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ToF-SIMS of Polyphosphate Glasses
No full textPhosphate glasses have been investigated in detail, due to their interesting optical properties: low dispersion, high refractive index and high transparency for ultraviolet light. More recently, the low processing temperature of zinc and iron phosphates as well as their chemical durability opened the possibility for new applications, such as immobilization of nuclear wastes and biomechanical properties. Polyphosphate glasses have also been investigated for their anti-wear properties, since almost all common engine oil additives contain phosphorus and are able, under sliding conditions, to react on steel surfaces to form a protective tribofilm mainly constituted of glassy polyphosphates The characterization of the surface chemistry of such glasses turns out to be a key step for understanding their mechanism of action.
In this work zinc and iron phosphate glasses of different chain length were synthesized and characterized by means of ToF-SIMS. ToF-SIMS was able to successfully differentiate between the samples of different composition
The corrosion resistance of electroless deposited nano-crystalline Ni-P alloys
No full textCoatings of Ni–P alloys are increasingly used for their shiny appearance, the high degree of hardness and very good corrosion resistance. Electroless coatings are preferred in industry compared to the electroplated ones because they adhere well to different materials and form a homogeneous coating even on complicated geometries. Electroless Ni–P alloys produced as a coating on technical iron in a commercial autocatalytic hypophosphite-type plating bath were studied, with regard to their corrosion behaviour upon immersion in near neutral sulphate and chloride electrolytes. The anodic polarization curves of the alloys showed a current plateau at potentials E < +0.2 V SCE confirming their high corrosion resistance. The dynamic cathodic polarization curves in deaerated solutions showed a Tafel behaviour with a slope of ca. −0.4 V, indicating inhibition of oxygen reduction on “as received” and on mechanically polished surfaces. Corrosion rates of ca. 0.5–0.7 μA/cm2 were found during prolonged immersion in near neutral solutions open to air. Potentiostatic polarization at selected potentials in the range of the current plateau (−0.1 V SCE, +0.1 V SCE) showed a steady decrease of the current density following a power law with exponent −0.5, thus a diffusion controlled process. These results show that the suppression of the anodic dissolution and the low corrosion rates of Ni–P alloys cannot be associated with the oxide-film passivity. Despite this fact, black spots identified as localized corrosion appeared on mechanically polished and “as plated” electroless Ni–P deposits after prolonged potentiostatic polarization at potentials in the range of dissolution suppression
Electroless deposited Ni-P alloys: corrosion resistance mechanism
No full textElectroless Ni–P alloys are produced as coatings on a broad variety of substrates. They exhibit a corrosion resistance that is superior to pure nickel but do not form a NiO oxide film (passive film) as pure nickel does. Despite the fact that many mechanisms have been proposed to explain this superior corrosion behaviour, no consensus has yet been reached. In this work electrochemical and XPS surface analytical methods have been combined in order to gain a deeper insight into the mechanisms underlying the corrosion resistance of electroless deposited Ni–P alloys with phosphorus content between 18 and 22 at.%. The anodic polarization curves in acidic and neutral solutions confirm a broad current plateau followed by a region with increasing current density. During potentiostatic polarization in the plateau region the current decays according to a power law with exponent ca. −0.5 indicating diffusion-limited dissolution of nickel. XPS/XAES measurements performed after potentiostatic polarization show that phosphorus is present in three different chemical environments. Based on the Auger parameter concept and on the chemical state plot, the three phosphorus states were assigned to phosphorus in the bulk alloy, phosphates and an intermediate phosphorus compound attributed to elemental phosphorus. Angle-resolved XPS analysis has shown that the elemental phosphorus is enriched at the interface between the alloy and the outermost surface in contact with the corrosive solution. These results suggest the following conclusions: the high corrosion resistance of electroless deposited Ni–P alloys can be explained by a strong enrichment of elemental phosphorus at the interface which limits the dissolution of nickel via a diffusion mechanism. A complementary explanation––not yet advanced––for the high corrosion resistance may lie in the electronic state of nickel in the Ni–P alloys
ToF-SIMS of polyphosphate glasses
No full textThe increasing interest in phosphate glasses is due to the fact that they are considered to be bioactive functional materials in applications such as hard and soft tissue engineering and also that they possess interesting optical properties. These include
low dispersion, high refractive index and high transparency for ultraviolet light. They have also been found to be responsible for the good anti-wear properties of phosphorus-based engine-oil additives. More recently, the low processing temperature of
zinc and iron phosphates, as well as their chemical durability, has opened the possibility for new applications, such as the immobilization of nuclear wastes. Understanding the surface chemistry of such glasses turns out to be a key step for inferring
