Method for making a brake band for a brake disc of cast iron and brake band

Abstract

The present invention relates to a method for making a brake band of a cast iron brake disc and to a brake band, the method comprising the steps of: a) making a brake band of a cast iron brake disc; b) at least partially immersing the brake band into molten aluminum maintained at a predetermined temperature so as to cover at least a predetermined surface area of the brake band with the molten aluminum. The immersion is continued for a predetermined period of time to allow aluminum atoms to diffuse in the surface microstructure of the cast iron, forming iron-aluminum intermetallic compounds at the surface layer of the brake band, thereby creating a layer comprising iron-aluminum intermetallic compounds in the predetermined surface area of the brake band. The method further comprises: c) taking the brake band out of the molten aluminum; d) after taking out, removing the aluminum still adhering on the brake band so as to expose the layer of iron-aluminum intermetallic compounds on the surface, the layer of iron-aluminum intermetallic compounds exposed on the surface providing the cast iron brake band with higher corrosion and wear resistance at the predetermined surface area.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for making a brake band for a cast iron brake disc, and to the brake band thus obtained, having increased wear and corrosion resistance. BACKGROUND

[0002] The brake disc of a disc brake system of a vehicle comprises a ring-like structure, or brake band, and a central fixing element, called bell, by which the disc is fixed to the rotating part of the vehicle suspension, for example the wheel hub. The brake band is provided with opposite braking surfaces adapted to cooperate with friction elements, brake pads, housed in at least one caliper body placed astride the brake band and integrated with the non-rotating part of the vehicle suspension. The controlled interaction between the opposite brake pads and between the opposite braking surfaces of the brake band determines the braking action by friction, which allows the vehicle to slow down or stop.

[0003] Generally, the brake band of a brake disc is made of cast iron, in particular grey cast iron, or steel. In fact, cast iron allows to obtain good braking performances, especially in terms of wear containment, at relatively low cost. Brake bands made of carbon or carbon ceramic materials provide better performances, but at much higher cost.

[0004] Although cast iron is harder than many metals, during the operation of the brake disc it wears the cast iron, thus reducing the thickness of the brake band and limiting the service life of the brake disc.

[0005] Another limit of cast iron is related to the fact that, after exposure to water, in particular in the presence of salt (sodium chloride, potassium chloride), the cast iron corrodes rapidly. This forms an oxide layer on the surface of the brake band, which, before being removed, causes braking problems, in addition to being aesthetically unattractive.

[0006] To solve these problems, various strategies have been implemented to treat the brake band made of cast iron disc, which are outlined below:

[0007] - Application of surface coatings : application of a coating based on hard, wear-resistant materials (chromium and tungsten carbides, aluminum oxides, etc.) to the surface of the cast iron brake band. Various techniques exist to apply this coating; the most commonly used technique is "thermal spraying": a ceramic powder is mixed to a suitable metal matrix, which adheres the coating to the cast iron. The thermal spraying technique is preferred over other film deposition methods, as it allows to achieve higher productivity and use of the above-mentioned materials. The main limit of this technique is the relatively high cost if it is referred to the application of thermal spraying on cast iron brake discs;

[0008] - Reaction / diffusion processes: This is the process that makes cast iron react with other substances, which changes the chemical-physical state of the cast iron, thus increasing the surface properties of the cast iron. The reaction / diffusion processes include carburizing, nitriding and boronizing. The reaction can occur in the gas phase or in the liquid phase. The main limitations of this type of process are the limited improvement in performance, the long processing times and, in some cases, excessively high processing temperatures, which are not compatible with the need to control the deformation of the cast iron disc.

[0009] - "in situ" formation processes: these processes involve the formation of a surface layer made of a material with suitable properties on the brake band during the formation thereof, i.e. during the casting of the molten cast iron in the mould. Before casting, elements / substances that, in contact with the molten cast iron, form new compounds or simply add elements with the desired properties are introduced into the mould. The main limitations of this type of process are related to the fact that the manufacturing process of the cast iron brake band is very complex. Moreover, in some cases, heat treatments are required at temperatures that are not compatible with the temperatures required to control the deformation of the cast iron disc.

[0010] Therefore, there is a strong felt need for a method for making a brake band for a cast iron brake disc that allows the formation of a surface layer with higher wear and corrosion resistance properties on the surface of the brake band and which completely or at least partially overcomes the operating limitations of the prior art described above. SUMMARY

[0011] Therefore, the aim of the present application is to eliminate or at least reduce the problems described above related to the prior art by providing a method for making a brake band for a cast iron brake disc with increased wear and corrosion resistance that does not cause a significant increase in production costs.

[0012] Another aim of the present application is to provide a method for making a brake band for a cast iron brake disc with increased wear and corrosion resistance that does not cause a significant increase in production times.

[0013] Another aim of the present application is to provide a method for making a brake band for a cast iron brake disc with increased wear and corrosion resistance that can be carried out at processing temperatures compatible with the requirements to control the deformation of the brake band. BRIEF DESCRIPTION OF DRAWINGS

[0014] The technical features of the present application can be found clearly in the content of the attached claims and the advantages of these technical features will become more apparent from the detailed description below, with reference to the attached drawings, which show by way of non-limiting example one or more embodiments of the technical features, in which:

[0015] Figure 1a flow chart of the method according to the present application in accordance with the general embodiment is shown;

[0016] Figure 2 、 Figure 3 and Figure 4 a flow chart of the method according to the present application in accordance with the preferred embodiment is shown;

[0017] Figure 5 SEM images obtained with a scanning electron microscope on a cast iron sample provided with a layer of intermetallic compounds, the cast iron sample provided with a layer of intermetallic compounds being obtained according to the method of the present application without prior surface decarburization treatment by aluminizing treatment at 700°C for about 1 minute;

[0018] Figure 6 SEM images obtained with a scanning electron microscope on another cast iron sample provided with a layer of intermetallic compounds, the cast iron sample provided with a layer of intermetallic compounds being obtained according to the method of the present application with prior surface decarburization treatment by aluminizing treatment at 700°C for about 1 minute;

[0019] Figure 7 SEM images obtained with a scanning electron microscope at high magnification (2000x) on a brake band sample according to the present application.

