Chromium Plating

Abstract

A method of chromium coating a substrate is described, which method comprises: a) depositing, preferably electrodepositing, a metallic layer onto the substrate comprising chromium from a plating bath that includes trivalent chromium and no, or substantially no, hexavalent chromium; b) physical vapor depositing a layer comprising chromium cn the layer deposited from step a). In one embodiment, in step a), the layer is chromium and is electrodeposited from a plating bath that includes trivalent chromium and no, or substantially no, hexavalent chromium. The metallic layer deposited in step a) is preferably a metal or an alloy of two or more metals and may be selected from the group consisting of chromium, copper, nickel, cobalt, zinc, lead iron, palladium and tin and alloys thereof, e.g. copper tin, copper zinc, tin lead, tin nickel, tin cobalt, tin zinc, iron nickel, cobalt nickel, zinc nickel, zinc iron, zinc iron nickel, zinc iron cobalt or palladium nickel. Copper and nickel are especially preferred. The coating formed by the method has a blue sheen that is usually associated with chromium electrodeposited from hexavalent chromium baths without having to use a hexavalent chromium bath, which is highly toxic.

Description

TECHNICAL FIELD

The present invention relates to chromium plating of substrates, including metal and plastic substrates.

BACKGROUND ART

Chromium plating is a well known technique for protecting substrates and giving them an attractive appearance. It is known to electroplate both metallic and plastic substrates using an electroplating bath containing hexavalent chromium, typically chromic acid or chromates (H₂CrO₄ and CrO₄²⁻). Electroplated deposits from hexavalent plating baths provide a strong, abrasion-resistant layer that will protect the underlying substrate from corrosion and provide an attractive decorative finish. One feature that is seen to be particularly attractive is the “blue sheen” of the chromium layer that is electrodeposited from hexavalent chromium baths.

Hexavalent chromium, however, is believed to be toxic and carcinogenic and its use is being phased out. Accordingly, alternatives are being sought. One alternative that has been proposed is a bath based on trivalent chromium, which does not give rise to the same health problems as hexavalent chromium, see for example U.S. Pat. No. 5,415,763, U.S. Pat. No. 5,413,646 U.S. Pat. No. 524,326 and U.S. Pat. 5,196,109.

However, trivalent chromium electroplating baths are not stable and are difficult to control. Thus it is not easy to obtain a consistently smooth finish using a trivalent chromium bath; furthermore electrodeposits from trivalent chromium baths do not have the characteristic blue sheen of electrodeposits from hexavalent chromium baths, which is seen to be highly desirable.

Physical vapor deposition (often abbreviated “PVD”) is a well known technique for forming layers on substrates and involve the evaporation of the coating material by bombarding a target with a high energy source, such as a beam of electrons or ions. This beam dislodges atoms from the surface of the target thereby vaporising them. The vaporised atoms are transported from the target to the substrate, where they form a deposit on the substrate.

JP-63-230189 describes a cutter for a razor in which the outer part of the cutter blade has a nickel layer formed by electrodeposition from a nickel sulphamate bath, a layer of a phosphorus-containing alloy, e.g. Ni—P, and a layer of chromium less than 0.2 μm thick and formed by sputtering.

U.S. Pat. No. 4,906,533 describes a method of plating steel for use in making tin cans; the steel is electroplated with chromium, removing chromium oxide from the deposited layer and depositing a layer of aluminium by vacuum deposition.

CH 660278 describes a steel watch case that is electroplated with chromium and the a layer of TiC is applied by PVD.

JP 6-192853 and JP 3-197688 describe a method of applying a hardened surface to an object by electroplating it with chromium and applying a ceramic layer, e.g. TiN, by PVD.

JP 59-215483 describes a method of applying a hardened gold-coloured surface to an object by electroplating it with chromium and sputtering Ti onto in a nitrogen atmosphere to form a gold-coloured TiN layer.

DE-A-1514668 describes a method of making electrical contacts for semrconductors by depositing successively by PVD a Cr layer, a Cr and Ag layer and a layer of Ag alone; the layers can be thickened by electrodeposition.

