Alida Brentari

Slurry Coating of Environmental Barrier Coating (EBC) on Silicon Carbide Based Material

In order to increase the component lifetime of SiC based materials in combustion environments, environmental barrier coatings (EBCs) based on ceramic oxides are used to improve corrosion resistance of SiC, due to their high chemical stability. In this work, mullite and barium-strontium-aluminosilicate (BSAS) coatings were deposited by slurry dip coating on SiC substrates. Slurries were prepared by suspending commercial powders in water or ethanol, using appropriate dispersants. Substrates were dipped into the slurry and subsequently dried and heat treated at high temperature to promote densification. SEM observations were carried out to investigate the microstructure of the obtained coatings and to evaluate crack formation, porosity and adhesion.

Estimate of compressive strength of an unidirectional composite lamina using cross-ply and angle-ply laminates

In this work has been estimated the compressive strength of a unidirectional lamina of a carbon/epoxy composite material, using the cross-ply and angle-ply laminates. Over the years various methods have been developed to deduce compressive properties of composite materials reinforced with long fibres. Each of these methods is characterized by a specific way of applying load to the specimen. The method chosen to perform the compression tests is the Wyoming Combined Loading Compression (CLC) Test Method, described in ASTM D 6641 / D 6641M-09. This method presents many advantages, especially: the load application on the specimen (end load combined with shear load), the reproducibility of measurements and the experimental equipment quite simplified. Six different laminates were tested in compressive tests. They were realized by the same unidirectional prepreg, but with different stacking sequences: two cross-ply [0/90]ns, two angle-ply [0/90/±45]ns and two unidirectional laminates [0]ns and [90]ns. The estimate of the compressive strength of the unidirectional laminates at 0°, was done by an indirect analytical method, developed from the classical lamination theory, and which uses a multiplicative parameter known as Back-out Factor (BF). The BF is determined by using the experimental values obtained from compression tests.

Exploitation of Ceramic Wastes by Recycling in Alumina-Mullite Refractories

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.

Two-Step Pressureless Sintering of Silicon Carbide-Based Materials

Pressureless sintering of silicon carbide powder requires addition of sintering aids and high sintering temperature (>2100°C) in order to achieve high sintered density (>95% T.D.). The high sintering temperature normally causes an exaggerated grain growth which can compromise the mechanical properties. Two-step sintering (TSS) can be used to overcome this problem. By this method, high sintered density is obtained avoiding the grain growth associated to the last step of the sintering. Two-step sintering was successfully applied to different commercial silicon carbide powders with different sintering mechanism: solid-state and liquid-phase sintering. In both cases the sintering temperature was set nearly 100 °C below the temperature conventionally required. Microstructures of samples obtained by TSS and conventional sintering (CS) processes were compared. TSS-SiC showed finer microstructure consisted of equiaxed grains with very similar density. The beneficial effects of the two-step sintering process were more evident in the solid state sintering. In this case sintered density higher than 98% was achieved with T<2000 °C.

Cf/C composites: Correlation between CVI process parameters and Pyrolytic Carbon microstructure

Chemical Vapour Infiltration (CVI) technique has been long used to produce carbon/carbon composites. The Pyrolytic Carbon (Py-C) matrix infiltrated by CVI could have different microstructures, i.e. Rough Laminar (RL), Smooth Laminar (SL) or Isotropic (ISO). These matrix microstructures, characterized by different properties, influence the mechanical behaviour of the obtained composites. Tailoring the process parameters, it is possible to direct the infiltration towards a specific Py-C type. However, the factors, influencing the production of a specific matrix microstructure, are numerous and interconnected, e.g. temperature, pressure, flow rates etc. Due to the complexity of the physical and chemical phenomena involved in CVI process, up to now it has not been possible to obtain a general correlation between CVI process parameters and Py–C microstructure. This study is aimed at investigating the relationship between infiltration temperature and the microstructure of obtained Py-C, for a pilot - sized CVI/CVD reactor. Fixing the other process parameters and varying only the temperature, from 1100°C to 1300°C, the Py-C infiltration was performed on fibrous preforms. Polarized light microscopy, with quantitative measurements of average extinction angle (Ae), and Raman spectroscopy were used to characterize the obtained Py-C microstructures.

