IATERIALS LETTERS ELSEVIER Materials Letters 50(2001)358-363 www.elsevier.com/locate/matlet Analysis of mechanical properties and SEM for laminated SiC/W composites Yongyi Gaoa, Shiyuan Zheng Kaicheng Zhu ac , Department of Chemistry, Chongqing Adoanced Training School of Teacher, Chon Received 10 March 2000; received in revised form 3 January 2001; accepted 6 January 2001 Abstract In this letter, it is reported that a laminated SiC/w composite has been developed using the hot pressing method. It is found that a chemical reaction between W and Sic occurs during the preparation process. Making use of sEM, th components within the sandwiched-in metal and the fracturing crack for the laminated SiC/w composite are determined. esting mechanical properties of the laminated SiC/w composite indicates that fracture toughness increases while bending strength reduces, with an increase of the thickness of the sandwiched-in metal ranging from 10-50-Hm thickness. C 2001 Elsevier Science B. v. All rights reserve Keywords: SiC/W laminated composite; Sandwiched- in metal thickness; Fracture toughness; Bending strength 1. Introduction there have also been few works to study the lam nated composites with sandwiched-in metal. In this Making use of layered structure to increase mate- work, we investigate the dependence of fracture rial toughness is one of the most effective ways to toughness and bending strength on the thickness of improve fracture toughness of structured ceramic the sandwiched-in metal for the laminated materials materials. Therefore, there have been a large number f works focusing on the problem. Now, many bionic laminated composites have been made [1-3 that 2.Experimental exhibit the potential of wide application. It is found that laminated structure of material can greatly im- 2.1. Preparation of Sic/w laminated composites knowledge, these bionic laminated composites were generally made up with the sandwiched-in layer In this experiment, the ceramic slice is prepared consisting of BN, graphite and carbide fibre. But with the gel-casting method, in which 6 wt % Al2O3 and 4 wt %Y,O, were mixed in SiC (99.9%, 5 um) powder. Then, using the spraying process, the metal W powder (99.9%, 5 um) is smeared on the SiC ceramic slice, and the samples treated by this method ressgaowei_ims@sohu.com(y.Gao) are laminated. Finally, the Sic/w laminated mate- 00167-577X/01/$- see front matter C2001 Elsevier Science B V. All rights reserved PI:S0167-577X(01)00256-7
September 2001 Materials Letters 50 2001 358–363 Ž . www.elsevier.comrlocatermatlet Analysis of mechanical properties and SEM for laminated SiCrW composites Yongyi Gao a,), Shiyuan Zheng b , Kaicheng Zhu a,c a Department of Physics, Xiangtan Normal UniÕersity, Xiangtan 411201, Hunan, PR China b Department of Chemistry, Chongqing AdÕanced Training School of Teacher, Chongqing 402168, PR China c Department of Applied Physics and Heat Transfer Technology, Central South UniÕersity, Changsha 410083, Hunan, PR China Received 10 March 2000; received in revised form 3 January 2001; accepted 6 January 2001 Abstract In this letter, it is reported that a laminated SiCrW composite has been developed using the hot pressing method. It is found that a chemical reaction between W and SiC occurs during the preparation process. Making use of SEM, the components within the sandwiched-in metal and the fracturing crack for the laminated SiCrW composite are determined. Testing mechanical properties of the laminated SiCrW composite indicates that fracture toughness increases while bending strength reduces, with an increase of the thickness of the sandwiched-in metal ranging from 10–50-mm thickness. q 2001 Elsevier Science B.V. All rights reserved. Keywords: SiCrW laminated composite; Sandwiched-in metal thickness; Fracture toughness; Bending strength 1. Introduction Making use of layered structure to increase material toughness is one of the most effective ways to improve fracture toughness of structured ceramic materials. Therefore, there