Study On Preparation And Properties Of La₂(Zr_0.75Ce_0.25)₂O₇-based High-temperature Thermal Barrier Coatings

Thermal barrier coatings are commonly applied in gas turbines and aircraft engines to protect hot end components from high-temperature damage,which can increase the operating temperature and improve engine efficiency.However,the first generation of thermal barrier coating ceramic material(6-8 wt.%Y₂O₃ partially stabilized ZrO₂,YSZ)can no longer meet the requirements of advanced gas turbines due to the low service temperature(<1200?).Therefore,high-performance thermal barrier coating materials must be developed for working at higher temperatures and having lower thermal conductivity.La₂Zr₂O₇ pyrochlore has drawn a lot of attention attributed to its lower thermal conductivity as compared with other thermal barrier coating candidate materials.However,the thermal expansion coefficients of La₂Zr₂O₇ ceramics are small,which easily causes thermal mismatch with metal bond coat and substrate at high temperatures,and then coating failure.Doping a certain amount of CeO₂ in La₂Zr₂O₇can increase its thermal expansion coefficients and further reduce the thermal conductivity.In the CeO₂ doped La₂Zr₂O₇ series,La₂(Zr_0.75Ce_0.25)₂O₇ ceramic was found to remain a stable pyrochlore.And it possesses lower thermal conductivity and greater thermal expansion coefficients than La₂Zr₂O₇ ceramic.Thus,La₂(Zr_0.75Ce_0.25)₂O₇ ceramic is regarded as a very promising candidate material for high-temperature thermal barrier coating.However,the details for La₂(Zr_0.75Ce_0.25)₂O₇ceramic as high-temperature thermal barrier coating need further study.Based on the complex and harsh service environment of thermal barrier coatings,this paper launches the following research around high-temperature thermal barrier coating candidate material La₂(Zr_0.75Ce_0.25)₂O₇:(1)La₂(Zr_0.75Ce_0.25)₂O₇ ceramic coatings were successfully prepared by atmospheric plasma spraying and submitted to annealing and sintering experiments,respectively.The coating structure before and after heat treatment was characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD)and Raman spectroscopy.Raman spectra fitted by the Lorentz function were employed to intuitively show the structure evolution of La₂(Zr_0.75Ce_0.25)₂O₇ coatings at high temperatures.Results showed that as-sprayed La₂(Zr_0.75Ce_0.25)₂O₇coatings present a metastable fluorite phase, which gradually transforms into pyrochlore under high-temperature annealing.When the temperature increases to 1100?,the ceramic coating completely changes into pyrochlore.The structure order degree of La₂(Zr_0.75Ce_0.25)₂O₇ pyrochlore increases with the increase of annealing temperature to 1500?.Long-term sintering demonstrated that LCZ ceramic coating has a preferable sintering resistance at 1500?,desirable for thermal barrier coating applications.Besides,the associated thermal properties of La₂(Zr_0.75Ce_0.25)₂O₇ ceramics were obtained.Results showed that La₂(Zr_0.75Ce_0.25)₂O₇ ceramic has an ultralow thermal conductivity(0.65 W/m·K,1200?),which is only 1/3 of that of traditional thermal barrier coating materials YSZ.The thermal expansion coefficients of La₂(Zr_0.75Ce_0.25)₂O₇ ceramic increase from 9.68×10^-6K^-1 to 10.7×10^-6 K^-1 at temperatures of 300-1500?,which are relatively larger than those of La₂Zr₂O₇.(2)Three groups of high-temperature La₂(Zr_0.75Ce_0.25)₂O₇-based coatings (La₂(Zr_0.75Ce_0.25)₂O₇,La₂(Zr_0.75Ce_0.25)₂O₇/8YSZand La₂(Zr_0.75Ce_0.25)₂O₇/CYSZ)were designed and produced by atmospheric plasma spraying.Two cooling ways,water cooling and air cooling,were employed to conduct the thermal shock tests.Thermal shock lifetime characterized by the number of thermal cycles was used to evaluate the thermal shock resistance of these coatings.Besides,the failure mechanisms were analyzed in the framework of crack initiation and propagation.Moreover, the main reasons for the growth of these cracks were also discussed and analyzed in detail.Based on the present results,La₂(Zr_0.75Ce_0.25)₂O₇/8YSZ coating remains intact after 100 air-cooled thermal shock cycles,indicative of remarkable thermal shock resistance.The initiation and propagation of transverse cracks at the interface between the ceramic layer and bond coat result in interface debonding,which is the only failure mechanism of these coatings in air-cooled thermal shock.Except for the interface debonding,in water-cooled thermal shock,the spalling of the topcoat is another failure mechanism due to the intersection of vertical cracks and interlayer microcracks.(3)To better understand the hot corrosion behaviors of La₂(Zr_0.75Ce_0.25)₂O₇ ceramic coatings against molten salts,plasma-sprayed La₂(Zr_0.75Ce_0.25)₂O₇ coatings were submitted to hot corrosion tests in different molten salts:single Na₂SO₄,single V₂O₅ and V₂O₅+Na₂SO₄ mixtures.The hot corrosion behaviors of La₂(Zr_0.75Ce_0.25)₂O₇ coating depend on the molten salts.In single Na₂SO₄,molten content,corrosion duration and corrosion temperature have important effects on the hot corrosion behaviors.Different corrosion products of La₂O₂SO₄,La₂O₂S and Ce₇O₁₂ were identified depending on the hot corrosion conditions.The phase transition and decomposition of corrosion products are responsible for the different hot corrosion behaviors at 1050-1500?.In single V₂O₅,La₂(Zr_0.75Ce_0.25)₂O₇ coatings were corroded for different durations.It was found that(La,Ce)VO₄ and m-ZrO₂ were formed on the coating surface,independent on duration.Meanwhile,the thickness of the corrosion layer increased with increasing duration and then remained unchanged.In the mixed V₂O₅+Na₂SO₄ molten salts,the La₂(Zr_0.75Ce_0.25)₂O₇ coating surface was partially corroded and only(La,Ce)VO₄ was formed.The morphologies of(La,Ce)VO₄ particles have an important relationship with Na₂SO₄ content.Moreover,the corroded zone can be divided into three layers based on the microstructure and chemical composition.A model which well described the formation mechanism was proposed.These results demonstrated that single V₂O₅ was more corrosive to La₂(Zr_0.75Ce_0.25)₂O₇ coating than single Na₂SO₄ and mixed V₂O₅+Na₂SO₄ molten salts.