The Effect of Heat Treatment on the Microstructure and Phase Composition of Plasma Sprayed Al2O3 and Al2O3-TiO2 Coatings for Applications in Biomass Firing Plants

Abstract

This study presents the thermal and chemical resistance of plasma-sprayed Al2O3 and Al2O3 doped with 13 wt.% of TiO2 coatings and their suitability for the fire grate of straw pellet furnaces. Coatings were deposited on steel substrates using direct current atmospheric pressure plasma spraying. The surface structure, elemental, and phase composition of formed coatings were analyzed before and after the thermal treatment, imitating natural application conditions. For the experiment, the annealing temperature was 500 °C for twenty-five cycles (80 min each). It was found that the steel substrate oxidized after five thermal cycles, and the formation of iron oxides was observed. The elemental composition of the Al2O3 and Al2O3-13 wt.% TiO2 coatings remained unchanged even after 25 cycles of heat treatment. The X-ray diffraction (XRD) results revealed that the alpha-Al2O3 to gamma-Al2O3 phase ratio in the Al2O3-TiO2 coating was reduced by only 8.7% after 25 cycles.

Possibility of Phase Transformation of Al2O3 by a Laser: A Review

“…The phase transformation of amorphous Al 2 O 3 to crystalline α-Al 2 O 3 often requires heat treatment at elevated temperatures [57]. In one study, anodic aluminum oxide (AAO) films were formed in a 15% H 2 SO 4 solution by the anodizing method, and the prepared AAO films were heat-treated in the temperature range of 25 Tian et al [33] anodized Al alloys of 99.999% purity in an oxalic acid electrolyte using the two-step anodizing method. Before anodizing, the samples were annealed at 500 • C for 2 h in a nitrogen environment.…”

“…Several studies have demonstrated that the spraying of Al 2 O 3 using standard thermally sprayed techniques usually reduces the content of α-Al 2 O 3 , due to its transformation to the γ-Al 2 O 3 phase [49,50]. On the other hand, the phase transformation of α-Al 2 O 3 to γ-Al 2 O 3 may be significantly reduced by 8.7% using plasma-sprayed Al 2 O 3 coatings doped with 13 wt% TiO 2 [25]. Michalak et al [26] demonstrated that high velocity oxygen fuel spraying and plasma spraying using Al 2 O 3 aqueous suspensions resulted in αphase rich Al 2 O 3 coatings of up to 47 vol% and 62 vol%, respectively.…”

Influence of Plasma Torch Power on the Plasma Jet Properties and Microstructure of Alumina Coatings

“…Using a direct-current (DC) plasma torch developed at the Lithuanian Energy Institute (Kaunas, Lithuania), Al 2 O 3 coatings were applied to steel (P265GH) substrate at atmospheric pressure [26,27].…”