Mechanical characterization of thin films using nanoindentation techniques.

摘要

A fairly novel and convenient method to analyze and characterize thin films and coatings in situ is via nanoindentation. Due to the small length scale, traditional mechanical characterization techniques do not capture thin film mechanical behavior. In order to explore the nano to micron-scale, i.e. 10 nm thin films to 30 micron thick coatings, indenters with varying resolutions and limits to the applied load and depth were used such as a Hysitron Triboscope, MTS Nano Indenter II, IBM Micromechanical tester and a Perkin-Elmer Dynamic Mechanical Analyzer from high to low resolution and low load/small depth to high load/large depth respectively. Properties such as elastic modulus, viscosity, hardness, yield strength, adhesive strength and adhesion are predicted with contact mechanics models, a viscoelastic model and elastic fracture mechanics models depending on the film/substrate combinations. The investigated material combinations, include a hard ceramic and a hard film on a hard ceramic, specifically Diamond-Like carbon (DLC) on magnesium oxide (MgO), a hard film on a soft substrate such as Diamond-Like carbon on polysiloxane (PSO) polymer, as well as a soft coating on a hard substrate for a styrene-acrylate copolymer coating on aluminum. Indentation derived properties for the DLC on MgO were based on a contact mechanics approach. Plastic deformation and failure was investigated for various substrate surface treatments and film thickness combinations in terms of the critical shear stress and surface dislocation density. For DLC on PSO a mechanical model analogous to a bending drumhead was developed to explore the indentation behavior. Interaction of bending and compressive stresses limits the application of this model to thin coatings, in this study 100 nm thin DLC. Effect of process variables such as crosslink density, rheological additives, surface anodizes on the mechanical properties as well as performance mechanics of a styrene-acrylate copolymer electropaint was investigated with indentation. The generated algorithms indicated that indentation derived properties, measured as a function of temperature and pressure give insight to pressure and glass transitions otherwise not observable in situ for a thin polymeric coating on a substrate.