R Gnanamoorthy

Fretting wear resistance of uncoated and surface modified biomedical titanium alloys (Ti-6Al-7Nb) in air and Ringer fluid has been investigated in the present work. Laser nitrided specimen has shown superior performance with minimum surface damage and wear rate (<0.1 × 10-6 mm3/Nm) despite high friction coefficient in air (0.6) compared to uncoated and plasma nitrided (>12 × 10-6 mm3/Nm) specimens. This is due to presence of TiN dendrites (60-80%) in the laser nitrided layer. Plasma nitrided surface is relatively softer and hence wear rates and surface damage are comparable with uncoated alloys. Friction coefficient is high for uncoated (0.8) and plasma nitrided alloys (0.6) in air as well as Ringer fluid. Fretting induced electrochemical dissolution is responsible for higher wear rates in uncoated and plasma nitrided specimens. The fretting damage resistance is primarily governed by relative hardness and modified layer thickness of the mating components. © 2006 Elsevier B.V. All rights reserved.

Grain size refinement by severe surface plastic deformation is one way of improving the surface properties. This paper describes the microstructural evolution due to severe surface plastic deformation by oil jet peening in aluminium alloy, AA6063-T6. Detail characterization of the treated surfaces using X-ray diffraction analysis and transmission electron microscopy revealed the formation of submicron size grains at and near the surface. The nozzle-traveling velocity decides the peening intensity and coverage and affects the surface properties. The specimen peened at low nozzle-traveling velocity exhibited an ultrafine grain size (∼210. nm) with high surface hardness (∼0.88. GPa), compressive residual stress (-102 ± 7. MPa) and dislocation density. The hardness is high at the surface and the depth of hardened layer is ∼400μm. Formation of high-density dislocations and associated grain refinement resulted in increased surface hardness. Presence of surface modified layer will be beneficial in improving the fatigue and tribo behavior. © 2010 Elsevier Ltd.

One way of improving the surface properties of engineering material is by reducing the grain size at the surface. Controlled ball impact process is developed for producing surface nanocrystallization and improves the surface mechanical properties by inducing compressive residual stress on the metallic materials. Improvement in the surface mechanical properties will affect the tribological properties. This paper reports the influence of the surface nanocrystallization on the tribological properties of aluminium alloy. Tribological properties were evaluated under dry sliding conditions using a reciprocating wear test facility. The friction coefficient of the treated surface is lower than that of the untreated samples and treatment improves the wear resistance of aluminium alloys. The improvement in the friction and wear properties is due to enhancement of surface strength, due to grain refinement and induction of compressive residual stress. The worn surfaces observed using scanning electron microscope reveal the dominant adhesive nature of wear and mild abrasive wear. © 2009 Elsevier B.V. All rights reserved.

Oil jet peening makes use of high-pressure oil jet to impart compressive residual stresses on the surface of metallic materials in order to improve the surface properties. This article reports the dry sliding wear characteristics of oil jet peened aluminium alloy, AA6063-T6. The presence of compressive residual stress and high hardness improves the wear resistance of oil jet peened surfaces. Changes in the hardness, surface morphology, and residual stress distribution due to peening affect the tribological behaviour. The initial and steady-state coefficient of friction are less in the treated samples compared to untreated samples. The scanning electron microscope images of the worn surfaces reveal the dominant adhesive and mild abrasive form of wear.

A novel surface modification process namely controlled ball impact peening was developed for synthesizing a nanostructured surface layer and to impart compressive residual stresses on metallic materials in order to enhance the overall surface properties. This article demonstrates the microstructural evolution, surface hardening and introduction of the residual stresses in the ball impact peened aluminium alloy surfaces, AA6063-T6. Hardened steel balls were impinged in controlled manner inducing high strain rates on the aluminium samples which are precisely moved using independent programmable logic controlled linear actuators in the controlled ball impact peening process. Mechanical properties of the nanocrystalline surface layer were investigated using dynamic ultra micro-hardness tester. The hardness of the nanocrystalline surface layer is (~. 1.3. GPa) improved compared to the matrix (~. 0.58. GPa) and the depth of the hardened layer is about ~. 350 μm depending upon the peening conditions. The amount of compressive residual stress developed by the treatment is also studied using depth sensing indentation method. The surface compressive residual stresses induced in the ball impact peened samples is about 70-127% of yield strength of the target material depending upon the peening conditions. X-ray diffraction analysis and transmission electron microscope analysis revealed the formation of nanograin crystalline structure on the ball impact peened surface layer. The mean grain size of the peened sample determined by transmission electron microscope is about 8 ± 2. nm in the top surface layer. High strain rate and repeated directional loading imparted in the contact zone generates the various dislocation activities and microstructural features which were responsible for the formation of the randomly oriented nanostructured grains on the metallic materials. With increasing strain, the various microstructural features produced in the ball impact peened aluminium samples are deformation twins, multiple shear bands, high density dislocation and dislocation pile-up at the grain boundaries as investigated by transmission electron microscope. Grain refinement on the ball impact peened aluminium surfaces resulted in the formation of high density dislocation associated with the subdivision of original grains into subgrains. The peening coverage and number of overlapping impacts depend upon the sample travelling velocity, which in turn affects the hardness, compressive residual stresses induced and grain size formed for a given ball diameter and impact velocity. © 2012.

