(description)
FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. Title of the Invention
DOUBLE-LAYER BRAKE DISC IN NICKEL-FREE STEEL AND MANUFACTURING METHOD
2. Applicant(s)
Name Nationality Address
BREMBO S.P.A. Italian Via Brembo, 25 I-24035 Curno,
Bergamo, Italy
3. Preamble to the description
The following specification particularly describes the invention and the manner in which it is to be performed
2
DESCRIPTION
FIELD OF APPLICATION
[0001] The present invention relates to a method for making a brake disc and to a
brake disc for disc brakes.
5 PRIOR ART
[0002] A brake disc of a vehicle disc brake system comprises an annular structure, or
braking band, and a central fixing element, known as a bell, by which the disc is
attached to the rotating part of a vehicle suspension, for example a hub. The braking
band is provided with opposite braking surfaces suitable for cooperating with friction
10 elements (brake pads), housed in at least one gripper body placed astride of said
braking band and integral with a non-rotating component of the vehicle suspension.
The controlled interaction between the opposite brake pads and the opposite braking
surfaces of the braking band determine by friction a braking action which allows the
deceleration or stopping of the vehicle.
15 [0003] Generally, the brake disc is made of gray cast iron or steel. In fact, this
material allows good braking performance (especially in terms of limited wear) to be
obtained at relatively low costs. Discs made of carbon or carbon-ceramic materials
offer much higher performance, but at a much higher cost.
[0004] The limits of traditional discs of cast iron or steel are linked to excessive
20 wear. As far as gray cast iron discs are concerned, another very negative aspect is
linked to excessive surface oxidation, with the consequent formation of rust. This
aspect affects both the performance of the brake disc and its appearance, as the rust
on the brake disc is aesthetically unacceptable to the user. An attempt was made to
address these problems by making the discs in gray cast iron or steel with a
25 protective coating. The protective coating serves on the one hand to reduce the wear
3
of the disc, and on the other hand to protect the gray cast iron base from surface
oxidation, thus avoiding the formation of a layer of rust. The protective coatings
available today and applied on discs, although offering resistance to wear, are
however subject to flaking that causes the detachment thereof from the disc itself.
5 [0005] A protective coating of this type is described for example in patent
US4715486, relating to a low-wear disc brake. The disc, made in particular of cast
iron, has a coating made with a particle material deposited on the disc with an impact
technique with high kinetic energy. According to a first embodiment, the coating
contains from 20% to 30% of tungsten carbide, 5% of nickel and the balance of a
10 mixture of chromium and tungsten carbides.
[0006] In the case of application of the coating with flame spray techniques, a cause
of the detachment of traditional protective coatings from aluminum or aluminum
alloy discs is the presence of free carbon in the protective coating. This phenomenon
also affects gray cast iron or steel discs.
15 [0007] A solution to the aforesaid problems has been proposed by the same applicant
in international application WO2014/097187 as regards discs made of gray cast iron
or steel. It is composed in creating a protective coating on the braking surfaces of a
brake disc obtained by depositing a material in particle form composed of 70 to 95%
by weight of tungsten carbide, 5% to 15% by weight of cobalt and 1% to 10% by
20 weight of chromium. The deposition of the material in particle form is obtained by
the HVOF (High Velocity Oxygen Fuel) technique, or by the HVAF (High Velocity
Air Fuel) technique or by the KM (Kinetic Metallization) technique.
[0008] More in detail, according to the solution offered in WO2014/097187, the
combination of the HVOF, HVAF or KM deposition technique and the chemical
25 components used for the formation of the coating allows a protective coating with
4
high bond strength to be obtained, which guarantees a high degree of anchoring on
gray cast iron or steel. The above solution allows the flaking phenomena of the
protective coating recorded in the prior art to be significantly reduced, but not to
eliminate them completely. In fact, even in discs provided with a protective coating
5 made according to WO2014/097186, peeling and sagging of the protective coating
continue to occur - albeit less frequently than in the prior art.
[0009] The aforementioned flaking and sagging may contribute in particular to the
release by rubbing of nickel particles, a metal which contributes significantly to
sensitization phenomena in the population.
10 [0010] However, in the specific field of steel production for brake discs, to date, the
presence of nickel is considered essential as it increases the strength of the steel and
toughness. Furthermore, nickel increases the resistance of steel to oxidation and
corrosion, but, above all, nickel increases the abrasive resistance of the steel and the
heat resistance of that steel, aspects which are extremely relevant for the stresses they
15 are subjected to in brake discs. Therefore, to date, the presence of nickel is
considered an essential element for the production of a cast iron or steel brake disc.
[0011] Taking into account the advantages in terms of wear resistance guaranteed by
the protective coatings and the simultaneous need to maintain the presence of nickel
in the composition of the brake disc, the need to solve the drawbacks mentioned in
20 reference to the prior art is very much felt in the field.
[0012] In particular, the need is felt to have gray cast iron or steel discs capable of
reducing the release of nickel particles, but at the same time capable of guaranteeing
adequate or equivalent thermal and mechanical performance typical of the prior art
brake discs, including high wear resistance of the disc and reliability over time.
25 [0013] According to a further aspect, the need is also felt to make steel discs with
5
less consumption of resources necessary for production (and therefore also of costs),
while maintaining an adequate hardness of the coating and at the same time a
reduced (or even absent) release of nickel particles.
[0014]
5 DISCLOSURE OF THE INVENTION
[0015] The need for brake discs capable of reducing the release of nickel particles,
but at the same time capable of guaranteeing adequate or equivalent thermal and
mechanical performance, is met by a brake disc and by a method for making a brake
disc according to the appended independent claims.
10 DESCRIPTION OF THE DRAWINGS
[0016] Further features and advantages of the present invention will become more
apparent from the following description of preferred and non-limiting embodiments
thereof, in which:
- Figure 1 shows a top plan view of a disc brake according to an embodiment of the
15 present invention;
- Figure 2 shows a sectional view of the disc of Figure 1 according to the section line
II-II indicated therein, according to an embodiment of the present invention;
- Figure 3 shows a sectional view of the disc of Figure 1 according to the section line
II-II indicated therein, according to a further embodiment of the present invention;
20 - Figure 4 shows a sectional view of a half portion of a braking band according to an
embodiment of the present invention;
- Figure 5 shows a sectional view of a half portion of a braking band according to a
second embodiment of the present invention;
- Figure 6 shows a sectional view of a half portion of a braking band according to a
25 third embodiment of the present invention;
6
- Figure 7 shows a sectional view of a half portion of a braking band according to a
fourth embodiment of the present invention;
- Figure 8 shows a sectional view of a half portion of a braking band according to a
fifth embodiment of the present invention;
5 - Figure 9 shows a sectional view of a half portion of a braking band according to a
sixth embodiment of the present invention;
- Figure 10 shows a sectional view of a half portion of a braking band according to a
seventh embodiment of the present invention.
[0017] Elements or parts of elements common to the embodiments described
10 hereinafter will be indicated with the same reference numerals.
DETAILED DESCRIPTION
[0018] With reference to the above figures, reference numeral 1 globally denotes a
brake disc according to the present invention.
[0019] In the present discussion, where numerical percentage intervals are indicated,
15 the extremes of these intervals are always understood to be included, unless
otherwise specified.
