CN104457635A - Ultra-thin coating thickness uniformity lossless detection method based on Welch method spectral estimation - Google Patents

Abstract

The invention provides an ultra-thin coating thickness uniformity lossless detection method based on Welch method spectral estimation. The method includes the steps that an ultrasonic microscope system is used for conducting full-wave acquisition, for ultrasonic A scanning signals formed by aliasing coating upper surface reflection echoes and coating lower surface n-time reflection echoes acquired in each scanning point, coating upper surface echo signals in the A scanning signals are removed through the pulse duration of a focusing probe, and coating lower surface n-time reflection echo signals are acquired, wherein the acoustic beam reflection and transmission spreading principle is shown in an attached map; then, Welch method spectral estimation is conducted on the n-time echo signals, frequencies corresponding to various maximum values are read on a Welch power spectrum, the thickness of a coating is acquired through calculation in combination with the sound velocity of the coating, and the thickness values are converted into corresponding colors for representation; finally the thickness values of scanning points are acquired through calculation in sequence and represented through the corresponding colors, and then a C scanning imaging map for coating thickness uniformity evaluation can be formed. The method is simple, practical, high in measurement speed, and suitable for coating field measurement.

Description

Based on the microthin coating thickness evenness lossless detection method of Welch method Power estimation

One, technical field

What the present invention proposed is a kind of microthin coating thickness evenness lossless detection method based on Welch method Power estimation, belongs to material Ultrasonic NDT and assessment technique field.

Two, background technology

Process for surface coating is widely used in Aero-Space, information electronic, medicine and other manufacturing industry, and the thickness in expression coating and homogeneity thereof directly have influence on the quality of coating performance.Therefore, the quality quick and precisely Non-Destructive Testing of workpiece coating processing is necessary.

At present, utilize supercritical ultrasonics technology to detect coating thickness and mainly comprise ultrasonic pulse-echo, these two kinds of technology of ultrasonic surface wave.Ultrasonic pulse-echo is utilized to measure coating thickness mainly based on the various superonic spectrum analytical approachs of principle of interference, obtain in paper " The application ofbroadband ultrasonic spectroscopy to the study of layered media " as N.F Haines etc. the sound pressure reflection coefficient coefficient spectrum of steel surface corrosion layer and phase spectrum resonance frequency and and then determine the thickness of corrosion layer, patent (Lin Li, Hu Zhixiong etc. based on the thickness of thin layer supersonic detection method [P] of sound pressure reflection coefficient autocorrelation function. application number: 201310036748,2013.) autocorrelation function is utilized to carry out ultrasonic thickness measurement to aluminium matter thin layer specimen in, patent (Lei Mingkai, Lin Li etc. a kind of method [P] of ultrasonic signal spectral filtering technology in nondestructive mode coating thickness. application number: 201310577801,2013) sound pressure reflection coefficient method is utilized to obtain coating thickness in conjunction with spectral filtering technology, but above-mentioned ultrasonic measurement method is subject to the interference of upper surface echoed signal and the restriction of frequency spectrum analysis method thereof, accurately resonance frequency can not be obtained in some cases on spectrogram.Utilize ultrasonic surface wave technology for detection coating thickness, mainly according to sound wave dispersion equation in the coating, by measuring coating phase velocities dispersion curve, then coating thickness is calculated in conjunction with inversion technique, because coating thickness is many at some tens of pm to hundred micron order, therefore the surface wave frequency excited needed for is many in 40MHz-200MHz scope, and current over-borrowing helps laser to excite, but factor limits it quotes because optoacoustic conversion efficiency is low, echoed signal is weak and detection sensitivity is low etc.And up to now, not yet find that there is the correlative study that layer thickness uniformity is detected.

Three, summary of the invention

The object of this invention is to provide a kind of microthin coating thickness evenness lossless detection method based on Welch method Power estimation, may be used for quick, the Measurement accuracy of coating thickness and homogeneity thereof.

