Synthesis of Cubic Aluminum Nitride Coating from Al₂O₃ Powder in Reactive Plasma Spray Process

Abstract

Plasma spraying is a versatile technique for producing abradable and protective ceramic coatings. However, it was difficult to fabricate aluminium nitride (AlN) coating by conventional plasma spray processes. It is due to the thermal decomposition of the feedstock AlN powder during spraying without a stable melting phase. Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of AlN thermally sprayed coatings. This study investigated the feasibility of reactive plasma spraying of Al 2 O 3 powder in N 2 /H 2 plasma upon fabrication of AlN coating. It was possible to fabricate a cubic-AlN (c-AlN)/Al 2 O 3 composite coating and the fabricated coating consists of c-AlN, ¡-Al 2 O 3 , Al 5 O 6 N and £-Al 2 O 3 . Furthermore, the AlN content was improved with increasing the flight time (spray distance), due to increasing the reaction time between the Al 2 O 3 particles and the surrounding N 2 /H 2 plasma. It was possible to fabricate coating contains about 97% of c-AlN. However, it was difficult to clarify the in-flight reaction during the coating process, due to losing the particles shape and features after colliding and flattening on the substrate surface. The sprayed particles were collected into a water bath to maintain its particle features in order to investigate the in-flight reaction. It was clear from the microstructure and the cross section observation of the collected particles that, the nitriding reaction started from the surface. During the coating process, the sprayed particles were melted, spheroidized and reacted in the high temperature N 2 /H 2 plasma and formed aluminum oxynitride (Al 5 O 6 N) which have cubic structure. The particles collided, flattened, and rapidly solidified on a substrate surface. The Al 5 O 6 N is easily converted to c-AlN phase via continuous nitriding (both have the same cubic symmetry: cubic and closely packed) during the rapid solidification and plasma irradiation on the substrate. The high quenching rate of the plasma flame prevents the AlN crystal growth to form the hexagonal phase. Therefore, it was possible to fabricate c-AlN/Al 2 O 3 composite coatings through reactive plasma nitriding of Al 2 O 3 powder and the nitriding process of the Al 2 O 3 particle as well as the formation process of cAlN phase were investigated.

Influence of Transient Liquid Phase Promoting Additives upon Reactive Plasma Spraying of AlN Coatings and Its Properties

“…The Al 5 O 6 N phase is related to the reaction between the Al 2 O 3 feedstock with the surrounding active N 2 ‐H 2 plasma species. This composition agrees with the plasma nitriding of only Al 2 O 3 powder . Furthermore, the reaction between the AlN and Al 2 O 3 feedstock powders lead to the formation of the Al 5 O 6 N phase .…”

“…This is attributed to the decomposition of some AlN particles under the high temperature conditions of the plasma stream. The c ‐AlN is related to the plasma nitriding of the Al 2 O 3 particles in the N 2 /H 2 plasma [as explained before in the case of only Al 2 O 3 powder . The XRD diffraction peak at 2θ value of 44 corresponds to the c ‐AlN phase.…”

“…Reactive plasma spraying (RPS) is a promising solution to fabricate AlN based coatings . Several AlN based coatings were successfully fabricated through plasma nitriding of Al powders, Al–AlN mixtures, Al 2 O 3 powders, and Al 2 O 3 –AlN mixtures, in atmospheric N 2 –H 2 plasma. Although, it was possible to fabricate thick AlN coatings with uniform microstructure.…”

ANALISA PERMUKAAN MATERIAL ALUMINIUM PADUAN (AlSiCu) HASIL NITRIDASI ION

“…Menurut Shahien [10], untuk terbentuknya reaksi pada nitridasi dimulai pada permukaan partikel cairan berbentuk bola (molten spherical particle) untuk membentuk struktur kubik aluminium oxynitride (Al5O6N) Gambar 3. Lapisan nitrida aluminium akibat nitridasi ion [9].…”