PISTON RING WITH TRANSITION LAYER

Description

[0001] The present invention generally relates to a piston ring.

[0002] The piston ring has a wear protection layer and a running-in layer, between which there is a transition layer that provides a mechanically stable and thermally resistant layer bond.

[0003] Advances in large-bore engines are leading to ever-increasing combustion pressures and higher loads on piston rings in terms of temperature and surface pressure. The tribological properties of current coating systems lead to problems, particularly at the transition from a soft running-in layer to a hard wear-resistant layer. Thermal overload, scuffing, and layer flaking can occur.

[0004] The stresses to which certain materials, such as the running surfaces of piston rings, are subjected have steadily increased in recent years due to increased engine performance and simultaneous reduction in exhaust emissions. Therefore, piston rings must meet a wide range of requirements, such as lower oil consumption and be suitable for operation with sulfur-free lubricating oils and fuels, which are associated with increased material stress and wear.

[0005] Piston rings are manufactured from cast iron or steel according to requirements for mechanical strength, wear resistance, and elasticity. Coatings are applied to the rings to improve wear resistance. Various electrochemical processes are used, such as chromium plating, thermal spray coating, physical CrN, and chemical DLC coating. On steel rings, the surface layer adjacent to the cylinder bore is often hardened by nitriding.

[0006] From the German patent application DE 10 2007 025949 A1, a piston ring with a thermally sprayed wear protection layer and a running-in layer is known, in which there is a graduated transition between the wear protection layer and the running-in layer.

[0007] The international patent application WO 2019 / 015818 A1 discloses a piston ring with a shot-peened, work-hardened wear protection layer.

[0008] From the German patent specification DE 10 2008 056720 B3 a sliding element is known which comprises a thermally sprayed wear protection layer which is obtainable by thermal spraying of a powder comprising the element proportions 40-79.9 weight percent chromium, Cr; 10-30 weight percent nickel, Ni; 0.1-5 weight percent carbon, C; 10-20 weight percent nitrogen, N.

[0009] German patent DE 3421569 C1 discloses a coating for piston ring running surfaces, which has a structure or composition that continuously changes radially from the inside to the outside, for example, a porosity of the coating that increases radially from the inside to the outside. The coating can be produced, for example, using a plasma spraying process.

[0010] An abrasion-resistant coating for a piston ring is described in German patent application DE 41 12422 A1. The coating contains a chromium nitride, with the nitrogen concentration of the coating continuously increasing from the interface between the substrate and the coating toward the outer surface of the coating. The coating is achieved by vapor deposition of chromium with variable amounts of nitrogen.

[0011] German patent DE 10 2004 032 403 B3 relates to a coating for a piston ring and a corresponding manufacturing method. Several layers are applied one above the other on the outer circumferential surface of the base body of the piston ring. The bonding layer facing the outer circumferential surface is a PVD bonding layer based on Cr. A PVD CrN gradient layer with a nitrogen content increasing toward the outside, ranging from Cr through Cr-Cr2N mixtures, Cr2N, Cr2N-CrN mixtures, and finally CrN, is applied to the CrN gradient layer. At least one cover layer of constant composition based on CrN, Cr2N, or a mixture thereof is applied to the CrN gradient layer.

[0012] Piston rings with a 3-layer system are known from European patent EP 21 18534 B1. A bonding agent layer is applied to a non-circularly machined blank. This serves to compensate for differences in the thermal expansion coefficient between the base material and the wear protection layer and improves thermal shock behavior. A wear protection layer is deposited on the bonding agent layer. This layer generally consists of a metal e.g. Ni, NiCr, Fe, FeCr, NiCr and molybdenum (high burn mark resistance) with embedded hard materials (CrC or MoC). A soft running-in layer (AlCuFe) is applied to the wear protection layer, which enables rapid adaptation and short running-in of the ring surface / shape to the cylinder surface / shape. The layers are applied by plasma coating (bonding agent and wear protection layer) orArc wire spraying (running-in layer) is deposited, resulting in a rough and jagged transition between the individual layers due to the process. The layer change is characterized by an irregular transition between the two layer materials (running-in layer and wear-protection layer) and is defined by the surface roughness of the wear-protection layer. After the soft running-in layer areas are removed, the tribological function of the running surface is suddenly taken over by the wear-protection layer. The tribological conditions during the layer change are temporally determined by the surface roughness of the wear-protection layer.

