Patent Publication Number: US-9850778-B2

Title: Thermal barrier coating with controlled defect architecture

Description:
FIELD OF THE INVENTION 
     The invention relates to thermal barrier coatings, and particularly to such coatings on surfaces in the hot gas flow path of a gas turbine engine. 
     BACKGROUND OF THE INVENTION 
     Thermal barrier coatings (TBCs) are used to provide thermal protection for components in the hot gas flow of turbine engines. In addition to low thermal conductivity, these coatings require compliance, meaning flexibility or other strain tolerance, in order to withstand stresses from cyclic thermal expansion, vibration, and particle impacts. TBCs require strong adherence to the substrate. They are commonly made of ceramic materials such as yttria stabilized zirconia (YSZ) due to the refractory properties of ceramics. However, ceramic coatings do not readily adhere to metal surfaces, so a bond coat of a material such as MCrAlY (M=metal, Cr=chromium, Al=aluminum, Y=yttrium) is commonly applied between a metal substrate and the TBC. MCrAlY resists oxidation at high temperatures, and is compatible with a metal superalloy substrate and a ceramic TBC. 
     The TBC may be applied at less than full density to reduce thermal conductivity. However, present TBCs can densify during service asymptotically toward full density. This is due to tight conformance of ceramic splats to each other, resulting in small between-the-splat (inter-splat) gaps, which can close by sintering during service. As the splat interfaces disappear; the TBC becomes rigid and loses its ability to resist strains that occur during thermal cycling. This leads to spalling. Unmitigated cracking occurs, which allows the hot working gas to reach the bond coat directly, reducing its life. Since the inter-splat gaps reduce thermal conductivity, as they close, conductivity increases. 
     Various means have been proposed to overcome this problem, including inclusion in the TBC of hollow ceramic spheres, columnar cracking of the TBC, and surface grooving to provide compliance by segmentation. However, the TBC material can still sinter over time, thus increasing its conductivity and reducing its resistance to spalling. Materials that delay phonon propagation, such as low k Gadolinium, can be used, but they are more expensive than yttria stabilized zirconia. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a photomicrograph of porous particles of yttria stabilized zirconia as known in the art. 
         FIG. 2  is a diagram of a thermal spray system and process operating in accordance with aspects of the invention. 
         FIG. 3  is a conceptual sectional view of a prior art thermal barrier coating. 
         FIG. 4  is a sectional photomicrograph of a prior art thermal barrier coating before operational heating. 
         FIG. 5  is a sectional photomicrograph of a prior art thermal barrier coating after heating to 1400° C. for 10 hours. 
         FIG. 6  is a sectional view of a porous particle with a solid shell showing aspects of an embodiment of the invention. 
         FIG. 7  is a conceptual sectional view of a thermal barrier coating system showing aspects of an embodiment of the invention. 
         FIG. 8  is a sectional photomicrograph of a thermal barrier coating system showing aspects of an embodiment of the invention. 
         FIG. 9  is a stress/strain graph from tests of an embodiment of the invention, showing elastic hysteresis of the invented thermal barrier coating compared to prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventors devised a process that produces a thermal barrier coating having a particular architecture that provides reduced thermal conductivity, improved compliance, and long life span, all at low expense. This is done by starting with YSZ particles with within-the-particle (internal) porosity, and thermally spraying them onto a substrate using spray parameters that melt only an outer surface portion of each particle. This retains the internal porosity of the particles. It also increases inter-particle gaps by reducing the average aspect ratio of the splats compared to fully melted splats. 
       FIG. 1  is a photomicrograph of YSZ powder formed by agglomeration and/or other process that provides particles  19  with internal porosity. 
       FIG. 2  illustrates a thermal spray system  20  for producing a ceramic thermal barrier coating  22  on a substrate  24  by injecting  26  a ceramic powder feedstock  28  such as YSZ into a thermal jet  30 . A plasma gun  32  may be used to produce the thermal jet. The temperature of the substrate  24  may be controlled during the spray process by a temperature control unit  32 . A spray parameter controller  34  may execute control logic, and may input user parameters, to control the spray process, including the rate and temperature of the carrier gas  36 , the electric power +−, and the feedstock feed rate  38 , to produce a desired thermal jet with partly melted particles  40  of the powder in accordance with aspects of the invention. 
       FIG. 3  conceptually illustrates a prior art thermal barrier coating  42  on a substrate  24 . A bond coat  44  such as MCrAlY is applied to the substrate, and then a coating of ceramic such as YSZ is applied by thermal spray. This melts the ceramic particles and impacts them on the substrate, forming relatively thin splats  46   a - c  that highly conform to previous splats with high coherence of adjacent splats, high internal density of each splat, and small inter-splat gaps  48 .  FIG. 4  is a photomicrograph of a conventional YSZ TBC sprayed with full melting of the YSZ particles by the thermal spray. Gaps between splats are commonly 1 micron or less.  FIG. 5  is a photomicrograph of the TBC of  FIG. 4  after 10 hours at 1400° C., showing merging and densifying due to sintering at operating temperature levels. 
