De Chang Jia

Spark Plasma Sintered ZrO2(Y2O3)-Al2O3-SrSO4 Self-Lubricating Nanocomposites for High Temperature Tribological Applications

Spark plasma sintering is employed to synthesize a variety of self-lubricating ZrO2(Y2O3)- Al2O3 matrix nanocomposites by tailoring the chemical compositions and by adjusting the sintering parameters. Different additives are incorporated into the nanocrystalline ceramics of ZrO2(Y2O3)- 20wt.% Al2O3 to evaluate their potentials as effective high temperature solid lubricants from room temperature to 800oC by using a high temperature friction and wear tester in sliding against alumina ball in air. The density, microstructure, hardness and tribological properties of the sintered nanocomposites have been investigated, as contrasted with the unmodified ceramics, to obtain a better understanding of lubrication mechanisms over a wide temperature range. The ZrO2(Y2O3)-Al2O3-SrSO4 composite exhibits steady-state friction coefficients of less than 0.2 and wear rates in the order of 10-6 mm3/Nm over a broad temperature range from room temperature to 800oC.

Rheological Properties of Boron Nitride-Silica Composite Suspensions

In this paper, a novel colloidal forming process has been developed to prepare boron nitride-silica composites. The rheological properties of mixed boron nitride-silica powders in deionised water have been investigated by adjusting the concentration of dispersant, pH value of the solution and other process parameters. The isoelectric points for both boron nitride and silica powders are in the pH range of 2 to 3. Boron nitride doped with or without A-1 (an anionic dispersant) as a dispersant shows a similar electrophoretic behavior to silica powder, i.e. Zeta potential becomes more negative with the increase of pH value from 2 to 12. The sedimentation and rheological results of aqueous boron nitridesilica suspensions indicate that the optimized pH value is around 7.2. It depends upon the synthetic effects of dissociation of A-1 and sufficient positive adsorption sites on the particle surfaces in aqueous media. The optimum concentration of the dispersant at pH=6 is 0.6wt % (dry weight basis of composite powders). Boron nitride-silica composite suspensions with a solid content as high as 50vol% have also been achieved. A ball milling process can improve the fluidity of the suspensions.

Effect of pH Values on Conversion of Calcite Crystals into Calcium Phosphate Phases in Buffer Solutions

Calcium phosphate phases with laminar-plate structure were converted from calcite powders after soaking in phosphate buffer solutions of pH’s 6.0-8.0 at 37 °C for 9 days. The effect of pH values on the conversion of calcite crystals was investigated by X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectroscopy. If the pH value of a buffer solution is kept at 6.0, calcite powders are converted mainly to dicalcium phosphate dehydrate (DCPD) or octacalcium phosphate (OCP). If the pH value is kept at 6.4 or 7.0, calcite powders are converted mainly to OCP. Hydroxyapatite (HAP) with poorly crystalline can be obtained from calcite powders both by treatment of a basic buffer solution, and by treatment of an acid buffer solution without regulating its pH value during the reaction. The conversion mechanism of calcite crystals is a dissolution-precipitation reaction.

Formation Mechanism and its Pozzolanic Activity of Metakaolin

In this paper, the process of the transformation from kaolin to metakaolin was investigated. The kaolin was calcined at different temperatures and analyzed by Xray diffraction (XRD), Fourier transform infrared spectra (FTIR) and solid state nuclear magnetic resonance (NMR). The formation of metakaolin structure was based on the stacking polyhedrons changes, which originated from dehydroxylation of kaolinite. With increasing temperature, kaolin kept structure of kaolinite unchanged in the course of dehydroxylation and then structure of kaolinite transformed to metakaolin when the dehydroxylation was over. It was demonstrated that the essence of pozzolanic activity of metakaolin. The result revealed that the pozzolanic activity of metakaolin increased with increasing temperature at the range of 600~900 °C.

