Sliding contact material for motor brush, motor brush, and direct current motor

Abstract

The present application is a sliding contact material for a motor brush, the sliding contact material having dispersed in a matrix composed of pure Ag a metal boride at a content of 0.3% by volume or more and 12% by volume or less. The metal boride as the dispersed particles is specifically preferably a boride of Hf, Ta, Nb, Ti, Zr, Mo, Cr, or V. Furthermore, the matrix composed of pure Ag can contain inevitable impurity elements having a solid solubility limit with respect to Ag, but the content thereof is preferably 0.5% by mass or less. The sliding contact material according to the present application is suitable as a constituent material of a motor brush of a small DC motor. The present application is a sliding contact material that is excellent in mechanical wear resistance and spark erosion resistance and is cost-replaceable with respect to an Ag-Pd-based alloy.

Description

Technical Field

[0001] This invention relates to sliding contact materials suitable for use as brushes in small DC motors. In particular, it relates to sliding contact materials that exhibit excellent wear resistance and spark erosion resistance, are Pd-free, and whose material cost is also taken into account, and to motor brushes using these materials. Background Technology

[0002] Small DC motors have long been widely used in various fields such as automobiles, precision instruments, and household appliances. For example, automobiles are equipped with numerous electrical devices, including audio systems, air conditioning units (air conditioning dampers), power folding mirrors, steering locks, and door locks, all of which are powered by small DC motors. Furthermore, small DC motors are also used as key components in household appliances such as shavers, electric toothbrushes, and small vacuum cleaners.

[0003] Despite their different applications, DC motors share a common basic structure and configuration. A typical DC motor consists of a housing, permanent magnets mounted on the inner surface of the housing, and a rotor rotatably supported by a shaft inside the housing. The rotor includes an armature and a commutator, with brushes acting as current collectors and electrically connected to the commutator. The motor receives power from an external power source through the brushes and commutator to rotate the rotor. In this type of DC motor, the durability of the contact materials that make up the brushes and commutator is crucial for ensuring stable operation.

[0004] Brushes are components in a DC motor that work in conjunction with the commutator. The durability of their contact materials must be superior to that of the commutator's contact materials. During motor operation, the brushes are constantly in contact with the commutator while it is pressing against it, transferring current from the power supply to the commutator, thus operating under more demanding conditions. Furthermore, the contact materials used for the brushes of small DC motors are selected based on the load determined by the correlation between the motor's locked-rotor torque and locked-rotor current. Specifically, the contact materials for the brushes of small DC motors are selected based on the following required characteristics: (i) Brush material for high-load small DC motors For the motor brushes of high-load small DC motors with large locked-rotor torque and locked-rotor current, spark discharge and arc discharge are easily generated when the brushes separate from the commutator due to the large current. Therefore, in high-load small DC motors, in addition to durability against mechanical wear, durability against spark erosion is also required.

[0005] For the contact material of the motor brushes in high-load small DC motors, carbon-based materials with low dynamic friction coefficients against metals and excellent mechanical wear resistance are generally used. Among the carbon-based materials known are those obtained by mixing precious metals or metal powders such as copper, or ceramic powders with carbon powder (Patent Documents 1 and 2). Furthermore, in the motor brushes of high-load small DC motors, to compensate for mechanical wear and spark erosion, brush materials obtained by forming large-volume blocks of carbon-based materials in the millimeter range are used. In addition, the motor brushes of high-load small DC motors typically employ a structure in which the block-shaped brush material is pressed against the commutator using a spring. It should be noted that high-load small DC motors with this configuration are used in electrical equipment such as electrically folding rearview mirrors, steering locks, and door locks in automobiles.

[0006] (ii) Brush material for low-load small DC motors

[0007] In the brushes of low-load small DC motors with relatively low locked-rotor torque and locked-rotor current, the concern about spark erosion caused by discharge is less compared to high-load small DC motors. Therefore, in the brush materials of low-load small DC motors, the focus is on ensuring durability against mechanical wear, rather than durability against spark erosion.

[0008] As the contact material for the motor brushes of low-load small DC motors, precious metal-based materials, especially Ag-Pd alloys, are used. Examples of Ag-Pd alloys include Ag-50%Pd alloy and Ag-30%Pd alloy as described in Patent Document 3. Ag-Pd alloys have extremely high durability against mechanical wear and excellent resistance to welding.