[0020] According to a general embodiment of the invention, illustrated in the
accompanying figures, the brake disc 1 comprises a braking band 2, provided with
two opposite braking surfaces 2a and 2b, each of which at least partially defines one
20 of the two main faces of the disc.
[0021] The braking band 2 is made of gray cast iron or steel.
[0022] Preferably, the braking band 2 is made of gray cast iron. In particular, the
entire disc is made of gray cast iron. In the remainder of the description, reference
will therefore be made to a gray cast iron disc, without however excluding the
25 possibility that it is made of steel.
7
[0023] The disc 1 is provided with a base layer 30 which covers at least one of the
two braking surfaces 2a, 2b of the braking band and is preferably made in direct
contact with said braking surfaces 2a, 2b.
[0024] According to an aspect of the present invention, such base layer 30 is
5 composed of steel having a nickel content lower than or at most equal to 15%.
[0025] According to a further aspect of the present invention, such base layer 30 is
composed of steel having a nickel content lower than or at most equal to 7.5%, even
more preferably lower than or at most equal to 5%.
[0026] According to a further aspect of the present invention, such base layer 30 is
10 totally nickel-free. This allows limiting, if not even avoiding, the dispersion of nickel
particles during the life of the brake disc 1.
[0027] In general, in the present discussion, when reference is made to phrases such
as “nickel free” or “without nickel” or the like, it is meant exactly the total absence
of nickel but also an absence of nickel less than a small amount of nickel which may
15 be present due to traces or residual impurities due to the manufacturing process, but
in any case amounts of nickel lower than 1% or possibly at the most strictly lower
than 5%, for any layer.
[0028] It is clear that, to those skilled in the art, it is known what is meant when
referring to percentages of content of nickel or of any other component of the steel or
20 cast iron alloy. For example, reference is generally made to the percentage content
by mass with respect to the total content of the alloy. Therefore, in the continuation
of the present discussion, particular percentage calculations will be specified only
where they deviate from the aforementioned definition; where not specified, the
percentages indicated should be considered as understandable by those skilled in the
25 art.
8
[0029] According to an embodiment of the invention, the steel of the base layer 30 is
composed of 10% to 15% chromium Cr, at most 1% silicon Si, at most 4%
manganese Mn, between 0.16% and 0.5% of carbon C and for the balance of iron Fe,
i.e., for the remaining percentage by weight of iron. This allows obtaining a
5 martensitic steel, without nickel content.
[0030] Preferably, the content of carbon C of the steel of the base layer is comprised
between 0.16 and 0.25%.
[0031] Advantageously, the aforesaid composition allows a reduced percentage of
any carbides included in the steel to be used, without reducing the hardness of any
10 coating (described in more detail, later in the text).
[0032] According to a preferred embodiment variant, the chromium (Cr) content in
the steel of the base layer 30 is comprised between 11% and 14%, extremes included.
[0033] According to an embodiment variant of the invention, for example shown in
Figure 5, the base layer 30 also is composed of one or more carbides included in the
15 nickel-free steel. Such inclusion is obtained by means of techniques known to those
skilled in the art of inclusion of carbides in steel, for example the carbides are
dissolved in the alloy.
[0034] Preferably, the one or more carbides included comprise at least one carbide
selected from the group comprising: tungsten carbide (WC), chromium carbide
20 (preferably, but not limited to, Cr3C2), Niobium carbide (NbC), titanium carbide
(TiC). It is clear that more than one carbide selected from the aforementioned group
or all the carbides present in the present group may also be present.
[0035] The one or more carbides included comprise at least one carbide selected
from the group comprising: tungsten carbide (WC), chromium carbide (e.g., Cr3C2),
25 Niobium carbide (NbC), titanium carbide (TiC).
9
[0036] According to an advantageous embodiment, for example shown in Figure 6,
the brake disc 1 comprises a protective surface coating 3 which covers the base layer
30 at least on the side of one of the two braking surfaces 2a, 2b of the braking band.
Such protective surface coating 3 is arranged on one side of the base layer 30 which
5 does not face towards the braking surface 2a, 2b. Furthermore, the surface protective
coating 3 is composed of at least one carbide or more carbides in particle form which
may be deposited by the Thermal Spray deposition technique, for example by the
HVOF (High Velocity Oxy-Fuel) technique, or by the HVAF (High Velocity Air
Fuel) technique or by the APS (Atmosphere plasma spray) technique or by the Cold
10 Spray deposition technique, for example by the KM (Kinetic Metallization)
technique, or by the deposition technique using a laser beam, for example by the
LMD (Laser Metal Deposition) technique, or by HSLC - high speed laser cladding
technique, or by EHLA - Extreme High Speed Laser Application technique, or by
TSC - Top Speed Cladding technique.
15 [0037] The surface protective coating 3 is therefore obtained by depositing directly
on the disc 1 one or more carbides in particle form also by the HVOF technique, or
by the HVAF (High Velocity Air Fuel) technique or by the KM (Kinetic
Metallization) technique, preferably tungsten carbide (WC) or chromium carbide (for
example, Cr3C2) or niobium carbide (NbC) or titanium carbide (TiC).
20 [0038] According to a further embodiment variant, the surface protective coating 3 is
composed of steel having a nickel content lower than or at most equal to 15% or
lower or at most equal to 7.5%, or lower or at most equal at 5%, or even more
preferably totally free from nickel, and of one or more carbides included in the steel.
In this variant, in other words, the base layer 30 in nickel-free steel and above a
25 protective surface coating 3 composed of the aforementioned steel and one or more
10
carbides included in the steel are joined above the cast iron band in the order
indicated.
[0039] The presence of carbides deposited on the surface or included in the steel
substantially or totally without nickel allows imparting mechanical strength and wear
5 resistance, so as to compensate for the scarcity or total lack of nickel inside the steel.
[0040] According to a variant, the surface protective coating 3 is composed of one or
more of the following carbides: tungsten carbide (WC), niobium carbide (NbC),
chromium carbide (for example, Cr3C2), titanium carbide (TiC). Preferably, such
protective surface coating 3 is obtained by depositing on the base layer 30 one or
10 more of the aforementioned carbides in particle form by the Thermal Spray
deposition technique, for example by the HVOF (High Velocity Oxy-Fuel)
technique, or by the HVAF (High Velocity Air Fuel) technique or by the APS
(Atmosphere plasma spray) technique or by the Cold Spray deposition technique, for
example by the KM (Kinetic Metallization) technique, or by the deposition technique
15 using a laser beam, for example by the LMD (Laser Metal Deposition) technique, or
by HSLC - high speed laser cladding technique, or by EHLA - Extreme High Speed
Laser Application technique, or by TSC - Top Speed Cladding technique. It is
therefore clear that more than one carbide selected from the aforementioned group or
all the carbides present in the present group may also be present.
20 [0041] According to an advantageous embodiment, the surface protective coating 3
is composed of chromium carbide (for example, Cr3C2) and titanium carbide (TiC).
[0042] According to a variant, the surface protective coating 3 is composed of at
least one metal oxide or a mixture of metal oxides or a mixture of metals and ceramic
materials, preferably a mixture of aluminum oxides Al2O3, or a mixture of Al2O3
25 and intermetallic matrix Fe-Cr, for example Fe28Cr.
