J. Mater. Sci. Technol., 2011, 28(3), 234-240. Preparation of SiC Fiber Reinforced Nickel Matrix Composite Lu Zhang, Nanlin Shi, Jun Gong and Chao Sun Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received March 7, 2011, in revised form July 2, 2011] A method of preparing continuous (Al+Al 2 O 3 )-coated SiC fiber reinforced nickel matrix composite was presented, in which the diffusion between SiC fiber and nickel matrix could be prevented. Magnetron sputtering is used to deposit Ni coating on the surface of the (Al+Al 2 O 3 )-coated SiC fiber in preparation of the precursor wires. It is shown that the deposited Ni coating combines well with the (Al+Al 2 O 3 ) coating and has little negative effect on the tensile strength of (Al+Al 2 O 3 )-coated SiC fiber. Solid-state diffusion bonding process is employed to prepare the (Al+Al 2 O 3 )-coated SiC fiber reinforced nickel matrix with 37% fibers in volume. The solid-state diffusion bonding process is optimized and the optimum parameters are temperature of 870, pressure of 50 MPa and holding time of 2 h. Under this condition, the precursor wires can diffuse well, composite of full density can be formed and the (Al+Al 2 O 3 ) coating is effective to restrict the reaction between SiC fiber and nickel matrix. KEY WORDS: SiC fiber; Composite; Diffusion barrier layer; Precursor wire 1. Introduction Nickel alloys are widely used in aerospace and turbine engines due to their excellent mechanical properties at elevated temperatures. However, high density and poor creep-resistance limit their further applications [1]. Composites can reduce the density and meanwhile improve the high-temperature mechanical properties of the matrix [2 4], which makes it an effective way to overcome those problems in nickel alloys. With respect to SiC fiber reinforced nickel alloy composites, the diffusion reaction between SiC fiber and the matrix is so intense that the reinforced effect of SiC fiber in nickel matrix is degraded [5 7]. A diffusion barrier layer on the surface of fiber can prevent the diffusion between SiC fiber and matrix [8,9]. Lin et al. [10] deposited Al 2 O 3 coating on the surface of the short SiC fiber by arc ion plating, which alleviated the reaction between the fibers and Ni. However, macroparticles produced in the process had negative influences on the quality of the film. Larkin Corresponding author. Prof., Ph.D.; Tel.: +86 24 83978081; E-mail address: csun@imr.ac.cn (C. Sun). et al. [11] deposited yttria by chemical vapor deposition (CVD) to restrict the reaction between the SiC fibers and NiAl matrix. Nevertheless, their research was only focused on short SiC fibers, which cannot satisfy the requirement in practical applications. In order to solve these problems, (Al+Al 2 O 3 ) coating was deposited on the surface of continuous C-coated SiC fiber as diffusion barrier layer by reactive magnetron sputtering in our previous work [12]. It is feasible to use this kind of fiber to prepare nickel matrix composite. Solid-state diffusion bonding (SDB) is an important technology for preparing metal matrix composite. It is a micro-deformation process in which metal matrix and reinforcement is vacuum hot pressed together at an elevated temperature below the melting point of the matrix. The key process in this method is to prepare the preform of the composite. Physical vapor deposition (PVD) is widely employed to prepare precursor wire which can be easily arranged into preform [13 15]. During PVD, precursor wire is prepared by directly depositing matrix material on the fiber. PVD can be used to solve the problem in producing foil or powder with high melting point, e.g.
