|Title of Invention||
MULTI-LAYER COMPOSITE COATING WITH ULTRA-THIN FILM LAMINATES FOR SECORATIVE ARTICLE
|Abstract||(1) A process for coating a thermally resistant decorative article by sputtering method with a composite coating consisting of the following: (a) a monolithic gold coloured undercoat layer from a group consisting of TiN, ZrN, TaN, HfN, VaN, TiCN and VCN; (b) an intermediate composite layer consisting of a stack or stacks of alternate ultra-thin layers of the under coal material from the group mentioned in (a) and a topcoat material selected from a group of gold and an alloy of gold and. (c) monolithic layer of gold/gold alloy as the top coat.|
MULTI - LAYER COMPOSITE COATING WITH ULTRA-THIN FILM LAMINATES FOR DECORATIVE ARTICLE
A thermally resistant non-precious metal article is coated with a base coat of monolithic TiN followed by a stack(s) of ultra-thin Au/TiN layers and finally with a top coat of gold / gold alloy layer. The multi-layer composite coating of ultra-thin Au/TiN stack(s) offers good wear and corrosion resistance to the article. In case of wear-off of gold topcoat, the ultra-thin Au / TiN stack offers good brilliance like gold and hence the top coat gold layer wear-off is less conspicuous.
MULTI-LAYER COMPOSITE COATING WITH ULTRA-THIN FILM LAMINATES FOR
Field of Invention:
The present invention relates to a process sequence of deposition of ultra-thin multilayer coatings of a hard refractory metal nitride and a precious metal/metal alloy to enhance the life endurance of the aesthetic look of a decorative article. The ultra-thin layers of the refractory metal nitride and the precious metal/metal alloy are deposited alternately in between a relatively thick refractory metal nitride under coat and a precious metal/metal alloy top coat by Physical Vapour Deposition (PVD) . The application of the present invention includes decorative/protective coatings on watchcases, watch bands, eye-glass frames and other similar personal items.
Background of the Invention:
Decorative coatings on consumer products such as watch cases and bands or eye-glass frames provide two important functions, viz. , enhancing the product appearance through their brilliant colours and offering good resistance to wear and corrosion.
Traditionally, electroplating had been the natural choice for providing various decorative coatings on electrically conducting and thermally resistant substrates. However, electroplating has the following disadvantages,
(1) Ever raising gold price and stricter anti-pollution measures taken against electroplating have in consequence rendered galvanic coatings uncompetitive.
(2) It is difficult to coat multi-element alloys and hard refractory metallic compounds, such as TiN, TiCN, etc.
(3) In general, decorative electroplating involves a "barrier" undercoat of nickel (Ni). When the top precious metal/metal alloy coating is worn-off or punctured due to sharp edges or impact, the nickel layer is exposed and will come in contact with skin of the user. This is prone to give rise to skin allergy. Such products become unacceptable to the customers.
A consumer article such as wristwatch is akin to Jewellery since it is worn on the hand of a person. It would therefore be subjected to harsh user conditions in the following ways:
a) It may be used continuously by the user and hence the watch must be capable of being used for 365 days a year and 24 hours a day.
b) It is subjected to constant movements, jerks, shocks, swiveling motion and rotation of hand.
c) In the case of a watch with metallic bracelet, each link of the bracelet undergoes a constant movement with possible rubbing of the link element with one another leading to attrition of the links or rupture of the coating.
d) The watch and bracelet is constantly subjected to the body sweat of the wearer. The styling of a watch and a bracelet, where the designer calls for intricate shapes and crevices results in a very large scope for the sweat to accumulate. This coagulates with dust and saline water to cause corrosion on the film/substrate composite due to galvanic effect leading to eventual failure of the layers.
It is therefore essential to provide a hard, wear resistant and "Body Compatible" thin film coating on the watch components.
The PVD process has a distinct advantage of depositing hard coatings of nitrides, carbides, carbo-nitrides and almost any metal or its alloys in a clean vacuum atmosphere on any solid substrate. This new environment-friendly technology provides the highest standard of coating quality, reproducibility, and a wide variety of colours with brilliant and shiny finish. Also it offers good uniformity in colour over larger area of deposition and reliability in providing the same colour tone consistently.
