《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_creep behavior of SiC whisker -SiN4-BN.pdf_大学文库

E驅≈3S ournal of the European Ceramic Society 21(2001)841-845 www.elsevier.com/locate/jeurc Creep behavior of SiC whisker-reinforced Si3 N4/BN fibrous monolithic ceramics Shuqin Li*, Yong Huang, Changan Wang, Yongming Luo, Linhua Zou, Cuiwei Li state Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084 People's Republic of China Received 15 June 2000: received in revised form 11 August 2000: accepted 21 August 2000 The flexural creep behavior of Sic-whisker reinforced hot-pressed Si3N4/BN fibrous monolithic ceramics in the temperature range 1000-1200@C and stress range 250-600 MPa were characterized. Creep curves generally showed extensive primary and lack of tertiary creep. The creep and fracture mechanism were mainly controlled by the BN interlayer Bn began to oxidise at 1200 C and decreased the creep resistance. Meanwhile SiC whisker pulling-out improved the creep resistance. Microscopic damage mechanisms were investigated by SEM. C 2001 Elsevier Science Ltd. All rights reserved. Keywords: BN; Composites; Creep: SiC whiskers; Si3N4/ BN fibrous monoliths 1. Introduction 2. Experimental methods Silicon nitride-based ceramics possess excellent prop 2. 1. Raw materials and fabrication process erties. 1,2 which make them ideal candidate materials for high temperature applications. However, the nature of Si3N4(Founder High-Tech ceramic Corp, China) brittleness is one of the most crucial problems in their powders with 8 wt %Y2O3(purity >99.9%), 4 wt. applications. Al,O3(>99.9%) were ball milled with 20 wt. SIC Enlightened from studying some natural composites, e whisker(TWS-400, Tokai Carbon Co., Japan)in ethanol bamboo, mollusk shell etc, ceramic matrix composites for 24 h to achieve a homogenous mixture. This was containing weak interfacial layers have been considered mixed with organic binders and then produced green to offer a very important approach to improve the filaments using a coextrusion process. The green fila- property of ceramics. 3.4 Whisker reinforcement is a well ments were subsequently coated with a slurry of 25 established method of enhancing the creep resistance of wt. BN and 75 wt. Al2O3, dried and parallel packed ceramic materials. 5,6 For this reason, the present inves- into a graphite die. After dewaxing, the green body was tigation was designed to characterize the creep and hot pressed in a graphite resistance furnace under N2 creep fracture properities of Si, N4/BN fibrous monolithic 1820 C for 1.5 h and under a pressure of 22 MPa. A ceramics. The creep responses at various temperatures detailed description of the fabrication process can be and stress levels were measured. From the data, the found in the literature. The test specimens were cut into kinetics of deformation and the stress dependence of 4x x36 mm rectangular bars, then polished with dia- flexural creep were determined. A numerical analysis mond pastes down to 7um was also employed to estimate the power-law creep parameters. The influence of microstructure on the 2. 2. Creep testing creep deformation mechanisms was also investigated Creep testing was conducted using a creep tester which consisted of a three-point bend test fixture of ax Co SiC with a span of 30 mm. Loads were applied to the upper ram via a lever arm having a 5: 1 leverage ratio 0955-2219/01/S. see front matter C 2001 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(00)00251-X