       FIG. 6  illustrates a ceramic particle  40  in the thermal spray  30  of  FIG. 2  showing aspects of an embodiment of the invention. The spray parameters are selected to melt only an outer layer or shell  50  of the particle, leaving an interior portion  52  unmelted and porous  54 . The particle  40  may be 10-50% melted, or especially 10-25% melted by volume after melting. To achieve limited peripheral melting, a control methodology based on energy density in the thermal spray is useful. YSZ powders from different vendors, and in different batches from the same vendor, can vary in substantially in mass density and other properties. However, the inventors found that the melt percentage is a linear function of energy density, which may be expressed as watts per liter of carrier gas flow for a given powder mass feed rate in the thermal spray process. To adjust for a new batch of powder, test spraying may be done into a collection tank with small sample of the powder using an energy density such as 500 watts per liter. The collected particles may then be evaluated for melt percentage, and the energy density may be adjusted if needed. This results in at least most of the spray particles, or especially over 80% of them, having the desired melting percentage, with the outer shell  50  being essentially non-porous, meaning it has greater than 95% of theoretical density, and the interior portion being porous, meaning it has less than 90% of theoretical density. 
     The melt percentage may be evaluated using Archimedes&#39; Principle to find the powder density before and after test spraying, calculating the resulting densification percentage, and converting this to the melt percentage. Alternately, the melt percentage may be evaluated graphically in sectional photomicrographs of a sample of the test-sprayed particles. 
       FIG. 7  illustrates a thermal barrier coating system  56  on a substrate  24  showing aspects of an embodiment of the invention. A bond coat system  44 A-B of a material such as MCrAlY may be applied in two layers, the first layer  44 A being highly dense, for example having a mass density of at least 95%, and the second layer  44 B being rougher and less dense. For example, layer  44 A may be applied by a high velocity oxy-fuel process, and layer  44 B may be applied by air plasma spray as a rough flash coat. After application, the bond coat system  44 A-B may be heat-treated sufficiently for diffusion bonding of the two layers  44 A,  44 B to each other and to the substrate  24 . 
     A thermal barrier layer  58  is formed on the rough bond coat  44 B by a thermal spray process such as air plasma spray. Controlled melting renders the particles  40  partly malleable. The force of impact may cause some flattening, but the particles  40  do not conform to each other as closely, or cohere as completely, as fully melted splats. The particles may have an average aspect ratio in a range of 1-4, for example. This leaves larger inter-particle gaps  48 , which may have an average gap dimension (such as gap width) greater than 5 microns or especially 10-40 microns or 20-30 microns. This contrasts with prior art gaps averaging 1 micron or less. The thermal barrier layer  58  may have a porosity of greater than 12% or especially 14-17%, including porosity  54  in the particles thereof and the inter-particle gaps  48 . The particles have less contact area and coherence than prior art, which allows more relative motion among them, including sliding among some surfaces of some of the particles. This combination of micro-structural features in the coating system  56  provides low thermal conductivity; increased compliance, including increased elasticity; minimal sintering; mitigation of crack propagation; and negligible or reduced spalling compared to prior art. 
       FIG. 8  is a photomicrograph of a thermal barrier coating system  56  showing aspects of an embodiment of the invention, including a rough bond coat layer  44 B, and a thermal barrier layer  58  with controlled defects including inter-splat gaps  48 . 
       FIG. 9  shows an elastic hysteresis loop exhibited by a thermal barrier system in an embodiment of the invention as drawn on a stress/strain graph with linear/linear units. Within a given stress range SR, the thermal barrier starts at a beginning shape  60  and reaches a relatively distorted shape  62  along a first stress/strain curve  64 . Upon removal of the stress, the thermal barrier returns to its beginning shape along a different stress/strain curve  66 . A prior art TBC with fully melted splats and operational sintering follows a stress strain curve with a limited linear elastic portion  74  followed by a non-linear plastic portion  76  ending in spalling. The modulus of elasticity of such prior art is commonly over 30 GPa. In contrast, the overall modulus of elasticity of the present TBC after operational service may be in a range of about 15-25 GPa or especially 16-20 GPa, based on line  68 . 
     A magnitude of hysteresis is defined herein as the separation  70  between the two stress/strain curves  64 ,  66  divided by the distance  68  between the beginning and ending points  60 ,  62 . A more detailed description is as follows: The thermal barrier layer exhibits elastic hysteresis on a stress/strain graph with linear/linear units, wherein first  64  and second  66  stress/strain curves each span between a beginning point  60  on the graph and an ending point  62  on the graph, forming a hysteresis loop  64 ,  66 , wherein the distance  70  between the two stress/strain curves divided by the distance  68  between the beginning and ending points  60 ,  62  gives a hysteresis magnitude in a range of 0.05-0.10, wherein the distance between the two stress/strain curves is taken along a perpendicular  72  drawn from a midpoint of a line  68  between the beginning and ending points  60 ,  62 . 
     Elastic hysteresis of the invented thermal barrier layer appears to be caused by a proportion of slidable ceramic particles in the TBC retained by a 3D web of coherency chains among other particles. The slidable particles may have partial or no coherence to adjacent particles. The 3D web distorts elastically under stress, allowing non-coherent surfaces of the some particles, to slide against other particles, creating frictional heat, and thus producing the hysteresis loop. It takes more work to slide a particle out of its spray-nested position than to slide it back into that position. Each particle has a relatively thin, dense shell that can elastically distort slightly in a motion. The thinness of the shell enhances its elasticity. The porous interior of the particle fractures into a mobile filler that keeps the particle inflated, but is not rigid. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.