Production and characterization of titanium carbide and graphene nanoplatelets (GNP) reinforced zirconium carbide ceramics prepared by spark plasma sintering

Transition metal carbides, such as zirconium carbide (ZrC), are refractory ceramics with high melting points over 3000oC, low densities, high hardness (above 24GPa), excellent thermal stability and good corrosion resistance. They are promising for ultra-high-temperature structural applications, such as cutting tools, jet engine parts, nose caps and leading edges of re-entry space aircraft. However, low sinterability and poor fracture toughness of ZrC materials restrict their practical applications. Because of their high melting points and low self-diffusion coefficients, it is difficult to consolidate monolithic ZrC materials by conventional solid-state sintering. ManLabs-Air Force Materials Research Laboratory (AFML) began working on the ultra-high temperature ceramics in aviation and aerospace applications in 1960s. In the early 1990s, with the development in pressure-assisted sintering techniques, researches were conducted about the utilization of intermetallic ceramics on leading edge and nose cap parts of hypersonic systems and atmospheric reentry vehicles. Fully dense carbide-based materials have been hardly obtained and required high sintering temperature and time because of strong covalent bonding and high melting temperature. Densification problem of borides can be overcome by using advanced sintering techniques i.e., spark plasma sintering (SPS). SPS technique has some important advantages such as low sintering temperature, short sintering time compared to hot press, hot isostatic press and pressureless sintering systems. These properties make it possible to produce high-density carbides, borides and nitrides having high melting temperatures in several minutes without grain growth. Titanium carbide (TiC), another transition metal carbide such as zirconium carbide, has a higher fracture toughness value than zirconium carbide and has better sintering ability. Graphene is a nanosized carbon form that has gained considerable attention in recent years thanks to its mechanical, thermal and high electron mobility properties. Under the experimental studies, ZrC-TiC binary and ZrC-TiC-GNP ternary composites in different compositions were produced and then characterization was carried out in terms of density measurement, densification behavior, Vickers microhardness, fracture toughness, toughening mechanisms, phase analysis and microstructural investigations. Samples of 4 mm in height, 50 mm in diameter were sintered by SPS. Composites were produced in vacuum at 1700oC for 300 s under 40 MPa. Spark plasma sintering (SPS) is a pressurized sintering method which is a very effective sintering technique for material with high melting point and strong covalent bonds such as zirconium carbide and at the same time having low sintering ability. Materials with high melting points and low sintering ability require high sintering temperatures and long sintering times to reach a dense structure. Since advanced sintering techniques such as SPS have lower sintering temperatures and shorter synergistic times than hot isostatic presses, hot presses and pressureless sintering methods, these materials are highly effective at sintering almost 100% of the density. Thanks to the sintering process that takes place in a very short time with the SPS system, better grain growth can be avoided and better mechanical properties can be obtained. Experimental studies were carried out with an SPS system with a capacity of 20,000 A On phase analysis, it was determined that the identical peaks of ZrC and TiC are very close to eachother. To determine the peaks clearly, XRD was performed on each peak of the scales separetly. After this operation the peaks were identified and no chemical reaction was detected. Densification behaviors of 50mm diameter 4mm thick composites were determined, Vickers microhardness and fracture toughness values were measured and phase and microstructure analyzes were performed. In this study, ZrC-TiC-GNP triple composites were produced by adding TiC and Grafen nano plate (GNP) to the ZrC matrix to improve the mechanical properties and sintering behavior of ZrC. The production was carried out by using spark plasma sintering method at a temperature of 1700 ° C under a pressure of 40 MPa for 5 minutes with sintering. During the production phase, the GNP content of 0.25-3.00% by weight and the 80% ZrC- 20% TiC powder mixtures were milled for 24 hours in ball mills and the product was dried. In the next step, the powders mixed with the purpose and ultrasonic mixer to prevent GNP agglomeration and homogenous distribution of the powder mixture were prepared for sintering by filling 50mm diameter and 4mm thickness graphite mold. Relative density measurements were determined by using Archimedes principle. The density of the sintered sample was determined to between 96.33-99.43%. Decreased density values were observed with increasing amounts of graphene. When the XRD pattern of mechanically mixed 80% ZrC and 20% TiC powders and the composites produced after sintering were examined, the values of the lattice parameters were calculated by determining the peak positions, the distance between planes, the miller indices and the most intense peaks of each pattern. Lattice parameter values were found to change with decreasing latttice parameter values in the 80% ZrC- 20% TiC powder mixture before sintering. Vickers microhardness measurements showed that the highest density had ZrC-TiC binary composite without GNP. It is thought that the increase of fracture toughness causes the GNPs to disperse homogeneously in the structure, to prevent crack progression and to reduce the energy of the crack by changing the direction of the cracks. The fracture toughness results which was calculated according to Anstis equation showed that the fracture toughness starts to increase with more than zero point five percent graphene content, and doesn't change significantly more than 1 percent content The addition of GNP on %20 TiC content by volume ZrC-TiC composite, succesfully increased the fracture toughness from 3.92 up to 5,17 MPa.m1/2 in 1% GNP by mass composite. The fracture toughness of all ZrC-TiC-GNP composites above 3,9 MPa.m1/2. In order to determine the toughening mechanisms, interaction between propagating crack and microstructure was analysed. When the propagating crack met on C-rich zones on ZrC-TiC composite, the crack most frequently crossed through. The C-rich zones was unable to deflect the crack. But when analyse the ZrC-TiC-GNP composite , the crack was remarkably deflect when met on GNP and this behaviour consistent with high fracture toughness results. The highest fracture toughness value was found in the ZrC-TiC-GNP ternary composite containing 1% by weight of GNP.