Journal of Materials Processing Technology 89–90 (1999) 457–461

In-situ HIP synthesis of TiB2/SiC ceramic composites

Zhu Degui *, Liu Shikai, Yin Xiandong, Yang Liu, Xiao Chuanchun,Zhou Haoming, Zhang Jianyong

Department of Materials Engineering, Southwest Jiaotong Uni6ersity, Chengdu, Sichuan, 610031 People’s Republic of China

Abstract

Dense TiB2/SiC ceramic composites were fabricated by means of in-situ HIP synthesis processing from TiH2, Si, C and B4Cpowder blends. The microstructure and mechanical properties of the ceramic matrix composites were investigated using X-raydiffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy and microhardness. The grains ofTiB2 and SiC in the ceramic matrix composites are fine, the TiB2 grains being regular-platelet, and the SiC grains being irregularpolygon. The mechanical properties of the composite are mainly affected by their densities. As the density increases, the propertiesof the composites improve. The microhardness, fracture toughness KIC and flexural strength are, respectively, 14.80 GPa, 4.75MPa m1/2 and 408.62 MPa in the 96% density TiB2/SiC composites. The crack propagation path is deflected in these composites.TiB2/SiC composites also have an excellent oxidation resistance. The average oxidation rate of TiB2/SiC is about 10−10m s−1

from 1073 to 1473 K. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: In-situ synthesis; Hot isostatic pressing; TiB2/SiC; Composites

1. Introduction

The demand for materials exhibiting exceptionalhardness and stability at very high temperature has ledto a renewed interest in refractor materials such ascarbides, borides, beryllides, nitrides, and silicides.Other potential applications for this class of materialsinclude armor plates, cutting tools and dies [1,2]. How-ever, the use of these single-phase ceramics, even fullydensified, in structural or wear applications is limitedby the variability of their mechanical strength and theirpoor fracture toughness. Second phase additions canimprove the strength and the fracture toughness ofsingle ceramics. Full densities require extremely highsintering temperatures and pose a veritable challenge.

SiC is an extremely hard and wear resistant materialwhich has, furthermore, excellent corrosion, thermalshock and oxidation resistance. All of these propertiestogether with its good high temperature strength allowthe use of SiC for numerous structural and wear appli-cations, e.g. heat exchangers, metal working parts and

nozzles. However, the moderate fracture toughness ofSiC limits its use.

TiB2 is also an extremely hard and wear resistantmaterial which has a relatively high fracture toughness,good electrical conductivity and excellent chemical re-sistance to molten non-ferrous metals. These character-istics make TiB2 and TiB2-based composites attractivematerials for land-based armor, cutting tools, Heroult-Hail cells, wear-resistant and vacuum metallization ap-plications. However, the relatively poor oxidationresistance of TiB2 limits its use under severe conditions,e.g. in the atmosphere or in any oxidating gas.

TiB2 and SiC are representative of refractory com-pounds with very high melt points and high hardness.The addition of TiB2 particles was found to be effectivein terms of increasing the fracture toughness of SiC [3].Thus, TiB2/SiC composites could be tailored with verygood properties, depending on their respective goodcharacteristics.

Generally, TiB2/SiC composites are prepared by di-rectly mixing TiB2 and SiC powders and then hotpressing the powder mixture. The sintering temperatureof composites is very high due to their high meltingpoint and chemical stability. For example, the hot-pressing of TiB2 and SiC at 2251 K and 17 MPa for

* Corresponding author.E-mail address: nhuang@center2.swjtu.edu.cn (Z. Degui)

0924-0136/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 1 3 7 - 5

Z. Degui et al. / Journal of Materials Processing Technology 89–90 (1999) 457–461458

Table 1Synthesis processes and products of all samples

Reaction temperature (K) Hold time (s)Sample Pressure (MPa)Composition (mole ratio) Products

Si C B4CTiH2

1 0 1 1673 7200A1 502 2TiB2+SiC1 0 1 16732 7200A2 50 2TiB2+SiC

2A3 1 0 1 1673 7200 50 2TiB2+SiC2B1 8 7 1 1773 7200 50 2TiB2+8SiC

8 7 1 18732 7200B2 50 2TiB2+8SiC5 4C1 12 1773 7200 50 2TiB2+5SiC5 4 1 18732 5400C2 80 2TiB2+5SiC

2C3 5 4 1 1873 7200 50 2TiB2+5SiC

7200 s, yielded samples that were between 95.2 and99.6% dense [4]. Recently, self-propagating high-tem-perature synthesis (SHS), and special, dynamic com-paction (DC), has been applied in the fabrication ofTiB2/SiC composites, which contained a lot of cracksbecause of their high reactive temperature and largecooling rate [5]. TiB2/SiC composites are also producedby in-situ hot pressing synthesis at 2273 K, 30 MPa for360 s [6].

