Method for non-destructive evaluation of ceramic coatings
A method for evaluating the condition of a ceramic coating deposited on a substrate comprising illuminating the ceramic coating with light, measuring the intensity of light returned from the ceramic coating as function of depth in the coating and transverse position on the coating, and analyzing the measured light intensities to obtain one or more of intensity of the light returned from the exposed coating surface relative to the intensity of light returned from the coating/substrate interface, intensity of the light returned from the coating/substrate interface relative to the intensity of light returned from the bulk of the ceramic coating, determination of roughness at the exposed surface of the ceramic coating, and determination of roughness of the interface between the ceramic coating and underlying bond coat or substrate.
1. A method for evaluating the condition of a ceramic coating deposited on a substrate, the method comprising the steps of:
illuminating the ceramic coating with light;
measuring the intensity of light returned from the ceramic coating as function of depth in the coating and transverse position on the coating by utilizing low coherence interferometry; and
analyzing the measured light intensities to obtain one or more of the group consisting of:
intensity of the light returned from the exposed coating surface relative to the intensity of light returned from the coating/substrate interface;
intensity of the light returned from the coating/substrate interface relative to the intensity of light returned from the bulk of the ceramic coating;
determination of roughness at the exposed surface of the ceramic coating; and
determination of roughness of the interface between the ceramic coating and underlying bond coat or substrate.
2. The method of claim 1 wherein the measuring step utilizes optical coherence tomography.
3. The method of claim 2 wherein the measuring step utilizes Fourier domain optical coherence tomography.
4. The method of claim 2 wherein the measuring step utilizes a spectral domain optical coherence tomography instrument and the illuminating step utilizes a broadband light source.
5. The method of claim 2 wherein the measuring step utilizes a balanced photodetector and the illuminating step utilizes a swept source laser.
6. The method of claim 2 wherein the measuring step utilizes a time domain optical coherence tomography system with a scanning optical delay line.
7. The method of claim 1 wherein the analyzing step utilizes depth profiles with spatial resolution of about 25 μm or less.
8. The method of claim 1 wherein the analyzing step determines degree of microcracking of the coating.
9. The method of claim 1 wherein the illuminating step utilizes a broadband superluminescent diode.
10. The method of claim 1 wherein the measuring step utilizes a fiber optic interferometer.
11. The method of claim 1 wherein the measuring step utilizes an InGaAs linescan camera.
12. The method of claim 1 wherein the wavelength of the illuminating light is within the range of 1.5 μm to 2.2 μm.
13. The method of claim 1 wherein determination of roughness is according to ISO 4287-1997 Softgauge standard for arithmetic mean deviation (Ra), root mean square deviation (Rq), or tortuosity.
14. The method of claim 1 additionally comprising the step of providing an index-matching fluid of index of refraction greater than or equal to 1.6 and less than or equal to 2.0 to an outer surface of the ceramic coating.