Sliding element

Abstract

A sliding element, for example a piston ring for an internal combustion engine, may include a sliding face, an adhesive layer disposed on the sliding face, and a coating disposed over the adhesive layer. The coating may be composed of an amorphous carbon material. The coating may have a ratio between sp.sup.3 bonds and sp.sup.2 bonds that indicates a predominance of sp.sup.2 bonds. The coating may have a roughness profile including a value of Rpk of 0.15 m and a value of Rz of 0.7Rz1.5 m.

Claims

1. A sliding element comprising: a sliding face; an adhesive layer disposed on the sliding face; and a coating of amorphous carbon disposed over the adhesive layer; wherein the coating has a ratio between sp.sup.3 and sp.sup.2 bonds that indicates a predominance of sp.sup.2 bonds and a roughness profile including a Rmr(0.3/0.5) of 65% or greater and at least one of: (i) a value of Rpk of 0.15 m, and (ii) a value of Rz of 0.7 Rz 1.5 m; and wherein a contact surface of the coating has a peak and valley structure defining a plurality of plateaus facilitating contact between the sliding surface and a peripheral surface.

2. The sliding element according to claim 1, wherein the coating contains less than 2% by weight of hydrogen.

3. The sliding element according to claim 1, wherein the coating of amorphous carbon is of an a-C type and the ratio between sp.sup.3 and sp.sup.2 bonds includes a value of sp.sup.3 bonds of between approximately 25% and 45%.

4. The sliding element according to claim 1, wherein the roughness profile of the coating has the value of Rpk of 0.15 m and the value of Rz of 0.7 Rz 1.5 m.

5. The sliding element according to claim 1, wherein the coating has a thickness that exceeds 8 micrometers.

6. The sliding element according to claim 1, wherein the coating has a thickness between 8 micrometers and 15 micrometers.

7. The sliding element according to claim 1, wherein the adhesive layer includes at least one of a chromium metal having a body centred cubic structure, a nickel material and a cobalt material.

8. The sliding element according to claim 1, wherein the coating has a hardness ranging between 20 and 40 GPa.

9. The sliding element according to claim 1, wherein the sliding surface includes a base stainless steel material having approximately 10% to 17% chromium, and a remainder of cast iron and carbon.

10. The sliding element according to claim 1, wherein the adhesive layer includes a chromium metal having a body centred cubic polycrystalline columnar structure.

11. The sliding element according to claim 1, wherein the coating has a thickness between 5 micrometers and 8 micrometers.

12. The sliding element according to claim 1, wherein the coating is a plasma assisted chemical vapour deposition coating.

13. The sliding element according to claim 1, further comprising a plurality of plateau peaks projecting from the plurality of plateaus and defining a plurality of plateau valleys.

14. A sliding element for an internal combustion engine, comprising: a sliding face; a metal adhesive layer disposed on the sliding face; and a coating of a hydrogen-free amorphous carbon material disposed over the adhesive layer; wherein the coating has a ratio of sp.sup.a to sp.sup.2 bonds that ranges between 0.25 and 0.45, and a roughness profile including a Rmr(0.3/0.5) of 65% or greater, a value of Rpk of 0.15 m or less, and a value of Rz ranging from 0.7 to 1.5 m; and wherein a contact surface of the coating has a peak and valley structure defining a plurality of plateaus facilitating contact between the sliding surface and a peripheral surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will, hereinafter, be described in greater detail on the basis of 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 cross-section of the piston ring revealing the structure of the coating of the present invention;

(4) FIG. 3: a drawing representing the points of contact of a cylinder liner and the peaks of a coated piston ring of the state of the art;

(5) FIG. 4: a drawing representing the points of contact of a cylinder liner and the peaks of a coated piston ring of the present invention;

(6) FIG. 5: a graph exemplifying the results from a surface having peaks and valleys of the same magnitude;

(7) FIG. 6: a graph exemplifying the results from a surface having low peaks and deep valleys;

(8) FIG. 7: a graphic representing the results of friction for DLC as a function of the Rpk and Rz values between the state of the art and the present invention;

(9) FIG. 8: a graphic representing the wear of the DLC rings and of the cylinder liner for the present invention and the state of the art.

DETAILED DESCRIPTION OF THE DRAWINGS

(10) The present invention relates to a sliding element, in particular a piston ring 10 for internal combustion engines comprising a coating 15 of hard amorphous carbon of the a-C type presenting a roughness profile having Rpk values 0.15 m and 0.7Rz1.5 m.

