Effect of BN content on elastic modulus and bending strength of SiC BN in situ composites

Transcription

1 ARTICLES Effect of BN content on elastic modulus and bending strength of SiC BN in situ composites Guo-Jun Zhang and Tatsuki Ohji Synergy Ceramics Laboratory, National Industrial Research Institute of Nagoya, Nagoya, Aichi , Japan (Received 13 March 2000; accepted 15 May 2000) The effect of BN content on elastic modulus and bending strength of SiC BN in situ composites was investigated. The SiC BN in situ composites were prepared on the basis of the chemical reaction of Si 3 N 4,B 4 C, and C. The elastic modulus decreased markedly with increasing BN content and obeyed well with both of the exponential and polynomial rules. The bending strength increased slightly with increasing BN content up to 15 vol% and then gradually decreased; the decrease was accelerated above 35 vol%. The results were explained by a percolation network formation of the BN phase. The markedly low elastic modulus with the relatively high strength was indicative of excellent strain tolerance of this material. I. INTRODUCTION By the addition of hexagonal BN (h-bn) into a ceramic matrix, such as Si 3 N 4, 1 5 SiC, 5 9 Al 2 O 3, 10 and TiB 2, 11 fracture toughness and thermal shock resistance can be improved. The improvement mechanism was considered to be related to the large anistropy of thermal expansion of BN grains ( C 1 parallel to the c axis and 10 7 C 1 perpendicular to the c axis 4 ), which results in premicrocracking around BN grains during cooling after sintering. 5,12 The characteristic of easy cleavage perpendicular to the c axis of BN flakes assists in such microcracking. However, bending strength of the BN composites was not always improved, compared with that of the corresponding monolith. 1,6,7,13 For example, in Si 3 N 4 /BN microcomposites prepared by the conventional process of mechanically mixing Si 3 N 4 and BN powders, the bending strength monotonously decreased with increasing BN content. 13 In situ processed Si 3 N 4 /BN nanocomposites also showed a strength decrease when increasing BN content, though the strength slightly increased only in a range from 0 to 5 vol%. 13 It has been reported that Young s modulus of BN composites was reduced with increasing BN content, especially in a low content range. 6 Mazdiyasni and Ruh 1 have found that, for Si 3 N 4 -BN composites, Young s modulus of 5% BN composition is in agreement, but those of 10 and 15% BN compositions are 15 and 20% low, compared with the calculated upper bound, E U (E U E i V i, i stands for the component phase and V the volume percent of the i phase). Kusunose 13 has reported that Young s moduli of both conventional microcomposites and in situ nanocomposites of the Si 3 N 4 BN system were consistent each other and were between E U and the lower bounds, E L (E L 1 E i 1 V i ). These results suggest that the size and distribution of BN particles have no obvious effects on Young s modulus of the composites. The in situ process of ceramic composites is advantageous for obtaining high chemical and thermodynamic stability at high temperatures, homogeneous and fine microstructures, and good mechanical properties, compared to conventional process such as mechanical powder mixing procedures. For the BN composites, the in situ process has been applied to various systems, including SiO 2 BN, Al 2 O 3 BN, and mullite BN composites 14 by the reactions of Si 3 N 4 +B 2 O 3,A1N+B 2 O 3,orSi 3 N 4 + A1N+B 2 O 3, respectively, and Si 3 N 4 BN nanocomposites 13 by the reaction of H 3 BO 3 and urea to synthesize nano-sized BN particles dispersed in Si 3 N 4 matrix. Recently the authors proposed the following in situ reaction to produce SiC BN composites: Si 3 N 4 +B 4 C+2C 3SiC + 4BN. (1) The weight percents of SiC and BN in the in situ SiC BN composite according to reaction (1) are 54.79% and 45.21%, respectively, and the volume percents are 46.29% and 53.71%, respectively. The in situ formed SiC and BN are in cubic ( ) and hexagonal crystallization type, respectively. The BN content can be controlled by adding BN or SiC powders. This study is aimed to investigate effect of BN content on the bending strength and Young s modulus in the in situ synthesized SiC BN composite. The results are discussed in relation with the microstructural change. The optimized BN content of the in situ SiC BN composite realizes markedly low elastic modulus and relatively high strength, indicating excellent strain tolerance of this material J. Mater. Res., Vol. 15, No. 9, Sep Materials Research Society

