FIG. 1 is a perspective view of a turbine blade.
FIG. 2 is a cross-sectional view of the turbine blade ofFIG. 1 where a section has been taken at line 2-2 (shown inFIG. 1) and show a thermal barrier coating system overlying the airfoil of the turbine blade.
FIG. 3 is an exemplary cleaning process.
Fig. 4 is a phase diagram of an exemplary bond coat as a function of temperature.

DETAILED DESCRIPTION

FIG. 1 is a perspective view ofturbine blade 10 of a gas turbine engine. Turbineblade 10 includesplatform 12 and airfoil 14.Airfoil 14 ofturbine blade 10 may be formed of a nickel based, cobalt based, iron based superalloy, or mixtures thereof or a titanium alloy.Turbine blade 10 is exposed to high temperatures and high pressures during operation of the gas turbine engine. In order to extend the life ofturbine blade 10 and protect it from high stress operating conditions and the potential for oxidation and corrosion, a thermal barrier coating (TBC) (shown inFIG. 2) is applied overairfoil 14 andplatform 12 ofturbine blade 10.

The exact placement of the TBC system depends on many factors, including the type ofturbine blade 10 employed and the areas ofturbine blade 10 exposed to the most stressful conditions. For example, in alternate embodiments, a TBC may be applied over a part of the outer surface ofairfoil 14 rather than over the entire surface ofairfoil 14. Airfoil 14 may include cooling holes leading from internal cooling passages to the outer surface ofairfoil 14, and thesystem 16 may also be applied to the surface of the cooling holes.

FIG. 2 is a sectional view ofturbine blade 10, where a section is taken from line 2-2 inFIG. 1.TBC system 16 is applied to an exterior surface ofairfoil 14 andplatform 12.

TBC system 16 may includebondcoat 18 andceramic layer 20.Bond coat 18 overlays and bonds toairfoil 14 andplatform 12 whileceramic layer 20 overlays and bonds to the thermally grown oxide (TGO) on thebond coat 18. In the embodiment shown inFIG. 2,bond coat 18 may be applied toairfoil 14 andplatform 12 at a thickness ranging from about 0.5 mils (0.0127 mm) to about 10 mils (0.254 mm).Ceramic layer 20 may be any thermal barrier coating (or "topcoat") that is suitable for use on alumina forming bond coats and/or alloys. Non-limiting examples include zirconia stabilized with yttria (Y₂O₃), gadolinia (Gd₃O₃), ceria (CeO₂), scandia (Sc₂O₃), and other oxides known in the art.Ceramic layer 20 may be applied by electron beam physical vapor deposition (EBPVD) or by plasma spray.Ceramic layer 20 may be deposited in thickness sufficient enough to provide the required thermal protection forbondcoat 18 andsubstrate 10.

Bond coat 18 may be a MCrAlY coating where M may be Ni, Co, Fe, Pt, Ni-base alloy, Co-base alloy, Fe-base alloy or mixtures thereof. In an embodiment, M may include Hf or Si or mixtures thereof. In an exemplary embodiment, thebond coat 18 can comprise NiCoCrAlY.Bond coat 18 may be applied toairfoil 14 andplatform 12 by any suitable technique including, but not limited to, thermal spray processes such as low pressure plasma spray (LPPS) deposition and high velocity oxyfuel (HVOF) deposition, physical vapor deposition such as cathodic arc deposition or chemical vapor deposition, and the like. In an embodiment,bond coat 18 may be an aluminide bondcoat formed by techniques such as pack cementation, chemical vapor deposition, and others followed by appropriate diffusion heat treatments.

Theprocess 100 can be understood by referring also toFig. 3. At aninitial state 120, thebondcoat 18 includes asurface 32 covered withcontaminant materials 34, such as oils, oxides and the like. Within thebond coat 18, a combination of betaphase bcc structure 36 and gammaphase fcc structure 38 is shown distributed throughout thebond coat 18. The betaphase bcc structure 36 can act as an Al reservoir for thebond coat 18. The gammaphase fcc structure 38 can act as an Al diffusion media.

Atstep 130 cleaning and structural refinement takes place at and near thesurface 32 of thebond coat 18. The cleaning is done at high temperature and at an atmospheric pressure. In an exemplary embodiment, the cleaning temperature can be above 1350 degrees C, the point at which the NiCoCrAlY liquefies. Anenergy beam 40 is utilized to remove thecontaminants 34 while also melting thebond coat 18 proximate thesurface 32, forming a liquid 42 from thebond coat 18 material. The liquid 42 subsequently rapidly cools down into a fine grainedlayer 44. The grain refinement is achieved by very rapid melting and resolidification of thebond coat 18 material at thesurface 32 under very high intensity, short duration energy beam pulses. In an exemplary embodiment, theenergy beam 40 can utilize a 50-100 nano-second pulse duration at 5-50 J/cm^2. Thelocal surface 32 heating above the predetermined temperature, such as 1350 degrees Centigrade, can create the fine grained predominantlygamma phase layer 44 proximate thesurface 32. Raising the temperature this way causes the metal right at the surface to liquefy very quickly, and resolidify very quickly providing the desired fine grain structure.

Theenergy beam 40 can include a laser. The laser can include an yttrium aluminum garnet laser, ultraviolet, eximer, or carbon dioxide, fiber, or disc laser. The laser can have a power range up to about 1000 watts.

