Influence of grain boundary sliding on diffusion in yttria doped zirconia

Grain boundary sliding during high temperature deformation can lead to stress concentrations and an enhancement of diffusion in mobile boundaries. Experiments were conducted on a fine grained 3 mol% yttria stabilized tetragonal zirconia, under conditions associated with superplastic flow involving grain boundary sliding. Tracer diffusion studies under creep conditions and without load indicate that there is no enhancement in either the lattice or grain boundary diffusivities. The experimental creep data are consistent with an interface controlled diffusion creep mechanism. (C) 2011 Elsevier Ltd. All rights reserved.

Electron transfer reaction in the marcus inverted region: role of high frequency vibrational modes

A theoretical study of the dynamics of photo-electron transfer reactions in the Marcus inverted regime is presented. This study is motivated partly by the recent proposal of Barbara et al. (J. Phys. Chem. 96, 3728, 1991) that a minimal model of an electron transfer reaction should consist of a polar solvent mode (X), a low frequency vibrational mode (Q) and one high frequency mode (q). Interplay between these modes may be responsible for the crossover observed in the dynamics from a solvent controlled to a vibrational controlled electron transfer. The following results have been obtained. (i) In the case of slowly relaxing solvents, the proximity of the point of excitation to an effective sink on the excited surface is critical in determining the decay of the reactant population. This is because the Franck-Condon overlap between the reactant ground and the product excited states decreases rapidly with increase in the quantum number of the product vibrational state. (ii) Non-exponential solvation dynamics has an important effect in determining the rates of electron transfer. Especially, a biphasic solvation and a large coupling between the reactant and the product states both may be needed to explain the experimental results. ©1996 American Institute of Physics

Modes of binding of 2'-AMP to RNase T1. A computer modeling study

The modes of binding of adenosine 2'-monophosphate (2'-AMP) to the enzyme ribonuclease (RNase) T1 were determined by computer modelling studies. The phosphate moiety of 2'-AMP binds at the primary phosphate binding site. However, adenine can occupy two distinct sites--(1) The primary base binding site where the guanine of 2'-GMP binds and (2) The subsite close to the N1 subsite for the base on the 3'-side of guanine in a guanyl dinucleotide. The minimum energy conformers corresponding to the two modes of binding of 2'-AMP to RNase T1 were found to be of nearly the same energy implying that in solution 2'-AMP binds to the enzyme in both modes. The conformation of the inhibitor and the predicted hydrogen bonding scheme for the RNase T1-2'-AMP complex in the second binding mode (S) agrees well with the reported x-ray crystallographic study. The existence of the first mode of binding explains the experimental observations that RNase T1 catalyses the hydrolysis of phosphodiester bonds adjacent to adenosine at high enzyme concentrations. A comparison of the interactions of 2'-AMP and 2'-GMP with RNase T1 reveals that Glu58 and Asn98 at the phosphate binding site and Glu46 at the base binding site preferentially stabilise the enzyme-2'-GMP complex.

Modes of binding of α(1–2) linked manno-oligosaccharides to concanavalin A

Three-dimensional structures of the complexes of concanavalin A (ConA) with alpha(1-2) linked mannobiose, triose and tetraose have been generated with the X-ray crystal structure data on native ConA using the CCEM (contact criteria and energy minimization) method. All the constituting mannose residues of the oligosaccharide can reach the primary binding site of ConA (where methyl-alpha-D-mannopyranose binds). However, in all the energetically favoured complexes, either the non-reducing end or middle mannose residues of the oligosaccharide occupy the primary binding site. The middle mannose residues have marginally higher preference over the non-reducing end residue. The sugar binding site of ConA is extended and accommodates at least three alpha(1-2) linked mannose residues. Based on the present calculations two mechanisms have been proposed for the binding of alpha(1-2) linked mannotriose and tetraose to ConA.

Effect of sliding speed on friction and wear of uni-directional aramid fibre-phenolic resin composite

A previous study on the tribological performance of a compression-moulded aramid fibre-phenolic resin composite, containing 30% continuous fibre, showed that this composite provides a reasonable combination of the friction coefficient and wear rate to be used as a friction component, such as a brake shoe. In the present work, the effect of sliding speed on the friction and wear behaviour of this composite has been investigated. The sliding experiments were conducted in a speed range of 0.1-6 m s(-1) at two normal pressure levels of 1.0 and 4.9 MPa. The coefficient of friction was found to be stable over a wide range of sliding speeds and normal pressures. The wear of the composite was found to be insensitive to changes in the speed in the higher speed range. The results have been supplemented with scanning electron micrographs to help understand possible friction and wear mechanisms.

Effect of Speed and Pressure on Dry Sliding Interactions of Alumina against Steel

Sliding of alumina (87%) pins against a hardened steel disk over a range of pressures (3.3-30.0 MPa) and speeds (0.1-12.0 ms(-1)) has been studied. Four different regions (R1, R2, R3, and R4) of friction as a function of speed have been identified. R1 and RS exhibit single-valued friction while in R2 and R4 the friction exhibits dual behavior. The speed range over which these regions prevail is sensitive to the pressure. R1 and R2 are low-speed and low-temperature regions, and in both, metal transfer and formation and compaction of gamma-Fe2O3 occur. R3 and R4 are associated with high speeds and high interface temperatures. Formation of FeO, FeAl2O4, and FeAlO3 has been observed. The implications of the tribochemical interactions on friction and wear characteristics are discussed.