GB 2327091 describes a method of applying a decorative and protective coating on an object by electroplating a layer of Cu, Ni or Cr, e.g. from a hexavalent chromium bath, pulse blow drying the electrodeposited layer and depositing a refractory metal (Zr, Ti, Hf or Zr/Ti) by PVD. The method can produce a brass colour.

DISCLOSURE OF INVENTION

It has now been found that coatings having a blue sheen can be obtained without using hexavalent chromium baths.

According to the present invention, there is provided a method of coating a substrate to provide a decorative and protective chromium deposit having a blue sheen without having to use hexavalent chromium electrodeposition, which method comprises:

• • a) depositing a metallic layer onto the substrate, and;
  • b) physical vapor depositing a layer comprising chromium on the layer deposited from step a).

Preferably in step a), the layer is chromium and is electrodeposited from a plating bath that includes trivalent chromium and no, or substantially no, hexavalent chromium.

The metallic layer deposited in step a) is preferably a metal or an alloy of two or more metals and may be selected from the group consisting of chromium, copper, nickel, cobalt, zinc, lead iron, palladium and tin and alloys thereof, e.g. copper tin, copper zinc, tin lead, tin nickel, tin cobalt, tin zinc, iron nickel, cobalt nickel, zinc nickel, zinc iron, zinc iron nickel, zinc iron cobalt or palladium nickel. Copper and nickel are especially preferred. Preferably the layer deposited in step a) contains at most only traces of non-metals that are common in the electroplating art, e.g. nickel deposits often contain sulphur.

The invention is primarily directed at decorative and protective coatings rather than coatings to provide hardness and wear resistance that are used in cutting and tooling applications and so the layers deposited on the substrate, including in step a), preferably do not include a hard ceramic, e.g. a nitride or phosphide, or a phosphorus containing alloy.

It has also been found that the method of the present invention provides a chromium coating that is highly effective in preventing corrosion of the substrate.

Preferably, the electrodeposited chromium layer (step a) has a thickness of 0.1 to 1 μm, e.g. 0.3 to 0.9 μm, such as 0.5 to 0.8 μm. Preferably the step a) layer is greater than 0.2 μm thick. The preferred thickness of the PVD layer is 0.01 to 2 μm, e.g. 0.05 to 1.5 μm, such as 0.1 to 1 ∞m.

Before the article is coated in step a), e.g. electroplated with chromium, it may be provided with one or more protective or barrier layers, e.g. a base layer of nickel, preferably formed by electrodeposition. If the substrate is plastic, its surface should be rendered conductive, which can be achieved using well-known techniques for electroplating plastic and other non-conductive substrates.

It is not necessary that the chromium PVD deposit is formed directly on the metal, e.g. chromium, deposited in step a) and an intermediate layer, e.g. of white metal such as nickel, may be laid down between the step a) layer and the PVD layer.

Both electrodeposition from trivalent chromium baths and PVD are well-known techniques and so their operation will be entirely apparent to the skilled person.

The substrate may be plastic. Electroplating of plastic substrates is well-known and involves subjecting the insulating plastic to a series of known pre-treatment steps that render the surface electrically conductive. These steps involve the non-electric deposition of a conductive material, e.g. nickel, and the deposition of a layer of nichrome (nickel-chromium alloy). The metallic layer may be thickened through electroplating of cupronickel, using the following process steps:

• • pre-treatment→acid cleaning water→cleaning→pyrophosphoric copper→water cleaning→acid cleaning→bright copper→water cleaning→activation→water cleaning→semi-bright nickel→bright nickel→seal nickel→water cleaning→drying

The plastic has thereby become conductive and is suitable for general electroplating.

BEST MODE FOR CARRYING OUT THE INVENTION

Although electrodeposition from a trivalent chromium bath and PVD are well-known individually, we have found that the following are the optimum conditions for achieving the required blue sheen to a chromium deposit using the present invention. However, these conditions are not intended to limit the claims.

