CERAMIC LINED TURBINE SHROUD AND METHOD OF ITS MANUFACTURE

Abstract: An abradable ceramic coated turbine shroud structure (1) includes a grid of slant-steps (3) isolated by grooves (14) in a superalloy metal shroud substrate. A thin NiCrAlY bonding layer (8) is formed on the machined slant-steps. A stabilised zirconia layer (19) is plasma sprayed on the bonding layer (8) at a sufficiently large spray angle (18) to cause formation of deep shadow gaps (22) in the zirconia layer (19). The shadow gaps (22) provide a high degree of thermal strain tolerance, avoiding spalling. The exposed surface of the zirconia layer (19) is machined nearly to the shadow gap ends. The turbine blade tips (4) are treated to minimise blade tip wear during initial abrading of the zirconia layer (19). The procedure results in very close blade tip-to-shroud tolerances after the initial abrading.

Description: The present invention relates to turbine shrouds lined with insulative and abradable ceramic coatings, and a method of making such shrouds. Those skilled in the art know that the efficiency loss of a high pressure turbine increases rapidly as the blade tip-to-shroud clearance is increased, either as a result of blade tip wear resulting from contact with the turbine shroud or by design to avoid blade tip wear and abrading of the shroud. Any high pressure air that passes between the turbine blade tips and the turbine shroud without doing any work to turn the turbine obviously represents a system loss. If insulative shroud technology could be developed in order to provide blade tip clearances to be small over the life of the turbine, there would be an increase in overall turbine performance, including higher power output at lower operating temperatures, better utilisation of fuel, longer operating life, and reduced shroud cooling requirements. To this end, efforts have been made in the gas turbine industry to develop abradable turbine shrouds to reduce clearance and associated leakage losses between the blade tips and the turbine shroud. Attempts by the industry to produce abradable ceramic shroud coatings have generally involved bonding layer of yttria stabilised zirconia (YSZ) to a superalloy shroud substrate using various techniques. One approach is to braze a superalloy metallic honeycomb to the superalloy metallic shroud. The "pore spaces" in the superalloy honeycomb are filled with zirconia containing filler particles to control porosity. These techniques have exhibited certain problems, for example, the zirconia sometimes falls out of the superalloy honeycomb structure, severely decreasing the sealing effectiveness and the insulating characteristics of the ceramic coating. Another approach that has been used to provide an abradable ceramic turbine shroud liner or coating involves the use of a complex system typically including three to five ceramic and cermet layers on a metal layer bonded to the superalloy shroud substrate. A major problem with this approach, which utilises a gradual transition in thermal expansion coefficients from that of the metal to that of the outer zirconia layer, is that oxidation of the metallic components of the cermet results in severe volumetric expansion and destruction of the smooth gradient in the thermal expansion coefficients of the layers. The result is spalling of the zirconia, shroud distortion, variation in blade tip-to-shroud clearance, loss of performance, and expensive repairs. Yet another approach that has been used is essentially a combination of the two mentioned above, wherein on array of pegs of the superalloy shroud substrate protrude inwardly from areas that are filled with a YSZ/NiCrAlY graded system. This system has experienced problems with oxidation of the NiCrAlY within the ceramic and de-lamination of ceramic from the substrate, causing spalling of the YSZ. Another problem is that if the superalloy pegs are rubbed by the blades, blade tip wear is high, causing rapid loss of performance and necessitating replacement of the shroud and blades. GB-A-2061397 discloses a gas turbine engine having a metal substrate and a mechanically attached ceramic sealing layer which is plasma sprayed on to intermediate bonding layers. Another reason that ceramic turbine shroud liners have been of interest is the inherent low thermal conductivity of ceramic materials. The insulative properties allow increased turbine operating temperatures and reduced shroud cooling requirements. Thus, there remains an unmet need for an improved, highly reliable, abradable ceramic turbine shroud liner or coating that avoids massive spalling of the ceramic due to thermal strain, avoids weaknesses due to oxidation of metallic constituents in the shroud, and minimises rubbing of turbine tip material onto the ceramic shroud liner. Accordingly, it is an object of the invention to provide an improved high pressure gas turbine capable of operating at substantially higher efficiency over a longer lifetime than prior gas turbines. It is another object of the invention to provide an abradable turbine shroud coating