阅读说明：本技术 基于超声复合模型的含孔隙的涂层厚度及声速的测量方法 (Method for measuring thickness and sound velocity of coating containing pores based on ultrasonic composite model ) 是由 武通海 梁鹏 郑鹏 邹来胜 窦潘 于 2021-08-10 设计创作，主要内容包括：本发明公开了一种基于超声复合模型的含孔隙的涂层厚度及声速的测量方法,确定被测试样涂层密度；获取被测试样参考信号以及实测反射信号,得到实测声压反射系数；根据超声复合模型,从声压反射系数中分离出带有相位、幅值和衰减信息的指数项,从而提取层中的相位变化公式,通过最小二乘直线拟合原理,拟合出声阻抗和斜率的最优估计；利用密度、声速、声阻抗之间的数值关系,将已求解的密度和拟合的涂层声阻抗值代入,计算得到涂层的声速；根据波在涂层中穿越一次的相位变化原理以及拟合的最佳斜率值和声速,计算出涂层厚度。本发明原理明确、易于实现,能够实现含孔隙涂层厚度与声速的同步测量。(The invention discloses a method for measuring the thickness of a coating containing pores and the sound velocity based on an ultrasonic composite model, which is used for determining the coating density of a sample to be tested; acquiring a reference signal and an actually measured reflection signal of a tested sample to obtain an actually measured sound pressure reflection coefficient; according to the ultrasonic composite model, separating an exponential term with phase, amplitude and attenuation information from a sound pressure reflection coefficient, thereby extracting a phase change formula in a layer, and fitting optimal estimation of acoustic impedance and slope by a least square straight line fitting principle; substituting the solved density and the fitted coating acoustic impedance value by using the numerical relation among the density, the acoustic velocity and the acoustic impedance to calculate the acoustic velocity of the coating; and calculating the thickness of the coating according to the phase change principle that the wave passes through the coating once and the fitted optimal slope value and sound velocity. The invention has clear principle and easy realization, and can realize the synchronous measurement of the thickness of the coating containing the pores and the sound velocity.)

1. The method for measuring the thickness of the coating containing pores and the sound velocity based on the ultrasonic composite model is characterized by comprising the following steps of:

s1, determining the coating density of the tested sample based on the Archimedes principle;

s2, acquiring a reference signal and an actually measured reflection signal of the tested sample to obtain an actually measured sound pressure reflection coefficient;

s3, according to the ultrasonic composite model, separating an exponential term with phase, amplitude and attenuation information from the sound pressure reflection coefficient obtained in the step S2, extracting a phase change formula in the coating, and fitting optimal estimation of sound impedance and slope by a least square straight line fitting principle;

s4, substituting the coating density of the sample to be tested obtained in the step S1 and the coating acoustic impedance value fitted in the step S3 by using the numerical relation among the density, the acoustic velocity and the acoustic impedance, and calculating the acoustic velocity of the coating of the sample to be tested;

and S5, calculating the thickness of the coating of the tested sample according to the phase change principle that the wave propagates in the coating of the tested sample once and the optimal slope value and the sound speed which are fitted in the step S3.

2. The method of claim 1, wherein the density p of the coating of the sample being tested is_eComprises the following steps:

wherein G is₁For the weight of the coating of the test specimen suspended in air, G₂For the weight of the coating of the sample to be tested suspended in water, p_{Water (W)}Is the density of water.

3. The method according to claim 1, wherein step S2 is specifically:

collecting reflected waves of incident waves reflected from a water-stainless steel interface as reference signals of a tested sample; reflected waves of incident waves reflected from the water-coating-substrate three-layer structure are used as actual reflected signals of the tested sample; the ratio of the actual reflected signal to the reference signal is taken as the sound pressure reflection coefficient.

4. The method of claim 1, wherein in step S3, based on the ultrasound composite model, the exponential terms with phase, amplitude and attenuation information are separated from the sound pressure reflection coefficient, and the phase variation formula is extracted as follows:

wherein phi is the phase change quantity generated by the wave going back and forth once in the coating of the tested sample, and f is the wave frequency; alpha is the attenuation coefficient of the coating of the tested sample; r is₁₂And r₂₃Respectively, the sound pressure reflection coefficient of a water-coating interface and a coating-substrate interface, R (f) is the sound pressure reflection coefficient of a tested sample coating, and i is an imaginary number unit.