11
[0043] According to an advantageous embodiment variant, the surface protective
coating 3 is composed of one or more of the following carbides: tungsten carbide
(WC), niobium carbide (NbC), chromium carbide (for example, Cr3C2), titanium
(TiC), mixed with a mixture of metal oxides or mixed with a mixture of metals and
5 ceramic materials, preferably with a mixture of aluminum oxides Al2O3, or a
mixture of Al2O3 and intermetallic matrix Fe-Cr, for example Fe28Cr.
[0044] It is clear that the oxides or mixtures of oxides, or the metals or mixtures of
metals and ceramic materials, or the mixtures of carbides and metal oxides described
above are preferably deposited by the same deposition techniques of the carbides in
10 particle form described above and in the present discussion.
[0045] Preferably, the surface protective coating 3 has a thickness comprised
between 30 μm and 150 μm, and preferably between 50 μm and 90 μm.
[0046] According to an embodiment of the present invention, the steel of the base
layer 30 comprises between 10% and 20% of chromium (Cr).
15 [0047] According to an embodiment of the present invention, the steel of the base
layer 30 comprises at least 15% chromium (Cr), even more preferably between 16%
and 18% chromium.
[0048] According to an embodiment, the steel of the base layer 30 comprises at most
5% manganese (Mn), even more preferably, the manganese content is between 0.5%
20 and 5%, extremes included, so as to at least partially compensate for the lack of the
properties of the steel alloy generally imparted by the presence of nickel, increasing
the mechanical strength.
[0049] In particular, according to an embodiment, in order to compensate for the
scarce quantity or complete absence of nickel and to obtain adequate performances
25 for a brake disc, the steel of the base layer 30 is composed of 10 to 20% of
12
Chromium (Cr) by weight, preferably between 16% to 18% of chromium (Cr) by
weight, at most 1.5% by weight of silicon (Si), at most 2% by weight of manganese
(Mn), at most 0.03% by weight of carbon (C) and for the balance iron (Fe), i.e. for
the remaining percentage by weight of iron.
5 [0050] Preferably, the base layer 30 has a thickness comprised between 20 μm and
300 μm, and preferably equal to 90 μm.
[0051] According to a variant of the invention, in order to compensate for the low
quantity or complete absence of nickel and to obtain adequate performance for a
brake disc, the steel of the base layer 30 has a molybdenum content between 0.5%
10 and 10%, even more preferably between 0.5% and 4.5%, extremes included and a
manganese content between 0.5% and 5%. The presence of molybdenum and
manganese in the above percentages allows adequate resistance to corrosion and at
the same time adequate mechanical resistance to be obtained.
[0052] According to an embodiment, between the base layer 30 and at least one of
15 the two braking surfaces 2a, 2b of the braking band 2 there is interposed an
intermediate layer 300 of steel comprising nickel, preferably with a nickel content
higher than 5% in the case in which the base layer 30 is totally free from nickel, or,
even more preferably with a nickel content of at least 5%, and even more preferably
with a nickel content of at least 5% and less than 15%.
20 [0053] According to an embodiment, the intermediate layer 300 comprises a steel
with a nickel content of at most 15% or equal to 15%.
[0054] According to an embodiment, the intermediate layer 300 comprises a steel
with a nickel content of at most 7.5% or equal to 7.5%.
[0055] According to a further embodiment, an intermediate layer 300 of nickel- free
25 steel is interposed between the base layer 30 and at least one of the two braking
13
surfaces 2a, 2b of the braking band.
[0056] According to an embodiment, the intermediate layer 300 comprises a nickelfree steel composed of 10% to 15% of chromium (Cr), at most 1% of silicon (Si), at
most 4% of manganese (Mn), from 0.16% to 0.5% of carbon (C) and for the balance
5 iron (Fe). Preferably, the carbon (C) content is comprised between 0.16% and 0.25%.
[0057] The presence of the intermediate layer 300 allows a disc with adequate
mechanical features to be obtained, but at the same time with a reduced
environmental impact, by virtue of the presence of the base layer 30.
[0058] According to an embodiment, the intermediate layer 300 comprises steel
10 composed of 10% to 15% of chromium (Cr), at most 1% of silicon (Si), at most 4%
of manganese (Mn), from 0.16% to 0.5% of carbon (C) and for the balance iron (Fe).
Preferably, the carbon (C) content of the steel of the intermediate layer 300 is
comprised between 0.16% and 0.25%, extremes included.
[0059] According to an embodiment, the surface protective coating 3 comprises steel
15 composed of 10% to 15% of chromium (Cr), at most 1% of silicon (Si), at most 4%
of manganese (Mn), between 0.16% and 0.5% of carbon (C) and for the balance iron
(Fe), preferably without nickel.
[0060] Preferably, the carbon (C) content of the steel of the surface protective
coating is comprised between 0.16% and 0.25%, extremes included.
20 [0061] According to an embodiment, an auxiliary layer of ferritic-nitrocarburization
or an auxiliary ferroalumination layer is interposed between one of the two braking
surfaces 2a, 2b of the braking band and the base layer 30, or between one of the two
braking surfaces 2a, 2b of the braking band and the intermediate layer 300, or
between the base layer 30 and the surface protective coating 3, or between the
25 intermediate layer 300 and the base layer 30.
14
[0062] According to an embodiment, an auxiliary layer of ferritic-nitrocarburization
and an auxiliary ferroalumination layer is interposed between one of the two braking
surfaces 2a, 2b of the braking band and the base layer 30, or between one of the two
braking surfaces 2a, 2b of the braking band and the intermediate layer 300, or
5 between the base layer 30 and the surface protective coating 3, or between the
intermediate layer 300 and the base layer 30.
[0063] For simplicity of discussion, the brake disc 1 will now be described
contextually to the method according to the present invention. The brake disc 1 is
preferably, but not necessarily, made by the method according to the invention which
10 will now be described.
[0064] According to a first aspect of the present invention, a general embodiment of
the method according to the invention comprises the following operating steps:
a) preparing a brake disc, comprising a braking band and provided with two opposite
braking surfaces 2a, 2b, each of which defines at least partially one of the two main
15 faces of the disc, the braking band being made of gray cast iron or steel;
b) depositing a steel layer comprising at most 15% of nickel, preferably by laser
deposition technique, for example Laser Metal Deposition or Extreme High-Speed
Laser Material Deposition or by Thermal Spray deposition technique, or by Cold
Spray deposition technique, to form the base layer 30;
20 c) optionally depositing over said base layer 30 a material in particle form composed
of tungsten carbide (WC) or niobium carbide (NbC) or titanium carbide (TiC) or
possibly chromium carbide by a Thermal Spray deposition technique, e.g. by the
HVOF (High-Velocity Oxy-Fuel) technique, the HVAF (High-Velocity Air Fuel)
technique, the APS (Atmosphere Plasma Spray) technique or a Cold Spray
25 deposition technique, e.g. by the KM (Kinetic Metallization) technique, or by a laser
15
beam deposition technique, e.g. by the LMD (Laser Metal Deposition) technique, or
by the HSLC (High-Speed Laser Cladding) technique, or by the EHLA (Extreme
High-Speed Laser Application) technique, or by the TSC (Top Speed Cladding)
technique, forming a protective surface coating 3 which covers at least one of the
5 two braking surfaces of the braking band, for example which covers the base layer
30, preferably at least for the entire surface of one of the two braking surfaces 2a, 2b
of the braking band.
