EFFECTS OF WC OR Mo2C ADDITION ON TRIBO-LOGICAL AND MECHANICAL PROPERTIES OF COPPER BASED BEARING ALLOY K. SAKAI, S. KURIMOTO and K. YAMAMOTO Daido Metal Co.Ltd., Tendoh Shinden, Maehara, Inuyama 484-0061, JAPAN; e-mail: fwhf3037@mb.infoweb.ne.jp SUMMARY The plain bearing is one of the important tribological material parts for transportation machines, especially for automobiles. Three layers copper based bearings which consist of the lead alloy overlay, the copper-lead bearing alloy, and the steel backing are generally applied under high load automotive engines. Recently it is expected to develop the bearing without the lead which is poisonous element under the circumstance of preserving environment. The reasons of adding lead into the copper based bearing alloy are that lead have good anti-seizure against the steel shaft and have lubricant property. In order to keep the anti-seizure without the lead, we tried to study the effects of hard particles addition uniformly into copper based alloy. From this seizure test result, it was confirmed that the copper based alloy with Mo2C particles increased the anti-seizure property compared with that of the copper based alloy without hard particles. The reason of having better anti-seizure of this bearing alloy is considered to be effect of polishing the surface of steel shaft by uniformly dispersed Mo2C particles.

Keywords: Plain bearings, Copper based alloy, Lead free, Seizure, Hard particles

1 INTRODUCTION The plain bearing is one of the important tribological material parts for transportation machines, especially for automobiles. Three layers copper based bearings which consist of the lead alloy overlay, the copper-lead bearing alloy, and the steel backing are generally applied under high load automotive engines.

As the recent engines are designed for higher performance and better efficiency, three layers bearings must endure under the dynamic load and higher speed conditions of engines [1]. And it is also expected to develop the bearing without the lead which is poisonous element under the circumstance of preserving environ-ment. The reasons of adding lead into the copper based bearing alloy are that lead have good anti-seizure against the steel shaft and have lubricant property.

In this report, the lead free copper based bearing alloy layer, which has better anti-seizure and has higher fatigue strength was studied.

2 STUDY FOR KEEPING ANTI-SEIZURE PROPERTY

In order to keep the anti-seizure without the lead, we tried to study the effects of hard particles addition uniformly into copper based alloy. It is expected that hard particles prevent adhesion of copper based alloy against steel shaft.

However it is concerned that hard particles addition into copper based alloy reduces fatigue strength compared with that of copper based alloy without hard particles. Therefore we selected hard particle which has good wetting property against copper not to reduce fatigue strength and has higher hardness to prevent adhesion against hardened steel shaft. We researched that tungsten carbide (WC) or molybdenum carbide (Mo2C)

has good wetting property against copper and has higher hardness in several kinds of hard particles such as carbide, nitride, silicide and boride particles. In this reason, WC particles or Mo2C particles was selected for addition into the copper based alloy to keep the better anti-seizure without reducing the fatigue strength of the copper based alloy.

2.1 Preparation of Test Samples

Several kinds of copper based alloy with dispersed WC particles or Mo2C particles were made for evaluation in tribological properties and mechanical properties. CuSnNi alloy which has higher strength than conventional CuSnPb alloy was used for copper matrix alloy. The amount of WC or Mo2C added into CuSnNi alloy was 1.5 volume %. The method of hard particle addition into CuSnNi alloy was mixing CuSnNi alloy powder with hard particle fine powder. The particle size of CuSnNi alloy powder was less than 250 micro-meter and the average particle size of hard particles was 1.5 micro-meter.

These mixed powders were experimentally sintered and bonded onto steel back sheets. And then these two material layers were rolled. After then these sintering and rolling processes were repeated.

2.2 Observation of Hard Particles in Copper Based Alloy

Cross-section microstructure of these CuSnNi alloy with WC or Mo2C particles were observed. These photographs are shown in Figure 1. In CuSnNi alloy with WC particles, WC particles were observed between sintered CuSnNi powders. However WC particles cohere and don’t uniformly disperse. While in CuSnNi alloy with Mo2C particles, Mo2C particles exist

Specific load of seizure occurrence (MPa)

B

A

D

C

0 5 10 15 20 25

between sintered CuSnNi powders and disperse more uniformly than WC particles.

WC particles Mo2C particles

WC addition alloy Mo2C addition alloy

Figure 1: Cross-section microstructure of CuSnNi alloy with hard particles on steel backing

2.3 Seizure Test

In order to evaluate the effect of hard particles addition on anti-seizure properties, the bearing performance test was carried out by using the thrust type testing machine. Test alloys are shown in Table 1 and this seizure test conditions are shown in Table 2. The static load of every 3 MPa is accumulatively increased. The shape of test specimen and the outline of seizure test machine are shown in Figure 2. The seizure occurrence was assumed when the temperature of test specimen back rose over 200 °C or the friction torque exceeded 4.9 Nm. The seizure test results are shown in Figure 3.