Preparation of Amorphous Si-B-C-N Powders and Nano-Sized Ceramics

Amorphous nano-sized silicoboron carbonitride (Si-B-C-N) powders with average grain size <50 nm were fabricated by high energy shaker mill using hexagonal boron nitride, graphite and amorphous silicon powders as starting materials. The powders were consolidated by spark plasma sintering at 1900° and 1950°C. Amorphous phase were partially retained in ceramic sintered at 1900°C. For ceramic sintered at 1950°C, amorphous Si-B-C-N ceramic transferred to hexagonal BN and cubic SiC.

Preparation of BiFeO3 Nanopowders Using Acetylacetone as Stabilizer

The present investigation reports on the preparation of BiFeO3 nanopowders by a sol-gel method using acetylacetone as a stabilizer. Single-phase BiFeO3 nanopowders without any impurity or amorphous phases were obtained when the precursor was thermal treated at temperatures as low as 400 oC for 2 h. Acetylacetone (acac) plays an important role on lowering the formation temperature of pure phase BiFeO3 nanopowders. It is found that Bi/acac molar ratio of 1/30 was favorable for a stable sol and for the lowest crystallization temperature of pure BiFeO3 nanopowders. X-ray diffraction and Fourier transform infrared spectroscopy revealed that thermally induced crystallization process of BiFeO3 nanopowders from Bi-Fe polymeric precursor. When the thermal treated temperature was below 200 oC, only amorphous phase existed. With the temperature increasing up to 300 oC, crystallized phase, carbonate, were detected. After annealed at 400 oC, Bi-Fe precursor totally changed to rhombohedral BiFeO3 nanopowders. Scanning electron microscopy characterized morphologies of BiFeO3 nanopowders calcined at 400 oC and 500 oC. The ferroelectric transition of BiFeO3 nanopowders at 827 oC has been detected by differential thermal analysis.

Microstructures of ZrO2 (3Y) Ceramics after Dynamic Impact

ZrO2(3Y) ceramics were sintered by vacuum hot-pressing and impacted by split Hopkinson method. The microstructure of the materials was studied by X-ray diffraction and scanning electron microscopy. The results show that the phase transformation from tetragonal phase to monocline phase occurred in the ceramics during dynamic loading process, and the transformation rate is about 30%. From the results of scanning electron microscopy, it can be found that the microcracks formed in the materials under the dynamic loading, and there is a kind of liked-amorphous layer appeared on the fragment surface.

Microstructural Characterization of In Situ TiB Whiskers in Ti MMCs Fabricated by SPS

The in situ TiB whisker reinforced titanium matrix composites were prepared by mechanical alloying followed by spark plasma sintering. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to characterize the microstructure of the TiB whiskers. The effect of sintering temperature on morphologies of in situ TiB whiskers was evaluated. With the increase of spark plasma sintering temperature, the average diameter of in situ TiB whiskers increased. The in situ TiB whiskers exhibited a hexagonal shape with (100), (101) and (10 1 ) planes at the transverse section and a growth orientation of [010]TiB direction.

Influence of Nano-Silica on Microstructure and Mechanical Properties of SiO2/BN Ceramics

In this paper, nano-silica particles were incorporated to a fused silica-boron nitride dual phase ceramics originally using micrometer sized particles as starting powders. Effects of nano-silica content on density, sintering behavior, mechanical properties and microstructure of the dual phase ceramics were investigated. It was found that with the addition of nano-silica particles, the density and mechanical properties of the dual phase ceramics increased. Flexural strength and fracture toughness of them attained to 51.4MPa and 0.8MPa·m1/2, respectively. The increased density and spatial network structure of fused silica attribute to the improvement of the mechanical properties of the dual phase ceramics.

Properties of h-Bn/Si3N4 Composite Ceramics Fabricated by Gel-Casting

h-BN/Si3N4 composite ceramics were fabricated by gel-casting method and pressureless sintering using Si3N4 and h-BN as raw materials. The effect of h-BN on the microstructure and comprehensive properties of h-BN/Si3N4 ceramic were investigated. The results showed that the introduction of h-BN hindered the densification of h-BN/Si3N4 ceramic as well as the transformation of α-Si3N4 to β-Si3N4. The mechanical properties of Si3N4 based ceramics decreased with the increase of h-BN content, while the thermal property, dielectric properties and machinability increased.