[0020] Elements or components common to the described embodiments will be denoted hereinafter using the same reference numerals. DETAILED DESCRIPTION

[0021] The present application relates to a method for making a brake band for a cast iron brake disc having increased wear and corrosion resistance and to a brake band obtained thereby.

[0022] The making method will be described first, then the brake band obtained by this method.

[0023] According to the general embodiment of the present application, as shown in the chart of Figure 1 the method for making a brake band for a cast iron brake disc having increased wear and corrosion resistance comprises the following operating steps:

[0024] a) making a brake band for a cast iron, preferably gray cast iron, brake disc;

[0025] b) at least partially immersing the brake band in molten aluminum held at a predetermined temperature, so that the molten aluminum covers at least a predetermined surface area of the brake band.

[0026] the immersion in the molten aluminium is continued for a predetermined period of time to allow diffusion of aluminium atoms in the microstructure of the cast iron surface, thereby forming iron-aluminium intermetallic compounds at the surface layer of the brake band, thus creating a layer comprising iron-aluminium intermetallic compounds in said predetermined surface area of the brake band.

[0027] The method comprises the following further operational steps:

[0028] c) removing the brake band from the molten aluminium;

[0029] d) after removal, removing the aluminium that is still adhering to the brake band and that has not reacted with the cast iron, to expose the layer of iron-aluminium intermetallic compounds on the surface.

[0030] It has been found experimentally that the layer of iron-aluminium intermetallic compounds exposed on the surface provides the brake band, whose predetermined surface area is cast iron, with a higher resistance to corrosion and wear compared to the case in which the cast iron brake band has no coating and the cast iron is directly exposed on the surface.

[0031] The properties provided by the layer of iron-aluminium intermetallic compounds are maintained over time because such a layer combines resistance to corrosion and resistance to wear. In other words, the protective layer made of iron-aluminium intermetallic compounds is less attacked both chemically and mechanically - by the friction caused by the braking action exerted on the brake band. This synergistically contributes to protecting the brake band during its operating life.

[0032] The method according to the present application is operationally easier to implement compared to the traditional techniques described above for treating the brake band of a cast iron brake disc (application of surface coatings; reaction / diffusion processes; "in situ" forming processes) because the method of the present application does not require the use of complex equipment to be built and operated.

[0033] Furthermore, the method according to the present application requires a relatively short execution time at least on steps b), c) and d).

[0034] The method according to the present application therefore allows to manufacture a brake band of a cast iron brake disc with increased resistance to wear and corrosion in a technically simple manner without involving significant increases in costs and production times.

[0035] More in detail, the layer of iron-aluminium intermetallic compounds described above is formed at the surface layer of the cast iron brake band in contact with the molten aluminium through a diffusion process of aluminium atoms within the cast iron structure and the formation of iron-aluminium intermetallic compounds.

[0036] The aforementioned iron-aluminum intermetallic compound layer may include multiple iron-aluminum intermetallic compounds, particularly Fe3Al, FeAl, FeAl2, FeAl3, and Fe2Al5. The main intermetallic phase is FeAl3 because FeAl3 is thermodynamically more stable.

[0037] Preferably, the molten aluminum is maintained at a predetermined temperature of not less than 680°C.

[0038] Advantageously, the molten aluminum is held at a predetermined temperature not exceeding 750°C, and preferably between 690°C and 710°C, or even more preferably equal to 700°C. Such a temperature value represents a good compromise between the need to promote the formation of intermetallic compounds and the need to maintain the geometric characteristics of the brake band. In fact, at such a temperature of molten aluminum, the chemical reaction between iron and aluminum is fast enough to allow for the rapid formation of an intermetallic compound layer, and from an operational perspective, the deformation induced on the brake band is within acceptable tolerances.

[0039] In practice, the thickness of the intermetallic compound layer is primarily based on the temperature of the molten aluminum and the immersion time in the molten aluminum. With the same molten aluminum temperature, the thickness increases with increasing immersion time; with the same immersion time, the thickness increases with increasing molten aluminum temperature.

[0040] Advantageously, since the temperature of the molten aluminum is the most constrained parameter to avoid deformation of the brake band, the thickness of the intermetallic compound layer is essentially controlled by the duration of immersion in the molten aluminum. Taking into account the temperature of the molten aluminum, the predetermined immersion time is set according to the desired thickness of the intermetallic compound layer.

[0041] Preferably, if the temperature of the molten aluminum is maintained between 690°C and 710°C, the predetermined immersion time is between 5 minutes and 60 minutes, or even more preferably equal to 30 minutes, to obtain an intermetallic compound layer between 30 micrometers and 150 micrometers.

[0042] according to Figure 2 and Figure 3 A preferred embodiment of the method shown in the diagram includes step f): decarburizing at least one predetermined surface area (the area intended to contact molten aluminum) of the brake band to a predetermined depth.

[0043] Operationally, the decarburization step f) is performed before the immersion step b), and is adapted to form a substantially carbon-free, in particular graphite-free surface layer at least in the predetermined surface area, the surface layer having a thickness that extends from the outer surface of the brake band to the predetermined depth.

[0044] Advantageously, the depth of the surface layer which is decarburized is substantially equal to or greater than the desired depth of the iron-aluminium intermetallic compound layer. Operationally, the decarburization is performed up to a depth equal to or greater than the depth of the iron-aluminium intermetallic compound layer to be obtained from the outer surface of the brake band. Preferably, the predetermined decarburization depth is 30 microns or greater.

[0045] Advantageously, the purpose of step f) of decarburization is to prepare a surface layer in the brake band which is advantageous for the formation (by aluminizing) of a more uniform and compact intermetallic compound layer.

[0046] In fact, it has been found experimentally that by aluminizing (immersion in molten aluminium) a brake band which has not been subjected to surface decarburization, the intermetallic compound layer obtained is very non-uniform, with large separation zones and a strong variability in the growth thickness. Furthermore, the growth structure appears very granular.

