METHOD AND DEVICE FOR PRODUCING A WEAR-RESISTANT SURFACE ON A WORKPIECE

Abstract

A method including closing upper and lower ends of a bore with upper and lower closure element, respectively; introducing a cathode into the bore; and flowing an electrolyte through an annular space between a wall of the bore an outer surface of the cathode to provide an inner surface of the bore with a wear-resistant surface by electrolysis.

Claims

1. A method comprising: closing upper and lower ends of a bore with upper and lower closure elements, respectively; introducing a cathode into the bore; and flowing an electrolyte through an annular space between a wall of the bore an outer surface of the cathode to provide an inner surface of the bore with a wear-resistant surface by electrolysis.

2. The method of claim 1, wherein the cathode is a hollow cathode.

3. The method of claim 1, wherein the closing and introducing steps are carried out simultaneously.

4. The method of claim 1, further comprising producing the electrolysis by plasma electrolytic oxidation (PEO).

5. The method of claim 1, further comprising producing the electrolysis by plasma electrolytic deposition (PED).

6. The method of claim 1, wherein the flowing step included continuously flowing the electrolyte.

7. The method of claim 1, further comprising introducing the electrolyte into the bore from above the bore and through the cathode.

8. The method of claim 1, wherein the flowing step including flowing the electrolyte at a speed of 2 m/s to 5 m/s.

9. The method of claim 1, wherein the wear-resistant surface has a thickness of 20 μm to 50 μm.

10. A device comprising: upper and lower closure elements for closing a bore; a cathode extending within the bore from the upper closure element towards the lower closure element to form an annular space between a wall of the bore an outer surface of the cathode, the annular space configured to receive a flow of an electrolyte therethrough during electrolysis; and an outlet opening for discharging a gas formed during electrolysis.

11. The device of claim 10, further comprising an inlet line for feeding the electrolyte and opening into the cathode.

12. The device of claim 10, wherein a free end of the cathode is spaced apart from the lower closure element.

13. The device of claim 10, wherein the annular space between the bore and the cathode increases continuously in the direction of the lower closure element towards the upper closure element with a conically tapering configuration of the cathode from a free end in the direction of the upper closure element.

14. The device of claim 10, wherein the cathode is a hollow cathode.

15. The device of claim 10, further comprising a collecting space situated above the bore and on the upper closure element in which space gas that forms during the electrolysis collected.

16. A device comprising: upper and lower closure elements for closing a bore; and a hollow cathode extending within the bore from the upper closure element towards the lower closure element to form an annular space between a wall of the bore an outer surface of the hollow cathode, the annular space configured to receive a flow of an electrolyte therethrough during electrolysis.

17. The device of claim 16, further comprising an inlet line for feeding the electrolyte and opening into the cathode.

18. The device of claim 16, wherein the cathode is a central hollow cathode.

19. The device of claim 16, wherein the upper closure element includes multiple openings.

20. The device of claim 16, further comprising a collecting space situated above the bore and on the upper closure element in which space gas that forms during the electrolysis collected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Further features and advantages of the invention will become apparent from the following description of an illustrative embodiment of the present invention, which should not be interpreted as restrictive and which is explained in greater detail below about the drawing.

[0031] FIG. 1 shows an engine block in schematic view with a cylinder bore to be coated and, schematically, a coating device.

DETAILED DESCRIPTION

[0032] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0033] Although gray cast iron material is tribologically well-suited to stress imposed by piston rings because of the graphite flakes that are present in the gray cast iron, such cylinder liners have several disadvantages. In addition to the increased weight, problems arise with the differential thermal expansion of the gray cast iron material and the aluminum block material. Increasingly, therefore, use is being made of thermal spray coatings, for example, those disclosed in DE 10 2007 023 297 A1. With such thermal spray coatings, however, it is typically necessary to carry out an expensive surface pretreatment to obtain sufficient adhesion of the functional coating on the aluminum substrate. Either the sliding surface must be roughened by corundum or water jets, or a micro-profile with an undercut is introduced into the surface by a turning spindle process. A porous sprayed steel layer with a thickness of about 300 μm is then applied to a surface prepared in this way and subsequently machined down to a remaining final layer thickness of 80-150 μm by honing.

