Coating for estimating history of stress

Abstract

To provide a coating capable of stabilizing a structure even when exposed at a temperature of about 350-450°C and estimating the history of stress by observing the change of particle diameter caused by the action of repeated stress.SOLUTION: A coating for estimating the history of stress applied to a subject, consisting of nickel alloy containing phosphorus and boron and deposited on one or more areas in the subject by plating is subjected to heating at a temperature of more than 450°C.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The following disclosure relates to a film for estimating the history of stress applied to an object.

Background Art

[0002] As is well known, in many metallic materials, a phenomenon called fatigue occurs due to repeated stress. Fatigue is prominent, for example, at local areas where stress is concentrated. However, since stress concentration further occurs at the tip of a crack caused by fatigue, the crack easily propagates from that location to other areas, and its influence may quickly reach the entire structure. Therefore, in an actual structure, it is desirable to establish a technique not only for quickly detecting the occurrence of cracks but also for surely predicting their occurrence in advance. For that purpose, first, it is necessary to grasp the stress actually acting in the actual structure.

[0003] For stress measurement, for example, the phenomenon in which the electrical resistance changes due to strain can be utilized, and a strain gauge utilizing such a phenomenon is an easily available means. However, since a strain gauge requires a certain size, it is not necessarily suitable for applications where stress concentrated in a very limited area is measured.

[0004] Patent Document 1 discloses related techniques.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The inventors focused on the phenomenon in which metal crystal grains grow gradually due to repeated stress, and thought of using this phenomenon to evaluate repeated stress. The technology disclosed below provides a coating that can be applied in advance to a target location on an object and used to evaluate the repeated stress experienced by that location. [Means for solving the problem]

[0007] The present disclosure relates to a coating for estimating the history of stress applied to an object, the coating being made of a nickel alloy containing phosphorus and boron, deposited on one or more regions of the object by plating, and heat treated at a temperature exceeding 450°C.

[0008] Preferably, after the heat treatment, the coating has a Vickers hardness of 550 or more and 650 or less. Also preferably, in the nickel alloy, phosphorus is 1.5 mass % or more and 2.5 mass % or less, and boron is more than 0 mass % and 1 mass % or less. Also preferably, the coating further includes a base layer made of a phosphorus-nickel alloy not containing boron at the interface with the target object. Effect of the Invention

[0009] The coating structure remains stable even when exposed to temperatures of around 350°C to 450°C, and by observing the change in grain size, it is possible to estimate the history of stress applied to the object. [Brief description of the drawings]

[0010]

Figure 1

Figure 2

Figure 3

[0011] Several exemplary embodiments will be described below with reference to the accompanying drawings.

[0012] When a repeated stress acts on a metal, often, the crystal is induced by the stress and gradually grows. Therefore, it is possible to estimate the stress experienced by the metal by measuring the crystal growth by observing the microstructure with a metal microscope or by the electron backscatter diffraction (EBSD) method or the like. If the object is made of metal, it may be possible to estimate the stress by observing the structure of the object itself. However, the relationship between the repeated stress and the crystal growth strongly depends on the metal type, alloy composition, or its manufacturing process and heat treatment. If the influence of these parameters has not been sufficiently grasped in advance, it is impossible to estimate the stress history from the crystal growth. Therefore, in the present embodiment, instead of the object itself for which the stress history is to be estimated, a metal film having a certain composition and for which the relationship between the stress history and the crystal growth can be grasped in advance is used.

[0013] Referring mainly to FIG. 1, the film 1 for estimating the stress history is formed on the object 5. The film 1 can be limited to a specific region in which the stress history is to be estimated in the object 5, and the target region may be one or more. That is, the film 1 is pre-attached to one or more regions of the object 5. Since the purpose is to evaluate the fatigue of the metal, usually, the object 5 is made of metal, but it does not necessarily have to be metal. The film 1 may be formed directly on the object 5, but for the purpose of improving the adhesion, facilitating the formation of the film 1, or other purposes, the underlayer 3 may be further included at the interface with the object 5.

[0014] In principle, any metal can be applied to the film 1, but preferably it is a metal whose crystal growth is highly sensitive to repetitive stress. Examples of such metals include copper, copper alloys, nickel, and nickel alloys. More preferably, it is a metal whose microstructure is expected to be stable in the temperature environment in which the object is used. For example, a nickel alloy containing phosphorus and boron (Ni-P-B alloy) can be used even at a relatively high temperature of about 350 to 450 °C because its recrystallization temperature reaches 450 °C or higher.

[0015] For the formation of the film 1 and also for the formation of the underlayer 3, an appropriate thin film formation process can be used, and such processes include, for example, physical vapor deposition (PVD), spraying, and plating. Plating is relatively inexpensive, stable quality can be expected, and it can be applied to a complex structure, so it can be preferably used. Either a known electrolytic plating method or electroless plating method can be used, and an example by the electroless plating method will be described below.

[0016] The electroless plating bath is, for example, an aqueous solution containing an appropriate nickel salt, hypophosphite, and dimethylaminoborane (DMAB). The object 5 is immersed in such a bath, preferably with only the target area exposed. Hypophosphite and DMAB are reducing agents that reduce nickel ions to nickel and deposit them on the object 5, and the phosphorus and boron contained in each molecule are incorporated into the film, whereby a film 1 made of a nickel alloy containing phosphorus and boron is formed on the object 5. The composition of the film 1 is, for example, Ni: 97 to 98% by mass, P: 1.5 to 2.5% by mass, B: 1% by mass or less, but is not necessarily limited to such a range. The thickness of the film 1 is, for example, 7 to 9 μm, but is not necessarily limited to such a range.

