JP2007155368A - Flaw detection method of conductive coating film and flaw detector used therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flaw detection method capable of precisely and simply measuring the flaw such as peeling, a crack, voids, an oxide, a corrosion product, etc. produced by the flaw and secular change of a conductive coating film, and a flaw detector. <P>SOLUTION: In the flaw detection method for detecting the flaw such as voids, a crack, an enclosure, etc. produced in the conductive coating film formed by a surface treatment method such as flame spraying, plating, diffusion penetration treatment, etc., the electric resistance across two separate points (B-C) positioned inside two points (A-D) on a surface (3) of the film to be inspected is measured in a state that a predetermined microcurrent is allowed to flow across two points (A-D) to be compared with the electric resistance value of a normal film preliminarily measured under the same condition. A probe (11) includes a support structure for supporting respective probes (A-D) at a predetermined distance interval and contact pressure regulating means (15 and 16) for bringing the respective probes (A-D) into contact with the surface (3) of the film to be inspected under predetermined contact pressure. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a conductive coating film that can be used for inspection of defects, aging, etc. of conductive coating films used for parts and parts of various devices used under harsh conditions such as corrosive environments, durability life prediction, etc. The present invention relates to a defect detection method and an inspection apparatus used therefor.

  A coating film having a thickness of about several μm to several 1000 μm, such as thermal spraying and plating, has a tendency to form various film defects such as pores, cracks, and oxides in the production or use stage. Such a film defect is a problem in protecting the substrate from damage such as corrosion and wear, and securing the long-term durability of the film itself.

  For this reason, in the quality control of various devices used under severe conditions such as corrosive environments, the necessity of a method for nondestructively inspecting the presence or absence of these defects after film formation has long been recognized. Methods using various physical or chemical parameter changes such as impedance method, acoustic wave method, permeability change, electrochemical method, etc. have been studied, but there is no method currently in practical use. The following points are cited as reasons why it is difficult to put to practical use.

(A) Correspondence between changes in physical and chemical parameters and the size and type of defects appears clearly in artificial defects such as slits. It is difficult to verify experimentally because there are structural changes due to
(B) In the actual film, there are various forms of defects such as pores, layered oxides, crystal grain boundaries, cracks, etc., and there are many influencing factors such as the base material / film interface and film surface irregularities. Is difficult.
(C) Furthermore, when these coating films are used in an actual apparatus for a long period of time, the films gradually wear and deteriorate. In this case, the following consumption and deterioration forms are taken.

(C1) Decrease in film thickness: wear from outer surface, corrosion due to corrosion, etc. (c2) Generation of defects inside the film: Film breakage due to operating stress, etc., causing vertical cracks, horizontal cracks or partial peeling. In addition, corrosive gas penetrates into the interior through coating defects, and corrosion and scale formation occur inside the coating.
(C3) Separation of film / base material interface: In addition to the above-mentioned corrosion in the film, corrosion of the substrate or film occurs at the interface, and the film peels off.
For example, a report by the present inventor has been published on the deterioration mechanism of corrosion resistant coating such as thermal spraying used at high temperatures (see Non-Patent Document 1 and FIG. 8).

  Despite attempts at various methods as described above, film defects that occur in various forms can be easily and reproducibly eliminated by eliminating physical and chemical variation factors as film characteristics. It is difficult to detect well, and there is no method currently in practical use.

Yuzo Kawahara: Thermal Spray, Japan Thermal Spray Association, VoL38, No.2, (2001) p73

In general, a thin coating film (surface-modified film or surface-treated film) formed on the surface of a member according to the purpose introduces specific micro defects depending on the forming method. For example, the following defects are known in typical surface coatings.
(1) Metal spraying: pores, cracks, oxides, etc. (2) Weld overlay: voids, cracks, slag entrainment, component segregation, texture, shrinkage, etc. (3) Diffusion, penetration treatment: voids, cracks, etc. Aluminizing, chromizing, siliconizing, etc.)
(4) Plating: pores, cracks, impurities, etc. (chromium plating, nickel plating, etc.)

