Effect of SiC Content on the Microstructure and Properties of Zr2Al4C5/SiC Composites in situ Fabricated by Spark Plasma Sintering

Zr2Al4C5/SiC composite ceramics were successfully fabricated from the powder mixture of Zr, Al, C and SiC by spark plasma sintering method. The composition-dependent densification, phase evolution, microstructure and mechanical properties of the composite ceramics were studied. With increasing the SiC content in the starting powder mixture, the densification behavior of sintered samples was promoted. It is found that the as-sintered products are mainly composed of Zr2Al4C5 matrix and SiC reinforcement, and the fine SiC particles tend to disperse on the matrix grain boundaries. Besides, the addition of SiC particles can evidently hinder the coarsening of Zr2Al4C5 grains. Both the Vickers hardness and Young’s modulus of composites ceramics gradually increase as increasing the SiC content. The fracture toughness of the composites decreases first then increases with the increase in the SiC and the Zr2Al4C5/30vol%SiC sample possesses the maximum value of 4.4MPa·m1/2.


Introduction
Compared with the corresponding binary carbide ZrC, the Zr-Al-C compounds (including Zr 2 Al 4 C 5 , Zr 3 Al 4 C 6 and Zr 2 Al 3 C 5 )show superior oxidation resistance, high fracture toughness, and excellent room and high-temperature stiffness, being promising high-temperature structure ceramics [1][2][3][4]. The Zr-Al-C compounds also exhibit comparable high-temperature mechanical properties, the Young's modulus decreased slowly with increasing temperature and the Young's modulus was 293 GPa at 1580 o C, which is about 81% of that at room temperature. At the same time, the strength at 1400 o C was 371 MPa, which is about 10% higher than that at room temperature. These properties make it attractive candidate for high-temperature structural applications in aerospace.
However, as a high-temperature structural material, the fracture toughness, hardness and oxidation resistance of Zr 2 Al 4 C 5 ceramics at high temperature is still unsatisfactory, which restrict its wider application. Recently, a number of works have been published on improving the mechanical properties of Zr 2 Al 4 C 5 . Owing to the light weight, high hardness and strength, superior high temperature oxidation resistance [5][6], the incorporation of second phase SiC particles into Zr 2 Al 4 C 5 significantly improve the hardness, fracture toughness, and oxidation resistance [7][8]. However, to the best of the In the present work, therefore, the Zr 2 Al 4 C 5 /SiC composites were prepared using Zr, Al, graphite, and SiC powders as initial materials by spark plasma sintering process. The effect of SiC content on the densification, phase evolution, microstructure and mechanical properties of the composites were investigated in detail. The related mechanism was also discussed.
The molar ratios of starting powders were selected to 2Zr/6.2Al/4.8C, in order to compensate the loss of Al at higher temperatures, off-stoichiometry molar content of Al were used. Finally, the Zr 2 Al 4 C 5 /SiC composites with 0 vol%, 10 vol%, 20 vol% and 30 vol% SiC were obtained and designated as ZS0~ZS3, respectively.
The mixture of initial powders with the designed composition was mixed in an agate mortar, and put into graphite crucibles of 15 mm in diameter and finally spark plasma sintered in vacuum (model-1050, Sumitomo Coal Mining Co. Ltd., Tokyo), under a temperature of 1800 •C for 3 min with a uniaxial pressure of 20 MPa. The temperature was measured by means of an optical pyrometer focused on to the sintered sample through a small hole in the die. The detailed operation and sintering process of spark plasma sintering (SPS) was reported in the previous papers [9][10].
The open porosity of the sintered products was determined by Archimedes' immersion method with water as the immersing medium. The microstructure of ceramic samples was studied by using Xray diffraction (XRD, X'Pert PRO-PANalytical) with Cu Kα radiation. The polished surface and fracture morphologies of sintered ceramic samples and the surface and cross-sectional morphologies of the samples after high-temperature oxidation were observed by using scanning electron microscope (SEM). The grain sizes of samples were determined from SEM images using the average of length and width from 50 randomly selected grains. The testing details of the Young's modulus (E), the Vickers hardness and fracture toughness of the ceramic samples were reported in the previous papers [9][10].   Figure 1 shows the XRD patterns of the composites. As shown in Figure 1(a), the peaks of Zr 2 Al 4 C 5 was not detected in the SPS sintered sample, and the main peaks were related to the Zr 2 Al 3 C 5 phase. However, with the increase of the content of SiC, only a traces amount of Zr 2 Al 3 C 5 was detected, besides the main phase of Zr 2 Al 4 C 5 , which indicates SiC played a key role on the synthesis of Zr 2 Al 4 C 5 [11].

