《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Fracture behavior of laminated SiC composites

CERAMICS INTERNATIONAL SEVIER Ceramics International 30(2004)697-703 www.elsevier.com/locate/ceramint Fracture behavior of laminated SiC composites J.X. Zhang, D L. Jiang Sh.Y. Qin, Zh. R. Huang te Key Laboratory of High Performance Ceramics and Superfine Structure hai Institute of Ceramics hinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Received 25 May 2003: received in revised form 19 June 2003; accepted 25 July 2003 Available online 17 March 2004 Abstract Experimental investigation of the fexural properties of SiC/C laminates had been conducted. Effect of the interfacial thickness and compo- sition on the mechanical properties of SiC laminates was characterized. The diffusion of elements from adjacent Sic layers in the carbon-based interfacial layers was studied. Accordingly, the optimal thickness and composition of interfacial layers were determined experimentally o2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved eywords: B. Composites, C. Mechanical properties; C. Fracture; D SiC 1. Introduction The individual layers of laminates with weak interface are usually fabricated by extrusion [5,6, 8, 211, slip casting[19, Active research and development have been conducted electrophoretic deposition [22], non-aqueous tape casting to seek the application of ceramics as structural com- [10, 12, 14, 15, 17, 18, 20] and laminated object manufacturing ponents, however, the lack of damage tolerance abil 231, no paper had ever reported the using of aqueous tape has been a critical problem. It was reported that casting process. As far as we know, the interface layer of continuous-fiber-reinforced ceramic composites(CFCC) laminates are prepared by coating [5,6, 8, 10, 121, spraying are the most promising candidates and have shown a [21], electrophoretic deposition [22], screening [14]and tape non-brittle response [l], but the fabrication of these ad- casting [12, 15, 17, 18] vanced structural materials for high temperature applica- Understanding the fracture behavior of multilayered com- tions is time-consuming and expensive. The concept of posites requires the study of interface crack propagation laminated composite for improved performance of brittle mechanism[24-29]. Phillipps et al. [7, 301, Folsom [31, 32], materials is well established [2]. Such structure is found in and others proposed models of the fracture behavior of ce- many biological hard tissues, such as mollusk shell B3, 4] ramic laminates in bending test. The reliability and fric- and teeth. Clegg and co-workers [5-8] have produced lam- tional energy dissipation in laminates were also investigated inated Sic with graphite interface layers by rolling the [33-35 dough like powder mixture into sheets, coating the sheets Phillipps et al. proposed that the specimen size, in with graphite followed by laminating and sintering. These cluding the beam length, the interfacial toughness, the multilayer SiC composites showed stepped stress-strain lamina thickness will play important roles on the appar- behavior with the apparent toughness and work of fracture ent toughness, work of fracture of laminated composites as 15 MPam/ and 4625 Jm-, respectively. Multilayered [8]. In the present paper, SiC/C laminated composites ceramic materials with weak inter face have been evaluated were prepared through aqueous tape casting and lami- in other ceramic systems, such as Sic/Sic 191, Si]N4/Bn nation process. The properties of the thin C-SiC inter 10-15] or Al2O3/LaPO4 [16], etc. Recently, it was shown facial layers were adjusted by varying the composition that laminated composites without weak interfaces also of the C through the addition of SiC. Influence of the exhibited damage-tolerant behavior [9, 17-201 composition and thickness of interfacial layers on the me- chanical properties of Sic multilayers was studied. The s Corresponding author. microstructure and fracture behavior were investigated E-mailaddress:jxzh@yahoo.com(D.L.Jiang). 0272-8842/$30.00 0 2004 Elsevier Ltd and Techna Group S r l. All rights reserved doi:10.1016/ ceramist2003.07.016

