Abstract:
A vibration damped turbo-machine blade includes a shot peened metallic substrate which provides a shape for the blade. Carried on the metallic substrate and bonded to an outer surface of this substrate is a singular ceramic coating of a damping material. The substrate may be made of forged titanium, and the coating may be made of a ceramic material including cobalt at a weight percentage of from about 13% to about 21%, with the balance of the ceramic material being substantially all tungsten carbide.

Description:
This application claims the benefit of Stoker et al. provisional application Ser. No. 60/052,813 filed on Jul. 17, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to vibration damped blades for turbo-machinery. More particularly, the present invention relates to vibration damped fan and compressor blades for such turbo machinery, which blades include a metallic substrate and a vibration damping coating bonding with a surface of the metallic substrate and defining an exterior surface for the blades. 
     Turbo-machinery such as combustion turbine engines and air cycle machines include high-speed turbine wheels, compressor wheels, and fans that expand, compress, and move ambient air or other working fluids. Blades of the wheels and fans frequently encounter vibrations. The vibrations can affect fatigue life of the blades and, consequently, shorten the useful life of the blades. 
     U.S. Pat. No.3,301,530 to W. R. Lull and U.S. Pat. No.3,758,233 to Cross et al. both show vibration damping coatings applied to turbo-machine blades. The blades and coatings shown in the Lull and Cross et. al. patents both carry coatings of more than one layer. The Lull patent shows intermediate and overlying outer sub-layers that are both made of metals having differing coefficients of elasticity. Similarly, the Cross et. al. patent shows coating sub-layers that are selected from a ceramic, and from a mixture of the selected ceramic along with the metal from which the turbo-machine blade itself is formed. 
     Such damped blades and vibration damping coatings utilizing plural sub-layers can be both expensive and difficult to manufacture. Particularly, the vibration damping coatings can be difficult to apply successfully. Because of the necessity to control such factors as the thicknesses of the sub-layers, the interbonding of the sub-layers with the substrate of the blade and with one another, and other manufacturing parameters, the opportunities for error in the manufacture of such vibration damped blades is increased, and the opportunities for variability in the manufacturing process are multiplied. The differing materials of the sub-layers shown in the Lull and Cross et al. patents are likely to have differing coefficients of thermal expansion that may lead to separation of these layers during manufacturing or during use of the blade. Thus, manufacturing costs for vibration damped blades utilizing the known technology may be high, and scrap and error rates may also be excessive. 
     SUMMARY OF THE INVENTION 
     The present invention can be regarded as a vibration damped blade that overcomes one or more of these problems. The vibration damped blade includes a metallic substrate, and a singular ceramic vibration damping coating carried on an outer surface of the metallic substrate; with the singular coating forming both an interface with the outer surface of the metallic substrate and extending outwardly to define a respective outer surface of the ceramic coating, with intermediate material of the coating between the two interfaces of the coating being substantially homogeneous. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is an axial view of a turbo-machine fan having plural blades, each of which is damped in accord with the present invention; 
     FIG. 2 is a cross sectional view taken at line 2--2 of FIG. 1; and 
     FIG. 3 is a flow chart of a method of making a vibration damped blade according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an exemplary turbo-machine fan 10. It is understood that the present invention is not limited to embodiment in such a fan 10, but may also be applied to and embodied in, for example, compressor blades and turbine blades of turbo-machinery. The fan 10 includes a hub portion 12 defining a central bore 14, through which a tie bolt (not shown) may pass in order to secure the fan to other components (also not shown) of a turbo-machine. The hub portion 12 defines an outer circumferentially extending surface 16 from which plural fan blades 18 extend radially. The blades 18 are in this case integral with the hub portion 12, although such need not be the case. Each blade 18 includes a substrate having a root radius portion 20 which the blade blends into the hub portion 12, a leading edge 22, a trailing edge 24, and a radially outer tip surface 26. Generally the hub 12 and blades 18 are formed of metal. Particularly, the hub 12 and blades 18 may be formed of titanium metal. Additionally, the surfaces of the blade substrates are shot peened. The shot peening creates residual compressive strength in the substrates. A forged and shot peened form of titanium metal known as Ti 6Al-4V may be used for the fan 10 of an air cycle machine. 
     As is seen in FIG. 2, the metal blades 18 define an outer surface 28 (i.e., a metal surface) to which is bonded a singular homogeneous coating 30 of vibration damping material. The vibration damping coating 30 defines an interface at 32 with the metal surface 28, and extends outwardly to define an outer surface 34. It is the outer surface 34 of the coating on blades 18 which is shown in FIG. 1. In between the interface 32 and the outer surface 34, the material of coating 30 is substantially homogeneous, and has no internal interfaces or sub-layers. The material most favored for the coating 30 is tungsten carbide cobalt. This tungsten carbide cobalt material may be applied using a variety of available processes, but a process known as HVOF (high velocity oxygen-fuel) has been used successfully to practice the invention. Other available application processes such as CVD, PVD, thermal spray, detonation gun, and plasma spray application may be used to apply the coating 30. 
     Surprisingly, the tungsten carbide cobalt material actually has a coefficient of elasticity which is higher than that of the metal from which the fan 10 is formed, so that one might believe that any cracks which formed in the coating 30 would propagate into the underlying metal and result in a shortened service life for the fan 10. However, the improvement in fatigue life of the combined metal substrate forming the blades 18 along with the coating 30 and the residual compressive strength from the shot peening results in a longer life for the blades 18 (in contrast to a blade having only a titanium metal substrate). 
     Most preferably, the coating 30 is applied to the surface 28 of the metal blades in a thickness of from about 0.003 inch to about 0.008 inch everywhere except at the blade root radius area 20 and at the blade tip surface 26. At the blade root radius 20, the coating 30 is about 0.001 inch thick. No coating is required at the tip surface 26. The constituents of coating 30, by weight, are most preferably: 
     
