An article includes a substrate, a functional coating deposited over the substrate,
and a protective coating deposited over the functional coating. The functional
coating and the protective coating define a coating stack. The protective coating
provides the coating stack with an emissivity value higher than the emissivity
value of the functional coating alone. The protective coating can have a thickness
in the range of greater than 100 to less than or equal to 10 microns and
a refractive index in the range of 1.4 to 2. The protective coating can include
a first layer formed over the functional coating and a second layer formed over
the first layer. The first layer can include 50 wt. % to 100 wt. % alumina and
50 wt. % to 0 wt. % silica, and the second layer can include 50 wt. % to 100 wt.
% silica and 50 wt. % to 0 wt. % alumina. An alternating current power supply and
cathode target system includes a cathode target including aluminum in the range
of 5 wt. % to 100 wt. % and silicon in the range of 0 wt. % to 95 wt. %.
