The Microstructure And Properties Study Of Ni-Al-graphite Self-lubricating Material

Self-lubricating composite which possesses superior antifriction and wear resistance has been one of the central issues for advanced material research. Ni-Al-graphite alloy was produced by vacuum induction melting, and the effect of Al content on microstructure and properties of the composite were studied. The results show: the addition of Al component into the Ni-graphite alloy indicates that Al element is not only an element that strengthens the alloy matrix, but also nodularizer that promotes the transition from flaky graphite to spheroidal graphite. It reveals that the carbon atoms mainly exist in graphitic form in Ni-Al-C ternary alloy. The alloy with 4.25 mass percent Al content possesses high content of graphite with the main morphology of eutectic graphite and the matrix of the alloy is γ-Ni(Al) solution single. With the increment of Al content, under less and primary graphite there has a tendency of spherical transformation. γ-Ni(Al) solution and a small quantity of (γ+γ′)E binary eutectic make up the matrix of the alloy with 6.85 mass percent Al content. As the Al content is in the range of 8.13-9.94 mass percent, γ-Ni(Al) solution disappears, the solidifying structure of the alloy is composed of pro-eutectic γ′-Ni3Al(,γ+γ′)E and graphite. The matrix is γ′-Ni3Al and Ni3AlC0.5 phase with high hardness is observed on reticular intergranular in the alloy of 12.74 mass percent Al content. The effects of Al content on the physics, mechanical and tribological properties were investigated. The results show that the small thermal expansion coefficient of Ni-Al-graphite alloy makes it qualify for engine and piston ring material, and the impact toughness reaches about 2~17-fold compare to Ni-Ag-C prepared by PM method. With the increasing of Al content, the density and impact toughness decrease while the hardness increases. The transformation of thermal expansion coefficient presents nearly "N" shape with the increment of Al content. And the tensile strength and bend strength increase at first and reduce then. The extrusion and swearing course of graphite in surface & inferior surface is the formation dynamic force of dry-tribolayer. During the course of wear, the superficial friction film of Ni-Al-graphite composite follows the law of formation, service, delamination and regeneration. The dry friction coefficient and wear rate reduce at first and increase then with the increasing of Al content. Under the oil lubricating condition, the solid-fluid (graphite-oil) lubricant film is the basic reason why tribological performance of the composite can remarkably improved. With the increment of Al content, the wear rate reduces while the friction coefficient rises at first and descends then. The addition of RE-Mg nodularizer plays an effective spheroidized role to the graphite in the alloy, remarkably improves the impact toughness and dry friction properties of the composite. The spheroidal mechanism of graphite in Ni-Al-graphite alloy lies in: Al reacts with O in the liquid metal, which leads to the purification of liquid metal and the increasing of undercooling during solidification.