Specimen Mark Composition (mass%)

A Conventional Cu-3.5Sn-23Pb B Studied Cu-6Sn-1.5Ni

C Studied Cu-6Sn-1.5Ni-

2.5WC

D Studied Cu-6Sn-1.5Ni-

1.5 Mo2C

Table 1: Seizure test copper based alloys Test Conditions Dimensions Units

Bearing size OD27.2 x ID22 mm

Velocity 2.0 m/s

Lubricant SAE 30 - Oil inlet temperature

60 deg-C

Amount of oil supply

20x10-6 m3/min

Shaft : Material : Roughness

Hardened steel

1.0>

-

Ry (µm)

Table 2: Seizure test conditions

Load specimen shaft

sliding surface shape of specimen outline of machine

Figure 2: The shape of test specimen and the outline of seizure test machine

Figure 3: Seizure test results of copper based alloy From these seizure test results, it was found that the anti-seizure property of the CuSnNi alloy with Mo2C particles (D) increased compared with that of the CuSnNi alloy without hard particles (B) and that of the conventional CuSnPb alloy (A). On the other hand, the CuSnNi alloy with WC particles (C) showed similar anti-seizure property to the CuSnNi alloy without hard particles (B).

It is considered that the difference between effect of Mo2C particles addition and WC particles addition on anti-seizure property depends on microscopic hard particles dispersed condition. Because several WC particles which cohere between sintered CuSnNi powders attack the steel shaft, this CuSnNi alloy with WC particles don’t have good anti-seizure property.

In order to confirm the effect of Mo2C particles addition on anti-seizure property of CuSnNi alloy, we investi-gated a change of roughness and a microscopic observation of sliding steel surface. The results of the change of roughness are shown in Figure 4 and the results of the microscopic observation are shown in Figure 5.

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The change of roughness of sliding steel surface on the conventional CuSnPb alloy (A), CuSnNi alloy without hard particles (B) and CuSnNi alloy with WC particles (C) weren’t almost confirmed. On the other hand, the change of roughness of sliding steel surface on CuSnNi alloy with Mo2C particles (D) was definitely confirmed. The roughness of sliding steel surface decreased compared with that of the surface before seizure test. From the microscopic observation, it was also confirmed that sliding steel surface on CuSnNi alloy with Mo2C particles (D) was polished compared with that of the conventional CuSnPb alloy (A), CuSnNi alloy without hard particles (B).

It is considered that Mo2C particles have effect of polishing the surface of steel shaft by uniformly dispersed.

Figure 4: The change of roughness of sliding steel surface

2.4 Measurement of Mechanical Properties Tensile strength of CuSnNi alloys with hard particles were measured to confirm a demerit of hard particles addition on mechanical properties.

Tensile test was carried out for CuSnNi alloy with hard particles, CuSnNi alloy without hard particles and the conventional CuSnPb alloy at the room temperature and high temperature. Test results of tensile strength are shown in Figure 6.

From the tensile test results, it was confirmed that the CuSnNi alloy with WC particles (C) or Mo2C particles (D) showed the similar tensile strength to the CuSnNi alloy without hard particles (B) and that all CuSnNi alloys (B, C, D) showed about twice as tensile strength as the conventional CuSnPb alloy (A) at each test temperature.

before test (machined surface)

A after test B after test

C after test D after test

Sliding direction

Figure 5: Microscopic observation of sliding steel surface

○ : Conventional CuSnPb alloy (A) ◊ : CuSnNi alloy without hard particles (B) □ : CuSnNi alloy with WC particles (C) : CuSnNi alloy with Mo2C particles (D)

Figure 6: The results of tensile strength

It is also important to compare the fatigue strength between CuSnNi alloys with hard particles, CuSnNi alloy without hard particles and the conventional CuSnPb alloy. The fatigue test was carried out and the load of tension by means of the electro-hydraulic servo system was repeatedly applied to the test specimen at the room temperature. The tensile load pattern was sine-wave at a frequency of 10 Hz. Test shape of specimen is shown in Figure 7. The fatigue test was conducted in each load until the test specimen was broken and S-N curve of each alloy was measured. The results of the fatigue strength are shown in Figure 8.

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These fatigue strength results corresponded with static tensile strength. It was also confirmed that the CuSnNi alloy with WC particles (C) or Mo2C particles (D) showed the similar fatigue strength to the CuSnNi alloy without hard particles (B) and that all CuSnNi alloys (B,C,D) showed about twice as fatigue strength as the conventional CuSnPb alloy (A).

Figure 7: The shape of fatigue test specimen

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○ : Conventional CuSnPb alloy (A) ◊ : CuSnNi alloy wituout hard particles (B) □ : CuSnNi alloy with WC particles (C) : CuSnNi alloy with Mo2C particles (D)

Figure 8: The results of the fatigue test

3 CONCLUSION It was found that the anti-seizure property of the CuSnNi alloy with Mo2C particles increased compared with that of the CuSnNi alloy without hard particles and that of the conventional CuSnPb alloy. It is considered that Mo2C particles have effect of polishing the surface of steel shaft by uniformly dispersed.

It was confirmed that the CuSnNi alloy with WC or Mo2C particles showed the similar fatigue strength to the CuSnNi alloy without hard particles.

From these test results, it is expected that this CuSnNi alloy with Mo2C particles instead of lead could be applied for copper based bearing alloy layer having better tribological properties and no including poisonous element. 4 ACKNOWLEDGEMENTS The authors greatly acknowledge the financial support offered by the New Energy Industrial Technology Development Organization (NEDO). 5 REFERENCES [1] Y. Sato: THE TRIBOLOGY No.128 (April.1998) 56-58

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