This invention relates generally to composite compositions suitable for high pressure and elevated temperature environments. More particularly, the invention relates to composite compositions for abrasive tips of turbine blades, and methods for joining the abrasive tips to the turbine blades.
Typically, gas turbine engines are comprised of three major sections or components which function together to produce thrust for propulsion, or energy for generating power. The compressed air from the compressor section is delivered to a combustion section where fuel is added to the air and ignited, and thereafter delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor.
Each turbine section includes a rotor assembly comprising a plurality of rotor blades, each extending radially outward from a disk across the airflow path. More specifically, each rotor blade has a dovetail which engages with the disk, and an airfoil extending radially from the disk to a blade tip at the opposite end. In the initial sections of the engine, the compressor blades are generally made of titanium alloys, or a martensitic stainless steel. In the later sections, the high pressure turbine blades are generally made of ferrous or nickel base alloys.
A shroud encompasses the blade tips with as little radial gap (clearance) as possible, in order to minimize bypass flow of air or other gases past the tips of the blades. The purpose of the narrow gap is to minimize gas leakage and to allow the pressure of the gas to increase from one section or stage to the next. A narrower gap (or reduced clearance) between the tips and the adjacent shroud usually increases the engine efficiency and power output of the engine. For example, every 10 mil decrease in the clearance in the hot gas path can significantly improve the power output of the turbine engines.
The minimization of the gap is often limited by several factors, for example, manufacturing tolerances, differing rates of thermal expansion, mass inertia effect, and dynamic effects. If the gap is too narrow, there is the possibility of undesirable contact (e.g., rubbing) between the tip and the shroud. In these cases, the blade can heat up faster than the surrounding shroud, and thus come into contact with the shroud, due to thermal expansion and mass inertia differentials. There are likely other mechanisms (e.g., deformation, oxidation) that also cause this contact (e.g., rubbing), and in turn, damage or unduly wear the blade tips. For example, the inner diameter of the shroud is not usually concentric with the axis of rotation of the blades, and the blade tips can rub the surrounding shroud. Damage to the blade tips leads to widening the gap between the blade tip and the surrounding shroud, and allowing the gas to leak through, which can, in turn, degrade the efficiency of the turbine. Severe damage can even lead to cracking of the blade and possible blade failure.
Several approaches have been proposed to address the problem of the blade tip damage and air leakage within the gas-flow path. One approach is to apply a clearance sealing layer on the inner diameter of the shroud, so that the sealing layer can be abraded away by the blade tip, as disclosed in U.S. Pat. Nos. 4,540,336 and 4,280,975 or 5,704,759. Another approach is to incorporate a cutting edge (“squealer tip”) at the blade tip. As an example, a cutting edge of a blade tip typically includes an abrasive tip coating. As disclosed in U.S. Pat. Nos. 5,704,759 and 4,169,020, commonly used abrasive tips or tip coatings often entailed abrasive particles dispersed in a metallic matrix.
Much emphasis has been placed on developing suitable combinations of metal matrix materials with abrasive particles, as well as methods for their manufacture. Suitable metal matrix materials must exhibit acceptable environmental resistance (e.g., oxidation and hot corrosion resistance) to the operating temperature of a gas turbine engine. Various materials have been proposed, including nickel-base alloys (e.g, NiCoCrAlY) dispersed with an abrasive material such as nitrides (e.g., cubic boron nitride), carbides, and oxides. Furthermore, a variety of methods, such as spraying, soldering, electroplating etc, have been proposed to provide the tip coatings on the blade. However, currently used tips or tip coatings have several issues, for example, depletion of elements such as aluminum and chromium, spallation of the coatings, fatigue or other defects, and hydrogen embrittlement (usually in case of electroplated tips).
Therefore, there is a need for a material for a blade tip, and a blade tip thereof with improved characteristics to meet performance requirements for turbine applications. In some preferred embodiments, manufacturing the tip should be less complicated and less expensive than existing tip coating methods.