[0013] In addition, piston rings have a crowned running surface profile or a barrel-shaped outer surface. In the area of ​​high crowning (ring center, high surface pressure), the running-in layer is worn away very quickly, while in the area of ​​low crowning, the running-in layer material can remain for a longer period.

[0014] To reduce thermal overload / scuffing during layer changes, the wear protection layer on new ring designs is ground, i.e., the surface roughness is reduced. This design variant requires a critical pre-clamping process for the rings or a complex grinding process for the wear protection layer surface. Furthermore, grinding or smoothing the surface also reduces the adhesive strength of the running-in layer. To avoid flaking of the running-in layer with this design, the layer thickness of the running-in layer must not exceed 100 µm.

[0015] However, no methods are currently known for creating multi-layer coatings with sufficient mechanical and thermal resilience that would satisfactorily fulfill their function throughout the entire service life of an internal combustion engine. Furthermore, coatings on substrate materials, such as the running surfaces of piston rings, must not only exhibit good, temperature-resistant adhesion to the substrate and good, temperature-resistant cohesion within the layer, but also be wear-resistant. Furthermore, the coatings must exhibit little or no thermal fatigue behavior (cracking in the coating) after long service lives.

[0016] An object of the present invention is therefore to further improve the mechanical strength and wear resistance of piston rings, so that the running-in behavior of piston rings under operating conditions is improved and an extended operating life is ensured.

[0017] This object is achieved by a piston ring having the features of the present claim 1, preferred embodiments are described in dependent claims.

[0018] A wear protection layer is applied to a carrier material of a piston ring, with a running-in layer provided. Furthermore, a transition layer is present between the wear protection layer and the running-in layer, which ensures a largely continuous transition between the materials. The wear resistance of the coating according to the invention is provided by the wear protection layer, whereas favorable running-in properties are achieved by the running-in layer. The wear protection layer prevents excessive wear under extreme operating conditions over the entire service life of the engine. The running-in layer does not require remachining, but acquires its final shape through the abrasion that occurs during the running-in phase. Furthermore, a transition layer is present between the wear protection layer and the running-in layer, which results in improved tribological properties during the transition from the running-in layer to the wear protection layer.

[0019] The piston ring comprises a wear protection layer made of Ni, Cr, Mo and / or Cr3C2 on a carrier material and a running-in layer made of AlFeCu; a transition layer is provided on the wear protection layer, specifically between the wear protection layer and the running-in layer.

[0020] It has now been discovered that substrate materials coated in this way, and in particular piston rings, exhibit improved mechanical resilience during the transition from the running-in layer to the wear-resistant layer. Furthermore, the coatings according to the invention exhibit improved tribological properties overall due to the "smooth" material transition. Another advantage is that no stabilization times occur during the manufacturing process, such as thermal spraying of multilayer coatings, and the required material quantities can be reduced. According to the invention, the piston ring comprises a wear-resistant layer with a composition of 52-62 wt% molybdenum, 12-22 wt% chromium, 0.5-2 wt% carbon, and 12-20 wt% nickel, as well as unavoidable impurities. The components Mo, Cr, C, and Ni, as well as unavoidable impurities, add up to 100 wt% of the wear-resistant layer.According to the invention, the running-in layer comprises a composition of 5 - 7 wt% aluminum, 1 - 3 wt% iron, the remainder being 100 wt% copper and unavoidable impurities.

[0021] The piston ring contains a transition layer with 40-60% AlFeCu and 60-40% Ni, Cr, Mo, and / or Cr3C2. Good transition behavior has been observed.

[0022] The piston ring preferably has a transition layer formed by molten powder particles (called splats) that are evenly distributed.

