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 SunInstitute 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+Al2O3)-coated SiC ber reinforced nickel matrix composite was pre-sented, in which the diusion between SiC ber and nickel matrix could be prevented. Magnetron sputteringis used to deposit Ni coating on the surface of the (Al+Al2O3)-coated SiC ber in preparation of the precursorwires. It is shown that the deposited Ni coating combines well with the (Al+Al2O3) coating and has littlenegative eect on the tensile strength of (Al+Al2O3)-coated SiC ber. Solid-state diusion bonding processis employed to prepare the (Al+Al2O3)-coated SiC ber reinforced nickel matrix with 37% bers in volume.The solid-state diusion 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 diuse well, com-posite of full density can be formed and the (Al+Al2O3) coating is eective to restrict the reaction betweenSiC ber and nickel matrix.

KEY WORDS: SiC ber; Composite; Diusion barrier layer; Precursor wire

1. Introduction

Nickel alloys are widely used in aerospace andturbine engines due to their excellent mechanicalproperties at elevated temperatures. However, highdensity and poor creep-resistance limit their furtherapplications[1]. Composites can reduce the densityand meanwhile improve the high-temperature me-chanical properties of the matrix[24], which makes itan eective way to overcome those problems in nickelalloys.

With respect to SiC ber reinforced nickel alloycomposites, the diusion reaction between SiC berand the matrix is so intense that the reinforced ef-fect of SiC ber in nickel matrix is degraded[57]. Adiusion barrier layer on the surface of ber can pre-vent the diusion between SiC ber and matrix[8,9].Lin et al.[10] deposited Al2O3 coating on the surfaceof the short SiC ber by arc ion plating, which alle-viated the reaction between the bers and Ni. How-ever, macroparticles produced in the process had neg-ative inuences on the quality of the lm. 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 deposi-tion (CVD) to restrict the reaction between the SiCbers and NiAl matrix. Nevertheless, their researchwas only focused on short SiC bers, which cannotsatisfy the requirement in practical applications. Inorder to solve these problems, (Al+Al2O3) coatingwas deposited on the surface of continuous C-coatedSiC ber as diusion barrier layer by reactive mag-netron sputtering in our previous work[12]. It is feasi-ble to use this kind of ber to prepare nickel matrixcomposite.

Solid-state diusion bonding (SDB) is an impor-tant technology for preparing metal matrix compos-ite. It is a micro-deformation process in which metalmatrix and reinforcement is vacuum hot pressed to-gether at an elevated temperature below the meltingpoint of the matrix. The key process in this methodis to prepare the preform of the composite. Physicalvapor deposition (PVD) is widely employed to pre-pare precursor wire which can be easily arranged intopreform[1315]. During PVD, precursor wire is pre-pared by directly depositing matrix material on theber. PVD can be used to solve the problem in pro-ducing foil or powder with high melting point, e.g.

L. Zhang et al.: J. Mater. Sci. Technol., 2011, 28(3), 234240. 235

Fig. 1 Schematic diagram of procedure of preparing composite

nickel alloys, and it is also easy to control the vol-ume ratio of ber in composite. Magnetron sputter-ing (MS) is one of the most widely applied methodsof PVD, which can deposit high-quality lm and haslittle inuence on mechanical properties of sputteredsamples since the process is preformed at lower tem-perature.

In this paper, MS was used to prepare precursorwire by depositing nickel on the surface of continuous(Al+Al2O3)-coated SiC ber and SDB was employedto prepare the composite. The inuence of MS onmechanical property of (Al+Al2O3)-coated SiC berwas examined. Meanwhile, preparation technology ofcomposite, the eect of (Al+Al2O3) coating to restrictthe diusion between SiC ber and nickel matrix, andthe plastic ow mechanism of nickel matrix in SDBprocess were discussed.

2. Experimental

Continuous (Al+Al2O3)-coated SiC ber with Alcoating (50 nm in thickness) and Al2O3 coating(900 nm in thickness) on the surface of continu-ous C-coated (2.5 m in thickness) SiC ber (IMR,China) of 100 m in diameter was used. The pu-rity of Ni used as sputtering target was 99.99% andthe target was rectangular with a size of 272 mm68mm. The processing chamber was evacuated to a basepressure of 3103 Pa by mechanical pump and mole-cular pump before sputtering. Argon was introducedto the chamber by ow controller to keep the workingpressure 0.5 Pa during the sputtering. The sputteringpower was supplied by a pulsed power supply with apower of 670 W and a pulse frequency of 30 kHz.