5. The method of claim 4, wherein the sound pressure reflection coefficient R (f) of the coating is:

wherein alpha is the attenuation coefficient of the coating, h is the coating thickness of the tested sample, exp (i phi) is the phase term of the ultrasonic expression, I (f) is the incident wave sound pressure, and P (f) is the reflected wave sound pressure.

6. The method according to claim 1, wherein in step S3, the minimum fitting error D is_eminTaking outGet the optimal estimationAndfitted layer phase shift slope k_eAnd fitting error D_eComprises the following steps:

wherein phi is_e(f_i；Z_2e) Estimating acoustic impedance Z for input_2eThe layer phase shift in time; phi is a_e(f_i；Z_2e) (ii) a layer phase shift obtained for least squares fitting; i is the ith FFT frequency value.

7. The method of claim 6, wherein if no minimum value of the fitting error is found, increasing Z_2maxAnd repeating the above steps until the best estimation value is obtained.

8. The method of claim 1, wherein in step S4, the fitted acoustic impedance and the density p calculated in step S1 are calculated according to the numerical relationship among sound velocity, density and acoustic impedance_eSubstituting to obtain the sound velocity c of the tested sample coating₂Comprises the following steps:

wherein the content of the first and second substances,fitting an acoustic impedance; rho₂Is the coating density.

9. The method of claim 8, wherein the numerical relationship between sound velocity, density, and acoustic impedance is:

Z＝ρc

where Z is impedance, ρ is density, and c is speed of sound.

10. The method of claim 1, wherein in step S5, the coating thickness d of the test sample is:

wherein the content of the first and second substances,is the optimal estimation of the slope; c. C₂Is the coating sound velocity of the sample tested.

Technical Field

The invention belongs to the technical field of ultrasonic nondestructive testing, and particularly relates to a method for measuring the thickness of a coating containing pores and the sound velocity based on an ultrasonic composite model.

Background

With the rapid development and wide application of coating technology in the fields of aviation and the like, the detection and monitoring of the delivery and service performance of the coating are concerned. In the main characteristic parameters for representing the quality of the coating, the thickness of the coating is one of key technical indexes for reflecting the structural performance of the coating, has great influence on the characteristics of the coating, such as heat insulation, wear resistance, conductivity and the like, and is also related to the material consumption and cost of the coating in production. However, due to the non-uniform structure inside the coating, a certain amount of micropores with different sizes and random distribution are formed inside the coating. The incidence of ultrasonic waves into the non-uniform structure can show complex scattering behavior, so that acoustic parameters such as sound velocity and attenuation coefficient of the coating are difficult to accurately extract, great trouble is caused to the measurement of the thickness of the coating, and the introduction of the parameters can cause non-uniqueness of an analytic solution. It is of great significance to how to reduce or eliminate the introduction of acoustic parameters into the calculation for the measurement of non-uniform coating thickness.

The existing technical scheme is mostly a parameter inversion method, the attenuation coefficient of the coating needs to be fitted in advance, then multi-parameter inversion is carried out by means of a Reflection Coefficient Amplitude Spectrum (RCAS) and a Reflection Coefficient Phase Spectrum (RCPS), and the iterative computation efficiency and precision are low, so that a computation method which does not need to introduce acoustic parameters such as the attenuation coefficient is needed to measure the thickness and the sound velocity of the coating containing the pores.

Disclosure of Invention

The technical problem to be solved by the invention is to provide a method for measuring the thickness and the sound velocity of a coating containing pores based on an ultrasonic composite model, aiming at the problem that acoustic parameters such as the sound velocity and the attenuation coefficient are difficult to obtain in the measurement of the thickness of the coating containing pores, and the thickness and the sound velocity of the coating containing pores are synchronously measured by using the phase and amplitude changes of the ultrasonic composite model layer. The method has the advantages of clear principle, easy realization, good elimination of the influence of the attenuation coefficient on the thickness measurement, improvement of the calculation efficiency and the measurement precision of the thickness of the coating and the sound velocity measurement, and good popularization and application prospects.

