Magnesium-based composite material, method for manufacturing the same, and sliding member

Abstract

To provide: a Mg-based composite material that has excellent strength characteristic because of particle dispersion into a parent phase, and also has high friction and wear characteristics; a method for manufacturing the Mg-based composite material; and a sliding member.SOLUTION: An Mg-based composite material of the invention is a particle dispersed Mg-based composite material. Particles of titanium dioxide having a mean diameter of 0.05 μm or more and particles of an optional oxide or nitride are dispersed in a metallographic structure of the Mg-based composite material. Therefore, strength characteristic of pure magnesium is maintained, and a coefficient of friction of a part subjected to friction and wear is lower than that of pure magnesium, thereby showing more excellent friction and wear characteristics than pure magnesium.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an Mg-based composite material, a method for producing the same, and a sliding member.

Background Art

[0002] Magnesium (Mg) is abundant in the ground and is the lightest among practical metal materials, so its application to moving structural members such as automobiles has been actively studied. On the other hand, when used as a member, it is inevitable to come into contact with other parts, and there is a need to develop Mg and Mg alloys with excellent strength and friction and wear characteristics. Generally, refinement of the crystal grain size is an effective means for strengthening metal materials, and the same effect is exhibited for Mg and Mg alloys. However, it is known that refinement of the crystal grain size of Mg and Mg alloys is not effective as a means for improving friction and wear characteristics (Non-Patent Document 1). Due to the large grain boundary diffusion coefficient of Mg, grain boundary slip easily occurs. Therefore, as the crystal grain size is refined, the grain boundary volume fraction increases, grain boundary slip is promoted, and work softening occurs during friction and wear. Therefore, in order to maintain and further improve the friction and wear characteristics of Mg and Mg alloys, it is preferable to coarsen the crystal grain size of the matrix phase, but this causes a problem of deterioration of strength characteristics.

[0003] In addition to refinement of the crystal grain size, particle dispersion in the matrix phase is often used to strengthen the material. Among them, in the case of metal materials, dispersing intermetallic compounds precipitated or crystallized from the matrix phase is effective for strengthening. Also, particle dispersion in the matrix phase has an effect of suppressing grain boundary slip. A Mg alloy with excellent friction characteristics in which spherical or non-acute-angled intermetallic compounds are dispersed in the Mg matrix phase, invented by the present inventors, is disclosed in Patent Document 1. Dispersing intermetallic compounds in the Mg matrix phase is also an effective means for improving strength. However, in Patent Document 1, since intermetallic compounds are precipitated and crystallized from the casting material during the production of the Mg alloy, the crystal grain size of the Mg matrix phase is coarse, and further strengthening is desired.

[0004] In the case of metal materials, not only the dispersion of precipitated or crystallized intermetallic compounds but also the dispersion of particles made of substances or materials that do not dissolve in the metal (for example, graphite, ceramics, etc.) in the matrix phase, that is, compounding, is also an effective method for improving strength. However, since Mg has extremely poor wettability with additive particles for the purpose of compounding, it is impossible to create a composite material by a casting method. Therefore, as disclosed in Patent Documents 2 and 3, a mechanical alloying method or a repeated shear strain application method that requires the strain application process to be repeated dozens of times or more is used to solidify Mg powder and additive powder to create an Mg-based composite material. However, since any of these methods is complex and requires many working steps, an increase in material cost is inevitable.

[0005] On the other hand, in order to maintain the strength characteristics of the material itself and improve the friction and wear characteristics, surface modification of the surface layer of Mg alloys is known. Patent Document 4 discloses that forming a surface-modified structure on the surface of an Mg alloy by anodic oxidation treatment and impregnating this surface-modified structure with molybdenum disulfide, which is a solid lubricant, is an effective method for improving the friction and wear characteristics of Mg. Since this method does not depend on the grain size of the Mg matrix phase and is only a modification of the surface layer, it is possible to maintain the strength characteristics. However, since it is necessary to perform anodic oxidation treatment as an additional process when using the material, there is concern about an increase in cost.

[0006] The present inventors have focused on a simpler production method as disclosed in Patent Document 5, mixed SiC powder and Mg powder, and created an Mg-based composite material by warm and hot working. In these composite materials, it has been found that SiC particles are dispersed in the Mg matrix phase, and when subjected to friction and wear, SiC particles re-aggregate at the portion subjected to the friction and wear to exhibit self-film-forming ability while maintaining excellent strength characteristics. In this specification, a material in which specific particles are dispersed in the matrix phase is also referred to as a "particle dispersion type", and the particles are also referred to as "dispersed particles".

[0007] When aiming to further modify the properties and enhance the versatility of Mg-based composites, it is necessary to consider not only focusing solely on carbides such as SiC as the particles dispersed in the Mg matrix phase, but also examining the use of other compounds. The inventors of the present invention have disclosed in Patent Document 6 that by paying attention to the surface energy of the additive powder with respect to the metal matrix phase and using particles with a large surface energy difference with respect to magnesium as the additive powder, similar properties can be obtained. That is, when using oxides and nitrides represented by Al2O3, CuO, MnO2, Si3N4, SiO2, and Y2O3, it is possible to create an Mg-based composite material without cracks or fractures and with self-film-forming ability.

[0008] On the other hand, in Patent Documents 5 and 6, self-film formation occurs at the sites subject to friction and wear. To obtain good friction and wear properties, a certain amount of time must elapse (although it is also affected by the test conditions, for example, 1000 seconds or more). In view of practical application, it is extremely desirable to reduce the friction coefficient in a shorter time. In the above-mentioned patent documents, in addition to SiC, examples related to oxides and nitrides of Al2O3, CuO, MnO2, Si3N4, SiO2, and Y2O3 are disclosed, but it is not necessary to be limited to these, and the oxide may be titanium dioxide (TiO2).

