CU-PB System Alloy Composite Bearing Having Overlay

1. Field of the Invention

The present invention relates to a heavy-duty bearing material which is increasingly required with a recent increase in the power of an internal combustion engine.

2. Description of the Related Art

A conventional copper-lead bearing alloy material generally used for an internal combustion engine consists of a Cu-Pb system or Cu-Sn-Pb system alloy. This alloy has an overlay (surface layer) provided on the surface thereof and consisting of a Pb-Sn alloy, a Pb-Sn-Cu alloy, a Pb-Sn-In alloy or the like. Particularly, such a copper-lead bearing alloy material often has an intermediate layer (barrier layer) consisting of nickel or the like and interposed between the alloy and the overlay in order to prevent the diffusion of tin or indium contained in the overlay into the alloy.

A sliding member has a sintered copper alloy layer to which 1 to 20% by weight of Ni-B compound is added for improving the wear resistance of a cylindrical bush without an overlay, as disclosed in Japanese Patent Application of Serial No. 2-333283 which had not been published at the filing time of the parallel patent application in Japan corresponding to the present application.

In order to prevent the diffusion of tin, indium or the like into the copper-lead bearing alloy, which is contained in an overlay provided on the surface of a copper-lead bearing alloy, a barrier nickel layer (anti-diffusion layer) is generally interposed between the copper-lead bearing alloy and the overlay. However, when the barrier nickel layer is present, there is a problem that if the overlay is worn out during slide-contacting with a shaft of mating sliding member, the barrier nickel layer having poor anti-seizure property is exposed and causes seizure with the mating sliding member. In addition, the production cost is increased by providing the barrier layer. On the other hand, when the barrier nickel layer is lacking, there is a problem that tin, indium or the like contained in the overlay diffuses into the copper-lead alloy and brings about decreases in strength of the overlay and corrosion resistance property.

A bearing alloy material having excellent fatigue resistance, wear resistance and higher strength than that of conventional Cu-Sn-Pb system alloy materials is increasingly demanded with recent increases in the power and performance of engines.

In order to improve the strength and fatigue resistance of a lead-bronze alloy which is known as an engine bearing material, tin content may be increased while decreasing lead content. However, although this expedient prevents diffusion of tin contained in the overlay into the lead-bronze alloy, it causes problems that the heat conductivity of the lead-bronze alloy is significantly deteriorated by an increase in the tin content, and that the anti-seizure property and embeddability of foreign substances (hereinafter referred to as "embeddability") are deteriorated by a decrease in the lead content.

A primary object of the present invention is to solve the above problems of conventional bearing alloys and provide a copper-lead alloy bearing material as a sliding material which has excellent fatigue resistance, anti-seizure property and embeddability and which prevents the components such as tin, indium or the like contained in an overlay from easily diffusing and entering a copper-lead alloy even if a nickel intermediate layer deteriorating the anti-seizure property is absent between the copper-lead alloy and the overlay.

In order to achieve the object, according to one aspect of the present invention, there is provided the following copper-lead alloy bearing:

A composite bearing comprising three layers including a backing metal, a copper-lead system bearing alloy layer formed on the backing metal and a tin-containing lead alloy overlay formed on the bearing alloy layer, the copper-lead bearing alloy containing, by weight, 2 to 10% of nickel and 0.05 to 1.0% of boron (B). The use of a copper-lead system bearing alloy having the above composition effectively prevents the tin contained in the overlay lead alloy from diffusing into the copper-lead system bearing alloy. In this case, the copper-lead system bearing alloy must have a hardness above HV80.

A preferable example of the copper-lead system bearing alloy consists of, by weight, 0.5 to 8% of tin, 15 to 30% of lead, 0.2% or less of phosphorus, 2 to 10% of nickel, 0.05 to 1.0% of boron and the balance of copper and unavoidable impurities.

Examples of tin-containing lead alloys that can be preferably used for forming the overlay include an alloy consisting of, by weight, 3 to 18% of tin and the balance of lead and unavoidable impurities, and an alloy consisting of, by weight, 3 to 18% of tin, at least one of 15% or less of indium and 5% or less of copper, and the balance of lead and unavoidable impurities.

