Steel piston for an internal combustion engine and method for its production

Abstract

A steel piston for an internal combustion engine may include a piston crown and a protective layer disposed on the piston crown. The protective layer may include an adhesion layer of Cr or CrN, which is present on a surface of the piston crown. The protective layer may include a functional layer, which is present on the adhesion layer. The functional layer may have at least one of (i) at least one layer (A) of CrN and (ii) at least one layer (B) of CrOn in the form of [(A)/(B)].sub.a, and a may represent 1 to 100.

Claims

1. A steel piston for an internal combustion engine, comprising: a piston crown and a protective layer disposed on the piston crown, wherein the protective layer includes: a) an adhesion layer of Cr or CrN, which is present on a surface of the piston crown, and b) a functional layer, which is present on the adhesion layer, wherein the functional layer has at least one of (i) a layer (A) of CrN and (ii) a layer (B) of CrON, wherein the functional layer has a layer sequence in the form of [(A)/(B)].sub.a, and wherein a=1 and the layer of (B) of CrON is present as a gradient layer with increasing oxygen content in a direction of a surface of the layer which faces away from the adhesion layer.

2. The steel piston according to claim 1, wherein the adhesion layer is CrN.

3. The steel piston according to claim 1, wherein the protective layer has a thickness of 1 m to 15 m.

4. The steel piston according to claim 1, wherein the adhesion layer has a thickness of 0.5 m to 5 m.

5. The steel piston according to claim 1, wherein the functional layer has a thickness of 0.5 m to 10 m.

6. The steel piston according to claim 1, wherein the layers (A) and (B) independently respectively have a thickness of 0.04 m to 0.25 m.

7. The steel piston according to claim 1, wherein the layers (A) and (B) respectively have the same thickness.

8. The steel piston according to claim 1, wherein the protective layer is present only on a bowl edge of the piston crown.

9. The steel piston according to claim 1, wherein the protective layer further includes a plurality of alternating layers of CrN and CrON disposed on the functional layer.

10. A steel piston for an internal combustion engine, comprising: a piston crown and a protective layer disposed on the piston crown, wherein the protective layer includes: a) an adhesion layer of Cr or CrN, which is present on a surface of the piston crown, and b) a functional layer, which is present on the adhesion layer, wherein the functional layer has at least one of (i) a layer (C) of AlCrO and (ii) a layer (C) of AlCrO, wherein the layer (C) of AlCrO differs in oxygen content from the layer (C) of AlCrO, and wherein the functional layer has a layer system in the form of [(C)/(C)].sub.a, wherein a is a number of sequenced layers of the layer system and ranges from 1 to 100.

11. The steel piston according to claim 10, wherein a=1.

12. The steel piston according to claim 11, wherein the layer (C) of AlCrO has a thickness of 0.04 m to 0.25 m.

13. The steel piston according to claim 11, wherein the layer (C) is present as a gradient layer with increasing oxygen content in a direction of a surface of the layer which faces away from the adhesion layer.

14. The steel piston according to claim 10, wherein a=2 to 100.

15. The steel piston according to claim 10, wherein at least one of (i) the adhesion layer has a thickness of 0.5 m to 5 m, and (ii) the functional layer has a thickness of 0.5 m to 10 m.

16. The steel piston according to claim 10, wherein the layers (C) and (C) each respectively have a thickness of 0.04 m to 0.25 m.

17. A method for producing a steel piston for an internal combustion engine, comprising the steps of: providing a steel piston, which has a piston crown, and applying a protective layer onto the piston crown via a physical vapour deposition (PVD) method, wherein applying the protective layer includes the steps of: applying an adhesion layer of Cr or CrN to a surface of the piston crown; and depositing a functional layer onto the adhesion layer, the functional layer including one of: (a) a layer of CrN and a layer of CrON disposed on the layer of CrN, wherein the layer of CrON is present as a gradient layer with increasing oxygen content in a direction of a surface of the layer which faces away from the adhesion layer; and (b) at least one of a layer of AlCrO and a layer of AlCrO, the layer of AlCrO differing in oxygen content from the layer of AlCrO, wherein the functional layer has a layer system in the form of [AlCrO/AlCrO].sub.a, wherein a is a number of sequenced layers of the layer system and a=1 to 100.

