Production and characterization of zirconium carbide based composites prepared by spark plasma sintering

Zirconium carbide (ZrC) is a potential candidate for ultra high-temperature materials, such as nuclear fuel elements, cutting tools, jet engine parts, leading edges of re-entry space aircraft due to its high melting point (3420 °C), high hardness (25.5 GPa), high electrical and (78.10-6 Ω.cm) thermal (20.5 W/m·°C) conductivity. Despite having such outstanding properties, low fracture toughness, low oxidation resistance and low sinterability due to strong covalent bonding and low self-diffusion rates restrict the potential industrial applications of this material. Therefore, to promote ZrC based composites has been an important research area in recent years and different methods have been developed to improve the densification and microstructural properties of ZrC. The most effective method for improving the sintering behaviour and obtaining ZrC ceramics with high densities in relatively low temperatures is to use advanced sintering techniques such as spark plasma sintering (SPS), reactive spark plasma sintering and optimize the sintering parameters. It could be possible to enhance the oxidation resistance and mechanical properties of ZrC by incorporating one or more carbide, boride, nitride or oxide phases into the ZrC matrix. Within the scope of experimental studies, ZrC-TiB2, ZrC-ZrB2, ZrC-ZrB2-CNT and ZrC-ZrB2-GNP composites were produced with the addition of different amounts of titanium diboride (TiB2), zirconium diboride (ZrB2), carbon nanotube (CNT) and graphene nano platelets (GNP) in ZrC matrix, by using the Spark Plasma Sintering technique at different temperatures and pressures. ZrC-based composites in 50 mm diameter and 4 mm thick were produced by using an SPS apparatus (SPS 7.40 MKVII, SPS Syntex Inc.) with a capacity of 20.000 A in Department of Metallurgical and Materials Engineering, Istanbul Technical University. The composites of ZrC-TiB2 were produced at 1685 °C and 1700 °C temperatures under 40 MPa, 50 MPa and 60 MPa pressures for 5 minutes. ZrC-ZrB2 composites were sintered at 1700 °C under 40 MPa and 60 MPa pressures for 5 minutes; and ZrC-ZrB2-CNT and ZrC-ZrB2-GNP composites were produced at 1700 °C under 60 MPa pressure. Duruing the sintering process, the temperatures were measured with an optical pyrometer (Chino, IR-AH) focused on the graphite die and linear shrinkages of the specimens during sintering were monitored through the displacement of a punch rod. After cleaning the surfaces of the sintered samples extracted from the graphite dies; they were cut by diamond wheel in order to achieve appropriate dimensions for characterization. The density of the samples were measured by using Archimedes' principle. Hardness and fracture toughness values of the samples were determined by indentation method, after polishing the samples using diamond suspensions. Crystalline phases were identified by X-ray diffraction using Cu-Kα radiation. A micro-Raman spectroscopy was utilized to determine whether the carbon nanotubes were preserved after the SPS process for ZrC-ZrB2-CNT composites. The microstructure of the starting powders, polished and fracture surfaces of the sintered specimens were observed with field emissive scanning electron microscopy (FE-SEM). In the oxidation studies, the samples were placed in an alumina crucible with a minimum contact area, so that the maximum area of the samples could be exposed to oxidation. Composites were exposed to stagnant air at 900°C for 2h and oxidation behavior of the composites were investigated. For ZrC-TiB2 composites, XRD results showed that all samples contain ZrC peaks while they do not contain TiB2, as thermodynamically expected. TiC peaks are also absent in the XRD patterns of all compositions. When the XRD patterns analyzed in detail, it is seen that the diffraction peaks shift to higher 2θ degrees as the content of Ti increases. Because the ionic radius of Zr is larger than Ti, the introduction of Ti atoms into ZrC decreases the lattice parameter. The calculated lattice parameters of XRD patterns are 4.68 A°, 4.67 A° and 4.66 A° for %10, %20 and %30 TiB2 containing composites, respectively. Thus, the shift of (Zr,Ti)C diffraction peaks towards larger angles can be observed. By the determination of the lattice parameters of composites, it is proved that substituted solid solutions were formed in ZrC-TiB2 composites. The increase in TiB2 addition decreased the initiation and completion temperatures of shrinkages probably due to the volumetric diffusion of Ti and Zr atoms. Increase in pressure also enhanced the densification by means of increasing the number of contact points between particles and reducing the effective diffusion distance. The hardness values of the composites increased with increasing TiB2 content. The highest value of hardness, 21.64 GPa, was achieved in the composite prepared with 30 vol % TiB2 addition. Oxidation behavior was also improved with the addition of TiB2 and the composite prepared by adding 30 vol % TiB2 has the lowest weight gain after oxidation test. For ZrC-ZrB2 composites, the relative density values are in the range from 94.73% to 97.34% and the composite containing 30 vol % ZrB2, sintered under 60 MPa pressue, has the highest relative density. The monolithic ZrC produced at the same sintering conditions has a relative density of 95.52% and started to shrink at 1700 °C while the initiation temperature of shrinkage is 1565 °C for composite containing 30 vol. %ZrB2. The hardness of the composites decreased with increasing ZrB2 content and the decrement is attributed to the lower theoretical hardness of ZrB2 phase compared to ZrC. On the other hand, the fracture toughness showed an increase with increasing ZrB2 content and varied from 2.32 to 3.89 MPa.m1/2. The weight gain could not be determined after oxidation test at 900 °C for 2 h, because of the spallations. For triple ZrC-ZrB2-CNT composites, the addition of CNT decreased the initiation and completion temperatures of shrinkages and the improvement in sintering behavior could be attributed to the perfect combination of the sintering mechanism of SPS and the high electrical and thermal conductivity of CNTs. The high pulse current in SPS is preferentially transported through CNTs due to their low resistance, which locally increases the temperature near CNTs. Thereby, CNTs become the dominant heat generator in the composites. The hardness measurement results revealed that addition of 0.25 wt% CNTs slightly decreased the hardness of composite from 17.67 to 16.75 GPa, and when the amount of CNT reinforcing increased, hardness values continued to decrease. This could be related to the agglomeration of CNTs. The addition of CNT to ZrC-ZrB2 composites significantly increased the fracture toughness, and the highest toughness, 5.16 MPa·m1/2, was obtained with the addition of 1 wt% CNT as it is well known that CNTs improve the fracture toughness of the composite through a range of toughening mechanisms. The toughening mechanisms were also investigated through crack propagation by using SEM. For triple ZrC-ZrB2-GNP composites, the addition of GNP decreased the relative densities due to the overlapping of GNP in the composites with porosites. Anisotropic microstructure was observed by microscopic analysis due to preferential orientation of GPLs perpendicular direction to SPS pressure. The hardness values decreased with the addition of GNP while fracture toughness values increased up to 5,66±0,29 MPa·m1/2. The toughening mechanisms were also investigated through crack propagation by using SEM.