In-situ sintering reaction of Al2O3–LaAl11O18–ZrO2 composite

    In-Situ Sintering Reaction of Al 2 O3 - LaAl11 O18 - ZrO2 Composite S.M. Naga, A.M. Hassan, H.F. El-Maghraby, M. Awaad, H.S. Abd-Elwahab PII: DOI: Reference:

S0263-4368(15)30100-1 doi: 10.1016/j.ijrmhm.2015.07.026 RMHM 4126

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Revised date: Accepted date:

21 June 2015 18 July 2015 20 July 2015

Please cite this article as: Naga SM, Hassan AM, El-Maghraby HF, Awaad M, Abd-Elwahab HS, In-Situ Sintering Reaction of Al2 O3 - LaAl11 O18 - ZrO2 Composite, International Journal of Refractory Metals and Hard Materials (2015), doi: 10.1016/j.ijrmhm.2015.07.026

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ACCEPTED MANUSCRIPT In-Situ Sintering Reaction of Al2O3- LaAl11O18- ZrO2 Composite S.M. Nagaa, A.M. Hassanb*, H.F. El-Maghrabya, M. Awaada, H.S. Abd -

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Elwahaba

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a National Research Centre, Ceramics Dept., El-Bohous Str.,12622 Cairo,

b

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Egypt

Zagazig Uni., Faculty of Engineering, Materials Engineering Dept.,

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44519 Zagazig, Egypt.

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Corresponding author: Ahmed M. Hassan, Zagazig University, Materials Engineering Dept., Zagazig, Egypt. e-mail: [email protected],

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+201009884668

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Tel.: +20552304987

Abstract

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Al2O3- LaAl11O18- ZrO2 composites were prepared by in situ sintering reaction of different proportions of Al2O3 and La2Zr2O7. The studied

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batches were uniaxially pressed and pressureless sintered at 1600ºC up to 1725ºC for 1 h. Phase composition study reveals that the only present phases are alumina, lanthanum hexaluminates and zirconia. No other intermediate phases are present. Rodlike LaAl11O18 was observed in the sintered bodies containing more than 25 wt% LaAl11O18. The effect of rodlike particles on the densification and mechanical behavior was discussed. It was found that increasing the LaAl11O18 content more than 25 wt% enhances the fracture toughness, but reduces both the bending strength and the hardness of the sintered composites.

Keywords: In situ sintering reaction; Lanthanum hexaluminates; Microstructure; Mechanical properties; Origin of fracture.

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1- Introduction The brittleness of the alumina-based ceramics hinders its structural

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application where high strength and fracture toughness are required.

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Many attempts have been made to improve alumina mechanical

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properties. The dispersing of a second phase; such as whiskers [1] or platelets [2] in the microstructure can improve the mechanical properties by promoting crack deflection and crack bridging [3]. Elongated grains;

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like rare earth aluminates; can improve the mechanical properties of the alumina-based materials. It is well known that Al 2O3 compatibles with

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many layer aluminates, hexaluminates. Hexaluminates are highly stable as their crystal structure is composed of spinel blocks separated by layers

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of cations and oxygen ions [4]. Such structure together with the platelet-

interfaces [5].

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like morphology enable their use for adjustment of the fiber-matrix

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Lanthanum aluminate is prepared by conventional solid – state reaction of alumina and lanthanum oxide in a temperature range of 1500 – 1700 ºC [6,7]. Ropp and Carrol [8] found that Al2O3 and La2O3 are reacted to form

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perovskite – type aluminate at 800 ºC, then at higher temperature the formed aluminate reacts with alumina to form layered hexagonal aluminate according to the following equations: La2O3 + Al2O3 LaAlO3 + 5 Al2O3

2 LaAlO3

(1)

La Al11O18 (2)

Due to the heterogeneous distribution of LaAl11O18 (LA6) phase in the alumina matrix, as well as the high temperature needed for the solid-state reaction, Wu et al [9] used co-precipitation method. They prepared Al2O3/ 25 vol% LaAl11O18 by co-precipitation method from Al(NO3)3.9H2O and La(NO3)3.6H2O. They showed that the formed composite; obtained after

