Availableonlineatwww.sciencedirect.com ScienceDirect materials letters ELSEVIER Materials Letters 61(2007)551-555 www.elsevier.com/locate/matlet Fracture toughness of alumina /lanthanum titanate laminate composites with a weak interface Ali Ceylan a, *, Paul A. Fuierer b Department of Materials Science and Engineering, Anadolu University, Eskisehir: Turkey b Department of Materials and Metallurgical Engineering. New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA Received 19 July 2005: accepted 5 May 2006 Available online 6 June 2006 Abstract Layers of lanthanum titanate (La,O,)and a-alumina(a-AlO3) were employed to form a layered composite in order to improve the fracture toughness of monolithic alumina. The composites were produced by two different processing methods. In the first, individually presintered pellets of a-Al2O3 and La2Ti2O7 were stacked together and hot-forged. In the second, tape cast molten salt La2Ti2O7 and dense a-Al2O3 were stacked together and hot-forged. The forged composite samples were investigated by optical microscopy, scanning electron microscopy (SEM), Vickers indentation and three-point bending. During the hot-forging process, an interphase, aluminum titanate(Al2TiOs)was found to form as a result of the reaction between a-Al2O3 and La] Ti2O7. The flexural strength and the fracture toughness of the resulting laminate composites were found to be 320 MPa and 7. 1 MPa m 2, respectively. Indentation experiments showed that the newly formed Al TiOs at the interface is sufficiently weak to promote crack deflection and hence increase the fracture energy and mechanical properties of the composite. o 2006 Elsevier B V. All rights reserved. Keywords: Alumina; Lanthanum titanate; Interphase; Layered composite; Fracture toughness 1. Introduction AlPO4/AlO3[12]. It has been shown that the fracture toughness can be enhanced by introducing inter-layers or interfaces that are Single crystal or high density sintered polycrystalline a-Al2O3 sufficiently weak to deflect cracks, yet strong enough to give has remarkable bending and tensile strength in comparison with useful compressive and the shear properties [ 13]. Laminar com- other refractory single oxide ceramics [1]. However, just like all posites can be synthesized by a variety of processing schemes, ceramics, a-Al O3 has the tendency to show catastrophic fracture including tape casting, sequential slip casting, electrophoteric behavior rather than gradual fracture under applied stresses deposition and colloidal techniques [14-17 ure [o oughness of a-Al2O3 Here, we present a new combination of oxides with poten- ceramics in the presence of randomly dispersed second phases tial as a tough, high temperature oxide composite, a-Al2O3/ has been reported [2]. Ceramic/ceramic lamellar structures have La2Ti2O7. Lanthanum titanate is a refractory oxide with a emerged in the last few years as promising candidates to over- layered perovskite structure, which has previously been shown come the inherent brittleness of the ceramics for use in structural to exhibit a pronounced, platelet grain structure when hot-forged applications. These structures bring a unique opportunity for [18, 19]. Lanthanum titanate was chosen as the second oxide due loring the mechanical properties by stacking layers of dif- to the cleavage planes that exist between perovskite slabs in the ferent thickness and composition in a suitable sequence [3]. structure. It was thought that if properly oriented, these planes Many composite systems have been studied so far, such as Sic/c could function as crack deflection sites similar to micaceous [4, 5], SiC/SiC [6], Al2O3/Al2O3[7], Si3 N4/BN [8], Si3 N4/Si3 N4 cleavage. In a recent paper [201, we reported the reaction whiskers [9], ZrO2 toughened Al2O3[101, Al2O3/SiC [ll], and thermodynamics of mixtures of Al2O3 and La2Ti2O7. An inter- mediate phase, AlTiOs was found to form at temperatures Corresponding author above 1100 oC. the in situ formation of aluminum titanate E-mail address: aceylan2(@ anadolu.edu. tr(A. Ceylan). (Al2TiOs) in pressed and sintered disks, as well as forged 0167-577X/S-see front matter o 2006 Elsevier B V. All rights reserved. doi:10.1016/ malet200605.0
