Researchs Of Preparation Methods Of High Temperature Insulating Coatings Based On MEMS Technology

With the development of the aviation engine,the measurements of key parameters also put forward higher requirements.Because the aviation engine is running at high temperature,monitoring engine operating status at high temperature is significant for the safety and reliability.Thin film sensors are thought to be a superior choice compared to conventional wire and foil sensors since they are directly deposited onto the test part with a micrometers scale,which minimized the disturbance of the operating propulsion system.The aviation engine blades are typically metal-based superalloy,a high quality insulator between the substrate and the sensor is required in order to integrate thin film temperature sensor and high temperature strain sensors directly on the engine blades,therefore the development of reliable high temperature insulation layer becomes one of the key processes in thin film sensors.Thus preparation of high temperature insulation layer with good insulating properties on a superalloy substrate is common problem in domestic and foreign research institutions.The non-silicon MEMS technology which can integrate different functional materials provides a new technical approach to solve this problem.In this thesis,two preparation methods were investigated,one method is thermal oxidation plus sputtered aluminum oxide coating for high temperature electrical insulation,and the other is plasma sprayed aluminum oxide on MEMS microstructures which can strengthen the adhesion force.In order to deal with the issues such as adherence and integrity of oxide layer in thermal oxidation of NiCoCrAlY coatings plus sputtered aluminum oxide,the thermal oxidation process was optimized.In our study,the effects of different thermal oxidation conditions on morphology and composition of the thermal oxide layer were discussed.And the results showed that heat treatment at 950?for 4h in argon atmosphere help Al element diffuse toward the samples surface.And with the extension of the thermal oxidation time,more-Al₂O₃ was formed on the surface and the surface became rougher.When the thermal oxidation time was more than10h,NiO would appear in thermal oxide layer which seriously undermined the high temperatures insulating property.Through comparing the test results it was found that the thermal oxide layer after 10h thermal oxidation was composed of high-purity-Al₂O₃,the thermal oxide layer plus sputtered 2?m alumina had optimal insulating property at high temperature,the resistance was 6.4k?at 800?.Thin film resistance senor was prepared on the surface of the insulating layer,and the sensor could work normally in the range of 100-800?.However,this preparation method was complex and cost long time,and the high temperature insulating resistance is not adequate enough.Another method was proposed which is plasma sprayed aluminum oxide on MEMS microstructures which can strengthen the adhesion force.Through micromachining square pillar array on substrate surface,it could increase the binding force and reduce thermal stress.The effect of different dimensions of the micro-array configuration on thermal stress was examined and the difference between the new preparation method and the traditional method of sandblasting preparation was compared using COMSOL software.The simulation results showed that when the groove width between the micro-pillars was 500?m,the thermal stress at the interface was relatively minimal,and the thermal stress was smaller than that of conventional sandblasting.The final experimental results showed that high-temperature insulating layer produced by this method shows good thermal stability and insulating property at high temperature.When the coating thickness was 300?m,the insulating resistance was 281.32k?at 1000?.Thin film resistance senor was prepared on the surface of the insulating coating,and the sensor can work normally in the range of100-1000?.The insulating layer formed by this method has better insulation performance and this method can simplify the process.