Reactive Plasma-Sprayed Aluminum Nitride-Based Coating Thermal Conductivity

Abstract

Recently, thick aluminum nitride/alumina (AlN/Al2O3) composite coatings were successfully fabricated through the reactive plasma spraying of fine Al2O3/AlN mixture in the N2/H2 atmospheric plasma. The coatings consist of AlN, Al5O6N, γ-Al2O3, and α-Al2O3 phases. This study will evaluate the thermal conductivity of these complicated plasma-sprayed coatings and optimize the controlling aspects. Furthermore, the influence of the process parameters on the coatings thermal conductivity will be investigated. The fabricated coatings showed very low thermal conductivity (2.43 W/m K) compared to the AlN sintered compacts. It is attributed to the phase composition of the fabricated coatings, oxide content, and porosity. The presence of Al2O3, Al5O6N and the high coating porosity decreased its thermal conductivity. The presence of oxygen in the AlN lattice creates Al vacancies which lead to phonon scattering and therefore suppressed the thermal conductivity. The formation of γ-Al2O3 phase in the coating leads to further decrease in its conductivity, due to its lower density compared to the α-phase. Moreover, the high porosity of the coating strongly suppressed the conductivity. This is due to the complicated microstructure of plasma spray coatings (splats, porosity, and interfaces, particularly in case of reactive spray process), which obviously lowered the conductivity. Furthermore, the measured coating density was lower than the AlN value and suppressed the coating conductivity. In addition, the spraying parameter showed a varied effect on the coating phase composition, porosity, density, and therefore on its conductivity. Although the N2 gas flow improved the nitride content, it suppressed the thermal conductivity gradually. It is attributed to the further increase in the porosity and further decrease in the density of the coatings with the N2 gas. Furthermore, increasing the arc did not show a significant change on the coating thermal conductivity. On the other hand, the influence of spray parameters was optimized by investigating the effect of simple heat treatment (at 1100 °C) as a function of the arc current and N2 gas flow. The heat treatment improved the coating thermal conductivity at the different spray parameters. Thus, after heat treatment, the coating porosity, γ-Al2O3, Al5O6N strongly decreased and therefore the conductivity improved. On the other hand, the N2 gas flow and/or arc current did not show any difference on the conductivity after heat treatment. Therefore, using higher N2 gas flow and higher arc current is economically useless. Finally, although the obtained conductivity of the coating was not so high (compared to the AlN value), the main factors that govern the conductivity of such complicated plasma-spraying composite coating was realized. Herein, in order to fabricate high thermal conductivity AlN plasma spray coating, adjusting the phase composition, oxide content, porosity, and microstructure (at low N2 gas flow rate and low arc current) through the post-heat treatment is the key factor.