Patent Description:
Carbon seals are commonly used to seal between relatively rotating components in gas turbine engines (used in propulsion and power applications and broadly inclusive of turbojets, turboprops, turbofans, turboshafts, industrial gas turbines, and the like). These include shaft seals (i.e., where the sealing surfaces of seal and seat face radially (e.g., within <NUM>° or essentially <NUM>°) and extend axially) and face seals (i.e., where the sealing surfaces face axially (e.g., within <NUM>° or essentially <NUM>°) and extend radially).

In typical face seal situations, the annular carbon seal is axially spring biased into engagement with an annular seat (typically metallic such as a steel). Typical bias springs are bellows springs, coil springs (multiple circumferentially-distributed springs), or wave springs. The spring(s) act axially between a seal housing (seal support) and a seal carrier (carbon carrier) carrying the seal. An example seal carrier is a full annulus metal component carrying the seal and intervening between the seal and the spring. For example, a carrier may have: an outer sidewall surrounding a portion of the seal in interference fit; and a radial flange engaged by the spring (e.g., to which a bellows spring is welded). The seal may be in a radial interference fit (e.g., thermal interference fit) with the carrier outer sidewall. The sprung mass of such a seal includes the mass of the carbon ring, the carrier, and effectively half of the spring(s).

The seal may be a single-piece full annulus carbon member or may be segmented (formed by an end-to-end circumferential array of segments in arch bound relation via the carrier interference fit). For either type, the interference fit is advantageously tight enough, in view of the coefficient of friction between the seal and carrier, to maintain the seal seated in the carrier so that the seal does not shift circumferentially or axially (local or overall) relative to the carrier. An example static coefficient of friction between the carbon of the seal and the steel or other metallic substrate is ≤ <NUM>, often much less. Alternative proposed seals use an adhesive film (e.g., epoxy) to reduce or eliminate the required interference fit. At the macroscale retaining the seal seated in the carrier is important because if the seal rotates or moves axially it can break or liberate resulting in seal failure. On a microscale if the retention method does not retain the seal it can move after assembly so that the flatness of the sealing face no longer meets the strict requirements, thereby making the seal ineffective.

Often, the carbon seal is on non-rotating static structure and the seat rotates with one of the engine shafts. The sliding engagement causes frictional heating. The heat must be dissipated. With a rotating seat, it is common to use oil cooling. Generally, oil cooled carbon seals are divided into two categories: "dry face" seals wherein the oil passes through passageways in the seat without encountering the interface between seal face and seat face; and "wet face" seals wherein the oil passes through the seat to the interface so that the oil that flows through the seat cools the seat but then lubricates the interface to further reduce heat generation.

For both wet face and dry face seals, the oil may be delivered through a nozzle and slung radially outward by the rotating component and collected in a radially outwardly closed and inwardly open collection channel from which the passageways extend further radially outward.

Prior art includes <CIT>, <CIT>, <CIT> and <CIT>. <CIT> discloses an apparatus according to the preamble of claim <NUM>.

One aspect of the invention involves an apparatus as claimed in claim <NUM>.

Optionally, the radial interference fit provides a compressive stress in the seal of <NUM> MPa to <NUM> Mpa.

Optionally, the coating increases a static coefficient of friction between the seal and the seal carrier.

Optionally, additionally and/or alternatively, the coating comprises at least <NUM>% by weight nickel.

Optionally, the seal carrier inner diameter (ID) surface is formed by a steel or iron-nickel alloy body of the seal carrier.

Optionally, the coating comprises at least <NUM>% by weight silver.

Optionally, the coating has a thickness of <NUM> micrometers to <NUM> micrometers.

Optionally, the coating has an outer friction layer and an inner strike layer wherein a largest by weight elemental component of the outer friction layer is different from a largest by weight elemental component of the inner layer.

Optionally, the coating has an outer friction layer and an inner strike layer wherein a largest by weight elemental component of the outer friction layer is sliver or nickel and a largest by weight elemental component of the inner strike layer is copper.

Optionally, the seal carrier inner diameter (ID) surface is formed by a coating on a substrate of the seal carrier.

