Sliding bearing comprising an aluminium bearing metal layer

Abstract

The invention relates to a sliding bearing composite comprising a carrier layer made of steel, an intermediate layer arranged on the carrier layer and made of aluminum or an aluminum alloy that is lead-free except for impurities, and a bearing metal layer arranged on the intermediate layer and made of an aluminum alloy that is lead-free except for impurities. Said aluminum alloy contains 6.0-10.0 wt. % tin, 2.0-4.0 wt. % silicon, 0.7-1.2 wt. % copper, 0.15-0.25 wt. % chromium, 0.02-0.20 wt. % titanium, 0.1-0.3 wt. % vanadium and optionally less than 0.5 wt. % other elements, the remaining portion being aluminum.

Claims

1. A sliding bearing composite comprising a carrier layer made of steel, an intermediate layer arranged on the carrier layer and made of aluminum or an aluminum alloy that is lead-free except for impurities, and a bearing metal layer arranged on the intermediate layer and made of an aluminum alloy that is lead-free except for impurities, which comprises 6.0-10.0% by weight tin, 2.0-4.0% by weight silicon, 0.7-1.2% by weight copper, 0.15-0.25% by weight chromium, 0.02 to 0.20% by weight titanium, 0.1 to 0.3% by weight vanadium, 0.01-0.08% by weight strontium, and optionally less than 0.5% by weight of other elements, remainder aluminum, wherein the tin is present in the bearing metal layer in the form of particles of such a distribution that on a surface of 1.42 mm2, no more than 50 Sn particles with a surface area of more than 100 m2 are present, wherein the silicon is present in the bearing metal layer in the form of particles of such a distribution that 30-70 Si particles >5 m are present on a surface of 0.04 mm2, the average Si particle size of all measured Si particles having a diameter of >5 m is 6.0-8.0 m, wherein the aluminum alloy of the bearing metal layer comprises a 0.2-%-yield strength Rp, 0.2 of more than 90 MPa and a tensile strength of more than 145 MPa.

2. The sliding bearing composite according to claim 1, wherein the aluminum alloy of the bearing metal layer comprises at least one element selected from the group 0.1-0.2% zirconium and 0.1-0.2 scandium.

3. The sliding bearing composite according to claim 1, wherein the proportion of tin in the aluminum alloy of the bearing metal layer is 8.0-10.0% by weight.

4. The sliding bearing composite according to claim 1, wherein the proportion of silicon in the aluminum alloy of the bearing metal layer is 2.0-3.0% by weight.

5. The sliding bearing composite according to claim 1, wherein the proportion of titanium in the aluminum alloy of the bearing metal layer is 0.04-0.10% by weight.

6. The sliding bearing composite according to claim 1, wherein the size distribution of the silicon particles in the bearing metal layer is set through a cooling rate after the casting process of less than 75 K/s.

7. The sliding bearing composite according to claim 1, wherein the intermediate layer comprises a thickness d2 of 25-70 m.

8. The sliding bearing composite according to claim 1, wherein the intermediate layer comprises a microhardness of 40 HV 0.01-90 HV 0.01.

9. The sliding bearing composite according to claim 1, wherein a polymer-based cover layer is arranged on the bearing metal layer.

10. The sliding bearing composite according to claim 1, wherein the silicon in the bearing metal layer is present in the form of particles, and the size distribution of the silicon particles in the bearing metal layer is set through a cooling rate after the casting process of less than 50 K/s.

Description

THE DRAWINGS

(1) FIG. 1 shows a basic layer structure of a first exemplary embodiment of a sliding bearing composite according to the invention;

(2) FIG. 2 shows a basic layer structure of a second exemplary embodiment of a sliding bearing composite according to the invention;

(3) FIG. 3 shows an illustration of the determination of the Si particle size distribution;

(4) FIG. 4 shows a graph comparing the strength values and elongation at break of the bearing metal alloy in dependence on the content of vanadium and tin, and

(5) FIG. 5 shows a graph comparing the size distribution of the tin phases in the bearing metal alloy.

