Abstract:
The invention relates to a method for producing a friction element ( 3 ) comprising the steps: providing a metal main body ( 10 ), hardening the main body ( 10 ) on at least part of its surface ( 11, 12 ) in a salt bath, wherein the salt bath hardening is the final method step, and no further processing of the hardened surface ( 11, 12 ) is performed. Furthermore, the invention relates to a friction element produced according to this method and a friction component comprising the latter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of and Applicant claims priority under 35 U.S.C. §§120 and 121 of U.S. application Ser. No. 13/309,093 filed on Dec. 1, 2011, which claims priority under 35 U.S.C. §119 from Austrian Patent Application No. A 2015/2010 filed on Dec. 3, 2010, the disclosures of each of which are hereby incorporated by reference. A certified copy of priority Austrian Patent Application No. A 2015/2010 is contained in parent U.S. application Ser. No. 13/309,093. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The invention relates to a method for producing a friction element comprising the steps: providing a metal main body and hardening the main body on at least one part of its surface in a salt bath, a friction element comprising a metal main body, which has a plurality of surfaces, wherein at least one of the surfaces is hardened by salt bath at least partly, a friction component comprising at least one first friction element and at least one second friction element, wherein the two friction elements can be arranged so as to be brought into friction closure with one another as well as the use of the friction element. 
         [0003]    Such friction elements are used for example in disc couplings as so-called mating discs. The disc coupling usually comprises a plurality of discs. In this case a first group is arranged on a disc carrier in a rotationally and displacement secure manner and a second group on a hub. The two groups can be displaced relative to one another in axial direction and can be moved into and out of engagement in pairs. 
         [0004]    The friction discs are designed in general to be annular and sandwich-like. They comprise a support sheet which is made for example of steel and which supports a friction lining on its annular surfaces. The latter usually consists of a fiber layer which is made e.g. of paper or a paper-like material and is soaked in an organic resin provided with friction particles. The mating discs are produced differently from a single piece of friction material. A mating disc of this kind is described e.g. in WO 97/32678 A1. 
         [0005]    In said disc packets the lubrication of the individual components plays an important role. In particular, it must be ensured that lubricant oil reaches the friction surfaces. Therefore, reference is also made to so-called wet-running applications of the discs. The friction lining usually has in addition a porous structure, whereby a certain amount of oil saving is achieved. The saved oil is removed again during the friction closure of the disc with the mating disc and the resulting compression. In order to improve the oil delivery the friction lining normally comprises grooves for guiding the lubricating oil. The lubricant oil also performs the function of cooling the friction surfaces, in that the oil absorbs the frictional heat resulting from frictional closure and removes it via the grooves. 
         [0006]    Mating discs usually do not have such grooves, whereby said mating discs are exposed to higher temperatures. It may occur in this case that the strength of the mating disc is reduced owing to the high temperature, in particular if the latter is made from an aluminum alloy. The use of aluminum alloys is advisable as the motor industry demands the reduction in weight of individual assemblies. 
         [0007]    In order to overcome this problem of overheating the mating discs, in DE 199 06 980 A1 it is proposed to structure the surfaces of the discs so that oil pockets are formed. The oil pockets ensure that the friction surface at risk is covered by a residual lubricant oil supply even when the adjacent disc moved out of its axial perpendicular position bears in a grinding manner with its end edge against the said friction surface or centrifugal forces try to spin off the oil film. The structuring of the surface in this case only needs to be minimal, with minimal elevations and depressions. The structure only has to be designed so that an effective oil supply is available on or in the relevant friction surface. The structures of the surface and thereby of the said oil pockets can be microscopically small. However, they can also lie within the macro range. They can have the appearance of a fabric imprint. They can be formed in a different way, for example by laser processing of the disc-surfaces or also a chemical treatment, for example in an etching process in sintered linings. 
         [0008]    It is also known in the prior art to harden the surfaces of mating discs, in particular the mating discs which do not have separate friction linings. One of the hardening methods used is salt bath hardening. 