[0047] On the contrary, by aluminizing (immersion in molten aluminium) a surface-decarburized brake band made of cast iron, the intermetallic compound layer obtained is uniform and substantially continuous, without being interrupted by graphite flakes.

[0048] By comparing Figure 5 and Figure 6 it can be seen that, Figure 5 and Figure 6 show SEM images obtained with a scanning electron microscope on two cast iron samples aluminized at 700°C for about 1 minute. Figure 5 refers to a sample aluminized without prior surface decarburization, while Figure 6 refers to a sample aluminized with prior surface decarburization. In Figure 5 and Figure 6 the intermetallic compound layer is indicated with L, the residual aluminium layer is indicated with Al, while the unmodified cast iron is indicated with G; the graphite flakes are indicated with LG.

[0049] Therefore, the different morphology of the intermetallic compound layer present on the non-decarburized brake band and on the surface-decarburized brake band can be attributed to the presence or absence of carbon, in particular in the form of graphite flakes, in the surface layer subjected to the diffusion of the aluminizing process. In particular, it was found that the presence of graphite flakes has a great influence on reducing the surface wettability of the liquid aluminium. In addition to this, the presence of graphite flakes further slows down the growth of the intermetallic compound layer, which in the non-decarburized cast iron indeed appears "mottled" and is far from being a continuous and compact layer.

[0050] It has been found that the presence of carbon in the surface layer of the brake band subjected to the diffusion penetration of aluminium atoms (induced by aluminizing) also causes the formation of iron carbides and intermetallic compounds. The presence of iron carbides creates discontinuities in the intermetallic compound layer, which can cause corrosion phenomena and cracking. Advantageously, surface decarburization thus allows to avoid (or at least greatly reduce) the formation of iron carbides, thus causing the formation of an intermetallic compound layer which is more resistant to corrosion and less prone to cracking.

[0051] Preferably, in said step f) the decarburization of said at least one predetermined surface area is carried out by means of an electrolytic process.

[0052] More in detail, said electrolytic process is carried out by immersing the predetermined surface area of the brake band in a bath of molten salt and applying a potential difference between the bath and the brake band.

[0053] Upon application of the potential difference, the brake band is connected to the positive pole (cathode), while the above-mentioned bath of molten salt is connected to the negative pole (anode). The carbon, in particular in the form of graphite flakes, is oxidized to carbon dioxide by releasing electrons and atomic oxygen released at the anode. The carbon mainly reacts with the oxygen and is finally bound as carbon dioxide.

[0054] The oxidation of the surface of the brake band induced by the electrolytic process is not limited to the carbon present on the surface of the brake band, but also extends to the metallic matrix of the cast iron (iron), thus causing the formation of a surface film of metal oxides. Reversing the polarity causes a reduction of the surface film of metal oxides, thus allowing the metal oxides to return to the initial metallic state.

[0055] Preferably, said electrolytic process can thus provide for reversing the polarity to return the film of metal oxides to the initial metallic state of the film of metal oxides after a predetermined period of time in which the surface of the brake band is connected to the cathode to oxidize the carbon.

[0056] Advantageously, the reversal of the polarity can be repeated several times in succession during the execution of the electrolytic process.

[0057] Operationally, the decarburization depth is controlled by adjusting the duration of the electrolytic process, which can be divided into a number of cycles of reversal of the polarity. By increasing the duration of the decarburization process (brake band oxidation step; cathode connection), the decarburization depth increases, all other conditions being equal.

[0058] Dcarburization can be achieved by alternative processes to the above-mentioned electrolytic process, for example by means of laser treatment or chemical treatment. However, decarburization by means of electrolytic process is preferred because:

[0059] - the electrolytic process is more effective and more rapid than laser treatment, thus ensuring a more complete and uniform removal of carbon in a shorter time;

[0060] - the electrolytic process is more effective than chemical treatments (for example with potassium permanganate) (thus ensuring more complete and uniform removal of carbon in a shorter time) and does not leave oxidized areas of the cast iron metal matrix on the treated portion.

[0061] More in detail, it has been observed that at the oxidized areas on the cast iron metal matrix the wettability of the molten aluminum is greatly reduced, which negatively affects the aluminizing process and the characteristics of the intermetallic layer. For this reason too, the electrolytic decarburization process is preferred to the alternative processes described above.

[0062] As mentioned above, the growth thickness of the intermetallic layer described above is mainly affected by the temperature of the molten aluminum and the immersion time in the molten aluminum. However, it has also been found that another factor that affects the thickness of the intermetallic layer is the silicon content in the molten aluminum. The higher the weight content of silicon in the molten aluminum, the thinner the intermetallic layer, under otherwise identical conditions. Preferably, the molten aluminum has a content of silicon less than 1% by weight.

[0063] Preferably, the impurity content of the molten aluminum does not exceed 1% by weight. In particular, aluminum with a maximum purity of 99.7% by weight can be used, with the following impurity content (in % by weight): Si < 0.30%; Fe < 0.18%; Sr < 0.0010%; Na < 0.0025%; Li < 0.0005%; Ca < 0.0020%; P < 0.0020; Sn < 0.020%.

[0064] During the aluminizing process two opposite diffusion processes occur: on the one hand, iron migrates from the cast iron metal matrix to the aluminum bath, on the other hand, aluminum migrates from the aluminum bath to the cast iron metal matrix. The dissolution of iron in aluminum (and the consequent migration of iron into the aluminum bath) is very fast in terms of kinetics, of the order of magnitude of the reaction that forms the intermetallic compound, in particular FeAl3 (Fe + 3Al -> FeAl3). Therefore, both processes are very fast and occur simultaneously.

[0065] In some cases it has been found that, despite the decarburization treatment having been carried out on the brake band, and therefore the graphite flakes having been eliminated from the surface layer in which the intermetallic layer should have formed, the intermetallic layer formed still includes graphite flakes, as if the graphite flakes had never been eliminated. Such a phenomenon can be explained by the fact that the dissolution of iron in aluminum is too fast, so that the decarburized layer is quickly consumed and therefore the intermetallic compound is formed in the layer below the decarburized layer, i.e. in the layer in which the graphite flakes are present.

