Effects of oxidation on the strength of debound SiC parts by powderinjection moulding

Zhen Lu a,⁎, Kaifeng Zhang b, Changrui Wang a

a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, Chinab National Key Laboratory for Precision Heat Processing of Metal, Harbin Institute of Technology, Harbin, 150001, China

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

Article history:Received 10 September 2010Received in revised form 12 November 2010Accepted 3 December 2010Available online 10 December 2010

Keywords:Powder injection mouldingSiCDebindingOxidationBending strength

In order to increase the production efficiency of powder injection moulding for SiC parts, thermal debindingwas performed in air furnace without gas shield. Bending tests were performed to evaluate the strength ofsamples debound under different temperatures. The effects of oxidation on debinding process were alsoanalyzed. Analysis indicates that air could accelerate the debinding rate of green parts without defectsoccurring. The bending strength of debound samples increases from 6.55 MPa to 11.58 MPa as the pre-sintering temperature increases from 550 °C to 850 °C. On the other hand, the bending strength of the samplespre-sintered at 1200 °C in argon atmosphere is only 11.52 MPa. It was found that, blanks have enoughstrength for transport after being pre-sintered in air atmosphere at 850 °C. The technology could reduce therequirement for heating equipment and enhance the efficiency of debinding for SiC parts.

Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction

As a high-temperature structural material, silicon carbide (SiC)ceramic has many advantages, including high melting temperatures,low density, high elastic modulus and strength, and good resistanceto wear [1–5]. However, the industrial applications for these SiCparts with complex shape are limited due to their poor machinabilityand high manufacturing cost. Powder injection moulding (PIM) isjust a near-net shape processing technique which could manufacturecomplex parts using ceramic or metal powders [6–11]. PIM containsfour process stages, i.e. feedstock preparation, injection moulding,debinding (followed by pre-sintering) and sintering [12–15].Thereunto, debinding stage needs the longest time. On the otherhand, a lot of defects usually occur during the debinding stage.Despite a large number of advanced debinding methods (microwavetreatment, supercritical debinding, solvent extraction, catalyticdebinding, etc.) have been developed, which speed up this processand make it more effective, thermal debinding is still the mostpopular method until now [16–18].

In order to avoid the oxidation of SiC powder, thermal debinding isalways performed in inert atmosphere or vacuum. Debound samplesmust have enough strength which is convenient for the transport. So,a pre-sintering at 1200–1300 °C for 1–2 h with inert gas shieldingusually follows the thermal debinding. This increases the formingcycle and enhances the production cost. Until now, many studies have

been carried out on the oxidation of SiC, and several models have beendeveloped to explain its mechanism. Among the researches of SiCmaterials, the oxidation of SiC powder is much more poorlyinvestigated, especially for sub-micrometer SiC powder in airatmosphere under lower temperature [19–23]. Otherwise, commonviewpoint thinks that oxidation data in low temperature of SiCpowder is rarely available. So the influence of low temperatureoxidation on the forming properties of SiC components by powderinjection moulding is not clear. The purpose of this study is toinvestigate the effects of oxidation on the strength of SiC partsdebound in air atmosphere.

2. Experimental section

The silicon carbide powders from Mudanjiang Jingangzuan BoronCarbide Co., Ltd in China were used in this study, as shown in Fig. 1.Thermo-gravimetric analysis (TGA) was performed for SiC powderswith a heating rate of 5 °C/min from room temperature to 1000 °C inair atmosphere. SiC (90 wt.%) powders with a mean particle size of0.8 μmand Y2O3/Al2O3 (10 wt.%) in the ratio 3/5 as sintering aidsweremixed. Feedstockwas prepared bymixing ceramic powders (55 vol.%)and binder system (paraffin wax+polypropylene+stearic acid) in adouble star mixer. After compounding, the feedstock was granulatedby a single-screw extrusion machine.

Two kinds of samples were moulded on a Babyplast6/10 injectionmoulding machine, as shown in Fig. 2. Dimensions of the bendingsample are 3 mm×4 mm×40 mm. On the other hand, the diameterand thickness of the disk are 25 mm and 4 mm, respectively. Differingfrom the conventional method, debinding was carried out in an air

Powder Technology 208 (2011) 49–53

⁎ Corresponding author. Tel.: +86 451 86413681.E-mail address: Luzhen-hit@163.com (Z. Lu).

0032-5910/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2010.12.002

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furnace from room temperature to 500 °C followed by pre-sintering.Samples were supported in SiC powders to prevent slumping and toassist in partitioning of melting wax. Pre-sintering is the last phase ofthe debinding process. In this paper, five kinds of pre-sinteringtemperatures were fixed on, i.e. 550 °C, 650 °C, 750 °C, 850 °C, and950 °C. The holding time is 2 h. Otherwise, some samples weredebound and pre-sintered at 1200 °C in a tube furnace with argon gasshield. The heating rate is 1 °C/min to 80 °C, 0.5 °C/min to 500 °Cfollowed by 2 °C/min to the pre-sintering temperature. All the holdingtime at 180 °C, 380 °C, 500 °C and the pre-sintering temperature are2 h. Debound samples were sintered at 1900 °C for 1 h with Ar shield.

