Effects of pyrolysis temperature and fillers on joining of ceramics via silicone resin

Vol. 15 No. 6Trans. Nonferrous Met. Soc. ChinaDec.2005Article ID: 1003 - 6326(2005)06- 1322 - 06.Effects of pyrolysis temperature and fillers onjoining of ceramics via silicone resinSUO Jun(所俊)，CHEN Zhao- hui(陈朝辉),ZHENG Wen-wei( 郑文伟)，HAN Wei-min(韩卫敏)(Key Laboratory of Advanced Ceramic Fibers and Composites,College of Aerospace and Materials Engineering，National University of Defense Technology, .Changsha 410073, China)Abstract: The joining of graphite，ceramic SiC and C:/ SiC composites via preceramic silicone resin(SR) at high .temperature (800- 1400 "C) was studied. The curing and pyrolysis process of SR, pyrolysis temperature, inert andactive fllers were especially discussed. The results show that the curing process of SR was accomplished by consu-ming Si- OH. The temperatureof 1 200 C is the appropriate treating temperature for graphite and SiC ceramic, andthe temperature of 1 400 C is suitable for C/ SiC composites. Inert fller SiC powder(5%，mass fraction) has muchpositive influence on the shear strength of the joints. Active fller nano Ai, Si powder can greatly improve the prop-erties of the joints treated at high temperature. The improvement is over 700%.Key words: preceramic; silicone resin; pyrolysis temperature; inert fller; active fller; joiningCLC number: TB323Document code: A1 INTRODUCTION(Si powder and Al/Si nano-powder respectively)were researched. In the mean while, the joining ofCeramic matrix composites(CMC) have manygraphite and SiC ceramic as the components of the .advantages, such as high strength at high tempera-3D-C/ SiC composites was discussed.ture，high rigidity， high wearability， high endur-ance of chemical erode, and low coefficient of ther-2 EXPERIMENTALmal expansion-1.2l. But it is difficult to manufac-ture complex ceramic or CMC components. So2.1 Main raw materialsjoining simple components into complex ones isPreceramic SR (chemical pure) is produced bynecessary under such a situation. Joining tech-Dow Corning， USA. SiC powder ( industrial pure) .niques of ceramics and CMC have been researchedwith average grain diameter of 0. 4 μ is made byin the areas of solid phase diffusion, high tempera-Zhengzhou No. 2 Grinding Wheel Works， China.ture welding and the methods by the similar mech-Al nano-powder (average diameter is 30 - 70 nm)anisms. The disadvantages of these methods lie inwith purity of 98%- 99. 9% is produced by Zhong-huge energy waste at high temperature, high facili-kangda Nanotech Institute. Si nano- powder ( aver-ty cost of the process， and severe damage to theage diameter ≤30 nm) with purity over 99% issubstratesi3.4. Pyro-joining ceramics and CMCproduced by Institute of Metal Research, Chinesewith preceramic at high temperature is a relativelyAcademy of Sciences. Graphite is produced bynew joining processC5-8 which relies on the joiningXiangbei Graphite Works. SiC ceramic ( relativeby the ceramic pyrolyzed from the preceramic withdensity 99%) is fabricated by hot press method inenhancing the joining properties of the materials atthe laboratory. 3D C/ SiC composites are preparedhigh temperature. Compared with other methods,by precursor infiltration process with carbon fiberit needs low joining temperature, short heat pres-(Jilin Carbon Corp.，China) as reinforcement andervation，no pressure and vacuum， which can be8 :1 : 1 as the fiber volume fraction in X, Y, Z di-widely applied with relatively simple process. Therections. The fiber volume fraction is 45%.method can also maintain the similar joining phasecompared to the substrates. In this paper，joining2.2. Experimental process and testing characteriza-between 3D-C/ SiC composites by ceramics pyro-中国煤化工lyzed from SR was realized. The influences oMHCNMHGOr Al/Si nano- powderpyrolysis temperature, inert and active fillerpartrally) and etnanol were mixed in solution and①Foundation item: Project(04JJ Y3002) supported by the Natural Science Foundation of Hunan Province, ChinaReceived date: 2005 - 03 - 03; Acepted date: 2005 - 08 - 24CorrespondenetUO Jun, PhD candidate; Tel: + 86-731-4576441; E mail: mrsuo@ 163. comVol. 