FORMING A METAL COATING

Abstract: This invention relates to a method of forming a metal coating on a surface. According to one version of the invention, the method comprises decomposing at the surface a vapour comprising: a first volatile compound between a first metal and a polydentate reagent, and a second volatile compound between a second metal and the same or another polydentate reagent, the reagent(s) being volatile, whereby the metals are co-deposited on the surface. In other versions of the invention, the compounds can be applied to the surface in the liquid phase, in the form of a paint. Such a paint need contain only one metal.

Description: FORMING A METAL COATING This invention relates to a method of forming a single-metal or mixed-metals coating on a surface, to a metallic paint, to a method of painting and to a method of spray-forming powders using the paint. Metallic paints are known which comprise metal (e.g. zinc or aluminium) particles suspended in binder, pigment and volatile solvent. Such paints are easy to apply but cannot give a seamless coherent metal coating, which can be required in demanding and advanced technical applications. According to the present invention, a metallic paint comprises a compound between the metal or each of the metals of the paint and a polydentate reagent, said compound(s) being dissolved in a paintable solvent. An object is painted, according to the invention, by applying thereto a paint as set forth above, allowing the solvent to evaporate, and applying a gaseous or volatile substance which can decompose said compound(s) to release said metal(s). Metal powder may be made by atomising the paint and applying a said gaseous compound. According to another aspect of the Invention, the method of forming mixed-metal coatings on a surface comprises decomposing at the surface a fluid e.g. vapour comprising: a first preferably volatile compound between a first metal and a polydentate reagent, and a second preferably volatile compound between a second metal and the same or another polydentate reagent, wherein the reagent(s) is/are volatile, whereby the metals are co-deposited on the surface. The reagent(s) and the compounds are preferably stable in air. The decomposition preferably yields the reagent(s) directly, which accordingly may be recovered for re-use. Transport of the 'bulk' complex in vapour phase and its reaction on the heated substrate can in certain cases result in interactions between the regenerating ligand and substrate material, i.e. removal of surface oxide, or the formation of a volatile intermediate which itself is reduced later. This would particularly be enhanced where the chelating ligands are extremely 'active' toward the base material. Thus, in one option, the said second metal is the surface and the said second volatile compound is formed from reagent liberated upon decomposition of the first volatile compound. By this technique, interpenetratlon of metals can be achieved at lower temperatures than if, say, diffusion was being relied upon. Preferably the decomposition of the polydentate reagent(s) to metal is by reduction. Preferably the reduction is performed by a gas or gases. Preferably the donor atoms of the said reagent(s) are any selection from N, 0 and S. Preferably the said reagent (or one of said reagents) is a Schiff base or a β-diketone. Both these- are air-stable and non-toxic, unlike for example metal alkyls and metal carbonyls. Where the reagents are different for the two metals, at least one may comprise ballasting substituents. Thus, by exploiting the resulting differential volatilities of the two compounds, their relative vapour pressures can be varied so as to adjust the composition of the resulting metal mixture if other ways of altering the vapour composition are not available. One of the metals may be copper and the other may be nickel. The substrate may be a metal or glass or ceramic (e.g. alumina) or a membrane requiring to be metallised. UK Patent GB 2135984B, the disclosure of which is imported by reference, claims a method of winning metal from ore, and discloses for that purpose compounds which may find use in this invention, such as, in the case of Cu(II), tetradentate Schiff base reagents. β-diketones which may be used include for example 2,2,6,6 tetramethyl 3,5 heptadione (trivially called tert-butyl acetyl acetone), two molecules of which complex each copper atom: The removal of surface oxide, mentioned earlier, takes place as follows. Complex ML + Decomposing agent say H 2 •»• Deposited metal M + Polydentate reagent LH 2 Regenerated LH 2 + Surface oxide MO •* ■ H 2 0 vapour + ML Note that surface oxide can be removed at low temperature (e.g. 300°C), below the onset of rapid diffusion. This feature improves the adhesion of deposited metal. The invention will now be described by way of example. The co-deposition of metals, e.g. nickel and copper on various substrates, was carried out using physically mixed 'powdered' proportions of respective metal complexes. A range of metallic complex mixtures was prepared by physical mixing, and each mixture was shaken and volatilised prior to reduction and deposition. The volatilisation