Sliding element for an internal combustion engine

Abstract

A sliding element for an internal combustion engine may include a base material having an annular external surface. The external surface may include a bonding layer, a first layer of coating, and a second layer of coating sequentially disposed thereon. The first layer of coating and the second layer of coating may include hard amorphous carbon (DLC) of a combined matrix having a plurality of sp3/sp2 bonds. The first layer of coating may include 45% sp3 bonds or less and may have a thickness of at least 10 micrometers. The second layer of coating may include at least 55% sp3 bonds and may have a thickness of at least 3 micrometers.

Claims

1. A sliding element for an internal combustion engine comprising a base material having an annular external surface including a bonding layer, a first layer of coating and a second layer of coating sequentially disposed thereon, the first layer of coating and the second layer of coating including hard amorphous carbon (DLC) of a combined matrix having a plurality of sp3/sp2 bonds, wherein: the first layer of coating includes 45% sp3 bonds or less and has a thickness of at least 10 micrometers; and the second layer of coating includes at least 55% sp3 bonds and has a thickness of at least 3 micrometers.

2. The sliding element as claimed in claim 1, wherein a thickness of a total layer of coating defined by a combination of the first layer of coating and the second layer of coating is at least 15 micrometers.

3. The sliding element as claimed in claim 1, wherein the second layer of coating has a roughness profile having at least one of a Rmr(0.3-0.5) equal to or exceeding 50% and a Rmr(0.4-0.5) equal to or exceeding 70%.

4. The sliding element as claimed in claim 1, wherein the second layer of coating has a roughness profile having a Rpk of less than 0.07 micrometers.

5. The sliding element as claimed in claim 1, wherein the second layer of coating has a roughness profile having a Rz of less than 1 micrometer.

6. The sliding element as claimed in claim 1, wherein the second layer of coating has a hardness of 25 GPa to 50 GPa.

7. The sliding element as claimed in claim 1, wherein the bonding layer includes at least one of metallic chromium, nickel, cobalt, tungsten, and chromium carbide.

8. The sliding element as claimed in claim 1, wherein the bonding layer is deposited via physical vapor deposition (PVD).

9. The sliding element as claimed in claim 1, wherein the base material is composed of a ferrous metal.

10. The sliding element as claimed in claim 1, wherein the sliding element is structured as a piston ring.

11. An internal combustion engine comprising at least one sliding element including a base material having an annular external surface, the external surface including a bonding layer, a first layer of coating, and a second layer of coating sequentially disposed thereon, the first layer of coating and the second layer of coating including hard amorphous carbon (DLC) of a combined matrix having a plurality of sp3/sp2 bonds, wherein: the first layer of coating includes 45% sp3 bonds or less and has a thickness of at least 10 micrometers; and the second layer of coating includes at least 55% sp3 bonds and has a thickness of at least 3 micrometers.

12. The internal combustion engine as claimed in claim 11, wherein the second layer of coating has a roughness profile having at least one of a Rmr(0.3-0.5) equal to or exceeding 50% and a Rmr(0.4-0.5) equal to or exceeding 70%.

13. The internal combustion engine as claimed in claim 11, wherein the second layer of coating has a roughness profile having a Rpk of less than 0.07 micrometers and a Rz of less than 1 micrometer.

14. The internal combustion engine as claimed in claim 11, wherein the second layer of coating has a hardness of 25 GPa to 50 GPa.

15. A sliding element for an internal combustion engine comprising a base material having an annular external surface including a bonding layer, a first layer of coating, and a second layer of coating sequentially disposed thereon, the first layer of coating and the second layer of coating including hard amorphous carbon (DLC) of a combined matrix having a plurality of sp3/sp2 bonds, wherein: the first layer of coating includes 45% sp3 bonds or less and has a thickness of at least 10 micrometers; the second layer of coating includes at least 55% sp3 bonds and has a thickness of at least 3 micrometers; and the second layer of coating has a roughness profile having a Rmr(0.3-0.5) equal to or exceeding 50% and a Rmr(0.4-0.5) equal to or exceeding 70%.

16. The sliding element as claimed in claim 15, wherein the roughness profile of the second layer of coating has a Rpk of less than 0.07 micrometers.

17. The sliding element as claimed in claim 15, wherein the roughness profile of the second layer of coating has a Rz of less than 1 micrometer.

18. The sliding element as claimed in claim 1, wherein the second layer of coating has a roughness profile having an Rmr(0.3-0.5) equal to or exceeding 50%.