18. The method according to claim 17, wherein applying the protective layer via the PVD method occurs at a temperature of 150 C. to 550 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) There are shown here, respectively diagrammatically,

(2) FIG. 1 a coating, applied according to Example 1, in deposition state,

(3) FIG. 2 a microscopic micrograph of a sectional area through a steel piston coated according to the invention, after the engine test in the region of the bowl edge according to Example 1,

(4) FIG. 3 a microscopic micrograph of a sectional area through an uncoated steel piston after the motor test in the region of the bowl edge according to the comparative example,

(5) FIG. 4 a protective layer according to the first embodiment on a polished hard metal substrate with a magnification of 25000, and

(6) FIG. 5 a protective layer according to the second embodiment on a polished hard metal substrate with a magnification of 50000.

DETAILED DESCRIPTION

(7) An example is described in detail below for the application of the protective layer according to the invention onto a piston crown by means of a known PVD method.

(8) As example for a PVD method, a coating process is described below, which is based on reactive cathodic spark evaporation. This is not, however, to be understood as a restriction to solely this PVD method. Other PVD methods, such as for example sputtering, electron beam evaporation or laser ablation can be used for coating, which, however, have a lower cost effectiveness and require a greater technical effort in the monitoring of the coating process.

(9) Firstly, the substrates (pistons) which are to be coated are introduced into the rotatable mounts, provided for this, of a vacuum coating plant. The vacuum coating plant is then evacuated to a pressure of approximately 10.sup.4 mbar.

(10) To set the process temperature, a low-voltage arc plasma (LVA), supported by radiation heating, is ignited between a hot cathode and the anodically connected workpieces in an argon-hydrogen atmosphere.

(11) In this process, the following parameters were set:

(12) TABLE-US-00001 Discharge current LVA 110 A Argon flow 50 sccm Hydrogen flow 300 sccm

(13) Under these conditions, a process pressure of 1.410.sup.2 mbar occurs. The heating devices and the LVA were regulated so that a substrate temperature of 230 C. was maintained. The process duration of this pre-treatment step was 100 min.

(14) As the next process step, the etching of the substrate surfaces takes place, in order to free the substrate surfaces of any impurities which may be present. For this, the LVA is operated between the hot cathode and an auxiliary anode provided in the coating plant. Here, a DC- a pulsed DC- or a MF- or RF supply, operated with alternating current, can be applied between the substrates and the earth. Preferably, however, the workpieces are acted upon with a negative bias voltage.

(15) For this pre-treatment step, the following process parameters were set:

(16) TABLE-US-00002 Argon flow 60 sccm Discharge current LVA 150 A Bias voltage 60 V (DC)

(17) Under these conditions, a process pressure of 2.410.sup.3 mbar occurs in the coating system. The process parameters were selected again so that the substrate temperatures of 230 C. were not exceeded. The duration of this pre-treatment was 45 min.

(18) In the next process step, the coating of the substrate with the CrN adhesion layer takes place. This process step was carried out with four Cr targets. Various aspects are taken into consideration in the number of targets. The coating duration can be reduced when the number of targets is increased, wherein then, however, the thermal load of the substrates increases. In the present process, likewise the 230 C. with regard to the substrate temperature should not be exceeded with the coating. The process parameters for the coating with the adhesion layer were:

(19) TABLE-US-00003 Nitrogen flow regulated to 3 Pa total pressure Current per Cr target 140 A DC substrate bias voltage U = 20 V

(20) Thereby again substrate temperatures below 230 C. were able to be guaranteed. The duration of the process step for the application of the adhesion layer was 60 min.