Technical scheme of the present invention is: a kind of coating thickness ultrasonic measurement method based on Welch method Power estimation, comprises the ultrasonic micro-scanning system of supersonic detection device, water immersion focusing probe, coating sample, drive unit, ondograph and computing machine composition.The measuring process adopted is as follows:

(1) the distance d of probe and sample is first calculated according to formula 1 _w, then focusing probe is placed in sample upper surface, and ensures that probe Main beam axis and specimen surface are disposed, regulate the distance of probe and sample upper surface to d _w, formula 1:

d _wb＝d _f-(C _T/C _w)t

In formula: d _wbrepresent probe and surperficial distance, C _tfor the substrate velocity of sound, C _wfor the velocity of sound in water, t is collective's thickness, d _ffor probe focal length.

(2) utilize ultrasonic microscopic system to coating sample Vertical Launch compressional wave, and utilize ondograph gather coating any upper surface reflection echo and the A that formed of lower surface n echo aliasing sweep signal.

(3) according to the duration of pulse of adopted focusing probe, the A that removal (2) collects sweeps the upper surface echo of waveform, obtains lower surface n echoed signal.

(4) by n echoed signal x in described (3) _nn () substitutes into formula 2, obtain the Welch power spectrum of coating sample, formula 2:

$P_{\mathrm{per}}\left(f\right)=\frac{1}{\mathrm{MUL}}\underset{i=1}{\overset{L}{\mathrm{\Σ}}}{\left|\underset{n=0}{\overset{M-1}{\mathrm{\Σ}}}{x}_N^i\left(n\right)d_2\left(n\right)e^{-\mathrm{jwn}}\right|}^2$

In formula: P _perf () represents the Welch power spectrum of sample, L represents the hop count that data are divided into, and M represents the data length of each section, and U represents normalized factor, d ₂n () represents Hamming window.

(5) from formula 3, P _perf () figure there will be maximum value, the frequency of its correspondence is relevant to coating thickness.The P asked in (4) _perfrequency f corresponding to two adjacent maximum value is read in (f) ₁, f ₂, and calculate Δ f (Δ f=f ₂-f ₁).Formula 3:

(6) by the Δ f that calculates in (5) and coating velocity of sound c ₂substitute into formula 3 and just can try to achieve any thickness d of coating.Formula 4:

$d=\frac{c_2}{2\mathrm{\Δf}}$

(7) ultrasonic micro-scanning system C scanning collection is utilized to obtain the all-wave data of coating sample, the thickness distribution C that the Data Post program of then writing according to (1) ~ (6) described Welch method Power estimation method obtains corresponding coating sweeps figure, utilize the thickness range that discrete color value represents different, for assessment of the homogeneity of coating thickness.

Four, accompanying drawing explanation

The ultrasonic micro-scanning system schematic of Fig. 1

Fig. 2 coating sample A sweeps waveform

The duration of pulse of Fig. 3 probe

The lower surface n secondary reflection echoed signal of Fig. 4 coating

The Welch power spectrum of Fig. 5 ceramic coat sample

The C of Fig. 6 coating sample thickness distribution sweeps figure

Five, embodiment

Below the specific embodiment of the present invention is described in detail:

As shown in Figure 1, for measured coating sample, (ceramic coat that coating sample adopts plasma spraying to prepare records its one-tenth-value thickness 1/10 d (μm) by digit microscope, velocity of sound c ₂(m/s)), adopt water immersion focusing probe launch to coating sample and accept ultrasonic signal, utilize oscillograph to complete waveform observation and data acquisition, obtain A and sweep waveform as shown in Figure 2.

