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
METHOD FOR PRODUCING A CERAMIC COATING ON THE SURFACE OF AN ALUMINUM
ALLOY SUBSTRATE BY MEANS OF PLASMA ELECTROLYTIC OXIDATION
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 producing a ceramic coating on the
surface of an aluminum alloy substrate by means of plasma electrolytic oxidation.
5 [0002] The method according to the invention is applied in particular to aluminum and
silicon alloy substrates.
[0003] The method according to the invention finds particular application in the
automotive field, in the production of protective surface coatings for components of
braking systems, since it allows ceramic coatings having high resistance to wear and
10 corrosion to be made.
Prior art
[0004] Aluminum alloys, and in particular aluminum-silicon alloys, are widely used in
automotive and aerospace applications due to their high strength/density ratio, their
machinability and also their excellent castability.
15 [0005] As is known, anodizing is the preferred method for obtaining a corrosionresistant coating on aluminum alloys.
[0006] However, anodizing has some operational limitations.
[0007] With anodizing it is difficult to obtain coatings with high thicknesses on
aluminum-silicon alloys. This is essentially due to the presence of silicon which tends to
20 inhibit the formation of the anodized coating.
[0008] Furthermore, anodizing requires a pickling pre-treatment which greatly affects
the fatigue life of the machined aluminum alloy.
[0009] As an alternative to anodizing, a process of formation of coatings by means of
plasma electrolytic oxidation has been proposed for several years.
25 [0010] Plasma electrolytic oxidation, known as PEO, MAO (Micro Arc Oxidation) or
EPO (Electrolytic Plasma Oxidation), is an electrochemical surface treatment which
allows different alloys, such as Magnesium, Aluminum and Titanium, to be coated.
[0011] The principle underlying plasma electrolytic oxidation is the formation of an
oxide layer with dielectric properties on the substrate to be coated. The substrate is
30 immersed as an electrode together with a counter-electrode in an alkaline electrolytic
aqueous solution. By applying a sufficient electrical voltage, the treatment works in a
discharge regime, creating multiple sparks on the surface. The local temperature of the
3
sparks allows the local remelting of the oxide layer, which reacts with the electrolyte in
which the substrate is immersed. The coating layer that is formed has a high adhesive
strength, since it penetrates the substrate for a few micrometers, and has a high
resistance to corrosion.
5 [0012] There are many applications of the PEO process to aluminum alloys, and in
particular the application of Al-Si alloys to aluminum-silicon alloys.
[0013] The main problem of the PEO process is the formation of a considerable porous
outer layer of low micro-hardness and with numerous micro and macro defects (pores,
micro-cracks, flaky patches). The thickness of the defective layer is equal to 25-55% of
10 the total thickness of the ceramic coating, depending on the chemical composition of the
substrate and the method of carrying out the electrolysis.
[0014] The SEM image of Figure 3 shows a cross section of a ceramic coating obtained
on an aluminum-silicon alloy substrate by means of a traditional PEO process. The
lower band represents the aluminum-silicon alloy substrate (indicated with a in the
15 figure); the central band immediately above the substrate (indicated with b1 in the
figure) represents the most compact and homogeneous layer of the ceramic coating; the
large granular band above the layer b1 (indicated with b2 in the figure) represents the
porous surface layer of the ceramic coating; the upper band (indicated by c in the figure)
above the ceramic coating b represents the resin layer used to incorporate the sample in
20 order to polish the sample and consequently perform the SEM scan.
[0015] Expensive precision equipment is used to remove the porous layer. If the
substrate is complex in shape, with surfaces that are difficult to reach for abrasive and
diamond tools, the problem of removing the defective layer becomes difficult to solve.
This limits the application scope of the process.
25 [0016] This problem has been addressed in particular in patent GB2386907. The PEO
process described in such patent allows the quick and efficient formation of uniformly
colored wear-resistant, corrosion-resistant, heat-resistant and dielectric ceramic coatings
on the surfaces of these articles. The coatings obtained with this process are
characterized by a high degree of uniformity of thickness, low surface roughness and by
30 the virtual absence of the aforementioned outer porous layer.
