Method for extracting thickness of multiple coatings on anisotropic substrate

Abstract

The invention discloses a method for extracting the thickness of a multi-layer coating on an anisotropic substrate, and the method comprises the steps: S1, projecting terahertz waves at a position of a to-be-detected substrate through a first TPX lens, thereby obtaining first information; s2, using a second TPX lens to project terahertz waves at the same position of the substrate to be detected so as to obtain second information; s3, creating an equivalent matrix value irrelevant to the test position for the anisotropic substrate; and S4, repeating the step S1 and the step S2 on the substrate with the coating so as to obtain the coating thickness of the substrate. According to the method for extracting the thickness of the multi-layer coating on the anisotropic substrate, the coating thickness measuring process is greatly simplified, and the difficult and time-consuming step that a sample or testing equipment needs to be rotated to collect a large amount of data during measurement is effectively omitted. Equivalent parameters of the anisotropic substrate are obtained, and the multi-layer thickness extraction process is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of thickness extraction, and in particular relates to a method for extracting the thickness of a multilayer coating on an anisotropic substrate. Background Art

[0002] In many industries, such as automotive, aerospace, and turbines, surface coatings are critical to the performance of various products. In addition, surface coatings of various products must reach an optimal thickness to work properly. In the case of aircraft, the thickness of the paint is critical to the safety and performance of the aircraft. It determines the extent of damage to the aircraft in the event of a lightning strike. Paint thickness is particularly important in areas near fuel tanks, where lightning can produce sparks that ignite the fuel. There are several reasons that may cause the thickness of the coating to be thinner at a later stage.

[0003] Environmental exposure: UV radiation and adverse weather conditions

[0004] Chemical Exposure: Exposure to corrosive chemicals or contaminants

[0005] Mechanical stress: Physical wear or abrasion and poor flexibility can lead to cracking or delamination

[0006] Substrate compatibility: poor coating adhesion and high stress

[0007] Microbiological growth: mold or mildew, causing discoloration and degradation.

[0008] Extreme temperatures: Repeated exposure to temperature fluctuations and overheating

[0009] Aging and material breakdown: Coatings may break down chemically or physically, reducing their effectiveness.

[0010] Aircraft are therefore regularly repainted to ensure that the paint thickness is adequate. Paint thickness measurement is an important step when repainting an aircraft fuselage. Accurate measurement of paint thickness helps ensure that the paint thickness is kept to a minimum to reduce unnecessary payload on the aircraft, but not below the limits set by safety regulations. Too thin a coating puts the aircraft at risk of damage from lightning strikes or UV radiation, while too thick a coating can hamper the aircraft's aerodynamic performance. It is therefore crucial to assess the paint thickness of an aircraft during maintenance and manufacturing.

[0011] During the measurement process, problems arise if the material to be measured has anisotropy, such as uneven refractive index in different directions. The problem with anisotropic samples is that various parameters such as the refractive index can be very different at every point in the sample. In previous work, the terahertz frequency range of 0.1THz to 1.2THz was used to determine the thickness of paint. However, the above method is difficult to apply to real life because it is difficult and time-consuming to collect a large amount of data by rotating the sample or testing equipment. Therefore, the substrate problem hinders the commercialization of the technology. In addition, when the beam size becomes larger, the signal-to-noise ratio decreases significantly.

[0012] Therefore, further improvements are made to the above problems. Summary of the invention

[0013] The main purpose of the present invention is to provide a method for extracting the thickness of multilayer coatings on anisotropic substrates, which greatly simplifies the coating thickness measurement process and effectively eliminates the difficult and time-consuming step of rotating the sample or test equipment to collect a large amount of data during measurement. Using this method, equivalent parameters of anisotropic substrates are obtained and the multilayer thickness extraction process is improved. The algorithm developed for multilayer thickness extraction can be further extended to anisotropic substrates.