Optionally, the coating on the substrate of the seal carrier is a metal or an alloy of the same base as a base of a metal or an alloy forming the coating on the substrate of the seal.

Optionally, the base of the coating on the seal substrate and the base of the coating on the seal carrier substrate are both silver.

Optionally, the seal carrier substrate comprises a steel or iron-nickel alloy.

Optionally, the seal substrate is a single piece.

Optionally, the apparatus is a gas turbine engine.

Optionally, a method for manufacturing the apparatus comprises: applying the coating to the substrate; and thermal interference fitting the seal to the carrier.

Optionally, the applying comprises plasma spray.

Optionally, the thermal interference fitting comprises: heating the seal carrier to a temperature of <NUM> to <NUM>; and inserting the seal into the heated seal carrier.

Optionally, cooling of the seal carrier leaves a radial interference fit with a compressive stress in the seal of <NUM> Mpa to <NUM> Mpa.

Optionally, the method further comprises grinding of the applied coating.

Another aspect (not claimed) involves an apparatus comprising: a first member; a shaft rotatable relative to the first member about an axis; and a seal system. The seal system comprises a seal carrier having: an axially-extending wall having an inner diameter (ID) surface; and a radially-extending wall having a first surface. A seal is carried by the seal carrier in a radial interference fit with the seal carrier axially-extending wall ID surface and has: an outer diameter (OD) surface; and a seal face. A seat is carried by the shaft and has a seat face in sliding sealing engagement with the seal face. One or more springs bias the seal carrier relative to the first member so as to bias the seal face against the seat face. The seal system further includes means for increasing a static coefficient of friction between the seal and the seal carrier. The means may comprise one or more coatings. The one or more coatings may be according to the foregoing embodiments.

All things being equal, at a given level of interference fit contact pressure between the seal and the carrier, increasing the coefficient of friction between the seal and the carrier will increase resistance to seal displacement. Thus, for example, increased coefficient of friction may be used to provide better seal retention or maintain seal retention while reducing contact pressure. Reduced contact pressure may allow lightening of the seal and/or carrier while keeping seal compressive stresses and carrier tensile stresses within material capabilities.

Due to carbon lubricity, the material pairing of the seal carbon material and the metallic carrier will have a relatively low static coefficient of friction µ (e.g., ≤ <NUM>, often much less). Accordingly, a coating (friction coating) on the seal carbon material intervening between the carbon material and the carrier may increase the static coefficient of friction (e.g., to ≥ <NUM> or ≥ <NUM> or an example <NUM> to <NUM>).

Discussed further below, example coatings according to the invention are metallic resulting in a metal-on-metal pairing having a higher static coefficient of friction than the pairing of the carbon material and carrier material.

Example metallic coating materials include silver, nickel, aluminum, and copper (including their respective alloys and mutual alloys) and steels. Example coating application techniques include spray and physical vapor deposition. In optional multi-layer coatings, the outer layer of the layer stack may be such a material acting as a friction coating; whereas other layer(s) may be for bonding or other purposes.

One particular area of candidate materials are nickel alloys commonly used for thermal spray repairs (of steel or other alloys) in applications such as shafts, bearing journals, and the like. One example of such a nickel alloy is a nickel-aluminum alloy such as used in wire spray. Example by weight aluminum content is <NUM>% to <NUM>% with nickel being substantially the balance plus impurities and minor alloying elements (if any). Example impurities and minor alloying elements are less than <NUM> weight percent total.

Another particular area of candidate materials are silver or silver alloys commonly used for lubricity in applications such as coating the steel cages of ball bearings and the like. Even though such alloys are typically used for lubricity, they increase the static friction coefficient from that provided by the carbon material. They also offer a particularly increased/favorable static coefficient of friction when coating both seal and carrier. This reflects the particular low relative lubricity of silver on silver (or, more broadly of silver or silver-base alloys with each other or itself). One example of essentially pure silver is AMS <NUM>. More broadly, a relatively pure silver may be at least <NUM> weight percent silver or at least <NUM> weight percent. The purity limits melting point depression to maintain desirable service temperatures. Particularly when lower service temperatures are involved, alloys with lower silver levels may be used. One group of such alloys are existing braze alloys (e.g., Ag-Cu-Ni alloys). One example of such a silver alloy is AMS <NUM>, a silver braze alloy of nominal by weight percent composition Ag56, Cu42, Ni2. Thus, example silver alloys are at least <NUM>% silver by weight.

