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Process engineering of ceramic composites and coatings modified with chemically bonded composite sol-gel Yao, Yu Chen

Abstract

This work explores the possibilities of the application of chemical bonding to process ceramic-ceramic composites by employing ceramic sol-gel as the core processing technique. The principal rationale is that chemical bonding produces ceramic bond at substantially lower temperatures under 600°C, opening new processing possibilities. Versatility of the chemically bonded composite sol-gel alumina is demonstrated by the two final product forms: coatings and wear resistant composites. In Thermal Barrier Coating (TBC) application, an additional ceramic layer is sandwiched between two layers of Plasma Sprayed TBC, to improve overall thickness and thermal insulation. Such combination creates an unique Hybrid Thermal Barrier Coating (HTBC) with the chemically bonded ceramic layer ranging between 200-500μm between two ~100μm traditional air plasma sprayed barrier films. The HTBC has dual microstructure, adjustable porosity and varying composition, including alumina and yttria stabilized zirconia. The chemically bonded ceramic layer provides vertical microstructure beneficial in enhancing thermal strain tolerance. Thermal cyclic tests (30-1050 °C) showed good performance of the hybrid coating up to 1000 cycles. In the Carbon-Carbon Composites (CCC), pores are infiltrated with alumina particles, which are hardened to a wear resistant phase through chemical bonding. The infiltration process is facilitated by vacuum impregnation followed by heat treatment in air at 300-500°C, to create a Ceramic Carbon-Carbon Composite (C⁴). Meanwhile, the infiltration also achieves densification at rate faster than traditional Chemical Vapour Infiltration (CVI) process. The time saving of ceramic vacuum impregnation process compared to the tradition CVI promises significant reduction of the processing costs, possibly up to a factor of 10 (from the current ~$1,000/kg, to the target ~$100/kg). The CCC as high performance light weight braking pads are almost exclusively used in aerospace and racing vehicles due to prohibitively high cost. The faster vacuum impregnation process could potentially migrate the C⁴ to general automotive applications. The C⁴ were tested for abrasive and sliding wear resistance. The results show a reduction in volume loss as compared to similar porosity CCC. The process of chemically bonding alumina-based ceramics is thus effective in introducing alumina-based ceramic composite phase to both a coating system and a porous monolith ceramic.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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