[0066] In other words, the excessive solubility of iron in molten aluminum can totally or partially offset the beneficial effects of decarburization of the surface of the brake band.

[0067] Advantageously, the step b) of immersion in a molten aluminium bath in which iron has been dissolved can be performed to slow down the dissolution of iron in the aluminium bath. In this way, the inhibition of the dissolution of iron in aluminium kinetically promotes the formation of FeAI3, thus forming an intermetallic compound at the decarburized layer.

[0068] Preferably, the content of iron in the dissolved state in the aluminium bath is not greater than 5% by weight (solubility limit of iron in aluminium; iron-saturated aluminium), even more preferably comprised between 3% and 5%, quite preferably equal to 4% by weight, to ensure a significant effect of slowing down the process of dissolution of iron of the cast iron in aluminium.

[0069] For example, an aluminium bath having the following composition (in percentage by weight): Al < 97%; Fe 3-5%; with the following impurities: Si < 0.30%; Fe < 0.18%; Sr < 0.0010%; Na < 0.0025%; Li < 0.0005%; Ca < 0.0020%; P < 0.0020; Sn < 0.020% can be used.

[0070] According to experimental observations, by aluminizing with an aluminium bath having iron in the dissolved state, a greater layer of porous intermetallic compound can be obtained, especially in the case where the iron content is close to the solubility limit. This can be explained by the fact that the viscosity of the molten aluminium bath containing iron is higher compared to the cast iron, therefore the wettability of the molten aluminium bath is reduced.

[0071] Advantageously, as shown in the graph in Figure 3 , the above immersion step b) is performed in the following two sub-steps to form a compact and uniform and therefore not porous intermetallic compound layer, while preventing such a layer from extending below the decarburized layer and joining the graphite sheets present:

[0072] - a first sub-step bl) of immersion in a first bath of molten aluminium substantially free of iron in the dissolved state (or iron is present at most as an impurity; for example, iron content less than 0.20% by weight) to obtain on said predetermined surface area an initial layer comprising iron-aluminium intermetallic compounds; and

[0073] - a second sub-step b2) of immersion in a second bath of molten aluminium containing iron in the dissolved state to increase said initial layer until obtaining on said predetermined surface area a final layer comprising iron-aluminium intermetallic compounds having a predetermined thickness.

[0074] The immersion time of the brake band in the first bath is less than the immersion time of the brake band in the second bath.

[0075] Preferably, the immersion of the braking band in the first bath lasts for a period of time which is as short as possible but sufficient to obtain on the predetermined surface area an initial layer of iron-aluminium intermetallic compound comprising a thickness of no more than 10 microns. In particular, if the first bath is at a temperature of about 700°C, the immersion time in the first bath is comprised between 3 and 5 minutes. The immersion time must be reduced as the bath temperature increases.

[0076] Operationally, it is observed that the initial intermetallic compound layer formed during the immersion of the first sub-step bl) is increased in wettability of the braking band by the second bath containing iron in dissolved state. This allows to significantly reduce the porosity of the intermetallic compound layer grown from the initial layer in the subsequent second sub-step b2).

[0077] Advantageously, the immersion time of the braking band in the second bath is determined as a function of the thickness to be obtained for the final intermetallic compound layer.

[0078] More in detail, with equal temperature of the second bath, the thickness increases with the increase of the immersion time; with equal immersion time, the thickness increases with the increase of the temperature of the second bath.

[0079] Advantageously, both the first molten aluminium bath and the second bath have an impurity content of no more than 1% by weight. In particular, both the two molten aluminium baths have a silicon content of less than 1% by weight.

[0080] Preferably, in the second aluminium bath, the content of iron in dissolved state is no more than 5% by weight (solubility limit of iron in aluminium at 700°C equal to 4%; iron-saturated aluminium), even more preferably comprised between 3% and 5%, quite preferably 4% by weight. The iron content must be no less than 3% to ensure a significant effect of slowing down the process of dissolution of iron from cast iron into aluminium.

[0081] Advantageously, both the first bath and the second bath are kept at a temperature of no more than 680°C, preferably at a temperature of no more than 750°C, more preferably at a temperature comprised between 690°C and 710°C, even more preferably at a temperature of 700°C.

[0082] Advantageously, the method can comprise a surface pre-treatment step of the braking band, carried out at least at the predetermined surface area before the step b) of immersion. Preferably, the surface pre-treatment step comprises grinding, degreasing, sandblasting and / or chemical removal of surface oxides.

[0083] Preferably, the method comprises a step of removing the surface oxide layer from the molten aluminium bath, before carrying out said step b) of immersion. Such a surface oxide removal step can be carried out, whether the immersion is carried out in a single bath, or in two successive steps, in a first and a second bath.

[0084] Advantageously, the above-mentioned step (c) of extracting the brake band from the molten aluminium is carried out by controlling the extraction speed as a function of the viscosity of the bath, so as to regulate the amount of molten aluminium which remains adhered to the brake band.

[0085] If the brake band is immersed in a first bath (containing no iron, or iron present at most as an impurity), and then immersed in a second bath having iron in dissolved state, the process of extraction from the first bath in dissolved state is not critical, since the excess aluminium will re-melt upon immersion in the second bath.

[0086] According to a particular embodiment of the application, Figure 4 in the preferred embodiment shown in the diagram of

[0087] - a first sub-step dl) of removal, carried out on the brake band just extracted from the molten aluminium, to remove the molten aluminium still adhered to the brake band; and

[0088] - a second sub-step d2) of removal, carried out on the brake band extracted from the molten aluminium and cooled, to remove the solidified residual aluminium remaining after the removal of said first sub-step d2).

[0089] Preferably, the method comprises a step e) of quenching the brake band, between said first sub-step dl) of removal and said second sub-step d2) of removal.

[0090] Advantageously, said first sub-step dl) of removal can be carried out by mechanical shaving of the aluminium still in liquid state.

[0091] Advantageously, said second sub-step d2) of removal can be carried out by chemical removal of the solidified aluminium not removed mechanically.

