Patent claim: Method according to claim 3 characterized in that the zirconia is stabilized with Y2O3 CaO MgO or any combinations thereof.

Related passage: For completeness various aspect of the invention are set out in the following numbered clauses1 A method for smoothing the surface of a ceramicbased protective coating which exhibits roughness comprising the steps ofa applying at least one ceramicbased slurrygel coating over the surface of the protective coating b heating the slurrygel coating under temperature and time conditions sufficient to remove substantially all volatile material from the slurrygel coating and then c further heating the slurrygel coating under temperature and time conditions sufficient to cure the slurrygel coating and bond it to the protective coating.2.The method of clause 1 wherein the slurrygel coating as cured has a thickness sufficient to provide a surface roughness Ra for the protective coating of less than about 7 microns.3.The method of clause 2 wherein the slurrygel coating as cured has a thickness sufficient to provide a surface roughness Ra for the protective coating of less than about 3 microns.4.The method of clause 1 wherein the slurrygel coating is a slurry which comprises a refractory filler and precursors of an oxide matrix.5.The method of clause 4 wherein the refractory filler is zirconia.6.The method of clause 5 wherein the zirconia is in the form of spheres.7.The method of clause 5 wherein the zirconia is chemicallystabilized by a material selected from the group consisting of yttrium oxide cerium oxide calcium oxide magnesium oxide scandium oxide and mixtures of any of these oxides.8.The method of clause 5 wherein the zirconia is present at about 20 to about 70 by volume based on the total volume of the slurry coating.9.The method of clause 4 wherein the oxide matrix comprises an oxide selected from the group consisting of aluminum oxide aluminosilicate calcium oxide magnesium oxide barium oxide silicon dioxide titanium dioxide zirconium dioxide yttrium oxide and mixtures of any of these oxides.10.The method of clause 9 wherein the oxide matrix comprises alumina aluminosilicate or mixtures of alumina and aluminosilicate.11.The method of clause 1 wherein the slurrygel coating is polished or ground after step b.12.The method of clause 1 wherein the cured slurrygel coating is polished or ground after step c.13.The method of clause 1 wherein the slurrygel coating of step a is a single slurry base layer having a composition which comprises precursors of an oxide matrix and about 0 to about 40 by weight of a refractory filler based on the weight of the base layer composition and wherein at least one slurrygel topcoat is applied on top of the base layer between steps b and c to form a final overcoat wherein at least one of the topcoats is formed of a composition comprising the refractory filler.14.The method of clause 13 wherein the refractory filler comprises zirconia.15.The method of clause 13 wherein the application of the base layer is followed by the application of about 2 to about 10 of the slurrygel topcoats wherein each topcoat contains a greater amount of refractory filler than that which is contained in the base layer.16.The method of clause 15 wherein each successive topcoat contains a greater amount of refractory filler than that which was contained in the prior topcoat.17.The method of clause 13 wherein each topcoat is heattreated after it has been applied according to a timetemperature schedule sufficient to remove substantially all volatile material in each topcoat while preventing excessive bubbling.18.The method of clause 13 wherein a ceramicbased finishcoat is applied over the lastapplied topcoat.19.The method of clause 18 wherein the finishcoat has a composition substantially identical to the base layer.20.The method of clause 1 wherein the surface of the protective coating comprises pits or cracks or a combination of pits and cracks and the cured slurrygel coating substantially fills all of the pits and cracks.21.The method of clause 1 wherein the surface of the protective coating comprises peaks and valleys and the cured slurrygel coating covers the surface of the protective coating and fills all of the valleys.22.The method of clause 21 wherein the cured slurrygel coating is polished or ground to remove substantially all of the cured slurrygel coating from the peaks while leaving the cured slurrygel coating in the valleys.23 The method of clause 1 wherein the protective coating is applied over a metalbased substrate.24.The method of clause 23 wherein a bond coat contacts the substrate and the protective coating is applied over the bond coat.25.The method of clause 23 wherein the substrate is made of a material comprising a superalloy.26.A method for