Combustion Flame Diamond Coating Method (Hirose Method); from its Birth to the Present

Abstract

Abstract Since the announcement of a reproducible CVD synthesizing method of diamond thin films, by the group of Setaka in Japan National Institute for Research in Inorganic Materials, which utilized decomposition of hydrocarbon (for example CH4), several other synthesizing methods have been proposed. Typical ones include, for example, hotfilament CVD, microwave plasma CVD, rf plasma CVD, dc plasma CVD, and arc discharge plasma CVD. The author, in fact, has found an interesting common element for these CVD methods during various research efforts since autumn of 1987 in developing a new synthesizing method. This was a finding that all these conventional methods included introducing a carbon source as a precursor into a reducing plasma space, decomposing a carbon source, then producing radicals such as CH3, C2, etc., thereby depositing diamond onto substrate through the radical reaction. It is noted here that the abovementioned plasma can include not only the generally known ones of glow discharge and arc discharge but also one of combustion flame in an atmosphere. To be more specific, the flame consists of a combustion substance including chemically reacted hydrocarbon (CH3, C3H8, etc.) gas and oxygen gas; the temperature being as high as 2000°C and more. Furthermore, the combustion flame comprises two areas, one being an oxidizing area called “outer flame” (oxidizing one) and the other being a reducing area called “inner flame” (reducing one). Finally, it is a well known experimental fact that a key point to successful diamond synthesis is to produce a radical which is made in a reducing plasma space from the decomposition therein of a gas including a carbon precursor. In view of the above, the author assumed that if diamond could be synthesized through a use of combustion flame, the synthesizing area would be within the abovementioned inner flame. This is the background why the author attempted a diamond synthesis by means of a substrate placed within an inner flame. This finally resulted, in the end of November 1987, in a first successful synthesis of diamond using a CH4 + H2 combustion flame. This was substantially followed by the reconfirmation of another combustion flame, i.e., a C2H2 + 0 2 flame in its ability again to synthesize diamond with a very good reproducibility. The present combustion flame method is characterized in that (1) it needs neither reaction chamber nor expensive electric power because it depends on a combustion flame within the air atmosphere; (2) the apparatus involved can be very simple and inexpensive as well: (3) it can deposit diamond film at a rate of 100 ∼ 170 μm/h which is higher than the other CVD methods; and (4) it enables coating to a substrate having large area and/or curved surface. In this paper, further details of the development with time of the diamond film synthesis through the use of combustion flame will be described as well as update of this research.