Materials Chemistry and Physics 67 (2001) 256–262

Multilayer composites in Al2O3/MoSi2 system

Tadahiko Watanabe∗, Guo-Jun Zhang, Xue-Mei Yue, Yu-ping Zeng,Kazuhisa Shobu, Naoufal Bahlawane

Kyushu National Industrial Research Institute, Shuku Tosu Saga 841-0052, Japan

Abstract

A new multilayer composite with a super-plastic layer, a hard layer and a weak interface was proposed. The hard layer can providethe strength of the multilayer composites at high temperature, the plastic layer can deform plastically at high temperature and dispersethe applied stress and stop the advance of the crack, and the weak interface can deflect the propagating crack at room temperature. Suchmultilayer composites were prepared by tape casting in the Al2O3/TiC/MoSi2–Mo2B5 system. It was found that such design is effective onthe increase of fracture energy both at room temperature and at high temperature, and the strength at high temperature could be remainedin a relatively high revel. © 2001 Published by Elsevier Science B.V.

Keywords:Multilayer; Composite; Molybdenum; Silicide; Alumina

1. Introduction

One of the most important environments for the applica-tion of structural ceramics is high temperature to room tem-perature. Also, much work has been conducted to overcomethe brittleness of ceramics. Making a composite is a com-monly used method to improve the toughness of ceramics,via the use of reinforcements such as particles, whiskers,platelets, and fibers.

It has been demonstrated that multilayer composites(MLCs) with weak interfaces can sustain stresses afterthe onset of fracture [1–3]. This observation means thatMLCs with weak interfaces are flaw tolerant and reliablefor structural applications. However, generally, only theroom-temperature properties have been evaluated.

We propose for one of the MLC a new MLC with asuper-plastic layer, a hard layer and a weak interface. Thehard layer can provide the strength of the MLCs at hightemperature, the plastic layer can deform plastically at hightemperature and disperse the applied stress and stop the ad-vance of the crack, and the weak interface can deflect thepropagating crack at room temperature.

It is shown in this paper that MoSi2–Mo2B5 ceramics havesuch properties as high oxidation resistance and strength,and moreover, have plasticity at the high temperature. Al2O3ceramics at large have the rigidity at high temperature. So,MLC ceramics of MoSi2–Mo2B5/Al2O3 is thought for thetoughened ceramics at high temperature.

∗ Corresponding author.

Also, for improving the room temperature fracture beha-vior of this MLC, TiC was used as the inter-phase materialfor getting a relatively weak interface. Moreover, TiC willbecome ductile at about 1073 K and also maybe have a goodrole for the high-temperature fracture behavior of this MLC.

The authors attempt to develop multilayer of MoSi2–Mo2B5 and Al2O3 by using Doctor-blade. Also it is shownthat this MLC ceramics have high fracture energy at hightemperatures and room temperature.

2. Experimental procedures

In the experiment, the Mo2B5, MoSi2, Al2O3 and TiCraw powders were high-purity products. Particle sizes ofMo2B5, MoSi2, Al2O3 and TiC was 0.6, 1.1, 0.2 and 1.2mm,respectively.

In the experiment of MoSi2–Mo2B5 properties, the com-pacted powders were pressed in the metal mold and CIP at294 MPa, and then the compacted powders were sintered ata desired temperature for 90 min in a vacuum.

The Al2O3/MoSi2–Mo2B5 MLC system was selectedas the first experimental system for demonstrating thisidea; a tape-casting method was used to produce thisMLC. Fig. 1 shows flow chart for the preparation ofAl2O3/MoSi2–Mo2B5 MLC. Also the slurry formulation islisted in Table 1. The organic-based slurries of Al2O3andMoSi2–20% Mo2B5 were prepared using ball milling. Af-ter de-airing, the slurries were tape cast. After drying, theobtained tapes were punched and then stacked alternatively

0254-0584/01/$ – see front matter © 2001 Published by Elsevier Science B.V.PII: S0254-0584(00)00448-X

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Fig. 1. Flow chart for the preparation of Al2O3/TiC/MoSi2–Mo2B5 MLC.

