CH708829A1 - An aluminum-lithium alloy component comprising a ceramic coating and a 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

Description Technical area

[0001] The present invention relates to a component comprising an alloy of aluminum and lithium coated with a ceramic coating. As well as the process for forming the coating.

State of the art

[0002] 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, it is possible to reduce the density of aluminum by 3% and to increase its elastic modulus by 5%. . This type of alloy also has great resistance to fatigue and corrosion, thus helping to extend 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,

[0003] However, these alloys exhibit poor resistance to wear and corrosion. Indeed, despite the addition of lithium, this alloy, like all crude aluminum alloys, exhibits weaknesses in corrosion and wear and impact.

Brief summary of the invention

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

[0005] According to the invention, these goals 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 plasma micro-arc oxidation process making it possible 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 method 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 to apply 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.

[0007] This solution has the particular advantage over the prior art of providing a component having high hardness, excellent resistance to wear, impact, and corrosion.

[0008] The plasma micro-arc oxidation process is also a technology so the environmental impact is low, in particular in view of conventional anodization techniques, so acid baths are strongly discouraged for environmental protection.

Brief description of the figures

[0009] Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: FIG. 1 illustrates an electrolysis installation; fig. 2 shows a sectional view of the component with the coating formed by the plasma microarc oxidation process; fig. 3 shows a coating formed by the oxidation process of the invention; fig. 4 illustrates a perspective of such a watch case; fig. 5 shows the coating formed on a lever; fig. 6 shows the coating formed on a bezel; fig. 7 shows the coating formed on a middle part; fig. 8 shows the coating formed on the caseband horn; fig. 9 shows a sectional view of a threaded portion of the middle lug; fig. 10 is a graph which reports the measured thicknesses of the coating formed on the different parts of the watch case;

2 in fig. 1 1 shows the locations of the coating thickness measurement; fig. 12 is a graph which reports the measured thicknesses of the coating formed on the different parts of the watch case. fig. 13 shows the condition of the surface of the aluminum-lithium alloy watchband on which the coating has been formed, after wear; fig. 14 shows the condition of the surface of the aluminum-lithium alloy watchband on which the coating was formed, after impact tests; fig. 15 shows the surface condition of the aluminum-lithium alloy watchband on which the coating has been formed, after corrosion tests; and fig. 16 shows a photograph of an aluminum-lithium alloy watch case having undergone the oxidation process of the invention.

Example (s) of embodiment of the invention

[0010] 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 the acronym “MAO”) so as to obtain a ceramic coating 2 on the surface of the aluminum alloy.

[0011] The lithium-containing aluminum alloy can be one of the commercial alloys available in the market. For example, the aluminum-lithium alloy can include one of the alloys mentioned in table 1. In the table, the first row gives the name of the alloy and the left column gives the element as well as the content. of the element in% by weight in 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. 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

If 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 rest

Table 1

[0012] FIG. 1 illustrates an arrangement of an installation, in which a tank 3 contains an electrolytic bath 4. Inside the electrolyte 4 is immersed a counter-electrode, or cathode, 5 and an anode which corresponds to component 1 to be coated. In fig. 1 are also shown a current supply unit 6 capable of generating an alternating current 31.

3 [0013] According to one embodiment, the plasma micro-arc 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 component 1 and cathode 5.

[0014] The electrolyte 4 can 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.

[0015] Preferably, the applied current comprises positive and negative current taps alternating with a frequency between 10 Hz to 10,000 Hz. The amplitude of the current taps is between 2 and 200 A / dm2 so as to apply a voltage between component 1 and cathode 5 of the order of 100 V to 1000 V. In fact, a voltage between 100 and 1000 V makes it possible to create an electrolytic plasma necessary for the formation of coating 2 on component 1 .

[0016] In one embodiment, the current taps 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 draw. The duration of the dead time is such that the voltage drops to zero. For example, each of the positive and negative current taps may have a maximum amplitude followed by a decrease in current to zero. The duration of the pulse where the current value is zero is approximately 10% of the total duration of the current draw.

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 maximum (ceiling line). The minimum base voltage is preferably adjusted to a voltage 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 emission of micro electric arcs visible to the naked eye, while a more consistent 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 retinal perception from 0.1 to 0.2 seconds). The influence of the choice of the basic minimum mean voltages compared to the maximum voltage and therefore of the type of micro-arcs obtained therefore makes it possible to control 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. In fact, in the first case the growth of the nanoporous layer will take place while in the second 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 cathode and anode current (and vice versa) and the current amplitude (and therefore the applied voltage). For example, the coating growth rate 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 positioning used to keep the component in the bath is suitable and does not modify the formation of micro arcs and do not extinguish them,) to a few hundred microns. The plasma micro-arc oxidation process is described, for example, in document WO03 / 083181.

[0019] FIG. 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 hard layer 21 forming approximately two-thirds of the total thickness of the coating 2, and a porous outer layer 22 forming approximately one-third of the total thickness of the coating 2 In addition, during its growth, the coating comprises coating 2 is formed partly by the transformation of the substrate material 7 and partly by growth beyond the initial surface 8 of the component (shown by the hatched line in Fig. 2). In fig. 2, the extra 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.