[0014] To achieve the above objectives, the present invention discloses a method for extracting the thickness of a multilayer coating on an anisotropic substrate, comprising the following steps:

[0015] Step S1: Use a first TPX lens to project a terahertz wave at a position of a substrate to be tested (without coating), and the beam size of the first TPX lens matches the characteristic size of the position (the beam size is slightly larger), thereby obtaining first information including optical path length L0, refractive index n0, and peak frequency f0;

[0016] Step S2: Use a second TPX lens to project a terahertz wave at the same position of the substrate to be tested (without coating), the beam size of the second TPX lens is larger than the beam size of the first TPX lens (2 times or larger), so as to obtain second information including optical path length L1, refractive index n1, and peak frequency f1;

[0017] Step S3: analyzing the spectral data measured by the first information and the second information, creating an equivalent matrix value for the anisotropic substrate that is independent of the test position, and using the equivalent matrix value as an equivalent index of the substrate;

[0018] Step S4: Repeating steps S1 and S2 on the substrate with the coating, thereby obtaining the coating thickness of the substrate.

[0019] As a further preferred technical solution of the above technical solution, for step S3:

[0020] Since the projection is performed at the same position of the substrate and the beam size of the second TPX lens is larger than the beam size of the first TPX lens, an overlapping area is generated; the first information and the second information are respectively converted into frequency domain signals of two sizes using Fourier transform, and then the frequency domain signals are integrated, and then the power per unit area is calculated and normalized. The specific implementation is as follows:

[0021] After obtaining the time domain spectrum through the first information, Fourier transform is performed to obtain the first spectrum F1;

[0022] After obtaining the time domain spectrum through the second information, Fourier transform is performed to obtain the second spectrum F2;

[0023] Integrate the first spectrum F1 and the second spectrum F2 respectively to obtain the first spectrum intensity I1 and the second spectrum intensity I2 respectively;

[0024] Based on the measurement results of the two TPX lenses, the integrated intensity achieves a constant I2-I1 (subtraction), and this result is used as an equivalent indicator of the substrate (the signals of the two circles are sensitive to the information of the center point, the signal of the small circle (i.e., the aperture projected by the first TPX lens) is greatly affected by the anisotropy of the substrate, and the outer ring signal is stable after subtraction).

[0025] As a further preferred technical solution of the above technical solution, in step S4, the parameters of the substrate and each coating material are input through a numerical simulation method, and the thickness of each coating is obtained through simulation and calculation.

[0026] As a further preferred technical solution of the above technical solution, the light beam of the terahertz wave projected by the first TPX lens is smaller than the characteristic size of the (anisotropic) substrate.

[0027] As a further preferred technical solution of the above technical solution, the beam size of the second TPX lens is twice the beam size of the first TPX lens. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of projecting a light beam through the first / second TPX lens of the present invention.

[0029] Figure 2 : This is the relationship between the amplitude and frequency of the two light beams of the 5 mm signal and the 8 mm signal of the present invention.

[0030] Figure 3 It is the signal processing result of the 5 mm spot size, the 8 mm spot size and the comprehensive information of the 5 mm and 8 mm spot sizes of the present invention. DETAILED DESCRIPTION

[0031] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only examples, and those skilled in the art can think of other obvious variations. The basic principles of the present invention defined in the following description can be applied to other embodiments, variations, improvements, equivalents, and other technical solutions that do not deviate from the spirit and scope of the present invention.

[0032] In the preferred embodiments of the present invention, those skilled in the art should note that the substrates and the like involved in the present invention may be regarded as prior art.

[0033] Preferred embodiments.

[0034] like Figure 1-3 As shown, the present invention discloses a method for extracting the thickness of a multilayer coating on an anisotropic substrate, comprising the following steps:

[0035] Step S1: Use a first TPX lens to project a terahertz wave at a position of a substrate to be tested (without coating), and the beam size of the first TPX lens matches the characteristic size of the position (the beam size is slightly larger), thereby obtaining first information including optical path length L0, refractive index n0, and peak frequency f0;

[0036] Step S2: Use a second TPX lens to project a terahertz wave at the same position of the substrate to be tested (without coating), the beam size of the second TPX lens is larger than the beam size of the first TPX lens (2 times or larger), so as to obtain second information including optical path length L1, refractive index n1, and peak frequency f1;

