Production and characterisation of SiC based ceramics by using spark plasma sintering (SPS) method

Silicon Carbide (SiC) ceramic has good physical, chemical and mechanical properties such as high melting point, high hardness, high Young's modulus, good corrosion resistance, and low density. Such properties give to SiC materials a wide application area such as advanced engineering ceramics, aerospace materials, nuclear energy processing materials, and ballistic protection materials. One of the greatest drawbacks in the manufacture of SiC is the difficulty in sintering dense samples without the use of additives and/or pressure. This is because the covalent nature of the Si-C bonds and the low self-diffusion coefficients require very high sintering temperatures and pressures. Usually, common sintering techniques, such as pressureless sintering, hot pressing (HP), and hot isostatic pressing (HIP) were employed to sinter the monolithic SiC ceramic, however, due to the above-mentioned methods present high sintering temperature, long sintering and cooling time, the monolithic SiC ceramicexhibits coarse-grained microstructure. As a consequence, the strength and toughness of the monolithic SiC ceramic synthesized by these sintering methods referred above is very low. Thus, the applications of SiC ceramics are rather limited.Spark plasma sintering (SPS) method is a newly developed technique that enables the compacted powder to be fully densified at a comparatively low temperature, and in very short time. the grain growth is prohibited and the densification is accelerated by rapid heating. Hence, fully dense ceramics with higher performance can be achieved using SPS technique at lower sintering temperature in comparison with the conventional sintering process.In this study, in order to achieve fully densified monolithic SiC ceramic, experiments were conducted by the SPS technique at four different temperatures in the range of 1800-1950 °C under three different pressures, 40, 60 and 80 MPa without using sintering additives. The objective is to highlight the contribution of the SPS temperature and pressure on the microstructure, densification behaviour and some mechanical properties.The starting α-SiC powder (UF-10 H.C Starck, Germany) with its average particle size of 0.7 μm, was wet-mixed in ethanol with SiC balls in a plastic bottle to ensure homogeneity. Then, the slurry was dried at 100 °C for 24 hours in an oven and ethanol was removed. After drying and sieving to 150 μm, a graphite die with a 50 mm inner diameter was filled with the powder, and they were sintered by the SPS technique (SPS-7.40MK-VII, SPS Syntex Inc.) under a vacuum atmosphere by applying 40 and 80 MPa of pressure. The used DC pulse sequence is 12:2, implying that the current is ON during 12 pulses (3.3 ms each) and OFF during two time intervals.The sintering process was carried out at four different temperatures in the range of 1800-1950 °C with a heating rate of 100 °C/min under a vacuum atmosphere. The temperature of the SPS process was measured with an optical pyrometer that was focused on the surface of the die. The current was controlled manually. Linear shrinkage of the specimens during the SPS process was continuously monitored by displacement of the punch rods. After soaking the powder at a desired temperature for 5 min, the applied current was reduced, the pressure was released, and the specimen was cooled down to room temperature. The densified samples were in the form of disks 50 mm in diameter and 5 mm thick and sand-blasted to remove the graphitic sheet. The densities of the sintered bodies were measured by Archimedes' method in distilled water. Samples were polished by using diamond polishing solution. The hardness and fracture toughness (KIC) of the samples were evaluated by the Vickers indentation technique at a load of 49 N (Struers, Duramin A300) from the polished surfaces. Fracture toughness values were determined by measuring the half-length of a crack formed around the indentations.The densification of specimens during the SPS process was evaluated by the displacement of punch rods due to the shrinkage of samples. To get the actual sample shrinkage blank runs were carried out at identical SPS conditions and the displacement data from the blank runs were subtracted from the total displacement. Under higher pressure values such as 80 MPa the graphite die (without SiC powder) expanded up to approximately 1600 °C and between 1600 °C and 1820 °C displacement remained constant and from 1820 °C to SPS temperature the die started to shrink significantly. Under lower pressure values such as 40 MPa only expansion of the graphite die (without SiC powder) was observed. Up to the SPS temperature the graphite die continued expanding and during the soaking time the displacement remained constant.For most published results on SPS, the recorded temperature is obtained from the pyrometer directly. The recorded temperature values differ significantly from the temperature that should be obtained from the sample. Since cylindrical geometry is used in SPS, it is possible to make a simple model to obtain the temperature of the sample. When the sample is small, thin and centrally positioned, the temperature distribution can be assumed to be according to the graphite die only. The temperature difference between the edge of the die and the centre of the sample is therefore strongly dependent on both the die surface temperature and the properties and geometry of the die. Using the the high temperature properties of the graphite die (thermal conductivity 100Wm–1) which was utilised in our