ZRIP/Po; 13.12.2007 C56065WOIP

Sliding bearing

Field of invention

The present invention refers to a sliding bearing comprising a back metal layer, a bearing layer applied to the back metal layer, a diffusion barrier layer applied to the bearing layer, and an overlay applied to the diffusion barrier layer.

Background art

Sliding bearings manufactured as lead free composite multilayer bearings are demanded for launching of new engines, especially for medium to heavy duty engine applications. This demand goes worldwide as it is difficult to consider the development of world platforms with variations of internal components for captive markets.

A common solution being developed to the medium and heavy duty market is a composite sliding bearing based on a multilayer construction that comprises a steel backing, a lead free copper based bearing layer, an intermediate layer or diffusion barrier layer and a lead free tin-based overlay. Such bearing system is relatively close to the existing lead containing system and attempts to preserve some important functional properties of the bearings system, like the conformability and embedability given by the soft tin-based overlay with relatively preserved fatigue strength and the emergency running property of the bearing layer for the protection of the engine in the case that the overlay is completely worn out in operation. This kind of sliding bearing is for example described in the German patent application DE 103 37 030 A1.

A problem with regard to this kind of sliding bearing is the control of the tin migration or diffusion from the overlay to the copper based bearing layer that gives rise to a brittle intermetallic phase of copper-tin that leads to spalling of the overlay and seriously jeopardize the bearing load carrying capacity. DE 103 37 030 A1 discloses a

diffusion barrier layer located between the overlay and the bearing layer which comprises a pure nickel layer with a thickness of 4 to 6 μm.

Object of the invention

It is the object of the present invention to provide a lead free sliding bearing with a tin-based overlay, which exhibits an effective control of the tin migration or diffusion from the tin-based overlay to the bearing layer. A further object of the present invention is to provide a lead free sliding bearing with a tin-based overlay, having a good conformability and good embedability properties for foreign particles during the run- ning-in period and high load carrying capacity of the medium, especially to heavy duty engines.

Summary of the invention

The object of the present invention is achieved by a sliding bearing with a diffusion barrier layer which consists of one pure nickel layer applied to the bearing layer and one silver-based layer applied to the pure nickel layer. The object of the present invention is further achieved by a sliding bearing with a diffusion barrier layer which consists of one pure iron layer applied to the bearing layer and one silver-based layer applied to the pure iron layer. The object of the present invention is also achieved by a sliding bearing with a diffusion barrier layer which consists of one pure nickel layer applied to the bearing layer, one layer of a tin/nickel alloy applied to the pure nickel layer and one silver-based layer applied to the layer of a tin/nickel alloy.

Brief Description of Drawings

Figure 1 shows a cross-section of a first embodiment of a sliding bearing according to the present invention just after manufacturing.

Figure 2 shows a cross-section of a first embodiment of a sliding bearing according to the present invention after operating or after heat treatment.

Detailed description of the invention

Figure 1 shows a cross-section of a preferred embodiment of a sliding bearing 10 according to the present invention. This preferred embodiment of the sliding bearing 10 comprises a back metal layer 12, preferably made of steel, a copper-based bearing

layer 14 which is for example based on a copper-tin alloy or copper-tin-nickel alloy or copper-tin-bismuth alloy or copper-tin-bismuth-nickel alloy, having a copper content of at least 85 wt%, a diffusion barrier layer consisting of two layers 16a and 16b and a tin-based overlay 18.

The overlay 18 of the present invention comprises pure tin or tin and one or more elements selected form the group comprising copper, silver, nickel, cobalt, zinc, gold, bismuth, lead or indium, preferably in a total amount of maximum 20% by weight. Tin is the main component of the overlay.

The diffusion barrier layer consists of a first layer 16a applied on the bearing layer 14. The first layer 16a is made of pure nickel or of pure iron. The first layer 16a can further be made of a layer consisting of pure nickel applied to the bearing layer 14 and of a layer consisting of a tin/nickel alloy applied on the pure nickel layer (not shown in the drawings). The diffusion barrier layer further consists of a second layer 16b. The second layer 16b is manufactured by applying a silver-based layer, preferably a pure silver layer as shown in figure 1 , onto the surface of the first layer 16a, for example made of pure nickel. In a further embodiment (not shown) the second layer 16b consists of silver with up to 30 weight % in total of one or more elements selected from the group consisting of tin, lead, cadmium and zinc. Instead or additionally, the second layer 16b contains up to 2 weight % in total of one or more elements selected from the group consisting of antimony, iron, cobalt and nickel.

