Research On Fabrication And Properties Of High-temperature Composite Insulating Layer On Metal Substrate

Electrical insulation of nickel-based superalloy substrate,especially at high temperature range,is one of the major challenges for the reliability and stability of the integrated thin film sensors.Thin-film sensor technology is an advanced measurement technique for the measurement of working conditions of aero-engines.Thin-film sensors fabricated by this technique usually have a multilayer film structure.From the bottom to the top,Ni-based alloy substrate,NiCrAlY buffer layer,thermally grown Al₂O₃ layer,insulating layer,sensitive functional layer,and protective layer are sequentially arranged.As a transitional insulating layer,its performance is directly related to the performance of the thin film sensor.This article mainly aims at improving the high-temperature insulation between Ni-based superalloy substrate and thin-film sensor.Firstly,the influence of the thickness of Ni CrAl Y buffer layer and Al₂O₃ thermal growth layer on insulation performance was studied.Then,starting from the preparation method and system structure of the insulating layer,the high-temperature insulating properties,adhesion,thermal shock resistance and thermal fatigue resistance of the insulating layer were mainly studied.Firstly,NiCrAlY transition layers with different thicknesses?24⁴2?m?were prepared on two groups of nickel-base alloys by polishing and sandblasting using DC magnetron sputtering technology.Then,they were vacuum annealed and oxidized at high temperature,and the Al₂O₃ thermal growth layer was obtained after the procedure was completed.It shows that the longitudinal resistance of the two groups of samples at room temperature is only a thousand euros in the insulation test,and the insulating performance of the polished substrate is better than that of the sandblasted substrate,but all the insulating layers are conducted at 800°C.Then,an Al₂O₃ layer with a thickness of 2?m was prepared on the samples surface by electron beam evaporation and annealed at 800°C for 2 hours.The insulation resistance of all samples at room temperature was greater than 10 M?,but when the test temperature reached 800°C,the resistance of the samples was reduced to 1K euros in the insulation test with little difference.The two results show that the thickness of the Ni CrAl Y transition layer has little effect on the insulating properties of the Al₂O₃ thermal growth layer.In order to improve the high temperature insulation performance,a multilayer Al₂O₃ thin films were prepared on the nickel-based superalloy substrate after oxidation of aluminum by sol-gel method.And two types of composite insulating layers were prepared on the nickel-based superalloy substrate.After determining the preparation process of the Al₂O₃ film by sol-gel method,two kinds of composite insulating layers were successively prepared on the nickel-based alloy.One composite insulating layer was composed of an Al₂O₃ thermal growth layer and a solution processed Al₂O₃ thin film layer.Another one was with a sandwich structure composed of an Al₂O₃ thermal growth layer,an Al ON amorphous layer and a solution processed Al₂O₃ layer.The high-temperature insulation test,adhesion test,thermal shock resistance and thermal fatigue test were performed on the insulating layers of these two structures.And the results showed that the composite insulation layers of the two structures all had good adhesion,thermal shock resistance and thermal fatigue properties.The former had a longitudinal resistance of 3.5 M?at 800°C and the latter reached 10 M?.Finally,in order to verify the high temperature insulation performance of composite insulation layer prepared by sol-gel method,an S-type thin film thermocouple was prepared on a nickel-based superalloy substrate for calibration verification.The calibration results showed that thermoelectric output of the S-type thin film thermocouple was of stable output,the average Seebeck coefficient is 8.0?V/°C,high sensitivity.And the sensitivity is as high as 0.8 to 0.83,the difference is only 3%,which shows good linearity and repeatability.And it can work stably for 30 hours in a temperature range of 300 to 900°C.These demonstrate the superior high-temperature performance of the composite insulating layer designed and prepared.