Indentation-derived elastic modulus of multilayer thin films: Effect of unloading-induced plasticity
- PDF / 954,617 Bytes
- 12 Pages / 584.957 x 782.986 pts Page_size
- 46 Downloads / 220 Views
Yu-Lin Shen Department of Mechanical Engineering, The University of New Mexico, Albuquerque, New Mexico 87131, USA (Received 5 March 2015; accepted 10 June 2015)
Nanoindentation is useful for evaluating the mechanical properties, such as elastic modulus, of multilayer thin film materials. A fundamental assumption in the derivation of the elastic modulus from nanoindentation is that the unloading process is purely elastic. In this work, the validity of elastic assumption as it applies to multilayer thin films is studied using the finite element method. The elastic modulus and hardness from the model system are compared to experimental results to show validity of the model. Plastic strain is shown to increase in the multilayer system during the unloading process. The indentation-derived modulus of a monolayer material shows no dependence on unloading plasticity while the modulus of the multilayer system is dependent on unloading-induced plasticity. Lastly, the cyclic behavior of the multilayer thin film is studied in relation to the influence of unloading-induced plasticity. It is found that several cycles are required to minimize unloading-induced plasticity.
I. INTRODUCTION
Natural and engineered multilayer structures can exhibit intriguing properties and functionalities.1 As an example, lamellar structures (a ferrite/Fe3C cementite) on the microscopic scale are the foundation for the combination of strength and toughness of steels. Some structures in nature, such as mollusk shells, derive their high strength and toughness from ceramic layers bonded together by an organic glue.2 Multilayer thin films may now actually be viewed as a new class of materials. With a thickness of up to tens of micrometers and layer thicknesses of a few to hundreds of nanometers, these structures have a wide range of applications including optical devices, high-performance capacitors, ultrahighstrength materials, thermo-electric materials, high wear resistance and low friction coatings for gears, bearings, cutting tools, and thermal protective layers in aircraft and automobile engines.3–8 In addition, certain systems, such as the aluminum (Al)-silicon carbide (SiC) nanolayers, are being considered as reflective coatings in ultraviolet applications as well as other applications.9–11,10 Compared to traditional material systems, these materials can offer higher strength to weight ratios, less friction and wear, higher operation temperatures, corrosion resistance, and fracture toughness. Therefore, designing and manufacturing multilayered structures at the micro- and Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.200
nanoscales are attractive strategy for developing a new generation of protective and infrastructure materials, and thus have been subjects of intensive research.12–18 The scale and complexity of these multilayered structures often make it difficult to characterize mechanical behavior. Nanoindentation is a commonly used experimental technique to determ
Data Loading...
