Patent Application
20170211611
The present invention relates to a sliding member and a sliding bearing each having a sliding surface on which a counterpart shaft slides.
Technologies of forming a cover layer of Bi on a Cu alloy to slide a counterpart material on the cover layer are known (see Patent Literature 1). In Patent Literature 1, an intermediate layer of Ag is formed under the cover layer of Bi. This layer can improve the conformability derived from Bi and prevent fatigue fracture by virtue of Ag.
Patent Literature 1: JP 2006-266445 A
In Patent Literature 1, however, the crystalline structure of Bi in the cover layer forms a columnar crystal, and disadvantageously tends to cause cleavage fracture in grain boundaries of the columnar crystal. Especially, when Bi has a small crystalline grain diameter, cleavage fracture in grain boundaries disadvantageously tends to occur at the time of fatigue.
The present invention has been made in light of the above problems, and an object thereof is to provide a technology with which cleavage fracture of the sliding surface at the time of fatigue can be prevented.
In order to attain the object, in a sliding member and a sliding bearing according to the present invention, a cover layer of a cover material having a sliding surface for a counterpart material is formed on a base layer. The cover layer has a crystalline structure of the cover material including crystalline grains having a grain diameter of 3 μm or more and 7 μm or less. The crystalline grain diameter of the cover material is defined as 3 μm or more in this manner, thereby making it possible to promote the transition in the crystalline grains at the time of fatigue. Specifically, the promotion of the movement of the transition in the crystalline grains can increase the ductility of the cover layer and prevent cleavage fracture at the time of fatigue. On the other hand, the crystalline grain diameter of the cover material is defined as 7 μm or less, leading to reduction in strength so that the promotion of fatigue fracture can be prevented conversely.
Also, the cover material may be Bi, Sn, Pb, In or Sb. All of Bi, Sn, Pb, In and Sb have small hardness (for example, Mohs hardness), and are suitable as the cover material. The effects of the present invention explained above are obtained also in a sliding bearing having the characteristic features of the present invention.
Embodiments of the present invention will now be described in the following orders.
(1-1) Constitution of Sliding Member:
(1-2) Measurement Method:
(1-3) Method for Producing Sliding Member:
(1-1) Constitution of Sliding Member
The sliding member 1 has a structure in which the back metal 10, lining 11 and overlay 12 are laminated in an order of being away from the center of curvature. Hence, the back metal 10 constitutes the outermost layer of the sliding member 1, and the overlay 12 constitutes the innermost layer of the sliding member 1. The back metal 10, lining 11 and overlay 12 each have a certain thickness in the circumferential direction. The thickness of the back metal 10 is 1.3 mm; that of the lining 11 is 0.2 mm; and that of the overlay 12 is 12 μm. The radius of the surface on a side of the center of curvature of the overlay 12 (inner diameter of the sliding member 1) is 40 mm. Hereinafter, the “inner (side)” means a side of the center of curvature of the sliding member 1, and the “outer (side)” means a side opposite to the center of curvature of the sliding member 1. The inner surface of the overlay 12 constitutes the sliding surface for the counterpart shaft 2.
The back metal 10 is formed of steel containing 0.15 wt % of C, 0.06 wt % of Mn and the balance Fe. The back metal 10 is preferably formed of a material which can support the load from the counterpart shaft 2 via the lining 11 and the overlay 12, and is not necessarily required to be formed of steel.
The lining 11 is a layer laminated on the inner side of the back metal 10, and constitutes the base layer of the present invention. The lining 11 consists of 10 wt % of Sn, 8 wt % of Bi, and the balance Cu with inevitable impurities. The inevitable impurities contained in the lining 11 are Mg, Ti, B, Pb, Cr and the like, and are impurities mixed in smelting or scrapping. The content of the inevitable impurities is 1.0 wt % or less, as a whole.
