Research On Fabrication Of Noble-Metal Thin-Film Thermocouples On Ni-Based Superalloy And It's Antioxidant Properties

The value and distribution of surface temperature is vital for the mechanical property and working life of the hot end parts of aeroengine.Real-timely and precisely measuring the surface temperature is of great importance to design verification of aeroengine.Traditional temperature measuring method has the disadvantages of disturbing flow field,slow response,bringing defects to the hot end parts and so on.However,through thin-film deposition technology the thin-film thermocouple is fabricated on the surface of the device,integrated with the device,which has the advantages of small size,minimal environmental impact on the surface of the measuring devices,small thermal capacity and quick response speed.Noble-metal thin-film thermocouple?S type and R type?,the electrode material of which is composed of Pt and PtRh,has great stability of thermoelectric properties and wide temperature measurement range.Thus,that kind of thermocouple is very suitable for the temperature measurement of the hot end parts of an aeroengine working at high temperature.In this paper,as for the above application,and based on the basis of the previous work,the fabrication,antioxidant properties and thermoelectric properties of the noble metal thin-film thermocouple are studied.First of all,Pt-10%Rh films and Pt-13%Rh films are prepared on Al₂O₃ ceramic substrate by magnetron sputtering,then treated with the atmosphere annealing,in order to research on the change of surface morphology,crystal structure,antioxidation and electrical properties of PtRh thin films caused by high temperature oxidation environment.Through the calibration of the thermal electromotive force of the S type and R type thin film thermocouples preparated on the same piece of Al₂O₃ ceramic substrate,combined with the influence on the composition and structure of PtRh thin film materials caused by high temperature oxidation environment,this paper researchs on differences of antioxidation properties and thermoelectric properties betwteen S type thin-film thermocouple and R type thin-film thermocouple and analyzes its internal mechanism.Results show that,in high temperature oxidation environment,the segregation and oxidation of Rh and the crystallization of Rh2O3 appear in PtRh thin films.When the temperature is 700⁹00?,these phenomena will become more obvious with the increase of temperature.When the temperature is 900¹000?,the oxide of Rh is changed from crystalline state to amorphous state.The change of surface morphology,element composition and valence of Pt-13%Rh films caused by high temperature oxidation environment is smaller than that of Pt-10%Rh films.The thermal electromotive force of R type thermocouple is higher than the S type thermocouple and the average Seebeck coefficient of the former is higher adout 1?V/? than the latter.Meanwhile,the variation of K value of the former with temperature is also more gentle than the latter.Therefore,the antioxidation properties and thermoelectric properties of the R type thin-film thermocouple are better than that of the S type,and the thermal electromotive force of which has better linearity and repeatability.Secondly,preparating noble metal thin-film thermocouple on Al₂O₃ ceramic substrate,with static calibration method,this paper studies on the improvement of antioxidation properties and thermoelectric properties of noble metal thermocouple caused by different thickness of functional layer and Al₂O₃ protective layer.Results show that,appropriately increasing the Pt/PtRh functional layer thickness can improve the antioxidation properties and thermoelectric properties of noble metal thin-film thermocouple,but when the thickness is more than 1?m,the effect is not significant.Therefore,the appropriate value of the thickness of the functional layer of noble-metal thin-film thermocouple is 1?m.Finally,preparating type R noble-metal thin-film thermocouple on Ni-based superalloy,with static calibration method,this paper studies on the improvement of insulating property and thermoelectric properties of noble-metal thin-film thermocouple caused by different thickness of Al₂O₃ insulating layer.Results show that,when the thickness of the Al₂O₃ insulating layer is about 3?m,part of the composite insulation layer between the thin-film thermocouple and the Ni-based superalloy is connected.But when the thickness of the Al₂O₃ insulating layer is 6?m,the resistance of the composite insulation layer is more than 104? during the whole calibration process.Besides,according to the outputs of thermal electromotive force of the thin-film thermocouples,there is no big difference betwteen the 6?m and 9?m samples.Hence the appropriate thickness of Al₂O₃ insulating layer is 6?m.