L. Zhang et al.: J. Mater. Sci. Technol., 2011, 28(3), 234 240. 235 Fig. 1 Schematic diagram of procedure of preparing composite nickel alloys, and it is also easy to control the volume ratio of fiber in composite. Magnetron sputtering (MS) is one of the most widely applied methods of PVD, which can deposit high-quality film and has little influence on mechanical properties of sputtered samples since the process is preformed at lower temperature. In this paper, MS was used to prepare precursor wire by depositing nickel on the surface of continuous (Al+Al 2 O 3 )-coated SiC fiber and SDB was employed to prepare the composite. The influence of MS on mechanical property of (Al+Al 2 O 3 )-coated SiC fiber was examined. Meanwhile, preparation technology of composite, the effect of (Al+Al 2 O 3 ) coating to restrict the diffusion between SiC fiber and nickel matrix, and the plastic flow mechanism of nickel matrix in SDB process were discussed. 2. Experimental Continuous (Al+Al 2 O 3 )-coated SiC fiber with Al coating ( 50 nm in thickness) and Al 2 O 3 coating ( 900 nm in thickness) on the surface of continuous C-coated ( 2.5 µm in thickness) SiC fiber (IMR, China) of 100 µm in diameter was used. The purity of Ni used as sputtering target was 99.99% and the target was rectangular with a size of 272 mm 68 mm. The processing chamber was evacuated to a base pressure of 3 10 3 Pa by mechanical pump and molecular pump before sputtering. Argon was introduced to the chamber by flow controller to keep the working pressure 0.5 Pa during the sputtering. The sputtering power was supplied by a pulsed power supply with a power of 670 W and a pulse frequency of 30 khz. In the experiment, SDB was used to prepare composite. As shown in Fig. 1, the precursor wires (Fig. 1(a)) were arranged tightly in a plane and fixed to be a precast slab (Fig. 1(b)) by a special binder. The slab was then cut into smaller pieces and piled up to be a preform (Fig. 1(c)) which fitted for the size of the die in vacuum hot press equipment (Fig. 1(d)). The preform was finally compressed to a composite by SDB (Fig. 1(e)). To avoid oxidation of the metal, the vacuum hot pressing chamber was evacuated to 5 10 2 Pa by mechanical pump and oil diffusion pump during the heating stage. The temperature in SDB process is in the range of 850 880 C, the pressure is in the range of 30 70 MPa, and the holding time is in the range of 1 2 h. The morphologies and microstructures of the precursor wires and composites were observed by scanning electron microscopy (SEM) (Hitachi S- 3400N) and X-ray diffractometer (XRD, SHIMADZU, D/Max-2500PC). Line-scanning results of cross section were inspected by electron dispersive spectroscopy (EDS) (Hitachi S-3400N). Tensile strength of the precursor wires and fibers was measured by using a miniature tensile machine, and the number of the test samples was forty in one group. 3. Results and Discussion 3.1 Precursor wire In our previous work [12], (Al+Al 2 O 3 ) coating was deposited as a diffusion barrier layer on the continuous SiC fiber by MS. The results showed that (Al+Al 2 O 3 ) coating protected C-rich layer of SiC fiber and it is beneficial of reducing surface residual stress of SiC fiber. Meanwhile, it has little influence on mechanical
236 L. Zhang et al.: J. Mater. Sci. Technol., 2011, 28(3), 234 240. Fig. 2 Surface and cross section morphology of precursor wire: (a) surface, (b) cross section) property of SiC fiber. Thus, continuous (Al+Al 2 O 3 )- coated SiC fiber is desirable to prepare precursor wire of SiC fiber reinforced nickel matrix composite. The surface and cross section morphology of precursor wire are shown in Fig. 2. In Fig. 2(a), the deposited nickel coating by MS process is in the form of columnar crystal growing in radial direction. Fig. 2(b) reveals that the nickel coating combines well with the (Al+Al 2 O 3 ) fiber surface layer. Good combination between (Al+Al 2 O 3 )-coated SiC fiber and nickel matrix coating avoids abscission of matrix coating and is beneficial to prepare composite in SDB process. In addition, the diameter of SiC fiber is about 100 µm and the thickness of nickel coating is about 32 µm. Therefore, the volume ratio of SiC fiber in precursor wire was about 37% by calculation. At room temperature the average tensile strength of (Al+Al 2 O 3 )-coated SiC fiber was 3.39 GPa. Thus, the theoretical tensile strength of the precursor wire was 1.38 GPa, according to the rule of mixture in composite: σ c = σ f V f + σ m (1 V f ) (1) where, σ c is the tensile strength of precursor wire, V f is volume ratio of SiC fiber in precursor wire, and σ m is the tensile strength of nickel target (σ m =0.205 GPa in our experiment). Tensile testing results showed that the tensile strength of the precursor wires prepared in the experiment was 1.18 GPa, about 85.5% of the theoretical value. After Ni coating of the precursor wire was removed by corrosion, the tensile strength of (Al+Al 2 O 3 )-coated SiC fiber was 3.30 GPa, which was near to the value before the deposition process. Therefore, the process of depositing nickel matrix has little negative influence on tensile strength of (Al+Al 2 O 3 )-coated SiC fiber. In sum, the MS process is a proper way to prepare the precursor wires for (Al+Al 2 O 3 )-coated SiC fiber reinforced nickel matrix composite; the deposited Ni coating combines well with the (Al+Al 2 O 3 ) coating and does not degrade the tensile strength of (Al+Al 2 O 3 )-coated SiC fiber. 