  Furthermore, these coating surfaces and the base material / film interface necessarily have irregularities of several tens to several hundreds of micrometers due to the manufacturing method or pretreatment, and these are inspected by physical and chemical methods. This is a factor that causes variation. Since removing the surface irregularities consumes the film and takes time and effort, it is desirable to inspect the film as it is.

  The present invention has been made in view of such a situation, and its purpose is to accurately detect defects such as peeling, cracks, pores, oxides, corrosion products, and the like caused by defects in the conductive coating film and aging. The present invention provides a method and an apparatus that can be easily measured in the field.

In order to solve the above-described problems of the prior art, the present inventor has conducted extensive research and found that a film defect can be detected with sufficient accuracy using a direct current electric resistance method for detecting a minute current.
That is, the present invention is a method for detecting defects such as pores, cracks, inclusions, etc. generated in a conductive coating film formed by a surface treatment method such as thermal spraying, plating, diffusion penetration treatment, etc. Conduction to measure the electrical resistance between two other points located inside the two points with a predetermined minute current flowing between the two points, and to compare with the electric resistance value of the normal film measured in advance under the same conditions The method is to detect defects in the conductive coating film.

  In the present invention, it is preferable that the electrical resistance is measured at two other points that are equidistant inside the two points and on a straight line connecting the two points with respect to the two points through which the minute current flows. It is. Further, two styluses for passing a minute current between the two points and two styluses for measuring the electrical resistance between the two inner points are respectively applied to the surface of the film to be inspected at a predetermined contact pressure. It is further preferable to measure the electrical resistance in the state of contact with

  Moreover, this invention measures the electrical resistance between the two styluses for passing a minute electric current between the two points, and the two points on the inside as an inspection device used for the defect detection method of the conductive coating film. Two contact styluses, a support structure for supporting the styluses at a predetermined distance, and contact pressure adjusting means for bringing the styluses into contact with the surface of the film to be inspected at a predetermined contact pressure. An inspection apparatus including a probe, a constant current control device that supplies a predetermined minute current between the two styluses for flowing the minute current, and a measuring device that measures an electrical resistance between the two inner styluses is configured. did.

  In another aspect of the present invention, as a defect detection method for the conductive coating film, the electrical resistance between the two points is measured in a state where a predetermined minute current is passed between the two points on the surface of the film to be inspected. It can also be compared with the electrical resistance value of a normal film measured in advance under the same conditions. Furthermore, in another aspect of the present invention, as the defect detection method, the potential of the one point is set in a state where a base material of the film to be inspected is grounded and a predetermined minute current is supplied to one point on the surface of the film to be inspected. It is also possible to measure the electric resistance from the measured value and compare it with the electric resistance value of the normal film measured in advance under the same conditions.

  The method for detecting a defect in a conductive coating film according to the present invention is a method of detecting electrical resistance between two points located inside the two points in a state where a predetermined minute current is passed between the two points on the surface of the film to be inspected. Since a defect detection method was used to measure and compare with the electrical resistance value of the normal film measured in advance under the same conditions, peeling, cracks, pores, oxides, corrosion products caused by defects and aging of the conductive coating film Defects such as can be measured accurately and easily on site.

  The above effect is obtained by measuring the electrical resistance at two other points that are equidistant inside the two points and on a straight line connecting the two points with respect to the two points through which a minute current flows. It will be remarkable. Further, two styluses for passing a minute current between the two points and two styluses for measuring the electrical resistance between the two inner points are respectively applied to the surface of the film to be inspected at a predetermined contact pressure. By measuring the electrical resistance in the state of contact with the contact point, the influence of the contact resistance due to the contact pressure difference of each stylus on the electrical resistance value can be eliminated, and more accurate defect detection can be performed.

Hereinafter, the present invention will be described in detail based on illustrated embodiments.
FIG. 1 shows a configuration of an inspection apparatus 10 that performs the method (four-needle electric resistance method) according to the present invention. In the figure, an inspection device 10 includes a probe 11 having four styluses A, B, C, and D, a constant current control device 12 for passing a predetermined minute current between outer styluses A-D, and an inner stylus B. It is mainly composed of a measuring device 13 that measures a minute resistance (voltage) between −C.

