Wen Jie Si

Kinetics for Supercritical CO2 Debinding of Injection Molded ZrO2

Mercury porosimeter and scanning electron microscope (SEM) were used to analyze the pore structures evolution and distributions for supercritical CO2 debinding of injection molding ceramics. Classical diffusion equation was used to describe the mass transfer of supercritical CO2 debinding of the injection molded ZrO2 ceramics. The behavior and kinetics of the debinding were studied and analyzed. Results show that the solubility and diffusivity of soluble binder are the key factors in supercritical CO2 debinding, while the diffusivity is a dominant factor. The calculation data from the theoretical model are consistent with the experiment under the condition of enough long debinding time. It is shown that the diffusivity can be obtained by simple theoretical model combined with experimental data. The extraction rate and the extraction kinetics of the process can be predicted using the theoretical model.

Bending Strength of Ceramics Implanted by Titanium, Zirconium and Chromium Ions with MEVVA Source

ZTA (alumina toughened by 20 wt.% zirconia), hot-pressed silicon nitride (with totally 10 wt.% Y2O3 and Al2O3 as additives) and TZP (pressureless-sintered yttria stabilized zirconia) ceramics were implanted by various doses (5 × 1016 ions/cm2 ~ 1 × 1018 ions/cm2) of Ti, Zr, and Cr ions with a MEVVA (metal vapor vacuum arc) source implanter. The bending strength of these ceramics was investigated. It was discovered that, for different ceramics, different behaviors were presented with the same doses of implantation ions. For alumina and zirconia ceramics, the bending strength increased with increasing implantation doses of Ti and Zr ions, but decreased with high dose of Cr ions. For silicon nitride ceramics, however, the bending strength originally increased with smaller doses of metals implanted, and decreased with higher doses of metals of Ti, Zr, and Cr ions. The different behaviors are correlated to the different variations in compositions and microstructures of ceramics after ion implantation.

Dynamic Fatigue of Y-TZP Ceramics at the Room Temperature

Dynamic fatigue behavior of two kinds of Y-TZP ceramics was investigated at room temperature. The results showed that dynamic fatigue and slow crack growth behaviors exist in zirconias. Crack growth exponents of original and pre-cracked samples were obtained. Crack growth exponents of the two original zirconias are 15.60 and 21.00, respectively. Crack growth exponents of annealed pre-cracked samples are close to the original samples, indicating that the pre-crack experimental method is reasonable and effective. Because of the influence of residual stress indcued by indentation, crack growth exponents of unannealed samples are different from that of original samples. So residual stress should be removed.

Preparation of Layered Potassium Titanate

Layered potassium titanate is an important intermediate in preparing titania nanosheets. Three methods including method of potassium stearate, method of stearic acid and method of high-temperature solid-state reaction were used to produce layered potassium titanate in this paper. XRD and SEM were used to characterize the crystal structure and shape. The result of experiments showed that method of potassium stearate was the most effective way to prepare the layered potassium titanate.

Microstructure and Mechanical Properties of SiC-Al2O3 Nanocomposite Prepared by Surface Modified SiC Nanopowders

SiC nanopowders were surface modified by low temperature plasma polymerization. 5vol% SiC-Al2O3 nanocomposite was prepared by adding the surface modified SiC nanopowders into the matrix of Al2O3.The influence of surface modified SiC nanopowders on the properties and structure of SiCAl2O3 nanocomposite was studied. The surface modified SiC-Al2O3 nanocomposite had higher density and nicer mechanical properties than untreated SiC- Al2O3 nanocomposite because of the improvement of dispersion. The hardness and toughness of the SiC- Al2O3 nanocomposite were higher than monolithic Al2O3 but the ultimate bending strength was not perfect because of retardation of the densification and formation of unsintered pores caused by the induced oxygen during the process of powder treatment.

Microwave Sintering of Alumina-Silicon Carbide Nanocomposites

Densification behavior, microstructural developments and mechanical properties of 5 wt% SiC nano particulate toughened Al2O3 composites were prepared by using microwave sintering at different sintering temperatures. Densities of up to 97.5% of the theoretical value were achieved at temperature as low as 1380°C. Improvements in fracture toughness and bending strengths are of the order of 99% theoretical dense conventionally sintered composites. Surface heating was observed at the final stages of densification. No thermal runaway was observed in the nanocomposites due to presence of SiC spheroids.

A New Dewaxing Method and its Effect on the Properties of Low-Pressure Injection Molded Ceramics

A new dewaxing method for low-pressure injection molded ceramics is presented. Supercritical extraction with carbon dioxide was used to remove paraffin wax from the ceramic green parts. The composition of organic additives for low-pressure injection molding feedstock and the extraction condition for the green parts were investigated. Moreover, the properties of sintered ceramic samples dewaxed by supercritical carbon dioxide were compared with those by thermal dewaxing. The results show that the new binder system containing 50wt% paraffin wax, 35% bee wax and 15% stearic acid fulfills the requirements of both low-pressure injection molding feedstocks and supercritical dewaxing, where the feedstock has high fluidity, low viscosity and quick solidification. The efficient extraction condition for supercritical dewaxing from the green parts is at 30MPa pressure and 45°C. Under this condition, defect free ceramic green parts can be obtained. Dewaxing methods have significant influence on the properties of sintered parts. The mechanical properties of the sintered sample can be improved by supercritical dewaxing. With this method, the bending strength of sintered samples (σ = 331.6 MPa) is higher than that obtained by thermal treatment (σ = 312.3MPa). The sintered samples dewaxed by supercritical CO2 have shown the property of higher density and less distortion compared to the thermal dewaxing method. Moreover, with supercritical extraction the dewaxing time can be reduced to about one tenth of the time required by thermal dewaxing.

Structure of the Coatings onto Ceramic Cutting Tools Deposited by Pulsed High Energy Density Plasma

With a newly-developed technique, pulsed high energy density plasma (PHEDP), TiN, TiCN, and (Ti,Al)N coatings were deposited onto silicon nitride and cemented carbide cutting tools. The structures of these coatings were systematically investigated in this paper. The average surface roughness (Ra) of the coated tools were ranged in 20~150 nm. The smooth surface of coated tools means that the coatings are promising candidate for cutting tools of high precision and it is in favor of reducing the fiction coefficients and flank wear of tools. The coating thickness varied, in the range of 3~20 µm, with the deposition conditions of the shot number of pulsed plasma, and the voltages between the inner and outer electrodes of the coaxial gun. The coating has a densified structure compared to the substrate structure and almost no pores and cracks exist in the coating surface. The grain sizes of the coating were small (<100nm), much finer than those of the substrate (>2 µm). Except for TiN-Si3N4 system, no apparent columnar grain structure as presented predominantly in typical vapor deposited coatings was observed. In fact, an equiaxed structure was presented, due to the pulsed mode of plasma bombardment and solid solution strengthening of C or Al into TiN lattices, resulting in disruption, through renucleation, of epitaxy on individual columns. A continuous and densified interface was observed. All these characteristics in structures promised an excellent performance of the coated tools.