Research On Microstructure And Properties Of Al₂O₃ Reinforced Mo-base Composites

Molybdenum is a refractory metal with high boiling point, melting point, high hardness, high temperature strength, good thermal shock resistance and excellent corrosion resistance. Molybdenum also be easy mechanical processing and pressure processing. All of the fine properties above make molybdenum and its alloys to be used widely in aerospace, electronics, chemicals and other key areas. However, molybdenum is easy to be oxidized at high temperature, it has low recrystallization temperature, low temperature brittleness, easy to brittle fracture after recrystallization. The plastic-brittle transition temperature of molybdenum and its alloys is high. All of the shortcomings above limit the processing of molybdenum and the range of its application.This issue focuses on the development of molybdenum alloy as a high temperature and wear resistant material, mainly by adding the second phase Al₂O₃ ceramics particles to improve the friction and wear performances. Ultrafine powders of Al₂O₃/Mo mixture prepared successfully by sol-gel, and the Al₂O₃/Mo composite materials were prepared by powder metallurgy processing technology. At the same time, its microstructure and wear properties are investigated. The facts as following are important guides for the application and development of molybdenum alloys.1. In the sol-gel, the better quality of the powders were abtained under the conditions of pH<2 and about 1.5 of quality ratio of citric acid to ammonium molybdate. Colloidal organic matter eventually completely decompounded under the temperature of 550℃, and Al₂O₃ remained in mixed powder. Mo particles is about hundreds of nanometers, and Al₂O₃ particles several tens of nanometers or so. Al₂O₃ particles play the role of refining the basic particles in all stages of the process. The refining effect is enhanced with the increase of Al₂O₃ volume.2. The sintered tissues is composed of Mo, Al₂O₃, Al₃Mo and Al₅Mo. Adding Al₂O₃ is helpful of grain refinement of the molybdenum. With increasing of Al₂O₃, the micro-hardness of Mo-base increased gradually and the sintering shrinkage and density of composite first increased and then decreased. There was an intersection of the measured density and theoretical density and the extreme appeared at the range of 7% to 9% of the Al₂O₃.3. When the Al₂O₃ content is 3%, the specific wear rate of composites increased gradually with the particle size of abrasive decreasing. However, at higher Al₂O₃ content (5%, 10%, 15%), the specific wear rate increased firstly and then decreased with abrasive grain diameter increasing. The wear volume increased with the increasing of load and travel. The abrasion mechanism is mainly the failure mode of grooves and furrows caused by rolling and cutting effects of abrasive. With the load increasing, the depth and width of furrow was increased. With increasing of volume fraction of Al₂O₃, the quantity and depth of groove and furrow produced in wear process have a certain degree of reduction.4. Under argon atmosphere at 400℃, the wear of the composite material dry sliding to Cr26 steel decreased with the increasing of Al₂O₃, the friction coefficient increasing. Adhesive and scaly wear morphology appeared.