  The probe 11 is devised as follows in order to ensure the reproducibility and accuracy of measured values against surface contact during repeated use. That is, the four styluses A, B, C, and D that protrude from the distal end surface 14 of the probe 11 and are arranged in a straight line contact and separate from the surface 3 to be inspected at the distal end surface 14 of the probe 11. Each is supported so as to be slidable in the direction, and each is biased by a spring 16 in a direction protruding from the tip portion 14. Further, spacers 15 are provided on both sides or around the tip surface 14 of the probe 11 (or around each of the four styluses A, B, C, and D), and the probe 11 is placed on the surface to be inspected 3 by the spacer 15. In this state, the distal end surface 14 is supported in parallel with the inspection surface 3 at a predetermined interval. The tips of the stylus A, B, C and D are moved by the contact pressure adjusting means including the pressure means (spring 16) of the stylus A, B, C and D and the support means (spacer 15) of the probe 11. It is pressed against the surface to be inspected 3 with a predetermined contact pressure. Note that at least a contact portion of the spacer 15 with the surface to be inspected 3 is made of an insulating material, and the four styluses A, B, C, and D are supported on the probe tip surface 14 via the insulating material.

  In the illustrated example, the spring 16 is interposed between the proximal end portions of the stylus A, B, C, D and the distal end portion 14 of the probe 11, and exerts an urging force by elastic recovery of the tensioned spring 16. However, the biasing force may be obtained by elastic recovery of the compressed spring. Further, as a pressurizing means for the stylus A, B, C, D, in addition to an elastic body such as a spring, an air cylinder, a bellows frame, or the like using a fluid pressure can be used. The optimum contact pressure of the stylus A, B, C, D is determined by adjusting the displacement of the spring 16, that is, the connecting position of either end of the spring 16 with an adjusting means (not shown) such as a screw. Preferably, it can be set and adjusted.

  The tip shape of the stylus A, B, C, D is tapered to ensure good contact even if minute irregularities are present on the coating 1, but the tip is rounded so as not to damage the coating 1. It is preferable to use a spherical surface with a radius of about 0.05 mm to 0.3 mm. Further, as the material of the stylus A, B, C, D, a high-hardness material that does not easily wear or deform even when repeatedly measured with the above-described tip shape, and has good electrical conductivity is preferable. As such a material, for example, tungsten is preferably used.

The interval between the styluses A, B, C, and D varies depending on the object, such as the thickness of the coating 1, the size of the inspection area, and the shape of the inspection object. In consideration of workability when performing inspection while moving the probe 11, it is preferable that the probe 11 is small and light. From these points, in the general-purpose form, the distance L ₀ between A and D is preferably about 100 mm or less. Further, the distance L ₁ between B and C is related to the detectability in detecting the voltage change, that is, the resistance change, variation, and physical properties of the inspection region, and the coating 1 having a thickness of about 50 to 600 μm such as a thermal spray coating. Is preferably 0.2 to 0.8 times the A-D distance L ₀ . The stylus A, B, C, D and the corresponding terminal (connector) of the probe 11 are connected by a flexible wire 17, and the terminals of the stylus A, D are connected to the constant current control device 12. The terminals of the needles B and C are connected to the measuring device 13, respectively.

  The constant current control device 12 is a device that includes a circuit that performs control to keep a current flowing in the coating 1 constant. The constant current control device 12 applies a voltage to the stylus A and D, and adjusts the applied voltage to be about 1 μA to 10 A regardless of the contact resistance between the stylus A and D and the object to be inspected. A constant current that can be adjusted within the range of. Measurement can also be performed using low-frequency alternating current, which can provide substantially the same effect as direct current.