Ceramics International 30 (2004) 697–703 Fracture behavior of laminated SiC composites J.X. Zhang, D.L. Jiang∗, Sh.Y. Qin, Zh.R. Huang The State Key Laboratory of High Performance Ceramics and Superfine Structure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Received 25 May 2003; received in revised form 19 June 2003; accepted 25 July 2003 Available online 17 March 2004 Abstract Experimental investigation of the flexural properties of SiC/C laminates had been conducted. Effect of the interfacial thickness and compo￾sition on the mechanical properties of SiC laminates was characterized. The diffusion of elements from adjacent SiC layers in the carbon-based interfacial layers was studied. Accordingly, the optimal thickness and composition of interfacial layers were determined experimentally. © 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Composites; C. Mechanical properties; C. Fracture; D. SiC 1. Introduction Active research and development have been conducted to seek the application of ceramics as structural com￾ponents; however, the lack of damage tolerance abil￾ity has been a critical problem. It was reported that continuous-fiber-reinforced ceramic composites (CFCC) are the most promising candidates and have shown a non-brittle response [1], but the fabrication of these ad￾vanced structural materials for high temperature applica￾tions is time-consuming and expensive. The concept of laminated composite for improved performance of brittle materials is well established [2]. Such structure is found in many biological hard tissues, such as mollusk shell [3,4] and teeth. Clegg and co-workers [5–8] have produced lam￾inated SiC with graphite interface layers by rolling the dough like powder mixture into sheets, coating the sheets with graphite followed by laminating and sintering. These multilayer SiC composites showed stepped stress–strain behavior with the apparent toughness and work of fracture as 15 MPa m1/2 and 4625 J m−2, respectively. Multilayered ceramic materials with weak interface have been evaluated in other ceramic systems, such as SiC/SiC [9], Si3N4/BN [10–15] or Al2O3/LaPO4 [16], etc. Recently, it was shown that laminated composites without weak interfaces also exhibited damage-tolerant behavior [9,17–20]. ∗ Corresponding author. E-mail address: jx zh@yahoo.com (D.L. Jiang). The individual layers of laminates with weak interface are usually fabricated by extrusion [5,6,8,21], slip casting [19], electrophoretic deposition [22], non-aqueous tape casting [10,12,14,15,17,18,20] and laminated object manufacturing [23], no paper had ever reported the using of aqueous tape casting process. As far as we know, the interface layer of laminates are prepared by coating [5,6,8,10,12], spraying [21], electrophoretic deposition [22], screening [14] and tape casting [12,15,17,18]. Understanding the fracture behavior of multilayered com￾posites requires the study of interface crack propagation mechanism [24–29]. Phillipps et al. [7,30], Folsom [31,32], and others proposed models of the fracture behavior of ce￾ramic laminates in bending test. The reliability and fric￾tional energy dissipation in laminates were also investigated [33–35]. Phillipps et al. proposed that the specimen size, in￾cluding the beam length, the interfacial toughness, the lamina thickness will play important roles on the appar￾ent toughness, work of fracture of laminated composites [8]. In the present paper, SiC/C laminated composites were prepared through aqueous tape casting and lami￾nation process. The properties of the thin C–SiC inter￾facial layers were adjusted by varying the composition of the C through the addition of SiC. Influence of the composition and thickness of interfacial layers on the me￾chanical properties of SiC multilayers was studied. The microstructure and fracture behavior were investigated too. 0272-8842/$30.00 © 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2003.07.016

J.x. Zhang et al. /Ceramics International 30(2004)697-703 3000 3000 illustration of (a) e Interfacial thickness( m)F wing the notch orientation he layered Work of Fracture 2. Processing, mechanical testing and characterization methods 2.1. Lamination process Thickness of interfacial layers (um) Silicon carbide sheets were prepared using commercial Fig. 2. Influence of the thickness of interfacial layers on the mechanical SiC powders(FCP-15, Norton com. )as starting materi- properties of Sic laminates 6620.2. Fig. 3. Fractured surface of the laminated SiC composites(a) interfacial layer,(b) left layer, (c)right layer

