MATERIALS LETTE.S ELSEVIER Materials Letters 46(2000)65-69 www.elsevier.com/locate/matlet Mechanical properties and fracture behavior of Al,O3 laminates with different architectures Jihong She a ", Takahiro Inoue, Masato Suzuki, Satoshi Sodeoka, Kazuo Ueno Institute of Materials Research, German Aerospace Center(DLR, D-51147 Cologne, Germany Department of Energy Conersion, Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received 18 April 2000, accepted 28 April 2000 Abstract Fibrous, multilayered and hybrid Al,O3 laminates were fabricated by extrusion and hot-pressing techniques. The effects of different architectures on the mechanical properties and fracture behavior are investigated. It is shown that hybrid laminates may provide a considerable flexibility and tailorability in mechanical behavior. An alternative arrangement of the fibrous and monolithic layers in hybrid laminates gives an average strength of up to 450 MPa, an apparent toughness of 11 MPa m/ and a fracture energy in excess of 2600 J/m2, with a non-catastrophic fracture behavior. 02000 Elsevier Science B v. All rights reserved PACS:81.05e;8105Mh8120.Hy,81.40.Np6220Mk Keywords: Mechanical properties; Fracture; Al,2O3; Laminates; Delamination 1. ntroduction Following Coblenz's method, Baskaran et al. ( 2-4 fabricated a variety of fibrous ceramics such as It has previously been shown that brittle ceramics Sic/C, SiC/BN and Si, N4/BN. Similarly, Clegg can be toughened by the introduction of crack-de- et al. [5, 6] developed multilayer SiC/C ceramics flecting interfaces. A well-known example is fiber Lately, multilayer Si, NA/BN [7, 8]ceramics were reinforced ceramic composites. However, expensive also successfully fabricated. Due to the deflection of ceramic fibers and some time-consuming processes cracks at the weak c or bn interfaces these fibrous such as chemical vapor deposition and or chemi- or layered ceramics exhibited a fracture behavior cal vapor deposition are required for the fabrication similar to that of fiber-reinforced cer of such composites. To overcome this limitation, Ites Coblenz [1] demonstrated a new concept of"fibrous ceramics" which can be manufactured from com- The objective of the present work is to show that a non-catastrophic fracture behavior can be attained mercially available ceramic powders via conven- for fibrous, multilayered and hybrid Al2O3 lami- tional ceramic- and polymer-processing technology nates, which were fabricated by stacking the mon lithic and/ or fibrous Al,, layers in different fash- Corresponding author. Tel: +49-2203-6012560; fax: +45 ion The effects of different architectures on the 2203-696480 mechanical properties and fracture behavior were E-mail address: jhshe hotmail com(. She) investigated 00167-577X/00/S-see front matter o 2000 Elsevier Science B V. All rights reserved 144-0
November 2000 Materials Letters 46 2000 65–69 Ž . www.elsevier.comrlocatermatlet Mechanical properties and fracture behavior of Al O laminates 2 3 with different architectures Jihong She a,), Takahiro Inoue b , Masato Suzuki b , Satoshi Sodeoka b , Kazuo Ueno b a Institute of Materials Research, German Aerospace Center DLR , D-51147 Cologne, Germany ( ) b Department of Energy ConÕersion, Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received 18 April 2000; accepted 28 April 2000 Abstract Fibrous, multilayered and hybrid Al O laminates were fabricated by extrusion and hot-pressing techniques. The effects 2 3 of different architectures on the mechanical properties and fracture behavior are investigated. It is shown that hybrid laminates may provide a considerable flexibility and tailorability in mechanical behavior. An alternative arrangement of the fibrous and monolithic layers in hybrid laminates gives an average strength of up to 450 MPa, an apparent toughness of ;11 MPa m1r2 and a fracture energy in excess of 2600 Jrm2 , with a non-catastrophic fracture behavior. q 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Je; 81.05.Mh; 81.20.Hy; 81.40.Np; 62.20.Mk Keywords: Mechanical properties; Fracture; Al O ; Laminates; Delamination 2 3 1. Introduction It has previously been shown that brittle ceramics can be toughened by the