Future Trends

In the future, there are certain to be new applications for sputtering technology and new thin-film coatings tailored to special needs. Recently there has been a move away from simple binary alloy coatings into multicomponent coatings to provide increased film "tailorability." This trend can be observed both in microelectronics, with investigations into ternary and quaternary II-V and II-VI systems, and in the area of hard, wear-resistant coatings of TiAlN, TiZrN, and TiVN (Ref 86). For example, for enhanced corrosion resistance for turbine blades, (Ti,Al)N alloys appear very promising as a replacement for the current TiN coatings (Ref 85), especially because the high-temperature oxidation resistance of (Ti,Al)N coatings is also superior.

Multilayer coatings are another area in which there is likely to be continued development in the next decade. These materials are already beginning to see commercial use in optical and wear-resistant applications. Very recently, work involving polycrystalline metal-nitride superlattice materials has produced films with hardness values in excess of 5000 HV.

It is foreseeable that sputtered chromium nitride coatings will also see increased use, because CrN has mechanical properties and color much like those of traditional electrodeposited hard chrome coatings and, in many applications, provides superior performance. In the past its cost has been prohibitively high, but with growing concern over process safety and environmentally hazardous wastes, sputter deposition processes are becoming increasingly attractive. Hence, the CrN sputter deposition process is an excellent candidate to replace traditional electroplating or chemical vapor deposition processes, at least in high-performance applications of hard chrome.

Improvements in the thermal stability of hard coatings for temperatures up to 800 °C and beyond seem possible in the near future. Development of more stable high-temperature coatings would open up a whole range of new applications.

In the area of decorative coating, there is a demand for distinctive new colors to supplement the well-known golden and black color tones that are currently available.

Although sputtered thin-film materials will continue to be developed, there will be some notable changes in the techniques used to deposit these films. One approach currently under investigation is to combine specific positive aspects from various deposition techniques. For instance, it has been shown that during the etching phase of the arc deposition process, titanium ions penetrate into regions below the substrate surface, forming intermetallic-like compounds and developing a gradually decreasing titanium atom density with depth into the steel substrate (Ref 87). This effect is thought to strengthen the adhesion performance of arc ion-plated coatings. On the other hand, unbalanced magnetron sputtering offers the possibility of depositing dense but rather low-stress coatings (Ref 88). A combination of arc etching and unbalanced magnetron sputter deposition may open new avenues (Ref 89, 90).

Another combined process, sputtering and plasma nitriding, can be used to increase the life of sputter-coated tools. Ti05Al05N sputter coating of hobs has been shown to produce a sixfold improvement in tool life (Ref 91). After resharpening, however, the improvement decreased to a twofold increase in tool life when compared to the uncoated hobs. Instead of recoating the hobs by sputter deposition after each resharpening step, a simple plasma nitriding treatment was successful in restoring a fourfold increase in tool life. The Ti05Al05N sputter-deposited coatings did not deteriorate appreciably under repeated plasma nitriding.

Examination of these trends suggests that sputtering processes should continue to hold a strong position among the competing deposition techniques. The environmentally friendly nature of sputtering processes should also make them increasingly attractive in the future.

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