[0017] The process described in GB2386907 comprises the following steps: i)
supplying the electrodes with bipolar pulses at a high current frequency having a
4
predetermined frequency range (at least 500Hz); and ii) generating acoustic vibrations
in the electrolyte in a predetermined sound frequency range so that the frequency range
of the acoustic vibrations overlaps the frequency range of the current pulses. The
acoustic vibrations cause the aero-hydrodynamic saturation of the electrolyte with
5 oxygen. For this purpose, the electrolyte is fed with oxygen or air. The process also
involves the introduction of ultra-dispersed solid particles into the electrolyte to create a
stable hydrosol through acoustic vibrations.
[0018] The process described in GB2386907, while leading to appreciable results, is
nevertheless complex to control.
10 [0019] Therefore, the need is felt - in particular in the field of braking systems - to have
a method for producing a ceramic coating on the surface of an aluminum alloy substrate
by means of plasma electrolytic oxidation, which allows ceramic coatings having low
roughness, high hardness and high corrosion resistance to be obtained more easily on
aluminum alloy substrates.
15 Disclosure of the invention
[0020] Therefore, the object of the present invention is to eliminate, or at least reduce,
the aforementioned problems relating to the prior art, by providing a method for
producing a ceramic coating on the surface of an aluminum alloy substrate by means of
plasma electrolytic oxidation which allows ceramic coatings with low roughness, high
20 hardness and high resistance to corrosion to be obtained more easily.
[0021] A further object of the present invention is to provide a method for producing a
ceramic coating on the surface of an aluminum alloy substrate by means of plasma
electrolytic oxidation which allows a coating substantially free of a porous surface layer
to be obtained.
25 [0022] A further object of the present invention is to provide a method for producing a
ceramic coating on the surface of an aluminum alloy substrate by means of plasma
electrolytic oxidation which allows a very homogeneous coating to be obtained.
Description of the drawings
[0023] The technical features of the invention are clearly identified in the content of the
30 claims set out below and its advantages will become more readily apparent in the
detailed description that follows, made with reference to the accompanying drawings,
which represent one or more embodiments provided purely by way of non-limiting
5
examples, in which:
[0024] - Figure 1 shows an SEM image of a cross section of a ceramic coating obtained
on an aluminum-silicon alloy substrate by means of a traditional PEO process;
[0025] - figure 2 shows the trend of the electrical potential applied to an aluminum5 silicon alloy substrate according to a preferred embodiment of the method according to
the invention;
[0026] - Figure 3 shows an SEM image of a cross section of a ceramic coating obtained
on an aluminum-silicon alloy substrate by the method according to the invention; and
[0027] - Figure 4 shows an SEM image of the surface of a ceramic coating obtained on
10 an aluminum-silicon alloy substrate by the method according to the invention.
[0028] Elements or parts of elements common to the embodiments described
hereinafter will be indicated with the same numerical references.
Detailed description
[0029] The present invention relates to a method for producing a ceramic coating on the
15 surface of an aluminum alloy substrate by means of plasma electrolytic oxidation.
[0030] The method according to the invention is generally applied on aluminum alloy
substrates, and in particular on aluminum and silicon alloy substrates.
[0031] The method according to the invention finds particular application in the
automotive FIELD, in the production of protective surface coatings for components of
20 braking systems, since it allows ceramic coatings having high resistance to wear and
corrosion to be made.
[0032] According to a general embodiment of the invention, the method for producing a
ceramic coating on the surface of an aluminum alloy substrate by means of plasma
electrolytic oxidation comprises the following steps:
25 [0033] - immersing the substrate as an electrode together with a counter-electrode in an
alkaline electrolytic aqueous solution;
[0034] - applying an electrical potential sufficient to generate spark discharges on the
surface of the substrate for a predefined period of treatment time so as to lead to the
formation of said coating.
30 [0035] The coating thus obtained consists mainly of aluminum oxides and oxides of any
alloying agents of said alloy.
[0036] In particular, the aforesaid substrate is made of aluminum and silicon alloy, and
6
even more particularly of aluminum alloy with a high silicon content (˃ 7% by weight).
In this case, the ceramic coating obtained is a layer consisting mainly of a mixture of
aluminum oxides, silicon oxides and mixed aluminum-silicon oxides.
