Andreas Kailer

Phase transformations of silicon caused by contact loading

Combining hardness indentation tests and micro-Raman spectroscopy it is shown that metallic Si-II is produced near the interface of a diamond indenter and silicon to a depth of about 0.5 μm, where the highest stresses (hydrostatic and deviatoric) exist. At fast unloading rates Si-II transforms to the amorphous state, whereas a mixture of the r8 high pressure polymorph Si-XII and the bc8 phase Si-III forms upon a slow load release. The region of Si-III+Si-XII is surrounded by the wurtzite structured Si-IV, where the stresses during the indentation had not been high enough to cause the transition to the metallic state. Thus, because of shear deformation a direct transformation to Si-IV takes place. Outside the phase-transformed regions the classical aspects of indentation-induced deformation by dislocation glide, twinning and crack formation are observed. Annealing of the high pressure phases leads to the formation of Si-IV at moderate temperatures and to the reversal to the original diamond structure (Si-I...

Rheological characterization of ionic liquids and ionic liquid crystals with promising tribological performance

Several rheological studies of ionic liquids and ionic liquid crystals were performed to describe the measured low coefficients of friction (μ ≈ 0.02) in reciprocating sliding tests (cylinder-on-disc and ring-on-disc). The main mechanisms which are assumed to lead to low friction values are the chemical composition of the anion and the orientation of the molecules under shear. The ionic liquid crystals show strong non-Newtonian viscosity behavior and viscoelastic properties in the liquid crystalline phase. In addition, the viscosity depends on the molecule orientation, which can be influenced by shear. At the transition from the liquid crystalline to the isotropic phase a strong decrease of viscosity is observed.

Ultralow Friction Induced by Tribochemical Reactions: A Novel Mechanism of Lubrication on Steel Surfaces

The tribological properties of two steel surfaces rubbing against each other are measured while they are in contact with 1,3-diketones of varying structure. Such systems show after a short running-in period ultralow friction properties with a coefficient of friction of as low as μ = 0.005. It is suggested that the extremely favorable friction properties are caused by a tribochemical reaction between the 1,3-diketones and the steel surfaces, leading to formation of a chelated iron-diketo complex. The influence of temperature and the molecular structure of the 1,3 diketo-lubricants onto the friction properties of the system is elucidated under both static and dynamic conditions. With progression of the tribochemical reaction, the sliding surfaces become very conformal and smooth, so that the pressure is greatly reduced and further wear is strongly reduced. All iron particles potentially generated by wear during the initial running-in period are completely dissolved through complex formation. It is proposed that the tribochemical polishing reaction causes a transition from boundary lubrication to fluid lubrication.

1,3-Diketone fluids and their complexes with iron.

Tribological experiments with 1,3-diketone fluids in contact with iron surfaces show ultralow friction, which was suggested to be connected to the formation of iron complexes. In order to support this assumption, we calculate infrared and optical spectra of various substituted 1,3-diketones and their iron complexes using gradient-corrected density functional theory (DFT). The description of the complexes requires the application of the DFT+U scheme for a correct prediction of the high spin state on the central iron atom. With this approach, we obtain excellent agreement between experiment and simulation in infrared and optical spectra, allowing for the determination of 1,3-diketone tautomeric forms. The match in the spectra of the complex strongly supports the assumption of iron complex formation by these lubricants.

Ultralow Friction of Steel Surfaces Using a 1,3-Diketone Lubricant in the Thin Film Lubrication Regime

Ultralow friction (coefficient of friction μ ≈ 0.005) is observed when two steel surfaces are brought into sliding contact in the presence of a particular 1,3-diketone lubricant (1-(4-ethyl phenyl) nonane-1,3-dione). We investigate the friction process of such a system both experimentally and theoretically and show that the superlubricity is caused by a novel, unique mechanism: The formation of iron-1,3-diketonato complexes during frictional contact leads to a self-limiting, tribochemical polishing process while at the same time a self-assembled monolayer of the diketone is formed on the employed steel surfaces. This polishing process reduces the contact pressure and at the same time leads to formation of a boundary lubricant layer. During sliding the system transits from the original boundary lubrication regime toward hydrodynamic lubrication. Conductivity measurements across the friction gap during sliding show that the lubricant layer present in the gap between the two shearing surfaces is a only few 10 nanometers thick, so that the molecules experience under typical sliding conditions shear rates of a few 10(6) s(-1). Simulations show that under such strong shear the molecules become strongly oriented in the friction gap and the effective viscosity in sliding direction is significantly reduced so that the system is in the thin film lubrication regime and superlubricity is observed. The results of the experiments suggest that such diketones are promising lubricants to achieve a decrease of energy loss and frictional damage in steel based mechanical devices.