[0092] Preferably, the above-mentioned chemical removal is carried out by exposing the aluminium to ferric chloride for at least 4 minutes, to cause the following reaction:

[0093] Al + FeCl3-> AlCl3+ Fe

[0094] The chemical removal by ferric chloride must take place after the solidification of the aluminum. Ferric chloride boils at 315°C, and therefore cannot be in contact with molten aluminum. Preferably, the chemical removal is therefore carried out after the step e) of quenching is carried out.

[0095] A brake band with increased wear and corrosion resistance of a cast iron brake disc according to the present application will now be described.

[0096] In particular, such a brake band can be made according to the method of the present application, in particular as described above. The making method will therefore not be described again, and for the sake of brevity, reference should be made to the method described above.

[0097] According to a general embodiment of the present application, the disc brake band comprises a brake band body made of cast iron (gray cast iron or compacted graphite cast iron).

[0098] The brake band body has at least one braking surface comprising a protective surface layer at at least a portion of the braking surface.

[0099] According to the present application, such a protective surface layer is a surface layer comprising one or more iron-aluminum intermetallic compounds.

[0100] It has been experimentally found that the above iron-aluminum intermetallic compound layer exposed at the surface provides a higher corrosion and wear resistance to the cast iron brake band at the predetermined surface area compared to a case where the cast iron brake band does not contain a coating and the cast iron is directly exposed at the surface.

[0101] The above iron-aluminum intermetallic compound layer can comprise a plurality of iron-aluminum intermetallic compounds, in particular Fe3Al, FeAl, FeAl2, FeAl3, Fe2Al5. The intermetallic main phase is FeAl3, as FeAl3 is thermodynamically more stable.

[0102] According to a preferred embodiment, the above protective surface substantially does not contain carbon, in particular graphite flakes.

[0103] In particular, the above iron-aluminum intermetallic compound layer is a diffusion layer, so as to interpenetrate the microstructure of the cast iron.

[0104] As Figure 7 shown, the figure represents a high magnification (2000x) SEM image obtained by scanning electron microscopy on a brake band sample according to the present application, said diffusion layer interpenetrating the cast iron microstructure having a jagged interpenetration profile, in particular a jagged interpenetration profile having a shark tooth shape. This profile is a typical feature of all diffusion phenomena in solids, in which the diffusion front encounters a crystal structure that locally blocks diffusion, as is the case for cast iron.

[0105] In Figure 7 In the following, the intermetallic compound layer is indicated with L, the residual aluminum layer is indicated with Al, and the unmodified cast iron is indicated with G.

[0106] Preferably, the iron-aluminum intermetallic compound layer has an average thickness comprised between 30 microns and 200 microns. This thickness is chosen according to the wear resistance properties that the brake band must guarantee during use.

[0107] Preferably, the braking surface has a hardness at the layer of iron-aluminum intermetallic compound comprised between 400 HV and 1000 HV. This hardness can vary according to the specific composition of the protective layer. The hardness of Fe2Al5 is 600 HV to 700 HV, while the hardness of FeAl3 is 900 HV to 1000 HV.

[0108] Below, some application examples of the method according to the present application are described, carried out on a sample consisting of a gray cast iron disc. Similar results have also been obtained on a sample of sheet cast iron.

[0109] Example 1

[0110] A sample of gray cast iron, consisting of a disc having a diameter of 50 mm and a thickness of 6 mm, is first subjected to an electrolytic decarburization treatment which allows the removal of the graphite flakes to a depth of at least 100 microns.

[0111] Then the sample is immersed in 37% by volume hydrochloric acid for 30 seconds, after which it is washed with demineralized water.

[0112] Immediately after the pickling, the sample is immersed in two different baths of molten aluminum, in sequence.

[0113] The first bath has the following composition: aluminum 99.7% by weight, containing the following impurities (in weight percent): Si < 0.30%; Fe < 0.18%; Sr < 0.0010%; Na < 0.0025%; Li < 0.0005%; Ca < 0.0020%; P < 0.0020; Sn < 0.020%.

[0114] The second bath has the same composition as the first bath, but with the addition of 4% by weight of iron powder with a purity of 98.5%.

[0115] Both baths are kept at a temperature of 700°C. The temperature is controlled to within ± 10°C, so there is no significant difference between 690°C and 710°C.

[0116] The immersion time in the first bath is 3 minutes, while the immersion time in the second bath is 30 minutes.

[0117] The method of removal from the first bath is irrelevant, since even a relatively thick layer of aluminium (1 to 2 mm) remains adhering to the sample, which is remelted in the second bath. In contrast, the removal from the second bath is fast, in the order of 1 second. The slower the removal process, the greater the thickness of the aluminium that remains adhering to the surface.

[0118] Once the sample is removed from the second bath, it is planed very quickly by means of a steel blade, while the aluminium is still molten on the surface. The aluminium is immediately solidified after only one pass of the blade. The thickness of the remaining aluminium is about 0.1 to 0.3 mm.

[0119] Subsequently, the sample is immersed in a 40% by weight aqueous solution of ferric chloride for 40 minutes. It is then washed with demineralised water. In this way, the aluminium is almost completely removed from the surface.

[0120] At the end of the above process, the sample has a layer of iron-aluminium intermetallic compounds having an average thickness of about 100 microns, with a variation of ± 30 microns.

[0121] Under the electron microscope, the intermetallic compound layer appears very compact and uniform.

[0122] The average hardness value of the intermetallic compound layer is not less than 400 HV.

[0123] The sample was tested in a climatic chamber in compliance with UNI EN ISO 6270-2 CH (condensed water). The entire duration of the test was 120 hours. The surface sample was free from corrosion in about 75% of the surface area. The sample had a clear corrosion in the only central area that triggered after 1 hour. Scanning electron microscope analysis showed no signs of penetration of the condensed water down to the cast iron. It can therefore be concluded that the intermetallic compound layer protects the iron in the cast iron from oxidation.

[0124] Example 2

[0125] A grey cast iron sample - comprising a disc having a diameter of 50 mm and a thickness of 6 mm - was subjected to the same process as described in Example 1, except that it was not subjected to an initial decarburisation process.