The measuring device 13 measures the voltage (V ₁ ) detected by the stylus B and C under a constant current passed through the stylus A and D using the constant current control device 12, and the electrical resistance ( It has a function of a resistance meter (voltmeter) that converts a measured value into an electric resistance (R) by R) = voltage (V) / current (I). FIG. 2 shows a measurement circuit diagram (equivalent circuit diagram) when a film defect is detected by the four-needle electric resistance method. 2, the electrical resistance R of the device under test, the electrical resistance R ₁ of the film 1, the electric resistance R ₂ of the interface of the film 1 / base material 2, which corresponds to a combined resistance of the electrical resistance R ₃ of the base material 2. The electrical resistance R is very small in the healthy film 1 and shows good electrical conductivity. However, due to defects in the film 1, the electrical conductivity of the film 1 is reduced, and the electrical resistance R ₁ of the film ₁ and the electrical resistance R _{2 of the} interface are low. Increase.

Internal resistance R ₀ of the measuring device 13, the electrical resistance R _1, R _2, R ₃ and probe B, the contact resistance r _b and C, for sufficiently large compared to r _c, little current in the measuring device 13 flows Absent. Therefore, probe B, the contact resistance r _b of C, r _c is negligible, the electrical resistance R is measured in the measuring device 13 is such as to reflect the status of the film 1 inside the defect. Therefore, by measuring the minute resistance between the stylus B-C using the inspection apparatus 10 configured as described above, a minute resistance change due to a defect in the film 1 can be detected with high accuracy.

  The defect detection of the film 1 using the four-needle type inspection device 10 has been described above. However, the two-needle type inspection device 20 shown in FIG. 3 or the one-needle type inspection device 30 shown in FIG. It is possible to detect a defect in the film 1 by measuring a proper electric resistance.

  In FIG. 3, the two-needle type inspection device 20 includes a probe 21 having two styluses E and F, a constant current control device 22 for passing a predetermined minute current between the stylus EFs, and the constant current control device 22. It is comprised from the measuring device 23 which measures the minute resistance (voltage) between the stylus EF based on the flowing microcurrent. In FIG. 4, a single-needle inspection device 30 includes a probe 31 having one stylus H, a constant current control device 22 that is grounded at one end and a predetermined minute current is passed between H-G, and one end at the same end. It is composed of a measuring device 33 that measures a minute resistance (voltage) between H and G based on a minute current that is grounded and supplied by a constant current control device 32. The base material 2 must also be grounded. Although not shown, the contact pressure adjusting means for keeping the contact pressure of each stylus E, F or H with respect to the surface 3 to be inspected is the same as in the case of the 4-needle type inspection apparatus 10 described above. It is the same.

  In these two-needle type inspection apparatus 20 and single-needle type inspection apparatus 30, the basic principle of detecting defects of defects in the film 1 from the minute resistance (voltage) measured by the measuring apparatuses 23 and 33 is described above. This is the same as in the case of the needle type inspection apparatus 30. However, in the two-needle type inspection device 20 and the one-needle type inspection device 30, since the contact resistances of the stylus E and F and the stylus H are added, it is necessary to obtain information about the correlation between the film defect and the measured value in advance. There is. In addition, the single-needle inspection device 30 does not reach the four-needle inspection device 10 and the two-needle inspection device 20 in terms of local defect detection accuracy, but obtains a wide range of inspection information about the inspection target film 1. It can be used in combination with other methods. When judging the state of a defect from the resistance value in each method including the four-needle method, it is compared with a microphotograph of a defect as shown in FIGS. It is necessary to create and investigate the correspondence.

The method of the present invention can be applied not only to a conductive single layer film but also to a multilayer film having a two-layer or three-layer structure. In principle, there is no application limit to the film thickness, and information in the film can be obtained by expanding the distance L ₁ between the styluses and spreading the current distribution. However, when the distance L ₁ between the styluses is large or the film 1 is thin, the ratio of the inflow current into the base material 2 increases, and the sensitivity of detecting information in the film relatively decreases. narrowing the distance L ₁ between it is necessary to appropriately set.

