Abstract:
Disclosed is a piston ring for engine that includes chromium (Cr) coating layer coated on the surface of a base material of the piston ring of an engine; a chromium nitride (CrN) coating layer; and a silicon-incorporated diamond-like carbon (Si-DLC) coating layer, which is formed on the CrN coating layer, and contains Si component of about 3˜10 at %, and a method for manufacturing thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2012-0027221, filed on Mar. 16, 2012, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     (a) Technical Field 
     The present disclosure relates to a piston ring for engine wherein low friction Si-DLC is coated on the peripheral surface of the piston ring to reduce friction loss in an engine cylinder and to improve fuel efficiency, and a method for manufacturing thereof. 
     (b) Background Art 
     A piston ring is a pair of rings that fits into a groove on the outer diameter of a piston to maintain air tightness between a piston of a vehicle engine and inner cylinder wall, and to prevent entering lubricating oil to a combustion chamber by scraping out the lubricating oil of the cylinder wall. 
       FIG. 1  is a drawing representing coating condition of the existing piston ring for engine. In this conventional piston ring design, the piston ring has difficulty maintaining durability due to its low friction efficiency. Generally, Cr (Chrome) plating  30  or nitriding (gas nitriding) is being used on the peripheral surface of the piston ring  10 , and recently, due to high fuel cost and CO 2  regulation, various surface treatment techniques including CrN (Chrome Nitride) are also becoming more popular for reducing friction loss and improving durability. 
     Furthermore, methods like DLC (Diamond Like Carbon), which is an intermediate phase of diamond and graphite, and has low friction coefficient of graphite, high rigidity of diamonds and excellent chemical resistance, can further reduce friction loss of the engine when it is applied to the peripheral surface of the piston ring and can improve fuel efficiency of a vehicle in the end. However, the friction and durability of DLC becomes worse when it is exposed to higher temperatures for a long period of time, and coating is exfoliated due to high residual stress in the coating as the coating becomes thicker. 
     The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
     SUMMARY OF THE DISCLOSURE 
     The present invention has been made in an effort to solve the above-described problems associated with prior art. It is an object of the present invention to provide a piston ring for an engine, which provides low friction properties and high durability at the same time by coating Si-DLC (Silicon doped Diamond Like Carbon) on the peripheral surface of the piston ring to reduce friction loss in an engine cylinder and to improve fuel efficiency. 
     In order to accomplish the above objectives, the piston ring of the present invention is characterized by comprising: a chromium (Cr) coating layer coated on the surface of a base material; a chromium nitride (CrN) coating layer coated on the Cr coating layer; and a Si-DLC coating layer, which is formed on the CrN coating layer, and alternately laminated with low content layers containing a first Si component of about 3 at % or less (not including 0 (zero)) and high content layers containing a second Si component of about 3˜10 at %. 
     The low content layer and the high content layer may be characterized as having a thickness of 50 nm or less (not including 0 (zero)), respectively. Furthermore, the high content layer is in contact with an inner cylinder wall due to the coating being located on the outer-most layer of the Si-DLC coating layer. 
     The Si-DLC coating layer may be formed by a chemical reaction of hydrocarbon gas (C x H y ) and TMS (Tetra-methylsilane, Si(CH 3 ) 4 ) gas, or hydrocarbon gas and HMDSO (Hexamethyldisiloxane, O(Si(CH 3 ) 3 ) 2 ) gas. In addition, the Cr coating layer and the Si-DLC coating layer may be coated only on the outer peripheral surface of the base material contacted with the inner cylinder wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a drawing representing conventional coating conditions of the existing piston ring for engine; 
         FIG. 2  is a drawing representing the piston ring for engine according to an exemplary embodiment of the present invention; 
         FIG. 3  is a drawing representing a cross section of the coating of the piston ring for engine illustrated in  FIG. 2 ; 
         FIG. 4  is a device for manufacturing the piston ring for engine illustrated in  FIG. 2 ; and 
         FIGS. 5 to 7  are drawings comparing performances of Examples and Comparative Examples of the present invention. 
     
    
    
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. 
     In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
     DETAILED DESCRIPTION 
     Hereinafter, a piston for engine and a method for manufacturing thereof according to the preferred embodiments of the present invention now will be described in detail with reference to the accompanying drawings. 
     It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, plug-in hybrid electric vehicles. As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. 
       FIG. 2  is a drawing representing a piston ring for engine according to an exemplary embodiment of the present invention, and  FIG. 3  is a drawing representing a cross section of the coating of the piston ring for engine illustrated in  FIG. 2 . 
