To clarify the formation micromechanism of plasma spray deposits, internal microstructures have been examined by transmission electron microscopy on the thin films prepared from different depths of the plasma sprayed Al 2 O 3 deposits. Microstructures in the top layer have consisted of both the coarse grains of 1 µm and the fine ones of 0.2 µm in average diameter. Point defects were contained mostly in γ phase grains. In the mid-layer, columnar structures often developed in parallel or in radial directions which adjoined with γ phase grains. From the SAD analyses, columnar grains were identified as α phase and lay in a twin crystal orientation relationship to each other between the joined grains. The bottom layer was composed of a mixture of the coarse grains of 0.2 µm and the fine ones of 0.02 µmin average grain diameter. It was found that the spray deposit was composed of different microstructures in crystallization, depending on depth to the thickness direction of the spray deposit.

The SUS316L stainless steel rod specimen coated with plasma-sprayed Al 2 O 3 deposits has been fatigued in a physiological saline solution (0.9 % NaCl solution) to evaluate the potential of its application to prosthetic implant materials. Push-pull loading fatigue tests were conducted at the stress ratio of R = -1, and at the frequency of 2 Hz. Pure titanium powder was selected for undercoat. Fatigue damage was examined on longitudinal section of the specimen and fracture surface by optical and electron microscopy from the microstructural viewpoints. The plasma spraying of Al 2 O 3 powder has significantly improved fatigue properties of the substrate metal in the longer range of fatigue lives, compared with the results of the non-coated steel specimen. It was found from electrochemical experiments that titanium for undercoat metal has acted as sacrificial anode to protect the substrate metal from corrosive attack and under lower stress amplitudes the plasma sprayed Al 2 O 3 coating kept the solution out at an early stage of fatigue lives. Fatigue cracks preferentially originated from flaws, which had been caused on the substrate metal surface through grit blasting, and extended into the bulk of substrate metal. Fatigue cracks appear not to develop into plasma-sprayed deposits while the deposits could accommodate themselves to the crack opening displacement at the surface of substrate metal. It was understood that the plasma sprayed coating has enhanced fatigue properties in the solution both by keeping the solution out during the early stage of fatigue lives and by electrochemical effects of the undercoat metal when the topcoat was cracked in macroscopic scale.