Preparation of platy Co/Al hydrotalcites using aluminum hydroxide and investigation of their tribological properties in base oil

In this study, the synthesis of Co/Al hydrotalcite (layered double hydroxides: LDHs) using insoluble Al[(OH).sub.3] by a hydrothermal method is described. The syntheses were conducted under various conditions. The as-prepared Co/Al-C[O.sub.3.sup.2-]-LDH was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The tribological properties of the products were evaluated in base oil using four-ball and Plint friction testers. The LDH thus synthesized displays perfect hexagonal platelike morphology having a disk diameter in the range of 0.2-1.5 µm with a thickness of 40 nm. The addition of the product Co/Al-C[O.sub.3.sup.2-]-LDH, as an additive to the base oil, significantly reduces the friction coefficient (22.3%) and wear (26.1%) compared to base oil alone. The morphology and chemistry of the worn surface were characterized by SEM, atomic force microscopy (AFM), and X-ray absorption near-edge structure (XANES) spectroscopy. The results show that the structural distortion of LDH induced by mechanochemical processes during friction consumes the friction force between the rubbed surfaces, and as a result, the friction and the wear is reduced. Thus, our results show that the LDH sheets absorbed on the worn surface can prevent direct contact between the friction pairs (surfaces) and decreases the roughness of the worn surface. Key words: Co/Al hydrotalcites, antifriction, tribochemistry, X-ray absorption near-edge structure (XANES). La presente etude decrit la synthese d'hydrotalcite de Co/Al (hydroxydes doubles lamellaires ou HDL) a l'aide d'Al[(OH).sub.3] insoluble par methode hydrothermale. Les syntheses ont ete realisees dans diverses conditions. Les Co/Al-C[O.sub.3.sup.2-]-HDL ainsi prepares ont ete caracterises par diffractometrie de rayons X (DRX) et microscopie electronique a balayage (MEB). Les proprietes tribologiques des produits ont ete etudiees dans de l'huile de base a l'aide de bancs d'essai de frottement a quatre billes et de tribometres Plint. Les HDL ainsi synthetises presentent une morphologie hexagonale parfaite en plaque dont le diametre de disque est compris entre 0,2 et 1,5 µm et l'epaisseur egale a 40 nm. L'ajout du produit Co/Al-C[O.sub.3.sup.2-]-HDL, comme additif a l'huile de base, reduit considerablement le coefficient de frottement (22,3%) et l'usure (26,1%), par rapport a l'huile de base seule. La morphologie et la chimie de la surface usee ont ete caracterisees par MEB, microscopie a force atomique (MFA) et spectroscopie de structure pres du front d'absorption de rayons X. Les resultats montrent que la distorsion structurale des HDL provoquee par les processus mecanochimiques ayant lieu lors des frottements reduit la force de frottement presente entre les surfaces enduites, ce qui entraine une diminution du frottement et de l'usure. Nos resultats demontrent ainsi que les feuilles d'HDL absorbees sur la surface usee peuvent empecher les paires de frottement (surface) d'etre en contact et reduisent la rugosite de la surface usee. [Traduit par la Redaction] Mots-cles: hydrotalcites de Co/Al, antifriction, tribochimie, spectroscopie de structure pres du front d'absorption de rayons X.
Introduction Layered double hydroxides (LDHs) are a class of layered anionic clays having the general formula of [[M.sub.(1-x).sup.2+][M.sub.x.sup.3+][(OH).sub.2]][([A.sup.n-]).sub.x/n] x z[H.sub.2]O, where [M.sup.2+] (Mg, Mn, Fe, Co, Ni, Cu, Zn, etc.) [...]