Due to molybdenum’s Body-Centered Cubic (BCC) crystalline structure, its ductile–brittle transition temperature is sensitive to shaping, purity and microstructure. Dense molybdenum parts are usually shaped by the powder metallurgy process. The aim of this work concerns the spark plasma sintering of high-purity powders prepared by inductively coupled plasma. The influence of carbon diffusion and its interaction with oxygen on the density (i.e., the densification stage) and on the microstructure (i.e., the grain growth stage) during spark plasma sintering was investigated. The formation of carbide is usually expected for a sintering temperature above 1500 °C leading to grain growth (e.g., more than 10 times larger than the initial powder grain size after sintering at 1900 °C for 10 min). The brittleness was also affected by the segregation of molybdenum carbides at the grain boundaries (i.e., intergranular brittle fracture). Consequently, to reduce the sintering temperature to below 1500 °C, mechanically activated powders were used. From these milled powders, a dense molybdenum disc (60 mm in diameter and 10 mm in thickness) sintered at 1450 °C under a pressure of 70 MPa for 30 min was obtained. It is composed of a fine microstructure without carbide and oxide, its ductility is close to 13% with a maximum resistance of 550 MPa.
Spark plasma sintering technique is used for the fabrication of dense materials with a fine-grained microstructure. In this process, a powder is placed into a graphite mold and a uniaxial pressure is applied by two graphite punches. A graphite foil is inserted between the punches and the powder and between the mold and the powder to ensure good electrical, physical and thermal contact. One of the major drawbacks during sintering of metal powders is the carburization of the powder in contact with the graphite foils. In this study, a PVD coating of titanium was applied on the graphite foils in contact with the metal powder (pure iron). The results are promising, as the investigations show that the application of a Ti PVD film of 1.5 and 1.1 µm thickness is effective to completely avoid the carburization of iron powder. Carbon diffuses inside the PVD film during sintering. In parallel, iron diffusion was revealed inside the Ti coating of 1.5 µm thickness. On the other hand, a Ti PVD film of 0.5 µm thickness provides a protection against carbon diffusion just on the sides in contact with the mold, proving that the coating thickness represents an important parameter to consider.
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