Method for manufacturing a part coated with a protective coating

Abstract

A method of fabricating a part coated with a protective coating, the method including using micro-arc oxidation treatment to form a protective coating on the outside surface of a part, the part including a niobium matrix having metallic silicide inclusions present therein, the current passing through the part being controlled during the micro-arc oxidation treatment in order to subject the part to a succession of current cycles, the ratio of (quantity of positive charge applied to the part)/(quantity of negative charge applied to the part) lying in the range 0.80 to 1.6 for each current cycle.

Claims

1. A method of fabricating a part coated with a protective coating, the method comprising: using micro-arc oxidation treatment to form a protective coating on an outside surface of a part, the part comprising a niobium matrix having metallic silicide inclusions present therein, a current passing through the part being controlled during the micro-arc oxidation treatment in order to subject the part to a succession of current cycles, a ratio of (quantity of positive charge applied to the part)/(quantity of negative charge applied to the part) lying in the range 0.80 to 1.6 for each current cycle, wherein each current cycle includes: a positive current rise stage during which the current passing through the part is positive and increasing, the duration of the positive current rise stage lying in the range 3% to 15% of the total duration of said cycle, and a positive stabilization stage during which a constant positive current passes through the part, the positive stabilization stage being performed after the positive current rise stage, the duration of the positive stabilization stage lying in the range 15% to 50% of the total duration of said cycle.

2. A method according to claim 1, wherein each current cycle includes a negative stabilization stage during which a constant negative current passes through the part, a duration of the negative stabilization stage lying in the range 30% to 80% of a total duration of said cycle.

3. A method according to claim 1, wherein the part is present in an electrolyte, and wherein prior to the beginning of the micro-arc oxidation treatment the electrolyte includes a silicate.

4. A method according to claim 1, wherein the part is present in an electrolyte, and wherein throughout all or part of the micro-arc oxidation treatment, the electrolyte is maintained at a temperature less than or equal to 40 C.

5. A method according to claim 1, wherein the part is present in an electrolyte, and wherein during the micro-arc oxidation treatment, the current passes through the part and through a counter-electrode present in the electrolyte, the counter-electrode having a same shape as the part.

6. A method according to claim 1, wherein the duration during which the part is subjected to micro-arc oxidation treatment is greater than or equal to 10 minutes.

7. A method according to claim 1, wherein the part is subjected to a micro-arc oxidation treatment enabling self-regulation conditions to be achieved, said self-regulation conditions then being maintained for a duration lying in the range 3 minutes to 10 minutes.

8. A method according to claim 1, wherein, for all or part of the current cycles, the ratio (quantity of positive charge applied to the part)/(quantity of negative charge applied to the part) lies in the range 0.8 to 0.9.

9. A method according to claim 1, wherein the part is initially subjected to a succession of current cycles for which the ratio (quantity of positive charge applied to the part)/(quantity of negative charge applied to the part) lies in the range 0.9 to 1.6, the part subsequently being subjected to a succession of current cycles for which the ratio (quantity of positive charge applied to the part)/(quantity of negative charge applied to the part) lies in the range 0.8 to 0.9.

10. A method of fabricating a part coated with a protective coating, the method comprising: using micro-arc oxidation treatment to form a protective coating on an outside surface of a part, the part comprising a niobium matrix having metallic silicide inclusions present therein, a current passing through the part being controlled during the micro-arc oxidation treatment in order to subject the part to a succession of current cycles, a ratio of (quantity of positive charge applied to the part)/(quantity of negative charge applied to the part) lying in the range 0.80 to 1.6 for each current cycle, wherein each current cycle includes: a positive stabilization stage during which a constant positive current passes through the part, the duration of the positive stabilization stage lying in the range 15% to 50% of the total duration of said cycle, and a positive current descent stage during which the current passing through the part is positive and decreasing, the duration of the positive current descent stage lying in the range 1% to 10% of the total duration of said cycle, the positive current descent stage being performed after the positive stabilization stage.

