CN112630120A - Method for establishing model for measuring porosity of coating and using method of model - Google Patents
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Abstract
The invention provides a method for establishing a model for measuring the porosity of a coating and a using method of the model, wherein the model for measuring the porosity of the coating is established according to an SEM photograph in-situ modeling principle, is closer to the actual appearance of pores in the coating compared with other methods, the appearance of the pores of the model with different porosities is basically kept consistent, and the principle of controlling a single variable is followed, so that the porosity is only changed along with the sound velocity of longitudinal waves, and the research result is more pertinent, strict and persuasive. The application method of the model for measuring the porosity of the coating comprises the steps of analyzing the amplitude spectrum of the ultrasonic reflection echo signal of the coating to obtain the change rate of the longitudinal wave sound velocity of the coating under different porosities, and determining the porosity of the coating according to the relation model. The invention has no damage to the coating in the measuring process and small measuring error.
Description
Technical Field
The invention relates to the field of protective coatings, in particular to a method for establishing a model for measuring the porosity of a coating and a using method of the model.
Background
The coating is a film that is sprayed onto a substrate of metal, plastic, fabric, etc. for protection, insulation, etc. In the field of aerospace, thermal barrier coatings play a key role in protecting the thermal structure of aerospace turbine engines. The thermal barrier coating has good heat insulation effect, high-temperature oxidation resistance and thermal shock resistance, and has high-temperature corrosion resistance according to special requirements in a corrosion medium. Porosity occurs during the preparation of the thermal barrier coating, and the thermal conductivity, insulation and durability of the thermal barrier coating depend to a large extent on the magnitude of the porosity. Porosity is a necessary condition for thermal insulation and for accommodating operational thermal stresses, and an increase in porosity of a certain magnitude effectively impedes heat transfer to the coating. However, high porosity can reduce the overall mechanical properties of the coating and increase corrosion of the coating, mainly due to the entry of corrosive media, leading to corrosion products on the surface area of the protected substrate, and cracking and peeling off of the thermal barrier coating. It is necessary to make an accurate measurement of the porosity of the coating.
The prior method for measuring the porosity of the coating is usually mainly based on anatomical verification, but the method needs to destroy the coating to measure the porosity, has complicated testing steps and increases the testing cost. The prior art also provides a method for measuring the porosity of a coating by detecting the porosity of the coating by an ultrasonic method. However, a certain amount of pores and microcracks are randomly distributed in the thermal barrier coating, and a complex dispersion phenomenon occurs when ultrasonic waves propagate in the coating, so that the signal waveform is distorted, the energy attenuation is serious, and the porosity cannot be effectively predicted by an ultrasonic detection method.
In view of the foregoing, the prior art cannot effectively measure the porosity of the coating directly by the ultrasonic inspection method, and therefore, there is a need for improvement of the prior art.
Disclosure of Invention
The invention provides a method for establishing a model for measuring the porosity of a coating and a using method of the model, which are used for solving at least one problem. According to the method, the amplitude spectrum of the ultrasonic reflection echo signal of the coating is analyzed to obtain the change rate of the longitudinal wave sound velocity under different porosities, a fitting curve of the change rate of the longitudinal wave sound velocity of the coating and the porosity is further drawn, a relation model of the change of the longitudinal wave sound velocity of the coating along with the porosity is summarized, and the porosity of the coating is determined.
The invention provides a method for establishing a model for measuring the porosity of a coating, which comprises the following steps:
step S01, establishing a series of physical coating models with different porosities;
step S02, performing ultrasonic numerical simulation calculation on each coating physical model to obtain a time domain signal, performing frequency spectrum analysis processing on the time domain signal to obtain a sound pressure reflection coefficient amplitude spectrum, and obtaining a coating resonant frequency corresponding to each coating physical model from the sound pressure reflection coefficient amplitude spectrum;
step S03, calculating and obtaining the coating longitudinal wave sound velocity of each coating physical model based on the coating resonance frequency corresponding to each coating physical model;
step S04, obtaining the coating longitudinal wave sound velocity change rate corresponding to each coating physical model based on the coating longitudinal wave sound velocity of each coating physical model and the preset compact coating longitudinal wave sound velocity;
and step S05, fitting the porosity corresponding to each coating physical model and the change rate of the longitudinal wave sound velocity of the coating to obtain a measured coating porosity model.