An electronic balance namely sartorius BS124Swas used to test themass of different samples. XRD measurements were conducted on aD8 DISCOVER X-ray Diffractometer. Bending tests for debound andsintered samples were performed by the Instron5569 universaltesting machine. The fracture morphology of bending samples wasobserved by an S-3400 scanning electron microscope.

3. Results and discussion

Bending samples in moulded and debound states are shown inFig. 3. As can be seen, samples present different colors. The sampledebound in argon atmosphere presents approximate color withoriginal SiC powder. However, samples debound in air atmospherewhiten. In addition, the whitening becomes more marked with theincrease of pre-sintering temperature from 550 °C to 950 °C.

Fig. 4 displays the TGA test result of SiC powders in air atmosphere.As shown, the weight gain starts at about 500 °C and rises after 900 °C.This indicates that, the extremely high surface areas of SiC lead to asignificant adsorption of oxygen even at low temperature duringheating stage. So, the whitening of debound parts in air atmosphere isinduced by oxidation. Test results show that the weight gain of SiCpowders at 1000 °C is about 0.8%.

The deformation resistance of the debound parts during transportdepends on their bending strength. There are large numbers of factorwhich affect the bending strength of debound part, such as powdershape, powder content, pre-sintering temperature, debinding atmo-sphere, defects in blank, and so on. The irregular shape of SiC powdersas shown in Fig. 1 helps to enhance the inherent friction amongdifferent powders. This is in favor of the green part to retain compactshape and attain uniform shrinkage during the debinding stage. Onthe other hand, the powder content of feedstock is 55 vol.% which isfavorable for the increase of debound part density. Theoretically,relatively higher density would enhance the bending strength ofdebound part. The reason is that, powders in blank could achievepoint-contact easily with the binders removing. Pre-sintering willenhance the point-contact strength. As we know, SiC powders havelow sintering activity under low temperature. So, the pre-sinteringtemperature for SiC blank usually reaches to more than 1200 °C withgas shield. It's a common view that [20], oxidation of SiC forms acoherent, dense SiO2 layer on the surface and SiO2 has better sinteringactivity. Oxidation will affect the point-contact of SiC powders. Inorder to investigate the effect of oxidation on the strength, the

Fig. 1. Scanning electron micrograph of SiC powder.

Fig. 2. Moulded samples.

Fig. 3. Moulded and debound parts.

Fig. 4. TGA of SiC powder.

50 Z. Lu et al. / Powder Technology 208 (2011) 49–53

bending strength of different samples was tested. As shown in Fig. 5,the bending strength increases from 6.55 MPa to 11.58 MPa as thepre-sintering temperature increases from 550 °C to 850 °C. Thebending strength enhances to about 26.2 MPa when the temperaturereaches 950 °C. On the other hand, the bending strength of samplespre-sintered at 1200 °C in argon atmosphere is only 11.52 MPa. So, thepre-sintering temperature could be decreased to 750–850 °C byheating in air furnace. This decreases the pre-sintering temperature,saves the cost of argon, and reduces the requirement of heatingequipment. On the other hand, the occurrence of defects duringthermal debinding is one of the major problems that greatly affect theproperties of debound parts, such as crack, pore, and warp. So theeffects of air atmosphere on the forming quality of debound sampleswere analyzed in this paper. No defects such as crack, distortion andwarp were found for all samples debound in air atmosphere under500 °C. The mass of single sample debound under 500 °C is almost thesame with the powder content by theoretical calculation. Otherwise,the final decomposition temperature of binders used in this study islower than 500 °C. It can be concluded that all binders havedecomposed completely at 500 °C. Fig. 6 shows the fracturemorphology of the debound sample. On the other hand, it waspreviously reported that the high-molecular-weight binders wereeasier to remove in air because of the continuous oxygen supply [24].So the debinding rate could be enhanced effectively by heating in airfurnace.