Thus, in one group of examples, the mating surface of the carrier is uncoated substrate material. However, other embodiments may include a coating on the mating surface of the carrier. Such carrier coatings may further increase the static coefficient of friction and/or provide anti-corrosion or other benefit.

<FIG> shows a seal system <NUM> having a sealing element (seal) <NUM> and a seat <NUM> (seal plate). As is discussed further below, the seal system is used in a turbomachine such as a gas turbine engine for a purpose such as isolating a bearing compartment <NUM>. The seal is mounted to a first structure such as an engine static structure and the seat is mounted to rotate relative thereto (e.g., mounted to a shaft) about an axis A which may be the engine centerline or central longitudinal axis. As discussed below, the example seal system includes the seat <NUM> as one piece and the seal <NUM> as part of a cartridge subassembly (cartridge) <NUM>. <FIG> further shows an outward radial direction <NUM> and a forward direction <NUM>.

The example seal <NUM> is a carbon seal (carbon element) having an axially-facing/radially-extending seal surface or face <NUM>. The example seal <NUM> is formed as single-piece body circumscribing a central axis normally coincident with the centerline A when installed.

The seat <NUM> has an axially-facing/radially-extending seat surface or face <NUM> engaging the seal face <NUM>. This engagement may allow relative radial displacement of seal and seat.

The seal system <NUM> (<FIG>) isolates a space or volume <NUM> from a space or volume <NUM>. The example space or volume <NUM> is a bearing compartment. The example seal system is at an aft end of the bearing compartment. A similar or other seal system (not shown) may be at a forward end of the bearing compartment (e.g., oppositely oriented). The example bearing compartment <NUM> contains a bearing supporting the shaft for rotation relative to the static structure about the axis A. The example second space or volume <NUM> is a buffer air chamber.

In the example engine configuration and position, a case component <NUM> (e.g., a strut ring/frame) of the static structure is positioned radially inboard of a gas path (core flowpath) C (<FIG>). An example seal system is an oil-cooled dry-face seal system wherein an array of passageways (not shown) extend through the seat from respective inlet ports (not shown) at a plenum (between the seat and a portion of a shaft) through outlet ports (not shown) on the seat to an outer diameter (OD) rim for carrying oil. An alternative is a dry face uncooled seal system. The seal system may alternatively be a wet face seal system in that there are oil passageways to outlets on the seat face <NUM>.

The seal system <NUM> cartridge <NUM> further includes a seal housing (seal support) <NUM> and one or more bias springs <NUM> (e.g., a bellows spring or an array of coil springs) biasing the seal <NUM> into engagement with the seat <NUM> in the assembled engine. The seal housing <NUM> is mounted to the case component <NUM> such as via interference fit and/or fasteners. Example fasteners <NUM> (<FIG>) are screws extending through apertures <NUM> in mounting ears or a radial flange <NUM> of the housing and then into threaded bores <NUM> of the case component <NUM> (or through the case component to engage nuts (not shown)). The example seal housing <NUM> is machined or cast/machined of an alloy. The example mounting ears or flange <NUM> protrudes from one end of a sidewall <NUM> of the seal housing <NUM>. An end wall <NUM> extends radially inward from the opposite end of the sidewall <NUM>. The adjacent end(s) of the spring(s) <NUM> contact the interior radial face <NUM> of the end wall <NUM>. For a bellows spring <NUM>, the spring end may be welded, brazed, or otherwise secured to the face <NUM>. For coil springs, coil spring (not shown) ends may be captured in bores in the face <NUM> or may capture projections from the face <NUM>.

