EP2878712A1 - Aluminium-lithium alloy component including a ceramic coating and method for forming the coating - Google Patents

Abstract

The invention relates to a component (1) comprising an aluminum alloy comprising between 0.1 and 10% by weight of lithium, characterized in that said component (1) is treated using a microorganism oxidation process. plasma arc for obtaining a ceramic coating (2) on the surface of the aluminum alloy. The invention also relates to a method for growing the ceramic coating (2) on the surface of the component.

Description

Technical area

The present invention relates to a component comprising an aluminum and lithium alloy coated with a ceramic coating. As well as the process for forming the coating.

State of the art

Aluminum alloys containing lithium have interesting properties. Among them we can mention their lightness compared to other conventional aluminum alloys. Indeed, lithium is the lightest of the metallic elements and, for each 1% of lithium added to the aluminum alloy makes it possible to reduce by 3% the density of the aluminum and to increase by 5% its elastic modulus . This type of alloy also has a high resistance to fatigue and corrosion, thus extending the life of the product. These alloys are 100% recyclable. Aluminum alloys containing lithium find their applications in the fields of aeronautics, aerospace and the military.

However, these alloys have poor resistance to wear and corrosion. Indeed, despite the addition of lithium, this alloy, like all raw aluminum alloys, has weaknesses in corrosion and wear and impacts.

Brief summary of the invention

An object of the present invention is to provide a coated component free from the limitations of known components.

According to the invention, these objects are achieved in particular by means of a component comprising an aluminum alloy comprising between 0.1 and 10% by weight of lithium, characterized in that said component is treated using a process of oxidation by micro-arc plasma to obtain a ceramic coating on the surface of the aluminum alloy.

The invention also relates to a method for growing a ceramic coating on the surface of the component, the process being a plasma micro-arc oxidation process and comprising the steps of immersing the component to be coated in an electrolytic bath composed of an aqueous solution of alkali metal hydroxide, the component forming one of the electrodes; and applying an alternating current having a frequency of between 10 Hz to 10,000 Hz, so as to apply a voltage between the component and another electrode varying between 0 V and a value between 100 V and 1000 V.

This solution has the advantage over the prior art of providing a component having a high hardness, excellent resistance to wear, impact, and corrosion.

The oxidation process by micro-arc plasma is also a technology so the environmental impact is low, especially in view of conventional anodizing techniques so acidic baths are strongly discouraged for the protection of the environment.

Brief description of the figures

• la figure 1 illustre une installation d'électrolyse;
• la figure 2 montre une vue en coupe du composant avec le revêtement formé par le procédé d'oxydation par micro-arc plasma;
• la figure 3 montre un revêtement formé par le procédé d'oxydation de l'invention;
• la figure 4 illustre une perspective d'une telle boite de montre;
• la figure 5 montre le revêtement formé sur un levier;
• la figure 6 montre le revêtement formé sur une lunette;
• la figure 7 montre le revêtement formé sur une carrure;
• la figure 8 montre le revêtement formé sur la corne de carrure;
• la figure 9 montre un vue en coupe d'une portion filetée de la corne de carrure;
• la figure 10 est un graphique qui rapporte les épaisseurs mesurée du revêtement formé sur les différentes parties de la boite de montre;
• la figure 11 montre les endroits de mesure d'épaisseur du revêtement;
• la figure 12 est un graphique qui rapporte les épaisseurs mesurée du revêtement formé sur les différentes parties de la boite de montre.
• la figure 13 montre l'état de la surface de la carrure de montre en alliage d'aluminium-lithium sur laquelle le revêtement a été formé, après usure;
• la figure 14 montre l'état de la surface de la carrure de montre en alliage d'aluminium-lithium sur laquelle le revêtement a été formé, après des tests de chocs;
• la figure 15 montre l'état de la surface de la carrure de montre en alliage d'aluminium-lithium sur laquelle le revêtement a été formé, après des tests de corrosion; et
• la figure 16 montre une photographie d'un boîtier de montre en alliage aluminium-lithium ayant subi le procédé d'oxydation de l'invention.

