Instrumented-Indentation for Mechanical Characterization of Boron Carbide Nano-Composite Coatings
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Instrumented-Indentation for Mechanical Characterization of Boron Carbide NanoComposite Coatings Mark Walter, Srikant Nekkanty, Elizabeth Cooke1, and Gary Doll1 The Ohio State University, Department of Mechanical Engineering, Columbus, OH 43212; 1 The Timken Company, Canton, Ohio 44706. ABSTRACT Boron carbide (BC) is well known as a coating material that is important for a wide range of technological applications. The applicability of boron carbide stems from the fact that it is a very hard material with high lubricity, high elastic modulus, low specific gravity, and good chemical stability. Disadvantages, however, include extreme brittleness and sometimes poor adhesion. Recently, a reactive sputtering involving boron carbide targets and hydrocarbon gases has been used to produce novel nano-composite boron carbide thin films comprised of BC nanocrystals embedded in a matrix of hydrogenated amorphous carbon (DLC). The microstructure of these thin films is similar to that of other metal carbide/DLC nano-composite films. The present paper discusses the results of Vickers indentation experiments carried out on four different samples of boron carbide/DLC coatings that were sputtered deposited onto 52100 steel disks. The four different samples resulted from four different levels of hydrocarbon gas flow during processing. Acoustic emission data was recorded simultaneously with the indentation experiments. The indentations and the associated crack patterns were observed using scanning electron microscopy. INTRODUCTION Diamond like carbon (DLC), also known as tetrahedral or hydrogenated amorphous carbon, is an amorphous state of carbon consisting mainly of sp3 carbon atoms. In recent years DLCs have attracted significant attention because they can be produced in coating form at relatively low temperatures and have been found to possess high elastic modulus, hardness, atomic smoothness, and chemical inertness. These noteworthy properties originate from the fact that DLCs form a network of sp3 carbon atoms that is continuous, highly oriented, and very rigid. With their diamond like properties, DLCs have tremendous potential for application in wear coatings, microelectronics, microtribology, and biomedical technology. The properties of boron carbide coatings vary significantly with the coating process and process conditions. Techniques such as plasma enhanced chemical vapor deposition (PECVD) [1-3], DC sputtering or rf sputtering have been used to synthesize boron carbide films. Various studies have been carried out on the effects of the process parameters like processing gas pressure, heating during deposition, and bias voltage during deposition. The microstructure, chemistry and the properties of the coatings depend on the process parameters. Hu et al. [4], for instance, have discussed the effects of bias voltage on the microstructure and the coating properties. Knotek et al. [5] have concluded that an increase in hardness is observed with both the heating and the applied negative bias voltage level at the substrate. Recently
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