       SUMMARY OF THE INVENTION 
       [0009]    The objective of the present invention was to create a friction element with improved thermal stability. 
         [0010]    This objective is achieved by means of the said method, in which the salt bath hardening is the final method step hardening, and there is no further processing of the hardened surface, and also by the friction element, in that the salt bath hardened surface is at least partly oxidized, where several different oxides are formed on the surface in discrete areas, by a friction component comprising the friction element according to the invention, and by the use of a salt bath hardened friction element as mating disc in a wet-running friction component with at least one additional friction element, wherein the additional friction element comprises an organic friction lining on a support element. 
         [0011]    It is an advantage that by means of salt bath hardening a structured surface is formed, which as in DE 199 06 980 A1 serves as an oil storage area, wherein however, owing to the substances formed on the surface during salt bath hardening, in particular oxides, an adhesive bond is formed between the lubricant oil or additives of the lubricant oil, which are added to improve the tribological properties of the lubricant oil, i.e. the lubricant oil or its additives are available over a longer period also in the engaged position of the friction surfaces of the coupling. An additional effect of the method is that the production of the friction element is less expensive, as the final processing steps usually performed after the salt bath hardening, such as e.g. washing out the residues of the salt bath or polishing the surface of the friction element to remove the oxides formed, are omitted. By adding additives to the lubricant oil or oil molecules the coefficient of friction, the coefficient of friction path and the noise or vibration behavior can be improved, and there is also a reduction in wear, in particular in mating discs made of aluminum or aluminum alloys. By means of the at least partial formation of an oxide layer from different oxides the surface of the friction element in the region of the friction surface is given a different polarity, so that the adhesive binding of oil molecules and oil additives, which usually also have a different polarity, is improved. With said friction element the wear of a friction disc with an applied friction lining can be reduced. 
         [0012]    Preferably, a salt bath is used which contains at least one salt of an inorganic acid, in particular a salt is selected from a group comprising or consisting of oxides, nitrates, nitrites, hydrogen nitrates, chlorides, carbonates and cyanides, so that the salt bath hardened surface comprises salt residues from the salt bath. Owing to the ionic bonding they are very polar salts, whereby the adhesive bonding of polar additives added to the lubricant oil to improve the tribological properties can be improved considerably. 
         [0013]    Preferably, the salt bath hardening is performed at a temperature selected from a range with a lower limit of 150° C. and an upper limit of 450° C., as in this temperature range the rapid formation of oxidic bonds on the surface of the main body is achieved, from which a reduction of the processing time and thereby also a reduction in the production costs of the friction element is achieved. Furthermore, it was observed that by hardening in said temperature range the forming individual metal oxides are limited to discrete areas, whereby it should be noted that the whole friction surface of the friction element is covered with oxides after the salt bath hardening, which are of a size that is advantageous for the formation of lubricant areas, in particular also with respect to the noise reduction of the entire disc packet, i.e. the friction component. Said discrete areas in this case have a maximum diameter in plan view which is selected from a range with a lower limit of 50 μm and an upper limit of 300 μm. 
         [0014]    Said discrete areas are preferably in the form of craters, which means that the amount of lubricant oil or lubricant additives stored therein can be increased. 
         [0015]    For the same reasons alternatively or additionally the main body can be heated prior to salt bath hardening, in particular to a temperature selected from a range with a lower limit of 200° C. and an upper limit of 600° C. 
         [0016]    As already mentioned above, by means of the method preferably friction elements are made from steel or aluminum or an aluminum alloy. 
         [0017]    According to one embodiment variant of the friction element the oxides are arranged at least partly in several layers, whereby the storage capacity of the friction surface for oil molecules and lubricant additives can be improved. 
         [0018]    Preferably, the friction element according to the invention is used in a motorbike coupling which, owing to the reduced amount of space available for the coupling and the pressure caused by high speed motors, is exposed to increased temperatures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    For a better understanding of the invention the latter is explained in more detail with reference to the following Figures. 
