Piston ring

Abstract

A piston ring for an internal combustion engine may include a metal base and a protective coating disposed on the base to cover at least part of a surface of the base. The coating may be composed of at least one nitride phase formed from at least one metal element and aluminum, and the coating may have an internal stress of less than 1.2 GPa.

Claims

1. A piston ring for an internal combustion engine comprising: a metal base and a protective coating disposed on the base to cover at least part of a surface of the base, wherein the protective coating has a composition including at least one nitride phase having at least one metal element and aluminium (Al), and an internal stress of less than 1.2 GPa; and wherein the protective coating has less than 4×10.sup.4 macroparticles per square millimeter of an average cross-sectional area of the coating.

2. The piston ring according to claim 1, wherein the at least one metal element in the nitride phase is from group B of the periodic table that comprises aluminium (Al), boron (B) and gallium (Ga).

3. The piston ring according to claim 1, wherein the protective coating has a multilayer structure including a layer composed of the at least one nitride phase having the at least one metal element doped with said aluminium, and another layer composed of a metal nitride.

4. The piston ring according to claim 1, wherein the nitride phase has a composition including at least one of CrN, TiN and NbN.

5. The piston ring according to claim 1, wherein the coating includes a multilayer structure and the at least one nitride phase is interposed in the multilayer structure via superimposing a plurality of sublayers, and wherein at least one of the plurality of sublayers is composed of CrN.

6. The piston ring according to claim 1, wherein the coating is a physical vapour deposition layer deposited via a high-power impulse magnetron sputtering (HiPIMS) source.

7. The piston ring according to claim 1, wherein said macroparticles have a diameter of less than 0.9 μm.

8. The piston ring according to claim 1, wherein the coating includes an element selected from the group of elements in the periodic table of which oxygen (O) and carbon (C) belong, in a quantity of not exceeding ten percent by weight.

9. The piston ring according to claim 1, wherein the coating has a hardness between 1500 HV and 2500 HV.

10. The piston ring according to claim 1, wherein the coating has a thickness between 10 μm and 100 μm.

11. The piston ring according to claim 1, further comprising an intermediate layer, principally composed of chromium, nickel or cobalt, wherein the intermediate layer is disposed between the base and the coating.

12. The piston ring according to claim 1, wherein the metal base is a compression ring.

13. The piston ring according to claim 1, wherein the protective coating is free of macroparticles.

14. A piston ring for an internal combustion engine comprising: a metal base and a protective coating disposed on the base to cover at least part of an external surface of the base, wherein the coating is multilayer including a nitride phase and a phase of a chromium nitride with a metal element selected from group IIIA of the periodic table, and wherein the coating is substantially free of macroparticles and has an internal stress of less than 1.2 GPa and a periodicity of the multilayers of the coating is less than 10 nm.

15. The piston ring according to claim 14, wherein the coating has a multilayer structure including a layer of the nitride phase and a layer of the phase of chromium nitride with the metal element arranged one on top of the other.

16. The piston ring according to claim 14, wherein the nitride phase is composed of CrN, NbN or TiN.

17. The piston ring according to claim 14, wherein the metal element of the phase of the chromium nitride is aluminium (Al).

18. The piston ring according to claim 14, wherein the coating is a physical vapour deposition coating disposed via an HiPIMS source.

19. The piston ring according to claim 14, wherein the coating has a hardness between 1500 HV and 2500 HV.

20. The piston ring according to claim 14, wherein the coating has a thickness between 10 μm and 100 μm and is free of macroparticles.

21. A piston ring for an internal combustion engine comprising: a metal base and a physical vapour deposition protective coating disposed on the metal base; the coating having a composition including CrAlN and macroparticles in a quantity ranging between 5×10.sup.2 and 3×10.sup.4 macroparticles per square millimeter of an average cross-sectional area of the coating, wherein the coating is disposed on the base via an HiPIMS source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a ring of a piston having three rings comprised in the prior art, within a cylinder liner.

(2) FIG. 1A is a detailed view of one of the rings of the piston illustrated in FIG. 1.

(3) FIG. 1B is a detailed schematic view of the ring illustrated in FIG. 1A, showing the base metal and the ceramic coating.

(4) FIG. 2 is a detailed schematic view of the occurrence of fragmentation in a ring comprised in the prior art without any kind of coating.

(5) FIG. 3 is a detailed schematic view of the occurrence of fragmentation in a ring comprised in the prior art having an already known ceramic coating.

