Patent Publication Number: US-7581933-B2

Title: Airfoil having improved impact and erosion resistance and method for preparing same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of Non-Provisional U.S. application Ser. No. 10/898,755 filed Jul. 26, 2004 now U.S. Pat. No. 7,186,092. The entire disclosure and contents of the above non-provisional patent application is hereby incorporated by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   This invention was made with Government support under Contract No. N00019-96-C-0176 awarded by the Office of Naval Research. The Government may have certain rights to the invention. 

   BACKGROUND OF THE INVENTION 
   This invention relates to turbine airfoils, especially turbine compressor airfoils, having improved impact and erosion resistance. This invention further relates to a method for forming improved erosion resistance coatings on these turbine airfoils. 
   Airfoils used in gas turbine engines can suffer from erosion and impact caused by particles ingested into the engine, especially in helicopter turboshaft engines. This is particularly true of the airfoils that comprise the turbine engine compressors. The effects of such ingestion can result in power loss, increased fuel consumption, increased gas turbine generator temperatures, as well as scrapping of compressor components long before the expected fatigue life limits. This performance loss can be sufficient to force these engines to be removed from the aircraft for compressor overhaul to regain lost performance. 
   Turbine compressor performance can be degraded because of impact damage to the leading edges of the compressor airfoils, as well as erosion of portions of the side or surface of the airfoil beyond the leading edge. Erosion and impact damage to airfoils can occur relatively quickly in desert environments due to sand ingestion. The impact of large sand particles can cause burr formation where the leading edge of the airfoil gets rolled or curled over, thus disturbing the airflow and degrading compressor performance. Additionally, erosion on the pressure surface or side of the airfoil contributes to early replacement or removal as the effective surface area of the airfoil decreases and the cross section (i.e., thickness) becomes too thin. 
   Both impact and erosion resistance needs to be addressed to increase the durability and longevity of gas turbine compressors, especially in environments such as the desert where particle ingestion is a significant issue. However, impact and erosion damage is the result of different problems created by the ingestion of these particles. Impact damage is primarily caused by high kinetic energy particle impacts on the leading edge of the airfoil. The particle flow, traveling at relatively high velocities, strikes the leading edge or section of the airfoil at a shallow upward angle of from about 30° below the concave or pressure surface or side of the airfoil, to angle directly or head on to leading edge (0°), i.e., at an angle perpendicular or 90° to the leading edge of the airfoil. Because the airfoil is typically made of relatively ductile metals, this shallow upward to direct or head on striking of the leading edge is what causes burrs to form where the portion of the leading edge struck by the particle deforms and then rolls over or curls. In addition to disturbing the airflow and degrading compressor efficiency, these burrs constrain the airflow, necessitating the engine to compensate by consuming more fuel for the required power. 
   Erosion damage is primarily caused by glancing or oblique particle impacts on the concave or pressure side or surface of the airfoil, and tends to be focused in the area in front or forward of the trailing edge, and secondarily in the area aft or beyond the leading edge. These glancing impacts on the concave airfoil surface can cause portions thereof to be eroded. This erosion typically occurs proximate or at or around the trailing edge nearest the tip of the airfoil. As a result, the airfoil steadily loses its effective surface area due to significant chord length loss, as well as becoming thinner, resulting in a decrease in compressor performance of the engine. 
   Hard coatings, such as titanium nitride and other nitride coatings, have been applied to the metal airfoil to alleviate or minimize such erosion. See, for example, U.S. Pat. No. 4,904,528 (Gupta et al), issued Feb. 27, 1990 (titanium nitride coating for turbine engine compressor components to reduce erosion); U.S. Pat. No. 4,839,245 (Sue et al), issued Jun. 13, 1989 (zirconium nitride coating for turbine blades to provide erosion resistance). However, standard titanium nitride coatings are less capable of resisting impact damage caused by particles that strike the leading edge of the airfoil directly or head on. Titanium nitride coatings can also adversely impact the high-cycle fatigue (HCF) strength of the airfoil. Thicker coatings, such as HVOF applied tungsten carbide coatings, can provide greater impact resistance than titanium nitride coatings. See, for example, U.S. Pat. No. 4,741,975 (Naik et al), issued May 3, 1988 (tungsten carbide or tungsten carbide/tungsten coating applied to a layer of palladium, platinum or nickel on a turbine compressor blade for erosion resistance). However, such tungsten carbide coatings are often too thick and heavy to be applied to fast rotating airfoils, especially for helicopter turboshaft engines, and are generally too thick to be implemented with existing airfoil designs. 
