Papers by Keyword: Sintering Behaviour

Abstract: The production of ferrous powder metallurgical parts by the press-and-sinter route becomes more and more attractive. Today, parts are produced for loading requirements that until now only could be fulfilled by conventional produced steel components. The high mechanical properties that must be attained require the use of alloying elements so far not common in powder metallurgy because of their high affinity for oxygen. The sintering of chromium containing steels is a challenge for the whole production process, because the reduction of the surface oxides is critical for successful sintering.Dilatometry can be a useful instrument to control the sintering behaviour of the materials, especially the combination with mass spectrometry allows analysing the very complex sintering process and simultaneously monitoring the solid-gas reactions. This work shows that the sintering atmosphere plays a major role in the entire process. Degassing and deoxidation processes during sintering are demonstrated for different alloying systems (Fe, Fe-C, Fe-Mo-C, Fe-Cr-Mo-C). Dilatometry coupled with MS is shown to be a very good instrument for process control of the sintering process. The generated analytical data can be related to the mechanical properties of the sintered steels if the size of the specimen is large enough.

Abstract: The effect on densification and mechanical properties of 3 mol% Y-TZP ceramics doped with zinc oxide (0.1 wt.% to 1 wt.%), has been investigated in this study. Green samples were compacted by uniaxial pressing and cold isostatically pressed at 200 MPa. All samples have been sintered over the temperature range of 1250°C to 1500°C with a ramp rate of 10°C/ minute and 2 hours of holding time. In addition to that, the tetragonal phase stability of the samples was also studied in superheated steam at 180°C/10 bar for up to 50 h. The sintered bodies were examined to determine the phase content, bulk density, Young’s modulus, Vickers hardness and fracture toughness. The results showed that the addition of 1 wt.% ZnO was effective in aiding densification (~99% theoretical density). It also improved the matrix stiffness (~208 GPa) and Vickers hardness (~13 GPa), when the samples were sintered below 1300°C comparing to the undoped Y-TZP sintered at the same temperature. In contrast, ZnO addition to the Y-TZP matrix displayed a tendency to change hydrothermal ageing behavior as it was noticed that the monoclinic content increased with increasing content of dopant. The undoped samples showed a better hydrothermal ageing behavior at 1300°C compared to the doped samples.

Abstract: CaB2O4 was added into hexagonal boron nitride (hBN) to improve the sintering behaviors of hBN. CaB2O4 and hBN were mixed and then pressed into plates. The plates were sintered at 2000°C for 5h under a N2 ambience. The phase compositions with different CaB2O4 contents were examined with X-ray diffraction analysis. The fracture cross-sections of the hBN plates were investigated by SEM. The apparent density and Rockwell hardness were also measured. The results show that the hBN particles had a plate-like shape and the grain sizes of hBN increased with increasing CaB2O4 contents. The apparent density and Rockwell hardness decreased with increasing CaB2O4 contents. When the CaB2O4 content was 15(wt) %, the hBN has the average grain sizes of 3μm in diameter and 200nm in thickness, the apparent density of 1.06 g/cm3 and the Rockwell hardness of 3, respectively.

Abstract: A high temperature HRTEM holder equipped with a W-coil heater was used to make insitu observation of high temperature behavior of Al2O3 very thin (about 1 nm in thickness) protective film on AlN particles. The film was used to prevent AlN particles from damages by moisture. Rapid melting and rapid solidification of very small Al2O3 particles of about 2 nm in diameter were found within about 0.2 seconds. Therefore we concluded that the Al2O3 protective film worked as the sintering additives in the high temperature heating process. In the present study, very small Al2O3 particles were identified by the space between observed lattice fringe images. It was found that a tilt boundary was instantaneously formed and annihilated in an Al2O3 particle. There was also evidence that showed the formation and annihilation of edge dislocations within seven seconds during sintering.

Abstract: A series of physical and chemical changes, including the shrinkage and the phase transformation, will continuously take place during the sintering process of ceramic green body, which is important for designing a scientific and economic sintering schedule. The sintering behavior of two electrical porcelains and a structural tile were investigated using thermal analysis techniques. The dehydration, phase changes and densification of ceramic green body during sintering process were characterized by DSC-TG and thermal expansion curves. The sintering temperature range of ceramic green body was measured by thermal dilatometer with different heating rates. The sintering temperature Ts is linear to the logarithm of heating rate V. The results were compared with that measured by conventional refractoriness tester. The Ts is much more accurate to evaluate the sintering behavior of ceramic green body from the thermal analysis technique than that from the traditional method. The projected sintering schedule is very concordant to the experimental sintering results.

Abstract: Spark plasma sintering (SPS) method was used to consolidate ultra-fine and mixed 93W-5.28Ni-1.32Fe-0.4Y2O3 (wt pct) powders, and the effects of sintering parameters on the densification degree and the performance of the as-sintered materials were investigated. Results showed that the SPS densification process could be divided into four stages. Compacts were densified rapidly at the stage III and a proper holding time promoted W diffusing into fcc-type Ni-Fe based solution. Heating rate and sintering time had a great influence on densification. When the powders were heated at a rate of 100°C/min to 1230°C and then held for 5min, the density, hardness and transverse rupture strength of the as-sintered material reached the optimum, being 17.18×103kg/m3, 41.4HRC and 935.1MPa, respectively. Its corresponding fracture morphology was characterized as intergranular rupture of W-W, and accompanied with some transgranular cleavage of tungsten grain.

Abstract: Three series of ZnO-based materials with different doping levels were prepared. The correlation between the composition and microstructure, and the roles of main dopants, Bi2O3 and Sb2O3, in the sintering behaviors were proposed. Both Bi2O3 and Sb2O3 evaporated at 1115°C, but the amount of them, in which bismuth is the majority, is not significant. Bi2O3 functioned mainly as liquid during sintering to promote the sintering of ZnO, but it doesn’t mean the materials will be denser. The bismuth-rich phase retracted into small pores during cooling, leaving the big pores as voids at room temperature. More Bi2O3 added would result in less increase in material densities and dramatic decrease in relative densities, and a little bit increase in grain sizes of matrix ZnO. Sb2O3 would react with ZnO matrix into spinel, Zn7Sb2O12, which will pin at the grain boundary of ZnO to control the ZnO grain growth. The more Sb2O3 added, the smaller the grain sizes of ZnO. Appropriate amount of Sb2O3 added will yield denser materials.

Abstract: Microwave sintering behaviors of four different compositions of YSZ electrolyte materials were investigated. The samples were sintered in 2.45GHz microwave furnace. For comparison, conventional sintering was performed at 1821K.The densities of sintered samples showed considerable enhancement in the densification process under the influence of microwave fields. The samples with lower Y2O3 content are easy to sinter. The influence of the composition and sintering methods on the final phase composition and microstructure were investigated by X-ray diffraction and scanning electron microcopy. Finer and more uniform microstructures were observed in the microwave sintered samples comparing to the conventionally sintered samples.

Abstract: Si-containing tricalcium phosphate (TCP) ceramics are expected to be useful biodegradable bone substitutes that enhance bone regeneration because TCP is a biodegradable ceramic and Si is the trace element that enhances bone formation. We successfully synthesized the Si-containing TCP with different Si contents through a wet process. The Si addition lowered the temperature of phase transition from β-TCP to α-TCP. After the sintering the compacts of Si-containing TCP powders at 1400 oC, the sintered ceramics showed higher density than those of Si-free TCP. The Si addition was effective to promote the sintering of TCP.