21 - Hard thin films: Applications and challenges

Abstract

Thin films have high surface-to-volume ratios and behave differently from bulk materials of the same chemical composition in several different ways. Those films that are hard in terms of the traditional tribological connotation (i.e. anti-abrasive under contact sliding) as well as those that are anti-abrasive to operating conditions are characterized as hard thin films. This chapter will first present an overview of the production, properties, and application of three types of hard thin films: diamond and diamond-related, semi-conductor and metallic glass thin films. In the case of diamond/diamond-like/nanocrystalline diamond films, the discussion will be on their potential use as protective coatings in mechanical, optical, biomedical, thermal and electronic applications. For semiconductor thin films, the focus will be mainly on the pressure and temperature resistant AlN films and radiation resistant GaAs films with applications in micro-electromechanical systems (MEMS), light emitting diodes (LED), solar cells and terahertz wave emitters. With metallic glass thin films, the discussion will be on their unique properties such as ultrahigh strength and excellent elasticity as well as their characteristics that are not readily achievable with other thin films and their applications in micro- and nano-electromechanical systems and biomedical tools. The second part of the chapter will outline the challenges encountered as a result of residual stress, surface roughness, defects and delamination that prevent the realization of the abovementioned potential applications of hard thin films. Finally, the chapter will provide a summary of the present status and future prospects that are expected to enter the stage of practical application.