[0009] It should be noted that the brushes in low-load small DC motors use brush materials that bond these precious metal-based materials to a substrate made of Cu-based materials such as beryllium copper and phosphor bronze. Furthermore, applications of low-load small DC motors with this configuration include automotive applications such as vehicle air conditioning (HVAC) systems (e.g., air conditioning dampers) and audio equipment. Additionally, low-load small DC motors are also used in general-purpose motors integrated into household appliances such as shavers and toys.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 11-4563

[0013] Patent Document 2: Japanese Patent Application Publication No. 2005-176492

[0014] Patent Document 3: Japanese Patent Application Publication No. 5-277762

[0015] Patent Document 4: Japanese Patent Application Publication No. 2023-095274 Summary of the Invention

[0016] The problem that the invention aims to solve

[0017] The sliding contact materials for motor brushes used in the aforementioned high-load and low-load small DC motors have been used without problems in their respective applications to date. However, due to the increasing demand for small DC motors and the requirement to reduce product costs, a change in the contact material is needed for the motor brushes of both types of small DC motors.

[0018] Regarding carbon-based materials used in the brushes of high-load small DC motors, the primary reason for changing the material is to meet the requirements of motor miniaturization. Carbon-based materials, due to their low minimum arc current, are prone to spark and arc discharges during sliding. To date, carbon-based materials have been used as bulk brush materials to compensate for spark erosion and mechanical wear caused by discharges; however, brush materials in the millimeter range are insufficient for miniaturization.

[0019] Furthermore, the spark discharge and arc discharge easily generated by carbon-based materials can contribute to noise and rotational noise. In recent years, with the increasing trend towards higher-end automotive products, there has been a growing demand for noise reduction during motor operation. Therefore, for sliding contact materials in high-load small DC motors, there is a need for a sliding contact material that can replace carbon-based materials, possesses excellent wear resistance, is not prone to spark discharge, and is resistant to ablation even when spark discharge occurs.

[0020] On the other hand, Ag-Pd alloys, used as contact materials for motor brushes in low-load small DC motors, have also seen a surge in Pd prices recently, leading to demands for material changes. While Pd prices were relatively low among precious metals in the past, they have risen sharply since about 2-3 years ago, now reaching levels comparable to or even higher than gold (Au). Some Ag-Pd alloys used in motor brushes contain up to 50% Pd by mass, thus the price of Pd directly impacts the material cost of motor brushes. For motor brushes used in general-purpose motors in household appliances, the rising material costs also affect product costs.

[0021] As mentioned above, Ag-Pd alloys exhibit extremely high durability against mechanical wear, making them unparalleled as sliding contact materials. However, due to the aforementioned surge in Pd prices, there is a demand for a Pd-free sliding contact material that strikes a good balance between cost and performance, even if it does not demonstrate the same level of durability as Ag-Pd alloys.

[0022] While the necessity of material changes in both high-load and low-load small DC motors has been recognized, as stated above, concrete measures have been few and far between. In particular, considering the aforementioned material costs, material changes are practically difficult to implement.

[0023] To address these issues, the applicant has disclosed a low-cost contact material that exhibits excellent mechanical wear resistance and spark erosion resistance, and is Pd-free (Patent Document 4). This contact material uses pure Ag as a matrix and employs ZnO and Ta2O5 particles as dispersed particles. The applicant's contact material achieves excellent spark erosion resistance by using a pure Ag matrix with spark erosion resistance and selecting suitable oxide particles. Furthermore, while its mechanical wear resistance is not as good as Ag-Pd alloys, it is still excellent, and its Pd-free nature results in a superior cost-performance ratio.

[0024] However, in the field of sliding contact materials, there is always a demand for improved durability. While the contact materials described above by the applicant are excellent in terms of balancing mechanical wear resistance, spark erosion resistance, and material cost, further improvements in durability could contribute to the cost reduction and miniaturization of small DC motors.

[0025] This invention was made against the above background and relates to contact materials for motor brushes of small DC motors, providing a sliding contact material for motor brushes that is useful in both high-load and low-load small DC motors. Specifically, for high-load small DC motors, a sliding contact material is provided that exhibits excellent durability against mechanical wear and excellent resistance to spark erosion. Furthermore, for low-load small DC motors, a Pd-free sliding contact material that can replace Ag-Pd alloys and exhibits good wear resistance is provided.