The overlay 12 is a layer laminated on the inner surface of the lining 11, and constitutes the cover layer of the present invention. The overlay 12 consists of Bi and inevitable impurities. The content of the inevitable impurities is 1.0 wt % or less. In this embodiment, the overlay 12 has a crystalline structure composed of crystalline grains of Bi, and the average grain diameter of the crystalline grains of Bi constituting the overlay 12 was 4 μm.
As a result of a fatigue test on the sliding member 1 explained above, the fatigue damage area rate was good, 10%. The grain diameter of the crystalline grains in the overlay 12 is defined as 4 μm, thereby making it possible to promote the transition in the crystalline grains at the time of fatigue and to prevent cleavage fracture in the crystalline grain boundaries. Specifically, the movement of the transition in the crystalline grains is promoted, thereby making it possible to increase the ductility of the entire overlay 12 and to prevent cleavage fracture at the time of fatigue. Also, the grain diameter of the crystalline grains in the cover material is defined as 7 μm, leading to reduction in strength, thereby making it possible to prevent the promotion of fatigue fracture conversely.
(1-2) Measurement Method
The respective numerical values presented in the above embodiment were measured by the following technique. The masses of the elements constituting the respective layers of the sliding member 1 were measured by an ICP emission spectrometer (ICPS-8100 manufactured by Shimadzu Corporation).
The thicknesses of the respective layers were measured through the following procedures. Firstly, the cross section, in the diameter direction, of the sliding member 1 was polished by a cross section polisher (IB-09010CP manufactured by JEOL Ltd.). Then, the cross section of the sliding member 1 was photographed at a magnification of 7000 by an electron microscope (JSM-6610A manufactured by JEOL Ltd.), thereby obtaining image data on observed images (reflected electron images). Then, the observed images were analyzed by an image analyzer (LUZEX AP manufactured by NIRECO) to measure the film thickness.
The average grain diameter of the crystalline grains of Bi in the overlay 12 was measured through the following procedures. Firstly, an arbitrary cross section of the overlay 12 was polished by a cross section polisher. An arbitrary observation visual field range (rectangular range having length: 0.1 mm×breadth: 0.2 mm) having an area of 0.02 mm2, in the cross section of the overlay 12, was photographed at a magnification of 7000 by an electron microscope (JSM-6610A manufactured by JEOL Ltd.), thereby obtaining image data on observed images (reflected electron images). Then, the observed images were subjected to a section method, thereby measuring the grain diameter of the crystalline grains of Bi. In this section method, the grain diameter of the crystalline grains on a line segment formed on the observed images was measured by dividing the length of the line segment by the number of the crystalline grains through which the line segment passed. Further, the arithmetic average value (total value/number of line segments) of the grain diameters of the crystalline grains measured for a plurality of line segments, respectively, was measured as an average grain diameter.
The fatigue damage area rate was measured through the following procedures. Firstly, a fatigue test was conducted by a fatigue test device as shown in
The above state was maintained over 50 hours to conduct the fatigue test on the overlay 12. After the fatigue test, the inner surface (sliding surface) of the overlay 12 was photographed from a position on a straight line orthogonal to the surface in such a manner that the straight line was the main light axis, thereby obtaining evaluation images which were the photographed images. Then, damaged portions in the surface of the overlay 12 projected on the evaluation images were observed and identified by a binocular (magnifier), and the percentage of the value obtained by dividing the damaged part area, as an area of the damaged portions, by the area of the entire surface of the overlay 12 projected in the evaluation images was measured as the fatigue damage area rate.
(1-3) Method for Producing Sliding Member
Firstly, a flat plate of low carbon steel having the same thickness as that of the back metal 10 was provided.
Next, a powder of the material constituting the lining 11 was dispersed on the flat plate formed of low carbon steel. Specifically, a Cu powder, a Bi powder and a Sn powder were mixed, and the mixed powder was dispersed on the flat plate of low carbon steel so as to attain the mass ratio among the respective components in the lining 11 described above. Any powder may be used so long as the mass ratio among the respective components in the lining 11 can be satisfied, and alloy powders of Cu—Bi, Cu—Sn and the like may be dispersed on the flat plate of low carbon steel. The grain diameter of the powder was adjusted to 150 μm or less by use of a sieve for testing (JIS Z8801).