[0020] 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, impact and corrosion. Coating 1 is colored corresponding to the natural coloring of oxidized aluminum. For example, coating 1 has a dark brown coloration in the case of aluminum-lithium alloy 2050 (see table 1).

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

[0022] In order to obtain an optimum surface condition and to remove the last microns of the porous ceramic layer 22, a tribofinishing step can be carried out at the end of the formation of the coating 1 by the oxidation process. This tribofinishing step can comprise, for example, microsandblasting.

[0023] FIG. 3 shows a coating 1 formed by the oxidation process of the invention. In particular, Figs. 3a and 3c show the coating 2 seen from the front, respectively after its formation (fig. 3a) and after the tribofinishing step (fig. 3c). Fig. 3b is a sectional view showing the Al-Li alloy 7 substrate and coating 2.

[0024] Component 1 to be coated can also be obtained from a conventional shaping process such as machining, turning or shaping by liquid aluminum casting process (Cobapress process type).

[0025] For example, the plasma micro-arc oxidation process has been applied to various parts of a watch case made from an aluminum-lithium alloy. Fig. 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 caseband 91, a caseband 92, a lever 93, a fixing bridge 94, a back (not visible in the figure), a bezel 95 and a crown cover 96.

[0026] Figs. 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, FIG. 5 shows the coating 2 formed on the lever 93. FIG. 6 shows the coating 2 formed on the bezel 95. FIG. 7 shows the coating 2 formed on the middle part 91. FIG. 8 shows the coating 2 formed on the middle horn 92. In these figures, the layer visible in pale gray corresponds to a copper coating layer 10 deposited on the coating for protection purposes during the preparation of the metallographic cut.

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

[0028] In one embodiment, the oxidation process comprises a step of forming the coating 2 on the area or areas having the fine structuring. The zone or zones with the fine structuring are then masked so as to be able to form the coating 2 on the rest of the component 1 without affecting the zone or zones having the fine structuring. Then the coating is formed on the rest of the surface of component 1. The thickness of the coating in the area (s) with the fine patterning will depend on the size of the patterning. For example, the coating can be formed on the area (s) with the fine patterning with a thickness of about 10% of the dimension of the patterning.

In the case of the example of the watch case 9 above, the area or areas with the fine structuring may include threads, holes, tapping, etc. Fig. 9 shows a sectional view of a threaded portion of the middle lug 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 lug 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 pm). The threaded portion was then masked and the rest of the middle lug 92 was oxidized again 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 as as illustrated in fig. 8. Since the threaded portion is hidden during this step, the coating 2 does not grow more there 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 zone or zones with the fine structuring can be carried out using silicone gaskets or any other means of protection resistant to the oxidation treatment by plasma micro-arc and which can be removed at the end of the process.

[0031] The graph of FIG. 10 reports 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 50 µm thick coating 2 is formed on the middle 91 and the bottom of the case 9. The coating 2 is 45 µm thick 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 bridge fixing 94, for fixing the fixing bridge 94 on the middle part 91. The three lines 1, 2 and 3 in table 2 correspond to the thickness of the covering 2 measured at the top of three successive threads, as shown in fig. . 11.

5 Coating thickness measurements [pm]

Measure case middle thread axis Bridge axle fixing lever Bridge fixing case middle

1 42 23 43

2 48 20 46

3 59 18 45 Average 50 20 45

Residual coating thickness measurements at the bottom of the thread rpm]

Measure Top bottom thread Case middle Bridge fixing hole at 3 o'clock

1 43 77 10.2

2 56 73 6.0

3 48 70 Average 49 73.35 5.4

Table 2

[0033] The graph of FIG. 12 summarizes the different thickness measurements of the coating 2 at the holes and threads of the different elements of the box 9.

[0034] FIG. 16a shows a photograph of an aluminum-lithium alloy watch case 9 which has undergone the oxidation process of the invention. Fig. 16b shows a detail of the housing 9, in particular a portion of the fixing bridge 94, of the lever 93 and of the bezel 95. These photographs make it possible to assess the surface condition of the component with the coating and, in this case, the final step of microsandblasting.

Wear tests carried out under ball wear conditions at a speed of 46 rev / min and with a mixture of 2 kg of ceramic balls with a diameter of 3 mm in half a liter of water and 10 cc of wetting agent. The duration of the test was 36 hours. After 6 hours of this test, a shine of the surface is observed. After 36 hours of this test, no further development was observed. The ceramization process therefore provides excellent resistance to wear. The condition of the surface after wear of the ceramic coating on lithium aluminum is shown in fig. 1 1 in the case of an initial unworn surface and after 6 hours of the test, 12 hours of the test and 36 hours of the test.

[0036] FIG. 13 shows the state of the surface of the lithium aluminum alloy watchband on which the coating was formed by the oxidation process of the invention, after 6 hours of wear according to the above test.

[0037] Fine scratch tests were also carried out. The conditions of these tests included the rotation of the sample to be tested 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 felts and 10 g of “bremor BR 650” glass powder. The duration of the test was 24 hours. Fig. 17 shows tables 3 and 4 which represent the observations made for different durations of stress from 1 min to 24 hours.