temperatures were dependent upon the 'mixtures' used, generally in the region of 180-200°C for β-diketonoate complexes, and around 250°C for Schiffs base complexes. The relative percentage of respective metals in the deposit depends as mentioned on the relative volatilities of the complexes. Thus a mixture of nickel and copper 2,2,6,6-tetramethyl-3,5-heptandioate complexes in the proportions Ni 70:Cu 30 gave a deposit containing the same ratio of metals. With a mixture of the same ratio, but 2,4-pentandione as the che te, a higher proportion of copper was detected. In the following examples, deposition temperatures of between 350 - 450 ± 10°C and total gas flow ( carrier and reductant) of 75 ± 5 cm 3 /min were used. Prior to any deposition all substrates were washed in 0.1 M HC1 , followed by an acetone rinse. EXAMPLE 1: NiCu alloy is deposited on mild steel. A physically mixed composition of Ni/Cu (heptandioate> 2 complex mentioned above, in the metals ratio 70:30, was weighed in a sample boat and placed in a volatilization zone. (Total complex 0.4273g; Ni 0.0415g; Cu 0.019g>. The system was initially flushed with nitrogen. Then hydrogen (15 ± 5 cm 3 /m1n) was' introduced into the system. The substrate, mounted on a heating probe, was brought to a steady state temperature (400 ± 10°C) before the complex mixture was heated to its volatilization temperature of 180°C and held for a period of 1 hr. Porosity of the resulting NiCu alloy deposit on the mild steel substrate was determined using a chemical staining technique, incorporating a sensitivity test for the substrate metal. A piece of filter paper was dipped into a solution of hexacyanoferrate (III), then pressed lightly on to the thin deposit. Although a quantitive result was not obtained from the test, it gave an indication as to the porosity of the deposit, in that any holes in the deposit resulted in a blue stain on the filter paper. EXAMPLE 2: NiFe alloy is deposited on mild steel by depositing Ni and relying on the freshly liberated ligand to react with the substrate iron, the iron complex then being decomposed alongside the nickel complex. In this case, the nickel complex used was nickel bis 1 ,1 ,l-trifluoro-2,4-pentanedioate, Ni(tfa) 2 for short. Ni(tfa) 2 , 0.4344 g, was weighed in a sample boat and placed in a volatization zone. The system was initially flushed with nitrogen. Then hydrogen (10 ± 5 cm 3 /min) was introduced into the system. The mild steel substrate, mounted on a heating probe, was heated to 400 ± 10°C before the complex was vapourised over a period of 1 hr at a temperature of 190°C. Cross-sectional analysis of the coated substrate, using an energy dispersive X-ray analyser, showed interpenetration of the base metal (steel) into the coating (nickel). Quantϊtive results have indicated the migration of iron to the nickel to be as high as 11% in cases where the coating has been 'built-up' during a number of separate runs. EXAMPLES 3 - 5: NiCu alloy is deposited on silica glass/on alumina/on aluminium. In each case a thin yet non-porous coherent and adherent film of alloy was achieved, at a temperature low enough not to damage the substrate, even with substrates having awkward grooves and undercuts; that is, the method has good throwing power. In Example 5 (aluminium substrate) 1n particular, the operating conditions were identical to Example 1. Initial mlcrohardness measurements of cross-sectional pieces of coated substrate (mounted in plastics) have indicated that the deposit 1n some cases is harder than the substrate. However, these results are tentative where the deposit is extremely thin, such as 10 μm. EXAMPLE 6: Coating powder particles can radically alter their properties. The coatings may only be a few atoms thick - less than 1% of the weight of the powder, yet be effective. In this example, metallic copper is deposited on supermagnetic flakes, 7imm x %mm x /lomm for example, of iron neodymium boride Fe 14 NdB, known as Magnequench, or equally successfully on 5-micron Fe 14 NdB powder known as B14. This copper provides a non-magnetic 'insulation'; on compressing the flakes (or powder particles), a small-domain highly magnetic material is obtained. This task would be difficult to achieve using conventional metal paint, and is conventionally performed by tumbling the flakes with copper powder in a process known as 'tumble-co-milling', which cannot yield the same uniformity of magnetic insulation without greater volumetric dilution of the supermagnetic material. A copper complex consisting of the compound 2,2,6,6-tetramethyl-3,5-heptadione described above, two molecules of which complex each copper atom, was used. B14 is very pyrophoric and is therefore stored under cyclohexane. The copper complex is dissolved directly into this, in an amount depending on the thickness of copper coating required after calculating the particle surface area; for magnetic purposes the coating thickness should be the minimum which will survive compression without rupturing. In this example, the complex was calculated to amount to 1% (based on copper) by mass of the B14. The same procedure, using cyclohexane, was also used with Magnequench. The mixture