Effect of foaming pressure on the properties of porous Si 3 N 4 ceramic fabricated by a technique combining foaming and pressureless sintering Xiangming Li, a,b Pute Wu b,⇑ and Delan Zhu a,b a College of Water Resources and Architecture Engineering, Northwest A&F University, Yangling, Shaanxi 712100, People’s Republic of China b Institute of Water Saving Agriculture in Arid Areas of China, Northwest A&F University, Yangling, Shaanxi 712100, People’s Republic of China Received 7 January 2013; accepted 8 February 2013 Available online 21 February 2013 To fabricate porous Si 3 N 4 ceramics with a complex shape easily, a simple and low-cost technique combining foaming and pres- sureless-sintering is explored. The foaming pressure has a decisive effect on the microstructure, pore size distribution, porosity and mechanical properties of the porous Si 3 N 4 ceramics. As the foaming pressure increases, the porous Si 3 N 4 ceramics become uniform in microstructure, change gradually in pore size distribution from bimodal to unimodal, slowly reduce in porosity and pore size, and improve significantly in mechanical properties. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Si 3 N 4 ; Porous; Foaming; Microstructure; Mechanical properties Recently, Si 3 N 4 ceramics with high porosity have attracted much attention for their potential use in spe- cific applications [1–9]. For engineering applications, such as separation membranes, catalyst supports and gas filters, porous Si 3 N 4 ceramics with a high porosity of channels, good mechanical properties, well-controlled pore size distribution and a tailored microstructure are required [4,5]. For functional applications, especially as electromagnetic wave transparent materials used on carrier rockets, airships, missiles and return satellites, an effective way to improve the dielectric properties of an Si 3 N 4 ceramic is by increasing its porosity – and par- ticularly so when the porosity is higher than 35% [6–9]. In the past two decades, various techniques have been developed to fabricate porous Si 3 N 4 ceramics [2–6,8]; of these, cold-pressing is the most common technique for fabricating the green body of porous Si 3 N 4 ceramics. However, whereas cold-pressing is suitable for fabricat- ing ceramic green bodies with a simple shape, it is not so suitable for fabricating ones with more complex shapes. In order to make porous Si 3 N 4 ceramics with a complex shape, grinding is commonly used technique, but it is expensive and difficult to operate. In the present paper, to fabricate porous Si 3 N 4 ceramics with a complex shape easily, a simple and low-cost technique combining foaming and pressureless sintering (F-PS) is explored. The effect of foaming pres- sure on the microstructure and pore size distribution of Si 3 N 4 -(F-PS) is investigated in detail. The porosity and mechanical properties of Si 3 N 4 -(F-PS) are discussed and compared with those of the porous Si 3 N 4 ceramic fabricated in our previous work by the techniques of cold-pressing combined with oxidation bonding (CP- OB) [7] and cold-pressing combined with pressureless sintering (CP-PS) [9]. Si 3 N 4 powder (a phase > 90 wt.%, b phase < 10 wt.%, Si < 0.1 wt.%) was mixed with 5 wt.% Yb 2 O 3 and 10 wt.% dextrin, then ball milled for 10 h. At 5 °C, the as-obtained powder blend was mixed with ammonium bicarbonate (NH 4 HCO 3 ) solution (15 wt.%) in a weight ratio of 1:0.8, then ball milled for 10 min into a slurry. The slurry was poured into an Al 2 O 3 mould and then foamed by (1) putting the Al 2 O 3 mould in a closed con- tainer, (2) raising the temperature in the container to 60 °C and holding it for 2 h, then (3) raising the temper- ature in the container to 90 °C and holding it for 10 h. 1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.02.033 ⇑ Corresponding author. Tel.: +86 029 87092860; fax: +86 029 87012210; e-mail: [email protected]Available online at www.sciencedirect.com Scripta Materialia 68 (2013) 877–880 www.elsevier.com/locate/scriptamat
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Available online at www.sciencedirect.com
Scripta Materialia 68 (2013) 877–880
www.elsevier.com/locate/scriptamat
Effect of foaming pressure on the properties of porous Si3N4
ceramic fabricated by a technique combining foaming andpressureless sintering
Xiangming Li,a,b Pute Wub,⇑ and Delan Zhua,b
aCollege of Water Resources and Architecture Engineering, Northwest A&F University, Yangling, Shaanxi 712100,