   Accordingly, it would also be desirable to improve the impact resistance properties of turbine airfoils, in particular turbine compressor airfoils used in helicopter turboshaft engines. It would also be desirable to be able to decrease erosion, and especially improve the erosion resistance of such turbine airfoils. It would be further desirable to improve such erosion resistance without adversely affecting other desirable properties of the turbine airfoil such as high-cycle fatigue strength. 
   BRIEF DESCRIPTION OF THE INVENTION 
   An embodiment of this invention relates to a turbine airfoil having improved impact resistance, as well as improved erosion resistance. This airfoil has a leading edge, a trailing edge, a convex curved surface suction side extending from the leading to the trailing edge, and a concave curved surface pressure side extending from the leading to the trailing edge, the pressure side having a leading edge portion aft of the leading edge and a trailing edge portion forward of the trailing edge. This airfoil comprises:
         a. a base segment having an impact resistant leading edge section proximate to the leading edge comprising a material having a yield strength of at least about 250 ksi and an elongation percentage of about 12% or less; and   b. an erosion resistant coating overlaying the base segment at least in the leading and trailing edge portions of the pressure side, the erosion resistant coating comprising at least one ceramic layer having at least one of the following properties: (1) an erosion value of at least about 200 g. of erodent to cause a thickness loss of about 15-20 microns; (2) an erosion volume loss value (V) of about 1.9 or less as defined by the equation V=H −0.18 ×E 0.75 ×F −1.65 , wherein H is hardness, E is elastic modulus and F is fracture toughness; and (3) an F value of at least about 6.0 MPa*m 1/2 .       

   Another embodiment of this invention relates to a method for forming an erosion resistant coating comprising alternating ceramic and metallic layers on the base segment of the airfoil. This method comprises the step of forming on the pressure side of the base segment in an alternating fashion at least one ceramic layer and at least one metallic layer. 
   The airfoils of this invention having the improved impact resistant leading edge section and erosion resistant coating, as well as the method of this invention for forming the erosion resistant coating, provide several benefits and advantages. The impact resistant leading edge section improves the impact resistance properties of turbine airfoils, in particular turbine compressor airfoils used in helicopter turboshaft engines. The erosion resistant coating improves the erosion resistance of such airfoils, but without adversely affecting other desirable properties of the turbine airfoil such as high-cycle fatigue strength. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top plan view of a typical turbine compressor airfoil. 
       FIG. 2  is a sectional view along line  2 - 2  of  FIG. 1 . 
       FIG. 3  is a fragmentary view of an alternative embodiment to that shown in  FIG. 2 . 
       FIG. 4  is a fragmentary view of another alternative embodiment to that shown in  FIG. 2 . 
       FIG. 5  is an enlarged fragmentary view of  FIG. 2  showing an embodiment of the erosion resistant coating. 
       FIG. 6  shows an apparatus for determining the erosion value of ceramic coatings. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As used herein, the term “elongation” refers to the increase in gage length of the specimen measured after fracture of the specimen within the gage length, and expressed as the percentage of the original gage length or “elongation percentage” (E p ) according to the following equation:
 
 E   p =[( L   x   −L   o )/ L   o ]×100
 
   wherein L o  is the original gage length and L x  is the final gage length. See Davis, ASM Materials Engineering Dictionary (1992), p. 114 
   As used herein, the term “fracture toughness” refers to the measurement of the resistance of the specimen being tested to extension of a crack. Fracture toughness of a specimen is measured herein by the Charpy impact test. See Davis, ASM Materials Engineering Dictionary (1992), p. 72. 
