| The growth mechanism of MoSi₂ coatings formed by the pack siliconizing of molybdenum was investigated in the temperature range of 1223K∼1473K. Experiments were conducted under an argon atmosphere using 40Si-5NaF-55Al₂O₃ in weight percents packs. The results clearly revealed the following novel phenomenon : when packs were kept at a constant temperature, the growth rate of MoSi₂ coating was proportional to time at early stage, but at later stage the growth was changed to parabolic function of time. In order to interpret the phenomenon, a model was proposed under the assumption that chemical reaction for MoSi₂ formation at MoSi₂/Mo interface were responsible for the early stage growth. As a result, it was understood that the growth at early stage was controlled by chemical reaction and the later stage growth controlled by the mass balance conditions of gas and solid diffusions. Thus, two kinds of activation energy were obtained ; one with the value of 240 KJ/mole was considered as the linear growth rate constant, k₁, (㎝/sec), and the second with the value of 1.93 KJ/mole was about the parabolic growth rate constant, k_p(㎠/sec). |
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