introduction of crack-deflecting interfaces. A well-known example is fiberreinforced ceramic composites. However, expensive ceramic fibers and some time-consuming processes such as chemical vapor deposition andror chemical vapor deposition are required for the fabrication of such composites. To overcome this limitation, Coblenz 1 demonstrated a new concept of w x Afibrous ceramicsB, which can be manufactured from commercially available ceramic powders via conventional ceramic- and polymer-processing technology. ) Corresponding author. Tel: q49-2203-6012560; fax: q49- 2203-696480. E-mail address: jhshe@hotmail.com J. She . Ž . Following Coblenz’s method, Baskaran et al. 2–4 w x fabricated a variety of fibrous ceramics such as SiCrC, SiCrBN and Si N3 4rBN. Similarly, Clegg et al. 5,6 developed multilayer SiC w x rC ceramics. Lately, multilayer Si N rBN 7,8 ceramics were w x 3 4 also successfully fabricated. Due to the deflection of cracks at the weak C or BN interfaces, these fibrous or layered ceramics exhibited a fracture behavior similar to that of fiber-reinforced ceramic composites. The objective of the present work is to show that a non-catastrophic fracture behavior can be attained for fibrous, multilayered and hybrid Al O lami- 2 3 nates, which were fabricated by stacking the monolithic andror fibrous Al O layers in different fash- 2 3 ion. The effects of different architectures on the mechanical properties and fracture behavior were investigated. 00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0167- 577X 00 00144-0 Ž
J. She et al./ Materials Letters 46(2000)65-69 2. Experimental procedure 3. Results and discussion Fig. I shows the cross-sections of three represen- Al2O, sheets of 0.35 mm in thickness and Al O3 tative laminates, where the grey regions are Al., fibers of 0.5 mm in diameter were prepared by extrusion using a submicron a-Al2O, powder(0.22 and the dark regions correspond to the SiC-contain- um, TM-D, Taimei Chemicals, Japan). To introduce ing interphase. As shown. the fibrous and or mono- fibrous layers into laminated composites, the Al,O fibers were uniaxially arranged into rectangular sheets with fibers aligned in the length direction. Subse (a) quently, the monolithic and fibrous sheets were coated by spraying with a 65 vol. SiC-containing slurry. Fibrous, multilayered and hybrid laminates were assembled from these coated sheets in different fashion. Fibrous laminates were formed by stacking the fibrous Al2O3 sheets, while multilayered lami- nates were composed of the monolithic Al,O, sheets In hybrid laminates, every two monolithic Al,O rr sheets were followed by two fibrous Al,O, sheets ot pressing was performed in vacuum under a 400 pressure of 25 MPa at 1500C for I h, and a low heating rate of 5 C/min was selected below 600C to pyrolyze the polymers that were used for extru- sion. After hot pressing, the specimens were cut and ground into rectangular bars of 3 mm (width)x 4 nm(height)X 40 mm (length), with the prospective tensile surface normal to the hot-pressing direction Density was measured by water displacement. Flexural strength was determined by a three-point bending test with a support distance of 30 mm at a cross-head speed of 0.5 mm/min. The tensile sur- faces were polished and the edges were chamfered Fracture toughness was measured by a four-point SENB technique with an inner span of 10 mm and an outer span of 30 mm at a loading rate of 0.1 mm/min. A straight notch was introduced at the center part of the test bar using a 100-km-wide diamond blade, and the notch depth was about 1.5 mm. Strength and toughness measurements were conducted in a universal testing machine with the tensile surface perpendicular to the direction of the applied load. Data were recorded with a computer- ized data-acquisition syst obtained from toughness measurements by dividing the area under the load-displacement curve by the ross-section area of the notched bar Cross-sections and fracture surfaces were observed by optical and Fig. 1. Cross-sections of (a)fibrous, (b)multilayered and(c) scanning electron microscopy, respectively hybrid Al,O, laminates
