This invention refers to a sliding member for use in an internal combustion engine, such as in a piston ring or a cylinder liner, among others. Description of the State of the Art

There is currently a growing demand for materials and/or components that offer greater resistance to wear and to corrosion, which has fostered interest for the area of surface engineering. As far as the automobile sector is concerned, and more specifically, to the coating of piston rings and cylinder liners / bores of combustion engines, recent research is being developed with a view to achieving materials that offer greater resistance to wear and tear, thus avoiding the rupture/breakage of the coating material, which would severely compromise its functional requirement.

Specifically regarding piston rings, they undergo severe wear and tear processes, and chemical attacks from sub-products from the burning of the air-fuel mixture, inside the combustion chamber of an engine.

In order to develop products that find a greater resistance to wear and tear, the ring manufacturers usually apply a wear resistance coating on the surface of the piston rings.

In this sense US patent 5,743,536 describes a piston ring for an internal combustion engine, whose surface is coated by a wear/friction material mainly comprised of Chromium Nitride. The coating is formed by the deposit of chromium nitride on the surface of the part desirable to apply using a process called PVD or physical vapor deposition. The invention described in this document suggests the use of Chromium Nitride in the form CrN or Cr2N. The proposed coating can also comprise a mixture of CrN and Cr2N and is more resistant to the delamination of the surface of the ring, and to abrasion.

Patent US 5,316,321 describes a piston ring for internal combustion engine, made from a titanium alloy, whose outer friction surface is coated by a hard film, especially TiN (Titanium Nitride) and CrN (Chromium Nitride), using the process of physical vapor deposition for this deposition. Patent US 5,154,433 describes a piston ring for internal combustion engine made of martensitic stainless steel and coated with a Titanium Nitride film. One of the preferred embodiments has a coating of Titanium Nitride in which a concentration of Nitrogen gradually decreasing on its surface, conferring the film a Vickers hardness of around 1800 HV or less.

Another proof of developments regarding coatings of piston rings to reduce wear is patent US 7,267,344, which describes a piston ring made of steel, whose contact surface has a coating of aluminum nitride and/or silicone and/or zircon. The proposed coating presents a thickness less or equal to 70 pm, and is applied by physical vapor deposition.

It is noted, from the above mentioned prior art documents, that the Chromium Nitride film is widely used in the composition of an outer coating for the combustion engine piston ring, in order to confer greater resistance to wear and tear. Recently, however, it was noted that in engines working in a high load regime, or engines working under high combustion pressure levels, degradation of the Chromium Nitride layer occurs, with the consequent formation of micro-cracks on their surface. The micro-cracks propagate until the covering starts to peel (a phenomenon known as delamination). The hard particles, resulting from delamination, loose in the system, may lead to undesirable wear of the piston ring and to a catastrophic failure of the engine.

The higher wear found at the ring tips, usually 3 times more than in other regions, means that a larger, more expensive, coating thickness is applied on the complete ring.

In a similar way, designers seek ways to reduce friction and wear on engine cylinders. As a result, several solutions applied to piston rings, liners/cylinders and other internal components have been developed in order to reduce the working friction of internal combustion engines.

One possible solution is presented in the patent document WO2009069703 A1 , which describes Ring-Liner combination structure where the liner internal surface has Rz 0.5 to 1.0 pm, Rk 0.2 to 0.4 pm, Rpk 0.05 to 0.1 pm and Rvk 0.08 to 0.2 pm. Ring running face also having a specific roughness and face pressure against the liner. The components in friction contact have an extremely smooth, difficult to produce, surface. However, to assure engine durability lube oil has to be provided in sufficient amounts for preventing piston seizure, which is difficult to obtain by such smooth surfaces.

Another solution is disclosed in European patent EP 2 229 467, which describes a manufacturing method for a mechanical element, such as an engine component, having a reduced friction surface, which has a step of tribocherfiical deposition of a substance, such as solid lubricant, which covers the surface of the element.

Canadian patent document CA 2 704 078 is very similar to the European patent above and refers to the process for manufacturing a low- friction element by depositing solid lubricant, specifying the application of solid lubricant to an engine cylinder liner.

Finally, patent document US 2010/0272931 , of the same family of both documents abovementioned, refers to a similar manufacturing process, highlighting the use in cylinder liners and cylinders of engine block.

Purposes of the Invention

The purpose of this invention is to provide a sliding set for use in an internal combustion engine, assuring a reduction in engine fuel consumption and preventing piston seizure.

