Introduction

For materials engineers, modern aircraft engine design and materials selection has been an extremely challenging area. Gas turbine engines, also called jet engines, work on the principle that air passes into the turbine, is compressed, mixed with fuel and is then ignited. The gas mixture is then ejected from the rear of the engine after passing though the turbine stage. The gas stream enters the high pressure turbine section and turns a number of rows of blades called stages which are connected to the main rotor shaft of the engine.

With heat engines, the efficiency is related to the maximum and minimum temperatures in the cycles. Currently today’s engines are running at temperatures of the order of 1350°C, which is achievable due to blade cooling introduced in the 1960s. However the increasing temperature requires new measures and technology to sustain reliability and safety.

The majority of gas turbines are made from nickel-based alloys. However current running temperature of the gas turbine (1350°C) is often in excess of the melting point of these Nickel alloys. In order to overcome this problem, two main advancements have been adopted. One is sophisticated cooling of the blades, using air that bypasses the combustion chamber after the compressor, and the second is low thermal conductivity coatings on the surface of the blade.

 

Gas turbines obviously require protection to prevent the nickel alloy melting at high temperature but also require protection to prevent corrosion at high temperature. Corrosion is defined as the unwanted reaction of a material that results in the dissolution or consumption of the material e.g. the rusting of iron. Corrosion is accelerated by high temperature and impurities present in the air due to the combustion of fuel in the engine. Therefore coating the gas turbine components will protect against both kinds of attack i.e. high temperature and corrosion.

Investment Casting

Investment casting is an industrial process based on and also called lost-wax casting used in the aerospace and power generation industries to produce turbine blades with complex shapes and cooling systems. Blades produced by investment casting can include single-crystal (SX), directionally solidified (DS), or conventional equiaxed blades. The castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys.

 

The ceramic cores are held by the ceramic shell and will generate the inner cavity of the blade once the body of the blade is leached just after the casting process. The solidification of the metal starts in the gap between the ceramic shell and the ceramic core.