Patent Description:
Some components, such as hot gas path components of gas turbines, are subjected to high temperatures while in service. At least some such components include a coating system, including a thermal barrier coating and bond coat, on an exterior surface exposed to the high temperatures. The microstructure of many known thermal barrier coatings is dependent on the process parameters of the coating application process. For example, thermal barrier coatings made of the same material may nevertheless have varying microstructures due to variations in the process parameters of the respective coating application processes. Some microstructures are effective at protecting components from exposure to high temperatures, while other microstructures may have a comparatively reduced effectiveness and shorter service life. However, at least some known methods for determining the microstructure of a particular coating are time-consuming and expensive to implement.

The article of<NPL>, discloses a fast terahertz reflective confocal scanning imaging system employing a fast terahertz quantum-well photodetector (QWP) and a fast rotating translational platform. The terahertz QWP can rapidly detect the THz radiation generated from a pulsed electrically pumped terahertz quantum cascade laser, which is used as the terahertz source of the imaging system, achieving a lateral resolution better than <NUM> and an axial resolution of about <NUM>. The terahertz two-dimension images of some objects are obtained within <NUM> with a high contrast. The three-dimensional sections of the object demonstrate the great axial selectivity of the terahertz imaging system.

The utility model <CIT> discloses a non-contact terahertz multi-coating thickness detection device, which comprises a rack, a pulsed terahertz source, a terahertz camera, a control and signal processing module and a computer, wherein the pulse terahertz source and the terahertz camera are fixedly arranged on the lower surface of an upper working platform, and the upper working platform is arranged on the rack. The pulse transmitting direction of the pulse terahertz source and the receiving direction of the terahertz camera are arranged symmetrically with respect to the normal on the upper working platform, and a translation rotating mechanism is arranged between the rack and the upper working platform. The non-contact terahertz multi-coating thickness detection device can provide real-time detection of multiple coatings and does not scratch the coating.

The publication of <NPL>, describes nondestructive quality inspection with terahertz waves, especially in the automotive and aviation industries. Depending on the specific application, different terahertz systems - either fully electronic or based on optical laser pulses - cover the terahertz frequency region from <NUM> THz up to nearly <NUM> THz and provide high-speed volume inspections on the one hand and high-resolution thickness determination on the other hand. Different industrial applications are presented, namely three-dimensional imaging of glass fiber-reinforced composites and foam structures, and thickness determination of multilayer plastic tube walls. Then, the characterization of known and unknown multilayer systems down to some microns and the possibility of measuring the thickness of wet paints is described.

<CIT> discloses a terahertz time-domain spectroscopic ellipsometry system including a sample stage, a terahertz emitter configured to provide pulses of terahertz radiation with preselected polarization components to illuminate a sample on the sample stage along an incident direction, and a coherent terahertz detection system arranged to coherently detect pulses of terahertz radiation from the terahertz emitter along an emerging direction after at least one of reflecting from or passing through the sample. The sample stage is rotatable to vary a relative angle between the incident direction and the emerging direction, and the coherent terahertz detection system substantially maintains alignment for amplitude and polarization detection as the relative angle is varied.

The present invention is defined in claims <NUM> and <NUM>. In one aspect, a system for use in inspecting a coating on a substrate is provided. The system includes a platform configured to receive a sample including the substrate having the coating deposited thereon, and 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 is 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, and a second portion of each electromagnetic pulse is reflected from a surface of the substrate. An actuator is 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.

In another aspect, a method of inspecting a coating on a substrate is provided. The method includes 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. A first portion of each electromagnetic pulse is reflected from the surface of the coating, and a second portion of each electromagnetic pulse is reflected from a surface of the substrate at an interface between the coating and the substrate. The method also includes assessing 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 time delays to assess a microstructure of the coating.

In yet another aspect, a method of assessing a coating microstructure is provided. The method includes 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 the respective sample, and obtaining time delay data associated with the plurality of electromagnetic pulses being reflected from the plurality of samples. 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. The method also includes 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.

Embodiments of the present disclosure relate to non-destructive examination of single or multi-layer coating structures deposited on substrates. In the exemplary embodiment, the inspection system described herein includes a platform that receives a sample thereon, a light source for directing electromagnetic pulses towards the sample, and a light detector for receiving electromagnetic pulses reflected from the sample. The electromagnetic pulses directed towards the sample have a wavelength in the terahertz frequency range. At this frequency range, the electromagnetic pulses are partially reflected from the air/coating interface at a surface of the coating on the sample, and the rest of the wave travels through the coating and is reflected from the coating/substrate interface. A time delay realized as a result of the difference in travel length of the reflected electromagnetic pulses facilitates estimating a thickness of the coating as well as its refractive index.