TIN film deposited by PVD techniques is universally employed for decorative coating applications because of its gold-like aesthetic appeal in combination with a high degree of wear and corrosion resistance. Though TiN exhibits gold yellow colour, it can not be substituted for gold/gold alloy in decorative coating field as the brilliance of TiN is less compared to gold. Therefore, an additional topcoat of gold is required to meet the necessary brilliance. The brilliance of the gold coating depends on the coating thickness. A minimum thickness of 0.1 micrometer of gold is required to have brilliance like that of a solid gold.
The physical vapour deposited (TiN + gold/gold alloy) shows an extremely good wear resistance as compared to gold. It has been reported that a typical film of (0.5 micrometer TiN + 0.1 micrometer gold) will have at least the same wear resistance as a 5 micrometer electroplated gold coating.
Gold is a soft metal and hence it is vulnerable to easy wear-off compared to hard materials. During the service life of a decorative article, the gold topcoat on it may wear-off faster as a result of various wears mechanisms, notably on the sharp ridges. In the regions where the gold layer is removed, the TiN undercoat is exposed. As the colour of TiN is less brilliant yellow colour with greenish tinge, the gold layer removal becomes more apparently visible and exhibits an optical contrast which would be readily noticeable by the user. Thus the wear of the top gold layer reduces the dazzling appeal of the decorative article.
In the present invention, a new deposition process sequence has een developed which reduces the inferior wear behavior of gold alloy ayer, enhances the decorum of the decorative article and avoids the lumsy exposure of dull TiN. Thus the present invention provides a onger endurance life to the aesthetic appearance of the decorative rticle against wear and corrosion.
'ascription of the invention:
It is possible to prepare tailor-made coatings with required iroperties by selection of suitable material(s), selection of a uitable deposition process and controlling the deposition parameters 'hich in turn control the growth process of the films.
It is known that ultra-thin films have different characteristics rhich are unusual to their parent materials or thick films. Also .aminate composites with ultra-thin films show better strengths and .oughness compared to as-sputtered monolithic metallic films.
In accordance with the present invention, a thermally resistant ion-precious metal article, such as a watch case or watch band is :oated with a relatively thick base coat of TiN film (the thickness .ies in the range of 0.3 micrometer to 1.5 micrometer) by reactive sputtering technique. A stack or stacks of ultra-thin films of Au/TiN ire deposited on the monolithic TiN coating. The thickness of the .ndividual ultra-thin films in an Au/TiN stack can be varied from J.005 to 0.05 micrometer. After the deposition of Au/TiN stack(s), a ;opcoat of gold/gold alloy layer of thickness ranging from 0.05 - 0.2 micrometer is deposited.
The deposition of TiN and gold alloy are effected through a sputter ion plating equipment by employing two or more number of cathodes in a single vacuum chamber. This enables to deposit 'multilayer" coatings or stack(s) of ultra-thin layers of different :oating materials.
The alternate ultra-thin layers of Au and TiN in an Au/TiN stack nay be continuous or non-continuous with islands and empty channels. The average thickness at which a continuous film forms depends on the coating material, substrate material and deposition parameters which influence the surface mobility of the adatoms. Since the laminate composite may contain micro or macro pores, an undercoat of dense and acre-free monolithic TiN coating is very much essential for a decorative article such as a watch case or a watch band as the article is always in direct contact with the user's skin. In the absence of TiN undercoat, the human sweat can cause galvanic corrosion on the plated decorative article. Also, by virtue of its high hardness, the base TiN layer provides good support to the applied load to the composite and hence offers a good wear resistance.
Apart from micro/macro pores, ultra-thin films usually have high ntrinsic stress, high point defect concentration, extremely fine [rain size and oriented crystallite leading to the formation of mique microstructure with large surface to volume ratio.
Because of high surface to volume ratio, the pinning of Lislocation by the free surface leads to the high tensile strengths md hence high hardness. As a result, ultra-thin films exhibit better lechanical properties and tribological behavior.