Brief description of the invention

The purposes of this invention are achieved by a sliding set for use in an internal combustion engine, having at least a liner/cylinder of an engine block which defines an internal sliding surface and at least a piston ring, wherein:

(i) at least part of the external sliding surface of the ring has ceramic material coating; and

(ii) at least part of the internal sliding surface of the liner/cylinder comprises a solid lubricant nanolayer coating wherein ^0,5 < ^R Pk ⁺ + ^R k ^{1 5}

Rpk + Rk

, , 0.5 <— < 2.0

and/or Rvk

The external sliding surface cooperates with the internal sliding surface of the liner/cylinder. Summarized description of the drawings

This invention will be described, next, in more details based on an example of implementation represented in the drawings. The figures show: Figure 1 - is a schematic view of the sliding set for use in an internal combustion engine covered by the invention herein.

Figure 2 - is a schematic view of the sliding set for use in an internal combustion engine covered by this invention in a reciprocating tribological test.

Figure 3 - is a graph which links the average coefficient of friction of three specimens, when varying the rotation, under determined load.

Figure 4 - is a graph which presents the coefficient of friction of three specimens at each moment of a condition that simulates the 360° cycle of a crankshaft rotation of an engine.

Figure 5 - is a graph which presents the electrical resistance of three specimens under identical conditions of trial.

Figure 6 - is a graph which presents the values of acoustic emission for three specimens under identical conditions of trial.

Figure 7 - is a graph which presents the values of wear suffered by the three specimens, under identical conditions of trial, after four hours of reciprocating test.

Detailed description of the figures

In internal combustion engines, there is a goal to reduce friction amongst their internal components, such as piston rings and liners or cylinders of the engine and, as a consequence, increase the durability and useful life of such components. In the specific case of piston rings, the most critical wear is observed in their free tips, adjacently to the opening, and can be even three times higher than the wear that affects other portions/regions of the ring.

Due to the more intense wear of the ring in its tips, it is necessary to apply thicker layer of coating all over the ring in order to keep the engine's durability at satisfactory levels. However, those skilled in the art know that thick layers are expensive to manufacture and largely redundant in ring regions in which wear is not so intense. Thus, this invention is based on the concept and experimental results that the proposed solution brings, in an addition to the reduced friction losses, a substantial reduction of ring wear. Therefore, it is possible to reduce the cost to manufacture the ring, due to the lower thickness required, keeping the life cycle of the engine at compensatory levels.

The present findings contemplate a sliding set for use in an internal combustion engine comprising a piston ring having a ceramic material coating, preferably a chromium nitride layer, in combination with a cylinder liner or cylinder wall which presents a solid lubricant nanolayer coating, preferably comprising a transition metal dichalcogenide, more preferably tungsten disulfide (WS2), with roughness values within a determined and specific range. The solid lubricant nanolayer coating is preferably produced via a mechanochemical surface finishing process.

In prior art, there is no technology which combines the use of a piston ring having e.g. a chromium nitride liner with a layer or cylinder wall presenting e.g. a transition metal dichalcogenide nanolayer coating, produced via a mechanochemical finishing process. In particular, such sets presenting roughness values within ranges as defined here below are not previously known. The determined and specific roughness ranges provide an adequate lube reservoir in the cylinder surface, resulting in lower friction.

With reference to Fig. 1 , an embodiment of this invention refers to a sliding set for use in an internal combustion engine, having at least a liner or cylinder of an engine block 1 which defines an internal sliding surface 2 and at least a piston ring 3 substantially shaped as a ring which defines an external sliding surface 4 that cooperates with the internal sliding surface 2 of the liner/cylinder 1 , wherein (i) at least part of the external sliding surface 4 of the ring 3 has ceramic material coating; and (ii) at least part of the internal sliding surface 2 of the liner/cylinder 1 comprises a solid lubricant nanolayer coating, in this embodiment a transition metal dichalcogenide nanolayer coating, produced by a mechanochemical process, with roughness substantially under certain parameters. Preferably, the solid lubricant nanolayer coating comprises tungsten-disulfide-like components, and most preferably, at least to a part, tungsten disulfide (WS2).

Evidently, the constitution of the main part of these components may vary freely. In the case of the liners, there is no limitation to the shape or material constituting them as long as it is possible to apply to them such transition metal dichalcogenide nanolayer coating. It also may be possible that the cylinder does not refer to a liner, but actually to a bore provided directly in the block, in which situation, as mentioned previously, the contact surface is directly made onto the part's casting material. This nanolayer coating has very low thickness, usually less than 1pm, and is known by those skilled in the art as a tribofilm.

