Measurement of material creep parameters of amorphous selenium by nanoindentation and the relationship between indentation creep and uniaxial creep

by 1978- LaManna, James Anthony

The purpose of this work was to measure the indentation creep parameters of amorphous selenium and to begin to develop a relationship between indentation creep and uniaxial creep. The data for this study was obtained by creating three different indentation creep experiments to test amorphous selenium under multiple loading conditions at multiple temperatures. The testing temperatures included 40°C, 35°C, 30°C, and 25°C. These specific temperatures were chosen to collect creep data above and below the glass transition temperature of 31°C. This indentation data was used to accomplish several objectives. The first objective was to prove the loading history independence of amorphous selenium. This was accomplished by showing that at each temperature the data from the different loading conditions all fit on one curve when the indentation strain rate is plotted as a function of the mean pressure applied by the indenter. The second objective was to calculate the creep exponent, n, from the indentation creep equation, ?& I = BP n m , where ?& I is the indentation strain rate, Pm is the mean pressure applied by the indenter, and B is a material constant. The results show that the creep exponent is a strong function of temperature and strain rate and that linear creep is displayed at temperatures above Tg. The activation energy for creep, Qc, was also calculated from the indentation data. These results were obtained from the strain rate data at constant values of mean pressure and temperature. The calculated values of Qc ranged from 340 to 380 kJ/mole at 37.5°C and 515 to 530 kJ/mole at 32.5°C. These values were compared to literature data from both uniaxial and indentation experiments. Another objective of this work was to analyze the indentation load-displacement behavior of amorphous selenium. Below the glass transition temperature it is shown that during the loading portion of a constant loading rate test the load is a function of the displacement squared. This result is compared to a theoretical model suggested in the literature. Experimental results show that above the glass transition temperature the load iv is a linear function of displacement. A model was derived for the load displacement behavior during linear creep from the indentation creep equation. The viscosity of amorphous selenium was also calculated from the indentation data. A model found in the literature was used to calculate the viscosity for the linear creep data from the slope of the load -displacement curves. The results show an estimated viscosity of 5 GPa-s at 35°C and 4 GPa-s at 40°C. These values are compared to literature data from both indentation and uniaxial creep tests. The final objective of this study was to investigate the relationship between the material constant B in the indentation creep equation and the material constant A in the n uniaxial creep equation, & ? = A? u , where ?& u is the uniaxial strain rate, ? is the uniaxial stress and n is the creep exponent. The values for A were taken from the literature and B was calculated from the indentation load-displacement data and the model developed for linear creep. The A/B relationship was calculated to be 0.256 +/- 0.001 at 40°C and 0.266 +/- 0.009 at 35°C. The results were compared to a theoretical model presented in the literature. v vi