have been a large number of works focusing on the problem. Now, many bionic laminated composites have been made 1–3 that w x exhibit the potential of wide application. It is found that laminated structure of material can greatly improve fracture toughness of ceramic material. To our knowledge, these bionic laminated composites were generally made up with the sandwiched-in layer consisting of BN, graphite and carbide fibre. But ) Corresponding author. E-mail address: gaowei lms@sohu.com Y. Gao . Ž . – there have also been few works to study the laminated composites with sandwiched-in metal. In this work, we investigate the dependence of fracture toughness and bending strength on the thickness of the sandwiched-in metal for the laminated materials. 2. Experimental 2.1. Preparation of SiCrW laminated composites In this experiment, the ceramic slice is prepared with the gel-casting method, in which 6 wt.% Al O2 3 and 4 wt.% Y O were mixed in SiC 99.9%, 5 2 3 Ž . mm powder. Then, using the spraying process, the metal W powder 99.9%, 5 Ž . mm is smeared on the SiC ceramic slice, and the samples treated by this method are laminated. Finally, the SiCrW laminated mate- 00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0167-577X 01 00256-7 Ž
Y Gao et al/ Materials Letters 50(2001)358-363 rial is obtained after putting the laminated material sandwiched-in metal W for a fixed thickness of SiC into a hot pressing stove, which lasted for 2 h under ceramics(Fig. 1), which represents that at A direc argon atmosphere at 25 MPa pressure and a treat- tion, fracture toughness increases from 4.5 to 8.9 ment temperature of 1800C MPa m/2. with an increase in the thickness of the sandwiched-in metal W, that is, fracture toughness of 2. 2. Test of the mechanical properties the SiC/w laminated composite depends undoubt- edly on the thickness of the metal w layer. This The SiC/W laminated composite is made into result is unanimous with those of the Al,2O3/Ni standard samples. Test to bending strength at roor laminated composites [4, 5] temperature is performed with the three-point-bend It is well known that although laminated structure g testing, while the one-side-cut method is em- can greatly improve fracture toughness of materials, ployed to determine fracture toughness at room tem- the whole uniform character of block materials will perature be lost. Therefore, we also investigate the relation between fracture toughness of the SiC/w laminated 2. 3. Analysis method of sEM and the phase diagram composite and the direction along which the external of the SiC/w laminated material force acts. These, denoted by Nos. 6 and 7 in Table 1, show that when the thickness of the metal layer is The cross-sectional microstructure of the sample 50 um, fracture toughness Kic(Sic /w) along the A made in this experiment was determined by means of direction is 2. 12 Kic(sic), while the Kic(sic/w) along scanning electron microscopy(Opton CSM950, Ger- the B direction is only 1.62 KIc(sic), which indicates many, containing EDAX). With X-ray diffraction that fracture toughness is very sensitive to the acting analysis (XRD; Rigaku D/MAX111A, Japan), the direction of the external force material phase of the sample was investigated On the other hand. the results listed in table 1 show that bending strength is reduced from 662 MPa (0.97 times bending strength of Sic block material) 3. Results and discussions to 624 MPa (0.92 times bending strength of SiC block material), with the increase from 10 to 50 um 3. 1. Relation between mechanical properties and the of the thickness of the metal W layer. Hence, in- layer thickness creasing the thickness of the sandwiched-in w would lead to a reduction of bending strength bendis he dependence of fracture tough Similarly, we also determine the dependence of trength on the thickness of the bending strength of Sic/w laminated composites on Table I Measured results of fracture toughness and bending resistance for a fixed thickness(150 um) of Sic ceramics at room temperature tested at the b direction, while the others were tested at the a direction(see Fig. 1). During the bending tests, all the samples used have the same cross-sectioning Resistance to Ceramics Metal thickness (um) Ku(MPa/2) bending(MPa) ssss wwwwwww 00000 6.8