[0026] Methods for solving problems

[0027] In the prior art of the applicant (Patent Document 4), spark erosion resistance is ensured by using pure Ag as the matrix. Compared with carbon-based materials and Ag alloys, Ag is a metallic material that is less prone to discharge and has good spark discharge characteristics in high-load areas. Therefore, Ag can also avoid the noise problem that occurs with carbon-based materials. For the inventors, who consider improving the spark erosion resistance of contact materials for motor brushes a problem that must be solved, using pure Ag as the matrix is ​​also a preferred configuration. Therefore, while using pure Ag as the matrix, the inventors conducted research on dispersed particles that can achieve mechanical wear resistance and spark erosion resistance superior to those of existing technologies. And as a result, they found that the above-mentioned problems can be solved by applying metal borides, thereby completing the present invention.

[0028] That is, the present invention is a sliding contact material for motor brushes, wherein the sliding contact material has metal borides dispersed in a matrix composed of pure Ag, characterized in that the content of the metal borides is 0.3% by volume or more and 12% by volume or less.

[0029] The sliding contact material for motor brushes disclosed in this invention is composed of a pure Ag matrix and metal borides, resulting in excellent durability against both mechanical wear and spark erosion. Furthermore, this invention uses a contact material containing a Pd-free noble metal, which is free of expensive Pd as the constituent metal. Therefore, this invention is also useful as a substitute for Ag-Pd alloy sliding contact materials for motor brushes. The sliding contact material for motor brushes disclosed in this invention will be described in more detail below.

[0030] (A) The present invention relates to the composition and manufacturing method of sliding contact materials.

[0031] (I) Pure Ag matrix

[0032] The sliding contact material for motor brushes according to the present invention has a matrix composed of pure Ag. Considering spark resistance, the spark characteristics deteriorate when metallic elements other than Ag are mixed into the matrix, i.e., when the matrix is ​​an Ag alloy or the like. Therefore, using pure Ag as the matrix is ​​essential for the contact material of the motor brushes according to the present invention.

[0033] The matrix refers to the substrate / parent phase in which metal boride particles are dispersed. A matrix composed of pure Ag refers to a metal containing no other elements besides Ag and unavoidable impurities. Unavoidable impurities include elements such as Fe, Cr, Pd, Cu, Ni, Mn, Al, Si, Mg, Zn, Sn, In, and Bi. These unavoidable impurity elements are elements that are mixed in trace amounts into the Ag powder and metal boride powder used as raw materials for sliding contact materials, or they are mixed in from the constituent materials of the manufacturing equipment (crushers, mixers, grinding media, etc.).

[0034] The aforementioned elements that are unavoidable impurities are classified according to whether they have a solid solubility limit relative to Ag. Examples of unavoidable impurity elements with a solid solubility limit relative to Ag include Pd, Cu, Mn, Al, Mg, Zn, Sn, In, and Bi. Elements with a solid solubility limit relative to Ag alloy with Ag, causing the matrix to become an Ag alloy. This Ag alloy matrix alters the melting point and contact resistance of the contact material, potentially reducing its resistance to spark erosion. Therefore, elements with a solid solubility limit relative to Ag are unavoidable impurities that should be particularly limited. Specifically, the total content of these elements in the matrix is ​​preferably 0.5% by mass or less, more preferably 0.3% by mass or less, even more preferably 0.2% by mass or less, and particularly preferably 0.1% by mass or less.

[0035] On the other hand, Fe, Cr, Ni, and Si are examples of unavoidable impurity elements that do not have a solid solubility limit relative to Ag. These elements do not alloy with Ag and form a matrix together with pure Ag in the form of atomic states or compounds such as oxides. Therefore, these elements are not as strictly limited as the elements that can be dissolved in Ag mentioned above, and can be contained within a range that does not hinder the effects brought about by the metal boride particles that are a feature of this invention. However, Fe, Cr, and Ni may increase the contact resistance by forming oxides and promote commutator wear as a brush mating material, so they should not be actively contained. Regarding the content of unavoidable impurity elements that do not have a solid solubility limit relative to Ag, in terms of total amount calculated by metal conversion, it is preferably 3% by mass or less, more preferably 1% by mass or less, further preferably 0.5% by mass or less, and particularly preferably 0.3% by mass or less.

