Mechanical Property Improvement andMicrostructure Observation of SiCw±AlNCompositesXin Jiang,* Yuan Chen, Xin-wei Sun and Li-ping Huang

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, People's Republic of China

(Received 20 May 1998; accepted 7 August 1998)

Abstract

SiCw±AlN composites were produced with di�erentsintering additives. Eight wt% Y2O3 was found asuitable sintering additive for mechanical propertyimprovement of SiCw-AlN composites. With thesintering additive, SiCw±AlN composites with whis-ker content ranging from 10±30wt% were hot-pres-sed to high density, and the ¯exural strength andfracture toughness were signi®cantly improved withthe increase of SiC whisker content. SEM and TEMobservation indicated that several kinds of toughen-ing mechanism contributed to the increasing tough-ness, including crack de¯ection, crack branching andwhisker bridging process. The interfacial boundariesof the composites were also discussed in detail.# 1999 Elsevier Science Limited. All rights reserved

Keywords: whiskers, mechanical properties, com-posites, SiC, AlN.

1 Introduction

AlN ceramics attract growing interest for theirhigh thermal conductivity and good electricalproperties in recent years. The theoretical thermalconductivity of AlN single-crystal was estimated at320Wmÿ1Kÿ1, which is 80% of the thermal con-ductivity of copper metal. According to the micro-structure and chemical composition of the sinteredceramics, thermal conductivity of AlN ceramicsvaries between 80 and 260Wmÿ1 Kÿ1.2±5 Besideshigh thermal conductivity, AlN ceramics exhibitgood electrical insulation, low dielectric constantand low thermal expansion coe�cient. It is com-monly recognized that AlN ceramics are con-siderably potential substrate materials.

In addition, AlN ceramics have various struc-tural and refractory applications. They are alsoexpected to be used as engine components, such asheat exchanger in gas turbine or a matrix for ther-mal shock resistant ceramic±ceramic compositesdue to their corrosion resistance and high-tem-perature stability. However, in many cases, therelatively low ¯exural strength and fracture tough-ness of AlN ceramics limit their application inmodem industries and high-technologies.6

In the past years, a lot of work were done on SiCwhisker reinforcement of ceramics. It was foundthat SiC whisker reinforcement of ceramics couldresult in substantial improvement in ¯exuralstrength, fracture toughness and wear resistance.7±10 For example, the fracture toughness of Al2O3

was doubled or tripled to approximately9.5MPam1/2 with the addition of 30 vol% SiCwhiskers.11,12 The substantial improvement in ¯ex-ural strength and fracture toughness was achievedby load transfer and other mechanisms, such ascrack de¯ection, crack bowing, whisker bridgingand whisker pullout.Through carefully controlling the processing

parameters, interface reaction and microstructuredevelopment, incorporating a ductile metal, suchas Al, was proved e�ective in enhancing themechanical properties of AlN ceramics.13 Based onthe strengthening and toughening mechanism,incorporation of SiC whisker should also be apotential method in improving the mechanicalproperties of AlN ceramics. AlN has great com-patibility with SiC, and a ¯exural strength of up to1GPa was achieved in AlN±SiC material becauseof the formation of AlN±SiC solid solution with adense, equiaxed grain structure.14 However, littlework has been done in reinforcing AlN ceramicswith SIC whiskers. In this work, we tried toimprove the mechanical properties of AlN ceramicsby the incorporation of SiC whiskers, and analyze

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*To whom correspondence should be addressed.

the toughening mechanism and interfacial bound-aries in the composites through SEM and TEMobservation.