(21) Following the coating of the substrates with the adhesion layer, the coating with the functional layer takes place. As already described above, this is particularly simple and economical in the case of the CrNCrON multi-layered coat. The four Cr targets are continued to be operated unchanged with respectively 140 A per target. For the first CrON layer, 300 sccm oxygen is then admitted into the coating system for 2 min. Subsequently, the oxygen flow is set to zero again for 2 min, i.e. is switched off. Then the same sequence takes place as previously described: 2 min oxygen input of 300 sccm, 2 min switch-off of the oxygen flow, wherein a CrN coating is obtained. In the present process, this sequence was carried out 18 times, i.e. a total of 36 individual layers were produced. With the application of this functional layer, therefore also the application of the entire protective layer was completed. After the substrates were subsequently cooled to approximately 150 C., the coating system was vented for substrate removal. FIG. 4 shows such a protective layer, on a polished surface, however, but which was also coated during the coating of the steel pistons under identical conditions. The rupture cross-section was taken by a scanning electron microscope with a magnification of 25000. The approximately 2.4 m thick adhesion layer on the substrate, which consists of CrN, can be seen. In addition, the approximately 3.0 m thick functional layer of CrN/CrON on the adhesion layer can be seen as a multi-layered coat.

(22) In the process described above, the coating temperature was limited to 230 C. Such a limit can be expedient if, for example, the steel pistons have undergone special pre-treatments which are temperature-sensitive. If, however, the steel pistons merely have steel which permits higher temperatures, it is preferred to select somewhat higher temperatures during the coating, because then the pre-treatment steps become more effective and small cavities in the piston can be cleaned better by outgassing processes. Here, substrate temperatures of between 300 C. and 400 C. are preferred.

(23) FIG. 5 shows in analogy to FIG. 4 a protective layer which consists of a CrN adhesion layer and a single AlCrO functional layer according to the second embodiment. The layer cross-section viewed in the scanning electron microscope (magnification 50000) shows a total layer thickness of approximately 3 m, consisting of a 1.6 m thick CrN adhesion layer and a 1.4 m thick AlCrO functional layer. In this case, operations were carried out with Cr targets and AlCr targets in the coating system. The pre-treatment steps corresponded to those which were already described above.

(24) Further important features and advantages of the invention will emerge from the subclaims, from the drawings and from the associated figure description with the aid of the drawings.

(25) It shall be understood that the features named and explained in the present application are able to be used not only in the respectively indicated combination, but also in other combinations or in isolation, without departing from the scope of the present invention.

Example 1

(26) The bowl edge of the piston crown of a steel piston was coated by the PVD method, as has been described in detail above, firstly with a 3.9 m thick adhesion layer of CrN and subsequently with a functional layer of 23 layers of CrN and 23 layers of CrON, which respectively had a layer thickness of approximately 0.06 m and were applied alternately. The first layer of the functional layer applied on the adhesion layer was a CrN layer here. The total thickness of the functional layer was approximately 2.9 m. FIG. 1 shows the obtained coating in the deposition state, wherein FIG. 1(b) shows a diagrammatic cross-section through the coated piston and FIGS. 1(a) and 1(c) show respectively microscopic micrographs of sectional areas through the steel piston according to FIG. 1(b).

(27) The obtained coated steel piston was installed into an engine and a test run was carried out (passenger car diesel engine with steel piston, 150 kW output, 120 hours endurance test, temperature at the bowl edge approximately 600 C.). After the test run, the piston was dismantled again (cf. FIG. 2(a)) and a microscopic micrograph of a sectional area through the steel piston in the region of the bowl edge was produced, which is shown in FIG. 2(b).

(28) As can be seen from FIG. 2(b), the protective layer applied onto the bowl edge is completely intact and the material of the steel piston shows no scale notches whatsoever. This shows that the steel piston according to the invention has an excellent resistance to damage by oxidation.

Comparative Example

(29) The same test run as in Example 1 was carried out with the steel piston as was used in Example 1, onto which, however, no protective layer has been applied.

(30) As can be seen from FIGS. 3(a) and (b), the material of the uncoated steel piston has scale notches, which leads to the disadvantages described above.