The invention adopts the following technical scheme:

the method for measuring the thickness and the sound velocity of the coating containing pores based on the ultrasonic composite model comprises the following steps:

s1, determining the coating density of the tested sample based on the Archimedes principle;

s2, acquiring a reference signal and an actually measured reflection signal of the tested sample to obtain an actually measured sound pressure reflection coefficient;

s3, according to the ultrasonic composite model, separating an exponential term with phase, amplitude and attenuation information from the sound pressure reflection coefficient obtained in the step S2, extracting a phase change formula in the coating, and fitting optimal estimation of sound impedance and slope by a least square straight line fitting principle;

s4, substituting the coating density of the sample to be tested obtained in the step S1 and the coating acoustic impedance value fitted in the step S3 by using the numerical relation among the density, the acoustic velocity and the acoustic impedance, and calculating the acoustic velocity of the coating of the sample to be tested;

and S5, calculating the thickness of the coating of the tested sample according to the phase change principle that the wave propagates in the coating of the tested sample once and the optimal slope value and the sound speed which are fitted in the step S3.

In particular, the density ρ of the coating of the sample to be tested_eComprises the following steps:

wherein G is₁For the weight of the coating of the test specimen suspended in air, G₂For the weight of the coating of the sample to be tested suspended in water, p_{Water (W)}Is the density of water.

Specifically, step S2 specifically includes:

collecting reflected waves of incident waves reflected from a water-stainless steel interface as reference signals of a tested sample; reflected waves of incident waves reflected from the water-coating-substrate three-layer structure are used as actual reflected signals of the tested sample; the ratio of the actual reflected signal to the reference signal is taken as the sound pressure reflection coefficient.

Specifically, in step S3, based on the ultrasound composite model, the exponential terms with phase, amplitude and attenuation information are separated from the sound pressure reflection coefficient, and the phase change formula is extracted as follows:

wherein phi is the phase change quantity generated by the wave going back and forth once in the coating of the tested sample, and f is the wave frequency; alpha is the attenuation coefficient of the coating of the tested sample; r is₁₂And r₂₃Respectively, the sound pressure reflection coefficient of a water-coating interface and a coating-substrate interface, R (f) is the sound pressure reflection coefficient of a tested sample coating, and i is an imaginary number unit.

Further, the sound pressure reflection coefficient r (f) of the coating is:

wherein alpha is the attenuation coefficient of the coating, h is the coating thickness of the tested sample, exp (i phi) is the phase term of the ultrasonic expression, I (f) is the incident wave sound pressure, and P (f) is the reflected wave sound pressure.

Specifically, in step S3, the minimum fitting error D is determined_eminTo obtain the optimal estimationAndfitted layer phase shift slope k_eAnd fitting error D_eComprises the following steps:

wherein phi is_e(f_i；Z_2e) Estimating acoustic impedance Z for input_2eThe layer phase shift in time; phi is a_e(f_i；Z_2e) (ii) a layer phase shift obtained for least squares fitting; i is the ith FFT frequency value.

Further, if the minimum value of the fitting error cannot be found, Z is increased_2maxAnd repeating the above steps until the best estimation value is obtained.

Specifically, in step S4, based on the numerical relationship among the sound velocity, the density, and the acoustic impedance, the acoustic impedance obtained by fitting and the density ρ calculated in step S1 are combined_eSubstituting to obtain the sound velocity c of the tested sample coating₂Comprises the following steps:

wherein the content of the first and second substances,fitting an acoustic impedance; rho₂Is the coating density.

Further, the numerical relationship among the sound velocity, the density and the acoustic impedance is as follows:

Z＝ρc

where Z is impedance, ρ is density, and c is speed of sound.

Specifically, in step S5, the coating thickness d of the test sample is:

wherein the content of the first and second substances,is the optimal estimation of the slope; c. C₂Is the coating sound velocity of the sample tested.

Compared with the prior art, the invention has at least the following beneficial effects:

the invention provides a method for measuring the thickness of a coating containing a pore and the sound velocity based on an ultrasonic composite model, which aims at the measurement of the thickness of the coating containing the pore and the sound velocity, provides a new calculation method based on the ultrasonic composite model, does not need to know the acoustic impedance value and the coating attenuation coefficient of a matrix before measurement, overcomes the problems of low calculation efficiency and low precision caused by introducing acoustic parameters such as the attenuation coefficient and the acoustic impedance of the matrix by the existing inversion method, optimizes the calculation process and improves the accuracy of the measurement of the thickness and the sound velocity.