Y. Gao et al.rMaterials Letters 50 2001 358–363 ( ) 359 rial is obtained after putting the laminated material into a hot pressing stove, which lasted for 2 h under argon atmosphere at 25 MPa pressure and a treatment temperature of 18008C. 2.2. Test of the mechanical properties The SiCrW laminated composite is made into standard samples. Test to bending strength at room temperature is performed with the three-point-bending testing, while the one-side-cut method is employed to determine fracture toughness at room temperature. 2.3. Analysis method of SEM and the phase diagram of the SiCrW laminated material The cross-sectional microstructure of the sample made in this experiment was determined by means of scanning electron microscopy Opton CSM950, Ger- Ž many, containing EDAX . With X-ray diffraction . analysis XRD; Rigaku D Ž . rMAX111A, Japan , the material phase of the sample was investigated. 3. Results and discussions 3.1. Relation between mechanical properties and the layer thickness Table 1 shows the dependence of fracture toughness and bending strength on the thickness of the sandwiched-in metal W for a fixed thickness of SiC ceramics Fig. 1 , which represents that at A direc- Ž . tion, fracture toughness increases from 4.5 to 8.9 MPa m1r2 , with an increase in the thickness of the sandwiched-in metal W; that is, fracture toughness of the SiCrW laminated composite depends undoubtedly on the thickness of the metal W layer. This result is unanimous with those of the Al O rNi 2 3 laminated composites 4,5 . w x It is well known that although laminated structure can greatly improve fracture toughness of materials, the whole uniform character of block materials will be lost. Therefore, we also investigate the relation between fracture toughness of the SiCrW laminated composite and the direction along which the external force acts. These, denoted by Nos. 6 and 7 in Table 1, show that when the thickness of the metal layer is 50 mm, fracture toughness KICŽSiCrW. along the A direction is 2.12 KICŽSiC., while the KICŽSiCrW. along the B direction is only 1.62 K , which indicates ICŽSiC. that fracture toughness is very sensitive to the acting direction of the external force. On the other hand, the results listed in Table 1 show that bending strength is reduced from 662 MPa Ž . 0.97 times bending strength of SiC block material to 624 MPa 0.92 times bending strength of SiC Ž block material , with the increase from 10 to 50 . mm of the thickness of the metal W layer. Hence, increasing the thickness of the sandwiched-in W would lead to a reduction of bending strength. Similarly, we also determine the dependence of bending strength of SiCrW laminated composites on Table 1 Measured results of fracture toughness and bending resistance for a fixed thickness 150 Ž . mm of SiC ceramics at room temperature ) The data with A B is tested at the B direction, while the others were tested at the A direction see Fig. 1 . During the bending tests, all the Ž . samples used have the same cross-sectioning. No. SiCrW system Metal layer Fracture toughness, Resistance to 1r2 thickness Ž. Ž . Ž . mm K MPa m bending MPa Ceramics Metal IC 1 SiC W 0 4.2 680 2 SiC W 10 4.5 662 3 SiC W 20 6.9 651 4 SiC W 30 7.4 643 5 SiC W 40 8.2 630 6 SiC W 50 8.9 624 ) ) 7 SiC W 50 6.8 625
