A Study On Acoustic Impedance Matching Criteria For Coating Thickness Measurement By Ultrasonic Interferometry

Thickness is an important quality indicator of coating,which closely relates to the service life of the coating,the consumption of material,stress,combined strength and cost.At present,ultrasonic interferometry method has become one of the hot researches as it is not limited by the type and thickness of coating.There are many kinds of materials of substrate and coatings,and the acoustic impedance difference between them has a direct influence on signal spectrum.So the principles of coating thickness measured by ultrasonic interferometry and the model of ultrasonic propagating in layered medium are investigated.Then the acoustic impedance matching criterion of ultrasonic interferometry for coating thickness measurement is proposed,and the experimental verification on the criteria is performed.In addition,the influencing factors,such as the number of reflection echoes received by probe n, ultrasonic incidence angleθ₁,frequency f and coating thickness d,of aluminum reflection coefficient are investigated.The main results are as following:(1) In this paper,the criteria of choosing ultrasonic echoes,including or excluding first echo P₁,is proposed when the interfacial reflection coefficient between the coating and its adjacent medium meets |r₁₂|＜1/2^1/2or|r₁₂|＞1/2^1/2,respectively.Then,the law that secondary maxima and minima occurs in the reflection coefficient spectrum when |r₁₂|＜1/2^1/2or|r₁₂|＞1/2^1/2 is analyzed.Base on the law,the minima and maxima are chosen respectively in thickness measurement when |r₁₂|＜1/2^1/2or|r₁₂|＞1/2^1/2.In order to verify the principle and its effectiveness,the Airborne Radome Skin coating and ZrO₂ ceramic coating sprayed on steel substrate are chosen for thickness measurement while the coating and its adjacent medium meets |r₁₂|＜1/2^1/2or|r₁₂|＞1/2^1/2,respectively.Comparing with the SEM (Scanning Electron Microscope) results,the maximum mean relative error is less than 10%. The experimental results meet the need of engineering.(2) The influencing factors,such as the number of reflection echoes received by probe n,ultrasonic incidence angleθ₁,frequency f and coating thickness d,of aluminum reflection coefficient are investigated.With increasing the value of n,the secondary extreme of reflection coefficient spectrum decreases,the spectrum become more smooth and extreme are easy to obtain.Whenθ₁ is greater than 13°,total reflection is occured.Whileθ₁ is less than 13°,the reflection coefficient spectrum sharply reduced at certain incidence angle with the increase of incidence angle.This brings significant deviation to the thickness measurement results.In the range of 0-25MHz,the phenomenon of cyclical rapid emergence to the reflection coefficient spectrum is observed with the increase of f,and the changes are more sensitive in the range of high frequency whenθ₁ dose not equal to zero.Maxima and minima of reflection coefficient spectrum occur cyclically with the increase of d.The influence of d on the spectrum becomes smaller when the coating is thinner.In addition,the best coating and its adjacent acoustic impedance matching situation can be obtained by choosing coupling medium through theoretical calculations.So it can provide theory and experiment basis for practical engineering applications.It is concluded that the acoustic impedance matching criteria are of great value in the engineering applications.