Next, the flat plate of low carbon steel and the powder dispersed on the flat plate were sintered. The sintering temperature was controlled to 700° C. to 1000° C. for sintering in an inert atmosphere. Sintering was followed by cooling.
After completion of cooling, a Cu alloy layer is formed on the flat plate of low carbon steel. This Cu alloy layer would contain soft Bi particles deposited during cooling.
Next, the low carbon steel on which the Cu alloy layer was formed was press-machined into a shape such that a hollow cylinder was bisected in the diameter direction. At this time, press-machining was conducted so that the outer diameter of the low carbon steel coincided with the outer diameter of the sliding member 1.
Then, the surface of the Cu alloy layer formed on the back metal 10 was cut. At this time, the quantity of cutting was controlled so that the thickness of the Cu alloy layer formed on the back metal 10 was equal to that of the lining 11. This allows for formation of the lining 11 by the Cu alloy layer after cutting. Cutting was conducted by use of a lathe set with a cutting tool material formed, for example, of sintered diamond.
Next, Bi serving as a cover material was laminated, at a thickness of 12 μm, on a surface of the lining 11 by electroplating, thereby forming an overlay 12. The Bi-electroplating procedures were conducted as will be described below. Firstly, the surface of the lining 11 was defatted by flowing current onto the surface of the lining 11 in an electrolyte solution. Then, the surface of the lining 11 was washed with water. Further, the surface of the lining 11 was washed with an acid to remove an unnecessary oxide. Thereafter, the surface of the lining 11 was washed with water again. After completion of the above pretreatment, Bi-electroplating was conducted by supplying current to the lining 11 immersed in a plating bath.
The conditions for Bi-electroplating in the overlay 12 were as will be described below. The plating bath was designed so as to have a bath composition including Bi (concentration): 40 to 60 g/L; an organic sulfonic acid: 25 to 100 g/L; and an additive: 0.5 to 50 g/L. The bath temperature of the plating bath was adjusted within the range between 40° C. and 60° C. Further, rectangular pulse current having a duty ratio of 50% was employed as the current to be supplied to the lining 11, and its average current density was defined within 4 A to 8 A/dm2.
The lamination of the overlay 12 was followed by water-washing and drying, thereby completing a sliding member 1. Further, two sliding members 1 were combined to form a cylindrical shape, thereby forming a sliding bearing A.
Table 1 indicates the results of measurement of the fatigue damage area rate for each average grain diameter of Bi in the overlay 12. Samples 1 to 5 (Sample 3 for the above embodiment) having different average grain diameters of Bi were produced by a method similar to the production method described above. However, the average grain diameter of Bi was adjusted by adjusting the current density in the electroplating of the overlay 12. Due to the property that the average grain diameter of Bi becomes larger as the current density in the electroplating of the overlay 12 increases, the current density was adjusted to be greater as the target average grain diameter became larger.
Although the above embodiment has illustrated the sliding member 1 constituting the sliding bearing A for bearing the crankshaft of an engine, the sliding bearing A for other applications may be formed by the sliding member 1 of the present invention. For example, gear bushing, piston pin bushing/boss bushing, etc. for transmissions may be formed by the sliding member 1 of the present invention. Also, the matrix of the lining 11 is not limited to the Cu alloy, and the material for the matrix is preferably selected according to the hardness of the counterpart shaft 2. Also, any material may be used as the cover material so long as the material is softer than the lining 11, and any of Pb, Sn, In, Sb and the like may be used. Also in Pb, Sn, In and Sb, the ductility can be improved by increasing the grain diameter of the crystalline grains.
1 . . . Sliding member
2 . . . Counterpart shaft
10 . . . Back metal
11 . . . Lining
11
b . . . Bi particles
12 . . . Overlay
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-083511 | Apr 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/061347 | 4/13/2015 | WO | 00 |
