Using a variety of depth-sensing indentation techniques, the creep response of high-purity indium, from room temperature to 75 ЊC, was measured. The dependence of the hardness on the variables of indentation strain rate (stress exponent for creep (n)) and temperature (apparent activation energy for creep (Q)) and the existence of a steady-state behavior in an indentation test with a Berkovich indenter were investigated. It was shown for the first time that the indentation strain rate ( /h) could ⅐ h be held constant during an experiment using a Berkovich indenter, by maintaining the loading rate divided by the load ( /P) constant. The apparent activation energy for indentation creep was found ⅐ P to be 78 kJ/mol, in accord with the activation energy for self-diffusion in the material. Finally, by performing /P change experiments, it was shown that a steady-state path independent of hardness ⅐ P could be reached in an indentation test with a geometrically similar indenter.

No abstract

No abstract

This paper describes experimental measurements of the linear viscoelastic behavior of the surface of low-density (LD) polyethylene in contact with a pyramidal Berkovich diamond indenter. The experiments were carried out at two different temperatures, 15.9 and 27.2 °C, between frequencies of 0.1 and 800 Hz. Using the shift of the loss tangent between the two temperatures at frequencies lower than 20 Hz and an Arrhenius equation, an activation energy of 105 ± 2 kJ/mol was obtained. This value is in good agreement with the bulk value of the a relaxation of LD polyethylene reported in the literature.

Depth-sensing indentation involves applying a specific force-time history on a rigid indenter while continuously monitoring the displacement of the indenter into the surface. Frequency specific depth-sensing indentation testing entails adding a small harmonic force on the indenter and measuring the harmonic response of the indenter at the excitation frequency. While often taken for granted, understanding the dynamics behind these frequency specific measurements is of vital importance in the determination of quantitative mechanical properties. This paper will focus on the dynamics of a variety of depth-sensing indentation systems and how these dynamics affect such parameters as detecting the point of surface contact, environmental sensitivity, dynamic frequency range, and the range over which contact stiffnesses and moduli can be accurately measured.

Constant loading rate/load indentation tests (i/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.

Diamond-like-carbon (DLC) thin films have been deposited at room temperature on Si substrates by ablation of a graphite target using a KrF excimer laser at intensities ranging from 0.9×108 W/cm2 to 6.0×109 W/cm2. The microstructure of the films was studied by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The macroscopic properties were evaluated by measurement of their optical constants using in-situ laser reflectometry and their hardness using the continuous stiffness measurement technique. Analysis of the XPS C 1s core level spectra of the DLC films shows that their sp3 hybridized carbon atom content increases with laser intensity up to a maximum value of about 60% obtained at 7.0×108 W/cm2. At higher laser intensities, the sp3 content appears to stabilize at about 53%. Such an evolution of the sp3 content can be understood in terms of the subsurface carbon ions implantation model which has been proposed for ion beam deposited films. On the other hand, Raman analysis indicates that an increase in laser intensity leads to the establishment of some long range order of the sp2 domains in the deposited layers. The extinction coefficient k of the deposited layers was found to be correlated to their sp3 content. Finally, it is shown that hardness values as high as 47 GPa can be obtained and that hardness is also correlated to the sp3 content of the films.

No abstract