Thin-film fracture during nanoindentation of a titanium oxide film–titanium system

Abstract

Nanoindentation testing of the titanium oxide/titanium system with electrochemically grown oxide films exhibits permanent deformation prior to a yield excusion, indicating that the occurrence of this suddent discontinuity is predominantly controlled by oxide film cracking rather than dislocaton nucleation and multiplication. Observations of circumferential cracking also lend support to this explanation. A model has been developed to predict the mechanical response prior to oxide fracture for the case of a hard coating on a soft substrate. During loading contact, the hard coating undergoes elastic deflection which may include both bending and membrane stretching effects, while the substrate is elastoplastically deformed. The model works well for surface films thicker than 20 nm. Additionally, the maximum radial tensile stress in anodically grown titanium oxide, which is responsible for film cracking at the critical load, is approximately 15 GPa.

Rate Dependence of Serrated Flow During Nanoindentation of a Bulk Metallic Glass

“…For continuum plastic yielding such pop-ins are not expected, and in crystalline materials their presence is invariably associated with discrete plastic deformation phenomena, such as cracking or dislocation nucleation. [17][18][19][20][21] In BMGs, these discontinuities are likened to the serrated flow observed in other modes of loading and are associated with the nucleation and motion of shear-bands. 2,14 As observed in Fig.…”

Influence of pre-existing dislocations on the pop-in phenomenon during nanoindentation in MgO

“…This abrupt increase in the penetration depth marks the transition from the initial elastic behaviour to an elasto-plastic behaviour [7,8]. This phenomenon is observed in many different materials [1,7,9], and is associated with dislocation activity: homogeneous nucleation of dislocations [10], source activation [11], point defect source activation [12] or thin oxide film perforation by dislocation pile-up [13][14][15]. In most cases, the shear stress below the indenter before the pop-in is close to the theoretical shear strength, suggesting a homogeneous dislocation nucleation mechanism, which has also been reported in different simulations [10,16,17].…”

Dislocation Nucleation and Source Activation during Nanoindentation Yield Points

“…These observations were thus related to dislocation motion in general, and in particular to dislocation nucleation [3][4][5][6][7] or to dislocation source activation. [8,9] One of the major challenges in assessing the behavior of the elastic-plastic transition during nanoindentation is the difficulty in ascribing the behavior to homogeneous dislocation nucleation, [10] the activation of well-spaced dislocation sources, the activation of a point defect source (i.e., a vacancy), [9] and the tensile fracture of a surface film, [11][12][13][14] or some other surface film relationship with the underlying dislocation structure. [15] For instance, in a well-annealed metallic single crystal, it is not unreasonable to assume a dislocation density on the order of 10 11 m -2 .…”