ACCEPTED MANUSCRIPT firing at 1500C for 1h; is composed of rodlike LaAl11O18 grains homogeneously distributed in the Al2O3 matrix. Negahdari and Porada [10] synthesized homogeneously distributed (28-

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80 vol%) LaAl11O18 in alumina matrix by coating the alumina particles with lanthanum aluminate via electrostatic technique. Solano and

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Esquivias [11] reported the effect of preparation conditions on the densification, phase formation and microstructure of lanthanum hexaluminates /Al2O3 composites. They showed that the anisotropic

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plate-like La-β-Al2O3 phase obtained by sol-gel method is formed at

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1050C, which is lower by 300C than solid-state reaction. They claimed that the formation of plate-like grains via sol-gel method restrained the composites densification, as the platelet formation is accompanied with

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Al2O3 grain growth, which results in poor densification. In contrary, some

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authors [5] claimed that LaAl11O18/ Al2O3 composites could sinter to high density.

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ZrO2 is compatible with Al2O3 and LaAl11O18; accordingly it might be used as a second reinforcing phase for LaAl11O18/Al2O3 composites.

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Many researchers incorporated rodlike LaAl11O18 phase into ZTA composites [12-15]. They concluded that the addition of LaAl11O18 decreases the sintering temperature of ZTA composites and increases their fracture toughness. The main toughening mechanisms are crack bridging and crack deflection together with residual thermal stresses. According to their studies, residual stresses led to the increase in the Al2O3 transgranular fracture. Several authors found that it is difficult to prepare pure LaAl11O18 phase by the calcination of La2O3 and Al2O3 or by the reaction between Al(OH)3 and La(NO3)3.6H2O [11,16]. Guo et al [12] and Kern [13] studied the effect of in situ formation of LA6 rodlike particles in ZTA ceramics on their mechanical properties. They showed that the presence of the rodlike particles enhances the sintering process at

ACCEPTED MANUSCRIPT low sintering temperature. The produced composites showed high fracture and bending strength together with a high resistance to subcritical crack growth.

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The aim of the present study is to develop tough ceramic composite via in

rodlike

particles.

Accordingly,

LaAl11O18/Al2O3/ZrO2

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towards

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situ formation of a second phase that possesses anisotropic growth

(LA6/Al/Zr) composites with different LaAl11O18, Al2O3 and ZrO2 concentrations were prepared via in situ solid-state reaction of different

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proportions of La2Zr2O7 and alumina. Microstructure, physical and

2. Materials and methods

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mechanical properties of the prepared composites were investigated.

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2.1. Materials and processing

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Chemically pure lanthanum oxide (La2O3), 99.99%-La of particle size

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ranged from 100 – 120 nm, (Strem Chemicals, Newburyport, MA, USA), zirconium n-(IV) butoxide Zr(OC4H9)4, 80 wt. % in 1-butanol (Strem Chemicals, Newburyport, MA, USA) and aluminum oxide (Al2O3), purity

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> 99.98% and particle size ranged from 135 – 150 nm, (Almatis Gmbh Ludwigshafen/RH, Germany) were used as starting materials. 2.1.1. Preparation of lanthanum zirconate Predetermined amounts of Zr(OC4H9)4 and La2O3 were carefully weighed. Zirconium alkoxide (Zr(n-C3H7O)4) amount was rapidly dissolved in absolute ethanol with stirring till complete mixing. An amount of HNO3 just sufficient to break the metal-alkoxide bonds (3-5 ml) was added to the mixture. The (Zr(n-C3H7O)4) and HNO3 mixture was then hydrolyzed with the addition of 1:1 water/ethanol mixture and vigorous stirring for 2 h at 80°C. The hydrolyzed (Zr(n-C3H7O)4) was peptized with the addition of 3 ml HNO3, which led to a transparent sol,

ACCEPTED MANUSCRIPT and then was left to cool. Meanwhile, La2O3 was dissolved in hot diluted HNO3. The formed lanthanum nitrate (La(NO3)3 solution was evaporated to almost dryness to get rid of the excess HNO3, and was mixed with the

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previously hydrolyzed (Zr(n-C3H7O)4). The transparent sol mixture was

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left at room temperature till gel formation. The gel was dried at 110 °C

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till complete dryness and then calcined in an electric oven for 2 h at 700 °C to get rid of all organic and nitrate species, then, calcined again at 900, 1000 and 1100°C for 2 h to follow up the La 2Zr2O7 phase stability. The free of all particle agglomerations.