[0023] To achieve a uniform splat structure in the transition layer, powder materials with a particle size of 5–70 µm are preferred. Powder materials with a particle size of 5–70 µm are used to achieve a uniform splat structure of 10–150 µm in the transition layer. The splat size or splat structure refers to the average diameter of individual splats in the plane of, or tangential to, the piston ring's running surface.

[0024] Piston ring according to one of the preceding embodiments, wherein powder materials with a particle size of 5 - 70µm are used to achieve a uniform splat structure of 10 - 150µm in the transition layer (8, 24).

[0025] The area distribution of the transition layer on the surface is approximately 50 - 75% CrCNiC+Mo and 50% - 25% AlCuFe.

[0026] The transition layer is preferably formed by plasma coating. The transition layer is preferably produced by plasma spraying.

[0027] The thickness of the transition layer is preferably between 50 µm and 250 µm, particularly preferably between 100-180 µm.

[0028] The microhardness of the AlCuFe material used is preferably between 150 - 300 HV0.05.

[0029] The microhardness of Mo is preferably between 300 - 720 HV0.05.

[0030] The microhardness of CrC-NiCr is preferably between 300-1200 HV0.05.

[0031] A bonding agent or adhesive layer is preferably formed between the carrier material or substrate and the wear protection layer.

[0032] The piston ring can be manufactured using a process that includes the following steps: (b) applying a wear-resistant layer of Ni, Cr, Mo, and / or Cr3C2 to a substrate; (c) applying a transition layer to the wear-resistant layer; and (d) applying a running-in layer of AlFeCu to the transition layer, wherein the method preferably comprises, before applying the wear protection layer, (a) may comprise applying an adhesive layer to the carrier material.

[0033] The application of the layers (b) to (d) or (a) to (d) is preferably carried out by a process selected from plasma spraying, HVOF coating (high-velocity oxyfuel spraying), arc spraying or a combination of the processes.

[0034] First, an adhesion promoter layer is preferably applied to the carrier material.

[0035] The transition layer is preferably characterized by a certain material proportion of wear protection layer and a certain material proportion of running-in layer.

[0036] The figures illustrate the invention using representations of microsections. Fig. 1 shows a reflected light micrograph of a layer structure according to the invention. Fig. 2A and 2B show reflected light micrographs of a layer structure according to the invention at 100x and 200x magnification, respectively. Figure 3 shows a cross-section of a layer structure without a transition layer. Figure 4 shows a top view of a running surface after removal of a running-in layer. Figure 5 represents a cross-section of a layer structure with a transition layer. Figure 6 shows a top view of a running surface after a running-in process.

[0037] In the following, both in the description and in the figures, the same or similar reference symbols are used to refer to the same or similar elements and components.

[0038] Fig. 1shows a reflected-light micrograph of a layer structure according to the invention. The bottom layer is formed by a wear-resistant layer 6 made of a Mo base material. A transition layer 8 is arranged on top of the wear-resistant layer 6, which in turn is covered by a running-in layer 10.

[0039] Fig. 2 A and 2B show reflected light micrographs of a layer structure according to the invention in 100x ( Fig. 2a ) or 200x ( Fig. 2b ) Enlargement. An adhesive layer 4 is applied to the gray cast iron substrate material 2, which ensures optimal adhesion of the wear protection layer 6 to the gray cast iron substrate 2. A transition layer 8 is applied to the wear protection layer 6, and a running-in layer made of AlCuFe is applied to the transition layer 8. The transition layer 8 is located between the wear protection layer made of a Mo base material and the running-in layer 10 made of AlCuFe.

[0040] The following examples serve to illustrate the invention without limiting it.

[0041] In the present invention, a transition between the wear-resistant layer and the running-in layer is implemented with an additional transition layer. This transition layer consists of a material mixture (volume fraction approximately 40-60:60-40 V%) of running-in layer material (AlFeCu) and wear-resistant layer material (Ni, Cr, Mo, Cr3C2).

