Multilayer coatings for improved performance

Although single-layer coatings are finding a range of applications in many sectors of engineering, there are an increasing number of applications where the properties of a single material are not sufficient. One way to surmount this problem is to use a multilayer coating that combines the attractive properties of several materials, each chosen to solve a problem in the application. Simple examples of this include the use of interfacial bonding layers to promote adhesion, or thin inert coatings on top of wear-resistant layers to reduce the corrosion of cutting tools. There is, however, mounting evidence that the multilayer structure produced when many alternating layers of two materials are deposited can lead to improvements in performance over a mixed coating (by virtue of the introduction of new interfaces) even if the two materials do not have specific functional requirements in the intended application. In this paper results for multilayer coatings in the Ti-N system will be presented showing the advantages of compositionally and structurally modulated coatings over conventional single-layer titanium nitride coatings in tribological applications. The results will be discussed in light of the microstructure and properties of the coatings produced by the two different multilayer coating techniques.