B. N. Lucas

On the measurement of stress–strain curves by spherical indentation

Abstract It has been proposed that with the appropriate models, instrumented indentation test (IIT) data can be reduced to yield the uniaxial stress–strain behavior of the test material. However, very little work has been done to directly compare the results from uniaxial tension and spherical indentation experiments. In this work, indentation and uniaxial tension experiments have been performed on the aluminum alloy 6061-T6. The purpose of these experiments was to specifically explore the accuracy with which the analytical models can be applied to IIT data to predict the uniaxial stress–strain behavior of the aluminum alloy. Despite not being able to reproduce the physical shape of the uniaxial stress–strain curve, the results do indicate that spherical indentation can be successfully used to establish an engineering estimate of the elastic modulus and yield strength of 6061-T6.

Time dependent deformation during indentation testing

Constant loading rate/load indentation tests (1/P dP/dt) and constant rate of loading followed by constant load (CRL/Hold) indentation creep tests have been conducted on high purity electropolished indium. It is shown that for a material with a constant hardness as a function of depth, a constant (1/P dP/dt) load-time history results in a constant indentation strain rate (1/h dh/dt). The results of the two types of tests are discussed and compared to data in the literature for constant stress tensile tests. The results from the constant (1/P dP/dt) experiments appear to give the best correlation to steady-state uniaxial data.

Scratch durability of automotive clear coatings: A quantitative, reliable and robust methodology

Scratch and mar durability of clear coatings are issues of concern to the automobile manufacturer and paint supplier. Scratching of clearcoats is a consequence of tribological events encountered by painted exteriors during normal service life. Several subjective methods to assess scratch durability have been proposed. These methods offer little insight into scratch mechanisms. More recently, single scratch methods have been proposed to probe clearcoat scratch mechanisms. This paper outlines a reliable and robust scratch methodology for evaluating scratch durability of automotive clear coatings. It is shown that, with appropriate characterization of tip geometry, quantitative and reproducible critical load values can be obtained. A suggested test method for scratch durability is described.

Mechanical Characterization of Sub-Micron Polytetrafluoroethylene (PTFE) Thin Films

This study reports the results of an investigation of the mechanical properties of polytetrafluoroethylene (PTFE) thin films on silicon substrates in the 0.5 to 15 {micro}m thickness regime using frequency specific depth-sensing indentation. All measurements were conducted at an excitation frequency of 45 Hz using a constant (1/P dP/dt) load ramp of 0.1 s{sup {minus}1}. The modulus of the PTFE at a depth of 5% of the film thickness was measured to be approximately 1 GPa ({nu} = 0.46) independent of film thickness. These values are somewhat higher than the values obtained from free-standing 15 {micro}m film measurements of 0.4 GPa for the tensile modulus and 0.49 GPa for the storage modulus {at} 1.1 Hz. The film hardness at these depths was observed to range between 30 and 55 MPa with no correlation observed between the hardness and respective film thickness. While reliability modeling for interconnects currently uses interlayer dielectric mechanical properties data determined from free-standing films with thicknesses of several microns, these in-situ results should more closely mimic the constrained deformation that occurs during service and perhaps lead to a better understanding of the electromigration resistance of PTFE.

The elastic, plastic, and time dependent properties of thin films as determined by ultra low load indentation

Using a highly spatially resolved mechanical properties microprobe, the elastic, plastic and time dependent mechanical properties of sapphire and a 1.9 {mu}m amorphous alumina film on a sapphire substrate have been studied. Youngs modulus, hardness, and stress-exponent data are reported. The technique for characterizing time dependent properties via indentation (hardness versus displacement rate/displacement) are directly compared to standard uniaxial compressive techniques (stress vs strain rate) for a bulk Pb-In alloy to further quantify the relationships between the two techniques.

Mechanical Characterization Using Indentation Experiments

Ultra-low load indentation experiments offer a flexible technique for characterizing the mechanical properties of thin films and small volumes. Techniques now exist for the measurement of strength (hardness), modulus, and the stress exponent for creep. Improved techniques for calculating hardness and modulus using the load-displacement data from ultra-low load indentation experiments are presented. Results from six materials with a wide range of hardnesses and moduli are used to compare contact areas calculated from load-displacement data to imaged areas. In addition, moduli are compared with literature values. The results show that the model used is quite accurate for the six materials investigated. A comparison of results from compression testing and indentation creep experiments on a low melting Pb-In alloy suggest that such indentation creep experiments reflect transient creep properties. Results from a study of the mechanical properties, including hardness, modulus and stress exponent of amorphous Al2O3 and sapphire are also presented.

Mechanical Characterization Of Ultra-Thin, Hard-Disk Overcoats Using Scratch Testing And Depth-Sensing Indentation

Two series of five diamond-like carbon (DLC) coatings were sputtered under nominally identical conditions, but to different film thicknesses of 20 nm and 105 nm. First, the hardness of each sample was determined by depth-sensing indentation. Hardness measurements were substrate-affected to some extent for all samples but especially so for the 20 nm coatings. Two types of scratch tests were performed in an attempt to isolate and characterize the top coatings. The first was a wear test, which consisted of moving the sample back and forth repeatedly under a small constant load. The residual damage was inconsistent, but appeared to be a function of the composite, or substrate-affected hardness. The second test was a single-pass scratch in which the normal load was ramped linearly. For all samples, the friction coefficient was approximately constant as a function of load. Furthermore, samples with the same top coats yielded similar friction coefficients, regardless of the coating thicknesses. Friction coefficient decreased with hydrogen content and to some extent, increased with hardness, as measured on the 105 nm samples. The friction coefficient measured during a ramp-load scratch offers an alternative for characterizing ultra-thin films, when indentation alone yields measurements that are significantly affected by the substrate.

Microstructural and Mechanical Property Changes in Model Fe-Cu Alloys

This paper describes a technique developed to determine values for the dislocation barrier strength of the defects believed to be responsible for the embrittlement of light water reactor (LWR) pressure vessel steels. Microstructures consisting of a single defect type were introduced by ion irradiation or thermal annealing, and the defect distributions were determined by TEM. Hardness changes were measured using a nano indenter and the dislocation barrier strengths for the defects involved were computed based on a dispersed barrier hardening model.

On the Measurement of Stress-Strain Curves by Spherical Indentation

It has been proposed that with the appropriate models, instrumented indentation test (IIT) data can be reduced to yield the uniaxial stress-strain behavior of the test material. However, very little work has been done to directly compare the results from uniaxial tension and spherical indentation experiments. In this work, indentation and uniaxial tension experiments have been performed on the aluminum alloy 6061-T6. The purpose of these experiments was to specifically explore the accuracy with which the analytical models can be applied to IIT data to predict the uniaxial stress-strain behavior of the aluminum alloy.

Computing Thin Film Mechanical Properties with the Oliver and Pharr Method

A commonly used technique to compute mechanical properties from indentation tests is the Oliver and Pharr method. Using dimensional analysis and finite element modeling, this paper investigates errors when the Oliver and Pharr method is used to compute thin film properties.