Non-metallic coating and method of its production

Abstract

A method of forming a non-metallic coating on a metallic substrate involves the steps of positioning the metallic substrate in an electrolysis chamber and applying a sequence of voltage pulses of alternating polarity to electrically bias the substrate with respect to an electrode. Positive voltage pulses anodically bias the substrate with respect to the electrode and negative voltage pulses cathodically bias the substrate with respect to the electrode. The amplitude of the positive voltage pulses is potentiostatically controlled, whereas the amplitude of the negative voltage pulses is galvanostatically controlled.

Claims

1. A method of forming a non-metallic coating on a surface of a metallic or semi-metallic substrate comprising the steps of, positioning the substrate in an electrolysis chamber containing an aqueous electrolyte, the aqueous electrolyte being an alkaline solution, and an electrode, at least the surface of the substrate and a portion of the electrode contacting the aqueous electrolyte, and electrically biasing the substrate with respect to the electrode by applying a sequence of voltage pulses of alternating polarity for a predetermined period of time, positive voltage pulses anodically biasing the substrate with respect to the electrode and negative voltage pulses cathodically biasing the substrate with respect to the electrode, in which the voltage pulses have a pulse repetition frequency of between 0.1 and 20 KHz, and in which the amplitude of the positive voltage pulses is potentiostatically controlled, and the amplitude of the negative voltage pulses is galvanostatically controlled, in which the amplitude of each of the positive voltage pulses is constant over the predetermined period of time, and in which the amplitude of successive negative voltage pulses increases over the predetermined period of time.

2. A method according to claim 1 in which both the positive and negative voltage pulses are substantially trapezoidal in shape.

3. A method according to claim 1 in which the amplitude of each of the positive voltage pulses is between 200 volts and 2000 volts.

4. A method according to claim 1 in which the amplitude of each of the positive voltage pulses is between 250 volts and 900 volts.

5. A method according to claim 1 in which the amplitude of each of the positive voltage pulses is about 600 volts or about 650 volts or about 700 volts.

6. A method according to claim 1 in which the amplitude of successive negative pulses increases from about 1 volt to a maximum of up to 1000 volts over the predetermined period of time.

7. A method according to claim 1 in which the amplitude of successive negative pulses increases from about 1 volt to a maximum of up to 400 volts.

8. A method according to claim 1 in which the voltage pulses have a pulse repetition frequency of between 1.5 and 15 KHz.

9. A method according to claim 1 in which the voltage pulses have a pulse repetition frequency of between 2 and 10 KHz.

10. A method according to claim 1 in which each positive voltage pulse comprises an interval during which the voltage is increased (T.sub.ai) and an interval during which voltage is decreased (T.sub.ad) and/or each negative voltage pulse comprises an interval during which the voltage is increased (T.sub.ci) and an interval during which voltage is decreased (T.sub.cd), wherein each of the intervals during which voltage is increased or decreased comprises between 3% and 30% of the total pulse duration.

11. A method according to claim 10 in which each positive voltage pulse further comprises an interval (T.sub.ac) during which voltage is maintained to be substantially constant and/or each negative voltage pulse further comprises an interval (T.sub.cc), during which voltage is maintained to be substantially constant.

12. A method according to claim 11 in which the, or each, interval during which voltage is maintained to be constant comprises between 40% and 94% of the total pulse duration.

13. A method according to claim 10 in which the, or each, interval during which voltage is increased or decreased is not shorter than 10 microseconds.

14. A method according to claim 11 in which the, or each, interval during which voltage is maintained to be constant is within the range of 10 to 9000 microseconds in duration.

15. A method according to claim 1 in which the electrolyte has a pH of 9 or greater.

16. A method according to claim 15 in which the electrolyte has an electrical conductivity of greater than 1 mS cm.sup.1.

17. A method according to claim 1 in which the electrolyte comprises an alkaline metal hydroxide.

18. A method according to claim 1 in which the electrolyte comprises potassium hydroxide or sodium hydroxide.

19. A method according to claim 1 in which the electrolyte is colloidal and comprises solid particles dispersed in an aqueous phase.