Y Gao et al/ Materials Letters 50(2001)358-363 Fig. 1. Testing direction of fracture toughness and bending strength the direction along which the external force acts. The and wc do exist in the Sic/w laminated compos- data given by Nos. 6 and 7 in Table I represents that ites obtained in our preparation. Therefore, we can the bending strengths at the a and b directions are deduce that a chemical reaction between w and sic almost the same. This reflects that bending strength occurs, and produces WsSi3 and wC. In addition, of SiC/w laminated composites is almost isotropic, because the height of the diffraction peak of wsSi3 which is contrasted to fracture toughness and could and wC increases with an increase of the thickness provide some reference during designing of the ma- of the sandwiched-in metal, this makes it clear that terial structure the thicker the metal layer, the larger the amount of the metal W taking part in the chemical reaction, and 3.2. Analysis of material phase therefore, the larger the amount of WsSi3 and wC being produced Fig. 2 shows the results of material pha ase an Based on the technological condition, prepared sis. From it, we can see that there is no diffraction SiC/W laminated composites in this experiment, as peak of metal w, but the diffraction peaks of WsSi, well as that combined with the results given in other SiC 35um ·,·h 20r ·叉A Fig. 2. XRD results of SiC/W laminated composites for the sandwiched-in metal w with different thicknesse
360 Y. Gao et al.rMaterials Letters 50 2001 358–363 ( ) Fig. 1. Testing direction of fracture toughness and bending strength. the direction along which the external force acts. The data given by Nos. 6 and 7 in Table 1 represents that the bending strengths at the A and B directions are almost the same. This reflects that bending strength of SiCrW laminated composites is almost isotropic, which is contrasted to fracture toughness and could provide some reference during designing of the material structure. 3.2. Analysis of material phase Fig. 2 shows the results of material phase analysis. From it, we can see that there is no diffraction peak of metal W, but the diffraction peaks of W Si 5 3 and WC do exist in the SiCrW laminated composites obtained in our preparation. Therefore, we can deduce that a chemical reaction between W and SiC occurs, and produces W Si and WC. In addition, 5 3 because the height of the diffraction peak of W Si 5 3 and WC increases with an increase of the thickness of the sandwiched-in metal, this makes it clear that the thicker the metal layer, the larger the amount of the metal W taking part in the chemical reaction, and therefore, the larger the amount of W Si and WC 5 3 being produced. Based on the technological condition, prepared SiCrW laminated composites in this experiment, as well as that combined with the results given in other Fig. 2. XRD results of SiCrW laminated composites for the sandwiched-in metal W with different thicknesses
Y Gao et al/ Materials Letters 50(2001)358-363 works [6, 7), we may see that the resultant products grains are visible, while in the latter, the grains are of the reaction of Sic and w are WsSi, and wc, clearly evident on the fracture surface respectively. This is agreement with the result deter Fig. 4 shows the cross-sectional field emission mined by XRD SEM micrographs of the interface, from which we can deduce the information enumerated below (1) The crystal type in SiC/w laminated compos- 3.3. SEM analysis of the SIC/W laminated compos- ites is that SiC and WsSi3 is in grained-crystal form while WC is in sliced-crystal form. Moreover, WsSi3 crystallite is bigger than SiC crystallite(see A-D in From the energy distribution analysis displayed in (2) There exhibits the secondary laminated struc- Fig. 3, we could deduce that in the sandwiched-in re in SiC/w laminated composites, which is metal, W-sliced crystal is carbide. Combined with formed through overlapping of these sliced crystals XRD analysis, we can infer that the 'sliced crystal wC, and there are chinks between the slices(see vC, while the 'grained cry WSi, in th A-F in Fig. 4) layer. Here, 'sliced crystal or'grained crystal'means (3)In SiC/W laminated composites, the fractur- those structures pictured in Fig. 4, in the former, no form of base layer (Sic layer)is along the 506.0708090 MC:11 Avolt: 15.00( kev) Quantity Result Atoic Nel Inten AF Corr 4931 Fig. 3. The EDAX results of sliced crystal
