EP2417278A1 - Sliding element having adjustable properties - Google Patents

Abstract

The invention relates to a sliding element, particularly a piston ring for an internal combustion machine, comprising a substrate; and a wear protection coating obtained by thermal spraying of a powder comprising the element proportions 2-50 percent by weight of iron, FE; 5-60 percent by weight of tungsten, W; 5-40 percent by weight of chromium, Cr; 5-25 percent by weight of nickel, Ni; 1-5 percent by weight molybdenum, Mo; 1-10 carbon, C; and 0.1-2 percent by weight silicon, Si; and a run-in coating, obtained by thermally spraying a powder comprising the element proportions 60-95 percent by weight nickel; 5-40 percent by weight carbon.

Description

 Sliding element with adjustable properties

The present invention relates to a sliding element, in particular a piston ring, with adjustable properties, in particular with respect to the wear behavior, and a method for its production.

Today, the customer requirements for the wear behavior on the piston ring and the cylinder bore are different. On the one hand, the lowest possible wear is required, on the other hand, engine manufacturers also require higher wear rates in order to obtain the best possible intake behavior for the system "piston ring / cylinder liner lubricated." This task is increasingly in the range of 2-stroke engines (ring diameter > 430mm).

Iron-based coatings, applied by means of thermal spraying, are not yet used on the piston ring. In the area of the crank mechanism, only iron-based coatings on the cylinder bore have been known so far which are produced by means of arc wire spraying (LDS) (EP 1 055 351 B2). The production of wear protection layers by means of the thermal spraying process is a known method. Today's powder materials used for it are based on Mo, WC, NiCr and Cr ₃ C ₂ .

The invention is therefore based on the following objects. On the one hand, an improvement in the tribological properties of thermally sprayed piston rings with a previously unused material system as a coating material in comparison to conventional Mo-ring piston ring coatings. Furthermore, the production of customer-specific, coated piston rings, which are tailored in terms of wear behavior and residual stresses, the Coating takes place via thermal spraying. Furthermore, an optimization of the run-in behavior should be achieved. The base material matrix should preferably have similar physical properties (thermal expansion coefficient and thermal conductivity) as the underlying substrate and sufficient mechanical properties (hardness, ductility).

According to a first aspect of the invention, a sliding element is provided, in particular a piston ring for an internal combustion engine, comprising a substrate; and a wear protection layer obtainable by thermally spraying a powder comprising the elementary parts

2-50 weight percent iron, FE;

5-60 weight percent tungsten, W;

5-40 weight percent chromium, Cr; 5-25 weight percent nickel, Ni;

1-5 weight percent molybdenum, Mo;

1-10 carbon, C; and

0.1-2% by weight of silicon, Si; and an inlet layer obtainable by thermal spraying a powder comprising

element proportions

60-95 weight percent nickel;

5-40 weight percent carbon.

In order to achieve the object described above, it is necessary to produce a layer system consisting of a base system with similar physical properties to the substrate to be coated and sufficient strength, combined with a wear-resistant portion, depending on the proportion of wear rates on the ring and liners in the lubricated State is coming. Likewise, the nature and strength of the residual stresses can be adjusted by the addition of defined amounts of the wear-resistant portion. Basically, no residual stresses in thermally sprayed Layers desirable because they can not reduce or even promote the crack growth of a resulting crack. The solution is a new Fe-based system reinforced with carbides combined with an inlet layer adapted to the requirements of the engine manufacturer.

With regard to the physical properties (thermal conductivity, thermal expansion coefficient), a minimum proportion of the iron-containing base system of 25% by weight results in a quasi-homogeneous system between the substrate and the coating. As a result, the thermal energy generated during the mixed friction, in particular in the OT or UT range, can be better dissipated and a uniform thermal relaxation process can be ensured by the temperature fluctuations present in the motor. The application of Fe base alloys as a piston ring base coating material together with a carbide system and a run-in layer (graded or not graded) produced by thermal spraying results in a new type of piston ring. The piston ring to be coated can be a cast but also a steel piston ring.

According to one embodiment, the new material system consists of the following elements: iron (Fe), tungsten (W, as WC), chromium (Cr, as Cr and Cr ₃ C ₂ ), nickel (Ni), molybdenum (Mo), silicon ( Si) and carbon (C, partially bound in Fe, W and Cr as carbide or in pure form, electrochemically coated with nickel).

In one embodiment, the amount of carbides is 10-75 weight percent composed of 0-60 weight percent tungsten carbide, WC, and 0-50 weight percent chromium carbide, Cr ₃ C ₂ .

The iron-base alloy without carbides is not recommended because the wear resistance (measured as described below) does not meet today's requirements. An increase of the total carbide content above 75 wt.% Is not recommended for use as a piston ring coating, because too high a carbide content, the layer gets too strong ceramic character (too high modulus) and thus the thermal cycling in the engine does not withstand. According to one embodiment, the sliding element further comprises a transition layer between the wear protection layer and the inlet layer, wherein the chemical composition of the transition layer has a graduation ratio of 20:80 to 80:20, based on the wear protection layer and the inlet layer.

The chemical composition is in the grading ratio 20:80 to 80:20 for the single-layer types Wear protection layer: inlet layer adjustable.

