Application of various coating technologies and methods

Application of various coating technologies and methods

Chemical vapor deposition is to supply one or several compounds and simple gases which constitute the coating elements to the substrate, and generate the required film by the gas phase action or chemical reaction on the surface of the substrate. The CVD reaction mechanism is that the reactant reaches the surface of substrate first, then the reactant gas molecules are adsorbed on the surface of substrate, and generate chemical reaction and nucleation on the surface of substrate, and the product diffuses to form film on the surface of substrate.

Ordinary gas molecules are electrically neutral substances that show insulation. However, in the ionized state, gas molecules produce a large number of electrons and positive and negative ions, which become conductive ionized gas. In the ionized gas, the positive and negative nuclei interact with each other violently. As a result, the positive and negative charge densities are almost the same, and become the neutral state, which is called plasma gas. When the electric field is applied to the gas in the low-pressure vessel, a few electrons in the gas are accelerated. Under the low-pressure condition, the average free path is large, and it is easy to be added to the high speed, collide with the neutral atoms or molecules, and lose energy. If the collision is elastic, the kinetic energy of the neutral atom or molecule will increase and the gas temperature will rise; if the collision is inelastic, the atom and molecule will be intensified, dissociated and ionized, resulting in many different ions and atomic clusters. These particles are all chemically active and can generate new particles and participate in the reaction. The new electrons are accelerated, collided with other atoms and molecules, repeated, and the gas is ionized rapidly, forming a plasma state.

The principle that plasma gas can promote chemical reaction is that after the raw gas becomes plasma state, it becomes chemically very active excited state molecules, atoms, ions and atomic groups, which makes the chemical reaction with slow reaction speed easier and faster. Therefore, PECVD can be used to produce good films at a lower temperature, with less heat loss, which can inhibit the change of substrate and form films on non heat resistant substrate. From the thermodynamics point of view, in the case that the reaction can occur or may occur but the speed is slow, the reaction can be promoted by means of the plasma excited state, so that new materials with composition ratio structure that can not be formed in the equilibrium state can be developed.

In order to compare the barrier properties of different oxide coating materials, we explored the influence of various coating technology methods on the final product quality in the laboratory, and found that under the same evaporation condition, different materials and different evaporation methods have the most obvious influence on the barrier properties of non-metallic coating microwave food packaging materials. Composite evaporated SiO coating has excellent barrier performance, but the coating is not stable in the atmospheric environment. According to the principle that all free chemical reactions are in the direction of Gibbs free energy reduction, SiO tends to produce SiO2 with lower free energy. The factors that affect the barrier performance of non-metallic vacuum coating materials include the surface treatment of substrate, vacuum degree, evaporation speed, etc., but the most important one is the raw materials and methods of evaporation, which have the same regularity.

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