19. The sliding element as claimed in claim 1, wherein the second layer of coating has a roughness profile having an Rmr(0.4-0.5) equal to or exceeding 70%.

20. The sliding element as claimed in claim 1, wherein the second layer of coating has a topography including a plurality of peaks and a plurality of valleys, the plurality of peaks having a height of less than 0.07 micrometers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention shall be described hereinbelow in greater detail, based upon an example of embodiment represented in the drawings. The figures show:

(2) FIG. 1: a representation of the disposition of a piston ring within the interior of a cylinder of an internal combustion engine;

(3) FIG. 2: a representation of a cross-section of the piston ring, revealing the structure of the coating of the present invention;

(4) FIG. 3: a graphic comparing the values of Rmr obtained in the state of the art and in the present invention;

(5) FIG. 4: a graphic comparing the values of Rmr obtained in the state of the art and in the present invention;

(6) FIG. 5: a graphic comparing the values of Rz and Rpk obtained in the state of the art and in the present invention;

(7) FIG. 6: a graphic comparing the values of Rz and Rpk obtained in the state of the art and in the present invention;

(8) FIG. 7: photographs comparing the surfaces obtained in the state of the art and in the present invention; and

(9) FIG. 8: graphs comparing the roughness profiles obtained in the state of the art and in the present invention.

DETAILED DESCRIPTION

(10) The present invention relates to a sliding element, such as a piston ring 10 for internal combustion engines, provided with a coating comprising two layers 14, 15 of hard amorphous carbon and having a roughness profile capable of bringing about a good running in condition and better contact between two sliding surfaces, in addition to reducing the friction and the wear, ensuring high hardness.

(11) The sliding element of the present invention may be an engine component maintaining contact with a thin layer of lubricant fluid, experiencing wear, such as, for example and inter alia, a cylinder liner, an oil scraper piston ring, a first groove piston ring.

(12) Firstly, it must be noted that the sliding element of the present invention is preferentially a piston ring 10 for internal combustion engines operating under high load and/or power, as illustrated in FIG. 1. These piston rings usually operate with very fine tolerances and a thin film of oil to ensure excellent performance and low emissions of CO.sub.2.

(13) The piston ring 10 of the present invention comprises a base material 12 provided with an annular external surface 11, being composed, preferentially, by a ferrous metal, for example a stainless steel having 10% to 17% of chromium or a carbon steel. The annular external surface 11 corresponds to the portion of the ring parallel to and facing the region of contact with a cylinder liner 20, in other words it is the surface acting as interface between the ring and a cylinder liner 20, it being upon the external surface 11 that the coating is deposited.

(14) In a preferential configuration, illustrated in FIG. 2, the external surface 11 receives, sequentially, a bonding layer 13 and, upon the bonding layer 13, a first layer of coating 14 and a second layer of coating 15, the first and second layers of coating 14, 15 being composed by hard amorphous carbon of the a-C/ta-C type.

(15) The bonding layer 13 has the objective of bringing about the relieving of the stresses between the metal structure of the ring and the coating of hard amorphous carbon, in this manner ensuring an excellent adhesion between the coating and the base material 12. In a preferential manner, not however obligatory, the bonding layer 13 is formed by metallic chromium (Cr). In an alternative preferential configuration, the bonding layer 13 is composed by one from among the materials such as nickel (Ni), cobalt (Co), tungsten (W), chromium carbide (CrC) or by a ceramic material. The bonding layer 13 comprises a thickness of less than 3 micrometers, preferentially between 1 micrometer and 3 micrometers, and is deposited by a physical vapor deposition (PVD) process.

(16) In turn, the first and second layers of coating 14, 15 are formed of hard amorphous carbon (DLC), the carbon being totally amorphous carbon free, or substantially free, of hydrogen, comprising less than 2% by weight of hydrogen.

(17) In a manner differing from the solutions found in the state of the art, the present invention presents a coating of hard amorphous carbon comprising a dual layer, that is to say two deposited layers, each layer comprising matrices of sp.sup.2/sp.sup.3 bonds and differing thicknesses, in addition to utilizing a specially developed topography upon the surface of contact of the coating capable of presenting good compatibility with the contact surface of the cylinder liner 20.

(18) In order to achieve the objects of the present invention, the first layer of coating 14, deposited upon the bonding layer 13, is composed by hard amorphous carbon, comprising up to 45% of sp.sup.3 bonds and having a thickness equal to or exceeding 10 micrometers. In this preferential configuration of the first layer 14, the low percentage of sp.sup.3 bonds, equal to or less than 45%, has as objective the optimization of the rate of deposition of the coating of the first layer, in addition to relieving and reducing internal stresses.

