  The Casimir free energy and pressure of thin metal films deposited on
metallic plates are considered using the Lifshitz theory and the Drude and
plasma model approaches to the role of conduction electrons. The bound
electrons are taken into account by using the complete optical data of film and
plate metals. It is shown that for films of several tens of nanometers
thickness the Casimir free energy and pressure calculated using these
approaches differ by hundreds and thousands percent and can be easily
discriminated experimentally. According to our results, the free energy of a
metal film does not vanish in the limiting case of ideal metal if the Drude
model approach is used in contradiction with the fact that the fluctuating
field cannot penetrate in its interior. Numerical computations of the Casimir
free energy and pressure of Ag and Au films deposited on Cu and Al plates have
been performed using both theoretical approaches. It is shown that the free
energy of a film can be both negative and positive depending on the metals
used. For a Au film on a Ag plate and vice versa the Casimir energy of a film
changes its sign with increasing film thickness. Applications of the obtained
results for resolving the Casimir puzzle and the problem of stability of thin
films are discussed.
