CrN coating and sliding components

Abstract

To provide a CrN film excellent in peel resistance, namely, hardly generating peeling of a film starting a crack, even under an environment having a severer lubrication environment.SOLUTION: In a CrN film, priority orientation by XRD is 200, an X-ray diffraction intensity ratio (200) / (111) of (200) plane to (111) plane is 5.5 or more, and in a distribution of a crystal grain size measured by EBSD analysis, the ratio of crystal grains of 1 μm or less is 85% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a CrN film and a sliding member coated with the CrN film. [Background technology]

[0002] CrN coatings are applied to the sliding surfaces of sliding components used in harsh sliding environments, and are required to have good sliding properties and wear resistance. For example, piston rings used in internal combustion engines are experiencing increasing loads on their surfaces due to factors such as rising cylinder pressure, direct injection, and reduced viscosity of lubricating oils. As a result, the CrN coatings covering the surfaces of piston rings can sometimes crack or peel off due to sliding.

[0003] To solve these problems, a coating has been proposed that has a composition in which one element selected from the group consisting of carbon, phosphorus, nitrogen, boron, and silicon is dissolved in metal chromium, and that has high hardness, hydrogen embrittlement, toughness, and fatigue resistance (see Patent Document 1). It has also been disclosed that a coating made of CrN-type chromium nitride, in which the crystal lattice constant and Cr content are within specific ranges, improves the sliding properties and peeling resistance (see Patent Document 2). Furthermore, it has been disclosed that a coating having a specific diffraction peak in a coating composed of a mixture of metal chromium in which nitrogen is dissolved and CrN can be provided, which has excellent wear resistance and, in particular, high toughness with resistance to cracking and peeling (see Patent Document 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 58-144473 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-335878 [Patent Document 3] International Publication No. 2013 / 136510 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, CrN coatings with excellent spalling resistance have been proposed. However, during sliding under more severe lubrication environments, cracks are likely to initiate peeling of the coating. An object of the present invention is to provide a CrN coating with excellent spalling resistance that is less likely to cause crack-initiated peeling of the coating even under such more severe lubrication environments, and a sliding member coated with the CrN coating. [Means for solving the problem]

[0006] The present inventors have conducted research to solve the above problems and have found that the above problems can be solved by reducing the size of the crystals that form the CrN coating and by setting the preferred orientation within a specific range, thereby completing the present invention.

[0007] The present invention relates to a CrN coating, wherein the preferred orientation of the CrN coating is 200 as determined by XRD (X-ray diffraction), the X-ray diffraction intensity ratio (200) / (111) of the (200) plane to the (111) plane is 5.5 or more, and the preferred orientation of the CrN coating is 200 as determined by EBSD (Electron Backscatter Diffraction). In the distribution of crystal grain size measured by the CrN coating pattern analysis, the proportion of crystal grains of 1 μm or less is 85% or more. It is preferable that no particles of 2.0 μm or larger are present. Desirable. Furthermore, a preferred embodiment has a micro Vickers hardness of 800 HV or more and 1300 HV or less. By keeping the coating hardness low while maintaining a dense coating, the coating becomes non-brittle and has improved peel resistance, which is preferable. If the micro Vickers hardness is less than 800 HV, the abrasion resistance may be insufficient, and if it is more than 1300 HV, chipping and chipping tend to occur easily when handled during processing. In addition, a preferred embodiment is one in which the plastic power is 61% or more and 69% or less, as measured using a Vickers indenter in accordance with the international standard for nanoindentation testing, ISO 14577-1. Here, the plastic power refers to the proportion of plastic deformation work to the total indentation work in an indentation test. A coating with a high plastic power improves the coating's resistance to peeling from cracks. If the plastic power is less than 61%, the hardness tends to be higher than 1300 HV, and if it is higher than 69%, the hardness tends to be lower than 800 HV.

