Patent Publication Number: US-2021170482-A1

Title: Metal object and manufacturing method thereof having solid lubricating surface layer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Taiwan Patent Application No. 108144743, filed on Dec. 6, 2019, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a metal object and a manufacturing method thereof having a solid lubricating surface layer, and in particular, to a metal object and a manufacturing method thereof having improved mechanical properties of the metal object and simultaneously forming with a solid lubricating surface layer. 
     Related Art 
     Powder metallurgy (PM) and metal powder  3 D printing have been widely used in the manufacture of industrial components because of various advantages, for example, high material utilization, ability of forming complex components and low power consumption. However, the metal components manufactured by using PM or  3 D printing, such as gears, have microvoids on their surface layers, and are prone to metal fatigue failures or brittle fractures because of loads, thereby posing threats to the normal operation of a system or even endangering personal safety. 
     The transmission element may cause power consumption and heat generation due to friction during operation. There are three conventional methods for decreasing the friction on a surface of a sliding element, including: (1) surface grinding and polishing, (2) lubricating liquids, and (3) low friction coefficient coating. However, with the trend of precision and miniaturization of products and the needs of green manufacturing, the mechanical surface grinding, polishing and conventional coating technology cannot meet the demanding needs, and the use of the lubricating liquids is prone to causing environmental issues. 
     The patent document (TW patent No. M582091) discloses a solid lubricating chain. The solid lubricating chain disclosed in the patent document mainly has an internal surface and an external surface of each of the linings coated with solid lubricating layers. By using the solid lubricating layers effectively adhered to the internal surfaces and the external surfaces of the linings, not only the frictional resistance is decreased, but also the abrasion caused by the friction between shafts, rollers and the linings is slowed down, thereby obtaining a better lubricating effect, and improving the smoothness of transmission. In this patent document, the solid lubricating layers formed on the internal surface and the external surface of each lining by a coating method, can decrease the frictional resistance, however, under the action of forces between the shafts, the rollers and the linings, the solid lubricating layers are prone to falling off because of poor adhesion. 
     The patent document (TW patent No. 1550210) discloses a method for manufacturing a pin shaft of a chain, including: mixing raw material powder with a solid lubricant, coating them on a peripheral surface of the pin body, and then sintering them to form a lubricating layer having the solid lubricant. In this patent document, by using the sintering process, the raw material powder and the solid lubricant are fixed to the peripheral surface of the pin body, thereby solving the problem of the poor adhesion of the lubricating layer formed by coating method. However, the use of the sintering process increases process time and cost. 
     Therefore, there is a need to provide a metal object and a manufacturing method thereof having a solid lubricating surface layer to resolve the foregoing problems. 
     SUMMARY 
     An object of the present disclosure is to provide a metal object and a manufacturing method thereof having a solid lubricating surface layer, capable of improving both the mechanical properties and the tribological properties of the metal object. 
     In order to achieve the foregoing objective, the present disclosure provides a method for manufacturing a metal object having a solid lubricating surface layer, comprising the following steps of: providing a metal blank having a surface; providing a plurality of microparticles and solid lubricating powder, and mixing them together, wherein the plurality of microparticles have a hardness greater than that of the surface; and projecting the plurality of microparticles and the solid lubricating powder onto the surface, wherein the plurality of microparticles cause plastic flow on the surface to form a compressive stress layer, and the solid lubricating powder adheres to the compressive stress layer to form a solid lubricating surface layer. 
     The present disclosure further provides a metal object having a lubricating surface layer, comprising: a metal substrate comprising a compressive stress layer and a solid lubricating surface layer adhered to the compressive stress layer. 
     A plurality of microparticles and solid lubricating powder are projected onto a surface of a metal blank of the present disclosure. The microparticles cause plastic flow of a material, forming a denser compressive stress layer to improve the mechanical properties. While the material is plastically flowing, the solid lubricating powder adheres to the compressive stress layer, so as to have a better adhesion compared with the lubricating surface layer formed by conventional spraying or coating methods, and a thinner lubricating surface layer can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart of a method for manufacturing a metal object having a solid lubricating surface layer according to a preferred embodiment of the present disclosure; 
         FIG. 2  is a schematic diagram of the operation of forming a solid lubricating surface layer according to the present disclosure; 
         FIG. 3  is a schematic cross-sectional view of a metal object having a solid lubricating surface layer according to the present disclosure; and 
         FIG. 4  shows a corresponding relationship between the thickness of a stress layer and the compressive stress of the metal substrate according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     To make the foregoing or other objects, features, and characteristics of the present disclosure more comprehensibly, the related embodiments of the disclosure are described in detail below with reference to the accompanying drawings: 
       FIG. 1  is a flowchart of a method for manufacturing a metal object having a solid lubricating surface layer according to a preferred embodiment of the present disclosure,  FIG. 2  is a schematic diagram of the operation of forming a solid lubricating surface layer, and  FIG. 3  is a schematic cross-sectional view of a metal object having a solid lubricating surface layer according to the present disclosure. Referring to  FIG. 1 ,  FIG. 2  and  FIG. 3 , the method for manufacturing a metal object having a solid lubricating surface layer according to a preferred embodiment of the present disclosure includes the steps as follows: 
     A step of providing a metal blank (S 10 ): a metal blank  10 , having a surface  11  is provided. 