[0009] Regarding Mg-based composites using titanium dioxide as the composite material, they are disclosed in Patent Document 7 and Patent Document 8. In Patent Document 7, an attempt is made to improve the strength characteristics. However, it is important to synthesize titanium-aluminum intermetallic compounds, and it is unclear what effect these intermetallic compounds have on the friction and wear properties. This is because the intermetallic compounds formed by in-situ reaction synthesis are mainly due to precipitation, and it is unclear whether their forms are spherical or have sharp corners. In addition, the size (diameter) of the intermetallic compounds formed by precipitation is considered to be on the order of several tens of nm to several hundreds of nm. In Patent Document 8, a composite material composed of titanium dioxide and hydroxyapatite is coated, and it is characterized by suppressing corrosion dissolution. Similar to Patent Document 7, the influence of titanium dioxide particles on the friction and wear properties is unclear.

[0010] Within the knowledge of the present inventors, the surface energy of titanium dioxide with respect to magnesium is unknown, and the influence and effect on the friction and wear characteristics of the Mg-based composite material in a state where titanium dioxide particles are dispersed alone or together with other particles in the matrix phase are completely unknown.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Non-Patent Documents

[0012]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0013] The present invention has been made in view of the above circumstances, and an object thereof is to provide an Mg-based composite material having excellent strength characteristics due to particle dispersion in a matrix phase and having high friction and wear characteristics, a method for producing the same, and a sliding member.

Means for Solving the Problems

[0014] In order to solve the above problems, the Mg-based composite material of the present invention is a particle-dispersed Mg-based composite material, in which titanium dioxide particles with an average diameter of 0.05 μm or more and optional oxide or nitride particles are dispersed in the Mg matrix phase in the metal structure of the Mg-based composite material, and the friction coefficient of the portion subjected to friction and wear is lower than that of pure magnesium. It is preferable that the crystal grain size of the Mg matrix phase of this Mg-based composite material is 200 μm or less. For this Mg-based composite material, the ratio of the average diameter of the titanium dioxide particles to the crystal grain size of the Mg matrix phase is in the range of 1:4 to 1:10, and the ratio of the average diameter of the optional oxide or nitride particles to the crystal grain size of the Mg matrix phase is preferably in the range of 1:4 to 1:10. It is preferable that the total content of the titanium dioxide particles and the optional oxide or nitride particles in this Mg-based composite material is less than 65% by mass. For this Mg-based composite material, it is preferable that the friction coefficient of the portion of the sliding surface that has undergone friction and wear, obtained within 10 seconds from the start of the test by the dry friction and wear test, is less than 0.20, and more preferably 0.15 or less. It is preferable that the optional oxide or nitride in this Mg-based composite material is Al2O3, CuO, MnO2, Si3N4, SiO2 or Y2O3.

[0015] The sliding member of the present invention is a sliding member containing the above Mg-based composite material, having a sliding surface made of the Mg-based composite material, and the friction coefficient of the portion of this sliding surface that is subjected to friction and wear is lower than that of pure magnesium. For this sliding member, it is preferable that the friction coefficient of the portion of the sliding surface that has undergone friction and wear, obtained within 10 seconds from the start of the test by the dry friction and wear test, is less than 0.20, and more preferably 0.15 or less.

[0016] The manufacturing method of the Mg-based composite material of the present invention includes a step of filling and enclosing a mixed powder containing Mg powder or Mg alloy powder, titanium dioxide powder with an average diameter of 0.05 μm or more, and optionally, oxide or nitride powder with an average diameter of 0.05 μm or more into a billet, and a step of subjecting the billet filled and enclosed with the mixed powder to warm or hot plastic working with a cross-sectional reduction rate of 50% or more at a temperature of 50°C or higher and 550°C or lower. In this manufacturing method of the Mg-based composite material, it is preferable that the total content rate of the titanium dioxide powder and the optional oxide or nitride powder in the mixed powder is less than 65% by mass with respect to the total amount of the Mg powder or Mg alloy powder and the titanium dioxide powder. In this manufacturing method of the Mg-based composite material, it is preferable that the optional oxide or nitride is Al2O3, CuO, MnO2, Si3N4, SiO2, or Y2O3. In this manufacturing method of the Mg-based composite material, it is preferable that the warm or hot plastic working is extrusion, forging, rolling, or drawing.

Effects of the Invention

[0017] According to the present invention, there are provided an Mg-based composite material having excellent strength characteristics due to particle dispersion in the matrix phase and high friction and wear characteristics, a manufacturing method thereof, and a sliding member.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 5

Figure 6A

Figure 6B

Figure 7

Embodiments for Carrying out the Invention

[0019] Regarding the Mg-based composite material, its manufacturing method, and the sliding member of the present invention, they will be described in the order of 1. Preparation of raw material powder, 2. Billet preparation and filling of mixed powder, 3. Warm or hot strain-imparting processing, 4. Microstructure of the Mg-based composite material, and the sliding member.

[0020] In the present invention, the average diameter of the raw material Mg powder and Mg alloy powder (hereinafter referred to as Mg-based powder), the average diameter of the titanium dioxide powder (hereinafter also referred to as additive powder), the average diameter of an arbitrary oxide or nitride powder (hereinafter also referred to as auxiliary additive powder), the average diameter of the additive powder particles (and auxiliary additive powder) (hereinafter collectively referred to as dispersed particles) in the metal structure of the Mg-based composite material, and the crystal grain size (crystal grain diameter) of the Mg matrix phase of the Mg-based composite material can be measured by the following method. <Average Diameters of Mg-Based Powder, Additive Powder, and Auxiliary Additive Powder> Using a laser diffraction / scattering particle size distribution measuring device, from the measured values of the particle size distribution by the laser diffraction / scattering method, the median diameter (d50, volume basis) by the cumulative distribution is taken as the average diameter. <Average diameter of added powder particles and auxiliary added powder particles> The particle sizes of the individual particles of the added powder and the auxiliary added powder are determined using the formula D = (L1 + L2) / 2 (where D is the particle size, L1 is the major axis of the particle, and L2 is the minor axis of the particle) from the images observed with an SEM or an optical microscope. The average diameter is obtained by extracting 100 or more particles from the images observed with an SEM or an optical microscope, determining the particle sizes of the individual particles from the above formula, and calculating the average value thereof. <Grain size of Mg matrix phase> It is measured and calculated by the sectioning method described in JIS H 0542:2008 "Test Method for Grain Size of Magnesium Alloy Rolled Sheets".