Results and discussion
The density and open porosity of samples sintered at 1800 o C for 3 min as a function of SiC content is shown in table 1. The density of Zr 2 Al 4 C 5 /SiC composite gradually decreases with the content of SiC increasing. The density increases due to a smaller density of 4.5 g/cm 3 for Zr 2 Al 4 C 5 than that of 3.22 g/cm 3 for SiC [2,6]. It can be seen that the open porosity decreased with the increase of SiC volume content and all the composite ceramics have much lower open porosities only around 0.06-0.21%, indicating that the desification of the composite ceramics are easier to achieve due to the existence of SiC which is densified.  Table 1 compares the diameter and aspect ratio of elongated Zr 2 Al 4 C 5 grains for Zr 2 Al 4 C 5 /SiC samples. The aspect ratio of Zr-Al-C grains of pure Zr-Al-C ceramic is 4.10 ± 0.94. As the volume content of Zr-Al-C decreases from 90 vol.% to 70 vol.%, the aspect ratio of Zr-Al-C first decreases from 3.15 ± 0.85 for ZS1 and then apparently increases to 4.35±1.15 for ZS3. With the increase of SiC content, the diameter of elongated Zr 2 Al 4 C 5 grains decreased as a consequence of the pinning effect of SiC on the grain-boundary. As discussed above, the grain size of Zr 2 Al 4 C 5 matrix decreases with the increase in the SiC content, which suggests that the addition of SiC particles can evidently hinder the coarsening of Zr-Al-C grains, and the restriction in grain growth could further improve the densification and the material's stability at elevated temperature during sintering [12].
Vickers hardness and Young's modulus of Zr-Al-C-based composite as function of SiC content are listed in Table 1. It can be seen that the Vickers hardness and Young's modulus increase gradually with the increase of SiC volume content. Compared to ZrB 2 and SiC, Zr 2 Al 4 C 5 has a low hardness of about 11 GPa [3], the incorporation of SiC could effectively improve the Vickers hardness of the composites. The intrinsic Young's modulus of SiC was higher than Zr-Al-C. In addition, the ceramics with the lowest porosity and lower grain size of the matrix. The ZS3 sample exhibits relatively higher values of Vickers' hardness and Young's modulus. Figure 2 shows the polished surface morphologies from SEM backscattered electron images of samples with different compositions of Zr-Al-C and SiC volume contents. It is clearly observed that there are two distinct phases which have different grain shape and sizes. Combining with the analysis of XRD and EDS (not shown), the columnar or plate-like gray grains are identified as Zr-Al-C grains and the dark particles are SiC grains. As shown in the figures, the SiC particles disperse homogenously among the Zr-Al-C grains, which was helpful to reinforce the mechanical properties. The matrix grain size of the products decreases significantly as the SiC content increase, and no any pore could be observed in the SiC containing composites.     omposition-d volume con ugh SEM su /SiC compos trans-granula k propagation osite ceramic racture morp of ceramic m y finer and s values of ents of SiC l 4 C 5 /SiC com dependent fr ntents, the c urface morph site ceramic ar and intern during the cs [13], which phologies sho matrix [10].