Either an unplated steel plate or a copper-plated steel plate may be used as the backing metal for the bearing alloy layer.

In order to improve the corrosion resistance of the bearing, it is effective to plate both the outer and inner surfaces of the copper-lead alloy bearing, i.e., the surfaces of the overlay and of the backing metal, with tin, lead or an alloy thereof. It is recommended that the thickness of each of the plated layer is 3 .mu.m or less.

In accordance with another aspect of the present .invention, there is provided a copper-lead bearing alloy which can be used in combination with a tin-containing lead alloy overlay and which consists of, by weight, 0.5 to 8% of tin, 15 to 30% of lead, 0.2% or less of phosphorus, 2 to 10% of nickel, 0.05 to 1% of boron, and the balance of copper and unavoidable impurities.

FIG. 1 shows an example of the structure of a bearing having a semicircular sectional form and comprising three layers including a backing metal, a bearing alloy layer and an overlay in accordance with an embodiment of the present invention;

FIG. 2 is an example of a bearing of the present invention in which both surfaces of the bearing shown in FIG. 1 are subjected to outermost surface plating;

FIG. 3 is a table showing the results of fatigue tests conducted in examples; and

FIG. 4 is a graph showing the diffusion state of tin in an overlay.

The reasons for defining the composition of a copper-lead alloy bearing of the present invention are as described below.

(1) Nickel: 2 to 10% by weight

With a nickel content of less than 2% by weight, the strength of the alloy is insufficient, and it is impossible to sufficiently prevent the diffusion of tin contained in the overlay into the copper alloy. With a nickel content over 10% by weight, the alloy becomes excessively hard, and the conformability of the bearing with a shaft serving as a mating member deteriorates.

(2) Boron: 0.05 to 1.0% by weight

Boron is present as a compound with nickel, promotes the sintering property and contributes to improvement in the wear resistance. With a boron content of less than 0.05%, the amount of the Ni-B compound is small. With a boron content over 1.0%, the Ni-B compound is present in a large amount and thus does not contribute to the above effect. In addition, the hardness of the alloy is excessively increased, the toughness deteriorates, and the production cost is increased.

(3) Tin: 0.5 to 8% by weight

With a tin content of less than 0.5%, the strength of the alloy is insufficient. With a tin content over 8% by weight, the alloy becomes excessively strong, and the heat conductivity and anti-seizure property deteriorate.

(4) Lead: 15 to 30% by weight

With a lead content of less than 15%, the bearing properties such as the anti-seizure property and the embeddability deteriorate. With a lead content over 30%, the strength of the alloy is decreased.

(5) Phosphorus: 0.2% by weight or less

With a phosphorus content over 0.2%, the alloy becomes excessively hard, and the conformability with a shaft, which is a property required to a bearing, deteriorates.

(6) Hardness of copper-lead system bearing alloy:

The copper-lead system bearing alloy is required to have Vickers hardness (HV) of 80 or more. With a hardness of less than HV 80, the strength and hardness of the alloy are insufficient, and the fatigue resistance and wear resistance deteriorate.

(7) Outermost surface plated layer: 3 .mu.m or less

An outermost surface plated layer is provided for preventing corrosion of the surface of the backing metal and improving the corrosion resistance of the surface of the bearing alloy. Even if the thickness of the layer exceeds 3 .mu.m, the effects are the same, and the production cost is increased.

(8) Tin: 3 to 18% by weight

With a tin content of less than 3%, the strength of the overlay is insufficient, and the corrosion resistance deteriorates. With a tin content over 18%, the melting point of the overlay is decreased, thereby easily causing the wiping phenomenon (melt flowing phenomenon).

(9) Indium: 15% by weight or less

With an indium content over 15%, the melting point of the overlay is excessively decreased.

(10) Copper: 5% by weight or less

With a copper content over 5%, the overlay becomes hard and brittle.

