阅读说明：本技术 基于光栅激光超声声谱的薄膜涂层厚度无损测量方法 (Method for nondestructive measurement of thickness of thin film coating based on grating laser ultrasonic acoustic spectrum ) 是由 裴翠祥 钱程 寇兴 于 2021-07-12 设计创作，主要内容包括：本发明公开了一种基于光栅激光超声声谱的薄膜涂层厚度无损测量方法,强脉冲激光束经过光学掩模板或微阵列透镜照射到被测涂层表面形成周期分布的微小栅状激光光斑,在被测区域激发固定波长的超声表面波,由于超声表面波在多层介质中的频散特性,涂层厚度的变化会引起所激发表面波频率的变化,采用激光超声检测单元在激励位置附近接收所产生的超声表面波信号,对被接收的信号进行傅里叶变换获取其中心频率,最后根据超声表面波中心频率与涂层厚度的单调关系实现涂层厚度的无损测量。(The invention discloses a method for nondestructively measuring the thickness of a thin film coating based on grating laser ultrasonic sound spectrum, which comprises the steps of irradiating a strong pulse laser beam onto the surface of a measured coating through an optical mask plate or a micro-array lens to form periodically distributed micro-grating-shaped laser spots, exciting an ultrasonic surface wave with fixed wavelength in the measured area, receiving a generated ultrasonic surface wave signal near an excitation position by a laser ultrasonic detection unit due to the frequency dispersion characteristic of the ultrasonic surface wave in a multi-layer medium, carrying out Fourier transform on the received signal to obtain the central frequency of the ultrasonic surface wave signal, and finally realizing the nondestructive measurement of the thickness of the coating according to the monotonic relation between the central frequency of the ultrasonic surface wave and the thickness of the coating.)

1. A method for nondestructive measurement of the thickness of a thin film coating based on grating laser ultrasonic sound spectrum is characterized in that: firstly, forming a grid-shaped laser spot (4) with a period of d by using an equidistant optical mask plate or a micro-array lens (2) to irradiate a high-energy pulse laser beam (1) on the surface of a measured thin film coating structure (3), and exciting an ultrasonic surface wave (5) with a corresponding wavelength of lambda on the surface of the measured thin film coating structure under the excitation of periodic thermal expansion, wherein lambda is d; then, the laser ultrasonic detection unit (6) receives the ultrasonic surface wave signal (7) near the detected area, and carries out fast Fourier transform to obtain a frequency domain signal (8) thereof, and the central frequency of the frequency domain signal is measured; due to the frequency dispersion characteristic of the ultrasonic surface wave in the multilayer medium, the change of the coating thickness of the measured film coating structure (3) can cause the change of the center frequency of the excited ultrasonic surface wave signal, and finally, the nondestructive measurement of the coating thickness is realized according to the corresponding relation between the center frequency of the received ultrasonic surface wave signal and the coating thickness.

2. The method for nondestructive measurement of the thickness of a thin film coating based on grating laser ultrasonic acoustic spectroscopy according to claim 1, wherein the method comprises the following steps: the high-energy pulse laser beam (1) is a collimated laser beam with the pulse length of nanosecond level and is used for exciting the ultrasonic surface wave; the period d of the grid-shaped laser spot (4) is larger than the thickness of the measured coating.

3. The method for nondestructive measurement of the thickness of a thin film coating based on grating laser ultrasonic acoustic spectroscopy according to claim 1, wherein the method comprises the following steps: the grating-shaped laser spot (4) with the period d excites an ultrasonic surface wave (5) with a fixed wavelength lambda on the surface of the measured thin film coating structure (3), the change of the coating thickness of the measured thin film coating structure (3) can cause the change of the wave velocity v of the excited ultrasonic surface wave (5) due to the frequency dispersion characteristic of the ultrasonic surface wave (5) in a multilayer medium, and the change of the coating thickness of the measured thin film coating structure (3) can inevitably cause the change of the central frequency (8) of the excited surface wave due to the v ═ lambadaf existing among the ultrasonic wave velocity v, the wavelength lambda and the frequency f.