     A step of providing a plurality of microparticles and solid lubricating powder (S 20 ): a plurality of microparticles  20  and solid lubricating powder  30  is provided, and they are mixed together, wherein the plurality of microparticles  20  have a hardness greater than that of the surface  11 . 
     A step of forming a solid lubricating surface layer (S 30 ): the plurality of microparticles  20  and the solid lubricating powder  30  are projected onto the surface  11 , wherein the plurality of microparticles  20  cause plastic flow on the surface  11  to form a compressive stress layer  411 , and the solid lubricating powder  30  adheres to the compressive stress layer  411  to form a solid lubricating surface layer  42 , thereby forming a metal object  40  having the solid lubricating surface layer  42 . 
     In the step of providing a metal blank (S 10 ), the surface  11  of the metal blank  10  has processing features left during processing and forming, such as tool marks left during cutting, and pore marks left during powder metallurgy (PM), metal powder  3 D printing or metal injection molding (MIM). 
     In the step of providing a plurality of microparticles and solid lubricating powder (S 20 ), the material of the microparticles  20  can be selected to be a ceramic material, such as zirconia (ZrO 2 ). The material of the microparticles  20  can also be selected to be high-speed steel. To enable the surface  11  of the metal blank  10  to produce plastic deformation after the projection of the microparticles  20 , the microparticles  20  have a hardness greater than that of the surface  11 . The diameter of the microparticle  20  can be selected according to the needs of implementations, and is not limited. 
     In the step of providing a plurality of microparticles and solid lubricating powder (S 20 ), the material of the solid lubricating powder  30  is selected from a group consisting of tungsten disulfide (WS 2 ), molybdenum disulfide (MoS 2 ), hexagonal boron nitride (h-BN) and graphite. The diameter of the solid lubricating powder  30  is preferably between 0.5 μm and 5 μm. 
     In the step of providing a plurality of microparticles and solid lubricating powder (S 20 ), the plurality of microparticles  20  and the solid lubricating powder  30  are mixed uniformly after preparation. For example, the plurality of microparticles  20  and the solid lubricating powder  30  are placed in a mechanical mixer to be mixed. 
     In the step of forming a solid lubricating surface layer (S 30 ), the plurality of microparticles  20  and the solid lubricating powder  30  can be projected onto the surface  11  at a high speed, for example, but not limited to, by providing a airflow (using siphon principle). 
     In the step of forming a solid lubricating surface layer (S 30 ), when the plurality of microparticles  20  are projected onto the surface  11  of the metal blank  10 , the surface  11  is subjected to the compressive stress of the plurality of microparticles  20  to generate plastic deformation and material flow, so that the metal blank  10  forms a metal substrate  41 . The local region on the metal substrate  41  where the plastic deformation and the material flow are generated because of the compressive stress is a compressive stress layer  411 , and the compressive stress layer  411  has a plurality of dimples  4111  formed by impacts of the plurality of microparticles  20 . When the plastic deformation and the material flow are generated, the processing features (tool marks and pore marks) of the surface  11  left during processing and forming will be destroyed, and a denser surface layer (the compressive stress layer  411 ) will be formed, so as to improve the mechanical properties (such as elastic limit, yield strength, fatigue strength and hardness). Referring to  FIG. 4 , it shows the corresponding relationship between the thickness of the stress layer and the compressive stress of the metal substrate obtained in a specific embodiment. In this embodiment, the thickness of the compressive stress layer  411  is less than 100 μm, and when the thickness is less than 40 μm, the compressive stress layer  411  has a compressive stress between 1000 MPa and 1450 MPa. 
     In the step of forming a solid lubricating surface layer (S 30 ), the solid lubricating powder  30  and the plurality of microparticles  20  are simultaneously projected onto the surface  11 . When the surface  11  undergoes the plastic deformation and the material flow, the solid lubricating powder  30  is embedded in the plurality of dimples  4111  formed by the impacts of the plurality of microparticles  20  on the surface  11 . In addition, when the plurality of microparticles  20  impact the surface  11 , and the surface  11  is subjected to compressive stress to generate the plastic deformation and the material flow, a predetermined temperature (local high temperature) can be generated, facilitating the adherence of the solid lubricating powder  30  to the compressive stress layer  411 . In an embodiment of the present disclosure, the solid lubricating surface layer  42  has a thickness between 0.1 μm and 1 μm. 
     It should be particularly noted that the solid lubricating powder  30  embedded in and adhered to the compressive stress layer  411  have an opportunity to be refined and/or embedded deeper by the following impacts of the microparticles  20 , so that a solid lubricating surface layer  42  with a thin thickness and good adhesion may be obtained. 