[0036] It should be noted that the unavoidable impurity content in the Ag matrix of the sliding contact material can be easily determined using analytical methods such as electron probe microanalysis (EPMA), energy-dispersive X-ray analysis (EDX), wavelength-dispersive X-ray analysis (WDX), and X-ray fluorescence analysis (XRF). Furthermore, the sliding contact material of this invention is sometimes bonded to a substrate such as a Cu-based material to form a motor brush. Where it is possible to separate part or all of the contact material from the substrate, the amount of impurities in the Ag matrix can be estimated by performing inductively coupled plasma optical emission spectrometry (ICP-OES) on the separated contact material. In this case, by measuring the content of each metal component in the contact material and thus considering the content of dispersed particles, the impurity content in the Ag matrix can be estimated.

[0037] (II) Dispersed particles (metal borides)

[0038] In the sliding contact material for motor brushes disclosed in this invention, particles composed of metal borides are dispersed in the aforementioned pure Ag matrix. Since metal borides are high-hardness ceramic materials, their use as dispersed particles in composite formulation helps improve the mechanical wear resistance of the contact material. Furthermore, metal borides possess high melting points and relatively high thermal and electrical conductivity. Dispersed particles with high melting points and good electrical conductivity impart spark erosion resistance to the contact material. In this respect, conventional brush contact materials (Patent Document 4, etc.) mostly use metal oxides as dispersed particles. In contrast, this invention, by using metal borides as dispersed particles, achieves durability superior to existing technologies.

[0039] Furthermore, in this invention, the preferred metal borides used as dispersed particles are metal borides with the metal composition Hf, Ta, Nb, Ti, Zr, Mo, Cr, and V (HfB2, TaB2, NbB2, TiB2, ZrB2, MoB, CrB2, VB2). These metal borides also have high melting points and high electrical conductivity, thus contributing to improved resistance to spark erosion.

[0040] The content of metal boride (dispersed particles) in the sliding contact material of the present invention is set to 0.3 vol% or more and 12.0 vol% or less. If it is less than 0.3 vol%, the above-mentioned effect cannot be expected; if it exceeds 12.0 vol%, the processability will be significantly reduced. Since the contact material for motor brushes is manufactured through multiple plastic processing steps, the quality of processability is also important. The content of metal boride is preferably 0.5 vol% or more and 11.0 vol% or less, more preferably 1.0 vol% or more and 11.0 vol% or less.

[0041] It should be noted that the content of metal borides in sliding contact materials can be determined using the analytical methods described above, such as those employing EPMA. Alternatively, the content of metal borides can also be determined by directly analyzing the contact material using ICP-based fluorescence spectrophotometry to measure the content of each metal component and boron.

[0042] The particle size of the dispersed particles composed of metal borides is preferably 1.0 μm or more and 100 μm or less, based on the average particle size. Dispersed particles with excessively small average particle sizes have a low effect on improving wear resistance. Furthermore, when the average particle size of the metal borides is too large, their dispersion becomes sparse, making it difficult to contribute to wear resistance. It should be noted that the average particle size of the dispersed particles in the contact material can be determined by observing the metal microstructure using SEM or similar methods, measuring the particle size of the metal borides in the image using biaxial methods, and calculating the average value.

[0043] It should be noted that the dispersed particles of the sliding contact material for motor brushes involved in this invention may consist solely of metal borides, without the need for other dispersed particles. However, other dispersed particles may be included as long as they do not impair the effects of the invention brought about by the aforementioned metal borides. Examples of other dispersed particles include metal oxides, nitrides, and carbides. These other dispersed particles should be limited according to the same principles as the aforementioned metals that do not dissolve in Ag (such as Fe).

[0044] The sliding contact material for motor brushes involved in this invention can be manufactured using powder metallurgy. In powder metallurgy, the contact material can be obtained by sintering a mixture of Ag powder as a matrix and metal boride powder (HfB2 powder, TaB2 powder, NbB2 powder, TiB2 powder, ZrB2 powder, MoB powder, CrB2 powder, VB2 powder) as dispersed particles. In this case, the average particle size of the Ag powder is preferably 1.0 μm or more and 15 μm or less, and the average particle size of the metal boride powder is preferably 1.0 μm or more and 100 μm or less. Furthermore, before sintering, it is more preferable to press and compress the mixed powder to form a blank (compressed body). The blank is preferably manufactured at a particle size of 2.5 × 10⁻⁶. 2 MPa or higher and 15×10 2 The mixed powder is pressurized to a pressure below MPa. Furthermore, the heating temperature during sintering is preferably set to 700°C or higher and 950°C or lower. The contact material made from the sintered body obtained by this powder metallurgy method can be appropriately processed.

