {110}, (c) {100}, (d) {112}) of the -SiC whisker ((F) first layer atom, (f) second layer atom, () third layer atom, (Œ) fourth layer atom, () fifth layer atom, and () sixth layer atom). Fig. 3. Side-surface crystal models of the -SiC whiskers (“T” denotes twins). Table I. Calculation Results of the Surface Energy for Four Types of Crystal Planes in the fcc Structure of -SiC Whisker Crystal plane Surface area, S Number of broken atomic bonds per crystal cell, n Surface energy per unit area, {111} (3/2)a2 6 23(Ua/a2 ) {110} a2 8 4(Ua/a2 ) {100} (2/2)a2 6 32(Ua/a2 ) {112} (6/2)a2 9 36(Ua/a2 ) November 2002 Communications of the American Ceramic Society 2865
Communications of the American Ceramic Sociery Vol. 85. No. 11 Table Il. Calculation Results of the Surface Area and Total Surface Energy of B-Sic Whisker Model Surface area, Shy Total surface area,a n1=(7V6/30h,S10=(2V3/5)h,S1=(3V2/5he[29/30)V2+(65V6(U2)h 1=(3V2/4h (3/2)V6(Uda) SiC 111 10nm Fig. 5. High-resolution TEM image showing the zigzag side surface of the B-SiC whisker. References J. Petrovic. and S. R “ Growth of Whisker by the VLS Process, T. Christman and S. Suresh,"Microstructural Develop Aluminu 1111l Alloy-SiC Whisker Composite, Acta Metall, 36, 1691(1988 BL. Geng and C. K. Yao, "Development of Some Fundamental Research of SiCW/Al Composites in HIT, "J. Mater. Sci. Technol, 9, 431(1993)- F. Ye, T. C. Lei, and Y. Zhou, "Interface Structure and Mechanical AL,O-20 vol% SiCw Ceramic Matrix Composite, Mater. Sci. Eng. A, A281, 305 Z.Y. Ma, Y. X. Lu, M. Luo, and J Bi, "Effect of Solution Tempera Properties of SiCw/2024Al Composite, "J. Mater. Sci. TechnoL, 11, 29 SiCw/6061Al Composites by Squeeze Casting Method", p. 29 in MRS Intermational nm Meeting on Advanced Materials, Vol. 4(Tokyo, Japan). Materials Research Society R Nutt and J. M. Duva,"Failure Mechanism in Al-SiC Composites,"Scr. Fig 4. High-resolution TEM images of the B-SiC whisker in longitudinal Metall20,1055(1986) section(a) triangular whisker and(b) hexagonal whisker ). eL. Cao, L Gene C K. Yao, and T Q Lei, "Interface in Silicon Carbide Whisker terface Bonding Mechanism in a Squeeze Casting SiCw/Al Composite, " J. Mater. Sci. Left, 14, 606(1995). Ⅳv. Conclusions IoL Geng and C. K. Yao, "SiC-Al Interface Structure in Squeeze Cast Sicw/ Composite, Scr: Metall. Mater, 33, 949(1995) The side surface of the commercial TwS-100 B-SiC whiskers Q. Liu, K. Wu, L. Geng, and C. K. Yao, "Microdiffraction Study of B-Sic display a form of zigzag structure. For the hexagonal whisker, the ter in a SiCw/Al Composite, Mater. Sci. Eng. A, A130, 113(1990) ide surfaces are composed of (111) crystal planes. For the R. Nutt, "Microstructure and Growth Model for Rice-Hull-Derived SiC Whiskers,J.Am. Ceram Soc., 71 3]149(1988) triangular whisker, the side surfaces are composed of (1111 IS R Nutt, "Defects in Silicon Carbide Whiskers, "J. Am. Ceram. Soc., 67(61428 11101, and( 100) crystal plar
IV. Conclusions The side surface of the commercial TWS-100 -SiC whiskers display a form of zigzag structure. For the hexagonal whisker, the side surfaces are composed of {111} crystal planes. For the triangular whisker, the side surfaces are composed of {111}, {110}, and {100} crystal planes. References 1 J. V. Milewski, F. D. Gac, J. J. Petrovic, and S. R. Skaggs, “Growth of beta-Silicon Carbide Whiskers by the VLS Process,” J. Mater. Sci., 20, 1160 (1985). 2 T. Christman and S. Suresh, “Microstructural Development in an Aluminum Alloy–SiC Whisker Composite,” Acta Metall., 36, 1691 (1988). 3 L. Geng and C. K. Yao, “Development of Some Fundamental Research of SiCw/Al Composites in HIT,” J. Mater. Sci. Technol., 9, 431 (1993). 4 F. Ye, T. C. Lei, and Y. Zhou, “Interface Structure and Mechanical Properties of Al2O3–20 vol% SiCw Ceramic Matrix Composite,” Mater. Sci. Eng. A, A281, 305 (2000). 5 Z. Y. Ma, Y. X. Lu, M. Luo, and J. Bi, “Effect of Solution Temperature on Tensile Properties of SiCw/2024Al Composite,” J. Mater. Sci. Technol., 11, 291 (1995). 6 L. Geng, W. Zhao, and C. K. Yao, “A Study of Fabrication Technique of SiCw/6061Al Composites by Squeeze Casting Method”; p. 29 in MRS International Meeting on Advanced Materials, Vol. 4 (Tokyo, Japan). Materials Research Society, Pittsburgh, PA, 1988. 7 S. R. Nutt and J. M. Duva, “Failure Mechanism in Al-SiC Composites,” Scr. Metall., 20, 1055 (1986). 8 L. Cao, L. Geng, C. K. Yao, and T. Q. Lei, “Interface in Silicon Carbide Whisker Reinforced Aluminum Composite,” Scr. Metall., 23, 227 (1989). 9 L. Geng and C. K. Yao, “SiC-Al Interface Bonding Mechanism in a Squeeze Casting SiCw/Al Composite,” J. Mater. Sci. Lett., 14, 606 (1995). 10L. Geng and C. K. Yao, “SiC-Al Interface Structure in Squeeze Cast SiCw/Al Composite,” Scr. Metall. Mater., 33, 949 (1995). 11Q. Liu, K. Wu, L. Geng, and C. K. Yao, “Microdiffraction Study of -SiC Whisker in a SiCw/Al Composite,” Mater. Sci. Eng. A, A130, 113 (1990). 12S. R. Nutt, “Microstructure and Growth Model for Rice-Hull-Derived SiC Whiskers,” J. Am. Ceram. Soc., 71 [3] 149 (1988). 13S. R. Nutt, “Defects in Silicon Carbide Whiskers,” J. Am. Ceram. Soc., 67 [6] 428 (1984). Fig. 4. High-resolution TEM images of the -SiC whisker in longitudinal section ((a) triangular whisker and (b) hexagonal whisker). Fig. 5. High-resolution TEM image showing the zigzag side surface of the -SiC whisker. Table II. Calculation Results of the Surface Area and Total Surface Energy of -SiC Whisker Model Surface area, Shkl Total surface area, t I S112 lw 36(Ua/a2 )lw II S001 (76/30)lw, S110 (23/15)lw, S111 (32/5)lw [(29/30)2 (6/5)]6(Ua/a2 )lw III S111 (32/4)lw (3/2)6(Ua/a2 )lw 2866 Communications of the American Ceramic Society Vol. 85, No. 11