adherent , thermal shock - resistant protective coatings for nickel base super alloys are obtained by applying to the base metal a thin bond coat of an alloy of chromium aluminum and yttrium with materials selected from the group consisting of iron , cobalt , nickel and nickel - cobalt and applying thereover a continuously graded mixture of this material with a zirconia - based ceramic , the concentration of zirconia - based ceramic increasing from the bond coat to the outer layer . the zirconia ceramic may be stabilized by the addition thereto of amounts of magnesium oxide or other materials .