Using Weibull Statistics to Analyze Ion Beam Enhanced Adhesion as Measured by the pull Test
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USING WEIBULL STATISTICS TO ANALYZE ION BEAM ENHANCED ADHESION AS MEASURED BY THE PULL TEST J.E. PAWEL', W.E. LEVER', D.J. DOWNING', C.J. McHARGUE**, L.J. ROMANA', AND J.J. WERT**" "OakRidge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6376 USA "The University of Tennessee, Knoxville, TN 37996-2200 USA "'Vanderbilt University, Nashville, TN 37235 USA ABSTRACT The adhesion of iron films to single crystal A120 3 substrates was investigated using a pull test. Chromium (300 keV) or nickel (340 keV) ions were implanted to a fluence of 1 X 10" ions-cm 2 after film deposition. The adhesion test results were widely scattered due to a random distribution of interfacial flaw sizes controlling the failure nucleation. Because Weibull statistics were developed to describe the failure probability due to a population of flaw-initiated cracks, the Weibull distribution was chosen to analyze the data. Modifications in the adhesion strength due to the ion implantation were reflected in the failure distributions. It was found that the chromium ions improved the adhesion of the Fe/AI 20 3 system while the implantation of nickel did not. INTRODUCTION The strength of the bond between coatings and substrate materials is an important limitation in a variety of applications. The interest in and importance of this subject is indicated by a recently published bibliography on adhesion measurements that includes over 300 references [1]. Progress in determining the fundamental parameters that govern the strength of such bonds has been hindered by the lack of quantitative test procedures. Even results of the so-called quantitative tests, the scratch test, the pull test, and the peel test, can be difficult to interpret due to large scatter of the individual strength measurements [2-4]. In this study, the Weibull distribution is used to analyze pull test measurements taken from as-deposited and ion implanted films and the applicability of this model is assessed. For the pull test, pins are attached to the film surface with an epoxy and then pulled off with a normal force. The force required to remove the film from the substrate is referred to as the pull strength. The test is an indirect measure of the adhesive bond strength between the film and its substrate. The failure is nucleated at interfacial defects and the propagation of the cracks is controlled by the bond strength. For a system with a low bond strength or work of adhesion, a small defect can initiate failure of the film. Because Weibull statistics were developed to describe the failure probability due to a population of flaw-initiated cracks [5, 6], the Weibull distribution was chosen to analyze the data. EXPERIMENTAL PROCEDURE High purity A120 3 single crystals were coated with 100 nm of iron by electron beam evaporation [3]. After the film deposition, the specimens were implanted with Cr (300 keV) or Ni (340 keV) ions. The energies chosen were those required to place the peak of the ion concentration at the interface [3]. The fluence in each case was 1 X 1015 ions-cm"2 and t
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