Research On Quality Of Thermal Barrier Coating By Ultrasonic Testing

With the improvement of gas turbine temperature, the thrust-weight ratio ofaviation turbine engines is enhanced. So the turbine blade which is working in hightemperature and high pressure environment needs thermal barrier coating to protect itnot being oxidated and corroded. Coating materials and substrate material havedifferent physical properties and thermal physical parameters, so some defects such ascracks, pore, weak bond of interface and uneven thickness may appear during thepreparation of coating, the life and reliability of turbine blade are affected by quality ofcoating.In the paper, on the basis of analysis of acoustic propagation characteristics ofthermal barrier coating and ultrasonic testing principle of water extraction focus andmeasurement principle of coating thickness by spectrum analysis, method of ultrasonicpulse reflecting with water extraction focus, ultrasonic pulse launch and receivingapparatus of OLYMPUS5077and oscilloscope of TDS3032were used to acquireultrasonic signals, and combined with ultrasonic microscope and spectrum analysis ofultrasonic signal to evaluate coating quality.The coatings of specimens are ceramic coatings which contain Al2O3and SiO2,andthe substrates are ni-based high-temperature alloy and stainless steel, and the thicknessof coatings ranged from20to140μ m.In the paper, method of spectrum analysis ofultrasonic signal were used to measure the thickness of coatings with nondestructive,the results of the study show that, frequency interval between two maximum value canbe got from phase spectrum of ultrasonic echo signal, and the thickness value ofcoatings can be got from the quotient of sonic speed and two times frequency interval.Compare the thickness values with measurement values of SEM, it can be known fromthe comparing results that the maximum absolute error is less than6micron and themaximum relative error is less than8percent, these are all meet the needs of project.Vickers hardness tester were used to manufacture surface crack and weak bonding area，and the loading parts were observed by SEM. The results show that coatings whichcontain more brittle material of Al2O3are easier to crack, and have weaker bonds withsubstrate, substrates of ni-based high-temperature alloy have better bonds with the samecoating than substrates of stainless steel. The surface damage of5kg and10kg loading position can not be detected by ultrasonic microscope with water extraction focusingprobe of which the frequency is100MHz, but20kg load position and above can bedetected. It is known from the waveform amplitude distribution of different loadingposition that the larger the load, the badder the surface damage, and the lower the signalamplitude of interface echo and bottom echo, the coating material which contains moreAl2O3has higher brittleness, and has weaker bond with substrate after loading.In the paper, coating thickness can be measured by spectrum analysis of ultrasonicsignal, surface damage and bonds between coatings and substrates can be qualitativecharacterized by method of signal amplitude of ultrasonic microscope. These methodsand results provide new direction for quality study of coating.