People’s Republic of ChinabInstitute of Water Saving Agriculture in Arid Areas of China, Northwest A&F University,
Yangling, Shaanxi 712100, People’s Republic of China
Received 7 January 2013; accepted 8 February 2013Available online 21 February 2013
To fabricate porous Si3N4 ceramics with a complex shape easily, a simple and low-cost technique combining foaming and pres-sureless-sintering is explored. The foaming pressure has a decisive effect on the microstructure, pore size distribution, porosity andmechanical properties of the porous Si3N4 ceramics. As the foaming pressure increases, the porous Si3N4 ceramics become uniformin microstructure, change gradually in pore size distribution from bimodal to unimodal, slowly reduce in porosity and pore size, andimprove significantly in mechanical properties.� 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Recently, Si3N4 ceramics with high porosity haveattracted much attention for their potential use in spe-cific applications [1–9]. For engineering applications,such as separation membranes, catalyst supports andgas filters, porous Si3N4 ceramics with a high porosityof channels, good mechanical properties, well-controlledpore size distribution and a tailored microstructure arerequired [4,5]. For functional applications, especiallyas electromagnetic wave transparent materials used oncarrier rockets, airships, missiles and return satellites,an effective way to improve the dielectric properties ofan Si3N4 ceramic is by increasing its porosity – and par-ticularly so when the porosity is higher than 35% [6–9].
In the past two decades, various techniques have beendeveloped to fabricate porous Si3N4 ceramics [2–6,8]; ofthese, cold-pressing is the most common technique forfabricating the green body of porous Si3N4 ceramics.However, whereas cold-pressing is suitable for fabricat-ing ceramic green bodies with a simple shape, it is not sosuitable for fabricating ones with more complex shapes.In order to make porous Si3N4 ceramics with a complex
1359-6462/$ - see front matter � 2013 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2013.02.033
shape, grinding is commonly used technique, but it isexpensive and difficult to operate.
In the present paper, to fabricate porous Si3N4
ceramics with a complex shape easily, a simple andlow-cost technique combining foaming and pressurelesssintering (F-PS) is explored. The effect of foaming pres-sure on the microstructure and pore size distribution ofSi3N4-(F-PS) is investigated in detail. The porosity andmechanical properties of Si3N4-(F-PS) are discussedand compared with those of the porous Si3N4 ceramicfabricated in our previous work by the techniques ofcold-pressing combined with oxidation bonding (CP-OB) [7] and cold-pressing combined with pressurelesssintering (CP-PS) [9].
Si3N4 powder (a phase > 90 wt.%, b phase < 10 wt.%,Si < 0.1 wt.%) was mixed with 5 wt.% Yb2O3 and10 wt.% dextrin, then ball milled for 10 h. At 5 �C, theas-obtained powder blend was mixed with ammoniumbicarbonate (NH4HCO3) solution (15 wt.%) in a weightratio of 1:0.8, then ball milled for 10 min into a slurry.The slurry was poured into an Al2O3 mould and thenfoamed by (1) putting the Al2O3 mould in a closed con-tainer, (2) raising the temperature in the container to60 �C and holding it for 2 h, then (3) raising the temper-ature in the container to 90 �C and holding it for 10 h.
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During the foaming process, the pressure in the closedcontainer was controlled at 0.2, 0.3, 0.4 or 0.5 MPa.After the foaming process, the Si3N4 green body to-gether with the Al2O3 mould was removed from the con-tainer and pre-oxidized in air at 800 �C for 5 h, beforeundergoing pressureless sintering in a furnace at1680 �C for 2 h under a nitrogen atmosphere pressureof 0.3 MPa with a heating and cooling rate of10 �C min�1. For convenience, the porous Si3N4 ceramicfabricated by foaming under n (n means 0.2, 0.3, 0.4 or0.5 MPa) pressure is denoted as Si3N4-n.
The porosity and density were measured by Archime-des’ method. The microstructure was observed by scan-ning electron microscopy. Phase analyses wereconducted by X-ray diffraction (XRD). The flexuralstrength (r) was evaluated by a three-point bending test,with a support distance of 30 mm and a loading speed of0.5 mm min�1. The fracture toughness (KIC) was testedby the single-edge notched beam method. The averagevalues of flexural strength and fracture toughness wereobtained by testing five specimens.