that allows reduced blade tip-to-shroud clearances and consequently results in substantially higher efficiency. It is another object of the invention to increase the oxidation resistance of an abradable turbine shroud and to avoid massive spalling of the ceramic layer due to high thermal strain between the ceramic layer and the superalloy turbine shroud substrate. It is another object of the invention to provide an abradable ceramic turbine shroud liner or coating that results in high density at a metal bonding interface and lower density and higher abradability at the gas path surface. It is another object of the invention to provide a rub tolerant ceramic turbine shroud coating that reduces the shroud's cooling requirements, decreases shroud and retainer stresses and associated shroud distortion, minimises leakage, and delays the onset of blade tip wear. It is another object of the invention to provide an insulative coating which avoids spalling on a substrate that is subjected to severe high temperature cycling. According to the invention, there is provided a lined turbine shroud comprising a shroud substrate whose inner surface is lined with a layer of ceramic material, characterised in that the inner surface of the substrate is formed with an array of surface discontinuities, each including a steep edge, the ceramic layer including a plurality of shadow gaps, each shadow gap comprising a region of loosely consolidated particles of ceramic material and/or a void region, and extending from a respective steep edge through a substantial portion of the thickness of the ceramic layer. The shadow gaps segment the ceramic layer to minimise spallation by accommodating strains and stresses in the ceramic layer, and the surface is abradable. The array of surface discontinuities may be regular or irregular. Preferably, however, the discontinuities are in the form of an array of steps, each step including a first face having a relatively small slope and a second face adjoining the first face at a corner and having an approximately vertical slope thereby constituting the steep edge, respective steps being separated by an array of intersecting grooves in the inner surface of the shroud. Preferably each step is a slant-step, the maximum height which is approximately 200 mils (5mm) while the maximum depth of the grooves is also approximately 200 mils (5mm). Preferably, each of the first faces has a lower edge adjoining a lower edge of the second face of another of the steps. Preferably, each of the shadow gaps extends along the entire length of a corner of a step or groove. Preferably, the shroud substrate has circular cross-sections and each of the grooves lies in a separate plane intersecting an axis of the circular cross-sections. Thus, in accordance with one embodiment the invention provides an abradable turbine shroud coating including a shroud substrate, wherein an array of steps is provided on the inner surface of the shroud substrate, and a segmented coating is provided on the steps such that adjacent steps are segmented from each other by shadow gaps or voids that propagate from the steps upward entirely or nearly through the coating. The shadow gaps may be produced by plasma spraying ceramic material onto the steps at a plasma spray angle that prevents the coating from being deposited directly on steep faces of the steps, which in the described embodiment are slant-steps. In the described embodiment of the invention, longitudinal, circular parallel grooves and slant-steps having the same or similar heights or depths are formed (by machining, casting, etc.) in the inner surface of the shroud substrate. Shadow gaps propagate upwards into the coating during deposition and segment adjacent steps from each other. The ceramic layer may be attached to the substrate via a bonding layer which may be composed of NiCrAlY. The ceramic material is preferably zirconia for example yttria-stabilised zirconia. Preferably, the bonding layer is less than about 0.1 inches (2.5mm) thick and wherein the ceramic layer is less than approximately 0.5 inches (12.7mm) thick. More preferably, the bonding layer is approximately 3-5 mils (0.076 to 0.127mm) thick and wherein the ceramic is approximately 40 to 60 mils (1.0 to 1.5mm) thick. Preferably therefore, after a suitable cleaning operation, a thin layer of bonding metal is plasma sprayed onto the slant-steps. The ceramic material then is plasma sprayed onto the metal bonding layer at a deposition angle that causes the shadow gaps to form. The metal boding layer is preferably composed of NiCrAlY (or some other suitable oxidation resistant metallic layer), while the ceramic layer is preferably composed of yttria-stabilised zirconia. The preferred height of the slant-steps is 20 mils (0.51mm) and the preferred spray angle of the plasma is 45 degrees, which results in the shadow gap height being approximately twice the height of the slant-steps, or approximately 40 mils (1.0mm). The preferred thickness of the ceramic layer, after machining to provide a smooth cylindrical surface, is