(11) As will be seen hereinbelow, the roughness profile satisfying the values of Rpk and Rz as described above, when utilised in conjunction with a coating of amorphous carbon such as that of the present invention, generates a performance superior to the sliding elements of the state of the art.

(12) At the outset, it should be stated 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. Usually these piston rings 10 work with very small clearances and a thin film of oil to ensure excellent performance and low emissions of CO.sub.2.

(13) As principal characteristics, the piston ring 10 of the present invention has as principal characteristics the low friction of a coating 15 based upon a carbon structure having high durability, the result whereof derives from a specially developed topography in the surface of the coating.

(14) The ring 10 of the present invention, comprising a metal base provided with a sliding face 11, is preferably of cast iron, steel, stainless steel containing from 10% to 17% of chromium or carbon steel. The sliding face 11 is that orientated towards the region of contact with a cylinder liner 20 and it is upon the sliding face 11 that the coating 15 is deposited. In a preferential configuration, the sliding face 11 receives, from inside outwards, an adhesive layer 14 and a coating 15 of hard amorphous carbon of a-C type (see FIGS. 1 and 2).

(15) The adhesive layer 14 has as objective the promotion of the accommodation of the stresses between the metal structure of the ring 10 and the coating 15 of amorphous carbon, the internal stresses whereof are very high, consequently ensuring an excellent adhesion between the functional coating 15 and the metal base. In a preferential manner, however not obligatory, the adhesive layer 14 is formed by a body centered cubic (bcc) polycrystalline columnar structure of chromium, the thickness whereof lies between 0.5 and 1 micrometer. In an alternative preferential configuration, the adhesive layer may be of nickel or cobalt. Furthermore, the adhesive layer 14 is deposited by a vapor deposition process from a metal source.

(16) In turn, the anti-friction layer, shown in FIG. 2 as coating 15 and hereinafter so designated, is composed of totally amorphous carbon free or substantially free of hydrogen (DLC) having a predominant quantity of sp.sup.2. With respect to the absence of hydrogen, it is important to note that the quantity thereof by weight is less than 2%.

(17) In respect of the ratio of sp.sup.3 to sp.sup.2 it should be noted that, in a preferential configuration, the ratio between sp.sup.3 and sp.sup.2 comprises sp.sup.3 bonds present at between 25% and 40%, characterising the coating 15 of a-C. In other words, in this alternative preferential configuration the ratio of proportions between sp.sup.3 and sp.sup.2 ranges between 0.25 and 0.45, wherein, in a second alternative preferential configuration, this ratio lies between 0.35 and 0.85. Additionally, the total thickness of the film of the present invention lies between 8 and 15 microns. In an alternative preferential configuration the thickness of the coating 15 ranges between 5000 and 30 000 nanometers.

(18) In respect of the hardness, the coating 15 of amorphous carbon substantially free of hydrogen comprises a hardness lying between 20 and 40 GPa.

(19) It should furthermore be noted that the process of deposition of the coating 15 occurs by a plasma assisted chemical vapour deposition (PACVD) process.

(20) In a manner differing from the automotive solutions of the state of the art, the present invention utilizes a specially developed topography in the contact surface of the coating 15 capable of being adapted to the contact surface 21 of the cylinder liner 20. The development of the present invention has permitted it to be understood that in many situations of tribological contact of sliding and abrasion, the coatings having high roughness yield inferior results.

(21) In this manner, the present invention has as object a piston ring 10 provided with a coating 15 comprising a hardness in the band from 2000 to 4000 HV (Vickers hardness), avoiding however high wear of the cylinder liner 20 the hardness whereof is considerably lower, generally less than 700 HV (Vickers hardness), or even 250 to 300 HV.

(22) In order to achieve the objects of the present invention, the coating 15 of the piston ring 10 comprises a surface having a controlled quantity of valleys with a view to maximizing the supply of oil during the operation of the starting of the engine, also increasing the oil film and reducing the points of solid contact, yielding a lower coefficient of friction in the tribological system (piston ring/cylinder liner).

(23) Contributing greatly to the excellent results of the present invention, the surface of the coating 15 of the present invention comprises a topography the contact whereof with the contact surface 21 of the said cylinder liner 20 occurs principally through plateaus 18.

(24) The standard ISO 4287 defines the roughness parameters Rpk and Rz, representing the average height of the peaks and the average distance from the highest peaks to the deepest valleys. For a better understanding, in terms of an example, a comparison of FIGS. 3 and 5 with FIGS. 4 and 6 permits it to be perceived that, when a given topography comprises many peaks of the same magnitude 16 (see FIGS. 3 and 5), the empty spaces for entry of oil are reduced, rendering difficult the presence of oil in the interface of the piston ring and of the liner. That is to say, rendering difficult the formation of a lubricating film.

