Materials and Design 33 (2012) 231–235

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Materials and Design

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Short Communication

The effects of pre-oxidation on the sintering and mechanical property ofpowder injection moulded SiC material

Zhen Lu a,⇑, Zhenlong wang b, Kaifeng Zhang a, Changrui Wang a

a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, Chinab School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 May 2011Accepted 11 July 2011Available online 12 August 2011

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.07.025

⇑ Corresponding author. Tel.: +86 451 86413681.E-mail address: luzhenhit@hit.edu.cn (Z. Lu).

Usually, injection moulded SiC green parts are debound in inert atmosphere or vacuum, which inducesthe residual carbon and increases forming cycle and production cost. In this paper, injection mouldedSiC with Al2O3 and Y2O3 as sintering assistant was thermal debound in air and Ar, respectively. The paperinvestigates the effects of pre-oxidation during debinding stage on the sintering and mechanical propertyof SiC material. During sintering, the oxide SiO2 is in favour of the shrinkage of debound samples at lowertemperature. After sintering, the linear shrinkage of sintered samples with pre-oxidation is bigger thanthe sample without pre-oxidation. Test results by TEM and XRD indicate that SiO2 disappear from theinside of the sintered samples. The loss of SiO2 decreases the content of Al2O3, which affects the formationof YAG (Y3Al5O12). Sintered Sic samples contain a-SiC phase and intergranular phase. There is no hetero-phase between the boundaries of a-SiC phase and intergranular phase. The bending and compressionstrength values of sintered samples with pre-oxidation reach to 537 MPa and 2.89 GPa, respectively.These values approach the strength of sintered samples without pre-oxidation (594 MPa and 3.0 GPa).

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1. Introduction

Silicon carbide (SiC) is a well-known structural ceramic, whichhas many excellent high-temperature properties, such as strength,hardness, chemical stability [1–3]. However, its machinability is sopoor, that the engineering applications are limited especially forthese parts with complex shape. As we know, powder injectionmoulding (PIM) is an appropriate technique for manufacturing ofcomplex parts using ceramic or metal powders [4–6]. Someresearches have been focused on the effects of PIM process onthe forming and property of SiC material. Zhang manufacturedSiC rotor with complex shape by PIM [7]. The irregular shaped partof silicon carbide with a relative density of 98.2% has been pro-duced by Zhang using PIM process [8]. PIM contains four processstages, i.e. feedstock preparation, injection moulding, debindingand sintering [9,10]. Ceramic or metal powders are mixed togetherwith binder system by feedstock preparation process to formhomogeneous materials. During injection moulding, the mouldedgreen part is injection moulded in the mould cavity. Debinding isa binder removal process from PIM green samples. During the re-moval of binder, defects such as cracking, blistering, warping,and delamination usually occur in the samples. There are so manyfactors affecting these defects, such as size and shape of samples,heating-up rate and holding-time and shielding gas atmosphere.

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Sachanandani and Lombardo focused their study on the effect ofgreen body size and heating rate on failure during thermal debind-ing and on the debinding cycle time [11]. For SiC, shielding gasatmosphere is more important, as it could be oxidated in air atmo-sphere at lower 1000 �C [12,13]. In order to avoid the oxidation ofSiC, the thermal debinding is always performed in inert atmo-sphere or vacuum for moulded SiC green parts. Despite in inertatmosphere, different kind of gas has different effects on the sinter-ing and property of SiC. For example, Rodríguez-Rojas studied theeffect of Ar or N2 sintering atmosphere on the oxidation behaviourof SiC [14]. A pre-sintering at 1200 �C with inert gas shieldingusually follows the thermal debinding to ensure the strength ofdebound products during transportation. This not only increasesthe forming cycle but also enhances the production cost. On theother hand, residual carbon during thermal debinding with inertgas shielding is a common phenomenon for powder injectionmoulding, which may affects the sintering and property of product.In order to remove the residual carbon and increase the productionefficiency of powder injection moulded SiC with metal oxide asadditives, thermal debinding in air were performed in our previousinvestigations [15]. The effects of oxidation on the forming prop-erty during debinding stage and strength of debound samples havebeen studied. It was found that, SiC blanks have enough strengthfor transport after being debound in air atmosphere at lowertemperature than that without oxidation. However, how the pre-oxidation affecting the sintering property of SiC material is noclear. On the other hand, the density and microstructure affected

232 Z. Lu et al. / Materials and Design 33 (2012) 231–235

by the sintering have a remarkable relation with the mechanical,thermal, and chemical properties of SiC material. So, the purposeof this study is to investigate effects of pre-oxidation duringdebinding stage on the sintering and mechanical property ofpowder injection moulded SiC.