It has been observed in many cases that the mechanical strength lecreases as the film thickness increases from a range of ipproximately 0.02 - 0.04 micrometer to a range about 0.2 - 0.4 licrometer. At higher thickness range, the strength is about the same as that of heavily worked base material. Thus an ultra-thin film of even a soft material such as gold would exhibit better mechanical ind tribological performance compared to thick monolithic gold film.
The individual ultra-thin films in an Au/TiN stack synergically help each other. As ill tra thin f i 1ms are semi - transparent, they exhibit the colour of the undercoat partially. Hence a stack of Au/TiN ultra-thin films would exhibit a far better brilliance compared to TiN as result of high brilliance of gold ultra-thin film. [n Au/TiN ultra-thin film composite, the function of the "TiN" layer is several fold; it supports the applied load and restricts the plastic deformation by either inhibiting dislocation sources from becoming operative or restricting slip transfer across the adjacent gold metallic layers. Hence Au /TiN ultra-thin film composite exhibit higher hardness and elastic modulus. Usually ultra-thin film composite of a metal/metal nitride shows an increase in wear [resistance with decreasing layer thickness.
The final topcoat of gold/gold alloy is deposited to obtain the colour and brilliance that of a solid gold. Such a top gold coating on a stack or stacks of ultra-thin films of TiN and gold is expected to have a longer durable life compared to electroplated gold or gold alloy. During the service life of the decorative article, if the gold topcoat wears off gradually, the underneath Au/TiN stack offers good brilliance like gold and the removal of the top gold layer appears less conspicuous. Also Au/TiN ultra-thin film composite exhibits better tribological performance compared to monolithic gold film and hence offers a longer endurance life to the decorative article.
The new process in accordance with the present invention is expected to be more economical than the substantially thicker electroplated gold coating having the same visual appearance and wear resistance performance because of the fact that this includes less gold.
Co-deposition of titanium and gold in a nitrogen atmosphere produces films that consists of a mixture of TiN and gold. These films exhibit brilliances that are roughly equal to the weighted averages of the brilliances of gold and TiN with the weighting
factors proportional to the relative amounts of gold and titanium present in the films. Hence high brilliances can only be achieved by using high gold contents, making these types of coatings uneconomical.
In order to further explain the present invention, a few examples are described below.
A watch case of stainless steel material is ultrasonically cleaned in alkaline solution, rinsed in de-mineralised water, dipped in diluted acid, rinsed in de-mineralised water and dried in a hot oven. The dried watch case is loaded onto the substrate carrier of the vacuum system. The vacuum chamber has two pairs of facing target cathodes (Fig. 1} so that the watchcase arranged in the middle between the cathodes is equally coated from both sides.
One pair of facing target cathode arrangements is loaded with titanium targets and the other pair has targets of gold alloy comprising 98.3 wt% of gold and 1.7 wt% of Ni.
Evacuation of the vacuum chamber is accomplished by a turbo-molecular pump backed by a combination of roots pump and rotary pump. A base vacuum of the order of several times 10~5 mbar is obtained in 20 min. duration. During the final stage of the vacuum pumping, substrates are heated using a resistance heater to enhance the degassing.
The vacuum chamber is evacuated to a base vacuum of 5xl0~5mbar. The substrates are further cleaned by sputter etching technique by applying a negative D.C. potential of 1400 V until all the contaminants are removed. Cleaning of titanium targets are carried out by placing a grid in between the titanium cathodes and sputtering both the cathodes at 8 kW at a argon flow rate of 300 seem.
A gas discharge created by applying a high negative voltage to the opposing cathodes generates plasma concentrated around negatively biased parts to be coated. As a result of the sputtering action of the positive ions in the plasma, the target material is removed and condenses on the substrates. Ion bombardment of the growing film is accomplished by applying a bias of 180 V to the substrate carrier. A metallic binding layer of 0.05 micrometer of titanium is deposited on the substrates. Now nitrogen gas is admitted into the chamber gradually to reach a flow rate of 80 seem. As a result of reactive sputtering TiN gets deposited onto the substrates. This process goes on until a thickness of 0.5 micrometer of TiN deposited. Then the titanium cathodes are disabled and the nitrogen gas admittance is stopped. Gold targets are enabled to deposit an ultra-thin layer of 0 .025 micrometer and then disabled. Again nitrogen gas is admitted into the chamber and TiN film of 0.02 micrometer is sputtered onto the gold film. If required, a number of stacks of Au/TiN can be
deposited. Generally 2 to 3 stacks of Au/TiN are found to be adequate. Finally a topcoat of gold layer of o.l to 0.2 micrometer thickness is deposited.
