V.K. Sarin

SYNTHESIS OF MULLITE COATINGS BY CHEMICAL VAPOR DEPOSITION

Formation of mullite on ceramic substrates via chemical vapor deposition was investigated. Mullite is a solid solution of Al{sub 2}O{sub 3} and SiO{sub 2} with a composition of 3Al{sub 2}O{sub 3}{center_dot}2SiO{sub 2}. Thermodynamic calculations performed on the AlCl{sub 3}{endash}SiCl{sub 4}{endash}CO{sub 2}{endash}H{sub 2} system were used to construct equilibrium chemical vapor deposition (CVD) phase diagrams. With the aid of these diagrams and consideration of kinetic rate limiting factors, initial process parameters were determined. Through process optimization, crystalline CVD mullite coatings have been successfully grown on SiC and Si{sub 3}N{sub 4} substrates. Results from the thermodynamic analysis, process optimization, and effect of various process parameters on deposition rate and coating morphology are discussed. {copyright} {ital 1996 Materials Research Society.}

The development of CVD mullite coatings for high temperature corrosive applications

Adherent crystalline mullite coatings have been chemically vapor deposited on SiC and Si 3 N 4 -based substrates to enhance its corrosion/ oxidation resistance. Thermodynamic and kinetic considerations have been utilized to produce mullite coatings with a variety of growth rates, compositions, and morphologies. The flexibility of processing can be exploited to produce coated ceramics with properties tailored to specific applications and varied corrosive environments. The coating and been tested in several corrosive environments including oxidation, jet burner exhaust, and Na 2 SO 4 .

Structure and high-temperature stability of compositionally graded CVD mullite coatings

Abstract Dense, uniform and crack-free mullite (3Al 2 O 3 ·2SiO 2 ) coatings were deposited on SiC by chemical vapor deposition. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. The coatings were found to start out as a nanocrystalline layer, which is an intimate mixture of γ-Al 2 O 3 nanocrystallites imbedded in a vitreous silica-rich matrix at the substrate/coating interface. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied. These phase transformations, which include a tetragonal-to-orthorhombic transformation, mullitization and devitrification of silica in the nanocrystalline layer, and α-alumina precipitation and twinning of the alumina-rich mullite, are discussed in light of the adhesion and corrosion resistance of the coatings.

Formation of mullite coatings on silicon-based ceramics by chemical vapor deposition

Abstract Dense, uniform, mullite coatings have been deposited by chemical vapor deposition on SiC substrates, using a AlCl3-SiCl4-CO2-H2 system. The typical coating microstructure consisted of a thin layer of nanocrystallites of γ-Al2O3 in vitreous silica at the coating-substrate interface, with columnar mullite grains over this interfacial layer. The composition of the coating was graded such that the outer surface of the coating was highly alumina rich. The changes in the coating microstructure with processing parameters are discussed. The ability of mullite to incorporate such large composition variations is discussed in the light of vacancy formation as theAl/Si ratio is increased and the ordering of these vacancies leads to changes in lattice parameters. The formation of domains was studied by measuring the spacing of superlattice spots in electron diffraction patterns and the relationship between domain size andAl/Si ratio is discussed.

Development of CVD WC–Co coatings ☆

To address the problem of high speed machining titanium and its alloys, a CVD process to deposit WC–Co coatings on high-temperature deformation resistant substrates was developed. The WF6–CH4–H2 and CoCl2–H2 systems were used to deposit the WC and Co phases, respectively. Characterization by scanning electron microscopy revealed a microstructure similar to that of commercially available sintered WC–Co. X-Ray diffraction confirmed the presence of two distinct phases in the co-deposited coating. Although adhesion was not a problem on cemented carbide substrates, a functionally graded interfacial coating was used to improve adhesion to ceramic substrates. Turning tests were performed on Ti–6% Al–4% V alloy to evaluate the machining characteristics of these coatings. These tests demonstrated that the coatings considerably enhanced chemical wear resistance and therefore can be effectively used for high speed machining of Ti alloys.

