Research On Thin Film Strain Gauge Integrated With Aeroengine Turbine Blade

The next generation of aeroengines is developing towards high thrust-weight ratio,long life,and reduced fuel consumption.The internal working temperature is close to the critical operating temperature of high temperature components,such as turbine blades and rotating shafts.As servicing time increases,the blades may have fatigue cracks and even fractures,which may cause breakdowns in aeroengines.Monitoring the stress/strain of blades and other high temperature components can effectively detect their fatigue damages and early warning of engine failures.Therefore,it is vital to develop a stable and reliable strain measuring sensor,which can work in extreme environments involving high temperature,high stress and strong vibration.As an advanced surface strain test technology,thin film strain gauges?TFSGs?integrated with aeroengine turbine blades should be light,good accuracy,fast response,and only a few micrometers thick,meanwhile they have less disturb to the surface air flow and even maintain the integrity of the mechanical properties of blades.In order to satisfy the strain measurement of blades in harsh environment,such as high temperature,high stress and strong vibration,the multilayered TFSGs including graded buffer layer,insulating layer,strain sensitive element and protective layer are deposited by physical vapor deposition technology.This dissertation breaks through the technical difficulties of structure matching and thermal matching technology,the complex surface patterning preparation technology,the high temperature insulation and protection technology through the architecture design and processes optimization of the multilayered structure.Additionally,the resistance-strain response performance of TFSGs at high temperature was investigated systematically via the calibration technique.Finally,the high cycle vibration strain characteristics of a certain type of engine turbine blade were studied by using TFSG.Therefore,this dissertation provides an advanced strain measurement technology for aeroengine turbine blade test and fault diagnosis,which may make a contribution to the development of high-performance aeroengines.The specific research contents are as follows:?1?The segregation and oxidation processes of NiCrAlY film were studied,and then the graded buffer layer from metal to oxide was realized,to improve the structural and thermal matching between Ni-based superalloy substrate and multilayered TFSG.The graded buffer layer could effectively enhance the adhesion strength between Ni-based superalloy substrate and TFSG.Moreover,the dense thermal grown oxidation?TGO?layer formed by thermal oxidation is beneficial to improve the electrical insulation between the TFSG and superalloy substrate.?2?The multilayered oxide films insulating layers were fabricated to improve high temperature insulation performance.Firstly,the single-layered Al₂O₃ film,bilayered YSZ/Al₂O₃ film and four-layered YSZ/Al₂O₃/YSZ/Al₂O₃ film were investigated.The superior electrical insulation performance of the insulation resistance about 200 k?at 800?are observed in the four-layered YSZ/Al₂O₃/YSZ/Al₂O₃ films structure.There are many defects such as grain boundary in the single oxide insulating layer and the ionic conductive channels are easily formed at high temperature,which would result in the decrease of insulation resistance.While in multilayered insulating layers,the introduced insulating layer interface can effectively block ion conduction channels and promote high-temperature insulation performance,and thus the insulation performance gradually improved by increasing layer interfaces.Besides,the MgO films were employed as insulting materials to form the TGO/MgO double insulating layer,which showed an excellent insulation performance in the temperature range of 25-1000?.The insulation resistance of this structure could reach up to 1.5 M?at 1000?.The low conductivity at room temperature and the low conductivity temperature dependence of TGO/MgO insulting layer contribute to the improved insulation characteristic at high temperature.After four cycles?holding at 1000?for 2 hours in each cycle?,the insulation resistance still remaines 0.55 M?at 1000?,which showed good high temperature stability and would meet the demand of the electrical insulation of TFSGs at higher temperatures.?3?The effect of thickness on the square resistance,temperature coefficient of resistance,resistivity and the stability of PdCr film as strain sensitive layer were discussed.It was found that the electrical properties of PdCr films tended to be stabilized with the increase of thickness,as a result of relatively fewer dislocations and other structure defects,which would weaken the stability of the high temperature performance,in the thicker films had.Moreover,the thicker PdCr films were treated in the vacuum to further reduce the resistance drift and improve the stability of PdCr film at high temperature.The results showed that the PdCr film has better crystallinity after vacuum heat treatment,and thus obtaining a better stability in high temperature environment.The average resistance drift rate of PdCr films decreased from-0.126%/h to-0.065%/h at 800?,which was only a half of the untreated sample.?4?The protective performance of the heterolayered Al₂O₃-ZrO₂/Al₂O₃,monolayer Al₂O₃ and composite Al₂O₃-ZrO₂ overcoat were analyzed to improve the oxidation resistance of strain element at high temperature.The heterolayered Al₂O₃-ZrO₂/Al₂O₃ showed an outstanding protective property than the remaining two overcoats,which can attribut to the dense,stable structure of the Al₂O₃ and Al₂O₃-ZrO₂films,and the interfacial potential barriers further blocked the defect channels resulting from the homogeneous films to suppress the permeation of oxygen atoms.?5?The gauge factor?GF?of PdCr TFSG was calibrated by static strain test system.The resistance of PdCr TFSG was linearly related to strain,and the GF exhibited excellent repeatability.Meanwhile,the GF was linearly increased with the temperature,which was 1.78 and 2.13 at room temperature and 800?,respectively.?6?The PdCr TFSG was fabricated on a certain type of turbine blade,and the high temperature and high cycle dynamic strain of the turbine blade was studied.And the dynamic strain analysis method based on frequency domain processing was established.The results displayed that PdCr TFSG had a fast response,high working temperature and excellent reliability,which would meet the requirements of operating temperature25⁸00?,vibration accelerations 0¹0 g,variation frequency 0¹700 Hz.?7?The reliability assessment of PdCr TFSG was carried out by the thermal shock test and the altitude simulation test.The structure of PdCr TFSG remained intact without obvious phenomenon of cracking and peeling after multiple cycles and high intensity thermal shock.The results suggested that PdCr TFSG had a strong bonding strength with turbine blade,which exhibits promising reliability at high temperature,high pressure and high speed environment of aeroengine.?8?The strain sensitivity of 90°PdCr thin film strain rosette,which could measure the magnitude and direction of strain,was preliminarily studied.Moreover,the indium tin oxide?ITO?TFSG with higher GF was fabricated,and the fabrication processes of ITO strain sensitive material were researched,but the stability of ITO TFSG at high temperatures was poor,requiring further improvement.