Ultrasonic Characterization For The Pore Condition Of Plasma Sprayed Al/BN Abradable Seal Coating

High-thrust, high efficiency is the core objective of aircraft engine design. Abradable seal coating can ensure the efficiency of the engine while also greatly reducing the substrate temperature of the working end and the gas losses, now it has been widely used in aviation engines. Abrasion resistance, erosion resistance and thermal shock resistance is seal coating three key performance indicators, the pore condition of the internal coating influences these three indicators a lot.Accurate and reliable nondestructive testing methods porosity for seal coating become urgent in the field of engineering requirements. Porosity, pore size and pore distribution of Al/BN seal coating prepared by plasma spraying were studied by ultrasonic method. The principal component of this seal coating is aluminum, boron and a certain percentage of pores.The difficulty of characterization focused on two aspects:One is the thinner the abradable seal coating is the more serious the aliasing is, the old theoretical model doesn’t fit this situation very well, a new model should be created; the high porosity and the serious attenuation of the coating make the traditional time-domain signal is not feasible, improving echo amplitude becomes very important as well as effective characterization of attenuation.The ultrasonic propagation of the coating containing pores has been researched and analysised, besides a mathematical model has been established. Conclusions were obtained:Theoretically, expression of the sound pressure reflection coefficient of the power spectrum of the coating is given, both the change’s rate of speed of sound and sound attenuation relations with the porosity of coating have been established, and respectively, which has been tested and verified.Experimentally, Porosity information of the coating is obtained by metallographic experiments. Produce Al/BN abradable seal coating metallographic sample and obtain the microscopic morphology of the pore by SEM, detailed information of the pores. The overall porosity of the coating is21.04%, the part porosity change from16.20%to29.17%; Obtain the luminance image of the entire coating by using C-scan apparatus anastomosis, the results of the qualitative analysis shows that the luminance image and porosity data fits each very well. Finish the scan on the specified path (the cutting line in the process of sample preparation) and the spacing is0.5mm. Get the power spectrum of the received ultrasound signal to extract the desired parameters, and draw curves of the sound velocity change rate and attenuation. The results show that the rate of change of velocity of the coating’s sound increases with porosity, the amplitude of the power spectrum of the difference Δy decreases, which fits the theoretical model very well.In the simulation, create the original and situ model according to the metallographic photo and simulate, draw curves of the sound velocity change rate and attenuation. The results show the same law as before, which fits the theoretical model very well.