Zirconium oxide formation and surface hardening behavior by nitrogen implantation under oxygen atmosphere in Zircaloy-4

Abstract

Zircaloy-4 has been implanted with nitrogen at 120 keV and various total ion doses under an oxygen partial pressure of 1.5×10 −5 to 4×10 −5 torr. The substrate temperature was varied from 300 °C to 600 °C by controlling the ion-beam current density. It was observed from Auger electron spectroscopic analysis that oxygen penetration was three to five times deeper than nitrogen penetration depth for all implantation conditions at elevated temperatures above 300 °C. In addition, a composition ratio of oxygen to zirconium of approximately 2:1 was reached in a layer thickness range 0.3 to 0.5 μm with increasing total ion dose up to 5×10 17 ions cm −2 below 500 °C. Another noted phenomenon in nitrogen implantation under oxygen atmosphere was deep penetration of oxygen to >20 μm at 500 °C and 600 °C. The corresponding ion current density was above 70 μm cm −2. However, deep penetration of oxygen decreased the oxygen concentration to about 10 at.% over the whole penetration depth from the surface. Therefore ZrN as well as ZrO 2 formation was observed by X-ray diffraction below 500 °C while ZrO 2 formation was almost suppressed owing to low oxygen concentration above 500 °C. Surface hardness was highly enhanced at all implantation conditions. The hardness range of implanted Zircaloy-4 was 500 to 1800 HK (0.01 N) depending on implantation conditions, which corresponds to a two- to sixfold increase compared with the hardness (300 HK) of unimplanted Zircaloy-4. The improvement of hardness was found to be closely associated with ZrN as well as ZrO 2 formation and depended on total ion dose, temperature and O 2 partial pressure.