Thanks to their extremely high melting point above 3000°C and their unique combination of thermo-physical and engineering properties, ultra-high temperature ceramics (UHTCs) and coatings on their basis are envisaged as promising candidates for application at very high temperature. Among various compounds, ZrB2 has several advantages over other metal-based borides and carbides. One critical issue in the production of ZrB2-based ceramics is the achievement of fully dense bodies/coatings, which is the base-line condition to achieve good thermo-mechanical properties and oxidation resistance. Owing to the strong covalent bonds featuring all UHTCs, temperature above 2000°C and the application of pressure are required to densify these compounds. However, the microstructure deriving from such extreme processing conditions are coarse, with trapped porosity and hence with poor oxidation and mechanical performances. The addition of specific sintering additives has the twofold effect of decreasing the sintering temperatures down to 1650°C, and of notably modifying the performances at high temperature. This work aims to the development of ablation resistant UHTC coatings for the protection of future reusable space vehicles that should survive harsh earth and/or planetary re-entry conditions. For this purpose, selected UHTC compositions on the basis of ZrB2 (i.e. ZrB2-5WC-3SiC and ZrB2-15MoSi2) have been deposited by means of a new Shrouded Plasma Spray (SPS) technique under atmospheric conditions. A preliminary stage of the work consisted on the development of feedstock powders suitable for thermal spray processing, thus the morphology and size distribution of the powder particles were optimized to reach a good flowability and optimal processing conditions with the SPS system. Relevant SPS process parameters have been optimized based on the resulting coating microstructure and composition. Collected results led to the conclusion that the developed coatings are essentially composed of ZrB2 as main phase, indicating the low decomposition of the different feedstock powders in the plasma plume. Coatings have been applied on different type of Ceramic Matrix Composites (CMCs) (i.e. C-C, C/C-SiC) with dense structures, good adhesion to CMC base material and apparently good thermo-cycling resistance. Moreover, preliminary ablation and high temperature cycling tests have demonstrated the potentiality of the developed UHTC coatings to successfully withstand extreme thermal shock conditions without peel-off (critical delamination failure).
Development of UHTC coatings using a Shrouded Plasma Spray (SPS) technique
L Silvestroni;
2016
Abstract
Thanks to their extremely high melting point above 3000°C and their unique combination of thermo-physical and engineering properties, ultra-high temperature ceramics (UHTCs) and coatings on their basis are envisaged as promising candidates for application at very high temperature. Among various compounds, ZrB2 has several advantages over other metal-based borides and carbides. One critical issue in the production of ZrB2-based ceramics is the achievement of fully dense bodies/coatings, which is the base-line condition to achieve good thermo-mechanical properties and oxidation resistance. Owing to the strong covalent bonds featuring all UHTCs, temperature above 2000°C and the application of pressure are required to densify these compounds. However, the microstructure deriving from such extreme processing conditions are coarse, with trapped porosity and hence with poor oxidation and mechanical performances. The addition of specific sintering additives has the twofold effect of decreasing the sintering temperatures down to 1650°C, and of notably modifying the performances at high temperature. This work aims to the development of ablation resistant UHTC coatings for the protection of future reusable space vehicles that should survive harsh earth and/or planetary re-entry conditions. For this purpose, selected UHTC compositions on the basis of ZrB2 (i.e. ZrB2-5WC-3SiC and ZrB2-15MoSi2) have been deposited by means of a new Shrouded Plasma Spray (SPS) technique under atmospheric conditions. A preliminary stage of the work consisted on the development of feedstock powders suitable for thermal spray processing, thus the morphology and size distribution of the powder particles were optimized to reach a good flowability and optimal processing conditions with the SPS system. Relevant SPS process parameters have been optimized based on the resulting coating microstructure and composition. Collected results led to the conclusion that the developed coatings are essentially composed of ZrB2 as main phase, indicating the low decomposition of the different feedstock powders in the plasma plume. Coatings have been applied on different type of Ceramic Matrix Composites (CMCs) (i.e. C-C, C/C-SiC) with dense structures, good adhesion to CMC base material and apparently good thermo-cycling resistance. Moreover, preliminary ablation and high temperature cycling tests have demonstrated the potentiality of the developed UHTC coatings to successfully withstand extreme thermal shock conditions without peel-off (critical delamination failure).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