15 No. 6Effects of pyrolysis temperature and fillers on joining of ceramics●1323 .daubed on the substrates whose surfaces had beenthe breaks of the side chains of SR and the emittedpolished and ultrasonically cleaned. Then themolecular fragments. With the temperature goingjoints were cured at 250 "C for 2 h and pyrolyzed inup to about 1 000 C，the mass-loss ratio reduces toAr atmosphere with heating speed of 2 "C/min to aa low level and the former SR changes toSi- O- Ccertain temperature. They were cooled down at theceramic structure.same speed. Shear strengths were tested， and in-frared ray (IR，Hitachi270- 30) spectrum, thermo-100 [gravimetry analysis(TGA，TAS10-0)，X-ray dif-fraction( XRD，Siemens-D500) and scanning elec-tron microscope ( SEM，JSM- 5600LV) information .95were got from the joints.)0 t3 RESULTS AND DISCUSSION3.1 Curing and pyrolysis of SR853.1.1 Curing of SRTheIRspectraofSRcuredat250Cfor2hare shown in Fig. 1.80L200400600800 1 000t/CFig.2 TGA curves of SRIn Fig.3，the XRD curves of Si- 0- C ceram-以cs pyrolyzed at 800 and 1 000 C are smooth with aprotuberant shape at about 20=21°，correspondingto the amorphous C. No SiO2 and SiC crystals ex-ist under thus conditions. Small peaks appearing inthe XRD curve at 1 200 C means SiO2 and SiCcrystals begin to appear, but most of these mattersare amorphous. Under this condition, the crystalsare generally small, which is beneficial to the prop-4 00030002 0001 000erty of the joints-9l.Wavenumber/cm-1Fig. 1 IR spectrums of SR before(a) and。一B-SiCafter(b) curing at 250 °C for 2 h●一Si02As illustrated in Fig. 1，Si- O- Si bonds arejiVinm i出the main chain in general， and - - CHs，一OH,- Ph are in the side chains. Evident Si一 OH peakexists in 3 600 - 3 000 cm^“1. Comparing curves (a)with (b) in Fig. l, it can be seen that the curing ofSR is accomplished by the method of consuming(bSi一OH. The reaction is-Si- 0H+HO- Si-- Si- -O- Si-+H2O(1)(a)Si- O- - Si structure is formed by dehydrationreaction in SR.' The poly molecules construct a20304050670structure of highly cross-linked reticulation that is20/<*)not melted and dissolved, and gain higher stability中国煤化工。SR pyrolyzed at .and ceramic yield at high temperature.3.1.2 Pyrolysis of SRMHC N M H GeraturesTGA of SR pyrolyzed at 5 C/min is shown in .a)- 800 "C; (b)- -1000 C; (c)- -1 200 C; (d)- -1 400 CFig.2. As shown in Fig. 2，SR emits molecularfragments and loses its mass gradually with the in-3.2 Influence of pyrolysis temperaturecreasing. heating temperature.The remarkable .Different structures and properties of thmass lossbe数据400 and 600 'C is mainly due tojoints are achieved at different pyrolysis tempera-1324.Trans. Nonferrous Met. Soc. ChinaDec.2005 .tures. Table 1 lists the properties of the joints.°Cisabout40μm，whileitisabout5-6μmatachieved from the SR/ ethanol solution treated at1 400 C. The thickness of the bonding layerdifferent temperatures for 5 h.between C:/SiC treated at l 200 C is about 25 μum,while it is about 1μm at 1 400 C. Suitable thinTable 1 Shear strengths of joints made bybonding layer is good for the properties of the4 g/ mL SR/ ethanol solution treated atjoints.different temperatures for 5 h( MPa)Fig.5 shows the SEM photographs of theJoint800 'C1000C1200C1400Cjoints treated at different temperatures for 5 h.SiC/SiC1.319. 1714. 144. 27The bonding layer treated at 1 200 C is more com-pact than that at 1 400 "C as shown in Fig. 5. Al-Graphite/ graphite 0. 