2 II. EXPERIMENTAL PROCEDURE Starting powders of Si 3 N 4 (E-10 grade, Ube Industries, Ube, Japan), B 4 C (F1 grade, particle size 1 m, Denki Kagaku Kogyo Co. Ltd., Tokyo, Japan), C (2600 no. grade, Mitsubishi Chemical Co., Tokyo, Japan), and SiC (A2 grade, mean particle size 0.63 m, Showa Denko Co., Tokyo, Japan) with 10 wt% Al 2 O 3 Y 2 O 3 sintering additives related to the SiC content in the composites (7:3 mixture of Al 2 O 3 and Y 2 O 3 by weight) according to the designed composition were mixed with ZrO 2 (Y 2 O 3 ) balls in ethanol is a plastic bottle for 24 h and then dried. Hot pressing of the mixed powders was conducted under a pressure of 30 MPa in a graphite die with BN coating at 2000 C for 60 min in an Ar atmosphere. The sample designations are given in Table I. The calculated BN content was changed by adding SiC powder into the reactants of reaction (1). The obtained billets with a diameter of 25 mm and thickness of 5 mm were cut to specimens and then ground and beveled. The density was tested by water immersion. Theoretical densities were calculated using density values (g/cm 3 ) of 3.22 for SiC, 2.29 for BN, and for YAG. 15 The three-point bending strength was measured using bars of mm; the span was 16 mm, and the crosshead speed was 0.5 mm/min. The strength data was an average of 5 measurements. Young s modulus E, parallel to the hot pressing direction, was measured by the pulse echo method. 16 The microstructures of the composites were observed by scanning electron microscopy (SEM). III. RESULTS AND DISCUSSION A. Densification behavior and microstructure It is well known that BN powder compact is hardly densified by pressureless sintering due to their poor sinterbility. For this reason, a hot press is generally employed to obtain dense BN ceramics. Similarly, the BN composites become more difficult to sinter at a higher BN content. 1,4,6 The relative densities of the in situ SiC BN composites as a function of BN content are illustrated in Fig. 1. It can be seen that the densification is not affected by the BN addition through the in situ reaction when the BN content is lower than 5 vol%. As the BN content further TABLE I. Sample designation Sample B0 B5 B15 B25 B35 B45 B55 BN content vol%) FIG. 1. Relative density of in situ SiC BN composites versus BN content. increased, the relative density decreased but was still higher than 95% in the studied range of BN content. Particularly, when the BN content was lower than 35 vol%, the relative density was as high as 97%. This high sinterability of the in situ SiC BN composite is very likely attributable to fine and homogeneously dispersed BN flakes of this composite. The SEM micrographs of the fracture surfaces of the in situ SiC BN composites are shown in Fig. 2. The in situ formed hexagonal BN is in flake shape as arrowed in the pictures, and the SiC phase is in equiaxial shape. The hot press direction was vertical in these pictures. It should be noted that the in situ formed BN flakes are homogeneously and isotropically distributed. These BN flakes are about 1 m in length and 0.1 m in thickness and are located at boundaries between SiC grains. The size and shape of the BN flakes do not depend on the BN content nor on the initial particle size of reactants of the in situ reaction. For SiC phase, there is not obvious change in grain size (2 3 m) when the BN content is lower than 15 vol%. However, as the BN content increased in a range about 25 vol%, the grain size of SiC decreased and reached about 1 m in B55 composite. That is, above a certain value of BN content, the grain boundary BN flakes inhibit grain growth of SiC. This phenomenon is plausibly attributable to a percolation network formation of the BN phase. 17 It is well known that when identical spherical inclusions are uniformly dispersed into a volume, at least one continuous network percolates from one side of the volume to the other at an inclusion volume of 16%. 17 Holm and Cima 18 have simulated the twodimensional whisker percolation in ceramic matrix ceramic whisker composites by computer. They found that, with the increase of the whisker aspect ratio, the percolation threshold for two-dimensional whisker percolation decreased. At whisker aspect ratio of 10, the percolation threshold is about 24%. J. Mater. Res., Vol. 15, No. 9, Sep