[0065] According to a second aspect of the present invention, in a further general
embodiment of the method according to the invention comprises the following
10 operating steps:
a) preparing a brake disc, comprising a braking band and provided with two opposite
braking surfaces 2a, 2b, each of which defines at least partially one of the two main
faces of the disc, the braking band being made of gray cast iron or steel;
b) depositing a steel layer completely free from nickel, preferably by laser deposition
15 technique, for example Laser Metal Deposition or Extreme High-Speed Laser
Material Deposition or by Thermal Spray deposition technique, or by Cold Spray
deposition technique, to form the base layer 30;
c) optionally depositing over said base layer 30 a material in particle form composed
of tungsten carbide (WC) or niobium carbide (NbC) or titanium carbide (TiC) or
20 possibly chromium carbide by a Thermal Spray deposition technique, e.g. by the
HVOF (High-Velocity Oxy-Fuel) technique, the HVAF (High-Velocity Air Fuel)
technique, the APS (Atmosphere Plasma Spray) technique or a Cold Spray
deposition technique, e.g. by the KM (Kinetic Metallization) technique, or by a laser
beam deposition technique, e.g. by the LMD (Laser Metal Deposition) technique, or
25 by the HSLC (High-Speed Laser Cladding) technique, or by the EHLA (Extreme
16
High-Speed Laser Application) technique, or by the TSC (Top Speed Cladding)
technique, forming a protective surface coating 3 which covers at least one of the
two braking surfaces of the braking band, for example which covers the base layer
30, preferably at least for the entire surface of one of the two braking surfaces 2a, 2b
5 of the braking band.
[0066] According to a third aspect of the present invention, in a further general
embodiment of the method according to the invention comprises the following
operating steps:
a) preparing a brake disc, comprising a braking band and provided with two opposite
10 braking surfaces 2a, 2b, each of which defines at least partially one of the two main
faces of the disc, the braking band being made of gray cast iron or steel;
a1) after step a), depositing on at least one of the two opposite braking surfaces 2a,
2b, an intermediate layer 300 composed of nickel-free steel;
b) after step a1), depositing a steel layer completely free from nickel, preferably by
15 laser deposition technique, for example Laser Metal Deposition or Extreme HighSpeed Laser Material Deposition or by Thermal Spray deposition technique, or by
Cold Spray deposition technique, to form the base layer 30;
c) optionally depositing over said base layer 30 a material in particle form composed
of tungsten carbide (WC) or niobium carbide (NbC) or titanium carbide (TiC) or
20 possibly chromium carbide by a Thermal Spray deposition technique, e.g. by the
HVOF (High-Velocity Oxy-Fuel) technique, the HVAF (High-Velocity Air Fuel)
technique, the APS (Atmosphere Plasma Spray) technique or a Cold Spray
deposition technique, e.g. by the KM (Kinetic Metallization) technique, or by a laser
beam deposition technique, e.g. by the LMD (Laser Metal Deposition) technique, or
25 by the HSLC (High-Speed Laser Cladding) technique, or by the EHLA (Extreme
17
High-Speed Laser Application) technique, or by the TSC (Top Speed Cladding)
technique, forming a protective surface coating 3 which covers at least one of the
two braking surfaces of the braking band, for example which covers the base layer
30, preferably at least for the entire surface of one of the two braking surfaces 2a, 2b
5 of the braking band.
According to an embodiment, step a1) provides for depositing an intermediate layer
300 composed of nickel-free steel and from 10% to 15% of chromium (Cr), at most
1% of silicon (Si), at most 4% of manganese (Mn), from 0.16% to 0.5% of carbon
(C), preferably from 0.16% to 0.25% of carbon (C), extremes included, and for the
10 balance of iron (Fe). According to a further aspect of the present invention, in a
further general embodiment of the method according to the invention comprises the
following operating steps:
a) preparing a brake disc 1, comprising a braking band 2 provided with two opposite
braking surfaces 2a, 2b, each of which defines at least partially one of the two main
15 faces of the disc, the braking band being made of gray cast iron or steel;
b) depositing a base layer 30 composed of steel totally free from nickel and from
10% to 15% of chromium (Cr), at most 1% of silicon (Si), at most 4% of manganese
(Mn), from 0.16% to 0.5% of carbon (C), preferably from 0.16% to 0.25%, and for
the balance iron (Fe).
20 According to a further aspect of the present invention, a general embodiment of the
method according to the invention comprises the following operating steps:
a) preparing a brake disc, comprising a braking band and provided with two opposite
braking surfaces 2a, 2b, each of which defines at least partially one of the two main
faces of the disc, the braking band being made of gray cast iron or steel;
25 a1) after step a), depositing on at least one of the two opposite braking surfaces 2a,
18
2b, an intermediate layer 300 composed of steel comprising nickel, preferably
according to the features described in the previous paragraphs of the present
discussion;
b) after step a1), depositing a steel layer completely free from nickel, preferably by
5 laser deposition technique, for example Laser Metal Deposition or Extreme HighSpeed Laser Material Deposition or by Thermal Spray deposition technique, or by
Cold Spray deposition technique, to form the base layer 30;
c) optionally depositing over said base layer 30 a material in particle form composed
of tungsten carbide (WC) or niobium carbide (NbC) or titanium carbide (TiC) or
10 possibly chromium carbide by a Thermal Spray deposition technique, e.g. by the
HVOF (High-Velocity Oxy-Fuel) technique, the HVAF (High Velocity Oxy-Fuel)
technique, the APS (Atmosphere Plasma Spray) technique or a Cold Spray
deposition technique, e.g. by the KM (Kinetic Metallization) technique, or by a laser
beam deposition technique, e.g. by the LMD (Laser Metal Deposition) technique, or
15 by the HSLC (High-Speed Laser Cladding) technique, or by the EHLA (Extreme
High-Speed Laser Application) technique, or by the TSC (Top Speed Cladding)
technique, forming a protective surface coating 3 which covers at least one of the
two braking surfaces of the braking band, for example which covers the base layer
30, preferably at least for the entire surface of one of the two braking surfaces 2a, 2b
20 of the braking band.
[0067] In addition to the aforementioned general embodiment variants of the method
according to the present invention, the method preferably provides for the further
steps which will be described below.
[0068] Preferably, in step c) the tungsten carbide (WC) or the niobium carbide
25 (NbC) or the titanium carbide (TiC) or possibly the chromium carbide is dispersed in
19
a metal matrix.
[0069] According to a preferred embodiment, in step c), the material in particle form
is composed of chromium carbide and titanium carbide.
[0070] Advantageously, the brake disc is arranged with a portion suitable for fixing
5 the disc to a vehicle, composed of an annular portion 4 arranged centrally to the disc
1 and concentric to the braking band 2. The fixing portion 4 supports the connecting
element 5 to the wheel hub (i.e. the bell). The bell may be made in one piece with the
annular fixing portion (as illustrated in the accompanying figures) or it may be made
separately and, therefore, fixed through suitable connecting elements to the fixing
10 portion.
