Mohammed Shahien

Challenges Upon Reactive Plasma Spray Nitriding: Al Powders and Fabrication of AlN Coatings as a Case Study

Reactive plasma spraying (RPS) is a promising technology for the in situ formation of several ceramic coatings. The focus of this paper is to summarize the state of our current knowledge about the RPS process and using the nitriding of Al particles and the fabrication of aluminum nitride coatings, as a case study. The aspects and challenges in this process such as the influence of the plasma power, in-flight time, particle size, nitriding mechanism, splat morphology, in-flight particle diagnostics, N2 plasma gas, and the feeding rate on the RPS process are analyzed and discussed.

Fabrication of AlN/Al2O3 Coatings via Atmospheric Reactive Plasma Nitriding of Al2O3 Powders

Plasma sprayed alumina (Al2O3) coatings has showed a great effect in service the life of engineering tools, while its low thermal conductivity limits its application in heat exchange applications. Aluminum nitride (AlN) is a promising material to improve the thermal conductivity of Al2O3 coatings. This paper proposed a new way to fabricate AlN/Al2O3 coatings through reactive plasma nitriding of Al2O3 powders. It was possible to fabricate cubic-AlN/Al2O3 coatings by spraying Al2O3 powders in atmospheric plasma spray process (APS). During plasma Al2O3 powder reacted in high temperature N2/H2 plasma to form aluminum oxynitride and it easily nitride to produce the cubic AlN phase. Thus, both of Al5O6N and c-AlN phrases have the same cubic symmetry (cubic and closely packed crystal structure). Then, the particles collide, flatten, and rapidly　solidified on substrate surface. The high quenching rate of the plasma flame prevents the crystal growth and formation of hexagonal AlN phase. The fabricated coatings consist of c-AlN, α-Al2O3, Al5O6N and γ-Al2O3. The AlN content was improved with increasing the flight time (spray distance) due to increasing the reaction time between Al2O3 particles and the surrounding N2 plasma. It was possible to fabricate AlN/Al2O3 coating consist of 97 wt. % of AlN phase at the spray distance of 300 mm.

REACTIVE PLASMA NITRIDING OF AL2O3 POWDER IN THERMAL SPRAY

Among advanced ceramics, aluminum nitride (AlN) had attracted much attention in the field of electrical and structural applications due to its outstanding properties. However, it is difficult to fabricate AlN coating by conventional thermal spray processes directly. Due to the thermal decomposition of feedstock AlN powder during spraying without a stable melting phase (which is required for deposition in thermal spray). Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of AlN thermally sprayed coatings. In this study the possibility of fabrication of AlN coating by reactive plasma nitriding of alumina (Al2O3) powder using N2/H2 plasma was investigated. It was possible to fabricate a cubic-AlN (c-AlN) based coating and the fabricated coating consists of c-AlN, α-Al2O3, Al5O6N and γ-Al2O3. It was difficult to understand the nitriding process from the fabricated coatings. Therefore, the Al2O3 powders were sprayed and collected in water. The microstructure observation of the collected powder and its cross section indicate that the reaction started from the surface. Thus, the sprayed particles were melted and reacted in high temperature reactive plasma and formed aluminum oxynitride which has cubic structure and easily nitride to c-AlN. During the coatings process the particles collide, flatten, and rapidly solidified on a substrate surface. The rapid solidification on the substrate surface due to the high quenching rate of the plasma flame prevents AlN crystal growth to form the hexagonal phase. Therefore, it was possible to fabricate c-AlN/Al2O3 based coatings through reactive plasma nitriding reaction of Al2O3 powder in thermal spray.

Controlling of Nitriding Process on Reactive Plasma Spraying of Al Particles

Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of aluminum nitride (AlN) thermally sprayed coatings. To fabricate thick A lN coatings in RPS process, controlling and improving the in-flight nitriding reaction of Al particles is required. In this study, it was possible to control the nitriding reaction by using ammonium chloride (NH4Cl) powders. Thick and dense AlN coating (more than 300 μm thickness) was successfully fabricated with small addition of NH4Cl powders. Thus, addition of NH4Cl prevented the Al aggregation by changing the reaction pathway to a mild way with no explosive mode (relatively low heating rates) and it acts as a catalyst, nitrogen source and diluent agent.

Reactive atmospheric plasma spray of Al powder

Reactive plasma spraying (RPS) is based on using reactive plasma to react with feedstock powders. RPS is a promising technology of material processing and coating manufacturing, especial for ceramic coatings like AlN which is expected to contribute to semiconductor equipments and so on, due to its outstanding properties [1]. However, fabrication of AlN coatings by conventional thermal spray processes was impossible due to thermal decomposition of AlN powder during spraying. In our previous study we fabricated AlN coatings through nitriding of Al powder in evacuated Radio Frequency (RF) plasma spray equipments using Ar/N2 mixture as a plasma gas [2,3]. In this study we fabricated AlN coatings through nitriding of Al powder in the air by Atmospheric plasma spray (APS) process, using N2/H2 as plasma gases. Al particles are injected in the plasma jet created by the passage of N2/H2 gases through an electric arc under high current conditions. Al particles continuously heated up (At high temperatures: above its melting point) and interacts with the plasma jet during flight to reach a certain energy state which is composed of thermal energy and kinetic energy. Al particles are fused and simultaneously accelerated by the plasma flow and gain their velocity and temperature by thermal and kinematic transfers towards SS400 substrate to be coated. Firstly we used carrier gas Ar and we found that the spray distance is an important parameter for nitriding reaction of Al powder. At small spray distance the coatings contain both Al and AlN but by increasing the spray distance the amount of AlN increases gradually, due to increase the reaction time so the possibility of Al nitriding increases, that at 300 mm spray distance the coatings is pure AlN. However, the coatings thickness decreased gradually from 100 to 20 µm with increasing the spray distance from 150 to 300 mm, thus particles temperature decrease by increasing the spray distance so its deposition efficiency decreased. To fabricate thick AlN coatings we used N2 as carrier gas and the coating mainly AlN with thickness 100 µm at spray distance 100 mm.