Microstructural Characterization of Activated Carbon Obtained from Waste Tires

SOREME project (LIFE 11 ENV/IT/109) is aimed at synthesizing an innovative sorbent based on activated carbon obtained from the carbonization of waste tires. Microstructural characterization was mainly performed in order to define crystallinity, morphology and porosity of the activated carbon powders obtained in different conditions. In particular, XRD analysis always revealed a partially crystalline structure with different crystallite size of the nanographitic structure. The disorder of these structures was determined by Raman spectroscopy. This evaluation was made on the basis of the ratio of the integrated area of the D and G bands typical of the graphitic structure. Finally, SEM was used to put in evidence the mesopores and macropores.

Poly-Siloxane Impregnation and Pyrolysis of Basalt Fibers for the Cost-Effective Production of CFCCs

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.

Optimization of a Pyrolysis Procedure for Obtaining SiC-SiCf CMC by PIP for Thermostructural Applications

Polymer Impregnation Pyrolysis (PIP) is a cost effective technique for obtaining Ceramic Matrix Composites (CMC) modified with nanoparticles. Commercial UBE polymeric precursor (Tyranno polymer VL-100, diluted in xylene) of a SiC ceramic matrix (with 11 wt% O and 2 wt% Ti) was used to infiltrate 100x85x3 mmSuperscript text3 SiC felts (Tyranno ZM fibers, diameter 14 microns, 800 filament/yarn, 270 g/mSuperscript text2, with 9 wt% O and 1 wt% Zr), applying different pyrolysis procedures. In particular, pyrolysis was performed in two conditions: 1) at 1000 °C for 60 min; 2) at 900 °C for 120 min. A pyrolysis at 900 °C could be more convenient since it can be easily performed in a steel furnace, without a refractory lining. The SiC felts were pretreated by CVD (Chemical Vapour Deposition) in order to deposit a pyrolytic carbon interphase (about 0.1 microns). Impregnation was performed under vacuum, and drying was carried out in an explosion-proof heating oven. Pyrolysis at 900°C was performed in a AISI 310S austenitic steel furnace, under nitrogen flow. Geometric density was monitored during densification. Mechanical characterisation (bending tests at room temperature, following UNI EN 658-3:2002) was performed after 11 PIP cycles. The results were used to compare the influence of pyrolysis temperature on densification.

Microstructural and Thermo-Mechanical Characterization of Yttria Ceramic Cores for Investment Casting, with and without Particulate Reinforcement

Yttria is an interesting material for the production of high performance ceramic cores for Directionally Solidified investment casting. DS casting of superalloys of the last generation challenge conventional silica ceramic cores, because of the high temperatures and the long times involved. Compared to silica, yttria is characterized by improved mechanical properties and higher chemical resistance at high temperatures. Submicronic and nanometric SiC reinforcements were tested in order to increase compression creep resistance of yttria. Oxidation resistance of both reinforcements was tested in conditions simulating the industrial process. The experimental results demonstrated the increase of compression creep resistance and showed that the industrial application of yttria ceramic cores reinforced with submicronic SiC is possible with minimal changes to current practices, thanks to a high enough oxidation resistance.

Alumina-Mullite Refractories: Prototypal Components Production for Thermal Shock Tests

Alumina-mullite refractory tiles are used as liners in gas turbines for power production, for the thermal insulation of the combustion chambers. The typical microstructure is characterized by a coarse fraction, in order to increase porosity (and hence thermal insulation) and improve thermal shock resistance (by grain bridging mechanism). A mixture of alumina and ceramic wastes was optimized to manufacture prototypal components, by cold isostatic press (CIP). On sintering at 1600°C, a final composition of 40% mullite and 60% alumina was obtained. For the production of refractory tiles with dimensions similar to the commercial ones, an appropriate mould was CAD-CAM designed and produced, using aluminium and silicone. 115 x 95 x 30 mm3 tiles were obtained and utilized for thermal shock tests. Samples were heated up to 1000°C and quenched to 20°C: this cycle was repeated 30 times before inspecting the macroscopic cracks. The results were compared with those obtained with a standard test for advanced technical ceramics based on MOR measurements. Two different pressing conditions were tested (60 and 150 MPa) in order to get data about a possible industrial production by uniaxial pressing.