Novel Powder Processing
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Background There has been considerable interest in mullite (3Al2O3× 2SiO2) and mullite-matrix composites in recent years due to their desirable properties. For high temperature applications, mullite has long been recognized for its excellent resistance to creep and thermal shock in refractories. Recent studies have shown that phase-pure mullite can also retain its strength to temperatures as high as 1500 ° C. Combined with its intrinsic thermal stability under oxidizing conditions, mullite then stands out as a unique material for high temperature applications. As a substrate, mullite, with a dielectric constant, e =6.7, results in about 17% lower signal transmission delay time than alumina (e =9.8). Furthermore, mullite has a low-thermal-expansion coefficient (4´ 10-6 K-1 in the 20° to 200° C range), which is almost that of silicon. Consequently, mullite and mullite-based glass-ceramics have now emerged as candidates in high-performance packaging applications. Interest in optical applications mainly centers on its applicability as a window material within the mid-infrared range. However, the relative poor mechanical properties have prevented a more widespread use of this ceramic material. ZrO2 addition is effective in improving the strength and fracture toughness at intermediate temperatures, where the properties of mullite ceramics are not as good as those of alumina ceramics, taken as the reference materials. Although they are still unclear, several toughening mechanisms of ZrO2 in mullite matrix were reported. The phase stability of tetragonal (t) phase is of importance in toughening effect and is influenced by a series of factors such as grain size, strain field, type and concentration of the stabilizer, etc. The present study attempts to form ZrO2-toughened mullite by mechanical alloying (MA) and plasma spraying of high pure alumina and zircon mixtures and investigates the stability of t-ZrO2 in mullite matrix. Emphasis is also put on the clear understanding of toughening mechanisms in ZrO2-mullite composite. Another main factor that prevents the development of high-tech mullite ceramics is the difficulty in processing. This is partially related to the absence of commercially available mullite powders, and explains the numerous works that are presently carried out to produce or to use such powders. It is possible to reaction sinter Al2O3 and ZrSiO4 mixtures to get ZrO2-mullite composite according to the following reaction: 2ZrSiO4 + 3Al2O3 = 3Al2O3× 2SiO2 + 2ZrO2 However, mullitization normally requires relatively high temperatures and long duration because of the low bulk and grain-boundary diffusion coefficients for mullite. In order to reduce diffusion distances and to increase the driving force for sintering, preparation of mullite powders which have fine particle size and homogeneity is well recognized. This study starts from powder preparation to form ZrO2-toughened mullite by plasma spraying. This approach enables rapid solidification rates of ~ 106 K/sec. The feedstock is prepared by a combination of mechanical alloying which allows formation of fine grained, homogeneous solid solution mixtures and following plasma spheroidization that yields rapid solidified microstructures and enhanced compositional homogeneity. The homogeneity is further realized when the plasma spray coatings are formed at last, and mullite formation temperature is expected to decrease by the unique preparation technique adopted in this study, which may be used to produce mullite large scale in industries. Although mullite and mullite matrix composites have been synthesized by various methods in the past twenty years, systematic studies on mechanical alloying and plasma spraying as an unique technique of preparation of ZrO2-mullite composite were scarcely reported, especially in comparison with other techniques. In this project, the author intends to confirm experimentally and systematically the preparation and characterization of ZrO2-toughened mullite by mechanical alloying and plasma spraying of Al2O3 and ZrSiO4 mixtures. Also formation of the non-equilibrium phases in MA and plasma spraying will be discussed from the thermodynamic and kinetic points of view. Mullitization routine and the kinetics of mullite formation by this unique technique will also be investigated, about which few reports have been found.
Research Objectives and Scope The aim of this research work is to fabricate ZrO2-mullite protective coatings by mechanical alloying and plasma spraying Al2O3/ZrSiO4 mixtures. The main contents of investigation are listed below.
Spheroidized composite powders BSE image of the coatings
Surface of the laser-treated coatings
TEM micrograph of the coatings heat treated at 1400° C. (Information provided by Dr. Li Ying) |
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