JPH0518833A - Residual stress measuring method and lens used for the same - Google Patents

Abstract

PURPOSE:To enable distribution of a residual stress on a surface of a material to be evaluated quantitatively by obtaining an amount of change in a speed of a surface acoustic wave under presence of a stress for the speed of the surface acoustic wave while a surface of an object to be measured is not subjected to stress. CONSTITUTION:An ultrasonic wave is emitted while an acoustic lens 112 is scanned in depth direction of a measurement material 111 in stress state, an interference wave which is generated due to a mirror-surface reflection wave and an excitation surface acoustic wave at an irradiation region and an internal excitation wave are transmitted to a computer 211 through a circulator 113 and an interface 114. and then a periodical change in interference wave intensity from an amplified irradiation excitation wave is displayed 212 as a V(Z) curve. Then, this curve is subjected to a high-speed Fourier transformation by a conversion portion 213, thus obtaining a surface acoustic wave speed VSAW. A difference in reference to the speed VSAW when no stress is being applied to namely an amount of change, is obtained for each scanning position, thus obtaining a distribution in axial direction of a junction body and that in interface direction. Therefore, a residual stress distribution can be measured highly accurately since a distribution of the speed VSAW can be obtained accurately.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a residual stress measuring method and a lens used therefor.

[0002]

2. Description of the Related Art Ceramics are characterized by being lightweight and having high hardness, and high strength and wear resistance due to this hardness are positively utilized. Furthermore, the advantages of ceramics are exhibited in the high temperature range of 1000 ° C or higher. For example, silicon nitride has a heat resistance up to 1200 ° C, and silicon carbide has a heat resistance up to around 1500 ° C, and it can be expected to greatly improve the thermal efficiency when used in a temperature range where a metal cannot withstand.

However, since ceramics are inherently brittle materials, it is difficult to use them alone, and it is rational to use them in combination with other materials, such as using the ceramics only in the parts where the characteristics are required. As such a joining technique of ceramics, a method using mechanical bonding or an organic adhesive, or a chemical joining method involving some reaction such as diffusion of constituent elements at the interface, solid solution, formation of reaction product, etc. Etc. have been tried.

As the chemical bonding method of ceramics, in the case of structural ceramics, a solid phase bonding method of directly bonding without inclusions, or an inserter that easily reacts with the ceramics.
The active metal method using a metal material is the mainstream, and the metallizing method is used for semiconductor substrates. Then, in order to investigate the residual stress distribution of such a bonded structure, methods such as a micro-bundle X-ray method, an indentation fracture method, and a strain gauge method are usually used.

[0005]

By the way, the coefficient of thermal expansion of ceramics is smaller than that of metals, and among them, particularly useful as structural ceramics, silicon nitride, silicon carbide and the like having excellent heat resistance have a very high coefficient of thermal expansion. Is small. Therefore, when the ceramic and the metal material are joined, residual stress is generated due to the difference in thermal expansion during the cooling process after joining, which causes various problems.

That is, a large residual stress is generated in the vicinity of the joint, particularly in the vicinity of a singular point at the joint interface, and the joint strength is significantly reduced due to the synergistic effect with the external stress, or the main stress is caused by the cooling process after the joint or the thermal cycle. There was a problem that cracks were generated from the maximum point and the ceramics was destroyed.

Under these circumstances, a method for quantitatively evaluating the residual stress on the ceramic surface from a microscopic viewpoint is desired.

The present invention has been made in order to solve such a problem, and provides a residual stress measuring method capable of quantitatively evaluating the distribution of residual stress on the material surface, and a lens used therefor. The purpose is to

[0009]

According to the residual stress measuring method of the present invention, a smooth surface of an object to be measured is provided with a distance between the surface and an acoustic lens that emits ultrasonic waves having anisotropy in an irradiation area on the surface. The ultrasonic wave is radiated while being changed, and the intensity of the interference wave between the specular reflection wave of the ultrasonic wave, the surface acoustic wave excited, and the internal excitation wave in the irradiation area with respect to the irradiation distance in the depth direction of the ultrasonic wave. V (Z) curve representing the relationship between the heights is calculated, and the surface acoustic wave velocity in the irradiation excitation region of the surface of the measured object in the stress state is calculated from the curve period of the V (Z) curve.
Using the surface acoustic wave velocity in the stress-free state of the measured object surface as a reference velocity, the amount of change between the surface acoustic wave velocity in the presence of stress and the reference velocity is obtained, and based on the distribution of this amount of change, calibration is performed. It is characterized in that a curve is used to convert into a residual stress value in the long side direction of the ultrasonic wave irradiation region to obtain a distribution in a specific direction.

Further, the lens of the present invention is used for measurement by an ultrasonic microscope, and gives anisotropic ultrasonic waves having an aspect ratio of 1.5 or more at the time of focusing on the surface of an object to be measured,
It is characterized by a rectangular or elliptical shape.