was shaken thoroughly at room temperature. The solvent cyclohexane was evaporated under nitrogen at 100°C before increasing the temperature of the system and changing to a hydrogen atmosphere to deposit the copper on the Fe 14 NdB at operating temperatures of 210°C for 1 hour. At much lower temperatures, the complex will not give up the copper at an adequate speed, while at much higher temperatures, the liberated dione may attack and extract the substrate Fe 14 NdB. Good coverage of the Fe 14 NdB with copper was established by visual inspection, and by noting in the case of B1 that the product was not pyrophoric. In some cases it may be advantageous as a final step to dry tumble the coated material to assist good overall coverage. The hydrogen gas reduces the complex, yielding elemental solid metal and liberating, in the gas phase, the initial chelating ligand. Initial n.m.r. and i.r. studies on the collected products have shown that whilst a number of ligands can be regenerated to some 'purity', especially straight chain alkane β-diketones, their fluorinated derivatives are prone to some decomposition. EXAMPLE 7: In other applications, the copper complex of Example 6 dissolved in cyclohexane or diethyl ether was brush painted onto a substrate, which was heated to 210°C for 1 hour in hydrogen. A continuous copper coating was left on the substrate, and the liberated ligand could be recycled to make further paint. EXAMPLE 8: In another application, the copper complex of Example 6 in solution was jetted, through an atomising nozzle, as a fine spray into a chamber containing hydrogen at 250°C. Copper powder was recovered from the chamber. The liberated ligand could be recycled. Care must be taken to avoid an undue proportion of the complex from decomposing on the chamber walls and simply plating them, for example a cyclonic gas flow path may be established within the chamber so that the complex does not contact the chamber wal1. A mixture of such compounds may be used, in the same or separate sprays, to yield a mixture of liberated metals, in precalculated volumetric proportions of liquid to yield the metals in the desired ratios. Preferably the metal is one or more of a mixture of copper and nickel . A mixture of such metal powders may permit alloys to be made by pressures-sintering which would otherwise be unobtainable or only obtainable by extraordinary techniques such as implantation by nuclear bombardment.

Claims: CLAIMS 1. A method of forming a mixed-metals coating on a surface, comprising decomposing at the surface a fluid comprising: a first compound between a first metal and a polydentate reagent, and a second compound between a second metal and the same or another polydentate reagent, wherein the reagent(s) is/are volatile, whereby the metals are co-deposited on the surface. 2. A method according to Claim 1, wherein the said second metal is the surface and wherein the said second compound is formed from reagent liberated upon decomposition of the first compound. 3. A method according to either preceding claim, wherein the decomposition is by reduction. 4. A method according to Claim 3 wherein the reduction is performed by a gas or gases. 5. A method according to any preceding claim, wherein the donor atoms of the said reagent are any selection from N, 0 and S. 6. A method according to anypreceding claim, wherein the said reagent (or one of said reagents) is a Schiff base reagent or a β-diketone. 7. A method according to any preceding claim, wherein one of the metals is copper. 8. A method according to any preceding claim, wherein one of the metals 1s nickel. 9. A method according to any preceding claim, wherein said first compound is volatile. 10. A method according to any preceding claim, wherein said second compound is volatile. 11. A method according to any preceding claim, wherein said fluid is a vapour. 12. A method according to any of Claims 1-8, wherein said fluid is a liquid. 13. A method according to any preceding claim, wherein the surface is of metal, glass or ceramic or is a membrane. 14. A metallic paint, comprising a compound between the metal or each of the metals of the paint and a polydentate reagent, said compound(s) being dissolved in a paintable solvent. 15. A paint according to Claim 14, wherein the donor atoms of said reagent(s) are any selection of N, 0 and S. 16. A method of painting an object, comprising applying thereto a paint according to any preceding claim, allowing the solvent to evaporate, and applying a gaseous or volatile substance which can decompose said compound(s) to release said metal (s). 17. A method of painting according to Claim 16, wherein said compound or at least one of said compounds is dissolved in said solvent after the object has been placed in the solvent. 18. A method of making metal powder, comprising atomising a paint according to " Claim 14, and applying a gaseous substance which decomposes said compound(s) to release said metal(s).

Inventors: COX MICHAEL, JOSHI RAJENDRA, COX, MICHAEL, JOSHI, RAJENDRA
Applicants: NAT RES DEV, NATIONAL RESEARCH DEVELOPMENT CORPORATION
Application year: 1990
Main classification: C23C18
IPC codes: B22F9, C23C16, C23C18