PVD Ion Plating Decorative Film

- Sep 10, 2019-

PVD ion plating decorative film


The development of PVD decorative coating technology has made rapid progress, ion plating decorative film has been widely used in watchcase, watchband, mobile phone case, glasses frame, golf clubs, accessories, architectural hardware, daily hardware and other products. Ion plating decorative film gives the surface of the product a metal texture and rich color, and improve wear resistance, corrosion resistance, extend the service life. With the increasing market of ion plating decorative film, the film performance requirements are getting higher and higher.


1. A new challenge to the process of ion plating decorative film


Stainless steel substrate will usually use polishing, wire-drawing, sandblasting, chemical etching, laser engraving, CNC machine engraving and other surface art treatment, some also use laser welding, water electroplating nickel chromium layer.


Polishing operations will leave a hard to remove hardened polish wax on the workpiece; Nylon wheel wire-drawing ditch will remain transparent polymer compounds not easy to find; Grinding wheel drawing may have residual abrasive particles and binders, or may produce oxide skin due to overheating of friction. Sandblasting will have ceramic sand, glass sand, alumina sand residue embedded in the surface; Chemical etching may have corrosion products remaining in corrosion pits or other non-corrosive surfaces used for corrosion protection; Laser engraving can produce high temperature oxide skin and ablative matrix debris residue. CNC machine carving will have high temperature oxide skin and cutting oil residue, and there will be substrate fragments pressed into the matrix; Laser welding can also lead to high temperature oxidation; Water electroplating chromium layer will generate complex oxides that are difficult to remove, and the incomplete cleaning of pollution caused by these treatments will lead to ignition in ion plating process, false adhesion of coating, film dropping, and uneven color.


In ion plating products, the art treatment is often not a single, but a variety of work combinations, which increases the difficulty of cleaning before plating. The traditional ultrasonic method of oil removal and wax removal is not suitable, so the cleaning process must be adjusted. For different pollution, mechanical cleaning, gas phase cleaning, vacuum heating and then cleaning, cathode electrolysis, anode electrolysis, or the use of targeted high-efficiency cleaning agent, etc. These comprehensive measures can achieve better results, but some pollution has not yet found ideal solutions. The recent application of plasma polishing method to pre-plating cleaning is a worthy of attention.


2. The ion plating decorative film products ,How to overcome local corrosion


Ion plating decorative film is mostly used transition metal (such as Ti, Zr, Cr) or its alloy of nitrogen, carbon, oxygen compounds, and amorphous carbon (diamond-like) film. Generally speaking, these compounds have high chemical inertia and good corrosion resistance. But in recent years, some decorative coating products, including watch, mobile phone shell, golf head, but failed to pass the artificial sweat corrosion test or neutral salt spray corrosion test, and caused people's attention!


In fact, this problem has existed for a long time, but has not been noticed. Corrosion of decorative coating products is not only the corrosion resistance of coating itself, but also the corrosion of decorative coating + substrate system. A lot of research work show that the ion plating decorative coating itself good corrosion resistance, it is in the metal plating, such as stainless steel substrate, uniform corrosion resistance improved for matrix as a whole, but decorative coating is very thin, inevitably there are surface defects, such as micro cracks, pores, pinhole, columnar grain boundary, arc deposition of macroscopic particles, etc. Corrosion media can pass through the channels formed by these defects, through the coating to the substrate.



Coating → corrosive medium → substrate consists of corrosion battery, coating and substrate become electrode corner, thus producing electrochemical corrosion. General coating chemical inertia is higher than the substrate, coating electrode potential is higher than the substrate, the substrate is oxidized and corroded. Local corrosion or pinhole corrosion occurs on coated products, even more serious than no coated products. Local corrosion has also become the basis for determining whether coating products are qualified. Market evaluation of the corrosion resistance of coating products, generally random sampling for artificial sweat corrosion test and salt spray corrosion test, if there is a local corrosion exceeds the standard judged batch of products unqualified. In view of the current decorative coating so thin (1 micron or so) and the existing ion coating technology level, coating pinhole rate is high, so the coating product corrosion test does not pass the risk is always there.


