GB2301110A - Abrasive medium comprising silicon carbide coated with a barrier material - Google Patents

Abstract

An abrasive medium suitable for use in the cutting tip of an aeroengine turbine rotor stage comprises a dispersion of silicon carbide particles in a supporting matrix. Individually coated silicon carbide particles are protected from the deteriorating effects of chemical reaction and interdiffusion by a barrier material in the form of a layer preferably of aluminium nitride. Thus, the cutting ability of the abrasive particles is preserved by the protective layer until a tip rub occurs.

Description

ABRASIVE MEDIUM The invention relates to an abrasive medium. In particular, the invention concerns an abrasive medium suitable for use in an abrasive tip for a turbine blade in a gas turbine aeroengine.

In a turbine stage an abradable liner is provided encircling the rotor assembly with a nominal clearance over the tips of the turbine blades. The abradable liner carries a radially inward facing surface layer of relatively soft material, such as a porous ceramic material, and the tips of the turbine blades are adapted to cut a wear track in the liner surface whenever a tip rub occurs. It is known to employ in a blade tip cubic boron nitride for its cutting ability.

In the prior art, there is known from EP 0517463 an abrasive medium comprising cubic boron nitride grit dispersed in a metallic matrix, for example a nickel alloy layer applied to the tip of a turbine blade in a gas turbine aeroengine. The cubic boron nitride grits are individually coated in oxidation resistant material in the form of aluminium nitride to a depth of about 1520 mm. The aluminium nitride coating is intended to protect the cubic boron nitride grits from the destructively harsh effects of the hot exhaust gas of the engine.

When a tip rub occurs the cubic boron nitride grits at the radial end surface of the blade tip cut into the liner surface. Clearly in the first stage of this event the oxidation resistant coating on the surface of individual cutting grits is worn away exposing the surface facets of the grit itself. However, once the cubic boron nitride is exposed to the harsh, hot gas of the engine exhaust it has a limited life span before it too oxidises. Thus, when a tip rub occurs the grits fulfil their immediate function and remain capable of further cutting for a short period afterwards, but over a period of hours at exhaust gas temperatures the grits progressively oxidise until after, say, ten hours they have disappeared.This represents a temporary loss of cutting ability in this known arrangement although the blade tip still retains an incipient cutting ability because unexposed grits remain embedded in the tip matrix layer and protected by their oxidation resistant coatings. After the loss of previously exposed cutting grits a further tip rub must first remove some of the supporting alloy matrix material before fresh cutting grits are exposed, and this material is transferred to the surface of the abradable liner. The present invention is intended to overcome this drawback.

It is also known from the prior art that silicon carbide has a cutting ability nearly equal to that of boron nitride, and has a longer lifetime compared to boron nitride when exposed to turbine exhaust gas. However, it is a more difficult material to use because it tends to dissolve in matrix materials typically used to tip blades and forms useless compounds. US Patent 4741973 attempts to tackle this problem by employing a multi-layer coating system which chemically isolates the silicon carbide from the matrix material and bonds it to the substrate. It suffers an inherent drawback in that this system is costly in so far as it requires three different layers to be applied in three different operations.

According to the present invention, therefore, there is provided an abrasive medium comprising a dispersion of silicon carbide grits coated in a barrier material.

Preferably the barrier material comprises aluminium nitride. The barrier material is also required to provide a good bond with a substrate material.

The invention and how it may be carried into practice will now be described with particular reference to an exemplary embodiment.

As mentioned above particles of the abrasive medium are embedded in a supporting metal matrix on the radially outer end of a shroudless blade tip. Some of the particles protrude from the surface of the matrix and others are completely embedded in the matrix. Encircling the blades and spaced therefrom by an overtip clearance is an abradable liner. In normal construction the liner comprises a plurality of circumferentially abutting metal backing segments arranged in an annular array. The inner surface of each liner backing segment carries a layer of abradable, for example porous, ceramic material.

Under normal running conditions and at normal engine speeds a running clearance is maintained between the blade tips and the surface of the abradable liner. Above a certain engine speed, or under an exceptional transverse loading for example, the blade tips may make contact with the liner. When this occurs it is intended that the blade tips shall cut a wear track into the surface of the abradable liner material. It is a prerequisite, therefore, that whenever such circumstances occur the blade tips shall possess abrasive or cutting ability.Whereas cubic boron nitride particles as discussed in EP 0517463 are sufficiently hard to be an effective abrasive and its ready-for-use life is significantly, and usefully, improved by an oxidation resistant coating of aluminium nitride, it has been found in practice that the cutting ability of exposed particles is lost rapidly within relatively few hours running time after a tip rub. Silicon carbide has, for practical purposes, much the same hardness as boron nitride but has a slower oxidation rate in a gas turbine exhaust environment. Silicon carbide in contact with the surrounding metal matrix material will lead to the formation of deleterious silicide compounds. However, the multi-layer protective coating system of US Patent 4741973, which is designed to be a barrier and bonding system is a multi-step preparation process so that it is commercially unattractive.

The proposed use of silicon carbide abrasive particles individually coated with barrier material, in the disclosed embodiment a single layer of aluminium nitride, overcomes these drawbacks and provides a substantial improvement in the useful cutting life of abrasive particles after their protective coatings have been breached by a tip rub. The aluminium nitride coating permits good bonding between the ceramic, silicon carbide particles and the metal matrix material without dissolution of the silicon carbide. Unprotected silicon carbide particles may be expected to react with the base metal of a nickel based alloy or an MCrAlY matrix material to form nickel or metal silicides. However, it has been found that a single layer coating of aluminium nitride of thickness of the order of 15-20mm remains stable and preserves the silicon carbide protecting it from the deteriorating effects of chemical reaction and interdiffusion. Furthermore any oxidation of the aluminium nitride which takes place will not cause reaction between the silicon carbide and the metal matrix material, or a nickel based alloy of the substrate.

Silicon carbide particles may be coated using the chemical vapour disposition process described in EP 0517463, in particular in conjunction with the fluidised bed procedure. The disclosure of EP 0517463 (corresponding US Patent No 5355637) is therefore incorporated herein by reference.

Claims (6)

1 An abrasive medium comprising a dispersion of silicon carbide particles coated in a barrier material.

2 An abrasive medium as claimed in claim 1 wherein the barrier material comprises aluminium nitride.

3 An abrasive medium as claimed in claim 1 or claim 2 wherein the silicon carbide particles are dispersed in a metallic matrix.

4 An abrasive medium as claimed in any preceding claim wherein the metallic matrix comprises a nickel or cobalt based alloy.

5 An abrasive medium substantially as hereinbefore described with reference to the particular embodiment.

6 An abrasive tip for a gas turbine engine blade comprising an abrasive medium as claimed in any of the preceding claims.