  We present the results of mechanical characterizations of many different
high-quality optical coatings made of ion-beam-sputtered titania-doped tantala
and silica, developed originally for interferometric gravitational-wave
detectors. Our data show that in multi-layer stacks (like high-reflection Bragg
mirrors, for example) the measured coating dissipation is systematically higher
than the expectation and is correlated with the stress condition in the sample.
This has a particular relevance for the noise budget of current advanced
gravitational-wave interferometers, and, more generally, for any experiment
involving thermal-noise limited optical cavities.