Fracture toughness of alumina/lanthanum titanate laminate composites with a weak interface Ali Ceylan a,⁎, Paul A. Fuierer b a Department of Materials Science and Engineering, Anadolu University, Eskisehir, Turkey b Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA Received 19 July 2005; accepted 5 May 2006 Available online 6 June 2006 Abstract Layers of lanthanum titanate (La2Ti2O7) and α-alumina (α-Al2O3) were employed to form a layered composite in order to improve the fracture toughness of monolithic alumina. The composites were produced by two different processing methods. In the first, individually presintered pellets of α-Al2O3 and La2Ti2O7 were stacked together and hot-forged. In the second, tape cast molten salt La2Ti2O7 and dense α-Al2O3 were stacked together and hot-forged. The forged composite samples were investigated by optical microscopy, scanning electron microscopy (SEM), Vickers indentation and three-point bending. During the hot-forging process, an interphase, aluminum titanate (Al2TiO5) was found to form as a result of the reaction between α-Al2O3 and La2Ti2O7. The flexural strength and the fracture toughness of the resulting laminate composites were found to be 320 MPa and 7.1 MPa m1/2, respectively. Indentation experiments showed that the newly formed Al2TiO5 at the interface is sufficiently weak to promote crack deflection and hence increase the fracture energy and mechanical properties of the composite. © 2006 Elsevier B.V. All rights reserved. Keywords: Alumina; Lanthanum titanate; Interphase; Layered composite; Fracture toughness 1. Introduction Single crystal or high density sintered polycrystalline α-Al2O3 has remarkable bending and tensile strength in comparison with other refractory single oxide ceramics [1]. However, just like all ceramics, α-Al2O3 has the tendency to show catastrophic fracture behavior rather than gradual fracture under applied stresses. Some improvement in the fracture toughness of α-Al2O3 ceramics in the presence of randomly dispersed second phases has been reported [2]. Ceramic/ceramic lamellar structures have emerged in the last few years as promising candidates to overcome the inherent brittleness of the ceramics for use in structural applications. These structures bring a unique opportunity for tailoring the mechanical properties by stacking layers of different thickness and composition in a suitable sequence [3]. Many composite systems have been studied so far, such as SiC/C [4,5], SiC/SiC [6], Al2O3/Al2O3 [7], Si3N4/BN [8], Si3N4/Si3N4 whiskers [9], ZrO2 toughened Al2O3 [10], Al2O3/SiC [11], and AlPO4/Al2O3 [12]. It has been shown that the fracture toughness can be enhanced by introducing inter-layers or interfaces that are sufficiently weak to deflect cracks, yet strong enough to give useful compressive and the shear properties [13]. Laminar composites can be synthesized by a variety of processing schemes, including tape casting, sequential slip casting, electrophoteric deposition and colloidal techniques [14–17]. Here, we present a new combination of oxides with potential as a tough, high temperature oxide composite, α-Al2O3/ La2Ti2O7. Lanthanum titanate is a refractory oxide with a layered perovskite structure, which has previously been shown to exhibit a pronounced, platelet grain structure when hot-forged [18,19]. Lanthanum titanate was chosen as the second oxide due to the cleavage planes that exist between perovskite slabs in the structure. It was thought that if properly oriented, these planes could function as crack deflection sites similar to micaceous cleavage. In a recent paper [20], we reported the reaction thermodynamics of mixtures of Al2O3 and La2Ti2O7. An intermediate phase, Al2TiO5 was found to form at temperatures above 1100 °C. The in situ formation of aluminum titanate (Al2TiO5) in pressed and sintered disks, as well as forged Materials Letters 61 (2007) 551–555 www.elsevier.com/locate/matlet ⁎ Corresponding author. E-mail address: aceylan2@anadolu.edu.tr (A. Ceylan). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.05.021
