Microstructural Studies of Iad and PVD Cr Coatings by Cross Section Transmission Electron Microscopy
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MICROSTRUCTURAL STUDIES OF LAD AND PVD Cr COATINGS BY CROSS SECTION TRANSMISSION ELECTRON MICROSCOPY C. C. CHENG, R. A. ERCK, AND G. R. FENSKE Materials and Components Technology Division, Argonne National Laboratory, 9700 S. Cass Ave, Argonne, IL 60439 ABSTRACT Cross section transmission electron microscopy was used to study the microstructure and interface structure of Cr films deposited by ion-assisted deposition as a function of the incident ion energy. High-energy ion bombardment (1 keV) was found to enhance the adhesion of the deposited film owing to the formation of an intermixed layer, whereas deposition with lowenergy ions (100 eV) was found to reduce or eliminate grain boundary porosity. A tailored Cr film with excellent adhesion and no grain boundary porosity was deposited by combining highand low-energy ion bombardment. INTRODUCTION Coatings are applied to materials for a large and growing number of purposes, including increased wear resistance, reduced friction, corrosion protection, optical requirements, electrical conduction or insulation, or simply for decorative effect. Conventional chemical and physical vapor depositions (CVD and PVD) still play the major roles in fulfilling most of these purposes. However, the use of energetic-ion beams to modify the near-surface region of materials has been increasingly explored in recent years, especially for tribological purposes. Among ion beam processes, ion-assisted deposition (lAD) is a relatively new technique involving ion bombardment and concurrent PVD to produce controlled films. Many investigators have shown that ion bombardment can be used to increase nucleation rates [1], enhance adatom mobilities [2], promote the adhesion of films [3], increase the film density [4,5], decrease the average grain size [6], inhibit the formation of a columnar structure [7], reduce the internal stress [8], and increase the overall defect density [9].
The tribological and chemical properties of films strongly depend on the structure of the film and in particular, on the grain morphology and the structure of the substrate/coating interface. Studies of the film microstructure are necessary to understand the kinetics of film formation and the resulting in-use performance. Transmission electron microscopy (TEM) is a powerful tool for studying the microstructure of thin films, which often possess small-size grains and many crystalline defects. Observations by TEM are most commonly performed with the electron beam normal to the plane of the film. Because the film microstructure changes during film growth, cross section TEM studies give a more complete picture of the film structure and the film/substrate interface. Thus, it is possible to study the relationship between the film and substrate microstructures. This is especially important for lAD because the grain microstructure and the substrate/film interface are strongly influenced by the energetic ions. Grain and interface structures depend on a number of lAD process parameters, including the incident ion energy, the ion/va
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