Influence of SiAlON addition on the microstructure development of hot-pressed ZrB2–SiC composites (original) (raw)
Related papers
ZrB2–SiC composites doped with 0, 1, 3 and 5 wt.% SiAlON were prepared by hot pressing (under a low pressure of 10 MPa) and pressureless sintering processes at 1900 °C for 2 h. A fractographical approach was employed to investigate the effects of SiAlON addition on the densification behavior in such ceramic composites. Several reported chemical reactions were investigated to indicate the probable mechanisms in the progression of the sintering process and also the potential unfavorable effects of SiAlON in ZrB2–SiC composites. On one hand, results show that the SiAlON addition promotes the densification in the hot pressed samples by the liquid phase formation, as a fully dense composite can be obtained by hot pressing of 5 wt.% SiAlON doped ZrB2–SiC composite. On the other hand, in the pressureless sintering process, more SiAlON content intensifies the formation of gasses which leads to more porosity. In both processes, investigation of fracture surfaces demonstrates the presence of thin layers of glassy phases which act as the sintering aid. The steps of densification process in both hot pressing and pressureless sintering processes were presented by a graphical model.
A Processing–Microstructure Correlation in ZrB2–SiC Composites Hot-pressed under a Load of 10 MPa
Universal Journal of Materials Science, 2015
Monolithic ZrB 2 ceramic and its composites, with 5 to 30 vol. % SiC, has been prepared by hot pressing at temperatures of 1700, 1850 and 2000 °C, for 30 minutes under relatively low pressure of 10 MPa. Densification behavior of ZrB 2-based composites is improved by the addition of SiC particulates. The fracture surface of monolithic ZrB 2 ceramics shows a grained structure, with faceted ZrB 2 grains, as the fracture appears to spread prevalently along an intergranular path. The ZrB 2 /ZrB 2 boundary interface is seemingly free of any secondary phases. The microstructure of ZrB 2-30 vol. % SiC composite, hot-pressed at 1700 °C, is consistent with measured porosity for the sample that has ~8% open pores, nearly without closed pores. It seems that mechanical interlocking between ZrB 2 and SiC is an important mechanism for densification. In the microstructure of specimens consolidated at 1850 °C, neck formation between ZrB 2 particles is visible. In contrast, relatively fully dense samples are obtained by hot-pressing at 2000 °C. Intergranular SiC particles inside ZrB 2 grains show the occurrence of mass transfer among ZrB 2 particles, which in effect brings the elimination of pores to a fortunate ending. Efficient mixing of starting powders is very critical in order to achieve a fine-grained homogenous microstructure.
International Journal of Refractory Metals and Hard Materials, 2015
In this paper, the effects of the hot pressing parameters and SiC content on the densification of ZrB 2-based composites have been studied. This research reports a design of experiment approach, the Taguchi method, employed to analyze the processing of ZrB 2-SiC composites based on four processing parameters: the hot pressing temperature, the soaking time, the applied pressure and SiC content. In this way, an L9 orthogonal array, including nine experiments for four parameters with three levels, was used to optimize the processing factors. The analysis of variance identified the applied pressure as the most impressive parameter affecting the densification and hardness of ZrB 2-SiC composites. A relative density of~96% and a Vickers hardness of 15.2 GPa were achieved for ZrB 2-25 vol.% SiC composite with the sintering temperature of 1850°C, the soaking time of 90 min and the applied pressure of 16 MPa. The confirmation test, fulfilled under the optimal conditions, disclosed that the result of the experiment and the Taguchi prediction were alike.
Effect of Si 3 N 4 Addition on Compressive Creep Behavior of Hot-Pressed ZrB 2 -SiC Composites
Journal of the American Ceramic Society, 2014
Compressive creep studies have been carried out on hot-pressed ZrB 2 -SiC (ZS) and ZrB 2 -SiC-Si 3 N 4 (ZSS) composites in air under stress and temperature ranges of 93-140 MPa and 1300°C-1425°C, respectively for time durations of %20-40 h. The results of these studies have shown the creep resistance of ZS composite to be greater than that of ZSS. As the temperature is increased from 1300°C to 1425°C, the stress exponent of ZS decreases from 1.7 to 1.1, whereas that of ZSS drops from 1.6 to 0.6. The activation energies for these composites have been found as %95 AE 32 kJ/mol at temperatures ≤1350°C, and as %470 AE 20 kJ/mol in the range of 1350°C-1425°C. Studies of the postcreep microstructures using scanning and transmission electron microscopy have shown the presence of glassy film with cracks at both ZrB 2 grain boundaries and ZrB 2 -SiC interfaces. These results along with calculated values of activation volumes suggest grain-boundary sliding as the major damage mechanism, which is controlled by O 2diffusion through SiO 2 at ≤1350°C, and by viscoplastic flow of the glassy interfacial film at temperatures ≥1350°C. Studies by transmission electron microscopy have shown formation of crystalline precipitates of Si 2 N 2 O near ZrB 2 -SiC interfaces in ZSS tested at ≥1400°C, which along with stress exponent values <1 suggests that grain-boundary sliding involving solution-precipitation-type mechanism is operative at these temperatures.
Microstructure evolution of ZrB2–30 vol.% SiC composites, prepared by hot pressing at different processing temperatures (1700, 1850 and 2000 °C) for 30 min under 10 MPa, were investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The microstructures of the fabricated composites were compared with and the effects of the processing temperature on the sintering process and densification behavior during the hot pressing were found. The amount and the orientation of dislocations which were indicated by TEM analysis in the sample hot pressed at 1700 °C showed that no plastic deformation and atomic diffusion occurred. But the presence of amorphous phases and rearrangement of particles are signs of the fact that liquid phase sintering and particle fragmentation/rearrangement is the main densification mechanism. On the other hand, in the sample hot pressed at 1850 °C, aggregation of dislocations behind the grain boundaries and the presence of twinnings addressed wide plastic deformations which were introduced as the main densification mechanism at 1850 °C. Finally in the sample hot pressed at 2000 °C, lower amounts of un-oriented dislocations and also some annealing twinnings were observed in TEM micrographs together with fractographical SEM analysis and showed that the atomic diffusion is the dominant densification mechanism of hot pressed ZrB2–30 vol.% SiC composite.
Effect of Si3N4 Addition on Oxidation Resistance of ZrB2-SiC Composites
Coatings
The oxidation behavior of ZrB 2-20 vol % SiC and ZrB 2-20 vol % SiC-5 vol % Si 3 N 4 composites prepared by hot-pressing and subjected to isothermal exposure at 1200 or 1300 • C for durations of 24 or 100 h in air, as well as cyclic exposure at 1300 • C for 24 h, have been investigated. The oxidation resistance of the ZrB 2-20 vol % SiC composite has been found to improve by around 20%-25% with addition of 5 vol % Si 3 N 4 during isothermal or cyclic exposures at 1200 or 1300 • C. This improvement in oxidation resistance has been attributed to the formation of higher amounts of SiO 2 and Si 2 N 2 O, as well as a greater amount of continuity in the oxide scale, because these phases assist in closing the pores and lower the severity of cracking by exhibiting self-healing type behavior. For both the composites, the mass changes are found to be higher during cyclic exposure at 1300 • C by about 2 times compared to that under isothermal conditions.