[0021] 1. Preparation of raw material powder The raw material powder used in the production of the Mg-based composite material of the present invention contains Mg powder or Mg alloy powder (Mg-based powder) and titanium dioxide powder (added powder), and optionally further contains oxide or nitride powder (auxiliary added powder). In a preferred embodiment, the raw material powder consists of Mg-based powder and titanium dioxide powder, excluding impurities (hereinafter also referred to as inevitable impurities) that may be unavoidably mixed in during the preparation process. In another preferred embodiment, the raw material powder consists of Mg-based powder, titanium dioxide powder, and oxide or nitride powder, excluding inevitable impurities.

[0022] The Mg powder is composed of pure magnesium. Further, the Mg alloy powder has Mg as the main component, and the other components may be any elements that are soluble in Mg, such as aluminum, zinc, and rare earths. The average diameter of the Mg-based powder is preferably 1 μm or more. Since Mg has a high reactivity with oxygen, when the average diameter of the Mg-based powder is 1 μm or more, the heat generation and ignition risks during the mixing of the Mg-based powder and the added powder can be reduced, and the safety of the working process can be enhanced. The Mg-based powder may be, in addition to ordinary powder, cutting powder generated from Mg bulk material or Mg alloy bulk material by machining typified by milling and turning, and these are also broadly referred to as Mg-based powder in this specification. The upper limit of the average diameter of the Mg-based powder is not particularly limited, but considering the bonding and sintering of the Mg-based powders, it is preferably 1000 μm or less.

[0023] The average diameter of the titanium dioxide powder is 0.05 μm or more, preferably 0.1 μm or more. When the average diameter is less than 0.05 μm, the surface area of the powder per unit volume increases, and the proportion of oxygen contacting the surface of the added powder particles increases. Therefore, the formation and incorporation of oxides composed of Mg occur at the interface and boundary between the Mg-based powder and the added powder, which inhibits the reduction of the friction coefficient. The upper limit of the average diameter of the added powder is not particularly limited, but considering the strengthening of the Mg-based composite material, it is preferably 1000 μm or less, more preferably 100 μm or less.

[0024] The mass of the added powder to be used, that is, the content of the added powder in the Mg-based composite material, is preferably less than 65% by mass, more preferably less than 60% by mass, and even more preferably less than 50% by mass with respect to the total mass of the mixed powder with the Mg-based powder. When the mass of the added powder is 65% by mass or more with respect to the total mass of the mixed powder, particles of the added powder with an area ratio of 50% by mass or more will be dispersed in the Mg matrix phase of the Mg-based composite material after warm or hot strain-imparting processing, and it is difficult to call it an Mg-based material.

[0025] The oxides or nitrides of the auxiliary additive powder are not particularly limited, and examples include Al2O3, CuO, MnO2, Si3N4, SiO2, Y2O3, etc. The average diameter of the auxiliary additive powder is subject to the same conditions as the above-mentioned additive powder (titanium dioxide). Also, when using the auxiliary additive powder, the total mass of the additive powder and the auxiliary additive powder used should satisfy the above-mentioned content conditions. Here, the ratio of the auxiliary additive powder to the total content of the additive powder and the auxiliary additive powder in the Mg-based composite material is set to 50% or less.

[0026] The mixing method of the Mg-based powder and the additive powder (and the auxiliary additive powder) and the state of the mixed powder will be described. The mixed powder is preferably in a state where the Mg-based powder and the additive powder (and the auxiliary additive powder) do not segregate from each other. In the state of the mixed powder, if any of the powders is segregated, when the Mg-based composite material is subjected to wear and friction, the portion subjected to friction and wear becomes a site of stress concentration, and it may be difficult to obtain the desired wear and friction characteristics. In order to make the Mg-based powder and the additive powder (and the auxiliary additive powder) not segregate from each other, it is preferable to mix the additive powder (and the auxiliary additive powder) little by little, that is, the mass of the additive powder (and the auxiliary additive powder) added at one time is 50 g or less. If it exceeds 50 g, mixing becomes difficult and there is a concern that powder segregation may occur.

[0027] The container used for mixing is not particularly limited as long as it is a container capable of mixing powders, typified by a mortar. The mixing of the Mg-based powder and the additive powder (and the auxiliary additive powder) is preferably carried out in the air using a mortar within 10 minutes from the viewpoint of simplifying the working process. Since the Mg-based powder easily reacts with oxygen, if the mixing exceeds 10 minutes, it is difficult to obtain a sound mixed powder because oxides and the like are incorporated into the mixed powder. Of course, considering the safety of the operation, the mixed powder may be mixed using a stirrer in an argon atmosphere or in a vacuum by a mechanical alloying method.

[0028] 2. Billet preparation and filling of the mixed powder Fill the mixed powder into a billet for warm or hot deformation processing. The material (stock material) used for the billet is preferably a metallic material that can be subjected to warm or hot deformation processing, such as Mg or Mg alloy. Of course, a metallic material other than Mg or Mg alloy, for example, Al or Al alloy, may also be used.

