title: Surface coating from phosphonate ionic liquid electrolyte for the enhancement of the tribological performance of magnesium alloy.
abstract: A chronoamperometric method has been applied for the growth of a surface coating on AZ31B magnesium alloy, using the imidazolium alkylphosphonate room-temperature ionic liquid 1-ethyl-3-methylimidazolium ethylphosphonate ([EMIM][EtPO3H]) as electrolyte. A surface coating layer is obtained after 4 h under a constant voltage bias of -0.8 V with respect to the standard electrode. The coating nucleation and growth process correlates well with a 3D progressive mechanism. X-ray photoelectron spectrometry (XPS) analysis of [EMIM][EtPO3H] shows new P 2p and O 1s peaks after its use as electrolyte, as a consequence of reaction between the phosphonate anion and the magnesium substrate. Angle-resolved XPS (ARXPS) analysis of [EMIM][EtPO3H] did not show any change in the composition of the surface before and after chronoamperometry, since the sampling depth (1.5 nm at the highest emission angle) is larger than the cation and anion sizes (ca. 7 and 5 Å, respectively). Characterization of the coating was made by scanning electron microscopy (SEM), focussed ion beam SEM, energy dispersive X-ray spectroscopy, XPS, and ARXPS. FIB-SEM shows that the coating presents a mean thickness of 374 (±36) nm and contains magnesium and aluminum phosphates. Linear reciprocating tribological tests under variable load show that the presence of the coating can reduce friction coefficients of the coated AZ31B against steel up to 32% and wear rates up to 90%, with respect to the uncoated alloy.
authors.affiliation: †Departamento de Ingeniería de Materiales y Fabricación, Universidad Politécnica de Cartagena, 30202 Cartagena, Spain.
authors.forename: Ana Eva
authors.initials: AE
authors.lastname: Jiménez
authors.affiliation: ‡Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, Switzerland, Vladimir-Prelog-Weg 5, CH-8093 Zürich, Switzerland.|§Dipartimento di Scienze Chimiche e Geologiche, Cittadella Universitaria di Monserrato, Università degli Studi di Cagliari, I-09100 Cagliari, Italy.
authors.forename: Antonella
authors.initials: A
authors.lastname: Rossi
authors.affiliation: §Dipartimento di Scienze Chimiche e Geologiche, Cittadella Universitaria di Monserrato, Università degli Studi di Cagliari, I-09100 Cagliari, Italy.
authors.forename: Marzia
authors.initials: M
authors.lastname: Fantauzzi
authors.affiliation: †Departamento de Ingeniería de Materiales y Fabricación, Universidad Politécnica de Cartagena, 30202 Cartagena, Spain.
authors.forename: Tulia
authors.initials: T
authors.lastname: Espinosa
authors.affiliation: †Departamento de Ingeniería de Materiales y Fabricación, Universidad Politécnica de Cartagena, 30202 Cartagena, Spain.
authors.forename: Joaquin
authors.initials: J
authors.lastname: Arias-Pardilla
authors.affiliation: †Departamento de Ingeniería de Materiales y Fabricación, Universidad Politécnica de Cartagena, 30202 Cartagena, Spain.
authors.forename: Ginés
authors.initials: G
authors.lastname: Martínez-Nicolás
authors.affiliation: †Departamento de Ingeniería de Materiales y Fabricación, Universidad Politécnica de Cartagena, 30202 Cartagena, Spain.
authors.forename: María-Dolores
authors.initials: MD
authors.lastname: Bermúdez
keywords: ARXPS; FIB-SEM; chronoamperometry; coatings; phosphonate ionic liquids; reciprocating friction and wear; surface analysis