In the present invention, the measurement sample is set as shown in FIG. 4, for example. Sample 2 placed on stage 1
The surface of is coupled with a coupler 3 such as water, and ultrasonic waves S are transmitted to the sample through the coupler 3. If ultrasonic waves are directly passed through the air without such a coupler, the speed of the ultrasonic waves is attenuated and the measured values vary, so that good measurement results cannot be obtained. The specular reflection wave R reflected from the sample passes through the acoustic lens 4 and is converted into a signal by the transducer 5 together with the surface acoustic wave and the interference wave excited on the sample surface at this time.

Further, at this time, as shown in FIG. 5, it is preferable to irradiate the ultrasonic wave S with the measurement sample 2 placed in a thermostat. The thermostatic device shown in FIG. 5 includes a stage 11 with a built-in heater, a heat insulating mantle 12, and a temperature measuring thermocouple 13.
It is mainly composed of Stage 1 with built-in heater
For example, water is contained as a heat transfer medium 14 in the heat insulation mantle 12 placed on the measurement sample 2, and the measurement sample 2 is arranged in the water 14. This water 14 also functions as a coupler (3). The acoustic lens 4 and the temperature measuring thermocouple 1 are located above the heat insulating mantle 12.
It is covered with a heat retaining lid 15 having three insertion openings 15a and 15b. Then, the temperature of the heat transfer medium 14 is measured at any time by the temperature measuring thermocouple 13, and the stage 11 having the heater therein is PID-controlled on the basis of the measured temperature, whereby the surface of the measurement sample 2 arranged in the heat transfer medium 14 is measured. The temperature distribution of is controlled to be substantially constant. Measurement sample 2 at this time
The surface temperature is preferably controlled within ± 0.05 ° C.

In the present invention, the ultrasonic waves applied to the object to be measured are anisotropic ultrasonic waves, and are adjusted so that the aspect ratio of the object to be measured at the surface focus is 1.5 or more, more preferably 5 or more. . Further, in the present invention, the surface of the measurement sample is preferably a mirror surface. This is because if the surface of the sample is rough, the sound waves are diffusely reflected and the measurement accuracy is reduced.

Further, in the present invention, ultrasonic waves of about 200 MHz to 1500 MHz are used as ultrasonic waves for irradiating the bonded structure, and the distance Z between the acoustic lens for generating the ultrasonic waves and the object to be measured and each irradiation. The relationship between the intensity of the interference wave between the specular reflection wave of the ultrasonic wave and the surface acoustic wave (expressed as the output V (dB)) in the region is graphed as a V (Z) curve. Here, for example, signal processing of a V (Z) curve obtained from a bonded structure such as a ceramic-metal bonded body is performed by FFT (fast Fourier t).
ransformation) spectral analysis method is used. As in the case of general spectrum analysis, this procedure is as follows: (1) The background, that is, the trend with a period larger than the fluctuation of interest is removed, (2) the fine fluctuation, that is, noise, The window function is selected and the integration segment is determined, and (4) FFT calculation and the period are calculated from the peak position of the power spectrum to obtain the sound velocity.

It is carried out by: Also, the attenuation is obtained from the half width of the spectrum.

FIG. 6 shows an example of a spectrum analysis procedure for such a V (Z) curve. In the figure, (a) is the measured raw data, (b) is the result of removing the ripple due to the interference with the lens reflected wave by the moving average filter, and (c) is the acoustic velocity of the surface acoustic wave is small and it contributes to the V (Z) curve. No component V
(Z) curve, (d) is the result of subtracting the V (Z) curve of (c) as the background, and (e) is the power spectrum of the waveform of (d) obtained by FFT.

This power spectrum changes as in the example shown in FIG. 7 depending on the distance between the acoustic lens and the sample to be measured. The period ΔZ peculiar to the stress state of the sample is obtained from the V (Z) curve, and , The surface acoustic wave velocity is determined based on the following equation.

Formula: V _R = V _W / {1- (1-V _W / 2fΔZ) ² } ^1/2 (1) (where V _W is the speed of sound of the medium (water) (about 40 ° C.) 1525
m / sec), and f is the frequency) and the lens used in the present invention is used for the measurement by an ultrasonic microscope, and the aspect ratio of the ultrasonic wave at the time of the surface focus of the measured object is 1.5 or more. It is characterized by having a rectangular or elliptical shape by giving an ultrasonic wave of direction, and by using such an elongated lens, it is possible to obtain directional data.

[0019]

According to the residual stress measuring method of the present invention, any substance capable of injecting an ultrasonic wave can be used as a measuring sample, and a diffraction peak cannot be obtained by a conventional X-ray inspection, so that measurement is impossible. It can also be applied to glass and the like.