Despite this, air is in favor of removing the binders. The effects ofoxidation on the forming property of debound parts should be

disclosed. Usually, the pre-sintered parts present small shrinkagecompared with the moulded states. In this study, test results indicatethat the line shrinkage of samples debound in argon atmosphere isabout 0.1%. On the other hand, there is no shrinkage of all samples pre-sintered in air atmosphere. Samples pre-sintered at 550 °C and 650 °Chave the same dimensions with the moulded green parts. Further-more, the volume of sample pre-sintered at 750 °C starts to expand, asshown in Fig. 7. Correspondingly, the mass of pre-sintered samplesincreases with the increasing of pre-sintering temperature. Comparedwith the powder content in single sample by theoretical calculation,the weight gain could be calculated. The weight gain increases from0.31% to 1.47% with the temperature increasing from 550 °C to 850 °C.As shown in Fig. 3, no defect was found in all the samples pre-sinteredbelow 850 °C, despite oxidation induces volume expand and weightgain. Linear expansion and weight gain increase sharply to 2.1% and3.47% when the pre-sintering temperature reaches to 950 °C.Simultaneously, warp induced by excessive oxidation is found forthe sample pre-sintered at 950 °C, as displayed in Fig. 3. So, it'simportant to control the content of SiO2. On the other hand, excessiveoxidation of SiC is not favorable for the subsequent sintering. As shownin Fig. 8, the sample pre-sintered at 950 °C in air dehisces obviouslyafter sintered at 1900 °C. However, a small quantity of SiO2 is favorablefor the densification of SiC during the final sintering stage. Samplespre-sintered in air furnace have bigger shrinkage than the samplespre-sintered in Ar atmosphere after the final sintering process, whichcan be seen in Fig. 8. So, the pre-sintering temperature should be

Fig. 5. Bending strength of different samples.

Fig. 6. Fracture morphology of sample debound under 500 °C.

Fig. 7. Linear expansion and weight gain of debound samples.

Fig. 8. Sintered samples and fracture morphology of the sintered part.

51Z. Lu et al. / Powder Technology 208 (2011) 49–53

below 850 °C in air furnace. The pre-sintering of the debound part is toincrease the strength of the part to facilitate handing and ensure thatall binder components are completely removed from the part.However, the mechanical properties of the final sintered part arestill very important. After the final sintering at 1900 °C, the bendingstrength of the samples pre-sintered at 550 °C, 650 °C, 750 °C, 850 °Cin air furnace and 1200 °C in Ar atmosphere increases to 475 MPa,537 MPa, 492 MPa, 506 MPa and 594 MPa, respectively. These valuesare approximate with the test results by some other researchers, suchas K. Suzuki [25] (490–540 MPa) and N. Zhang [26] (300–450 MPa).

The weight gain of debound samples is induced by the oxidation ofSiC powders. As we know, all values of Gibbs free energy for possiblereactions between SiC and O2 are negative with the temperatureincreasing from room temperature. The oxidation behavior could bedivided into two categories: passive and active. Passive oxidationforms a coherent, dense SiO2 layer on the surface, which reducesoxidation rate later. So, samples pre-sintered at 550 °C still present alittle weight gain after a long heating time. However, the diffusion ofoxygen from surface layer to inner of SiC powders is enhanced by theincreasing of heating time and temperature. So, the weight gain ofsamples pre-sintered at 950 °C reaches to about 3.47%, which is higherthan the test value by TGA. The content of SiO2 could be controlled byadjusting the pre-sintering time and temperature. Nevertheless, theamount of oxide generation during the pre-sintering stage is still notenough to be found by XRD test. XRD test results shown in Fig. 9indicate that there is no obvious diffraction apex of other oxide exceptAl2O3 and Y2O3. There is no obvious difference among these XRD testresults for samples pre-sintered at different temperatures.

The oxidation during debinding stage decreases the pores andenhances the contact area of SiC powders, as shown in Fig. 10. So,enough strength for the transport could be obtained by pre-sinteringsamples in air atmosphere at low temperature. The suitabletemperature should be confirmed by the requirement of strengthfor samples with different structure and volume.

4. Conclusions

No defects were found for SiC parts debound under 500 °C in airatmosphere. The debinding rate could be enhanced effectively byheating in air furnace. The bending strength of different samplesincreases from 6.55 MPa to 11.58 MPa with the pre-sinteringtemperature increase from 550 °C to 850 °C. On the other hand, thebending strength of samples debound and pre-sintered at 1200 °C inargon atmosphere is only 11.52 MPa. Despite the linear expansion,weight gain increases from 0 and 0.31% to 0.43% and 1.47% with thepre-sintering temperature increase from 550 °C to 850 °C. No defectwas found for all these samples pre-sintered below 850 °C. Defectnamely warp is found for the samples pre-sintered at 950 °C, althoughit's bending strength is high to 26.2 MPa. So, blanks with enoughstrength for transport could be obtained by debinding in airatmosphere below 850 °C. The suitable temperature should beconfirmed by the requirement of strength for samples with differentstructure and volume.

Acknowledgments

The financial support from the “Fundamental Research Funds forthe Central Universities” (Grant No. HIT.NSRIF.2009034), “OpeningFunding of Key Laboratory of Micro-system and Micro-structuresManufacturing (Harbin Institute of Technology), Ministry of Educa-tion (HIT.KLOF.2009004)”, “Opening Funding of State Key Laboratoryof Materials Processing and Die & Mould Technology, HuazhongUniversity of Science and Technology” and “China PostdoctoralScience Foundation funded project” are greatly acknowledged.

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