Non-contact And Nondestructive Testing Of Coating Thickness By Using Infrared Thermography

The coating can add additional properties such as stealth,high temperature resistance and antifouling to the substrate without affecting the performance of the substrate material.Coatings occupy an increasingly important position in modern industry.Currently coating preparation process and service environment make the coating have defects such as uneven thickness,pores,debonding and delamination after production or use,which directly affects the performance of the coating.Therefore,the detection of coating thickness becomes particularly important.Infrared thermography has the advantages of non-contact,large-area imaging,and fast detection speed,and has great potential in the non-destructive testing industry.In this paper,the changing law of the coating surface temperature and the relationship between the coating thickness and surface temperature under different excitation methods are studied theoretically and experimentally.Finally,the non-contact measurement of coating thickness is realized.Based on the classical heat conduction equation,the physical model of the pulsed infrared heating for translucent coatings is established and solved by the mathematical method and finite element method.Theoretical and numerical results show that the thickness of the translucent coating is linearly proportional to the peak time of the surface temperature in logarithmic coordinates.The higher the transmittance of the translucent coating,the higher the degree of adhesion of the linear relationship between the coating thickness and the peak time of the surface temperature on the logarithmic coordinate,and the greater the coating thickness range that conforms to the linear relationship;The thickness of the substrate does not affect the linear relationship between the peak times of the surface temperature and the thickness of the coating on the logarithmic scale.The separation time of the surface temperature feature and the substrate thickness also shows a linear relationship on the logarithmic coordinates;The greater the thermal conductivity of the base material,the earlier the peak surface temperature and the lower the peak surface temperature.Therefore,a method for measuring the thickness of the translucent coating based on the peak time of the surface temperature is proposed.A translucent coating specimen with continuously varying thickness was fabricated and tested with a flash thermographic system.The experimental results show that the thickness of the translucent coating and the peak time of the surface temperature present a linear relationship on the logarithmic coordinate.Under the current experimental conditions,the measurement range of the thickness of the translucent coating based on the peak time of the surface temperature is 80μm to 220μm.The result of the measured thicknesses is within 5%of the actual values.Based on the classical heat conduction equation,the physical model of the laser scanning infrared heating for coatings is established and solved by the mathematical method and finite element method.Theoretical and numerical results show that the time variation curve of the opaque coating surface temperature and the space variation curve of the coating surface temperature have an equivalent relationship within a certain range;The thickness of the opaque coating is monotonous relationship to the peak time of the surface temperature in logarithmic coordinates;The thickness of the substrate does not affect the peak time of the surface temperature;Coating material and substrate material affect the peak time of the surface temperature;The laser scanning speed affects the application range of the monotonic relationship between the coating thickness and the peak time of the surface temperature.The slower the scanning speed,the larger the application range.Therefore,a method for measuring the thickness of opaque coatings based on the peak surface temperature is proposed.A laser scanning infrared non-destructive testing system was established and specimens with different coating thicknesses were fabricated.Taking into account the frame rate limit of the thermal imager,the surface temperature spatial variation curve is used instead of the time variation curve in the experiment.The experimental results show that the thickness of the opaque coating and the peak position of the surface temperature present a monotonic relationship in the logarithmic coordinate.Under the current experimental conditions,the laser scanning speed is fixed at 3mm/s,the measurement range of the thickness of the opaque coating based on the peak position of the surface temperature is 50μm to 400μm.The result of the measured thicknesses is within 10% of the actual values.The research results can provide theoretical and experimental reference for coating thickness detection based on infrared thermography.