Sliding component for use in an internal combustion engine

Abstract

A sliding component for an internal combustion engine may include a ferrous base having a peripheral sliding surface covered with a protective layer including at least one of a nitride applied via physical vapor deposition and a nitrided layer. The base may include a coating of carbon of a diamond-like carbon type. The coating may include at least one of (i) a transition layer including a composition of WC.sub.1-X and (ii) a layer of metallic chromium of crystalline structure disposed between the ferrous base and an outer layer of amorphous carbon. The coating may also include an intermediate layer of a nanocrystalline phase of carbides in a multilayer structure having a first sub-layer including a composition different than a second sub-layer disposed under the outer layer.

Claims

1. A sliding component for an internal combustion engine, comprising: a ferrous base having a peripheral sliding surface covered with a protective surface layer including at least one of a nitride applied via physical vapor deposition and a nitrided layer, the base including a coating of carbon of a diamond-like carbon type, wherein the coating comprises an outer layer of amorphous carbon, at least one of (i) a transition layer including a composition of WC.sub.1-x and (ii) a layer of metallic chromium of crystalline structure disposed between the ferrous base and the outer layer, and an intermediate layer of a nanocrystalline phase of carbides in a multilayer structure having a first sub-layer including a composition different than a second sub-layer disposed under the outer layer; wherein at least the outer layer of amorphous carbon has a ratio of total intensity in a Raman spectrum of band D to band G of 0.2 to 1.0.

2. The sliding component according to claim 1, wherein the outer layer of carbon includes hydrogen.

3. The sliding component according to claim 1, wherein the transition layer includes nanocrystalline precipitates of tungsten carbide.

4. The sliding component according to claim 1, wherein at least one of the transition layer, the layer of metallic chromium, the intermediate layer and the outer layer is completely amorphous.

5. The sliding component according to claim 1, wherein the transition layer includes a thickness between 100 nm and 500 nm.

6. The sliding component according to claim 1, wherein the coating has an aggregate thickness of 1 m to 5 m.

7. The sliding component according to claim 1, wherein the hardness of the coating is between 1500 HV to 2000 HV.

8. The sliding component according to claim 1, wherein the first sub-layer of the intermediate layer includes a composition of a-C:H:W and the second sub-layer includes a composition of a-C:H.

9. The sliding component according to claim 1, wherein the ratio of total intensity in the Raman spectrum of band D to band G has values between 0.55 to 0.65.

10. A sliding component of an internal combustion engine, comprising: a ferrous base having a sliding surface covered with a protective layer including at least one nitride, the sliding surface having a coating of a diamond-like carbon, the coating including: an outer layer of amorphous carbon; an intermediate layer of a nanocrystalline phase of carbides in a multilayer structure having a first sub-layer having a composition of a-C:H:W and a second sub-layer having a composition of a-C:H; and at least one of (i) a transition layer including a composition of WC.sub.1-x and (ii) a layer of metallic chromium of crystalline structure disposed on the protective layer; wherein the coating has a ratio of total intensity value in Raman spectrum of band D to band G between 0.2 and 1.0.

11. The sliding component according to claim 10, wherein the outer layer of carbon includes hydrogen.

12. The sliding component according to claim 10, wherein the transition layer includes nanocrystalline precipitates of tungsten carbide.

13. The sliding component according to claim 10, wherein at least one of the transition layer, the layer of metallic chromium, the intermediate layer and the outer layer is completely amorphous.

14. The sliding component according to claim 10, wherein the transition layer has a thickness between 100 nm and 500 nm.

15. The sliding component according to claim 10, wherein the ratio of total intensity in the Raman spectrum of band D to band G has values between 0.55 to 0.65.

16. The sliding component according to claim 10, wherein the ferrous base includes is a piston ring.

17. A method of forming a sliding component surface, comprising: providing a ferrous basing having a peripheral sliding surface; nitriding the sliding surface with at least one nitride to form a protective layer; applying at least one of (i) an adhesive layer of metallic chromium of crystalline structure and (ii) a transition layer including a composition of WC.sub.1-x to the protective layer; depositing an intermediate layer on the at least one adhesive layer and transition layer, the intermediate layer including a nanocrystalline structure of carbides in a multilayer structure having sublayers including a composition of a-C:H:W and a-C:H; and overlaying an outer layer of amorphous carbon having a composition of a-C:H on the intermediate layer; wherein at least the outer layer includes a wavelength intensity ratio value in Raman spectrum of band D to band G between 0.2 and 1.0.