In this paper, high density TiB2/SiC ceramic com-posites were fabricated by means of an in-situ HIPsynthesis process from TiH2, Si and B4C powders. Themicrostructure, mechanical properties, tougheningmechanism, and oxidation behavior were investigated.

2. Experimental procedure

The raw materials are TiH2 powder (purity\99.5%,particle size B50 mm), Si powder (purity\99.5%), Cpowder (purity\98%) and B4C powders (purity\99%, particle size B3.5 mm). These powders withdifferent mixing ratio were mixed in alcohol, ball milledfor 14 400 s, and then dried at 353K for 43 200 s. Thepowder mixtures were uniaxially pressed into 20×20×50 mm green compacts, and then isostatically pressed at200 MPa. Further, TiB2/SiC composites were producedby means of in-situ synthesis under hot isostatic press-ing (HIP) with glass coating at 1673–1873 K under50–80 MPa Ar atmosphere for 7200 s. The synthesisprocesses of all samples are listed in Table 1.

XRD was used to determine the phase types of theproducts. Optical microscopy (OM) and scanning elec-tron microscopy (SEM) were used to investigate theirmorphologies. The densities of the products were testedby water displacement. The fracture strength was mea-sured on bars (3×4×25 mm) cut from the hot iso-static processing specimen by the three-point bendingmethod, and the fracture toughness of the samples bythe Vickers hardness method. The Vickers hardness wastested at a load of 9.8 N. Transmission electron mi-

croscopy (TEM) and SEM were conducted to observethe microstructures of the composites.

The oxidation behavior of the composites was testedby oxidized specimens in atmosphere. The weights anddimensions of all specimens were measured after dryingthe specimens at 673 K for 18 000 s. Then each speci-men was heated to 1073–1473 K in 2400 s and held for7200 s. The changes of weights were measured, and theaverage oxidation rate was evaluated.

3. Results and discussion

3.1. Microstructure

Fig. 1 shows the XRD pattern of sample A1. Peaksdue only to SiC and TiB2 phases appeared. Therefore,the suggested combustion reaction of:

2Ti+Si+B4C=SiC+2TiB2

seemed to be completed.

Fig. 1. XRD pattern of A1 sample.

Z. Degui et al. / Journal of Materials Processing Technology 89–90 (1999) 457–461 459

Fig. 2. Optical micrograph of the polished cross-section of thecomposites: (a) A3; (b) B2; (c) C2.

Fig. 3 is the TEM micrograph of the composite. Itcan be seen that the grains of TiB2 and SiC are fine.The interfaces of the composites are clean. The crystalstructure of TiB2 is polycrystalline. TiB2 grain is regu-lar-platelet and less than 5 mm. SiC has an interstitialNaCl crystal structure in TiB2/SiC composites. The SiCgrain is irregular polygonal. The grain size of SiC in theA-series samples is less than 10 mm, but some large SiCgrains formed in B- and C-series samples.

3.2. Mechanical properties

Table 2 summarizes the density, Vickers hardness,bend strength, and fracture toughness of the TiB2/SiCcomposites. It can be seen that the Vicker’s hardnessimproves with increase in density. Comparing withZhang’s result [6], the composites have higher fracturetoughness and bend strength and similar hardness anddensity. However, all of the mechanical properties ofthe composites are much lower than those of the TiB2/TiC composites. The densities of the composites de-crease with increase of the SiC. This shows that thesintering ability of SiC is lower than that of TiB2.

3.3. Toughening mechanism

TiB2/SiC composites have higher fracture toughnessthan TiB2 and SiC [6]. Fig. 4 is a SEM micrograph ofthe polished surfaces of the composite. It can be seenthat the crack propagation path is deflected in the

Fig. 3. TEM micrographs of: (a) TiB2; and (b) SiC; in the composites(A3).