A Ceylan, P.A. Fuierer/Materials Letters 61(2007)551-55 electron microscope(Hitachi HI-Scan HHS-2R). Examination of the interface between laminates was done with optical(Uni- tron Model 7297) and scanning electron microscopes hot-forged lamina Mechanical properties of the composites were determined sing the three-point bend test(Engineering Systems CT5)and an accepted method for the measurement of strength and toughness hol-forged Al, O, of ceramics was employed [21-23]. This method involves im- plantation of an artificial flaw on the surface of a flexure specimen in a three-point bending. A Vickers indentation was used to create the flaw Indented specimens were loaded into the 3-point fixture with 30 mm span, taking care that the indent was loaded in tension directly under the middle load pin. It was necessary to measure the 110012001300 Temperature°C) ubsequent loading in response to the load and the residual stress field associated with the indentation [23] Fig. 1. Densities of conventionally sintered pellets, hot-forged pellets and hot- The flexural strength was calculated using the standard formula Flexural strength=3FL 2Wh2 (1) laminates, was confirmed by X-ray diffraction and energy dis- pensive spectroscopy [20]. It is believed that both La]Ti2O where Fis the fracture load, L is the distance between the two outer lavage planes and the Al TiOs interphase layer have the points,W is the width of the specimen, and h is the height of the potential to enhance crack deflection and to improve the tough- specimen. The modulus of elasticity in bending or the flexural ness of laminate composite. The present paper reports the effects modulus, was calculated by the following formula [23]: mechanical properties of a-Al,O,/La Ti,0, layered composites. Flexural modulus=- FZ3 of hot-forging and the formed interface between laminates on where d is the deflection of the beam when a force Fis applied. Starting materials were powders of alumina(at-Al2O32 ( cture toughness was calculated by the following formula 2. Experimental procedure 99.999, Sigma-Aldrich and titania(TiO2299.999, Cerac). AIC-059/E)1/8 99.999%, Sigma-Aldrich) and lanthanum titanate La203> A/(ap/3)3A phase lanthanum titanate. Individual pellets were prepared from where E is the flexural modulus, Hv is the vicke ers hardness, ac alumina and lanthanum titanate, co-pressed together, and then co- is the flexure strength, and P is the indentation load.The sintered. Two different methods were employed to produce lami- Vickers hardness was calculated according to the formula (231 (60 to 65% of theoretical density) pellets of alumina(AO)and Hv=1.854P nate composites by hot-forging(HF). First, individually presintered lanthanum titanate(Lt) were alternately stacked and hot-forged at two different temperatures(1300 and 1400C). In the second where 2a is the length of Vickers indentation diagonal. The method, the lanthanum titanate layer was produced by tape casting Vickers hardness of the composite was calculated by taking the of aqueous slurries of molten salt synthesized powder [18-20 These tapes were stacked together with dense alumina in an 500 LT La altemating fashion and hot-forged at temperatures ranging from LO Lanthanum oxide 0411LT 1300 to 1500C. The hot-forging operation involved loading . T Aluminum titanate 212AO samples between zirconia rams in a vertical clamshell fumace, 4001 AO Alumina heating at 9C/min to the soak(max) temperature, gradually loading to a maximum of 20 MPa pressure, holding for 3 h, and unloading in 30 min, followed by furnace cooling. Densities were measured by the Archimedes method, using 20 water as the submerging fluid. The composites were sectioned to expose the plane parallel to the hot-forging axis(a plane perpendicular to the layers)and polished with a l um diamond. Microhardness indentation and crack propagation tests were performed using a Leco M400 hardness tester, with 1 kg load for 20 S A Vickers pyramid was indented to both the polished 2 Theta surfaces of Al2O3 and LazTiz07 near the interface region. Fig. 2. XRD result taken from the laminate composite cross-section, plane Observation of the indent and crack was done with scanning parallel to the HF axis(hot-forged at 1400C)