3.2 SiC fiber reinforced nickel matrix composite In SDB process, the important technology parameters are temperature, pressure and holding time. The temperature should be between 0.5 and 0.7 of melting point of pure nickel (in absolute temperature), which is 1453 K [16]. Thus the recommended SDB temperature is in the range of 590 935 C. On one hand, high temperature helps to accelerate diffusion rate of nickel matrix and shorten the time of preparation. On the other hand, if the selected temperature is too high, it will induce the undesirable reaction at interface. With respect to the pressure, higher pressure increases the plastic flow of nickel matrix and decreases the amount of voids, while too high pressure will damage either the coating on the surface of the fibers or the fibers themselves. In addition, holding time should be appropriate to achieve the required density but avoid the degradation of properties of the interface and the composite. To these regards, these three parameters should be optimized according to practical SDB process. Fig. 3 shows the morphology of the composites produced with different combinations of temperature, pressure and holding time, including 850 C 30 MPa 1 h in Fig. 3(a), 850 C 50 MPa 2 h in Fig. 3(b), 870 C 30 MPa 2 h in Fig. 3(c), 870 C 50 MPa 2 h in Fig. 3(d), 880 C 50 MPa 2 h in Fig. 3(e), and 870 C 70 MPa 2 h in Fig. 3(f). Voids with different shapes and sizes are observed at the interface between precursor wires in Fig. 3(a), Fig. 3(b) and Fig. 3(c), although the fibers keep intact. The occurrence of these voids at the interface between precursor wires indicates that precursor wires did not diffuse adequately. Compared with the quantity of voids in composite prepared at 850 C 30 MPa 1 h (Fig. 3(a)), the quantity of voids in the composite prepared under higher pressure and
L. Zhang et al.: J. Mater. Sci. Technol., 2011, 28(3), 234 240. 237 Fig. 3 Morphology of composite prepared at: (a) 850 C 30 MPa 1 h, (b) 850 C 50 MPa 2 h, (c) 870 C 30 MPa 2 h, d) 870 C 50 MPa 2 h, (e) 880 C 50 MPa 2 h, and f) 870 C 70 MPa 2 h longer holding time, i.e. 850 C 50 MPa 2 h (Fig. 3(b)), does not obviously decrease. However, most of the voids became smaller, and their shape changes from polygonal to triangular. In contrast, when the temperature improved, i.e. 870 C 30 MPa 2 h, only some quite small voids are observed, as shown in Fig. 3(c). With both improved temperature and improved pressure, i.e. 870 C 50 MPa 2 h, no void is observed (Fig. 3(d)) and furthermore fibers are arranged in order, revealing that precursor wires diffused completely under this condition. With further increased temperature, i.e. 880 C 50 MPa 2 h, Fig. 3(e) shows that the diffusion of precursor wires was adequate, but the margin of SiC fibers was not as smooth as that before SDB process. At 870 C 70 MPa 2 h (Fig. 3(f)), the fibers lost the original shape and (Al+Al 2 O 3 ) coating was not intact. These two samples revealed that there are temperature and pressure limits in the SDB process. When the temperature or the pressure is higher than the limit, the effect of diffusion barrier would be weakened and lead to undesirable diffusion between SiC fiber and nickel. Fig. 4 shows the SEM line-scanning of the interface between SiC fiber and nickel matrix in the composite prepared at 870 C 50 MPa 2 h. C-rich layer and (Al+Al 2 O 3 ) coating were both as intact as that in precursor wire, showing that Si did not diffuse to the matrix and Ni did not react with the fiber. Calculated by thermodynamics formula: G φ 298 = Hφ 298 T Sφ 298 < 0 (2) Al 2 O 3, SiC and Ni can coexist under 1000 C. Thus, this intact (Al+Al 2 O 3 ) coating could restrict interdiffusion of Ni and SiC, and protect SiC fiber. As analyzed above, the optimum SDB parameters for the nickel matrix composite with 37% volume ratio of SiC fiber is obtained as 870 C 50 MPa 2 h in the present experiment, under which adequate diffusion of precursor wires is obtained and the effect of the barrier layer (Al+Al 2 O 3 ) maintains. Fig. 5 shows the XRD spectrum of nickel in precursor wire and in composite prepared at 850 C 50 MPa 2 h. There are three crystal orientations of nickel in XRD spectrum, including (111), (200) and (220). In precursor wire, intensity ratio of (111), (200) and (220) is 100:30.8:20.1, which is away from 100:43.2:18.0 in PDF card (No.65-2865). It indicates that in precursor wire, Ni grains grow in preferred orientation (111), which accords with the result shown in Fig. 2(a). After SDB process, the intensity ratio changes to 100:34.6:17.04, showing that the trend to grow on (111) weakens. Meanwhile, the morphologies of tensile fracture surface in composite prepared at 850 C 50 MPa 2 h (Fig. 6) indicated that the columnar crystal of nickel existed near the void that formed at the interface between precursor wires, and disappeared if the precursor wires diffused completely. Therefore, adequate diffusion of precursor wires during SDB process weakens the preferred orientation of Ni coating that forms during the preparation of precursor wires and will favor the mechanical properties of the composite. 3.3 Plastic flow mechanism of nickel matrix Derby and Wallach [17,18] developed a model of