[0126] At the end of the above process, the sample has a layer of iron-aluminium intermetallic compounds having an average thickness of about 100 microns, with a variation of ± 30 microns.

[0127] Under scanning electron microscope, the intermetallic compound layer appeared less uniform than the layer of intermetallic compounds obtained in the sample of example 1. In fact, the intermetallic compound layer presented large, very uniform and compact areas alternating with some (not so large) broken and defective (porous) areas. It is hypothesized that, possibly due to the growth of the coating on the non-decarburized cast iron, the first aluminum bath would have left some graphite flakes exposed on the surface. The areas of the sample with graphite flakes failed to be well wetted by the second bath and created porosities in the coating.

[0128] The average hardness value of the intermetallic layer was not less than 400 HV.

[0129] The sample was also tested in a climatic chamber in compliance with UNI ENISO 6270-2 CH (condensed water). The entire duration of the test was 120 hours. After the test, it was found that the above-mentioned porous areas were more subject to corrosion than the rest of the layer. Apart from these areas, the coating appeared very uniform and compact.

[0130] Despite the presence of areas of the coating subject to early corrosion, the scanning electron microscope analysis showed no signs of penetration of the condensed water down to the cast iron. It can therefore be concluded that, despite the early localized corrosion phenomena in the porous areas, the intermetallic compound layer protected the iron in the cast iron from oxidation.

[0131] Example 3

[0132] A gray cast iron sample - comprising a disc with a diameter of 50 millimeters and a thickness of 6 millimeters - was subjected to the same process as described in example 2. The only difference was that the immersion time in the first bath was 30 seconds instead of 3 minutes. At the end of the process, the sample was subjected to the same tests as the samples in the previous examples.

[0133] Also in this case, the sample had a layer of iron-aluminum intermetallic compounds with an average thickness of about 100 microns, with a variation of ± 30 microns.

[0134] Unlike the previous example, the intermetallic compound layer appeared to have a two-layer structure under the scanning electron microscope. The first layer (the innermost one) had a large number of porosities and defects and had a mottled conformation. Above this first layer was a second layer, which was a more uniform, more compact layer, however, the second layer remained highly defective internally, due to the structure of the first intermetallic compound layer. It is hypothesized that this phenomenon, in addition to being related to the presence of graphite flakes on the surface (without surface decarburization), was also due to the significantly shorter time of immersion in the first bath.

[0135] The average hardness value of the intermetallic layer was not less than 400 HV.

[0136] In addition, the sample was tested in a climatic chamber in compliance with UNI EN ISO 6270-2 CH (condensed water). The entire duration of the test was 120 hours. After the test, it was found that the porous areas were corroded earlier than the rest of the layer.

[0137] Despite the presence of areas subject to early corrosion on the coating, the scanning electron microscope analysis showed no signs of penetration of the condensed water down to the cast iron. It can therefore be concluded that, despite the presence of early localized corrosion phenomena in the porous areas, the intermetallic layer protected the iron in the cast iron from oxidation.

[0138] Example 4

[0139] A sample of grey cast iron - comprising a disc with a diameter of 50 millimetres and a thickness of 6 millimetres - was subjected to the same procedure as described in Example 1, with the following differences:

[0140] - the sample was not previously subjected to decarburization treatment; and

[0141] - immersion in a single molten bath of aluminium having the composition of the first bath of Example 1, for a time of 30 minutes and at a temperature of 700°C.

[0142] At the end of the procedure, the sample was subjected to the same tests as the sample in the previous example.

[0143] Also in this case, at the end of the procedure described above, the sample had a layer of iron-aluminium intermetallics with an average thickness of about 100 microns, with a variation of ± 30 microns.

[0144] Under the electron microscope, the layer of intermetallics appeared to be uneven. In fact, the intermetallic layer alternated very uniform and compact areas with fractured zones with defects (pinholes). Also in this case, it is hypothesized that the surface porosity is related to the presence of graphite flakes on the surface (no surface decarburization was performed) in addition to the absence of immersion in the second saturated bath of iron.

[0145] The average hardness value of the intermetallic layer was not less than 400 HV.

[0146] In addition, the sample was tested in a climatic chamber in compliance with UNI EN ISO 6270-2 CH (condensed water). The entire duration of the test was 120 hours. After the test, it was found that the porous areas were corroded earlier than the rest of the layer. In addition to these areas, the coating appeared to be very uniform and compact.

[0147] Despite the presence of areas subjected to early corrosion on the coating, scanning electron microscope analysis revealed no signs of condensation water penetrating downwards into the cast iron. It can therefore be concluded that, despite the presence of early localized corrosion phenomena in the porous areas, the intermetallic compound layer protects the iron in the cast iron from oxidation.

[0148] Example 5

[0149] A sample of grey cast iron - comprising a disc having a diameter of 50 mm and a thickness of 6 mm - was subjected to the same procedure as described in Example 1, except that the sample with the decarburized surface was immersed in the first bath (substantially free of iron) for only 30 minutes.

[0150] At the end of the procedure described above, the sample had a layer of iron-aluminium intermetallic compounds, the average thickness of which was about 100 microns, with a variation of ± 30 microns.

[0151] Under the scanning electron microscope, the intermetallic compound layer appeared to be as uneven as the intermetallic compound layer obtained in the sample of Example 1. In fact, the intermetallic compound layer alternated large, very uniform and compact areas with some (less large) broken and defective (porous) areas. It is hypothesized that, despite the coating growing on the decarburized cast iron, the aluminium bath (not saturated with iron) dissolved the iron coming from the decarburized surface, thus causing the underlying graphite flakes to emerge and thus counteracting the decarburization effect. The areas of the sample with the graphite flakes that emerged after decarburization were not well wetted by the second bath and created porosities in the coating.

[0152] The average hardness value of the intermetallic layer was not less than 400 HV.

[0153] In addition, the sample was tested in a climatic chamber in compliance with UNI EN ISO 6270-2 CH (condensation water). The entire duration of the test was 120 hours. After the test, it was found that the above-mentioned porous areas were subjected to corrosion earlier than the other parts of the layer. Apart from these areas, the coating appeared to be uniform and compact.

