PAD , AND RELATED BRAKE PAD" DESCRIPTION

Field of the invention

[0001] The present invention relates to a method for manufacturing a brake pad preform, in particular for disc brakes , a method for manufacturing a brake pad and the aforesaid brake pad .

Prior art

[0002] The use of disc brake pads made of carbon-based materials , so-called carbon-carbon or "C/C" composites , is known . These are pads made of composite materials composed of reinforcing carbon fibers within a carbonaceous matrix . Said pads are adapted to cooperate with a disc for disc brakes , generally also made of "C/C" material .

[0003] Carbon-carbon composites are lightweight materials which are used for the manufacture of brake pads when the dissipation of a large amount of energy is required ( for example racing cars or aerospace vehicles ) . The frictional behavior of these materials may be greatly improved by introducing a small amount of abrasive particles , which act as tribological modi fiers by increasing the friction coef ficient at low temperatures . For this reason, controlling the amount , distribution and si ze of the abrasive is of paramount importance in the manufacture of carbon-carbon friction components .

[0004] Current manufacturing methods for carbon-carbon composites doped with abrasive particles provide for the infiltration of the carbonaceous part with a silica colloid, which is subsequently converted into silicon carbide ( SiC ) through a carbothermic reaction . An example of such manufacturing methods is described in document EP1763644A1 .

[0005] Disadvantageously, the brake pads obtained by means of the aforesaid methods of the prior art suf fer from an uneven distribution of the particles in the carbonaceous part , since the concentration o f silicon carbide ( S iC ) progressively decreases from the surfaces towards the core of the brake pad . This results in an undesired variable performance of the brake pad over time , as a consequence of the wear of the pad which exposes regions of the brake pad that are increasingly depleted of silicon carbide ( i . e . of the abrasive portion) .

[0006] Furthermore , inconveniently, the manufacturing methods of the prior art do not allow for adequate control of the si ze of the abrasive particles ( of silicon carbide ) . Furthermore , the morphology of the particles depends on the si ze of the starting particles of the sil ica, and also on their agglomeration after the infiltration process and on the contact surface between the silica and carbon . This typically produces a wide particle si ze distribution, which is however contrary to the need to keep the abrasive performance of the brake pad constant ( the friction coef ficient of the material requires particles with uni form shape and narrow range diameters ) .

[0007] Furthermore , in the manufacturing methods of the prior art , as a consequence of the high operating temperatures ( greater than 1400 ° C ) in the step of heat conversion of silica into silicon carbide, silicon monoxide ( SiO) is produced which condenses and accumulates on the cold areas of the furnaces ( for example on gaskets , pumps , filters and pipes ) . This requires careful and expensive cleaning operations after the heat treatment and considerably reduces the li fe of some components of the furnace .

[0008] Therefore , the problem underlying the present invention is that of providing a disc brake pad which overcomes the drawbacks of the prior art and which maintains high performance characteristics even in extreme applications , and a process for obtaining it which is conveniently feasible . More speci fically, the present invention proposes to modi fy the braking action of the pads themselves through a more uni form distribution of the quantity and si ze of the abrasive particles , in order to obtain abrasive performance of the brake pad that is as constant as possible .

Summary of the invention

[0009] The need is therefore felt in the sector to have a method for manufacturing brake pad preforms and brake pads which are capable of solving the aforementioned drawbacks of the prior art . In particular , the need is felt to implement a brake pad which has a performance that is as uni form as possible as a function of wear over time and which at the same time may be manufactured in a more ef ficient and convenient manner ; all this while maintaining or improving the high performance characteristics of "C/C" composites .

[0010] The above needs are met by a method for manufacturing a brake pad preform, by a method for manufacturing a brake pad and by a brake pad for disc brakes according to the appended independent claims , the definitions of which form an integral part of the present description .

