Patent Application
20110268944
The invention refers to a sliding element with one substrate and at least one layer of a sliding layer material applied to the substrate. The invention also refers to preferred applications of such sliding elements.
Sliding elements as sliding bearing elements in engines mostly consist of multiple-layer materials with especially modified surfaces, which optimise the sliding characteristics. Usually, the surfaces of sliding bearing elements are metal layers, possibly based on lead, tin or aluminium, which are applied by galvanic processes, vaporisation or mechanical plating.
Furthermore, non-metal sliding layers are known having an artificial resin base, which have been modified with regard to their properties, load capacity and resistance to wear.
Although the known coatings have a relatively high load capacity, this capacity is extremely limited, such that rapid failure of the sliding function results if the load capacity limit is exceeded. The associated exposure of the substrate material, which does not have sufficient emergency operating features, then causes the total failure of the sliding bearing element due to seizure.
Resin-based sliding bearing coatings have been used for many years as tool for the reduction of the friction in mechanical constructions. Usually, metal, plastic and rubber components are coated, which need to be permanently easily movable without further lubrication. In the typical applications the loads are rather low and the boundary conditions such as temperature and media are non-critical. It is known from various patent applications, in particular from EP 0 984 182 A1 that applications in the engine, i.e. e.g. crank shaft bearings are also possible with such sliding bearing elements. In this document, an overlay with a matrix of PI, PAI, epoxy resin or a phenolic resin is also described, to which Fe3O4 can be added, amongst others, to reduce wear.
DE 196 14 105 A1 reveals a wear- and cavitation-resistant plastics sliding layer consisting of a matrix material made of PTFE or thermoplastic fluoropolymers Fe2O3 and solid lubricants. This material is used for bearings e.g. as a guide element for shock absorbers, and is only suitable for low sliding speeds and low loads due to its structure and the soft fluoropolymer matrix.
From EP 1 775 487 A2, a sliding bearing is known, which has a metal carrier material and an aluminium alloy and a plastic sliding layer applied to it. In order to improve the bonding strength and cavitation resistance of the plastic sliding layer, a material is suggested that has a bonding agent made of PI, PAI, PBI, EP and FP and a solid lubricant such as MoS2, graphite, PTFE and BN.
The task of the invention is to provide a sliding element that has a higher peak load capacity with improved wear resistance and that can be used at high temperatures and sliding speeds such as e.g. on moving components within combustion engines.
This task is solved by means of a sliding element in which the sliding layer material consists of a sliding coating with at least one cross-linkable bonding agent or at least one high-melting thermoplastic material or which consists of a material with a matrix of at least one high-melting thermoplastic material or at least one duroplastic material and that has the sliding layer material Fe2O3
A sliding coating is used in accordance with a first alternative.
A sliding coating is understood to be a liquid or powdery coating material containing additives for the improvement of the sliding capability of the surface, which is thinly applied to a substrate and which is formed into a continuous thin film due to chemical or physical processes such as e.g. evaporation of the solvent or hardening by UV radiation.
The cross-linkable bonding agent of the sliding coating preferably consists of PAI (polyamideimide), PI (polyimide), epoxy resin, PBI (polyben-zimidazole) and/or silicone resin. These polymers are characterised by high temperature resistance and excellent media resistance.
In another type, the bonding agent can be a bonding agent hardening by means of UV radiation. Such bonding agents are preferably unsaturated polyester resins and/or silicones.
In another type, the sliding coating can contain at least one high-melting thermoplastic material.
In accordance with a second alternative, a material with a matrix of at least one high-melting thermoplastic material or at least one duroplastic material is used.
High-melting thermoplastic materials are understood to be materials with a melting point above 230° C.
Polyacrylates, PEEK (polyetheretherketone) and/or PES (polyether sulfone) can be primarily used as preferred high melting thermoplastics.
Preferred duroplastic materials are PAI (polyamideimide), PI (polyimide), epoxy resin, PBI (polyben-zimidazole) and/or silicone resin.