[0154] Despite the presence of areas subjected to early corrosion on the coating, scanning electron microscope analysis revealed no signs of condensation water penetrating downwards into the cast iron. It can therefore be concluded that, despite the presence of early localized corrosion phenomena in the porous areas, the intermetallic compound layer protects the iron in the cast iron from oxidation.

[0155] ***

[0156] The present application allows a number of advantages to be obtained, which have been illustrated throughout the description.

[0157] In particular, the application examples illustrated above highlight how the method according to the present application allows to make a brake band of a cast iron brake disc provided with a coating which confers to the brake band a higher resistance to wear and corrosion.

[0158] In fact, in all the examples, the coating obtained, consisting of layers of intermetallic iron / aluminum compounds, shows higher hardness values than bare cast iron, not less than 400 HV.

[0159] The coating obtained, although having different uniformity and porosity characteristics in the different examples, is always able to protect the underlying cast iron from corrosion. This demonstrates the ability of the coating obtained by the method according to the present application to increase the corrosion resistance of the brake band of the brake disc.

[0160] The examples illustrated above highlight how the embodiments of the method according to the present application comprise the decarburization of the surface to be coated and how the immersion is carried out in two different aluminum baths, wherein the first aluminum bath is substantially free of iron or has an amount of iron which can be classified as impurities, thus guaranteeing the best results in terms of corrosion resistance.

[0161] The method for making a brake band according to the present application can be carried out without the need for complex equipment and with very little preparation time. All this makes the increase in costs and production times of the cast iron brake band very limited.

[0162] Finally, the method according to the present application is feasible at processing temperatures compatible with the need to control the deformation of the brake band.

[0163] Therefore, the invention thus designed achieves the set objectives.

[0164] It is clear that in the practice, shapes and configurations different from those disclosed above can also be adopted, without thereby departing from the scope of protection.

[0165] Furthermore, all the details can be replaced with technically equivalent elements and any dimensions, shapes and materials can be used as required.

Claims

1. A method for manufacturing a brake band for a cast iron brake disc, the brake band having increased wear resistance and corrosion resistance, the method comprising the following steps: Manufacturing steps: Manufacturing the brake band for the cast iron brake disc; Immersion step: The brake band is at least partially immersed in molten aluminum maintained at a predetermined temperature, such that the molten aluminum covers at least the predetermined surface area of ​​the brake band, and the immersion is continued for a predetermined immersion time period to allow the diffusion of aluminum atoms in the surface microstructure of the cast iron, thereby forming an iron-aluminum intermetallic compound at the surface layer of the brake band, thereby generating a layer comprising the iron-aluminum intermetallic compound in the predetermined surface area of ​​the brake band; Removal step: Remove the brake band from the molten aluminum; Removal Step: After removal, the aluminum still adhering to the brake band is removed to expose the layer of the iron-aluminum intermetallic compound on the predetermined surface area. The layer of the iron-aluminum intermetallic compound exposed on the predetermined surface area gives the cast iron brake band high corrosion resistance and wear resistance at the predetermined surface area.

2. The method of claim 1, wherein, The manufacturing process includes manufacturing a brake band for a gray cast iron brake disc.

3. The method of claim 1 or 2, wherein, The iron-aluminum intermetallic compound layer is formed at the surface layer of the cast iron brake band that is in contact with the molten aluminum through the diffusion process of aluminum atoms within the structure of the cast iron and the formation of the iron-aluminum intermetallic compound.

4. The method of claim 1 or 2, wherein, The iron-aluminum intermetallic compound layer includes FeAl3 as the main phase of the iron-aluminum intermetallic compound.

5. The method according to claim 1 or 2, wherein, The molten aluminum is maintained at a predetermined temperature of not less than 680°C.

6. The method according to claim 1 or 2, wherein, The molten aluminum is maintained at a predetermined temperature not exceeding 750°C.

7. The method according to claim 1 or 2, wherein, The molten aluminum is maintained at a predetermined temperature between 690°C and 710°C.

8. The method according to claim 1 or 2, wherein, The molten aluminum is maintained at the predetermined temperature of 700°C.

9. The method according to claim 1 or 2, wherein, The predetermined immersion time period is determined based on the required thickness of the intermetallic compound layer, wherein the thickness increases with increasing immersion time when the temperature of the molten aluminum is constant.

10. The method according to claim 1 or 2, wherein, The predetermined immersion time period is between 5 minutes and 60 minutes.

11. The method according to claim 1 or 2, wherein, The predetermined immersion time period is 30 minutes.

12. The method according to claim 1 or 2, comprising a decarburization step: decarburizing at least one of the predetermined surface areas of the brake band to a predetermined depth, the decarburization step being performed prior to the immersion step.

13. The method according to claim 12, wherein, The predetermined depth for decarburization is equal to or greater than the depth of the iron-aluminum intermetallic compound layer from the outer surface of the brake band.

14. The method according to claim 12, wherein, The predetermined depth is equal to or greater than 30 micrometers.

15. The method according to claim 12, wherein, In the decarburization step, at least one of the predetermined surface areas of the brake band is decarburized by an electrolysis process.

16. The method according to claim 15, wherein, The electrolysis process is carried out by immersing the predetermined surface area of ​​the brake band in a bath of molten salt and applying a potential difference between the bath and the brake band, wherein the brake band is connected to the positive electrode and the bath is connected to the negative electrode.

17. The method according to claim 15, wherein, The electrolysis process is carried out by immersing the predetermined surface area of ​​the brake band in a bath of molten salt and applying a potential difference between the bath and the brake band, wherein the brake band is connected to the positive electrode and the bath is connected to the negative electrode, and wherein the polarity is reversed after a predetermined time period.

18. The method according to claim 15, wherein, The electrolysis process is carried out by immersing the predetermined surface area of ​​the brake band in a bath of molten salt and applying a potential difference between the bath and the brake band, wherein the brake band is connected to the positive electrode and the bath is connected to the negative electrode, wherein the polarity is reversed repeatedly in sequence.

19. The method according to claim 1 or 2, wherein, The molten aluminum has a silicon content of less than 1% by weight.

20. The method according to claim 1 or 2, wherein, The immersion step is carried out in a bath of molten aluminum containing iron in a molten state.