         ______________________________________Cobalt               13% to 21%Tungsten carbide     balanceOther                max of 1%______________________________________ 
    
     Microhardness of the applied coating 30 is preferably 900 HV300 minimum when tested according to the ASTM E 384 standard. A bond strength of the coating 30 to the surface 28 of the titanium metal of 10,000 psi minimum is preferred, when tested in accord with ASTM standard C 633. Testing of a fan embodying the present invention as described herein has shown an improvement in fatigue life of about two and half to one over the life of a fan made only from the forged titanium metal alone with no vibration damping coating on it. 
     A method of making the present vibration damped blade, as illustrated in FIG. 3, includes steps of forming a blade substrate of a metallic material having a metallic surface (block 100); shot peening the metallic surface of the substrate (block 102), applying and bonding to this metallic surface a singular layer of damping material (block 104); and using the single layer of damping material to reduce vibrations of the substrate of metallic material (block 106). Additionally, it is seen that the single layer of vibration damping material is utilized to define an interface with the metallic substrate, and that the layer of damping material extends homogeneously outwardly of the metallic surface of the substrate to define an outer surface for the blade. 
     The metallic substrate for the blade may be formed of forged titanium selected as Ti 6Al-4V alloy. This forged titanium form for the blade is shot peened all over (including the root portion) before the single layer of vibration damping material is applied. A single layer of vibration damping material having a thickness between about 0.001 inch and about 0.008 inch is then applied and bonded to the metallic surface of the substrate by using a process such as thermal spraying. 
     A specific embodiment of the invention has been described and illustrated above. However, the invention is not limited to the specific forms or arrangements of parts so described and illustrated. For example, the substrate could be made of aluminum or steel bar stock or casting instead of forged titanium. Residual compressive strength can be created in a substrate by ways other than shot peening. Accordingly, the invention is construed according to the claims that follow.