20. A method according to claim 19 in which the electrolyte comprises a proportion of solid particles having particle dimensions of less than 100 nanometers.

21. A method according to claim 19 in which the solid particles have a characteristic isoelectric point and the pH of this isoelectric point differs from the pH of the electrolyte by 1.5 or greater.

22. A method according to claim 19 in which the solid particles are ceramic particles.

23. A method according to claim 19 in which the solid particles are metallic oxides or hydroxides.

24. A method according to claim 23 in which the solid particles are oxides or hydroxides of an element selected from the group comprising silicon, aluminium, titanium, iron, magnesium, tantalum and the rare earth metals.

25. A method according to claim 19 comprising a step of incorporating solid particles from the electrolyte into the non-metallic coating.

26. A method according to claim 1 in which localised plasma micro-discharge is not generated during formation of the non-metallic coating.

27. A method according to claim 1 in which the predetermined period of time is between 1 minute and 2 hours.

28. A method according to claim 1 in which the predetermined period of time is between 2 minutes and 30 minutes.

29. A method according to claim 1 comprising the further step of maintaining the electrolyte at a temperature of between 10 and 40 degrees Centigrade.

30. A method according to claim 1 further comprising the step of circulating the electrolyte.

31. A method according to claim 1 in which the substrate comprises a metal selected from the group consisting of aluminium, magnesium, titanium, zirconium, tantalum, beryllium, or an alloy or intermetallic of any of these metals.

32. A method according to claim 1 in which the substrate is a semiconductor, for example silicon, germanium or gallium arsenide.

33. An apparatus for forming a non-metallic coating on the surface of a metallic or semi-metallic substrate comprising, an electrolytic chamber for containing an aqueous alkaline electrolyte, an electrode locatable within the electrolytic chamber, and a power supply capable of applying a sequence of voltage pulses of alternating polarity between the substrate and the electrode, the voltage pulses have a pulse repetition frequency of between 0.1 and 20 kHz, the power supply comprising a first pulse generator for generating a potentiostatically controlled sequence of positive voltage pulses to anodically bias the substrate with respect to the electrode and a second pulse generator for generating a galvanostatically controlled sequence of negative voltage pulses to cathodically bias the substrate with respect to the electrode.

34. An apparatus according to claim 33 further comprising a colloidal electrolyte comprising solid particles dispersed in an aqueous phase.

35. An article of manufacture comprising a non-metallic coating formed by a method according to claim 1.

Description

PREFERRED EMBODIMENTS OF THE INVENTION

(1) Preferred embodiments of the invention will now be described with reference to the figures, in which;

(2) FIG. 1 is a schematic illustration of a first embodiment of an electrolytic apparatus suitable for use with a method of forming a non-metallic coating on the surface of a substrate according to one or more embodiments of the invention,

(3) FIG. 2 is a schematic illustration of a second embodiment of an electrolytic apparatus suitable for use with a method of forming a non-metallic coating on a substrate according to one or more embodiments of the invention,

(4) FIG. 3 is a schematic diagram of an electronic power supply suitable for use with the apparatus of FIG. 1 or FIG. 2,

(5) FIG. 4 illustrates a preferred voltage waveform in for use in a method of forming a non-metallic coating on a substrate according to one or more embodiments of the invention,

(6) FIG. 5 is a schematic illustration of a current waveform corresponding to the voltage waveform illustrated in FIG. 4,

(7) FIG. 6 illustrates details of one positive voltage pulse and one negative voltage pulse from the waveform of FIG. 4,

(8) FIGS. 7 and 8 are typical scanning electron micrographs of a non-metallic coating formed on an aluminium alloy according to a specific embodiment of the invention described in Example 1,

(9) FIGS. 9 and 10 are typical scanning electron micrographs of a non-metallic coating formed on an aluminium alloy by a plasma electrochemical oxidation (PEO) process, showing the significant pore size associated with such a process,

(10) FIG. 11 is an X-ray diffraction (XRD) pattern of a non-metallic coating formed on an aluminium alloy according to a specific embodiment of the invention described in Example 1,

(11) FIG. 12 is an XRD pattern of a non-metallic coating formed on an aluminium alloy according to a specific embodiment of the invention described in Example 2.

