Surface porosity development on metal substrates by helium implantation and annealing
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Surface porosity development on metal substrates by helium implantation and annealing. R. Escobar Galindo1, A. van Veen1, 2, H. Schut1, J.H. Evans3, A.V. Fedorov1, P.Y. Hou4, J.Th.M. de Hosson2 1 Defects in Materials, IRI, Delft University of Technology, Delft, The Netherlands 2 Materials Science Centre, University of Groningen, Groningen, The Netherlands 3 Camrose Consultants, Abingdon, United Kingdom 4 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California ABSTRACT The effects of helium implantation parameters and heating schemes on the formation of surface porosity have been studied. Confinement of the implanted helium by a coating on the metal leads to a prolonged development of the bubble structure. Finally, delamination and blistering occurs and rather open surface structures remain. INTRODUCTION During helium implantation into metals at ambient temperature, it is well known that the helium is trapped at the implantation depth in implantation induced vacancies at low doses [1] and forms small bubbles at higher doses [2]. For doses below the threshold of blistering, annealing of such material leads to a clear surface release of the helium, as individual atoms at low doses, while in samples containing bubbles, bubble migration leads to coalescence of the bubbles, bubble swelling and the loss of bubbles at nearby surfaces. For helium in copper, Johnson and Mazey have reported transmission electron microscopy observations [3] showing such bubble behaviour starting at near 0.44 Tm (598 K) while in analogous work in copper containing krypton bubbles [4] there were clear surface effects consistent with the arrival of bubbles and the associated release of gas. A novel aspect studied here was the confinement of the bubbles by the presence of a coating on top of the surface. For the helium case the presence of a surface coating would lead to the agglomeration of the implanted gas at the film-metal interface during annealing and thus to interface blister formation. An increase in the blister pressure in a controlled way would then induce delamination as described above. These interface blisters must not be confused with the surface blistering that occurs on uncoated metal surfaces at high helium doses [5] during ambient temperature implantation. This blister formation requires a peak helium content of ~ 30 at. %. In the present work the chosen helium levels are some 15 to 30 times lower, but these levels will be sufficient to investigate the expected annealing induced loss of bubbles to the surface that might be relevant to interface blister effects if samples were coated. HELIUM IMPLANTATION AND THERMAL ANNEALING For copper, single crystalline material with (111) orientation was implanted with 30 keV helium ions. In order to apply neutron depth profiling NDP [6] the isotope 3He was implanted. The implanted fluences were controlled by NDP. The helium implantation depth was 130±57 nm. Peak concentrations were 0.83, 1.25 and 1.68% for fluences of 1.2, 1.8
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