approximately 50 mils (1.27mm). Preferably, the exposed surface of the ceramic layer is a smooth cylindrical surface. Accordingly, the invention may be considered to reside in a lined shroud comprising in combination: a shroud substrate having an inner surface; an array of surface discontinuities on the inner surface, each surface discontinuity including a plurality of grooves separating an array of raised areas, each discontinuity having a steep edge; a ceramic layer attached to the raised areas; and a plurality of shadow gaps in the ceramic layer, each shadow gap extending from a steep edge a substantial portion of the way through the ceramic layer and effectively segmenting the ceramic layer. Thermal expansion mismatch strain between the ceramic material and the substrate causes propagation of segmenting cracks from the tops of the shadow gaps to the machined ceramic surface. The shadow gaps accommodate thermal expansion mismatch strain between the metal and ceramic, preventing massive spalling of the ceramic layer. The plasma spray parameters are chosen to provide sufficient microporosity of the outer surface of the ceramic layer to allow abradability by the turbine blade tips. If necessary, spray parameters are selected to provide a higher density at the ceramic-metal interface as needed to provide adequate adhesion. The turbine blade tips may be hardened to provide effective abrading of the ceramic surface and thereby establish a very close shroud to blade tip clearance, without smearing blade material on the ceramic layer. Very high efficiency, low loss turbine operation is thereby achieved without risk of spalling of the ceramic due to thermal strains. The invention also extends to a gas turbine including a shroud substrate having an inner surface; an array of raised areas on the inner surface, each raised area having a steep edge; an array of grooves between the respective raised areas and separating the respective raised areas; a layer of ceramic material attached to the inner surface, the array of grooves effectively segmenting the inner surface; a plurality of shadow gaps in the ceramic layer, each shadow gap extending from a steep edge a substantial portion of the way through the ceramic layer, the layer of ceramic material and the shadow gaps therein forming a segmented abradable ceramic turbine shroud liner; a plurality of turbine blades surrounded by the segmented abradable ceramic turbine shroud liner; and hardened means disposed on an outer tip of each of the turbine blades for abrading the major surface of the ceramic layer. According to another aspect of the invention, there is provided a method of making a lined turbine shroud which comprises lining a substrate with a ceramic material characterised by forming on the inner surface of the substrate an array of discontinuities including an array of intersecting grooves separating an array of raised areas, each discontinuity including a steep edge, and performing a line of sight deposition of the ceramic material uniformly on the inner surface at a spray angle that prevents the ceramic material from being directly deposited on the steep edges so that a plurality of shadow gaps are formed in the ceramic layer as it is deposited, each shadow gap comprising a region of loosely consolidated particles of ceramic material and/or a void region, and extending from a steep edge through a substantial portion of the ceramic layer and segmenting the ceramic layer. The method may include machining a major exposed surface of the ceramic layer to provide a smooth inner ceramic surface. Preferably, the method includes applying a layer of bonding material to the inner surface of the shroud substrate to coat each of the raised areas prior to deposing the ceramic material. Preferably both the bonding layer and the ceramic layer are applied by plasma spraying. Plasma spraying the ceramic material may produce a sufficiently high microporosity in the ceramic layer that the ceramic layer is abradable by tips of turbine blades during operation of the turbine. Plasma spraying the ceramic material may produce a lower level of microporosity in the portion of the ceramic layer adjacent to the slant-steps than at the outer surface of the ceramic layer thereby providing a combination of high abradability of the outer surface of the ceramic layer and high adherence of the ceramic layer to the first faces of the steps. The method may also include rotating a plurality of turbine blades within the shroud to abrade a precisely predetermined amount of ceramic material from the ceramic layer in order to produce a minimum precise clearance between the tips of the turbine blades and the ceramic layer. In such a case it may be necessary to provide a hardened coating on the outer tip of each of the turbine blades capable of abrading the ceramic without smearing superalloy metal of the turbine blades on the ceramic. Accordingly, a preferred method of making a gas turbine shroud comprises the steps of: providing a shroud substrate having a smooth inner surface; forming an