Ternary Ni-P-Al alloy coating was fabricated by magnetron sputtering technique with a Ni-P/Al composite target source. The effects of thermal treatments, including deposition process heating and postannealing, on phase transformation phenomenon and related mechanical properties were investigated. The as-deposited coatings produced under process temperature below showed an amorphous/nanocrystalline microstructure. Significant crystallization of Ni matrix and precipitation of and compounds were observed for the coatings manufactured under high sputtering temperatures above . The amorphous Ni-P-Al coatings were postannealed from 500 to in vacuum environment for comparison. The amorphous feature of the Ni-P-Al coating remained unchanged under a high annealing temperature of , showing a superior thermal stability as compared to those fabricated under high process temperatures. Superior hardness was obtained for the post-annealed Ni-P-Al coatings due to volumetric constraint of crystallization and precipitation. On the other hand, the overaging phenomenon and subsequent degradation in hardness were found for the Ni-P-Al coatings fabricated under high-temperature deposition processes. The phase transformation mechanisms of the coatings through different thermal treatments were intensively discussed. 1. Introduction Nickel-Phosphorus (Ni-P) coating is frequently adopted as a hard alloy coating due to its various merits, such as corrosion resistance, high hardness and toughness, and wear resistance [1–4]. Under adequate heat treatment, the binary Ni-P can be strengthened by precipitation of intermetallic compounds, including Ni3P, Ni5P2, Ni12P5, and NiP hard phases [5]. In order to further enhance its mechanical and thermal properties, the introduction of the third elements, including Al [6], Cr [7], Cu [8, 9], and so forth, has been proposed. Especially for Ni-P-Al coatings, it has been pointed out that not only the crystallization temperature is increased but also the related mechanical properties are enhanced owing to the addition of Al [6]. Ni-P-based coatings are usually fabricated through wet chemical methods, including electroless and electroplating techniques [4, 10]. The codeposition of third element into Ni-P alloy through wet chemical procedures is sometimes complicated and not easy to control. Recently, the sputtering technique is successfully adopted to produce multicomponent Ni-P-based alloy coatings with good composition control [6, 9, 11, 12]. Since the Ni-P-based material is sensitive to thermal history, the process temperature during sputtering
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