Rolf Wäsche

Experimental study on zeta potential and streaming potential of advanced ceramic powders

Abstract 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 α-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.

Sintering behaviour of nanocrystalline titanium nitride powders

Abstract The sintering behaviour of one microcrystalline and several nanocrystalline TiN powders was investigated by means of a gas pressure furnace with an integrated dilatometer. The investigated nanocrystalline powders strongly differ in the sintering activity. It can be concluded from the experiments, that small grain size, narrow grain size distribution and a low oxygen content are the most important prerequisites for high sintering activity of the nanocrystalline TiN powders. The extraordinary sintering activity of the best TiN powder makes it possible to produce a dense (

Ceramic particulate composites in the system SiC-TiC-TiB2 sliding against SiC and Al2O3 under water

Abstract The tribological behaviour of SiC, SiC–TiC and SiC–TiC–TiB 2 was determined in oscillating sliding against SiC and α-Al 2 O 3 in water at room temperature. The tribo-systems with the composite materials containing TiC and TiB 2 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.

Wear of multiphase SiC based ceramic composites containing free carbon

Abstract Ceramics are of increasing interest for application in many branches of technique. In order to improve the tribological performance of ceramic materials, composites on the basis of silicon carbide with different amounts of titanium carbide, boron carbide, titanium diboride and free carbon in the form of graphite were manufactured by a reaction sintering process. The friction and wear behaviour of these composites was investigated with oscillating sliding motion in unlubricated tests at room temperature on different levels of relative humidity. Additional tests were performed under water lubrication, revealing the superior behaviour of composites for special test conditions. A major influence of the free carbon on friction or wear behaviour was not observed. The tribological behaviour in air as well as under water is mainly dependant on the amount of titanium phases.

Tribological performance of SiC and TiB2 against SiC and Al2O3 at low sliding speeds

Abstract The friction and wear behaviour of a new developed, pressureless sintered TiB 2 ceramic was studied in comparison to SiC against SiC and Al 2 O 3 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 Al 2 O 3 /SiC system and by 2 orders of magnitude for SiC/SiC system. For TiB 2 wear rates are nearly not affected by humidity against both ball materials and are much smaller than for SiC.

Physical chemistry of the femtosecond and nanosecond laser–material interaction with SiC and a SiC–TiC–TiB2 composite ceramic compound

The interaction of nanosecond laser pulses in the ultraviolet wavelength range and femtosecond laser pulses in the near-infrared region with the semiconductor SiC and the composite compound SiC–TiC–TiB2 was investigated. Surface analytical techniques, such as XPS, depth profile (DP), and micro-Raman spectroscopy (μ-RS) were used to identify the chemical changes between untreated and laser-treated areas. Single-pulse irradiation led to material modifications in the condensed state in most instances. Multi-pulse results differed depending on the pulse duration. Crystal structure changes were observed as a consequence of laser-induced melting and resolidification. In air contact all components underwent oxidation reactions according to thermodynamic expectations. Exceptions were observed under exclusion of oxygen, SiC was reduced to elemental Si.

In situ formation of tribologically effective oxide interfaces in SiC‐based ceramics during dry oscillating sliding

The tribological behaviour of single‐phase SiC as well as both SiC–TiC and SiC–TiC–TiB2 composite materials sliding against aluminium oxide has been investigated at room temperature with regard to the formation of wear‐reducing interfaces. The experiments were carried out in dry air for reasons of excluding the strong influence of water vapour. The introduction of the titanium phases into the SiC microstructure reduces the system wear by more than a factor of ten. The coefficient of friction is only slightly reduced but stabilised with time. The relatively abrasive oxide interface in the Al2O3/SiC pairing is altered to a soft and malleable oxide interface in the Al2O3/SiC–TiC–TiB2 pairing. The wear reduction is mainly caused by a change of the wear mechanism so that the formed soft oxide wear debris is transferred to the counterbody, thus shifting the sliding plane further into the layer.

Influence of the dispersants in gelcasting of nanosized TiN

Abstract The gelcasting process for shaping of plasma synthesized nanosized TiN with regard to the dispersion behaviour and the resultant microstructures has been investigated. The dispersion behaviour of the powder and rheology of the slurries under the influence of three different dispersants have been studied as a function of pH. Solid content of the slurries was up to 22 vol%. The isoelectric points as measured by the streaming potential method were found to be at a pH of 3.7 and lower. Sintering of the dried and debindered green bodies was carried out in a N 2 atmosphere with the application of 50 bar gas pressure in the final sintering stage leading to densities of 95.5%. Resulting microstructures are strongly dependent on the dispersant used.

Influence of Polymer Filler on Tribological Properties of Thermoplastic Polyurethane Under Oscillating Sliding Conditions Against Cast Iron

The tribological behaviour of unfilled thermoplastic polyurethane (TPU) and a polymer sphere filled (TPUG) thermoplastic polyurethane have been studied under oscillating sliding condition against cast iron as a counterpart. In the case of unfilled TPU, the wear mechanisms are dominated by particle detachment and roll formation. In principle, TPUG also showed a similar wear mechanism as that of unfilled TPU; in addition, particle pull-out and delamination are also observed. Wear volume of TPUG was significantly higher than that for the unfilled TPU and this is attributed to the different material removal processes taking place in the material during sliding. The polymer spheres as a filler material deteriorated the wear resistance of TPU because of improper adhesion and bonding of filler in the TPU matrix and therefore it contributed to more wear. In case of TPU the friction behaviour was strongly dependant on the temperature and surface roughness of the counter body. The results showed that below the glass transition temperature higher friction values are observed with higher counter body surface roughness. However, above the glass transition temperature, higher friction values are observed with a smoother surface roughness of the counter body. In case of TPUG, the friction behaviour was not significantly dependent on surface roughness of the counter body.

Tribological studies on polyetherketone composite under reciprocating sliding condition against steel cylinder

Tribological studies on neat polyetherketone (PEK) and glass fiber reinforced PEK composite were carried out at room and elevated temperature (120 ℃). 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.