[0029] The size of the billet varies depending on the total cross-sectional reduction rate used during the deformation processing, but it is set to a size such that the total cross-sectional reduction rate can preferably be 50% or more, more preferably 60% or more, and even more preferably 70% or more.

[0030] The size of the voids for filling the mixed powder is preferably 50% or more and 95% or less, more preferably 50% or more and 90% or less, and even more preferably 55% or more and 85% or less with respect to the total volume of the billet. When the size of the voids is less than 50%, the amount of the mixed powder that can be filled is small, so after the deformation processing, most of the obtained processed material becomes the material used for the billet, and it may not be possible to say that it is an Mg-based composite material. When the size of the voids exceeds 95%, the billet may crack during the warm or hot deformation processing, and the powder may leak to the outside.

[0031] As a method of putting the mixed powder into the billet, a compact may be produced by a hand press and put into the billet. Of course, it may be put into the billet using a container that can scoop up the powder, typified by a spoon. At that time, all operations are preferably carried out in an argon atmosphere or in a vacuum in order to suppress the reaction between the mixed powder and oxygen, but it may also be carried out in the atmosphere for work simplicity. Further, in order to control and improve the filling rate of the mixed powder, it is preferable to apply pressure using a hand press after filling the billet with the mixed powder. However, in order not to segregate the Mg-based powder and the additive powder (and auxiliary additive powder) from each other, it is not desirable to tap the billet excessively or apply vibration. After filling the billet with the mixed powder, it is sealed using an upper lid made of the same material as the billet so that the mixed powder does not spill out.

[0032] The filling rate of the mixed powder is preferably 60% or more, more preferably 70% or more, still more preferably 80% or more with respect to the size of the voids. The lower the filling rate, the shorter the time required to produce the Mg-based composite material of the present invention. However, when the filling rate is less than 60%, the proportion of defects present in the composite material increases, making it difficult to use as a structure or a member.

[0033] 3. Warm or hot plastic working The purpose of warm or hot plastic working is to bond and sinter Mg-based powder to form a sound Mg matrix phase, and to uniformly disperse the particles of the added powder (and auxiliary added powder) without segregation within the Mg matrix phase. The temperature of warm or hot working is preferably 50°C or higher and 550°C or lower. If the working temperature is less than 50°C, since the working temperature is low, the Mg-based powders may not bond and sinter together. Also, the metal material used for the billet may crack during working, making it impossible to produce a sound composite material. If the working temperature exceeds 550°C, the Mg-based powder is exposed to high temperatures, raising concerns about the risk of ignition due to local melting. Also, it can cause a decrease in the die life used in the case of extrusion working.

[0034] The plastic strain applied during warm or hot working is preferably a total cross-sectional reduction rate of 50% or more, more preferably 60% or more, still more preferably 70% or more. If the total cross-sectional reduction rate is less than 50%, since the applied strain is insufficient, the bonding between the powders is not promoted, and it may be impossible to produce a sound composite material. Representative methods of warm or hot working include extrusion working, forging, rolling, drawing, etc., but any plastic working method capable of applying strain may be used.

[0035] 4. Microstructure of Mg-based composite material and sliding member The microstructure of the Mg-based composite material of the present invention will be described. The Mg-based powder binds and sinters during warm or hot plastic working to form an Mg matrix phase. In order to maintain the strength characteristics of the Mg-based composite material and obtain excellent friction and wear characteristics, the size of the Mg matrix phase, that is, the grain size, is preferably 200 μm or less, more preferably 100 μm or less, and still more preferably 50 μm or less. When the grain size of the Mg matrix phase is coarser than 200 μm, the proportion of grain boundaries in the composite material is small, so dislocation movement is not inhibited by the grain boundaries, and it is difficult to maintain the strength characteristics.

[0036] Further, the titanium dioxide particles (added powder particles) are preferably homogeneously dispersed without segregating in the Mg matrix phase. The average diameter of the added powder particles (dispersed particles) in the metal structure of the Mg-based composite material is 0.05 μm or more, preferably 0.1 μm or more. When the average diameter of the added powder particles is 0.05 μm or more, an Mg-based composite material having excellent friction and wear characteristics while having high strength characteristics is obtained. In the metal structure of the Mg-based composite material, that the particles dispersed in the Mg matrix phase are titanium dioxide particles can be confirmed (identified) from the images observed by SEM, optical microscope, and / or transmission electron microscope (TEM). It is also possible to confirm (identify) the type of dispersed particles using X-ray diffraction method (XRD).

[0037] Further, the ratio of the average diameter of the titanium dioxide particles to the grain size of the Mg matrix phase is preferably in the range of 1:4 to 1:10, and more preferably in the range of 1:4 to 1:9. When the ratio of the average diameter of the titanium dioxide particles to the grain size of the Mg matrix phase is within the above range, an Mg-based composite material having excellent friction and wear characteristics while having high strength characteristics is obtained.

[0038] It is also preferable that the particles of the auxiliary additive powder, which are oxides or nitrides, are homogeneously dispersed without segregating in the Mg matrix phase, similar to the above-described additive powder. Further, it is preferable that the average diameter of the auxiliary additive powder particles and the ratio of the average diameter to the grain size of the Mg matrix phase also satisfy the same conditions as those of the above-described additive powder. As a result, an Mg-based composite material having excellent friction and wear characteristics while having high strength characteristics is obtained. When titanium dioxide particles and auxiliary additive powder particles are dispersed in the Mg matrix phase in the metal structure of the Mg-based composite material, the types of the dispersed particles can be confirmed (identified) by SEM (SEM-EDX), TEM (TEM-EDX), or XRD, etc., which are configured to enable elemental analysis.