With regards to the ring, its constitution and shape may also vary freely, as long as it is possible to apply ceramic chromium coating. Thus, the ring may take on the constitution of a compression ring (first channel), a second groove ring or also an oil control ring. Preferably, the entire external sliding surface 4 of the ring receives chromium nitride (CrN) applied by the process of physical vapor deposition (PVD) in a plurality of successive layers, in an architecture known as such by those skilled in the art as multilayer coating. In order to demonstrate the advantages of the proposed invention, a test was carried out in a reciprocating tribometer. In the test, illustrated in figure 2, normal load (C) is applied to a segment of ring with CrN coating applied by PVD and the drag movement (R) of this ring is forced over the internal surface of a liner using a closed loop servomechanism, and the normal loads and friction forces are measured with strain-gauges.

In this test, carried out for components extracted from a Diesel cycle engine, the following comparative results were obtained between two variations of liners containing the teachings of this invention and one liner manufactured according to the prior art (control).

Roughness values (pm) Rpk Rk Rvk Rz Ra

Control 0.16 0.52 1.91 4.68 0.47

Invention 1 0.11 0.30 0.62 2.13 0.15 Invention 2 0.12 0.22 0.66 2.19 0.17

It can be easily verified that no matter what form/standard for roughness measurement is used the proposed invention has lower values than the control sample, but much higher than the ones described in e.g. the mentioned WO2009069703 A1. Therefore, it is possible to attest the efficacy verified with the application of the proposed invention.

It is important to discuss the parameters of surface roughness profile contained in the above mentioned table. The parameters are defined in the ISO 13565-2:

The parameter Ra is the roughness average.

The parameter Rz is the average maximum height in the roughness profile. The parameter Rpk is the reduced peak height. A high value of Rpk implies a surface composed of high peaks providing small initial contact area and thus areas of high contact stress when the surface is contacted.

The parameter Rvk is related to the reduced valley depths, being a measure of the valley depths below the core roughness and may be related to lubricant retention.

The parameter Rk is the core roughness depth.

At a surface that is well adapted for low friction contacts, a number of roughness considerations have to be met. In general, the average roughness should be kept small, which means that the parameter Ra cannot be too high, preferably smaller than 0.20 μιτι. Also the Rz value should be kept small, preferably smaller than 2.5 μιτι. Similarly, high values of, in particular, Rpk, but also of Rk, are connected to increased friction, since high values indicate peaks on the surfaces and consequently high contact stress areas. On the other hand, the parameter Rvk is not as important concerning the contact stress. If one only considers solid-solid contact aspects, an as smooth surface as possible would be of benefit. Advantageously, a sum of Rpk, Rk and Rvk should be smaller than 1.5 μητι.

However, in a typical situation of use, other substances, such as lube oils, fuel residues etc., are also present in the volume between the surfaces in contact. To this end, it is also requested that there are micro volumes within the surface structures that may accommodate such substances. The sum of

Rpk, Rk and Rvk should therefore not be too small either, and 0.5 pm is presently considered as a lower limit for the preferred range for the sum of

Rpk, Rk and Rvk. In this context, it can also be noted that the valleys of the surface probably are the most suitable structures for comprising additional substances. Therefore, the Rvk should not be too small compared to the Rpk and Rk values. The ratio between on one hand the sum of Rpk and Rk and on the other hand the Rvk was found to preferably be lower than 2.0 pm. At the same time, too large valley volumes may cause too large volumes of e.g. lube oil to be kept close to the friction interaction, which e.g. may cause disadvantageous collection of contaminants. Therefore, the above mentioned ratio between on one hand the sum of Rpk and Rk and on the other hand the

Rvk was also found to preferably be kept higher than 0.5 pm.

Whatever is the preferred embodiment of the present invention, the preferred relationship between the parameters below are observed,

(j) 05 < Rpk + Rvk + Rk < 1.5 ·

Rpk + Rk

.5 <— < 2.0;

(ll) Rvk

(jjj) Ra < 0.20;

^jy^ RZ

As a result, the sliding set object of one embodiment of the present invention has the following characteristics:

- at least a liner or cylinder of an engine block 1 which defines an internal sliding surface 2

- at least a piston ring 3 substantially shaped as a ring which defines an external sliding surface 4 that cooperates with the internal sliding surface 2 of the liner/cylinder 1 , wherein

- at least part of the external sliding surface 4 of the ring 3 has ceramic material coating; and

- at least part of the internal sliding surface 2 of the liner/cylinder 1 comprises a solid lubricant nanolayer coating, in this particular embodiment comprising tungsten disulfide (WS2), Rpk + Rk wherein 0.5 < Rpk + Rvk + Rk < 1.5 and/or ^< fl fc ^{< 2"} ° ^"

Additionally, the sliding set object of preferred embodiments of the present invention comprises a solid lubricant nanolayer coating wherein the roughness parameters Ra < 0.20 μητι and/or Rz < 2.5 pm.