4. The method for nondestructive measurement of the thickness of a thin film coating based on grating laser ultrasonic acoustic spectroscopy of claim 1 comprising the steps of:

step 1: firstly, preparing n test pieces with different coating thicknesses, the serial numbers of which are respectively 1,2, … and n, the test pieces are the same as the structure and the material of the film coating to be measured, measuring the different test pieces by adopting a film coating thickness nondestructive measurement method based on grating laser ultrasonic spectrum, and obtaining the surface wave center frequency f of the film coating test pieces with different thicknesses_iWherein i is 1,2, …, n;

step 2: dissecting and micro-measuring the n test pieces to obtain the accurate coating thickness h_iData;

and step 3: according to the thickness h of the coating_iAnd the measured surface wave center frequency f_iEstablishing a fitting relation curve between the center frequency f of the surface wave and the coating thickness h as a measurement standard curve;

and 4, step 4: measuring a film coating test piece with unknown thickness by adopting a grating laser ultrasonic acoustic spectrum-based film coating thickness nondestructive measurement method to obtain the surface wave center frequency f_xAccording to the established f-h standard curve, the coating thickness h is measured_xQuantitative evaluation was performed.

Technical Field

The invention relates to a nondestructive measurement method of coating thickness, in particular to a nondestructive measurement method of the coating thickness of a thin film based on grating laser ultrasonic acoustic spectrum.

Background

The coating is a solid continuous film structure which is sprayed on the surface of a base material for achieving the purposes of protection, heat insulation, wear resistance and the like, is widely applied to high-temperature structural components such as aircraft engines, heavy-duty gas turbines and the like, is deposited on the surface of a high-temperature-resistant alloy base material, reduces the working temperature of the components and prolongs the service life of the components. Monitoring of the coating thickness is essential during production manufacturing and long-term service. The reduction in coating thickness is a significant cause of coating failure. For an aircraft engine, the thickness of the coating is generally 100-400 μm, the thicker the coating is, the better the heat insulation effect is, but the bonding strength with a substrate material is reduced along with the increase of the thickness, so that the aircraft engine is easier to peel off; if the coating is too thin, the heat insulation effect is affected, and the service life is reduced. Therefore, the thickness of the coating is one of the key factors influencing the service life of the coating, and accurate measurement of the thickness of the coating is an important problem to be solved urgently.

The ultrasonic detection is one of the main nondestructive detection methods at present, and has the advantages of easy operation, strong detection capability, wide application range and the like. When the thickness of a coating is measured by the traditional ultrasonic detection method, the thickness is measured by a time difference calculation method by generally utilizing body waves and reflected waves thereof. The method has the advantages that the phenomenon of signal confusion of initial waves and reflected waves can occur when the coating is thin, the signals are difficult to distinguish on time domain signals, the surface blind area occurs in the measurement of the thickness of the coating, the effective measurement of the thickness of the coating cannot be realized, the defects that the complete non-contact measurement cannot be realized, the detection cannot be realized in severe environments such as high temperature and the like exist, and the method cannot be widely applied in the production, manufacturing and service processes of the existing coating.

Disclosure of Invention

The invention aims to provide a nondestructive measurement method for the thickness of a thin film coating based on grating laser ultrasonic acoustic spectroscopy, which can form a grating laser heat source by regulating and controlling laser beams so as to excite ultrasonic surface waves with narrow frequency bands. The high sensitivity and high spatial resolution of the surface wave are utilized, the wave velocity is accurately measured through ultrasonic sound spectrum, and the coating thickness is determined by combining the frequency dispersion characteristic of the surface wave in a multilayer medium, so that the aim of accurately detecting the coating thickness is fulfilled. The method avoids the signal confusion phenomenon which may occur in the traditional ultrasonic method aiming at the thin film coating, effectively overcomes the surface blind area problem of the traditional ultrasonic method, and has the advantages of complete non-contact type, realization of working in severe environments such as high temperature and the like compared with the traditional ultrasonic detection means.

