Alumina-mullite (AM) refractories are widely used as liners in gas turbines for power production, because of their peculiar properties, appropriate for the thermal insulation of combustion chambers, characterized by turbine inlet temperature around 1400 °C. The typical tiles are made with a mixture of alumina and mullite with different granulometries, including a coarse fraction. In this work the feasibility of recycling of ceramic wastes, which come from other industrial processes, into AM refractories was assessed. The effects of their addition on phase composition, microstructure and thermomechanical properties of AM refractories were investigated. MOR and Young’s modulus were determined at room temperature and up to 1500 °C by four point flexural tests; thermal shock resistance was evaluated by MOR measurements after quenching tests. The comparison with a typical AM refractory used as liners shows that thermomechanical properties and thermal shock resistance were not significantly compromised by ceramic waste additions up to 20%, and, on the contrary, were improved.
In this work, the optimisation of basalt fiber CFCCs (Continuous Fiber Ceramic Composites) production is presented, focusing on the development of a silicon-oxycarbide matrix by PIP (Polymer Impregnation Pyrolysis). The use of low cost poly-siloxanes and basalt fibers is particularly promising for transports and constructions, where thermostructural CFCCs would be interesting for vehicle weight reduction and fire-resistant panels, but only on the condition that production costs are kept really low. The basalt/SiCO composites are suitable for mechanical applications up to 600°C and stand up temperatures up to 1200°C, also in oxidative environments. The key parameters to keep the production costs low are the furnace and moulds type, being steel probably the best material for both, since it withstands the pyrolysis temperature and can be easily cleaned, by oxidation, from any residue. Regarding the pyrolysis environment, two conditions were compared, nitrogen flow and vacuum, being perhaps the vacuum procedure less expensive and so potentially more appealing for a large scale production. The microstructure and the thermomechanical characteristics of the obtained composites were compared, Another key parameter in determining the production costs is the number of PIP steps, which has to be minimised. The present results support the conclusion that one PIP step in nitrogen or two PIP steps in vacuum can provide CFCC with satisfactory mechanical characteristics for thermomechanical applications in oxidative environments.
Composite materials usage in several industrial fields is now widespread, and this leads to the necessity of overcoming issues that are still currently open. In the aeronautic industry, this is especially true for Barely Visible Impact Damage (BVID) and humidity uptake issues. BVID is the most insidious kind of impact damage, being rather common and not easily detectable. These, along with the ageing that a composite structure could face during its operative life, could be a cause of fatal failures. In this paper, the influence of water absorption on impacted specimens compressive residual strength was studied. Specimens were impacted using a modified Charpy pendulum. Two different locations were chosen for comparison: Near-Edge (NE) and Central (CI). Accelerated hygrothermal ageing was conducted on impacted and reference nonimpacted coupons, placing them in a water-filled jar at 70 °C. Compressive tests were performed in accordance with the Combined Loading Compression (CLC) test method. A Dynamic Mechanical Analysis (DMA) was performed as well. The results showed the influence of hygrothermal ageing, as expected. Nevertheless, the influence of impact location on compressive residual strength is not clearly noticeable in aged specimens, leading to the conclusion that hygrothermal ageing may have a greater effect on composite compressive strength than the analysed BVI damage.
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