The graph of figure 3 properly illustrates the average coefficient of friction in the test with the reciprocating tribometer, when varying the rotation, at a load of 50N. Once again, it is clear the reduction of the coefficient of friction of samples "invention 1 " and "invention 2" when compared to "control." If a rotation of 100 rpm is applied, for instance, the coefficient of friction displayed by "control" is 0.08, whereas, the values presented by "invention 1 " and "invention 2" are approximately 0.03-0.04, that is, almost 50% lower than the control.

Figure 4 illustrates a graph which shows the coefficient of friction in each moment of a condition that simulates the 360° cycle of a crankshaft rotation of an engine. For whatever moment taken, except for 0°, 180°, 360° and adjacencies, when there is no translational movement of the piston/rings set, the coefficient of friction of "control" is considerably higher than that of "invention 1 " and "invention 2."

For example, considering a rotation of 25 rpm and load of 50N, in the position 270°, the coefficient of friction of the "control" sample is approximately 0.13, whereas the values of "invention 1" and "invention 2", in identical situation, are approximately 0.7-0.9. Once again, they are considerably lower values. In figure 5, the graph illustrates the electrical resistance of the three specimens "control," "invention 1 " and "invention 2" in identical conditions of trial (rotation of 25 rpm and load of 50N). The electrical resistance is a parameter that somewhat describes the surface separation, a higher separation cause an increase in the electrical resistance. When the surfaces enter in more contact, the electrical resistance reduces. The analysis of the graph allows us to conclude that, as it gets far from the top and bottom edges of the course of movement of the piston (top dead center - TDC and bottom dead center - BDC), the value of electrical resistance of samples "invention 1" and "invention 2" shows bigger differences in relation to the sample "control," making evident the advantage of the solution covered by this invention in terms of separating the surfaces, hence reducing wear and friction.

Acoustic emission (noise) testing also attests that the performance of this invention is better if compared to the samples "control." Figure 6 discloses the acoustic emission values for the three specimens "control," "invention 1" and "invention 2" under identical conditions of trial (rotation of 25 rpm and load of 50N). It is possible to clearly realize that the emission values of samples "invention 1" and "invention 2" are considerably lower in comparison to the sample "control," corroborating the lower friction caused by the application of a solid lubricant tribofilm.

Finally, the wear testing is illustrated in figure 7, in which one measures the wear suffered by samples "control," "invention 1" and "invention 2" after four hours of reciprocating test.

In the sample "control," there was the wear of the whole liner (about 2.2 pm) and the additional wear of about 1.4 pm of the ring substrate, which adds up to a total wear of 3.6 pm.

In samples "invention 1" and "invention 2," wear was considerably lower, for the same conditions of testing. In the case of "invention 1 ," there was wear merely of 1.7 pm of the liner. As it has total thickness of about 2.2 pm, considerable thickness of this liner was still left and, therefore, there was no wear of the ring substrate.

In the case of "invention 2", there was the wear of the liner (about 2 pm) and the additional wear of only approximately 0.1 pm of the ring substrate.

The present invention, when providing an internal sliding surface 2 of a liner/cylinder which offers reduced friction conditions for the external sliding surface 4 coated with chromium nitride of a piston ring, manages not only to increase durability of the piston rings (especially their edges), but also to reduce fuel consumption in the internal combustion engine. The reduction of friction, besides being advantageous with regards to reduction in consumption, enables the use of lubricating oils with lower viscosity, which, in turn, reduces the power required by the oil pump and the loss by viscosity in up to 25%. As a result, combining the use of a sliding set as described by this invention with the use of lower viscosity lubricant can enable a reduction of approximately 2% to 4% in the fuel consumption for the operation of the engine.

Having described examples of preferred embodiments, it should be understood that the scope of the present invention encompasses other possible variations, and is limited only by the content of the appended claims, other possible equivalents being included therein.