  Furthermore, for films that have deteriorated over a long period of time, not only increases in defects in the film as described above, but also phenomena such as a decrease in film thickness, scale formation at the film / matrix interface, and peeling are observed. It occurs at the same time. Such aged deterioration phenomenon increases resistance in the film and decreases the inflow current to the base metal, so it shows a large change in resistance as the deterioration progresses. Can be determined.

(1) Correspondence between electric resistance value and thermal spray coating defect (4-needle method)
Using the above-described four-needle method and two-needle method inspection devices 10 and 20, the distance L ₀ between the styluses A and D is 50 mm (four needle method), and the distance L ₀ between B-C or EF is 32 mm. The defects generated in the plasma sprayed coating of 50Cr50Ni alloy applied to the boiler evaporator tube were investigated by setting (four needle method, two needle method). After measuring the resistance, the portions showing various resistance values were cut, and the defect occurrence state of the film was investigated by microstructural observation.
(2) Test piece and test method A test for determining use conditions in an actual machine was performed using the following test piece.
(2.1) Tubing tube Using a spray tube that has been used for one year, the relationship between electrical resistance and film defects was investigated by the 4-needle method and the 2-needle method, and the measurement conditions and criteria were determined.
(2.2) New spray tube Using a 50Cr50Ni spray tube before use, the electrical resistance value of a normal coating having a coating thickness of 100 to 400 μm was measured, and the influence of the coating thickness was investigated.
(3) Test results Table 1 shows the test results of the extubated tube by the 4-needle method and Table 2 shows the 2-needle method. Moreover, the photograph of the typical microstructure of a measurement position is shown in FIG. 6, FIG. The resistance values in FIGS. 6 and 7 are based on the 4-needle method. 1-4, no. It corresponds to 6-9. Table 3 shows the measurement results of the new spray tube by the 4-needle method. The main results obtained are as follows.

(3.1)
The test using the 2-needle method showed a resistance value of about 40 mΩ to 1500 mΩ due to the effects of contact resistance, current, etc., but the 4-needle method showed a very small film resistance of 13 to 3600 μΩ, which was about 1/10 ³ of the 2-needle method. The change could be measured. Moreover, the tendency of the resistance increase by the increase in a defect can be measured clearly with both methods, and it turns out that it has sufficient practicality.
(3.2)
Regarding the correspondence between the resistance value and the film defect, the resistance value of the sound film shows a value of about 10 to 20 μΩ before and after use in the four-needle method. On the other hand, the resistance value increases corresponding to the presence of defects such as large defects, layered oxides, and cracks, and large resistance values of about 1000 μΩ or more are observed for large cracks on the surface that can be visually observed. From such a tendency, the threshold value of the defective film and the normal film in this experiment is about 70 μΩ. When this is expressed by the index α of the resistance value of the defective film relative to the resistance value of the normal film (defect index α = defective film resistance value / normal film resistance value), α ≧ 4.0 to 5.3 is Threshold value. The following is a summary of defect occurrence and resistance thresholds.
(A) Resistance value is 70 μΩ or less (α <4.0): Small defects such as normal / sputtering exist (No. 1).
(B) Resistance value of 70 to 1000 μΩ (4.0 ≦ α <60): There are harmful defects (not visible) / large layered oxides, fine vertical cracks, and partial peeling (No. 2 to No. .6).
(C) Resistance value of 1000 μΩ or more (60 ≦ α): large defect (visible) / large crack, large amount of oxide, large peeling (No. 7 to No. 9).
(3.3)
Regarding the effect of film thickness on the resistance value, the measurement results of the new thermal spray tube (Table 3) show that there is almost no effect on the resistance value of the film thickness in the case of a healthy film, and within the normal film thickness range, It shows that the defect can be detected without being affected by the thickness.

  The influence of film defects on the resistance value depends on the presence of various defects such as oxide amount, microcracking, collective defects such as spatter, bonding interface state, and other factors. to be influenced. For this reason, it is difficult at present to detect the degree of influence of each defect on the microresistance value, but harmful cracks and large oxides that affect the durability of the film by measuring microresistance on the order of μΩ. The presence and size of the can be detected. The greater the resistance value, the greater the proportion of defects in the film cross-section (current passing area cross-sectional area), which makes it easier to generate corrosive gas from the outside, cracks penetrating the film, etc. It is thought that the rate of decrease in the increase. Therefore, this method can be used for predicting the life of the coating.

