PLASMA SPRAYING OF TITANIUM ALUMINIDE COATINGS - PROCESS PARAMETERS AND MICROSTRUCTURE

Titanium aluminides, Ti3Al and TiAl, have been actively researched for potential applications in aerospace and gas turbine engines due to th e attractive high temperature properties. Other advantages it can offe r are low density (3.76 g/cm(3)), high temperature oxidation resistant and a relatively high modulus of elasticity. Titanium aluminides have the potential to substitute current Ni-based alloys used at intermedi ate temperatures. This can result in significant weight savings. Plasm a spraying is an economical as well as effective process for producing TiAl coatings. The powders are injected into the high temperature pla sma where they become molten droplets. The molten droplets are acceler ated towards the substrate where rapid solidification takes place on i mpact. The coating is formed by individual splats solidifying on top o f each other, resulting in a lamellae structure. This paper reports th e process parameters that are critical to the preparation of plasma sp rayed TiAl coatings. Among these are the plasma gas compositions, spra ying distance, spraying atmosphere (normal or low pressure) and are po wer. The coatings prepared are examined using a scanning electron micr oscope and image analyser. Fractured surfaces are also studied The res ults showed that the coatings have the typical lamellae structure of t hermally sprayed coatings and within each lamella there is evidence of columnar grain growth. The coatings were found to contain defects suc h as pores and ummelted or partially melted particles. Post-spray trea tment such as hot isostatic pressing (HIP) was used to improve the mic rostructure of the coatings. The preliminary results showed improvemen t in the adhesion between the lamellae and other physical properties s uch as hardness and strength.