Furthermore, according to the Archimedes drainage method principle, the density of the coating can be quickly and accurately obtained without a precise and complex instrument.

Furthermore, the sound pressure reflection coefficient of the coating can be directly obtained through experiments, and accurate measurement of the subsequent coating thickness and the sound velocity is facilitated.

Furthermore, the influence of the attenuation coefficient of the coating on the thickness measurement is eliminated by extracting a phase change formula in the sound pressure reflection coefficient of the coating, and the accurate measurement of the thickness of the coating and the sound velocity is facilitated.

Furthermore, a sound pressure reflection coefficient mathematical expression of the coating is obtained through a wave superposition principle, and the relation between the coating thickness and the phase is explained from a mechanism level, so that a phase change formula is deduced.

Furthermore, accurate acquisition of the acoustic impedance of the coating is ensured by setting the layer phase shift slope and the fitting error, so that accurate calculation of the acoustic impedance of the coating is ensured, and accurate measurement of the acoustic velocity of the coating containing the pores based on the composite model is realized.

Furthermore, the maximum value of the acoustic impedance of the coating can be set, so that the range of acoustic impedance fitting calculation can be enlarged, and the acoustic impedance of the coating can be accurately obtained.

Furthermore, the sound velocity of the coating of the tested sample is calculated by fitting the acoustic impedance and the density, so that the accurate measurement of the thickness of the coating is ensured.

Furthermore, the sound velocity is accurately obtained through the numerical relation among the sound velocity, the density and the acoustic impedance.

Furthermore, the layer phase shift slope value and the sound velocity value are substituted into the expression through the expression of the thickness of the tested sample coating, so that the accurate measurement of the thickness of the coating containing the pore space is ensured.

In conclusion, the principle of the invention is clear and easy to realize, and the synchronous measurement of the thickness of the coating containing the pore and the sound velocity can be realized.

The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.

Drawings

FIG. 1 is a flow chart of the present invention;

FIG. 2 is a graph of the phase shift of waves in the coating as a function of frequency for different acoustic impedances of the coating;

FIG. 3 is a cross-sectional view of a coating and measurement area of a real aviation compressor blade in accordance with the present invention;

FIG. 4 is a diagram of acoustic impedance measurement results of compressor blade coatings (test points 1-4);

FIG. 5 is a graph of the results of measuring the layer phase shift slope of the compressor blade coating (points 1-4).

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

In the description of the present invention, it should be understood that the terms "comprises" and/or "comprising" indicate the presence of the stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Various structural schematics according to the disclosed embodiments of the invention are shown in the drawings. The figures are not drawn to scale, wherein certain details are exaggerated and possibly omitted for clarity of presentation. The shapes of various regions, layers and their relative sizes and positional relationships shown in the drawings are merely exemplary, and deviations may occur in practice due to manufacturing tolerances or technical limitations, and a person skilled in the art may additionally design regions/layers having different shapes, sizes, relative positions, according to actual needs.

Under the influence of preparation method, process parameters and the like, a certain amount of micropores distributed randomly are formed in the thermal barrier coating. When ultrasonic waves are incident into the non-uniform structure, complex scattering behavior is shown, so that acoustic parameters such as sound velocity and attenuation coefficient of the coating are difficult to accurately extract. The traditional parameter inversion method needs to fit the attenuation coefficient of the coating in advance and then carry out multi-parameter inversion by means of RCAS and RCPS. However, the interaction between the coating pores makes the attenuation coefficient expression difficult to determine, which leads to non-uniqueness of the analytical solution in the parametric inversion process.

On the basis of the composite model, the ultrasonic complex reflection coefficient is further deduced, separated and sorted, and the introduction of the attenuation coefficient in the thickness and sound velocity measurement process is directly eliminated, so that the thickness and the sound velocity of the coating containing the pores can be quickly and accurately obtained.

The invention provides a method for measuring the thickness and the sound velocity of a coating containing pores based on an ultrasonic composite model.

