Synthesis and properties of B x C y N z coatings
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Some of the most interesting compounds from both technological and a scientific viewpoint can be found within the B–C–N composition triangle. Despite all the attention that some phases on the vertices and sides of the triangle have attracted, few works have focused inside the triangle itself. A laser-assisted chemical vapor deposition system was used to deposit B–C–N phases over fused quartz substrates. Two sets of gaseous precursors were used, namely B2H6 + NH3 + C2H4 and B2H6 + (CH3)2NH. The coatings were characterized regarding chemical composition, structure, and morphology. Hardness measurements were also carried out with a depth-sensing indentation instrument. It was found that depending on the gas phase, different regions of the BCN solid composition triangle are accessed. Coatings ranging from pure h-BN to pure B4C were obtained, as well as mixtures of these with BxCyNz compounds.
I. INTRODUCTION
Because of the highly interesting mechanical, optical, and semiconducting properties presented by some of the phases located on the vertices and sides of the B–C–N composition triangle, great effort has centered on the study of these phases for thin-film applications. Special emphasis has been given to the study of the cubic phase of carbon (diamond) and of boron nitride (c-BN) and, to a lesser extent, to the rhombohedral conformation of boron carbide. Nevertheless, the formation of ternary compounds can be expected because B–N and C–C units display isoelectric properties and graphite and h-BN, as well as diamond and c-BN, have similar crystal structures. When considering B–C–N materials, three classes should be considered: (i) those materials consisting of a mixture of primary phases, i.e., phases on the sides and vertices of the triangle; (ii) compounds that preserve the crystallographic structure of one of the primary phases and allow for the tuning of one or more of its properties through the incorporation of the third element; and (iii) new materials with phases and properties different from those observed in limit phases. For instance, it has been shown1 that the electrical properties of h-BN may be controlled by the incorporation of carbon in its structure, thus allowing for the electrical tuning of the ternary compound between the insulating characteristics of h-BN and the quasi-metal behavior of graphite. In their pioneering approach to B–C–N materials, Kosolapova et al.2 simply heated boron and carbon in the presence of either nitrogen or ammonia. However, the resulting products were not well characterized. Since 734
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J. Mater. Res., Vol. 16, No. 3, Mar 2001 Downloaded: 19 Mar 2015
then, several research groups3–6 have attempted to produce this type of material by using chemical vapor deposition (CVD). BCl3 and NH3 were the main precursors for boron and nitrogen, whereas various carbon precursors such as CCl4, CH4, and C2H2 were used. These experiments generally yielded ternary compounds, in a wide composition range. A detailed review of the work conducted before 1992 has
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