Influence of pressure-assisted polymerization on the microstructure and strength of polymer-infiltrated ceramics

A method of manufacturing polymer-infiltrated ceramics (PICs) is to pre-infiltrate a porous ceramic with a liquid monomer and subsequently polymerize the organic component inside the ceramic structure. The volume reduction during polymerization leads to the formation of pores (defects), which has a detrimental effect on the mechanical properties of PICs. To avoid the generation of defects, a new polymerization method that uses pressure during polymerization was developed. To investigate the influences of pressure and heating rate on strength and microstructure, both parameters were varied. The influences of both parameters on the strength of PICs were studied using a biaxial test. The influence on the microstructure was investigated through microscopy. Fracture toughness and R-curve behavior of the manufactured PICs were determined with the SEVNB method. The process parameters have a strong influence on strength and microstructure of PICs. Defect-free PICs with improved strength could be manufactured using elevated pressure during polymerization. As expected, a distinct R-curve behavior and enhanced fracture toughness relative to composites manufactured using conventional methods was found. The developed manufacturing method leads to defectless PICs with increased mechanic behaviors.

Influence of Electrochemical Potentials on the Tribological Behavior of Silicon Carbide and Diamond-Coated Silicon Carbide

Due to their high corrosion stability in combination with advantageous tribological performance, sintered silicon carbide ceramics are widely used in industrial applications. Both the corrosion stability and the tribological behavior can be affected by electrochemical processes. Tribological investigations were carried out using an electrochemical three electrode setup. The influence of electrochemical potentials on the friction and wear behavior of different sintered SiC materials in 1 M NaCl-solution was investigated to analyze the complex interplay between mechanical, chemical, and electrochemical interactions during tribo-corrosion. The results revealed that friction and wear of SiC were decreased under cathodic electrochemical polarization depending on the material composition. In addition, tribological tests at different electrochemical potentials showed that the coefficient of friction can be switched immediately. The results indicated that the tribological behavior is strongly affected by the increased double-layer repulsion due to the electrochemical potentials, which supports the hydrodynamic lubrication.

Macroscopic Superlow Friction of Steel and Diamond-Like Carbon Lubricated with a Formanisotropic 1,3-Diketone

Energy dissipation due to friction and wear is reducing the energy efficiency and reliability of mechanical systems. Thus, great efforts are being made to minimize friction for technical applications. In our present work, we investigate the tribological behavior of stainless steel 100Cr6 with a-C:H and a-C:H:Si coating lubricated with a surface-active formanisotropic 1,3-diketone. The results show that superlow friction can be achieved on the macroscale using a steel 100Cr6 self pairing (COF ∼ 0.005) and with 100Cr6 in combination with a-C:H coating (COF ∼ 0.008). Furthermore, the replacement of steel with a-C:H coating leads to a considerable decrease of wear. The reduced COF arises from the chemical interaction of the lubricant with the surface and nascent iron ions. It was found that interfacial parameters correlate with tribological results. In addition, the alignment of the formanisotropic molecules in the tribological contact at thin-film lubrication leads to an anisotropic viscosity with a minimum shear resistance in sliding direction. Atomistic simulation of tribochemical interactions was conducted to derive a friction model based on the thin-film lubrication theory. This investigation indicates the potential to substantially reduce friction and wear using this fluid in real technical applications.

Potential Controlled Tribological Behavior of Water-Based Ionic Liquids

Increase of energy efficiency, reliability and durability of technical systems in combination with resource conservation using environmentally friendly water-based lubricants would be an overarching goal in all tribological applications. According to this aim the objective of this work is to investigate and identify new water-based lubricants containing ionic liquids (ILs) to reduce friction and wear. Therefore the tribological behavior of different water-based ionic liquid mixtures, compared with a standard water based cooling lubricant emulsion, was studied using a ball-on-disk test. A three electrode setup was used to analyze the influence of different electric potentials. The results show that friction and wear can be improved by using ionic liquid. In addition, the tribological behavior can be strongly influenced by electric potentials. As tribological mechanism the attraction of cations and the formation of a triboactive layer is assumed, due to charging of the surface.

On the feasibility of the Chevron Notch Beam method to measure fracture toughness of fine-grained zirconia ceramics

OBJECTIVE The fracture toughness determination of fine-grained zirconia ceramics using the chevron notched beam method (CNB) was investigated to assess the feasibility of this method for quality assurance and material characterization. METHODS CNB tests were performed using four different yttria-stabilized zirconia ceramics under various testing modes and conditions, including displacement-controlled and load-rate-controlled four point bending to assess the influence of slow crack growth and identify most suitable test parameters. For comparison, tests using single-edge V-notch beams (SEVNB) were conducted. RESULTS It was observed that the CNB method yields well-reproducible results. However, slow crack growth effects significantly affect the measured KIC values, especially when slow loading rates are used. To minimize the effect of slow crack growth, the application of high loading rates is recommended. SIGNIFICANCE Despite a certain effort needed for setting up a sample preparation routine, the CNB method is considered to be very useful for measuring and controlling the fracture toughness of zirconia ceramics.