Film Fracture Phenomena During Indentation
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Film Fracture Phenomena During Indentation D.F. Bahr, M. Pang, and D. Rodriguez-Marek Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920 ABSTRACT Discontinuities during both load and depth controlled continuous indentation tests have been ascribed to dislocation nucleation or multiplication and film fracture. In materials which exhibit permanent deformation prior to a discontinuity in loading, it is more likely that the phenomena is indeed controlled by film fracture, and not the rapid generation of dislocations. The current study has been undertaken to examine the properties of passivating films on engineering alloys. An electrochemical cell coupled with a scanning probe microscope and nanoindentation system allows growth and mechanical testing of passive films on an austenitic stainless steel as well as a titanium alloy. A complementary set of ex situ experiments shows the presence of deformation prior to film fracture with both load – depth sensing techniques as well as imaging the surface topography. The occurrence of excursions is shown in these materials to be linked directly with film fracture, rather than dislocation multiplication. INTRODUCTION Indentation techniques that record load and penetration in the µN load and nm distance range are capable of identifying several phenomena that are not easily measured using traditional macroscopic indentation methods. With these new testing methods, it becomes possible to extend the type of information elucidated during an experiment. As traditional hardness is related to either yield or flow stress, nanoindentation techniques which can measure other materials properties will allow researchers to extend the testing of small features into the realm of fracture and other failure modes which dominate small scales, such as thin film fracture. Page and Hainsworth [1] have referred to the shape and description of a load – depth curve during nanoindentation as a “mechanical fingerprint” of the material. When testing metallic or ceramic materials (those which do not have strong time dependent properties), and particularly those systems in which a film exists on top of a substrate, there are three main mechanisms which generate observable discontinuities during loading in the “fingerprint”. Dislocation nucleation [2-4], film delamination [5], and through thickness film fracture all generate non-uniform deformation during indentations. In the case of a load controlled indentation system, these non-uniform events manifest themselves as a sudden increase in displacement at a constant load. This paper will demonstrate the similarities and differences between these phenomena, and describe in detail a specific example of through thickness film fracture which is easily confused with dislocation nucleation; the case of thin oxide films on titanium. EXPERIMENTAL PROCEDURES Indentations were performed on two engineering alloys. The first material used was a polycrystalline grade II titanium, which was annealed to produce a primarily HCP crystal st
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