Gas turbine engines and other axial flow turbomachines have rows of rotating blades contained within a generally cylindrical case. It is very desirable to minimize the leakage of the gas or other working fluid around the tips of the blades where they come close to the case. As has been known for some time, this leakage is minimized by blade and sealing systems in which the blade tips rub against a seal attached to the interior of the engine case. Generally, the blade tip is made to be harder and more abrasive than the seal; thus, the blade tips will cut into the seal during those parts of engine operation when they come into contact with each other.
In the earlier systems of the type just described the blade tip was a superalloy material, possibly even having a hard face, and the seal was a metal which had a suitable propensity for wear. For instance, porous powder metals were used. Now however, ceramic containing seals are finding favor, such as those shown in U.S. Pat. No. 3,975,165 to Elbert et al, U.S. Pat. No. 4,269,903 to Klingman et al and U.S. Pat. No. 4,273,824 to McComas et al. The ceramic faced seals are considerably harder than the prior art metal seals and as a result, the prior art blade tips were deficient in being able to wear away the seal with little wear to themselves.
Consequently, there have been developed improved blade tips, most particularly of the type described in U.S. Pat. No. 4,249,913 to Johnson et al "Alumina Coated Silicon Carbide Abrasive" of common ownership herewith. In the Johnson et al invention silicon carbide particulate of 0.20-0.76 mm average nominal diameter is coated with a metal oxide such as alumina and incorporated by powder metal or casting techniques in nickel or cobalt base alloys. A powder metal compact containing 30-45 volume percent particulate may be made and this part is then bonded, such as by diffusion bonding, liquid phase bonding or brazing to the tip of a blade.
However, there are certain inherent characteristics of an abrasive tip made by the foregoing technique. Specifically, the metal part can only be made in a practical minimum thickness, typically of the order of 1-2 mm thick. Usually, the abrasive tip part is made in the cross sectional shape of the tip of the turbine blade substrate. After being compacted or cast it is machined to a flat surface. Likewise, the blade tip is machined to a planar surface to receive the abrasive. Such planar machining is a practical limitation necessary to get good faying fit and minimum weld joint thickness, of the order of 0.05 mm. Unless this is done adequate bond strength in the 1100.degree. C. operating temperature range will not be attained. After bonding of the abrasive on a blade tip, a multiplicity of blades are assembled in a fixture which is adapted to rotate much like the disc of the engine in which they are used. They are then ground to a cylindrical or conical surface which corresponds with the interior surface of the engine case seals. As a result of this procedure, the abrasive will initially have a substantial thickness which will have to be ground to a substantial degree. The particulates are often costly and thus the approach is costly. Second, because practicality dictates a planar joint surface and because the final finished surface of the abrasive tipped blade will be cylindrical or conical, there will be a varying thickness of abrasive across the blade tip, as shown in FIG. 9 herein. While the prior art blade tips are useful, it is more desirable that the abrasive portion of the tip be uniform in thickness across the curved surface. It is also very desirable to minimize the quantity of grits which must be used in the manufacturing process since they must be of the highest quality and their manufacture, including the oxide coating process, is expensive.
An object of the present invention is to provide on the tip of the blade a thin and uniform layer of abrasive coating adapted for use in the vicinity of 1100.degree. C. and higher. Thin layers of particulate bearing abrasive, although not adapted to operate at such high temperatures, have been known. For example, coated abrasives made from alumina, silica and silicon carbide are common products, as are metal bonded diamond and cubic boron nitride grinding wheels. Fused and unfused layers of sprayed metal are well known in the metallizing field. See for example U.S. Pat. No. 3,248,189 to Harris, Jr. and U.S. Pat. No. 4,386,112 of Eaton and Novak, the present applicants. However, any process of metal spraying grits and matrix metal is inherently inefficient in that only a fraction of the sprayed material actually hits and adheres to the surface. These difficulties are especially significant in light of the relatively small size, e.g., about 6 by 50 mm, of a typical turbine blade tip.
Of particular interest in the context of the present invention is the following art. Silicon carbide particles are bonded to a fabric using an organic binder and then overcoated with aluminum, and other metals, according to Fontanella U.S. Pat. No. 3,508,890 and Duke et al U.S. Pat. No. 3,377,264. Fisk et al in U.S. Pat. No. 3,779,726 describe a method of making metal-abrasive tools containing silicon carbide and other grits which comprises encapsulating grit in a porous metal coating and then impregnating the encapsulating layer with other metal to unite the particles. Palena in U.S. Pat. No. 4,029,852 describes how a non-skid surface is made by laying grits on a surface and spraying molten metal droplets over them. The Palena invention involves a relatively crude product, such as a stairway tread, in contrast to the finer product which characterizes metal bonded abrasives and the invention herein. Wilder in U.S. Pat. No. 3,871,840 describes how encapsulating grits in a pure metal envelope improves the properties of a metal bonded abrasive made in various ways.
When an abrasive is used on a superalloy turbine blade tip, its method of application must be metallurgically compatible with obtaining or maintaining the desired properties of the superalloy substrate. Since turbine blade superalloys reflect a highly refined metallurgical art there are real limits on cycles associated with abrasive tip formation. The abrasive is not a structural material and its weight imposes stresses on the blade substrate during use wherein the blade rotates at high speed. Thus it is highly desirable that the minimum thickness abrasive be applied. But since blades are finished to length tolerances of 0.05 mm or less this means both the preparation of the substrate and the application of an abrasive layer must be carried out with high precision. All these considerations place severe restraints on the kinds of material and processing which are useful and considerable research and development has gone into the making of the present invention.