A new and simple micromechanical approach to the stress-strain characterization of thin coatings

Abstract

The mechanical properties of thin coatings are of great importance in the design, fabrication and use of micromechanical devices. Many different test methods and test structures have been previously suggested. These methods either preassume knowledge of the elastic properties of the coating and/or the substrate material, data that are generally unknown or unreliable for micromachined structures, or are based on thin, free-standing structures (released coatings), the mechanical properties of which are not representative for thin coatings in well-adhered film-substrate composites. The present work gives a theoretical basis for a new micromechanical experimental procedure, by which a complete elastoplastic stress-strain characterization of thin coatings on elastic cantilever beams can be performed. The novelty of this procedure lies in the fact that no previous information on elastic constants or plasticity parameters is required (neither for the film material nor for the substrate), and that the procedure works equally well in the plastic as in the elastic film-strain interval. Geometric parameters and experimental load-deflection data are the only necessary input data in the model, and a complete elastoplastic sigma - epsilon characteristic of the film is the output. The effect of residual stresses on the results is discussed, and the problem of inhomogeneous film stresses is briefly considered. Finally, various errors and uncertainties in the suggested test procedure are discussed, and some requirements on the sensitivity and accuracy of the test equipment are indicated.