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S. Liet al. Journal of the European Ceramic Society 21(2001)841-845 Load-point deflections to center-point deflections of stantially to the observed creep response, although most creeping specimens were measured directly using linear of the primary strain reportedly derived from redis variable displacement transducer (LVDT). In the tribution of the intergranular matrix glass. In the pre- and before the load was applied, the time of constant BN interlayer facilitated the redistribution of stress l, c experiment, the rate of rising temperature was 500C/h ent study, the weak interfacial bond provided by temperature was 15 min. Samples were creep tested similar fashion during primary creep, particularly in the between 1000 and 1200C at stresses between 250 and vicinity of the stress concentration, such as micro 600 MPa under an air atmosphere. After testing, the cracks. Moreover, the glassy phases concentrated in the ensile zones of the specimens were examined using X- BN layer and which might happen to plastic flow on the ray diffraction (XRD) to determine the phases in the interface layer. The two mechanisms may, meanwhile materials and by scanning electron microscopy to char- effect the material creep Tertiary, or accelerated, creep acterize the creep deformation mechanisms. is usually attributed to 'distributed" damage caused by nucleation, growth, and coalescence of cavities microcracks, which gradually deteriorates the load 3. Results and discussion bearing capacity of the specimen. The lack of tertiary creep may suggest that fracture was dominated by 3.1. Creep response localized damage due to the growth of preexisting defects, e.g. macrocracks or voids, even though other In this study behavior of Si3 N4/BN fibrous damaging mechanisms may operate concurrently. In monolithic ceramic systematically investigated in other words, the growth of a defect to its critical size the temperature 1000-1200 C. Test conditions may usually happen before other damaging mechanisms and rupture times are shown in Table 1 and typical start to have a significant effect on creep. However creep curves at 1200C are shown in Fig. I there was a difference between monolithic ceramic and An obvious feature exhibited by these creep curves is fibrous monolithic ceramic. We know that fibrous mono- their extensive primary creep and the lack of tertiary liths are not governed by weak link statistics like mono- creep. There was a substantial primary creep response, lith materials. For example, a few cells of a fibrous which, in fact, accounts for most of the measured strain. monolith can fracture without catastrophic failure The large primary creep response related to stress redis- occurring if the remaining cells can support the applied tribution processes occurring in the composite and to the stress, so the tertiary creep of the Si3N4/BN fibrous intrinsic response of glass-ceramic. 8. 9 This redistrution ceramic does not appear of stress may involve one or more of the following pro- There are many creep laws and thus many creep cesses: redistribution of residual intergranular glas equations, none of which can describe the whole creep phase and/ or compliance of interface. Mayer et al. process. Usually, the creep strain rate(e)can be described systematically explored the role of the interface response in by the power law: the composites. From their experiment, they concluded that plastic flow of an interface layer of siliconous glass emulated a debonded interface and contributed sub Where A is a constant, o the stress, n the stress exponent The rupture times of the composites at different temperatures under a for creep, Q the activation energy for creep, r the gas variety of stresses constant, and T the absolute temperature. The stress Temperature (C Creep stresses(MPa) Rupture times(h) 350MP .2 1200 Creep Time(h) 0.7 Fig 1. The creep curves at 1200.C under different stresses

S. Liet al. Journal of the European Ceramic Society 21(2001)841-845 exponent for the material is 1.69. It is usually considered The microstructure of the materials observed by that a number of deformation mechanisms, depending scanning electron microscopy(SEM) was illustrated in on the presence of grain boundaries, account for the creep Fig. 4. It is observed that in the less glassy phase in behavior. I These include diffusion creep processes, grain Si3 n4 grains, the cavities and voids were less on the boundary sliding controlled by the viscosity of the grain boundary in Si3 N4/BN fibrous monoliths, which impurity phases at crystal boundaries, dissolution and could ascribe to the bn layers. The Bn layers have redeposition of material and transfer of the viscous pha- capacity to absorb the glass and improve the creep ses from boundaries under compression to those under resistance of the materials, but, as is well known, BN is tension. But the structure of Si3 N4 fibrous monoliths was unstable above 900 C, so the conclusion could be made unique and obviously different from Si3 N4 monoliths. that bN will lose that capacity. In order to examine the Fibrous monoliths consisted of"soft"and"hard"pha- change in BN, we analyzed the XRD results obtained In Si3 N,/BN fibrous monolithic ceramic, the"soft" from the specimens after creep tests at high phases were BN and the "hard"phases were Si3 N4. The under 400 MPa. It was found that Bn stayed intact glassy phase was present in the boron nitride and the less until 1200oC, so the bn interface improved the resis glassy phase was present in the silicon nitride grains tance to oxidation in the fibrous monoliths. At 1200oC, within the cells of fibrous monoliths as compared to sill n began to oxidise and formed B2O3. This reacted con nitride grains in a monolithic specimen. 2, 13 There- with Al2O3 and resulted in 2Al2O3B2O fore, the Si3 N4/BN fibrous monoliths possess a low stress ponent and high creep resistance. 4BN(s)+302→2B2O3()+2N2(g) 2AlO3+B2O3- 2Al2O3.B,O 3. 2. Microstructure observations and analysis The 2Al2O3 B2O3 formed strengthened the combina- There are differences between the Si3N4/BN fibrous tion between BN and Si3 N4. So with the temperature monolithic ceramic and monolithic Si,N4 ceramic in increased, although the creep rate was increased rapidly, structure. The microstructure of Si3 N4/BN fibrous the strength at 1200C did not rapidly decrease monolithic ceramic is shown in Fig. 2(a) and (b). The The fracture surface of composite samples is illu polycrystalline Si3N4 cells and Bn cell boundaries are strated in Fig. 5. Whisker pullout and crack deflection viewed in the hot-pressing direction and normal to the along the interface during fracture were indicated and hot-pressing direction. the traces after the whisker pulled-out were observed. In Si3N4/BN fibrous monolith, a major crack propagated The Si3N4/BN fibrous monolithic ceramic was made by major crack progressed through fibrous cells to the next became aligned such that the major axes were pre interface. Sometimes it extended along the interface ferentially oriented perpendicular to the direction of between fibrous cells. a branch-like crack could be pressing. Whisker orientation affects the strength and observed in Fig. 3. The crack path was tortuous instead fracture toughness of this material; therefore, its effect of line shaped. The branch-like crack makes the on creep deformation vestigated. 4 In the case of surface irregular and the surface area is increased. whisker-oriented alignment, most of the whiskers were Therefore, it consumes more energy, which benefits perpendicular to the crack plane, i.e. parallel to the creep resistance. direction of stress, so that whiskers can effectively 3456 85 8s° ig. 2. The microstructure of Si3 N4/BN fibrous monolith (a) Viewed in hot-pressing direction.(b) Viewed normal to the hot-pressing

