Síntesis y estudio del comportamiento mecánico, térmico y de durabilidad ambiental del ortosilicato de itrio para su aplicación como recubrimiento de barrera ambiental.

Resumo

Silicon based ceramics, such as SiC and Si3N4, are promising materials as a structural material for high temperature applications (>1200ºC) due to their high melting point, excellent thermomechanical stability and low density. Their application for producing the electrical energy in critical components in gas turbine engines would enhance their thermodynamic efficiency and reduction of gas emissions. However, a drawback of these materials is their low oxidation resistance in combustion environments. Under these conditions, a silica protective layer formed in dry environments is not stable in the presence of water steam and reacts, leading to a resulting volatile hydroxide layer. That causes the degradation and recession of these materials in gas turbine components. Thus, environmental barrier coatings (EBC) have been developed to protect these silicon based materials from erosion and corrosion, mitigating these aforementioned effects. Rare earth silicates are a new generation of environmental barrier coatings, since they present a high erosion protection, low volatilization rate and low oxygen permeability. In particular, the yttrium monosilicate Y2SiO5, is one of the most potential candidates for this purpose because of its high melting point, low oxygen permeability and low volatilization rate. Nevertheless, due to the difficulties in synthesis and sintering process of yttrium silicates polycristals, the fabrication of a pure and dense bulk of this material becomes complicated and their bulk properties have not been investigated yet. For this reason, fundamental data regarding the high-temperature thermal and mechanical behavior or environmental durability are not yet available. These key properties are necessary to assess the long-term behavior and lifetime of yttrium silicate based environmental barrier coatings. Hence, the main aim of this work is to develop a feasible procedure to produce a pure and dense Y2SiO5 bulk material in order to evaluate these aforementioned properties and the influence of this fabrication-properties relationship. The first part of this work was focused on the fabrication of the Y2SiO5 ceramic material by means of different synthesis and sintering methods. With regard to the synthesis process, it was undergone by two different methods: solid-state reaction (with and without additives) and a freeze-drying synthesis. Final synthesized powders were characterized by an X-ray diffraction (XRD) and a particle size distribution to analyze final crystalline products and their particle size. As well as, a thermodynamic study of different synthesis was performed by differential scanning calorimetry (DSC). After the synthesis process, in order to obtain a dense material, Y2SiO5 powders were sintered by two different techniques: conventional sintering and Spark Plasma Sintering, SPS. The sintered pellets were characterized by an X-ray diffraction to verify final crystalline bulk products after the sintering process. Additionally, a microstructural study of resulting pellets was carefully observed by scanning and transmission electron microscopy (SEM and TEM). The second part of this work is devoted to the fundamental properties of Y2SiO5 material and its application as an environmental barrier coating. Firstly, a high temperature thermomechanical behavior was assessed. For this purpose, compression resistance and compressive creep at constant load tests were carried out in the range of temperature from 1200 to 1400ºC. This made it possible to determine plastic deformation mechanisms involved, as well as a comparison with the existing models. A microstructural complementary study of deformed samples was performed by scanning and transmission electronic microscopy. Subsequently, a high temperature thermal conductivity study of Y2SiO5 was undergone by means of the thermal diffusivity measurement performed by a Laser Flash technique. Finally, Y2SiO5 environmental durability assessment for corrosion behavior of typical engine deposits was studied. These are based on highly reactive calcium-magnesium-aluminosilicate (CMAS) deposits that melt into a highly reactive glass caused by a high temperature gas engine performance. Resulting final phases from Y2SiO5-CMAS interaction were studied by X-ray diffraction, scanning electronic microscopy and differential scanning calorimetry.