Y. Gao et al.rMaterials Letters 50 2001 358–363 ( ) 361 works 6,7 , we may see that the resultant products w x of the reaction of SiC and W are W Si and WC, 5 3 respectively. This is agreement with the result determined by XRD. 3.3. SEM analysis of the SICrW laminated composites From the energy distribution analysis displayed in Fig. 3, we could deduce that in the sandwiched-in metal, W-sliced crystal is carbide. Combined with XRD analysis, we can infer that the ‘sliced crystal’ is WC, while the ‘grained crystal’ is W Si in this 5 3 layer. Here, ‘sliced crystal’ or ‘grained crystal’ means those structures pictured in Fig. 4, in the former, no grains are visible, while in the latter, the grains are clearly evident on the fracture surface. Fig. 4 shows the cross-sectional field emission SEM micrographs of the interface, from which we can deduce the information enumerated below. Ž . 1 The crystal type in SiCrW laminated composites is that SiC and W Si is in grained-crystal form, 5 3 while WC is in sliced-crystal form. Moreover, W Si 5 3 crystallite is bigger than SiC crystallite see A–D in Ž Fig. 4 .. Ž . 2 There exhibits the secondary laminated structure in SiCrW laminated composites, which is formed through overlapping of these sliced crystals WC, and there are chinks between the slices see Ž A–F in Fig. 4 .. Ž . 3 In SiCrW laminated composites, the fracturing form of base layer SiC layer is along the Ž . Fig. 3. The EDAX results of sliced crystal
Y Gao et al./ Ma (a) W 非6g825 WcL 68826 85° WaSi Si 非6983 番6g834 6148 8sn° cture surface sweeping photos for SiC/W laminated composite
362 Y. Gao et al.rMaterials Letters 50 2001 358–363 ( ) Fig. 4. Fracture surface sweeping photos for SiCrW laminated composites
Y Gao et al/ Materials Letters 50(2001)358-363 crystalline planes. However, the fracture of the sand- References wiched-in layer has two types: one with the crack along the WsSi3 interface, and another with the crack that crosses through the sliced crystal(see [Il K. Clegg, w.J. Kendallk et al., Nature (London) 347(4) A-F in Fig 4) 2]H. Liu, S M. Hsu, J. Am. Ceram. Soc. 79(9)(1996)2452 In conclusion, analyzing the microscopic structure and the fracture features for the SiC/w laminate BY Huang, Acta Metall. Sin. 9(1996)479-484 composites shows that fracture toughness is clearly 4]Z Chen, J.J. Mecholsky, J. Mater. Res. 8(9)(1993)2362- improved by prolonging the crack halt, depleting the [5]Z Chen, J.J. Mecholsky, J. Mater Res. 11(8)(1996)2035 energy when the crystal slices are withdrawn out [6] Kubashewski et al, Metallurgical Thermochemistry. Perga- from the secondary laminated structure, and prevent on, Oxford, 1976, p. 303 ing the crack expansion using micro-cracking in the [7 F. Goesman, R. Schmid-Fetzer, Mater. Sci. Eng, B 34(1995) secondary laminated structure 224-231
Y. Gao et al.rMaterials Letters 50 2001 358–363 ( ) 363 crystalline planes. However, the fracture of the sandwiched-in layer has two types: one with the crack along the W Si interface, and another with the 5 3 crack that crosses through the sliced crystal see Ž A–F in Fig. 4 .. In conclusion, analyzing the microscopic structure and the fracture features for the SiCrW laminated composites shows that fracture toughness is clearly improved by prolonging the crack halt, depleting the energy when the crystal slices are withdrawn out from the secondary laminated structure, and preventing the crack expansion using micro-cracking in the secondary laminated structure. References w x 1 K. Clegg, W.J. Kendallk et al., Nature London 347 4 Ž . Ž. Ž . 1990 455. w x 2 H. Liu, S.M. Hsu, J. Am. Ceram. Soc. 79 9 1996 2452– Ž .Ž . 2457. w x 3 Y. Huang, Acta Metall. Sin. 9 1996 479–484. Ž . w x 4 Z. Chen, J.J. Mecholsky, J. Mater. Res. 8 9 1993 2362– Ž.Ž . 2369. w x 5 Z. Chen, J.J. Mecholsky, J. Mater. Res. 11 8 1996 2035. Ž .Ž . w x 6 Kubashewski et al., Metallurgical Thermochemistry. Pergamon, Oxford, 1976, p. 303. w x 7 F. Goesman, R. Schmid-Fetzer, Mater. Sci. Eng., B 34 1995 Ž . 224–231