High Temperature Erosion Performance of Nanostructured and Conventional TiAlN Coatings on AISI-304 Boiler Steel Substrate

According to modern design philosophy better overall performance can be obtained with the modification of the surface structure and their properties without damaging underlying bulk material or substrate. The surface engineering can be classified in two broad classes: surface modification and surface coating. In the present research TiAlN coating was deposited on AISI-304 grade boiler steel using three different techniques, out of which two were thin nano coatings deposited at different temperatures of 500 and 200 °C developed by Oerlikon Balzers rapid coating system machine under a reactive nitrogen atmosphere. One conventional coating of TiAlN was deposited by plasma spraying method. The coated samples were characterized relative to their coating thickness, microhardness, porosity and micro structure. The optical microscopy, the X-ray diffraction analysis and field emission scanning electron microscope (FESEM with EDAX attachment) analysis have been used to identify various phases formed after coating deposition on the surface of AISI-304 grade boiler steel. The erosion studies were conducted on uncoated as well as coated specimens in simulated coal fired boiler environment using an air jet erosion test rig at various impingement angles of 30°, 60° and 90°. The alumina particles of average size of 50 µm were used as erodent at a velocity of 35 m/s. The eroded samples were analysed with SEM/EDAX and optical profilometer. The main objective of this research work was to increase the life of boiler tubes by using nanostructured and conventional TiAlN coatings and at the same time to compare the performance of coatings with respect to bare AISI-304 grade boiler steel. The nanostructured TiAlN coatings has shown minimum erosion rate as compared to conventional TiAlN coating and uncoated AISI-304 grade boiler steel. Maximum erosion was observed at an angle of 30° as compared to 60° and 90° indicative ductile behaviour.