(as Al2O3 layer and then a MoSi2 + Mo2B5 layer) intothe graphite die to make the MLC green body. Accordingto the thermo-gravimetric analysis (TGA) results, the or-ganic components in the green body were burned out ina high-purity (99.999%) argon atmosphere up to 873 K,with a temperature increase rate of 0.5 K min−1. Then,the green body was hot pressed at 1773 K for 60 min un-der vacuum using the direct-current hot press machine. Inthe case of Al2O3/TiC/MoSi2–Mo2B5, process of brush-ing TiC interface was added as shown in Fig. 1. Also,the hot press temperatures were 1673, 1773 and 1873 K,and the corresponding specimens were marked as MC14,MC15 and MC16, respectively. The obtained samples wereground to a size of 2 mm× 4 mm × 12 mm to evaluatethe room temperature bending behavior and to a size of1.5 mm×3.5 mm×12 mm to evaluate the high-temperaturebending behavior in an argon atmosphere. The span for thethree-point bending test was 10 mm, and the cross-headspeed for room- and high-temperature testing was 0.05and 0.07 mm/min, respectively. The microstructures wereobserved by using scanning electronic microscopy (SEM).

Table 1Slurry for tape casting Al2O3and MoSi2–Mo2B5

Constituent Function For Al2O3

(vol.%)For MoSi2 +Mo2B5 (vol.%)

Al2O3 orMoSi2–Mo2B5

Ceramic powder 21.4 25

MEK Solvent 38.4 352-Propanol Solvent 27.3 25Dilbutyl Plasticizer 5.6 6.5Polyvinyl butyral Binder 6.8 8Fish oil Deflocculant 0.5 0.5

Fig. 2. Relationship between Mo2B5 content and bending strength, andrelationship between Mo2B5 content and Vickers hardness. MoSi2–x%Mo2B5 powders were sintered in vacuum with heating at a rate of3 K min−1 from 1273 up to 1773 K.

3. Result and discussion

3.1. Properties of sintered MoSi2–Mo2B5 ceramics

Fig. 2 shows the relationship between Mo2B5 contentand bending strength. Sintered MoSi2 bodies have a bend-ing strength of only 200 MPa, but MoSi2–20 wt.% Mo2B5bodies have 550 MPa. Fig. 3(A) and (B) shows back scat-tered images of (A) MoSi2 ceramics and (B) MoSi2–20 wt.%Mo2B5 ceramics. Both ceramics have many round blackphases of SiO2. Also the both ceramics have the porosity of0.2–0.3 vol.%. Fig. 4 shows the X-ray diffraction patterns ofMoSi2–40% Mo2B5 ceramics. It is found that the compo-nents are MoSi2, Mo2B5 and MoB.

Fig. 5(A) and (B) shows the relationship between timeand true strain under each uni-axial compression load at atemperature of 1673 K. MoSi2 and MoSi2–Mo2B5 ceramicshave plasticity at 1673 K. The behaviors for example undersame 13 MPa and same loading time of 60 min show thatMoSi2–20 wt.% Mo2B5 ceramics have larger true strain thanMoSi2 ceramics, i.e. MoSi2–20 wt.% Mo2B5ceramics areeasier deform.

3.2. Fracture behavior of Al2O3/MoSi2–Mo2B5 multilayersceramics

Fig. 6 shows the side surfaces of the specimens after frac-ture at room temperature, and 1673 K. In all SEM picturesthe dark layer is Al2O3 and the light layer is MoSi2–Mo2B5.Fig. 6(A) shows that there is a small crack deflection whenthe main propagating crack meets the MoSi2–Mo2B5 lay-ers, because of the relatively lower mechanical-propertyindices of these layers, compared with those of Al2O3 lay-ers. However, when the specimen was fractured at 1573 Kas shown in Fig. 6(B), there were many cracks (the largestcrack was about 10mm wide), which were perpendicular to

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Fig. 3. Back scattered images of (A) MoSi2 ceramics sintered at 1773 K for 90 min in a vacuum, and (B) MoSi2–20 wt.% Mo2B5 ceramics sintered in avacuum, at rate of 3 K min−1 from 1273 K up to 1773 K.