[0039] According to the present invention, in the metal structure of the Mg-based composite material, a particle-dispersed Mg-based composite material can be produced in which titanium dioxide particles and optional oxide or nitride particles are homogeneously dispersed without segregating in the Mg matrix phase. In particular, in an embodiment where the raw material powder consists of an Mg-based powder, a titanium dioxide powder, and inevitable impurities, in the metal structure of the Mg-based composite material, an Mg-based composite material in which only the titanium dioxide particles are homogeneously dispersed without segregating in the Mg matrix phase can be produced. Further, in an embodiment where the raw material powder consists of an Mg-based powder, a titanium dioxide powder, an oxide or nitride powder, and inevitable impurities, in the metal structure of the Mg-based composite material, an Mg-based composite material in which only the titanium dioxide particles and the oxide or nitride particles are homogeneously dispersed without segregating in the Mg matrix phase can be produced. In the Mg-based composite material of the present invention, the titanium dioxide particles (and optional oxide or nitride particles) are also present in the surface layer (outer peripheral portion) of the material at a stage before being subjected to friction and wear. And, the Mg-based composite material of the present invention has a friction coefficient at the portion subjected to friction and wear lower than that of pure magnesium. That is, the Mg-based composite material of the present invention exhibits excellent wear and friction characteristics by showing a friction coefficient significantly lower than that of pure magnesium from the start of the test in a dry friction and wear test as described later and then showing a substantially constant friction coefficient. Although the degree of the friction coefficient can vary depending on the test conditions, for example, the friction coefficient of the portion of the sliding surface subjected to friction and wear obtained within 10 seconds from the start of the test is less than 0.20, more preferably a value of 0.15 or less, and then stabilizes at a substantially constant value.

[0040] Thus, according to the present invention, a material excellent in friction and wear characteristics can be provided, and the Mg-based composite material of the present invention can be suitably used as a sliding member. This sliding member includes the Mg-based composite material of the present invention, has a sliding surface made of the Mg-based composite material, and this sliding surface has a friction coefficient at the portion subjected to friction and wear lower than that of pure magnesium. Thus, since the characteristics of the sliding surface constantly subjected to friction and wear can be improved, the sliding member of the present invention is suitable for mechanical parts made of magnesium in the portion subjected to sliding, and applications in various fields such as automotive parts, space equipment parts, and aircraft parts can be expected.

[0041] In addition, Mg and Mg alloy wrought materials such as extrusion are caused by the crystal structure of Mg (hexagonal crystal), and the bottom surfaces are aligned in the processing direction. Therefore, in tensile deformation and compressive deformation, a large difference occurs in the yield stress, and it is known that three-dimensional isotropic deformation is difficult. This yield anisotropy is caused by the deformation stress generated at a low deformation stress. On the other hand, by dispersing fine particles in the Mg matrix phase, the formation of deformation twins is suppressed, or the stress for forming twin deformation becomes higher. Therefore, according to the present invention, it is possible to provide Mg and Mg alloys with reduced yield anisotropy and capable of three-dimensional isotropic deformation.

Example

[0042] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples in any way.

[0043] <Example 1> Commercially available pure Mg powder (powder diameter: 180 μm) and commercially available titanium dioxide: TiO2 powder (powder diameter: 2 - 3 μm) were used. TiO2 particles were weighed so that the content rate in the Mg matrix phase of the Mg-based composite material was 10%, and the Mg powder and TiO2 powder were dry-mixed in a mortar.

[0044] In order to fill the mixed powder of Mg and TiO2, a commercially available Mg alloy (Mg - 3Al - 1Zn; AZ31) material with an outer diameter of 40 mm and a length of 70 mm was used, and a hole with an inner diameter of 20 mm and a depth of 55 mm was made by machining to produce an extrusion billet having a cup shape. After filling the mixed powder into the extrusion billet, it was sealed using an Mg alloy (AZ31) material with a diameter of 20 mm and a thickness of 5 mm. At that time, in order to control the filling rate, the volume of the mixed powder was filled to be 95% with respect to the volume of the inner diameter 20 mm × depth 55 mm. Then, after holding for 30 minutes or more in a container set at 250°C, hot strain application processing by extrusion was performed at an extrusion ratio of 16:1 to produce an extruded material having a diameter of 10 mm and a length of 500 mm or more (hereinafter referred to as an Mg-based extruded composite material).

[0045] <Example 2> A mixed powder was prepared in exactly the same procedure as in Example 1, except that the TiO₂ particles dispersed in the Mg matrix phase of the Mg-based composite material were weighed so that the content was 5%. After filling the extrusion billet so that the volume of the mixed powder was 95%, extrusion processing was carried out in the same manner as in Example 1 to produce a Mg-based extruded composite material.

[0046] <Example 3> A mixed powder was prepared in exactly the same procedure as in Example 1, except that the TiO₂ particles dispersed in the Mg matrix phase of the Mg-based composite material were weighed so that the content was 20%. After filling the extrusion billet so that the volume of the mixed powder was 95%, extrusion processing was carried out in the same manner as in Example 1 to produce a Mg-based extruded composite material.

[0047] <Example 4> A mixed powder was prepared in exactly the same procedure as in Example 1, except that pure Mg powder was replaced with commercially available Mg alloy powder (WE43: Mg-rare earth-yttrium-based alloy powder) and the extrusion processing temperature (set temperature of the extrusion container) was set to 300°C. After filling the extrusion billet so that the volume of the mixed powder was 95%, extrusion processing was carried out in the same manner as in Example 1 to produce a Mg-based extruded composite material.

[0048] <Example 5> A mixed powder was prepared in exactly the same procedure as in Example 1, except that pure Mg powder was replaced with commercially available Mg alloy powder (AZ31: Mg-3Al-1Zn). After filling the extrusion billet so that the volume of the mixed powder was 95%, extrusion processing was carried out in the same manner as in Example 1 to produce a Mg-based extruded composite material.

