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Influence of Slip Viscosity on the Mechanical Properties of High Purity Alumina made by Gelcasting
(1997)
The isoelectric point is a material property for the characterization of acid-base behavior of the powder surface. The IEP of two alumina powders (AKP-30, Grade A) with different particle size characteristics in aqueous suspension has been determined by colloid vibration current (CVI) and particle charge detection (PCD) methods in moderately concentrated suspensions in order to qualify these methods for a future standardization. The solid loading of the suspensions were 1 and 5 vol%. The comparison of the IEP determined by PCD and CVI was in good agreement. However, the solid loading dependence of IEP and a large IEP hysteresis (at 1 vol%) were observed, which do not allow a proper determination of the IEP. The recording of the potential curve and therfore the determination of the IEP with CVI was difficult for the suspension of Grade A powder due to formation of agglomerates at pH values near the pHIEP.
Ceramic particulate composites in the system SiC-TiC-TiB2 sliding against SiC and Al2O3 under water
(1999)
The tribological behaviour of SiC, SiC-TiC and SiC-TiC-TiB2 was determined in oscillating sliding against SiC and -Al2O3 in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB2 differ significantly from the systems with the single phase SiC: The wear is reduced and the friction is increased. The wear reduction up to a factor of 10 is mainly due to the formation of an oxide film containing titanium oxides which is soft, stable in water and well adhering to the bulk material. This oxide film is transferred to the alumina ball but not to the silicon carbide ball.
Zeta potential as measured with the electrokinetic sonic amplitude (ESA) method has been compared to streaming potential as measured with the particle charge detection (PCD) method. The measurements have been carried out on identical suspensions of a-Al2O3, SiC and Si3N4 with a solids volume fraction of 1% and 10%. Zeta potential and streaming potential are related by a linear function. The correlation coefficients for this relationship are better than 0.99 for the suspensions with 1% solids loading. For the suspensions with 10% solids loading, correlation coefficients are slightly lower due to the influence of increasing viscosity. Regardless of lower correlation at the higher solids loading, the isoelectric pH is constant within the limits of accuracy of the pH measurements.
With respect to their excellent material properties ceramics are of interest as candidate materials for tribological applications. The friction and wear behaviour of ceramics is often superior to that of metallic materials. Furthermore, an essential improvement of tribological performance will be possible by tailoring of ceramics in the process of powder preparation and sintering technique. Silicon Carbide (SiC) shows interesting friction and wear behaviour at room temperature, even under unlubricated running conditions but is very sensitive against effects of environmental humidity and shows high friction and high wear in the absence of water vapour. The tribological performance of SiC can be improved considerably by adding substantial amounts of TiC and TiB2 to the microstructure thus creating ceramic particulate composite materials.
Various composites in the quasi ternary system SiC-TiC-TiB2 were prepared by a ceramic manufacturing process including sintering temperatures up to 2180 °C. Comparative tribo tests were performed under unlubricated oscillating sliding motion in dry, normal, and moist air and under water lubrication with SiC balls and Al2O3 balls as counter bodies. The friction is affected by the relative humidity (RH) but barely at all by the composition of the composites. The wear resistance of the composites was found to be improved considerably by addition of TiB2 in the range from 20 to 60 %. The highest wear resistance of the system wear was found when Al2O3 was used as counter body material.
The friction and wear behaviour of a new developed, pressureless sintered TiB2 ceramic was studied in comparison to SiC against SiC and Al2O3 balls under unlubricated conditions at room temperature. Special attention was paid to the influence of relative humidity on friction and wear results. For both ceramics the coefficient of friction against both ball materials decreases with increasing humidity and is higher in tests against alumina than against SiC. The wear rate is affected significantly by humidity and decreases by one order of magnitude for Al2O3/SiC system and by 2 orders of magnitude for SiC/SiC system. For TiB2 wear rates are nearly not affected by humidity against both ball materials and are much smaller than for SiC.
Untersucht wurden Verschleißpartikel als elektronen-transparente Proben auf dünnen Trägerfolien mit einem hochauflösendem Rasterelektronenmikroskop und energiedispersiver Röntgenspektrometrie im Transmissionsmodus. Die hohe laterale Auflösung dieser modernen Methoden ermöglicht eine morphologische Charakterisierung der Verschleißpartikel bis in den nm-Bereich. Die hochauflösende Elementanalyse erlaubt eine Interpretation der Zusammensetzung und damit Rückschlüsse auf die Verschleißpartikelbildung.
Die tribologischen Untersuchungen sind an einem Modell-Verschleißtribometer nach DIN 51834-2 geschmiert, aber ohne Ölkreislauf, durchgeführt worden. Die Präparation der Verschleißpartikel über eine Entnahme und Übertragung einer geringen Schmierstoffmenge mit einer Mikropipette auf eine TEM-Trägerfolie ist mit einem geringen Aufwand möglich. Der präparative Aufwand ist relativ gering. Das benötigte Schmierstoffvolumen beträgt wenige Mikroliter (0,001ml).
Die Untersuchung zeigt die geringe geometrische Größe der Verschleißpartikel und bestätigt indirekt die These der Reaktionsschichtbildung mit einer geringen Schichtdicke, die ständig verschleißt und sich stetig neu bildet. Die beschriebenen Verschleißprozesse sind in der Modell-Verschleiß-Prüfung mit sehr hohen Pressungen am Versuchsbeginn nachweisbar.
A new method for sampling wear particles directly from the lubricant reservoir has been developed and applied successfully for analyzing wear particles by high-resolution scanning electron microscopy in transmission mode having coupled energy-dispersive X-ray spectroscopy. The lubricated tribological testing was carried out with fully formulated as well as with non-formulated synthetic base oil. It was possible to analyze individual particles with dimensions as small as about 5–30 nm which are likely the 'primary' wear particles. A majority of the particles, however, are agglomerated and, thus, lead to the formation of larger agglomerates of up to a few micrometers. Chemical analysis led to the conclusion that most of the observed particles generated in formulated oil, especially the larger ones, are composed of the additives of the lubricant oil. In non-formulated base oil, the primary particles are of similar dimensions but contain only iron, chromium and oxygen, but most likely stem from the mating materials. This finding points to the fact that the main wear mechanism under lubricated conditions with fully formulated oil is more like a continuous shearing process rather than a catastrophic failure with the generation of larger primary particles. When the oil is non-formulated, however, several wear mechanisms act simultaneously and the wear rate is increased significantly. Generated larger primary particles are milled down to the nanoscale. When the oil is fully formulated, wear mainly takes places at the additive layer or tribofilm; thus, the steel surface is protected.
Tribological studies on neat polyetherketone (PEK) and glass fiber reinforced PEK composite were carried out at room and elevated temperature (120 °C). The objective of this study was to characterize for friction and wear properties under dry reciprocating sliding condition at different experimental conditions. The polymer specimens were made to oscillate against steel cylinder as a counterpart. This kind of contact condition are frequently found in bushes, sliding bearing, electronic parts, seals, etc. The friction and wear behavior of neat PEK and composite was quite different at room temperature and elevated temperature. It was observed that glass fiber reinforcement is beneficial in controlling the wear of PEK matrix at room temperature than at elevated temperature. The test results are discussed by considering the surface properties i.e. material removal and film transfer formation. Scanning electron micrographs and optical micrographs of the worn polymer and steel cylinder was used to study the wear mechanisms.