Referring to fig. 1, the method for measuring the thickness of a coating containing pores and the sound velocity based on an ultrasonic composite model of the present invention includes the following steps:

s1, determining the coating density of the tested sample based on the Archimedes principle;

measuring the weight G of the sample coating suspended in air using an electronic spring balance₁Measuring again the weight G of the coating of the sample to be tested suspended in water₂It is required that the coating be completely submerged in water. According to the Archimedes principle, the volume of the coating of the tested sample is calculated:

and substituting the mass formula G-rho gV into the formula (1) to solve the density of the coating of the tested sample:

s2, acquiring a reference signal and an actually measured reflection signal of the tested sample to obtain an actually measured sound pressure reflection coefficient;

collecting a reflected wave of an incident wave reflected from a water-stainless steel interface as a reference signal of a tested sample; reflected waves of the incident waves reflected from the three-layer structure of the water-coating-matrix are actual reflected signals of the tested sample; the ratio of the actual reflected signal to the reference signal is the sound pressure reflection coefficient.

S3, according to the ultrasonic composite model, separating an exponential term with phase, amplitude and attenuation information from the sound pressure reflection coefficient, thereby extracting a phase change formula in the layer, and fitting the optimal estimation of the sound impedance and the slope by a least square straight line fitting principle;

for a water-coating-matrix three-layer medium structure, the acoustic impedance is Z₁、Z₂、Z₃。

The sound pressure reflection coefficient of the coating is deduced according to the wave superposition principle as follows:

where φ is the phase change (phase shift of layer)/rad caused by a wave going back and forth once in the coating, φ 4 π df/c₂D is the thickness of the coating/. mu.m, c₂For coating sound velocity/m.s^-1F is wave frequency/MHz; alpha is coating attenuation coefficient/Np cm^-1；r₁₂And r₂₃The sound pressure reflection coefficients of the water-coating interface and the coating-substrate interface, respectively.

The exponential terms with phase, amplitude and attenuation information are separated from the above equation and are represented after rearrangement as:

if ln is taken simultaneously for both sides of the equation, then:

wherein n is₂For uncertain integers representing the ambiguity of the phase shift of the ultrasound layer, the atan2 function is used to fix the phase change to 0,2 π]In between, so take n₂＝0。

Equation Right (R (f) -r₁₂)/(1-r₁₂R (f) is a complex quantity, assuming its amplitude and phase are r and v, respectively, then:

ln(r₂₃)-2αd+iφ＝ln(r)+iv (6)

based on the ultrasonic composite model, separating the exponential terms with phase, amplitude and attenuation information from the sound pressure reflection coefficient, thereby extracting a phase change formula in the layer:

where φ is the phase change (phase shift of layer)/rad caused by a wave going back and forth once in the coating, φ 4 π df/c₂D is the thickness of the coating/. mu.m, c₂Coating sound velocity/m.s for the sample to be tested^-1F is wave frequency/MHz; alpha is coating attenuation coefficient/Np cm^-1；r₁₂And r₂₃The sound pressure reflection coefficients of the water-coating interface and the coating-substrate interface, respectively.

The left part of equation (7) equal sign is expressed as:

wherein，k_eIs the slope of the phase shift of the layer as a function of frequency.

The phase shift of the layer is linear with frequency, the slope k of the line_eDepending on the thickness of the coating and the speed of sound. R (f) in the right part of the equal sign of equation (7) is the measured complex reflection coefficient, then the measured phase shift of the layer will only be related to r₁₂Is related to the value of (A). Since the acoustic velocity of the porous coating is unknown, the reflection coefficient of the water-coating interface is expressed as r_12e＝(Z_2e-Z₁)/(Z_2e+Z₁)。

When Z is_2eLess than Z₂Time, estimated reflection coefficient r_12eWill be less than true r₁₂The curve of the layer phase shift changing along with the frequency changes into an inward concave arc line, and if a straight line with the intercept of 0 is used for fitting the layer phase shift at the moment, a larger linear fitting error exists; when Z is_2eGreater than Z₂The same situation can occur; only when Z is_2eEqual to true value Z₂The curve of the phase shift of the layer with the frequency is a straight line, as shown in fig. 2. Therefore, when the linear fitting error reaches the minimum, the least square straight line fitting principle is fused, and the minimum fitting error D is obtained_eminTo obtain the optimal estimationAnd

fitted layer phase shift slope k_eAnd fitting error D_eComprises the following steps:

wherein phi is_e(f_i；Z_2e) Estimating acoustic impedance Z for input_2eLayer phase shift/rad in time; phi is a_e(f_i；Z_2e) Layer phase shift/rad obtained for least squares fitting, with value equal to k_ef; i is the ith FFT frequency value.

