Patent ID: 6916542

Claim:
An article, comprising: a substrate; a functional coating deposited over at least a portion of the substrate; and a protective coating deposited over at least a portion of the functional coating, wherein the functional coating and the protective coating define a coating stack, wherein the protective coating provides the coating stack with an emissivity higher than the emissivity of the functional coating alone, wherein the protective coating has a thickness in the range of greater than 100 Å to less than or equal to 10 microns and the protective coating has a refractive index in the range of 1.4 to 2, and wherein the protective coating comprises a first layer formed over at least a portion of the functional coating and a second layer formed over at least a portion of the first layer, wherein the first layer comprises 50 wt. % to 99 wt. % alumina and 50 wt. % to 1 wt. % silica, and the second layer comprises 50 wt. % to 99 wt. % silica and 50 wt. % to 1 wt. % alumina.