laminates, was confirmed by X-ray diffraction and energy dispersive spectroscopy [20]. It is believed that both La2Ti2O7 cleavage planes and the Al2TiO5 interphase layer have the potential to enhance crack deflection and to improve the toughness of laminate composite. The present paper reports the effects of hot-forging and the formed interface between laminates on mechanical properties of α-Al2O3/La2Ti2O7 layered composites. 2. Experimental procedure Starting materials were powders of alumina (α-Al2O3≥ 99.999%, Sigma-Aldrich) and lanthanum titanate (La2O3≥ 99.999, Sigma-Aldrich and titania (TiO2≥99.999, Cerac.). Lanthanum oxide and titania were prereacted to produce single phase lanthanum titanate. Individual pellets were prepared from alumina and lanthanum titanate, co-pressed together, and then cosintered. Two different methods were employed to produce laminate composites by hot-forging (HF). First, individually presintered (60 to 65% of theoretical density) pellets of alumina (AO) and lanthanum titanate (LT) were alternately stacked and hot-forged at two different temperatures (1300 and 1400 °C). In the second method, the lanthanum titanate layer was produced by tape casting of aqueous slurries of molten salt synthesized powder [18–20]. These tapes were stacked together with dense alumina in an alternating fashion and hot-forged at temperatures ranging from 1300 to 1500 °C. The hot-forging operation involved loading samples between zirconia rams in a vertical clamshell furnace, heating at 9 °C/min to the soak (max) temperature, gradually loading to a maximum of 20 MPa pressure, holding for 3 h, and unloading in 30 min, followed by furnace cooling. Densities were measured by the Archimedes method, using water as the submerging fluid. The composites were sectioned to expose the plane parallel to the hot-forging axis (a plane perpendicular to the layers) and polished with a 1 μm diamond. Microhardness indentation and crack propagation tests were performed using a Leco M400 hardness tester, with 1 kg load for 20 s. A Vickers pyramid was indented to both the polished surfaces of Al2O3 and La2Ti2O7 near the interface region. Observation of the indent and crack was done with scanning electron microscope (Hitachi HI-Scan HHS-2R). Examination of the interface between laminates was done with optical (Unitron Model 7297) and scanning electron microscopes. Mechanical properties of the composites were determined using the three-point bend test (Engineering Systems CT5) and an accepted method for the measurement of strength and toughness of ceramics was employed [21–23]. This method involves implantation of an artificial flaw on the surface of a flexure specimen in a three-point bending. AVickers indentation was used to create the flaw. Indented specimens were loaded into the 3-point fixture with 30 mm span, taking care that the indent was loaded in tension directly under the middle load pin. It was necessary to measure the initial crack size, since the crack would extend stably during subsequent loading in response to the load and the residual stress field associated with the indentation [23]. The flexural strength was calculated using the standard formula Flexural strength ¼ 3FL 2W h2 ð1Þ whereF is the fracture load, L is the distance between the two outer points, W is the width of the specimen, and h is the height of the specimen. The modulus of elasticity in bending or the flexural modulus, was calculated by the following formula [23]: Flexural modulus ¼ FL3 2Wdh3 ð2Þ where δ is the deflection of the beam when a force F is applied. Fracture toughness was calculated by the following formula [23]: KIC ¼ 0:59 E Hv 1=8 ðrcP1=3 Þ 3=4 ð3Þ where E is the flexural modulus, Hv is the Vickers hardness, σc is the flexure strength, and P is the indentation load. The Vickers hardness was calculated according to the formula [23]: Hv ¼ 1:854P ð2aÞ 2 ð4Þ where 2a is the length of Vickers indentation diagonal. The Vickers hardness of the composite was calculated by taking the Fig. 1. Densities of conventionally sintered pellets, hot-forged pellets and hotforged composites. Fig. 2. XRD result taken from the laminate composite cross-section, plane parallel to the HF axis (hot-forged at 1400 °C). 552 A. Ceylan, P.A. Fuierer / Materials Letters 61 (2007) 551–555