[0011] According to the invention, the method for manufacturing a brake pad preform for disc brakes comprises the following operational steps :

[0012] a ) preparing a thermosetting mixture by mixing a polymer resin in liquid form or particle powder form and ceramic particles in powder form;

[0013] b ) combining the thermosetting mixture obtained in step a ) with a carbonaceous material composed by carbon fibers to obtain a molding compound;

[0014] c ) molding the molding compound obtained in step b ) , for example in a mold for brake pads preforms , by means of compacting and heat treatment to obtain a crude preform;

[0015] d) subj ecting the crude preform obtained in step c ) to a pyrolysis treatment to obtain a brake pad preform .

[0016] An alternative variant of the method provides a step al ) , replacing step a ) , wherein a thermosetting mixture is prepared by mixing a polymer resin in liquid or solid particle form and a liquid preceramic resin, as a precursor of ceramic particles . In this alternative variant of the method, ceramic particles are formed starting from the preceramic liquid resin during the heat treatment of step c ) or during step d) .

[0017] Furthermore , a brake pad for disc brakes according to the present invention is composed of a carbon-carbon composite which is composed of a matrix of carbonaceous material ( or carbonaceous matrix ) and carbon fibers .

Furthermore , ceramic particles are uni formly dispersed in the carbonaceous matrix . [0018] Further features and advantages of the invention will appear more clearly from the description of some preferred embodiments thereof , given hereinafter by way of nonlimiting example only .

Brief description of the figures

[0019] Fig . 1A shows an axonometric view from above of a portion of a disc brake pad according to an embodiment of the invention;

[0020] Fig . IB shows an image obtained with a scanning electron microscope ( SEM) of a cross section of the brake pad of Fig . 1A;

[0021] Fig . 1C shows an image obtained with an optical microscope of a portion of the cross section of Fig . IB, in which carbon fibers , carbon formed by CVI process ( indicated as a CVI matrix ) , carbon formed by pyrolysis of polymers ( indicated as pyrolyzed resin) and residual porosity are indicated .

[0022] Fig . 2A shows an image obtained with a scanning electron microscope ( SEM) of a portion of a brake pad preform close to the outermost surface of the preform, obtained according to a manufacturing method of the prior art ;

[0023] Fig . 2B shows an image obtained with a scanning electron microscope ( SEM) of a portion of the brake pad preform of Fig . 2A, but in proximity to the innermost core of the preform;

[0024] Fig . 3A shows an image obtained with a scanning electron microscope ( SEM) of a portion of a brake pad preform obtained according to the method for manufacturing a brake pad preform according to an embodiment of the invention, close to the outermost surface of the preform; [0025] Fig . 3B shows an image obtained with a scanning electron microscope ( SEM) of a portion of the brake pad preform of Fig . 3A, but in proximity to the innermost core of the preform;

[0026] Fig . 4 shows a summary block diagram of the method for manufacturing a brake pad preform according to a first embodiment of the invention (blocks Al , Bl , Cl , D and E ) and according to a second embodiment of the method according to the invention (blocks A2 , B2 , C2 , D and E ) , which is an alternative to the first embodiment ;

[0027] Fig . 5A to 5E show, each in an illustrative manner, a step of the sequence of steps of the method for manufacturing a brake pad preform according to a first embodiment of the present invention;

[0028] Fig . 6A to 6E show, each in an illustrative manner, a step of the sequence of steps of the method for manufacturing a brake pad preform according to a second embodiment of the present invention .

Detailed description of the invention

[0029] With reference to the above figures , reference numeral 6 globally denotes a brake pad preform for a disc brake according to the present invention .

[0030] The present invention also relates to a brake pad 1 for a disc brake that is directly obtained from the brake pad preform 6 , once any dry and/or wet finishing steps have been carried out on the brake pad preform, for example turning and milling to achieve the desired geometric design .

[0031] The brake pad 1 for disc brakes according to the invention is composed of a carbon-carbon composite , composed of a matrix of carbonaceous material and carbon fibers . In particular, ceramic particles are uni formly dispersed in the matrix of carbonaceous material .

[0032] According to an embodiment of the carbon-carbon composite , the volumetric concentration of ceramic particles in a volume of 5 mm ³ varies within a limit of

±20% between two di f ferent randomly identi fied areas of the pad .