A substrate may be pre-treated with one or more base layers, e.g. at least one nickel layer; in the case of a substrate made of plastic, an electrically conductive surface layer must be applied to the insulating plastic surface as described above. The substrate may be, for example, a plastic moulding, a zinc die cast, a cast or machined brass part or a machined steel part. The substrate is then subjected to trivalent chromium plating followed by PVD of chromium under the conditions described below:

Trivalent Chromium Plating
Bath composition:                trivalent chromium
Temperature:                     20° C.-50° C.
Bath Acidity (pH):               2.8-3.8
Current Density:                 5-20 A/dm2
Electroplating Time:             2-6 mins
Thickness of the electrodeposit: 0.1-1 μm

The resulting chromium electrodeposit 99.99% Metallic Chromium meets the following tests as a result of the trivalent chromium plating process:—

	(1)	Temperature test:	72° C.→ 25° C.→ −30° C. (3 cycles)
	(2)	CASS test:	8-16 HR

Physical Vapor Deposition (PVD)

Technique:                    Electric Arc Style Physical
                              Vapor Deposition
Material being deposited:     Metallic Chromium
Deposit Content:              99% Metallic Chromium
Deposition Temperature:       <70° C. (plastic substrate)
                              70-120° C. (Zinc die cast substrate)
                              120-180° C. (Brass, Steel or other
                              materials substrate)
Deposit thickness:            0.01-2 μm, preferably 0.1-0.5/μm
Pressure:                     2 × 10−5− 2 × 10−7 Torr
Colour:                       Silver-white
Process Steps:
Venting (2 × 10 Torr)         20-30 mins
glow discharge                5-10 mins
high bias                     1-3 mins
metal layer (99.99% Chromium) 1-100 mins

The resulting deposit had a blue sheen. The “L*” (black to white), “a*” (green to red), “b*” (blue to yellow) values of the colour measurement for the blue sheen effect of chromium are:

	i.	Measurement Instrument:	Spectrophotometer
	ii.	Setting of the Instrument:	D65/10, SCI
	iii.	L*	82.4 to 88.4
	iv.	a*	−1.5 to 0
	v.	b*	−2 to 0

Claims

1. A method of chromium coating a substrate to provide a chromium deposit having a blue sheen, which method comprises:

a) depositing a metallic layer on the substrate; and

b) physical vapor depositing a layer comprising chromium on the layer deposited from step a).

2. A method as claimed in claim 1 wherein step a) comprises electrodepositing a layer comprising chromium from a plating bath that includes trivalent chromium and no, or substantially no, hexavalent chromium.

3. A method as claimed in claim 1 wherein the metal deposited in step a) is a single metal or an alloy of two or more metals.

4. A method as claimed in claim 3, wherein the metal deposited in step a) is selected from the group consisting of chromium, copper, nickel, cobalt, zinc, lead iron, palladium and tin and copper tin, copper zinc, tin lead, tin nickel, tin cobalt, tin zinc, iron nickel, cobalt nickel, zinc nickel, zinc iron, zinc iron nickel, zinc iron cobalt or palladium nickel.

5. A method as claimed in claim 3, wherein the metal deposited in step a) is selected from chromium, copper and nickel.

6. A method as claimed in claim 1, wherein the metal deposited in step a) is electrodeposited.

7. A method as claimed in claim 1, wherein the metal deposited in step a) has a thickness of 0.05 to 1 μm.

8. A method as claimed in claim 7, wherein the metal deposited in step a) has a thickness of 0.3 to 0.9 μm.

9. A method as claimed in claim 8, wherein the metal deposited in step a) has a thickness of 0.5 to 0.8 μm.

10. A method as claimed in claim 1, wherein the vapor deposited chromium layer (step b) has a thickness of 0.05 to 1.5 μm.

11. A method as claimed in claim 11, wherein the vapor deposited chromium layer (step b)) has a thickness of 0.05 to 1 μm.

12. A method as claimed in claim 12, wherein the layers vapor deposited chromium layer (step b)) has a thickness of 0.1 to 1 μm.

13. A method as claimed in claim 1, wherein the layers formed in steps a) and b) consist substantially wholly of chromium and the layer formed in step a) is formed by electrodeposition.

14. A method as claimed in claim 1, wherein step b) deposits the chromium layer directly on top of the layer formed in step a).

15. A product having a chromium coating made by the method of claim 1.