18. The method according to claim 17, wherein the transition layer includes nanocrystalline precipitates of tungsten carbide.

19. The method according to claim 17, wherein applying the transition layer is performed via a sputtering process and the transition layer includes a thickness between 100 nm and 500 nm.

20. The method according to claim 17, wherein the ratio of total intensity in the Raman spectrum of band D to band G has values between 0.55 to 0.65.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be described in more detail below, on the basis of an embodiment example shown in the drawings. The figures show:

(2) FIG. 1 is a perspective view of a preferred configuration of the sliding component according to the present invention, in the form of a piston ring.

(3) FIG. 2 is a top view of the piston ring illustrated in FIG. 1.

(4) FIG. 3 is a schematic view of the cross section of the piston ring illustrated in FIG. 2 along section A-A, showing, schematically, the coating applied to its sliding peripheral surface.

(5) FIG. 4 is a detail view of a preferred variant of the coating applied to the sliding peripheral surface of the sliding component according to the present invention.

(6) FIG. 5 is a detail view of the intermediate layer consisting of (a-C:H:W) and (a-C:H) of the coating applied to the sliding peripheral surface of the sliding component according to the present invention.

(7) FIG. 6 is a photograph of the coating applied to the sliding peripheral surface of the sliding component according to the present invention at 1000 magnification.

DETAILED DESCRIPTION

(8) According to a preferred embodiment and as can be seen from FIG. 1, the present invention relates to a sliding component 1 for use in internal combustion engines, which has a ferrous base 10, on whose sliding peripheral surface 2 a coating 12 of carbon of the diamond-like carbon (DLC) type is applied.

(9) The present coating 12 developed by the present applicant is novel and inventive compared to the others currently existing, as will be explained below, and offers significant advantages such as ease of deposition, excellent mechanical and structural properties and competitive cost of application.

(10) Firstly, it should be noted that, preferably, the sliding component 1 is a piston ring, but it may assume any other necessary or desirable configuration such as for example a bearing, a bush, a sleeve or any other component.

(11) Still as a preliminary, it should be noted that the sliding component 1 according to the present invention possesses the coating applied at least to its sliding peripheral surface 2, but there is nothing to stop the coating being applied additionally to other parts and surfaces of the component.

(12) When component 1 has the preferable constitution of a piston ring, the present coating 12 is applied to the lateral peripheral surface 2, which is that which comes in contact with the wall of the cylinder, relative to which the ring slides as the piston executes its reciprocating motion.

(13) However, there is nothing to stop other parts of the ring, for example the top, bottom and inside surfaces, which rub against the respective ring groove present in the piston, from also receiving the present coating.

(14) In a conceptual description, the sliding component 1 is provided with a ferrous base 10 covered with a protective surface layer 11 consisting of chromium nitride applied by the process of physical vapor deposition (PVD), also known as ion plating and/or a nitrided layer as is the case presented in FIG. 6. As mentioned above, the sliding peripheral surface 2 is given a coating 12 of carbon of the DLC type, which comprises at least one transition layer 122 consisting of WC.sub.1-x and/or a layer of metallic chromium of crystalline structure (bccbody-centered cubic) 121 located between the ferrous base 10 and an outer layer of amorphous carbon 124. A schematic illustration of the coating, with the layers clearly illustrated, can be seen in FIGS. 3 and 4.

(15) The ferrous base 10 may have the most varied constitutions, but preferably it consists of a substrate of carbon steel, cast iron or else stainless steel (the latter preferably containing 17% of chromiumCr).

(16) This ferrous base 10 is given the aforementioned protective surface layer 11 consisting of at least one nitride (preferably chromium nitride), by the PVD process. The chromium nitride preferably has the preponderant constitution of CrN but obviously Cr.sub.2N may be used predominantly if necessary or desirable. Moreover, as an alternative, the use of a nitrided layer may be envisaged.

(17) The application of a nitrided layer 11 or covering of CrN by the PVD process on a ferrous substrate 10 is quite well known by persons skilled in the art, and therefore the novelty of coating 12 developed by the applicant resides in the layers applied on the nitrided layer and/or CrN layer.