As shown in Figure 4, the burst length (shown in Fig. 3) according to probe utilizes computing machine to sweep to the A obtained the removal that waveform carries out coating upper surface echoed signal, thus obtains the n secondary reflection echoed signal of coating lower surface.The Power estimation utilizing the n secondary reflection echoed signal of software to acquisition to carry out Welch method calculates the Welch power spectrum of coating sample, as shown in Figure 5.Then the frequency reading on Welch power spectrum two maximum value corresponding is f ₁(MHz), f ₂, thus Δ f (MHz) can be calculated (MHz).By Δ f (MHz) and c ₂(m/s) namely substitution formula 3 can calculate the thickness of ceramic coat sample is d (μm), and the absolute error between digit microscope measured value is Δ d (μm), and relative error range is 4% ~ 6%.

In order to the homogeneity realizing coating thickness detects, ultrasonic micro-scanning system is utilized to carry out C scanning to sample, and the full-time domain waveform data of the A sweep signal storing each scanning point form the all-wave data of sample, carry out obtaining its frequency spectrum based on the algorithm of Welch Power estimation to echo in data gate in the A sweep signal of each point, obtain spectrogram appreciate two maximum value respective frequencies and calculated the one-tenth-value thickness 1/10 of this point by publicity 4, its numerical value is converted to corresponding color to characterize, the last one-tenth-value thickness 1/10 calculating each analyzing spot successively, and represent by corresponding color, just the C scanning imagery figure of coating thickness distribution as shown in Figure 6 can be completed, this figure can be used for the assessment of layer thickness uniformity.

Claims (1)

1. the microthin coating thickness evenness lossless detection method based on Welch method Power estimation, comprise the ultrasonic microscope system of supersonic detection device, water immersion focusing probe, coating sample, drive unit, ondograph and computing machine composition, it is characterized in that: the measuring process that described method adopts is as follows:

(1) the distance d of probe and sample is first calculated according to formula 1 _w, then focusing probe is placed in sample upper surface, and ensures that probe Main beam axis and specimen surface are disposed, regulate the distance of probe and sample upper surface to d _w, formula 1:

d _wb＝d _f-(C _T/C _w)t

In formula: d _wbrepresent probe and surperficial distance, C _tfor the substrate velocity of sound, C _wfor the velocity of sound in water, t is collective's thickness, d _ffor probe focal length.

(2) utilize ultrasonic microscopic system to coating sample Vertical Launch compressional wave, and utilize oscillograph gather coating any upper surface reflection echo and the A that formed of lower surface n echo aliasing sweep signal.

(3) according to the duration of pulse of adopted focusing probe, the A that removal (2) collects sweeps the upper surface echo of waveform, obtains lower surface n echoed signal.

(4) by n echoed signal x in described (3) _nn () substitutes into formula 2, obtain the Welch power spectrum of coating sample, formula 2:

$P_{\mathrm{per}}\left(f\right)=\frac{1}{\mathrm{MUL}}\underset{i=1}{\overset{L}{\mathrm{\Σ}}}{\left|\underset{n=0}{\overset{M-1}{\mathrm{\Σ}}}{x}_N^i\left(n\right)d_2\left(n\right)e^{-\mathrm{jwn}}\right|}^2$

In formula: P _perf () represents the Welch power spectrum of sample, L represents the hop count that data are divided into, and M represents the data length of each section, and U represents normalized factor, d ₂n () represents Hamming window.

(5) from formula 3, P _perf () figure there will be maximum value, the frequency of its correspondence is relevant to coating thickness.The P asked in (4) _perfrequency f corresponding to two adjacent maximum value is read in (f) ₁, f ₂, and calculate Δ f (Δ f=f ₂-f ₁).Formula 3:

(6) by the Δ f that calculates in (5) and coating velocity of sound c ₂substitute into formula 3 and just can try to achieve any thickness d of coating.Formula 3:

$d=\frac{c_2}{2\mathrm{\Δf}}$

(7) ultrasonic micro-scanning system C scanning collection is utilized to obtain the all-wave data of coating sample, the thickness distribution C that the Data Post program of then writing according to (1) ~ (6) described Welch method Power estimation method obtains corresponding coating sweeps figure, utilize the thickness range that discrete color value represents different, for assessment of the homogeneity of coating thickness.