Patent Application
20110027573
The disclosure relates to turbine engine abradable coatings. More particularly, the disclosure relates to abradable coatings for interfacing with blade tips or vane airfoil tips.
A wide variety of abradable coatings have been proposed for use in gas turbine engines. A particular location of concern is the interface between blade tips and adjacent non-rotating structure (e.g., blade outer air seals (BOAS)). A recent proposal was seen in US20090136740 (the '740 publication). The '740 publication discloses a coating layer comprising a metallic phase, a first ceramic phase and a second ceramic phase. The first ceramic phase may comprise at least one of boron nitride or graphite and the second ceramic phase may comprise at least one of clay or metal oxide.
One aspect of the disclosure involves a coated article having a metallic substrate. A coating layer comprises at least 33% by volume ceramic, at least 5% by volume of a lubricant selected from the group consisting of hexagonal boron nitride (hBN), clay, graphite, and mixtures thereof. The coating layer lacks a metal phase of more than 5% by volume.
In various implementations, there may be a bondcoat (e.g., metallic) between the metallic substrate and the coating layer. The coating layer may comprise 35-50% said ceramic and 20-40% said lubricant by volume. The ceramic may consist essentially of a yttria-stabilized-zirconia. The lubricant may comprise said hBN and said hBN may form 5-30% by volume of the coating layer. The lubricant may consist essentially of said hBN, clay, and mixtures thereof. The coating layer may consist essentially of said ceramic and said lubricant.
The article may be in a machine further having a moving component having a motion path extending so that the coating layer forms a rub surface. The coating/rub surface may be on the moving component and/or a fixed component or a second moving component against which the moving component rubs. The coated article may be a blade outer air seal (BOAS) and moving component may be a blade.
Another aspect of the disclosure involves a coated article comprising metallic substrate and a coating layer. The coating layer includes a ceramic matrix and a lubricant within the matrix. The lubricant is selected from the group consisting of hBN, clay, graphite, and mixtures thereof.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
Similarly,
An exemplary bondcoat thickness is 0.005 inch (0.13 mm), more broadly, 0.1-0.2 mm or 0.5-3.0 mm. Exemplary ceramic coating layer thickness is 0.015 inch (0.38 mm, more broadly, 0.3-0.5 mm or 0.2-0.8 mm). Exemplary ceramic density is 33-70% (the remainder occupied by the lubricant and porosity), more narrowly 35-50%. Exemplary lubricant content is 5-50%, more narrowly 20-40% as measured by optical metallography (which treats porosity within the lubricant as if it was lubricant). Exemplary porosity (e.g., not including porosity within lubricant particles as just noted) is 5-30%, more narrowly 10-20%. Such compositions and porosities may be measured as a depth-wise average at a given location or average over an area. Such compositions may also be measured at any individual depth at such location or over such area (as in the case of a coating having a depth-wise gradient in composition or porosity). Overall, the ceramic coating layer may consist essentially of the ceramic, lubricant, and porosity (or porosity-formers prior to bake-off as is discussed further below). An exemplary content of such ceramic, lubricant, and porosity is at least 95% by volume. More particularly, essentially 100%. This, for example, is distinguished from metal-based coatings or coatings with substantial metal phases as are disclosed in US20090136740. Thus, separate metal phases may account for an exemplary no more than 5% of the volume, more particularly essentially none.
The exemplary airfoils may comprise metallic substrates either with or without coatings along the airfoil tips. One example is a nickel-based superalloy with an uncoated tip. The presence of the solid lubricant reduces the friction of the tip-to-seal (or rotor) rub and may reduce metal galling/transfer from the airfoil to the seal (or rotor). This can directly improve efficiency and life of both the airfoil and seal (or rotor). To the extent that the presence of lubricant reduces porosity, blow-by flow (of air or other gas) through the porosity may be reduced, thereby also improving efficiency. Heat generation and transfer to the seal substrate may similarly be reduced from the reduced friction and from the reduced porosity. The reduced heat transfer and reduced flow of air through the coating may reduce the formation of thermally grown oxides (TGO) atop the bond coat. An alternative exemplary airfoil is a titanium alloy having an abrasive tip coating (e.g., an aluminum oxide). In such a system the presence of the lubricant may have similar benefits to those described above including increasing life of the tip coating. Yet other embodiments might involve use of a similar lubricated coating on the tip in addition to the seal (or rotor). Particular benefits may be present where the coated surface is a rotor. Metal transferred from the airfoil is has a lower tendency to adhere to the ceramic/solid lubricant coating contrasted with a coating having a substantial metallic phase. Centrifugal action may cause transferred metal to harmlessly shed (whereas a buildup could cause centrifugal loading to spall/delaminate the rotor coating). The reduction of buildup can also permit tighter design clearances.
A variety of application techniques are possible. These include various thermal spray techniques, high velocity oxy-fuel (HVOF) techniques, and the like. Particular exemplary bond coat application techniques include air plasma spray, low pressure plasma spray (LPPS), vacuum plasma spray (VPS), and HVOF. Particular exemplary application techniques for the ceramic coating layer are air plasma spray and VPS. A more particular exemplary coating application combination is via air plasma spray, first of the bondcoat and then of the ceramic coating layer. Exemplary application of the ceramic coating layer may be via a blend of the ceramic, lubricant, and porosity-formers. In the exemplary lubricant blend of hBN and bentonite clay, the hBN and bentonite may initially be mixed and dispersed in water to form a slurry. The slurry may be spray dried to produce a mixture of particles. The mixture of particles (subject to appropriate size sorting) may provide improved flowability in spray equipment. Exemplary porosity-formers are polyester or methylmethacrylate or other polymer. As-deposited, the porosity-formers produce polymer phases within the ceramic coating layer. These polymer phases may be baked out to leave porosity (e.g., via a separate baking stage or upon use). Exemplary baking temperatures are at least 500° F. (260° F.) for methylmethacrylate and 900° F. (482° F.) for polyester. Alternative application techniques for the ceramic coating layer may be via separate introduction of the powders into the spray plume or codeposition of the powders with separate respective spray torches.
One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied to an existing engine, details of the engine configuration and materials may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.