A. Ceylan, PA. Fuierer /Materials Letters 61(2007)551-555 (b) 42O3 New Pha 100um200pm 1. a2T12O7 Fig 3.(a)Optical and(b) SEM micrographs taken from the plane parallel to the HF axis from tape cast La2Ti2O7 and dense AlO, sheets, hot-forged at 1400C. mean value of 10 different Vickers hardness value from 10 dif- pattern taken from the plane parallel to the hF axis from a laminate ferent regions of the laminated compo forged at 1400C. Panels a and b of Fig 3 are the optical and scanning electron micrographs respectively taken from that same plane. Fig 4a 3. Results and discussion and b shows the dot-mapping results for Al and Ti respectively. One can easily see the new interphase formation between A Hot-forging was found to increase the densities of both alumina and La2Ti2O7 laminates. According to these results, this newly formed lanthanum titanate significantly, as illustrated in Fig. 1. At a hot-forging interphase was found to be certainly Al2 TiOs. The reaction layer temperature of 1400oC, the alumina reached 98.5% and lanthanum thickness ranged from 10 um to several hundred microns, depending nate 99.4% of theoretical densities. The densities of the composites on sample type and forging conditions. Forged samples made from are between hot-forged alumina and lanthanum titanate. There was also dense and tape cast lanthanum titanate exhibited thinner reaction layers a corresponding increase in hardness values when compared to con- as seen in Fig. 5. The densified alumina limited the reactivity between ventionally sintered samples. oxides at the high forging temperatures, as opposed to the pellet stacks In our previous paper [20], it was shown that Al,O3 and Laz T1,O, which utilized only partially sintered alumina. always react to form an additional phase, whether heated as powder Fig 5a shows a Vickers indentation into the alumina forming radial mixtures, co-sintered as pellets or co-forged. This newly formed phase cracks near the interface region. Fig 5b shows a Vickers indentation was identified to be Al?TiOs, which formed over the entire range of into the lanthanum titanate side. The motion of radial cracks that from 1100 to 1500C. Fig. 2 shows the XRd propagate towards the interface identifies whether the interface is weak Fig. 4. EDS dot-map of the laminate composite surface seen in Fig. 5.(a)taken for aluminum(Al) and (b)taken for titanium (Ti)
mean value of 10 different Vickers hardness value from 10 different regions of the laminated composite. 3. Results and discussion Hot-forging was found to increase the densities of both alumina and lanthanum titanate significantly, as illustrated in Fig. 1. At a hot-forging temperature of 1400 °C, the alumina reached 98.5% and lanthanum titanate 99.4% of theoretical densities. The densities of the composites are between hot-forged alumina and lanthanum titanate. There was also a corresponding increase in hardness values when compared to conventionally sintered samples. In our previous paper [20], it was shown that Al2O3 and La2Ti2O7 always react to form an additional phase, whether heated as powder mixtures, co-sintered as pellets or co-forged. This newly formed phase was identified to be Al2TiO5, which formed over the entire range of reaction temperatures from 1100 to 1500 °C. Fig. 2 shows the XRD pattern taken from the plane parallel to the HF axis from a laminate forged at 1400 °C. Panels a and b of Fig. 3 are the optical and scanning electron micrographs respectively taken from that same plane. Fig. 4a and b shows the dot-mapping results for Al and Ti respectively. One can easily see the new interphase formation between Al2O3 and La2Ti2O7 laminates. According to these results, this newly formed interphase was found to be certainly Al2TiO5. The reaction layer thickness ranged from 10 μm to several hundred microns, depending on sample type and forging conditions. Forged samples made from dense and tape cast lanthanum titanate exhibited thinner reaction layers as seen in Fig. 5. The densified alumina limited the reactivity between oxides at the high forging temperatures, as opposed to the pellet stacks which utilized only partially sintered alumina. Fig. 5a shows a Vickers indentation into the alumina forming radial cracks near the interface region. Fig. 5b shows a Vickers indentation into the lanthanum titanate side. The motion of radial cracks that propagate towards the interface identifies whether the interface is weak Fig. 3. (a) Optical and (b) SEM micrographs taken from the plane parallel to the HF axis from tape cast La2Ti2O7 and dense Al2O3 sheets, hot-forged at 1400 °C. Fig. 4. EDS dot-map of the laminate composite surface seen in Fig. 5, (a) taken for aluminum (Al) and (b) taken for titanium (Ti). A. Ceylan, P.A. Fuierer / Materials Letters 61 (2007) 551–555 553