[0045] (B) The morphology of the sliding contact material involved in this invention

[0046] There are no particular limitations on the form in which the sliding contact material of the present invention can be used as a motor brush. It can be processed into a corresponding shape and size according to the specifications of the motor in which it is mounted. However, since the motor brush needs to maintain contact with the commutator with appropriate contact pressure, it is required to have spring characteristics. Therefore, the sliding contact material of the present invention is preferably used in the form of a composite material combined with a substrate having spring characteristics.

[0047] Cu-based materials are preferred as the substrate. Examples of Cu-based materials include pure Cu, cerium copper, beryllium copper, phosphor bronze, and CuNi alloys. Furthermore, by bonding a sliding contact material to at least a portion of a substrate made of Cu-based materials, a composite material for motor brushes can be manufactured.

[0048] It should be noted that while the sliding contact material involved in this invention is less expensive than Ag-Pd alloys, it is more expensive than Cu-based materials. Therefore, the application of composite materials also helps to reduce the cost of motor brushes. Furthermore, by selecting a substrate with appropriate spring properties, motor miniaturization can be achieved using composite material motor brushes. In conventional high-load small DC motors using carbon-based materials, brushes obtained by combining bulk carbon-based materials with spring materials are used, while the present invention enables even more miniaturized motors.

[0049] There are no particular limitations on the size of the aforementioned composite material. Strip, plate, or chip-shaped sliding contact materials are bonded to a portion or the entire surface of a strip or plate-shaped substrate. In addition to press-fitting (overlapping bonding), the sliding contact material can also be bonded to the substrate using seam welding, brazing, or other bonding methods. Furthermore, by appropriately cutting and processing this composite material, motor brushes can be manufactured.

[0050] Furthermore, the motor brushes using the contact material of the present invention can be applied to the aforementioned high-load, low-load small DC motors. The small DC motor is configured as described above, and motor brushes and a commutator are essential components. The motor brushes supply power from an external power source to the commutator.

[0051] For motor brushes using the sliding contact material of the present invention, Ag alloys are preferred as the constituent material of the commutator. Examples of Ag alloys include those obtained by adding one or more of Pd, Cu, Zn, and Ni in total of 1% to 12% by mass to Ag. Specifically, Ag-Cu alloys, Ag-Cu-Ni alloys, and Ag-Pd-Cu-Zn-Ni alloys are examples.

[0052] In addition to the aforementioned Ag alloys (solid solution alloys), oxide-dispersion Ag alloys are sometimes used as commutator materials. For example, there are oxide-dispersion Ag alloys in which MgO and ZnO are dispersed in an AgCu alloy matrix. Furthermore, there are materials that use an Ag alloy (containing one or more of Fe, Co, Ni, and Cu) as a matrix and disperse Ta2O5 particles or oxide particles of Mg, Fe, Co, Ni, and Zn therein.

[0053] The commutator material composed of Ag alloy described above is applied according to the specifications of small DC motors with low and high loads. It should be noted that in small DC motors for any application, the commutator of the small DC motor can have the above-mentioned commutator material on the surface that contacts the motor brushes. Therefore, the commutator can also be made of a composite material in which contact material is clad onto a substrate. In this case, the substrate can be the same Cu-based material as the composite material used for motor brushes described above.

[0054] Invention Effects

[0055] The sliding contact material of this invention is designed for use in motor brushes operating under harsh conditions of mechanical wear and discharge / sparking, exhibiting high wear resistance and spark erosion resistance. Furthermore, these properties are achieved without using Pd as a constituent metal. This invention improves upon both these performance characteristics and cost, and holds promise as a replacement for carbon-based contact materials conventionally used in motor brushes for high-load, small DC motors. The contact material of this invention also reduces noise while maintaining durability.