(12) FIG. 1 illustrates a typical electrolytic apparatus suitable for use with a method of forming a non-metallic coating on a substrate according to one or more embodiments or aspects of the invention. The apparatus comprises a chemically inert tank 2, for example a tank formed from a stainless steel alloy, which contains an electrolyte solution 3. The electrolyte solution 3 is an aqueous alkaline electrolyte solution, for example an aqueous solution of sodium hydroxide or potassium hydroxide, and has an electrical conductivity of greater than 5 mS cm.sup.1. The electrolyte may be a colloidal electrolyte comprising solid particles, with a proportion of those particles having a particle size lower than 100 nanometers.

(13) A substrate 1 on which it is desired to form a non-metallic coating is electrically connected to a first output 50 of a pulse power supply 4. An electrode 5 is connected to a second output 55 of the pulse power supply 4, and both the electrode 5 and the substrate 1 are immersed in the electrolyte solution 3 contained within the tank 2. The pulse power supply 4 is capable of supplying electrical pulses of alternating polarity in order to electrically bias the substrate 1 with respect to the electrode 5.

(14) FIG. 2 illustrates an alternative electrolytic apparatus suitable for use with a method of coating a substrate according to one or more aspects or embodiments of the invention. In common with the apparatus described above in relation to FIG. 1, the apparatus of FIG. 2 comprises a chemically inert tank 2 for containing an electrolyte solution 3. A substrate 1 is coupled to a first output 50 of a pulse power supply 4. A second output 55 of the power supply 4 is electrically connected to first and second electrodes 5 and 5, and the substrate 1 and the electrodes 5 and 5 are immersed in the electrolyte 3. The two electrodes 5, 5 are disposed on either side of the substrate 1 in order to generate a more even electric field over the surface of the substrate and produce a more even coating on both sides of the substrate.

(15) It is noted that more than two electrodes may be coupled to an output of the pulse power supply 4 should this be desired. Likewise, more than one substrate may be simultaneously coupled to an output of the pulse power supply 4 so that more than one substrate may be coated at any one time.

(16) The apparatus of FIG. 2 further comprises a heat exchanger 6 through which the electrolyte 3 is circulated. The heat exchanger 6 allows circulation of electrolyte 3 within the tank 2, and furthermore allows control of the temperature of the electrolyte.

(17) A preferred pulse power supply for use with one or more embodiments of the invention is capable of supplying separate positive and negative voltage pulses between the substrate and an electrode. A schematic diagram of a preferred pulse generator is illustrated in FIG. 3.

(18) The pulse power supply of FIG. 3 comprises two separate insulated gate bipolar transistor (IGBT) based generators and is a preferred pulse power supply for the apparatus of FIG. 1 or 2. A first generator, or anodic generator, 30 acts a generator of anodic pulses, i.e. pulses that anodically bias the substrate, or substrates, with respect to the electrode, or electrodes. A second generator, or cathodic generator, 35 acts as a generator of cathodic pulses, i.e. pulses that cathodically bias the substrate, or substrates, with respect to the electrode, or electrodes.

(19) The anodic pulse generator 30 and the cathodic pulse generator 35 are independently controlled and synchronised by means of a controller 40. The anodic pulse generator 30 generates trapezoidal-shaped pulses having a fixed voltage amplitude, i.e. the voltage amplitude of the pulses generated by the anodic pulse generator 30 is potentiostatically controlled.

(20) The cathodic pulse generator 35 provides trapezoidal-shaped pulses in which the mean cathodic current is maintained at a fixed value over successive pulses, i.e. the cathodic pulse generator 35 generates pulses that are galvanostatically controlled.

(21) An output switch 45 comprising an H-bridge electronic circuit, couples the anodic pulse generator 30 and the cathodic pulse generator 35 to a first output 50 and a second output 55. During use, the first output 50 is electrically coupled to a substrate and the second output 55 is electrically coupled to one or more electrodes. The controller 40 synchronises the output of the anodic pulse generator 30 and the cathodic pulse generator 35 and allows the output switch 45 to produce an output waveform comprising a sequence of positive and negative trapezoidal-shaped voltage pulses as illustrated in FIG. 4.