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calcined powder was ground in an automatic agate mortar and pestle to be

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2.1.2. LaAl11O18/ Al2O3/ZrO2 (LA6/Al/Zr) composites formation The amounts of La2Zr2O7 addition were adjusted to form LaAl11O18/

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Al2O3/ZrO2 (LA6/Al/Zr) composites by In-situ sintering reaction,

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according to the equation:

La2Zr2O7 + (11+x) Al2O3

2 LaAl11O18 + 2 ZrO2 + x Al2O3

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Table. 1 illustrates the different composites composition. La2Zr2O7 and Al2O3 powders were mechanically mixed for 5 h using a ball mill with 5 mm zirconia balls and isopropyl alcohol as grinding media in

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polypropylene container at constant speed of 300 rpm. The obtained powders were formed by uniaxial pressing at 220 MPa into discs of 13 mm—diameter and 4mm—height (for physical and microstructural characterization) and rectangular bars of dimensions of 6 × 6 × 60 mm (for mechanical evaluation). The green bodies were dried at air overnight and then at 110ºC for 24 h. Specimens were pressureless sintered in an electric furnace (KSL-1800X-KA-S) and air atmosphere at 1600º, 1650º, 1700º and 1725ºC for one -hour soaking time at the maximum firing temperature. Heating and cooling rates were conducted with 5 ºC/min. 2.2. Characterization

ACCEPTED MANUSCRIPT The densification parameters of the fired samples in terms of bulk density and apparent porosity were evaluated with the liquid displacement method (ASTM C-20). The different phases of the powdered samples

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developed during firing were identified by means of X-ray diffraction

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analysis (XRD) with a Philips X-ray diffractometer, model PW1730, with

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a Cu target and Ni filter. The XRD patterns were obtained at room temperature with goniometric range of 5 – 70° 2θ, a scanning rate of 0.005˚s-1 and a step size of 0.02˚. The microstructure of the polished

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surfaces of the as-sintered specimens was examined with a scanning electron microscope (SEM-Jeol JSM-T20). The samples were thermally

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etched at 50 °C lower than the sintering temperature for 15 min in air and coated with gold (15 nm thickness) by means of electro-deposition in

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order to impart electric conduction. The Vickers hardness of the sintered

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samples was measured with a hardness tester (Omnimet automatic MHK system Model Micro Met 5114, Buehler USA). The composite samples

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were polished down to 0.25 μm surface finish with diamond paste and thermally etched at 1000ºC for 1h in air atmosphere to get rid of any C residues. Indentations were made on the polished surfaces with a load of

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10 kg for 15-second dwell time. 30 indents were made for each sample and average hardness was calculated according to the following equation [17]: H = 1.8544 (P/d2) Where p is the load and d is the length of the impression diagonal. Bending strength and fracture failure origin were determined according to the ASTM B528 standard, using three point bending method on a universal testing machine (Model LLOYD LRX5K of capacity 5KN). The samples were diamond polish to mirror surface. The measurements were carried out under a crosshead speed of 0.5 mm/min crosshead speed and support distance of 25 mm. At least 10 specimens were measured for

ACCEPTED MANUSCRIPT each data point. The following equation is used to evaluate the bending strength from the fracture load obtained in the 3 point bending test: σ = 3PfL / 2wt2

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Where σ is the bending strength, Pf the load at the fracture, L the

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specimen length, w the specimen width, and t is the specimen thickness.

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The fracture origins were examined with (SEM- Joel JSM- T20) scanning electron microscope. The fracture toughness was determined with the single- edge v- notched beam (SEVNB) technique [18]. For the SEVNB

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method, ground and polished rectangular specimens (3 × 4 × 45 mm3) were notched on the surface (3 × 45 mm2) using a diamond charged

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cutting wheel, perpendicular to the length of the rectangular bars. The depth of the notches was approximately 0.7 mm, i.e. ≤ 20% of the height

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of the specimen in accordance with DIN 51109 [19]. The fracture

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toughness was determined by applying the following equation [20]: K1c = [Lmax / t (h1/2)] × [L0- Li/h] × [3RM (d/h)1/2 / 2(1 – d/h)3/2]

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Where, Lmax is the maximum load, L0 and Li are the outer and inner roller spans; respectively, t and h are thickness and height of the specimen, d is the depth of the sharpened notch.

