High-energy particles are involved in the coating. On the one hand, they can make the film dense, form a good transition layer with the substrate, and have the advantage of increasing the adhesion of the film. On the other hand, the internal stress of the film is increased and resulting the film breakage. This is particularly evident when the DLC film is plated. There are many factors that cause stress release phenomenon, and it is also more complicated. The most important and direct factors are the surface state of the substrate, the thermal conductivity of the substrate, the substrate temperature, the deposition rate and the thickness of the film.
With the increase of substrate temperature, the adhesion of film increases. The DLC film is plated by the pulsed arc ion plating process, which is usually deposited at a low temperature, and the film layer presented tensile stress. When deposited at high temperatures, the density of the film increases and compressive stress appears. At this time, the film is damaged and the adhesion rapidly decreases. Therefore, there is an upper limit for the increase of substrate temperature during the plating process.
There is a different optimum deposition rate threshold for each substrate. In this threshold, the adhesion is the best; below this value, the film is slightly loose and the adhesion is slightly poor and above this value, the internal stress in the film layer increases sharply, and the adhesion is the worst. In severe cases, the film layer will have an overall peeling phenomenon.
The DLC film was coated by pulsed arc ion plating. The internal stress increased correspondingly with the increase of film thickness. Corresponding to different substrate materials and different plating processes, the maximum thickness has a threshold value. Within this threshold, the adhesion is very good, exceeding this threshold, the adhesion is extremely poor, and the film is falling off.
When the DLC film is plated by the pulsed arc ion plating method, different plating processes are selected according to different substrates materials and different film thicknesses, otherwise as much as possible to improve the substrate surface finish and cleanliness; reasonable choice of substrate temperature and deposition rate. When the substrate material has good thermal conductivity and thin film layer, the higher deposition rate can be selected. On the contrary, the deposition rate needs to be reduced to ensure a high degree of firmness between the film and the substrate.