[0056] Furthermore, the sliding contact material of the present invention can also be used for brush applications in low-load small DC motors. In these DC motors, Ag-Pd alloys have historically been used as brush materials, and the present invention is useful as a substitute. Since the contact material of the present invention does not use Pd, it can provide general-purpose motors and the like at a low cost, considering the recent surge in Pd prices. Attached Figure Description

[0057] [ Figure 1 [ ] is the measurement result of the starting voltage of a DC motor equipped with the contact material (Ag-7 volume % HfB2) manufactured in the first embodiment during initial operation and after placement.

[0058] [ Figure 2 [Image] is a photograph showing the material structure observation results of contact materials with different HfB2 contents manufactured in the second embodiment. Detailed Implementation

[0059] Implementation Method 1 The preferred embodiments of the present invention will be described based on the following examples. In this embodiment, a sliding contact material having 7 vol% HfB2 dispersed in a pure Ag matrix and a composite material containing the same material were manufactured. The composite material was then processed into a motor brush and assembled into a small DC motor, and its wear resistance and spark erosion resistance were evaluated.

[0060] [Manufacturing of sliding contact materials]

[0061] In this embodiment, the sliding contact material is manufactured using powder metallurgy. Pure Ag powder (average particle size 7 μm, Ag concentration 99.95% by mass) is mixed with 7% by volume HfB2 powder (average particle size 3 μm) in a ball mill for 5 hours. The powder mixture is then placed in a cylindrical container and subjected to a 5 × 10⁻⁶ mm pressure from the longitudinal direction. 2The material is compressed under pressure of MPa to form a cylindrical blank with a diameter of 50 mm. This cylindrical blank is then sintered in a vacuum at 850°C for 8 hours. This compression and sintering process is repeated three times to produce the sliding contact material. The sliding contact material is then hot-extruded into a coarse wire with a diameter of 6 mm, and repeatedly drawn and annealed to produce a wire with a diameter of 1 mm. Further, this wire is rolled using a rolling mill to obtain a strip of sliding contact material.

[0062] Manufacturing of composite materials for motor brushes

[0063] Next, the strip-shaped sliding contact material is overlaid and bonded to the substrate using a rolling mill (interlocking bonding) to produce a composite material. The substrate is a copper alloy for springs (C1741). In this composite material used as a motor brush, the thickness of the sliding contact material is 40 μm, and the total thickness including the substrate is set to 75 μm.

[0064] [Reference Example]

[0065] To confirm the durability of the sliding contact material (Ag-7 vol% HfB2) manufactured in this embodiment, a composite material using Ag-50 wt% Pd alloy (an Ag-Pd alloy) as the sliding contact material was manufactured. Ag-Pd alloy was used as a reference example to evaluate the contact material involved in this embodiment because, as mentioned above, Ag-Pd alloy has extremely good wear resistance. Furthermore, Ag-Pd alloy is also a good noble metal contact material considering its resistance to spark erosion under high loads, and therefore can also serve as a reference for evaluating spark erosion resistance. However, in this application, the thickness of the sliding contact material (Ag-50 wt% Pd alloy) of the motor brush in this reference example is set to 10 μm. This is because one of the main problems addressed in this application is improving material cost, and the price of Pd is skyrocketing. In this embodiment, the material cost of the motor brush is kept approximately the same, and durability is evaluated and the possibility of substitution is explored based on this.

[0066] [Fabrication and Durability Evaluation Tests of Small DC Motors]

[0067] The composite material manufactured in this embodiment and the reference example was processed into motor brushes, and assembled with a commutator into an actual small DC motor. Durability evaluation tests were conducted on the sliding contact material. The durability tests were performed in three modes: (a) a wear life test primarily evaluating durability against mechanical wear; (b) an ablation life test primarily evaluating durability against spark erosion; and (c) a placement test evaluating start-up performance after placement at high temperatures. It should be noted that the commutator of the small DC motor in each evaluation test appropriately used contact materials that met the test conditions. Furthermore, a composite material was prepared by coating each contact material onto a substrate made of Cu-based material, and the commutator was manufactured from this composite material.

[0068] (A) Wear life test (mechanical wear resistance evaluation)

[0069] Evaluations were conducted under the following two test conditions, A-1 and A-2. These test conditions simulated a low-load small DC motor used in a vehicle air conditioning system (HVAC), as described above, and aimed to evaluate durability against mechanical wear. It should be noted that in these durability tests, a combination of counter-clockwise rotation (CCW) and stop (OFF) and clockwise rotation (CW) and stop (OFF) was set as one cycle, and the number of cycles until the motor stopped was evaluated. At this time, twice the specification value was used as the target value; meeting this target value was considered excellent (◎). Furthermore, although the target value was not met, it was above the specification value and was considered good (〇). Furthermore, those judged as excellent or good were considered acceptable.