[0037] Advantageously, the substrate consists of a component of a braking system, in
5 particular of a disc braking system. Preferably, the substrate consists of a brake caliper,
a brake caliper piston or a brake disc bell.
[0038] In particular, the method according to the invention is of the electrolytic type. In
particular, such method comprises an anode, represented by the substrate, and a counterelectrode.
10 [0039] In particular, the method according to the invention comprises a counterelectrode which may be a secondary electrode. Alternatively, such counter-electrode is
represented by a cathode. Alternatively, such counter-electrode is represented by the
container containing the alkaline electrolytic aqueous solution.
[0040] According to the invention, the electrolytic aqueous solution comprises:
15 [0041] - from 9 to 14 g/l of Na2SiO3;
[0042] - from 2.2 to 2.8 g/l of K3PO4;
[0043] - not less than 5 g/l of Na2WO4·2H20;
[0044] - from 0.4 to 1.5 g/l of Na3AlF6; and
[0045] - NaOH at a concentration such that the electrolytic solution has a pH between
20 11.8 and 12.0, and a conductivity between 9.5 and 10.5 mS/cm.
[0046] Preferably, the electrolytic aqueous solution contains only the electrolytes
indicated above: Na2SiO3; K3PO4; Na2WO4·2H20; Na3AlF6; NaOH.
[0047] It has been experimentally verified that, using alkaline electrolytic solutions
having the above composition, the ceramic coatings obtainable on aluminum alloy
25 substrates by means of plasma electrolytic oxidation have the following features:
[0048] - high resistance to corrosion;
[0049] - high hardness (HV0.01 < 1,400);
[0050] - low surface roughness (Ra < 2 µm);
[0051] - high morphological homogeneity of the coating layer;
30 [0052] - porous surface layer absent or substantially absent, or at least having a
thickness not exceeding 5% of the total thickness of the ceramic coating.
[0053] Such results were obtained using the electrolytic solution described above in a
7
traditional PEO process, and then setting the usual electrical process parameters, such as
applied electrical potential, current density, frequency and duration of the plasma
discharge process.
[0054] The method according to the invention therefore does not require particular
5 adjustments of the electrical process parameters, nor the adoption of particular control
methods of the PEO process.
[0055] In particular, as will be resumed hereafter, the method according to the invention
may be advantageously applied in conditions of low energy consumption, without
particular limitations to the speed of formation of the ceramic coating on the substrate.
10 [0056] It has been experimentally verified that the sodium silicate (Na2SiO3) present in
the electrolytic solution (in the concentrations indicated above) contributes to
significantly improving the density of the ceramic coatings obtained through the PEO
process and therefore, consequently, the resistance to corrosion.
[0057] It has also been experimentally verified that the absence of Na2SiO3 in the
15 electrolytic solution inhibits the correct growth of the coating and in most cases the
plasma discharges do not start correctly.
[0058] As already mentioned, the alkaline electrolytic solution contains from 9 to 14 g/l
of Na2SiO3. Preferably, the electrolytic solution comprises from 9 to 11 g/l of
Na2SiO3, and more preferably 10 g/l.
20 [0059] With such concentrations of Na2SiO3 a compromise was surprisingly found
between a high growth rate of the coating and a reduction in surface roughness. In fact,
it has been observed that concentrations lower than the values indicated above (2, 5, 3,
5, 8 g/l of Na2SiO3) have determined an insufficient growth rate of the coating, while
higher concentrations (15 or 20 g/l of Na2SiO3) resulted in a higher growth rate of the
25 coating (and therefore a greater thickness), associated however with a considerable
increase in the aggressiveness of the process on the substrate and in the roughness of the
coatings. In fact, at higher concentrations, more heterogeneous and less dense coatings
were obtained, with a worsening of corrosion resistance.
[0060] Advantageously, it was also possible to verify that Na2SiO3 may be introduced
30 into the electrolytic solution both in solid powder and already in solution.
[0061] As already mentioned, the alkaline electrolytic solution contains from 2.2 to 2.8
g/l of K3PO4. Preferably, the electrolytic solution comprises from 2.4 to 2.6 g/l of
8
K3PO4, and more preferably 2.5 g/l.