Further, in the step S03, the resonance frequency and the preset coating thickness are input into the sound pressure reflection coefficient spectrum resonance frequency formula fnObtaining longitudinal wave speed of the coating layer by n c/4d, wherein fnAnd d is the resonant frequency, d is the thickness of the coating, n is the order of the resonant frequency, and the value is a positive integer.
Further, in the step S05, the porosity and the coating longitudinal wave sound velocity exist according to the formula c ═ c0(1.05-2.09P), wherein c is the coating longitudinal wave sound velocity, c0Is the longitudinal wave sound velocity of the dense coating, and P is the porosity of the coating.
Further, in the step S01, the physical model of the coating is constructed according to the SEM photo in-situ modeling principle.
Further, the coating physical model adopts a time domain finite difference method to carry out ultrasonic numerical simulation calculation.
Further, the parameters used by the ultrasonic numerical simulation calculation include: the center frequency of the pulse sound source was 5MHz, and the thickness of the coating was 288. mu.m.
A method of using a model for measuring porosity of a coating, comprising the steps of:
step S11, obtaining the porosity model of the measured coating;
step S12, obtaining a coating of the area to be measured to obtain an SEM picture;
step S13, obtaining the coating thickness based on the SEM picture;
step S14, obtaining an ultrasonic reflection echo signal based on the coating, and carrying out frequency spectrum analysis on the ultrasonic reflection echo signal to obtain the resonant frequency of the coating;
step S15, obtaining the change rate of the longitudinal wave sound velocity of the coating based on the resonance frequency and the coating thickness;
and step S16, inputting the change rate of the longitudinal wave sound velocity of the coating into the relation model to obtain the porosity.
Further, in step S13, the thickness of the coating layer is calculated by performing image processing on the SEM photograph of the coating layer and using mathematical software.
Further, in the step S15, the resonance frequency and the coating thickness are input into a sound pressure reflection coefficient spectrum resonance frequency formula fnObtaining longitudinal wave speed of the coating layer by n c/4d, wherein fnAnd d is the resonant frequency, d is the thickness of the coating, n is the order of the resonant frequency, and the value is a positive integer.
Further, in the step S16, the coating longitudinal wave sound velocity and the previously obtained dense coating longitudinal wave sound velocity are input into the formula c ═ c0(1.05-2.09P) obtaining the porosity, wherein c is the coating longitudinal wave sound velocity, c0Is a solid coatingLayer longitudinal wave speed, P is the coating porosity.
The invention has at least the following beneficial effects: the invention has no damage to the coating in the measuring process and small measuring error. The physical model is built according to the in-situ modeling principle of the SEM photo, the physical model is closer to the actual appearance of the pores in the coating compared with other methods, the appearance of the pores of the model with different porosities is basically kept consistent, and the principle of controlling a single variable is followed, so that the porosity is only changed along with the longitudinal wave sound velocity, and the research result is more pertinent, strict and persuasive.
Drawings
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
FIG. 1 is a flow chart of a method for establishing a model for measuring the porosity of a coating according to the present invention.
FIG. 2 shows ZrO of examples of the present invention2SEM photograph of coating cross section.
FIG. 3 shows ZrO at different porosities for examples of the present invention2The acoustic pressure reflection coefficient magnitude spectrum of the coating.
FIG. 4 is a graph of the rate of change of longitudinal wave sound velocity versus coating porosity for an embodiment of the present invention.
FIG. 5 is a flow chart of a method of using a model for measuring porosity of a coating according to the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Specific implementations of the present invention are described in detail below with reference to specific examples, although the following examples refer to ZrO2Coatings are used as examples, but it is clear that the usual ceramic coatings are equally suitable for the method according to the invention, whereby this example uses ZrO2The coating is exemplary and not intended to limit the invention.