After oxidation at 800 �C for 5 h, the dextrin in thegreen body has been removed completely and about1.8–2.5 wt.% SiO2 is produced, as calculated accordingto the weight change in the green body. During the sin-tering process, the reaction between Yb2O3 and nativeSiO2 on the surface of Si3N4 forms Yb4Si2O7N2 [5,10],which is helpful for the a! b phase transformation ofSi3N4 [11,12]. In the green body, 1.8–2.5 wt.% SiO2 issufficient to react with Yb2O3 to produce Yb4Si2O7N2
according to the following reaction equation [10]:
2Yb2O3 þ 0:5SiO2 þ 0:5Si3N4 ! Yb4Si2O7N2 ð1ÞWhen Yb4Si2O7N2 has been produced, the phase trans-formation of Si3N4 from a to b takes place by (1) disso-lution of a phase and saturation of Yb4Si2O7N2, (2)transport of Si and N ions through the Yb4Si2O7N2
and (3) attachment onto existing b particles [13].Figure 1 shows the XRD patterns of raw Si3N4 pow-
der and porous Si3N4 ceramics. The raw Si3N4 powder iscomposed of a phase and a small amount of b phase.During the sintering process, Yb4Si2O7N2 is producedquickly according to Eq. (1). With the help of Yb4-
Si2O7N2, most a-Si3N4 transformed into b-Si3N4, so
Figure 1. XRD patterns of (a) raw a-Si3N4 powder and (b) porousSi3N4 ceramics.
the porous Si3N4 ceramics are composed of a primaryphase of b-Si3N4 and a secondary phase of Yb4Si2O7N2,with a small amount of a-Si3N4.
Figure 2 shows the micrographs of porous Si3N4
ceramics foamed under different pressures. During thepressureless-sintering process, the Yb4Si2O7N2 produceddistributes evenly throughout the whole sample. Yb4-
Si2O7N2 has a great effect on the a! b phase transfor-mation of Si3N4, so many large and long b-Si3N4
particles are present in the porous Si3N4 ceramics, asshown in Figure 2. However, the different pressures usedto foam the porous Si3N4 ceramics produce widely dif-fering microstructures. During the foaming process,the NH4HCO3 in the green body decomposes accordingto the following reaction:
NH4HCO3 ! NH3 " þH2OþCO2 " ð2ÞEq. (2) is a gas volume expansion process which causes alot of pores to be produced in the green body. When thefoaming pressure is 0.2 MPa, the pores produced in thegreen body are large and numerous; thus, there aremany large pores in Si3N4-0.2, and its microstructureis non-uniform, as shown in Figure 2(a). As the foamingpressure increases from 0.2 to 0.5 MPa, the pores pro-duced in the green body gradually shrink and the micro-structure becomes uniform. As shown in Figure 2(d),
Figure 3. Pore size distributions of porous Si3N4 ceramics foamedunder different pressures.
Table 1. Volume shrinkage, porosity and mechanical properties of porous Si3N4 ceramics foamed under different pressures.
Porosity (%) 43 47 52 53r (MPa) 23 178 116 143KIC (MPa�cm1/2) 0.4 2.9 2.1 2.3Reference [7] This work [9]
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Si3N4-0.5 possesses much smaller pores and a more uni-form microstructure than Si3N4-0.2.
Figure 3 shows the pore size distributions of porousSi3N4 ceramics foaming under different pressures. Whenthe foaming pressure is 0.2 MPa, Si3N4-0.2 has a bimo-dal pore size distribution, with more large pores thansmall ones. The mean pore sizes of the large and smallpores are about 6 and 2 lm, respectively. At a foamingpressure of 0.3 MPa, Si3N4-0.3 also has a bimodal poresize distribution, and the mean pore sizes are un-changed, though the amount of large pores decreasesgreatly while the amount of small pores increases onlyslightly. With an increase in foaming pressure from 0.3to 0.4 MPa, the pore size distribution of the porousSi3N4 ceramic changes from bimodal to unimodal, withthe mean pore size of the small pores remaining un-changed at 2 lm. As the foaming pressure increasesfrom 0.4 to 0.5 MPa, the pore size distribution of theporous Si3N4 ceramic remians unimodal, but the meanpore size decreases from 2 to 1 lm.