(6) FIG. 4 is another detailed schematic view of the occurrence of fragmentation in a ring comprised in the prior art having the ceramic coating.

(7) FIG. 5 is a schematic view of a ring of a piston having three rings being an objective of the present invention, within a cylinder liner.

(8) FIG. 5A is a detailed view of a ring of a piston having three rings being an objective of the present invention.

(9) FIG. 5B is a detailed schematic view of the ring illustrated in FIG. 5A, showing the base metal and the innovative coating developed by the inventor.

(10) FIG. 6A is a graph of the percentage of macroparticles/droplets in the area of the coating, comparing the piston ring of the prior art and of the present invention.

(11) FIG. 6B is a graph of the diameter of macroparticles/droplets in the coatings of the piston ring of the prior art and of the present invention.

(12) FIG. 6C is a graph of the number of macroparticles/droplets per area of the coating, comparing the piston ring of the prior art and of the present invention.

(13) FIG. 6D is a graph of the average wear of the coatings of the piston ring of the prior art and of the present invention.

(14) FIG. 7 is a schematic view of the Atlas apparatus for the execution of the scratch test, in accordance with the standard ASTM C 1624 05.

(15) FIG. 8 is a comparative graph of the delamination load supported by the ring of the prior art and of the present invention coated with CrN.

(16) FIG. 9 shows two images comparing the appearance of the coating of the ring of the prior art with the ring of the present invention, following the scratch test.

DETAILED DESCRIPTION

(17) The present invention relates, in essence, to a piston ring 3 for an internal combustion engine comprising a metal base 4 whereupon a specific new protective coating 5 is applied in such a manner as to cover at least part of the surface of the base 4. More preferably, the sliding member is a compression piston ring.

(18) The metal base 4 of the member is preferably composed of a ferrous metal, such as cast iron or stainless steel, however it is clear that another type of material may be specified if necessary or desirable, provided that the same is technically feasible, without departing from the scope of protection of the invention.

(19) As may be observed in FIGS. 1 to 4 the piston rings 30 already known have a base 40 whereupon a coating 50 may be applied.

(20) FIG. 2 shows, schematically, a ring without any type of coating. The lack of a coating to protect the base causes rapid wear and the occurrence of fragmentation.

(21) FIGS. 3 and 4 show, also schematically, rings having coatings 50 of CrN and/or CR.sub.2N. In some engines operating under conditions of extremely high load or having a high combustion pressure level, fragmentation occurs even with such a type of protective coating.

(22) Describing essentially the piston ring 3, which is the objective of the present invention, it comprises a metal base 4 whereupon a protective coating 5 is applied to cover at least part of the surface of the base 4, wherein the coating 5 comprises at least one phase of nitride produced from at least one metal element and aluminium (Al) and comprises an internal stress of less than 1.2 GPa. The hardness of the coating lies between 1500 HV and 2500 HV.

(23) Preferably, the coating 5 is a monolayer structure and the metal element in the nitride phase is from group B of the periodic table, and more particularly composed of CrN, NbN or TiN.

(24) In an alternative embodiment, the coating 5 is formed by a multilayer structure and the surface coating is interposed, originated by superimposing a plurality of sublayers wherein one among such sublayers is composed of CrN. More particularly, the coating 5 comprises at least a first sublayer 11 and at least a second sublayer 12, wherein the first sublayer 11 is composed of at least one metal ceramic and the second sublayer 12 is composed of at least one nitride compound and, additionally, a metal element of group IIIA of the periodic table.

(25) As already known to those skilled in the art, the first sublayer 11 is preferably composed of chromium nitride, including Cr.sub.2N and CrN, or TiN, by virtue of the good tribological properties and overall performance thereof. The chromium nitrides demonstrate an excellent level of abrasion resistance and the TiN may also be used due to its good properties of high oxidation temperature and abrasive wear resistance.

(26) In this embodiment, the most innovative step is the presence of at least a second sublayer 12, composed of at least one nitride compound plus a metal element of group IIIA of the periodic table, being preferably aluminium. More preferably, the second sublayer 12 is composed of a CrN doped with aluminium (Al), ensuring a reduction in internal stress allied with the construction of multilayers at nanometric scale, improving wear and fragmentation resistance. The chromium may be substituted by other elements such as, inter alia, titanium (Ti), molybdenum (Mo), niobium (Nb), vanadium (V).

(27) The coating 5 is applied on the metal base 4 by the PVD (physical vapour deposition) process, generated by an HiPIMS source, by codeposition on at least two (and preferably four) opposing targets (positioned at a spacing of 180° (or) 90° in relation to each other).

