고에너지 전자빔 투사법에 의한 (TiC, TiB₂, VC)/Steel 표면합금화 재료의 특성 연구
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To determine 1) the best ceramic particles, 2) the optimum fabrication conditions, and 3) the correlation of microstructural evolution and mechanical property improvement in the surface alloying of 1) TiC, 2) TiB₂, and 3) VC using High Energy Electron Beam, surface alloys of each type of ceramic particles with carbon steel were fabricated and the examined. The mixtures of the ceramic particles of ~ 2μm with a flux of Al and MgF₂ are spread over low carbon steel, which were then melted and alloyed by the 1.4MeV HEEB scanning. All the three surface alloyed layers were about 2mm thick and consisted of 1) melted, 2) interface, and 3) heat affected regions. The surface regions consisted of ferrite of BCC, a=~2.86Å with TiC or VC particles, whose respective structure and lattice parameter were found as FCC, ~4.33Å, and FCC, ~4.16Å. The surface alloying of TiB₂ appeared to be relatively incomplete showing particles of TiB₂ with evidence of incomplete melting, which was attributed to the high melting point of TiB₂. In the TiC surface alloyed layer, carbides consisted of primary cuboids of 5~10㎛ and eutectic plate-likes. In the TiB₂ surface alloyed layer, TiB₂ showed up as 1~2㎛ in diameter. In the VC surface alloyed layer, well developed dendritic structure was observed with carbides of 0.5~5㎛ long and 500nm thick which were mostly in the dendrite boundaries. TEM observation of the specimen additionally showed fine particles of 50nm, which were observed only in this specimen, of VC were abundant in the matrix. The Vikers microhardness test showed the highest hardness at the surface, which started to decrease sharply at the interface and reached to lowest at the matrix. The hardness of the VC alloyed layer showed 2~3 times of improvement relative to the unalloyed specimen. However, those of the TiC or TiB₂ alloyed specimens did not show much improvement of the hardness. SEM analysis of the cross-section of the wear-tested specimens showed deformed ferrite matrix and the cracks developed along the interface of the ferrite matrix and the strengthening particles. Though there was no change in the wear modes of the three types of surface alloyed specimens, the VC surface alloyed specimen showed the highest wear resistance among the three types, due to the fine high density VC particles in the matrix of that specimen.