(19) Furthermore, upon the first layer of coating 14 there is deposited a second layer of coating 15 composed by hard amorphous carbon comprising at least 55% sp.sup.3 bonds and having a thickness equal to or exceeding 3 micrometers. Differing from the first layer of coating 14, the second layer of coating 15 comprises a high percentage of sp.sup.3 bonds, equal to or exceeding 55%, and has the objective of ensuring an excellent finish of the coating, in addition to high hardness.

(20) The thickness of the total layer of coating, the first layer of coating 14 being added to the second layer of coating 15, is at least 13 micrometers, preferentially 15 micrometers, wherewith a better correlation is achieved between rate of deposition, internal stress, finish, hardness and thickness of coating. In terms of hardness, the second layer of coating 15 is of between 25 GPa and 50 GPa and utilizes a specially developed finish upon the surface of contact of the coating capable of preventing high wear and reducing the probability of scoring the cylinder liner, the hardness whereof is considerably lower. Both the hardness and the optimized finish are observed upon the second layer of coating 15, by virtue of the fact that this represents the most external layer of the coating, entering into direct contact with the contact surface of the cylinder liner 20.

(21) The process of deposition of the coating, comprising the first layer of coating 14 and the second layer of coating 15, occurs through a process of vapor deposition (PVD).

(22) Contributing greatly to the excellent results of the present invention, the surface of the second layer of coating 15 comprises a topography the contact whereof with the contact surface of the cylinder liner 20 occurs, principally, by means of peaks of less than 0.1 micrometers, presenting high support and better contact between the sliding surfaces.

(23) The standard ISO 4287 defines the parameters of roughness for a proportion of material of a surface (Rmr), reflecting the percentage of material at a given depth measured from a reference line, and represents how much contact exists between two surfaces. For a better understanding, in terms of an example for a topography comprising many peaks the Rmr tends to be lower, in detriment to the support and contact between the surfaces. On the other hand, a topography comprising low peaks, for example less than 0.1 micrometers, tends to present a high Rmr, indicating that the proportion of material in the surface is greater. In this manner, the values of Rmr are of great importance for characterizing the roughness profile of a surface, by virtue of the fact that such values permit the comprehension of the specific topography and obtainment of a solution having low friction and wear and high durability.

(24) Consequently, in order to improve the support and the contact between the sliding surfaces it is necessary to obtain a high Rmr, whether through the reduction in the number of peaks or the reduction of the magnitude/height of these peaks. Rpk is the roughness parameter indicating the quantity and the height of the peaks of a surface.

(25) In addition to the support and the contact between the surfaces, the presence of peaks brings about scoring and the critical wear upon the surface of contact of the cylinder liner 20 by virtue of the high contact pressure. Furthermore, this local high pressure renders difficult the formation and the maintenance of the film of lubricant oil even though there is oil present in the valleys of the second layer of coating 15 of hydrogen free amorphous carbon (DLC).

(26) Consequently, in order to resolve the problems of the state of the art, a surface has been developed possessing peaks of low magnitude/height, of less than 0.1 micrometers, having the objective of ensuring good support in the contact between the piston ring 10 and the cylinder liner 20. For the same load, this surface generates a low contact pressure when compared with a topography containing high peaks. In this manner the finish of the piston ring 10 is of great importance for the realization of the contact between the ring 10 and the liner 20, in this way contributing to good operation of the engine. The good compatibility between the piston ring 10 and the cylinder liner 20 is obtained through processes of smoothing and lapping to reduce the height of the peaks upon the second layer of coating 15, which process occurs subsequent to the deposition of the coating.

(27) In this respect the coating of the present invention presents the second layer of coating 15 having an Rpk of less than 0.07 micrometers, that is to say peaks having a magnitude/height of less than 0.07 micrometers.

(28) Consequently, the present invention has as characteristic of the topography of the second layer of coating 15 the presentation of an Rmr(0.3-0.5) equal to or exceeding 50% and/or an Rmr(0.4-0.5) equal to or exceeding 70%.

(29) In addition to the Rpk and Rmr, the second layer of coating 15 presents an Rz of less than 1 micrometer.

(30) The laboratory results obtained during the development of the sliding element of the present invention are presented in FIGS. 3 to 8 and clearly show the differences and advantages of the present invention in relation to the state of the art.