[0034] Thermal spray coatings of this kind may have one or more technical and cost problems. Owing to the porous layer, for example, there can be sub-surface corrosion in the presence of fuels, especially ethanol/methanol fuels. On the other hand, there can be significant overspray forming dust that must be extracted and disposed of. The method may give rise to rough layers, which then must be removed in an expensive honing operation until a surface suitable for operation is obtained.

[0035] Thin PEO coatings based on aluminum oxide or PED coatings based on titanium oxide therefore appear to be more suitable. These do not require expensive surface pretreatment and are distinguished by the very high adhesive strength of the functional coating. At the interface with the aluminum substrate, a thin barrier layer forms, providing good protection for the material against corrosive attack. The quantity of waste generated is negligible and only a very short honing operation for final machining is required, with a thin, smooth coating.

[0036] Although the entire engine block, including all the available cylinder bores, could also be coated in a treatment bath, for this coating process, there are not only higher costs for power but also technical disadvantages. Thus, different layer thicknesses and zones of different porosity are formed within the sliding surface owing to the nonuniform flow through the bore by the electrolyte used.

[0037] CA 2,556,869 A1 discloses that the electrolyte is sprayed against the inside of the cylinder sliding surface by a hollow rotating spindle in the PEO process using the “electro-jet plating” method. In this case, there is a horizontal outflow nozzle at the lower end of the spindle. This nozzle rotates at an adjustable speed around the spindle and is moved backward and forward vertically until the entire surface has been treated. During this process, the electrolyte can run off into an electrolyte collecting trough. The disadvantage with this coating arrangement may also be regarded as the fact that the engine sliding surface cannot be hermetically sealed in the presence of the vapors and spray mist which occur.

[0038] In one or more embodiments, improved methods and devices for producing a wear-resistant surface including a uniform layer thickness within a cylinder bore are disclosed.

[0039] With respect to one embodiment, FIG. 1 shows a workpiece 1, which is embodied as an engine block or crankcase. By way of example, the workpiece is produced from an aluminum or an aluminum alloy. A bore 2, e.g. a cylinder bore 2, is arranged in the workpiece 1. A coating device 3, can be used to electrolytically coat the wall 4 of the cylinder bore 2. As an electrolytic coating method, plasma electrolytic oxidation (PEO) or plasma electrolytic deposition (PET) is carried out. Four cylinder bores 2, for example, of which only one is visible, can be arranged in the workpiece 1. The coating device 3 has an upper closure element 6 and a lower closure element 7.

[0040] If the workpiece 1 has a plurality of cylinder bores 2, the upper closure element 6 covers all the cylinder bores 2. To secure the upper closure element 6, use can be made of screw holes 8 which are present in any case, these being provided for securing a cylinder head, for example. Suitable screws 9, of which only the screw heads are indicated in FIG. 1, are screwed into the screw holes 8. The medium-tight sealing can be achieved by sealing elements, e.g. by means of O-rings 11, which can be arranged between the surface of the workpiece 1 and the upper closure element 6 at each cylinder bore 2. The upper closure element 6 can be composed of a suitable material, e.g., a plastic material. Thus, the upper closure element 6 can be secured on the sealing surface of the subsequent cylinder head in such a way that medium-tight sealing is achieved.

[0041] The lower closure element 7 is provided at the bottom, although a separate lower closure element 7 is provided for each cylinder bore 2. The lower closure element 7 closes the cylinder bore 2 in a medium-tight manner in a suitable way. In one embodiment, provision is made to secure the lower closure element 7 on a central hollow cathode 12. For this purpose, a screw 13 can be provided, which passes through the lower closure element 7 and is screwed into a free end 14 of the central hollow cathode 12. Of course, it is also possible for a plurality of screws to be provided, which ensure that the lower closure element 7 is secured relative to the lower end of the respective cylinder bore 2. By use of sealing elements, for example, interposed O-ring seals, medium-tight sealing is ensured in this way.