11. A method of fabricating a part coated with a protective coating, the method comprising: using micro-arc oxidation treatment to form a protective coating on an outside surface of a part, the part comprising a niobium matrix having metallic silicide inclusions present therein, a current passing through the part being controlled during the micro-arc oxidation treatment in order to subject the part to a succession of current cycles, a ratio of (quantity of positive charge applied to the part)/(quantity of negative charge applied to the part) lying in the range 0.80 to 1.6for each current cycle, wherein each current cycle includes: a negative current descent stage during which the current passing through the part is negative and decreasing, the duration of the negative current descent stage lying in the range 1% to 10% of the total duration of said cycle, and a negative stabilization stage during which a constant negative current passes through the part, the negative stabilization stage being performed after the negative current descent stage, the duration of the negative stabilization stage lying in the range 30% to 80% of the total duration of said cycle.

12. A method of fabricating a part coated with a protective coating, the method comprising: using micro-arc oxidation treatment to form a protective coating on an outside surface of a part, the part comprising a niobium matrix having metallic silicide inclusions present therein, a current passing through the part being controlled during the micro-arc oxidation treatment in order to subject the part to a succession of current cycles, a ratio of (quantity of positive charge applied to the part)/(quantity of negative charge applied to the part) lying in the range 0.80 to 1.6 for each current cycle, wherein each current cycle includes: a negative stabilization stage during which a constant negative current passes through the part, the duration of the negative stabilization stage lying in the range 30% to 80% of the total duration of said cycle, and a negative current rise stage during which the current passing through the part is negative and increasing, the negative current rise stage being performed after the negative stabilization stage, the duration of the negative current rise stage lying in the range 1% to 10% of the total duration of said cycle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the present invention appear from the following description of particular implementations of the invention, given as non-limiting examples, and with reference to the accompanying drawings, in which:

(2) FIG. 1 is a diagrammatic and fragmentary section of a part coated with a protective coating obtained by performing a method of the invention;

(3) FIG. 2 is a diagrammatic and fragmentary view of an experimental set-up for performing a method of the invention;

(4) FIG. 3 is a diagrammatic view showing an example of a current cycle suitable for use in micro-arc oxidation treatment of the invention;

(5) FIG. 4 is a diagrammatic and fragmentary view of a variant embodiment of a counter-electrode usable in the context of a method of the invention:

(6) FIG. 5 is a photograph of the results obtained after using a method of the invention to treat a part having a niobium matrix with inclusions of metallic silicides present therein; and

(7) FIGS. 6A and 6B are scanning electron microscope section views of the protective coating formed at the surface of the FIG. 5 part.

DETAILED DESCRIPTION OF IMPLEMENTATIONS

(8) FIG. 1 is a section view of a part 1 having a protective coating. A protective coating 3 is formed on the outside surface S of the part 2 comprising a niobium matrix having metallic silicide inclusions present therein.

(9) The thickness e of the coating 3 that is formed may lie in the range 20 m to 150 m, for example.

(10) FIG. 2 shows an experimental set-up for performing micro-arc oxidation treatment that is usable in the context of the present invention. The part 2 is immersed in an electrolyte 10 including silicates. A counter-electrode 6 is present facing the part 2 and it is likewise immersed in the electrolyte 10. In a variant that is not shown, counter-electrodes are present on both sides of the part. By way of example, this counter-electrode 6 may be cylindrical in shape, and by way of example it may be constituted by a 304L stainless steel. The part 2 and the counter-electrode 6 are connected to a generator 5 that subjects them to a succession of current cycles.

(11) While performing the method of the invention, a first oxide layer is formed initially on the outside surface S of the treated part 2. Sufficient current is applied to reach the electrical breakdown point of the first oxide layer initially formed on the surface S of the part 2. Electric arcs are then generated and lead to a plasma being formed at the surface S of the treated part 2. The protective coating 3 is then formed by converting the elements contained in the part 2, and also by incorporating elements contained in the electrolyte 10. The experimental set-up used also includes a cooling system (not shown) for limiting the heating of the electrolyte during the micro-arc oxidation treatment.