[0008] Another aspect of the present invention is a sliding member having a sliding surface coated with the above-mentioned CrN coating. [Effects of the Invention]

[0009] The present invention can provide a CrN coating with excellent peeling resistance, which is less likely to peel off due to cracks even under harsher lubrication environments, and a sliding member coated with the CrN coating. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional schematic view of a piston ring coated with a CrN film according to one embodiment of the present invention. [Figure 2] 1 is a schematic diagram of an apparatus for depositing a CrN coating on a piston ring by an ion plating method. [Figure 3] 1 is an enlarged image (photograph substituting a drawing) of crystal particles forming a CrN film obtained in an example. [Figure 4] 4 is a graph showing the distribution of crystal grain sizes in the CrN coating of Example 1. [Figure 5] FIG. 2 is a cross-sectional schematic view of a pin-disk testing device used in a peel resistance test. [Figure 6] 1 is an image (photograph substituting a drawing) showing a CrN film after a peeling resistance test. DETAILED DESCRIPTION OF THE INVENTION

[0011] One embodiment of the present invention is a CrN coating. The CrN coating is a coating containing CrN as its main component and may contain CrN, metallic chromium with nitrogen as a solid solution, unavoidable impurities, and the like. The phases of the CrN coating can be evaluated by XRD (X-ray diffraction). The composition of the CrN coating can be analyzed by EPMA (Electron Probe Microanalyzer). In the CrN coating, Cr may be 45 at% or more, 50 at% or more, or 60 at% or less. The nitrogen content in the coating may be 40 at% or more, 50 at% or more, or 55 at% or less.

[0012] In this embodiment, the CrN coating has a preferred orientation of 200 as determined by XRD, an X-ray diffraction intensity ratio (200) / (111) of the (200) plane to the (111) plane of 5.5 or more, and in the crystal grain size distribution measured by EBSD analysis, the proportion of crystal grains of 1 μm or less is 85% or more.

[0013] The CrN coating has a preferred orientation of 200 as determined by XRD, and the X-ray diffraction intensity ratio (200) / (111) of the (200) plane to the (111) plane is 5.5 or more, preferably 6 or more, more preferably 111 or more. The upper limit is not limited, but is usually 20 or less, and may be 10 or less. In the CrN coating, when the proportion of crystal grains of 1 μm or less in the crystal grain size distribution measured by EBSD analysis is 85% or more, preferably 86% or more, and more preferably 90% or more, the CrN coating becomes dense, and even if cracks occur, they are less likely to join together, thereby improving peeling resistance. The upper limit is not limited and may be 100% or less, 99% or less, or 95% or less.

[0014] The CrN coating preferably has a plastic power of 61% or more, more preferably 64% or more, and preferably 69% or less, as measured using a Vickers indenter in accordance with the international standard for nanoindentation testing, ISO 14577-1. The CrN of this embodiment is a dense CrN coating and has a high plastic power. Furthermore, the CrN coating preferably has a micro Vickers hardness of 800 HV to 1300 HV, more preferably 1100 HV or less, and even more preferably 1000 HV or less. If the micro Vickers hardness of the coating is not too high, the coating will not be brittle and will have improved peeling resistance. To obtain the CrN coating of this embodiment, it is preferable to form the CrN coating by the ion plating method described below. In particular, by changing the position and shape of the control magnet arranged around the cathode, the behavior of the arc spot formed on the surface of the target material during discharge can be changed, thereby controlling the physical properties of the CrN coating.

[0015] FIG. 1 is a cross-sectional view of a portion of a piston ring as an example of this embodiment. The upper and lower surfaces and the sliding surface (left side surface in the figure) of the piston ring 10 have a CrN coating 12. In this embodiment, the piston ring 10 has the CrN coating 12 on at least the sliding surface, but other surfaces, such as the outer peripheral surfaces of the upper and lower surfaces, may also have a CrN coating. The thickness of the CrN coating on the sliding surface is not particularly limited, and is usually 3 μm or more, and may be 5 μm or more, and is usually 50 μm or less, and may be 30 μm or less. Note that a piston ring is one form of sliding member, and other examples of sliding members include pistons, bearings, washers, and valve lifters.

[0016] In the case of piston rings, the material of the substrate 11 of the piston ring 10 is not particularly limited as long as it is a material that has been conventionally used as a piston ring substrate. For example, stainless steel material, steel material, etc. are preferably used, and specifically, martensitic stainless steel, silicon chromium steel, etc. are preferably used.

[0017] A Cr plating film, a chromium nitride film, a titanium nitride film, etc. may be further provided between the CrN coating and the piston ring substrate, or the CrN coating may be formed directly on the piston ring substrate. In addition, when the substrate is stainless steel, the substrate may be subjected to a nitriding treatment.

