| In the fields of aircraft engine blades,thermal barrier coating(TBC)is the important protection technique in order to protect substrate metal and improve the service life of blade.Nanostructure coating and zirconate with pyrochlore structure are considered as two promising ways of high performance thermal barrier coatings.Now suspension thermal spraying technology has been gradually recognized,which was used to prepared efficient nano coatings especially in nano/sub-micron surface engineering field.Suspension plasma spraying is an important development direction of suspension thermal spraying.In this paper,nanosized La2Zr2O7(LZ)powder was successfully prepared,nanostructured LZ coating was fabricated by suspension plasma spraying technology,in order to improve the service life of nanometer LZ coating,nanosized 8 wt % Y2O3 stabilized ZrO2(8YSZ)was joined as a buffer layer,the design and preparation of a new type of LZ/nano double 8YSZ ceramic thermal barrier coatings and LZ/8YSZ graded thermal barrier coatings,three kinds of high temperature properties of the coating was studied,and finite element analysis was used to evaluate the influence mechanism of the graded structure on the performance of the coating.Thermal conduction and stresses in the coatings system were compared by finite element analysis,and the results show that the graded coatings structure can reduce the stress variation between LZ and 8YSZ layers,and reduce the stress extremum value in the coatings system,while the properties of its thermal insulation did not have no effect.The thermal stress of graded coating were optimized as comparison with different structure coatings,which show that the graded structure can effectively reduce interface stress between the LZ and 8YSZ layers,and can reduce the extreme stress in the coating system,at the same time,will have little impact on the thermal insulation performance.Nano-sized LZ powders,which fit for feedstock materials of graded coatings fabricated by suspension plasma spraying,was successfully prepared by combustion and hydrothermal method.The results show that in combustion process paraffin combustion agent provided higher combustion value to obtain higher temperature to the benefit of formation LZ compound from La and Zr ions,and average particle size of prepared LZ particles was 22 nm.In hydrothermal process,LZ powder particle size was more thin,crystallization was better,and the average size of particles was less than 20 nm.Moreover,the nano powder had better resistance to high temperature corrosion.A comparation show that combustion process was low cost and more efficient,and hydrothermal process obtained higher quality of LZ crystal.Suspension plasma spraying was applied to fabricate nanostructured LZ coating.Suspension plasma spraying device was modified based on the existing atmospheric plasma spraying equipped with atomizer nozzle which can control the better granularity of droplets.The parameters,such as spraying spraying distance,power,and feeding speed,were analyzed in detail.Results showed that spraying distance of suspension plasma spraying was 40-50 mm,greatly shorter than one of traditional atmospheric plasma spraying.Microstructure analysis showed that LZ coating maintained nanostructure,and LZ phase composition did not change before and after suspension plasma spraying.According to the results of the suspension atomization process analysis and substrate single sweep analysis,the mechanism of the suspension plasma spraying process was investigated.Process of suspension plasma spraying is composed of evaporating,agglomeration,melting,and deposition.Evaporation of suspension dispersion medium consumed a large amount of heat,which was main reason of the shorter spraying distance.Graded component variation between the LZ and 8YSZ layer in graded thermal barrier coatings was realized using suspension plasma spraying process.Graded spraying device equipped with the self-made suspension feed coordinated control system,realized the constant total feed speed and the suspension feed simultaneous coordination variation between 8YSZ and LZ,finally completed the composition graded transition in the graded coating.Microstructure and composition analysis showed that the obvious graded composition ramp region existed in the graded thermal barrier coatings.Bond strength tests of three kinds of coating showed that the bonding strength of the graded structure were 29.66 MPa,higher than the double layer structure and single ceramic layer structure,in agreement with the results of the analysis of residual stress.Heating and water quenching method was used to evaluate thermal shock resistance of thermal barrier coating of three different structure coatings fabricated by suspension plasma spraying.Thermal shock performance of graded coating was obvious superior.The thermal shock life of graded coatings was prolonged 50% than one of the double layer coatings.Through the macroscopic and microscopic analysis of coating,failure occured mainly at the edge and center,the crack extension inside the coating was the main form of failure.Finite element analysis results showed thatthe edge and center were the locations of the thermal stress concentration in the process of temperature change,and the crack tip stress were significantly different in three kinds of coating.The crack tip in the graded coating of tensile stress and compressive stress variation were smallest,the two values of the single ceramic layer structure coatings were biggest.The finite element analysis results were consistent with the results of the thermal shock test,which illustrated the graded coating can effectively reduce impetus of the crack propagation. |
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