[0042] The transition layer is applied between the wear protection layer and the running-in layer. The thickness of the transition layer is between 50 and 250 µm. The transition layer is applied by plasma coating. Through targeted process control, the transition layer is designed to achieve a uniform lamellar distribution of the different material components, see Figure 1. Figures 1 and 2The molten powder particles are deposited in a splat-like pattern and evenly distributed. 90% of all splats have a diameter of 10 µm to 150 µm. The structure of the transition layer is created by a uniform grain size distribution of the two powder sizes. To achieve a uniform splat structure in the transition layer, powder materials with a particle size of 5–70 µm are used.

[0043] The existing structure is characterized by a surface distribution of 50-75% CrCNiC+Mo and 25-50% AlCuFe, preferably 50% CrCNiC+Mo to 50% AlCuFe. The surface distribution should be carried out on microsections parallel or tangential to the transition layer. The splats form flat spots or blobs that are relatively flat compared to their lateral extent. In microsections, the splats result in a two-tone tam spot pattern. The surface distribution of the splats essentially corresponds to the area ratio of the materials of the CrCNiC+Mo wear protection layer and the AlCuFe running-in layer in the thin section.

[0044] The macroscopic hardness (HV1) of the transition layer is between 200 and 400 HV1. The microhardness of the layer components is: AlCuFe: 150 - 300 HV0.05; Mo: 300 - 720 HV0.05 and CrC-NiCr: 300-1200 HV0.05.

[0045] The transition layer is further characterized by a running surface porosity of 2-8%. The uniform, finely distributed structure of the transition layer, consisting of soft (AlCuFe) and hard (Ni, Cr, Mo, Cr3C2) material particles, improves the tribological properties after the layer change, i.e., removal of the pure AlCuFe running-in layer. The transition between the pure running-in layer and the pure wear protection layer is realized more smoothly by the transition layer. Furthermore, the layer thickness of 50µm to 250µm achieves uniform running-in across the ring circumference (at different wear rates) and across the axial ring height. This significantly improves the resistance to burn mark formation and scuffing.

[0046] Figure 3shows a cross-section of a layer structure without a transition layer. The cross-section shows a Mo / CrC / Ni / Cr wear protection layer 22, which is applied to a substrate 20. On this, an AlCuFe running-in layer 26 is applied, forming a transition region 25. The transition region 25 is defined by a roughness of the wear protection layer 22, wherein in the Figure 3 The transition area 25 has a thickness of 10-40µm. The transition area 25 is not created as a separate layer, but rather is formed when the running-in layer 26 is sprayed onto the rough, sprayed surface of the wear-protection layer 22. The particles / splats of the running-in layer 26 fill the surface irregularities of the wear-protection layer, resulting in the transition area 25. In the transition area 25, a single, non-smooth but simply connected contact surface is created between the wear-protection layer 22 and the running-in layer 26.

[0047] Figure 4 shows a top view of a running surface 40 after removal of a running-in layer 26. After removal of the running-in layer 26, the transition region 25 has ALCuFe regions of the running-in layer 26 and Mo / CrC / Ni / Cr regions of the wear protection layer 22 next to each other. The transition region 25 is Figure 4defined by the roughness of the wear protection layer 22, which here is 10-40 µm. The irregular distribution of the surfaces or regions of the running-in layer 26 and the wear protection layer 22 are clearly visible in the plan view in the transition region 25. The splats of the wear protection layer 22 in the transition region 25 only lie on other splats of the wear protection layer 22. A first layer of splats of the material of the running-in layer 26 covers the wear protection layer 22, and all further splats form a pure and essentially homogeneous running-in layer. In the transition region, no splat of the wear protection layer 22 lies over a splat of the material of the running-in layer 26. As the running-in layer is removed, larger contiguous regions of the material of the wear protection layer 22 appear in the region of the transition region 25 alongside larger contiguous regions of the material of the running-in layer 26.

[0048] Depending on the crowning of the tread profile, the relatively softer running-in layer 26 is worn away at different rates and to different degrees across the axial ring height. A sudden increase in hard wear layer material increases the tendency / likelihood of thermal overload. Thermal overload can lead to cracking in the layer, which can cause failure and detachment of the wear protection layer.