their underlying mechanisms of action. In this work, zinc phosphate and iron phosphate glasses of different chain lengths were synthesized and characterized by means of time-of-flight secondary ion mass spectroscopy (ToF-SIMS). ToF-SIMS was
able to successfully differentiate between the samples of different composition. A characteristic pattern of phosphate fragments was observed in the negative-mode ToF-SIMS spectra. The most intense peaks could be assigned to the phosphate
fragments PO, PO2, PO3 and PO4. Toward higher masses, fragments containing up to four phosphorus atoms with a periodicity of PO2 were observed: FePO3, ZnPO3, FePO4, ZnPO4, FeP2O6, ZnP2O6, FeP2O7, ZnP2O7; FeP3O8, ZnP3O8,FeP3O9and ZnP3O9. This pattern characterizes the spectra of all the glasses under investigation. For the first time, a method is proposed that allows the discrimination between polyphosphates of different chain lengths, by comparing the intensities of the relevant ToF-SIMS peaks
Effect of Chain-Length and Countersurface on the Tribochemistry of Bulk Zinc Polyphosphate Glasses
No full textZinc polyphosphate glasses are considered to be chiefly responsible for the anti-wear efficiency of ZnDTP tribofilms. In this work, the tribochemical properties of amorphous bulk zinc polyphosphates of different chain lengths (ranging from zinc metaphosphate to zinc pyrophosphate) have been investigated. Tribological tests on bulk polyphosphate discs have been carried out using steel and quartz balls as counter-surfaces in a poly-α-olefin (PAO) bath at room temperature. The composition in the wear track and on the contact region of the balls has been monitored by small-area and imaging X-ray photoelectron spectroscopy (i-XPS). The XPS analysis revealed that the composition of short-chain-length polyphosphates remained unchanged following tribological stress. Long-chain-length polyphosphates are depolymerized in the wear track as a consequence of a tribochemical reaction. By comparing the results obtained using quartz and steel balls, it could be observed that while the reaction of iron oxide with the polyphosphates certainly plays a role in the depolymerization of the samples under sliding conditions, pressure and shear stress alone and also in the presence of water or oil-oxidized species are able to depolymerize the glass when an inert material is used as counterpart; the composition of the wear track, in this case, is dependent on the applied load. All samples were able to form an adhesive, glassy transfer film on both steel and quartz balls, but the short-chain-length polyphosphates showed a lower friction coefficient and wear coefficient. The results suggest a third-body mechanism with the polyphosphates acting as a solid lubricant. Differences in tribological behavior of the different-chain-length polyphosphates are attributable to their mechanical and rheological properties
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Tribochemistry of bulk zinc metaphosphate glasses
No full textZinc polyphosphate glasses are the principal component of the antiwear tribofilms formed on steel surfaces in the presence of additives, such as zinc dialkyldithiophosphates. In this work amorphous, zinc metaphosphate glasses have been synthesized and characterized by means of X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), elemental analysis and X-ray photoelectron spectroscopy (XPS). Tribological tests were performed by rubbing steel balls against the zinc metaposphate discs in a poly-α-olefin (PAO) bath at room temperature. XPS was used in order to characterize the tribostressed areas on both metaphosphate discs and steel balls. A transfer film, constituted of iron and zinc polyphosphates, was formed on the contact area of the balls. This transfer film was found to reduce friction and prevent ball wear. A reduction in the relative intensities of XPS signals related to bridging-oxyg..
Chain-Length-Identification Strategy in Zinc Polyphosphate Glasses by Means of XPS and ToF-SIMS
No full textThe surface chemistry of amorphous zinc polyphosphates of different compositions (ranging from zinc metaphosphate to zinc orthophosphate) has been investigated by means of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectroscopy (ToF-SIMS). The identification of the chain length of zinc polyphosphates by XPS could be achieved on the basis of the integrated intensity ratio of the bridging (P-O-P) and non-bridging (P=O and P-O-M) oxygen peaks used for fitting the oxygen 1s signal, the shift of the P 2p3/2 signal towards lower binding energies, and of the modified Auger parameter towards higher values as the zinc content increases. The discrimination of the polyphosphate chain lengths could be also achieved by ToF-SIMS, by comparing the intensities of selected characteristic phosphate fragments. Both techniques appear to be suitable for the investigation of polyphosphate glasses in applications such as tribology, where there is a need to identify the chain length present in the outermost monolayer of the film.
Fourier-transform infrared (FT-IR) spectroscopy was used in order to characterize the bulk compounds. The FT-IR studies showed that long-chain structures linked through P-O-P bonds predominate in the metaphosphate composition, while upon increasing the Zn content, the chains become shorter, ultimately being replaced by PO4 monomers in the orthophosphate composition
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