array of steps on the inner surface so that each step includes a first face having a small slope and a second face adjoining the first face at a corner and having an approximately vertical slope, and also forming an array of intersecting grooves which separate the steps; performing a line of sight deposition of a ceramic material uniformly over the steps at a spray angle that prevents ceramic from being directly deposited on the second faces so that a plurality of shadow gaps are formed in the ceramic layer as it is deposited, each shadow gap extending above an edge of a step through a substantial portion of the ceramic layer; and machining a major exposed surface of the ceramic layer to provide a smooth, cylindrical, conical or toroidal inner ceramic surface to provide an abradable ceramic liner on the inner surface of the shroud substrate. The invention may be carried into practice in various ways and some embodiments will now be described with reference to the accompanying drawings, in which:- Figure 1 shows a turbine shroud substrate; Figure 2 is an enlarged perspective view of the shroud substrate of Figure 1 showing a pattern of slant-steps and longitudinal isolation grooves in its inner surface; Figure 2A is a section along section line 2A-2A of Figure 2; Figure 2B is a section along section line 2B-2B of Figure 2; Figure 3 is an enlarged section similar to Figure 2A showing plasma spraying of a NiCrAlY bonding layer onto the slant-steps and grooves; Figure 4 is a section similar to Figure 3 showing plasma spraying of a zirconia layer onto the NiCrAlY bonding layer of Figure 3; Figure 5 is a section similar to Figure 3 showing the structure as in Figure 4 after machining of the upper surface of the zirconia layer to a smooth finish; Figure 6 is a diagram showing the results of experiments to determine shadow gap height as a function of step height and groove depth for different ceramic plasma spray angles; and Figure 7 is a partial perspective view illustrating a hardened turbine blade tip to abrade the ceramic turbine shroud coating of the present invention. Referring now to Figure 1, the insulative abradable ceramic shroud coating is applied to a high temperature structural metallic (i.e., HS 25, Mar-M 509) or ceramic (i.e., silicon nitride) ring or ring segment 1 which has a pattern of slant-steps and/or grooves on the inner surface 2 to be coated. Depending upon the structure material, the steps and grooves may be formed by a variety of techniques such as machining, electrodischarge machining, electrochemical machining, and laser machining. If the shroud is produced by a casting process, the step and groove pattern may be incorporated into the casting pattern. If the shroud is manufactured by a rolling process, the step-and-groove pattern may be rolled into surface to be coated. If the shroud is manufactured by a powder process, the step-and-groove pattern may be incorporated with the moulding tool. Referring next to Figure 2, 2A and 2B, the inner surface of the turbine shroud 1 is fabricated to provide a grid of slant-steps 3 covering the entire inner surface 2 of the turbine shroud. The length 6 of the sides of each of the slant-steps 3 is approximately 100 mils (2.5mm). The vertical or nearly vertical edge 4 of each step is approximately 20 mils high (0.51mm) as indicated by reference numeral 5 in Figure 2A. The sides of the slant-steps 3 are bonded by continuous, spaced, parallel V-grooves 14, which are also 20 mils (0.51mm) deep, measured from the peaks 4A of the slant steps. (The grooves 14 need not necessarily be V-shaped, however.) After a conventional grit cleaning operation, thin layer of oxidation resistant metallic material, such as NiCrAlY having the composition 31 parts chromium, 11 parts aluminium, 0.5 parts yttrium and the rest nickel is plasma sprayed onto the slant-stepped substrate 1, as indicated in Figure 3, thereby forming a metallic layer 8. This is carried out by means of a plasma spray gun 10 oriented in the direction of the dotted line 12 at an angle 13 relative to a reference line 11 that is approximately normal to the plane or the substrate 1 (or radial to its overall curvature). In the embodiment described, the spray angle 13 is approximately 15 degrees to ensure that the vertical walls 4 of the slant-steps 3 and the 100mil (2.5mm) square slant-steps are coated with the oxidation resistant metal (NiCrAlY) bonding layer materials as the shroud substrate is rotated at a uniform rate. The thickness of the NiCrAlY bonding layer 8 is 3 to 5 mils (0.076 to 0.127mm). A suitable material for the NiCrAlY metal bonding layer 8 is made by various vendors, for example, Chromalloy. The NiCrAlY layer 8 provides a high degree of adherence to the metal substrate 1, and the subsequent layer of stabilised zirconia ceramic material is highly adherent to NiCrAlY bonding layer 8. Next, as indicated in Figure 4, a layer of yttria stabilised zirconia approximately 50 mils (1.27mm) thick is plasma sprayed by a gun 15 onto the upper surface of the NiCrAlY bonding layer 8 as the shroud