Mechanical behavior of ceramic/SAM bilayer coatings

Ceramic/self-assembled monolayer (SAM) bilayer films can provide adequate protection and/or act as a multipurpose coating for microelectronics and MEMS applications, due to synergistic effects of the hybrid film structure. The organic SAM acts as a "template" for the growth of the ceramic film while the hard ceramic can provide protection from environmental and mechanical impact. To process the bilayer films, a low-temperature solution deposition technique (biomimetic process) is employed using phosphonate-based SAM and zirconium oxide precursors. A particulate zirconium oxide film is formed by enhanced hydrolysis of zirconium sulfate solutions in the presence of HCl at about 80°C, and its particle size and thickness effects are discussed. In addition, microstructure and micromechanics involved in the synthesis of both zirconia films and SAM are systematically assessed. Especially, mechanical properties such as Young's modulus and hardness are analyzed using a nanoindenter, as well as with the aid of theoretical models. Further, the substrate effects resulting from a large indentation depth relative to the film thickness are eliminated to obtain the "film-only" properties. This study also highlights the role of compliant SAM layer in forming a strain-tolerant bilayer film.