[0033] In order to obtain the aforesaid brake pad preform 6 , the main obj ect of the present invention is a method for manufacturing the brake pad preform 6 for disc brakes , which method comprises a series of operational steps which will be described in detail below . A general embodiment of the method for manufacturing the brake pad preform 6 for disc brakes provides for :

[0034] a ) preparing a thermosetting mixture 2 by mixing a polymer resin in liquid form 21 or particle powder form 22 and ceramic particles 25 in powder form;

[0035] b ) combining the thermosetting mixture 2 obtained in step a ) with a carbonaceous material 3 composed of carbon fibers to obtain a molding compound 4 ;

[0036] c ) molding the molding compound 4 obtained in step b ) , for example in a mold for brake pads preforms , by means of compacting and heat treatment to obtain a crude preform 5 ( or polymeri zed composite ) ;

[0037] d) subj ecting the crude preform 5 obtained in step c ) to a pyrolysis treatment to obtain a brake pad preform 6 .

[0038] According to an embodiment of the method, the polymeric resin in liquid form 21 or particle solid form 22 is composed of one or more of the res ins selected from the group comprising : phenolic resin, acrylic resin, furan resin, isocyanate resin, polystyrene .

[0039] Preferably, the ceramic particles 25 of step a ) comprise silicon carbide ( SiC ) and/or silicon nitride

( S13N4 ) . [0040] According to an advantageous embodiment of the method, the ceramic particles 25 o f step a ) have an average particle si ze comprised between 0 . 5 and 100 micrometers , preferably between 1 and 50 micrometers , even more preferably between 2 and 30 micrometers .

[0041] Preferably, step a ) comprises the step of also mixing a dispersing agent , such as a polyacrylic acid compound or a polyethyleneimine compound, so as to improve the dispersion of the ceramic particles .

[0042] According to an embodiment , step a ) further provides for the use of mechanical mixing treatments or ultrasound treatments to improve the disaggregation and dispersion of the ceramic particles 25 .

[0043] According to an embodiment , the thermosetting mixture 2 is composed of 3% to 20% by weight of ceramic particles 25 in powder form relative to the total weight of the thermosetting mixture 2 , preferably between 9 and 15% by weight of ceramic particles 25 in powder form relative to the total weight of the thermosetting mixture 2 . On the basis of the content of ceramic particles 25 , it is possible to modulate the frictional behavior of the material and optimi ze it for each application .

[0044] In particular, an embodiment in which the thermosetting mixture 2 is composed of at least 10% by weight of ceramic particles 25 in powder form relative to the total weight of the thermosetting mixture 2 allows friction to be favored at temperatures below 300 ° C at the expense of higher temperatures .

[0045] According to a further embodiment in which the thermosetting mixture 2 is composed of at least 3% and at most 10% ( extreme excluded) by weight of ceramic particles 25 in powder form relative to the total weight of the thermosetting mixture 2 , the friction effect has a more balanced behavior even at temperatures higher than 300 ° C . [0046] It is clear that step b ) of j oining the thermosetting mixture 2 obtained in step a ) with a carbonaceous material 3 composed of carbon fibers may take place in di f ferent ways , for example by immersion or by compaction or by mixing and the like .

[0047] According to an embodiment , the carbonaceous material

3 is composed of two-dimensional or substantial ly two- dimensional fabric layers 31 , preferably layers of carbon fiber fabric . In this embodiment, step b ) comprises the operational step of impregnating the two-dimensional fabric layers 31 with the thermosetting mixture 2 and j oining the layers together to form the molding compound

4 to be molded in step c ) . In this case , the molding compound is also referred to as a prepreg . [0048] According to an embodiment variant , the carbonaceous material 3 is composed of chopped carbon fibers 32 . Preferably, in step b ) , after the chopped carbon fibers 32 have been j oined to the thermosetting mixture 2 of step a ) , the molding compound 4 thus obtained is then subsequently formed by hot molding in step c ) to form a crude preform 5 which is preferably polymeric (polymeri zed composite ) .

