The porosity measurement of gas-thermal coatings with the laser optoacoustic method

The porosity is one of the basic properties of gas-thermal coatings and serves as the monitored factor of spraying process quality, and for some types of antifriction, corrosion-proof and heat-resistant coatings the mandatory control of porosity is required. For quantitative evaluation of porosity the method of hydrostatic weighing is usually applied, that is quite attractive due to relative simplicity and availability of testing equipment. However because of the possible systematic errors of this method there is the necessity to develop the alternative methods of porosity measurements of gas-thermal coatings. In the present work the laser optoacoustic method is proposed and realized experimentally for determination of porosity (volume fraction of pores) of ceramic (aluminum oxide) and metal (self-fluxing alloys) coatings manufactured by plasma gas-thermal spraying on a steel substrate. The technique is based on laser thermooptical excitation of probe ultrasonic pulses, the measurement of the phase velocity of longitudinal acoustic waves in a coating sample impregnated with an immersed liquid (distilled water, ethanol or kerosene). The porosity is calculated with the theoretical model of ultrasound propagation in a two-phase medium with the assumption that the volume concentration of the immersed liquid in the impregnated coating coincides with the volume fraction of pores in the 'dry' coating. The thickness of coatings is 45÷540 microns, the measured porosity values are in the range 1-23 %, the best relative accuracy of measurements is 8-10%. The experimental results coincide within inaccuracy limits with the hydrostatic weighing data. Therefore the laser optoacoustic method can be useful for operative and reliable monitoring of porosity of products with gas-thermal coatings during their manufacturing and operation activity. (authors)