[0062] It has been experimentally verified that the potassium phosphate (K3PO4)
present in the electrolytic solution further contributes to improving the density of the
ceramic coatings obtained through the PEO process and therefore, consequently, the
5 resistance to corrosion.
[0063] Electrolytic solutions containing K2HPO4 or KH2PO4 in place of K3PO4 were
tested. In both cases the results were poor. A white precipitate (composed mainly of
phosphorus) was detected on the coatings obtained.
[0064] As already mentioned, the alkaline electrolytic solution contains not less than 5
10 g/l of Na2WO4·2H20.
[0065] It has been experimentally verified that sodium tungstate dihydrate
(Na2WO4·2H2O) significantly increases the ratio of the thickness of the dense layer to
the total thickness of the coating (see fig. 5). The presence of such component in the
electrolytic solution leads to an increase in the hardness of the ceramic coatings
15 obtained. This is attributable to the fact that the ceramic coatings obtained contain
tungsten.
[0066] Advantageously, it is possible to use electrolytic solutions with higher
concentrations of sodium tungstate dihydrate (Na2WO4·2H2O), in order to further
increase the hardness of the coatings obtained.
20 [0067] Preferably, the electrolytic solution comprises 5 g/l of Na2WO4·2H20. With this
concentration, in fact, already satisfactory results have been obtained.
[0068] As already mentioned, the alkaline electrolytic solution contains from 0.4 to 1.5
g/l of Na3AlF6.
[0069] It has been experimentally verified that sodium hexafluoroaluminate (Na3AlF6)
25 allows the surface roughness of the ceramic coatings obtained to be decreased.
[0070] Preferably, the electrolytic solution comprises from 0.4 to 0.6 g/l of Na3AlF6,
more preferably 0.5 g/l. In fact, with these concentrations, coatings with higher
morphological homogeneity were obtained.
[0071] As already mentioned, the alkaline electrolytic solution comprises NaOH at a
30 concentration such that the electrolytic solution has a pH between 11.8 and 12.0, and a
conductivity between 9.5 and 10.5 mS/cm.
[0072] Preferably, the alkaline electrolytic solution comprises NaOH in a concentration
9
such that the alkaline electrolytic solution has a pH of 11.9, and a conductivity of 10.0
mS/cm.
[0073] Preferably, the electrolytic solution comprises from 0.8 to 1.2 g/l of NaOH,
more preferably from 0.9 to 1.1 g/l, and even more preferably 1.0 g/l.
5 [0074] Sodium hydroxide (NaOH) is introduced into the solution to bring the pH and
conductivity of the electrolytic solution to the values indicated above.
[0075] Surprisingly, it was found that sodium hydroxide (NaOH), unlike potassium
hydroxide (KOH), which could be used as an alternative to adjust the pH of the
solution, allows plasma discharges to start without leading to an excessive dissolution
10 of the substrate to be coated, also leading to more homogeneous coatings.
[0076] The replacement of NaOH with KOH has in fact been studied, but obtaining
negative results, such as excessive dissolution of the metal substrate without starting the
plasma, very aggressive processes or heterogeneous coatings. Furthermore, the degree
of dispersion of the electrolyte was also altered, forming a white precipitate.
15 [0077] The pH and electrical conductivity values depend on the chemical composition
of the electrolytic solution. The electrical conductivity of the electrolytic solution plays
a more relevant role as it is directly related to the final and stable voltage value of the
PEO process which will determine, among other things, the energy consumption of the
process.
20 [0078] For dilute electrolytic solutions, the final voltage value will be very high, while
concentrated electrolytic solutions may lead to excessive dissolution of the metal
substrate, preventing plasma initiation and inhibiting the growth rate of the coating.
[0079] Similar pH and conductivity values to those indicated above could be obtained
by modifying the chemical composition and concentration of the electrolytic reagents.
25 However, changing the composition and concentrations of the electrolytic solution
would alter the morphology, composition and properties of the PEO coatings obtained.