           [0020]    In a schematically simplified representation: 
           [0021]      FIG. 1  shows a section of a disc packet in side view; 
           [0022]      FIGS. 2 to 4  shows different images of the frictional surface of a friction element; 
           [0023]      FIG. 5  shows a diagram of the path of the coefficients of friction as a function of the number of shifting cycles; 
           [0024]      FIG. 6  shows a diagram of the NVH behavior, 
           [0025]      FIG. 7  shows a diagram of the wear of a friction disc which was used in a friction packet with a friction element according to the invention as mating disc. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    First of all, it should be noted that in the variously described exemplary embodiments the same parts have been given the same reference numerals and the same component names, whereby the disclosures contained throughout the entire description can be applied to the same parts with the same reference numerals and same component names. Also details relating to position used in the description, such as e.g. top, bottom, side etc, relate to the currently described and represented figure and in case of a change in position should be adjusted to the new position. 
         [0027]      FIG. 1  shows a section of friction component  1  of a disc coupling in side view. Said friction component  1  comprises at least one first friction element  2  and at least one second friction element  3 . The at least one first friction element  2  is arranged on a shaft  4  and connected to be latter in a non-rotatable manner, the at least one second friction element  3  is arranged on a pin  5  and connected to the latter in a non-rotatable manner. The pin  5  is connected in a non-rotatable manner to an additional, not shown shaft. The shaft  4  and the pin  4  are adjustable relative to one another in axial direction, so that the friction elements  2 ,  3  enter into frictional closure with one another. 
         [0028]    The number of shown friction elements  2 ,  3  should not be considered to be restrictive. Rather the number of friction elements  2 ,  3 , i.e. friction discs, corresponds to the frictional power required. For example for each disc packet three, four, five, six, seven, eight, nine, ten to twenty etc. friction elements  2  or  3  can be combined into a friction component. 
         [0029]    The friction element or elements  2 , i.e. the friction discs, can be designed according to the prior art. They comprise or consist e.g. of a support element  6 , which is formed in particular by a metal element, for example a steel ring, which is provided on at least one surface  7 , preferably also on a surface  8  opposite said surface  7  in axial direction, with a friction lining  9  and joined thereto. The friction lining  9  or friction linings  9  can be made from an organic resin, which contains friction particles or friction-modifying particles and possibly fibers and hard materials and is mounted at least partly on a woven or non-woven fiber layer. As such embodiments of friction elements  2  are known from the prior art, reference is made to the relevant literature. 
         [0030]    The friction elements  3  however, which are also defined in technical terms as mating discs, are only formed as a metal element without friction lining. In this case a metal main body  10  has at least on one surface  11 , preferably also on a surface  12  opposite said surface  11  in axial direction, a friction surface  13 , which is preferably as large as the respective surface of the friction lining  9  bearing against said friction surface  13  in the engaged state of the disc packet in frictional closure. 
         [0031]    The main body  10  of the friction element  3  can either be made of a sintered material or preferably of a solid material, wherein in the latter case the main body  10  is cut or stamped from a corresponding plate-like raw material, however, also a casting method is possible in principle. Preferably, the main body  10  is made of steel or a steel alloy or aluminum or an aluminum alloy. In principle however, other materials can also be used which are accessible to the salt bath hardening. 
         [0032]    Preferably, the friction elements  3 , i.e. the mating discs, are used in couplings, in particular motorbike couplings. In principle, said friction elements  3  can also be used in other friction components e.g. such as single and multiple synchronizations, uncontrolled and controlled distributor gears, brakes, differential locks, torque vectoring applications, etc. as mating discs. 
         [0033]    The friction element  3  is intended for so-called wet running applications, i.e. applications in which the latter is wetted with a lubricant and coolant, for example in particular a synthetic oil. In their basic form such friction discs are known from the prior art in all kinds of different geometric shapes which correspond with the respective application. As this would be known to a person skilled in the art, reference is made to the relevant literature. 