A Ceylan, P.A. Fuierer/Materials Letters 61(2007)551-55 1. azli,O7 C Interlace Al O3 Fig 5 SEM micrographs of a forged laminate composite made from tape cast La2 11207 and dense Al2O sheets. (a)A Vickers indentation into the Al2O3 side and (b)a Vickers indentation into the La2 Ti,O, side of the interface. or strong. If the interface bonding is strong, the radial crack will The crack redirection is a positive sign for possible improvement in propagate through the interface region to the next layer without fracture toughness(Kic) and specific work of fracture [21]. changing its direction. If the interface is weak, the radial crack will be Only the laminates made from dense alumina and tape cast lanthanum deflected in the interface region. Both micrographs demonstrate crack titanate yielded geometries favorable for mechanical testing. Stacked- deflection and arrest at the interface region. This suggests the formation pellet laminates contained layers that were too thick and too few, and it of a weak interface between alumina and lanthanum titanate laminates. was presumed that the al TiOs reaction layer thickness was too great, and would dominate the mechanical properties. Therefore, bend bars were only machined from the dense Ao/tape cast LT composites. These test samples contained 4 layers of AlO, and 3 layers of La2Ti2O7. Dense alumina layers were each 1 mm thick, while lanthanum titanate layers were roughly 1 mm thick depending on forging conditions. E Table 1 cal properties of monolithic ceramics and AO/LT laminate processed at 400 Process Vick. hardness Flex 1300°C1400° strength toughness m2 990 kg/mm2 200 MPa 2.9 MPa m/2 001260-1350280 3.2 50 a?, Sintered 2. Temperature(C) 610-650190 AlTiOs Sintered Fig.6. The effect of forging temperature on the fracture toughness of the Al2O,/ Laminate Forged 7 La2Ti2O7 laminate compo
or strong. If the interface bonding is strong, the radial crack will propagate through the interface region to the next layer without changing its direction. If the interface is weak, the radial crack will be deflected in the interface region. Both micrographs demonstrate crack deflection and arrest at the interface region. This suggests the formation of a weak interface between alumina and lanthanum titanate laminates. The crack redirection is a positive sign for possible improvement in fracture toughness (KIC) and specific work of fracture [21]. Only the laminates made from dense alumina and tape cast lanthanum titanate yielded geometries favorable for mechanical testing. Stackedpellet laminates contained layers that were too thick and too few, and it was presumed that the Al2TiO5 reaction layer thickness was too great, and would dominate the mechanical properties. Therefore, bend bars were only machined from the dense AO/tape cast LT composites. These test samples contained 4 layers of Al2O3 and 3 layers of La2Ti2O7. Dense alumina layers were each 1 mm thick, while lanthanum titanate layers were roughly 1 mm thick depending on forging conditions. Fig. 5. SEM micrographs of a forged laminate composite made from tape cast La2Ti2O7 and dense Al2O3 sheets. (a) AVickers indentation into the Al2O3 side and (b) a Vickers indentation into the La2Ti2O7 side of the interface. Fig. 6. The effect of forging temperature on the fracture toughness of the Al2O3/ La2Ti2O7 laminate composites. Table 1 Mechanical properties of monolithic ceramics and AO/LT laminate processed at 1300 and 1400 °C Material Process Vick. hardness Flex. strength Frac. toughness 1300 °C 1400 °C Al2O3 Sintered 900 kg/mm2 990 kg/mm2 200 MPa 2.9 MPa m1/2 Forged 1200–1300 1260–1350 280 3.2 La2Ti2O7 Sintered 400 450 155 2.1 Forged 600–620 610–650 190 2.4 Al2TiO5 Sintered 200–250 107 1.2 Forged – – –– Laminate Forged – – 320 7.1 554 A. Ceylan, P.A. Fuierer / Materials Letters 61 (2007) 551–555