In the experiment, SDB was used to prepare com-

posite. As shown in Fig. 1, the precursor wires(Fig. 1(a)) were arranged tightly in a plane and xedto be a precast slab (Fig. 1(b)) by a special binder.The slab was then cut into smaller pieces and piled upto be a preform (Fig. 1(c)) which tted for the size ofthe die in vacuum hot press equipment (Fig. 1(d)).The preform was nally compressed to a compos-ite by SDB (Fig. 1(e)). To avoid oxidation of themetal, the vacuum hot pressing chamber was evacu-ated to 5102 Pa by mechanical pump and oil dif-fusion pump during the heating stage. The temper-ature in SDB process is in the range of 850880C,the pressure is in the range of 3070 MPa, and theholding time is in the range of 12 h.

The morphologies and microstructures of theprecursor wires and composites were observed byscanning electron microscopy (SEM) (Hitachi S-3400N) and X-ray diractometer (XRD, SHIMADZU,D/Max-2500PC). Line-scanning results of cross sec-tion were inspected by electron dispersive spec-troscopy (EDS) (Hitachi S-3400N). Tensile strengthof the precursor wires and bers was measured by us-ing a miniature tensile machine, and the number ofthe test samples was forty in one group.

3. Results and Discussion

3.1 Precursor wire

In our previous work[12], (Al+Al2O3) coating wasdeposited as a diusion barrier layer on the continuousSiC ber by MS. The results showed that (Al+Al2O3)coating protected C-rich layer of SiC ber and it isbenecial of reducing surface residual stress of SiCber. Meanwhile, it has little inuence on mechanical

236 L. Zhang et al.: J. Mater. Sci. Technol., 2011, 28(3), 234240.

Fig. 2 Surface and cross section morphology of precursor wire: (a) surface, (b) cross section)

property of SiC ber. Thus, continuous (Al+Al2O3)-coated SiC ber is desirable to prepare precursor wireof SiC ber reinforced nickel matrix composite.

The surface and cross section morphology of pre-cursor wire are shown in Fig. 2. In Fig. 2(a), the de-posited nickel coating by MS process is in the form ofcolumnar crystal growing in radial direction. Fig. 2(b)reveals that the nickel coating combines well with the(Al+Al2O3) ber surface layer. Good combinationbetween (Al+Al2O3)-coated SiC ber and nickel ma-trix coating avoids abscission of matrix coating andis benecial to prepare composite in SDB process. Inaddition, the diameter of SiC ber is about 100 mand the thickness of nickel coating is about 32 m.Therefore, the volume ratio of SiC ber in precursorwire was about 37% by calculation.

At room temperature the average tensile strengthof (Al+Al2O3)-coated SiC ber was 3.39 GPa. Thus,the theoretical tensile strength of the precursor wirewas 1.38 GPa, according to the rule of mixture incomposite:

c = fVf + m(1 Vf) (1)where, c is the tensile strength of precursor wire, Vfis volume ratio of SiC ber in precursor wire, and mis the tensile strength of nickel target (m=0.205 GPain our experiment).

Tensile testing results showed that the tensilestrength of the precursor wires prepared in the ex-periment was 1.18 GPa, about 85.5% of the theoret-ical value. After Ni coating of the precursor wirewas removed by corrosion, the tensile strength of(Al+Al2O3)-coated SiC ber was 3.30 GPa, whichwas near to the value before the deposition process.Therefore, the process of depositing nickel matrixhas little negative inuence on tensile strength of(Al+Al2O3)-coated SiC ber.

In sum, the MS process is a proper way to preparethe precursor wires for (Al+Al2O3)-coated SiC berreinforced nickel matrix composite; the deposited Nicoating combines well with the (Al+Al2O3) coat-

ing and does not degrade the tensile strength of(Al+Al2O3)-coated SiC ber.