     The piston ring of the illustrative embodiment of the present invention includes a Cr coating layer  200  coated on the surface of a base material  100 , and a Si-DLC coating layer  600 , which is coated on the outer-most layer of the base material  100 , and alternately mixed with low content layers  610  containing a first Si component of about 3 at % or less (not including 0 (zero)) and high content layers  630  containing a second Si component of about 3˜10 at %. The exemplary piston ring of the illustrative embodiment of the present invention also includes a CrN coating layer  400  coated between the Cr coating layer  200  and the Si-DLC coating layer  600 . 
     The piston ring of the illustrative embodiment of the present invention consisting of the structure described above reduces friction loss of the piston ring and improves fuel efficiency by about 0.2˜0.5% since friction coefficient of the Si-DLC is 23% lower than Cr plating and nitriding and 11% lower than CrN. Additionally, oil film destruction can be suppressed and durability of the piston ring can be improved since resistance to scuffing of the Si-DLC is 50% or more better than Cr plating and nitriding and 30% or more better than CrN. Further, the low-friction properties and high temperature wear resistance of the DLC can be improved by doping Si on the DLC, and the durability of the piston ring can be improved since a Cr+CrN sub-multi-layer is provided in case the Si-DLC is worn away. 
     Meanwhile, the Si-DLC coating layer  600  is characterized that the thickness of the low content layer  610  and the high content layer  630  may be 50 nm or less (not including 0 (zero)), respectively, and preferably, the thickness of the low content layer  610  and the high content layer  630  are same. 
     The high content layer  630  can improve durability and reduce friction but has weak rigidity. Therefore, in order to make up for its weak rigidity, a low content layer  610  having lower content of a second Si component than the high content layer  630  is formed together one on top of the other alternately, and in this case, it is preferred to form the low content layer  610  and the high content layer  630  with the same thickness to ensure the piston ring has an uniform rigidity over the entire region. 
     The low content layer  610  and the high content layer  630  are alternately laminated one by one from the CrN coating layer  400 , and the high content layer  630  having excellent durability and friction reduction is coated on the outer-most layer of the Si-DLC coating layer  600  to be in contact with the inner cylinder wall. As a result, the Si-DLC coating layer  600  can maintain its low friction and durability even at higher temperature according to the Si content ratio. 
     Meanwhile, the Si-DLC coating layer  600  may be formed by a chemical reaction of hydrocarbon gas (C x H y ) and TMS (Tetra-methylsilane, Si(CH 3 ) 4 ) gas, or hydrocarbon gas and HMDSO (Hexamethyldisiloxane, O(Si(CH 3 ) 3 ) 2 ) gas. Furthermore, it may also be effective to coat the Cr coating layer  200  and Si-DLC coating layer  600  only on the peripheral surface of the base material  100  contacted with the inner cylinder wall. 
     Thus, the Si-DLC applied in the present invention is an effective coating material to improve low friction, wear resistance and resistance to scuffing of the piston ring since it has a lower friction coefficient and higher rigidity than CrN, and its low friction and durability can be maintained even at higher temperature unlike a general DLC coating (e.g., due to the present invention&#39;s low content layer  610  and the high content layer  630  according to the Si content ratio). 
       FIG. 4  is a device for manufacturing the piston ring for engine illustrated in  FIG. 2 , and the method for manufacturing the piston ring for engine of the present invention is as follows. 
     The method for manufacturing the piston ring for engine according to the present invention includes a Cr coating step of coating a Cr coating layer  200  on a base material  100  of the piston ring; and a Si-DLC coating step of coating a Si-DLC coating layer  600  by a chemical reaction of hydrocarbon gas (C x H y ) and TMS (Tetra-methylsilane, Si(CH 3 ) 4 ) gas, or hydrocarbon gas and HMDSO (Hexamethyldisiloxane, O(Si(CH 3 ) 3 ) 2 ) gas, and to have low content layers  610  containing a first Si component of about 3 at % or less (not including 0 (zero)) and high content layers  630  containing a second Si component of about 3˜10 at %. 
     Herein, the method may further comprises a CrN coating step of coating a CrN coating layer  400  by chemically reacting N 2  gas with sputtered Cr ions between the Cr coating step and the Si-DLC coating layer. Further, in the Si-DLC coating step, the Si content may be controlled by controlling the injection amount of TMS or HMDSO gas. 
     Namely, the low content layer  610  can be controlled to contain the first Si component in an amount of about 3 at % or less (not including 0 (zero)) and the high content layer  630  can be controlled to contain the second Si component in an amount of about 3˜10 at %. 