Acoustic impedance Z to coating_2eIncrease from 0 to a set maximum value Z_2maxAnd recording each Z_2eSlope k obtained by fitting guessed values_eAnd error D_eOptimal estimationAndwill be at a minimum fitting error D_eminAnd (4) obtaining.

If the minimum value of the fitting error cannot be found, the upper limit of the acoustic impedance guess is not large enough, so that it is necessary to increase Z_2maxAnd repeating the above steps until the best estimation value is obtained.

S4, substituting the solved density and the fitted coating acoustic impedance value by using the numerical relation among the density, the acoustic velocity and the acoustic impedance, and calculating to obtain the acoustic velocity of the coating of the tested sample;

based on the numerical relationship (Z ═ ρ c) between the sound velocity, density, and acoustic impedance, the acoustic impedance obtained by fitting and the density ρ calculated in step S1 are compared_eSubstituting, the sound velocity of the test sample coating can be calculated:

wherein the content of the first and second substances,fitting an acoustic impedance; rho₂Is the coating density.

And S5, calculating the thickness of the coating according to the phase change (layer phase shift) principle of the wave once propagating in the coating and the fitted optimal slope value and sound velocity.

Substituting the fitted optimal slope value and sound velocity into formula (8), calculating the coating thickness d of the tested sample as follows:

wherein the content of the first and second substances,is the optimal estimation of the slope; c. C₂Is the coating sound velocity of the sample tested.

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Verification example:

real aero-engine compressor blades have a complex profile (see fig. 3) with dimensions of about 50mm x 70 mm. The specific materials of the substrate and coating are not known to be confidential. The density of the matrix was measured to be 7840kg m^-3The sound velocity is 6240m · s^-1The density of the coating is 2300kg · m^-3. Two measuring points are respectively selected at the leaf back and the leaf pot, and four measuring points are used for collecting ultrasonic signals, and are marked as a measuring point 1, a measuring point 2, a measuring point 3 and a measuring point 4.

After the measurement is finished, the blade measurement area is subjected to linear cutting and digital microscope (OM) observation. The thickness averages for the regions at stations 1, 2, 3 and 4 are 117 μm, 98 μm and 97 μm, respectively. It was found that the coating was denser and the thickness of the coating at the leaf back was slightly greater than the thickness of the coating at the leaf basin.

The measurement was performed using a water immersion focusing probe with a nominal center frequency of 10 MHz. And after obtaining the reference signal, measuring to obtain that the effective bandwidth (-6dB) of the reference signal is 7.06-13.02 MHz, and performing coating acoustic impedance fitting in the effective bandwidth.

Referring to fig. 4 and 5, the acoustic impedance of the coating (fig. 4) and the slope of the fitted line (fig. 5) are obtained, and then the acoustic velocity and the thickness of the coating are calculated by combining the equations (11) and (12).

The frequencies of the first-order resonance points in the RCPS of each measuring point are respectively 12.84MHz, 12.96MHz, 14.83MHz and 14.89MHz, and the sound velocity is obtained by combining the thickness calculation observed by a digital microscope (OM). The results of the thickness and sonic velocity measurements at each point of the coating are shown in table 1.

TABLE 1 measurement results of thickness and sound velocity of coating (measurement points 1-4) of compressor blade

The relative error in the measurement of the parameters at stations 1 and 2 is generally smaller than at stations 3 and 4. The maximum values of the relative errors of the thickness measurement and the sound velocity measurement of the four measuring points are respectively 8.2 percent and-5.7 percent, and the engineering measurement requirements are met. The effectiveness of the method for measuring the thickness of the coating and the sound velocity of the real aviation blade is verified through experiments.

In conclusion, the method for measuring the thickness of the coating and the sound velocity containing the pores based on the ultrasonic composite model does not need to know the acoustic impedance value and the coating attenuation coefficient of the substrate before measurement, overcomes the problems of low calculation efficiency and low precision caused by introducing acoustic parameters such as the attenuation coefficient and the acoustic impedance of the substrate by the existing inversion method, can realize accurate measurement of the thickness of the coating and the sound velocity by deducing the sound pressure reflection coefficient of the coating and obtaining the optimal fitting slope and the sound impedance by the least square method, optimizes the calculation process and improves the accuracy of measurement of the thickness of the coating and the sound velocity.

The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.