The prior art measurement of the porosity of a coating is usually based on anatomical verification, however, the method belongs to destructive detection and the porosity of the coating needs to be measured by destroying the coating. The prior art also provides a method for non-destructive measurement of the porosity of a coating, i.e. the porosity of a coating is detected by ultrasonic methods. However, a certain amount of pores and microcracks are randomly distributed in the thermal barrier coating, which causes an obvious dispersion phenomenon of an ultrasonic signal directly obtained in ultrasonic detection of the coating, causes distortion of signal waveform, serious energy attenuation and the like, and cannot effectively and directly measure the porosity by an ultrasonic detection method.
The method for measuring the porosity of the coating mainly comprises the steps of carrying out spectrum analysis on ultrasonic reflection echo signals of the coating, calculating the longitudinal wave sound velocity of the coating based on the result of the spectrum analysis, and determining the porosity of a coating material according to the calculated longitudinal wave sound velocity. The ultrasonic detection method is greatly limited due to the large randomness of the porosity and the distribution state of the coating. Therefore, the invention constructs the ZrO through the in-situ modeling method of the SEM photo2The physical model with higher similarity of the actual pore morphology of the coating avoids the influence of randomly distributed pores on the simulation, and the relation between the porosity of the coating and the longitudinal wave sound velocity is researched by combining an ultrasonic numerical simulation technology. The modeling method provided by the invention can extract more characteristic parameters such as coating resonant frequency, coating longitudinal wave sound velocity and the like, and improves the accuracy of measured porosity by multi-parameter limitation.
The invention provides a modeling method for measuring the porosity of a coating, which is a flow chart of the modeling method for measuring the porosity of the coating, as shown in figure 1, and mainly comprises the following steps:
step S01, establishing a series of physical coating models with different porosities;
step S02, performing ultrasonic numerical simulation calculation on each coating physical model to obtain a time domain signal, performing frequency spectrum analysis processing on the time domain signal to obtain a sound pressure reflection coefficient amplitude spectrum, and obtaining a coating resonant frequency corresponding to each coating physical model from the sound pressure reflection coefficient amplitude spectrum;
step S03, calculating and obtaining the coating longitudinal wave sound velocity of each coating physical model based on the coating resonance frequency corresponding to each coating physical model;
step S04, obtaining the coating longitudinal wave sound velocity change rate corresponding to each coating physical model based on the coating longitudinal wave sound velocity of each coating physical model and the preset compact coating longitudinal wave sound velocity;
and step S05, fitting the porosity corresponding to each coating physical model and the change rate of the longitudinal wave sound velocity of the coating to obtain a measured coating porosity model.
In a specific one of the modeled examples, ZrO prepared using an atmospheric plasma spray method (APS method)2The coating is used as a detection object, the spraying material range of the APS method is wide, materials from a low melting point to a high melting point can be sprayed, the requirement on the granularity of spraying powder is not high, the porosity of the coating is low, and oxide inclusions are few, so that the method is one of effective methods for preparing the metal-based amorphous/nanocrystalline coating. As shown in FIG. 2, ZrO produced by the plasma spraying method2And (3) SEM pictures of the coating, wherein microcracks and pores are randomly distributed in the coating, and the organization structure presents certain heterogeneity. Due to the difference of the random appearance of the pores at different positions of the coating, the local medium density and the elastic modulus of the coating have certain fluctuation, so that the fluctuation of the ultrasonic sound velocity is caused. Thus, the longitudinal acoustic velocity is a multivariate function of porosity related to pore morphology, pore distribution, and the like. In order to conveniently obtain the relation between the longitudinal wave sound velocity and the porosity of the coating, a plurality of variables are avoided in the construction of a physical model. In step S01, a retained ZrO is used2The SEM photograph in-situ modeling method of the actual appearance and distribution of the pores in the coating is characterized in that a series of physical models with the same pore appearance and pore distribution but different porosities are constructed by taking an SEM photograph as a reference and combining with picture processing software. Through a computer program algorithm, the ratio of the pore area in the image to the whole image area is read, and the change range of the porosity of the physical model constructed in the embodiment is 5.7% -28.5%, and the porosity of the common ceramic coating is covered by 7% -25%, so that the physical model constructed in the embodiment has a wide application range and has great theoretical and practical significance for measuring the porosity of the coating. Compared with the traditional regular pore model and the random medium model, the physical model constructed by the in-situ modeling principle of the SEM photograph is used in the embodimentThe model is closer to ZrO2The actual appearance of the coating enables the measurement result to be more accurate; the pore morphology and pore distribution of the constructed model are the same, but the porosity is different, and the principle of controlling a single variable is followed, so that the change of the longitudinal wave sound velocity is only related to the porosity, and the measurement method disclosed by the invention is ensured to be more targeted and more rigorous.