[0037] Step S3: analyzing the spectral data measured by the first information and the second information, creating an equivalent matrix value for the anisotropic substrate that is independent of the test position, and using the equivalent matrix value as an equivalent index of the substrate;

[0038] Step S4: Repeat steps S1 and S2 on the substrate with coating to obtain the coating thickness of the substrate. Repeat steps S1 and S2 on the substrate with coating, and use the same calculation method as step S3 for the coating. These values ​​will be further input into the thickness extraction algorithm. According to the obtained information such as optical path length and refractive index, according to the formula:

[0039] L = n × d

[0040] Where L is the optical path length, n is the refractive index, and d is the thickness, the thickness data can be initially obtained. At this point, it can be observed that the signals collected from small focal points will be very different. For larger focal points, although the coverage area is larger, the signals collected at different positions on the anisotropic substrate are still different, but the changes are smaller.

[0041] like Figure 2As shown, with this information, two smaller focal spot information (5 mm and 8 mm) can be used to obtain stable equivalent information, similar to the 13 mm focal spot size information, which is independent of the anisotropic substrate position. To obtain a stable result for the 13 mm spot size information, the sample must be rotated 360 degrees to obtain an average value. The new measurement scheme makes it possible to achieve the same effect without rotating the sample.

[0042] like Figure 3 As shown, the relationship between the THz TDS amplitude and frequency of the 5 mm signal and the 8 mm signal is shown. Figure 3 Processed data for three measurement points are shown. For these three measurement points, the 5 mm spot size signal and the 8 mm spot size signal are also plotted together in the figure for comparison. The same legend is used for three measurement points with the same spot size but different measurement points to illustrate that stable and similar results can be obtained for the same spot size but different measurement points.

[0043] The results show that for the spot sizes of 5 mm and 8 mm, the signal varies greatly. However, for the processed data containing the spot size information of 5 mm and 8 mm, the signal becomes very stable with very small variations. These processed values ​​can be regarded as the average information of the anisotropic substrate, thereby obtaining the equivalent parameters of the anisotropic CFRP substrate and further input into the thickness measurement algorithm.

[0044] Specifically, for step S3:

[0045] Since the projection is performed at the same position of the substrate and the beam size of the second TPX lens is larger than the beam size of the first TPX lens, an overlapping area is generated; the first information and the second information are respectively converted into frequency domain signals of two sizes using Fourier transform, and then the frequency domain signals are integrated, and then the power per unit area is calculated and normalized. The specific implementation is as follows:

[0046] After obtaining the time domain spectrum through the first information, Fourier transform is performed to obtain the first spectrum F1;

[0047] After obtaining the time domain spectrum through the second information, Fourier transform is performed to obtain the second spectrum F2;

[0048] Integrate the first spectrum F1 and the second spectrum F2 respectively to obtain the first spectrum intensity I1 and the second spectrum intensity I2 respectively;

[0049] Based on the measurement results of the two TPX lenses, the integrated intensity achieves a constant I2-I1 (subtraction), and this result is used as an equivalent indicator of the substrate (the signals of the two circles are sensitive to the information of the center point, the signal of the small circle (i.e., the aperture projected by the first TPX lens) is greatly affected by the anisotropy of the substrate, and the outer ring signal is stable after subtraction).

[0050] More specifically, in step S4, the parameters of the substrate (equivalent matrix value) and each coating material (including n, k and dielectric constant, etc.) are input through a numerical simulation method, and the thickness of each coating is obtained through simulation and calculation, where:

[0051] Furthermore, the beam of the terahertz wave projected by the first TPX lens is relatively small relative to the characteristic size of the (anisotropic) substrate.

[0052] Furthermore, the beam size of the second TPX lens is twice the beam size of the first TPX lens.

[0053] Inverse modeling: Use a common optimization procedure to compare the experimental results with the simulation model. Input the parameters of the substrate and the materials of each layer, material parameters including n, k and dielectric constant, etc. To minimize the difference: Use the particle swarm optimization (PSO) method. Once the error is minimized, the thickness can be derived from this set of optimal parameters.