In order to achieve the above purpose, the invention adopts the following technical scheme:

a method for nondestructively measuring the thickness of a film coating based on grating laser ultrasonic sound spectrum comprises the steps of firstly utilizing an equidistant optical mask plate or a micro-array lens 2 to enable a high-energy pulse laser beam 1 irradiated on the surface of a film coating structure 3 to be measured to form a grating-shaped laser spot 4 with a period of d, exciting an ultrasonic surface wave 5 with a corresponding wavelength of lambda (lambda is d) on the surface of the film coating structure to be measured under the excitation of periodic thermal expansion, receiving an ultrasonic surface wave signal 7 near a measured area by a laser ultrasonic detection unit 6, carrying out fast Fourier transform to obtain a frequency domain signal 8 of the ultrasonic surface wave signal, and measuring the corresponding central frequency of the frequency domain signal. Due to the frequency dispersion characteristic of the ultrasonic surface wave in the multilayer medium, the change of the coating thickness of the measured thin film coating structure 3 can cause the change of the center frequency of the excited ultrasonic surface wave, and finally, the nondestructive measurement of the coating thickness is realized according to the corresponding relation between the center frequency of the received ultrasonic surface wave signal and the coating thickness.

The high-energy pulse laser beam 1 is a pulse length nanosecond-level collimation laser beam and is used for exciting an ultrasonic surface wave; the period d of the grid-shaped laser spot 4 should be larger than the thickness of the measured coating.

The method for nondestructively measuring the thickness of the film coating based on the grating laser ultrasonic acoustic spectrum comprises the following steps:

step 1: firstly, preparing n test pieces (the serial numbers are respectively i-1, 2, … and n) which have the same structure and material with the thin film coating to be measured and have different coating thicknesses, measuring the different test pieces by adopting a thin film coating thickness nondestructive measurement method based on grating laser ultrasonic spectrum, and obtaining the surface wave center frequency f of the thin film coating test pieces under different thicknesses_i(i＝1,2,…,n)；

Step 2: dissecting and micro-measuring the n test pieces to obtain the accurate coating thickness h_i(i ═ 1,2, …, n);

and step 3: according to the thickness h of the coating_iAnd the measured surface wave center frequency f_i(i is 1,2, …, n), and establishing a fitting relation curve between the center frequency f of the surface wave and the coating thickness h as a measurement standard curve;

and 4, step 4: the thickness of the film coating based on the grating laser ultrasonic acoustic spectrum is not thickMeasuring a film coating test piece with unknown thickness by a loss measuring method to obtain the surface wave center frequency f_xAccording to the established f-h standard curve, the coating thickness h is measured_xQuantitative evaluation was performed.

According to the invention, the narrow-band ultrasonic surface wave with known wavelength is excited by the laser-induced transient grating, the wave velocity of the narrow-band ultrasonic surface wave is determined by the spectral analysis, and the coating thickness is determined by utilizing the frequency dispersion characteristic of the surface wave in a multilayer medium. Meanwhile, compared with the traditional ultrasonic detection means, the method has the advantages of complete non-contact type, capability of working in severe environments such as high temperature and the like.

Drawings

FIG. 1 is a general schematic diagram of a method for nondestructive measurement of the thickness of a thin film coating based on grating laser ultrasonic acoustic spectroscopy.

FIG. 2 is an ultrasonic acoustic spectrum signal for thin film coating measurement at different thicknesses according to the present invention.