3.2 SiC ber reinforced nickel matrix composite

In SDB process, the important technology para-meters are temperature, pressure and holding time.The temperature should be between 0.5 and 0.7 ofmelting point of pure nickel (in absolute tempera-ture), which is 1453 K[16]. Thus the recommendedSDB temperature is in the range of 590935 C. Onone hand, high temperature helps to accelerate dif-fusion rate of nickel matrix and shorten the time ofpreparation. On the other hand, if the selected tem-perature is too high, it will induce the undesirablereaction at interface. With respect to the pressure,higher pressure increases the plastic ow of nickel ma-trix and decreases the amount of voids, while too highpressure will damage either the coating on the surfaceof the bers or the bers themselves. In addition,holding time should be appropriate to achieve the re-quired density but avoid the degradation of propertiesof the interface and the composite. To these regards,these three parameters should be optimized accordingto practical SDB process.

Fig. 3 shows the morphology of the compos-ites produced with dierent combinations of tem-perature, pressure and holding time, including850 C30 MPa1 h in Fig. 3(a), 850 C50 MPa2 h in Fig. 3(b), 870 C30 MPa2 hin Fig. 3(c), 870 C50 MPa2 h in Fig. 3(d),880 C50 MPa2 h in Fig. 3(e), and 870 C70 MPa2 h in Fig. 3(f). Voids with dierent shapesand sizes are observed at the interface between precur-sor wires in Fig. 3(a), Fig. 3(b) and Fig. 3(c), althoughthe bers keep intact. The occurrence of these voidsat the interface between precursor wires indicates thatprecursor wires did not diuse adequately. Comparedwith the quantity of voids in composite prepared at850 C30 MPa1 h (Fig. 3(a)), the quantity of voidsin the composite prepared under higher pressure and

L. Zhang et al.: J. Mater. Sci. Technol., 2011, 28(3), 234240. 237

Fig. 3 Morphology of composite prepared at: (a) 850 C30 MPa1 h, (b) 850 C50 MPa2 h, (c) 870 C30 MPa2 h, d) 870 C50 MPa2 h, (e) 880 C50 MPa2 h, and f) 870 C70 MPa2 h

longer holding time, i.e. 850 C50 MPa2 h(Fig. 3(b)), does not obviously decrease. How-ever, most of the voids became smaller, andtheir shape changes from polygonal to triangular.In contrast, when the temperature improved, i.e.870 C30 MPa2 h, only some quite small voidsare observed, as shown in Fig. 3(c). With bothimproved temperature and improved pressure, i.e.870 C50 MPa2 h, no void is observed (Fig. 3(d))and furthermore bers are arranged in order, reveal-ing that precursor wires diused completely underthis condition. With further increased temperature,i.e. 880 C50 MPa2 h, Fig. 3(e) shows that the dif-fusion of precursor wires was adequate, but the mar-gin of SiC bers was not as smooth as that beforeSDB process. At 870 C70 MPa2 h (Fig. 3(f)), thebers lost the original shape and (Al+Al2O3) coat-ing was not intact. These two samples revealed thatthere are temperature and pressure limits in the SDBprocess. When the temperature or the pressure ishigher than the limit, the eect of diusion barrierwould be weakened and lead to undesirable diusionbetween SiC ber and nickel.

Fig. 4 shows the SEM line-scanning of the interfacebetween SiC ber and nickel matrix in the compositeprepared at 870 C50 MPa2 h. C-rich layer and(Al+Al2O3) coating were both as intact as that inprecursor wire, showing that Si did not diuse to thematrix and Ni did not react with the ber. Calculatedby thermodynamics formula:

G298 = H298 TS298 < 0 (2)

Al2O3, SiC and Ni can coexist under 1000 C.

Thus, this intact (Al+Al2O3) coating could restrictinterdiusion of Ni and SiC, and protect SiC ber.

As analyzed above, the optimum SDB parametersfor the nickel matrix composite with 37% volume ra-tio of SiC ber is obtained as 870 C50 MPa2 hin the present experiment, under which adequate dif-fusion of precursor wires is obtained and the eect ofthe barrier layer (Al+Al2O3) maintains.

