Hot Corrosion Behavior Of Yb₂Si₂O₇ Bulk Ceramic By NaVO₃ And Other Molten Salts

Yb₂Si₂O₇ is an ideal material for environmental barrier coatings（EBCs）due to its relatively low coefficient of thermal expansion（CET）,phase stability and low recession rate in water vapor.At present,many researchers have focused on the water-vapor corrosion and calcium-magnesium-aluminosilicates（CMAS）corrosion of the silicates EBCs materials.However,few reports about the hot corrosion of molten salts have been reported yet.Therefore,the hot corrosion behavior of Yb₂Si₂O₇ bulk ceramic by NaVO₃,V₂O₅ and Na₂SO₄ molten salts is studied.It will provide reference for corrosion protection of Yb₂Si₂O₇EBCs.The main research results are as follows:（1）The weak acid of NaVO₃ has a strong corrosion effect on bulk ceramic.Hot corrosion products of YbVO₄ and a small number of SiO₂ were identified at a temperature range of 500℃-900℃.The shape of YbVO₄ is rod with a tetrahedral cone at each end.Morphology of SiO₂ exhibits lamellar agglomeration.The size of YbVO₄ gets bigger with the increase of temperatures from 500℃to 600℃and no obviously crystals grow when the temperature increased from 700℃to 900℃.At 600℃-900℃,the size of SiO₂ gets bigger with the increase of temperature.At the same time,the corrosion depth increased with temperature increasing.The corrosion depths are 3.5μm,3.9μm,4.7μm,4.8μm and 5.2μm from 500℃to 900℃.YbVO₄ and other corrosion products were not detected at or below480℃and the weight of Yb₂Si₂O₇ buck ceramic did not change before and after corrosion,indicating that the critical thermal corrosion temperature for NaVO₃ on Yb₂Si₂O₇ buck ceramics was 500℃below the NaVO₃ melting point.The weight of the corrosion samples increased slightly compared with that of Yb₂Si₂O₇ ceramic due to the appearance of YbVO₄product after corrosion at 500-900℃range.（2）At 1000℃-1250℃range,the corrosion product of YbVO₄ was formed on the bulk surface.The shape of YbVO₄ was rod with a tetrahedral cone at each end.YbVO₄ phase decreased with the increase of temperatures.At the same time,the corrosion depth decreased gradually.The corrosion depths were 4.6μm,3.9μm and 3μm.At 1300℃-1500℃,the corrosion product became into Yb₂SiO₅ and its shape is granular polygons.Yb₂SiO₅ phase increased with temperature increasing.（3）The reaction between strong acid salt V₂O₅ and Yb₂Si₂O₇ bulk ceramic occurred from 700℃to 900℃.Hot corrosion product was YbVO₄ and YbVO₄ had a granular shape.The size of YbVO₄ got bigger and the number of crystals decreased with the increase of temperatures.The corrosion depth increased with temperature increasing.The corrosion depths are 3.5μm,2.6μm,3.4μm and 3.8μm.Morphology of YbVO₄ exhibited significant difference resulted in the difference of acidity of corrosion mediums.Under the condition of NaVO₃ of the weak acid,the YbVO₄ has a low nucleation rate and grows well-developed crystals.Then the shape of YbVO₄ is rod with a tetrahedral cone at each end.Under the condition of V₂O₅ of the strong acid,YbVO₄ has a higher nucleation rate which suppresses the growth of YbVO₄.The shape of YbVO₄ is granular.At the same time,experiments were carried out at 600℃and below.No corrosion reaction occurred.It is shown that the critical temperature of hot corrosion of Yb₂Si₂O₇ bulk ceramic by NaVO₃ is 700℃.（4）No chemical reaction was found between Yb₂Si₂O₇ and Na₂SO₄ at the temperature of equal to or less than 900℃.When the temperature is higher than or equal to 1000℃,Na₂SO₄ had a hot corrosion effect on the Yb₂Si₂O₇ bulk ceramic.The corrosion products were Yb₂SiO₅ and Na₂O·xSiO₂ and their morphology is polygon accompanied with pits in molten state.In addition,the temperature had an influence on both the number of crystals and the corrosion depth of Yb₂Si₂O₇ bulk ceramic.At the temperature range of 1000℃to1300℃,with temperature increasing,the driving force of the corrosion reaction increased and the corrosion product Yb₂SiO₅ increased as well.At the temperature range of 1400℃to1500℃,because the boiling point of Na₂SO₄ is 1404℃,the concentration of Na₂SO₄decreased due to its volatilization and the corrosion effect decreases.Therefore,the corrosion product Yb₂SiO₅ decreased with the increase of temperature.At temperatures from1000℃to 1500℃,the corrosion depth of Yb₂Si₂O₇ ceramic bulk increased first and then decreased.The corrosion depths were 2.4μm,4.7μm,6.9μm,8.6μm,7.2μm and 6.8μm.