(28) Considering the use of four targets positioned at 90° spacing between each other, there are preferably two Cr targets positioned at 180° in relation to each other, and two targets of CrAl, also positioned at 180° in relation to each other and at 90° in relation to the Cr targets. Solely one type of target is applied for the monolayer structure. For example, four targets of TiAl or CrAl.

(29) The sliding member (that is to say, the piston ring), is positioned on a support base which rotates during the deposition procedure with the objective of alternatively exposing the substrate to the two or four targets. As the substrate rotates, the metal species of both the targets will be deposited and a mixed layer will be formed. The speed of rotation may be modified with the objective of permitting control over the thickness of all sublayers and grain size.

(30) The coating preferably has a thickness between 10 μm and 100 μm, however it may vary freely if necessary or desirable.

(31) The coating 5 also contains, not of necessity, an element selected from the group of which oxygen (O) and carbon (C) form part, in a quantity not exceeding ten percent by weight.

(32) A further optional step of the coating 5 is an intermediate layer principally composed of chromium, nickel or cobalt, disposed between the base 4 and the coating 5.

(33) Also essentially, the piston ring 3, being the objective of the present invention, may be described as comprising a metal base 4 whereupon a protective coating 5 is applied to cover at least part of the external surface of the base 4, wherein the coating 5 is composed of a nitride (metal element+nitrogen (N)) and at least one additional metal element selected from group IIIA of the periodic table, comprising less than 3×10.sup.4 macroparticles per square millimeter of the average cross-sectional area of analysis of the material coating. Preferably, the additional metal element is aluminium (Al), however it may be another element from the aluminium IIIA family, such as indium (In), boron (B) and gallium (Ga).

(34) The graphs illustrated in FIGS. 6A to 6D show the analysis of droplets of two versions of the piston ring, which is the objective of the present invention, and the advantages thereof may be easily perceived in comparison with a piston ring belonging to the state of the art having a regular coating of CrN.

(35) The graph in FIG. 6A clearly shows the reduction in the area of macroparticles/droplets of the two versions of the present invention (respectively approximately 0.08% (version 2 of the invention) and approximately 0.75% (version 1 of the invention)) when compared with the almost 1.2% of the CrN ring of the prior art.

(36) In an identical manner, the graph in FIG. 6B shows the reduction in diameter of the macroparticles/droplets (in μm) of version 2 of the present invention (approximately 0.57 μm against approximately 0.81 μm of a CrN ring of the prior art). In this specific analysis, version 1 of the present invention does not demonstrate advantages in relation to the ring of the prior art, having a marginally greater diameter (approximately 0.82 μm).

(37) The graph in FIG. 6C shows the reduction in the ratio of the number of macroparticles/droplets to the area of the coating (in numbers of droplets per μm2). Whereas the coating of CrN of the prior art shows that ratio as being approximately 0.058, versions 1 and 2 of the present invention show much smaller values, respectively approximately 0.014 and 0.003.

(38) Finally, the graph in FIG. 6D relates to the principal wear (in μm), which is reduced from approximately 5.3 μm on a ring coated with regular CrN to 3 μm (version 1 of the invention) and to 2.5 μm (version 2 of the invention).

(39) A scratch test was executed to analyse the performance of the piston ring 3 of the present invention in comparison with a piston ring having a regular coating of CrN. The scratch test, schematically illustrated in FIG. 7, was executed on an Atlas apparatus in conformity with the standard ASTM C 1624 05.

(40) In such a test, a stylus is forced to slide over a sample having a coating which must be tested whilst a load F.sub.N is applied perpendicularly to the displacement. Whilst the stylus slides, the load applied is gradually increased until delamination of the coating occurs. Here the advantages of the coating 5 of the ring object of the present invention are clearly observed.

(41) The graph illustrated in FIG. 8, which illustrates the delamination load (in N) supported by the coating during displacement of the stylus, shows that the ring coated with CrN of the prior art supported a load of approximately 100 N prior to delamination.

(42) In terms of comparison, version 1 of the present invention supported 150N prior to delamination (approximately 50% more in relation to the ring of the prior art). Version 2 of the invention performed even better, by virtue that it supported a load of 180N (the maximum capacity of the equipment) without delamination.

(43) Having described examples of preferred embodiments, it shall be understood that the scope of the present invention embraces other possible variations and is solely limited by the content of the claims appended hereto, potential equivalents being included therein.