Preparation before PVD Evaporation
1. Preparation before PVD Evaporation
This process concludes cleaning plating parts, selecting membrane material, cleaning vacuum chamber and plating fixture, as well as installing evaporation sources and plating parts.
- Chemical Degreasing
Chemical degreasing is employing hot alkali solution to remove the surface oil of the parts, since the surface oil will have saponification and emulsification reactions with alkali solution.
- Electrostatic Dust Removal
During the shaping of the plating parts, they are likely to carry static electricity, which will reduce the binding force of the coating film and lead to pinholes. Electrostatic dust removal is a method of dust collection by using electrostatic field to ionize the dust particles to the electrodes.
- Painting Primer
Generally the thickness of the coating film is 0.05~0.1um. However, the surface roughness of the plating parts can reach 0.5um. In order to reduce the surface roughness, the plating parts should be painted with a special primer of 7~10um.
(1) Cleaning of Plating Parts
The surface binding energy between coating and the plating part is an important index. It can be influenced by many factors. If the surface of the plating parts has grease, water, or dust, the binding energy will be different, influencing roughness of the surfaces. Therefore, the surface cleaning is one basic step before coating.
(2) Selection of Membrane Material
According to the application requirements of the product and the material type of the plating, it’s essential to choose appropriate evaporation material for producing high quality film. When selecting the membrane materials, there are several principles. Membrane materials should have good thermal and chemical stability, high mechanical strength, and low internal stress. Besides, good compatibility with primer, high reflectivity, various sources with low price are also qualities that membrane material should have.
(3) Cleaning Vacuum Chamber and Plating Fixture
The vacuum chambers with enclosures can be taken apart to be cleaned. The ones without enclosures can be first cleaned with calcium carbonate and then wiped with water, at last with anhydrous ethanol. Common aluminum fixtures can be put into 20% NaOH solution, then wiped with water, and soaked in HNO3 until its brown color fades.
(4) Installing Evaporation Sources
Pay attention to wearing degreasing gloves and degrease the tools in advance. Be careful that the evaporation source and the electrode have good connection.
(5) Placement of Plating
The plating parts should be firmly placed on plating fixtures in case the plating will run away due to whirling during the evaporation. The workers must wear degreasing gloves, and keep the plating fixtures clean.
The vacuuming steps of evaporation coating can be listed as bellow. First, open the cooling water valve, adjust the valve to suitable hydraulic pressure, turn on total power supply, close the air valve to the vacuum chamber, close the pipe valve, start the mechanical pump power, and open the pre-vacuum valve. Second, evacuate the chamber with mechanical pump, energize diffusion pump heating power, close the pre-vacuum valve, and close the vacuum valve when diffusion pump reach the requirements of working. Last, diffusion pump and mechanical pump evacuate the chamber. When the vacuum degree reaches a certain value, later processes can continue, like baking, pre-melting and evaporation.
3. Ion Bombardment
During glow discharge, ions reach high speeds after bombarding electrons. Because the electrons have high mobility, they become negatively charged quickly around the plating. Under attraction of the negative charge, positive ions bombard the surface of the plating with energy exchanges. Then there are chemical reactions between the absorbed layer and the active gas, which realizing the cleaning effects.
This procedure can accelerate spilling the gas absorbed by plating and fixture. Besides, the vacuum degree and film binding force can be improved. During heating, non-metallic baking temperature should be lower than the hot deformation temperature of plating. While the heating temperature for metal is higher over 392℉.
Pre-melting can remove the impurities with low melting points from the evaporation materials as well as the air absorbed by evaporation sources. This procedure benefits the next procedure, evaporation. The vacuum degree of pre-melting is generally 6.6 × 10-3P. This process won’t stop until the materials are completely melted. Material with high hygroscopicity should be pre-melted repeatedly. In general, the vacuum degree should not go down after the evaporation material reach the evaporation temperature.
Therefore, different metals have different requirements of evaporation. For example, some metals need to be rapidly evaporated, which may not be suitable for other metals. The choosing of heating method and shape of the evaporation sources depend on the type of evaporation materials.