Laser-induced reaction alumina coating on ceramic composite

Silicon carbide ceramics are susceptible to corrosion by certain industrial furnace environments. It is also true for a new class of silicon carbide-particulate reinforced alumina-matrix composite (SiC{dollar}sb{lcub}rm (P){rcub}{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar}) since it contains more than 55% of SiC particulate within the composite. This behavior would limit the use of SiC{dollar}sb{lcub}rm (P){rcub}{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar} composites in ceramic heat exchangers. Because oxide ceramics corrode substantially less in the same environments, a laser-induced reaction alumina coating technique has been developed for improving corrosion resistance of the SiC{dollar}sb{lcub}rm (P){rcub}{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar} composite. Specimens with and without the laser-induced reaction alumina coating were subjected to corrosion testing at 1200{dollar}spcirc{dollar}C in an air atmosphere containing Na{dollar}sb2{dollar}CO{dollar}sb3{dollar} for 50 {dollar}sim{dollar} 200 hours. Corroded specimens were characterized via x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS).; The uncoated SiC{dollar}sb{lcub}rm P{rcub}{dollar}/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} composite samples experienced an initial increase in weight during the exposure to Na{dollar}sb2{dollar}CO{dollar}sb3{dollar} at 1200{dollar}spcirc{dollar}C due to the oxidation of residual aluminum metal in the composite. There was no significant weight change difference experienced during exposure times between 50 and 200 hours. The oxidation layer formed on the as-received composite surface consisted of Si and Al{dollar}sb2{dollar}O{dollar}sb3{dollar} (after washing with a HF solution). The oxidation layer grew outward and inward from the original surface of the composite. The growth rate in the outward direction was faster than in the inward direction. The formation of the Si/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} oxidation layer on the as-received composite was nonuniform, and localized corrosion was observed.; The coated samples experienced very little mass increase. The laser-induced reaction alumina coating effectively provided protection for the SiC{dollar}sb{lcub}rm P{rcub}{dollar}/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} composite by keeping the corrodents from contacting the composite and by the formation of some refractory compounds such as Na{dollar}sb2{dollar}OAl{dollar}sb2{dollar}O{dollar}sb3{dollar}SiO{dollar}sb2{dollar} and Na{dollar}sb2{dollar}Al{dollar}sb{lcub}22{rcub}{dollar}O{dollar}sb{lcub}34{rcub}.{dollar} After exposure to 1200{dollar}spcirc{dollar}C for 200 hours, the bonding between the laser-induced reaction alumina coating and the composite appeared to be unattacked. For both the coated and uncoated samples, porosity within the composite increased after exposure due to the reaction: 3SiO{dollar}sb2{dollar}(s) + 4Al(l) {dollar}to{dollar} 2Al{dollar}sb2{dollar}O{dollar}sb3{dollar}(s) + Si(s).