[0071] The annular fixing portion 4 may be made of the same material as the braking
band, that is, of gray cast iron, or of another suitable material. The bell 5 may also be
made of gray cast iron or of another suitable material. In particular, the whole disc
(i.e. braking band, fixing portion and bell) may be made of gray cast iron.
15 [0072] Preferably, the braking band 2 is made by casting. Similarly, when made of
gray cast iron, the fixing portion and/or the bell may be made by casting.
[0073] The annular fixing portion may be made in a single body with the braking
band (as illustrated in the accompanying figures) or be made as a separate body,
mechanically connected to the braking band.
20 [0074] As regards the HVOF, HVAF or KM, or LMD or HSLC techniques, these are
three deposition techniques which are known to those skilled in the art and will
therefore not be described in detail.
[0075] HVOF (High Velocity Oxygen Fuel) is a powder spray deposition technique
which uses a spray device provided with a mixing and combustion chamber and a
25 spray nozzle. The chamber is supplied with oxygen and fuel. The hot combustion gas
20
which forms at pressures close to 1 MPA passes through the converging-diverging
nozzle into the powdered material reaching hypersonic speeds (i.e. higher than
MACH 1). The powder material to be deposited is injected into the hot gas stream,
where it melts rapidly and is accelerated to speeds of the order of 1000 m/s. Once
5 impacted on the deposition surface, the molten material cools rapidly and due to the
impact with high kinetic energy it forms a very dense and compact structure.
[0076] The High Velocity Air Fuel (HVAF) deposition technique is similar to the
HVOF technique. The difference is that in the HVAF technique the combustion
chamber is fed with air instead of oxygen. The temperatures involved are therefore
10 lower than those of the HVOF. This allows for greater control of the thermal
alteration of the coating.
[0077] The KM (Kinetic Metallization) deposition technique is a solid-state
deposition process in which metal powders are sprayed through a two-phase sonic
deposition nozzle which accelerates and triboelectrically charges metal particles
15 within an inert gas stream. Thermal energy is expected to be supplied to the transport
stream. The process transforms the potential energy of the compressed inert gas
stream and the thermal energy supplied into the kinetic energy of the powders. Once
accelerated to high speed and electrically charged, the particles are directed against
the deposition surface. The high-speed collision of the metal particles with this
20 surface causes a large deformation of the particles (approximately 80% in the
direction normal to impact). This deformation results in a huge increase in the
surface area of the particles. Upon impact, the effect is therefore intimate contact
between the particles and the deposition surface, which leads to the formation of
metal bonds and a coating having a very dense and compact structure.
25 [0078] Advantageously, as an alternative to the three deposition techniques listed
21
above, which share the fact that they are high kinetic energy impact deposition
techniques, other techniques may also be used which exploit different deposition
methods, but which are able to generate coatings having a very dense and compact
structure.
5 [0079] The combination of the HVOF or HVAF or KM or LMD or HSLC deposition
technique and the chemical components used for the formation of the base layer 30
and the surface protective coating 3, allows both high bond strength on the lower
material on which they are deposited and the deposition of powders with high
carbide content to be obtained.
10 [0080] As already mentioned above, the base layer 30 and the protective surface
coating 3 cover at least one of the two braking surfaces of the braking band.
[0081] Hereinafter, the term “coating” will refer to both the set given by the base
layer 30 and the protective surface coating 3, and to the base layer 30 alone, in the
variant which does not provide for the surface protective coating 3, but which
15 provides for the inclusion of carbides in the base layer 3.
[0082] Preferably, as illustrated in Figure 2 and Figure 3, the disc 1 is provided with
a coating 3, 30 which covers both the braking surfaces 2a and 2b of the braking band
2.
[0083] In particular, the coating 3, 30 may cover only the braking band, on a single
20 braking surface or on both.
[0084] According to embodiments not illustrated in the appended figures, the coating
3, 30 may also extend to other parts of the disc 1 such as the annular fixing portion 4
and the bell 5, up to cover the entire surface of the disc 1. In particular, the coating 3,
30 may cover - in addition to the braking band - only the fixing portion or only the
25 bell. The choice is dictated by essentially aesthetic reasons, in order to have a
22
homogeneous coloring and/or finish on the whole disc or between some portions
thereof.
[0085] Advantageously, the deposition of the particulate material for the formation
of the coating 3, 30 may be carried out in a differentiated manner on the surface of
5 the disc at least in terms of thickness of the coating.
[0086] At the braking band, the coating 3, 30 may be made with the same thickness
in the two opposite braking surfaces. Alternative solutions may be provided in which
the coating 3, 30 is made by differentiating the different thicknesses between the two
braking surfaces of the braking band.
10 [0087] According to an embodiment of the method, the step b) of depositing the base
layer 30 provides for depositing a composition in particle form composed of steel
having a nickel content of at most 15% or at most 7.5% or at most 5% or totally
nickel-free steel, by laser deposition technique, preferably LMD (Laser Metal
Deposition) or EHLA (Extreme High-Speed Laser Material Deposition), or by
15 Thermal Spray deposition technique, or by the Cold Spray deposition technique.
[0088] In an advantageous embodiment, in step b) the composition in particle form
further comprises carbides mixed in a percentage not exceeding 50% by weight of
the total particulate composition.
[0089] In an advantageous embodiment, in step b) the composition in particle form,
20 in addition to steel, also includes metal oxides or a mixture of metals and ceramic
materials, preferably a mixture of aluminum oxides Al2O3, or a mixture of Al2O3
and intermetallic matrix Fe-Cr, for example Fe28Cr.
[0090] According to an embodiment, in step b) the composition in particle form, in
addition to steel, also includes metal oxides or a mixture of metals and ceramic
25 materials, preferably a mixture of aluminum oxides Al2O3, or a mixture of Al2O3
23
and intermetallic matrix Fe-Cr, for example Fe28Cr, and also one or more carbides
selected from the group comprising: tungsten carbide (WC), niobium carbide (NbC),
titanium carbide (TiC), chromium carbide.
[0091] It is therefore clear that, by virtue of the aforementioned variants of the
5 method, it is possible to obtain a braking band 2 in which the base layer 30
comprises a mixture of steel and metal oxides described above, or, in another variant,
a mixture of steel and metal oxides and carbides described above.
[0092] The preferred embodiment variants of the braking band and the arrangement
order of the base layer 30, of the intermediate layer 300 and of the surface coating
10 layer 3, are also more understandable with reference to the appended figures.
[0093] Preferably, the step a1) of depositing the intermediate layer 300 provides for
depositing a composition in particle form composed of steel having a nickel content
between 5% and 15%, by means of a laser deposition technique, preferably LMD
(Laser Metal Deposition) or EHLA (Extreme High-Speed Laser Material
15 Deposition), or by the Thermal Spray deposition technique, or by the Cold Spray
deposition technique.
[0094] According to an advantageous embodiment variant of the method, the step
e1) is provided of depositing an auxiliary layer of ferritic-nitrocarburization between
one of the two braking surfaces 2a, 2b of the braking band and the base layer 30,
20 and/or between one of the two braking surfaces 2a, 2b of the braking band and the
intermediate layer 300, and/or between the base layer 30 and the protective surface
coating 3, and/or between the intermediate layer 300 and the base layer 30.
