Shankar K. Venkataraman

The injection of plasticity by millinewton contacts

Abstract Diamond indentation of a surface with a thin passive film require s loads an order of magnitude smaller for Ni〈100〉 crystals than for Fe-3 wt% Si〈100〉 crystals. The load bearing capacity of the Fe-3 wt% Si can be reduced by two orders of magnitude by removing the 1- nm thick native oxide film. These phenomena can be explained by considering the equilibrium of forces associated with tip, image and friction stresses acting on dislocations emitted from the indenter tip. The key ingredient to this model is the nucleation and growth of dislocation loops at loads of only tens of micronewtons. Three types of critical contact experiments demonstrate that dislocations can be initiated at loads well below those previously thought to represent elastic loading only.

Microscopy and microindentation mechanics of single crystal Fe-3 wt. % Si: Part I. Atomic force microscopy of a small indentation

Atomic force microscope measurements of elastic-plastic indentation into an Fe--3wt.%Si single crystal showed that the volume displaced to the surface is nearly equal to the volume of the cavity. The surface displacement profiles and plastic zone size caused by a 69 nm penetration of a Vickers diamond tip are reasonably represented by an elastic-plastic continuum model. Invoking conservation of volume, estimates of the number of dislocations emanating from the free surface are reasonably consistent with the number of dislocations that have formed in the plastic zone to represent an average calculated plastic strain of 0.044.

Microscratch analysis of the work of adhesion for Pt thin films on NiO

The adhesion of as-sputtered Pt thin films to NiO single crystals has been characterized by a continuous microscratch technique. In these experiments, a conical indenter was driven into a 1.2 μm thick Pt film at a rate of 15 nm/s, and across the sample surface at a rate of 0.5 μm/s, until a load drop was observed indicating that the film had delaminated. Using the width of the scratch track at the point at which the film delaminated from the substrate, the critical load required for delamination, and the area of the delaminated region, a model has been developed to determine the work of adhesion of the Pt/NiO system. This model uses an elastic contact mechanics approach to relate the stresses acting in a scratch experiment to the strain energy released during film delamination. Using this model, the work of adhesion and hence the interfacial fracture toughness have been determined to be 0.023–0.06 J/m 2 and 0.07–0.11 MPa

Continuous microindentation of passivating surfaces

\sqrt m

Metal-ceramic interfacial fracture resistance using the continuous microscratch technique

, respectively. These values are in reasonable agreement with those determined by other methods for metal-ceramic systems.

Dislocation distribution under a microindentation into an iron-silicon single crystal

Continuous microindentation tests performed on the electropolished surfaces of single crystal Fe (3 wt.% Si), known to have a thin passivation film, show a sharp discontinuity at a load of 1.8 mN. To this point, there was no apparent plastic deformation in the metal in that the loading and unloading curves exactly overlay each other. Stresses at the discontinuity were close to the theoretical strength of the metal. Elastic contact theories of Hertz and Love reproduced the elastic portion of the load-displacement curves. On removing the passivation film with a HCl solution, indentation tests yielded strengths nearly two orders of magnitude smaller. The strength recovered to near its initial value after the liquid evaporated and the passivation film re-formed.

Continuous microscratch measurements of the practical and true works of adhesion for metal/ceramic systems

Abstract To investigate the adhesion of thin films to substrates, continuous microscratch tests were carried out on carbon or nickel thin films bonded to silicon, NiO, or Al2O3 substrates. In the present study, continuous microscratch experiments were carried out by driving a conical diamond indenter into the sample at a rate of 15 nm s−1 and across the sample surface at a rate of 0.5 μm s−1, until a load drop occured, indicating film delamination. The critical load for film delamination provides a qualitative measure of the adhesion strength of the bi-material system. From the area of the debonded film and the critical load to failure, a quantitative measure of the strain energy release rate and the interfacial fracture resistance of the bi-material systems, was calculated using a model developed in a previous paper. The strain energy release rate and the interfacial fracture toughness were in the range 0.05–0.5 J m−2 and 0.1–0.55 MPa m 1 2 respectively. These values are in reasonable agreement with those determined by other methods for metal-ceramic systems. In general, the fracture resistance for brittle films was found to be much larger than that for ductile films. Preliminary investigation of the contributions of film and substrate modulus to the work of adhesion indicate that the work of adhesion increases non-linearly with the moduli.

Continuous microscratch measurements of thin film adhesion strengths

Abstract A transmission electron microscopy study of the dislocation distribution under an indentation has revealed first that a complex sequence of loop nucleation, cross-slip and renucleation results in a cross-like network at the free surface and a square-like network underneath. These dislocation distributions are made up of two types of dislocation: redundant dislocations responsible for general plasticity and work hardening, and shielding dislocations responsible for the equilibrium of forces. The former are roughly an order of magnitude more prevalent. The total number of dislocations are consistent with continuum plasticity theory while the shielding density is consistent with a proposed superdislocation pile-up model.

Micromechanical toughness test applied to NiAl

Using a continuous microscratch technique, the adhesion strengths of Pt, Cr, Ti, and Ta{sub 2}N metallizations to NiO and Al{sub 2}O{sub 3} substrates have been characterized. The practical work of adhesion was determined as a function of both thickness and annealing conditions. For all except the Ta{sub 2}N films, the practical work of adhesion increases nonlinearly from a few tenths of a J/m{sup 2} to several J/m{sup 2} as the thickness of the thin film is increased, indicating that a greater amount of plastic work is expended in delaminating thicker films. Further, the practical work of adhesion also increases with increasing annealing temperature, indicating stronger bonding at the interface. In the limit that the film thickness tends to zero, the plastic energy dissipation in the film tends to zero. As a result, the extrapolation to zero thickness yields the {ital true} work of adhesion for that system. {copyright} {ital 1996 Materials Research Society.}

Non-Isothermal Kinetic Analysis of the Crystallization of Metallic Glasses Using the Master Curve Method

The adhesion strengths of metal/ceramic, metal/polymer, and polymer/polymer interfaces have been characterized using the continuous microscratch technique. In these experiments, a conical diamond indenter was driven simultaneously into a thin film at a rate of 15 nm/s and across the film surface at a rate of 0.5 μm/s until a load drop or other discontinuity occurred, indicating film failure. The critical load at failure of the thin film was taken as a measure of the adhesion strength. For metal/ ceramic systems such as Cr thin films on Al2O3, and for diamond-like-carbon (DLC) films on glass, clear load drops provided an accurate measure of the adhesion strengths. For metal/polymer systems such as Cu thin films on PET, a change in the loading pattern and periodic cracking events along the scratch track provided evidence of film delamination. For DLC films on polycarbonate substrates, the carbon thin film cracked before it delaminated. For bulk polymers such as polycarbonate and polystyrene/polypropylene, c...