substrate is rotated at a uniform rate. The spray direction is indicated by the dotted line 16, and is at an angle 18 relative to a reference line 17 that is perpendicular to a plane tangential to shroud substrate 1. Presently, a spray angle of 45 degrees in the direction shown in Figure 4 has been found to be quite satisfactory in causing "shadow gaps" or voids 22 in the resulting zirconia layer 19. The voids occur because the plasma spray angle 18 is sufficiently large that the sprayed-on zirconia does not deposit or adhere effectively to the steeply sloped surfaces 9 of the metal bonding layer or to one of the nearly vertical walls of each of the grooves 14. This type of deposition is referred to as "line of sight" deposition. Thus, high integrity, bonded zirconia material builds up on and adheres to the slant-stepped surfaces 8A of the NiCrAlY metal bonding layer 8, but not on the almost-vertical surfaces 9 nor on one nearly vertical wall of each of the grooves 14. This results in the formation of either shadow gaps, composed of voids and regions of weak, relatively loosely consolidated ceramic material. These "shadow gaps" propagate upwardly most of the way through the zirconia layer 19, effectively segmenting the 100 mil square slant-steps. The zirconia of the above-indicated composition is stabilised with 8 percent yttria to inhibit the formation of large volume fractions of monoclinic phase material. This particular zirconia composition has exhibited good strain tolerance in thermal barrier coating applications. Segmentation of the ceramic layer will make a large number of ceramic compositions potentially viable for abradable shroud coatings. Chromalloy Research and Technology can perform the ceramic plasma spray coating of the shroud, using the 45 degree spray angle, and selecting plasma spray parameters to apply the zirconia coating with specified microporosity to assure good abradability. In Figure 4, reference numeral 25 represents a final contour line. The rippled surface 20 of the zirconia layer 19 is subsequently machined down to the level of machine line 25, so that the inner surface of the abradable ceramic coated turbine shroud of the present invention is smooth. In the present embodiment of the invention, the shadow gaps 22 have a shadow gap height of approximately 40 mils, (1.0mm) as indicated by the dimension 23 in Figure 4. Figure 5 shows the final machined, smooth inner surface 25 of the abradable ceramic shroud coating of the present invention. A number of experiments have been performed with different zirconia plasma spray parameters to determine a suitable spray angle, stand-off distance, and zirconia layer thickness. Figure 6 is a graph showing the shadow gap height as a function to step height 5 (figure 2). The experiments showed that the depths of the longitudinal V-grooves 14 (Figure 2) should be at least as great as the step height 5. In Figure 6, reference numerals 27,28 and 29 correspond respectively to zirconia plasma spray angles 18 (Figure 4) of 45 degrees, 30 degrees and 15 degrees. The experimental results of Figure 6 show that the heights of the shadow gap 22 (Figure 4) are approximately proportional to the step height and groove depth and also are dependent on the spray angle 18. For the experiments performed, the 45 degree spray angle and step heights (ad groove depths) of 20 mils (0.51mm) (the maximum values tested) resulted in shadow gaps heights of 40 mils (0.1mm) or greater, which was adequate to accomplish the segmentation desired. It is expected that larger spray angles and greater step heights will result in effective segmentation of much thicker insulative barrier coatings and shroud coatings than described above. Changing the distance of the plasma spray gun from the substrate during the plasma spraying of the yttria stabilised zirconia did not appear to affect the shadow gap height for the ranges investigated. In order to test adequately the above-described abradable, segmented ceramic turbine shroud coating, it was necessary to modify the tips of the blades of a turbine engine used as a test vehicle by widening and hardening the blade tips to minimise wear of the turbine blade tip metal on the ceramic shroud coating. In Figure 7, the blade 34 has a thin tip layer 40 of hardened material. Hardened turbine blade tips are well-known, and will not be described in detail. A series of two tests were run with the above-described structure. The first test included several operating cycles, totalling approximately 25 hours. The purpose of this test was to verify that the morphology of the segmented ceramic layer would resist all of the thermal strains without any spalling, and would be highly resistant to high velocity gas erosion under operating temperatures. Clearances were sufficiently large to avoid rubbing in this initial test. As expected, there was no evidence of gas erosion, and no evidence of spalling of any of the 100mil (2.5mm) square zirconia segments isolated by the shadow gaps. Also, there was no evidence of distortion of the metallic shroud