Low-Temperature Cyclic Oxidation Behavior of MoSi 2 /SiC Nanocomposite Coating Formed on Mo Substrate Jin-Kook Yoon 1 , Kyung-Hwan Lee 2 , Gyeung-Ho Kim 3 , Jun-Hyun Han 1 , Jung-Mann Doh 1 and Kyung-Tae Hong 1 1 Metal Processing Research Center, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Republic of Korea 2 Division of Materials and Engineering, Korea University, Ahnam-dong, Sungbuk-ku, 136-701, Republic of Korea 3 Nano-Materials Research Center, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Republic of Korea The low-temperature cyclic oxidation resistance of MoSi 2 /19.3 vol% SiC nanocomposite coating formed on a Mo substrate in air at 500 C was investigated and compared with that of the monolithic MoSi 2 coating using field emission-scanning electron microscopy (SEM) and cross- sectional transmission electron microscopy (XTEM). The nanocomposite coating was produced by a prior carburizing process followed by chemical vapor deposition of Si on a Mo substrate. While the accelerated oxidation behavior was observed for the monolithic MoSi 2 coating after the incubation time of about 454 cycles, no pest oxidation was observed in the nanocomposite coating. The excellent low-temperature cyclic oxidation resistance of nanocomposite coating resulted from the deceleration of further inward diffusion of oxygen by formation of relatively dense SiO 2 and Mo 9 O 26 composite oxide scale through the preferential oxidation of SiC particles followed by oxidation of MoSi 2 phase. (Received February 9, 2004; Accepted May 14, 2004) Keywords: MoSi 2 /SiC nanocomposite coating, low-temperature cyclic oxidation resistance, molybdenum 1. Introduction Molybdenum disilicide (MoSi 2 ) has long been known as an attractive coating material for the protection of Mo and Mo-based alloys used in an oxidative atmosphere at high temperatures because of its high melting point (2020 C), good high-temperature oxidation resistance, and low density (6.24 g/cm 3 ). 1,2) The excellent oxidation resistance of MoSi 2 coating at high temperatures results from the exclusive formation of a slow-growing continuous SiO 2 scale accord- ing to eq. (1). MoSi 2 (s) þ (7/2)O 2 (g) ! 2SiO 2 (s) þ MoO 3 (g) ð1Þ Any MoO 3 that forms at high temperature rapidly evaporates to produce an uncontaminated SiO 2 film. However, the major obstacle of MoSi 2 coating is the structural disintegration during oxidation at temperatures between about 400 and 600 C, which is known as the ‘‘pest oxidation’’. 3–10) This phenomenon results from the build-up of MoO 3 in the oxide product according to eq. (2), which inhibits the formation of a protective silica scale since MoO 3 is less volatile at the low temperatures, and SiO 2 does not grow quickly enough to exclude MoO 3 scale formation. MoSi 2 (s) þ (7/2)O 2 (g) ! 2SiO 2 (s) þ MoO 3 (s) ð2Þ Some workers reported that the pest oxidation occurred through transport of oxygen into the interior of MoSi 2 along pre-existing cracks, grain boundaries, and/or pores, where it reacted to form MoO 3 and SiO 2 . The amount of volume expansion incurred during the pest oxidation of MoSi 2 (assuming complete conversion into SiO 2 and MoO 3 ) was calculated to be about 250%. 7) This substantial volume expansion produces local wedging stresses at defects such as cracks, pores, and grain boundaries and then results in the catastrophic disintegration of MoSi 2 . MoSi 2 coatings with the thickness of several tens hundreds micrometer were produced by reactive diffusion processes such as solution growth into molten Si-In alloy, 11) pack siliconizing, 12) or chemical vapor deposition (CVD) of Si 13) at high temperatures. Many cracks are formed in MoSi 2 coating due to the mismatch of coefficient of thermal expansion (CTE) between MoSi 2 coating (8:5 10 6 K 1 ) and Mo substrate (5:8 10 6 K 1 ) during cool-down from the deposition temperature or from the service temperature to the room temperature. This implies that the MoSi 2 coating is susceptible to occur pest disintegration. A number of methods have been proposed for protecting MoSi 2 coating against pest oxidation, but no satisfactory results have been obtained. Betztiss et al. 4) have shown that preoxidation at 1000 C to form a Mo-oxide free SiO 2 scale delayed the onset of accelerated oxidation. After some incubation period, spalling of the scale was observed with the subsequent growth of the dual MoO 3 and amorphous SiO 2 scale in the spalled regions. Mueller et al. 14) and Cockeram et al. 15,16) reported that germanium additions improved the pest oxidation resistance of MoSi 2 coating, but a fairly rapid rate of low-temperature oxidation was observed and that the MoSi 2 coatings formed by a NaF-activated pack siliconizing process or by the application of superficial alkali-salt layers did not pest or undergo the accelerated oxidation for 2000 2500 h in air at 500 C since the sodium-rich by-product layer produced by the NaF-activated pack passivated the MoSi 2 coating by forming a fast growing Na-silicate scale. Fitzer 17) reported that the pest reaction was also avoided by applying a solid MoO 3 layer to MoSi 2 at 300 to 700 C in an oxidizing atmosphere. However, all of these coatings are either quickly degraded by high-temperature oxidation, or involve a com- Materials Transactions, Vol. 45, No. 7 (2004) pp. 2435 to 2442 #2004 The Japan Institute of Metals EXPRESS REGULAR ARTICLE
Low-Temperature Cyclic Oxidation Behavior of MoSi2/SiC Nanocomposite Coating Formed on Mo Substrate
Jun 16, 2023
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