Further, by using a line-shaped lens such as a rectangle or an ellipse, information of a certain area can be obtained, and information having directionality that cannot be obtained by a point sound source can be obtained, and the information on the sample surface can be obtained. The residual stress distribution can be quantitatively evaluated in a specific direction. This allows
The quality control accuracy of the bonded structure can be improved, and the reliability can be improved. Further, by accurately controlling the surface temperature distribution of the measurement sample to be within ± 0.05 ° C., the measurement accuracy of the surface acoustic wave is improved, and thus the residual stress distribution can be obtained more accurately.

[0021]

EXAMPLES Next, examples of the present invention will be described.

FIG. 1 is a diagram showing an outline of an example of a residual stress measuring system to which the residual stress measuring method of the present invention is applied. The residual stress measurement system (acoustic microscope analysis system) shown in the figure is connected to an ultrasonic microscope unit 100 capable of varying the frequency in the range of 200 MHz to 1500 MHz and an output end of an amplifier of ultrasonic waves generated by interference. It is mainly composed of a computer unit 200 which is a computer.

In the ultrasonic microscope 100, the measurement sample 111 is irradiated with ultrasonic waves by the acoustic lens 112, and specular reflection waves of ultrasonic waves in the irradiation area of the measurement sample 111, surface acoustic waves excited, and internal excitation. The interference wave generated by the wave is converted into a signal through the circulator 113 and transmitted from the interface 114 to the computer unit 200.

In the computer unit 200, the periodic change in the intensity of the interference wave from the amplified irradiation excitation region is
It is displayed on the display 212 as a V (Z) curve by the computer 211. This V (Z) curve is then subjected to FFT processing in the conversion unit 213, and is accurately obtained as the surface acoustic wave velocity V _SAW within the irradiation area.

In this embodiment, the measurement sample 111 is 30
mm × 40mm × 3mmt Si ₃ N ₄ / Cu / S45C series ceramics −
A metal bonded body was used. In this ceramics-metal bonded body, Cu was inserted between the silicon nitride ceramics and the metal as a buffer material, and the surface of the bonded body was subjected to polishing treatment.

As an acoustic lens, 2.6 during surface focusing
μm x 0.86mm line focus lens (line focus l
ens) at a frequency of 400 MHz and a water temperature of 60 ° C. to an effective depth of about 50 μm. At this time, the measurement sample 111 was placed in a thermostat as shown in FIG. 5, and the temperature of the water in the thermostat was controlled to be 40 ° C. ± 0.05 ° C.

Here, the shapes of the line focus lens and the sound field in this embodiment are shown in FIG. The line focus lens 21 irradiates the measurement sample 23 with ultrasonic waves via the coupler (water) 22, and a sound field is excited and formed in the area A of the measurement sample 23 by this irradiation. And
The lens 21 is scanned in the sample depth direction (Z direction). Note that if the lens 21 is brought too close to the bonding interface, ultrasonic waves will be reflected from other adjacent members, so it is preferable to set the scanning start point at a certain distance or more.

After that, the sound velocity (V _SAW ) of the surface acoustic wave is obtained by FFT processing, and the surface acoustic wave velocity V _SAW without stress is obtained.
Difference from (0) ΔV _SAW = V _SAW −V _SAW (0), that is, the change amount was obtained for each scanning position, and the axial distribution of the bonded body and the interfacial direction distribution were obtained. The result is shown in FIG. In the figure, the surface acoustic wave velocity distribution (ΔVy) and interface direction distribution (ΔVx) obtained in this example are shown.
Is shown by a solid line, and at the same time, vertical stress values (σRy, σRx), which are residual stress measured by the micro X-ray method, are shown by a chain line.

As is apparent from FIG. 3, the measurement results using the ultrasonic microscope according to this example and the measurement results using the micro X-ray method tend to be in good agreement, and the distribution of the residual stress of the bonded structure is shown. It could be evaluated quantitatively along with the directionality.

According to this result, the higher the residual stress, the faster the surface acoustic wave. This is also supported by the formula for obtaining the speed of sound. That is, the sound velocity V is

[0031]

[Equation 1]

It is shown by. In equation (2) above, ρ that represents the density
The value of changes with changes in the stress remaining in the sample. In other words, if tensile stress acts on the sample, the density of that portion becomes smaller, and from the above equation (2), it is understood that the sound velocity V increases as the density decreases. Conversely, if compressive stress acts on the sample, the density of that portion increases and the sound velocity V decreases.

Therefore, in the silicon nitride-S45C material joined body, since the silicon nitride side has a smaller thermal expansion coefficient than the S45C material, a large tensile stress is applied in the axial direction at the time of joining. Remains in. Then, the density on the silicon nitride side becomes smaller than the original value, and when such a surface is measured using an ultrasonic microscope, the surface acoustic wave becomes faster. That is, the difference (ΔV _SAW ) from the reference speed when there is no stress increases. Since the residual stress increases as it approaches the bonding interface, the velocity of the surface acoustic wave becomes slower as the ultrasonic irradiation area moves away from the bonding interface, and the difference from the reference velocity when there is no stress becomes smaller.

