Formation Mechanism of Sintering Crack in Mo–W–ZrO₂ Sensor for Molten Steel Temperature Caused by Water Vapor

抄録

Mo–W–ZrO₂ cermet, which is usually sintered in H₂, has been used for molten steel temperature sensor. However, the crack caused by water vapor during its sintering limits the industrial application. Thus, the mechanism of crack formation has been investigated. Cracks are mainly caused by a significant volume expansion which is attributed to that the metal powders of the green body of cermet are oxidized by water vapor. The obvious expansion of approximately 17% in diameter occurs below 1200°C, reducing the bonding strength. Meanwhile, the support frame for the sintering restricts the expansion. Both of them are the main reasons for the crack formation. At higher temperature, the volume of above oxidized metal powders decreases caused by the reduction of H₂. The volume of the metal powders increases first and then decreases, which leads to a more difficult sintering densification. Moreover, the ceramic powders are sintered earlier and form a skeleton. As a result, the sintering densification is restrained and the pores form. As the water vapor is eliminated through the H₂ filling and pumping vacuum, the problems of cracks and pores are addressed, which is helpful to solve the main contradiction in industrial application of the Mo–W–ZrO₂ sensor.