[0080] According to a preferred embodiment of the method according to the invention,
the electrolytic solution has a pH equal to 11.9 and a conductivity equal to 10.0 mS/cm
and comprises:
30 [0081] - 10 g/l of Na2SiO3;
[0082] - 2.5 g/l of K3PO4;
[0083] - 5 g/l of Na2WO4·2H20;
10
[0084] - 0.5 g/l of Na3AlF6;
[0085] - 1.0 g/l NaOH,
[0086] Preferably, during the PEO process the alkaline electrolytic aqueous solution is
cooled by means of a cooling system, preferably to maintain said alkaline electrolytic
5 aqueous solution at a temperature between 25°C and 45°C during said predefined period
of treatment time.
[0087] As already mentioned above, the ceramic coatings described above were
obtained using the electrolytic solution described above in a traditional PEO process,
and then setting the usual electrical process parameters, such as applied electrical
10 potential, current density, frequency and duration of the plasma discharge process.
[0088] The method according to the invention therefore does not require particular
adjustments of the electrical process parameters, nor the adoption of particular control
methods of the PEO process.
[0089] Preferably, the electrical potential is kept substantially constant for the
15 aforementioned predefined period of time, preferably at a value between 300 and 400 V,
more preferably equal to 350V, as illustrated in Figure 2.
[0090] Preferably, during the aforementioned predefined period of treatment time an
electrical current with a current density between 20 and 25 A/dm3, preferably 25
A/dm3, is applied to the substrate.
20 [0091] Preferably, an electrical current having a frequency of at least 50 Hz, even more
preferably equal to 50 Hz, is applied to the substrate. It has been experimentally verified
that by using the electrolytic solution described above already at frequencies of 50 Hz,
very dense coatings (substantially free of porosity) are obtained, without the need to
increase the frequency of the current.
25 [0092] Preferably, the electrical current is applied continuously (not pulsed). However,
the current may also be applied in a pulsed mode.
[0093] Preferably, the predefined period of treatment time is comprised between 20 and
40 min, more preferably equal to 30 min.
[0094] According to a preferred embodiment of the invention, the electrical potential is
30 kept substantially constant for said predefined period of treatment time at a value of
350V. During a period of treatment time of 30 min, an electrical current with a current
density of 25 A/dm3 and a frequency of 50 Hz is continuously applied to the substrate.
11
[0095] As already mentioned, the method according to the invention may therefore be
advantageously applied in conditions of low energy consumption, without particular
limitations to the speed of formation of the ceramic coating on the substrate.
[0096] Advantageously, coating formation rates of between 0.5 and 1 µm/min were
5 observed.
[0097] Advantageously, the method for producing a ceramic coating on the surface of
an aluminum alloy substrate according to the invention may comprise a step c) of pretreatment of the substrate, to be carried out before said steps a) and b).
[0098] Such pre-treatment step c) consists in subjecting the substrate to caustic attack
10 and subsequently in washing the substrate itself with distilled water.
[0099] Preferably, the aforementioned caustic attack is obtained by immersing the
substrate for a predefined period of time in an aqueous solution of NaOH, preferably
containing 50 g/l of NaOH, maintained at a temperature between 60°C and 70°C,
preferably at 60°C. The aforesaid predefined immersion time being comprised between
15 5 and 15 min, preferably equal to 10 min. It has been experimentally found that the
above NaOH concentration allows an adequate pickling of the aluminum alloy substrate
to be carried out, reducing the release of Al3+ ions from the substrate (and therefore the
aggressiveness on the substrate itself) and the formation of slurry in the caustic bath.
[00100] It has been verified that the caustic attack (and subsequent washing) of
20 the substrate leads to an improvement in the aesthetic finish of the final ceramic coating
obtained, in terms of greater homogeneity of the coating.
[00101] However, the caustic attack may also not be carried out if the substrate
already has a clean surface, substantially free of dirt and impurities.
[00102] Advantageously, in the aforesaid step c) of pre-treatment of the substrate,
25 after the caustic attack and subsequent washing with distilled water, the substrate may
be immersed in an acid bath for a predefined period of time and then washed with
distilled water.
[00103] Preferably, the aforementioned acid bath consists of an aqueous solution
of nitric acid. The predefined time of immersion in said acid bath is between 5 and 15 s,
30 preferably equal to 10 s.
[00104] Operatively, the immersion in an acid bath is carried out to have a
desmutting treatment of the substrate after the caustic attack.