[0018] The CrN coating can be formed by physical vapor deposition such as ion plating or sputtering. An example of forming a CrN coating by ion plating will be explained with reference to the drawings. 2 is a cross-sectional schematic diagram showing an example of an apparatus 20 for forming a CrN coating by ion plating. A gas inlet pipe 22 and a vacuum exhaust system pipe 23 are connected to a vacuum chamber 21, and the temperature inside the vacuum chamber 21 can be controlled by a heater (not shown). The apparatus also includes a cathode 24 and an anode 25. A control magnet 26 is disposed at the tip of the cathode 24 (the right end of the cathode in the figure), and a target material 27 is ionized into plasma by arc discharge.

[0019] A piston ring is placed on a rotary table (not shown) in the vacuum chamber 21, and a gas introduction The target material, chromium, is ionized and deposited on the surface of the piston ring while nitrogen gas is introduced from the inlet pipe 22. The operating conditions of the apparatus at this time can be an arc current of 100 to 200 A, a bias voltage of 0 to 50 V, a pressure inside the chamber of 1 to 4 Pa, and a heating temperature by the heater of 300 to 400°C. The nitrogen content in CrN can be controlled by the internal pressure of the introduced gas and the nitrogen partial pressure.

[0020] The properties of the CrN coating can also be controlled by changing the position and shape of the control magnet placed around the cathode. For example, by placing a magnet around the tip of the cathode, the arc spots become smaller, the speed at which each arc spot moves across the cathode surface increases, and the generated plasma extends close to the piston ring, improving the ionization rate and making it easier to form a denser CrN coating. [Example]

[0021] Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited only to the following examples. The physical property values of the coating were measured using the following equipment. <X-ray diffraction measurement> The preferred orientation of the coating by XRD was measured using an XRD apparatus (D8 DIS COVER, manufactured by Bruker AXS). For the X-ray tube and X-ray used in XRD, Cu Kα rays were used, and measurements were taken in the range of 2θ = 30 to 90° at a tube voltage of 40 kV and a tube current of 40 mA. The sample used was a piston ring with a CrN coating on its outer peripheral surface, which was cut and irradiated with X-rays from the outer peripheral sliding surface side for measurement. From the obtained XRD pattern, the peak intensities of the (111) and (200) planes of CrN were determined, and their ratio was calculated.

[0022] <Measurement of crystal grain size (EBSD analysis)> The crystal grain size of the coating was measured using a FE-SEM (JSM-7100F, manufactured by JEOL Ltd.) and EBSD analysis software (DigiviewIV, manufactured by TSL). Measurements were taken at an acceleration voltage of 15.0 kV, a measurement interval of 0.02 μm, and a measurement area of 20 × 20 μm. The sample used was a piston ring with a CrN coating on its outer peripheral surface, which was cut, polished with diamond slurry on its outer peripheral sliding surface, ultrasonically cleaned, and then subjected to Ar ion milling to remove polishing marks before irradiating the outer peripheral surface side with an electron beam for measurement. The sample was tilted and irradiated with an electron beam, and the reflection electron diffraction pattern (kikuchi lines) was measured from the scattered electron beam. The kikuchi lines were analyzed to create an inverse pole figure along each crystal orientation. From the inverse pole figure, crystal grains were defined as continuous measurement points within an azimuthal difference of 5° or less and grouped together, and an inverse pole figure azimuth map within the measurement area was created. The grain size of each crystal particle was measured from the inverse pole figure azimuth map, and the area ratio with respect to the entire measurement area was calculated in 0.1 μm increments. From the histogram of the crystal grain size distribution created in 0.1 μm increments, the ratio (area ratio) of the crystal grain size of 1 μm or less with respect to the entire measurement surface was calculated.

[0023] <Coating composition> The coating composition was measured using an EPMA. The EPMA measurement was performed using a Shimadzu EPMA-1720HT. Quantitative analysis was performed using an accelerating voltage of 15 kV, a probe current of 50 nA, an electron beam diameter of 100 μm, and pure Cr as the standard sample and BN as the standard sample. The samples were prepared using the same procedure as used for EBSD. The intensity obtained for the standard sample was set to 100%, and the weight percentage of the sample was calculated by comparing this with the intensity of the unknown sample. The weight percentages of the elements being measured were normalized so that the sum of the obtained weight percentages was 100%, and the atomic percentage was calculated.