S Liet al. Journal of the European Ceramic Society 21(2001)841-845 transfer stress and develop well. Consequently, debond- ing and whisker pulling-out will be observed in this case The bridging ligaments carry part of the applied load by bridging the crack faces, leading to the reduction of effective driving stress of crack growth. The whisker improved the creep resistance greatly 4. Conclusions 1. Si3 N4/BN fibrous monolithic ceramics existed a ubstantial transient response in creep curve because the bn weak interlayer may facilitate the redistribution of stress Fig 3. a branch like crack in fibrous monolithic ceramics. 2. BN interlayers had the capacity to absorb glass and purify the Si3 N4 grain boundary. The creep deformation and fracture mechanisms were mainly controlled by the Bn interlayer 3. At 1200oC, the bn layer began to oxidise and form 2Al,O3.B,O3. The cavities and voids were increased more rapidly at 1200 C than at 1000 and 1 100oC, which led to a decrease in creep resistance 4. SiC whisker pulling-out greatly improved the creep resistance Acknowledgements This work was supported by the National Natural Science Foundation of China(NSF) 83138120KU134*1N References Fig. 4. The microstructure of Si, Na/BN fibrous monolith after creep under 350 MPa at 1200oC for 100 h I. Ziegler, G, Heinrich, J and Wotting, G, Relationships between reaction- onded silicon nitride. J. Mater Sci. 1987. 22. 3041-3080 2. Mecholsky, J. J. Jr, Engineering needs of advanced ceramic- matrix composites Ceramic Bulletin, 1989, 68, 1083-1099. 3. Clegg, w.J., Kendlaa, K. and Alford, N. M, A simple way to make tough ceramics. Nature. 1990. 347 445-447. Huang, Y, Hao, H. N, Chen, Y. L and Zhou, B L, Design and preparation of silicon nitride composite with high fracture toughne and nacre structure. Acta metall. sinica. 1996.9 479-484. 5. Goto, Y. and Tsuge, A, Mechnical properties of unidirectionally oriented Sic-whisker-reinforced Si,Na fabricated by extrusion and hot-pressing. J. Am. Ceram. Soc., 1993, 76, 1420-1424 6. Matsui, T, Komora, O. and Miyake, M, The effects of surface ating and orienting of whiskers on mechanical properties of SC(w)/Si3N4.J. Ceram.Soc.Jp,1991,99,1103-110 7. Guo, H, Huang, Y and Wang, C. Preparation and properties of fibrous monolithic ceramics by in-situ synthesizing. J. Mater Sci 1999,34,2455-2459 8. Wu, X and Holmes, J. W. Tensile creep and creep-strain recovery behavior of silicon carbide fiber/calcium aluminosilicate matri 8313162eK838*2NM ceramic composites. J.Am. Ceran. Soc., 1993, 76, 2659-2700. 9. Meyer, D. w,Cooper, R. F and Plesha. M. E, High-tempera- Fig. 5. Fracture surfaces after creep test under 500 MPa at 1100.C for ture creep and the interfacial respo ceramIc-matrIx com- posites. J. Am. Ceram. Soc., 1996, 79, 539-543

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《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_creep behavior of SiC whisker -SiN4-BN

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_creep behavior of SiC whisker -SiN4-BN