         [0034]    According to the invention the main body  10  of the friction elements  3  is hardened by salt bath at least in the area of the friction surface(s)  13 , which forms the last processing step for the production of the friction element  3 —with the exception of quality control and packaging—in the method according to the invention. In addition, said main body  10  is immersed in a salt bath after the main shaping and possible deburring or degreasing. 
         [0035]    In principle, the salt bath can have a composition which corresponds to the prior art in relation to salt bath hardening. Preferably, the salt bath contains at least one salt of an inorganic acid, in particular a salt, for example an alkaline salt or alkaline earth salt, selected from a group comprising or consisting of oxides, nitrates, hydrogen nitrates, nitrites, chlorides, carbonates, cyanides. 
         [0036]    For example, the salt bath can be composed according to the following data or comprise said components: 
       EXAMPLE 1  
       [0037]    25 wt. % to 35 wt. % barium chloride 
         [0038]    35 wt. % to 45 wt. % potassium chloride 
         [0039]    15 wt. % to 25 wt. % sodium chloride 
         [0040]    5 wt. % to 15 wt. % lithium chloride 
       EXAMPLE 2  
       [0041]    60 wt. % barium chloride 
         [0042]    10 wt. % sodium cyanide 
         [0043]    30 wt. % sodium carbonate 
       EXAMPLE 3  
       [0044]    55 wt. % barium chloride 
         [0045]    15 wt. % potassium cyanide 
         [0046]    30 wt. % sodium carbonate 
         [0047]    In particular the salt bath consists of nitrates 
         [0048]    For the salt bath hardening the salt bath is heated to the temperature at which the salts are melted. Preferably however, a temperature is chosen which is selected from a range with a lower limit of 150° C., in particular 200° C., and an upper limit of 450° C., in particular 300° C. 
         [0049]    Alternatively or preferably, in addition the main body  10  can be heated to a temperature, which is selected from a range with a lower limit of 200° C., in particular 250° C., and an upper limit of 600° C., in particular 500° C. For example, the temperature of the main body  10  can be between 300° C. and 600° C., if the latter is made of steel, or between 200° C. and 250° C., if the latter is made of aluminum or an aluminum alloy, such as e.g. Aludur (aluminum alloy with Si, Mn and Mg), Aluman (aluminum alloy with 1.1 wt. % Mn), Duraluminum (aluminum alloy with 2.5 wt. % to 5.5 wt. % Cu, 0.2 wt. % to 5 wt. % Mg). 
         [0050]    The main body  10  is left in the salt bath for a period of between 5 minutes, in particular 10 minutes, and 40 minutes, in particular 20 minutes. 
         [0051]    Afterwards the main body  10  is removed from the salt bath again, wherein excess liquid salt bath contents are removed by letting them run off. The main body is then cooled in air at room temperature, i.e. ambient temperature, or below inert gas atmosphere (N 2  and/or He, etc.), or in an oil bath. 
         [0052]    The salt bath hardened main body  1  now has one or two salt bath hardened friction surface(s)  13 , which are oxidized at least partly, but preferably completely, wherein several different oxides are formed on both the surface  11  and/or  12  of the main body  10  in discrete areas.  FIGS. 2 to 3  both show sections in different enlargements of a friction surface  13  of a steel mating disc which was hardened in a salt bath made of nitrates at a temperature of 200° C. for a period of 10 minutes. 
         [0053]    It can be seen clearly from these Figures, that the oxides formed, in particular Fe 2 O 3 , NaOH, K 2 O, KOH, K 2 CO 3 , are present in their own discrete areas  14 , wherein said areas comprise a single oxide or mixed oxide, i.e. the different oxides are formed on the surface  11 ,  12  of the main body  10  next to one another respectively, where the term “next to one another” does not mean “adjacent”. Said discrete areas  14  have in particular a maximum diameter in plan view, which is selected from a range with a lower limit of 50 μm, in particular 75 μm, and an upper limit of 300 μm, in particular 200 μm. 
         [0054]    Preferably, said areas  14  are designed to be at least partly crater-like, in particular with a crater depth of between 2 μm, in particular 5 μm, and 20 μm, in particular 10 μm. 
