Method for coating cooling channel with coating containing hexagonal boron nitride

Abstract

A method for coating a surface of a closed cooling channel, having a plurality of oil supply bores and a plurality of oil discharge bores, of a piston for an internal combustion engine, having a coating medium containing hexagonal boron nitride may include introducing a defined quantity of a coating medium comprising a suspension of hexagonal boron nitride with a solution on a basis of at least one thermally curable inorganic binder and at least one solvent into the cooling channel, spreading the coating medium over the surface of the cooling channel by moving the piston about at least two spatial axes, using a laminar air flow to dry the coating medium spread over the surface of the cooling channel, and thermally curing the coating medium to complete a coating adhering to the surface of the cooling channel.

Claims

1. A method for coating a surface of a closed cooling channel, having a plurality of oil supply bores and a plurality of oil discharge bores, of a piston for an internal combustion engine, comprising: a) introducing a defined quantity of a coating medium including a suspension of hexagonal boron nitride with a solution including at least one thermally curable inorganic binder and at least one solvent into the cooling channel; b) spreading the coating medium over the surface of the cooling channel by moving the piston about at least two spatial axes; c) using a laminar air flow to dry the coating medium spread over the surface of the cooling channel; and d) thermally curing the coating medium to complete a coating adhering to the surface of the cooling channel.

2. The method as claimed in claim 1, wherein prior to step a) a size of the surface of the cooling channel is determined.

3. The method as claimed in claim 1, wherein prior to step a) the surface of the cooling channel is cleaned with a cleaning substance.

4. The method as claimed in claim 3, wherein the cleaning substance includes one or more of methanol, ethanol, acetone, 1-propanol, and 2-propanol.

5. The method as claimed in claim 1, wherein in step a) at least one polysiloxane is used as the at least one thermally curable inorganic binder.

6. The method as claimed in claim 5, wherein ethanol is used as the at least one solvent.

7. The method as claimed in claim 1, wherein at least one of sodium silicate and potassium silicate is used as an additional binder.

8. The method as claimed in claim 1, wherein in step a) a quantity of 7 ml of the coating medium is used to coat the surface of the cooling channel per 190 cm.sup.2 of area.

9. The method as claimed in claim 1, wherein in step b) the piston is moved via a biaxial mixing device.

10. The method as claimed in claim 1, wherein in step c) the laminar air flow has a velocity of 1 to 2 meters per second.

11. The method as claimed in claim 1, wherein in step c) drying is carried out at room temperature.

12. The method as claimed in claim 1, wherein in step d) the thermal curing is carried out at a temperature of 180 C. to 220 C.

13. The method as claimed in claim 1, wherein the coating has a substantially uniform thickness over an entire surface of the cooling channel.

14. The method as claimed in claim 1, wherein the coating has a thickness of between 10 m and 100 m.

15. The method as claimed in claim 1, wherein a thickness of the coating is between 20 m to 40 m.

16. The method as claimed in claim 1, wherein a thermal conductivity of the coating is 40 W/mK to 50 W/mK.

17. The method as claimed in claim 1, wherein a coefficient of friction of the coating is 0.2 and is constant up to a temperature of 600 C.

18. The method as claimed in claim 1, wherein a surface area of the coating is 5 m.sup.2/g to 15 m.sup.2/g.

19. The method as claimed in claim 1, wherein the piston comprises steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows, in section, an exemplary embodiment of a piston according to the invention;

(2) FIG. 2 shows a photographic representation of the main body of a piston as per FIG. 1, with the coating that has been applied using the method according to the invention;

(3) FIG. 3 shows a further photographic representation of the main body of a piston, with a defective coating.

DETAILED DESCRIPTION

(4) The piston 10 has a piston head 11 with a piston crown 12, a combustion depression 13, a circumferential fire land 14 and a circumferential ring portion 15 with ring grooves for receiving piston rings (not shown).

(5) The piston 10 also has a piston skirt 16 which is provided, in a manner known per se, with piston bosses 17 in which are created boss bores 18 for receiving a piston pin (not shown). The piston bosses 17 are connected to one another by running surfaces 19.

(6) In the exemplary embodiment, the piston 10 is designed as a one-piece piston made of a steel material. In this context, a piston main body 21 and a piston upper part 22 are permanently connected to one another by welding or soldering. The piston main body 21 and the piston upper part 22 can be made of the same material or of different materials.

(7) The piston main body 21 and the piston upper part 22 together form a cooling channel 23 that is circumferential at the level of the ring portion 15, which channel has oil supply bores and oil discharge bores 23, 23. The surface 24 of the cooling channel 23 is provided with a coating 25 containing hexagonal boron nitride (hBN). The thickness of the coating 25 is preferably 20 m to 40 m. The thermal conductivity of the coating 25 is preferably 40 W/mK to 50 W/mK, depending on the degree of purity of the hexagonal boron nitride. The coefficient of friction of the coating 25 is constant up to a temperature of 600 C. and is 0.2. The specific surface area of the coating 25, depending on the degree of purity of the hexagonal boron nitride, is 5 m.sup.2/g to 15 m.sup.2/g.