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which:

• the figure 1 illustrates an electrolysis plant;
• the figure 2 shows a sectional view of the component with the coating formed by the plasma micro-arc oxidation process;
• the figure 3 shows a coating formed by the oxidation process of the invention;
• the figure 4 illustrates a perspective of such a watch case;
• the figure 5 shows the coating formed on a lever;
• the figure 6 shows the coating formed on a telescope;
• the figure 7 shows the coating formed on a middle part;
• the figure 8 shows the coating formed on the middle horn;
• the figure 9 shows a sectional view of a threaded portion of the middle horn;
• the figure 10 is a graph that reports the measured thicknesses of the coating formed on the different parts of the watch case;
• the figure 11 shows the places of thickness measurement of the coating;
• the figure 12 is a graph that reports the measured thicknesses of the coating formed on the different parts of the watch case.
• the figure 13 shows the condition of the surface of the aluminum-lithium alloy watchband on which the coating was formed, after wear;
• the figure 14 shows the condition of the surface of the aluminum-lithium alloy watchband on which the coating was formed, after shock tests;
• the figure 15 shows the condition of the surface of the aluminum-lithium alloy watchband on which the coating was formed, after corrosion tests; and
• the figure 16 shows a photograph of an aluminum-lithium alloy watch case that has undergone the oxidation process of the invention.

Example (s) of embodiment of the invention

According to the invention, a component 1 comprises an aluminum alloy comprising between 0.1 and 10% by weight of lithium. The component is treated using a plasma micro-arc oxidation process (also known as "micro-arc oxidation" and acronym "MAO") so as to obtain a ceramic coating 2 on the surface of the aluminum alloy.

The aluminum alloy containing lithium may be one of the commercial alloys available on the market. For example, the aluminum-lithium alloy may include any of the alloys listed in Table 1. In the table, the first line gives the name of the alloy and the left column the element as well as the content of the element in% by weight in the successive columns. Such an aluminum-lithium alloy will therefore contain up to 2.45% by weight of lithium, more particularly between 0.88 and 2.45% by weight of lithium. Table 1 alloy 2050 2090 2091 2099 2195 2196 2199 8090 Weldalite 049 Cu 4.3 2.7 2.1 2.4-3.0 3.05 2.90 2.3-2.9 1.3 5.4 Li 0.88 2.2 2.0 1.6-2.0 1.70 1.67 1.4-1.8 2.45 1.3 Zn <0.01 - - 0.4-1.0 0.01 0.01 0.2-0.9 - - mg 0.58 - - 0.1-0.5 0.39 0.40 0.05-0.4 0.95 0.4 mn - - - 0.1-0.5 0.01 0.31 0.1-0.5 - - Ag 0.34 - - - 0.35 0.38 - - 0.4 Zr 0.13 0.12 0.1 0.05-0.12 0.12 0.1 0.05-0.12 0.12 0.14 Fe 0.03 - - 0.07 max 0.05 0.04 0.07 max - - Yes 0.03 - - 0.05 max 0.03 0.03 0.05 max - - Ti 0.04 - - - 0.03 0.03 - - - al rest rest rest rest rest rest rest rest rest

The figure 1 illustrates an arrangement of an installation, in which a tank 3 contains an electrolytic bath 4. Inside the electrolyte 4 plunges a counter-electrode, or cathode, and an anode which corresponds to the component 1 to be coated. To the figure 1 are equally represented a power supply block 6 able to generate an alternating current 31.

According to one embodiment, the micro-arc plasma oxidation process comprises the steps of immersing the component 1 to be coated in the electrolyte 4 and of passing the alternating current 31 so as to apply a voltage between the component 1 and the cathode 5.

The electrolyte 4 may comprise an aqueous solution of alkali metal hydroxide, such as potassium or sodium, and an oxyacid salt of an alkali metal. The electrolyte 4 is typically maintained at a temperature between 10 ° C and 55 ° C.

Preferably, the applied current comprises positive and negative current pulses alternating with a frequency of between 10 Hz and 10,000 Hz. The amplitude of the current pulses is between 2 and 200 A / dm ² so as to apply a voltage between the component 1 and the cathode 5 of the order of 100 V to 1000 V. Indeed, a voltage of 100 and 1000V makes it possible to create an electrolytic plasma necessary for the formation of the coating 2 on the component 1.

In one embodiment, the current pulses are separated by a dead time where no current is applied. The duration of the dead time is preferably about 10% of the total duration of the current pulse. The duration of the dead time is such that the voltage drops to zero. For example, each of the positive and negative current pulses may have a maximum amplitude followed by a decrease of the current to a zero value. The duration of the pulse where the value of the current is zero is about 10% of the total duration of the current pulse.

