hot isostatic pressing ceramics

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J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 16 ( 1 9 9 7 ) 1 0 8 0 1 0 8 3

Preparation and characterization of nano-structured monolithic SiC and

Si3N4 SiC composite by hot isostatic pressing

S H A O M I N G D O N G , D O N G L I A N G J I A N G , S H O U H O N G T A N , J I N G K U N G U O

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, Peoples Republic of China

Nano-structured materials are now providing aprospective research field in ceramic engineering.Because of the decrement of particle size, some

physical and mechanical properties of the materialswill be changed greatly. In structural ceramics,

Niihara [1] has proposed a new design concept,ceramic nano-composite, and divided it into threecategories: intragranular nano-composite and nanonano composite. According to this classification,many intragranular and intergranular compositeshave been developed in both oxide and non-oxideceramics [26], such as Al2O3 SiC, Al2O3 Si3N4and Si3N4 SiC. In these nano-composites, thesecond nano-dispersions are mainly dispersed withinthe matrix grains, and the fracture toughness andstrength are improved significantly. The remarkableimprovement in hardness and strength even at hightemperature are still observed [5, 7]. Strengtheningand toughening mechanisms of nano-compositeshave also been summarized by Niihara [1, 6]. Theother attractive results of nano-structured ceramicsare the machinability and superplasticity. The nano-

scale turbostratic ribbon structure of carbon formedin the pores in -phase silicon carbide, providesgood machinability in addition to the porousstructure [8]. The superplasticity of Si3N4 SiCnano-composite can probably be related to the

presence of an intergranular liquid phase [9]. Boththese novel properties may allow development usefulfor applications.

Although nano-structured materials have beendeveloped extensively in recent years, nano nanocomposites still require much study. As indicated by

previous research results [10, 11], hot isostaticpressing (HIP) can depress the grain growth so thata fine and homogeneous microstructure can beotained. The present work studied the preparationof monolithic SiC and Si3N4 SiC nano-composite byencapsulation HIP. Characterization of the micro-structure development is presented.

The starting nano-powders used in this experimentwere prepared by the chemical vapour deposition(CVD) method in our laboratory. The characteristicsof the nano-powders are listed in Table I. X-raydiffraction pattern (XRD) analysis indicated that thenano-SiC powder was mainly -phase, while thenano-Si C N precursor powder was in the amor-

phous state. Some of the Si C N powder wassubjected to pretreatment. The pretreated powderexhibited the Si3N4 and SiC compositions, whichmaintained an average particle size of 50 nm. The

monolithic SiC, amorphous Si C N, and Si3N4SiCpowders were first mixed with adhesive and thendried, screened, and uniaxally pressed to thecylindrical green compacts, 15 cm long and 10 cmdiameter. The green compacts were glass-encapsu-lated under vacuum conditions. The HIP process wasconducted both at 1750 8C, 150 MPa for 1 h for

preparing SiC Si3N4 composite, and 18508C,200 MPa for 1 h for preparing monolithic SiCceramics. The phase compositions of the specimens

prepared by glass-encapsulation HIP were deter-mined by XRD. Microstructure characterization

was performed by transmission electron micro-scopy (TEM), high-resolution electron microscopy(HREM) and energy-dispersive X-ray spectroscopy(EDS).

Fig. 1 shows the XRD patterns of monolithic SiCprepared by HIP. It is indicated that in the sinteredspecimens, the phase is mainly -SiC with a trace of-SiC. A transmission electron migrograph of thespecimen is illustrated in Fig. 2, showing the fullydense and homogeneous microstructure withequiaxed grains. The average grain size is about100 nm. This result may indicate that grain growthoccurs during HIP of the monolithic nano-SiC

powder. However, the grain size remains at thenano-scale.

Fig. 3 shows an HREM image of the monolithicSiC sintered at 1850 8C, 200 MPa pressure. A very

0261-8028 1997 Chapman & Hall

TA BL E I Characteristics of SiC and Si C N nano-powders

Nano-SiC Nano-Si C Na

C Si molar ratio 0.994

N Si molar ratio 0.445

(0.75 N C) Si molar ratio 1.232

O content (wt %) 0.88 1.48

Particle size (nm) 60 50

aPart of the Si C N powder was pretreated.

10 20 30 40 50 60 70 80

2 (deg)

Intensity(Arb.units)

Figure 1 XRD patterns of monolithic SiC specimen by HIP at

1850 8C, 200 MPa for 1 h. (d ) -SiC, (j ) -SiC.

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thin amorphous intergranular film exists at the grainboundary. The occurrence of a grain-boundary filmin monolithic nano-SiC ceramics may be attributedto the presence of SiO2 in the starting powder

particles. Because nano-powders have a largespecific surface area, they are very reactive andeasy to oxidize. The oxide will be present on thesurface of the particles, and after the HIP process,this oxide becomes the amorphous grain-boundary

phase.Fig. 4 shows XRD patterns of HIP specimens

prepared using both amorphous Si C N nano-powder and pretreated Si3N4SiC nano-powder. Asindicated by the peak locations, the main phase ofHIP specimens using Si C N powder is Si2ON2, and-SiC and -Si3N4 are also present with very lowdiffraction peaks (Fig. 4a). The formation of Si2ON2may be attributed to oxidation during material

preparation. The very fine and active Si C Namorphous powder is also easy to oxidize when itis exposed to air. As a result, the oxidation processmay lead to the formation of SiOSi, NSiO and

CSiO, which have been confirmed from theinfrared (IR) spectrum [12]. The absorption ofoxygen into the lattice of SiC N will obviouslyaffect the final phase composition of the HIP

specimens, which makes the amorphous Si C Nnano-powder change to Si2ON2, SiC and Si3N4. Forthe Si3N4 SiC powder, the HIP specimen exhibitstwo phases: -Si3N4 and -SiC (Fig. 4b). During theHIP process, -Si3N4 in the nano-powder istransformed exclusively to phase, while -SiCremains in almost the same phase in the compositeas in the starting powder.