In birefringent materials, the refractive index is dependent on the direction of propagation of an incident beam. Birefringent materials are anisotropic in nature (i.e., they have a defined crystal structure periodicity). In the inspection system described herein, the platform and the light source are moved or rotated relative to each other to enable time delay data to be obtained with the light source positioned at different rotational positions relative to a scanning location on the coating. In many known thermal barrier coatings, two different types of microstructures are typically observed. One is a columnar type, which is more ordered and anisotropic in nature. The other is a cauliflower type, which is more random in structure (i.e., isotropic). It is expected that the time delay will vary in anisotropic materials, and that the time delay will be largely consistent in isotropic materials, as the sample is inspected from the different rotational positions. Using the time delay data obtained from the inspection process, the amount of birefringence contained within a particular sample is determinable, thereby enabling the type of microstructure to be identified in a fast, efficient, and non-destructive manner.

<FIG> is a schematic illustration of an exemplary inspection system <NUM> in a first mode of operation. In the exemplary embodiment, inspection system <NUM> includes a platform <NUM>, a light source <NUM>, and a light detector <NUM>. Platform <NUM> receives a sample <NUM> to be inspected thereon. Sample <NUM> includes a substrate <NUM> having a coating <NUM> deposited thereon. Substrate <NUM> and coating <NUM> may be fabricated of any material that enables inspection system <NUM> to function as described herein. Example substrate materials include, but are not limited to, metallic materials that are nickel-based, cobalt-based, and the like. An example coating material includes, but is not limited to, a ceramic material.

In operation, light source <NUM> directs electromagnetic pulses <NUM> towards a scanning location <NUM> on coating <NUM>. Light source <NUM> is oriented to direct the plurality of electromagnetic pulses at an oblique angle Θ relative to a surface <NUM> of coating <NUM>. Accordingly, light source <NUM> is oriented to accurately obtain time delay data from potentially birefringent material having anisotropic microstructures, as described above. The plurality of electromagnetic pulses <NUM> have a wavelength in the terahertz (THz) frequency range defined within a range between about <NUM> THz and about <NUM> THz. In an alternative embodiment, electromagnetic pulses <NUM> have a wavelength in any frequency range that enables inspection system <NUM> to function as described herein.

Light detector <NUM> receives electromagnetic pulses <NUM> reflected from sample <NUM>. As will be described in more detail below, a first portion of each electromagnetic pulse <NUM> is reflected from surface <NUM> of coating <NUM>, and a second portion of each electromagnetic pulse <NUM> is reflected from a surface of substrate <NUM>. Inspection system <NUM> also includes a computing device <NUM> in communication with light detector <NUM>. Computing device <NUM> determines a time delay between reception of the first portion and the second portion of each reflected electromagnetic pulse <NUM> at light detector <NUM>. Computing device <NUM> may then analyze time delay, for electromagnetic pulses <NUM> directed towards sample <NUM> from different rotational positions, to determine a microstructure of coating <NUM>.

For example, at least one of platform <NUM> and light source <NUM> is coupled to an actuator <NUM>. Actuator <NUM> moves platform <NUM> and/or light source <NUM> such that the plurality of electromagnetic pulses <NUM> are directable towards scanning location <NUM> from the different rotational positions. More specifically, actuator <NUM> may be coupled to platform <NUM> for rotating platform <NUM> relative to light source <NUM>. Alternatively, or additionally, actuator <NUM> may be coupled to light source <NUM> for moving light source <NUM> about platform <NUM>.

As shown in <FIG>, actuator <NUM> orients sample <NUM> at a first rotational angle (i.e., <NUM> degrees) relative to a reference axis <NUM>. In operation, first time delay data is obtained when sample <NUM> is oriented at the first rotational angle, and actuator <NUM> then orients sample <NUM> at a second rotational angle α (e.g., <NUM> degrees) relative to reference axis <NUM>, as shown in <FIG>. Second time delay data is obtained when sample <NUM> is oriented at second rotational angle α, and actuator <NUM> then orients sample <NUM> at a third rotational angle β (e.g., <NUM> degrees) relative to reference axis <NUM>, as shown in <FIG>. Third time delay data is obtained when sample <NUM> is oriented at third rotational angle β, and the birefringence of coating <NUM> may be determined based on an analysis of the time delay data by computing device <NUM>.