Fig. 1. Moulded, debound and sintered parts.

Fig. 2. Linear shrinkage and density of sintered parts at 1900 �C.

2. Experimental procedure

SiC (90 wt%) powders with a mean particle size of 0.8 lm andY2O3/Al2O3 (10 wt%) powders in the mole ratio 3/5 as sintering aidswere ball-milled for 10 h in ethanol using ZrO2 balls. Feedstock wasprepared by mixing ceramic powders (55 vol%) and binder system(paraffin wax (PW) + polypropylene (PP) + stearic acid (SA)) in adouble star mixer. Firstly, compound powders were heated up to180 �C for 30 min. Then, polymer based binder system were putinto and mixed. After compounding, the feedstock was granulatedby single-screw extrusion machine for several times. Plastictemperature and rotational speed of screw is 180 �C and 600 r/min. Bending samples green parts were injection moulded by theBabyplast6/10 microinjection moulding machine. The plastictemperature and mould temperature is 190 �C and 60 �C, respec-tively. Injection pressure and cooling time is 100 MPa and 10 S,respectively. Dimensions of the bending samples green parts are3 mm � 4 mm � 40 mm. Compression test samples were cut fromthe sintered bending samples. Differing from the conventionalmethod, moulded green parts were debound in air furnace under550 �C, 650 �C, 750 �C, 850 �C, and 950 �C, respectively. Corre-spondingly, some samples were debound from room temperatureto 1200 �C with argon gas shield in tube furnace. Then, deboundsamples were sintered at 1900 �C for 1 h with Ar shield. Theheating rate is 25 �C/min to 1000 �C, 15 �C/min to 1600 �C,followed by 10 �C/min to 1900 �C. The length of moulded parts(Lm) and sintered part (Ls) was measured. In order to investigatethe effects of pre-oxidation on the sintering and mechanical prop-erty, the density, microstructure, bending and compressionstrength of different sintered samples were analyzed.

An electronic balance namely sartorius BS124S was used to testthe mass changes of different samples. Densities of the sinteredceramics were measured using the Archimedes methods. The sur-face and fracture morphology of bending samples were observedby the S-3400 scanning electron microscope. The compositions oftest specimens were examined using the X-ray diffraction (XRD)in Rigaku D/max-rB using Cu Ka radiation. Microstructure observa-tion and energy dispersive spectroscopy (EDS) analysis of sinteredcompacts were carried out using the FEI TECNHI G2 F30 transmis-sion electron microscopy (TEM). Bending and compression testswere performed by the Instron5569 universal testing machine.The calculation of strength values were conformed based onmeasuring the dimensions of different sintered samples.

3. Results and discussion

The moulded, debound and sintered SiC samples are shown inFig. 1. As can be seen, all sintered samples shrink obviously com-paring with moulded and debound parts. The linear shrinkage isconformed by (Lm–Ls1900�C)/Lm: Lm – the length of moulded greenpart; Ls1900�C – the length of sintered samples at 1900 �C. Fivesamples were tested for each temperature. The average value anddimensional variation are displayed in Fig. 2. The linear shrinkageincreases with the increasing of debinding temperature from550 �C to 950 �C, as can be seen in Fig. 2. On the other hand, allsintered samples debound beforehand in air have bigger shrinkagethan the samples debound beforehand in Ar, which indicates thatpre-oxidation is advantageous for the shrinkage of SiC. Thephenomenon is different with the research result of some other