(31) FIG. 3 shows the values of Rmr obtained for rings of the state of the art and of the present invention. In this respect, rings were tested provided with a coating of hard amorphous carbon having a thickness equal to or exceeding 10 micrometers and up to 40% of sp.sup.3 bonds, as revealed by the document of the state of the art U.S. Pat. No. 9,086,148, referred to hereinbefore.

(32) The results of FIG. 3 show that the coating of hard amorphous carbon revealed by the present invention presents values of Rmr exceeding the values achieved in the state of the art. The rings of the state of the art present a roughness profile having an Rmr(0.3-0.5) of between 30% and 40% and an Rmr(0.4-0.5) of between 50% and 60%, whereas the rings of the present invention achieve values of Rmr(0.3-0.5) of between 80% and 90% and of Rmr(0.4-0.5) exceeding 90%.

(33) In the same manner, FIG. 4 shows results from a second test wherein the rings of the state of the art present an Rmr(0.3-0.5) of 35% and an Rmr(0.4-0.5) of 61%, whilst the rings of the present invention attain an Rmr(0.3-0.5) of 78% and an Rmr(0.4-0.5) of 89%.

(34) FIG. 5 shows the values obtained for the magnitude/height of peaks and valleys upon rings of the state of the art and of the present invention. It may be observed that the rings of the state of the art comprise values of Rz exceeding 0.8 micrometers and an Rpk of the order of 0.1 micrometers, whilst the rings of the present invention present low values of Rz and Rpk, having an Rz of between 0.6 and 0.8 micrometers and an Rpk of less than 0.1 micrometers.

(35) FIG. 6 shows the values of Rz and Rpk obtained in the present invention in comparison with the values revealed in the state of the art, represented by the document U.S. Pat. No. 8,123,227, referred to hereinbefore. The lighter points of the graphic of the present invention represent individual values and the darker point is the average of the values measured. It may be observed that it is not possible to compare values of Rmr between the present invention and the document U.S. Pat. No. 8,123,227 by virtue of the fact that that state of the art reveals a coating of hard amorphous carbon of a thickness of up to 2 micrometers, much less than the minimum thickness of 10 micrometers of the coating of the present invention. The values of Rmr are naturally greater for thin coatings, such that the greatest challenge lies in obtaining a thick coating having a high value of Rmr, this objective being achieved by the present invention.

(36) Finally, FIGS. 7 and 8 show, respectively, the surface aspect and the roughness profile of the coating of the state of the art and of the present invention. The coating of the present invention may be observed in FIG. 7, comprising a more homogenous surface having less obvious/high peaks. On the other hand, FIG. 8 clearly shows the differences between the roughness profile of the coating of the state of the art, having deep valleys and high peaks, and the roughness profile of the present invention, having shallower valleys and lower peaks.

(37) In summary, the sliding element of the present invention is provided with a coating of hard amorphous carbon offering the following advantages:

(38) i) a coating presenting high hardness, of between 25 GPa and 50 GPa, having a thickness exceeding 10 micrometers, bringing about a reduction in the friction and wear, ensuring high durability;

(39) ii) a good rate of deposition of the coating in combination with excellent finish, by virtue of the deposition of two layers of coating having differing percentages of sp.sup.3 bonds and thicknesses, ensuring an Rz of less than 1 micrometer;

(40) iii) good support between the contact surface of the sliding element and the contact surface of the liner 20, having high values of Rmr, Rmr(0.3-0.5) being equal to or exceeding 50% and/or Rmr(0.4-0.5) being equal to or exceeding 70%;

(41) iv) a reduction in the running in period of the sliding element and cylinder liner 20 assembly, by virtue of the lower magnitude/height of the peaks, and a lower probability of the occurrence of scoring upon the surface of contact of the liner 20 during the running in, having a value of Rpk of less than 0.07 micrometers.

(42) In this manner no doubt remains that the coating of substantially hydrogen free hard amorphous carbon of the present invention is capable of reconciling a good rate of deposition linked to an excellent finish, high hardness in combination with relief and reduction of internal stresses, thickness exceeding 10 micrometers in combination with a high Rmr, an Rz of less than 1 micrometer and an Rpk of less than 0.07 micrometers, and of bringing about better support and contact between two sliding surfaces, having good running in conditions, reducing the friction, the wear and the occurrence of scoring upon the sliding surfaces, ensuring excellent durability.

(43) An example of preferred embodiment having been described, it shall be understood that the scope of the present invention comprises other possible variations, being limited solely by the content of the appended claims, the possible equivalents being included therein.