2 Experiment Procedure

The starting powders included AlN powders, SiCwhiskers, SiO2 and Y2O3 powders. The averagegrain size of AlN powders was 1�m, nitrogencontent 34.01wt% and oxygen content 0.60wt%.SiC whiskers (Advanced Matrix Company) weredimensionally straight with smooth surface. Fewclusters were found in the whiskers. The meanlength and diameter of SiC whisker were measuredas approximately 40 and 0.5�m, respectively, andthe average aspect ratio was 80. They were notcoated. Oxygen, carbon and iron were the mainimpurity. The average particle size of both SiO2

and Y2O3 powders was 1�m, and the purity was99.99%.First, the SiC whiskers were dispersed by ultra-

sonication and a high-shear homogenizer. Then,the dispersed SiC whiskers, AlN powders and sin-tering additives of Y2O3 and /or SiO2 were pouredtogether and ball-milled for 12 h in alcohol usingSi3N4 pot with Si3N4 milling balls. The chemicalcomposition of the composites was listed in Table 1.The SiC whisker content in the mixture was 0, 10,20 and 30wt%, respectively. The mixture weredried, sieved through a 100 mesh sieve. Then, themixed powders were compacted into pellets underunidirectional pressure. The pellets were put intoBN coated graphite die and hot-pressed in nitrogengas at 1850�C for 1 h under 20MPa pressure. Theheating and cooling rates were both 10�Cmin.ÿ1

A three-point bending test was used to measurethe room temperature ¯exural strength, with thebar dimensions of 2.5�5�30mm and the cross-head speed of 0.5mmminÿ1. The fracture tough-ness was measured by the single edge notch beam(SENB), with a center notch 2.0mm deep in a testbar of 5�2.5�30mm and the cross-head speed of0.05mmminÿ1. For each batch, ®ve bars were tes-ted and the average values was reported. The den-sity was measured by the Archimedes displacement

method. X-ray di�raction was used to identify thephases present on the composites. Scanning elec-tron microscopy (SEM) and transmission electronmicroscopy (TEM) were used for microstructureanalysis. In order to study the whisker/matrixinterface, high-resolution electron microscopy(HRFM) was also used. Thin foils of the compo-sites were prepared by dimpling and subsequention-beam thinning.

3 Results and Discussion

3.1 Selecting sintering additive for SiCw±AlNcompositesIn order to study the e�ect of sintering additivecomposition on the mechanical properties of SiCw±AlN composites, 20wt% SiCw±AlN compositeswith di�erent sintering additive were produced.Table 1 lists the additive composition, relative den-sity, ¯exural strength and phase composition of20wt% SiCw±AlN composites. X-ray di�ractionanalysis shows that the main phase in the compo-sites was all SiC and AlN, but the secondary phasewas controlled by the additive composition. It wasreported that solid solution between SiC and AlNwas formed in a wide composition range becauseboth SiC and AlN had the similar wurtzite (2H)structure.14 In present work, however, no solidsolution was found on the di�raction patterns. Thereason was probably the low sintering temperature,which caused the atoms di�use lower than that inothers' work.14 During sintering, sintering addi-tives reacted with Al2O3 on the surface of AlN toform liquid, enhancing densi®cation behavior,therefore, the secondary phases were controlled bythe additives. When Y2O3 content was high in thesintering additive, YAM phase formed easily,however, when SiO2 content was high, 27R sialonformed.From Table 1, it was clearly seen that the com-

posite obtained high relative density and high ¯ex-ural strength with the addition of only Y2O3 if thesame amount of additives was added. When10wt% Y2O3 was added, the relative density of thecomposites was 99.0%, and the ¯exural strength

Table 1. Chemical composition, properties and phase composition of 20wt% SiCw±AlN composites

Chemical compositions Properties Phase compositionSample

AlN (wt%) SiCw (wt%) Y2O3 (wt%) SiO2 (wt%) Density (%) Strength (MPa) Main phase Minor phase

1 70 20 0 10 82 271(�37) AlN,SiC 27R sialon2 70 20 5 5 95 380(�71) AlN,SiC YAG,27R sialon3 70 20 10 0 99 508(�59) AlN,SiC YAM4 72 20 8 0 99 520(�67) AlN,SiC YAM5 74 20 6 0 97 480(�53) AlN,SiC YAM6 76 20 4 0 83 330(�48) AlN,SiC YAM,YAG