3 FIG. 2. Microstructures of the in situ SiC BN composites observed by SEM. The in situ formed BN flakes are arrowed in (b) with sharp arrows, and the BN flake pairs are arrowed in (b) and (c) with wide arrows. Key: (a) B0, (b) B5, (c) B15, (d) B25, (e) B35, (f) B45, and (g) B55. Lange et al. 17 also have shown that even before the percolation threshold is reached (inclusion volume 10%), many inclusion pairs exist within the composite and that every inclusion is part of the same percolating network at volumes 22%. In liquid-phase sintering of alumina coated with magnesium aluminosilicate glass, 19 Nakajima and Messing reported that the mullite and spinel crystallization retarded densification by forming a percolating network. For the oxidation behavior of silicon carbide/zirconia/mullite composites, 20 a dramatic increase of the oxidation rate at ZrO 2 contents >20 vol% was explained by a percolation theory for oxygen diffusivity in a randomly distributed two-phase medium. Also, it has been demonstrated that percolation of the inclusions is suppressed more when the matrix has finer grains but is enhanced more when the inclusions have higher aspect ratio. 18,21 It can be seen that the percolation threshold is different for materials with different microstructure, but that is around 20 vol% for a randomly distributed two-phase system. In the present case, even in 1878 J. Mater. Res., Vol. 15, No. 9, Sep 2000

4 the B5 specimen with only 5 vol% BN, two-flake pairs of BN can be seen from Fig. 2(b). In the B15 specimen, more BN flake pairs appeared; however, the percolation network has not formed yet [Fig. 2(c)]. In the specimens with BN content higher than 35 vol%, the percolation network can be clearly seen [Figs. 2(e) 2(g)]. Consequently, the percolation threshold V c for the BN phase in this in situ SiC BN composite can be reasonably taken as 25 vol%. It can be considered that the grain growth of SiC is inhibited by a percolation network of the BN phase, which presumable forms at volumes above the percolation threshold. B. Young s modulus and bending strength Figure 3 illustrates Young s modulus of the in situ SiC BN composites as a function of BN content. The upper bound (Voigt bound), E U, and the lower bound (Reuss bound), E L, of Young s modulus are also shown in this figure. Despite the porosity deviation, it can be seen that Young s modulus is reduced with increasing the BN content and the values are plotted in between E U and E L. For two-phase composite, variation of Young s modulus with volume fraction is expressed either by an exponential function as 22 E E 0 exp( bv). (2) or by a polynomial function as E E 0 (1 a 1 V + a 2 V 2 ), (3) where E 0, b, a 1, and a 2 are constants and V is the volume fraction of the second phase. For the present SiC BN in situ composites, the exponential and polynomial approximate curves are give by E = exp V, (4) E = V V 2, (5) as illustrated in Fig. 3. The respective reliabilities (R 2 ) are very high, and , indicating excellent fitness of both the approximations. Generally, as a second phase with low Young s modulus such as pores and BN is incorporated, the strength is degraded. 4,6,22 For instance, in the case of the BN nanoparticle-dispersed Si 3 N 4 composites, 13 while the strength increased slightly (less than 4%) as BN content increased up to 5 vol%, further increase of the BN content resulted in remarkable degradation of the strength. The SiC BN in situ composites, however, exhibited 9% strength increase at 5 vol% BN and 4.4% at 15 vol% BN, compared with the SiC monolith, as shown in Fig. 4. Moreover, the strength decreased only about 6% when the BN content was less than 35 vol%. Due to the thermal expansion mismatches of BN and SiC, the BN percolation network can facilitate defect formation at the boundaries, degrading the material strength. As discussed in the above section, because that the BN flake pairs appeared and increased with increasing BN content before the formation of BN percolation network, the strength was degraded slightly with increasing BN content below FIG. 3. Young s modulus of the in situ SiC BN composites versus BN content: (a) exponential approximation and (b) polynomial approximation. FIG. 4. Young s modulus, bending strength, and strain to failure of the in situ SiC BN composites versus BN content. J. Mater. Res., Vol. 15, No. 9, Sep