(22) Simultaneous use of potentiostatic control for positive (anodic) pulses and galvanostatic control for negative (cathodic) pulses enables a gradual increase in a ratio between the power of cathodic and anodic pulses over the duration of the process, and this creates conditions allowing high energy process without of generation of micro-discharges.

(23) A particularly preferred waveform for use in a method of producing a non-metallic coating on a substrate according to one or more embodiments of the invention is illustrated by FIGS. 4, 5 and 6.

(24) FIG. 4 illustrates a waveform consisting of a sequence of alternating positive and negative voltage pulses generated over a period of time. Positive voltage pulses are substantially trapezoidal in shape and have a positive pulse interval (T.sub.a) as indicated in FIG. 4. When applied between a substrate and an electrode, positive voltage pulses cause the substrate to be anodically biased relative to the electrode. Successive positive voltage pulses are controlled to have substantially the same voltage amplitude (V.sub.a).

(25) Negative voltage pulses are substantially trapezoidal in shape and have a negative pulse interval T.sub.c. When applied between a substrate and an electrode, negative voltage pulses cause the substrate to be cathodically biased relative to the electrode. Successive negative voltage pulses are controlled to have substantially the same current amplitude (I.sub.c in FIG. 5).

(26) The amplitude of each successive negative voltage pulse is controlled to be a voltage at which a constant level of current flows across the electrolyte. When used in a method according to an embodiment of the invention the application of the waveform results in a non-metallic coating being formed on the surface of the substrate. As the coating grows thicker its electrical resistance increases and the voltage required to pass the same amount of current increases. Thus, the amplitude of successive cathodic voltage pulses (V.sub.c) increases over a period of time.

(27) FIG. 5 is a diagram showing the current waveform that corresponds to the voltage waveform illustrated in FIG. 4. When a positive voltage pulse is applied a positive current is deemed to flow, and when a negative voltage is applied a negative current is deemed to flow. The positive voltage pulses are potentiostatically controlled, such that the amplitude of each successive pulse is substantially the same. Over a period of time the thickness of the coating on the surface of the substrate increases, and the current driven by this voltage decreases. Thus, the positive current pulse amplitude (I.sub.a) associated with the positive voltage pulses tend to decrease over the period of time.

(28) As discussed above in relation to FIG. 4, negative voltage pulses are controlled galvanostatically, and thus these pulses are controlled to have a constant current amplitude (I.sub.c).

(29) FIG. 6 illustrates a portion of the waveform of FIG. 4 showing one positive voltage pulse and one negative voltage pulse. Each positive voltage pulse is substantially trapezoidal in shape and has an interval (T.sub.ai) during which the voltage rises from zero to the positive or anodic voltage amplitude (V.sub.a). Each positive voltage pulse has an interval (T.sub.ac) during which constant voltage is applied. This constant voltage is applied at the voltage amplitude of the pulse (V.sub.a). Each positive voltage pulse further comprises an interval (T.sub.ad) during which the voltage decreases from the voltage amplitude (V.sub.a) to zero. The intervals (T.sub.ai) and (T.sub.ad) may be varied to control the current flow associated with the voltage pulse. It is highly undesirable that current spikes are generated during voltage pulses as current spikes promote the breakdown of the growing coating and cause micro-discharge or plasma generation. Micro-discharge events have a deleterious effect on the quality of the coating produced.

(30) Each negative voltage pulse is substantially trapezoidal in shape and comprises three intervals analogous to the three intervals described in relation to the positive voltage pulses. Each cathodic voltage pulse has an interval (T.sub.ci) during which voltage is increased from zero to the cathodic voltage amplitude (V.sub.c) of that pulse, an interval during which the cathodic voltage remains at the cathodic voltage amplitude (V.sub.c) and an interval (T.sub.cd) during which the voltage decreases from the voltage amplitude (V.sub.c) to zero. The voltage amplitude (V.sub.c) is determined with respect to the current flow at the voltage. Thus, the voltage amplitude (V.sub.c) tends to increase over a period of time, as illustrated in FIG. 4.