experiments, the temperature difference between the centre and the edge of a 5 cm diameter die was obtained as 262 °C and 346 °C at a surface temperature of 1800 °C and 1950 °C, respectively. The effects of different temperatures and pressures on the density, Vickers hardness, fracture toughness, densification behaviour and microstructure were examined. Under an applied pressure of 40 MPa, as the sintering temperature was increased from 1800 to 1950 °C, the relative density of the bulk compact increased from 87% to 97.4% due to the pore elimination and expedited rearrangement of the grains. Fully dense monolithic SiC ceramics with a relative density of approximately 99% were obtained. Applying higher pressure is a very effective way to achieve high relative densities for the SiC specimens without sintering additives. This condition is attributed to the difference of relative density values where the pressure (80 MPa) was applied above 1000 °C in one specimen and at 1950 °C in another specimen both SPSed at 1950 °C with a 5-minute soaking time. Higher sintering temperatures resulted in higher densification rates of spark plasma sintered SiC ceramics, which led to a decrease in porosity. Under an applied pressure of 80 MPa, as the sintering temperature was increased from 1800 to 1950 °C, the relative density increased from 95.5% to 99.7%, with much of the densification taking place by the time the temperature reached 1850 °C. The highest relative density value obtained was approximately 99.7% for the sample SPSed at 1950 °C while applying 80 MPa of pressure, revealing that the sintering pressure has a significant influence on the relative density of the SiC ceramics. The Vickers hardness value of 10.2 GPa was attained in the sample spark plasma sintered at 1800 °C under an applied pressure of 40 MPa; however, increasing the sintering temperature to 1950 °C caused it to reach 28.7 GPa. For the sample SPSed at 1800 °C while applying 80 MPa of pressure, the Vickers hardness value was measured as 28.5 GPa. As the sintering temperature increased to 1950 °C, the Vickers hardness value reached 31.9 GPa. It was concluded that a higher sintering temperature and pressure led to an increase in the Vickers hardness of the SiC ceramics due to accelerated densification results in the value of the relative density. The fracture toughness values varied between 3.3±0.2 and 3.6±0.3 MPa.m1/2. The highest fracture toughness that could be achieved, 3.6±0.3 MPa.m1/2, was with the sample SPSed at 1950 °C while applying 80 MPa of pressure. The most important mechanical weakness of SiC even it is sintered to high relative densities is its low fracture toughness. In this study, fracture toughness of SiC was tried to be increased and it was objected to lower the sintering temperature by using some sintering additives.Depending on the kinds of doped sintering aids, SiC may be densified to high density by either a solid-state or liquid phase sintering mechanisms. Sintering of SiC with the aid of B and C is regarded to take place by solid-state diffusion process. The use of yttria or alumina as sintering additives, which form together with the SiO2 existing on the surface of the starting SiC-powder, a liquid phase during the sintering, promotes the densification and microstructural development. Liquid phase sintering allows densification of SiC at temperatures close to 1700 °C. The liquid-phase sintered SiC ceramics have high fracture toughness compared with solid phase sintered SiC. However, a major problem associated with sintering of SiC in the presence of oxide additives is the reaction between the SiC and the oxides.Futhermore in this study, SiC ceramics were consolidated by both solid state and liquid phase sintering mechanisms using spark plasma sintering technique. For this purpose, SiC ceramics were fabricated by SPS technique with the use of some oxide (Al2O3, Y2O3), boride (TiB2) and carbide (B4C) additives. The sintering process was carried out at three different temperatures in the range of 1700-1800 °C applying three different pressures 40,60 and 80 MPa under vacuum atmosphere. The effect of additive, different temperatures and pressures on densification behaviour, density, vickers hardness, fracture toughness and microstructure were examined. The hardness and fracture toughness of the samples were evaluated by the vickers indentation technique. Microstructure of spark plasma sintered SiC samples were characterized by using Scanning Electron Microscope (SEM) technique.The addition of 5 vol.% B4C resulted in an increase in relative density from 87% to 99.2% for the samples SPSed at 1800 ºC under a pressure of 40 MPa in comparison with monolithic SiC SPSed at the same parameters. The addition of 5 vol.% TiB2 resulted in an increase in relative density from 87% to 90.2% for the samples SPSed at 1800 ºC under a pressure of 40 MPa in comparison with monolithic SiC SPSed at the same parameters. The use of non-oxides as a sintering additive, which reacts with the SiO2 existing on the surface of the starting SiC-powder, forms a liquid phase during the sintering process and the liquid promotes the densification. The addition of 5 vol.% Al2O3 resulted in an increase in relative density from 87% to 97.3% for the samples SPSed at 1800 ºC under a pressure of 40 MPa. This result is in a good correlation with the liquid phase sintering which enhances the densification. The addition of 5 vol.% Y2O3 resulted in an increase in relative density from 87% to 96.9% for the samples SPSed at 1800 ºC under a pressure of 40 MPa in comparison with monolithic SiC SPSed at the same parameters.