In step S02, the present embodiment uses a finite difference time domain method (FDTD method) to perform simulation calculation on the physical model, where the FDTD method is simple in use and fast in calculation speed, and can simulate various complex structures by only giving corresponding parameters to each grid. In addition, because the finite difference time domain method adopts a stepping method to calculate, the simulation of various complex time domain broadband signals can be easily realized, and the time domain signal waveform of a certain point in space can be conveniently obtained. And carrying out frequency spectrum analysis processing on the time domain signal of the coating physical model obtained by simulation calculation to obtain the acoustic pressure reflection coefficient magnitude spectra of the coating under different porosities. As shown in FIG. 3, the ZrO contents at different porosities are examples of the present invention2The amplitude spectrum of the sound pressure reflection coefficient of the coating layer, and the frequency point corresponding to the minimum value in figure 3 is ZrO2The resonant frequency of the coating.
In step S03, the resonant frequency formula f is based on the sound pressure reflection coefficient spectrumnCalculating to obtain the longitudinal wave sound velocity of the coating layer, wherein fnAnd the resonant frequency is c, the longitudinal wave sound velocity of the coating is c, the coating thickness is d, the order of the resonant frequency is n, and the value is a positive integer. Wherein the coating thickness was set to 288 μm.
In step S4, according to fig. 3, different porosities correspond to different curves, and the corresponding longitudinal wave sound velocity of the coating is calculated according to the minimum value of the acoustic pressure reflection coefficient amplitude spectrum, so as to obtain the longitudinal wave sound velocity change rate e, S ═ of (c-c) of the longitudinal wave sound velocity of the coating corresponding to different porosities0)/c0In the formula c0Is a dense ZrO2The coating sound velocity and the longitudinal wave sound velocity of the dense ZrO2 coating are 4686.15 m/s.
In step S05, the coating longitudinal wave sound velocity change rate and the coating porosity are fitted to obtain a relationship diagram of the coating longitudinal wave sound velocity change rate and the coating porosity as shown in fig. 4. From FIG. 4, the longitudinal soundThe velocity change rate and the porosity of the coating have better linear relationship, thereby summarizing the ZrO2A relation model of the change of longitudinal wave sound velocity of the coating along with porosity: c ═ c0(1.05-2.09P), wherein P is porosity.
Based on the established model, the invention provides a method for measuring the porosity of the coating continuously, so as to solve the problem that the porosity cannot be effectively measured by an ultrasonic detection method in the prior art. As shown in fig. 5, a flow chart of a method for measuring the porosity of a coating according to the present invention mainly includes the following steps:
step S11, obtaining the porosity model of the measured coating;
step S12, obtaining an SEM picture of the coating of the area to be measured;
step S13, obtaining the coating thickness based on the SEM picture;
step S14, obtaining an ultrasonic reflection echo signal based on the coating, and carrying out frequency spectrum analysis on the ultrasonic reflection echo signal to obtain the resonant frequency of the coating;
step S15, obtaining the change rate of the longitudinal wave sound velocity of the coating based on the resonance frequency and the coating thickness;
and step S16, inputting the change rate of the longitudinal wave sound velocity of the coating into the relation model to obtain the porosity.