698 J.X. Zhang et al. / Ceramics International 30 (2004) 697–703 (a) (b) Fig. 1. Schematic illustration of (a) the specimen orientations for three points bending, (b) a notched bend bar showing the notch orientation with respect to the layered structure. 2. Processing, mechanical testing and characterization methods 2.1. Lamination process Silicon carbide sheets were prepared using commercial SiC powders (FCP-15, Norton com.) as starting materi￾Fig. 3. Fractured surface of the laminated SiC composites (a) interfacial layer, (b) left layer, (c) right layer. Fig. 2. Influence of the thickness of interfacial layers on the mechanical properties of SiC laminates

J.x. Zhang et al. /Ceramics International 30(2004)697-703 699 als and Y203(2.85 wt %)and Al203(2.15%)as sintering (SENB) method on test bars of 4 mm high, 3 mm wide and additives. Details concerning the aqueous tape casting pro- 36 mm long by a three-point bend using a span of 30mm cess could be obtained from previous published papers and a cross head speed of 0.05 mm-min. A straight notch [36, 37]. The thickness of the Sic green tapes was adjusted with fine diamond saw was introduced with a depth at about to -150 um To introduce weak interface between SiC lay- 2 mm. Work of fracture was obtained by dividing the area un ers, thin C-SiC sheets were also prepared by tape casting der the load-displacement curve by twice the cross-section process using doctor blade equipment. The C powders(Op- area of the sample [381 tical pure, Shanghai Carbon Element Factory) were firstly dispersed in deionized water followed by the addition of Sic powders with controlled composition. PVAl788 was 23. Microstructure characterization selected as binder and glycerol as plasticizer. The thickness of C sheets was adjusted to 30 um. Much thinner layer Microstructure characterization was performed by op- were also prepared by the so-called"screen printing"pro- cal microscopy and SEM. Energy dispersive X-ray cess. The surface of each SiC sheets was coated by passing (EDX)spectra were obtained across the interfacial layer the aforementioned C slurries through a 200-mesh screen. to determine the diffusion of elements from adjacent 46-20 um range through the adjustment of solid content of characterize Crack pattern of fractured surface was also The thickness of the interfacial layers was varied in the said slurry. After coating, the sheets were dried and stacked in the repeating sequence of SiC and C-sic layers Subsequently, organic additives were removed by heating to 800C in a flowing argon atmosphere. a slow heating rate was selected to minimize bloating and cracking during py- rolysis, which might result in distortion of the layers. After pyrolysis, the billets were placed in a BN-coated graphite die. Consolidation was performed by hot pressing at 35 MPa under an argon atmosphere at temperature of 1850C for 0.5h 2. 2. Mechanical and microstructure evaluation of Denection Specimens for flexural test were cut and ground into rect- angular bars. A schematic illustration of bending test process for laminated SiC composites is shown in Fig. 1. Three-point bending strength was determined at room temperature on five to six 3 mm x 4 mm x 36 mm bars with a span of 30 mm and a cross head speed of 0.5- min-.Apparent toughness was measured by the single-edge-notched-beam 0.10.2030.40.5 Denection(mm) p→ Fracture toughness 5 三品 s Content of C(wt%)/12 6 20 0.30.4 Content of C in interfacial layers(wt%) Deflection (mm) Fig. 4. Influence of interfacial composition on the mechanical properties Fig. 5. Load-deflection curves of SiC laminates(a)SiC30-C70,(b) of Sic laminates SiC40-C60,(c)SiC50-C50

J.X. Zhang et al. / Ceramics International 30 (2004) 697–703 699 als and Y2O3 (2.85 wt.%) and Al2O3 (2.15%) as sintering additives. Details concerning the aqueous tape casting pro￾cess could be obtained from previous published papers [36,37]. The thickness of the SiC green tapes was adjusted to ∼150