380.443. 843.30.though in the bonding layer there are cracks,C/SiC/C/SiC01.403. 51which are vertical to the interface，they are not thesource of cracks in the shear strength test and doAs listed in Table 1, the shear strengths haveno harm to the properties of the joints. The rup-been enhanced until the temperature of 1 200 "C,tures of the samples treated at 1 400 'C often occurwhile decrease when the temperature is beyondin the interface between the bonding layer and the1 400 'C. The structures and properties of Si- -O-substrates. It is mainly because of the growth ofC ceramics achieved at different pyrolysis tempera-the crystals，loose bonding layer and defects crea-tures are the main reason of the tendency. The ce-ted at such temperature. So the temperature oframic yield alters from 84% at 800 C to 78% at. l 200 C is the appropriate treating temperature for1 400 C.graphite and SiC ceramic， and the temperature ofFig.4 shows the SEM photographs of the1 400 C is suitable for C;/SiC composites. Thejoints made by 6 g/ mL SR/ ethanol solution treatedreason is probably that the ceramic pyrolyzed fromfor 5 h at different temperatures. Fig. 4 shows thatSR is located on the complex and rough surface ofthe higher the pyrolysis temperature is, the thin-the composites to enhance the properties of thener the joints are. For instance, the thickness ofjoints. Its mechanism is to be further investigatedthe bonding layer between graphite treated at 1 000in the future research.aNeee58m治5e，1你284ek05exm 8507911a29以收中国煤化工20k0点T50Am 852815MAYk84HAY/e4fYHCNMHGFig. 4 SEM photographs of joints made at different temperatures(6 g/ mL SR/ethanol with heating rate 2 "C/ min)(a)一Graphite, 1 000 C; (b)一 Graphite, 1 400 C; (c)- C/SiC, 1000 C; (d)- C/SiC, 1 400 'CVol. 15 No. 6Effects of pyrolysis temperature and fillers on joining of ceramics●1325●[a(68505 MAY/8428kU5m8504+18MAY/04Fig.5 SEM photographs of joints made at different temperatures(Heating rate is 2 C/min, holding time is 5 h)(a)- Graphite, 1 000 C; (b)- Graphite, 1 400 C3.3 Influence of inert fillerker，which will increase the appearing probabilityAppropriate amount of inert filler SiC powderof cracks and defects，and decrease the propertiesis added into the SR/ ethanol solution to decreaseof the joints. Deduced from Ref.[ 11], the criticalthe volume shrinkage of the Si- -O一C ceramic dur-volume fractions ( which make the volume shrink-ing the process of the pyrolysis.age 0 when the preceramic is pyrolyzed) of the prece-The shear strengths of the joints with differ-ramic and the inert fller φ(SR，c) and φ(IF, c) areent contents of SiC powder treated at 1 400 C forφ(SR，c)=q/(1- - aβ)2 h with heating rate of 2 C/min are shown inφ(IF，c)=1- q/(1- a$)(3)Table 2.where q is the pore fraction, a is the mass ratio of .the ceramic and the preceramic, i. e. ceramic yield.Table 2 Shear strengths of jointsβis the density ratio of the preceramic and the ce-with SiC powderramic. In the paper, q≈0. 56，a≈0. 83, β≈0. 51，Mass ratio of SiC to SR/%Shear strength/ MPaso φ(IF，c)≈0. 02. The critical mass fraction isconverted to be 0. 05. The calculating result ac-03.51cords with the experimental test. That is to say,3. 43low shrinkage of the preceramic in the process ofthe pyrolysis is an important factor to enhance the5. 44properties of the joints.l01.45200. 