Mechanical behavior of artificially multilayered thin films

Robert C. Cammarata

Department of Materials Science and Engineering
The Johns Hopkins University
Baltimore, MD  21218

Artificially multilayered thin films such as those used for x-ray mirrors
have shown unusual and often enhanced mechanical behavior when the bilayer
repeat length is reduced below about 10 nm.  In many cases, this unusual
behavior can be understood as owing to the high density of interfaces
characteristic of these materials.  Elastic modulus variations of order 10
to 50% have been observed in certain metallic systems.  This  behavior is
intriguing as elastic properties are considered to be among the most
microstructurally insensitive.  The origin of the modulus variations
remains controversial.  Substantial enhancements in the yield strength and
hardness have been reported for many multilayered films.  There has been a
great deal of recent interest in modeling this behavior, with an eye toward
understanding how the enhancements scale with bilayer repeat length. It
appears that the hardness increases are results of the difference in
elastic modulus between neighboring layers that introduce image forces on
dislocations and impede their motion.  The enhanced hardness makes
artificially multilayered thin films of interest technologically as
protective coating materials. Although the elastic and hardness behavior
have been the most heavily studied, artificially multilayered thin films
have also displayed other interesting behavior including enhanced anelastic
and tribological properties.