m. To introduce weak interface between SiC lay￾ers, thin C–SiC sheets were also prepared by tape casting process using doctor blade equipment. The C powders (Op￾tical pure, Shanghai Carbon Element Factory) were firstly dispersed in deionized water followed by the addition of SiC powders with controlled composition. PVA1788 was selected as binder and glycerol as plasticizer. The thickness of C sheets was adjusted to ∼30
m. Much thinner layers were also prepared by the so-called “screen printing” pro￾cess. The surface of each SiC sheets was coated by passing the aforementioned C slurries through a 200-mesh screen. The thickness of the interfacial layers was varied in the 5–20
m range through the adjustment of solid content of the said slurry. After coating, the sheets were dried and stacked in the repeating sequence of SiC and C–SiC layers. Subsequently, organic additives were removed by heating to 800 ◦C in a flowing argon atmosphere. A slow heating rate was selected to minimize bloating and cracking during py￾rolysis, which might result in distortion of the layers. After pyrolysis, the billets were placed in a BN-coated graphite die. Consolidation was performed by hot pressing at 35 MPa under an argon atmosphere at temperature of 1850 ◦C for 0.5 h. 2.2. Mechanical and microstructure evaluation of laminated composites Specimens for flexural test were cut and ground into rect￾angular bars. A schematic illustration of bending test process for laminated SiC composites is shown in Fig. 1. Three-point bending strength was determined at room temperature on five to six 3 mm × 4 mm × 36 mm bars with a span of 30 mm and a cross head speed of 0.5 mm·min−1. Apparent toughness was measured by the single-edge-notched-beam 20 40 60 80 0 250 500 750 1000 20 40 60 0 1500 3000 4500 Work of fracture ( J.m-2) Content of C (wt%) Strength Fracture toughness Content of C in interfacial layers (wt%) Strength (MPa) 6 8 10 12 14 16 18 Fracture toughness (MPa.s-1 ) Fig. 4. Influence of interfacial composition on the mechanical properties of SiC laminates. (SENB) method on test bars of 4 mm high, 3 mm wide and 36 mm long by a three-point bend using a span of 30 mm and a cross head speed of 0.05 mm·min−1. A straight notch with fine diamond saw was introduced with a depth at about 2 mm. Work of fracture was obtained by dividing the area un￾der the load–displacement curve by twice the cross-section area of the sample [38]. 2.3. Microstructure characterization Microstructure characterization was performed by op￾tical microscopy and SEM. Energy dispersive X-ray (EDX) spectra were obtained across the interfacial layer to determine the diffusion of elements from adjacent layers. The crack pattern of fractured surface was also characterized. Fig. 5. Load–deflection curves of SiC laminates (a) SiC30-C70, (b) SiC40-C60, (c) SiC50-C50

J.x. Zhang et al/Ceramics International 30(2004)697-703 3. Results and discussion Table 1 Mechanical properties of interfacial layers 3.1. Interfacial thickness Sample SiC30-C70 SiC50-C50 SiC60-C40 733 1567 Efects of thickness of interfacial layers on the strength, Fracture toughness(MPam/2)0.30 parent toughness and work of fracture of Sic samples are Work of fracture (-) 35.4 shown in Fig. 2 The strength of SiC samples shows an increase trend with the decrease of the thickness of interfacial layers; while the 3.2. Interfacial composition and mechanical properties apparent toughness and work of fracture exhibit a differ- ent character(see Fig. 2). When the interfacial thickness The properties of interfacial layers are adjusted by con- is 3 um, SiC laminates exhibit a catastrophic fracture be- trolling their composition Effects of interfacial composition havior; on the other hand, when the interfacial thickness is on the properties of SiC laminates are shown in Fig 4.The higher than 25 um, graceful failure occurs, the mechanical C-SiC bulk samples with the same composition as interfa- properties of Sic laminates drop altogether. An optimal in- cial layers were also sintered under the same condition for terfacial thickness is shown to be around 10 um(see Fig. 2). comparison. The mechanical properties of these C-SiC bulk The microstructure of fractured surface