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RM = [1.9887– 1.326 (d/h) – [3.49 – 0.68 (d/h) + 1.35 (d/h)2] (d/h) (1(d/h)]/ (1 + (d/h)) 2. 3. Results and discussion 3. 1. Densification and phase composition Figure. 1a&b. indicates the positive correlation between the LaAl 11O18 (LA6) content and the densification behavior of the LA6/Al/Zr composites. The bulk density of samples L0, L1 and L2 increases with increasing their firing temperature from 1600 ºC up to 1725 ºC. Sample L3 densification temperature was 1700 ºC where L4 densified at 1650 ºC. The apparent porosity of the samples is the mirror image of their bulk density. It decreases with the firing temperature increase up to the

ACCEPTED MANUSCRIPT vetrification temperature, and then it starts to increase. Relative density of the studied samples is shown in Fig. 2. Relative density was measured on the basis of the theoretical density of pure Al2O3; 3.97 g/cm3; ZrO2; 6.012

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g/cm3; and LaAl11O18; 4.17 g/cm3. The presence of LA6 increases the

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relative density from 86.12 % (L0) to 95.32 % and 95.38 % (L1 and L2).

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Further increase in LA6 content to more than 25 wt% decreases the relative density of the samples. The in situ formation of the LA6 rodlike particles may be the main reason that retarded the densification of the

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LA6 composites [21] as the geometry of the particles having rodlike shape does not consolidate the particles packing [11]. The microstructure

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of the fired bodies confirms the formation of rodlike particles in the samples containing high LaAl11O18 content. Figure. 3 a&b compare the

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microstructures of L1 and L3 composite containing 12.5 and 37.5 wt%

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LA6 respectively. The elongated rodlike structure can easily recognize the aluminate phase in L3, while Al2O3 grains are almost present as

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equiaxed grains in L1 composites. The slight increase in the relative density of sample L4 is attributed to the increase in its ZrO2 content. It is stated by Moraes et al [22] that the introduction of small amounts of

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ZrO2; as a ternary phase, enhances the densification process. Zirconia promotes the sintering by both the migration of zirconia between fine and coarse particles and its hindering effect on the grain growth of the alumina grains [23]. The phase composition of the sintered bodies was determined by XRD and the patterns are shown in Fig.4. The figure shows that the samples displayed almost identical XRD patterns. They are composed of LaAl11O18, Al2O3 and ZrO2. No lanthanum zirconate or any other intermediate phases were detected. The XRD peak area of Al2O3 at 2θ =43.2º and ZrO2 at 2θ = 30.2º is shown in Fig.5. From L1 batch to L4 batch, the zirconia content increases, while the alumina content decreases.

ACCEPTED MANUSCRIPT According to Lakiza and Lopato [23], who plotted the isopleth of 20 mol% ZrO2 at 1500-1600 ºC, a very little content of lanthana was found as a solid solution in ZrO2 in addition to alumina and La hexaluminates.

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In agreement with the results obtained by Kern [13], we believe that

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lanthanum could release from lanthanum zirconate solid solution. As the

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batches contain high Al2O3 content, the released La could react with Al2O3 to form LaAl11O18. Jin and Gao [14] suggested that the formation

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of LaAl11O18 might be promoted by the intemperate of Al2O3 presence. 3.2. Microstructure

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Figure. 6a shows the SEM micrograph of the polished, thermally etched surface of L3 sintered sample. LaAl11O18 grains are sandwiched between

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alumina grains. They are tending to have elongated morphology. The

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figure shows that most of ZrO2 grains are characterized by an intergranular dispersion. Figure 6b shows that there are two different

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shapes of ZrO2 grains, nearly spherical, which might be monoclinic form, and nearly equiaxial, which might be tetragonal form. Zirconia undergoes tetragonal to monoclinic phase transformation by cooling if its size is

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above 1 µm. The figure shows that the average grain size of zirconia is > 1µm. This means that the possibility of t to m transformation is exist. The size of LaAl11O18 grains, L1 sample, is more than 4 µm. They exhibit a rippled surface, Fig. 6c. The transformation of the lanthanum hexaluminate rippled grains into rodlike particles is shown in Fig.6d.