     Specifically, as shown in  FIG. 3 , the Si-DLC coating applied in the present invention is coated on the peripheral surface of the base material  100  of the piston ring into a multi-layer structure of Cr (PVD, Physical Vapor Deposition method)+Corn (PVD method)+Si-DLC (PACVD method), and the outer-most layer of the Si-DLC is coated to alternately laminate the high content layers  630  containing the second Si component of about 3˜10 at % and the low content layers  610  containing the first Si component of about 3 at % or less (not including 0 (zero)) one by one from the CrN coating layer  400 . 
     At this time, the low content layer  610  and the high content layer  630  may be coated to the thickness of 50 nm or less (not including 0 (zero)), respectively, and the low content layer  610  and the high content layer  630  may be coated to the same thickness, preferably. 
     The piston ring of the present invention may be coated in a vacuum coating device as illustrated in  FIG. 4 , which uses a Cr target and a process gas of Ar, N 2  and hydrocarbon gas (C x H y ), TMS (Tetra-methylsilane, Si(CH 3 ) 4 ) or HMDSO (Hexamethyldisiloxane, O(Si(CH 3 ) 3 ) 2 ). 
     First, a plasmatic state is induced using Ar gas under vacuum condition, the surface of the piston ring is activated by heating a coating chamber to about 80° C., and the surface of the piston ring is cleaned by adding bias to force the Ar ions to crash into the surface of the piston ring (baking &amp; cleaning). 
     Next, the Cr layer is coated using only the Cr target to improve adhesion property between the coating layer and the base material (thickness: 0.1˜1.0 μm). The CrN layer is then coated by a chemical reaction of the discharged processing gas N 2  and the sputtered Cr ions on the Cr target (thickness: 0.1˜10 μm). 
     Once the chemical reaction has been conducted using hydrocarbon gas and TMS, or HMDSO gas without the Cr target, C and Si are combined with each other, and the Si-DLC layer is formed (e.g., having a thickness of preferably about 0.1˜10 μm). At this time, by controlling gas containing Si (TMS or HMDSO), a low content layer  610  containing a first Si component of about 3 at % or less (not including 0 (zero)) and a high content layers  630  containing a second Si component of about 3˜10 at % can be alternatively laminated one by one. 
     And,  FIGS. 5 to 7  are graphical illustrations comparing performances of Examples and Comparative Examples of the illustrative embodiment of the present invention. 
       FIG. 5  represents the comparison of friction coefficient. Friction coefficient between the piston ring and cylinder liner was measured using an oscillating friction-wearing tester. Test was conducted at load of 150 N, temperature of 150° C., oscillating period of 5 Hz and oil condition for 1 hour. As a result, friction coefficient of nitriding was the highest, and friction coefficient of Si-DLC was the lowest in order of Si-DLC&lt;DLC&lt;CrN&lt;nitriding as shown in  FIG. 5 . Further, fiction coefficient of the Si-DLC became lower according to doping Si while changing its content ratio as described above. 
       FIG. 6  represents the comparison of resistance to scuffing, and scuffing load between the piston ring and cylinder liner which were measured using an oscillating friction-wearing tester to compare resistance to oil film destruction. Test was conducted at temperature of 150° C., oscillating period of 5 Hz and oil condition while increasing load with 20 N every 20 min up to 440 N. As a result, nitriding generated scuffing most quickly in order of nitriding&lt;CrN&lt;DLC=Si-DLC, and scuffing load of the DLC and Si-DLC were the highest as shown in  FIG. 6 . 
       FIG. 7  represents the comparison of high temperature wear resistance, and piston ring wear between the piston ring and cylinder liner was measured using an oscillating friction-wearing tester. Test was conducted at load of 150 N, temperature of 25° C. and 200° C., oscillating period of 5 Hz and oil condition for 1 hour. As a result, wear of the DLC was largely increased but wear of the Si-DLC was not increased significantly at high temperatures. Further, high temperature wear resistance was further improved when Si was doped while controlling its content. 
     Namely, the piston ring for engine of the present invention consisting of the structure described above and the method for manufacturing thereof accordingly reduces friction loss of the piston ring and improves fuel efficiency 0.2˜0.5% since the friction coefficient of the Si-DLC is 23% lower than Cr plating and nitriding and 11% lower than CrN. 
     Further, because resistance to scuffing of the Si-DLC is 50% or more better than Cr plating and nitriding and 30% or more better than CrN, oil film destruction is suppressed and durability of the piston ring is improved. Furthermore, the low-friction properties and high temperature wear resistance of the DLC are improved by doping Si on the DLC. Additionally, durability of the piston ring can be improved since a Cr+CrN multi-layer is at lower layer when the Si-DLC is worn away. 
     While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.