Example 1 :

1st layer: wear protection layer

2nd layer: on the side of the wear protection layer, the chemical composition of the transitional layer is 80% like the composition of the wear protection layer, 20% like the inlet layer, a substantially linear transition up to the side of the inlet layer

Composition that corresponds to 20% of the composition of the wear protection layer, 80% of the composition of the inlet layer

3rd layer: inlet layer

Example 2:

1st layer: wear protection layer

2nd layer: chemical composition 20% as wear protection layer, 80% as inlet layer, transition linear to 80% as wear protection layer, 20% as inlet layer 3rd layer: inlet layer

According to one embodiment, the layer thickness of the wear protection layer is in the range of 100-800 μm, preferably 200-600 μm, and most preferably 300-500 μm.

According to one embodiment, the layer thickness of the inlet layer is in the range of 100- 500 μm, preferably 200-400 μm, and most preferably 150-300 μm.

According to one embodiment, the layer thickness of the transition layer, in which the wear protection and enema layer are graded, is in the range of 0-600 μm, and most preferably 0-250 μm.

According to one embodiment, the substrate is a ring with a diameter greater than 220 mm, preferably greater than 430 mm and a maximum of 980 mm.

According to one embodiment, the particle sizes of the powder are in the range of 1 to 100 μm.

According to one embodiment, the carbides are embedded in a nickel-chromium matrix and have a particle size of 0.5-5 microns.

Short description of the drawing

Fig. 1 shows the microstructure of a thermal sprayed wear protection / enema layer according to an embodiment of the invention;

Experiments performed:

The powder was thermally sprayed and for various variants the chemical composition (Table 1), the carbide content (Table 2), the microstructure (Figure 1), the porosity and hardness (Table 3) were tested. Experiments 1 and 2 differ in that layer type 1 was produced in experiment 1 and layer type 2 in experiment 2. For the experiments 1.1 to 1.4 or 2.1 to 2.4 different carbide concentrations were set. The respective top layer contains no carbides, as this layer is used for a controlled enema.

Table 1: Chemical composition of the wear protection / inlet layer type 1

Table 2: Carbide content of the wear protection / inlet layer type 1

The microstructure images (FIG. 1) show homogeneously distributed carbides for the wear protection layer, no unmelted particles and a very dense layer with a very low porosity of <2%. In the top layer, the graphite precipitations are clearly visible. The layer thickness of the wear protection layer is 330 microns, the inlet layer 180 microns.

Table 3: Hardness / Porosity of Wear Protection Layer Type 1

As shown in Table 3, initial tests have shown that the Type 1 wear protection layer has a porosity of <1-2% at a hardness of about 520HV 1 for the carbide-free Fe base material up to 71 OHV for the Fe base material with a carbide content of 60% by weight. The hardness of the inlet layer can not be determined due to the high graphite content.

The addition of carbides allows a targeted adjustment of the hardness of the ring and the cylinder liner. In addition, despite the high loads during the wear test, the microstructure is largely retained, which basically points to a wear-resistant piston ring produced by this coating according to the invention for the system "ring / sleeve lubrication" after the run-in process is completed.

Claims

claims 1. sliding element, in particular piston ring for an internal combustion engine, comprising - a substrate; and a wear protection layer obtainable by thermal spraying a powder comprising the elemental proportions 2-50 wt.% iron, FE; 5-60 weight percent tungsten, W; 5-40 weight percent chromium, Cr; 5-25 weight percent nickel, Ni; 1-5 weight percent molybdenum, Mo; 1-10 carbon, C; and 0.1-2 wt% silicon, Si; and an enema layer obtainable by thermal spraying a powder comprising the elemental proportions of 60-95 weight percent nickel; 5-40 weight percent carbon. 2. A sliding member according to claim 1, further comprising a transition layer between the wear protection layer and the run-in layer, wherein the chemical composition of the transition layer has a graduation ratio of 20:80 to 80:20, based on the wear protection layer and the run-in layer. 3. The sliding element according to claim 1 or 2, wherein the proportion of carbides 10-75 weight percent, composed of 0-60 weight percent tungsten carbide, WC, and 0-50 weight percent chromium carbide, Cr ₃ C ₂ . 4. Sliding element according to one of the preceding claims, wherein the layer thickness of Wear protection layer in the range of 100-800 microns, preferably 200-600 microns, and most preferably 300-500 microns. 5. Sliding element according to one of the preceding claims, wherein the layer thickness of the inlet layer in the range of 100-500 .mu.m, preferably 200-400 .mu.m, and most preferably 150-300 microns. 6. Sliding element according to one of claims 2 to 5, wherein the layer thickness of the transition layer, in the wear protection and inlet layer graded, in the range of 0-600 microns, and most preferably 0-250 microns. 7. Sliding element according to one of the preceding claims, wherein the substrate is a ring with a diameter greater than 220 mm, preferably greater than 430 mm and a maximum of 980 mm. 8. Sliding element according to one of the preceding claims, wherein the particle sizes of the powder are in the range of 1-100 microns. 9. Sliding element according to one of the preceding claims, wherein the carbides are embedded in a nickel-chromium matrix and have a particle size of 0.5-5 microns.