Vacuum is an environment where the gas pressure is less than the ambient. A plasma is a gaseous environment in which there are enough ions and electrons for there to be appreciable electrical conductivity. Vacuum coating is the deposition of a film or a coating in a vacuum (or low-pressure plasma) environment. Generally the term is applied to processes that deposit atoms (or molecules) one at a time such as physical vapor deposition (PVD) or low-pressure chemical vapor deposition (LP-CVD) processes or plasma-enhanced CVD (PECVD). In PVD processes, the material being deposited comes from the vaporization of a solid or liquid surface. In CVD processes, the material being deposited comes from a chemical vapor precursor species that is decomposed by reduction or thermal decomposition—mostly on a hot surface.
In some cases the material being deposited reacts with the gaseous environment or a codeposited species to form a film of a compound material such as an oxide, a nitride, carbide, or a carbonitride. In CVD processing, the use of a plasma to fragment the chemical vapor precursor in the vapor phase allows the decomposition or reduction processes to proceed at lower temperatures than with thermal activation alone. PECVD can be performed at pressures as low as those used in PVD processing (low-pressure PECVD, LP-PECVD), where the precursor vapor is decomposed mainly in the plasma. In some cases a hybrid deposition process of PVD and LP-PECVD is used to deposit alloys, composites, or compounds. An example is metal carbonitrides where the carbon comes from a chemical vapor precursor such as acetylene; the nitrogen comes from a gas; and the metal from evaporation, sputtering, or arc vaporization of a solid or liquid surface.
Decorative and Decorative/Wear Coatings
Metallization for strictly decorative purposes is a large market. Applications vary from coating polymer webs—which are then converted to decorative uses such as balloons and labels—to metallization of three-dimensional articles, such as sports trophies, zinc die cast and molded polymer decorative fixtures, and cosmetic containers. Often these coatings consist of a reflective aluminum coating that is deposited on a smooth base coat, then over coated with a dyed lacquer to give the coating the desired color and texture and also corrosion and wear resistance.
In some applications, in addition to the decorative aspects of the coating, the coating is required to withstand wear. For example, titanium nitride (TiN) is gold colored, and titanium carbonitride (TiCxNy) can vary in color from gold to purple to black depending on the composition. Zirconium nitride (ZrN) has the color of brass and is much more wear and scratch resistant than brass. Decorative/wear coatings are used on door hardware, plumbing fixtures, fashion items, marine hardware, and other such applications.
Hard and Wear-Resistant Coatings
Hard coatings are often called metallurgical coatings and are a type of tribological coating. The hard coatings are used to increase the cutting efficiency and operational life of cutting tools and to maintain the dimensional tolerances of components used in applications where wear can occur, such as injection molds. In addition, the coatings can act as a diffusion barrier where high temperatures are generated by motion between surfaces or corrosion protection in aggressive environments. There are various classes of hard coating materials. They include: ionically bonded metal oxides (Al2O3, ZrO2, and TiO2), covalently bonded materials (SiC, boron carbon [B4C], diamond, diamond-like-carbon [DLC], TiC, AlN, CrC, mixed carbide, nitride and carbonitride compound alloys, and cubic boron nitride), and some metal alloys (cobalt chromium aluminum yttrium [CoCrAlY], NiAl, NiCrBSi). In some cases the coatings may be layered to combine properties.
Hard coatings also are used to minimize fatigue-wear, such as is found in ball bearings. Wear-resistant coatings also may be applied to surfaces where there is a light or periodic load. For example, hard coatings are deposited on plastics to improve scratch resistance. Applications are on molded plastic lenses and plastic airplane canopies. In some cases wear coatings, such as SiO2 or Al2O3, may be applied to already hard surfaces, such as glass, to increase the scratch resistance.
Corrosion Protective Coatings
Protection from an aggressive chemical environment can be accomplished in several ways. The surface can be coated with an inert material or with a material that forms a protective surface after reacting with the environment or with a material that will be sacrificially removed to protect the underlying material. Tantalum, platinum, and carbon are inert in many chemical environments. For example, carbon coatings are used on metals that are implanted in the human body to provide compatibility. In the aerospace industry parts are aluminum coated by the PVD process of ion vapor deposition (IVD) so as to prevent galvanic corrosion of dissimilar materials in contact.
Chromium, aluminum, silicon, and the MCrAlY (where M is Ni, Co, Fe) alloys will react with oxygen to form a coherent protective oxide layer on the surface. If the metal ions (Fe, Cu) diffuse more rapidly than the oxygen through the oxide, a thick oxide will form on the surface. If the oxygen diffuses more rapidly through the oxide than the metal ions (Al, Si, Ti, Zr—the "valve" metals), oxidation will occur at the interface and a thin oxide will be formed. The MCrAlY alloy coatings are used as protective coatings on aircraft engine turbine blades. Cadmium, aluminum, and Al:Zn alloys are used as galvanic sacrificial coatings on steel. Vacuum cadmium ("vac cad") plating has the advantage over electroplated cadmium in that there is no possibility of hydrogen embrittlement of high-strength steel when vacuum deposition processing is used.