During operation of the sliding bearing for example in a combustion engine, or by heat treatment of the sliding bearing before operation, a diffusion of tin from the overlay 18 to the second layer 16b of the diffusion barrier layer takes place. Therefore, during operating, the second layer 16b is gradually transformed into a silver-tin layer 16c (see figure 2), mostly comprising of Ag ₃ Sn that replaces the layer 16b and has an increased thickness compared with the second layer 16b. Therefore, the diffusion of tin into the bearing layer 14 is effectively prevented. In addition this silver-tin layer 16c formed by diffusion of tin of the overlay 18 into the second intermediate layer 16b enhances the bonding between the diffusion barrier layer 16a plus 16c and the over-

lay 18. Further, the silver-tin layer 16c functions as an effective sliding layer in case that the overlay 18 is worn out completely.

In another embodiment of the invention, the sliding bearing is submitted to a heat treatment before operation. In this case the diffusion process of tin of the overlay 18 to the silver based layer 16b and the formation of the continuous silver-tin layer 16c takes place before operation. Preferably, at least 90% of the silver layer 16b is transformed into the silver-tin layer 16c. Such a heat treatment before operation is preferably done when the silver based overlay 16b is thicker than about 2 μm, because it is possible that a silver based overlay thicker than 2 μm is not completely transformed to Ag ₃ Sn when the transformation occurs only during operation. The heat treatment can preferably be performed at temperatures between 130 ⁰ C and 200 ⁰ C. The duration of the heat treatment depends on the temperature and the thickness of the silver based layer 16b. Higher thickness of the silver based layer 16b needs higher temperature or longer duration of heat treatment (see examples in table 1 for values of time and temperature needed for the transformation of a silver based overlay 16b into a Ag ₃ Sn-overlay 16c). A very long heat treatment is not preferable with regard to the costs. High temperatures also are not preferable due to the risk of further diffusion processes causing embrittlement of the layers. A too low temperature, for example 100 ⁰ C is not preferred, because in this case the transformation takes much more time. Also it is not preferable to transform a too small part of the silver based layer 16b, preferably less than 90%, into the Ag ₃ Sn-overiay 16c, because the remaining silver may be corroded, causing the failure of the bearing as a consequence. The heat treatment is preferably performed with the following parameters as shown in table 2.

For these reasons the silver based Iayer16b preferably has a thickness of 1 to 5 μm. Thick layers 16b take much more time for the transformation into the Ag ₃ Sn layer 16c. Thin layers 16b might be not tight enough and therefore less effective.

Table 1 : Thickness of Ag ₃ Sn-layer 16c formed by diffusion of Sn from the overlay 18 into the silver based layer 16b. Values are given in μm.

Table 2: Preferred heat treatment for transformation of silver based layer 16b of a certain thickness

For both embodiments - with or without heat treatment before operation - the silver- tin layer 16c is formed regardless of the composition of the tin-based overlay 18. Preferably, the overlay 18 contains not more than about 20 weight % in total of alloying elements. In case that the tin-based overlay 18 contains more than 20 weight % of alloying elements, the duration of formation of the silver-tin layer 16c is longer because the diffusion of tin from the overlay 18 to the silver based layer 16b is slower. Therefore, the present invention is useful with regard to every tin-based overlay 18, but preferably with an amount of alloying elements less than 20 wt%.

Preferably the silver-tin layer 16c can be described as the ε-phase of a tin-silver binary phase with a compound description as AgβSn and characterized by X-ray diffraction.

The presence of the silver-tin layer 16c that is formed by gradual transformation of the second layer 16b decreases the diffusion of tin to the first layer 16a and the bearing layer 14 hindering the formation of a brittle copper-tin phase at the interface of a copper-based bearing layer 14. As it is known from the prior art, such embrittlement process leads to overlay spading or to serious degradation of its fatigue resistance. With the formation of a continuous silver-tin layer by transformation of the second layer 16b on the top of the first layer 16a, it was observed a strong decrease of the tin migration to the first layer 16a. The silver-tin layer 16c obviously becomes an efficient diffusion controlling barrier.

An other function of a continuous silver-tin layer 16c is to become itself a hard high wear resistant sliding layer with improved scuffing and fatigue resistance after the worn out of the softer tin-based overlay 18.

Eventually, at the loaded areas of heavy duty applications there may happen a fatigue wear process of the tin-based overlay 18a with the exposure of the silver-tin layer 16c. Under such condition, the hard, highly seizure and fatigue resistant silver- tin layer 16c gives an enduring full protection to the bearing operation. The preservation of the tin-based overlay 18a at the regions less loaded gives a continuous em- bedability and represents a protection against wear of the silver-tin layer 16c exposed at the loaded area.