     In the step of forming a solid lubricating surface layer (S 30 ), a heating step can be further included. Before the plurality of microparticles  20  and the solid lubricating powder  30  are projected onto the surface  11 , the plurality of microparticles  20  and the solid lubricating powder  30  are heated to increase the energy of the plurality of microparticles  20  and the solid lubricating powder  30 . When the microparticles  20  impact the surface  11 , the work-hardening effect of the compressive stress layer  411  is further strengthened. Meanwhile, the solid lubricating surface layer  42  can be formed more efficiently, and the adhesion of the solid lubricating surface layer  42  can be further improved. Preferably, the microparticles  20  are made of a Fe-based metal material. After heating, the temperature of the microparticles  20  should not be higher than their tempering temperature, to prevent them from softening after tempering. In an embodiment, a heating unit  50 , such as a high-frequency heating unit, can be disposed outside the projection range of the plurality of microparticles  20  and the solid lubricating powder  30 , to heat the plurality of microparticles  20  and the solid lubricating powder  30  passing through. 
     The present disclosure further discloses a metal object  40  having a solid lubricating surface layer  42 , the metal object  40  includes a metal substrate  41  having a compressive stress layer  411  and a solid lubricating surface layer  42 , and the solid lubricating surface layer  42  is adhered to the compressive stress layer  411 . The material of the solid lubricating surface layer  42  is selected from a group consisting of WS 2 , MoS 2 , h-BN and graphite. 
     For example, a process of manufacturing a gear having a solid lubricating surface layer according to the method disclosed in the present disclosure is described as follows: 
     A step of providing a metal blank (S 10 ), the metal blank  10  is a gear blank manufactured by using PM, the material thereof is a Fe-based metal, and the surface  11  is a tooth surface of the gear blank. 
     A step of providing a plurality of microparticles and solid lubricating powder (S 20 ): the material of the microparticles  20  is selected is a zirconia ceramic material with a diameter between 10 μm and 30 μm, and the hardness of the microparticles  20  is 1100 HV to 1300 HV. The solid lubricating powder  30  selected is WS 2  having a diameter between 0.5 μm and 5 μm. After preparation, the plurality of microparticles  20  and the solid lubricating powder  30  are placed in a mechanical mixer to be mixed; 
     A step of forming a solid lubricating surface layer (S 30 ): the plurality of microparticles  20  and the solid lubricating powder  30  are projected onto the tooth surface with a high-speed airflow by using the siphon principle. The tooth surface is subjected to the compressive stress of the plurality of microparticles  20  to generate plastic deformation and material flow, forming a metal substrate  41  having a compressive stress layer  411 , and a plurality of dimples  4111  is formed on the compressive stress layer  411 . Meanwhile, pore marks of the tooth surface left during the forming of the gear blank (PM forming) will be compressed to be flattened because of the material flow, forming a denser compressive stress layer  411 . When the tooth surface undergoes the plastic deformation and the material flow, the solid lubricating powder  30  is embedded in a plurality of dimples  4111  on the compressive stress layer  411 , and meanwhile, when the plurality of microparticles  20  impact the tooth surface and the tooth surface undergoes the plastic deformation and the material flow, a local high temperature can be generated, so that the solid lubricating powder  30  adheres to the compressive stress layer  411 , forming a solid lubricating surface layer  42 . 
     The gear having a solid lubricating surface layer manufactured in this embodiment includes a gear substrate (a metal substrate  41 ) having a compressive stress layer  411 , a plurality of dimples  4111  are formed on the compressive stress layer  411 , and a solid lubricating surface layer  42  adhered to the compressive stress layer  411 . The compressive stress layer  411  has a thickness between 5 μm and 20 μm, and a stress at the thickness is between 1300 MPa and 1450 MPa. The plurality of dimples  4111  has a depth between 0.5 μm and 3 μm. The solid lubricating surface layer  42  is made of WS 2 , and has a thickness between 0.1 μm and 1 μm. 
     The present disclosure has the following effects: 1. The processing features (tool marks and pore marks) on the surface of the metal blank left during processing and forming are destroyed because of the projection of the microparticles, forming a denser surface layer (i.e., the compressive stress layer), thereby contributing to the improvement of mechanical properties (such as elastic limit, yield strength, fatigue strength and hardness). 2. The solid lubricating powder is embedded and adhered to the compressive stress layer when the plastic deformation and the material flow are generated, thereby obtaining a better adhesion than the lubricating surface layer formed by conventional spraying or coating method, and a thinner lubricating surface layer can be obtained. 3. By using the present disclosure to simultaneously improve the mechanical properties and form the lubricating surface layer, the process costs and process time can be reduced. 
     In conclusion, the foregoing descriptions are merely the preferred implementations or embodiments of the technical means adopted by the present disclosure to resolve the problems, and is not intended to limit the scope of the patent implementation of the present disclosure. That is, all changes and modifications accordant with the scope of the claims of the present disclosure, or made according to the scope of the claims of the present disclosure shall fall within the scope of the present disclosure.