The surface environment during thecleaning portion 130, can include air, inert gas, water, or combinations thereof.

In an exemplary embodiment, a laser based cleaning system for removing contaminants from thesurface 32 may include: alaser 40 capable of removing the oxide layer and producing an aluminum oxide layer on thesubstrate surface 32; an optical system capable of focusing the laser at the oxide layer; and a scanning system capable of directing the focused laser beam over thesurface 32 to remove contaminants to produce thegamma phase layer 44 on thesubstrate 32.

In an exemplary embodiment, the Al-richgamma phase layer 44 can be about 0.5 microns. In an exemplary embodiment thegamma phase layer 44 can range from about 0.25 microns to about 0.75 microns. In another exemplary embodiment, thegamma phase layer 44 can be about 1.5 microns thick. The thickness of thelayer 44 can be controlled by the amount of energy beam applied power to thesurface 32. In an exemplary embodiment, theenergy beam 40 applied power can range from about 500 Watts to about 1000 Watts. In an exemplary embodiment, thesurface 32 can be heated to about 1350 degrees C. Thesurface 32 can be heated to temperatures from about 1350 degrees C possibly much hotter, but not so hot as to start boiling off the Aluminum, so 2470 degrees C would be the upper limit to produce the desiredgamma phase layer 44. At the higher temperatures, as can be seen atFig. 4, thebond coat 18 is converted predominantly to gamma phase.

At 140 it can be seen that after cleaning and structural refinement, thebond coat 18 includes thegamma phase layer 44 with asmooth surface 32. The elements proximate thesurface 32 of themetallic bond coat 18 tend to be Ni and Al. Themolten alloy 42 freezes first at the location where it is in contact with the bulk of thesolid bond coat 18 and freezes last where themolten alloy 42 is in contact with air or process gases. Thus, the disclosed process promotes gamma-alumina formation proximate thesurface 32. Under ambient conditions, the gamma phase normally has lower Al content than the beta phase. However, because of the rapid melting and refreezing caused by theenergy beam 40, all of the Al is still present, so thegamma phase layer 44 is Al rich, or supersaturated with Al. Because the gamma phase is supersaturated with Al (i.e. it wants to get rid of Al) thegamma phase layer 44 has ability to create a dense initial alpha-alumina layer of TGO and avoid the non-alpha phase aluminum.

Thelayer 44 is Al oversaturated which can be beneficial for thenext step 150 of forming the aluminum oxide layer (TGO) 46 on thesurface 32. The extremelyfine grain structure 44 distributes the aluminum in thebond coat 18 very uniformly, so that the thermally grown oxide (TGO) 46 is even and has fewer internal stresses.

At 160, it can be seen that once the thermally grownoxide 46 is formed, theTGO 46 will be inhibited from growing rapidly, and will grow slowly as compared to a surface that was not treated by the disclosed process. Thegamma phase 44 proximate the surface is reduced in aluminum content and has poor diffusivity properties for aluminum. Theslower TGO 46 growth will result in good adhesion between theTBC 20 and themetallic bond coat 18. This also provides the benefit of longer life adhesion of theTBC 20, because theTGO 46 thickness is smaller and the stresses between theTGO 46 andTBC 20 will grow more slowly. The uniform gamma like fcc phase has an order of magnitude lower self-diffusion coefficient that the beta bcc phase material. Thegamma phase 44 impedes fast Al oxidation at thesurface 32, and allows for a very thin initial alpha-alumina scale to grow during the disclosed laser cleaning process. The rapid cooling and formation of the gamma phase layer inhibits a non-alpha aluminum oxide layer from growing responsive to said gamma phase layer proximate said surface. The non-alpha aluminum oxides can include aluminum oxide in other phases, including the cubic γ and η phases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase that can be tetragonal or orthorhombic. The aluminum oversaturation ensures that aluminum oxides are preferentially created proximate thesurface 32. Duringsubsequent TGO 46 growth in an EB-PVD chamber that initial alpha-alumina creates conditions for preferential alpha-alumina growth, which is less prone to further oxygen diffusion that causes TGO growth during part service life. TBC spallation happens when the TGO thickness reaches a certain value. The disclosed process provides for the initial TGO thickness and TGO growth rate coefficient to be smaller. The TGO growth coefficient for laser cleaned sample is about 2 times smaller than that for a grit blasted one.

The disclosed process relies upon surface chemical/phase composition modifications of the bond coat. In an exemplary embodiment, further protection can include adding other alloying elements to thebond coat surface 32, such as Pt, to further impeded the TGO growth.

The technical benefits of utilizing the disclosed process includes optimized surface chemistry for optimal TGO growth.

Another technical advantage of the disclosed process is the development of asmooth surface 32 without steps and ledges that mitigates geometric stress build-up.

Another technical advantage of the disclosed process is that surface treated material is given a diffusion barrier and controller for aluminum so that the TGO starts and continues to grow uniformly with a smooth, non-stepped geometry.

Another technical advantage of the disclosed process is that surface treating with a laser or other energy source (like e-beam) is a preferred approach.

Another technical advantage of the disclosed process can include adding supplemental elements to the surface by pre-coating (vapor deposition, sputtering, etc.) followed by laser processing that can also provide optimal surface material for TGO growth and subsequent diffusion barrier/control.

There has been provided a process. While the process has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.