Inspection system <NUM> also includes a scanning positioner <NUM> for providing visual positioning guidance on sample <NUM>, which facilitates ensuring the same scanning location <NUM> is evaluated by electromagnetic pulses <NUM> being directed towards sample <NUM> from the different rotational positions. Scanning positioner <NUM> may be any positioning device that enables inspection system <NUM> to function as described herein. In the exemplary embodiment, scanning positioner <NUM> is a laser projection device that provides a visual cue <NUM> on surface <NUM> of coating <NUM> at scanning location <NUM>. Accordingly, electromagnetic pulses <NUM> are directable towards the same location on coating <NUM> regardless of the relative rotational position of light source <NUM> to sample <NUM>.

<FIG> illustrate top and cross-sectional views of an exemplary first sample <NUM> that may be inspected with inspection system <NUM> (shown in <FIG>). In the exemplary embodiment, first sample <NUM> includes a coating <NUM> having a first microstructure type that is columnar, ordered, and thus inherently anisotropic in nature.

<FIG> illustrates test results obtained from inspection of first sample <NUM> (shown in <FIG>). In the exemplary embodiment, first sample <NUM> is inspected by directing a first electromagnetic pulse <NUM>, a second electromagnetic pulse <NUM>, and a third electromagnetic pulse <NUM> towards coating <NUM> (shown in <FIG>). First electromagnetic pulse <NUM> is directed when first sample <NUM> is oriented at the first rotational angle, as shown in <FIG>. First electromagnetic pulse <NUM> reflected from first sample <NUM> includes a first portion <NUM> received at a first point in time and a second portion <NUM> received at a second point in time. The difference in time that first portion <NUM> and second portion <NUM> are received defines a first time delay <NUM>. In addition, second electromagnetic pulse <NUM> is directed when first sample <NUM> is oriented at second rotational angle α, as shown in <FIG>, and third electromagnetic pulse <NUM> is directed when first sample <NUM> is oriented at third rotational angle β, as shown in <FIG>. A second time delay <NUM> is defined by a first portion <NUM> and a second portion <NUM> of reflected second electromagnetic pulse <NUM> received at different points in time, and a third time delay <NUM> is defined by a first portion <NUM> and a second portion <NUM> of reflected third electromagnetic pulse <NUM> received at different points in time.

<FIG> illustrate top and cross-sectional views of an exemplary second sample <NUM> that may be inspected with inspection system <NUM> (shown in <FIG>). In the exemplary embodiment, second sample <NUM> includes a coating <NUM> having a second microstructure type that is more random in structure when compared to the first microstructure type shown in <FIG>, and is thus more isotropic in nature than the first microstructure type.

<FIG> illustrates test results obtained from inspection of second sample <NUM> (shown in <FIG>). In the exemplary embodiment, second sample <NUM> is inspected by directing a fourth electromagnetic pulse <NUM>, a fifth electromagnetic pulse <NUM>, and a sixth electromagnetic pulse <NUM> towards coating <NUM> (shown in <FIG>). Fourth electromagnetic pulse <NUM> is directed when second sample <NUM> is oriented at the first rotational angle, as shown in <FIG>. Fourth electromagnetic pulse <NUM> reflected from second sample <NUM> includes a first portion <NUM> received at a first point in time and a second portion <NUM> received at a second point in time. The difference in time that first portion <NUM> and second portion <NUM> are received defines a fourth time delay <NUM>. In addition, fifth electromagnetic pulse <NUM> is directed when second sample <NUM> is oriented at second rotational angle α, as shown in <FIG>, and sixth electromagnetic pulse <NUM> is directed when second sample <NUM> is oriented at third rotational angle β, as shown in <FIG>. A fifth time delay <NUM> is defined by a first portion <NUM> and a second portion <NUM> of reflected fifth electromagnetic pulse <NUM> received at different points in time, and a sixth time delay <NUM> is defined by a first portion <NUM> and a second portion <NUM> of reflected sixth electromagnetic pulse <NUM> received at different points in time.