   As used herein, the term “hardness” refers to the measurement of the resistance of the material to surface indentation or abrasion. The hardness of a material is measured herein by units of indentation (in GPa) using Brinell or Rockwell hardness tests for metallic materials and Vickers or Knoop hardness tests for ceramic materials. See Davis, ASM Materials Engineering Dictionary (1992), p. 200. 
   As used herein, the terms “modulus of elasticity” or “elastic modulus” (E) refer interchangeably to the measurement of the rigidity or stiffness of the material, expressed as the ratio of stress, below the proportional limit, to the corresponding strain. See Davis, ASM Materials Engineering Dictionary (1992), p. 280. The E value (in GPa) of a material is determined herein by the 4 point bend test. 
   As used herein, the term “yield strength” refers to the stress at which a material exhibits a specified deviation from proportionality of stress and strain. See Davis, ASM Materials Engineering Dictionary (1992), p. 534. 
   As used herein, the term “comprising” means various compositions, compounds, components, layers, steps and the like can be conjointly employed in the present invention. Accordingly, the term “comprising” encompasses the more restrictive terms “consisting essentially of” and “consisting of.” 
   All amounts, parts, ratios and percentages used herein are by weight unless otherwise specified. 
   The embodiments of the turbine airfoils of this invention having improved impact and erosion resistance include turbine airfoils that can be removably attached to a central hub or disk, or can be turbine airfoils integral with the hub or disk, i.e., a turbine blisk. These turbine airfoils (e.g., turbine blades) can be used with a wide variety of turbine engine (e.g., gas turbine engine) components. These turbine engine components include turbine fans, turbine vanes, and turbine compressors. The embodiments of the turbine airfoils of this invention having improved impact and erosion resistance are particularly useful in turbine compressors and especially those turbine compressors used in helicopter turboshaft engines. However, while the following discussion of embodiments of the turbine airfoils of this invention having improved impact and erosion resistance will be with reference to turbine compressor airfoils or blades, it should also be understood that this invention can be useful for other turbine airfoils requiring impact and erosion resistance. 
   The various embodiments of the turbine airfoils of this invention are further illustrated by reference to the drawings as described hereafter. Referring to the drawings,  FIG. 1  depicts a typical turbine compressor blade indicated generally as  10 . Blade  10  has a leading edge indicated as  14 , a trailing edge indicated as  18 , a tip edge indicated as  22  and a blade root indicated as  26 . The span of blade  10  is indicated as  28  and extends from tip edge  22  to blade root  26 . 
     FIG. 2  shows the convex curved surface or side of blade  10  (also referred to as the “suction” side or surface of the blade) indicated generally as  30  that extends between leading and trailing edges  14  and  18 , as well as the concave curved surface or side of blade  10  (also referred to as the “pressure” surface or side of the blade) indicated as  34  that also extends between leading and trailing edges  14  and  18 . The dashed line indicated by  38  that extends from the leading edge  14  to the trailing edge  18  defines the width or chord of blade  10 . The double headed arrow indicated by  42  between suction side  30  and pressure side  34  defines the thickness (usually measured as the “maximum” thickness) of blade  10 . 
   Referring to  FIG. 2 , the leading edge section of blade  10  indicated generally as  46  is where the greatest impact damage tends to occur, especially at or proximate to leading edge  14 . Referring to  FIGS. 1 and 2 , the area of greatest erosion damage tends to occur in the tip edge portion or area of span  28  indicated generally as  50 , especially at or proximate to tip edge  22 , and also tends to be focused in the portion or area of pressure side  34  indicated generally as  54  that is directly forward of trailing edge  18  and to a lesser extent in the portion or area indicated generally as  58  of pressure side  34  that is directly aft of leading edge  14 . 
   To prevent or minimize impact damage, and as shown in  FIG. 2 , blade  10  is provided with a higher yield strength and lower elongation material in at least the leading edge section  46  of base segment or substrate  60  of blade  10 . As also shown in  FIG. 2  and to prevent or minimize erosion damage, an erosion resistant coating indicated generally as  62  is overlayed on base segment  60 . 