(18) Thus, beginning the description of the innovative aspects of the present invention, and describing the composition of the layers applied from the base toward the exterior, the nitrided layer is given the coating 12 of carbon of the diamond-like carbon (DLC) type, which comprises the metallic adhesive layer 121, the transition layer consisting of WC.sub.1-x 122, the intermediate layer 123 consisting of a nanocrystalline layer of carbides, in a multilayer structure (a-C:H:W) and (a-C:H) and, finally, the outer layer 124 of amorphous carbon of the type (a-C:H).

(19) The metallic adhesive layer 121 is preferably a layer of metallic chromium with the aforementioned crystalline structure (bccbody-centered cubic). Still preferable, but not obligatory, the thickness of the adhesive layer is from about 500 nm to 2000 nm.

(20) The main function of the adhesive layer 121, as the name suggests, is to increase the adhesiveness of the layers that are deposited thereon in relation to the nitrided layer 11 applied on the ferrous base 10, guaranteeing cohesion of coating 12 as a whole, avoiding spalling and build-up of stresses, phenomena which, if they occur, reduce the useful life of the sliding component 1.

(21) The transition layer 122 consisting of WC.sub.1-x is applied on top of the adhesive layer 121 and comprises tungsten carbide (which, in its turn, comprises the chemical elements in variable proportions). The composition WC.sub.1-x signifies that, if x=zero, the ratio is one carbon atom (C) to one tungsten atom (W). Similarly, if for example x=0.5, this signifies that there are two tungsten atoms to one carbon atom.

(22) If necessary or desirable, the metal tungsten may be replaced with other metallic elements.

(23) The intermediate layer 123, which as already mentioned consists of a nanocrystalline structure of carbides in a multilayer structure having sublayers (a-C:H:W) and (a-C:H), is applied on top of the transition layer 122, allowing subsequent application of the layer of amorphous carbon 124, of format (a-C:H), which is from the outermost layer of coating 12, the outer layer 124 contains hydrogen.

(24) The sublayers (a-C:H:W) and (a-C:H) are applied on top of one another, starting from manipulation of the amount of tungsten present at the moment of application, in the actual equipment in which it is carried out, forming the aforementioned multilayer structure as illustrated in FIG. 5. Such a constitution is clearly advantageous if compared with the existing coatings, in that the intermediate layer is not configured as multilayered, since this multiple structure reduces crack propagation enormously. Even if there is the beginning of a crack or fracture in one of the sublayers (a-C:H:W) or (a-C:H), its growth or propagation is interrupted at the instant that it reaches the immediately adjacent sublayer, due to the fact it has a structure that is sufficiently different in terms of dissipation/build-up of stresses.

(25) This characteristic displayed by the intermediate layer 123 of the coating according to the present invention, of preventing crack propagation, means that the sliding engine component according to the present invention has greater wear resistance, and consequently the engine is given a longer useful life, and so is much more desirable.

(26) The preferred thicknesses of the layers of the coating 12 according to the present invention are given in the following table, but the thicknesses may vary freely while the resultant invention remains within the scope of protection of the claims.

(27) TABLE-US-00001 Outer layer 124 1000 nm to 3000 nm Intermediate layer 123 (multilayer) 1000 nm to 3000 nm Transition layer 122 100 nm to 500 nm Adhesive layer 121 500 nm to 2000 nm

(28) Besides possessing a configuration not presented by any relevant coating of the prior art, coating 12 has a number of very desirable technical characteristics and properties.

(29) In some of the most recent of the assiduous studies that the applicant undertakes to remain in the vanguard of technology, he discovered that the morphology and microstructure of the present coating 12, as well as the process for deposition of the aforementioned layers that guarantees greater adhesion based on addition of two combined layers of metallic crystalline material (tungstenW), in combination with the amorphous structure of the outer layer 124, mean that the present coating 12 has lower rates of wear and an increase in toughness, properties that are extremely desirable for use in sliding components of engines.

(30) Although the increase in hardness could be achieved on the basis of grain refinement and control of the levels of tungsten present, even reaching the level displayed by ceramic components, it was possible to obtain increased hardness while maintaining reasonable elasticity, or toughness, and avoiding the typical defects displayed by coatings of the DLC type with large thicknesses.

(31) The present coating 12 is a very efficient alternative to a multilayer film with nanocrystalline structure because, contrary to the others that exist, it is tough enough to give good results according to two different approaches.

(32) In a first approach, due to the combination of resilience and toughness that are more compatible with those displayed by the base material (ferrous substrate), it is quite a lot more tolerant to wear compared to other coatings based on carbon.

