Systems and methods of assessing a coating microstructure

1. A system for use in inspecting a coating on a substrate, the system comprising:

a platform configured to receive a sample including the substrate having the coating deposited thereon;

a light source configured to direct a plurality of electromagnetic pulses towards a scanning location on the coating, wherein the light source is oriented to direct the plurality of electromagnetic pulses at an oblique angle relative to a surface of the coating;

a light detector configured to receive electromagnetic pulses reflected from the sample, wherein a first portion of each electromagnetic pulse is reflected from the surface of the coating at an interface between the coating and an ambient environment, and wherein a second portion of each electromagnetic pulse is reflected from a surface of the substrate at an interface between the coating and the substrate; and

an actuator coupled to at least one of the platform and the light source, wherein the actuator is configured to move the platform and the light source relative to each other such that the plurality of electromagnetic pulses are directable towards the scanning location from different rotational positions; and

a computing device in communication with the light detector, the computing device configured to:

determine a time delay between reception of the first portion and the second portion of each reflected electromagnetic pulse at the light detector, thereby defining a plurality of time delays; and

analyze the plurality of time delays to assess a microstructure of the coating by determining a standard deviation value for each of the plurality of time delays associated with each sample, grouping the plurality of standard deviation values based on relative differences in the plurality of standard deviation values, and assessing the microstructure of the coating of each sample based on the grouping.

2. The system in accordance with claim 1 , wherein the actuator is coupled to the platform, and is configured to rotate the platform relative to the light source.

3. The system in accordance with claim 1 , wherein the actuator is coupled to the light source, and is configured to rotate the light source about the platform.

4. A method of inspecting a coating on a substrate, the method comprising:

directing a plurality of electromagnetic pulses towards a scanning location on the coating, each electromagnetic pulse directed from a different rotational position relative to the scanning location, and each electromagnetic pulse directed at an oblique angle relative to a surface of the coating, wherein a first portion of each electromagnetic pulse is reflected from the surface of the coating, and wherein a second portion of each electromagnetic pulse is reflected from a surface of the substrate at an interface between the coating and the substrate;

determining a time delay between reception of the first portion and the second portion of each reflected electromagnetic pulse at a light detector, thereby defining a plurality of time delays; and

analyzing the plurality of time delays to assess a microstructure of the coating,

wherein analyzing the plurality of time delays to assess a microstructure of the coating comprises:

determining a standard deviation value for each of the plurality of time delays associated with each sample;

grouping the plurality of standard deviation values based on relative differences in the plurality of standard deviation values;

and assessing the microstructure of the coating of each sample based on the grouping.

5. The method in accordance with claim 4 , wherein analyzing the time delays comprises determining a standard deviation of the time delay for each of the plurality of electromagnetic pulses.

6. The method in accordance with claim 5 further comprising comparing the standard deviation to a predetermined threshold, wherein a first microstructure is determined when the standard deviation is greater than the predetermined threshold, and wherein a second microstructure is determined when the standard deviation is less than the predetermined threshold.

7. The method in accordance with claim 4 , wherein directing a plurality of electromagnetic pulses comprises directing the plurality of electromagnetic pulses having a wavelength in the terahertz frequency range.

8. The method in accordance with claim 4 , wherein directing a plurality of electromagnetic pulses comprises:

emitting the plurality of electromagnetic pulses from a light source; and

rotating at least one of the substrate and the light source.

9. The method in accordance with claim 8 , wherein rotating at least one of the substrate or the light source comprises positioning the light source within a rotational range defined between about 0 degrees and about 360 degrees relative to the scanning location.

10. The method in accordance with claim 4 further comprising:

providing a visual cue on the coating at the scanning location; and

directing the plurality of electromagnetic pulses towards the visual cue from each rotational position.

11. A method of assessing a coating microstructure, the method comprising:

providing a plurality of samples each having a substrate and a coating deposited thereon;

evaluating each sample with a plurality of electromagnetic pulses that are each directed from a different rotational position relative to each the respective sample;

obtaining time delay data associated with the plurality of electromagnetic pulses being reflected from the plurality of samples, wherein a time delay is defined by receiving a first portion and a second portion of each electromagnetic pulse reflected from each sample at different points in time; and

assessing a microstructure of the coating on the plurality of samples based on a comparison of the time delay data associated with the plurality of samples,

wherein assessing the microstructure of the coating on the plurality of samples based on a comparison of the time delay data associated with the plurality of samples comprises:

determining a standard deviation of the time delay data associated with each sample;

grouping the standard deviation values based on relative differences in the standard deviation values; and

assessing the microstructure of the coating of each sample based on the grouping.

12. The method in accordance with claim 11 , wherein grouping the standard deviation values comprises defining a first cluster and a second cluster, the first cluster and the second cluster being separated by a value that is greater than a maximum difference in standard deviation values contained in either the first cluster or the second cluster.

13. The method in accordance with claim 12 , the method further comprising wherein assessing the microstructure comprises:

determining the microstructure is anisotropic when the standard deviation of a sample is included in the first cluster; and

determining the microstructure is isotropic when the standard deviation of a sample is included in the second cluster.

14. The method in accordance with claim 11 , wherein evaluating each sample comprises directing the plurality of electromagnetic pulses towards a scanning location on the coating, each electromagnetic pulse directed at an oblique angle relative to a surface of the coating, wherein the first portion of each electromagnetic pulse is reflected from the surface of the coating, and wherein the second portion of each electromagnetic pulse is reflected from a surface of the substrate.

15. The method in accordance with claim 14 , wherein directing a plurality of electromagnetic pulses comprises directing the plurality of electromagnetic pulses having a wavelength in the terahertz frequency range.

16. The method in accordance with claim 14 , wherein directing a plurality of electromagnetic pulses comprises:

emitting the plurality of electromagnetic pulses from a light source; and

rotating at least one of the substrate and the light source.

17. The method in accordance with claim 16 , wherein rotating at least one of the substrate or the light source comprises positioning the light source within a rotational range defined between about 0 degrees and about 360 degrees relative to the scanning location.

18. The method in accordance with claim 11 further comprising:

providing a visual cue on the coating at a scanning location; and

directing the plurality of electromagnetic pulses towards the visual cue from each rotational position.