<FIG> further shows the cartridge <NUM> as including a seal carrier (carbon carrier) <NUM> intervening between the seal <NUM> and the spring(s) <NUM>. For forming a compartment (seal compartment) <NUM> for receiving the seal <NUM>, the seal carrier has a radial wall <NUM> and an axial wall <NUM> extending axially from the radial wall <NUM>. The radial wall <NUM> has, along the seal compartment <NUM>, a face <NUM> (an aft face of the seal compartment and forward face of the wall in the example or a forward face of the seal compartment and aft face of the wall if oppositely oriented). The axial wall <NUM> has, along the seal compartment, a face <NUM> (an inner diameter (ID) face). The example seal carrier <NUM> is machined or cast/machined of an alloy. In the example, the radial wall <NUM> has a face <NUM> axially opposite the face <NUM>. The example adjacent bellows spring end may be welded, brazed, or otherwise secured to the face <NUM>. In the example embodiment, the bellows spring <NUM> restricts rotation of the seal carrier <NUM> about the engine centerline A but also provides a relatively robust centering force. In alternative embodiments, additional anti-rotation and/or centering means may be provided. For example, when using an array of compression coil springs instead of the bellows spring the seal system may need such means. Example such means are one or more anti-rotation pins fixed relative to case structure received in holes in ears and/or a flange of the seal carrier to restrict rotation and limit radial excursions. Additionally, in various implementations, there may be secondary seals including labyrinth seals, C-seals, and the like.

<FIG> shows the seal <NUM> as having a main body section <NUM> and a nose <NUM> protruding axially therefrom to the seal surface <NUM>. The main body <NUM> has an inner diameter (ID) surface <NUM> and an outer diameter (OD) surface <NUM>. The main body has a first end face <NUM> (forward in the example) and a second end face <NUM> (aft in the example). The second end face <NUM> contacts the face <NUM>. The surface <NUM> contacts the seal carrier axial wall ID surface <NUM>. The nose <NUM> has an inner diameter (ID) surface <NUM> and an outer diameter (OD) surface <NUM> respectively radially recessed relative to the surfaces <NUM> and <NUM>.

The example seal <NUM> comprises a carbon-based substrate <NUM> (<FIG>) and a coating (coating system) (a single layer <NUM> in the example). Substrate surfaces form corresponding surfaces of the example seal with the coating outer surface forming the OD surface <NUM> (or section/portion thereof) atop a corresponding OD surface section <NUM> of the substrate.

The example seal system may represent a modification or reengineering of a baseline seal or configuration thereof (lacking the coating). The baseline may have a tight interference fit (e.g., press-fit and/or thermal interference fit) between the carbon seal and the seal carrier with direct seal carbon to carrier alloy contact. The tight interference fit may itself provide robust sealing between the seal and carrier. The modified or reengineered seal system or configuration may involve a lighter interference fit in some embodiments.

<FIG> further shows the coating <NUM> as having a thickness TM1 over a span of overlap S<NUM> of the coating <NUM> with the carrier ID surface <NUM>. In the illustrated example, the span of overlap extends to the faces <NUM> and <NUM>. However, in other embodiments, there may be an OD bevel to the substrate or other reduction in the span. An example thickness TM1 is <NUM> micrometer to <NUM> micrometers over a span S<NUM> of at least an example <NUM> millimeters (e.g., an average thickness over that span of contact). The particular span will depend on seal size with upper limits being in about <NUM> millimeters for larger seals in gas turbine engines. More particularly, example TM1 is <NUM> micrometers to <NUM> micrometers (or <NUM> micrometers to <NUM> micrometers or <NUM> micrometers to <NUM> micrometers over a span S<NUM> of <NUM> millimeters to <NUM> millimeters. As noted above, the material pairing of the coating <NUM> surface <NUM> against the carrier <NUM> ID surface <NUM> provides a static coefficient of friction greater than that otherwise offered between the substrate OD surface <NUM> and carrier ID surface <NUM>. The coating may axially extend over at least the entire contact span of seal and carrier.