(33) In a second approach, the nanocrystalline structure acts as a stress-relieving element and makes protection possible that is compatible with the coating of the engine component relative to which component 1 with the present coating 12 slides.

(34) The hardness of coating 12 is from 1500 HV to 2000 HV.

(35) It is also important to mention that, in coating 12 according to the present invention, the ratio of total intensity in the Raman spectrum of band D (associated with carbon disorder sp.sup.2) to band G (monocrystalline graphite) has values between 0.2 and 1.0, which will be discussed in detail later.

(36) Raman spectroscopy is a technique that uses a monochromatic light source which, on reaching an object, is scattered by it, generating light of the same energy or of different energy relative to the incident light. In the first case, the scattering is called elastic and is not of interest, but in the second case (inelastic scattering) it is possible to obtain a lot of important information on the chemical composition of the object from this energy difference. The technique is possible owing to the phenomenon by which, when a molecule is irradiated, the energy can be transmitted, absorbed, or scattered.

(37) The Raman effect can be explained by the inelastic collision between the incident photon and the molecule. This changes the levels of the vibrational and/or rotational energies of the molecule by a given increment and, by the law of conservation of energy, this means that the energies of the incident and scattered photons will be different.

(38) The Raman spectrum is the wavelength of the scattered radiation relative to the excitation radiation (laser). The readings are taken in the visible region and the NIR (near infrared).

(39) Explaining in greater detail, a beam of low-power laser radiation is used for illuminating small areas of the object of interest and, on impinging on the area defined, it is scattered in all directions, a small packet of this radiation being scattered inelastically (with frequency different from the incident frequency.fwdarw.E=hv or E=h.c.1).

(40) The energy difference between the incident radiation and the scattered radiation corresponds to the energy with which the atoms present in the area investigated are vibrating and this vibration frequency makes it possible to detect how the atoms are joined together, to provide information on molecular geometry, on how the chemical species present interact with one another and with the environment, among other things.

(41) Moreover, the technique of Raman spectroscopy allows differentiation of polymorphs, i.e. substances that have different structures and therefore different properties, despite having the same chemical formula.

(42) As there is not only one type of vibration, since generally the chemical species present are complex, the radiation scattered inelastically consists of a very large number of different frequencies that must be separated to obtain the measured intensity. The graph showing the intensity of the scattered radiation as a function of its energy (given in a unit called wavenumber and expressed in cm.sup.1) is called the Raman spectrum. Each chemical species, whether a pigment, colorant, substrate, agglutinant, vehicle or varnish, supplies a spectrum which is, as it were, its fingerprint, allowing unambiguous identification or, for example, detection of chemical changes arising from its interaction with other substances or with light.

(43) Returning to the present coating, analysis of the Raman spectrum makes it possible to define the bands D and G (carbon disorder sp.sup.2 and monocrystalline graphite, respectively).

(44) The Raman spectra of the various forms of carbon are well known. The first-order spectrum of diamond consists of a single peak at 1332 cm.sup.1. The corresponding spectrum of monocrystalline graphite also has a single peak (peak G), at 1580 cm.sup.1 (associated with graphitic carbon sp.sup.2). In polycrystalline graphite, besides peak G, the Raman spectrum has another peak near 1350 cm.sup.1 (peak D) (associated with carbon disorder sp.sup.2).

(45) Thus, by comparing the Raman spectrum of polycrystalline graphite with those obtained for the films of DLC, it is possible to investigate the changes in structure of the graphite caused by the presence of metallic elements in the films.

(46) The intensity ratio (ratio) between bands D and G (D/G) indicates the proportion of amorphous structure (outer layer 124) contained in the DLC coating. When the value of this ratio is higher (higher proportion of the amorphous layer), the amorphous structure tends to be transformed to graphite at the moment of sliding, which reduces the coefficient of friction and the resultant wear, but the wear resistance cannot be maintained owing to the points of weakness that exist.

(47) In the case of the present coating 12, the nanocrystalline structure of tungsten carbide is suitable for coatings of amorphous carbon that have a D/G ratio between 0.2 and 1.0 according to the analysis of the Raman spectrum, so that the characteristics of low friction and wear resistance can be improved.

(48) When the D/G ratio is below 0.55, the coefficient of friction cannot be reduced sufficiently, whereas when D/G is above 0.65, the wear resistance can no longer be maintained at interesting levels.

(49) A preferred embodiment example has been described, but it must be understood that the scope of the present invention comprises other possible variations, and is only limited by the content of the appended claims, including possible equivalents.