In other words, during the dead time the voltage must drop to zero, that is to say that the voltage is cycled between a base voltage (baseline) and a ceiling voltage, or ceiling (ceiling line). The minimum base voltage is preferably adjusted to a voltage of between 0 and 99.9% of the maximum peak of the ceiling voltage. The base voltage (for example 30% of the ceiling voltage) will promote the formation of micro-arcs emission visible to the naked eye, while a larger base voltage (for example 60% of the voltage ceiling), will promote the creation of a continuous plasma, also visible to the naked eye (relative to the retinal perception of 0.1 to 0.2 seconds). The influence of the choice of the basic minimum average voltages with respect to the maximum voltage and thus the type of micro-arcs obtained thus makes it possible to master a more or less dense and homogeneous layer. The densification of the layer is also subject to the frequency of alternation between the anode and cathode currents. Indeed in the first case the growth of the nanoporous layer will be realized whereas in the second case the densification of the nanoporosities will operate.

The growth rate of the coating 1 depends on the type of frequency and the shape of the pulse, in particular the passage between a cathodic and anodic current (and vice versa) and the current amplitude (and therefore the applied voltage) . For example, the growth rate of the coating is of the order of 1 micron / minute for an applied voltage of 100 to 400 V and a frequency of the order of 1000 Hz. The thickness of the coating thus obtained can range from a few microns, homogeneously on the part, as long as the setting used to maintain the component in the bath is adapted and does not modify the formation of micro arcs and do not extinguish them,) to a few hundred microns. The micro-arc plasma oxidation process is described, for example, in the document WO03 / 083181 .

The figure 2 shows a sectional view of the component with the coating 1 formed by the plasma micro-arc oxidation process. The coating comprises a thick functional ceramic layer 21 thick forming about two thirds of the total thickness of the coating 2, and a porous outer layer 22 forming about one third of the total thickness of the coating 2 Moreover, during its growth, the coating 2 is formed in part by the transformation of the substrate material 7 and partly by growth beyond the initial surface 8 of the component (represented by the hatched line in the figure 2 ). In the figure 2 , the excess thickness of the coating 2 is represented by the difference in thickness between the initial surface 8 and the upper surface of the layer 22.

The coating 1 formed by the plasma micro-arc oxidation process on the aluminum-lithium alloy component has a high hardness, close to 2000 Hv. It also has excellent resistance to wear, shock, corrosion. The coating 1 is of color corresponding to the natural color of the oxidized aluminum. For example, the coating 1 is dark brown in the case of the aluminum-lithium alloy 2050 (see Table 1).

The oxidation process may comprise a preliminary step of preparing the surface 8 of the component 1. This preparation step may comprise the cleaning and degreasing of the surface 8, for example with boiling water or with an alkaline cleaning agent such as a PARCO cleaner solution (product of Henkel Surface Technologies division of Henkel Corporation, Madison Heights, Michigan). The preparation step may be followed by a rinsing step, for example with distilled water.

In order to obtain an optimum surface state and remove the last microns of the porous ceramic layer 22, a tribofinishing step can be carried out after the formation of the coating 1 by the oxidation process. This tribofinishing step may include, for example, microsablage.

The figure 3 shows a coating 1 formed by the oxidation process of the invention. In particular, Figures 3a and 3c show the coating 2 seen from the front, respectively after its formation ( figure 3a ) and after the tribofinition stage ( figure 3c ). The figure 3b is a sectional view showing the Al-Li alloy substrate 7 and the coating 2.

The component 1 to be coated may also be derived from a conventional shaping process such as machining, bar turning or setting. form by molding process of liquid aluminum (Cobapress process type).

By way of example, the plasma micro-arc oxidation process has been applied to different parts of a watch case made of an aluminum-lithium alloy. The figure 4 illustrates a perspective of such a watch case 9. In particular, the coating 1 has been formed on parts of the watch case 9 comprising a middle part 91, a middle horn 92, a lever 93, a fastening bridge 94 , a bottom (not visible in the figure), a bezel 95 and a crown cover 96.

The Figures 5 to 9 show micrographs of views of a metallographic section of the coatings 2 formed by the oxidation process on the different parts of the watch case 9. In particular, the figure 5 shows the coating 2 formed on the lever 93. The figure 6 shows the coating 2 formed on the bezel 95. figure 7 shows the coating 2 formed on the middle part 91. The figure 8 shows the coating 2 formed on the middle horn 92. In these figures, the visible layer in pale gray corresponds to a copper plating layer 10 deposited on the coating for protection purposes during the preparation of the metallographic section.

In the case where the portion of the component 1 comprises one or more areas having a fine structuring, for example a thread, holes or tappings, it may be advantageous for these areas to be coated with the coating 2 having a lower thickness than on the rest of the surface of the component 1. Indeed, the formation of a thick coating in the fine structuring zones can result in a leveling of the structuring. This is particularly the case when the structuring has a dimension smaller than 100 μm.