[0049] According to an advantageous embodiment , the carbonaceous compound 4 is composed of from 50% to 80% by weight of this carbonaceous material 3 and from 20% to 50% by weight of said thermosetting mixture 2 , preferably from 65% to 75% by weight of said carbonaceous material 3 and from 25% to 35% by weight of said thermosetting mixture 2 . This allows a suf ficient quantity of resin 2 to be obtained to bind the carbonaceous material 3 and at the same time maintain as many carbonaceous fibers as possible to increase the mechanical resistance .

[0050] According to an embodiment , the step c ) of molding the molding compound 4 comprises the following operational step cl ) of compacting the molding compound 4 by means of a vacuum compaction technique , for example by means of vacuum bags .

[0051] Furthermore , preferably, the operational step cl ) also comprises the step of also subj ecting the molding compound 4 to an autoclave curing treatment . This embodiment is preferable in the case in which the carbonaceous compound 4 is composed of two-dimensional or substantially two-dimensional fabric layers 31 .

[0052] In this embodiment , in step cl ) , the autoclave treatment is carried out at a pressure between 5 and 15 bar, extremes included .

[0053] According to an embodiment variant , the step c ) of molding the molding compound 4 provides the operational step c2 ) of hot molding the molding compound 4 by means of molding in a uniaxial press . This embodiment is preferable in the case in which the carbonaceous compound is composed of chopped carbon fibers 32 .

[0054] According to an embodiment , in step c2 ) the hot molding is performed at a pressure between 5 and 50 bar, extremes included .

[0055] Preferably, step cl ) or step c2 ) are performed at a temperature comprised between 100 ° C and 160 ° C, extremes included, for at least 30 minutes . This allows a complete cross-linking of the polymeric resin to be obtained .

[0056] According to an embodiment , after step d) , the method further comprises an operational step e ) in which the brake pad preform 6 is subj ected to a carbon densi f ication process to obtain a densified pad preform 7, e.g., a densif ication process by means of a CVD (Chemical Vapor Deposition) technique, or CVI (Chemical Vapor Infiltration) , or PIP (Polymer Infiltration and Pyrolysis) , or PIP with pitch.

[0057] A first densif ication technique is CVD (Chemical Vapor Deposition) or CVI (Chemical Vapor Infiltration) , depending on whether there is only a coating or an infiltration of carbon in the form of vapor. Typically, if the material is fibrous and therefore has a high porosity, it is called Chemical Vapor Infiltration (CVI) . These methods involve the use of hydrocarbon mixtures (e.g. methane and propane) and the exposure of the material to be treated to these mixtures at high temperatures and low pressures. The operating temperatures are in the order of 900-1200 °C, preferably 1000-1100 °C, and pressures lower than 300 mbar, preferably from 10 to 100 mbar, are used. The hydrocarbon mixtures decompose, thus forming elemental carbon which is then deposited or infiltrated in the matrix of the material to be treated. This method, which requires the use of specially dedicated furnaces, involves the deposition of a thin layer (typically a few microns) on the fibers, whereby process times of tens to hundreds of hours are required to obtain the desired densif ication . In this way it is possible to achieve overall coverage on the fibers of more than ten microns (typically 10-20 microns) . [0058] A different method, known as LPI (Liquid Polymer Infiltration) or PIP (Polymer Infiltration and Pyrolysis) involves the infiltration of the matrix of the material to be treated with a liquid polymer and the subsequent high- temperature heat treatment (pyrolysis) which causes the carbonization of the polymer deposited on the carbon fibers. In this case, several infiltration and pyrolysis steps are required before obtaining an appropriate densif ication of the preform.

[0059] According to an embodiment, it is possible to use a combination of densif ication techniques, for example a combination of PIP and CVI techniques.

[0060] According to an embodiment, step e) comprises the step of densifying the brake pad preform 6 obtained in step d) until a density of the final material of at least 1.5 grams per cubic centimeter (g/cm ³ ) is obtained, preferably greater than 1.65 grams per cubic centimeter (g/cm ³ ) .