12
[00105] Advantageously, the method for producing a ceramic coating on the
surface of an aluminum alloy substrate according to the invention does not require
specific post-treatment steps after the aforementioned steps a) and b), i.e. at the end of
the PEO process.
5 [00106] In particular, there is no need for post-treatments to seal the surface
porosity in order to guarantee the anti-corrosion properties.
[00107] However, the method may comprise a post-treatment step d) of the
substrate, to be carried out after said steps a) and b), wherein said post-treatment
consists of: - washing said substrate with distilled water; - cleaning the surface of said
10 substrate with alcohol; and - allowing it to dry at room temperature.
[00108] The ceramic coatings obtained on aluminum alloy substrates with the
method according to the invention essentially consist of a compact non-porous layer,
which may possibly present on the surface a porous layer having a thickness not
exceeding 5% of the total thickness of said coating.
15 [00109] Preferably, as illustrated in the SEM image of Figure 3, the ceramic
coatings obtained on aluminum alloy substrates with the method according to the
invention consist only of a compact non-porous layer.
[00110] More in detail, in the SEM image of Figure 3 the darker band at the
bottom represents the aluminum-silicon alloy substrate (indicated with a in the figure);
20 the central band represents the PEO ceramic coating (indicated with b in the figure); the
upper band of variegated color (indicated with c in the figure) above the ceramic
coating represents the resin layer used to incorporate the sample in order to perform the
SEM scan. From this SEM image it may be seen that the ceramic coating obtained
consists of a single homogeneous and compact layer, substantially free of porosity. The
25 porous surface layer that is normally present in traditional coatings obtained with PEO
processes (visible instead in Figure 1) is practically absent.
[00111] The ceramic coatings obtained on aluminum alloy substrates with the
method according to the invention have roughness Ra≤ 2µm and hardness HV0.01 ˃
1,400.
30 [00112] From an aesthetic point of view, the ceramic coatings obtained on
aluminum-silicon alloy substrates with the method according to the invention have a
very homogeneous dark gray surface color.
13
[00113] These features may be seen from the SEM image of Figure 4 which
shows the surface of a ceramic coating obtained on an aluminum-silicon alloy substrate
by means of the method according to the invention.
[00114] This result (color homogeneity) confirms the fact that by the method
5 according to the invention it is possible in a homogeneous manner aluminum alloys
with a high silicon content, which are not easy to cover homogeneously. In fact, if a
standard anodizing process were used as an alternative, some defects could be detected
in areas with a high silicon content.
[00115] An example of application of the method according to the invention for
10 coating a substrate consisting of a disc brake caliper made of an aluminum alloy with a
high silicon content is given below.
[00116] More specifically, the caliper is made with an Al/Si7%/Mg/Ti
aluminum-silicon alloy.
[00117] An electrolytic aqueous solution was used having a pH equal to 11.9 and
15 a conductivity equal to 10.0 mS/cm and comprising: - 10 g/l of Na2SiO3; - 2.5 g/l of
K3PO4; - 5 g/l of Na2WO4·2H20; - 0.5 g/l of Na3AlF6; - 1.0 g/l of NaOH.
[00118] The substrate was immersed as an electrode together with a counterelectrode in the aforementioned electrolytic aqueous solution, thereby applying an
electrical potential sufficient to generate spark discharges on the surface of the substrate
20 for a period of treatment time of 30 min to lead to the formation of the coating. As
illustrated in Figure 2, the electrical potential was kept substantially constant for the
period of treatment time at a value of 350V. During such period of time, an electrical
current with a current density of 25 A/dm3 and a frequency of 50 Hz is continuously
applied to the substrate.
25 [00119] During the treatment, the alkaline electrolytic aqueous solution was
cooled by means of a cooling system to a temperature between 25°C and 45°C.
[00120] At the end of the treatment, a ceramic coating was obtained having an
average thickness of about 30µm, consisting of a mixture of aluminum oxide, silicon
oxide and aluminum silicate oxide. Sodium, Potassium, Phosphorus, Tungsten are
30 present in traces (< 2 atomic %). The elemental composition of the coating obtained
from an EDS spectroscopy analysis is reported below in Table 1.