         [0055]    The oxides can also be arranged at least partly in several layers, for example between one and four layers. This can be adjusted by the temperature of the salt bath, the temperature of the main body  10 , at which the latter enters the salt bath and by the period the main body  10  stays in the salt bath. 
         [0056]    Preferably, the salt bath hardened friction surface or surfaces  13  comprises or comprise at least partly salt residues  15  from the salt bath, as shown in  FIG. 4 , wherein said salt residues  15  are located in particular within the (crater-like) discrete areas  14 . 
         [0057]    In the following several examples of friction elements  3  and comparison examples are given together with the corresponding measurement values of the investigations. 
         [0058]    By means of an EDX analysis (Energy-dispersive X-ray spectroscopy) it could be established that on the friction surface  13  the salts NaNO 3 , KNO 3  and K 2 CO 3  are found in a concentration of 2 wt. % to 22 wt. % NaNO 3 , 1 wt. % to 18 wt. % KNO 3  and 1 wt. % to 2 wt. % K 2 CO 3 . The main body  10 , i.e. its friction surfaces  13 , is colored black or has black coloring. 
         [0059]      FIGS. 5 to 7  show different diagrams of the characteristic values of a friction element  3  made of steel according to the invention produced by using a salt bath according to Example 1 compared to a mating disc made of steel according to the prior art. Said mating discs were tested respectively in a friction component with the same friction disc according to the prior art. 
         [0060]      FIG. 5  shows the frictional behavior of the friction element  3  according to the invention as well as the mating disc according to the prior art over time, i.e. the number of shift cycles, which are entered on the x-axis. The coefficient of friction of is entered on the y-axis. It can be seen that the friction element  3  according to the invention (triangles) provides comparable values to the mating disc according to the prior art, wherein however as considered over the number of shift cycles after a running in phase the friction element  3  according to the invention has a more constant coefficient of friction path. Since there are no standardized testing methods, the measurements were made under the following test conditions: p spec. [N/mm 2 ]3-9, v [m/s] 0.18-0.45, theoretic friction moment [Nm] 200-1000. 
         [0061]      FIG. 6  shows a diagram of the NVH behavior (Noise, Vibration and Harshness behavior) of the friction element  3  according to the invention and the mating disc according to the prior art. The vibration intensity of the friction moment is shown (averaged values cf amplitude). In the NVH assessment a noise point is allocated at each pressure stage (max. 2), if cf-amplitude is greater than 0.025. It can be seen clearly that in this test the friction element  3  according to the invention (right bar) was significantly better than the mating disc according to the prior art. 
         [0062]    The abrasion resistance (wear) shown in  FIG. 7  was determined by a wall thickness measurement of a friction disc used with the mating disc before and after the test run, as the mating disc according to the invention has a positive effect on the wear resistance of the friction disc. In this case the wear is entered on the x-axis in [mm]. As shown clearly, the friction disc (lower bar) used with the mating disc according to the invention, i.e. the friction element  3 , under the same conditions shows much lower wear than a friction disc of the same composition, which was run with a mating disc according to the prior art (top bar). 
         [0063]    The exemplary embodiments show possible embodiment variants of the friction component  1  and the friction element  3 , whereby it should be noted at this point that the invention is not restricted to the embodiment variants shown in particular, but rather various different combinations of the individual embodiment variants are also possible and this variability, due to the teaching on technical procedure, lies within the ability of a person skilled in the art in this technical field. 
         [0064]    Finally, as a point of formality, it should be noted that for a better understanding of the structure of the friction component  1  or the friction element  3  the latter and its components have not been represented to scale in part and/or have been enlarged and/or reduced in size. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1 . Friction component 
           2 . Friction element 
           3 . Friction element 
           4 . Shaft 
           5 . Pin 
           6 . Support element 
           7 . Surface 
           8 . Surface 
           9 . Friction lining 
           10 . Main body 
           11 . Surface 
           12 . Surface 
           13 . Friction surface 
           14 . Area 
           15 . Salt residue