Manufacturing: Surface Finishing

Physical Vapor Deposition (PVD)

Physical Vapor Deposition (PVD) comprises a group of surface coating technologies used for decorative coating, tool coating, and other equipment coating applications. It is fundamentally a vaporization coating process in which the basic mechanism is an atom by atom transfer of material from the solid phase to the vapor phase and back to the solid phase, gradually building a film on the surface to be coated. In the case of reactive deposition, the depositing material reacts with a gaseous environment of co-deposited material to form a film of compound material, such as a nitride, oxide, carbide or carbonitride.

There are three basic process categories considered as PVD technologies: ion plating, evaporation, and sputtering. All utilize the same three fundamental steps to develop a coating. Each of the PVD technologies generate and deposit material in a somewhat different manner, requiring equipment unique to each process. The three fundamental steps include:

1. Vapor phase generation from coating material stock by -

• Evaporation
• Sputtering
• Arc Vaporization
• Chemical vapors and gases

2. The transfer of the vapor phase from source to substrate by -

• Line-of-sight
• Molecular flow
• Vapor ionization by creating a plasma

3. Deposition and film growth on the substrate

These steps can be independent or superimposed on each other depending on the desired coating characteristics. The final result of the coating/substrate composite is a function of each materials individual properties, the interaction of the materials and any process constraints that may exist.

The selection criteria for determining the best method of PVD is dependent on several factors;

1. The type of material to be deposited
2. Rate of deposition
3. Limitations imposed by the substrate, such as, the maximum deposition temperature, size and shape
4. Adhesion of the deposition to the substrate
5. Throwing power (rate and thickness distribution of the deposition process, i.e., the higher the throwing power, the better the process ability to coat irregularly-shaped objects with uniform thickness)
6. Purity of coating materials
7. Equipment requirements and their availability
8. Cost
9. Ecological considerations
10. Abundance of deposition material

PVD is a desirable alternative to electroplating and possibly some painting applications. PVD can be applied using a wide variety of materials to coat an equally diverse number of substrates using any of the three basic PVD technologies to deposit a number of desired finishes of variable thickness with specific characteristics.

The application of PVD surface coating technologies at large scale, high volume operations will result in the reduction of hazardous waste generated when compared to electroplating and other metal finishing processes that use large quantities of toxic and hazardous materials. PVD is a desirable alternative to electroplating and possibly some painting applications because it generates less hazardous waste and uses less hazardous materials (i.e., no plating baths).

Materials Compatibility: PVD coating processes are compatible with most metals and some plastics either as coatings or as substrates. However, temperature constraints may limit the degree to which dense coatings can be deposited on some plastics. Finally, PVD processes do not normally produce the kind of coatings that work well where lubrication is required. Thus, PVD coatings are not usually good choices for parts such as fasteners.

Advantages:

• PVD coatings are sometimes harder and more corrosion resistant than coatings applied by the electroplating process. Most coatings have high temperature and good impact strength, excellent abrasion resistance and are so durable that protective topcoats are almost never necessary.
• Ability to utilize virtually any type of inorganic and some organic coating materials on an equally diverse group of substrates and surfaces using a wide variety of finishes.
• More environmentally friendly than traditional coating processes such as electroplating and painting.
• More than one technique can be used to deposit a given film.

• Specific technologies can impose constraints; for example, line-of-sight transfer makes coating annular shapes practically impossible.
• Some PVD technologies typically operate at very high temperatures and vacuums, requiring special attention by operating personnel.
• Requires a cooling water system to dissipate large heat loads.
• Selection of the best PVD technology may require some experience and/or experimentation.
• High capital costs.

Economic Analysis: Economic considerations are probably the primary hindrance preventing conversion of plating operations to vapor deposition processes. A rough estimate of the capital cost for a new vapor deposition installation is several hundred thousand dollars. Operating costs are roughly equal to electroplating, although plating can be slightly less labor intensive.