We investigated the crystallinity, orientation, and stability of hydroxyapatite (HAp) thin films on structural materials including SiO2 and metals (Ru, Ti, and Pt). The HAp films were deposited by electron cyclotron resonance (ECR) plasma sputtering with an Xe sputtering gas under simultaneous flow of H2O vapor. The HAp films as-crystallized during deposition and solid-phase crystallized by post annealing were analyzed with X-ray diffraction, Fourier-transform infrared spectroscopy, and Raman scattering. The lowest solid-phase crystallization temperatures were 350 degrees C on Ru, 300 degrees C on Ti, and 300 degrees C on Pt. These crystallization temperatures are much lower than those on silicon and SiO2 substrates (550 degrees C). For HAp films as-crystallized at temperatures above 400 degrees C, the elements out diffused and reacted with substrates. Precipitation of CaO was observed for HAp films deposited on SiO2 and Ru substrates. Diffusion of POET into SiO2 and Ru-catalyzed decomposition of HAp crystals are suggested for the mechanism. When deposited on Pt substrates, CaPt2O4 alloy byproducts were created for solid-phase crystallized films and CaPt2 alloy byproducts were created for as-crystallized films, indicating reaction of Ca atoms with Pt substrates. (C) 2015 Elsevier B.V. All rights reserved.