Deformation and Fracture of Oxides Fabricated on 304L Stainless Steel via Pulsed Laser Irradiation
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Deformation and Fracture of Oxides Fabricated on 304L Stainless Steel via Pulsed Laser Irradiation Samantha K. Lawrence1,5, Douglas D. Stauffer2, Ryan C. Major2, David P. Adams3, William W. Gerberich4, David F. Bahr1, Neville R. Moody5 1. Mechanical and Materials Engineering, Washington State University, Pullman WA 2. Hysitron Inc., Minneapolis, MN 3. Sandia National Laboratories, Albuquerque, NM 4. Chemical and Materials Engineering, University of Minnesota, Minneapolis, MN 5. Sandia National Laboratories, Livermore, CA ABSTRACT Localized heating of metals and alloys using a focused laser beam in ambient atmosphere produces dielectric oxide layers that have characteristic optical appearances including different colors. Nanoindentation probed the deformation and fracture of laser-fabricated oxides on 304L stainless steel. Conductive nanoindentation measured electrical contact resistance (ECR) of the same colored oxides indicating a correlation between laser exposure, conductance during loading, current-voltage (I-V) behavior at constant load, and indentation response. Microscopy and X-ray diffraction examined the microstructure and chemical composition of the oxides. Combining techniques provides a unique approach for correlating mechanical behavior and the resulting performance of the films in conditions that cause wear. INTRODUCTION Nanoindentation is a powerful technique for investigating small material volumes [1,2]. With the development of instrumented indenters, which continuously monitor both the load and penetration depth of a tip into a sample, the phenomena of instantaneous load excursions have been observed. Sudden excursions in depth at low loads have been observed in film systems during load-controlled nanoindentation. Such load excursions in these systems have been ascribed to mechanisms such as oxide film fracture [3,4]. The small volumes of materials probed with nanoindentation make it an ideal technique for testing various thin films that can develop or be deposited on a substrate. The majority of previous thin film nanoindentation studies have involved films grown by physical vapor deposition, chemical vapor deposition, or electroplating. Almost no work has investigated the mechanical properties of oxides created by pulsed laser irradiation of metals in air. Coloring the exposed surfaces of metallic materials by forming an oxide or nitride layer through laser irradiation is a well-established concept [5]. Surface colorization of pure metals and alloys is a thermochemical growth process facilitated by the heat of absorbed laser light. Localized heating, provided by a focused laser beam, can lead to the growth of dielectric phases whose thickness is determined by the laser fluence and the adsorption and diffusion of reactive gas species (primarily O, N). Film color is intimately related to the layer thickness and the refractive index of the dielectric layer. Most previous experimental and theoretical work on laser colorization involves continuous wave (CW) laser exposure. Studies have correlated opti
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