Specifically, in this embodiment, in step S13, the SEM photograph obtained in step S12 is subjected to image processing, and the ZrO is automatically calculated by mathematical software2Average coating thickness of (2).
In step S14, analog calculation is performed by an ultrasonic reflection echo method, and a frequency spectrum analysis is performed on the ultrasonic reflection echo signal to obtain a sound pressure reflection coefficient magnitude spectrum of the coating at different porosities. The result of the frequency spectrum analysis shows that two minimum values appear, and the frequency point corresponding to the minimum values is the resonance frequency of the coating. The size of a probe crystal used in the ultrasonic simulation process is 1mm, the central frequency of a pulse sound source is 5MHz, the probe crystal and the ZrO2 coating are coupled through a water immersion method, the longitudinal wave sound velocity of a water layer is 1497m/s, water is used as a coupling agent, the probe is guaranteed to move fast and flexibly, and measurement errors caused by temperature changes can be reduced.
In step S15, the coating thickness obtained in step S13 and the resonance frequency obtained in step S14 are substituted into the sound pressure reflection coefficient spectrum resonance frequency formula fnAnd obtaining the longitudinal wave sound velocity of the coating layer by n c/4 d.
In step S16, the coating longitudinal wave speed obtained in step S15 is substituted into the relationship model: c ═ c0(1.05-2.09P) to obtain the porosity.
This example compares ZrO2The coating sample is subjected to experimental measurement to obtain the coating longitudinal wave sound velocity and the coating longitudinal wave sound velocity obtained by the physical model through simulation calculation, so that the reasonability and the effectiveness of the physical model constructed by the method and the accuracy of a simulation result are verified. Method for measuring ZrO sprayed by atmosphere plasma by using ultrasonic pulse reflection echo method in experimental verification2The longitudinal wave sound velocity of the coating, the experimental parameters and the numerical simulation parameters are kept consistent. And (3) performing analog calculation by an ultrasonic pulse echo reflection method aiming at a narrow pulse sound source with the frequency of 5 MHz. Performing frequency spectrum analysis on the ultrasonic pulse reflection echo signal obtained in the experiment, wherein two minimum values appear in the amplitude spectrum of the sound pressure reflection coefficient, and the corresponding resonant frequencies are f3＝5.12MHz，f58.56 MHz. The resonance frequency obtained from the spectral analysis is substituted into the sound pressure reflection coefficient spectrum resonance frequency formula fnCalculated as n c/4d, according to the resonance frequency f3The calculated longitudinal wave speed of the coating is 2594.1m/s according to the resonant frequency f5The calculated longitudinal wave sound velocity of the coating is 2602.2m/s, and the average longitudinal wave sound velocity of the coating is 2598.2m/s, which is the experimental result. For ZrO after grinding and polishing2The porosity of the coating cross section was measured by image analysis and the average porosity of the coating was found to be 22.9%. And obtaining resonance frequency in a sound pressure reflection coefficient amplitude spectrum corresponding to the porosity of 22.9%, and further obtaining the coating longitudinal wave sound velocity of 2598.2m/s through a sound pressure reflection coefficient spectrum resonance frequency formula, namely obtaining a simulation result. The relative error between the experimental result and the simulation result is 3.82%, the simulation result is well matched with the experimental result, and the rationality of the simulation model construction and the validity of the simulation conclusion are further verified.
Therefore, the invention provides a method for establishing a model for measuring the porosity of a coating and a using method of the model, wherein the model for measuring the porosity of the coating is used for establishing a physical model according to an SEM photo in-situ modeling principle, is closer to the actual appearance of pores in the coating compared with other methods, the appearance of the pores of the model with different porosities is basically kept consistent, and the principle of controlling a single variable is followed, so that the porosity is only changed along with the longitudinal wave sound velocity, and the research result is more pertinent, more strict and more persuasive. The application method of the model for measuring the porosity of the coating comprises the steps of analyzing the amplitude spectrum of the ultrasonic reflection echo signal of the coating to obtain the change rate of the longitudinal wave sound velocity of the coating under different porosities, and determining the porosity of the coating according to the relation model. The invention has no damage to the coating in the measuring process and small measuring error.