(31) The waveforms illustrated in FIGS. 4, 5 and 6 have a number of variables that may be controlled to influence the physical and electrical properties of the coating formed. The duration of both the positive and negative voltage pulses (T.sub.a & T.sub.c) may be independently controlled. The intervals (T.sub.a, T.sub.ac, T.sub.ad, T.sub.ci, T.sub.cc and T.sub.cd) associated with the positive and negative voltage pulses can be controlled in order to substantially eliminate current pulse sparks and micro-discharge. The amplitude of the positive voltage pulses (V.sub.a) may be controlled, as may the current flow at the peak voltage of each of the negative voltage pulses (I.sub.c). Furthermore, the frequency of the pulses may be varied within a range of 100 HZ to 20 KHZ.

(32) FIGS. 1 to 6 and the accompanying text describe apparatus and a preferred waveform suitable for generating a non-metallic coating on the surface of a metallic or semi-metallic substrate. Specific embodiments of the invention using apparatus as illustrated in FIG. 1 or 2, including the pulse generator illustrated in FIG. 3 and using the specific waveforms as illustrated in FIGS. 4 to 6, are described in the following examples. In all examples, the colloidal solutions comprise some solid particles with a particle size lower than 100 nanometers.

Example 1

(33) A substrate in the form of a plate of Al 6082 alloy having dimensions of 50 mm50 mm1 mm was treated in an apparatus as described above and illustrated in FIG. 1. The apparatus comprised a tank containing an electrolyte, and the substrate and an electrode were coupled to a pulse power supply as described above and illustrated in FIG. 3. The substrate and the electrode were arranged in contact with the electrolyte.

(34) The electrolyte was an aqueous solution containing 1.8 g/l of KOH and 1.0 g/l of Alumina particles, forming a stabilised colloidal solution.

(35) The Pulse Generator applied a sequence of trapezoidally-shaped voltage pulses of alternating polarity between the substrate and the electrode. Positive voltage pulses were applied having a fixed positive voltage amplitude (V.sub.a) of 700 V, and negative voltage pulses had a negative voltage amplitude (V.sub.c) continuously grown from 0 to 350 V. The pulse repetition frequency was 2.5 KHz.

(36) The pulses were applied for 8 minutes and a non-metallic coating was formed on the surface of the substrate.

(37) The non-metallic coating was characterised and had the following characteristics: The coating had a smooth surface profile. FIG. 7 illustrates an SEM micrograph showing a portion of the coating at a magnification of 60,000 times. It can be seen that the surface is substantially smooth at this magnification. FIG. 8 is a further SEM micrograph showing a portion of the coating at a magnification of 55,000 times. Pores in the coating having a size of between 50 an 150 nanometers can be seen. Pores of this dimension may be termed nano-pores.

(38) For comparison, FIGS. 9 and 10 show SEM micrographs of a coating formed on the surface of an aluminium alloy by means of a plasma electrochemical oxidation (PEO) process. These micrographs are at a magnification of 50,000 times. The surface of the PEO coating can be seen to be extremely rough at this magnification. Pores formed by plasma bulbs can be seen to have a size of greater than 500 nanometers, in great contrast to the coating illustrated in FIGS. 7 and 8.

(39) The coating thickness was 20 micrometers and its hardness was measured to be 1550 Hv. An XRD analysis of the coating (FIG. 11, revealed that the composition of the coating was aluminium oxide and that the coating having mean crystalline grain size of 40 nm. The average crystalline size was calculated on the base of the XRD data according to the Scherrer equation (B. D. Cullity & S. R. Stock, Elements of X-Ray Diffraction, 3.sup.rd Ed., Prentice-Hall Inc., 2001, p 167-171).

(40) The breakdown voltage of the coating was measured to be 1800 V DC and the dielectric strength was measured to be 90 KV/mm.

Example 2

(41) A substrate in the form of a plate of Al 5251 alloy having dimensions 25 mm25 mm2 mm was treated using the same apparatus as used for Example 1.

(42) The electrolyte contained 1.5 g/l of KOH and 2 g/l of Titania in stabilised colloidal solution.

