Effect of aluminum to alumina particles size ratio on the microstructural aspects of aluminide coatings by LTHA pack cementation

In this study, the effects of particle size ratio of filler particles (alumina) and metal source (aluminum) on the microstructural aspects of low-temperature high-activity (LTHA) aluminized coatings were investigated. For this purpose respectively, Al-a, Al-b, and Al-c powders with a mean particle size of 250, 20 and 3 mu m and alumina powder with a mean particle size of 44 mu m were used. According to analysis, the average thickness of the coatings created by Ala, Alb and Alc powders in the pack aluminizing process was measured to be 92.4, 89.6, 56.4 mu m, respectively. It was found that the thickness of aluminide coatings after the first step of LTHA process is directly influenced by the aluminum and nickel concentration gradient between the top and bottom of the coatings. The linear atomic distribution of Al, Ni, Cr, and Ti elements was compared throughout the coatings. According to the results, the coating produced by Alb had the highest Al concentration in its composition which shows the higher rate of AlCln production and deposition on the surface of the coating. The optimized aluminum to the alumina ratio can lead to enhancing gas-state diffusion due to inhibiting the local melting and sintering of Al particles.