   Suitable materials for preventing or minimizing impact damage in the leading edge section  46  of base segment  60  will have a yield strength of at least about 250 ksi, typically at least about 300 ksi and more typically at least about 320 ksi, and typically in the range of from about 300 to about 360 ksi, more typically in the range of from about 320 to about 360 ksi. Suitable metals or metal alloys will also have an elongation percentage or E p  value of about 12% or less, typically less than about 8%, more typically less than about 5%. Suitable materials having such higher yield strength and lower elongation percentage properties include metals and metal alloys such as iron (i.e., steel and steel alloys), nickel and nickel alloys, as well as cemented carbides and other composite materials. Representative metals and metal alloys having such higher yield strength and lower elongation percentage properties include 18Ni(350) (a steel alloy comprising 18.0% Ni, 4.2% Mo, 12.5% Co, 1.6% Tl, 0.1% Al), S7 (a steel alloy comprising 0.5% C, 0.5% Mn, 0.6% Si, 3.25% Cr, 1.5% Mo, 0.25% V), GE-1014-SS2 (a steel alloy comprising 0.25% C, 9.7% Co, 1.35% Al, 1.3% Mo, 13.1% Ni, 2.2% Cr), Elmax (a steel alloy comprising 1.7% C, 0.8% Si, 0.3% Mn, 18.0% Cr, 1.0% Mo and 3.0% V) and BG 42 (a steel alloy comprising 1.15% C, 0.3% Si, 0.5% Mn, 14.5% Cr, 4.0% Mo and 1.2% V). 
   These materials having higher yield strength and lower elongation percentage properties typically minimize burring of leading edge  14  to less than about 5 mils (127 microns), more typically to less than about 2 mils (51 microns), as well as minimize chord loss (i.e., relative to chord  38 ) to typically less than about 15 mils (381 microns), more typically less than about 12 mils (305 microns). The degree of burring and chord loss is typically measured by sending a stream of sand particles of particle size up to about 60 mils (1524 microns) at a velocity of about 1000 mph (1609 km/h) upwardly at an angle of about 20° against a test airfoil specimen having a thickness of about 10 mils (254 microns) at the leading edge  14 . Burring is measured optically as the height of the burr above the plane formed by the convex/suction side  30  of the test airfoil specimen at leading edge  14  (i.e., how much edge  14  is raised up); the degree of burring is determined by averaging the three highest measurements over each 1.5 inch (3.81 cm) section of the test airfoil specimen along leading edge  14 . The chord loss is determined by: (1) measuring the chord dimension  38  of the test airfoil specimen, both before and after testing, with calipers at 3 spots along the length of the test airfoil specimen; and (2) averaging the difference between the measured chord values before and after testing. 
   As shown in  FIG. 2 , the higher yield strength, lower elongation percentage material can comprise all or substantially all of the base segment  60  of blade  10 . An alternative embodiment of base segment  60  is shown in  FIG. 3 . In this alternative embodiment, base segment  60  comprises a forward impact resistant portion indicated generally as  66  proximate to the leading edge section  46  that comprises the higher yield strength, lower elongation percentage material, and a rearward main portion indicated generally as  70  that can comprise a material (e.g., metal or metal alloy) typically used in blades  10  that does not necessarily have higher yield strength and lower elongation percentage properties (e.g., can a have a yield strength of less than about 250 ksi and an elongation percentage of greater than about 7.5%), such as steel and steel alloys, nickel based alloys, titanium and titanium based alloys, for example, A286, AM355, IN718 and Ti6-4. Impact resistant portion  66  and main portion  70  can be joined, attached or otherwise associated together in a suitable manner such as by welding, using a suitable adhesive, mechanically locking together (e.g., a dovetail fit) or simply by the overlayed erosion resistant coating  62  if of suitable strength and durability for this purpose. 