(43) The Pulse power supply applied a sequence of trapezoidally-shaped voltage pulses of alternating polarity between the substrate and the electrode. Positive voltage pulses were applied having a fixed positive voltage amplitude (V.sub.a) of 600 V, and negative voltage pulses had a negative voltage amplitude (V.sub.c) continuously grown from 0 to 300 V. The pulse repetition frequency was 4 KHz.

(44) The pulses were applied for 3 minutes and a non-metallic coating was formed on the surface of the substrate. The coating thickness was 10 micron.

(45) FIG. 12 shows an XRD pattern of the produced coating. This pattern shows that the coating has both alumina and characteristic peaks of titania (TiO.sub.2) nanoparticles.

(46) TiO.sub.2 nanoparticles incorporated in the coating form a material that may efficiently absorb UV radiation. They also reveal catalytic activity in a number of redox processes. TiO.sub.2 nanoparticles are used as UV active pigments in paints and protective and self-cleaning coatings including bioactive and bactericidal coatings.

Example 3

(47) A substrate formed from an Al disk made of Al 7075 alloy and having a diameter of 30 mm and thickness of 2 mm was treated in an apparatus as illustrated in FIG. 2.

(48) The electrolyte solution contained 2 g/l of KOH and 3.5 g/l of Sodium Silicate Na.sub.2SiO.sub.3. The electrolyte temperature was maintained at 20 C. by use of a heat exchanger.

(49) The Pulse Generator applied a sequence of trapezoidally-shaped voltage pulses of alternating polarity between the substrate and the electrode. Positive voltage pulses were applied having a fixed positive voltage amplitude (V.sub.a) of 600 V, and negative voltage pulses had a negative voltage amplitude (V.sub.c) continuously grown from 0 to 450 V. The pulse repetition frequency was 3 KHz.

(50) The pulses were applied for a period of 18 minutes and the resulting coating had a thickness of 45 micrometers and a hardness of 1750 Hv. Such a coating may be of particular use in applications where a hard protective coating is required.

Example 4

(51) A substrate in the form of a Mg plate specimen made of AZ 91 alloy and having dimensions of 50 mm50 mm1 mm was treated in an apparatus as illustrated in FIG. 2.

(52) The electrolyte contained 2.5 g/l of KOH; 5 g/l Na.sub.4P.sub.2O.sub.7; 3 g/l NaF and 5 g/l of Alumina in stabilised colloidal solution.

(53) Electrolyte temperature was maintained at 20 C. by use of heat exchanger.

(54) The Pulse Generator applied a sequence of trapezoidally-shaped voltage pulses of alternating polarity between the substrate and the electrode. Positive voltage pulses were applied having a fixed positive voltage amplitude (V.sub.a) of 550 V, and negative voltage pulses had a negative voltage amplitude (V.sub.c) continuously grown from 0 to 300 V. The pulse repetition frequency was 1.25 KHz.

(55) The pulses were applied for a period of 4 minutes and the resulting coating had a thickness of 15 micrometers. Such a coating may be of particular use in applications where a protective coating is required.

Example 5

(56) A substrate consisting of a Ti threaded rod specimen made of Ti6Al4V alloy and having a diameter of 5 mm and a length 40 mm was treated in an apparatus as illustrated in FIG. 2.

(57) The electrolyte contained 4.5 g/l of Na.sub.4P.sub.20.sub.7; 5.5 g/l Na.sub.2B.sub.40.sub.7; 5.0 g/l NaF and 2 g/l of Titania in stabilised colloidal solution. Electrolyte temperature was maintained at 20 C.

(58) The Pulse Generator applied a sequence of trapezoidally-shaped voltage pulses of alternating polarity between the substrate and the electrode. Positive voltage pulses were applied having a fixed positive voltage amplitude (V.sub.a) of 500 V, and negative voltage pulses had a negative voltage amplitude (V.sub.c) continuously grown from 0 to 250 V. The pulse repetition frequency was 1 KHz.

(59) The pulses were applied for a period of 3 minutes and the resulting coating had a thickness of 10 micrometers. Such a coating may be of particular use in an anti-galling application.