[0024] <Plastic work rate> The plastic power of the coating was measured using a Fisher Instruments nanoindentation measuring instrument, model HM-2000. The measurement method was in accordance with ISO14577-1. Using a Vickers indenter, measurements were taken with an indentation load of 1000 mN and a time to reach the maximum indentation load of 30 seconds. The specimens were cut from piston rings with a CrN coating on the outer surface, embedded in resin, and the outer surface (measurement surface) was polished with emery paper and diamond slurry. The plastic power was calculated as the plastic deformation power η calculated from the load-indentation depth curve. plast It was decided.

[0025] <Examples and Comparative Examples> Steel material equivalent to JIS G3651 SWOSC-V was prepared as the piston ring substrate and machined into a piston ring shape (φ73.0 mm x thickness 1.0 mm). A CrN coating was formed on this using an apparatus for forming CrN coatings by ion plating, as outlined in Figure 2. The CrN coating was formed under the conditions shown in Table 1 below. Next, the physical properties of the formed CrN coating were measured. The results are shown in Table 2. Note that all CrN coatings had a preferred orientation of 200. Furthermore, no crystal grains of 2.0 μm or larger were present in the examples. Furthermore, the crystal grains of the CrN coating of Example 1 are shown in FIG. 3, and the distribution of the crystal grain size of the CrN coating of Example 1 is shown in FIG. 4.

[0026] <Peeling resistance test> In the peeling resistance test, a piece of piston ring was pressed against the side of a disk rotating at a constant speed, and after a certain period of operation, the quality of the ring was evaluated based on the presence or absence of damage (cracks or peeling) on ​​the sliding surface. Peeling resistance was judged as A if there was no peeling on the sliding surface, B if the maximum length of the peeling was less than 100 μm, and C if the maximum length of the peeling was 100 μm or more. The test conditions were a load of 40N, a speed of 5-10m / s, a time of 5 minutes, and 0W-20 lubricant. The disk material was S45C, and the surface roughness was 1.5μm in ten-point average roughness Rzjis according to JIS-B0601 (2001). The evaluation method was to take images of the sliding marks using a metallurgical microscope (Olympus inverted metallurgical microscope GX71) and measure the maximum length of the peel marks using image analysis software (Olympus industrial image analysis software OLYMPUS Stream).

[0027] [Table 1]

[0028] [Table 2]

[0029] A peeling resistance test was conducted on each of the CrN coatings obtained in Examples 1 to 8 and Comparative Examples 1 to 4. The peeling resistance test was conducted by observing the coating surface after conducting a pin-disk sliding test, the outline of which is shown in Figure 5. A portion of the observation results and an example of the evaluation are shown in Figure 6. The results are shown in Table 2. As a result of the observation, the CrN coating of the example had some cracks but no peeling of the coating, and the maximum size of the peeling was less than 100 μm, whereas the CrN coating of the comparative example had cracks and also had peeling with a maximum length of 100 μm or more. [Explanation of symbols]

[0030] 10 Piston rings 11 Piston ring base material 12 CrN coating 20 CrN film forming equipment 21 Vacuum chamber 22 Gas inlet pipe 23 Vacuum exhaust system piping 24 cathode 25 anodes 26 Control magnet 27 Target Materials 30-pin disk test equipment 31 Disc (lower test piece) 32 pin (upper test piece)

Claims

1. A sliding member having a sliding surface coated with a CrN coating, wherein the CrN coating has a preferred orientation of 200 as determined by XRD, an X-ray diffraction intensity ratio (200) / (111) of 5.5 or more for the (200) plane to the (111) plane, a proportion of crystal grains of 1 μm or less in a crystal grain size distribution measured by EBSD analysis of 85% or more, and a plastic power, which is the proportion of plastic deformation power in total indentation work measured using a Vickers indenter in accordance with ISO 14577-1, of 61% or more and 69% or less.

2. 2. A sliding member having a sliding surface coated with a CrN coating according to claim 1, wherein the CrN coating has a micro Vickers hardness of 800 HV or more and 1300 HV or less.

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