14
Element % by weight atomic %
C 6.70 12.14
O 42.69 58.09
Na 0.39 0.37
Al 15.64 12.62
Si 17.21 13.34
P 1.53 1.07
K 0.82 0.46
Ca 0.30 0.16
W 14.75 1.74
Table 1
[00121] The ceramic coating penetrates into the substrate for a few micrometers.
This feature ensures adhesion of the coating to the substrate.
[00122] The SEM images of figures 3 and 4 refer to the ceramic coating obtained
5 on the caliper treated according to this example. The ceramic coating consists only of a
compact non-porous layer. The coating obtained has a dark gray and very homogeneous
surface color.
[00123] The ceramic coating has a surface roughness Ra <2 m and a hardness
HV0.01 > 1,400.
10 [00124] Obtaining such a low roughness value is important in the application on
the brake caliper, and in particular inside the caliper piston seat, to reduce the risk of
abrasion of the caliper body on the piston.
[00125] Similarly, obtaining such high hardness values (difficult to obtain with
standard anodizing) is important for moving mechanical components.
15 [00126] The so-coated brake caliper was subjected to a corrosion resistance test
using an NSS (neutral salt spray) test. The caliper passed the test, reporting a minimum
resistance of 480 h without any damage to the substrate (black holes).
[00127] The invention allows numerous advantages to be obtained which have
been explained in the course of the description.
20 [00128] The method for producing a ceramic coating on the surface of an
aluminum alloy substrate by means of plasma electrolytic oxidation according to the
invention allows ceramic coatings having low roughness, high hardness and high
15
corrosion resistance to be obtained more easily.
[00129] The method according to the invention allows a coating substantially
devoid of a porous surface layer and very homogeneous to be obtained.
[00130] The method for producing a ceramic coating on the surface of an
5 aluminum alloy substrate by means of plasma electrolytic oxidation according to the
invention allows in particular a ceramic coating to be obtained on an automotive
component having:
[00131] - a smooth surface,
[00132] - high wear resistance;
10 [00133] - high resistance to corrosion;
[00134] - homogeneous morphology, substantially devoid of a porous surface
layer;
[00135] - high aesthetic features (homogeneous coloring)
[00136] These features are also associated with coating thicknesses not exceeding
15 50 µm.
[00137] The invention thus conceived therefore achieves its intended purposes.
[00138] Of course, in its practical implementation it may also assume different
forms and configurations from the one illustrated above, without thereby departing from
the present scope of protection.
20 [00139] Furthermore, all details may be replaced with technically equivalent
elements, and dimensions, shapes and materials used may be any according to the
needs.
16
WE CLAIM:
1. Method for producing a ceramic coating on the surface of an aluminium alloy
substrate by means of electrolytic plasma oxidation, comprising the following steps:
a) immerging the substrate as an electrode together with a counter-electrode in an
5 alkaline electrolytic aqueous solution;
b) applying an electrical potential sufficient to generate spark discharges on the
surface of the substrate for a predefined period of treatment time so as to lead to the
formation of said coating, consisting mainly of aluminium oxides and oxides of any
alloying elements of said alloy,
10 characterized in that the electrolytic aqueous solution comprises:
- from 9 to 14 g/l of Na2SiO3;
- from 2.3 to 2.8 g/l of K3PO4;
- not less than 5 g/l of Na2WO4·2H20;
- from 0.4 to 1.5 g/l of Na3AlF6;
15 - NaOH at a concentration such that the electrolytic solution has a pH between 11.8 and
12.0, and a conductivity between 9.5 and 10.5 mS/cm.
2. The method according to claim 1, wherein the electrolytic solution comprises
from 9 to 11 g/l of Na2SiO3, preferably 10 g/l.
3. The method according to claim 1 or 2, wherein the electrolytic solution
20 comprises from 2.4 to 2.6 g/l of K3PO4, preferably 2.5 g/l.
4. The method according to any of the preceding claims, wherein the electrolytic
solution comprises 5 g/l of Na2WO4·2H20.
5. The method according to any of the preceding claims, wherein the electrolytic
solution comprises 0.4 to 0.6 g/l Na3AlF6, preferably 0.5 g/l.
25 6. The method according to any of the preceding claims, wherein the electrolytic
solution comprises NaOH at a concentration such that the electrolytic solution has a pH
of 11.9, and a conductivity of 10.0 mS/cm.