(25) In turn, in a topography comprising low peaks and deep valleys the empty spaces are larger and the oil has the possibility of entering into these voids, facilitating the formation of the lubricating film. The formation of this film will prevent the contact between the piston ring and the liner. In other words, the graphs of FIGS. 5 and 6 reveal the surface condition present in this invention. In this type of surface, the percentage of oil between the surfaces is greater, providing favourable conditions for the lubrication, preventing the contact between the sliding element and the liner, reducing both fiction and wear. In this manner, FIGS. 5 and 6 permit the difference between a surface having few and having more oil reservoirs to be comprehended. The present invention makes use of a contact surface very similar to that found in the situation of FIG. 6, that is to say a surface formed substantially by small peaks and large valleys 18, ensuring good support in the region of contact between the piston ring 10 and the cylinder liner 20 and facility in forming the lubricating film.

(26) In this manner, the values of Rpk and Rz are of immense importance to be enabled to characterize the reduced peaks 18 and the presence of deep valleys of the present invention, by virtue of the fact that such values permit the comprehension of the specific topography permitting the obtainment of a solution having low friction, high durability, and reduced wear.

(27) In rings of the state of the art, the difficulty in forming the lubricating film 16 causes the scoring and critical wear of the wall 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 lubricating oil when there is a poor presence of oil between the coating of amorphous carbon free of hydrogen (DLC) and the liner.

(28) The solution of the present invention may be described as being similar to that found in FIG. 4. In this manner, to resolve the problems of the state of the art a surface has been developed having the objective of ensuring good support in the contact between the piston ring 10 and the cylinder liner 20 by virtue of the fact of rendering viable the formation of the lubricating film. For the same load, this surface generates little contact when compared with a topography having few oil reservoirs. In this manner, the finish of the piston ring 10 is of immense importance for the realization of the contact between the ring 10 and the liner 20 in order to contribute to the good operation of the engine. The good compatibility between the piston ring 10 and the cylinder liner 20 is obtained by a process of removal of the peaks 16 of the coating 15, this process occurring subsequent to the deposition of the coating 15, without totally eliminating the valleys.

(29) In this respect the present invention has as characteristic of the topography of the coating 15 of the piston ring 10 the fact that the value of Rpk0.15 m and 0.7Rz1.5 m. In an alternative preferential mode, the value of Rmr (0.3/0.5) is equal to or exceeds 65%.

(30) The laboratory results obtained during the development of the sliding element of the present invention clearly demonstrate the advantages thereof. In this respect, FIG. 7 shows the performance, in terms of friction and wear of the piston ring, as a function of the parameters Rpk and Rz, both for rings of the state of the art and for rings of the present invention. It should be noted that for the invention the tests were carried out with values of Rpk and Rz within the specification.

(31) The results of FIG. 7 show that the coatings of amorphous carbon substantially free of hydrogen and the coating of chromium nitride present different levels of Rpk and Rz. The coefficient of friction of the carbon coatings presents a reduction when compared with the Rpk and Rz of the state of the art. In turn, the values furthest to the right in the table of FIG. 7 reveal that the coating of the present invention of DLC and the coating of CrN present substantially the same Rmr values. Furthermore, the coating of DLC of the present invention presents a coefficient of friction substantially 25% lower than the coating of CrN.

(32) FIG. 8 furthermore shows the performance in terms of the wear of the coating of DLC of the present invention compared with a coating of DLC of the state of the art. The results demonstrate the advantage of the solution of the present invention by virtue of the revelation thereby that the presence of oil reservoirs of the present invention brings about less wear on the piston ring 10 (columns above zero) and less wear on the cylinder liner 20 (columns below zero). In other words, for amorphous carbon coatings, when the Rmr values attain the topographical profile of the present invention the wear is much lower both of the piston ring 10 and of the cylinder liner 20.

(33) Consequently, no doubts remain that the present invention, on combining the use of coatings 15 of amorphous carbon substantially free of hydrogen having a topology presenting oil reservoirs facilitating the formation of a lubricating film preventing the contact with the liner, the coefficient of friction is greatly reduced and the wear, both of the ring and of the cylinder, is also reduced, ensuring the correct behaviour at any stage of operation of the engine and ensuring a long life for the engines.

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