Fabbricazione e caratterizzazione di compositi a matrice ceramica ultra-refrattaria (UHTCMC) per applicazioni in ambienti severi

There is an increasing demand for advanced materials with enhanced temperature capability in highly corrosive environments. A new class of materials called Ultra-High Temperature Ceramics (UHTCMCs) have emerged as they combine the toughness of a composite with the high temperature stability of ultra-high temperature ceramics. This thesis deals with the fabrication and characterization of continuous fibre-reinforced UHTCMCs, using the Polymer Infiltration and Pyrolysis (PIP) technique re-adapted from the traditional CMC manufacturing technologies. The first part is aimed at the development of carbon fibre-reinforced ZrB2/SiC composites via aqueous slurry impregnation coupled with polymer infiltration and pyrolysis, using a allyl-hydrido polycarbosilane precursor. Ultra-high modulus pitch-based carbon fibres were selected for the first time for the PIP process investigating three different architectures (0/0°, 0/90°, and 2D), besides the cheaper and wider available high strength PAN-based carbon fibres, using simple (0/0°) and complex (2D and 2.5D) carbon fibre architectures. Microstructure and mechanical properties were investigated. As expected, the mechanical properties showed the tendency to decrease with increase of the preforms complexity, due to the higher amount of flaws and residual stresses. Moreover, the oxidation behaviour was investigated through short term oxidation tests in air performed for 1 min and 5 min at 1500 °C and 1650 °C in a bottom loading furnace. Microstructure, oxide scale thickness and composition were analysed by SEM/EDS/XRD. Results indicated that a good dispersion of ZrB2 particles in the polymer-derived SiC(O) matrix promoted the formation of compact scales of a viscous borosilicate glass filling surface holes left by fibre oxidation. The second part was focused on the improvement of performance at elevated temperature of PIP-ed UHTCMCs at mild conditions. A baseline ZrB2/SiC reinforced with 0/0° pitch-based Cf composite consolidated by PIP at 1000 °C was subjected to post-consolidation thermal treatments from 1100 to 1900 °C to explore the thermal stability. Microstructural characterization was performed to evaluate evolution of the matrix, highlighting the crucial aspects about crystallization and porosity. Mechanical properties at room and elevated temperature were investigated through bending tests up to 1500 °C. The composite consolidated at mild conditions exhibited good flexural strength up to 1400 °C, but at 1500 °C it showed a loss of performance. Post-treated composites resulted in a performance deterioration at room temperature, nevertheless at 1500 °C an improvement of the bending resistance was achieved after pyrolysis at temperature ? 1400 °C. The third part deals with the preparation of UHTCMCs by coupling water-based powder slurry infiltration, Polymer Infiltration and Pyrolysis (PIP) and Hot Pressing (HP) techniques. The best sequence of consolidation techniques to better integrate the carbon fibre cloths into an ultra-refractory sintered ceramic matrix of ZrB2-SiC was identified. Infiltrated preforms with UHTC powder slurry were densified via: a) a pre-sintering step by HP followed by two PIP cycles with polycarbosilane, and vice versa, b) two PIP cycles followed by a cycle of HP. Flexural strengths at room temperature and at 1500 °C were found to be significantly higher for composites obtained by the second route, suggesting that sintering of polymer-derived SiC during HP improves the structural properties of Cf/ZrB2-SiC composites. This represents an effective method for UHTCMC manufacturing in a shorter time than traditional PIP process. The fourth part is addressed to the development of simple ways to prepare polymer-derived ceramics (PDCs) with high thermal stability. Firstly, the effect pre-curing treatment and catalysts on the ceramic yield of commercial allyl-hydrido polycarbosilane was studied to understand its curing and pyrolysis behaviour. Then, a tentative of synthetise SiBCN(O) ceramic was carried out by coupling polycarbosilane, used as Si and C precursor, with ammonia borane, source of boron and nitrogen at the same time. The as-prepared precursor and obtained ceramic were investigated to identify chemical composition (FTIR, SEM-EDS and XRD). SEM analysis highlighted that the boron and nitrogen were not preferentially incorporated into the silicon carbide lattice but entered the SiC(O) forming segregated BNO-reach areas. Finally, a simple route was proposed for the synthesis of polymer precursors for zirconium carbide (ZrC), using zirconium tetrachloride (ZrCl4) as zirconium source and three different carbon sources: two phenolic resins and a type of bitumen. The results indicated that obtained organometallic compounds could be a Zr-O-C polymer. They were heat treated up to 1800 °C to achieve ZrC via carbo-thermal reaction. The pyrolysis behaviour and structural evolution of the carbon sources and the prepared precursors were analysed.