In one example of a manufacture process, the seal substrate and the carrier are made by conventional processes (e.g., the processes of a baseline being modified). For example, the seal substrate may be made via machining of a carbon blank and the seal carrier may likewise be machined from billet or other stock or cast and machined. The example coating <NUM> may be applied after masking (e.g., via covering by hard 8ixturing/tooling) portions of seal substrate <NUM> surfaces adjacent the OD surface <NUM> (and potentially portions of the OD surface <NUM>).

The coating <NUM> may be applied via spray. This may be applied by placing the seal on a mandrel and rotating the mandrel while spraying and continuously or incrementally axially traversing the spray gun. In some implementations, the mandrel may serve a dual purpose of mandrel and tooling/mask. This rotation/traversing may be repeated for multiple passes with offsets or other steps to limit surface variation. After spraying, the OD surface <NUM> may optionally be machined to tolerance such as via turning, abrasive grinding (e.g., with an abrasive wheel), or other machining. If an applied mask was used (e.g., a coating, pre-formed tape or film, or the like) demasking (e.g., solvent cleaning) may follow.

In an example of a nickel or nickel alloy coating <NUM>, the coating is applied via a thermal spray process such as air plasma spray. An initial as-sprayed coating thickness may be in an example range of <NUM> micrometers to <NUM> micrometers but is subsequently ground down to a more precise thickness and texture with an example post-grinding thickness in the range of <NUM> micrometers to <NUM> micrometers, more particularly <NUM> micrometers to <NUM> micrometers or <NUM> micrometers to <NUM> micrometers.

Alternate variations may avoid a post-spray machining (e.g., grinding). In such examples, the as-sprayed thickness may be less such as in the range of <NUM> micrometers to <NUM> micrometers. Variations in the thinner coating may have less influence on variations in the interference fit than would variations in a thicker coating. Thus, the thicker coating may be machined to provide a particular tolerance without necessarily becoming thin.

In further variations on the ground coating or unground coating, the application of the nickel coating layer <NUM> may be preceded by applying a strike/flash layer <NUM> (<FIG>) such as a copper strike. An example copper strike/flash is applied to a thickness TS1 of about <NUM> micrometers to <NUM> micrometers. Thus, in most implementations, the strike/flash may be thinner than the nickel alloy. But in some implementations, the strike/flash may be thicker than the nickel alloy. The strike/flash layer serves to improve bonding with the carbon. Example application is by electroplate or cold spray.

An example static coefficient friction between the nickel coating <NUM> and the steel of the seal carrier is <NUM> or greater at ambient conditions (e.g., temperature in the range of <NUM> to <NUM>).

In one example of an assembly process, the seal carrier <NUM> is preheated (e.g., by thermal convection in either an air oven or a liquid (e.g., water) bath). Example heating in an existing baseline range is to a temperature in the range of <NUM> to <NUM>. Example heating for a reduced temperature range for reduced stress is to a temperature of about <NUM>, more broadly, <NUM> to <NUM> or <NUM> to <NUM> or <NUM> to <NUM>.

The seal <NUM> may be inserted to the seal carrier via translation (e.g., held by a tool (not shown)). The seal may then be held in its fully seated condition while the carrier is allowed to cool (e.g., in ambient or forced air) to a threshold temperature (e.g., by at least <NUM>% of the peak temperature difference or at least <NUM>%). Thereafter, it may be released from the tool for any further cooling and subsequent assembly to additional components. Depending on configuration, prior to assembling the seal to the seal carrier the seal carrier may be assembled to the bellows spring.

In one example of reengineering from a baseline seal system, an interference fit of the baseline seal is replaced by a lighter interference fit plus the friction increase from the coating. Seal construction may otherwise be preserved. This interference reduction may be achieved by a slight increase in the diameter of the seal carrier ID surface or by a slight decrease in the diameter of the seal OD surface to at least partially offset the coating thickness. In some embodiments, the reduced tensile hoop stresses in the carrier enable the use of lower strength carrier materials that may have more favorable characteristics for seal performance such as lower coefficients of thermal expansion that more closely match that of the seal carbon. For example, an iron-nickel alloy such as ASTM F30 (e.g., Alloy <NUM> or UNS N94100), may replace a steel (e.g., <NUM>-4PH/AMS <NUM> stainless steel). Additionally, the lower stresses in the carbon and carrier may enable cross-sectional geometries that may be more favorable to seal performance but would otherwise not have sufficient structural strength to be acceptable.