DETAILED DESCRIPTION

(6) FIG. 1 shows a schematic cross section through a sliding bearing composite according to a first embodiment of the invention. It comprises a total of 3 layers. As the topmost layer in FIG. 1 is shown a bearing metal layer which comprises the claimed Al-based composition. The bearing metal layer 10 is applied over an intermediate layer 12 on a support or carrier layer 14 of steel. The intermediate layer serves as a bonding agent between the bearing metal layer 10 and the steel layer. It consists of pure aluminum or an aluminum alloy.

(7) In FIG. 1 is further shown symbolically a surface section 20 comprising the inner structure which is enlarged in the illustration of FIG. 3. In order to create an image of such a surface section, a flat cut at a suitable location of the bearing metal layer is preferably prepared. Contrary to the representation in FIG. 1, a surface section can also be considered which is, for example, parallel to the sliding surface.

(8) The thickness of the intermediate layer in the slide bearing composite according to the invention is preferably 25 m to 70 m and particularly preferably no more than 50 m.

(9) The second exemplary embodiment according to FIG. 2 comprises a different layer structure, to the effect that a polymer coating 16 is applied to the bearing metal layer 10, which is particularly advantageous in especially high stress bearing applications.

(10) The invention is not limited to the two embodiments shown. It is equally possible to provide a multilayer arrangement with further functional layers. Gradient layers are also not excluded. In principle, the number and shape of the layers is therefore not limited. However, primarily for the reason of cost-saving mentioned above, a slide bearing composite is preferred which has as few layers as safe operation allows.

(11) The method for determining the Si particle size distribution in the bearing metal layer will be explained below with reference to FIG. 3. After a flat surface grinding of the bearing metal layer is first prepared, which for example extends to the sliding surface, a surface section 20 of the bearing metal layer having a specific edge length and width is selected and marked under a microscope, for example at 500 magnification. If, for example, this is a rectangle with edge lengths of 500 m and 800 m, the measured surface is thus 400,000 m.sup.2. In this surface section can be seen a large number of Si particles 22, which according to the invention differentiate themselves optically through a specific gray or color value range from other inclusions, in particular from the soft phase, but also from foreign particles, both of which are not represented here. The detection of the Si particles preferably takes place automatically in an electronic imaging system. The Si particles 22 are measured in such a manner that regardless of form, the longest recognizable dimension thereof is determined. This dimension is designated as the diameter. According to their diameters, the Si particles are divided into classes, for example >5 m and/or <2 m, 2-4 m, 4-6 m, 6-8 m, etc.

(12) On this basis, two sizes can preferably be determined: The number of Si particles associated with this class is simply counted and then converted to a standard area of 0.04 mm.sup.2 to facilitate comparison. Alternatively or additionally, the particle surfaces of all particles associated with the class can also be determined and added up, and an average value calculated therefrom.

(13) FIG. 4 shows bar graphs comparing the strength values yield strength R.sub.p.0.2 and tensile strength R.sub.m and the elongation at break A for three different compositions of the aluminum alloy of the bearing metal layer at two different test temperatures. The alloys contain the compositions by weight percent which can be seen in table 1:

(14) TABLE-US-00001 TABLE 1 Sn Cu Fe Ti Si Cr V Sr Al 1. State of the Art 12.57 0.40 0.10 0.056 2.37 0.17 0.020 Remainder 2. Comparative 13.00 0.51 0.12 0.054 2.48 0.20 0.09 0.023 Remainder example with vanadium 3. inventive 10.00 0.84 0.15 0.085 2.07 0.22 0.22 0.010 Remainder embodiment

(15) A bearing metal alloy is selected as the prior art (1. Comparative Example) as is known from DE 10 2011 003 797 B3. On this basis, vanadium was added to the alloy, and this new alloy was tested as a second comparative example. Both examples were compared with an exemplary embodiment of the composition according to the invention with increased Cu content and reduced Sn content. The first comparative example is represented respectively by the left bar graph, the second comparative example by the center bar graph and the exemplary embodiment according to the invention by the right bar graph. Comparisons were carried out once at room temperature, left half of FIG. 4, and at a test temperature of 175 C., right half of FIG. 4.

(16) It was found that a composition of the alloy elements within the scope of the invention, in particular at an increased test temperature of 175 C., led to a significant increase in the tensile strength R.sub.m by more than 40% over the prior art, whereby the elongation of approximately 30% is still sufficiently high.