Observations on the role of the steel disc counterface during the sliding of various structural ceramics

Sliding wear characteristics and mechanisms of structural ceramics, namely Al2O3, zirconia-toughened alumina, tetragonal zirconia polycrystals (TZP) and Si3N4 against a steel counterface are influenced by mechanical and tribochemical interactions, specific to the combinations studied. The present paper studies the role of the disc in the sliding wear process of the above ceramics. Experiments were conducted at a pressure of 15.5 MPa between 0.1 and 12.0 m s(-1) with ceramic pins sliding against an EN-24 steel disc. Except in the case of TZP, the disc morphology is sensitive to variations in speed rather than to the pin material. The disc track is (i) mildly abraded at low speeds (about 0.1-0.75 m s(-1)), (ii) severely abraded at intermediate speeds (about 1.0-3.0 m s(-1)), (iii) covered with black patches at high speeds (about 4.0-6.0 m s(-1)) and (iv) completely black at very high speeds (about 7.0-12.0 m s(-1)). In the case of TZP, although black patches appear, transfer of TZP onto the disc surface and high wear of TZP occurs at 4.0 m s(-1). The order of the wear of the disc estimated from profilometric measurements is the same for all the ceramics. Except for Si3N4, the onset of wear of the ceramics is associated with the appearance of deep 'V' grooves on either side of the profile of the disc track. This can be explained on the basis of the thermal and hardness variations. Although other interaction products specific to the ceramic pin are present, the formation of iron oxides dominates the wear of the disc.

Sliding wear of YTZP ceramic against steel: observations on ceramic transfer and wear transition

Sliding tests were conducted, in air, of YTZP ceramic pins against steel discs at an applied pressure of 15.5 MPa over a speed range of 0.3 to 4.0 ms(-1). Pin wear was not detectable until 2.0 m s(-1), after which a finite but small wear rate was observed at 3.0 m s(-1), accompanied by a red glow at the contacting surface. A transition in wear behaviour and friction (mu) occurred at 4.0 ms(-1), increasing the former by over two orders of magnitude. Both mu and wear behaviour changed with time at 4.0 m s(-1). During initial periods mu was high and wear rate increased steadily with time accompanied by ceramic transfer onto the disc, which increased with time. When disc coverage exceeds a certain threshold value, mu decreased rapidly and the wear rate stabilized at a very high value. Metal transfer was not observed at any speed. High surface temperatures brought about significant adhesion between TZP and steel and this together with enhanced plastic deformation brought about a transition in wear behaviour.

Dry sliding wear of Al alloy 2024-Al203 particle metal matrix composites

In the present investigation, Al 2024-15vol.%Al2O3 particulate (average size, 18 mu m) composites were fabricated using the liquid metallurgy route. The wear and friction characteristics of Al alloy 2024 and Al 2024-15vol.%Al2O3p, composite in the as-extruded and peak-aged conditions were studied using a pin-on-disc machine (with a steel disc as the counterface material). The worn surfaces, subsurfaces and the debris were analysed in a scanning electron microscope.The performance of the composite in the as-extruded condition is slightly inferior to that of the unreinforced alloy. However, in the T6 condition, although the wear rates of two materials are initially comparable, the unreinforced alloy seizes while the composite does not within the tested range employed. In the as-extruded condition, the presence of Al2O3 particles is not particularly beneficial as they fracture and result in extensive localized cracking and removal of material from the surface. In the peak-aged condition, however, while the unreinforced alloy exhibits severe plastic deformation and undergoes seizure, there is no significant change in the mechanism in the case of the composite. Except in the case of the peak-aged unreinforced alloy, worn surfaces of all other materials show the presence of an iron-rich layer.

Mechanism of seizure of aluminium-silicon alloys dry sliding against steel

A steel ball was slid on aluminium-silicon alloys at different temperatures. After the coefficient of friction had been measured, the surface shear stress was deconvoluted using a two-term model of friction. The ratio of surface shear stress to bulk hardness was calculated as a function of temperature, silicon content and alloying additions. These results are qualitatively similar to those recorded for pre-seizure specimens slid against an En24 disc in a pin-on-disc machine. This similarity, when viewed in the context of the phenomenon of bulk shear, provides a model for seizure of these alloys.

Sliding wear of Al2O3-SiC-(Al,Si) composites against a steel counterface

The sliding-wear behavior of Al2O3-SiC-Al composites prepared by melt oxidation against a steel counterface has been recorded in a pin-on-disk machine. At high speeds and pressures (10 m/s, 20 MPa), friction and wear appear to be principally controlled by the in-situ formation of an interfacial film that consists of a layer of Fe3O4. The formation of this him is examined as a function of sliding speed, lubrication, and composite microstructure. A model is proposed in which high surface temperatures cause the preferential extrusion of aluminum from the composite onto the pin/disk interface. This promotes the adhesive pickup of iron and its oxidation to form a stable tribologically beneficial layer of Fe3O4.

Sliding wear studies in epoxy containing alumina powders

The effect of hard and refractory alumina additions on the mechanical properties of polymer in general and wear behavior in particular is not well studied. In this work, therefore, the changes in wear behavior of epoxy resin due to the additions of alumina powders have been looked into. Using a pin-on-disc set up, dry sliding wear tests were done on both filled (4, 8, & 11 wt. % alumina) and unfilled samples. A sliding velocity of 0.83 m/sec. and a sliding distance of 2 km were employed for the study. Load range used varied from 9.8 N to about 29 N. The experiments point to an increased resistance to wear with an increased presence of filler in the matrix. Further, higher loads result in larger loss of material irrespective of the filler level in the composite.