[0049] <Example 6> A mixed powder was prepared in exactly the same procedure as in Example 1, except that pure Mg powder was replaced with commercially available Mg alloy powder (Mg-6.3wt%Y alloy powder), the TiO₂ particles dispersed in the Mg matrix phase of the Mg-based composite material were weighed so that the content was 20%, and the extrusion processing temperature (set temperature of the extrusion container) was set to 300°C. After filling the extrusion billet so that the volume of the mixed powder was 95%, extrusion processing was carried out in the same manner as in Example 1 to produce a Mg-based extruded composite material.

[0050] <Example 7>

[0051] <Example 8>

[0052] <Example 9>

[0053] <Comparative Example 1>

[0054] <Comparative Example 2>

[0055] <Comparative Examples 3 to 7> The TiO2 powder was replaced with commercially available Al2O3 powder (Comparative Example 3), MnO2 powder (Comparative Example 4), Si3N4 powder (Comparative Example 5), SiO2 powder (Comparative Example 6), Y2O3 powder (Comparative Example 7) (all with a powder diameter of 2 to 3 μm), and a mixed powder was prepared in exactly the same procedure as in Example 1. After filling the extrusion billet so that the volume of the mixed powder was 95%, extrusion processing was carried out in the same manner as in Example 1 to produce a Mg-based extruded composite material.

[0056] Using an optical microscope, the microstructure of the produced Mg-based extruded composite material was observed. Figure 1 shows a typical example of the microstructure of the Mg-based extruded composite material. The bright region is the Mg matrix phase, and the dark region is the TiO2 particles. As a result of determining the grain size of the Mg matrix phase of the Mg-based extruded composite material by the sectioning method and calculating the ratio of the average diameter of the TiO2 particles (or auxiliary additive powder particles) to the grain size of the Mg matrix phase, it was confirmed that for the Mg-based extruded composite materials of Examples 1 to 9, it was within the range of 1:4 to 1:10.

[0057] Using a Vickers hardness tester, the hardness of the cut surface of the Mg-based extruded composite material was measured under the condition of an indentation load of 100 N. As a result, the hardness of Examples 1 to 3 was comparable to that of Comparative Example 2 (about 40 Hv). It can be said that the strength characteristics of Mg are maintained by the presence of additive particles in the Mg matrix phase. Also, for Examples 4 to 9, no decrease in the strength characteristics of the Mg alloy due to the presence of additive particles (and auxiliary additive powder particles) was observed.

[0058] Using a Ball-on-Disk type friction and wear tester, the dry friction and wear characteristics of the Mg-based extruded composite material were investigated. The surface of the Mg-based extruded composite material cut perpendicular to the extrusion direction was used as the measurement surface, and a ball with a diameter of 4.7 mm made of high-carbon chromium bearing steel (SUJ2) was used. A dry friction and wear test was carried out under the conditions of a distance from the center of the disk, that is, a rotational radius of 0.3 mm, an additional load of 0.5 N, linear velocities of 0.1 mm / s and 1.0 mm / s, and a sliding distance of 1000 mm or more. The relationships between the friction coefficient and the sliding distance obtained from the dry friction and wear test are shown in Figures 2 and 3.

[0059] The results shown in Figure 2 were obtained under the condition of a linear velocity of 1.0 mm / s. The legends in the figure indicate "Mg-based powder / additive powder (content of additive powder)", meaning Example 1, 4, 7, and 6 in order from top to bottom. The Mg-based extruded composites of Example 1, 4, 6, and 7 show a friction coefficient of about 0.1 from the start of the friction and wear test regardless of the linear velocity. The friction coefficients 2 seconds after the start of the test under the condition of a linear velocity of 1.0 mm / s were calculated to be 0.05, 0.06, 0.06, and 0.09 for Example 1, 4, 6, and 7, respectively.

[0060] Also, although not shown in the figure, it was confirmed that for the Mg-based extruded composites of Example 2, 3, and 5, the friction coefficient shows a value of less than 0.2 from the start of the friction and wear test regardless of the linear velocity. The friction coefficients 2 seconds after the start of the test under the condition of a linear velocity of 1.0 mm / s were calculated to be 0.11, 0.19, and 0.13 for Example 2, 3, and 5, respectively. Furthermore, regarding Example 6 using Mg-6.3 wt% Y alloy powder as the Mg alloy powder, when a Mg-based extruded composite was prepared with the content of TiO2 particles dispersed in the Mg matrix phase of the Mg-based composite being 0.5%, in the same dry friction and wear test as above, the friction coefficient 2 seconds after the start of the test under the condition of a linear velocity of 1.0 mm / s was calculated to be 0.13.

[0061] The results shown in Figure 3 were obtained for Example 9 under the condition of a linear velocity of 1.0 mm / s. Although not shown in the figure, similar results were obtained for Example 8. It was confirmed that for the Mg-based extruded composites of Example 8 and 9, the friction coefficient shows a value of less than 0.2 from the start of the friction and wear test regardless of the linear velocity. The friction coefficients 2 seconds after the start of the test under the condition of a linear velocity of 1.0 mm / s were calculated to be 0.13 and 0.11, respectively.

[0062] Figure 4A shows the relationship between the coefficient of friction and the sliding distance obtained by the dry friction and wear test for Comparative Examples 1 and 2. The results shown in Figure 4A were obtained under the condition of a linear velocity of 1.0 mm / s. For Comparative Example 1, the coefficient of friction showed about 0.1 after 2400 seconds from the start of the test (when the sliding distance reached about 2400 mm), but was about 0.4 at the initial stage of the test. Also, for Comparative Example 2, the coefficient of friction showed a constant value of 0.4 without decreasing during the test time. Even under the condition of a linear velocity of 0.1 mm / s, Comparative Examples 1 and 2 show the same tendency as in the case of a linear velocity of 1.0 mm / s. From the above, it can be seen that Examples 1 to 9 exhibit excellent coefficients of friction from the start of the friction and wear test.