scholars [16]. In their study, they believe that SiO2 is disadvanta-geous for the shrinkage of SiC material. However, in their studySiO2 was added in as particle form during the mixed process. Inthis study, SiO2 covered the surface of SiC particles during thedebinding stage in air furnace. As we know, SiO2 has better sinter-ing activity compared with SiC particle. It could be densified bysintering at 1200 �C. In this paper, a part of debound samples underdifferent temperature were sintered together at 1200 �C in tubefurnace with Ar shield. The linear shrinkage is conformed by theratio of (Lm–Ls1200�C) to Lm. These linear shrinkages of differentsamples (550–950 �C) are 0.7%, 0.7%, 1.0%, 1.5%, and 4.1%, respec-tively. So, it can be concluded that SiC with SiO2 covered thesurface starts to shrink at lower temperature. Nevertheless, thechange tendency of density belong to different samples is inconsis-tent with the linear shrinkage, as they have different weight lossafter the final sintering stage. The weight loss of different sinteredsamples is 5.00% (pre-oxidation at 550 �C), 5.10% (650 �C), 7.54%(750 �C), 10.12% (850 �C), 4.05% (without pre-oxidation), respec-tively. So, the density of sintered samples without pre-oxidation

Fig. 3. Surface morphology of sintered sample.

Table 1Components of the solidified liquid drop.

Element Weight (%) Atomic (%)

C 39.68 55.98O 16.58 17.56Al 2.66 1.67Si 41.08 24.79

Fig. 4. XRD results of debound and sintered samples: (a) debound at 550 �C; (b)debound at 550 �C and sintered at 1900 �C; (c) debound at 850 �C and sintered at1900 �C.

Table 2Components of the intergranular phase.

Element Weight (%) Atomic (%)

O 21.47 47.64Al 22.91 30.14Y 55.60 22.20

Fig. 5. TEM morphology in HAADF mode of sintered SiC sample (pre-oxidated at750 �C).

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is not the smallest, despite its linear shrinkage is the smallest.There is no the density of sintered sample with pre-oxidation at950 �C, because the defect as displayed in Fig. 1 affects the testresult.

The separation of liquid at high temperature during the sinter-ing sate induces the weigh loss. Fig. 3 displays the surface mor-phology of sintered sample with pre-oxidation at 850 �C. As canbe seen, some solidified liquid drops distributing on the surface.Test results by energy spectrum analysis indicate that Si, O, Aland C are the components of the solidified liquid drop, as displayedin Table 1. C and a part of Si come from the SiC under the drop. SiO2

and Al2O3 could form liquid based on the binary phase diagram.Some liquid will flow out from the samples along with the shrink-ing during sintering stage at high temperature, which induces theloss of SiO2 and Al2O3. As we know, the suited ratio of Al2O3 andY2O3 to form YAG (Y3Al5O12) is 5:3 or 3:2 [17,18]. The decreaseof Al2O3 affects the formation of YAG. So, there is no obviousdiffraction peak of YAG phase in the sintered samples, but occursa weak diffraction peak of YMP (YAlO3), as displayed in Fig. 4.The suited ratio of Al2O3 and Y2O3 to form YMP is 1:1. Moreover,the weight loss of Al2O3 increases with the increasing of SiO2

formed during the debinding stage. So, the diffraction peak ofexcessive Y2O3 occurs in the sintered sample when the pre-oxida-tion temperature increases to 850 �C. As we know, the oxidationbehaviour of SiC powder below 1000 �C belongs to passive oxida-tion. For the reaction of SiC and O2, its mechanism could includea series of intermediate steps, such as buck diffusion, physisorp-tion, chemisorption, surface penetration, diffusion within the layerand chemical reaction. Temperature and reaction time are two keyfactors affecting the rate of these steps. So, the content of SiO2

could be controlled by adjusting the debinding temperature andtime. Moreover, the type of glass phase in the sintered SiC samplescould be designed.