   Another alternative embodiment to provide impact resistance for blade  10  is shown in  FIG. 4 . As shown in  FIG. 4 , the impact resistant portion of leading edge section  46  is in the form of an impact resistant sheath indicated generally as  74  that comprises the higher yield strength, lower elongation percentage material. This sheath  74  has an upper or convex segment  76  and a lower or concave segment  80  that permit sheath to be joined, attached or otherwise associated with a complementary shaped forward portion  84  of main section  86  of base segment  60  that can comprise a material (e.g., metal or metal alloy) typically used in blades  10  that does not necessarily have higher yield strength and lower elongation percentage properties (e.g., can a have a yield strength of less than about 250 ksi and an elongation percentage of greater than about 7.5%), as previously described and defined for main portion  70  of the embodiment shown in  FIG. 3 . 
   Erosion resistant coating  62  can have a thickness of at least about 15 microns, with a thickness typically in the range of from about 15 to about 51 microns, more typically from about 20 to about 25 microns. Coatings  62  comprise at least one ceramic layer and optionally at least one metallic (metal) layer. This ceramic layer(s) typically comprises at least one ceramic material selected from the group consisting of tantalum carbide, niobium carbide, titanium carbide, titanium aluminum chromium carbide (TiAlCrC), titanium aluminum chromium nitride (TiAlCrN), titanium aluminum nitride (TiAlN), titanium aluminum carbide (TiAlC), boron carbide (B 4 C), as well as combinations thereof The metallic (metal) layer(s) typically comprises at least one metal selected from the group consisting of tantalum, niobium or titanium, as well as alloys thereof, including titanium aluminum alloys and titanium aluminum chromium alloys. 
   The ceramic materials for the ceramic layer(s) and metals/metal alloys for the optional metallic layer(s) are selected such that coating  62  imparts suitable erosion resistance properties to blade  10  at least in the portion or area  54  and  58  of pressure side  34 , typically over the entire or substantially the entire area of pressure side  34 , and more typically over the entire or substantially the entire area of pressure side  34  and suction side  30 . 
   More typically, suitable coatings  62  for use herein comprise a plurality of alternating ceramic and metallic (metal) layers formed from at least one ceramic layer and at least one metallic layer. Coatings  62  formed solely or substantially of ceramic layers tend to have increased stresses that could cause coating  62  to weaken and spall. Coatings  62  that comprise alternating metallic and ceramic layers tend to mitigate or minimize such stresses. The metallic layers typically comprise the metal atom(s) of the corresponding ceramic in ceramic layers. For example, where the ceramic layer comprises titanium carbide, the metallic layer typically comprises titanium. However, although less desirable, the metallic layers can comprise a different metal atom(s) from that of the ceramic in the ceramic layers. The ceramic layers can also comprise different ceramics, e.g., tantalum carbide in one ceramic layer and niobium carbide in another ceramic layer. 
   An embodiment of erosion resistant coating  62  comprising a plurality of alternating ceramic and metallic layers is shown in  FIG. 5 . As shown in  FIG. 5 , coating  62  comprises alternating metallic layers  90 ,  94  and  98  and ceramic layers  102 ,  106  and  110 . As shown in  FIG. 5 , a metal layer such as  90  is typically the innermost of coating  62  and is directly adjacent to substrate  60 , while the outermost layer of coating  60  is typically a ceramic layer such as  110 . Having metal layer  90  directly adjacent to substrate tends to promote the adherence of coating  62  to substrate. Having ceramic layer  110  be the outermost layer tends to provide a coating  62  having greater erosion resistance. However, although less desirable, the order of metallic and ceramic layers can be reversed such that layer  90  could comprise a ceramic layer directly adjacent to substrate  60 , while outermost layer  110  comprise a metal layer. 
   The erosion resistant coatings  62 , including those comprising a plurality of alternating ceramic and metallic layers, are typically formed by physical vapor deposition (PVD), such as EB-PVD, filtered arc deposition, and more typically by sputtering. Suitable sputtering techniques for use herein include but are not limited to direct current diode sputtering, radio frequency sputtering, ion beam sputtering, reactive sputtering, magnetron sputtering and steered arc sputtering. Typically, magnetron sputtering and steered arc used in the method of this invention. In forming the ceramic layers (e.g., layers  102 ,  106  and  110 ) comprising carbides or nitrides, sputtering is typically carried out in an atmosphere comprising a source of carbon (e.g., methane) or a source of nitrogen (e.g., nitrogen gas). In forming the metallic layers (e.g., layers  90 ,  94  and  98 ), sputtering is typically carried out in an inert atmosphere (e.g., argon). 
