Formation of Mo(Si,Al) ₂ coating on surface of Mo thin plate by dip-coating technique using Al-25mass%Si liquid at 1073K

A homogeneous layer of molybdenum alumininosilicide Mo(Si,Al) ₂ with C(40) structure was made on the sample surface of molybdenum thin plate with a thickness of 0.1 mm by dip-coating technique using Al-25mass% Si liquid at 1073 K. The layer was formed parallel to the specimen surface. Three regions with different compositions were found to be formed in the compound layer. Region A near the Mo(Si,Al) ₂/Mo interface had a constant composition of Mo(Si _0.83,Al _0.17) _2.33. In region B formed on the region A, Al concentration slightly increased. A remarkable increase in Al concentration was observed in region C near the specimen surface, whereas the concentration of Mo and Si decreased. The Mo(Si,Al) ₂ grains had two different shapes in the compound layer: needle-like grains near the Mo(Si,Al) ₂/Mo interface and granular grains near the specimen surface. The growth of the compound layer was found to exhibit three stages. In an early stage the thickness of the compound layer increased linearly as the dipping time increased, indicating that the growth was a reaction-controlled process. Its growth rate in the present study was much higher than that reported so far in the dip-coating using Al liquid with saturated Si content. In a long dipping-time stage an abnormal growth of the compound layer was observed. The compound region near the Mo(Si,Al) ₂/Mo interface had high hardness of 1400-1800 HV. The hardness of the compound layer decreased as the distance from the interface increased. Such a change of hardness in the compound layer was discussed in terms of shape and dense of Mo(Si,Al) ₂ grains.