Spectral reflectance studies were carried out to understand the effect of ultra-thin film composite coating of Au/TiN, stacked between a relatively thick TiN undercoat and gold topcoat (Fig,2), in the visible light wavelength region. From the Fig. 3, it can be seen that even without the top gold coat, the brilliance of the composite upto Au/TiN ultra-thin films stack is far better than that of a thick monolithic TiN film. Thus in the present invention, even if the top gold coating fails prematurely, the underlying stack of ultra-thin multi-layer coating is exposed. Since the brilliance of the stack is closer to gold, the failure of the top gold coating is not markedly seen. Also such a stack is expected to have a superior wear resistance compared to gold, hence it offers a longer durable life to the decorative article.
A watch case of brass material is electroplated with 5 micrometer Ni layer and 1 micrometer Pd/Ni layer. The electroplated watch case is then cleaned as mentioned in example 1. After the precleaning, the watch case is loaded into the coating chamber and the deposition is carried out as mentioned in example 1. In this example, the titanium targets are replaced with zirconium targets and hence, the ZrN is deposited wherever TiN was deposited in example 1.
(1) A process for coating a thermally resistant decorative article
with a composite coating consisting of the following:
(a) a monolithic gold coloured undercoat layer from a group consisting of TiN, ZrN, TaN, HfN, VaN, TiCN and VCN.
(b) an intermediate composite layer consisting of a stack or stacks of alternate ultra-thin layers of the under coat material from the group mentioned in (a) and a topcoat material selected from a group of gold and an alloy of gold.
(c) a monolithic layer of gold/gold alloy as the top coat.
(2) An article according to claim 1, in which the base material of the decorative article comprises atleast one material selected from the group containing steel, brass, German silver, Die cast zinc, aluminum or monel.
(3) An article according to claim 1, in which the gold coloured undercoat has a thickness range of 0.4 to 1.0 micrometer
(4) An article according to claim 1, in which the thickness of the ultra-thin layer of the under coat material or the top coat
(1) A process for coating a thermally resistant decorative article by sputtering
method with a composite coating consisting of the following:
(a) a monolithic gold coloured undercoat layer from a group consisting of TiN,
ZrN, TaN, HfN, VaN, TiCN and VCN;
(b) an intermediate composite layer consisting of a stack or stacks of
alternate ultra-thin layers of the under coal material from the group
mentioned in (a) and a topcoat material selected from a group of gold and
an alloy of gold and.
(c) monolithic layer of gold/gold alloy as the top coat.
(2) A process according to claim 1, in which the base material of the decorative
article comprises atleast one material selected from the group containing
steel, brass, German stiver, Die cast zinc, aluminum or monel.
(3) A process according to claim 1, in which the gold coloured undercoat has a
thickness range of 0.3 to 1.5 micrometer.
(4) A process according to claim 1, in which the thickness of the ultra-thin layer of the under coat material or the topcoat material in the intermediate composite layer is in the range of 0.005 to 0.05 micrometer.
(5) A process according to claim 1, in which the topcoat is either gold or gold alloy comprising atleast one of the elements Al, Ni, Pd, Zn, Sn, Co, Ti, Cd, Ga and with a layer thickness range of 0.05 to 0 2 micrometer.
(6) A process according to claim 1, in which the decorative article is a part of a
wrist watch or spectacle frame or writing implement or a Jewellery item.
(7) A process according to claim 1, which is carried out by any other Physical
Vapour Deposition (PVD) methods, other than sputtering method.
|Indian Patent Application Number||1705/MAS/1996|
|PG Journal Number||20/2006|
|Date of Filing||26-Sep-1996|
|Name of Patentee||TITAN INDUSTRIES LIMITED|
|Applicant Address||3, SIPCOT INDUSTRIAL COMPLEX, HOSUR 635126,|
|PCT International Classification Number||C23C14/34|
|PCT International Application Number||N/A|
|PCT International Filing date|