  In the above-described embodiment, the case where the four styluses A, B, C, and D of the probe 11 are supported so as to be slidable toward and away from the surface to be inspected 3 is shown. Instead of being freely supported, it may be supported so as to be swingable.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment, A various deformation | transformation and change are possible based on the technical idea of this invention.

It is a front sectional view showing the outline of an inspection device (4 needle type) which carries out the method of the present invention. It is a measurement circuit diagram in the case of detecting a film defect by the method of the present invention. It is a front view which shows the outline of the test | inspection apparatus (two needle type | mold) which implements this invention method. It is a front view which shows the outline of the test | inspection apparatus (1 needle type) which enforces this invention method. It is a figure which shows the relationship between the film | membrane defect based on this invention method, and resistance value, and a typical film | membrane structure | tissue. It is a tomogram which shows the microstructure of a thermal spray coating. It is a tomogram which shows the microstructure of a thermal spray coating. It is a figure which shows the mechanism of aged deterioration of a thermal spray coating.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Film | membrane 2 Base material 3 Surface to be inspected 10, 20, 30 Inspection device 11, 21, 31 Probe 12, 22, 32 Constant current control device 13, 23, 33 Measuring device (resistance meter / voltmeter)
15 Spacer (supporting means / contact pressure adjusting means)
16 Spring (Pressurizing means / Contact pressure adjusting means)
A, B, C, D, E, F, H Stylus G Ground

Claims (6)

  A method for detecting defects such as pores, cracks, inclusions, etc. generated in a conductive coating film formed by a surface treatment method such as thermal spraying, plating, diffusion penetration treatment, etc. The electric resistance between two other points located inside the two points in a state where a minute current is passed, and compared with the electric resistance value of a normal film measured in advance under the same conditions For detecting defects in conductive coatings.   2. The electrical conductivity according to claim 1, wherein the electrical resistance is measured at two other points that are equidistant inside the two points and on a straight line connecting the two points with respect to the two points through which a minute current flows. Defect detection method for coating film.   Two styluses for passing a minute current between the two points and two styluses for measuring the electrical resistance between the two inner points at a predetermined contact pressure on the surface of the film to be inspected, respectively. The method for detecting a defect in a conductive coating film according to claim 1 or 2, wherein the electrical resistance is measured in a contacted state. An inspection apparatus used in the defect detection method for a conductive coating film according to any one of claims 1 to 3,
Two styluses for passing a minute current between the two points, two styluses for measuring electrical resistance between the two inner points, and a support for supporting the styluses at a predetermined distance interval A probe having a structure and contact pressure adjusting means for bringing each stylus into contact with the surface of the film to be inspected at a predetermined contact pressure;
A constant current control device for supplying a predetermined minute current between two styluses for passing the minute current;
A measuring device for measuring electrical resistance between the two inner styluses;
Inspection equipment including   A method for detecting defects such as pores, cracks, inclusions, etc. generated in a conductive coating film formed by a surface treatment method such as thermal spraying, plating, diffusion penetration treatment, etc. A method for detecting a defect in a conductive coating film, wherein the electrical resistance between the two points is measured in a state where a minute current is passed, and is compared with the electrical resistance value of a normal film measured in advance under the same conditions.   A method for detecting defects such as pores, cracks, and inclusions generated in a conductive coating film formed by a surface treatment method such as thermal spraying, plating, and diffusion / penetration treatment, wherein the base material of the film to be inspected is grounded and covered. In a state where a predetermined minute current is applied to one point on the surface of the inspection film, the electric potential at the one point is measured, the electric resistance is obtained from the measured value, and compared with the electric resistance value of the normal film measured in advance under the same conditions. A method for detecting a defect in a conductive coating film.