3.3. Mechanical Properties 3.3.1. Three Point bending strength Figure. 7 indicates that the strength of the LA6/Al/Zr composites affected by the ZrO2 and LaAl11O18 contents. The figure shows a considerable increase in the bending strength with the existence of LaAl 11O18 up to 25

ACCEPTED MANUSCRIPT wt%. Higher concentration of LA6 reduces the bending strength of the bodies by 42.4%. The increase in the bending strength is attributed to the pinning effects of ZrO2 and LA6 on the grain growth of Al2O3 grains, Fig.

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8 a&b [14]. The figure shows an approximately Al2O3 isotropic structure

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with no abnormal grain growth. Furthermore, the thermal expansion 6

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coefficient differences between ZrO2 (10.5 X10-6/ºC), Al2O3 (8.5 X 10/ºC) and LaAl11O18 (7.7 X 10-6/ºC) impose compressive stress on Al 2O3

grain boundaries, which strengthen the grain boundaries and enhance the

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bending strength [14,24]. On the other hand, the noticeable degradation of samples L3 and L4 bending strength is due to the increase in their

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porosity content. Guo et al [12] found that the bending strength decreased when LaAl11O18 content is over 24%. They stated that it is due to the

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the Young’s modulus.

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partial transformation of t-ZrO2 to m- ZrO2, which led to the reduction in 3.3.2. Vickers Hardness

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The effect of lanthanum hexaluminate content on the composites hardness is shown in Fig.9. It is observed that the hardness increases with the increase in the LaAl11O18 content up to 25 wt%. These results are

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in agreement with Negahdari et al [25] results. They stated that the increase in the Vickers hardness of Al2O3/ LaAl11O18 composites (up to 20 vol% LA6) is due to the inhibition of alumina grain growth by LA6. We believe that the strong interface between the alumina grains and LA6 grains is an additional factor that improves the Vickers hardness of the composites. The above mentioned behavior is similar to that of 12CeTZP composites reinforced with in situ formed lanthanum hexaluminate studied by Kern [13]. The decrease in the hardness by further increase in LA6 up to 37.5 wt% could be attributed to the decrease in the relative density and the increase in the apparent porosity. On the other hand, the increased content of ZrO2 of L4 samples partially compromise the effect

ACCEPTED MANUSCRIPT of the increased porosity and improves slightly the hardness of the samples. 3.3.3. Fracture toughness

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It can be noticed from Fig.10 that the increase in the LA6 and ZrO2

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contents increases the fracture toughness of the composites. The increase

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in the LA6 content increases the rodlike particles. The rodlike particles strengthened and toughened the studied composites by the crack bridging similar to the crack bridging mechanism of SiC whiskers in SiC whiskers

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reinforced alumina composites [3]. Furthermore, the improvement in the fracture toughness can be due to the transformation of zirconia (t to m

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ZrO2), which aggravated by ZrO2 large grain size [13]. 3.3.4. Origin of the fracture failure determination.

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In the present study many features are detected as structure detrimental.

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They include cracks, pores and coarse grains. Figure 11a shows the origin of failure of Al2O3 sintered samples. It indicates that the failure originated

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from surface defects that associated with pores and clusters of coarse grains. Hotta et al [26] found that the only fracture origin of alumina bodies prepared by slip casting is the presence of coarse particles. The

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coarse particles result in high porosity. L1 sample, Fig. 11b, showed that it failed because of the pocket defects. Chao and Shetty [27] suggested that the cracks nucleated at the pores. They oriented around the pores perpendicular

to

the

direction

of

maximum

tangential

stress

concentration. Accordingly pores can consider as nuclei for penny-shaped cracks. Evan et al [28] considered pores as regions of stress concentrations. The preexisting cracks in such pores are subjected to different stress levels and together with the pores they are the origin of the failure. Another origin of failure is the long cracks associated with pockets, Fig. 11 c&d. We believe that the failure is occurred in the surface because of the long cracks located in the inner structure.

