| Thermal barrier coatings?TBCs?have been widely used to provide thermal protection for the hot-section metal components in advanced gas turbines and diesel engines in order to improve thermal efficiency and performance.In recent years,research on nanostructured YSZ?n-YSZ?based ceramic TBCs has attracted wide spread attention because of some extraordinary properties which are not found in conventional counterparts.While failure occurred during the service of n-YSZ.More terribly,there are a few cases about the pulverization of n-YSZ coatings for aero and space engines whose coatings were stored in natural environment for a few years before engine running,but the mechanism is not clear so far.Moreover,the YSZ coating cannot be long-term operated above 1200°C because of the phase transformation and serious sintering,which will result in early spallation of TBCs during thermal cycling.Therefore,alternative TBCs materials should be proposed and investigated.The thermal cycling performance,molten salt corrosion mechanism and hydrothermal degradation behavior of n-YSZ TBCs prepared by atmospheric plasma spraying?APS?were studied,and the failure mechanism of n-YSZ coating was analyzed.At same time,the ZrO2-CeO2-TiO2 has been also investigated,which showed good phase stability at1300°C,and had a potential application prospect.Bimodal structured n-YSZ coatings with different contents of nanostructure were fabricated by APS with different spraying powers.Results showed that both the porosity and the content of nanostructure in YSZ coatings decreased with increasing of the spraying power.The n-YSZ coating deposited by lower spraying power had a higher content of nanostructure,exhibiting better bond strength and superior thermal cycle life.After heat treatment at 1300°C,the porosity decreased and the hardness of the coating increased noticeably when compared with that of the as-sprayed coating,indicating the sintering effect of the nanostructured coating.However,the coating deposited by the lowest spraying powder?22 kW?still had a porous structure after annealing at 1300°C for 24 h,showing high sintering resistance ability.Although phase structures have certainly changed during heat treatment,phase compositions were more stable for the coatings deposited by the relatively low spraying powers.Hot corrosion behavior of both n-YSZ powders and their corresponding coatings with sodium metavanadate?NaVO3?has been investigated.During hot corrosion,NaVO3 reacts with Y2O3 in YSZ to form YVO4,leading to the tetragonal to monoclinic destructive phase transformation of YSZ.The crystalline grain size of n-YSZ powders appears not to affect the hot corrosion mechanisms of both n-YSZ powders and their corresponding coatings.However,compared with n-YSZ powders,their coatings were more easily corroded,coatings formed from starting powders with crystalline grain sizes smaller than 30 nm having better degradation resistance to hot corrosion.The hydrothermal degradation of plasma-sprayed n-YSZ coating was investigated.Results showed that the phase transformation was induced only by nucleation mechanism and the apparent activation energy was 67 kJ/mol at temperatures lower than 148°C.Intergranular cracking and transgranular cracking were found after hydrothermal degradation.Pulverization and failure of the n-YSZ coating would occur when the amount of m-phase achieved above 40%during the hydrothermal degradation process.The hydrothermal treatment can result in a significant amount of microcracks and holes in the coating,which had a detrimental impact on the bonding strength between the ceramic top coat and the metallic bond coat,resulting in reduction of the thermal cycling life of the coating.The effects of high-temperature heat treatment on the phase composition and the degree of hydrothermal degradation of n-YSZ coating were studied.Results showed that the phase composition of the coating did not change significantly when the coating was annealed under 1100°C.While the decomposition from tetragonal phase to monoclinic phase can be effectively reduced or inhibited after heat treatment.The metastable tetragonal phase in the n-YSZ is unstable upon high-temperature heat treatment and is driven to separate into yttrium-rich cubic and yttrium-lean tetragonal phases due to the stabilizer yttrium diffusion,and the yttrium-lean tetragonal phase was susceptible to the disruptive transformation to monoclinic phase during the hydrothermal degradation.The properties of ZrO2 co-stabilized by CeO2 and TiO2 ceramic bulks were investigated for potential TBCs applications.Results showed that the(Ce0.15Tix)Zr0.85-xO2?x=0.05,0.10,0.15?compositions containing single tetragonal phase were more stable at 1300°C than that of the traditional YSZ,and these compositions showed a large thermal expansion coefficient?TEC?and a high fracture toughness,which were comparable to those of YSZ.In addition,the phase stability,fracture toughness,the TEC and sintering resistance of the CeO2-TiO2-ZrO2 system showed a decline tendency with the increase of TiO2 content.The(Ce0.15Ti0.05)Zr0.8O2?CTZ?had the best comprehensive properties among the(Ce0.15Tix)Zr0.85-xO2 compositions as well as a low thermal conductivity.Therefore,it can be explored as a TBC candidate material for high-temperature applications.The microstructures,phase stability and thermo-physical properties of the CTZ coating were examined.Results showed that the CTZ coating containing single tetragonal phase was more stable than the YSZ coating during isothermal heat-treatment at 1300°C.The CTZ coating had a lower thermal conductivity than that of YSZ coating,decreasing from 0.89 W m-1 K-11 to 0.76 W m-1K-1 with increasing temperature from room temperature to 1000°C.The thermal expansion coefficients were in the range of 8.98×10-6 K-1 to 9.88×10-6 K-1.Samples were also thermally cycled at 1000°C and 1100°C.Failure of the TBCs was mainly a result of the thermal expansion mismatch between CTZ coating and superallloy substrate,the severe coating sintering and the reduction-oxidation of cerium oxide.The thermal durability of the TBCs at 1000°C can be effectively enhanced by using a YSZ buffer layer,while the thermal cycling life of CTZ/YSZ double-ceramic-layer TBCs at1100°C is still unsatisfying.The thermal shock resistance of the CTZ coating should be improved;otherwise the promising properties of CTZ could not be transferred to a well-functioning coating. |
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