[0042] The central hollow cathode 12 may extend from the upper closure element 6 in the direction of the lower closure element 7. The central hollow cathode 12 is arranged with its central axis centrally in the cylinder bore 2 and has an embodiment such that an annular space 17 between the outside diameter of the central hollow cathode 12 and the inner wall of the cylinder bore 2 can have a constant value of about 10 mm. Of course, the central hollow cathode 12 can also be embodied with a taper from the lower free end 18 in the direction of the upper end, with the result that the value of the annular space 17 increases continuously from about 10 mm. This has an advantageous effect on the layer thickness distribution, which can be influenced in this way. Of course, the configuration of the central hollow cathode 12 depends on the clear diameter of the respective cylinder bore 2. In this case, it is not essential to define the wall thickness, but it may be necessary to modify the outside diameter to enable the optimum annular space volume with the optimum flow speed to be set to produce the coating with the optimum thickness and density. The free end 18 of the central hollow cathode 12, e.g., the free end 14, is spaced apart from the lower closure element 7, e.g., spaced apart from the latter by about 10 mm.

[0043] In the coating process, an electrolyte is passed through the central hollow cathode 12 from above into the cylinder bore 2, which is closed in a medium-tight manner at both ends. For this purpose, an inlet line 19 is provided, passing through the upper closure element 6 and opening into the central hollow cathode 12. In one embodiment, the inlet line 19 is connected to the central hollow cathode 12 to be secured in position, and therefore the central hollow cathode 12 is held in a stable position in the cylinder bore 2 by the inlet line 19. The electrolyte enters the central channel of the central hollow cathode 12 and emerges from the free end 18 of the central hollow cathode 12, and reaches the lower closure element 7, with the result that the electrolyte is deflected in its flow direction and flows through the annular space 17 in the direction of the upper closure element 6. The flow of electrolyte is illustrated by arrows 21 in FIG. 1.

[0044] The workpiece 1 is connected as the anode. The central hollow cathode is connected as the cathode, thus allowing electrolysis, i.e. PED or PEO, to be carried out. The individual circuit elements, for example, cables, are not shown in FIG. 1. In this case, it is ensured that the electrolyte flows through the optimum annular space volume at a predetermined flow speed and is discharged at the top, e.g., by flowing through the upper closure element 6, together with the hydrogen formed during electrolysis. The hydrogen formed can be seen by the indicated circles in FIG. 1.

[0045] For this purpose, the coating device 1 in the illustrative embodiment shown has a collecting space 22, which is arranged above the respective cylinder bore 2. The collecting space 22 is arranged as a hat-like raised portion on the upper closure element 6. The collecting space 22 has an inflow opening 23, which corresponds approximately to the diameter of the cylinder bore 2 to be coated. At the top end, the collecting space 22 is closed by a cap 24. Outlet openings 16, which discharge accumulated hydrogen but also the electrolyte, are arranged in the cap 24. Also arranged in the cap 24 is the inlet line 19, which passes through the collecting space 22 and opens into the central hollow cathode 12. Through the inlet line 19, electrolyte is introduced into the cylinder bore 2, flowing through the central hollow cathode 12 in the direction of the free end thereof 18. Thus, the electrolyte is not only fed in from above but is also discharged at the top together with the hydrogen, which forms. Of course, a single outlet opening 16 can be arranged in the cap 24 of the collecting space 22, or a plurality of outlet openings 16, e.g., seven outlet openings 16. The electrolyte is passed into a collecting tank (not shown) with the hydrogen from the collecting space 22 and is cooled there. In the collecting tank, which is possibly open, the electrolyte can degasify, and the hydrogen can be removed safely by edge trough extraction.

[0046] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.