ACCEPTED MANUSCRIPT 4. Conclusions 1- LaAl11O18/ Al2O3/ ZrO2 composites are prepared by in situ sintering

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reaction of Al2O3 and La2Zr2O7. By adjusting the La2Zr2O7 and Al2O3

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contents, no lanthanum zirconate or any other intermediate phases are

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formed.

2- The presence of LaAl11O18 up to 25 wt% increases the relative density and the mechanical behavior of the LA6/Al/Zr composite. Further

density and mechanical behavior.

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increase in LaAl11O18 content has a negative effect on both the relative

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3- The in situ formation of the LaAl 11O18 rodlike particles may be the main reason that retarded the densification of the LA6/Al/Zr composites.

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4- The existence of small amounts of ZrO2, as a ternary phase enhances

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composites.

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the densification process and the mechanical behavior of LA6/Al/Zr

5- Rodlike particles are clearly identified at LaAl11O18 possessing LA6

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higher than 25 wt%.

6- Formation of LaAl11O18 rodlike particles have a converse effect on LA6/Al/Zr composites. Although it decreases the relative density, hardness and bending strength, it acts as a toughening agent and enhances the toughness. 5. References [1] Becher PF, Hsuch CH, Angelini P, Tiegs TN. Toughening behavior in whisker-reinforced ceramic matrix composites. J Am Ceram Soc 1988; 71 (12):1050-61. [2] Sakai H, Matsuhiro K, Furuse Y. Mechanical properties of SiC platelet reinforced ceramic composites. Ceram Trans 1991; 19: 765-71.

ACCEPTED MANUSCRIPT [3] Becher PF. Microstructural design of toughened ceramics. J Am Ceram Soc 1991; 74 (2): 255-69. [4] Iyi N, Inoue Z, Takekawa S, Kimura S. The crystal structure of

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[10] Negahdari Z, Porada MW. Tailoring the microstructure of reaction-

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sintered alumina/lanthanum hexaaluminate particulate composites. J Eur Ceram Soc 2010; 30 (6): 1381-9. [11] Solano CB, Esquivias L. Effect of preparation conditions on phase formation, densification, and microstructure evolution in La-β-Al2O3/ Al2O3 composites. J Am Ceram Soc 1999; 82 (5):1318-24. [12] Guo R, Guo D, Chen Y, Yang Z, Yuan Q. In situ formation of LaAl11O18 rodlike particles in ZTA ceramics and effect on the mechanical properties. Ceram Int 2002; 28 (7): 699-4. [13] Kern F. A comparison of microstructure and mechanical properties of 12Ce-TZP reinforced with alumina and in situ formed strontium- or

ACCEPTED MANUSCRIPT lanthanum hexaaluminate precipitates. J Eur Ceram Soc 2014; 34 (2): 413-23. [14] Jin X, Gao L. Effect of powder preparation method on the

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microstructure and mechanical performance of ZTA/ LaAl 11O18 composites. J Eur Ceram Soc 2004; 24 (4): 653-9.

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[15] Jin X, Gao L. Microstructure and mechanical performance of ZTA/ LaAl11O18 composite prepared by a heterogeneous precipitation method. Mater Sci Eng A 2003; 360: 275-9.

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direct crack measurement. J. Am. Ceram. Soc. 1981; 64:533-8. [18] Tang D, Lim H, Lee K, Lee C, Cho W. Evaluation of mechanical

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ACCEPTED MANUSCRIPT [24] Gadow R, Lischka M. Lanthanum hexaaluminate — novel thermal barrier coatings for gas turbine applications — materials and process development. Surf Coat Technol 2002; 151-152: 392-9.

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[25] Negahdari Z, Willert.-Porada M, Pfeiffer C. Mechanical properties

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of dense to porous alumina/lanthanum hexaaluminate composite

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101-6.

Figure Captions

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Fig.1: Physical properties of the fired LA6/Al/Zr composite fired at different firing temperatures, (a) Bulk density and (b) Apparent porosity. Fig.2: Relative density of the studied LA6/Al/Zr composite. Fig.3: SEM micrographs of (a) L1 composite containing 12.5 wt.% LaAl11O18 , (b) L3 composite containing 37.5 wt.% LaAl11O18. Fig.4: XRD patterns of the studied composites fired at their sintering temperatures.

