PVD vs. CVD: What’s the difference?
In recent years, physical vapor deposition (PVD) and chemical vapor deposition (PVD) have wide applications in various industries to increase the hardness of tools and molds or apply beautiful colors to the products. Thus these two methods are considered as the most attractive surface coating technologies. Then, using the example of cutting tools, let’s make a detailed comparison between these two methods.
Physical vapor deposition (PVD) uses low-voltage, high-current arc discharge technology under vacuum conditions to evaporate the target and ionize the vaporized material and the gas, and finally make the evaporated material and its reaction deposited on the workpiece.
Chemical vapor deposition (CVD) refers to a process in which a vapor, containing a gaseous reactant or a liquid reactant constituting a thin film element, and other gases required for the reaction are introduced into a reaction chamber to chemically react on the surface of the substrate to form a thin film.
From their definitions, we can clearly see the difference of PVD and CVD that the former involves evaporation of the material from gas phase to solid phase, while the latter includes chemical reaction. And that’s how they got their names—physical and chemical.
Process and Equipment
I put temperature in the first place because it is the main difference between PVD and CVD. The process temperature of CVD method is very high, usually exceeds the tempering temperature of the high speed steel. Thus in order to restore the hardness, the tools must be subjected to vacuum heat treatment after coating.
The cleaning requirements of PVD process for the tool entering the reactor are higher than that of CVD.
The CVD coating (about 7.5 μm) on the surface of the tool is thicker than the PVD coating (about 2.5 μm). So it is also a factor to be considered when you are choosing the coating method.
The PVD coating well reflects the surface of the tool and has a good metallic luster without grinding. However, the surface of the CVD coating is slightly rougher than the surface of the substrate.
The crafting process
Nearly all PVD technologies have poor coating performance both on the back and sides of the tool due to low air pressure. The PVD reactor must reduce the loading density to avoid shadow formation, and the loading and fixing are complicated. In a PVD reactor, the tool usually rotates continuously, and sometimes it needs to reciprocate while turning.
In contrast, CVD occurs in a low-vacuum gaseous environment and has good coating performance. Therefore, except for the support points, all the surfaces of the cutters sealed in the reactor, even deep holes and inner walls, can be completely coated.
The initial equipment investment of PVD is much higher–3 to 4 times of CVD, but the production cycle of PVD process is lower–1/10 of CVD. In addition, CVD can process a wide variety of workpieces, while PVD is greatly limited. In other words, PVD is sometimes more expensive than CVD.
PVD is a kind of “green engineering”, which produces little pollution during the process because it is “physical”. In contrast, the reactive gas and reaction tail gas of CVD may have certain corrosiveness, flammability and toxicity, and there may be powdery and fragmented substances in the reaction tail gas, so certain measures must be taken for equipment, environment and operators.
Both of physical vapor deposition and chemical vapor deposition have their own advantages. And you have to consider many factors while choosing the coating methods. For more information, please visit https://www.sputtertargets.net/.