2034 Xin Jiang et al.

508MPa. However, when 10wt%SiO2 was added,the relative density was 82% only, and ¯exuralstrength 271MPa. It was related with the role ofSiO2 in the sintering. When increasing the amountOf SiO2, 27R sialon formed. As we know, 27Rsialon has elongated shape, similar to SiC whisker,which inhibited the densi®cation. In addition, for-mation of 27R sialon absorbed liquid, which alsoreduced the densi®cation.Y2O3 was often used as sintering additive in the

sintering of AlN ceramics, and usually 2±3wt%was enough. But in SiCw±AlN composites, addi-tive should be more than 6wt% because of thewhisker bridging which made the densi®cation dif-®cult. The ¯exural strength reached the maximum520MPa when 8wt% Y2O3 added. Furtherincreasing the additive content decreased the ¯ex-ural strength. The reason was that strong interfacebetween AlN matrix and SiC whisker formed whentoo much Y2O3 was added, and the formed stronginterface limited the strengthening and tougheninge�ect of SiC whisker in the composites.

3.2 Mechanical properties of SiCw±AlN compositesBased on the above study, 8wt% Y2O3 was used assintering additive to fabricate SiCw±AlN compo-sites with SiC whisker content ranging from 0 to30wt%. The mechanical properties of monolithicAlN and SiCw±AlN composites are shown in Fig. 1The density of all the materials were larger than98% of theoretic density. The mechanical proper-ties of the composites increased with the increase ofSiC whisker content. The ¯exural strength andfracture toughness increased from 308MPa,3.12MPam1/2 for monolithic AlN to 646MPa,7.17MPam1/2 for 30wt%SiCw±AlN composites,respectively. The improvement in the mechanicalproperties of the composites was achieved by theincorporation of strong whiskers. The SiC whiskerscompacted in the matrix of AlN was supposed tohighly sustain the load and strengthen the grainboundary.

3.3 Microstructure observationFracture surfaces of monolithic AlN ceramics and20wt% SiCw±AlN composites are shown in Fig. 2The monolithic AlN ceramics were consisted ofwell crystallized hexagonal AlN grains. It was alsorevealed that some AlN grains were pulled out fromthe matrix, indicating the weak bonding of grainboundaries in monolithic AlN ceramics, whichresulted in low ¯exural strength. The intergranularfracture was considered as the main fracture mode.In SiCw±AlN composite, SiC whiskers were almostuniformly distributed within the matrix, orientingwith the long axes in planes perpendicular to thehot-pressing direction. The composites were free ofsigni®cant porosity, although voids associated withwhisker agglomerates were occasionally observed.The fracture surface of the composites were quiterough. Crack propagating around the whiskers andwhisker pulling out indicated the whiskerstrengthening e�ect. It exhibited regular inter-granular fracture. The fracture origin areas werelocated close to the tension surface. However, SEMobservation did not reveal the obvious ¯aw ordefects.Because the elastic modulus and hardness of SiC

whisker signi®cantly exceed that of AlN matrix,SiC whiskers were e�ective in inhibiting the grain

Fig. 1. Mechanical properties of SiCw±AlN composites as afunction of SiC whisker content.

Fig. 2. Fracture surface of (a) monolithic AlN and (b) 20wt%SiCw±AlN composites.

Improvement and microstructure observation of SiCw±AlN composites 2035

growth of the matrix and some whiskers wereembedded into AlN grains. The grain size of themonolithic was evidently larger than that of theSiCw±AlN composites. Matrix grain re®nement wasconsidered one of the reason for the improved ¯ex-ural strength. In SiC±AlN solid solution system,mechanical properties were greatly improvedbecause of the grain re®nement. Flexural strength of1GPa was obtained for 75SiC±25AlN compositeswhich contained a dense, equiaxed grain of 1�m.The following toughening mechanism were

revealed in the microstructure observation of thecomposites: cracking de¯ection, debonding, brid-ging, cracking branching and whisker pull out(Fig. 3). Two possible approaches existed when thecrack tip reached the whisker. First, crack de¯ec-tion occurred due to the mismatch in the elasticmodulus between SiC whisker and AlN matrix.The de¯ection length of the crack mainly dependedon the radius, length and strength of the whisker. Ifthe high stress on the crack tip was larger than thestrength of the whisker, it broke the whisker andthus resulted in crack branching. Such a process alsoreleased the high stress concentrated at the crack tipand inhibited crack propagation in the matrix. Thesecond approach was interfacial debonding. whenthe stress at the crack tip was not high enough tobreak the whisker, and the interfacial fracture

