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 the attractive high temperature properties. Other advantages it can offer are low density (3.76 g/cm³), high temperature oxidation resistant and a relatively high modulus of elasticity. Titanium aluminides have the potential to substitute current Ni-based alloys used at intermediate temperatures. This can result in significant weight savings. Plasma spraying is an economical as well as effective process for producing TiAl coatings. The powders are injected into the high temperature plasma where they become molten droplets. The molten droplets are accelerated towards the substrate where rapid solidification takes place on impact. The coating is formed by individual splats solidifying on top of each other, resulting in a lamellae structure. This paper reports the process parameters that are critical to the preparation of plasma sprayed TiAl coatings. Among these are the plasma gas compositions, spraying distance, spraying atmosphere (normal or low pressure) and are power. The coatings prepared are examined using a scanning electron microscope and image analyser. Fractured surfaces are also studied. The results showed that the coatings have the typical lamellae structure of thermally sprayed coatings and within each lamella there is evidence of columnar grain growth. The coatings were found to contain defects such as pores and ummelted or partially melted particles. Post-spray treatment such as hot isostatic pressing (HIP) was used to improve the microstructure of the coatings. The preliminary results showed improvement in the adhesion between the lamellae and other physical properties such as hardness and strength.