[0063] Figure 4B shows the relationship between the coefficient of friction and the sliding distance obtained by the dry friction and wear test for Comparative Examples 3 to 7. The results shown in Figure 4B were obtained under the condition of a linear velocity of 1.0 mm / s. The legend in the figure indicates the type of additive powder, and the results without additive powder (only pure Mg powder) are shown together as "PMg". Among the test subjects, for Comparative Example 6 (SiO2) and Comparative Example 4 (MnO2), the coefficient of friction decreased to 0.1 or less about 200 seconds after the start of the test (when the sliding distance exceeded about 200 mm), but was about 0.3 at the initial stage of the test, which is the same as that of pure magnesium (PMg). Also, for Comparative Example 5 (Si3N4), after about 300 seconds (when the sliding distance exceeded about 300 mm), for Comparative Example 7 (Y2O3), after about 500 seconds (when the sliding distance exceeded about 500 mm), and for Comparative Example 3 (Al2O3), after about 600 seconds (when the sliding distance exceeded about 600 mm), the coefficient of friction decreased to about 0.1 respectively, but were all about 0.3 at the initial stage of the test, which is the same as that of pure magnesium (PMg). Even under the condition of a linear velocity of 0.1 mm / s, Comparative Examples 3 to 7 show the same tendency as in the case of a linear velocity of 1.0 mm / s. From the above, it can be seen that Example 9 in which TiO2 particles and auxiliary additive powder particles (CuO particles) are dispersed in the Mg matrix exhibits an excellent coefficient of friction from the start of the friction and wear test, different from Comparative Examples 3 to 7.

[0064] After the friction and wear test, the surface roughness of the measurement surface was measured with a laser microscope, and the wear amount was determined by the following formula (1). However, since the wear amount varies depending on the applied load P, sliding distance D, etc., in this example, the specific wear rate K was used to evaluate the wear characteristics. K = A·b / P / D (1)

[0065] In formula (1), A is the cross-sectional area measured by a laser microscope or the like, and b is the circumferential rotation of the ball during the Ball-on-Disk test (0.3 mm in this example). A typical two-dimensional cross-sectional image of the measurement surface after the friction and wear test measured by a laser microscope is shown in Fig. 5. The part indicated by the arrow in the center of the figure was formed by the friction and wear test and corresponds to A in formula (1). The specific wear rate of the Mg-based extruded composite material of Example 1 was 0.010x10 -3 mm 3 / Nm, and the specific wear rates of Comparative Examples 1 and 2 measured under the same test conditions were 7.0x10 -3 mm 3 / Nm and 9.3x10 -3 mm 3 / Nm. Compared with this, it can be seen that the wear characteristics are excellent. Also, the specific wear rates of Examples 2 and 3 measured under the same test conditions were 1.71x10 -3 mm 3 / Nm and 1.81x10 -3 mm 3 / Nm. In addition, the specific wear rates of Examples 4, 5, 6 and 9 measured under the same test conditions were 0.021x10 -3 mm 3 / Nm, 0.040x10 -3 mm 3 / Nm, 0.030x10 -3 mm 3 / Nm, 0.020x10 -3 mm 3 / Nm, respectively.

[0066] Figures 6A and 6B show the results of observing the cross-sections of the Mg-based extruded composite material of Example 1 before and after the friction and wear test using SEM-EDS. The lower right image in each figure is the SEM image of the observed cross-section (scale bar: 8 μm), and Mg (upper left image), Ti (upper right image), and O (lower left image) are shown as the elemental analysis results of the area surrounded by the square in the image. Note that the cross-section observation here means observing from a direction perpendicular to the friction and wear test direction.

[0067] From the results shown in Fig. 6B (after the friction and wear test), it can be seen that the TiO2 particles are accumulated in the scratch marks formed by the friction and wear test. On the other hand, focusing on Fig. 6A (before the friction and wear test), it can be seen that a certain amount of TiO2 particles are present in the surface layer of the material at the stage before the friction and wear test. In other words, in the Mg-based extruded composite material of Example 1, it was confirmed that the added powder particles (dispersed particles) exist in the surface layer (the part that undergoes friction and wear) at a certain ratio at the stage before being subjected to friction and wear. This is a feature not seen in the case of the conventional added powder particles used in the comparative example. Although not shown, it was also confirmed that in the Mg-based extruded composite materials of Examples 2 to 9, a certain amount of TiO2 particles are present in the surface layer of the material at the stage before the friction and wear test, similar to Example 1.

[0068] Fig. 7 shows an example of the microstructure before the friction and wear test obtained by transmission electron microscope observation of Example 1. It is the microstructure near the interface between TiO2 particles and Mg, and there are no microcracks or gaps near the interface indicated by the arrow in the figure. Also, these interface contrasts are clear, suggesting the formation of coherent interfaces.

[0069] In the conventional Mg-based composite materials provided with the self-film-forming ability as described in Patent Documents 5 and 6, when observing the cross-section of the material before being subjected to friction and wear, it is confirmed that the surface layer (outer peripheral part) is made of Mg or an Mg alloy as the raw material, and the inside is made of a composite material composed of a mixed powder of Mg powder or Mg alloy powder and additive powder. In these Mg-based composite materials, the additive powder particles dispersed in the Mg matrix are characterized in that when the material is subjected to friction and wear, the additive powder particles re-aggregate at the portion subjected to the friction and wear to form a self-film. On the other hand, in the Mg-based composite material of the present invention, the titanium dioxide particles (and optional oxide or nitride particles) dispersed in the Mg matrix also exist in the surface layer (outer peripheral part) of the material at the stage before being subjected to friction and wear. Thus, while maintaining excellent strength characteristics, it is considered that a high friction and wear characteristic can be exhibited, in which the friction coefficient of the portion subjected to friction and wear is lower than that of pure magnesium and shows a low friction coefficient from the beginning when subjected to friction and wear.