the layers, distributed in Al2O3 layers near the main crack.The number of such cracks in each Al2O3 layer graduallydecreased from the tensile surface to the compressive sur-face of the specimen, as shown in Fig. 6(B). These crackswere separated by the MoSi2–MoSi2 layers, and there wasno interface debonding, as shown in Fig. 6(C), which in-dicates that the bonding strength between the Al2O3 layersand the MoSi2–Mo2B5 layers is strong at high tempera-tures up to 1573 K. This phenomenon is very different, withthe crack deflecting in the weak interface in the MLC, butis similar to the multiple cracking in the ceramic–metallaminate composites [4]. The MoSi2–Mo2B5 layers hadplasticity at 1573 K, and the stress at the maximum stressplane decreased by dispersing the applied stress to an areanear the maximum stress plane through super-plastic defor-mation. In this process, a large amount of energy should beabsorbed through crack re-nucleation in Al2O3 layers andplastic deformation in MoSi2–Mo2B5 layers.

When the fracture-test temperature was increased to1673 K, the MoSi2–Mo2B5 layers became more plasticand easier to deform. However, because the strength ofthe MoSi2–Mo2B5 and Al2O3 layers decreased greatly, theability of stress dispersion for MoSi2–Mo2B5 layers andthe ability of stress assumption for Al2O3 layers both de-creased evidently. Therefore, the number of cracks in theAl2O3 layers near the main crack was much less than that

Fig. 4. X-ray diffraction pattern of MoSi2–20 wt.% Mo2B5 ceramicssintered at 1773 K in a vacuum.

in the specimen that was fractured at 1573 K as shown inFig. 6(E). Fig. 6(E) and (F) also shows that, after the Al2O3layers were broken, the main crack also was connected bythe plastic MoSi2–Mo2B5 layers and the interfaces near thefractured surface became debonded. This debonding indi-cates that the strength of the MoSi2–Mo2B5 layers is higherthan the interfacial bonding strength (T) at 1673 K, and theshear or tensile stress at the interfaces surpassed theT be-cause of the decrease of theT with increases in temperature.

Fig. 7 shows the stress–displacement curves of the speci-mens fractured at room temperature (marked as “MB-RT”),

Fig. 5. The relationship between time and true strain for MoSi2 in (A)and for MoSi2–20 wt.% Mo2B5 in (B).

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Fig. 6. SEM images of the side surfaces of the Al2O3/MoSi2–Mo2B5 MLC specimens fractured at different temperatures ((A) room temperature; (B)1573 K,showing many cracks near the main crack; (C)1573 K, showing cracks connected via the super-plastic layer; (D) 1573 K, showing the absorption of acrack by the super-plastic layer and (E) 1673 K, showing the main crack connection via the super-plastic layer). (F) is a higher magnification view ofthe fractured side surface in (E), showing interfacial bonding.

1573 K (marked as “MB-1300”) and 1673 K (marked as“MB-1400”). According to the areas under these curves,the fracture energy of the specimen that was fractured at1573 K was about 40 times larger than that fractured at roomtemperature.

3.3. Fracture behavior of Al2O3/TiC/MoSi2–Mo2B5multilayers ceramics

Fig. 8 shows the micrographs of the side surfaces ofspecimens MC15 and MC16 which were broken at the roomtemperature. It can be seen at the room temperature that

there was large crack deflection in the specimen MC15, butthere was small crack deflection for MC16 specimen. InMC15, the crack deflection out of the main stress plane alongthe weak TiC interface which was vertical to the appliedstress. Then the deflected crack would deflect to the mainstress plane after certain propagation along the weak TiCinterface.

In this process either Al2O3 layer or MoSi2–Mo2B5 layerwould be broken as shown in Fig. 8(C). For the specimenMC14, the interface bonding strength was too weak to keepthe layers together to become a whole block as shown inFig. 9.

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Fig. 7. Stress–displacement curves of specimens fractured at room tem-perature (inset), 1573 and 1673 K.