However, further advantages may be achieved and may have a cumulative effect and any particular embodiment may involve tradeoffs among the possible advantages. For example, the reduced interference fit reduces stresses in the seal. This may allow a reduction in the cross-sectional area of the seal due to not having to withstand the stresses at a given level of interference. This cross-sectional reduction reduces the weight of the seal.

For thermal interference fits, reduced interference may reduce the heating temperature and thus decrease cycle time and energy used in heating. Similarly, reduced interference may be associated with reduced need for robustness of the seal carrier, allowing material removal from the seal carrier and, thereby, lightening of the seal carrier. Lightening of the seal carrier may have positive feedback by further reducing energy and cycle time for heating in the thermal interference fit.

Lightening of the seal and/or seal carrier and/or sealing ring also allows reduction in the needed bias force from the bias spring(s). This reduced bias force may be associated with reduced spring weight. However, the reduced biased force may have a number of other advantages. Reduced bias force will, all things being equal, reduce seal wear and heat generation. This may improve longevity.

As an example of temperature reduction for thermal interference fit, the baseline seal may use a heating temperature in the range of <NUM> to <NUM>; whereas the revised seal may use <NUM> to <NUM> or other ranges discussed above. The reduction may be of an example <NUM> to <NUM> or <NUM> to <NUM>.

As an example of interference and stress reduction, the baseline seal may have an example compressive stress (e.g., at ambient conditions of <NUM> and <NUM> atm (<NUM> Bar), more broadly <NUM> to <NUM> at <NUM> Bar to <NUM> Bar) of <NUM>,<NUM> psi (<NUM> Mpa), more broadly at least <NUM> Mpa or <NUM> Mpa to <NUM> Mpa; whereas the revised seal may have an example such a stress of <NUM>,<NUM> psi (<NUM> Mpa), more broadly at least <NUM> Mpa or <NUM> Mpa to <NUM> Mpa or <NUM> Mpa to <NUM> Mpa. The reduction may be of an example at least <NUM> Mpa, if present.

Additionally, in some embodiments the reduced interference may allow reduced tolerance requirements.

<FIG> shows an alternate seal cartridge <NUM> wherein the carrier <NUM> differs by having its ID surface <NUM> formed by the surface of a coating layer <NUM> atop an inner diameter surface <NUM> of a metallic substrate <NUM>. In this example, both coatings <NUM> and <NUM> are the same single-layer coating material with thicknesses TM1 and TM2 (which may be in the ranges discussed above). However, in other examples, they may differ. One particular example of the same coating is a silver coating where the silver-on-silver static coefficient of friction is greater than <NUM> at ambient conditions. As with the nickel coating, one or both of the silver coatings may be preceded by a copper strike/flash to provide a base layer.

As with the example nickel coatings, there may optionally be grinding post-spray. There may also be strike/flash layer(s) <NUM>, <NUM> (<FIG> thicknesses TS1 and TS2) for one or both the seal coating and the carrier coatings.

Component materials and manufacture techniques and assembly techniques may be otherwise conventional. For example, there are numerous commercially available annular carbon seal blanks. Such a stock blank may be lathed to profile and may then have material milled and drilled away to reveal any non-annular features such as anti-rotation features (not shown). These commercial blanks are available in a variety of base carbon materials (e.g., carbon graphite and electrographite) with various impregnants (e.g., for strength/cohesion and/or lubricity) suitable for particular operating environments and conditions. Example material is at least <NUM>% carbon by weight, more particularly, at least <NUM>% or <NUM> % or <NUM>% or even commercially pure carbon with inevitable impurities.

Example seats may be machined from an appropriate metal alloy (e.g., a stainless steel). This may be via lathing of an annular blank to a basic profile and then milling and drilling departures from annular (e.g., mounting splines, ID oil channels, and the like if present).