The above description is only exemplary of the present invention and should not be taken as limiting the invention, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A method for establishing a model for measuring the porosity of a coating is characterized by comprising the following steps:
step S01, establishing a series of physical coating models with different porosities;
step S02, performing ultrasonic numerical simulation calculation on each coating physical model to obtain a time domain signal, performing frequency spectrum analysis processing on the time domain signal to obtain a sound pressure reflection coefficient amplitude spectrum, and obtaining a coating resonant frequency corresponding to each coating physical model from the sound pressure reflection coefficient amplitude spectrum;
step S03, calculating and obtaining the coating longitudinal wave sound velocity of each coating physical model based on the coating resonant frequency corresponding to each coating physical model;
step S04, obtaining a coating longitudinal wave sound velocity change rate corresponding to each coating physical model based on the coating longitudinal wave sound velocity of each coating physical model and a preset compact coating longitudinal wave sound velocity;
and step S05, fitting the porosity of each coating physical model and the change rate of the coating longitudinal wave sound velocity to obtain the measured coating porosity model.
2. The method for constructing a model of porosity of a coating according to claim 1, wherein the step S03 is performed by inputting the resonance frequency and the predetermined thickness of the coating into the formula fnObtaining longitudinal wave speed of the coating layer by n c/4d, wherein fnAnd d is the resonant frequency, d is the thickness of the coating, n is the order of the resonant frequency, and the value is a positive integer.
3. The method for establishing the model for measuring the porosity of the coating according to claim 1, wherein in the step S05, the porosity and the coating longitudinal sound velocity exist according to the formula c-c0(1.05-2.09P), wherein c is the coating longitudinal wave sound velocity, c0Is the longitudinal wave sound velocity of the dense coating, and P is the porosity of the coating.
4. The method for building a model for measuring the porosity of a coating according to claim 1, wherein in the step S01, the physical model of the coating is built according to the in-situ modeling principle of SEM photos.
5. The method for building the model for measuring the porosity of the coating according to claim 1, wherein the physical model of the coating adopts a finite difference time domain method to perform ultrasonic numerical simulation calculation.
6. The method for building a model for measuring the porosity of a coating according to claim 5, wherein the parameters used in the ultrasonic numerical simulation calculation include: the center frequency of the pulse sound source was 5MHz, and the thickness of the coating was 288. mu.m.
7. A method of using a model for measuring porosity of a coating, comprising the steps of:
step S11, obtaining a measured coating porosity model according to any one of claims 1-6;
step S12, obtaining a coating of the area to be measured to obtain an SEM picture;
step S13, obtaining the coating thickness based on the SEM picture;
step S14, obtaining an ultrasonic reflection echo signal based on the coating, and carrying out frequency spectrum analysis on the ultrasonic reflection echo signal to obtain the resonant frequency of the coating;
step S15, obtaining the change rate of the longitudinal wave sound velocity of the coating based on the resonance frequency and the coating thickness;
and step S16, inputting the change rate of the longitudinal wave sound velocity of the coating into the relation model to obtain the porosity.
8. The method of using the measured coating porosity model of claim 7, wherein in step S13, the coating thickness is calculated by image processing of SEM pictures of the coating and using mathematical software.
9. The method of using the model for measuring porosity of coating according to claim 7, wherein the step S15 is implemented by inputting the resonance frequency and the coating thickness into the formula fnObtaining longitudinal wave speed of the coating layer by n c/4d, wherein fnAnd d is the resonant frequency, d is the thickness of the coating, n is the order of the resonant frequency, and the value is a positive integer.
10. The method as claimed in claim 7, wherein the step S16 is implemented by inputting the longitudinal sonic velocity of the coating and the pre-obtained longitudinal sonic velocity of the dense coating into a relation model c ═ c0(1.05-2.09P) obtaining the porosity, wherein c is the coating longitudinal wave sound velocity, c0Is the longitudinal wave sound velocity of the dense coating, and P is the porosity of the coating.
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