G. Halperin

Analytical and Experimental Investigation of Laser-Textured Mechanical Seal Faces

An analytical model is developed to predict the relation between the opening force and operating conditions in a mechanical seal with laser textured microsurface structure in the form of micropores. The model is valid for any desired shape of the micropores. An optimization is performed for spherical shape micropores evenly distributed on one of the mating rings face to maximize the opening force and fluid film stiffness. Results of a parametric study are presented showing the effect of the main design parameters on the seal performance. Some results of an experimental investigation with water-lubricated seal rings are also shown and compared with the theoretical model. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998

An improved wedge calibration method for lateral force in atomic force microscopy

An improved wedge calibration method for quantitative lateral force measurement in atomic force microscopy is presented. The improved method differs from the original one in several aspects. It utilizes a much simpler, commercially available, calibration grating and can be performed at any single specified applied load. It enables calibration of all types of probes, both integrated with sharp tips, and colloidal with any radius of curvature up to 2 μm. The improved method also simplifies considerably the calculation of the calibration factor by using flat facets on the calibration grating to cancel out system errors. A scheme for the data processing for on-line calibration of the lateral force is also presented.

Experimental Investigation of Laser Surface Textured Parallel Thrust Bearings

Performance enhancements by laser surface texturing (LST) of parallel-thrust bearings is experimentally investigated. Test results are compared with a theoretical model and good correlation is found over the relevant operating conditions. A comparison of the performance of unidirectional and bi-directional partial-LST bearings with that of a baseline, untextured bearing is presented showing the benefits of LST in terms of increased clearance and reduced friction.

Different aspects of the role of wear debris in fretting wear

Two different aspects of the role of oxide wear debris in fretting wear are studied by allowing them to escape from the interface during sliding. This is accomplished by laser surface texturing that forms regular micro-pores topography on the friction surfaces which enables this escape. It is found that the role of oxide wear debris depends on the dominant fretting wear mechanism. Their presence in the interface protects the friction surfaces when the dominant wear mechanism is adhesive and harms the friction surfaces when this mechanism is abrasive. The escape of oxide wear debris into the micro-pores results in up to 84% reduction in the electrical contact resistance of the textured fretting surfaces.

The effect of surface regular microtopography on fretting fatigue life

The possibility of increasing fretting fatigue life of a tribo-pair by laser surface texturing technology is studied. An appropriate experimental fretting fatigue device is used to conduct comparative experiments with a cylinder-on-flat type of contact. A relatively small number of long fretting fatigue experiments under seemingly partial slip condition are described and analyzed. From these preliminary tests, it is found that regular micropores created by laser texturing in the fretted zone on the cylinder almost double the fretting fatigue life compared to a common non-textured fretting zone.

Liposomes act as effective biolubricants for friction reduction in human synovial joints.

Phospholipids (PL) form the matrix of biological membranes and of the lipoprotein envelope monolayer, and are responsible for many of the unique physicochemical, biochemical, and biological properties of these supermolecular bioassemblies. It was suggested that phospholipids present in the synovial fluid (SF) and on the surface of articular cartilage have major involvement in the low friction of cartilage, which is essential for proper mobility of synovial joints. In pathologies, such as impaired biolubrication (leading to common joint disorders such as osteoarthritis), the level of phospholipids in the SF is reduced. Using a human-sourced cartilage-on-cartilage setup, we studied to what extent and how phospholipids act as highly effective cartilage biolubricants. We found that large multilamellar vesicles (MLV), >800 nm in diameter, composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or of a mixture of DMPC and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) are superior lubricants in comparison to MLV composed of other phosphatidylcholines. Introducing cholesterol into liposomes resulted in less effective lubricants. DMPC-MLV was also superior to small unilamellar vesicles (SUV), <100 nm in diameter, composed of DMPC. MLV are superior to SUV due to MLV retention at and near (<200 microm below) the cartilage surface, while SUV penetrate deeper into the cartilage (450-730 microm). Superiority of specific PL compositions is explained by the thermotropic behavior (including compressibility) of the lipid bilayer. Correlating physicochemical properties of the MLV with the friction results suggests that MLV having lipid bilayers in the liquid-disordered phase and having a solid-ordered to liquid-disordered phase transition temperature slightly below physiological temperature are optimal for lubrication. High phospholipid headgroup hydration, high compressibility, and softness are the common denominators of all efficient PL compositions. The high efficiency of DMPC-MLV and DMPC/DPPC-MLV as cartilage lubricants combined with their resistance to degradation at 37 degrees C supports further evaluation of these MLV for treatment of joint impairments related to poor lubrication. This work also demonstrates the relevance of basic physicochemical properties of phospholipids to their activities in biological systems.