[0049] Figure 5 shows a cross-section of another layer structure. A Mo / CrC / Ni / Cr wear-resistant layer 22 is applied to a substrate 20. A Mo / CrC / Ni / Cr + AlFeCu transition layer 24 is applied to the wear-resistant layer 22 in a mixing ratio of 50:50 wt.%, which has a layer thickness of 140-160 µm. An AlFeCu run-in layer 26 is arranged above the transition layer 24.

[0050] Figure 6shows a plan view of a running surface after a running-in process, in which the transition layer 24 of the Figure 5 is exposed. In the Figure 6the AlFeCu running-in layer 26 has been completely removed or has worn through. The transition layer 24 is present over the entire axial height of the piston ring. There are evenly distributed soft AlFeCu particles (40) or "splats" and hard MoCrCNiCr (42) particles or "splats". The splats each have a size distribution in the range of 50 to 100 µm. Compared to the conventional combination of a running-in layer on a wear protection layer, the result is a better, longer-lasting (defined by the layer thickness of the transition layer 24) and more uniform running-in process, and an improvement in friction is achieved after the running-in layer has passed through. In the area of ​​the transition layer 24, splats of the material of the wear protection layer 22 are located next to / underneath and above splats of the material of the running-in layer 26, since this transition layer 24 is sprayed in a spraying process in which both materials are used.Due to the transition layer 24, there is no clear interface between the running-in layer and the wear protection layer 22, and the micrograph appears small and disjointed. List of reference symbols

[0051] 2Substrate grey cast iron 4Adhesion layer 6Wear protection layer made of a Mo base material 8Transition layer 10Running layer 20Substrate or piston ring material 22Wear protection layer 24Transition layer 25Transition area 26Running layer made of AlFeCu 40AlFeCu particles or splats 42Mo / CrC / Ni / Cr particles or splats

Claims

1. A piston ring comprising: an anti-wear layer (6, 22), which is present on a substrate, made of Ni, Cr, Mo and / or Cr3C2; a running-in layer (10, 26) made of AlFeCu; characterized in that a transition layer (8, 24) is provided on the anti-wear layer (6, 22), namely between anti-wear layer (6, 22) and running-in layer (10, 26), wherein the anti-wear layer (6, 22) has a composition of Mo:52 - 62 % by weightCr:12 - 22 % by weightC:0.5 - 2 % by weightNi:12 - 20 % by weight, wherein the components Mo, Cr, C, and Ni as well as unavoidable contaminations add up to 100 % by weight in the anti-wear layer, wherein the running-in layer comprises a composition of Al:5 - 7 % by weightFe:1 - 3 % by weight Cu and unavoidable contaminations: remainder at 100 % by weight, and wherein the transition layer (8, 24) has 40 - 60 % of AlFeCu and 60 - 40 % of Ni, Cr, Mo and / or Cr3C2.

2. The piston ring according to claim 1, wherein the transition layer is formed by molten powder particles, which are separated in a splat-shaped manner and are distributed evenly.

3. The piston ring according to one of the preceding claims, wherein powder materials with a particle size of 5 - 70 µm are used to attain an even structure of molten powder particles with an average diameter in the plane of and tangentially to the running surface of the piston ring, respectively, of 10 µm - 150 µm in the transition layer (8, 24).

4. The piston ring according to one of the preceding claims, wherein the surface distribution of the transition layer (8, 24) on the surface is approximately 50 - 75 % of CrCNiC + Mo and 50 % - 25 % of AlCuFe.

5. The piston ring according to one of the preceding claims, wherein the thickness of the transition layer is between 50 µm and 250 µm.

6. The piston ring according to claim 5, wherein the thickness of the transition layer is between 100 µm and 180 µm.

7. The piston ring according to one of the preceding claims, wherein the micro hardness of the AlCuFe material is between 150 - 300 HV0.05.

8. The piston ring according to one of the preceding claims, wherein the micro hardness of the Mo material is between 300 - 720 HV0.05.

9. The piston ring according to one of the preceding claims, wherein the micro hardness of the CrC-NiCr material is between 300-1200 HV0.05.

10. The piston ring according to one of the preceding claims, wherein an adhesive layer is present between the substrate and the anti-wear layer.