providing a protective coating on a superalloy substrate comprising the steps ofi applying a thermal barrier coating over the substrate ii applying at least one ceramicbased slurrygel coating over a surface of the thermal barrier coating iii heating the slurrygel coating under temperature and time conditions sufficient to remove substantially all volatile material from the slurrygel coating and theniv further heating the slurrygel coating under temperature and time conditions sufficient to cure the slurrygel coating and bond it to the thermal barrier coating.27.The method of clause 26 wherein the slurrygel coating is polished or ground after step iv to further smoothen the coating.28.The method of clause 26 wherein the slurrygel coating comprises zirconia spheres and precursors of an oxide matrix.29.The method of clause 26 wherein a bond coat is applied to the substrate before the thermal barrier coating is applied.30.The method of clause 26 wherein the thermal barrier coating is applied by a plasma spray technique.31.The method of clause 26 wherein the slurrygel coating as cured has a thickness sufficient to provide a surface roughness Ra for the thermal barrier coating of less than about 7 microns.32.The method of clause 26 wherein the thermal barrier coating comprises yttriumstabilized zirconia.33.The method of clause 26 wherein the thermal barrier coating is smoothened to a partial degree after being applied but prior to step ii.34.The method of clause 33 wherein the smoothening step prior to step ii is carried out by a technique selected from the group consisting of grinding tumbling heavysanding heavypolishing or a combination of any of these techniques.35.A method for smoothing the surface of a zirconiabased thermal barrier coating which exhibits roughness comprising the steps ofa applying at least one zirconiabased slurrygel coating over the surface of the thermal barrier coating b heating the slurrygel coating under temperature and time conditions sufficient to remove substantially all volatile material from the slurrygel coating and then c further heating the slurrygel coating under temperature and time conditions sufficient to cure the slurrygel coating and bond it to the thermal barrier coating.36.The method of clause 35 wherein the slurrygel coating is a slurry which comprises a zirconiabased refractory filler and an oxide matrix which is selected from the group consisting of alumina aluminosilicate and mixtures of alumina and aluminosilicate.37.The method of clause 35 wherein the thermal barrier coating is applied over a superalloy substrate.38.The method of clause 37 wherein the superalloy substrate is a component of a turbine engine.39.An article comprising1 a metalbased substrate II a protective coating applied over the substrate and III a ceramicbased slurrygel overcoat over the surface of the protective coating.40.The article of clause 39 wherein component III comprises at least two slurrygel coatings which have been applied consecutively.41.The article of clause 39 wherein component III comprises zirconia and precursors of an oxide matrix.42.The article of clause 41 wherein the precursors of the oxide matrix comprise alumina and a silica source.43.The article of clause 42 wherein the precursors of the oxide matrix further comprise a liquid carrier.44.An article comprisingI a metalbased substrate II a protective coating applied over the substrate and III a cured ceramicbased slurrygel coating over the surface of the protective coating.45.The article of clause 44 wherein component 111 comprises a refractory filler contained within an oxide matrix.46.The article of clause 44 wherein the metalbased substrate is a superalloy.47.The article of clause 44 wherein a bond coat is disposed between the substrate and component II.48.The article of clause 44 wherein the protective coating surface comprises peaks and valleys and substantially all of the cured slurrygel coating is present in the valleys.49.The article of clause 44 wherein the protective coating is a zirconiabased thermal barrier coating.50.The article of 49 wherein the thermal barrier coating has been applied by plasmaspraying.51.The article of clause 49 exhibiting a surface roughness Ra of less than about 7 microns.52.An article comprisingI a superalloybased substrate II a zirconiabased thermal barrier coating applied over the substrate and III a cured ceramicbased slurrygel overcoat over the surface of the thermal barrier coating.53.The article of clause 52 wherein the overcoat comprises a zirconiabased refractory filler and an oxide matrix which is selected from the group consisting of alumina aluminosilicate and mixtures of alumina and aluminosilicate.54.The article of clause 52 wherein the superalloy substrate is a component of a turbine engine.