[0070] [Table 1]

[0071] (B) Ablation resistance life test (spark ablation resistance evaluation)

[0072] The evaluation was conducted under the following two test conditions, B-1 and B-2. These test conditions simulate the high-load small DC motor used in electrically folding automotive rearview mirrors, as described above, and are intended to evaluate the durability against spark erosion.

[0073] [Table 2]

[0074] (C) Placement test (initial evaluation)

[0075] The placement test for starting performance evaluation was conducted on a small DC motor (commutator: AgCuNi-Ta2O5+ZnO) used under test condition A-2. The small DC motor was placed at 85°C (without humidity control) and 85°C (95% RH) for predetermined times (500 hours without humidity control, 250 hours at 95% RH). After the specified placement time, the motor was driven and the starting voltage was measured. The starting voltage after placement was compared with the starting voltage before placement, and a starting voltage of DC 3.5V was used as the pass / fail value.

[0076] The results of the evaluation tests (A), (B), and (C) conducted in this embodiment are summarized and shown in Tables 3 and 4.

[0077] [Table 3]

[0078] [Table 4]

[0079] Regarding Table 3, in terms of the wear life evaluation as test (A), the contact material (Ag-7 vol% HfB2) of this embodiment exhibits durability that clearly exceeds the target value under any temperature conditions. Furthermore, compared to the contact material of the reference example (Ag-50 wt% Pd alloy), the contact material of this embodiment performs worse at the low temperature (-40°C) of test condition A-1, but its durability is higher than that of the reference example at other temperatures. Moreover, in terms of the spark erosion life evaluation as test (B), the contact material (Ag-7 vol% HfB2) of this embodiment also exhibits durability that clearly exceeds the target value under these test conditions. Furthermore, compared to the contact material of the reference example (Ag-50 wt% Pd alloy), the contact material (Ag-7 vol% HfB2) of this embodiment exhibits comparable spark erosion resistance.

[0080] In the (C) evaluation test on startability in Table 4, the contact material of this embodiment (Ag-7 volume%HfB2) and the contact material of the reference example (Ag-50 mass%Pd alloy) both meet the qualification criteria. Figure 1 These are the measurement results of the starting voltage of the contact material (Ag-7 volume % HfB2) of this embodiment after the initial stage and after a predetermined holding time. The starting voltage of the contact material of this embodiment did not change after being placed at high temperature, indicating that it has good starting performance.

[0081] The results of the above evaluation tests confirm that the Ag-HfB2 contact material of this embodiment exhibits suitable durability in applications where mechanical wear is a primary concern and spark erosion is a primary concern. Furthermore, it is confirmed that the Ag-HfB2 contact material of this embodiment can replace the Ag-Pd alloy contact material.

[0082] Implementation Method 2 In this embodiment, a motor brush material with varying HfB2 content dispersed in an Ag matrix was manufactured. Furthermore, a motor brush material was manufactured in which metal borides of metals other than Hf (Ta, Nb, Ti, Zr, Mo, Cr, V) were dispersed in an Ag matrix. Additionally, for comparison with the embodiments, motor brush materials using Hf oxides / carbides (HfO2, HfC) and tungsten carbides (WC) as dispersed particles were also manufactured. The durability of these motor brush materials was then evaluated.

[0083] The manufacturing method of the contact material in this embodiment is basically the same as that in the first embodiment. A powder mixture of pure Ag powder and metal boride / metal oxide powder constituting the dispersed particles is compressed to form a cylindrical blank, which is then sintered in a vacuum at 850°C. The resulting sliding contact material is then hot-extruded and drawn into wires, and further processed into strip-shaped sliding contact materials. Then, as in the first embodiment, it is coated with a copper alloy material as a base material and assembled into a small DC motor as a motor brush.

[0084] The evaluation test for this embodiment was conducted under the same conditions A-1 as in the first embodiment (Table 1), evaluating the wear resistance at room temperature. The evaluation results are shown in Table 5.