Fig 6 shows that a maximum toughness is expected at about [] M. Jimenez-Melendo, F. Gutierrez-Mona, A Domingez-Rodriguez, Acta 1400C. At higher temperatures, La2Ti2O7 and Al2O3 interdiffusion Mater:.48(20004715-4720 accelerated. As a result a very thick Al2TiOs reaction layer forms which 4R.A. Andrievski, J. Solid State Chem. 133(1997)249-253 begins to dominate the mechanical properties and weakens the 5]T Egama, T. Ichilkizaki, H. Tsukamoto, H. Tsunoda, T Shimoyama, IntJ omposite structure. The low mechanical strength of Al2TiOs makes Jpn.Soc. Precis.Eng.29(3)(1995)222-228. it useless for structural applications [24 However in small quantities [7 v. Winter, R. Knitter, Werkstoffwoche 98, Band VIl: Symp. 9. Keram as an interphase material, the micro-cracking tendency can be useful for Symp. 14 Simul. Keram, Wiley-VCH Verlag GmbH, 1999, pp 199-204 crack deflection and arrest by absorbing crack tip energy 8]A. Bellosi, G. De Portu, S Guicciardi, Eur. Ceram Soc. 10(4)(1992) he best a-AlO LazT12O7 laminate was found to have a fracture 307-315 toughness(Kc)of 7.1 MPa m". This is a very significant improvement [9] L. Zou, D.S. Park, B U. Cho, Y. Huang, H D. Kim, Mater. Lett. 58(10) over the toughness of the individual ceramic components(see Table 1). ( April2004)1587-1592 The fracture toughness attained is nearly triple the fracture toughnes [10] B. Basu, J. Vleugels, O. Van der Biest, J Alloys Compd. 372(1-2)(June 9 values for monolithic alumina [1]. 2004)278-284 [11]A. Demir, N. Altinkok, Compos. Sci. Technol. 64(13-14)(October 2004) 4. Conclusions [12] D.H. Kuo, W.M. Riven, Mater. Sci. Eng A Struct. Mater. Prop. Microstruct Process. 241(1998)241-250. As expected, hot-forging led to a significant increase in the [13] G M Gladysz, A Doctor of Philosophy Thesis in Materials Engineering New Mexico Tech(1997). density and the hardness of both Al2O3 and La2Ti2O7 compo- [14] JRequena, R Moreno, J.S.Moya, J Am. Ceram Soc. 72(1989)1511 nent layers. But another critical parameter affecting the fracture (15)T. Chartier, T. Rouxel, ]. Eur. Ceram. Soc. 17(1997)299 toughness is the character of the interface between these two [16] D B. Marshall, J.J. Ratto, J Am Ceram Soc. 74(1991)2979 quired weakness was achieved by in situ formation of an Al2 TiOs (S)PA aie a undamental of Ceramic Powder Processing and Synthesis. omponents. Interface bonding between laminates should be [17] T.A.Ring. relatively weak in order to deflect an imposing crack. The re- ess, San Diego, USA, 1996 reaction layer at the interface. Crack deflection from both sides [19)PA Fuierer, R.E. Newham, JAmCeram Soc. 74(11)(1991)2876-2881 was demonstrated by indentation tests 20]A Ceylan, P.A. Fuierer, Key Eng Mater. 264-268(2004)829-832. It is believed that this second mechanism may in fact be the [21] P. Chatikul, G.R.Anstis, B.R. Lawn, D.B. Marshall,J.Am. Ceram Soc.64 more influential one 9)(1981)539-543 The maximum fracture toughness(Kic) of the hot-forged a- 22] G D. Quinn, J. Salem, K. Cho, I Bar-on, M. Foley, H. Fang, NIST 97(5) Al2O3 and La2Ti2O7 laminate composite was measured to be (Semptember-October 1992) 7.1 MPa mat room temperature. This value is nearly triple [23] Japanase Industrial Standard, JIS r 1601(1981), Japanase Standards Association, Tokyo the fracture toughness of monolithic a-Al2O3 [1]. The optimum [24]E Kato, K. Daimon, J. Takahashi, J. Am. Ceram. Soc. 63(5-5)(1980) hot-forging temperature for this system, yielding the highest 355-356. fracture toughness composite was found to be about 1400C This Al2O3/La2Ti2O7 composite system appears to hold pro- mise as a novel, damage tolerant ceramic material. References [1] W.H. Gitzen, Alumina as a Ceramic Material, The American Ceramic Society, 735 Ceramic Place Westerville, Ohio, 43081, USA, 1970 [2]GM. Gladysz, K.K. Chawla, Z.R. Xu, M.K Ferber, D.L. Joslin, Presented at TMS/ASM Materials Week, Cincinnati, OH, October 1996