66 J. She et al.rMaterials Letters 46 2000 65–69 ( ) 2. Experimental procedure Al O sheets of 0.35 mm in thickness and Al O 23 23 fibers of 0.5 mm in diameter were prepared by extrusion using a submicron a-Al O powder 0.22 Ž 2 3 mm, TM-D, Taimei Chemicals, Japan . To introduce . fibrous layers into laminated composites, the Al O2 3 fibers were uniaxially arranged into rectangular sheets with fibers aligned in the length direction. Subsequently, the monolithic and fibrous sheets were coated by spraying with a 65 vol.% SiC-containing slurry. Fibrous, multilayered and hybrid laminates were assembled from these coated sheets in different fashion. Fibrous laminates were formed by stacking the fibrous Al O sheets, while multilayered lami- 2 3 nates were composed of the monolithic Al O sheets. 2 3 In hybrid laminates, every two monolithic Al O2 3 sheets were followed by two fibrous Al O sheets. 2 3 Hot pressing was performed in vacuum under a pressure of 25 MPa at 15008C for 1 h, and a low heating rate of 58Crmin was selected below 6008C to pyrolyze the polymers that were used for extrusion. After hot pressing, the specimens were cut and ground into rectangular bars of 3 mm width Ž .=4 mm height Ž. Ž. =40 mm length , with the prospective tensile surface normal to the hot-pressing direction. Density was measured by water displacement. Flexural strength was determined by a three-point bending test with a support distance of 30 mm at a cross-head speed of 0.5 mmrmin. The tensile surfaces were polished and the edges were chamfered. Fracture toughness was measured by a four-point SENB technique with an inner span of 10 mm and an outer span of 30 mm at a loading rate of 0.1 mmrmin. A straight notch was introduced at the center part of the test bar using a 100-mm-wide diamond blade, and the notch depth was about 1.5 mm. Strength and toughness measurements were conducted in a universal testing machine with the tensile surface perpendicular to the direction of the applied load. Data were recorded with a computerized data-acquisition system. Fracture energy was obtained from toughness measurements by dividing the area under the load–displacement curve by the cross-section area of the notched bar. Cross-sections and fracture surfaces were observed by optical and scanning electron microscopy, respectively. 3. Results and discussion Fig. 1 shows the cross-sections of three representative laminates, where the grey regions are Al O2 3 and the dark regions correspond to the SiC-containing interphase. As shown, the fibrous andror monoFig. 1. Cross-sections of a fibrous, b multilayered and c Ž. Ž. Ž. hybrid Al O laminates. 2 3
J. She et al./ Materials Letters 46(2000)65-69 lithic Al,O, elements are more or less separated by the continuous SiC interphase of 10 um in ness. Owing to the deformation of the soft green fibers along the compression axis during hot press- ing, the fibrous Al,O, elements appear as a roughly elliptic shape with an aspect of 2. Furthermore, Sem observations revealed that the fibrous and monolithic Al,O3 elements were almost free of pores, but a lot of pores were presented in the Sic inter- phase due to the poor sinterability of SiC. During grinding and polishing, such an interphase is easy to remove, and thus appears as grooves on the polished surfaces Fig. 2 shows the load-displacement curves of fibrous, multilayered and hybrid Al,O3 laminates under three-point bending tests. As illustrated, all the laminates exhibit a"graceful "fracture behavior, with an appearance of stepwise load decreases after the ak load this should be attributed to shear delani- ation along the weak Sic interphase in the direction parallel to the tensile surface, as described in detail elsewhere [9]. The flexural strengths of fibrous, mul- tilayered and hybrid Al,2O, laminates were calcu- ated from the maximum loads to be 283 + 41, 632 ±8and424±46MPa, respectively. Beyond the load maximum, the load-bearing ability is strongly NRTO 595mm的 dependent on the architectures of the laminates. As can be seen in Fig. 2, about 66-74% and 54-82% of their peak loads are retained for fibrous and hybrid AL,O3 laminates, but only 17-24% of its peak load is achieved for multilayered laminates after the first load drop. This is considered to be associated with Fibrous Multilayered NR I49725K Fig. 3. Fracture surfaces of (a) fibrous, (b)multilayered and(c) hybrid Al,O, laminates. the degree of shear delamination. During tests, long 00.1020.30.40.50.6 delamination cracks were observed between almost Crosshead Displacement (mm) every layer in fibrous and hybrid laminates, but Fig. 2. Load-displacement curves of fibrous, multilayered and between only several layers in multilayered lami- hybrid Al,O3 laminates under three-point bending tests nates. Fig. 3 shows the fracture surfaces of fibrous