[0095] According to an advantageous embodiment, the method comprises the step
e2) of depositing an auxiliary ferroalumination layer between one of the two braking
25 surfaces 2a, 2b of the braking band and the base layer 30, and/or between one of the
24
two braking surfaces 2a, 2b of the braking band and the intermediate layer 300,
and/or between the base layer 30 and the protective surface coating 3, and/or
between the intermediate layer 300 and the base layer 30.
[0096] Preferably, the ferroalumination step e2) comprises the steps of:
5 e21) immersing at least partially said braking band 2 into molten aluminum
maintained at a predetermined temperature so that the molten aluminum covers at
least a predetermined surface region of said braking band 2, said immersion being
protracted for a predetermined period of time to allow the diffusion of aluminum
atoms into the surface microstructure of said cast iron or steel with the consequent
10 formation of ferroaluminum intermetallic compounds in a surface layer of said
braking band 2, thus generating a layer comprising of ferroaluminum intermetallic
compounds in said predetermined surface region of said braking band 2;
e22) removing said braking band 2 from the molten aluminum;
e23) removing the aluminum remaining on said braking band 2 after extraction, so
15 as to expose said layer of ferroaluminum intermetallic compounds on the surface.
[0097] The layer of ferroaluminum intermetallic compounds exposed on the surface
imparts a superior resistance to corrosion and wear at said predetermined surface
region to the braking band 2 made of cast iron or steel.
[0098] Preferably, the layer of ferroaluminum intermetallic compounds comprises
20 FeAl3 as the prevailing phase of the ferroaluminum intermetallic compounds.
[0099] According to an advantageous embodiment, the predefined temperature at
which the molten aluminum is maintained is not higher than 750 °C, and is
preferably between 690 °C and 710 °C, and even more preferably equal to 700 °C.
[00100] According to an advantageous aspect of the method, the predefined
25 period of immersion time is determined according to the thickness to be obtained for
25
said layer of intermetallic compounds, at the same temperature of the molten
aluminum said thickness increasing as the immersion time increases, with the same
immersion time, said thickness increasing as the temperature of the molten aluminum
increases, preferably said predefined immersion time being between 5 and 60 min,
5 and even more preferably equal to 30 min.
[00101] According to an advantageous aspect, before the immersion step e21),
the method comprises a step f) of decarburizing said predefined surface region of
said braking band 2 up to a predefined depth.
[00102] It has been experimentally verified that the presence of carbon in the
10 surface layer of the braking band subject to penetration by diffusion of aluminum
atoms (induced by aluminization) also leads to the formation of iron carbide as well
as intermetallic compounds. The presence of iron carbide creates points of
discontinuity in the layer of intermetallic compounds, points which may trigger both
corrosive phenomena and cracks. Advantageously, the surface decarburization
15 therefore allows avoiding (or at least significantly reducing) the formation of iron
carbide, leading to the formation of a layer of intermetallic compounds more
resistant to corrosion and less subject to cracking.
[00103] Preferably, in said step f) the decarburization of said at least one
predefined surface region is carried out by means of an electrolytic process.
20 [00104] More in detail, said electrolytic process is carried out by immersing
the predefined surface region of said braking band in a bath of molten salts and
applying an electric potential difference between the bath and the braking band.
[00105] In applying the electric potential difference, the braking band is
connected to a positive pole (cathode), while the aforementioned bath of molten salts
25 is connected to a negative pole (anode). Carbon, particularly in the form of graphite
26
flakes, is oxidized to carbon dioxide by the release of electrons and atomic oxygen
released at the anode. Carbon reacts primarily with oxygen and is eventually bound
as carbon dioxide.
[00106] The oxidation of the surface of the braking band induced by the
5 electrolytic process is not limited to the carbon present therein, but also extends to
the metal matrix of the cast iron (iron), causing the formation of a surface film of
metal oxide. Reversing the polarity causes the reduction of the surface film of metal
oxide which is thus returned to the original metallic state.
[00107] Preferably, the aforementioned electrolytic process may therefore
10 provide that, after a predefined period of time in which the surface of the braking
band has been connected to the cathode to oxidize the carbon, the polarity is reversed
so as to return the metal oxide film to its original metallic state.
[00108] Operationally, the decarburization depth is controlled by adjusting the
duration of the electrolytic process, possibly divided into different polarity inversion
15 cycles. By increasing the duration of the decarburization process (oxidation phase of
the braking band; connection to the cathode), the decarburization depth increases, all
other conditions being equal.
[00109] The decarburization may be carried out with alternative processes to
the electrolytic process described above, for example by means of a laser treatment
20 or a chemical treatment.
[00110] Decarburization by electrolytic process is however preferred because:
- compared to a laser treatment it is much more efficient and rapid, ensuring a more
complete and uniform carbon removal in less time;
- compared to a chemical treatment (for example with potassium permanganate) it is
25 much more efficient (ensuring a more complete and uniform carbon removal in less
27
time) and does not leave oxidation areas of the metal matrix of the cast iron on the
treated portion.
[00111] More in detail, it has been observed that at the oxidized areas on the
metal matrix of the cast iron the wettability of the molten aluminum is very low and
5 this negatively affects the aluminization process and the features of the layer of
intermetallic compounds. Also for this reason, the electrolytic decarburization
process is preferred over the alternative processes indicated above.
[00112] As has already been pointed out above, the growth thickness of the
intermetallic compound layer is mainly influenced by the temperature of the molten
10 aluminum and the immersion time in the molten aluminum. However, it has been
found that a further factor affecting the thickness of the intermetallic compound layer
is the silicon content in the molten aluminum. The higher the weight content of
silicon in the molten aluminum, the lower the thickness of the intermetallic
compound layer under the same conditions. Preferably, the molten aluminum has a
15 silicon content lower than 1% by weight.
[00113] Preferably, the molten aluminum has an impurity content not higher
than 1% by weight. In particular, aluminum with a maximum purity of 99.7% by
weight may be used, with the following impurities (% by weight): Si ≤ 0.30%; Fe
≤0.18%; Sr ≤0.0010%; Na ≤0.0025%; Li ≤0.0005%; Ca ≤0.0020%; P ≤0.0020; Sn
20 ≤0.020 %.
[00114] In some cases it has occurred that, despite having subjected the
braking band to decarburization and therefore eliminated the graphite flakes from a
surface layer at which the layer of intermetallic compounds would have formed, the
resulting layer of intermetallic compounds still included graphite flakes, as if they
25 had never been eliminated. This phenomenon may be explained by the fact that the
28
dissolution of the iron in the aluminum is so rapid that the decarburized layer is
rapidly consumed and consequently the metal compounds are formed in the layer
below the decarburized layer, i.e. where there are graphite flakes.
[00115] In other words, the excessive solubility of iron in molten aluminum
5 may totally or partially cancel the beneficial effects of the surface decarburization of
the braking surface.
[00116] Advantageously, in order to slow down the dissolution of the iron in
the aluminum bath, the step b1) of immersion in a bath of molten aluminum in which
iron has been dissolved may be carried out. In this way, by inhibiting the dissolution
10 of the iron in the aluminum, the formation of FeAl3 is kinematically promoted, so as
to allow the intermetallic compounds to form at the decarburized layer.