[0054] In the inverse modeling process, the error function defines the error between the experimental output and the simulation model. If defined properly, the objective function can greatly improve the accuracy and precision of the optimization protocol. The error function defined in the method is the cross-correlation of the experimental results and the simulation model. The mathematical form of the cross-correlation operation is:

[0055]

[0056] and its Fourier transform pair:

[0057]

[0058] To better demonstrate the nature of the error function, an analytical model with two layers of paint on a substrate is assumed as the “experimental input” for the optimization process.

[0059] The “experimental input” is the amplitude of the wave reflected by the paint structure, swept as a function of THz frequency. The method uses the amplitude of the reflected wave |R input |, and use Fourier transform to map it to the time domain. |R input The Fourier transform of | is

[0060]

[0061] The method then removes components in the time domain that are several orders of magnitude lower than the components at t = 0. These components are not important and do not contribute to the reconstruction of the reflection curve in the frequency domain. This helps prevent inaccurate results due to these non-essential components.

[0062] The next step of the method is to calculate the cross-correlation error between the input function and the test function. The error is divided into two parts, magnitude and phase. The magnitude error between the input function and the test function is defined as:

[0063]

[0064] Phase Error:

[0065]

[0066] When the error reaches a minimum, the solution is found.

[0067] It is worth mentioning that the technical features such as the substrate involved in the patent application of this invention should be regarded as the prior art. The specific structure, working principle and possible control method and spatial layout method of these technical features can be selected by conventional methods in the field, and should not be regarded as the inventive point of the patent of this invention. The patent of this invention will not be further elaborated.

[0068] For those skilled in the art, it is still possible to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for extracting the thickness of a multilayer coating on an anisotropic substrate, characterized in that: The following steps are involved: Step S1: Use a first TPX lens to project a terahertz wave at a position of the substrate to be measured, and the beam size of the first TPX lens matches the characteristic size of the position, so as to obtain first information including optical path length L0, refractive index n0, and peak frequency f0; Step S2: using a second TPX lens to project a terahertz wave at the same position of the substrate to be measured, wherein the beam size of the second TPX lens is larger than the beam size of the first TPX lens, thereby obtaining second information including optical path length L1, refractive index n1, and peak frequency f1; Step S3: analyzing the spectral data measured by the first information and the second information, creating an equivalent matrix value for the anisotropic substrate that is independent of the test position, and using the equivalent matrix value as an equivalent index of the substrate; Step S4: Repeating steps S1 and S2 on the substrate with the coating, thereby obtaining the coating thickness of the substrate.

2. The method for extracting the thickness of a multilayer coating on an anisotropic substrate according to claim 1, characterized in that: For step S3: Since the projection is performed at the same position of the substrate and the beam size of the second TPX lens is larger than the beam size of the first TPX lens, an overlapping area is generated; the first information and the second information are respectively converted into frequency domain signals of two sizes using Fourier transform, and then the frequency domain signals are integrated, and then the power per unit area is calculated and normalized. The specific implementation is as follows: After obtaining the time domain spectrum through the first information, Fourier transform is performed to obtain the first spectrum F1; After obtaining the time domain spectrum through the second information, Fourier transform is performed to obtain the second spectrum F2; Integrate the first spectrum F1 and the second spectrum F2 respectively to obtain the first spectrum intensity I1 and the second spectrum intensity I2 respectively; Based on the results of the two TPX lens measurements, the integrated intensity achieves a constant I2-I1, and this result is used as an equivalent indicator of the substrate.

3. The method for extracting the thickness of a multilayer coating on an anisotropic substrate according to claim 1, characterized in that: In step S4, the parameters of the substrate and each coating material are input through a numerical simulation method, and the thickness of each coating is obtained through simulation and calculation.

4. The method for extracting the thickness of a multilayer coating on an anisotropic substrate according to claim 1, characterized in that: The terahertz wave beam projected by the first TPX lens is smaller than the characteristic size of the substrate.

5. The method for extracting the thickness of a multilayer coating on an anisotropic substrate according to claim 4, characterized in that: The beam size of the second TPX lens is twice the beam size of the first TPX lens.

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