J. She et al.rMaterials Letters 46 2000 65–69 ( ) 67 lithic Al O elements are more or less separated by 2 3 the continuous SiC interphase of ;10 mm in thickness. Owing to the deformation of the soft green fibers along the compression axis during hot pressing, the fibrous Al O elements appear as a roughly 2 3 elliptic shape with an aspect of ;2. Furthermore, SEM observations revealed that the fibrous and monolithic Al O elements were almost free of pores, 2 3 but a lot of pores were presented in the SiC interphase due to the poor sinterability of SiC. During grinding and polishing, such an interphase is easy to remove, and thus appears as grooves on the polished surfaces. Fig. 2 shows the load–displacement curves of fibrous, multilayered and hybrid Al O laminates 2 3 under three-point bending tests. As illustrated, all the laminates exhibit a AgracefulB fracture behavior, with an appearance of stepwise load decreases after the peak load. This should be attributed to shear delamination along the weak SiC interphase in the direction parallel to the tensile surface, as described in detail elsewhere 9 . The flexural strengths of fibrous, mul- w x tilayered and hybrid Al O laminates were calcu- 2 3 lated from the maximum loads to be 283"41, 632 "8 and 424"46 MPa, respectively. Beyond the load maximum, the load-bearing ability is strongly dependent on the architectures of the laminates. As can be seen in Fig. 2, about 66–74% and 54–82% of their peak loads are retained for fibrous and hybrid Al O laminates, but only 17–24% of its peak load 2 3 is achieved for multilayered laminates after the first load drop. This is considered to be associated with Fig. 2. Load–displacement curves of fibrous, multilayered and hybrid Al O laminates under three-point bending tests. 2 3 Fig. 3. Fracture surfaces of a fibrous, b multilayered and c Ž. Ž. Ž. hybrid Al O laminates. 2 3 the degree of shear delamination. During tests, long delamination cracks were observed between almost every layer in fibrous and hybrid laminates, but between only several layers in multilayered laminates. Fig. 3 shows the fracture surfaces of fibrous
J. She et al./ Materials Letters 46(2000)65-69 multilayered and hybrid Al,O3 laminates. Clearly, delamination occurs mostly at the interfaces between the fibrous Al,O3 layers or between the fibrous and monolithic layers, but to a lesser extent at the inter- Un-notche aces between the monolithic Al,O3 layers. As a Notched result, the load-carrying ability of fibrous and hybrid Al2O, laminates after the drop of the peak load is much higher than that of multilayered laminates Fig 4 shows the load-displacement responses of notched specimens in bending tests. Evidently, the fracture behaviors of notched specin to those of unnotched specimens, except that the Crosshead Displacement( peak load is relatively low for notched specimens Fig. 5. Load-displacement curves of notched and unnotched due to the decrease of the net cross-section. When specimens for a hybrid Al,O, laminate with alternating fibre tests were stopped, the total displacements were up to 0.8 and 0.7 mm for fibrous and hybrid laminates but only 0.36 mm for multilayered laminates. After and without a notch on the tensile surface. As ex testing, all the specimens did not fall apart. The pected, both specimens fail in a non-catastrophi apparent"toughness calculated from the maximum manner, with significant load-retaining capability af- load was 10.4+0.2 and 14.5+ 1. 1 MPa m/2 for ter the first fracture event. The flexural strength hybrid and multilayered Al,O3 laminates, but only apparent toughness and fracture energy were mea 6.0+0.5 MPa m/2for fibrous laminates. The frac- sured to be 451+5 MPa, 10.9+1. 