of SiC samples samples are shown in Table 1 is shown in Fig. 3. The belt between parallel lines is the As shown in Table 1, the addition of SiC powders has very interfacial layer, see Fig. 3(a), the left side(marked as"b) limited effects on the mechanical properties of C-Sic bulk and right side(marked as"")are shown in Fig 3(b)and samples. However, as shown in Fig 4, the mechanical prop- (c), respectively. The SiC layers(left side and right side)are erties of SiC laminates exhibit a considerable dependence well densified. However, it is difficult to detect exactly the on the composition of interfacial layers: the flexural strength layers. This may be due to the diffusion of elements from layers, similar to that of Si3N4 samples [13, 14]. The li,? interface between the interfacial layers and the adjacent decreases constantly with the increase of C in interfac Sic layers, which will be discussed later ture toughness and work of fracture first show an increase 250 (a)SiC40-C60 (b)SiC40-C60 250um 50um (c)SiC50-C50 d) siC60-C40 Fig. 6. Optical micrographs showing the side surface of fexural specimens containing (a)40 vol %,(b)40 vol %,(c)50 vol %, (d)60 vol. SiC in the

700 J.X. Zhang et al. / Ceramics International 30 (2004) 697–703 3. Results and discussion 3.1. Interfacial thickness Effects of thickness of interfacial layers on the strength, apparent toughness and work of fracture of SiC samples are shown in Fig. 2. The strength of SiC samples shows an increase trend with the decrease of the thickness of interfacial layers; while the apparent toughness and work of fracture exhibit a differ￾ent character (see Fig. 2). When the interfacial thickness is ∼3
m, SiC laminates exhibit a catastrophic fracture be￾havior; on the other hand, when the interfacial thickness is higher than 25
m, graceful failure occurs, the mechanical properties of SiC laminates drop altogether. An optimal in￾terfacial thickness is shown to be around 10
m (see Fig. 2). The microstructure of fractured surface of SiC samples is shown in Fig. 3. The belt between parallel lines is the interfacial layer, see Fig. 3(a), the left side (marked as “b”) and right side (marked as “c”) are shown in Fig. 3(b) and (c), respectively. The SiC layers (left side and right side) are well densified. However, it is difficult to detect exactly the interface between the interfacial layers and the adjacent SiC layers. This may be due to the diffusion of elements from SiC layers, which will be discussed later. Fig. 6. Optical micrographs showing the side surface of flexural specimens containing (a) 40 vol.%, (b) 40 vol.%, (c) 50 vol.%, (d) 60 vol.% SiC in the interfacial layers (after testing). Table 1 Mechanical properties of interfacial layers Sample SiC30-C70 SiC50-C50 SiC60-C40 Strength (MPa) 7.33 15.67 19.33 Fracture toughness (MPa m1/2) 0.30 – 0.34 Work of fracture (J m−2) 10 33.3 35.4 3.2. Interfacial composition and mechanical properties The properties of interfacial layers are adjusted by con￾trolling their composition. Effects of interfacial composition on the properties of SiC laminates are shown in Fig. 4. The C–SiC bulk samples with the same composition as interfa￾cial layers were also sintered under the same condition for comparison. The mechanical properties of these C–SiC bulk samples are shown in Table 1. As shown in Table 1, the addition of SiC powders has very limited effects on the mechanical properties of C–SiC bulk samples. However, as shown in Fig. 4, the mechanical prop￾erties of SiC laminates exhibit a considerable dependence on the composition of interfacial layers: the flexural strength decreases constantly with the increase of C in interfacial layers, similar to that of Si3N4 samples [13,14]. The frac￾ture toughness and work of fracture first show an increase

J.x. Zhang et al. /Ceramics International 30(2004)697-703 trend with the increase of content of C in interfacial lay- sition cannot be well correlated with the results shown in ers, up to approximately 50 vol % and decrease thereafter. Table 1. The only possible reason is the diffusion of ele- For the interfacial composition of 50 vol. SiC +50 vol. ments(Al,Y, Si, etc. )from adjacent SiC layers, as discussed C, the strength, work of fracture and fracture toughness later was 580MPa, 4282J-m-2, and KIC=10.8 MPa-m/2,re- The nominal stress on the tensile surface is d versus pectively. This strong dependence on interfacial compo- ross head deflection of notched specimens with varied C Interfacial layers X-stage(mm) Fig. 7. Diffusion of Al2O3, and Y203 in interfacial layers(a) schematic illustration, (b),(c),(d),(e) EDX characterization

