Researches On Hot Corrosion, High Temperature Oxidation And Thermal Shock Of The Plasma-sprayed High-temperature Protective Coatings

High temperature protective coatings on 310S stainless steel were prepared by plasma spraying technique. The surface coatings are respectively 8YSZ coating, ZrB₂-SiC coating, MoSi2-Al₂O₃ coating and Al₂O₃-TiO₂ coating. The bond coating is NiCrAlY. Based on the results of the experiments of hot corrosion, high temperature oxidation and thermal shock, the effect of the phase and the microstructure of coatings on their performance by using the X-ray diffraction（XRD）, standard electronic modules（SEM） and energy dispersive spectrometry（EDS）. The main results are summarized as following:（1） Plasma sprayed 8YSZ coating mainly consists of t-ZrO₂ phase, many cracks and pores exist in 8YSZ layer; the result show that 8YSZ coating in Na₂SO₄+NaCl at 700℃ still remained t-ZrO₂, show a good hot corrosion resistance. At 900 ℃ hot corrosion, the reaction will occur with Y₂O₃ after the decomposition of Na₂SO₄, destroy the stability of 8YSZ, lead to the change of phase, the oxygen decomposed from Na₂SO₄ diffused to the bonding layer can react with metal elements, the generated formation will precipitate from the cracks, can lead to the degradation of coating. The result show that there are thermal grown oxide（TGO）, the main components of which is alumina, between bonding layer and surface coating after 1100℃ high temperature oxidation.（2） Plasma sprayed ZrB₂-SiC coating mainly consists of ZrB₂, SiC, t-ZrO₂, t-ZrO₂, m-ZrO₂ and SiO₂ phase; the result show that ZrB₂-SiC coating in Na₂SO₄+NaCl at 700℃ due to the phase transformation of ZrO₂, which leads to coating volume change, part of the coating degradate. At 900 ℃ hot corrosion, Na₂ZrO₃ will generate after the decomposition of Na₂SO₄ with ZrO₂, the generated formation will precipitate from the cracks, lead to the degradation of coating, meanwhile the oxygen decomposed from Na₂SO₄ diffused to the bonding layer can react with metal elements, the generated formation will precipitate from the cracks, can lead to the degradation of coating. The result show that there are thermal grown oxide（TGO）, the main components of which is alumina, between bonding layer and surface coating after 1100℃ high temperature oxidation.（3） Plasma sprayed MoSi2-Al₂O₃ coating mainly consists of t-MoSi2, h-MoSi2, Al₂O₃, Mo5Sis phase, many cracks and pores exist in MoSi2-Al₂O₃ layer; After 700 ℃ hot corrosion experiment one cycle, MoSi2-Al₂O₃ coating fall off as a whole, the main reasons are that the difference of thermal expansion coefficients between coating and matrix is too large and fracture toughness is low under the low temperature.（4） Plasma sprayed Al₂O₃-TiO₂ coating mainly consists of α-Al₂O₃, γ-Al₂O₃ and TiO₂ phase, many cracks and pores exist in Al₂O₃-TiO₂ layer; the result show that Al₂O₃-TiO₂ coating in Na₂SO₄+NaCl at 700℃ have good hot corrosion resistance because surface of Al₂O₃-TiO₂ coating have no obvious corrosion products generated. At 900 ℃ hot corrosion, corrosion products generated from the cracks and pores, with the rise of corrosion products, can lead to the wide cracks between ceramic layer and bonding layer, eventually lead to the degradation of coating. The result of 1100 ℃ high temperature oxidation experiment show that there are thermal grown oxide（TGO）, the main components of which is alumina, between bonding layer and surface coating.（5） The result of thermal shock experiment show that the thermal shock resistance of protective coating（coating failure times） from high to low in the number:8YSZ coating, Al₂O₃-TiO₂ coating, ZrB₂-SiC coating, MoSi2-Al₂O₃ coating. The main reason for the thermal shock failure of coating, the coefficient of thermal expansion differences between metal matrix and ceramic surface is too large. In the process of repeated heating and cooling, cyclic thermal stress will lead to the expanding of cracks in the coatings, eventually lead to the coatings fall off, meanwhile the oxygen diffused to the bonding layer can react with metal elements via cracks, the generated formation will precipitate from the cracks, can lead to the degradation of coating.