1 MPa m/2 and ture energy measured from the area under the load- 2625+260 J/m2, respectively. The notable increase displacement curve was 349+ 109,1503# 34 and in fracture energy is probably due to extensive inter- 1904+99J/m- for fibrous, multilayered and hybrid facial delamination. As shown in Fig. 6, delamina- Al,O3 laminates, respectively tion occurs at all the interfaces between the fibrous In order to examine the mechanical characteristics and monolithic layers. This may not only cause a of hybrid laminates, another specimen was prepared high energy absorption during fracture, but also giv in such a way that the fibrous and monolithic Al2 O3 a large specimen deflection during tests. For exam- ayers were alternately stacked. Fig. 5 presents the ple, the notched specimen remains in one even load-displacement curves such a laminate after a total displacement of l mm Fig. 6. Fracture surface of a hy brid Al,O, laminate, showin Fig. 4. Load-displacement responses of fibrous, multilayered and pronounced delamination at the interfaces between the fibrous and hybrid Al,O3 laminates with a notched tensile surface
68 J. She et al.rMaterials Letters 46 2000 65–69 ( ) multilayered and hybrid Al O laminates. Clearly, 2 3 delamination occurs mostly at the interfaces between the fibrous Al O layers or between the fibrous and 2 3 monolithic layers, but to a lesser extent at the interfaces between the monolithic Al O layers. As a 2 3 result, the load-carrying ability of fibrous and hybrid Al O laminates after the drop of the peak load is 2 3 much higher than that of multilayered laminates. Fig. 4 shows the load–displacement responses of notched specimens in bending tests. Evidently, the fracture behaviors of notched specimens are similar to those of unnotched specimens, except that the peak load is relatively low for notched specimens due to the decrease of the net cross-section. When tests were stopped, the total displacements were up to 0.8 and 0.7 mm for fibrous and hybrid laminates, but only 0.36 mm for multilayered laminates. After testing, all the specimens did not fall apart. The AapparentB toughness calculated from the maximum load was 10.4"0.2 and 14.5"1.1 MPa m1r2 for hybrid and multilayered Al O laminates, but only 2 3 6.0"0.5 MPa m1r2 for fibrous laminates. The fracture energy measured from the area under the load– displacement curve was 1349"109, 1503"134 and 1904"99 Jrm2 for fibrous, multilayered and hybrid Al O laminates, respectively. 2 3 In order to examine the mechanical characteristics of hybrid laminates, another specimen was prepared in such a way that the fibrous and monolithic Al O2 3 layers were alternately stacked. Fig. 5 presents the load–displacement curves of such a laminate with Fig. 4. Load–displacement responses of fibrous, multilayered and hybrid Al O laminates with a notched tensile surface. 2 3 Fig. 5. Load–displacement curves of notched and unnotched specimens for a hybrid Al O laminate with alternating fibrous 2 3 and monolithic layers. and without a notch on the tensile surface. As expected, both specimens fail in a non-catastrophic manner, with significant load-retaining capability after the first fracture event. The flexural strength, apparent toughness and fracture energy were measured to be 451"5 MPa, 10.9"1.1 MPa m1r2 and 2625"260 Jrm2 , respectively. The notable increase in fracture energy is probably due to extensive interfacial delamination. As shown in Fig. 6, delamination occurs at all the interfaces between the fibrous and monolithic layers. This may not only cause a high energy absorption during fracture, but also give a large specimen deflection during tests. For example, the notched specimen remains in one piece even after a total displacement of ;1 mm. Fig. 6. Fracture surface of a hybrid Al O laminate, showing 2 3 pronounced delamination at the interfaces between the fibrous and monolithic layers