ACCEPTED MANUSCRIPT Fig.5: XRD peak area of Al2O3 and t- ZrO2 of LA6/Al/Zr composite fired at different firing temperatures.

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Fig.6: SEM micrographs of the studied composites microstructure, (a) L3

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elongated alumina particles of L3 composite, (b) t- and m- ZrO2 particles

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of L3 composite, (c) LA6 grains with rippled surface, (d) The transformation of rippled grains into rodlike particles. Fig.7: Effect of LaAl11O18 content on the bending strength of LA6/Al/Zr

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composite.

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Fig.8: SEM micrograph of the pinning effects of ZrO2 and LA6 on the grain growth of Al2O3.

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Fig.9: Effect of LaAl11O18 content on the Vickers hardness of LA6/Al/Zr

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composite

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Fig.10: Effect of LaAl11O18 content on the fracture toughness of LA6/Al/Zr composite

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Fig.11: The origin of fracture in the studied composites. Tables Caption

Table 1: Composition of LA6/Al/Zr batches, wt.%.

ACCEPTED MANUSCRIPT

L0

3.5

L1 L2 L3 2.5

L4

T 1600

1650

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1.5 1550

1700

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Bulk Density, g/ cm3

4.5

1750

Firing Temperature, ºC

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(a)

MA

40 30

L0 L1 L2 L3

D

20 10

AC

CE P

0 1550

TE

Apparent Porosity, %

50

1600

L4

1650

Firing Temperature, ºC

(b)

Figure. 1

1700

1750

IP

T

ACCEPTED MANUSCRIPT

SC R

92

84 0

12.5

MA

NU

88

25

CE P

TE

D

Lanthanum Aluminate, Wt.%

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Relative Density,%

96

Figure. 2

37.5

50

AC

CE P

TE

D

MA

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Figure 3

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IP

T

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Al2O3 LaAl11O18 m-ZrO2 t-ZrO2

L4

4000

L3

IP

T

5000

2000 1000 0 10

20

30

40

50

MA

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2θ

TE

D

Figure. 4

CE P

0

SC R

3000

AC

Intensity, Counts [a.u.]

6000

60

L2 L1 L0 70

80

ACCEPTED MANUSCRIPT

Peaks Area

7

Al2O3

0 L0

L1

L2

SC R

IP

T

t-ZrO2

L3

Lanthanum Aluminate, Wt.%

AC

CE P

TE

D

MA

NU

Figure. 5

L4

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 6

ACCEPTED MANUSCRIPT 350 300

T

250

IP

200

0

SC R

150 12.5

25

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Lanthanum Aluminate, Wt.%

CE P

TE

D

MA

Figure. 7

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Bending Strength, MPa

400

37.5

50

AC

CE P

TE

D

MA

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Figure 8

SC R

IP

T

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT 4000

3000

T

2500

IP

2000

SC R

1500 1000 0

12.5

25

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Lanthanum Aluminate, Wt.%

CE P

TE

D

MA

Figure. 9

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Vicker's Hardness

3500

37.5

50

ACCEPTED MANUSCRIPT

12

IP

T

8

SC R

4

0 0

12.5

25

NU

Lanthanum Aluminate, Wt.%

CE P

TE

D

MA

Figure. 10

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Fracture Toughness, MPa m1/2

16

37.5

50

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 11

ACCEPTED MANUSCRIPT Table 1

AC

CE P

TE

D

MA

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ZrO2, Wt.% _ 2 4 6 8

T

SC R

L0 L1 L2 L3 L4

LaAl11O18, Wt.% _ 12.5 25 37.5 50

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Al2O3, Wt.% 100 85.5 71 56.5 42

ACCEPTED MANUSCRIPT Highlights  In situ sintering reaction between Al2O3 and La2Zr2O7 was The effect of LaAl11O18 on the mechanical properties of

 Formation

of

LaAl11O18

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LA6/Al/Zr composite was studied.

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

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performed.

rodlike

particles

affects

the

densification and mechanical behavior of the composite.

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CE P

TE

D

MA

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 Introduction of ZrO2 enhances the behavior of the composite .