The seal housing and seal carrier may be formed of an appropriate metal alloy (e.g., stainless steel or a titanium alloy) and may be formed such as by pure machining/drilling of a blank or by casting and finish machining.

In further variations (not shown), the single-layer or multi-layer coating(s) may extend along the faces <NUM> and/or <NUM>.

<FIG> schematically illustrates a gas turbine engine <NUM> as one of many examples of an engine in which the seal system <NUM> may be used. The fan section <NUM> may include a single-stage fan <NUM> having a plurality of fan blades <NUM>. The fan blades <NUM> may have a fixed stagger angle or may have a variable pitch to direct incoming airflow from an engine inlet. The fan <NUM> drives air along a bypass flow path B in a bypass duct <NUM> defined within a housing <NUM> such as a fan case or nacelle, and also drives air along a core flow path C for compression and communication into the combustor section <NUM> then expansion through the turbine section <NUM>. A splitter <NUM> aft of the fan <NUM> divides the air between the bypass flow path B and the core flow path C. The housing <NUM> may surround the fan <NUM> to establish an outer diameter of the bypass duct <NUM>. The splitter <NUM> may establish an inner diameter of the bypass duct <NUM>.

The example engine <NUM> generally includes a low speed spool <NUM> and a high speed spool <NUM> mounted for rotation about an engine central longitudinal axis A (forming the axis <NUM>) relative to an engine static structure <NUM> via several bearing systems <NUM>.

The inner shaft <NUM> is connected to the fan <NUM> through a speed change mechanism, which in the example gas turbine engine <NUM> is illustrated as a geared architecture <NUM> to drive the fan <NUM> at a lower speed than the low speed spool <NUM>. The inner shaft <NUM> may interconnect the low pressure compressor (LPC) <NUM> and low pressure turbine (LPT) <NUM> such that the low pressure compressor <NUM> and low pressure turbine <NUM> are rotatable at a common speed and in a common direction. In other embodiments, the low pressure turbine <NUM> drives both the fan <NUM> and low pressure compressor <NUM> through the geared architecture <NUM> such that the fan <NUM> and low pressure compressor <NUM> are rotatable at a common speed. Although this application discloses geared architecture <NUM>, its teaching may benefit direct drive engines having no geared architecture. The high speed spool <NUM> includes an outer shaft <NUM> that interconnects a second (or high) pressure compressor (HPC) <NUM> and a second (or high) pressure turbine (HPT) <NUM>. A combustor <NUM> is arranged in the example gas turbine <NUM> between the high pressure compressor <NUM> and the high pressure turbine <NUM>.

The low pressure compressor <NUM>, high pressure compressor <NUM>, high pressure turbine <NUM> and low pressure turbine <NUM> each include one or more stages having a row of rotatable airfoils. Each stage may include a row of static vanes adjacent the rotatable airfoils. The rotatable airfoils and vanes are schematically indicated at <NUM> and <NUM>.

Claim 1:
An apparatus comprising:
a first member;
a shaft (<NUM>,<NUM>) rotatable relative to the first member about an axis (A); and
a seal system (<NUM>) comprising:
a seal carrier (<NUM>) having:
an axially-extending wall (<NUM>) having an inner diameter (ID) surface (<NUM>); and
a radially-extending wall (<NUM>) having a first surface (<NUM>);
a seal (<NUM>) carried by the seal carrier (<NUM>) in a radial interference fit with the seal carrier axially-extending wall ID surface (<NUM>) and having:
an outer diameter (OD) surface (<NUM>); and
a seal face (<NUM>);
a seat (<NUM>) carried by the shaft (<NUM>,<NUM>) and having a seat face (<NUM>) in sliding sealing engagement with the seal face (<NUM>); and
one or more springs (<NUM>) biasing the seal carrier (<NUM>) relative to the first member so as to bias the seal face (<NUM>) against the seat face (<NUM>),
wherein:
the seal outer diameter (OD) surface (<NUM>) is formed by a coating (<NUM>) on a substrate (<NUM>) of the seal (<NUM>), wherein the seal substrate (<NUM>) is optionally a single piece,
characterised in that the coating (<NUM>) is metallic.