It has shown that the materials used in combination with Fe2O3 have a significantly better peak load capacity than sliding layer materials that do not contain iron oxide. Up to 20% better load capacity values are achieved.
It is supposed that the efficiency of the lubricating film is improved by the combination of the Fe2O3 with the bonding agents or the matrix materials in accordance with the two alternatives, by means of which the increase of the wear rate with the specific bearing load is reduced. This increases the load capacity limit, which in turn significantly adds to the operational reliability of the bearings with loads below the load capacity limit.
These effects become evident with proportions of 0.1 to 15% by volume of Fe2O3. No improvement of the load capacity can be determined with lower proportions. Lower proportions however lead to a weakening of the matrix material or the cross-linkable bonding agent of the sliding coating.
The proportion of Fe2O3 related to the total sliding layer material is preferably 0.5 to 8% by volume.
It was able to be shown that the peak load capacity in crank shaft bearings can already be increased up to 120 MPa. These peak load capacity values are distinctly higher than those with a material combination acc. to EP 0 984 182 A1. The invention-related values are otherwise only reached by aluminium-based sputter coatings.
It has shown that the particle size of the Fe2O3 is also of significance. Fe2O3 with an average particle size of 0.01 to 5 μm is preferred. Powders with d50 of 0.1 to 0.5 μm are particularly advantageous.
The proportion of solid lubricants if the sliding layer material is preferentially up to about 30% by volume. The preferred range is approximately up to ≦9.5%. A particularly preferred range is ≧5 to 30% by volume.
Metal sulphides with layered structure, graphite, hexagonal boron nitride (hBN) and/or PTFE can be used as solid lubricants. Furthermore, the sliding layer material can contain hard materials with a proportion of up to 5% by volume, in particular a proportion of 3 to 5% by volume.
The hard materials are preferably nitrides, carbides, borides, oxides and/or metal powders, whereby the hard materials SiC, Si3N4, B4C3, cubic BN, TiO2 or SiO2 and metal powder made of Ag, Pb, Au, SnBi and/or Cu are preferred.
A particular type are multiple-layer systems made of sliding layers containing Fe2O3, whereby these multiple-layer systems can be designed such that an upper layer acts as a running-in layer e.g. due to the addition of hard particles for the conditioning of the shaft and the layer underneath acts as the permanent service layer.
A multiple-layer system can also be structured such that an additional layer of sliding is applied underneath the permanent service layer, which additionally increases the operational reliability of the bearings by optimisation, particularly with regard to wear resistance, thus delaying the full wearing down to the metal of the bearing.
An additional layer between the substrate and the sliding layer material can also be optimised with regard to the adhesion to the substrate a serve the purpose, similar to a primer, of improving the bonding of the layer of sliding coating or of the layer with a matrix of high-melting thermoplastics and duroplastics. This can be achieved, for example, by a few micrometre thick, less additived or non-additived layer of matrix material.
Multiple-layer systems can be implemented as discrete plies of layers and also as gradient layers, for which the layer properties continuously change via the thickness.
The Fe2O3 content in the bottom layer is preferably higher than in the top layer.
Two layers are preferably applied to the substrate, whereby one layer consists of a layer of sliding layer material with Fe2O3 and one layer of sliding layer material without Fe2O3. This type has the advantage that the geometrical adaptation is accelerated, as the top layer then wears more quickly and hence the maximum load capacity is more quickly provided. Further wear is then reduced by the Fe2O3 content of the bottom layer.
Another type of multiple-layer system provides that only the bottom layer contains the Fe2O3, while all layers above it have no Fe2O3.
If more than two layers of sliding layer material are applied to the substrate, it is advantageous if the Fe2O3 content is reduced from the bottom to the top layer. A lower proportion of Fe2O3 or none at all in the top layer has the advantage that the geometrical adaptation is accelerated, as the top layer then wears more quickly and hence the maximum load capacity is more quickly provided. Further wear is then reduced by the iron oxide content of the bottom layer.
It is also intended that the Fe2O3 content within a layer is continuously reduced from bottom to top.
The substrate can consist of one or several layers.
