Improvement of the tribological properties of Al2O3 reinforced duplex stainless steel MMC coating by the addition of TiS2 powder

Abstract

Abstract A self-lubricating, wear-resistant metal matrix composite (MMC) coating of significant thickness (approximately 1.2 mm) was produced by melting, using the plasma transferred arc technique, the surface of duplex stainless steel 2205 which was covered with a powder mixture of Al 2 O 3 , TiS 2 and Fe. During solidification, Al 2 O 3 forms and as the adjacent dissolved Al content in the melt decreases, the local Al and Ti contents reach a point when Al 2 TiO 5 becomes thermodynamically stable and forms on the existing Al 2 O 3 particles. As the melt cools further, the solubility of sulfur in molten steel decreases and the previously formed complex oxides spontaneously act as nucleation sites for sulfides (Ti 4 C 2 S 2 and TiS), whereas Al 2 TiO 5 is reduced to Al 2 O 3 . As the solidification proceeds, δ Fe begins to form by a eutectic reaction. During this stage, sulfides also form, which are also TiS and Ti 4 C 2 S 2 . After completion of the solidification, a partial solid-to-solid transformation δ Fe → γ Fe (ferrite to austenite) takes place. The balance between austenite and ferrite in the matrix was slightly disturbed (36 vol.% austenite and 64 vol.% ferrite) but within the acceptable limits. The friction coefficient and the wear rate of the coating against an Al 2 O 3 pin were lower compared to the corresponding values of the coating prepared without the addition of TiS 2 . The Al 2 O 3 reinforcement particles are surrounded by sulfides and thus they have better wettability by the molten stainless steel. As a result, they are less detachable compared to the Al 2 O 3 particles of the coating fabricated without the addition of TiS 2 . In addition, since there is a metallurgical bond between the Al 2 O 3 and the surrounding sulfide, it may be presumed that even when Al 2 O 3 particles eventually detach, they are always surrounded by sulfides which not only prevent the oxides' action as a third body abrasive but also act as lubricants.