(8) There follows a description of an exemplary embodiment of the method according to the invention for coating the cooling channel 23.

(9) First, the surface area of the cooling channel 23 in cm.sup.2 is determined in order to be able to optimally dose the coating medium.

(10) The surface 24 of the cooling channel 23 is thoroughly cleaned with ethanol. To that end, depending on the size of the surface 24, 10 ml to 30 ml of ethanol are introduced into the cooling channel 23 via one of the oil supply or oil discharge bores 23, 23, and the bores 23, 23 are closed with stoppers (preferably made of a rubber-elastic material). The piston 10 is moved in order to spread the ethanol inside the cooling channel and to ensure that the entire surface 24 is wetted with ethanol. For this, use can be made for example of a biaxial mixer. Then, the stoppers are removed so that the remaining ethanol runs out of the cooling channel 23. The surface 24 of the cooling channel 23 is dried via one of the bores 23, 23 using a laminar air flow having a flow velocity of 1 m/s to 2 m/s for five minutes at room temperature.

(11) As coating medium, use is made of a suspension of particles of hexagonal boron nitride in a polysiloxane dissolved in ethanol. In the exemplary embodiment, the content of hexagonal boron nitride in the suspension is 104 g/l, based on the volume of the pure polysiloxane solution. In the exemplary embodiment, the polysiloxane content is 61 g/l, based on the total volume of the suspension. In the exemplary embodiment, the ethanol content of the suspension is 647 g/l, based on the total volume of the suspension. A coating medium of that type is commercially available, for example under the name HeBoCoat400E from the manufacturer Henze Boron Nitride Products AG, Grundweg 1, 87493 Lauben. It is essential that the coating medium be free from halogen-containing substances, in particular free from fluorine-containing substances.

(12) Dosing is related to the size of the surface 24 of the cooling channel 23 in cm.sup.2. Optimal dosing of the suspension is 7 ml for a surface 24 of the cooling channel 23 with an area of 190 cm.sup.2. This corresponds, in the exemplary embodiment, to 4.53 g of ethanol, 0.43 g of polysiloxane and 0.73 g of hBN.

(13) A test with various doses of the coating medium for a cooling channel 23 having a surface 24 with an area of 190 cm.sup.2 yielded the following results, the results of the optimal dose and the excessive dose being illustrated in FIGS. 2 and 3:

(14) TABLE-US-00001 Optimal dose Excessive dose 7 ml/190 cm.sup.2 10 ml/190 cm.sup.2 Insufficient dose (FIG. 2) (FIG. 3) 4-5 ml/190 cm.sup.2 Layer thickness of 20 to 40 160 to 170 No coating in places the coating (25) Layer adhesion Very good Layer spalling and No crack formation after drying layer adhesion, marked crack Drying behavior Even drying Suspension gathers Very good drying behavior locally at the edges, properties crack formation there Flow behavior in Suspension Excess suspension Suspension reaches the cooling spreads evenly, runs back out via oil only some regions channel even layer discharge bore 23 of the cooling thickness or 23 channel 23

(15) The coating medium is introduced into the cooling channel 23 via one of the bores 23, 23, expediently with the aid of a dosing device, for example a metering pump. The bores 23, 23 are closed with stoppers, preferably made of a rubber-elastic material.

(16) Then, the piston 10 is moved about at least two spatial axes. This motion is essential for spreading the coating medium evenly over the surface 24 of the cooling channel. This is expediently done using a rotation unit, for example a biaxial mixer that is known per se, with which the piston 10 is rotated both about its longitudinal axis and also about an axis running perpendicular to the longitudinal axis.

(17) Then, the stoppers are removed. The coating medium adhering to the surface 24 of the cooling channel 23 is dried via one of the bores 23, 23 using a laminar air flow having a flow velocity of 1 m/s to 2 m/s for approximately five minutes at room temperature (approximately 20 C.). This removes the ethanol from the coating medium. This drying step is essential in order to ensure defect-free, even drying of the coating medium. The flow velocity of the laminar air flow may not be too high as this could cause coating medium adhering to the surface 24 of the cooling channel 23 in the vicinity of the bores 23, 23 to be displaced by the air pressure, which would result in a coating having an uneven thickness.

(18) Curing by heat treatment is used to produce the finished coating 25, in which the piston 10 is heated to 180 C. to 220 C. for a period of 25 min to 60 min. In this context, the polysiloxane is converted, in a manner known per se, to a SiO.sub.2 matrix in which the particles of hexagonal boron nitride are embedded.

(19) The resulting coating 25 has a surface energy of 15-17 mN/m and a layer thickness of 20 m to 40 m, which is constant over the entire surface 24 of the cooling channel 23. Owing to its small layer thickness, the coating 25 has no thermally insulating effect on the material of the piston 10. The coating 25 is heat-resistant up to 600 C.