J.X. Zhang et al. / Ceramics International 30 (2004) 697–703 701 trend with the increase of content of C in interfacial lay￾ers, up to approximately 50 vol.%, and decrease thereafter. For the interfacial composition of 50 vol.% SiC + 50 vol.% C, the strength, work of fracture and fracture toughness was 580 MPa, 4282 J·m−2, and KIC = 10.8 MPa·m1/2, re￾spectively. This strong dependence on interfacial compo￾Fig. 7. Diffusion of Al2O3, and Y2O3 in interfacial layers (a) schematic illustration, (b), (c), (d), (e) EDX characterization. sition cannot be well correlated with the results shown in Table 1. The only possible reason is the diffusion of ele￾ments (Al, Y, Si, etc.) from adjacent SiC layers, as discussed later. The nominal stress on the tensile surface is plotted versus cross head deflection of notched specimens with varied C

J.x. Zhang et al. /Ceramics International 30(2004)697-703 content(Fig. 5). Cracks are deflected in the interfacial layers rience a transition from graceful to catastrophic mode: de- until the Sic content is increased to 80 vol. lamination may still occur, but the failure of the subsequent The micrographs of SiC laminates are shown in Fig. 6. lamina will be determined by the delamination cracks across It is observed that some cracks in SiC layers initiate from the interface. In this case, the mechanical properties of Sic the growth of defects within the sic lamina, while other laminates are irrelevant to the thickness of the interfacial cracks from the growth of the delaminaton cracks propagat layers too. Further reduction in the thickness of interfacial ing along the interfacial layers. The latter would eliminate layers may result in a monolithic fracture mode. Clegg [6 the fracture toughness and WOF although graceful failure related this kind of behavior to the existence of gaps in the may still occur. During the failure process, frictional sliding interfacial layers. However, this explanation is not applica- at the debonded interface may also take effect [ 35], which ble to our case cannot be detected from the optical micrographs( Fig. 6) The influence of interfacial composition on the mechani- cal properties of SiC laminates can also be well correlated to 3.3. Diffusion effect the interfacial diffusion mechanism. The higher the content of c in interfacial layers, the weaker the interfacial layers As proposed by Cook and Gordon [2] and Clegg et al. after diffusion, consequently, the lower the strength. This 5, 61, the weak interfacial layers were deliberately intro- explanation is also applicable to the fracture toughness and duced as a toughening mechanism for deflecting growin work of fracture racks. Studies on the fracture behavior of laminated com- posites also showed that the critical ratio of strength and 4. Conclusion apparent toughness between interfacial layers and the adja cent matrix layers for crack deflection were relatively low The mechanical properties of Sic laminates depend on [2, 7, 8, 25, 26, 28, 31,32]. So it can be proposed that the inter- the thickness and composition of interfacial layers. For facial layers should be as thin as possible as long as these the interfacial composition of 50 vol % Sic +50 vol %C cracks could be deflected. However, specimens with the in- the strength, work of fracture and fracture toughness was terfacial thickness as 3 um did not show ure behavior (see Fig. 2).This might be due to the diffusion tively. Based on SEM and EDX observation, this depen- effects During sintering or other high temperature process, dence is mainly due to the diffusion of elements(Al,Y,si, elements in adjacent layers will diffuse to the interfacial etc. )from adjacent SiC layers layers due to the composition difference. In this case, the residual stress will not be a significant factor due to the very limited