Table 1 lists the volume shrinkage, porosity andmechanical properties of porous Si3N4 ceramics foamingunder different pressures. During the foaming process,when the pressure is as low as 0.2 MPa, the pores pro-duced in the green body are relatively large and numer-ous, so Si3N4-0.2 shrinks a relatively large amount involume (11.4%) and possesses a relatively high porosity(66%). As the foaming pressure increases, the pores pro-duced in the green body decrease gradually in size andamount, leading to a gradual decrease in the volumeshrinkage and porosity of the porous Si3N4 ceramics.Compared with Si3N4-0.2, Si3N4-0.5 shrinks only 6.2%in volume and possesses a relative low porosity of47%. The differences in the microstructure, pore size dis-tribution and porosity inevitably lead to differences inthe mechanical properties of the porous Si3N4 ceramics.When the foaming pressure is 0.2 MPa, the non-uniformmicrostructure, the large pores and the high porositymean that Si3N4-0.2 possesses low flexural strength(31 MPa) and fracture toughness (0.5 MPa cm1/2). Asthe foaming pressure increases, the increasingly uniformmicrostructure, decreasing pore size and reducing poros-ity gradually improve the mechanical properties of theporous Si3N4 ceramics. When the foaming pressure is
0.5 MPa, the uniform microstructure, relative smallpores and low porosity give Si3N4-0.5 a relative highflexural strength of 178 MPa and a fracture toughnessof 2.9 MPa cm1/2.
Though using the same starting Si3N4 powder, theporous Si3N4 ceramics fabricated by different techniquesshow different mechanical properties due to differencesin microstructure, pore size distribution and porosity[1,14]. Cold-pressing is currently the most common tech-nique for fabricating ceramic green bodies, and oxida-tion bonding and pressureless sintering are twocommon techniques for fabricating porous Si3N4 ceram-ics. Table 2 compares the porosity and mechanical prop-erties of Si3N4-(F-PS) with Si3N4-(CP-OB) and Si3N4-(CP-PS). As shown in the table, though Si3N4-0.5 pos-sesses a 5% higher porosity than Si3N4-(OB-IS), Si3N4-0.5 possesses flexural strength and fracture toughnessabout 6–7 times higher than Si3N4-(OB-IS) due to thegood load-bearing ability of rod-like b-Si3N4 particles.However, compared with the b-Si3N4 particles inSi3N4-(CP-PS), the b-Si3N4 particles in Si3N4-(F-PS)are relatively malformed, immaturely grown and short-er, perhaps because of the use of different sintering aidsand the lower sintering temperature. Thus, the load-bearing ability of b-Si3N4 particles in Si3N4-(F-PS) isrelatively lower than that of b-Si3N4 particles in Si3N4-(CP-PS). Therefore, though Si3N4-0.4 possesses almostthe same porosity as Si3N4-(CP-PS), the flexuralstrength and fracture toughness of Si3N4-0.4 are slightlylower than those of Si3N4-(CP-PS).
In this study, porous Si3N4 ceramics were fabricatedby a technique combining foaming and pressureless sin-tering that is suitable for fabricating porous ceramicswith a complex shape. The foaming pressure has a greateffect on the microstructure, pore size distribution andporosity of porous Si3N4 ceramics. As the foaming pres-sure increases, the porous Si3N4 ceramics become uni-form in microstructure, decrease in pore size andporosity, and change in pore size distribution from bi-modal to unimodal. Due to the above variations, themechanical properties of porous Si3N4 ceramics increasesignificantly with increasing foaming pressure. The por-ous Si3N4 ceramics fabricated in the present work arepromising electromagnetic wave transparent materials
880 X. Li et al. / Scripta Materialia 68 (2013) 877–880
due to their high porosity and good mechanicalproperties.
The authors gratefully acknowledge the financialsupport from the Reconstruction Project of NationalEngineering Technology Research Center(2011FU125Z27-1). This work was also supported bythe National Natural Science Foundation of China(No. 51209177) and the Basic Research Fund of North-west A&F University (No. QN2012024).
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