As shown in <FIG>, the anisotropic nature of coating <NUM> (shown in <FIG>) causes first time delay <NUM>, second time delay <NUM>, and third time delay <NUM> to each have different values. In addition, as shown in <FIG>, the isotropic nature of coating <NUM> causes fourth time delay <NUM>, fifth time delay <NUM>, and sixth time delay <NUM> to be substantially equal. Time delays <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> may be analyzed to assess the microstructures of coatings <NUM> and <NUM>. For example, computing device <NUM> (shown in <FIG>) may receive the values of time delays <NUM>, <NUM>, and <NUM>, and of time delays <NUM>, <NUM>, and <NUM>. Computing device <NUM> may then determine a standard deviation of the time delay values associated with each sample. In one embodiment, the microstructure of coatings <NUM> and <NUM> is determined by comparing the standard deviation associated with each sample to a predetermined threshold. A first microstructure is determined to be present when the standard deviation is greater than the predetermined threshold, and a second microstructure is determined to be present when the standard deviation is less than the predetermined threshold.

<FIG> illustrates test results obtained from inspection of a plurality of samples <NUM> using inspection system <NUM> (shown in <FIG>). As shown in <FIG>, eight samples <NUM> were fabricated and evaluated, but it should be understood that any number of samples <NUM> may be fabricated and evaluated that enables the method of assessing a coating microstructure to function as described herein. In the exemplary embodiment, each sample <NUM> was evaluated using the inspection system <NUM> as described above, and the microstructure of coating <NUM> on each sample <NUM> is determinable based on a comparison of the time delay data associated with each sample <NUM>. For example, a standard deviation of the time delay data associated with each sample <NUM> is determined for each sample <NUM>. The standard deviation values are then compared to each other and grouped based on relative differences in the standard deviation values.

In the exemplary embodiment, the standard deviation values of samples <NUM>, <NUM>, <NUM>, and <NUM> are greater than the standard deviation values of samples <NUM>, <NUM>, <NUM>, and <NUM>. A first cluster <NUM> and a second cluster <NUM> may be defined based on relative differences in the standard deviation values. For example, the difference between the highest and lowest standard deviation values contained in either first cluster <NUM> or second cluster <NUM> may be a first value, and the difference between the highest standard deviation value in first cluster <NUM> and the lowest standard deviation value in second cluster may be a second value. First cluster <NUM> and second cluster <NUM> are defined based on the first value being less than the second value. As such, the microstructure of coating <NUM> on each sample <NUM> is determined based on inclusion of associated standard deviation values in one of first cluster <NUM> or second cluster <NUM>. Thus, according to <FIG>, samples <NUM>, <NUM>, <NUM>, and <NUM> are determined to have an anisotropic microstructure, and samples <NUM>, <NUM>, <NUM>, and <NUM> are determined to have an isotropic microstructure.

Claim 1:
A system for use in inspecting a coating (<NUM>) on a substrate (<NUM>), the system comprising:
a sample (<NUM>) comprising a substrate (<NUM>) having a coating (<NUM>) deposited thereon;
a platform (<NUM>) configured to receive the sample (<NUM>);
a light source (<NUM>) configured to direct a plurality of electromagnetic pulses (<NUM>) towards a scanning location (<NUM>) on the coating (<NUM>), wherein the light source (<NUM>) is oriented to direct each of the plurality of electromagnetic pulses (<NUM>) at a different respective oblique angle relative to the scanning location (<NUM>), on a surface (<NUM>) of the coating (<NUM>);
a light detector (<NUM>) configured to receive electromagnetic pulses (<NUM>) reflected from the sample (<NUM>), wherein a first portion (<NUM>) of each electromagnetic pulse (<NUM>) is reflected from the surface (<NUM>) of the coating (<NUM>) at an interface between the coating (<NUM>) and an ambient environment, and wherein a second portion (<NUM>) of each electromagnetic pulse (<NUM>) is reflected from a surface (<NUM>) of the substrate (<NUM>) at an interface between the coating (<NUM>) and the substrate (<NUM>); and
an actuator (<NUM>) coupled to at least one of the platform (<NUM>) and the light source (<NUM>), wherein the actuator (<NUM>) is configured to move the platform (<NUM>) and the light source (<NUM>) relative to each other such that the plurality of electromagnetic pulses (<NUM>) are directed towards the scanning location (<NUM>) from different respective rotational angles relative to a reference axis (<NUM>) in a plane of the surface (<NUM>) of the coating (<NUM>); the system further comprising a computing device (<NUM>) in communication with the light detector (<NUM>), wherein the computing device (<NUM>) is configured to:
determine a time delay between reception of the first portion (<NUM>) and the second portion (<NUM>) of each reflected electromagnetic pulse at the light detector (<NUM>), thereby defining a plurality of time delays for the respective plurality of electromagnetic pulses; and
analyze whether the time delays for the different respective electromagnetic pulses are different to one another or are substantially equal to one another, to determine a microstructure of the coating (<NUM>).