strength and thickness of interfacial layers Acknowledgements To observe clearly the diffusion phenomena, samples The authors were grateful to the Science and see Fig. 7(aH(e). A schematic illustration of the diffusion ogy Committee of Shanghai for providing support under the phenomena is shown in Fig. 7(a) contract number 02Dj14065 As shown in Fig. 7. the distribution of Al. Y. Si etc. el- ements across the interfacial layers exhibits a similar trend References suggesting some diffusion of these elements from adjacent SiC layers. However, due to the low content of sintering dditives(AlO3 and Y2O3), the EDX curves show a high [I A.G. Evans, Perspective on the development of high-toughness ce- ramics, J. Am. Ceram Soc. 73(2)(1990)187-206. undulation near the interface. It is difficult to characterize [2]J. Cook, J.E. Gordon, A mechanism for the control of crack prop- exactly the diffusion distance of elements(Al,Y, Si, etc. )in ion in all-brittle materials, Proc. R. Soc. Lond. A282(1964) nterfacial layers. Based on estimation from Fig. 7, the dif- 508-520. fusion distance should be less than 10 um. Assuming 10 um 3IV.J. Laraia, A.H. Heuer, Novel composite microstructure and me- as the distance for diffusion of si. al and y into the interfa chanical behavior of mollusk shell, J. Am. Ceram. Soc. 72(11) (1989)2177-2179 cial layers, then the proper interfacial layer thickness should [4]J D. Currey, A.J. Ko ture in the crossed-lamellar structure of be about 20 um. For thicker interfacial layers(>>20 um) us shells, J. Mat such diffusion has negligible effect on the mechanical prop- 5] WJ. Clegg, K. Kene alford T.W. Button, J.D. Birchall, A erties of interfacial layers, SiC laminates might exhibit a cs, Nature(London)347(1990) 455-457 pronounced interfacial delamination failure behavior though on and failure of laminar ceramic compos- the mechanical properties are largely reduced. In this case ites, Acta Metall. Mater. 40(11)(1992)3085-3093 he mechanical properties of Sic laminates is irrelevant to [7AJ. Phillipps, wJ T.W. Clyne, Fracture behavior of ceramic the thickness of the interfacial layers. On the other hand laminates in bending-l. Modeling of crack propagation, Acta Metall. for very thin interfacial layers(<20 um), the diffusion of Mater..41(3)(1993)805-817 elements(Al, Y, Si, etc. )will make the interfacial layers [8]AJ. Phillipps, WJ. Clegg, T.w. Clyne, The correlation of interfacial d macroscopic toughness in SiC laminates, Composites 24(2) strong, therefore, the failure of Sic laminates may expe (1993)166-176

702 J.X. Zhang et al. / Ceramics International 30 (2004) 697–703 content (Fig. 5). Cracks are deflected in the interfacial layers until the SiC content is increased to 80 vol.%. The micrographs of SiC laminates are shown in Fig. 6. It is observed that some cracks in SiC layers initiate from the growth of defects within the SiC lamina, while other cracks from the growth of the delaminaton cracks propagat￾ing along the interfacial layers. The latter would eliminate the fracture toughness and WOF although graceful failure may still occur. During the failure process, frictional sliding at the debonded interface may also take effect [35], which cannot be detected from the optical micrographs (Fig. 6). 3.3. Diffusion effect As proposed by Cook and Gordon [2] and Clegg et al. [5,6], the weak interfacial layers were deliberately intro￾duced as a toughening mechanism for deflecting growing cracks. Studies on the fracture behavior of laminated com￾posites also showed that the critical ratio of strength and apparent toughness between interfacial layers and the adja￾cent matrix layers for crack deflection were relatively low [2,7,8,25,26,28,31,32]. So it can be proposed that the inter￾facial layers should be as thin as possible as long as these cracks could be deflected. However, specimens with the in￾terfacial thickness as ∼3
