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

Zircaloy-4 has been implanted with nitrogen at 120 keV and various total ion doses under an oxygen partial pressure of 1.5 x 10(-5) to 4 x 10(-5) torr. The substrate temperature was varied from 300 degrees C to 600 degrees 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 degrees 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 mu m with increasing total ion dose up to 5 x 10(17) ions cm(-2) below 500 degrees C. Another noted phenomenon in nitrogen implantation under oxygen atmosphere was deep penetration of oxygen to >20 mu m at 500 degrees C and 600 degrees C. The corresponding ion current density was above 70 mu m 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 ZrO2 formation was observed by X-ray diffraction below 500 degrees C while ZrO2 formation was almost suppressed owing to low oxygen concentration above 500 degrees 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 ZrO2 formation and depended on total ion dose, temperature and O-2 partial pressure. (C) 1997 Elsevier Science S.A.