structure. In the second test, blade tip-shroud clearances were reduced to permit a rub and cut into the surface of the zirconia coating to test its abradability. Visual examination of the ceramic coated shroud after that test indicated that it was abraded to a depth of about 10 mils (0.25mm). A sacrificial blade tip coating containing the abrasive particles was consumed during the cutting, and a small amount of blade tip metal then rubbed onto the abraded ceramic coating. The relatively severe rub did not result in any spalling, further verifying the superior strain tolerance of the above-described segmented ceramic turbine shroud coating. The above described segmented ceramic turbine shroud coating has been shown to increase substantially turbine engine efficiency by reducing the clearance and associated leakage loss problems between the blade tips and the turbine shroud. The above described technique allows establishment of significantly tighter initial blade tip-shroud clearances for improved engine performance, and permits that clearance to be maintained over a long operating lifetime, because the abradability of the ceramic coating layer prevents excessive abrasion of the turbine blade tips, which obviously increases the clearance (and hence increases the losses) around the entire shroud circumference. Use of a ceramic material insulates the shroud, and consequently reduces the turbine shroud cooling requirements and decreases the shroud and retainer stresses and associated shroud ring distortion, all of which minimise leakage and delay the onset of blade tip rubbing and loss of operating efficiency. More generally, the invention provides thick segmented ceramic coatings that can be used in other applications than those described above, where abradability is not a requirement. For example, the described segmented insulative barrier can be used in combustors of turbine engines, in ducting between stages of turbines, in exit liners, and in nozzles and the like. The segmentation provided by the present invention minimises spalling due to thermal strains on the coated surface. While the invention has been described with reference to a particularly embodiment, those skilled in the art will be able to make various modifications to the described structure and method without departing from the scope of the invention. For example, there are numerous other ceramic materials than zirconia that could be used. Furthermore, there are numerous elements other than yttria which can be used to stabilise zirconia. Although a single microporosity was selected in the zirconia layers tested to date, it is expect that increased microporosity can be obtained by further alteration of the a plasma spray parameters, achieving additional abradability. If necessary, a graded microporosity can be provided by altering the plasma spray parameters from the bottom of the zirconia layer to the top, resulting in a combination of good abradability at the top and extremely strong adhesion to the NiCrAlY bonding metal layer at the bottom of the zirconia layer. Furthermore, a wide variety of regular or irregular step surface or surface "discontinuity" configurations could be used other than the slant-steps of the described embodiment, which were selected because of the convenience of forming them in the prototype constructed. As long as steps on the substrate surface or discontinuities in the substrate surface have steep edge walls from which shadow voids propagate during plasma spraying at a large spray angle, so as to segment the ceramic liner into small sections, such steps or discontinuities can be used. A variety of conventional techniques can be used to form the steps, including ring rolling, casting the step pattern into the inner surface shroud substrate, electrochemical machining and electrical discharge machining, and laser machining. Alternate line of sight flame spray techniques and vapour deposition techniques (e.g. electron beam evaporation/physical vapour deposition) can also apply ceramic coatings with shadow gaps. NiCrAlY is only one of many possible oxidation resistant bonding layer materials that may be used. Alternative materials include CoCrAlY, NiCoCrAlY, FeCrAlY, and NiCrAlY. Non-superalloy substrates, such as ceramic, stainless steel, or refractory material substrates may be used in the future. A bonding layer may even be unnecessary if the structural substrate has sufficient oxidation resistance under service conditions and if adequate adhesion can be obtained between the ceramic coatings and the structural metallic or ceramic substrate. The substrate need not be of a superalloy material; in some cases ceramic material may be used. The shroud substrate can be a unitary cylinder, or comprised of part-cylindrical segments. The term "cylindrical" as used herein includes both complete shroud substrates in the form of a cylinder and cylindrical segments which when connected end to end form a cylinder. For radial turbine applications, the shroud may have a toroidal shape. For some applications, the shroud may be conical.