(12) A succession of periodic current cycles are applied to the part 2. The wave-form of one of the applied current cycles is shown in FIG. 3. The parameters are given in Table 1 below:

(13) TABLE-US-00001 TABLE 1 I.sub.p: current passing through T.sub.1: duration of the positive the part during the positive current rise stage stabilization stage T.sub.2: duration of the positive I.sub.n: current passing through stabilization stage the part during the negative T.sub.3: duration of the positive stabilization stage current descent stage Q.sub.p: quantity of positive T.sub.4: duration of the zero charge applied to the part current stabilization stage during the current cycle T.sub.5: duration of the negative Q.sub.n: quantity of negative current descent stage charge applied to the part T.sub.6: duration of the negative during the current cycle stabilization stage T: period of current cycles T.sub.7: duration of the negative F: frequency of current current rise stage cycles T.sub.8: duration of the zero current stabilization stage

(14) As shown in FIG. 3, each of the applied current cycles may comprise the following succession of stages: a positive current rise stage, then a positive stabilization stage, then a positive current descent stage, then optionally a zero current stabilization stage, then a negative current descent stage, then a negative stabilization stage, then a negative current rise stage.

(15) The total duration of the current cycle corresponds to the following sum:

(16) $\underset{i=1}{\overset{7}{.Math.}}{T}_i$
i.e. the duration between the beginning of the positive current rise stage and the end of the negative current rise stage. The frequency of the current cycles corresponds to the following magnitude:

(17) $\frac{1}{\underset{i=1}{\overset{8}{.Math.}}{T}_i}$

(18) FIG. 4 shows a variant implementation in which the counter-electrode 6 is of a shape that matches the shape of the part 2.

(19) As shown, the counter-electrode 6 may be similar in shape to the part 2 and it may fit closely around its shape. The part and the counter-electrode may also both be cylindrical or plane in shape.

EXAMPLE

(20) A substrate was treated by a method of the invention. Table 2 below gives the operating conditions (the times are expressed as a percentage of the total duration of the current cycle). The imposed cycle comprised the same succession of stages as the current cycle shown in FIG. 3.

(21) TABLE-US-00002 TABLE 2 Composition of the basic substrate before the beginning of the Composition of the micro-arc oxidation electrolyte before treatment the beginning of (% atomic): MASC Electrical the micro-arc alloy (described in parameters oxidation treatment U.S. Pat. No. 5,942,055) I (A) = 11 NaOH = 0.4 g/L Nb = 47% R = I.sub.n/I.sub.p = 55% Na.sub.2SiO.sub.2,5H.sub.2O = 15 g/L Ti = 25% Frequency = 100 Hz pH 12-13 Hf = 8% Q.sub.p/Q.sub.n = 0.87 solvent = water Cr = 2% T1 = 11% Al = 2% T2 = 20% Si = 16% T3 = 2% T4 = 1% T5 = 3% T6 = 61% T7 = 2%

(22) After about 30 minutes of treatment, self-regulation conditions were reached, characterized by progressive extinction of the electric arc. The samples continued to be treated for five additional minutes under self-regulation conditions so as to grow the oxide layer being formed and improve its compactness.

(23) The operating conditions advantageously enable a relatively dense protective coating to be formed having thickness equal to approximately 150 m at the surface of the treated test piece.

(24) After treatment, the bar appeared to be perfectly coated. Its macroscopic appearance is shown in FIG. 5.

(25) The layer formed on the surface of the substrate was characterized by scanning electron microscopy (see FIGS. 6A and 6B). The layer that was formed revealed a uniform appearance over the entire circumference of the bar and in the two zones analyzed.

(26) The coating formed by micro-arc anodic oxidation adhered perfectly.

(27) The term including/containing a should be understood as including/containing at least one.

(28) The term lying in the range . . . to . . . should be understood as including these limits.