The microstructures of TiC interfaces are shown in Fig. 10for the specimen MC14 and MC15. It can be seen that inthe MC14 specimen the sintering of TiC interface looks notto take place, and so resulted in the week interface in thisspecimen. For the MC15 the sintering of TiC had alreadytaken place, but the bonding strength was still not very high,i.e. it was a porous TiC interface with moderate bondingstrength. When the hot pressing temperature reached 1873 K,

Fig. 8. Micrographs of the side surfaces of specimens MC15 and MC16: (A) MC15 specimen; (B) MC16 specimen; (C) higher magnification of MC15showing the crack deflection.

Fig. 9. Micrograph of one debris of the specimen MC14.

the TiC interface was sintered to a dense layer and resultedin a strong bonding as shown in Fig. 11.

Fig. 12 shows the bending stress–displacement curves ofMC15 and MC16 specimens fractured at room temperature.For MC15 the large crack deflection showed in Fig. 8(A)was reflected in this curve. However for MC16, because ofthe strong bonding interface and almost no crack deflection,the bending stress–displacement curve is nearly a linear line.

The side surface of MC15 fractured at a high temperatureof 1573 K is shown in Fig. 13. This figure shows that the

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Fig. 10. Microstructures of the TiC interfaces of the specimens MC14 and MC15.

weak TiC interface still play the role in crack deflection at1573 K. Furthermore, when higher magnification was usedto observe the tensile part (the upper surface in Fig. 13) of theside surface of the fractured specimen, many cracks whichwere vertical to the layers could be found in the Al2O3 lay-ers as shown in Fig. 13(A). In MoSi2–Mo2B5 layers therewas not such a crack. This phenomenon was absolutelydifferent from that which appeared in specimen fracturedat room temperature as shown in Fig. 8(C). It was consi-dered that at 1573 K the MoSi2–Mo2B5 layers have alreadybecome super-plastic. Before the main crack propagatingthrough certain Al2O3 layer, the MoSi2–Mo2B5 layers nearthis Al2O3 layer would slide and disperse the applied stressin this Al2O3 layer. When the applied stress reached to somelimit, many new cracks in this Al2O3 layer would occur.The propagation of these cracks would be stopped by thenear MoSi2–Mo2B5 layer as shown in Fig. 13(B) and (C)because of the super-plastic MoSi2–Mo2B5 layer could de-form plastically and absorb the crack propagating energy or‘absorb’ the crack itself as shown in Fig. 13(D).

Fig. 14 is the bending stress–displacement curves ofMC15 fractured at 1573 and 1673 K. It can be seen that

Fig. 11. The side surfaces of MC16 specimen showing the microstructureof the TiC interface for MC16.

Fig. 12. Bending stress–displacement curves of MC15 and MC16 speci-mens fractured at room temperature.

the fracture energy increased obviously (about 10 times ofthe value at room temperature according to the area underthe bending stress–displacement curves) as well as thatmost of the strength of the material could maintain at thesetemperatures.

4. Summary

A new MLC of Al2O3/TiC/ MoSi2–Mo2B5 with asupper-plastic layer (MoSi2–Mo2B5), a hard layer (Al2O3)and a weak TiC interface was proposed in this paper. Thesuper-plastic layer of MoSi2–Mo2B5 can deform plasticallyat high temperature and disperse the applied stress, andthen stop the crack growth. The weak TiC interface can de-flect the propagating crack at room temperature. This kindof Al2O3/TiC/MoSi2–Mo2B5 MLC were prepared by tapecasting. The microstructures and the stress–displacementbehaviors of this MLC were investigated. It was found thatsuch design for MLCs was effective on the increase of frac-ture energy both at room temperature and high tempera-

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Fig. 13. Microstructures of cracks in Al2O3 layers fractured at 1573 K: (A) low magnification showing many cracks in Al2O3 layers; (B) highermagnification showing the distribution structure of cracks; (C) higher magnification showing ‘crack connecting’ by super-plastic MoSi2–Mo2B5 layer;(D) higher magnification showing ‘absorbing’ of crack by super-plastic MoSi2–Mo2B5 layer.

Fig. 14. Bending stress–displacement curves of MC15 fractured at 1573and 1673 K.

tures. In our present study of the Al2O3/TiC/MoSi2–Mo2B5MLC system, the mechanical properties of the Al2O3and MoSi2–Mo2B5 shall be improved further at hightemperature.

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