Diffusion couples of molybdenum with carbon and several carbides, i.e. B
4C, SiC, TiC, and TaC, respectively, were heated for up to 3.6×10
5 s at various temperatures ranging from 1373 to 2223 K. The couples were then examined for composition, growth rate, structure, and hardness of reaction layers. Main results obtained are as follows:
(1) In the Mo-C system, only Mo
2C layer was formed at below 1873 K, while two sub-layers consisted of Mo
2C and η (MoC
1−x), respectively, were found at above 1873 K. The activation energy for growth of total layer was 374 kJ/mol.
(2) In the Mo-B
4C system, two sub-layers consisted of Mo
2B and MoB, respectively, with dispersed carbon particles were formed.
(3) In the Mo-SiC system, Mo
2C layer, including η (MoC
1−x) phase at high temperature, mixture of Mo
2C and Mo
3Si
2 phases, and Mo
3Si
2 phase in order from the Mo side were formed. The activation energy for growth of total layer was 477 kJ/mol.
(4) In the Mo-TiC system, two kinds of TiC in point of view of free carbon content were used; one is with 0.2% free carbon and the other is with 0.01%. In the Mo-TiC with 0.2% free carbon system, two sub-layers, i.e. relatively thick Mo
2C layer and thin (Ti, Mo)C layer, were formed, while in the Mo-TiC with 0.01% free carbon system two thin sub-layers, Mo
2C and (Ti, Mo)C, were formed; the Mo
2C layer in the latter case was very thin and was not found after short time heating at low temperature. The activation energy for growth of Mo
2C layer in the former system was 393 kJ/mol.
(5) In the Mo-TaC with 0.02% free carbon system, two thin sub-layers, (Mo, Ta)
2C and (Ta, Mo)C, were observed.
(6) TEM studies on the interface between Mo(
bcc) and Mo
2C(
hcp) showed that there was the following orientation relation, called as the Burgers relation, between these two phases; {110}
Mo\varparallel(0001)
Mo2C, 〈111〉
Mo\varparallel〈11\bar20〉
Mo2C.
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