Metal-AlN cermet solar selective coatings deposited by direct current magnetron sputtering technology

A series of metal-aluminium nitride (M-AIN) cermet materials for solar selective coatings was deposited by a novel direct current (d.c.) magnetron sputtering technology. Aluminium nitride was used as the ceramic component in the cermets, and stainless steel (SS), nickel-based alloy Ni80Cr20 (NiCr), molybdenum-based alloy Mo99Ti0.5Zr0.1 (TZM) and tungsten were used as the metallic components. The aluminium nitride ceramic and metallic components of the cermets were deposited by simultaneously running both an aluminium target and another metallic target in a gas mixture of argon and nitrogen. The ceramic component was deposited by d.c. reactive sputtering and the metallic component by d.c. non-reactive sputtering. The total sputtering gas pressure was 0.8-1.0 Pa and the partial pressure of reactive nitrogen gas was set at 0.020-0.025 Pa which is sufficiently high to ensure that a nearly pure AIN ceramic sublayer was deposited by d.c. reactive sputtering. Because of the excellent nitriding resistance of stainless steel and the other alloys and metal, a nearly pure metallic sublayer was deposited by d.c. sputtering at this low nitrogen partial pressure. A multilayered system, consisting of alternating metallic and AIN ceramic sublayers, was deposited by substrate rotation. This multisublayer system can be considered as a macrohomogeneous cermet layer with metal volume fraction determined by controlling the thicknesses of metallic and ceramic sublayers. Following this procedure, M-AIN cermet solar selective coatings with a double cermet layer structure were deposited. The films of these selective surfaces have the following structure: a low metal volume fraction cermet layer is placed on a high metal volume fraction cermet layer which in turn is placed on an aluminium metal infrared reflection layer. The top surface layer consists of an aluminium nitride antireflection layer. A solar absorptance of 0.92-0.96 and a normal emittance of 0.03-0.05 at room temperature have been achieved for these M-AIN cermet solar selective coatings.