FIGS. 1 and 2 show two examples of the structure of a bearing according to the present invention. The bearing shown in FIG. 1 comprises a backing metal of steel 1, a copper-lead bearing alloy layer 2 integrally formed on a surface of the backing metal, and an overlay layer 3 integrally formed on the bearing alloy layer 2. The bearing shown in FIG. 2 further comprises a plated film 4 consisting of tin, lead or an alloy thereof and formed on both surfaces of the bearing shown in FIG. 1.

The present invention is described in detail below with reference to experiments.

A powder consisting of a copper-lead bearing alloy having each of the compositions shown in Table 1 was distributed on a steel plate having a copper-plated surface. The plate was then placed in a furnace of reducing atmosphere to be subjected to sintering treatment at a temperature of 700.degree. to 900.degree. C. for 10 to 30 minutes.

After sintering, the composite plate was rolled and then sintered and rolled again to obtain a sintered bimetal (dimensions: 1.58 mm of bimetal thickness, 1.23 mm of backing metal thickness, 130 mm of width). Sintering and rolling may be of course repeated several times as occasion demands.

The thus-obtained bimetal was then formed into a bearing having a semicircular sectional shape by pressing and machining to obtain each of invention specimens (Nos. 1 to 4) and comparative specimens (Nos. 5 to 8). The results of measurement of the hardness and tensile strength of the specimens are shown in Table 1.

Although the bearing of the present invention had an overlay, the measurement was made in a state without the overlay.

An overlay (20 .mu.m of thickness) having the composition, Pb-9%Sn-9%In, was formed on the bearing obtained by the same process as that described above in Experiment 1 by an electroplating method to obtain each of invention specimens (Nos. 1A and 2A) and comparative specimens (5A and 6A) (specimen Nos. respectively correspond to the specimen Nos. shown in Table 1). The results of fatigue tests of the specimens are shown in FIG. 3, and the test conditions are shown in Table 2.

A nickel layer (1.5 .mu.m of thickness) was formed on the bearing obtained by the same process as that described above in Experiment 1 by electroplating, and an overlay (20 .mu.m of thickness) having the composition, Pb-9%Sn-9%In (i.e. No Bi) was then formed on the nickel layer by electroplating to obtain a comparative specimen (No. 5B).

Each of the invention specimen No. 1A and comparative specimen Nos. 5A and 5B was subjected to a heat treatment diffusion test at a temperature of 165.degree. C. for a maintenance time which was set to 150, 300 and 1000 hours. The amount of tin decreased by diffusion into the copper-lead bearing alloy was measured by measuring the amount of tin remaining in the overlay after each of the maintenance times passed. The results of the measurement are shown in FIG. 4. Table 3 simply shows the types of the specimens used in the diffusion test.

As seen from the results of the diffusion test shown in FIG. 4, the invention specimens each formed by using a copper-lead alloy containing nickel and boron exhibit the effect of preventing diffusion of tin in the overlay into the bearing alloy. The effect is equivalent or superior to that of a material having nickel barrier layer formed at the boundary between the overlay and the bearing alloy. It is also found from the results of the fatigue test of the bearing alloy that the strength of the invention specimens is increased by adding nickel and boron thereto, and that the invention specimens have excellent fatigue resistance.

The results of the test shows that the direct formation of an overlay containing tin on a copper-lead alloy containing Ni-B effectively prevents tin diffusion, and an invention specimen can preferably be used as an engine bearing having a semicircular section shape. Namely, the addition of Ni-B to the copper alloy matrix decreases the affinity of the copper alloy for the tin contained in the overlay and thus prevents the tin diffusion. This effect causes the tin in the overlay to be maintained therein for a long term even if the nickel barrier layer is absent at the boundary between the copper alloy and the overlay. In addition, Ni-B uniformly distributes in the copper alloy matrix and thus has the effects of strengthening the matrix and improving the bearing characteristics such as the fatigue resistance and wear resistance.

As will be apparent from the above, the present invention enables a nickel barrier layer which deteriorates the anti-seizure property to be omitted by preventing the diffusion of tin contained in an overlay into a copper alloy. The prevent invention also realizes improvement in the fatigue resistance and anti-seizure property of a copper-lead bearing alloy material by addition of Ni-B.