238 L. Zhang et al.: J. Mater. Sci. Technol., 2011, 28(3), 234 240. Intensity / a.u. 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0 (111) (200) (220) Composite Precursor wire 20 30 40 50 60 70 80 2 / deg. Fig. 5 XRD spectrum of nickel in precursor wire and in composite prepared at 850 C 50 MPa 2 h Fig. 4 Line-scanning pattern of cross section of composite prepared at 870 C 50 MPa 2 h diffusion bonding and evaluated several mechanisms which close the voids in the process. Based on their model, Chen et al. [19] developed a model for matrix-coated fiber reinforced composite, in which the matrix-coated fibers are arranged in square hexagonal. This model shows a good agreement with the experimental results of the consolidation process of sapphire fiber-reinforced NiAl composites. The mechanisms developed in these two models were: (i) surface diffusion, (ii) volume diffusion from surface, (iii) evaporation-condensation, (iv) grain boundary diffusion, (v) volume diffusion from interfacial sources, (vi) power-law creep, and (vii) plastic flow. Fig. 7 shows the process of plastic flow mechanism of nickel matrix in our experiment. At the first stage (Fig. 7(a)) before the pressure was applied, precursor wires rearranged in hexagonal symmetry and a quadrangular void formed between the adjacent precursor wires. Surface diffusion, volume diffusion, and evaporation-condensation were the main mechanisms Fig. 6 Different morphologies of nickel matrix in tensile fracture surface of composite prepared at 850 50 MPa 2 h: (a) columnar crystal of nickel; (b) columnar crystal of nickel disappeared
L. Zhang et al.: J. Mater. Sci. Technol., 2011, 28(3), 234 240. 239 Fig. 7 Plastic flow mechanism of nickel matrix controlling this stage [20]. Among these mechanisms, volume diffusion was the most important mechanism which caused the apiciform voids in the composite. At the second stage (Fig. 7(b)) after pressure was applied, the two surfaces bond immediately contacted due to the very high contact stress and the quadrangle void turned into two triangular voids. Plastic deformation controlled this stage and it ceased when the contact area at the interface was sufficient to support the applied load, i.e. the local stress falls below the material s yield stress. At the third stage, the triangular void became smaller (Fig. 7(c), (d)) and finally disappeared (Fig. 7(e)). This stage was a time-dependent process, and most of the seven mechanisms contributed to the diffusion bonding, especially power-law creep and grain boundary diffusion. Power-law creep mechanism was developed from microcreep of asperities at high temperature, and stress had much influence on it. Grain boundary diffusion was affected by the grain size. During SDB process, all the mechanisms work together to influence the diffusion of precursor wires and these mechanisms are affected by temperature, pressure and holding time. Although higher temperature, higher pressure and longer time contribute to the diffusion of matrix, they are not of benefit to protect diffusion barrier layer (Al+Al 2 O 3 ) or SiC fiber. Thus, these three parameters should be optimized and controlled under the limit values, as discussed in Section 3.2. 4. Conclusions A method to prepare continuous (Al+Al 2 O 3 )- coated SiC fiber reinforced nickel matrix composite was presented in this paper. This method is advantageous to prevent the diffusion between SiC fiber and nickel matrix by using the MS in preparation of precursor wires and optimize SDB process in preparation of the composite. The main conclusions can be drawn as follows: (1) MS process is a proper way to prepare the precursor wire by depositing Ni coating in thickness about 32 µm on the surface of (Al+Al 2 O 3 )-coated SiC fiber; Ni coating combined well with (Al+Al 2 O 3 ) coating and it has little negative influence on tensile strength of (Al+Al 2 O 3 )-coated SiC fiber. (2) SDB process was optimized to prepare the (Al+Al 2 O 3 )-coated SiC fiber reinforced nickel matrix composite. The obtained optimum parameters were 870 C 50 MPa 2 h when the volume ratio of SiC fiber in precursor wires was about 37%. Under this condition, precursor wires diffused adequately and (Al+Al 2 O 3 ) coating effectively restricted the reaction between SiC fiber and nickel matrix. REFERENCES [1 ] S.T. Mileiko, V.M. Kiiko, A.A. Kolchin, A.V. Serebryakov, V.P. Korzhov, M. Yu Starostin and N.S. Sarkissyan: Compos. Sci. Technol., 2002, 62(2), 167. [2 ] J. Doychak: JOM, 1992, 44(6), 46. [3 ] Y.C. Fu, N.L. Shi, D.Z. Zhang and R. Yang: Mater. Sci. Eng. A, 2006, 426(1 2), 278. [4 ] X.H. Lu, and Y.Q. Yang: Trans. Nonferrous Met. Soc. China, 2006, 16(1), 77 [5 ] T. Yamada, H. Sekiguchi, H. Okamoto, S. Azuma, and A. Kitamura: in Proc. 2nd Int. Symp. on Ceramic Materials and Components for Engines, 1986, 441. [6 ] R.C.J. Schiepers, F.J.J. Van Loo and G. de With: J.
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