[0038] FIG. 14 shows a photomicrograph of the surface condition of the middle part after 12 hours of fine scratches. These tests show that the ceramization process also provides excellent scratch resistance.

[0039] Tests of falls into a gravel bed were also carried out using a test method according to the ISO 23160 standard comprising falls into a bed of gravel and ceramic chips of 8 cm by 500 cm <2> ceramic cartons with a diameter of 3 mm, a length of 12 mm and a hardness of 900HV ± 100Hv. The drop height was 40cm.

[0040] 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 ten falls on a gravel / ceramic bed, very low impacts are observed at the edges, as visible in fig. 14 which shows the surface condition after ten of these tests with drops in a gravel bed 40 cm high with a weighted caseband with a weight representing the weight of the mechanical movement. These tests show that the ceramization process also provides excellent resistance to shocks and impacts 40cm on gravel / ceramic bed.

[0041] Synthetic sweat tests were also carried out. These tests were carried out under the conditions of NIHS 96-50 and ISO 3160-2 standards, in which the tested parts are placed on a cotton support soaked in sweat in an environment of 40 ° C ± 2 ° C humidity 95 to 100 % relative humidity test duration over 6 days.

[0042] After 6 days, corrosion pitting appears on areas where the aluminum is exposed. The ceramic layer creates a protective layer of the aluminum substrate (see Fig. 15). These tests show that the ceramization process also provides excellent corrosion resistance.

6

Claims (1)

Overall, lithium aluminum provides the following properties: 25% lighter than conventional materials, thus making it possible to optimize the design of structural parts and reduce the weight of a watch case, for example, Better resistance to fatigue and corrosion, which makes it possible to increase the reliability and extend the life of the product, 100% recyclable, which makes a major contribution to a sustainable watch industry. [0044] The coating 2 obtained by the method of the invention is advantageous for the components of a mechanical watch subjected to friction or mechanical stress. It is also advantageous for treating watch components, such as the example of watch case 9 above, which are subjected to aggressive environmental constraints such as wear, humidity, salinity level (sea, tropical climates or others. ). Of course, component 1 comprising coating 2 is not only of interest in watchmaking but can also be used in various fields such as eyewear and writing instruments. Reference numbers used in the figures [0045] 1 component 2 coating 21 hard coat 22 second serving 3 bowl 31 alternating current 4 electrolytic bath 5 cathode 6 power supply unit 7 aluminum-lithium substrate 8 initial surface 9 watch box 91 caseband 92 middle horn 93 lever 94 fixing bridge 95 bezel 96 crown cover 10 copper plating Claims 1. 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 allowing to obtain a ceramic coating (2) on the surface of the aluminum alloy. 2. The component of claim 1, wherein the coating (2) has a thickness of between 1 µm and 100 µm. 3. The component according to claim 2, comprising at least a portion comprising a surface structuring having a dimension less than 100 µm; and wherein the coating has a thickness of between 1 µm and 5 µm. 4. The component according to claim 3, wherein the coating has a thickness between 5 µm and 100 µm on the remainder of the component. 5. The component of claim 3, wherein the coating has a thickness between 20 µm and 50 µm on the remainder of the component. 7 6. The component according to one of claims 1 to 5, comprising between 0.88 and 2.45% by weight of lithium. 7. The component according to one of claims 1 to 6, wherein the coating has a hardness between 1800 and 2000 Hv. 8. The component according to one of claims 1 to 7, wherein the coating is intact after a test of ten drops into a gravel bed at a height of 40 cm from the component, according to ISO 23160. 9. The component according to one of claims 1 to 8, wherein the coating shows a surface condition has slight alterations (index 4) after 36 hours of a test according to ISO 23160. 10. The component according to one of claims 1 to 9, being a watch component, a component for eyewear or a component for a writing instrument. 1 1. The component of claim 10, wherein the watch component is a trim component or a component of a movement. 12. 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 micro-oxidation process. plasma arc and comprises: 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; apply 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 1,000 V. 13. The method of claim 12, wherein the duration of the current pulse giving a voltage of 0 V is 10% of the total duration of the pulse giving a voltage between 100 V and 1000 V. 14. The method of claim 12 or 13, wherein the minimum average voltage) is adjusted to be between 0 and 99.9% of the maximum voltage. 15. The method of claim 14, the minimum average voltage is adjusted to be between 30% and 60% of the maximum voltage. 16. The method according to one of claims 12 to 15, wherein an area of component (1) has a fine structuring having a dimension of less than 100 µm; and wherein the method further comprises: growing the coating (2) to a thickness between 1 µm and 5 µm; masking said area so that the subsequent application of the method has no effect on said area; and growing the coating (2) having a thickness between 5 µm and 100 µm over the remainder of the surface of the component (2). 17. The method of claim 16, wherein the step of masking said area comprises applying a silicone sealant. 18. The method according to one of claims 12 to 17, further comprising a surface preparation comprising a cleaning and degreasing step. 19. The method according to one of claims 12 to 18, further comprising a step of microblasting. 8