(17) It was also found that this behavior is the result of a combination of the addition of vanadium with a simultaneous moderate increase of the Cu content and a reduction of the Sn content.

(18) Surprisingly, it was also found that a more refined tin distribution arises in the bearing metal alloy in the composition range according to the invention. This is evidenced by the two graphs of FIG. 5, showing the measured size distribution of the soft phase in the aluminum matrix in the three examples discussed above. The soft phase distribution was determined with the scanning electron microscope (SEM) by means of EDX measurement. Here, the Sn phase is first identified in the grinding based on its characteristic, defined gray value on a specified surface. The chemical composition of the Sn phase determined by means of its gray value is verified by means of EDX analysis. All particles consistent with the gray scale and EDX analysis are then recorded with respect to their size (area) and classified into freely selectable size classes. The result is a microstructure characterization in terms of Sn phase size and its distribution within the classes.

(19) The respective left bar in FIG. 5 shows the number of soft phase particles of a size falling under the class respectively indicated therebelow for the comparative example 1 according to table 1, the center for comparative example 2 according to table 1 and the right for the exemplary embodiment of the invention according to table 1. Below the entries for the size classes, the number is again respectively indicated in tabular form. In the upper diagram of FIG. 5, the classes are shown from <1 m.sup.2 to 20 m.sup.2 and in the lower diagram from 20 m.sup.2 to >150 m.sup.2, whereby it is to be noted that the lower diagram comprises a different scaling of the ordinate. The counting and measuring of the Sn phases refers respectively to a surface with a size of 1.42 mm.sup.2.

(20) It can be seen that in the alloy according to the invention, significantly more particles are present in the classes <10 m.sup.2, whereas particles in the classes >100 m.sup.2 are significantly reduced. This is responsible, among other things, for the improved strength. The reason for this is that larger, contiguous Sn areas or particles within the Al matrix lead to a weakening of the structure, as these are present as soft, separate phases (Sn or soft phases), which has a disadvantageous effect under mechanical stress, particularly at elevated temperatures. The tin is therefore preferably distributed in the bearing metal layer such that on a surface of 1.42 mm.sup.2, no more than 50 Sn particles can be recognized with a surface area of more than 100 m.sup.2.

(21) The specific choice of the alloy elements of the bearing metal alloy surprisingly also has an influence on the Si depositions in the bearing metal layer. The Si size distribution, which is determined as explained with reference to FIG. 3, has in turn a direct influence on strength and wear resistance. Si particles which are too coarse act as inner notches and reduce the strength. At the same time, however, sufficient Si particles in a size range between 2 and 8 m are required in order to ensure the known positive wear resistance of AlSnSi alloys, as Si particles >5 m are sufficiently large, which contribute as hard carrying crystals to the wear resistance of the material. This requirement can be suitably parameterized as follows: The silicon particles in the bearing metal layer are distributed with respect to their diameters such that 30-70 particles >5 m can be found on a surface of 0.04 mm.sup.2, the average Si particle size of all measured Si particles having a diameter of >5 m at 6.0-8.0 m.

(22) These alloys thus form an excellent compromise for a bearing metal alloy with increased strength, as a result of the special selection of the alloy elements, combined with a refined Sn distribution and a Si distribution which further ensures good wear resistance.

(23) Since the bearing metal surface comes into contact with the opposed piston, the feeding behavior and fatigue resistance are controlled to a first approximation by means of the bearing metal. The inventors have found, however, that the intermediate layer also contributes to the capacity of the bearing. During bearing failure in classic fatigue, cracks run from the surface to the weakest point of the composite. The intermediate layer ensures due to good adaptability that even during roll bonding of the bearing metal to the intermediate layer (cladding) and of the layer system of bearing metal and intermediate layer to the steel (bonding), no bonding issues arise. In addition, the intermediate layer improves the performance of the slide bearing in particular in highly loaded start-stop engines, because it does not experience aging effects, in particular temperature-induced formation of brittle intermetallic AlFe phases at the phase boundary between the steel of the carrier layer and the intermediate layer, for which reason its mechanical characteristics, which are ideally matched to the bearing metal layer in terms of strength and ductility, are permanently retained.