Experimental Investigation of the Elastic–Plastic Contact Area and Static Friction of a Sphere on Flat

An experimental investigation is presented to evaluate recently published models for the contact and sliding inception of a deformable sphere loaded against a smooth rigid flat. The effects of the normal load on the contact area, junction growth, and the static friction force in the elastic-plastic contact regime are presented. Very good correlation is found between the predicted and measured contact area. A dramatic decrease of the static friction coefficient with increasing normal loading is observed, similar to the trend predicted by the model. The quantitative agreement is, however, less satisfying. Some possible reasons for the poor agreement are pointed out.

Study of the wear behavior and adhesion of diamond films deposited on steel substrates by use of a Cr–N interlayer

Abstract In the present work, we report on the wear behavior and adhesion of diamond films deposited on steel coated by a Cr–N interlayer. The wear behavior of the diamond films was studied under fretting conditions. Fretting was used as a typical wear process occurring during vibration and friction. Wear grooves obtained after the fretting test under different loads and for different numbers of cycles were studied by profilometry, high-resolution scanning electron microscopy and electron probe microbeam analysis. The adhesion of the deposited diamond films was measured by using scratch and indentation tests. The fracture behavior of the diamond film/nitrided chromium interlayer complex under different conditions was studied and correlated to the interlayer nitridation conditions.

Experimental Study of Adhesive Static Friction in a Spherical Elastic-Plastic Contact

The elastic-plastic contact between a deformable sphere and a rigid flat during presliding is studied experimentally. Measurements of friction force and contact area are done in real time along with an accurate identification of the instant of sliding inception. The static friction force and relative tangential displacement are investigated over a wide range of normal preloads for several sphere materials and diameters. Different behavior of the static friction is observed in the elastic and in the elastic-plastic regimes of sphere deformation. It is found that at low normal loads, the static friction coefficient depends on the normal load in breach of the classical laws of friction. The presliding displacement is found to be less than 5% of the contact diameter, and the interface mean shear stress at sliding inception is found to be slightly below the shear strength of the sphere material. Good correlation is found between the present experimental results and a recent theoretical model in the elastic-plastic regime of deformation.

Microstructure and Tribological Properties of Ni-Based Claddings on Cu Substrates

Abstract Cu substrates were laser-clad with a Ni-based powder using a CW–CO 2 laser. In order to obtain sufficient absorption of the laser irradiation by the substrate a Ni–B–Si plasma-sprayed layer was deposited on the Cu substrate and remelted prior to the cladding process. High hardness values in the range of 5 to 5.4 GPa were achieved for the laser-clad Cu surfaces, compared with a Cu substrate hardness of ∼1.3 GPa. The high hardness of the Ni-based cladding is due to a microstructure consisting mainly of Ni dendrites and Ni and Ni 3 B lamellae. However, the large number of brittle boride phases at the interfaces between the cladding, the plasma-sprayed layer, and the substrate causes cracks during cooling owing to high thermal stresses. Tribological tests of the claddings reveal poor adhesive friction and wear properties with a tendency to seizure when sliding against steel under dry conditions. Abrasive wear coefficients of ∼0.1 were obtained for the claddings when abraded by #320 emery paper. Some correlation was found between the cladding thickness and the tribological properties due to thickness induced defects in the cladding.