443.4 Influence of active fillerActive filler can react with the production of30the pyrolysis and the protective atmosphere, andenhance the ceramic yield of the preceramic. TheTable 2 indicates that adding SiC powderresult of the reaction is to form new ceramic phasegreatly influences the shear strength of the joints.and change the volume. So adding active filler is anThe main mechanism of inert filler SiC powder en-effective method to increase the ceramic yield ofhancing the properties is to restrict the volumethe preceramic, and decrease volume shrinkage andshrinks of the Si- O- C ceramic during the pyroly-the pore fraction. SR/ ethanol solution( SR/AI-Si=sis. When the mass fractions of SiC powder are too1.67(mass ratio， %，the same below)，Al/Si=low，the powder can not control the volume80/20) with nano Al and Si powder is used to makeshrinks of the SR，but is just the impurity in thethe joints treated at 1 200 "C for 1 h with heating .ceramic which decreases the properties of therate中国煤化工joints. Too high mass fraction of SiC powder: with nano active fillerdecreases the mass fractions of the pyrolyzedis e.MYHCN M H G% measured by tests,ceramic, and increases the probability of SiC powwhich is beneficial to increase the densification andder existing at the surface of the joints, which de-decrease the pore fraction in the ceramic.creases the actual contacting area and the proper-Table 3 shows that the active filler enhancesties of the joints[10. Furthermore, too much SiCthe shear strengths of the joints. The reactionspowder al3o s报es the bonding layer much thic-that may occur during the pyrolysis of the SR with .●.1326●Trans. Nonferrous Met. Soc. ChinaDec.2005 .Table 3 Influence of inert and active fillerson the integrality and the density of the ceramic. .on shear strengthFig. 7 shows the XRD patterns of the ceramicsFillerNo fllerInert fllr Active fllepyrolyzed from SR treated at 1 200 C. There arepeaks of Si， Al4Cz and Al besides the peaks ofShear strength/MPa 1. 405.4411.25.Al2O3，SiC and SiO2 anticipated in the patterns.The chemical ratio of the active filler is inappropri-nano Al, Si powder asate to make some reactants directing the left-15] .4A1+ 3SiO2-→2Al2O3 +3Si(4)4Al+3C--→Al4Cz(5)3Si+ AlC3-→4Al+3SiC(6)Al2O3SiCSi+C--→SiC(7). SiOzAccording tol18]OG9=sH9一TOS9一◆- AldT/T2|" Oc,(T)dT(8)According to the raw data in Ref. [13]，thechanges of Gibbs functions of the reactions during800 to 1 400 "C are shown in Table 4.Table 4 Gibbs functions of reactions betweenSR and active fillers at differenttemperatures( kJ/ mol)1020304050607020/(°)ReactionOGPorsK OGP23K OGPs78K OGPergK4Al+ 3SiO22Al2O3 +3Si-690.55 -668.85 -647.15 - 625.45Fig. 7 XRD pattern for pyrolyzed SRadded with active fillers4Al+3C- + .AlC -184.16 -173.41 -162.66 - -151.914 CONCLUSIONS3Si+Al4Cs一-10.40 -16.48 -22. 57 - 28.664Al+ 3SiCThe curing of SR is accomplished by consu-3Si+3C一>-213.69 -199.17 - - 180.93 - 159.6ming Si- OH. The main mass loss occurs between3SiC400 C and 600 C. With the temperature going upto about 1 000 C，the mass-loss ratio reduces to aTable 4 indicates that Reactions (4) and (7)low level and the former SR changes to Si- O- Chave relatively great tendency to occur at appropri-ceramic structure. Effective joining at high tem-ate temperature. However, the reaction to produceperature is realized by Si一0- C ceramic pyrolyzedharmful phase Al4C3 is relatively not easy to occur.from SR/ethanol solution. The temperature ofFig. 6 shows the SEM image of the leftover of1 200 "C is the appropriate treating temperature forthe active fller in the ceramic pyrolyzed from SR.graphite and SiC ceramic, and the temperature ofIt can be seen from Fig. 6 that the Si- O- C ce-1 400 C is suitable for C:/SiC composites. Inertramic is very compact. Some leftover of the nanofiller SiC powder (5%) has much positive influencepowder ( be reunited) does not have much influenceon the shear strength of the joints. Active fillernano Al, Si powder can greatly improve the prop- ;erties of the joints treated at high temperature.The improvement is over 700%.REFERENCES[1] Kikuo N. Fabrication and characterization of three di-mensional carbon fiber reinforced silicon carbide and中国煤化工. 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