Patent Application
20240401544
The present invention relates to a coating for an engine piston made of steel or another iron-based alloy, which reduces or prevents oxidation or corrosion of the piston, where necessary.
Steel pistons become much hotter in the motor than aluminum pistons owing to their lower heat conductivity. The consequence of this is that different reactions are triggered on the steel surface. Thus, for example, an oxide layer is formed on the rim of the combustion bowl in the piston crown, which can become detached and thereby lead to a reduction of the material layer thickness at this point. In the worst case, the piston thereby becomes cracked which leads to a material failure.
So far, the consequences of this oxidation can only be countered by measures that are expensive or disadvantageous for other reasons. One possibility is a reinforced dimensioning of the piston, which makes it heavier. However, a low piston weight is advantageous for LVD pistons, i.e. pistons for diesel motors in passenger cars or light commercial vehicles.
A different possibility for reducing the oxidation is the use of special alloys having a high chrome and nickel content. However, such alloys are expensive and make surface treatment, such as manganese phosphating, more difficult. The application of a galvanic protection layer or a ceramic spray layer is complex and associated with high costs.
There is therefore a need for a coating for pistons made of steel or another iron-based alloy that provides good oxidation protection and may also serve as corrosion protection, where necessary.
US 8,6288,827 describes a coating for metal parts that is supposed to protect the surface from atmospheric corrosion at room temperature. The coating is obtained by applying a composition containing titanate or zirconate and curing. To give the coating a shiny appearance, a small amount of zinc, aluminum or zinc-aluminum alloy can be added to the composition. The use of this coating for pistons of internal combustion engines or other parts exposed to high temperatures of up to 550° C. is not disclosed.
Surprisingly, it has now been shown that the above problem can be solved by applying a layer to the piston, which can be obtained from a composition containing a titanate or a zirconate and which contains at least one metal pigment. In particular, it has been surprisingly shown that the cohesion of a layer obtained by means of a titanate or a zirconate can be improved if a metal pigment is added to this layer. In this way, a layer is obtained that is suitable for the piston of an internal combustion engine.
Consequently, the invention relates to a piston, in particular a piston for an internal combustion engine, which is made of steel or another iron-based alloy, wherein a layer is applied onto at least a part of the piston, which layer can be obtained from a composition containing titanate or zirconate and contains at least one metal pigment. Furthermore, the present invention relates to a method for producing this layer and its use for preventing high-temperature oxidation or bot gas oxidation of the piston.
Within the scope of the invention, a layer that can be obtained from a composition containing titanate is preferred.
In the production of the layer according to the invention, the titanate or zirconate contained in the composition is used as a precursor. A layer based on Ti—O or Zr—O units is formed by partial or complete dissociation of the organic residues.
This dissociation process can be initiated after the application of a suitable composition by exposure to moisture, including atmospheric moisture, or introduction of energy, for example by irradiation or heating. This dissociation process leads to curing of the layer.
Preferably, the layer that can be obtained from the composition containing titanate or zirconate contains 20 to 40% by weight of titanium oxide or 20 to 40% by weight of zirconium oxide, based on the entire layer, in the fully cured state. In this regard, the expression “in the fully cured state” relates to a layer obtained by application by means of a spray gun and subsequent heating in an oven at approximately 250° C. for 30 minutes.
The partial or complete dissociation of the organic residues causes the titanate or zirconate molecules to “shrink”, so that the produced layer can exhibit cracks and pores. This leads to problems with the cohesion of the layer.
It has now been shown that cohesion is significantly improved if at least one metal pigment is added to the coating. In this way, a coating can be obtained that leads to improved protection of the piston against oxidation, so that the problem underlying the invention is solved.
According to the invention, a compound of the following general formula (I) is used as the titanate or zirconate:
wherein R1, R2, R3 and R4 each independently represent H or an organic residue; preferably H, a phenyl group or a C1-C6 alkyl group; and particularly preferably a phenyl group or a C1-C4 alkyl group. R1, R2, R3 and R4 may optionally have substituents, but are preferably unsubstituted groups. M represents Ti or Zr, preferably Ti, n represents a natural number, preferably 1 to 4.
In a particularly advantageous embodiment of the invention, a titanate is used which is selected from the group consisting of tetramethyl titanate, tetraisopropyl titanate, tetrabutyl titanate and tetraphenyl titanate. The zirconate is preferably selected from the group consisting of tetramethyl zirconate, tetraisopropyl zirconate, tetrabutyl zirconate and tetraphenyl zirconate.
Within the scope of the invention, the titanates and zirconates can also be used in the form of chelate complexes.
The layer that can be obtained by means of the composition containing titanate or zirconate contains at least one metal pigment.
Alloys based on zinc, magnesium or aluminum are particularly suitable as metal pigments. Aluminum-magnesium and zinc-magnesium alloys are preferred.
Within the scope of the invention, the metal pigment is used, for example, in the form of a powder, preferably a powder with a particle diameter of 50 μm or less.
Preferably, the layer according to the invention has a thickness in the range of 5 to 100 μm, more preferably 10 to 50 μm, in particular 10 to 30 μm.
The layer according to the invention can also be part of a multilayer system. This means that other layers can be applied onto the piston in addition to the layer according to the invention in order to influence different properties of the piston. These layers can be applied under or on top of the layer according to the invention.
The layer according to the invention is suitable for all steels and iron-based alloys that are conventionally used in pistons for internal combustion engines.
A piston with the layer according to the invention can be manufactured by means of a method comprising the following steps:
In the method according to the invention, steps a) and b) can be carried out individually or together. Preferably, the composition containing titanate or zirconate and the metal pigment are applied together. Particularly preferably, the metal pigment is added to the composition containing titanate or zirconate before it is applied onto the piston. For this purpose, the metal pigment can be used in the form of a powder or a paste, for example.
Subsequently, the titanate or zirconate is dissociated as described and the layer is cured.
By using the layer according to the invention, high-temperature oxidation or hot gas corrosion of the piston can be reduced.
A composition containing a titanate and a zinc-magnesium alloy pigment was applied by spraying onto a piston made of 42CrMo4. Subsequently, the composition was cured by heating in an oven at approximately 250° C. for 30 minutes.
The coated piston and an uncoated piston were subjected to an engine running test for 516 hours, wherein the engine was operated alternately for 30 minutes under full load and 2 minutes at idling speed.
Micrographs of the two pistons after completion of the test are shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 210 995.0 | Sep 2021 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/076178 | 9/21/2022 | WO |