[00117] Preferably, the iron content in solution in the aluminum bath is not
higher than 5% by weight, and even more preferably it is comprised between 3% and
5%, and most preferably equal to 4% by weight to ensure a significant effect of
15 slowing of the melting process of the iron of the cast iron in the aluminum.
[00118] For example, an aluminum bath having the following composition (%
by weight) may be used: 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 %.
20 [00119] It has been experimentally observed that by carrying out
aluminization with an aluminum bath with iron in solution, especially if the iron
content is close to the solubility limit, more porous layers of intermetallic
compounds are obtained. This may be explained by a higher viscosity of the molten
aluminum bath containing iron and a consequent reduction of its wettability with
25 respect to cast iron.
29
[00120] Advantageously, in order to form a layer of intermetallic compounds
that is compact and uniform and therefore not very porous, while avoiding at the
same time that this layer develops below the decarburized layer and incorporates the
graphite flakes present therein, the aforementioned step b1) of immersion is carried
5 out in two sub-steps:
- a first sub-step b11) of immersion in a first bath of molten aluminum, substantially
free of iron in solution (or at least present at most as an impurity; for example with
an iron content lower than 0.20% by weight), to obtain on said predefined surface
region an initial layer composed of ferroaluminum intermetallic compounds; and
10 - a second sub-step b12) of immersion in a second bath of molten aluminum,
containing iron in solution, to increase said initial layer until a final layer is obtained
on said predefined surface region composed of ferroaluminum intermetallic
compounds having a predefined thickness.
[00121] The immersion time of said braking band in said first bath is less than
15 the immersion time of said braking band in said second bath.
[00122] Preferably, the immersion of said braking band in said first bath is
continued for a period of time as short as possible, but sufficient to obtain on said
predefined surface region an initial layer composed of ferroaluminum intermetallic
compounds having a thickness not exceeding at 10 µm. In particular, the immersion
20 time in said first bath is between 3 and 5 minutes if the first bath is at a temperature
of about 700 °C. As the bath temperature increases, the immersion time must
decrease.
[00123] More in detail, for the same temperature of the second bath, said
thickness increases as the immersion time increases and for the same immersion
25 time, said thickness increases as the temperature of the second bath increases.
30
[00124] Advantageously, both said first bath of molten aluminum and said
second bath have an impurity content not higher than 1% by weight. In particular,
said two molten aluminum baths have a silicon content lower than 1% by weight.
[00125] Preferably, the iron content in solution in the second aluminum bath is
5 not higher than 5% by weight (at 700 °C the solubility limit of iron in aluminum is
4% by weight; aluminum saturated with iron), and even more preferably it is
comprised between 3% and 5%, and most preferably it is equal to 4% by weight. The
iron content should not be less than 3% to ensure a significant slowing effect of the
melting process of the iron of the cast iron in the aluminum.
10 [00126] Advantageously, both said first bath and said second bath are
maintained at a temperature lower than 680 °C, preferably not higher than 750 °C,
more preferably between 690 °C and 710 °C, and even more preferably equal to 700
°C.
[00127] Advantageously, the method may comprise a step of surface
15 pretreatment of the braking band which is carried out before said immersion step
e21) at least at said predefined surface region. Preferably, said surface pretreatment
step comprises lapping, degreasing, sandblasting and/or chemical removal of the
surface oxides.
[00128] Preferably, the method comprises a step of removing a surface layer
20 of oxides from the molten aluminum bath before said immersion step e21). This step
of removal of the surface oxides is carried out both in the case in which immersion in
a single bath is contemplated, and in the case in which immersion is contemplated in
two successive steps in a first and in a second bath.
[00129] According to a preferred embodiment of the invention, the step of
25 removing the aluminum remaining adhered to said braking band after the extraction
31
is carried out in two sub-steps:
[00130] a first sub-step of removal is carried out on the braking band just
extracted from the molten aluminum to remove the molten aluminum still remaining
adhered to the braking band; and
5 [00131] a second removal sub-step is carried out on the braking band extracted
from the molten aluminum and cooled to remove the solidified residual aluminum
remaining after said first removal sub-step.
[00132] Preferably, the method comprises a quenching step of said braking
band carried out between said first removal sub-step and said second removal sub10 step.
[00133] Advantageously, said first removal sub-step may be carried out by
mechanical shaving of the still liquid aluminum.
[00134] Advantageously, said second removal sub-step may be carried out by
chemical removal of the solidified aluminum not removed mechanically.
15 [00135] Preferably, the aforementioned chemical removal is carried out by
exposing the aluminum to ferric chloride for at least 4 minutes so as to cause the
following reaction:
Al+ FeCl3 -> AlCl3 + Fe
[00136] Chemical removal by ferric chloride should necessarily take place
20 after the solidification of the aluminum. Ferric chloride boils at 315 °C and therefore
may not be brought into contact with molten aluminum. Preferably, said chemical
removal is then carried out after said quenching step.
[00137] The aforementioned steps of the method referred to ferroalumination
therefore allow a braking band, and therefore a brake disc, to be obtained with
25 increased resistance to wear and corrosion.
32
[00138] It should be noted that the layer of ferroaluminum intermetallic
compounds may comprise a plurality of intermetallic compounds between iron and
aluminum, in particular Fe3Al, FeAl, FeAl2, FeAl3, Fe2Al5. The prevailing
intermetallic phase is FeAl3 as it is thermodynamically more stable.
5 [00139] According to an embodiment, the method provides for depositing an
auxiliary ferritic-nitrocarburization and an auxiliary ferroalumination layer between
one of the two braking surfaces 2a, 2b of the braking band and the base layer 30,
and/or between one of the two braking surfaces 2a, 2b of the braking band and the
intermediate layer 300, and/or between the base layer 30 and the protective surface
10 coating 3, and/or between the intermediate layer 300 and the base layer 30.
[00140] As may be appreciated from the above description, the brake disc
according to the invention allows the drawbacks of the prior art to be overcome.
[00141] By virtue of the combination of a steel base layer with reduced nickel
content or even totally nickel-free with a cast iron band, the brake disc 1 according to
15 the invention is substantially not subject to the production and release of nickel
particles during operation.
[00142] Not only that, according to particularly advantageous variants, the
addition of a protective surface coating 3 which includes or is coated with carbides,
allows both the wear resistance properties to be improved, also compensating for the
20 lack of nickel in the steel of the base layer, and adequate and increased mechanical
strength to be provided.
[00143] Particularly advantageously, the base layer 30 composed of totally
nickel-free steel and 10% to 15% chromium (Cr), at most 1% of silicon (Si), at most
4% of manganese (Mn), from 0.16% to 0.5% of carbon (C), preferably from 0.16%
25 to 0.25% of carbon (C), and for the balance of iron (Fe), allows a martensitic steel
33
without nickel to be made with less brittleness during use at high temperatures and at
the same time an adequate anticorrosive coating. These advantageous aspects are
also synergistically combined with the possibility of using a reduced percentage of
any carbides included in the steel, thus reducing the resources necessary for
5 production, while maintaining an adequate hardness of the coating.
