American Institute of Aeronautics and Astronautics © Vikas Tomar 1 Role of Length Scale and Temperature in Nanoscale and Microscale Creep of Si-C-O Ceramics Ming Gan and Vikas Tomar 1 1 School of Aeronautics and Astronautics, Purdue University, West Lafayette, Indiana, 47906, USA This investigation presents nanoindentation and microindentation creep analyses on polymer derived Si-C-O ceramic coatings at temperatures ranging from room temperature to 500 degree-C. The properties of focus include elastic modulus, hardness, creep exponent, and creep strain rate. Analyses show that at the nanoscopic length scale the deformation mechanism is dominated by dislocation climb and diffusion. With increase in length scale to microscale the thermal activation volume increases by approximately 10 times. The increase in free volume leads to the deformation mechanism switching to volumetric densification and dislocation pile up. An important physical effect analyzed is the effect of increase in temperature on the observed deformation mechanism. At the nanoscale, with increase in temperature, both hardness and elastic moduli show an increase. At the microscale, however, hardness reduces with increase in temperature. The indentation size effect is observed at both scales. However, at the nanoscale the indentation size is linked with strain hardening. At the microscale, a strain softening behavior is observed. I. Introduction reep is non-recoverable high temperature plastic deformation occurring at low load regimes, constant stress, and small strain rates. Characterization of creep deformation is very important in the case of materials intended for use at high temperatures. Another important creep deformation is the one occurring at nanoscale contacts at moderate temperatures. Characterization of such creep deformation is important for applications related to operation of miniature devices, thermal stability of interfaces etc. Indentation creep experiments with typical holding times of 500 seconds are used to characterize such creep deformation, Li and Ngan 1, Chudoba and Richter 2, Li et al. 3 . Indentation creep is defined as the constant rate indenter displacement ( h ɺ ) after a finite time of load hold (~500 sec) once the indenter displacement is free of thermal drift (usually for depths ≥ 200 nm). Fundamentally, during nanoscale or microscale indentation tests contact stresses are high. Correspondingly, nominal pressure during nano- and microindentation can easily reach a few percent of the Young’s modulus of materials resulting in an ideal strength situation. Due to such high stress contacts indentation creep can occur at low homologous temperatures (Current Temperature (T)/Melting Temperature (T m )) even though the same material would not exhibit any creep under bulk condition at the same temperature. Nanoindentation creep has been observed in a wide range of materials including glasses, ceramics, and metallic materials, Li and Ngan 1, Chudoba and Richter 2, Li et al. 3 . The indentation creep rate for high melting point materials can be of the order of 5 × 10 -5 s -1 . This is a large value when compared to the corresponding bulk strain rate. The models for indentation creep are the same as those for bulk creep, with the equivalent stress exponent, n, used as an indicator of the creep mechanism, Li et al. 3, 4 . It is believed that when the value of n is 1, creep is controlled by vacancy diffusion as deformation mechanism, Herring 5, Coble 6 ; when the n value is 2, the creep mechanism the controlled by grain boundary sliding, Lifshitz 7 ; when n is 3, diffusion-controlled dislocation motion dominates as deformation mechanism, Weertman 8, Nix and Ilschner 9 ; and when n is 5, it is dislocation climb-controlled creep mechanism, Sherby and Burke 10 . During indentation creep tests on certain metals, alloys, and ceramics at room temperature, high n values up to hundreds have been observed, Li et al. 11, Li and Ngan 12, Feng and Ngan 13, Cao et al. 14, Mayo and Nix 15, Lucas and Oliver 16, Asif and Pethica 17 . The mechanism behind such high stress exponent values has been attributed to volumetric densification and dislocation pile up. Materials systems analyzed include metallic thin films, e.g. Cu, Al, Ni, Zong and Soboyejo 18, Durst et al. 19, 20 , thin films made of metallic alloys, e.g. Ni 3 Al, Li and Ngan 21 , and ceramics e.g. ZrO2, Jan et al. 22 , and SiC-based, Chollon et al. 23, Janakiraman and 1 Assistant Professor, Aeronautics and Astronautics, email: tomar@purdue.edu , Member AIAA. C 52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference<BR> 19th 4 - 7 April 2011, Denver, Colorado AIAA 2011-2157 Copyright © 2011 by Vikas Tomar. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.