The key measures to improve the corrosion resistance of the system are to improve the coating compactness and eliminate the surface defects directly to the substrate. A variety of processes and methods have been studied: thickening the coating, adopting high-corrosion resistant matrix material, depositing the corrosion resistant dense transition layer, additional plasma etching in the middle of the coating, multi-layer structure, plasma nitriding pretreatment of the substrate, etc., the above PVD method can improve the local corrosion resistance of the system at present. Water electroplating transition layer, more reliable but not environmental protection. Finally, the non - PVD method of UV oil coating, which is controversial but effective.


3.The influence factors, countermeasures and plasma sheath effect on the uniformity of ionic plating decorative film are discussed


The uniformity of membrane mainly includes the uniformity of composition, structure and thickness. The process conditions for the uniformity of ion deposition film are the same composition (species and proportion), energy (including incidence Angle and matrix temperature) and quantity of deposition particles deposited on the unit area of the workpiece. Coating equipment structure and process conditions are by influencing indoor air plasma with cloth plated inhomogeneity effect ion coating non-uniformity, plus the workpiece loading is not reasonable and install fixture and artifacts or block each other between artifacts, eventually leading to be plated workpiece parts deposition on unit area is equal and uneven coating.


In order to ensure uniformity, the following factors should be taken into account:

(1) plasma in coating space should be uniform;
(2) influence of space electric field and magnetic field on plasma uniformity;
(3) the distribution of reaction gas should be uniform;
(4) compensation of uniformity by workpiece movement;
(5) avoid occlusion;
(6) influence of inverse sputtering rate and membrane growth stress;
(7) influence of plasma sheath, etc.


The plasma sheath effect is related to the geometry of the workpiece. The irregular geometry of the plasma sheath varies the incidence Angle and deposition amount of charged particles on the corresponding surface, which will inevitably affect the heterogeneity of the deposition:

(1) the two-dimensional rectangular plane sheath generates ion aggregation effect at edges and corners;
(2) the sheath layer near the concave corner is opposite and produces ion divergence effect.
(3) at the wedge edge, the plasma sheath bent along the edge of the wedge, and the sheath narrowed towards the blade tip. As a result, ions were attracted to the blade tip from a large range due to the sheath bending, and the deposition of ions increased. As the thickness of plasma sheath is uneven, the boundary deformation of the sheath results in continuous variations of ion incidence Angle, the chance of the incident Angle being symmetric along the edge tip (included Angle) is increasingly small.
(4) the sheath of small holes and narrow slits produces shielding effect;
(5) the plasma sheath boundary in the blind hole is equivalent to the action of divergent lens. The special effect of ion plating needs to be further studied by technicians.


4. The possibility of obtaining new color system by ion plating

After more than 20 years of efforts, ion plating decorative film can produce imitation of gold, silver, rose, gun black, dark black, coffee, brown, blue and other color series. Currently, for mass production, it is mainly used as products of Ti, Zr, Cr, TiAl, graphite target and synthesis of nitrogen, acetylene (carbon) and oxygen. Some people also use silicon and silane. The market is constantly raising new color requirements, but the use of a limited number of elements to obtain more stable color coating, there are always limitations. Some people are testing niobium, hafnium, various rare earth elements, there has been no major breakthrough.


The color of matter is a stimulus that is selectively absorbed by visible light and reflected back to the human eye. Selective absorption depends on the electron structure of the material and the corresponding electron level transition. Electronic structure is too complex, complex compounds are the influencing factors of the electronic structure and too much, still can't do the color of the specified computing or design a corresponding electronic structure material, natural cannot expect to find the corresponding target material and reaction gas; at the same time, now in the process of ion plating process the physical and chemical behavior of the plasma is not well understood, the multielement substances in the decomposition in the process of ion plating, ionization, diverse intermediate form, the valence state, the number of all kinds of particles, scale, energy and its distribution in space, How they are related to the electrical parameters of plasma discharge, the emission source (target) and the state of the reaction gas supply is not well understood, so it is difficult to control the color aberration with high precision, only the color aberration can be controlled in a certain range.


Of course, for most electronic products with visual appearance, the current technology is quite acceptable in the market. Because of too many factors and not deep understanding, the production of some colors is very unstable, to accumulate experience and deep understanding before we can get out of trouble. Some people went the other way and used the method of optical interference film, through the calculation and design of the interference film system, and alternately plated the multi-layer film system with two kinds of materials with high refractive index and low refractive index, so as to obtain the needed interference color. With the method of plating optical film to do decorative film, finally the top layer of wear resistant transparent protective film, so that it does not affect the interference color, so it may be difficult to obtain the red, green problem solved.


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