IMECE2004-60070 FRACTURE SIZE EFFECT IN ULTRANANOCRYSTALLINE DIAMOND – WEIBULL THEORY APPLICABIILITY

Strength characterization and analysis of fracture size effect in ultrananocrystalline diamond (UNCD) thin films are presented. In this work, we report the changes in mechanical properties of UNCD by the addition of nitrogen gas to the Ar/CH4 microwave plasma. Both undoped and doped UNCD films show a decrease in fracture strength with an increase in specimen size. The strength data, obtained by using the membrane deflection experiment (MDE) developed at Nothwestern University, is interpreted using Weibull statistics. The capability of the theory is examined in conjunction with detailed fractographic analysis. The Weibull parameters are estimated by maximum likelihood estimation (MLE) based on 480 tests when the specimen volume varies from 500 to 16000 cubic microns. The results show that one can predict the fracture strength of a component possessing any arbitrary volume to within ±3 % from the fracture strength identified from the tested specimens. The failure mode of UNCD is suggested to be volume controlled