Mullite interfacial coatings for SiC fibers

A process for depositing CVD mullite on SiC fibers for enhanced oxidation and corrosion protection and/or to act as an interfacial barrier coating has been developed. Process optimization via systematic investigation of system parameters yielded uniform, crystalline mullite coatings on SiC fibers. Structural characterization has allowed for tailoring of coating structure and therefore properties. Preliminary high temperature oxidation/corrosion testing of the optimized coatings has shown that the coatings remain adherent and protective for extended periods.

Nucleation mechanisms in chemically vapor-deposited mullite coatings on SiC

Dense, uniform, and adherent chemically vapor-deposited mullite coatings were deposited on SiC substrates using the AlCl{sub 3}{endash}SiCl{sub 4}{endash}H{sub 2}{endash}CO{sub 2} system. Typical coating morphology consisted of a thin interfacial layer of {gamma}{endash}Al{sub 2}O{sub 3} nanocrystallites embedded within a vitreous SiO{sub 2}-based matrix. When a critical Al/Si ratio of 3.2{plus_minus}0.29 was reached within this nanocrystalline layer, mullite crystals nucleated and grew as columnar grains. The thickness of the nanocrystalline layer decreased as the input AlCl{sub 3}/SiCl{sub 4} ratio was increased. In all cases, the Al/Si composition in the coating increased from the coating/substrate interface to the coating surface. Critical factors leading to the nucleation and growth of mullite crystals are discussed in this article. {copyright} {ital 1999 Materials Research Society.}

A Ceramic Version of the LSO Scintillator

Although LSO is one of the most successful scintillator developments for medical diagnostics in the last two decades, good single crystals are not commercially available in any quantity. Consequently, we explored the feasibility of developing a ceramic version of the material, which requires a considerably lower temperature to consolidate the material to essentially crystalline density. Consolidation of the ceramic was achieved by hot pressing at temperatures up to 1700degC and pressure of 8000 psi. Hot pressing causes a loss of oxygen resulting in strong coloration of the ceramic, which had to be removed by heating (ldquobleachingrdquo) in air at approximately 1100degC. The resultant specimens were colorless and highly translucent, but the anisotropic nature of the crystal structure precluded the achievement of full transparency. The scintillation performance of the resulting ceramics was characterized and compared with that of high light-output LSO single crystals. The scintillation efficiency as measured by energy spectra generally fell in the range of 50-60% of that of the crystals. While this would be adequate for PET applications, the limited transparency provides a barrier to such use. An alternative application would be in signal integrating techniques, such as CT, where it could provide an alternative to GOS but with higher speed. Here, the problem of afterglow assumes major importance. The afterglow is a function of many factors, including conditions of excitation. Further work on improving the LSO ceramics is considered.

Comparison of WCl6-CH4-H2 and WF6-CH4-H2 systems for growth of WC coatings

Thermodynamic equilibrium calculations were performed using SOLGASMIX-PV at varying partial pressures of WCl 6 -CH 4 -H 2 and WF 6 -CH 4 -H 2 . Chemical vapour deposition (CVD) phase diagrams, which display equilibrium condensed phases, were then constructed from the results of these calculations. These results indicated that for the WF 6 and WCl 6 systems the phase transformation from W to WC occurs at a lower partial pressure of CH 4 with decreasing total pressure. However, the partial pressure of CH 4 needed to produce WC is lower for WF 6 than WCl 6 . Information from these CVD phase diagrams was used to determine process parameters for the deposition of monolithic hexagonal WC. It was observed that coatings grown at low temperatures contained phases other than those predicted in the CVD phase diagrams. Experimental results on the development of WC coatings based on CVD phase diagrams have been presented and discussed.

Fracture of Whisker-Reinforced Ceramics

Abstract This chapter provides an account of current understanding of the fracture of whisker-reinforced ceramics. First, a brief description of the preparation and characteristics of existing whisker-reinforced ceramics is given. This is followed by a presentation of theoretical models of micromechanics and fracture in fibre-reinforced microstructures appropriate to whiskers in brittle matrices. Crack bridging, whisker pull-out, crack-deflection and microcracking are shown to be feasible mechanisms of toughening in whisker-reinforced ceramics. Reported experimental studies of the fracture of such materials are summarized and seem to be consistent with the model predictions. Both high-temperature and fatigue fracture are discussed, although as yet few experimental observations of these have been reported.