In one embodiment, the oxidation process comprises a step of forming the coating 2 on the zone or zones having fine structuring. The zone or zones with fine structuring are then masked so as to form the coating 2 on the rest of the component 1 without affecting the zone or zones having fine structuring. Then, the coating is formed on the remainder of the surface of the component 1. The thickness of the coating in the area or areas with fine structuring will depend on the size of the patterning. For example, the coating can be formed on the area or areas with fine structuring with a thickness of about 10% of the dimension of the structuring.

In the case of the example of the watch case 9 above, the area or areas with fine structuring can include threads, holes, tapping, etc. The figure 9 shows a sectional view of a threaded portion of the middle horn 92. The thread has a dimension (distance between the valley and the top) of a few tens of microns. The threaded portion, as well as the rest of the middle horn 92, was first oxidized according to the oxidation process of the invention so as to form the coating 2 (black layer) with a thickness of a few microns (typically between 1 and 5 μm). The threaded portion was then masked and the rest of the middle horn 92 was again oxidized according to the oxidation process of the invention, making it possible to form the coating 2 with a thickness of several tens to hundreds of microns, such illustrated in figure 8 . The threaded portion being masked during this step, the coating 2 does not believe more in this place and its thickness remains unchanged. The coating 2 having a small thickness in the zone or zones with the fine structuring makes it possible to densify and harden these zones without however leveling the structuring.

The masking of the area or zones with fine structuring can be achieved using silicone gaskets or any other protective means resistant to oxidation treatment by micro-arc plasma and can be removed at the end of the process.

The graph of the figure 10 relates the measured thicknesses of the coating 2 formed on the different parts of the watch case 9 as discussed above. The bottom line of the graph gives the value of the thickness. For example, the coating 2 with a thickness of 50 μm is formed on the middle part 91 and the bottom of the box 9. The coating 2 has a thickness of 45 μm on the fixing bridge 94 and the crown cover 96, ect.

Table 2 reports the thickness measured for the coating 2 formed on the various threads present on the watch case 9, for example, in the threaded axis of the middle part 91, for fixing the lever 93 on the fixing bridge 94 for fixing the fixing bridge 94 on the middle part 91. The three lines 1, 2 and 3 in the table 2 correspond to the thickness of the coating 2 measured at the top of three successive threads, as shown in FIG. figure 11 .

The graph of the figure 12 summarizes different thickness measurements of the coating 2 at the holes and threads of the various elements of the box 9.

The figure 16a shows a photograph of an aluminum-lithium alloy watch case 9 which has undergone the oxidation process of the invention. The figure 16b shows a detail of the housing 9, in particular a portion of the fixing bridge 94, the lever 93 and the bezel 95. photographs allow to assess the surface state of the component with the coating and, in this case, the final step of microsablage.

Wear tests carried out under conditions of wear of balls at a speed of 46 rpm and with a mixture of 2 kg of ceramic beads with a diameter of 3 mm in one-half liter of water and cc of wetting. The duration of the test was 36 hours. After 6 hours of this test, a brightening of the surface is observed. After 36 hours of this test, no further evolution was observed. The ceramization process therefore provides excellent wear resistance. The state of the surface after wear of the ceramization coating on a lithium aluminum is shown in FIG. figure 11 in the case of an initial unworn surface and after 6h of the test, 12h of the test and 36h of the test.

The figure 13 shows the state of the surface of the watchband of lithium aluminum alloy on which the coating was formed by the oxidation process of the invention after 6 hours of wear according to the test above.

Fine scratch tests were also carried out. The conditions of these tests included the rotation of the test sample at 90 rpm, in a box with a volume of 0.6 liter with a diameter of 80 mm, height of 60 mm, blotter wall with 5 to 15 markers and 10g of bremor BR 650 glass powder. The duration of the test was 24 hours. The figure 17 shows tables 3 and 4 which represent the observations made for different durations of solicitations from 1 min to 24 hours.

The figure 14 shows a photomicrograph of the state of the surface of the middle part after 12 hours of fine scratches. These tests show that the ceramization process also allows excellent scratch resistance.

Gravel bed scrap tests were also performed using a test method according to ISO 23160 including falls in a bed of gravel and ceramic chips 8 cm by 500 cm ² of ceramics 3 mm diameter, with a length of 12 mm and hardness of 900HV ± 100Hv. The height of fall was 40cm.

The watch case to be tested is "loaded" with a weight representing the weight of the mechanical movement normally integrated in the watch head. After a dozen falls on a gravel / ceramic bed, there are very slight impacts on the edges, as can be seen in the diagram. figure 14 which shows the surface condition after ten of these tests to falls in gravel bed height 40cm of a weighted middle with a weight representing the weight of the mechanical movement. These tests show that the ceramization process also allows excellent impact and impact resistance 40cm on a gravel / ceramic bed.