Fig. 6 shows that a maximum toughness is expected at about 1400 °C. At higher temperatures, La2Ti2O7 and Al2O3 interdiffusion is accelerated. As a result a very thick Al2TiO5 reaction layer forms which begins to dominate the mechanical properties and weakens the composite structure. The low mechanical strength of Al2TiO5 makes it useless for structural applications [24]. However in small quantities as an interphase material, the micro-cracking tendency can be useful for crack deflection and arrest by absorbing crack tip energy. The best α-Al2O3/La2Ti2O7 laminate was found to have a fracture toughness (KIC) of 7.1 MPa m1/2. This is a very significant improvement over the toughness of the individual ceramic components (see Table 1). The fracture toughness attained is nearly triple the fracture toughness values for monolithic alumina [1]. 4. Conclusions As expected, hot-forging led to a significant increase in the density and the hardness of both Al2O3 and La2Ti2O7 component layers. But another critical parameter affecting the fracture toughness is the character of the interface between these two components. Interface bonding between laminates should be relatively weak in order to deflect an imposing crack. The required weakness was achieved by in situ formation of an Al2TiO5 reaction layer at the interface. Crack deflection from both sides was demonstrated by indentation tests. It is believed that this second mechanism may in fact be the more influential one. The maximum fracture toughness (KIC) of the hot-forged α- Al2O3 and La2Ti2O7 laminate composite was measured to be 7.1 MPa m1/2 at room temperature. This value is nearly triple the fracture toughness of monolithic α-Al2O3 [1]. The optimum hot-forging temperature for this system, yielding the highest fracture toughness composite was found to be about 1400 °C. This Al2O3/La2Ti2O7 composite system appears to hold promise as a novel, damage tolerant ceramic material. References [1] W.H. Gitzen, Alumina as a Ceramic Material, The American Ceramic Society, 735 Ceramic Place Westerville, Ohio, 43081, USA, 1970. [2] G.M. Gladysz, K.K. Chawla, Z.R. Xu, M.K. Ferber, D.L. Joslin, Presented at TMS/ASM Materials Week, Cincinnati, OH, October 1996. [3] M. Jimenez-Melendo, F. Gutierrez-Mona, A. Domingez-Rodriguez, Acta Mater. 48 (2000) 4715–4720. [4] R.A. Andrievski, J. Solid State Chem. 133 (1997) 249–253. [5] T. Egama, T. Ichilkizaki, H. Tsukamoto, H. Tsunoda, T. Shimoyama, Int. J. Jpn. Soc. Precis. Eng. 29 (3) (1995) 222–228. [6] Y.G. Gogotsi, F. Porz, J. Am. Ceram. Soc. 78 (1992) 2251–2259. [7] V. Winter, R. Knitter, Werkstoffwoche 98, Band VII: Symp. 9, Keram., Symp. 14 Simul. Keram, Wiley-VCH Verlag GmbH, 1999, pp. 199–204. [8] A. Bellosi, G. De Portu, S. Guicciardi, Eur. Ceram. Soc. 10 (4) (1992) 307–315. [9] L. Zou, D.S. Park, B.U. Cho, Y. Huang, H.D. Kim, Mater. Lett. 58 (10) (April 2004) 1587–1592. [10] B. Basu, J. Vleugels, O. Van der Biest, J. Alloys Compd. 372 (1–2) (June 9 2004) 278–284. [11] A. Demir, N. Altinkok, Compos. Sci. Technol. 64 (13–14) (October 2004) 2067–2074. [12] D.H. Kuo, W.M. Kriven, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 241 (1998) 241–250. [13] G.M. Gladysz, A Doctor of Philosophy Thesis in Materials Engineering, New Mexico Tech. (1997). [14] J. Requena, R. Moreno, J.S. Moya, J. Am. Ceram. Soc. 72 (1989) 1511. [15] T. Chartier, T. Rouxel, J. Eur. Ceram. Soc. 17 (1997) 299. [16] D.B. Marshall, J.J. Ratto, J. Am. Ceram. Soc. 74 (1991) 2979. [17] T.A. Ring, Fundamentals of Ceramic Powder Processing and Synthesis, Academic Press, San Diego, USA, 1996. [18] P.A. Fuierer, A Doctor of Philosophy Thesis in Solid State Science, The Pennsylvania State University, December, (1991). [19] P.A. Fuierer, R.E. Newham, J. Am. Ceram. Soc. 74 (11) (1991) 2876–2881. [20] A. Ceylan, P.A. Fuierer, Key Eng. Mater. 264–268 (2004) 829–832. [21] P. Chatikul, G.R. Anstis, B.R. Lawn, D.B. Marshall, J. Am. Ceram. Soc. 64 (9) (1981) 539–543. [22] G.D. Quinn, J. Salem, K. Cho, I. Bar-on, M. Foley, H. Fang, NIST 97 (5) (Semptember–October 1992). [23] Japanase Industrial Standard, JIS r 1601(1981), Japanase Standards Association, Tokyo. [24] E. Kato, K. Daimon, J. Takahashi, J. Am. Ceram. Soc. 63 (5–5) (1980) 355–356. A. Ceylan, P.A. Fuierer / Materials Letters 61 (2007) 551–555 555
