SILICA THIN-FILMS APPLIED TO NI-20CR ALLOY VIA COMBUSTION CHEMICAL-VAPOR-DEPOSITION

Amorphous silica thin films (<0.5 mu m) have been deposited onto Ni-20 Cr via combustion chemical vapor deposition (CVD) using two different nozzle assemblies, an oscillating capillary nebulizer (OCN) and a simp le nebulizing needle. The deposition conditions were similar for the t wo different nozzles, although slightly different coating morphologies were observed. The OCN-deposited material was deposited onto rotating samples (4 rpm), and showed a morphology striated in thickness, presu med to be a result of thermal gradients present across the rotating sa mple. The samples deposited with a nebulizing needle did not show thes e striations, but rather, showed a nodular morphology. Protection to N i-20Cr from the flame during deposition is reported, in addition to is othermal oxidation kinetic measurements which quantify the degree of p rotection provided by silica. Silica reduces the oxidation rate of Ni- 20Cr, with less transient oxidation of Ni reported for silica-coated s amples, as well as the rate constants of Ni-20Cr being reduced at all test temperatures by more than an order of magnitude. These reductions are attributed to the deposited oxides serving as reactive elements i n the chromia scale. (C) 1997 Elsevier Science S.A.