Synthetic sweat tests were also performed. These tests were carried out according to the conditions of the NIHS 96-50 and ISO 3160-2 standards, in which the tested parts are put on a cotton pad soaked in sweat in an environment of 40 ° C ± 2 ° C humidity 95 to 100 % relative humidity test duration over 6 days.

After 6 days, corrosion pits appear on areas of bare aluminum. The ceramizing layer makes it possible to create a protective layer of the aluminum substrate (see figure 15 ). These tests show that the ceramization process also allows an excellent resistance to corrosion.

Overall, a lithium aluminum provides the following properties: 25% lighter than conventional materials, thus optimizing the design of structural parts and reducing the weight of a watch case, for example, better resistance to fatigue and fatigue. corrosion, which makes the product more reliable and lengthier, 100% recyclable, making a major contribution to a sustainable watch industry.

The coating 2 obtained by the process of the invention is advantageous for the components of a mechanical watch subjected to friction or mechanical stress. It is also advantageous for treating watchmaking components, such as the example of the watch case 9 above, which are subject to aggressive environmental constraints such as wear, humidity, salinity (sea, tropical climates or other ). Of course, the component 1 comprising the coating 2 is not only of interest in the watch industry but can also be used in various fields such as eyewear and writing instruments.

Reference numbers used in the figures

1

component

2

coating

21

hard layer

22

second portion

3

tank

31

alternating current

4

electrolytic bath

5

cathode

6

power supply

7

aluminum-lithium substrate

8

initial surface

9

watch box

91

shoulders

92

horn of middle

93

the sink

94

fixing bridge

95

glasses

96

cover ring

10

copper plating

Claims (15)

Component (1) comprising an aluminum alloy
comprising between 0.1 and 10% by weight of lithium, characterized in that said component (1) is treated using a plasma micro-arc oxidation process to obtain a ceramic coating (2) on the surface aluminum alloy. The component of claim 1,
wherein the coating (2) has a thickness of between 1 μm and 100 μm. The component of claim 2,
comprising at least one portion having a surface pattern having a size smaller than 100 μm; and
wherein the coating has a thickness of between 1 μm and 5 μm. The component of claim 3,
wherein the coating has a thickness of between 5 μm and 100 μm on the rest of the component, and preferably between 20 μm and 50 μm on the rest of the component. The component according to one of claims 1 to 4,
comprising between 0.88 and 2.45% by weight of lithium. The component according to one of claims 1 to 5,
wherein the coating has a hardness of between 1800 and 2000 Hv. The component according to one of claims 1 to 6,
in which the coating is intact after a trial of ten falls in gravel bed at a height of 40 cm of the component, according to ISO 23160. The component according to one of claims 1 to 7,
in which the coating shows a surface condition has slight alterations (index 4) after 36 hours of a test according to ISO 23160. The component according to one of claims 1 to 8, being a watch component, in particular a dressing component or a component of a movement, a component for eyewear or a component for a writing instrument. A process for growing a ceramic coating (2) on the surface of a component comprising an aluminum alloy comprising between 0.1 and 10% by weight of lithium, characterized in that the process is a plasma micro-arc oxidation process and includes: immersing the component to be coated in an electrolytic bath (3) composed of an aqueous solution of alkali metal hydroxide, the component forming one of the electrodes; applying an alternating current having a frequency between 10 Hz and 10,000 Hz, so as to apply a voltage between the component and another electrode varying between 0 V and a value between 100 V and 1000 V. The process according to claim 10,
in which the duration of the current pulse giving a voltage of 0 V is 10% of the total pulse duration giving a voltage of between 100 V and 1000 V. The process according to claim 10 or 11,
wherein the minimum average voltage is adjusted to be between 0 and 99.9% of the maximum voltage and preferably between 30% and 60% of the maximum voltage. The process according to one of claims 10 to 12,
wherein an area of the component (1) has a fine structuring having a size less than 100 μm; and wherein the method further comprises: growing the coating (2) with a thickness of between 1 μm and 5 μm; mask said zone so that the subsequent application of the method has no effect on said zone; and growing the coating (2) having a thickness of between 5 μm and 100 μm on the rest of the surface of the component (2). The process according to one of claims 10 to 13,
further comprising a surface preparation comprising a cleaning and degreasing step. The process according to one of claims 10 to 14,
further comprising a microsablage step.

Images