5 percolation threshold. Although the BN incorporation above the percolation threshold leads to the boundary defect development, it also plays a role to make the fine matrix grain as discussed in the previous section and improve the material strength. These two contradictory factors may cause the gradual strength decrease in the BN content range from 15 to 35 vol%, as shown in Fig. 4. The strain to failure of the in situ SiC BN composites calculated from E 1 is also shown in Fig. 4. For improving the reliability of structural ceramics, particularly when they are used in conjunction with different materials like metals, large strain-to-failure, or strain tolerance, is an essentially important property. In the present case, the strain-to-failure increased remarkably with increasing the BN content and reached the maximum, which was about 2.5 times larger than that of the SiC monolith, at 45 vol% BN content. IV. CONCLUSION The effect of BN content on the densification behavior, microstructure, elastic modulus, and bending strength of the SiC BN in situ composites was studied. The experimental specimens were prepared on the basis of the in situ reaction of Si 3 N 4,B 4 C, and C by hot pressing at 2000 C for 60 min under 30 MPa. The change of elastic modulus with BN content obeyed both of the exponential and polynomial rules. The densification behavior of this in situ system was excellent even at high BN content. The grain growth of SiC was inhibited by the in situ formed BN flakes at high BN content above 25 vol%. The bending strength increased slightly with increasing BN content up to 15 vol% and then decreased gradually. Above 35 vol%, the strength decrease was accelerated. These results were explained according to the BN percolation network formation. The markedly low elastic modulus as well as the relatively high strength were indicative of excellent strain tolerance of this material. ACKNOWLEDGMENT This work has been supported by Agency of Industrial and Technology (AIST), Ministry of International Trade and Industry (MITI), Japan, as part of the Synergy Ceramics Project. REFERENCES 1. K.S. Mazdiyasni and R. Ruh, J. Am. Ceram. Soc. 64, 415 (1981). 2. K. Niihara, L.D. Bentsen, D.P.H. Hasselman, and K.S. Mazdiyasni, J. Am. Ceram. Soc. 64, C-117 (1981). 3. M. Iwasa and S. Kakiuchi, J. Ceram. Soc. Jpn. (Yogyo-Kyokai-Shi) 93, 661 (1985). 4. E.H. Lutz and M. V. Swain, J. Am. Ceram. Soc. 75, 67 (1992). 5. W. Sinclair and H. Simmons, J. Mater. Sci. Lett. 6, 627 (1987). 6. P.G. Valentine, A.N. Palazotto, R. Ruh, and D.C. Larsen, Adv. Ceram. Mater. 1, 81 (1986). 7. R. Ruh, A. Zangvil, and R.R. Wills, Adv. Ceram. Mater. 3, 411 (1988). 8. R, Ruh, L.D. Bentsen, and D.P.H. Hasselman, J. Am. Ceram. Soc. 67, C-83 (1984). 9. J. Hojo, W. Nabekura, K. Kishi, and S. Umebayashi, in Proceedings of the 6th International Symposium on Ceramic Materials and Components for Engines, edited by K. Niihara, S. Kanzaki, S. Hirano, and K. Morinaga (Technoplaza Co., Tokyo, Japan, 1998), pp R. Ruh, M. Kearns, A. Zangvil, and Y. Xu, J. Am. Ceram. Soc. 75, 864 (1992). 11. G.J. Zhang and Z.Q. Bao, Bull. Chin. Ceram. Soc. 9, 28 (1990). 12. D. Goeurito-Launay, G. Brayet, and F. Thevenot, J. Mater. Sci. Lett. 5, 940 (1986). 13. T. Kusunose, Ph.D. Thesis, Osaka University, Osaka, Japan (1999). 14. W.S. Coblenz and D. Lewis, J. Am. Ceram. Soc. 71, 1080 (1988). 15. N.P. Padture, J. Am. Ceram. Soc. 77, 519 (1994). 16. Japan Industrial Standard R F. Zok, F.F. Lange, and J.R. Porter, J. Am. Ceram. Soc. 74, 1880 (1991). 18. E.A. Holm and M.J. Cima, J. Am. Ceram. Soc. 72, 303 (1989). 19. A. Nakajima and G.L. Messing, J. Am. Ceram. Soc. 81, 1163 (1998). 20. C.Y. Tsai, C.C. Lin, A. Zangvil, and A.K. Li, J. Am. Ceram. Soc. 81, 2413 (1998). 21. A. Mortensen and S. Suresh, Int. Mater. Rev. 40, 239 (1995). 22. W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, Introduction to Ceramics, 2nd ed. (John Wiley & Sons, New York, 1976), pp J. Mater. Res., Vol. 15, No. 9, Sep 2000