Decorative PVD coatings have been a super success in replacing the traditional metal plating used for coating luxurious and commercial goods.
In decorative PVD process the raw material savings are 1000% higher than the traditional metal plating process.
For example, sputtering of gold needs only 100nm thin layer on a coating substrate where in traditional plating process minimum gold thickness is 10 μm. This thickness is necessary to obtain a similar eye-perceivience for golden colour.
Furthermore, using of Titanium or Zirconium target with C2H2 and Nitrogen to obtain the gold colour reduces the costs up to 7000% - even more than the traditional gold plating methods.
Ionized Physical Vapour Deposition is the best and efficient technology for decorative coating applications. In Ionized PVD (either Arc or Magnetron) the electric field can only influence the energetic impact of depositing ions on the coating film. However, the profile of magnetic field is so critical as it modulates the trajectory of depositing ions to the coating substrate. Hence, the magnetic field has more say in achieving the colour uniformity than the electric field.
Is the sputter target or the arc cathodes playing a role
in non-uniformity ?
Most of the decorative PVD coating industries face problems with the processes for these following five colours especially, rose gold, blue, brown, purple, and bronze (light or dark). When there is a complaint concerning the colour uniformity from the PVD coaters, the first doubt falls on the target material, spontaneously. On checking the targets’ quality, we understood that the target erosion profile and the surface texture of target throughout the racetrack is uniform.
If the target isn’t causing the trouble, then what could be the reason ?
The cooling system ?
After checking the complete drawing of every coating equipment and holding discussions with the respective system architects of the PVD machine building companies, we cleared the cooling system also from any possible doubt.
If the target and cooling system aren’t causing the issue -
Let's check the impact of the magnetic field !
The theoretical estimation from COMSOL simulations and the experimental magnetic field measurements using a gauss meter help to determine the parallel and perpendicular magnetic profile over the target cathodes, accurately. Of course, it is the magnetic field that differentiate the magnetron sputtering from the conventional diode sputtering, and that is why the accurate magnetic field profile is so critical in the coating processes.
The magnetic field controls the residence time and the lifetime of electrons in the cathode sheath and subsequently influences the energy efficiency of the discharge either in magnetron sputtering or in the cathodic arc.
Yes, the magnetic field has a very substantial influence on the uniformity of rose-gold colour.
However, apart from being influenced by the magnetic field profile of the machine, the other colours such as blue, brown, purple, and bronze are highly influenced by one more parameter – the uniformity of ion flux across the coating table.
What is the common item for all of these four colours?
target material Titanium or Zirconium
the reactive gases O2 and N2.
The interesting fact is the non-uniformity in the colour becomes less and less when reactive O2 gas flow is decreased, while the colours are becoming more non-uniform with increasing O2 gas supply.
We conclude from our experiments and analysis that the uniformity issues with blue, brown, purple, and bronze colours are almost common either in low or high degree with many coating machines except few models.
The thorough plasma diagnostics and monitoring was done using the optical emission spectroscopy from Gencoa Ltd and the quadrupole mass spectrometer attached to the machine via the appropriate flanges. The diagnosis at the different parts of the machine elucidates the major breakthrough to achieve the colour uniformity for these challenging blue, brown, purple, and bronze colours in the decorative PVD coating processes.
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