Process Heating and Postannealing Effects on Microstructure and Hardness of the Sputtered Ni-P-Al Coatings

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 475∘C showed an amorphous/nanocrystalline microstructure. Significant crystallization of Ni matrix and precipitation of Ni𝑥P𝑦 and Ni𝑝Al𝑞 compounds were observed for the coatings manufactured under high sputtering temperatures above 500∘C. The amorphous Ni-P-Al coatings were postannealed from 500 to 600∘C in vacuum environment for comparison. The amorphous feature of the Ni-P-Al coating remained unchanged under a high annealing temperature of 550∘C, 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.