Mechanical Characterization - Micromechanical Testing and Modeling

Monday, April 10, 2000 10:30 AM in Room California

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10:30 AM E4/F1-1 What is Indentation Hardness?
Y.-T. Cheng (General Motors Research and Development Center); C.-M. Cheng (Institute of Mechanics, Chinese Academy of Sciences, China)
For nearly one hundred years, indentation experiments have been performed to obtain the hardness of materials. Recent years have seen significant improvements in indentation equipment and a growing need to measure the mechanical properties of materials on small scales. It is now possible to monitor, with high precision and accuracy, both the load and displacement of an indenter during indentation experiments. However, questions remain, including what properties can be measured using instrumented indention techniques and what is hardness? @paragraph@ We address these basic questions using dimensional analysis and finite element calculations. We derive simple scaling relationships for loading and unloading curve, initial unloading slope, contact depth, and hardness. The relationship between hardness and the basic mechanical properties of solids, such as Young’s modulus, initial yield strength, and work-hardening exponent, is then revealed. It is shown that the hardness value is not necessarily 3 times the yield strength value. The conditions for “piling-up” and “sinking-in” of surface profiles during indentation are determined. The methods for estimating contact depth from initial unloading slope are examined. The work done during indentation is also studied. A relationship between the ratio of hardness to elastic modulus and the ratio of irreversible work to total work is discovered. This relationship offers a new method for obtaining hardness and elastic modulus. In addition, we demonstrate that stress-strain relationships may not be uniquely determined from loading/unloading curves alone using a single conical or pyramidal indenter. Finally, a scaling theory for indentation in power-law creep solids using self-similar indenters is developed. A connection between creep and “indentation size effect” is established.
11:10 AM E4/F1-3 Nanoindentation Behaviour of CNx
S.J. Bull, T. Malkow, I. Arce-Garcia (University of Newcastle, United Kingdom)
Thin CNx coatings deposited by magnetron sputtering and ion beam assisted deposition show very shallow residual impressions when investigated by Nanoindentation. The initial part of the loading curve has load proportional to displacement due to contact of rough surfaces and is followed by a period of elastic behaviour when load is proportional to displacement to the power of 1.5. There is a gradual transition to full plasticity at higher loads where load is proportional to displacement squared. The low work of indentation (i.e. small area enclosed by the loading and unloading curves) of these materials implies a very high hardness. However, analysis of the unloading curves by the Oliver and Pharr method generates hardness values which are often less than silicon. Very low E/H values are measured for CNx, particularly at high deposition temperatures. In this paper we will show how the properties of CNx vary with substrate, coating thickness, deposition temperature and deposition technology.
11:30 AM E4/F1-4 Ultramicrohardness of Amorphous and Crystalline GeBise Thin Films
G.B. Reddy, T. Rajagopalan, M. Agarwal (Thin Film Laboratory, Physics Department, Indian Institute of Technology, New Delhi, India-110016, India)
Chalcogenide thin films have been studied extensively in recent years because of their technological importance. Many applications of these materials are based on amorphous state to crystalline state transformation process and the resultant changes in basic material parameters. Variety of analytical techniques have been employed to monitor/control these processes. Recently, the authors have reported strong dependence of annealing conditions on the structure and orientation in annealed GeBiSe films. The study showed that it would be possible to obtain single crystal films directly from as deposited amorphous films.@@Since the surface hardness is a strong function of the structure, it should be possible to understand the structural transformation by systematically measuring the hardness of the films before and after annealing. In the present study the ultra-microhardness tester has been used to measure hardness of as-deposited amorphous and crystallized Ge@sub 15@Bi@sub 38@Se@sub 47@ and Ge@sub 25@Bi@sub 27@Se@sub 48@ films. The hardness of annealed films with different conditions has been correlated with the reported film structure. The denting parameters like indent load, indent speed and dwell time have been varied to understand the surface hardness and film quality. Film density variation has also been studied with random denting. In the paper the complete details of ultra-micro-hardness measurements and analysis will be presented.
11:50 AM E4/F1-5 Finite Element Modeling of the Stresses, Fracture and Delamination During the Indentation of Hard Elastic Films on Elastic-Plastic Soft Substrates
R.M. Souza, G.G.W. Mustoe, J.J. Moore (Colorado School of Mines)
In this work, the wear behavior of hard films on soft substrates was studied based on the finite element analysis of the indentation with normal forces. As an attempt to reproduce situations found in practice, defects were considered during the preparation of the finite element mesh, both in the film and at the interface. A sequence of steps was considered during the loading portion of the models. Initially, the deposition (intrinsic) and thermal (extrinsic) stresses were introduced to account for all residual stresses present in thin films deposited by processes such as sputtering. In a later step, an indentation with a rigid spherical indenter was applied. The influence of crack propagation through the film and along the interface was studied based on the normal and shear stresses that develop at the film surface and at the film/substrate interface.