energy was much lower than that of the matrix onthe whisker, the crack would propagate along theinterface and cause interface debonding. Theobservation also indicated that the development ofinterface debonding allowed the matrix crack toproceed without fracturing the whisker and thewhisker would bridge the crack behind the cracktip. The contribution of whisker bridging andinterface debonding to toughness were discussed indetail by Becher et al.15 It was found that bothdebonding and bridging would release the stressconcentration at the crack tip and inhibit crackpropagation. The toughening e�ect increased withincreasing the strength, volume fracture and radiusof whiskers.

3.4 Interfacial behaviorFor ceramic composites, the nature of whisker±matrix interface is clearly a key factor of thetoughness-related property. In the SIC±AlN sys-tem, as mentioned above, a solid solution formsover a wide composition range. Although no solid-solution phase was found by XRD analysis inSiCw±AlN composites, the whisker/matrix inter-action was detected by high-resolution electronmicroscope (HREM).Two kinds of interfacial layer between matrix

and whiskers were found in the composites. Most

Fig. 3. Toughening mechanisms shown in microstructure observation (a) crack de¯ection, (b) crack branching, (c) interfacedebonding, and (d) whisker pullout.

2036 Xin Jiang et al.

of the interfacial layers were amorphous layers.Figure 4 is a typical HREM image of grain-whiskeramorphous interfacial layer with the width of 10A.The HRFM image revealed that no interfacialreaction occurred. A more extensive examinationof some of the SiC/AlN interfaces was conductedusing analytical electron microscopy ®tted withEDAX. EDAX microanalysis revealed the exis-tence of Y in the interfacial layer. The interfacialthin layer was supposed to be a glass phase formedby sintering aids. On the other hand, a few crys-tallized layers were still found in the composites, asis shown in Fig. 5. EDAX microanalysis revealedthe absence of yttrium in the crystallized layers.The crystallized interfacial reaction layers seemedto be the products of solid solution between SiCwhisker and AlN matrix.It is con®rmed that interfacial debonding is a

prerequisite for e�ective whisker toughening. InSiCw±AlN composites, when the interface wasbonded by an amorphous layer, the weak inter-facial might be more likely to debond under stressthan a strong interface between SiC whisker and

AlN grains. When the interface was bonded by areaction layer, the whisker would be degraded andits contribution to toughening would be decreased.Figure 6 shows an unusual crack propagation inwhich the crack proceeded along the whiskergrowth axis instead of the interface bonding. Italso illustrated that the intensity stress of crack tipcaused stress stripes in the adjacent grains, but theinterfacial boundary was undamaged. One possibleexplanation of this process was that some solidsolution formed over the interface. Due to highstrength of SIC±AlN solid solution, this kind ofinterface had much stronger bonding strength thanconventional amorphous interface and decreasedthe whisker toughening e�ect.

4 Conclusions

1. SiCw±AlN composites were fabricated withdi�erent sintering additives. It was found thatthe mechanical properties of the compositesreached a maximum when Using 8wt% Y2O3

as sintering additive. It was also indicated thatthe mechanical properties increased sig-ni®cantly with the increase of SIC whiskercontent.

2. Microstructure observation of SiCw±AlNcomposite indicated SiC whiskers were welldispersed in AlN matrix. The main toughen-ing mechanisms revealed by microstructureobservation were crack de¯ection, whiskerbridging and interface debonding.

3. Two kinds of matrix/whisker interfaces werefound, one was amorphous interface, and theother was crystallized interface. The forma-tion of the crystallized interface depended onprocessing parameters, and it decreased thewhisker toughening e�ect.

Fig. 5. HREM shows the crystallized interface between SiCwhisker and AlN matrix.

Fig. 4. HREM shows the amorphous interface between SiCwhisker and AlN matrix.

Fig. 6. TEM shows an unusual crack propagation along thewhisker growth axis instead of the interface bonding.

Improvement and microstructure observation of SiCw±AlN composites 2037

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