A Consistent Rationale for the Superior Strength and Ultra-Hardness of Ceramic Nano-Composite Coatings
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A CONSISTENT RATIONALE FOR THE SUPERIOR STRENGTH AND ULTRA-HARDNESS OF CERAMIC NANO-COMPOSITE COATINGS A. S. Argon and S. Veprek1 Massachusetts Institute of Technology, Cambridge, MA 02139, USA 1 Institute for Chemistry of Inorganic Materials, Technical University of Munich, D-85747 Munich, Germany ABSTRACT Nano-structured composite ceramic coatings such as TiN with Si3 N4 or T iSi2 prepared by various forms of plasma assisted CVD, composed of crystalline components of equiaxed TiN of several nm diameter, surrounded by amorphous Si3 N4 intercrystalline layers of roughly 0.2 volume fraction have exhibited hardnesses in the range of 70-100 GPa-quite commensurate with polycrystalline diamond layers, and thermal stability up to 1000C. Preliminary considerations indicate that such ultra-hardness, uninfluenced by the usual artifacts of nano-indentations are not governed by processes of crystal plasticity in the crystalline component but by the characteristic flow mechanisms of the often topologically continuous amorphous component exhibiting “liquid-like” behavior in the constrained spaces between the crystalline components. INTRODUCTION The properties and behavior of nano-structured solids have been of intense interest for some time, with much effort being devoted to understand the underlying mechanisms and to capitalize for individual applications the generally perceived enhanced plastic resistance and hoped-for improvement of fracture toughness (for a recent overview see [1]). The materials of primary interest to us, here, are nano-structured ceramic composite coatings which exhibit extraordinary hardness and thermal stability making them of great interest for coating applications for cutting tools that would permit dry high speed machining and would do away with the need for coolants which are generally environmentally unfriendly and expensive to re-cycle. Savings in this application alone can result in economies in the billions of dollars [2]. Our interest is not merely from an industrial perspective but rather to develop some much needed fundamental understanding of the underlying mechanisms that govern the extraordinary behavior which can not be understood by the familiar mechanisms of plastic resistance that govern the behavior of crystalline substances of either metallic or covalent nature. Of particular interest will be the recently developed nano-structured ceramic coatings of TiN and Si3 N4 which have exhibited unambiguous hardnesses equal to or exceeding that of polycrystalline diamond films. Recent technological deelopments in the field of ultra-hard ceramic coating with nano-scale structure have been reviewed by one of us [2]. THE nc-TiN/a-Si3 N4 COMPOSITE COATING Structure. The thermodynamics and kinetics background for the formation of the nano-structured coatings and their method of production by variants of plasma assisted CVD have been discussed by us elsewhere [2, 3] including, characterization of the stoichiometry, crystal structure, scale and internal arrangement in coatings of micron level thicknesses.
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