J. She et al./ Materials Letters 46(2000)65-69 In view of the above results. it is obvious that the were measured to be 451 MPa. 10.9 MPa m /2 and hybrid laminate concept may offer a considerable 2625 J/'m, respectively. These properties have flexibility and tailorability in mechanical properties. rarely been observed in oxide laminates As explored in this work, hybrid Al,O3 laminates with some suitable arrangements of the fibrous and monolithic layers, can have an unique combination Acknowledgements of reasonable strength values and"tough" fracture Jihong She would like to thank the Agency of gate the mechanical properties and fracture behavior Industrial Science and Technology (AIST), Ministry of hybrid Al,O, laminates with different volume of International Trade and Industry (MITD) for grant fractions of the fibrous and monolithic layers ing him an AIST Research Fellowship at Osaka National research Institute 4. Conclusions References Fibrous, multilayered and hybrid Al2O3 laminates [1] w.S. Coblenz, US Patent 4 772 524, 20 September 1988 were fabricated by extrusion and hot pressing. The [2]SBaskaran, S.D. Nunn, D. Popovic,JW.Halloran,. Am mechanical properties and fracture behavior were Ceran.Soc.76(1993)2209 evaluated in bending with and without a notch on the [3] S. Baskaran, J W. Halloran, J. Am. Ceram. Soc. 76(1993) tensile surface. It is shown that the hybrid laminate concept may offer a new way to fabricate laminated 4]S. Baskaran, J W. Halloran, J. Am. Ceram. Soc. 77(1994) composites with unique and tailored properties. In [5]wJ. Clegg, K. Kendall, N.M. Alford, T.W. Button,JD this work, a novel Al,O, laminate was developed by Birchall Nature 347(1990)455 alternately stacking the fibrous and monolithic lay- 6 w.J. Clegg, Acta Metall. Mater. 40(1992)3085 ers. This hybrid laminate exhibits a non-catastrophic [7] H. Liu, S M. Hsu, J. Am. Ceram. Soc. 79(1996)2452 fracture behavior. The flexural strength, apparent [ 8] D Kovar, M D. Thouless, J.W.Halloran,J.Am. CeramSoc 81(1998)1004 toughness and fracture energy of such a laminate [9] J.H. She, T. Inoue, KUeno, Mater. Lett. 42(2000)155
J. She et al.rMaterials Letters 46 2000 65–69 ( ) 69 In view of the above results, it is obvious that the hybrid laminate concept may offer a considerable flexibility and tailorability in mechanical properties. As explored in this work, hybrid Al O laminates, 2 3 with some suitable arrangements of the fibrous and monolithic layers, can have an unique combination of reasonable strength values and AtoughB fracture behavior. Further work is clearly needed to investigate the mechanical properties and fracture behavior of hybrid Al O laminates with different volume 2 3 fractions of the fibrous and monolithic layers. 4. Conclusions Fibrous, multilayered and hybrid Al O laminates 2 3 were fabricated by extrusion and hot pressing. The mechanical properties and fracture behavior were evaluated in bending with and without a notch on the tensile surface. It is shown that the hybrid laminate concept may offer a new way to fabricate laminated composites with unique and tailored properties. In this work, a novel Al O laminate was developed by 2 3 alternately stacking the fibrous and monolithic layers. This hybrid laminate exhibits a non-catastrophic fracture behavior. The flexural strength, apparent toughness and fracture energy of such a laminate were measured to be 451 MPa, 10.9 MPa m1r2 and 2625 Jrm2 , respectively. These properties have rarely been observed in oxide laminates. Acknowledgements Jihong She would like to thank the Agency of Industrial Science and Technology AIST , Ministry Ž . of International Trade and Industry MITI for grant- Ž . ing him an AIST Research Fellowship at Osaka National Research Institute. References w x 1 W.S. Coblenz, US Patent 4 772 524, 20 September 1988. w x 2 S. Baskaran, S.D. Nunn, D. Popovic, J.W. Halloran, J. Am. Ceram. Soc. 76 1993 2209. Ž . w x 3 S. Baskaran, J.W. Halloran, J. Am. Ceram. Soc. 76 1993 Ž . 2217. w x 4 S. Baskaran, J.W. Halloran, J. Am. Ceram. Soc. 77 1994 Ž . 1249. w x 5 W.J. Clegg, K. Kendall, N.M. Alford, T.W. Button, J.D. Birchall, Nature 347 1990 455. Ž . w x 6 W.J. Clegg, Acta Metall. Mater. 40 1992 3085. Ž . w x 7 H. Liu, S.M. Hsu, J. Am. Ceram. Soc. 79 1996 2452. Ž . w x 8 D. Kovar, M.D. Thouless, J.W. Halloran, J. Am. Ceram. Soc. 81 1998 1004. Ž . w x 9 J.H. She, T. Inoue, K. Ueno, Mater. Lett. 42 2000 155. Ž