The substrate preferably has at least one aluminium or copper alloy layer. The following alloys are also suitable as substrate material: Ni-, Sn-, Zn-, Ag-, Au-, Bi- and Fe alloys. All alloys can be used not only as a metal bearing layer but also as a thin top layer, whereby the sliding layer can be formed as an additional running-in layer depending on its composition for the adaptation or conditioning of the shaft material or as an independent sliding layer with a long service life.
The use of the invention-related layer/layers as sliding layer/sliding layers on CuSn-, CuNiSi-, CuZn-, CuSnZn-, AlSn-, AlSi-, AlSnSi-metal alloy bearings is particularly preferred.
The sliding layers can be applied with or without an intermediate layer. Nickel, silver, copper and/or iron may be used as intermediate layers.
The thickness of the layer or layers in the case of a multiple-layer system lies within a range of 1 to 40 μm.
The sliding bearing element can be a bearing shell with up to 100 mm. In this case the thickness of the layer or layers is between 5 and 15 μm.
If the sliding bearing element is a bearing shell with a diameter of >100 mm, thickness values from >15 μm to 40 μm are preferred.
Basically, two types are possible. In the first type the invention-related sliding layer is directly applied to the metal bearing layer. The second type involves the coating of a substrate consisting of a bearing metal with an already existing metal sliding layer that is preferably applied by sputtering or galvanic deposition.
The substrate preferably has a roughness RZ from 1 to 10 μm, in particular from 3 to 8 μm. RZ is understood to be the average surface roughness acc. to DIN EN ISO 4287:1998.
The roughness improves adhesion and leads to the fact that in the case of wear, first only the peaks i.e. the very small surface proportions of the substrate are exposed, which increases the load-bearing capacity without having the proneness to seizure of larger exposed areas.
The required surface roughness levels can be achieved by mechanical procedures such as sand blasting or grinding, but also chemically by phosphating or slight etching.
Preferred uses are fluid-lubricated applications.
It is preferable to use the sliding elements as sliding bearings in combustion engines.
As the sliding elements are characterised by high a peak load, use of the sliding elements as sliding bearings for cranks shafts is particularly intended. Other preferred uses are sliding elements as piston skirts and piston rings, whereby in particular the ring flanks have the invention-related layer structure in order to prevent micro-welding with the piston groove surface.
Several examples with test results are listed below.
Table 1 contains only copper alloy substrates and table 2 aluminium substrates and examples for dual layers.
Underwood tests were performed for the assessment of the efficiency. For this, a shaft with eccentric weights rotates in rigidly mounted connecting rods. Mounting in the connecting rods is provided by the test bearings. The test bearings have a wall thickness of 1.4 mm and a diameter of 50 mm. The specific load is adjusted via the bearing width, the speed is 4000 U/m. Assessment criteria are sliding layer fatigue and wear after 100 h of continuous running. The limit load in MPa is stated where the layer is worn down to max. 5% of the sliding surface to the substrate or where there are signs of fatigue.
In order to substantiate the effect of the iron oxide, reference tests each designated with an R are listed in Table 1. The tests show that an increase of the load capacity of up to 20% is possible by the addition of Fe2O3.
For the aluminium substrates, examples 17-22, the fatigue strength of the substrate is respectively the limiting factor; however, an improvement in achieved here also by means of improved adaptability due to the invention-related sliding layer material. The main purpose of the invention-related sliding layer material is to optimise the sliding characteristics, if only a small proportion of the soft phase is contained in the alloy.
The double layer in example 23 is designed such that there is an increased adaptability due to the lower proportion of bonding agent and the higher proportion of solid lubricant in the top layer.
Example 24 is a layer construction for shafts with poor surfaces such as spheroidal grey cast iron that are additionally polished by the hard materials.
In example 25 the concentration of the Fe2O3 in the bottom layer rises and increases the wear resistance, whereby the top layer is preferably designed as an adaptation layer.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 055 194.5 | Dec 2008 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2009/067959 | 12/28/2009 | WO | 00 | 6/30/2011 |