21. The method according to claim 1 or 2, wherein, The immersion step is carried out in a molten aluminum bath containing molten iron in a dissolved state, the content of which is no more than 5% by weight.

22. The method according to claim 1 or 2, wherein, The immersion step is carried out in a molten aluminum bath containing molten iron in a dissolved state, the content of which is between 3% and 5% by weight.

23. The method according to claim 1 or 2, wherein, The immersion step is carried out in a molten aluminum bath containing molten iron in a dissolved state, the content of which is 4% by weight.

24. The method according to claim 1 or 2, wherein, The immersion step is performed in the following two sub-steps: - A first immersion sub-step (b1) is performed in a first bath of molten aluminum containing molten iron in a soluble state to obtain an initial layer comprising an iron-aluminum intermetallic compound on the predetermined surface area; - A second immersion sub-step (b2) is performed in a second bath containing molten aluminum with iron in a molten state to increase the initial layer until a final layer of iron-aluminum intermetallic compound of a predetermined thickness is obtained on the predetermined surface area. The immersion time of the brake band in the first bath is less than the immersion time of the brake band in the second bath.

25. The method according to claim 24, wherein, Both the first and second baths of molten aluminum contain impurities with a content not exceeding 1% by weight.

26. The method of claim 24, wherein, Both the first and second baths of molten aluminum contain less than 1% silicon by weight.

27. The method according to claim 24, wherein, The second bath of molten aluminum contains molten iron in a soluble state, comprising no more than 5% by weight.

28. The method according to claim 24, wherein, The second bath of molten aluminum contains molten iron in a soluble state at a content between 3% and 5% by weight.

29. The method according to claim 24, wherein, The second bath of molten aluminum contains 4% by weight of molten iron in a dissolved state.

30. The method according to claim 24, wherein, Both the first bath and the second bath are maintained at a temperature below 680°C.

31. The method according to claim 24, wherein, Both the first bath and the second bath are maintained at a temperature not exceeding 750°C.

32. The method according to claim 24, wherein, Both the first bath and the second bath are maintained at a temperature between 690°C and 710°C.

33. The method according to claim 24, wherein, Both the first bath and the second bath are maintained at a temperature of 700°C.

34. The method according to claim 24, wherein, The immersion of the brake band in the first bath is sustained for a period of time that is as short as possible but sufficient to obtain an initial layer of an iron-aluminum intermetallic compound with a thickness of no more than 10 micrometers on the predetermined surface area.

35. The method according to claim 24, wherein, If the first bath is at a temperature of approximately 700°C, the immersion time in the first bath is between 3 and 5 minutes.

36. The method according to claim 24, wherein, The immersion time of the brake band in the second bath is set according to the required thickness of the final layer of the intermetallic compound. When the temperature of the second bath is constant, the thickness increases with the increase of the immersion time.

37. The method according to claim 1 or 2, comprising a surface pretreatment step of pretreating the brake band, the surface pretreatment step being performed at least in the predetermined surface area prior to the immersion step.

38. The method according to claim 37, wherein, The surface pretreatment steps include grinding, degreasing, sandblasting, and / or chemical removal of surface oxides.

39. The method according to claim 38, wherein, The surface oxide layer is removed by the molten aluminum bath prior to the immersion step.

40. The method according to claim 1 or 2, wherein, The removal step, which removes the aluminum still adhering to the brake band after removal, is performed in two sub-steps: a first removal sub-step performed on the brake band just removed from the molten aluminum, to remove the molten aluminum still adhering to the brake band; A second removal sub-step is performed on the brake band after it has been removed from the molten aluminum and cooled, in order to remove the solidified aluminum that remains after the first removal sub-step.

41. The method of claim 40, comprising: A quenching step is performed on the brake band between the first removal sub-step and the second removal sub-step.

42. The method according to claim 40, wherein, The first removal sub-step is performed by mechanically planing the still liquid aluminum.

43. The method according to claim 40, wherein, The second removal sub-step is carried out by chemically removing the solidified aluminum that has not been mechanically removed.

44. The method according to claim 43, wherein, The chemical removal is carried out by exposing aluminum to ferric chloride for at least 4 minutes to induce the following reaction: Al + FeCl3 → AlCl3 + Fe.

45. The method according to claim 40, wherein, The method includes a quenching step of quenching the brake band between the first removal sub-step and the second removal sub-step, wherein the second removal sub-step is performed by chemically removing solidified aluminum that has not been mechanically removed.

46. ​​A brake band for a brake disc manufactured according to any one of claims 1 to 45, the brake band comprising a brake band body made of cast iron, the brake band having at least one braking surface, the braking surface comprising a protective surface layer corresponding to at least a portion of the braking surface, characterized in that, The protective surface layer is a layer comprising one or more iron-aluminum intermetallic compounds, which gives the cast iron brake band high corrosion resistance and wear resistance on at least a portion of the brake surface.

47. The brake band according to claim 46, wherein, The cast iron is gray cast iron.

48. The brake band according to claim 46 or 47, wherein, The iron-aluminum intermetallic compound layer includes FeAl3 as the main phase in the iron-aluminum intermetallic compound.

49. The brake band according to claim 46 or 47, wherein, The protective surface layer does not contain carbon.

50. The brake band according to claim 46 or 47, wherein, The protective surface layer does not include graphite sheets.

51. The brake band according to claim 46 or 47, wherein, The layer of the iron-aluminum intermetallic compound is a diffusion layer that interpenetrates with the microstructure of the cast iron.

52. The brake band according to claim 51, wherein, The diffusion layer has a serrated, interpenetrating profile.

53. The brake band according to claim 52, wherein, The serrated, interpenetrating contours have the shape of shark teeth.

54. The brake band according to claim 46 or 47, wherein, The iron-aluminum intermetallic compound layer has an average thickness between 30 micrometers and 200 micrometers.

55. The brake band according to claim 46 or 47, wherein, The braking surface has a hardness between 400 HV and 1000 HV at the layer of the iron-aluminum intermetallic compound.

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