| The wear-resistant coating applied to the cylinder liner can improve wear, corrosion resistance and prolong service life. With the wide application of the wear-resistant coating in the cylinder liner, it is a key that how to test and evaluate coating performance, predict coating life, which restricts the further development of the coating technology. The traditional methods for testing coating performance focus on appearance, while testing for the local mechanical properties such as hardness, bonding strength are generally destructive, which has an effect on normal running of the active device. In addition, most of defects exist in the coating and interface at random, there are still not any mature evaluation methods and criteria to test such defects completely. Therefore this paper focuses on evaluating comprehensive performance of coating such as the physical properties, defects and the critical load based on non-destructive testing.Taking a planar coating and a cylinder wall coating as the research objects, the method based on ultrasonic technology and acoustic emission technology is proposed for testing comprehensive properties of wear resistant coatings including the properties of the coating and interface, damage mode under applied loading. Fist of all, two mathematical models are established for evaluating the physical properties of coating based on frequency spectrum of ultrasonic echo. Next, based on detection mathematical model ultrasonic eigenvalue can be obtained. The relationship between ultrasonic eigenvalue and the coating properties and interface quality are built for the rubber coating and plasma spraying coating respectively. Finally, the coating damage mode is investigated under the simulative loading exerted by the self-designing radial expansion device. The relationship between the characteristic parameters of acoustic emission and different damage mode of the coating is established through recognizing acoustic emission parameters and signals according to wavelet energy theory.Taking simulative coating as the research object, models of the time delay spectrum and transfer function for evaluating character parameters of the coating are established by ultrasonic echo propagating in the coating and the substrate, from which the parameters relative to the coating such as ultrasonic propagation velocity, coating density, attenuation coefficient and reflection coefficient are obtained. Ultrasonic testing results for the rubber coating show that the attenuation coefficient increases with the increase of frequency. During ultrasonic propagating in the rubber coating acoustic beam diffusion, scattering, and media absorption leading to sound pressure diminishes. According to analyze time delay spectrum and transfer function obtained by two different sensors, the sawtooth distortion appears in frequency 2.2-2.4MHz by sensor of center frequency 1 MHz, the smoothing appears by sensor of center frequency 1.5 MHz.Ultrasonic testing results for the rubber coating with different bonding interfaces show that as regards to the reflection coefficient for the different bonding interface, the perfect interface is the lowest and weak interface is in the second place, and debonding interface is the highest. This is mainly because that for the perfect interface the reflection and transmission of the ultrasonic energy relate to physical properties of the substrate and the coating. While for the weak or debonding interface the reflection and transmission of the ultrasonic energy depend on not only physical properties of the coating and substrate, but the interfacial bonding. In addition, the reflection coefficient increase and sound velocity decrease with the increase of the porosity of the plasma spraying coating. Thereby, the reflection coefficient can be used to evaluate the coating character and reflect its quality. An ultrasonic C scan imaging system is designed for testing the cylinder wall coating interface based on ultrasonic spectrum evaluating the coating characteristics results which consists of the mechanical gearing, computer control and ultrasonic technology, and improves the automation of electromechanical control and testing speed. According to ultrasonic time delay spectrum theory the reflection coefficient can be obtained using time domain signal collected by ultrasonic immersion C scanning system. The reflection coefficient is employed to plot C scan image, which is able to characterize the bonding quality of the cylinder wall coating.A loading device is developed for simulating the damage of cylinder liner wear-resistant coating. The loading device which assembles acoustic emission equipment simulates damage process of the coating under the applied load and collects acoustic emission parameters and signal simultaneously. Acoustic emission testing results shows that for the coating with chromium plating process acoustic emission parameters such as counts, energy, and impact increase with the applied load increasing. In addition, based on acoustic emission recognition technology of the coating damage mode, there exits greater difference between the energy signals of the normal friction and coating damage through wavelet theory that is employed to analyze acoustic emission signal. The energy signal for normal friction mainly concentrates on the low frequency, energy signal of coating damage concentrates on the medium and high frequency. The simulation device is small, and has rapid detection, high accuracy.Under the simulative applied load of 54MPa the scanning test by the ultrasonic C scan system for chromium plating coating with thickness of 80μm,130μm and 180μm, respectively were conducted. The results show that the number of defects increases with the coating thickness increasing, it is because that cohesive strength and interface bonding strength for the thin coating is higher than those for thick coating. Comparing between acoustic emission and ultrasonic C scanning, it is concluded that both methods can be employed to locate and identify defects in the coating, the later is more accurate.
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