On the other hand, the internal structure of sintered sample wasobserved by TEM, as shown in Fig. 5. Sintered sample contains twophase, i.e. SiC and intergranular phase. Some SiC grains are sur-rounded by glass phase and some glass phase distributes in thetriple point of SiC grains. The intergranular phase prevents theatom diffusion among different SiC particle. So, the average grainsize of sintered SiC is about 1 lm. Composition analysis indicatesthat Al, Y, and O are the components of intergranular phase, asshown in Table 2. No Si element is found in the intergranularphase. There is no impurity between the boundaries of SiC andintergranular phase, as shown in Fig. 6. It can be concluded thatall SiO2 flow out from the inside of samples and some Al2O3 remain

Fig. 6. HREM images of boundary between SiC and intergranular phase.

Table 3Mechanical property of different sintered samples at 1900 �C.

Debinding temperature (�C) 550 in air 650 in air 750 in air 850 in air 1200 in Ar

Bending strength (MPa) Average 475 537 492 506 594Measured 477/491/457 556/524/540 507/486/483 516/514/489 563/616/603

Compression strength (GPa) Average 2.73 2.89 2.77 2.80 3.09Measured 2.79/2.68/2.72 2.82/2.86/2.99 2.79/2.70/2.82 2.81/2.75/2.84 3.11/3.22/2.94

Fig. 7. Fracture morphology of sintered SiC sample (pre-oxidated at 850 �C).

234 Z. Lu et al. / Materials and Design 33 (2012) 231–235

after final sintering. As we know, Y2O3 and the residual Al2O3 couldform liquid phase under 1900 �C during sintering stage, whichpromotes the shrinkage and densification much more.

Despite no residual SiO2 was found in the inside of sinteredsamples, linear shrinkage and density were affected by its occur-rence, which have close relation with the mechanical property ofsintered samples. On the other hand, the loss of SiO2 and Al2O3

affects the formation of YAG phase. As we know, the mechanicalproperties of the liquid phase sintered SiC materials depend onthe content and style of intergranular phase, which depends onthe mixture ratio of Y2O3 and Al2O3. So, the bending and compres-sion strength of sintered samples were tested to evaluate the effectof pre-oxidation on the mechanical property. Test results aredisplayed in Table 3, as can be seen the strength of sintered sam-ples with pre-oxidation are approximate with the test results bysome other researchers, such as Zhang [15] (300–450 MPa) andSuzuki [19] (490–540 MPa). The fracture type is dimple style formas can be seen from Fig. 7. Combining with the liquid distributionin Fig. 5, it can be found that the fracture occurs along the bound-aries between SiC and intergranular phase. As we know, the weightloss of sintered samples induces the decreasing of liquid phase. So,the strength values of sintered samples with pre-oxidation arelower than that without pre-oxidation. On the other hand, defectis a key factor affecting the mechanical property of sintered SiCmaterial, such as pore. The change of porosity induces the differ-ence on mechanical property among these sintered samples withpre-oxidation. The distinction of porosity belonging to differentsamples is induced by different linear shrinkage and weight loss.Among the sintered samples with pre-oxidation, the sample pre-oxidated at 650 �C has the smallest porosity. So, its bending andcompression strength is close to the samples without pre-oxidation.

4. Conclusions

(1) SiO2 covering on the surface of SiC particles is in favour ofthe shrinkage of debound samples at low temperature. Allsamples with pre-oxidation start to shrink at 1200 �C. Thelinear shrinkage of final sintered samples is bigger than thatwithout pre-oxidation.

(2) Test results by TEM show, SiO2 disappear from the inside offinal sintered samples. The sintered samples contain a-SiCphase and intergranular phase.

(3) The bending and compression strength of sintered sampleswith pre-oxidation are range from 475 to 537 MPa and from2.73 to 2.89 GPa.

(4) Based on the analysis of shrinkage, density, microstructure,and mechanical property of sintered SiC, it can be concludedthat thermal debinding in air furnace is feasible for SiC withoxide as sintering assistant. The suitable debinding temper-ature in air furnace should be conformed by the requirementof dimension, mechanical property, and so on.

Acknowledgments

The financial support from the ‘‘The Research Fund for the Doc-toral Program of Higher Education’’ (RFDP20102302120002),‘‘Opening Funding of State Key Laboratory of Materials Processingand Die & Mould Technology, Huazhong University of Science andTechnology’’ and ‘‘China Postdoctoral Science Foundation fundedproject’’ are greatly acknowledged.

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