Claims: A lined turbine shroud (1) comprising a shroud substrate whose inner surface (2) is lined with a layer (19) of ceramic material, characterised in that the inner surface (12) of the substrate is formed with an array of surface discontinuities (3), each including a steep edge (4), the ceramic layer (19) including a plurality of shadow gaps (22), each shadow gap (22) comprising a region of loosely consolidated particles of ceramic material and/or a void region, and extending from a respective steep edge (4) through a substantial portion of the thickness of the ceramic layer (19). A shroud as claimed in claim 1 characterised in that the discontinuities are in the form of an array of steps, (3) each step including a first face having a relatively small slope and a second face adjoining the first face at a corner (4A) and having an approximately vertical slope thereby constituting the steep edge (4), respective steps (3) being separated by an array of intersecting grooves (14) in the inner surface of the shroud. A shroud as claimed in claim 2 characterised in that each of the steps (3) is a slant-step. A shroud as claimed in claim 2 or claim 3 characterised in that each of the shadow gaps (22) extends along the entire length of a corner of a step (3) or groove (14). A shroud as claimed in any of Claims 2 to 4 characterised in that the shroud substrate has circular cross-sections and each of the grooves (14) lies in a separate plane intersecting an axis of the circular cross-sections. A shroud as claimed in preceding claim characterised by a bonding layer (8) attaching the ceramic layer to the substrate. A shroud as claimed in any preceding claim characterised in that the ceramic material is zirconia. A shroud as claimed in Claim 6 or Claim 7 characterised in that the bonding layer is composed of NiCrAlY. A method of making a lined turbine shroud which comprises lining a substrate with a ceramic material characterised by forming on the inner surface (2) of the substrate an array of discontinuties including an array of intersecting grooves (14) separating an array of raised areas (3), each discontinuity including a steep edge (4), and preforming a line of sight deposition of the ceramic material uniformly on the inner surface at a spray angle (18) that prevents the ceramic material from being directly deposited on the steep edges (4) so that a plurality of shadow gaps (22) are formed in the ceramic layer (19) as it is deposited, each shadow gap (22) comprising a region of loosely consolidated particles of ceramic material and/or a void region, and extending from a steep edge (4) through a substantial portion of the ceramic layer (19) and segmenting the ceramic layer (19). A method as claimed in Claim 9 characterised by machining a major exposed surface of the ceramic layer (19) to provide a smooth inner ceramic surface (25). A method as claimed in Claim 9 or Claim 10 characterised by applying a layer of bonding material (8) to the inner surface of the shroud substrate to coat each of the raised areas prior to depositing the ceramic material. A method as claimed in any of Claims 9 to 11 characterised in that both the bonding layer (8) and the ceramic layer (19) are applied by plasma spraying. A method as claimed in any of Claims 9 to 12 characterised by rotating a plurality of turbine blades (34) within the shroud to abrade a precisely predetermined amount of ceramic material from the ceramic layer (19) in order to produce a minimum precise clearance between the tips (40) of the turbine blades (34) and the ceramic layer (19).

Inventors: STRANGMAN, THOMAS E.
Applicants: ALLIED-SIGNAL INC.
Application year: 1987
Main classification: F01D11
IPC codes: F01D11