   The erosion resistant coatings  62  minimize volume loss due to erosion. Ceramic components of coatings  62  having potentially suitable minimum volume loss due to erosion can be selected by the following equation:
 
 V=H   −0.18   ×E   0.75   ×F   −1.65  
 
wherein V is the volume loss due to erosion, H is the hardness of the coating material (usually based on the ceramic material), E is elastic modulus of the coating material (usually based on the ceramic material), and F is fracture toughness of the coating material (usually based on the ceramic material). Suitable coatings  62  will typically have a V value of about 1.9 or less. Coating materials with such minimum V values usually have H values of at least about 19 GPa and typically in the range of from about 19 to about 30 GPa, more typically from about 25 to about 30 GPa; E values of at least about 200, and typically in the range of from about 200 to about 800 GPa, more typically from about 200 to about 750 GPa; F values of at least about 1.5 MPa*m 1/2 , and typically in the range of from about 1.5 to about 14 MPa*m 1/2 , more typically from about 6 to about 14 MPa*m 1/2 . In this equation for volume loss due to erosion, the F value or fracture toughness tends to predominate because of the wider range of F values for the various coating materials.
 
   Suitable coatings  62  also usually have minimum erosion values (based on the number of grams (g.) of erodent required to cause coating  62  to lose about 15-20 microns of its thickness). Suitable coatings  62  for use herein typically have erosion values of at least about 200 g. of erodent to cause about 15-20 microns of thickness loss in coating  62 , more typically at least about 800 g. of erodent. Erosion values herein are determined by testing carried out using a test apparatus (see  FIG. 6 ) indicated generally as  200 . Test apparatus  200  includes test stand indicated as  204  to support the test specimen indicated as  208 . The erodent in the form of ˜50 micron alumina particles is stored in a vibrator hopper indicated as  212 . Hopper  212  feeds the alumina particles in about 50 g. increments to a blast gun (“Port a Blast” gun, Lindberg Products Co.) indicated as  216 . Blast gun  216  fires the alumina particles at test specimen  208  at an incidence angle of 20° and a velocity of about 160 ft./sec. (48.8 m./sec) due to compressed air supplied from air supply line  220  that goes through a connecting air line indicated as  224 . Air from air supply line  220  enters connecting air line  224  through a shutoff valve indicated as  228  and passes through a filter indicated  232 . A solenoid valve  236  for opening and closing air line  224 , a pressure regulator  240  for controlling pressure at the set point and a precision bore flow meter  244  for measuring the flow of air through air line  224  are also provided. Connecting air line  224  supplies compressed air to blast gun  216  through a connector indicated as  248 . Apparatus  200  is also provided with a timer  252  for determining whether the alumina particles flowed out of hopper  212  within the desired time. 
   The erodent testing using apparatus  200  is carried out as follows: 
   1. Test Specimens. Test specimen  208  are in the form of a 1 inch (2.54 cm.) by 2 inch (5.08 cm.) panel having a thickness of from 50 to 185 mils (1.27 to 4.70 mm). The panels are cleaned, pretreated and coated with coating  62 ; a minimum of two coated panels are used in each evaluation. 
   2. Erosion Testing. Test specimen  208  is firmly supported by test stand  204  at an angle of 90°±4° to the axis of the blast nozzle of blast gun  216  and at a distance of 4.0±0.06 inches (101.6±1.5 mm) between the tip of the nozzle and the surface of specimen  208  to be tested. Alumina particles (average particle size of 50 microns) are used as the erodent. The pressure in connecting air line  224  is adjusted to provide a pressure of 25 psi (172 kPa) to blast gun  216  during erodent testing until the erodent particles in hopper  212  are consumed. 
   After specimen  208  is subjected to erodent testing with apparatus  200 , the deepest point of erosion of specimen  208  is measured with a ball point micrometer with at least 3 measurements being made and the results normalized and reported as microns eroded per grams (g.) of erodent. 
   While specific embodiments of this invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of this invention as defined in the appended claims.