[0085] [Table 5]

[0086] Target value: 470,000 cycles (Specification value: 235,000 cycles)

[0087] The evaluation results of this embodiment are discussed. First, regarding the content of HfB2 as the metal boride, good durability was observed within the range of 0.3 vol% to 11 vol% in this evaluation (No. 1 to No. 8). It should be noted that in this embodiment, in addition to these, a contact material with 13 vol% HfB2 was also attempted to be manufactured, but breakage occurred during the wire processing stage. Therefore, from the viewpoint of processability, it was confirmed that the upper limit of the metal boride content should be 12 vol%.

[0088] For the contact material with HfB2 as the dispersed particles manufactured in this embodiment, the results of observing the material microstructure are shown below. Figure 2 . Figure 2 The material microstructure is the result of observing the cross-section of the wire after processing. Furthermore, Figure 2 The measurement results of the hardness values ​​of each contact material are also shown. Figure 2 It has been confirmed that the hardness value of the contact material increases with increasing HfB2 content. Furthermore, with increasing HfB2 content, the dispersed particles tend to aggregate.

[0089] Furthermore, in this embodiment, contact materials containing 3.6 vol% to 9.1 vol% of borides of metals other than Hf (TaB2, NbB2, TiB2, ZrB2, MoB, CrB2, VB2) as dispersed particles were investigated (No. 9 to No. 16). As shown in Table 5, contact materials using these metal borides as dispersed particles also exceeded the specified values ​​and exhibited higher durability than the prior art (No. 20). Moreover, contact materials using TaB2, NbB2, TiB2, and VB2 as dispersed particles exhibited excellent durability exceeding the target value (twice the specified value), therefore, borides of Hf, Ta, Nb, Ti, and V are considered particularly preferred dispersed particles.

[0090] Furthermore, in this embodiment, the study focused on contact materials with HfB2 as the dispersed particle, and it was confirmed that contact materials with appropriately set HfB2 content all exhibited good durability. This result confirms the effectiveness of the metallic component Hf as the dispersed particle, but also confirms that Hf compounds are not always effective. This is because the durability of contact materials with dispersed Hf oxides (HfO2) and carbides (HfC) is lower than that of borides (HfB2) (Nos. 17 and 18).

[0091] It should be noted that tungsten carbide (WC), which is generally known as a hard particle, is unlikely to contribute significantly to improved durability (No. 19). Furthermore, in this embodiment, the contact material (Ag / TaO2+ZnO) of the applicant's prior art (Patent Document 4) was also evaluated (No. 20). Referring to this result, it can be seen that the contact material of the present invention, which disperses metal boride in an Ag matrix, exhibits higher durability than the prior art. Although the contact material of the prior art (No. 20) is below the target value (twice the specification value), it meets the specification value, and therefore its usability cannot be denied. However, it can be said that the contact material of the present invention is a contact material with better durability than the prior art.

[0092] Industrial availability

[0093] This invention provides a sliding contact material for brushes in small DC motors, exhibiting suitable wear resistance and spark erosion resistance. Furthermore, the contact material of this invention not only provides durability but also reduces noise. This invention is suitable for use in small DC motors in various fields, including automobiles, precision instruments, and household appliances. In automotive applications, examples include small DC motors in audio equipment, air conditioning vents, electrically folding rearview mirrors, and door locks. In household appliances, it can be applied to small motors in shavers, electric toothbrushes, and mini vacuum cleaners.

Claims

1. A sliding contact material for motor brushes, wherein the sliding contact material contains metal boride dispersed in a matrix composed of pure Ag, characterized in that, The content of the metal boride is 0.3% by volume or more and 12% by volume or less.

2. The sliding contact material for motor brushes according to claim 1, wherein, The metal borides are borides of Hf, Ta, Nb, Ti, Zr, Mo, Cr, and V.

3. The sliding contact material for motor brushes according to claim 1 or 2, wherein, The matrix composed of pure Ag contains Ag and unavoidable impurity elements with solid solubility limits relative to Ag, wherein the content of the unavoidable impurity elements is less than 0.5% by mass.

4. A composite material for motor brushes, comprising a substrate made of a Cu-based material and a sliding contact material bonded to at least a portion of the substrate, wherein, The sliding contact material as described in claim 1 or 2 is used as the sliding contact material for bonding.

5. A motor brush for a DC motor, comprising the composite material described in claim 4.

6. A DC motor comprising motor brushes and a commutator, wherein, The motor is equipped with the motor brush as described in claim 5.

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