From these facts, the graph of FIG. 3 decreases in the y component in the axial direction as the irradiation position of the sound wave moves away from the bonding interface, and in the x component in the interface direction, the velocity of the surface acoustic wave at the central portion of the interface. It is reasonable to draw a curve that makes the speed faster.

In the above embodiment, the measurement sample is
Since the measurement was performed by arranging in warm water of 40 ° C ± 0.05 ° C, the sound velocity V _H2O in water was 1525 m / sec ± 2 m / sec (at 40 ° C), so the error of the surface acoustic wave velocity V _SAW was ± 4 m / sec. It was suppressed within. This makes it possible to more accurately determine the distribution of surface acoustic wave velocities (ΔVy and ΔVx), so that the residual stress distribution can be measured with higher accuracy.

As a comparison with the line focus lens of this embodiment, the residual stress of the ceramic-metal bonded body was measured under isotropic sound waves using a point focus lens as shown in FIG. It was measured. The point focus lens 31 is a coupler 32.
An isotropic sound field B is formed in the measurement sample 33 via. In the case of such a lens, since it is a point sound source, the measurement area is limited to a small range and the values are averaged, so detailed data with directionality can be obtained. The accuracy was also not very good.

As described above, in this embodiment, the residual stress of the ceramic-metal bonded body is non-destructively evaluated as a quantitative and directional distribution by using the ultrasonic microscope using the line-shaped lens. Therefore, the reliability of the bonded structure could be improved.

[0038]

As described above, the residual stress measuring method of the present invention uses an ultrasonic microscope and a lens that gives anisotropic ultrasonic waves to the surface of an object to be measured, and utilizes the velocity change of surface acoustic waves. Thus, the residual stress distribution on the material surface can be regarded as a quantitative and directional value, and the residual stress evaluation method can be improved from a new perspective. Therefore, it greatly contributes to the quality control of various materials and, in turn, greatly improves the reliability of the bonded structure.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of a residual stress measuring system according to an embodiment of the present invention.

FIG. 2 is a diagram showing a line focus lens used in an example of the present invention.

FIG. 3 is a diagram showing a residual stress distribution obtained according to an example of the present invention.

FIG. 4 is a diagram showing an example of setting a measurement sample according to the present invention.

FIG. 5 is a diagram showing an example in which a measurement sample is placed in a thermostat.

FIG. 6 is a diagram showing an example of a spectrum analysis procedure of a V (Z) curve.

FIG. 7 is a diagram for explaining a cycle ΔZ peculiar to the stress state of the sample that changes depending on the distance between the acoustic lens and the measurement sample.

FIG. 8 is a diagram showing a point focus lens provided as a comparison with the present invention.

[Explanation of symbols]

1 ... stage 2 ... Measurement sample 3 ... Coupler 4 ... Acoustic lens 5 ... Transducer 21 ... Line focus lens 100 ... Ultrasonic microscope section 200 ... Computer unit

Claims (5)

[Claims] 1. An ultrasonic wave is radiated on a smooth surface of an object to be measured by changing the distance between this surface and an acoustic lens that emits ultrasonic waves having anisotropy in the irradiation area on the surface. V representing the relationship between the irradiation distance in the depth direction and the intensity of the interference wave between the specular reflection wave of the ultrasonic wave, the surface acoustic wave excited and the internal excitation wave in the irradiation area V
The (Z) curve is obtained, and the surface acoustic wave velocity in the irradiation excitation region of the surface of the measured object in the stress state is obtained from the curve period in the V (Z) curve. Using the velocity as the reference velocity, determine the amount of change between the surface acoustic wave velocity and the reference velocity in the presence of stress, and based on the distribution of this amount of change, determine the residual stress value in the long-side direction of the ultrasonic irradiation area. A method for measuring residual stress, which comprises converting and obtaining a distribution in a specific direction. 2. The residual stress measuring method according to claim 1, wherein the ultrasonic wave having anisotropy in the irradiation area has an aspect ratio of 1.5 or more when the surface of the object to be measured is in focus. 3. The temperature distribution on the surface of the object to be measured is ± 0.05.
The residual stress measuring method according to claim 1, wherein the residual stress is controlled to within ℃. 4. The residual stress measuring method according to claim 1, wherein the object to be measured is a ceramic, and the ceramic is a structure bonded to a metal. 5. An anisotropic ultrasonic wave, which is used for measurement with an ultrasonic microscope and has an aspect ratio of 1.5 or more when the surface of an object to be measured is in focus, is rectangular or elliptical in shape. Lens to be.