Fretting wear occurs when two contacting solid surfaces are subjected to a relatively small amplitude oscillatory motion in the order of few microns. In addition to the introduction of compressive residual stresses and increased substrate strength controlled ball impact treatment results in the formation of nanostructured grains at the surface. Fretting wear studies were performed on the untreated and controlled ball impact treated aluminium samples using a steel counterbody at constant slip amplitude and at different applied normal loads using a fretting wear test rig. Displacement amplitude and normal force determine the nature of the slip regime. The tangential force coefficient decreases with increasing normal loads under fretting conditions. The contact between the fretting surfaces makes the asperities interlock with each other at low applied normal loads, and results in a high tangential force coefficient, whereas at high applied normal loads tangential force coefficient decreases. Crack initiation and debris formation are the predominant types of damage observed in the fretting specimens due to micro-displacement between the junctions of two contacting members. The steady state tangential force coefficient, wear volume and specific wear rate of the ball impact treated samples were lower than those of the untreated coarse grain aluminium samples. The improvement in the tribological properties of the treated sample is attributed to high dislocation density, more number of grain boundaries, presence of compressive residual stresses and increase in substrate strength with associated grain refinement. The increased substrate strength and the presence of compressive residual stresses prolonged the crack initiation time and crack tip blunting retards the crack propagation resulting in decreased wear debris formation and wear volume. The surface morphology of the wear scars was analyzed using an optical microscopy and scanning electron microscopy, to identify the failure modes and fretting wear mechanisms. At low applied normal loads a complex adhesion and oxidation type of wear mechanism was observed and abrasion was found to be a dominant wear mechanism at high applied normal loads. © 2012 Elsevier B.V.

Use of aluminium alloy in aerospace and automotive industry has increased due to their high strength to weight ratio. Oil jet peening, a surface modification process is developed to impart compressive residual stresses on the surface of the metallic materials and resulted in significant surface hardening with associated grain refinement. Unlubricated fretting tests were performed on the oil jet peened and unpeened aluminium samples using ball-on-flat configuration at constant slip amplitude and at different applied normal loads. At low applied normal loads, the contact region between the mating surfaces makes the asperities interlock each other resulting in high tangential force coefficient. Due to micro-displacement between the interfaces of two mating members, cracks initiate and cause debris formation. The steady state tangential force coefficient, wear volume and specific wear rate of the oil jet peened samples were lower than those of the unpeened (as-received) samples for all the conditions tested and this is mainly attributed to increased substrate strength. A complex adhesion and oxidation type of wear mechanism was observed at low applied normal loads and at high applied normal loads abrasion was found to be a dominant wear mechanism. © 2012 Elsevier B.V.

Fretting damage is normally expected in biomedical implants due to body movements. Titanium alloys are most commonly used for biomedical devices and surface modified alloys have superior tribological properties to virgin materials. In the present study, the fretting wear resistance of physical vapour deposition (PVD) TiN coated, ion implanted and thermally oxidised biomedical alloys have been investigated and compared. PVD TiN coating has shown the best fretting wear resistance with minimum friction coefficient, less wear scar depth and diameter, and minimum wear rate compared to other coatings. The ion implanted specimen has undergone fretting assisted electrochemical dissolution to give higher wear loss after the modified layer has eroded away. Thermal oxidation has shown intermediate response to fretting. Fretting resistance can be improved with layers of high hardness and thickness. © 2007 Institute of Materials, Minerals and Mining.