Table 1 shows the results of the XRD analysis of allspecimens. Here, the mole ratios are calculating resultsaccording to their designed reactions. Fig. 2 shows themorphologies of the polished cross-sections of specimenA3, B2, and C2. The bright areas are the TiB2 phases,the light-gray areas are SiC grains, and the dark-grayareas are closed pores. The distribution homogeneity ofSiC is decreased by the increasing of SiC, and theporosity of the composite is increased. It indicates thatSiC leads the reduction of the sintering ability of TiB2/SiC composites.

Z. Degui et al. / Journal of Materials Processing Technology 89–90 (1999) 457–461460

Table 2Density and mechanical properties of the TiB2/SiC composites

DensitySample Relative density (%) Vicker’s hardness (GPa) Fracture toughness (MPa m1/2) Bend strength (MPa)(g cm−3)

91.55A1 11.123.75 4.76 –92.29 12.90A2 4.003.78 –96.22 14.803.64 4.75A3 408.62

3.06B1 87.30 – – –3.11B2 88.73 15.15 4.71 –

88.86 9.383.22 4.56C1 –91.35 17.72 5.36C2 304.113.3189.51 15.64 4.30 –C3 3.24

composite. Fig. 5 shows a SEM micrograph of thefracture surface of the composite. The crack propa-gated primarily in the intergranular mode with someevidence of the transgranular mode. It is different fromthe dynamically consolidated TiB2 and SiC [5]. Thefracture toughness is increased, since the crack propa-gation path is deflected in the intergranular mode.Additionally, the fracture toughness is improved by thesemi-infinite cracks [7] and the residual stress in thecomposite.

3.4. Oxidation beha6ior

SiC is an excellent oxidation resistant material be-cause a dense SiO2 glass film can be formed at itssurface. Thus TiB2/SiC composites may have betteroxidation resistance than TiB2/TiC composites. Theoxidation behavior of these composites at differenttemperature is listed in Table 3.

The weights of the composites are increased in speci-mens oxidized at different temperatures. Sample B2 hasa similar oxidation resistance as sample A3. The aver-age oxidation rate of TiB2/SiC composite is about10−10m s−1 from 1073 to 1473 K. At lower oxidationtemperatures, especially at 1073 K, the oxidation resis-tance of TiB2/TiC is as good as that of TiB2/SiC.However, at higher oxidation temperature; TiB2/TiChas a much poorer oxidation resistance whilst TiB2/SiCalso has an excellent oxidation resistance. Good me-chanical properties together with its excellent oxidationresistance allow the use of TiB2/SiC composite fornumerous structural and wear applications. However,TiB2/TiC composites cannot be used at high tempera-tures in the atmosphere.

4. Conclusions

In this study, higher density TiB2/SiC compositeswere fabricated from TiH2, Si, C and B4C powderblends by means of in-situ HIP synthesis processing.TiB2 and SiC are the final phases in the composites.

The grains of TiB2 and SiC are fine. The TiB2 grainsare regular-platelet, and the SiC grains are irregularpolygon. The mechanical properties of the compositesare mainly affected by their densities. Their Vicker’shardness, fracture toughness, and bend strength are,respectively, 14.80 GPa, 4.75 MPa m1/2, and 408.62MPa in the 96% density 2TiB2/SiC composite.

Fig. 4. The indentation crack in TiB2/SiC.

Fig. 5. Fractograph of TiB2/SiC.

Z. Degui et al. / Journal of Materials Processing Technology 89–90 (1999) 457–461 461

Table 3Oxidation behaviors of TiB2/SiC and TiB2/TiC composites

Specimen OxidizedWeighting (mg)

Before oxida- After 1073 K After 1273 K After 1473 K After 1073 K After 1273 K After 1473 K Totaltion

674.0 680.0 689.0A3 2.204672.0 6.611 8.955 17.770B2 559.0555.0 564.0 566.0 6.917 8.647 3.489 19.053

1090.0 1103.0 1146.0 2.5181087.0 10.911TiB2/TiCa 36.090 49.519

a TiB2/TiC has 100% density, and the mole ratio of TiB2:TiC is 2:1.

TiB2/SiC composites also have an excellent oxidationresistance. The average oxidation rate of TiB2/SiC isabout 10−10m s−1 from 1073 to 1473 K.

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