[0017] As described above, an underlayer 3 made of a nickel-phosphorus alloy may be interposed, which can be achieved, for example, by immersing the object 5 in a bath composed of a nickel salt and hypophosphite for a short time prior to the above-mentioned plating. The composition of the underlayer 3 is, for example, Ni: 90 to 92% by mass, P: 8 to 10% by mass, and the thickness is 1 to 3 μm, but it is not necessarily limited to such a range.

[0018] The as-plated film 1 contains certain microcrystals, and its microstructure is not necessarily stable and is easily affected by the subsequent temperature history. Referring to FIG. 2, the as-plated film 1 has a Vickers hardness of about Hv700, but it is known that the hardness tends to increase with heating from 100 °C to about 450 °C. The variation in crystal grain size due to the instability of such a structure is difficult to distinguish from crystal grain growth due to repeated stress. Preferably, the film 1 is heat-treated at a temperature exceeding 450 °C so as to lose the influence of the temperature history or to obtain a stable microstructure. For example, the heat treatment is carried out by holding the entire object 5 including the film 1 at 500 °C for 1 hour. By the heat treatment, the Vickers hardness generally stabilizes between 550 and 650 or less.

[0019] To investigate the influence of the thermal load on the microstructure, a thermal load test was carried out. A nickel alloy (or Inconel 600 of Special Metals Co., Ltd.) conforming to the JIS NCF600 standard, processed into a disk shape with a diameter of 10 mm and a thickness of 4 mm, was used as a sample. Samples a to j plated with Ni-P-B were prepared. Samples a to d were not heat-treated, and samples e to j were heat-treated (held at 500 °C for 1 hour), and thermal load tests were carried out on each of them.

[0020] The heat treatment was carried out using an electric furnace FO3010 (manufactured by Yamato Scientific Co., Ltd.). The test temperatures were three types: 350 °C, 400 °C, and 450 °C, and the samples were each held for 1 hour. For samples h to j, an additional 2 cycles of heat load were applied. The heat load was applied by holding the sample at 500 °C for 1 hour using the same furnace. For each sample, the hardness was measured 10 times each using a micro-Vickers tester (Mitutoyo Corporation) available under the name HM-102. The results are shown in Table 1.

[0021]

Table 1

[0022] For samples a to c that were not heat-treated, the Vickers hardness increased significantly due to the heat load. That is, it is understood that the as-plated structure is not stable and is susceptible to the influence of subsequent heat history. For samples e to j that were pre-heat-treated, the hardness was stable regardless of the temperature and number of cycles of the heat load, and it is understood that their structure is less susceptible to the influence of heat history.

[0023] To investigate the influence of cyclic stress on the structure, a fatigue test was conducted. The shape of the test piece is as shown in Figure 3. One side of this was sequentially polished with emery paper from #220 to #1200, then sequentially polished with 6 μm and 1 μm diamond paste to obtain a mirror finish, and an 8-μm-thick Ni-P-B plating was applied to the mirror-finished surface. A plurality of plated test pieces were prepared, each heat-treated by holding at 500 °C for 1 hour as described above, and subjected to a fatigue test.

[0024] The fatigue test was conducted using a hydraulic control type fatigue testing machine Model 810 (MTS Systems Corporation), while maintaining the temperature at least at the center of the test piece at 400 °C by high-frequency heating. The applied stress was a uniaxial tensile stress, and the total strain amplitudes corresponded to 0.15% and 0.2%. The particle size of the coating after the fatigue test was measured by the electron backscatter diffraction (EBSD) method. Also, the hardness was measured 10 points each before and after the test using a micro-Vickers testing machine. The results are shown in Table 2.

[0025]

Table 2

[0026] At a total strain amplitude of 0.2%, compared with Sample 1 to which a cyclic stress of 0.7×10 5 cycles was applied, in Sample 2 to which a cyclic stress of 1.0×10 5 cycles was applied, the particle size increased by approximately 10%. That is, the particle size of the coating reflects the number of cycles of the cyclic stress experienced by the object. Also, when Sample 2 is compared with Sample 3 to which a cyclic stress of 1.0×10 5 cycles was applied at a total strain amplitude of 0.15%, the particle size is approximately 10% larger. That is, the particle size of the coating reflects the magnitude of the cyclic stress experienced by the object. That is, the coating according to the present embodiment can be used to estimate the history of the cyclic stress experienced by the object by measuring its particle size.

[0027] Although several embodiments have been described, it is possible to modify or deform the embodiments based on the above disclosure.

Industrial Applicability

[0028] There is provided a coating that is stable in structure even when exposed to a temperature of about 350 °C to 450 °C, and by observing the change in particle size caused by the action of cyclic stress, the history of the stress can be estimated.

Explanation of Signs

[0029] 1 Coating 3 Substrate layer 5 Object

Claims

1. A coating for estimating a history of stress applied to an object, comprising: A coating comprising a nickel alloy containing phosphorus and boron, deposited on one or more areas of said object by plating, and heat treated at a temperature in excess of 450°C.

2. 2. The coating of claim 1, having a Vickers hardness of 550 or more and 650 or less after said heat treatment.

3. 2. The coating of claim 1 , wherein the nickel alloy has a phosphorus content of 1.5% to 2.5% by weight and a boron content of greater than 0% to 1% by weight.

4. A base layer made of a phosphorus nickel alloy not containing boron is formed on the interface with the object, The coating of claim 1 further comprising:

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