FIG. 3 is a standard curve of the present invention for thin film coating measurements at different thicknesses.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

The invention provides a method for nondestructively measuring the thickness of a thin film coating based on grating laser ultrasonic acoustic spectrum, which is shown in figure 1. Firstly, a high-energy pulse laser beam 1 irradiated on the surface of a measured thin film coating structure 3 forms a grid-shaped laser spot 4 with a period of d by using an equidistant optical mask plate or a micro-array lens 2, an ultrasonic surface wave 5 with a corresponding wavelength of lambda (lambda is d) is excited on the surface of the measured thin film coating structure under the excitation of periodic thermal expansion, an ultrasonic surface wave signal 7 is received near a measured area by a laser ultrasonic detection unit 6, a frequency domain signal 8 is obtained by performing fast Fourier transform, and the corresponding central frequency of the frequency domain signal is measured. Due to the frequency dispersion characteristic of the ultrasonic surface wave in the multilayer medium, the change of the coating thickness of the measured film coating 3 can cause the change of the center frequency 8 of the excited ultrasonic surface wave, and finally, the nondestructive measurement of the coating thickness is realized according to the corresponding relation between the center frequency of the received ultrasonic surface wave signal and the coating thickness.

As a preferred embodiment of the present invention, the high-energy pulse laser beam 1 is a collimated laser beam with a pulse length of nanometer order, and is used for excitation of an ultrasonic surface wave; the period d of the grid-shaped laser spot 4 should be larger than the thickness of the measured coating.

The basic detection principle of the method is as follows: the high-energy pulse laser beam 1 passes through an equidistant optical mask plate or a micro-array lens 2 and then irradiates the surface of a measured thin film coating structure 3 to form a grid-shaped laser spot 4 with a period d, the surface of the measured thin film coating structure 3 excites an ultrasonic surface wave 5 with a fixed wavelength lambda (lambda is d) under periodic thermal elastic excitation, the change of the coating thickness of the measured thin film coating structure 3 can cause the change of the wave velocity v of the excited surface wave 5 due to the frequency dispersion characteristic of the ultrasonic surface wave 5 in a multilayer medium, and the change of the coating thickness of the measured thin film coating structure 3 can inevitably cause the change of the center frequency 8 of the excited surface wave due to the v is lambda f among the ultrasonic wave velocity v, the wavelength lambda and the frequency f; acquiring data of the center frequency 8 of the ultrasonic surface wave frequency domain signal excited by different film coating structures to be measured 3, and establishing a fitting relation curve of the center frequency of the ultrasonic surface wave signal and the coating thickness as a measurement calibration curve; based on the calibration curve, the thickness of the coating of the test piece can be finally determined by measuring the center frequency of the ultrasonic surface wave signal propagated on the surface of the thin film coating structure with unknown coating thickness by the method.

The proposed measurement method is described in further detail below with reference to fig. 2, 3 and the detailed description.

The specific implementation mode of the invention comprises the following steps:

step 1: firstly, preparing n test pieces (the serial numbers are respectively 1,2, … and n) which have the same structure and material with the thin film coating to be measured and have different coating thicknesses, measuring the different test pieces by adopting the grating laser ultrasonic sound spectrum nondestructive measurement method, and obtaining the surface wave center frequency f of the thin film coating test pieces under different thicknesses_i(i-1, 2, …, n) such asFIG. 2 is a schematic illustration;

step 2: dissecting and micro-measuring the n test pieces to obtain the accurate coating thickness h_i(i ═ 1,2, …, n);

and step 3: according to the thickness h of the coating_iAnd the measured surface wave center frequency f_i(i is 1,2, …, n), and establishing a fitted relation curve between the center frequency f of the surface wave and the coating thickness h as a measurement standard curve, as shown in fig. 3;

and 4, step 4: measuring a film coating test piece with unknown thickness by adopting a grating laser ultrasonic acoustic spectrum-based film coating thickness nondestructive measurement method to obtain the surface wave center frequency f_xAccording to the established f-h standard curve, the coating thickness h is measured_xQuantitative evaluation was performed.