Fig. 5 shows the XRD spectrum of nickel in pre-cursor wire and in composite prepared at 850 C50 MPa2 h. There are three crystal orientationsof 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 from100:43.2:18.0 in PDF card (No.65-2865). It indicatesthat in precursor wire, Ni grains grow in preferred ori-entation (111), which accords with the result shownin Fig. 2(a). After SDB process, the intensity ratiochanges to 100:34.6:17.04, showing that the trend togrow on (111) weakens. Meanwhile, the morpholo-gies of tensile fracture surface in composite preparedat 850 C50 MPa2 h (Fig. 6) indicated that thecolumnar crystal of nickel existed near the void thatformed at the interface between precursor wires, anddisappeared if the precursor wires diused completely.Therefore, adequate diusion of precursor wires dur-ing SDB process weakens the preferred orientation ofNi coating that forms during the preparation of pre-cursor wires and will favor the mechanical propertiesof the composite.

3.3 Plastic ow 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), 234240.

Fig. 4 Line-scanning pattern of cross section of compos-ite prepared at 870 C50 MPa2 h

Fig. 5 XRD spectrum of nickel in precursor wire and incomposite prepared at 850 C50 MPa2 h

diusion bonding and evaluated several mechanismswhich close the voids in the process. Based ontheir model, Chen et al.[19] developed a model formatrix-coated ber reinforced composite, in which thematrix-coated bers are arranged in square hexago-nal. This model shows a good agreement with theexperimental results of the consolidation process ofsapphire ber-reinforced NiAl composites. The mech-anisms developed in these two models were: (i) sur-face diusion, (ii) volume diusion from surface, (iii)evaporation-condensation, (iv) grain boundary diu-sion, (v) volume diusion from interfacial sources, (vi)power-law creep, and (vii) plastic ow.

Fig. 7 shows the process of plastic ow mechanismof nickel matrix in our experiment. At the rst stage(Fig. 7(a)) before the pressure was applied, precur-sor wires rearranged in hexagonal symmetry and aquadrangular void formed between the adjacent pre-cursor wires. Surface diusion, volume diusion, andevaporation-condensation were the main mechanisms

Fig. 6 Dierent morphologies of nickel matrix in tensile fracture surface of composite prepared at850 50 MPa2 h: (a) columnar crystal of nickel; (b) columnar crystal of nickel disappeared

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Fig. 7 Plastic ow mechanism of nickel matrix

controlling this stage[20]. Among these mechanisms,volume diusion was the most important mechanismwhich caused the apiciform voids in the composite.

At the second stage (Fig. 7(b)) after pressure wasapplied, the two surfaces bond immediately contacteddue to the very high contact stress and the quadranglevoid turned into two triangular voids. Plastic defor-mation controlled this stage and it ceased when thecontact area at the interface was sucient to supportthe applied load, i.e. the local stress falls below thematerials yield stress.

At the third stage, the triangular void becamesmaller (Fig. 7(c), (d)) and nally disappeared(Fig. 7(e)). This stage was a time-dependent process,and most of the seven mechanisms contributed tothe diusion bonding, especially power-law creep andgrain boundary diusion. Power-law creep mecha-nism was developed from microcreep of asperities athigh temperature, and stress had much inuence onit. Grain boundary diusion was aected by the grainsize.

During SDB process, all the mechanisms work to-gether to inuence the diusion of precursor wires andthese mechanisms are aected by temperature, pres-sure and holding time. Although higher temperature,higher pressure and longer time contribute to the dif-fusion of matrix, they are not of benet to protectdiusion barrier layer (Al+Al2O3) or SiC ber. Thus,these three parameters should be optimized and con-trolled under the limit values, as discussed in Section3.2.

4. Conclusions

A method to prepare continuous (Al+Al2O3)-

coated SiC ber reinforced nickel matrix compositewas presented in this paper. This method is advanta-geous to prevent the diusion between SiC ber andnickel matrix by using the MS in preparation of pre-cursor wires and optimize SDB process in preparationof the composite. The main conclusions can be drawnas follows:

(1) MS process is a proper way to prepare theprecursor wire by depositing Ni coating in thicknessabout 32 m on the surface of (Al+Al2O3)-coatedSiC ber; Ni coating combined well with (Al+Al2O3)coating and it has little negative inuence on tensilestrength of (Al+Al2O3)-coated SiC ber.

(2) SDB process was optimized to prepare the(Al+Al2O3)-coated SiC ber reinforced nickel ma-trix composite. The obtained optimum parameterswere 870 C50 MPa2 h when the volume ratioof SiC ber in precursor wires was about 37%. Un-der this condition, precursor wires diused adequatelyand (Al+Al2O3) coating eectively restricted the re-action between SiC ber and nickel matrix.

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