[0061] These density values give the material of the densified brake pad preform 7 the appropriate characteristics of mechanical strength, thermal conductivity and wear resistance. [0062] For the manufacture of the brake pad preform 6 , the manufacturing method optional ly also provides the following steps :

[0063] i ) optionally, needling the two-dimensional or substantially two-dimensional layers of fabric, which are superimposed, to form an interwoven three-dimensional structure ;

[0064] i i ) optionally, needling the chopped fibers to form a three-dimensional interwoven structure .

[0065] Needling may be carried out with methods that provide for the use of special needles which engage part of the fibers by directing them axially to the pad, allowing three-dimensional structures to be obtained .

First embodiment example

[0066] An embodiment example of a brake pad preform 6 obtained according to an embodiment of the method according to the present invention is described below, for which some detailed portions obtained by the SEM are shown in Fig . 3A and 3B .

[0067] The brake pad preform 6 according to this embodiment example was obtained by means of the following operational steps :

[0068] - a thermosetting mixture 2 was prepared by mixing an isocyanate polymer resin 22 in solid form with ceramic particles 25 of silicon carbide ( Sic ) in powder form, each ceramic particle having an average particle si ze of about 2 micrometers ;

[0069] - the thermosetting mixture 2 obtained in step a ) was combined with a carbonaceous material 3 composed of chopped carbon fibers 32 to obtain a molding compound 4 .

[0070] The molding compound 4 obtained in this embodiment example is composed of 30% by weight of isocyanate polymer resin, 3% by weight of silicon carbide ( Sic ) ceramic particles and 67 % by weight of chopped carbon f ibers .

[0071] Subsequently, the aforementioned molding compound 4 was molded in a brake pad preform mold by compacting and heat treatment in a press , so as to obtain a crude preform 5 .

[0072] Subsequently, the crude preform 5 was subj ected to a pyrolysis treatment and a heat treatment to obtain a brake pad preform 6 in which a carbon-carbon compound is then formed .

[0073] Subsequently, the brake pad preform 6 was also subj ected to a densi f ication treatment by means of a CVI ( Chemical Vapor Infiltration) densi f ication technique , obtaining a densi fied brake pad preform 7 .

[0074] Fig . 3A and 3B show the SEM images of the present embodiment example , which are easily comparable with the respective Fig. 2A and 2B which show SEM images of corresponding portions of a brake pad preform 6 obtained with a method according to the prior art. In the images, the silicon carbide (SiC) particles correspond to the lighter points (in white/light gray) compared to the background .

[0075] The comparison between the images clearly demonstrates that in Fig. 3A and 3B (i.e. in the present invention) , there is no significant reduction in the density of silicon carbide (SiC) particles in the transition from the surface region of the preform (Fig. 3A) to the deeper area of the preform (Fig. 3B) .

[0076] On the contrary, in the preform made with the known technique, Fig. 2B shows an evident reduction in density of the silicon carbide particles with respect to Fig. 2A, relating to the most superficial portion of the preform.

[0077] As may be appreciated from what has been described, the brake pad preform 6, the brake pad 1 and the related manufacturing methods of this preform 6 and brake pad 1 of the present invention allow the drawbacks presented in the prior art to be overcome.

[0078] In particular, the present invention provides a method for manufacturing a carbon-carbon composite doped with ceramic particles uniformly dispersed in the matrix of carbonaceous material , which composite is capable of guaranteeing constant braking performance regardless of pad wear .

[0079] Furthermore , advantageously, since the method according to the present invention does not require high conversion temperatures of the silicon oxide into silicon carbide during the infiltration process , it is more ef ficient than the methods of the prior art , both s ince it does not require the use of high temperatures and since it reduces the risk of formation of silicon oxide deposit on the cold portions of the machinery used in the process . Furthermore , the use of SiC particles , instead of silica that has to be trans formed, allows the si ze of the introduced powders to be controlled, which are otherwise influenced by the heat treatment necessary for their conversion .

[0080] It is clear that a person skilled in the art may make several changes and adj ustments to the pad preform, to the pad and to the methods described above in order to meet speci fic and incidental needs , which changes all fall within the scope of protection defined in the appended claims .