Claims

Claim 1 A particulate-dispersed Mg-based composite material, wherein in the metallic structure of the Mg-based composite material, titanium dioxide particles having an average diameter of 0.05 µm or more and optional oxide or nitride particles are dispersed in an Mg matrix phase, the friction coefficient of the portion subjected to friction and wear is lower than that of pure magnesium, the friction and wear is a dry friction and wear test using a ball-on-disk type wear tester, and the friction coefficient of the portion of the sliding surface that has undergone friction and wear, obtained within 10 seconds from the start of the test by the dry friction and wear test, is less than 0.20, the dry friction and wear test using the ball-on-disk type wear tester uses, as a measurement surface, a disk surface obtained by cutting a Mg-based extruded composite material of the Mg-based composite material in a direction perpendicular to the extrusion direction, and uses a ball having a diameter of 4.7 mm made of high-carbon chromium bearing steel (SUJ2), and under the conditions of a rotational radius of 0.3 mm from the disk center, an additional load of 0.5 N, a linear velocity of 0.1 to 1.0 mm / s, and a sliding distance of 1000 mm or more, a dry friction and wear test is carried out, an Mg-based composite material in which the crystal grain size of the Mg matrix phase is 200 µm or less. Claim 2 A particulate-dispersed Mg-based composite material, wherein in the metallic structure of the Mg-based composite material, titanium dioxide particles having an average diameter of 0.05 µm or more and optional oxide or nitride particles are dispersed in an Mg matrix phase, the friction coefficient of the portion subjected to friction and wear is lower than that of pure magnesium, the friction and wear is a dry friction and wear test using a ball-on-disk type wear tester, and the friction coefficient of the portion of the sliding surface that has undergone friction and wear, obtained within 10 seconds from the start of the test by the dry friction and wear test, is less than 0.20, the dry friction and wear test using the ball-on-disk type wear tester uses, as a measurement surface, a disk surface obtained by cutting a Mg-based extruded composite material of the Mg-based composite material in a direction perpendicular to the extrusion direction, and uses a ball having a diameter of 4.7 mm made of high-carbon chromium bearing steel (SUJ2), and under the conditions of a rotational radius of 0.3 mm from the disk center, an additional load of 0.5 N, a linear velocity of 0.1 to 1.0 mm / s, and a sliding distance of 1000 mm or more, a dry friction and wear test is carried out, an Mg-based composite material in which the ratio of the average diameter of the titanium dioxide particles to the crystal grain size of the Mg matrix phase is in the range of 1:4 to 1:10, and the ratio of the average diameter of the optional oxide or nitride particles to the crystal grain size of the Mg matrix phase is in the range of 1:4 to 1:

10. Claim 3 The Mg-based composite material according to claim 2, wherein the crystal grain size of the Mg matrix phase is 200 μm or less.

4. The Mg-based composite material according to any one of claims 1 to 3, wherein the total content of the titanium dioxide particles and the optional oxide or nitride particles is less than 65% by mass.

5. wherein the optional oxide or nitride is Al ２ O ３ , CuO, MnO ２ , Si ３ N ４ , SiO ２ or Y ２ O ３ , and the Mg-based composite material according to any one of claims 1 to 4

6. A sliding member including the Mg-based composite material according to any one of claims 1 to 5, having a sliding surface made of the Mg-based composite material, wherein the friction coefficient of the portion of the sliding surface that undergoes friction and wear is lower than that of pure magnesium, wherein the friction and wear is a dry friction and wear test using a ball-on-disk type wear tester, and the friction coefficient of the portion of the sliding surface that has undergone friction and wear obtained within 10 seconds from the start of the test by the dry friction and wear test is less than 0.20, wherein the dry friction and wear test using the ball-on-disk type wear tester measures the disk surface obtained by cutting the Mg-based extruded composite material of the Mg-based composite material in a direction perpendicular to the extrusion direction as the measurement surface, and uses a ball with a diameter of 4.7 mm made of high-carbon chromium bearing steel (SUJ2), and performs a dry friction and wear test under the conditions of a rotation radius of 0.3 mm from the disk center, an additional load of 0.5 N, a linear velocity of 0.1 to 1.0 mm / s, and a sliding distance of 1000 mm or more.

7. A method for manufacturing the Mg-based composite material according to any one of claims 1 to 5, including a step of filling and enclosing a mixed powder containing Mg powder or Mg alloy powder, titanium dioxide powder having an average diameter of 0.05 μm or more, and optionally oxide or nitride powder having an average diameter of 0.05 μm or more into a billet, and a step of subjecting the billet filled and enclosed with the mixed powder to warm or hot strain application processing at a temperature of 50°C or higher and 550°C or lower with a cross-sectional reduction rate of 50% or more.

8. The method for manufacturing the Mg-based composite material according to claim 7, wherein the total content of the titanium dioxide powder and the optional oxide or nitride powder in the mixed powder is less than 65% by mass with respect to the total amount of the Mg powder or Mg alloy powder and the titanium dioxide powder.

9. The optional oxide or nitride is Al ２ O ３ , CuO, MnO ２ , Si ３ N ４ , SiO ２ or Y ２ O ３ The method for producing an Mg-based composite material according to claim 7 or 8, wherein the material is as described above.

10. The method for manufacturing the Mg-based composite material according to any one of claims 7 to 9, wherein the warm or hot strain application processing is extrusion processing, forging processing, rolling processing, or drawing processing.

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