[00144] Advantageously, the base layer 30, preferably nickel-free, also
performs a mechanical “cushioning” function for the protective surface coating 3
(anti-wear). The base layer 30, in fact, assumes an elastic behavior which allows the
stresses imparted to the disc to be attenuated at least in part when in use. The base
10 layer 30 therefore operates as a sort of shock absorber or cushion between the disc
and the protective surface coating 3. In this way, a direct transmission of stresses
between the two parts is avoided, thus also reducing the risk of initiation of cracks in
the protective surface coating 3.
15
34
WE CLAIM:
1. A brake disc (1) for disc brake, comprising a braking band (2) provided with two
opposite braking surfaces (2a, 2b), each of which defines at least partially one of the
two main faces of the disc (1), the braking band (2) being made of gray cast iron or
5 steel;
said disc being provided with a base layer (30) which covers at least one of the two
braking surfaces (2a, 2b) of the braking band, said base layer (30) being composed of
steel totally free from nickel and of one or more carbides included in the nickel-free
steel,
10 and wherein an intermediate layer (300) composed of nickel-free steel is interposed
between the base layer (30) and at least one of the two braking surfaces (2a, 2b) of
the braking band (2).
2. Brake disc (1) for disc brake according to claim 1, wherein in the base layer (30)
the one or more carbides included comprise at least one carbide selected from the
15 group comprising: tungsten carbide (WC), chromium carbide, niobium carbide
(NbC), titanium carbide (TiC).
3. Brake disc (1) for disc brake according to any one of the preceding claims,
comprising a protective surface coating (3) which covers the base layer (30) at least
on the side of one of the two braking surfaces (2a, 2b) of the braking band, said
20 protective surface coating (3) being arranged on a side of the base layer (30) which
does not face towards the one of the two braking surfaces (2a, 2b), said protective
surface coating (3) being composed of one or more carbides in particle form
deposited by a Thermal Spray deposition technique, e.g. by the HVOF (HighVelocity Oxy-Fuel) technique, or by the HVAF (High-Velocity Air Fuel) technique,
25 or by the APS (Atmosphere Plasma Spray) technique, or by a Cold Spray deposition
35
technique, e.g. by the KM (Kinetic Metallization) technique, or by a laser beam
deposition technique, e.g. the LMD (Laser Metal Deposition), or the HSLC (HighSpeed Laser Cladding) technique, or the EHLA (Extreme High-Speed Laser
Application) technique, or the TSC (Top Speed Cladding) technique.
5 4. Brake disc (1) for disc brake according to claim 3, wherein the one or more
carbides in particle form comprise tungsten carbide (WC) or chromium carbide or
niobium carbide (NbC) or titanium carbide (TiC).
5. Brake disc (1) for disc brake according to claim 4, wherein the carbides in particle
form are composed of chromium carbide and titanium carbide.
10 6. Disc according to any one of the preceding claims, wherein the steel of the base
layer (30) comprises at least 15% of chromium (Cr).
7. Disc according to any one of claims 1 to 6, wherein the steel of the base layer (30)
comprises between 10% and 20% of chromium (Cr), including the extremes.
8. Disc according to any one of the preceding claims, wherein the steel of the base
15 layer (30) has a content of molybdenum comprised between 0.5% and 10%,
including the extremes, and a content of manganese between 0.5% and 7%.
9. Disc according to any one of the preceding claims, wherein the steel of the base
layer (30) is composed of 10% to 20% of chromium (Cr), at most of 1.5% of silicon
(Si), at most of 2% of manganese (Mn), at most of 0.03% carbon (C) and for the
20 balance of iron (Fe).
10. Disc according to any one of the preceding claims, wherein the base layer (30)
has a thickness comprised between 20 μm and 300 μm, and preferably equal to 90
μm.
11. A method for making a brake disc comprising the following operating steps:
25 a) preparing a brake disc (1), comprising a braking band (2) provided with two
36
opposite braking surfaces (2a, 2b), each of which defines at least partially one of the
two main faces of the disc, the braking band being made of gray cast iron or steel;
a1) after step a), depositing on at least one of the two opposite braking surfaces (2a,
2b), an intermediate layer (300) composed of nickel-free steel;
5 b) after step a1), depositing a base layer (30) composed of steel totally free from
nickel.
12. Method according to claim 11, wherein the step b) of depositing the base layer
(30) comprises depositing a composition in particle form composed of nickel-free
steel, by means of a laser deposition technique, preferably Laser Metal Deposition or
10 Extreme High-Speed Laser Material Deposition, or by means of a Thermal Spray
deposition technique, or by means of a Cold Spray deposition technique.
13. Method according to claim 12, wherein, in step b) the composition in particle
form further comprises carbides mixed in a percentage not exceeding 50% by weight
of the total particulate composition.
15 14. Method according to any one of claims 11 or 12 or 13, further comprising the
step c) of depositing over said base layer (30) a material in particle form composed
of tungsten carbide (WC) or niobium carbide (NbC) or titanium carbide (Tic) or
chromium carbide by a Thermal Spray deposition technique, e.g. by the HVOF
(High-Velocity Oxy-Fuel) technique, the HVAF (High-Velocity Air Fuel) technique,
20 the APS (Atmosphere Plasma Spray) technique or a Cold Spray deposition
technique, e.g. by the KM (Kinetic Metallization) technique, or by a laser beam
deposition technique, e.g. by the LMD (Laser Metal Deposition) technique, or by the
HSLC (High-Speed Laser Cladding) technique, or by the EHLA (Extreme HighSpeed Laser Application) technique, or by the TSC (Top Speed Cladding) technique,
25 forming a protective surface coating (3) which covers the base layer (30), preferably
37
at least for the entire surface of one of the two braking surfaces (2a, 2b) of the
braking band.
15. Method according to any one of claims 12 to 14, further comprising the step e1)
of depositing an auxiliary layer of ferritic-nitrocarburization between one of the two
5 braking surfaces (2a, 2b) of the braking band and the base layer (30), and/or between
one of the two braking surfaces (2a, 2b) of the braking band and the intermediate
layer (300), and/or between the base layer (30) and the protective surface coating (3),
and/or between the intermediate layer (300) and the base layer (30), or
comprising the step e2) of depositing an auxiliary ferroalumination layer between
10 one of the two braking surfaces (2a, 2b) of the braking band and the base layer (30),
and/or between one of the two braking surfaces (2a, 2b) of the braking band and the
intermediate layer (300), and/or between the base layer (30) and the protective
surface coating (3), and/or between the intermediate layer (300) and the base layer
(30),
15 wherein the ferroalumination step e2) comprises the step of:
e21) immersing at least partially said braking band (2) into molten aluminum
maintained at a predetermined temperature so that the molten aluminum covers at
least a predetermined surface region of said braking band (2), said immersion being
protracted for a predetermined period of time to allow the diffusion of aluminum
20 atoms into the surface microstructure of said cast iron or steel with the consequent
formation of ferroaluminum intermetallic compounds in a surface layer of said
braking band (2), thus generating a layer comprising of ferroaluminum intermetallic
compounds in said predetermined surface region of said braking band;
e22) removing said braking band from the molten aluminum;
25 e23) removing the aluminum remaining on said braking band after extraction, so
38
as to expose said layer of ferroaluminum intermetallic compounds on the surface,
said layer of ferroaluminum intermetallic compounds exposed on the surface
imparting a superior resistance to corrosion and wear at said predetermined surface
region to said braking band made of cast iron or steel.