Aqueous Processing of WC-Co Powders Aqueous processing of WC-Co powders

Abstract: The object of this work is toobtain a fundamental understanding of the principal issuesconcerning the handling of WC-Co powders, in an aqueousmedium.The oxidation and dissolution oftungsten carbide powder dispersed in water was investigatedusing X-ray Photoelectron Spectroscopy (XPS) and leachingstudies. The WO3surface layer on the oxidised tungsten carbidepowder dissolves at pH>3 with the tungsten concentrationincreasing linearly with time. Adding cobalt powder to thetungsten carbide suspension resulted in a significant reductionof the dissolution rate at pH<10. Electrokinetic studiesindicate that the reduced dissolution rate may be related tothe formation of surface complexes; experiments showed that Cospecies in solution adsorb on the oxidised tungsten carbidepowder. The experimental data are discussed in relation totheoretical modelling of the WO3solution chemistry and the Co2+adsorption at oxide/water interfaces.The DLVO surface forces ofoxidised tungsten and cobalt surfaces were investigated usingthe atomic force microscope (AFM) colloidal probe technique.The colloidal probe consisted of an oxidised tungstenmicro-sphere and the flat surface was either oxidised tungstenor cobalt. The surfaces are representative as model systems forindustrial WC and Co powders. The surface chemistry andcomposition of the surfaces were analysed with XPS, X-rayDiffraction (XRD),and electrokinetic measurements. The effects of ionicstrength and pH, in the systems, were studied. The measuredinteractions are well described by theoretical Hamakerconstants, calculated from optical data for two WO3surfaces and one WO3and one CoOOH surface interacting over water,respectively.The effects on the surfaceforces of adsorption cobalt ions to tungsten oxide surfaces,and adsorption of polyethyleneimine (PEI) in both the symmetricand asymmetric systems, were investigated. Low concentrationsof cobalt ions to a WO3system resulted in an additional short-range forceand lower absolute value of the surface potential. It was shownthat PEI adsorbs to the WO3surfaces and induces an electrosteric repulsion,in both systems. The possible complexation of cobalt ions withPEI does not significantly influence the thickness of theadsorbed layer.Key words:Hard metal, cemented carbide, WC-Co, tungstencarbide, cobalt, oxidation, dissolution, surface complexation,XPS, AFM, colloidal probe, force curve, Hamaker constant,Cauchy, WO3, CoOOH, Co3O4, CoO, ESCA, zeta-potential, surface potential,polyethyleneimine, PEI, suspension, disperse, van der Waals,steric