m did not show the graceful fail￾ure behavior (see Fig. 2). This might be due to the diffusion effects. During sintering or other high temperature process, elements in adjacent layers will diffuse to the interfacial layers due to the composition difference. In this case, the residual stress will not be a significant factor due to the very limited strength and thickness of interfacial layers. To observe clearly the diffusion phenomena, samples with thick interfacial layers are characterized by EDX, see Fig. 7(a)–(e). A schematic illustration of the diffusion phenomena is shown in Fig. 7(a). As shown in Fig. 7, the distribution of Al, Y, Si, etc. el￾ements across the interfacial layers exhibits a similar trend, suggesting some diffusion of these elements from adjacent SiC layers. However, due to the low content of sintering additives (Al2O3 and Y2O3), the EDX curves show a high undulation near the interface. It is difficult to characterize exactly the diffusion distance of elements (Al, Y, Si, etc.) in interfacial layers. Based on estimation from Fig. 7, the dif￾fusion distance should be less than 10
m. Assuming 10
m as the distance for diffusion of Si, Al, and Y into the interfa￾cial layers, then the proper interfacial layer thickness should be about 20
m. For thicker interfacial layers ( 20
m), such diffusion has negligible effect on the mechanical prop￾erties of interfacial layers, SiC laminates might exhibit a pronounced interfacial delamination failure behavior though the mechanical properties are largely reduced. In this case, the mechanical properties of SiC laminates is irrelevant to the thickness of the interfacial layers. On the other hand, for very thin interfacial layers ( 20
m), the diffusion of elements (Al, Y, Si, etc.) will make the interfacial layers “strong,” therefore, the failure of SiC laminates may expe￾rience a transition from graceful to catastrophic mode: de￾lamination may still occur, but the failure of the subsequent lamina will be determined by the delamination cracks across the interface. In this case, the mechanical properties of SiC laminates are irrelevant to the thickness of the interfacial layers too. Further reduction in the thickness of interfacial layers may result in a monolithic fracture mode. Clegg [6] related this kind of behavior to the existence of gaps in the interfacial layers. However, this explanation is not applica￾ble to our case. The influence of interfacial composition on the mechani￾cal properties of SiC laminates can also be well correlated to the interfacial diffusion mechanism. The higher the content of C in interfacial layers, the weaker the interfacial layers after diffusion, consequently, the lower the strength. This explanation is also applicable to the fracture toughness and work of fracture. 4. Conclusion The mechanical properties of SiC laminates depend on the thickness and composition of interfacial layers. For the interfacial composition of 50 vol.% SiC + 50 vol.% C, the strength, work of fracture and fracture toughness was 580 MPa, 4282 J·m−2, and KIC = 10.8 MPa·m1/2, respec￾tively. Based on SEM and EDX observation, this depen￾dence is mainly due to the diffusion of elements (Al, Y, Si, etc.) from adjacent SiC layers. Acknowledgements The authors were grateful to the Science and Technol￾ogy Committee of Shanghai for providing support under the contract number 02DJ14065. References [1] A.G. Evans, Perspective on the development of high-toughness ce￾ramics, J. Am. Ceram. Soc. 73 (2) (1990) 187–206. [2] J. Cook, J.E. Gordon, A mechanism for the control of crack prop￾agation in all-brittle materials, Proc. R. Soc. Lond. A282 (1964) 508–520. [3] V.J. Laraia, A.H. Heuer, Novel composite microstructure and me￾chanical behavior of mollusk shell, J. Am. Ceram. Soc. 72 (11) (1989) 2177–2179. [4] J.D. Currey, A.J. Kohn, Fracture in the crossed-lamellar structure of conus shells, J. Mater. Sci. 11 (1976) 1615–1623. [5] W.J. Clegg, K. Kendall, N. McAlford, T.W. Button, J.D. Birchall, A simple way to make tough ceramics, Nature (London) 347 (1990) 455–457. [6] W.J. Clegg, The fabrication and failure of laminar ceramic compos￾ites, Acta Metall. Mater. 40 (11) (1992) 3085–3093. [7] A.J. Phillipps, W.J. Clegg, T.W. Clyne, Fracture behavior of ceramic laminates in bending-I. Modeling of crack propagation, Acta. Metall. Mater. 41 (3) (1993) 805–817. [8] A.J. Phillipps, W.J. Clegg, T.W. Clyne, The correlation of interfacial and macroscopic toughness in SiC laminates, Composites 24 (2) (1993) 166–176

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