Transmission electron micrographs of the nano-composite are shown in Fig. 5. It is shown that in theSi2ON2 Si3N4 SiC composite, many larger grains(more than 100 nm) are present in the microstruc-ture, probably because of the abnormal grain growth

Figure 2 Transmission electron micrograph of monolithic SiC speci-

men by HIP.

Figure 3 HREM image of monolithic nano-SiC sintered at 1850 8C,

200 MPa for 1 h, showing the very thin grain-boundary film.

10 20 30 40 50 60 70 80

2 (deg)

Intensity(Arb.units)

(a)

(b)

Figure 4 XRD patterns of HIP specimens formed by using

(a) amorphous Si C N nano-powder, (b) pretreated Si3N4SiC

nano-powder. (m) Si2ON2, (j ) -Si3N4, (d ) -SiC.

Figure 5 Transmission electron micrographs of HIP specimens using

(a) amorphous Si C N nano-powder, (b) pretreated Si3N4SiC nano-

powder.

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of Si2ON2 (Fig. 5a). In Si3N4 SiC composite, veryfine, homogeneous microstructure is obtained, asindicated in Fig. 5b. The average grain size is about50 nm and the morphology of the grains is ball-like.In this composite, grain growth is greatly depressed

by both the heterophase compositions and HIPconditions, which are at relatively low sinteringtemperature at high pressure.

Fig. 6 shows the HREM image of the Si2ON2

Si3N4 SiC composites. A large amount of residualamorphous phase is present at the grain boundariesin this composite. EDS analysis indicated that themain composition of this grain-boundary phase issilicon and oxygen, as shown in Fig. 7. This resultimplies that Si C N amorphous nano-powder mayreact with the absorbed oxygen and partly formSi2ON2, as mentioned previously, and the othersremain as amorphous phase in the grain boundaries,which coexist with Si3N4 and SiC to form thecomposite. Fig. 8 shows the HREM image ofSi3N4 SiC nano-composite. No residual amorphous

phase was found either at the grain boundary or atthe triple-grain junction. This is because the excesscarbon content in the precursor Si C N powder, aslisted in Table I, may remove the oxide during the

pretreatment. The reactions between carbon andoxidized powder may be described by the followingequations:

Si O Si 3C 2SiC CO (1a)

4N Si O 5C SiC Si3N4 4CO (1b)

C Si O C SiC CO (1c)

The reaction product of CO in Equations 1ac maybe substituted by CO2. Either way, in this process,part of the oxidized amorphous nano-powder may bepurified and crystallized to SiC and Si3N4.

Not only can carbon react with the absorbedoxygen in the Si C N lattice during pretreatment,

but, it can also react with the oxide, SiO2 during theHIP process. In addition, carbon may also play animportant role in separating particles during crystal-lization from amorphous Si C N nano-powder to thenano-powder composed of Si3N4 and SiC, so thatgrain growth is deeply depressed. The crystallizedSi3N4 and SiC powders will retain their originalgrain size after the HIP process, as indicated in Fig.5b, and therefore, the Si3N4 SiC nano nano com-

posite is obtained.In summary, nano-structured monolithic SiC

ceramics can be prepared by hot isostatic pressing.At 1850 8C, 200 MPa pressure for 1 h, dense andhomogeneous microstructure with a grain size ofabout 100 nm can be obtained. The starting powdermaterials may affect the microstructure of the finalcomposites significantly. The use of nano-Si C Namorphous precursor powder will lead to theformation of Si2ON2 because of the oxidation ofSi C N during material preparation. The formationof Si2ON2 can also lead to abnormal grain growth.Si3N4 SiC nano-composite can be prepared by usingthe pretreated Si3N4 and SiC nano-powder. At

1750 8C, 150 MPa pressure for 1 h, dense andhomogeneous microstructure with an average grainsize of 50 nm can be obtained. The depression ofgrain growth in this composite may be ascribed to

Figure 6 HREM image of HIP Si2ON2 Si3N4 SiC composite show-

ing a large amount of amorphous phase in the grain boundary.

10

5

0

0 1 2 3Energy (keV)

(Countss

2

1)

O

Si

Figure 7 EDS analysis of the grain-boundary phase in Si2ON2Si3N4 SiC composite.

Figure 8 HREM image of Si3N4 SiC composite showing the clean

grain boundaries and triple-grain junction.

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the heterophase compositions and HIP conditions.The existence of carbon in the nano-powder will

benefit the removal of the oxide in grain boundaries.

AcknowledgementsThe authors thank Ms M. L. Yan for her helpfulwork in microstructure analysis. The financialsupport by the National Natural Science Foundationof China is also appreciated.

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Received 27 November 1996and accepted 3 April 1997

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hot isostatic pressing ceramics

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