7. The method according to any of the preceding claims, wherein the electrolytic
solution comprises 0.8 to 1.2 g/l NaOH, preferably 0.9 to 1.1 g/l, even more preferably
30 1.0 g/l.
8. The method according to claim 1, wherein the electrolytic aqueous solution
comprises:
17
- 10 g/l of Na2SiO3;
- 2.5 g/l of K3PO4;
- 5 g/l of Na2WO4·2H20;
- 0.5 g/l of Na3AlF6;
5 - 1.0 g/l NaOH,
and wherein the electrolytic solution has a pH of 11.9 and a conductivity of 10.0
mS/cm.
9. The method according to any of the preceding claims, wherein the alkaline
electrolytic aqueous solution is cooled by means of a cooling system, preferably to
10 maintain said alkaline electrolytic aqueous solution at a temperature between 25°C and
45°C during said predefined period of treatment time.
10. The method according to any of the preceding claims, wherein the electrical
potential is kept substantially constant for said predefined period of time, preferably at a
value between 300 and 400 V, more preferably 350 V.
15 11. The method according to any of the preceding claims, wherein during said
predefined period of treatment time an electrical current with a current density between
20 and 25 A/dm3, preferably 25 A/dm3, is applied to the substrate.
12. The method according to any of the preceding claims, wherein an electrical
current with a frequency of at least 50 Hz, preferably 50 Hz is applied to the substrate.
20 13. The method according to claim 11 or 12, wherein the electrical current can be
applied continuously or in a pulsed mode.
14. The method according to any of the preceding claims, wherein the predefined
period of treatment time is between 20 and 40 min, preferably 30 min.
15. The method according to any of the preceding claims, wherein the electrical
25 potential is kept substantially constant for said predefined period of treatment time at a
value of 350V and wherein during said predefined period of treatment time an electrical
current with a current density equal to 25 A/dm3 and a frequency equal to 50 Hz is
applied continuously to the substrate, said predefined period of treatment time being 30
min.
30 16. The method according to any of the preceding claims, comprising a pretreatment step (c) of the substrate, to be carried out before said steps (a) and (b),
wherein the pre-treatment consists of subjecting said substrate to caustic attack and then
18
washing said substrate with distilled water.
17. The method according to claim 16, wherein said caustic attack is obtained by
immerging said substrate for a predefined period of time in an aqueous solution of
NaOH, preferably containing 50 g/l of NaOH, maintained at a temperature between
5 60°C and 70°C, preferably at 60°C, said predefined immersion time being between 5
and 15 min, preferably 10 min.
18. The method according to claim 16 or 17, wherein in said step (c) of pretreatment of the substrate, after the caustic attack and subsequent washing with distilled
water, the substrate is immersed in an acid bath for a predefined period of time and then
10 washed with distilled water.
19. The method according to claim 18, wherein said acid bath consists of an
aqueous solution of nitric acid, said predefined immersion time in said acid bath being
between 5 and 15 s, preferably 10 s.
20. The method according to any of the preceding claims, comprising a post15 treatment step (d) of the substrate, to be carried out after said steps (a) and (b), wherein
said post-treatment consists of:
- washing said substrate with distilled water;
- cleaning the surface of said substrate with alcohol; and
- allowing it to dry at room temperature.
20 21. The method according to any of the preceding claims, wherein said ceramic
coating consists essentially of a non-porous, compact layer, which may possibly have
on the surface, a porous layer with a thickness not exceeding 5% of the total thickness
of said coating.
22. The method according to claim 21, wherein said ceramic coating consists only
25 of said non-porous, compact layer.
23. The method according to claim 21 or 22, wherein said ceramic coating has a
roughness Ra≤ 2µm and a hardness HV0.01 ≥ 1.400.
24. The method according to any of the preceding claims, wherein said substrate is
in aluminium-silicon alloy and wherein the ceramic coating obtained is a layer
30 consisting mainly of a mixture of aluminium oxides, silicon oxides and mixed
aluminium-silicon oxides.
25. The method according to any of the preceding claims, wherein said substrate
consists of a component of a braking system, preferably a disc braking system, in
particular a brake caliper, a brake caliper piston or a brake disc bell.