Ductility Enhancement and Mechanical Properties of Ibad Ceramic thin films
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DUCTILITY ENHANCEMENT AND MECHANICAL PROPERTIES OF IBAD CERAMIC THIN FILMS J.K. HIRVONEN*, T.G. TETREAULT*, G. PARKER*, AND C.J. McHARGUE** * Spire Corporation, Bedford, MA 01730 ** ORNL, Oak Ridge, TN 37831 ABSTRACT Thin ceramic films (AIO 3, ZrO2, SiN 4, and BN) have been prepared by ion beam assisted deposition (IBAD) and their mechanical properties examined. The films exhibit extreme ductility and adhesion, with the former property possibly attributed to the very fine grained, quasi-amorphous grain structure noted for low temperature IBAD coatings. INTRODUCTION Recently, the IBAD process has become an increasingly studied method of applying highly adherent optical and tribological coatings [1,2]. The IBAD process used in this work, shown schematically in Figure 1, involved electron beam evaporation of a solid phase (e.g., aluminum oxide) onto a substrate while simultaneously bombarding the growing layer with energetic (300-1000 eV) oxygen or nitrogen ions. Ion beam bombardment results in a mixed zone interface between the substrate and coating, and characteristically gives IBAD-grown films superior adhesion and high density. IBAD ceramic coatings also exhibit enhanced ductility under compressive deformation which has been compared to the superplastic behavior seen for ultra-fine grained materials [3]. EXPERIMENTAL PROCEDURE AND RESULTS Figure 2 shows a schematic of the IBAD facility. The system consists of a two-chambered, differentially pumped vacuum system. The lower evaporation chamber contains the electron beam evaporator and is pumped by a four-inch diffusion pump. The upper target chamber contains the sample holder and is pumped by an eight-inch cryopump. The usual working base pressure for the system is 1 x 10-7 torr (1.3 x 10-3 Pa), with water being the predominant component as determined by an in-situ residual gas analyzer. For the coatings in this study, the evaporation rate of the A120,, ZrO2, Si, or B was set between 3 and 10 A/s and was controlled by a quartz crystal rate monitor while the oxygen or nitrogen ion (500-600 eV) flux at the substrate varied from 0- 500 j±A/cm . The ratio of ion flux to evaporant rate was varied per deposition to influence the stoichiometry of the resulting coatings. Coatings were deposited on Si (100) wafers for ellipsometric analysis, on carbon substrates for RBS analysis, and on glass cover slips for stress measurements. Elevated temperatures are normally used for producing dielectric optical coatings. However for mechanical applications, low temperature depositions were explored first to see how effective energetic ions are in providing activation for film growth by themselves. An oxygen background (3 x 10-' ton') was employed to counter the effect of molecular breakup of the aluminum and zirconium oxides during evaporation. It is estimated that the substrate temperature for these depositions due to beam heating did not exceed 150'C. A hot W filament was used for beam neutralization during the depositions, for the purpose of avoiding possible substrate charging. Ion
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