Shinji Hirai1 and Shigetomo Ueda2
1Graduate Student, Waseda University, Tokyo
The nonporous silicon diffusion coating using the reduction of silicon carbide with titanium has been applied to commercial iron (SPHC) and plain carbon steel (S45C). The results obtained are as follows:
(1) The optimum composition of the pack material was proved to be in a proportion of 18 mass% silicon carbide, 28 mass% titanium, 52 mass% titanium dioxide, and 2 mass% sodium fluoride. When a specimen was treated at 1373 K for 10.8 ks using this pack material, a nonporous silicon treated layer was produced. The SPHC treated layer had the thickness of 380 μm and the Si concentration of 13 mass%.
(2) In the case of SPHC, Fe3Si was identified from the additive layer and α-Fe with Si and Ti in solid solution from the diffusion layer. In the case of S45C, TiSi and Ti5Si3 were identified from the additive layer, TiC from the intermediate layer and α-Fe with Si in solid solution from the diffusion layer.
(3) In this experiment, sodium fluoride acting as an activator and titanium dioxide as a diluent were found to be highly effective in producing a nonporous silicon treated layer having a sufficient concentration of silicon. For example, alumina used as the diluent is reduced by titanium to form Al which suppresses the diffusion of Si.
(4) The cause for the formation of the nonporous silicon treated layer is considered as follows. The Ti which is first diffused in the steel suppresses the diffusion rate of Si which is diffused later and consequently relieves the non-uniform diffusion. Since Ti has a reducing reaction with the silicon carbide powder at the initial stage of the chemical reaction occurring in the pack material, Ti begins to diffuse in the steel while the titanium powder is being consumed for the reduction.
silicon diffusion coating, nonporous coating, silicon carbide, titanium, titanium dioxide, sodium flouoride, commercial iron, plain carbon steel
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