A universal characterization method on viscous materials using depth sensing indentation

Miniaturisation of devices and a paradigm shift toward using compliant material require small scale characterisation techniques such as nanoindentation [1]. Initial non-conformity of contact and delayed elasticity on the unloading curve are not currently taking into account in nanoindentation methods [2, 3], where the unloading curve is seen fully elastic. A different approach has been taken which considers actual localised deformation during nanoindentation, thus the proposed method [4] is able to acquire untainted elastic or viscoelastic response data. The method, validated for both viscous and non-viscous materials, takes into account the correction of the stiffness associated with the delayed elasticity at initial unloading and determines modulus with less variability even if testing conditions are non-quasi-static. The key step in the methodology is to account for initial conformity of the contact, the nose-out phenomena and delayed elasticity. Thus a full elastic point (FEP) is determined as seen in the figure 1. Traditionally to eliminate the initial delayed elastic response a multi-cycle approach was needed, whereas in our method any materials, even viscous, can be tested under any test conditions in a single loading-unloading cycle. The algorithm is universally applicable. Experiments were conducted on six different viscous materials under single and multi cycle loading conditions to validate our method to existing ones [2–3]. Multi-cycle test on PET & PEN are reported in figure 2. It was found that except for rubber all the different materials studied by using just a single-cycle, our method determined values equally well as previous Oliver and Pharr method [2] with less variation. For multi-cycle tests our method is capable of producing results as good as the fourth-cycle of Feng’s method [3], thus time and cost of experimentation can be reduced. So this method becomes appropriate as a standardised technique, and also for the characterisation of polymers which have been an issue in the past

Elimination of lubricants in industries in using self-lubricating wear resistant coatings based on MoS2

The problem that has been haunting for long manufacturing industries (automotive, aerospace, domestic appliance and electronics) is the lubrication. Most processes are impossible without lubrication, otherwise wear and galling will occur. On the other hand, many lubricants are toxic and some lubricants are even flammable, and therefore the use of lubricants is a big environment, safety and health concern. This paper demonstrates how innovative self-lubricated coating such as MoS2/Titanium composite coatings can be used to solve these problems and are suitable for the specific requirements with characteristics such as low frictional coefficient, high wear resistance and low cost. For example, demand coatings such as MoS2/Titanium composite coatings is increasing, therefore forming and cutting process can be performed “dry” without use additional lubricants, that is, the oil based lubricants can be eliminated from processes. The benefits of MoS2/Titanium composite coatings are both ecological and economical. The ecological side is obvious. The economical benefits can be obtained from the elimination of the direct costs of lubricants and the application apparatus, from longer tool life and less tool maintenance, from improved quality of machined parts as well as from the indirect costs of parts cleaning and waste disposal. Industrial results from forming and cutting processes will be presented and understanding analysis will be performed. Parameters such as life-time, cutting force, surface finishing… will be correlated to laboratory friction and wear tests as well as microstructure observations

A Review of Space Tribology Experiments in Low Earth Orbit: Challenges and Opportunities

Operating mechanical devices in low earth orbit (LEO) environment presents unique challenges due to adverse effects of the LEO environment on lubricants and materials in tribo-mechanisms. These challenges include corrosion due to atomic oxygen, molecular degradation of materials and fluids due to radiation, temperature extremes influencing lubricant viscosity, and rapid evaporative loss of fluids in vacuum conditions. Therefore, lubricants for mechanisms and components such as bearings and gears for spacecraft should be tested extensively in both air and vacuum to ensure their continuous and accurate function. Literature on ground based tribo-testing is extensive and well-established. However, tribological investigations conducted in LEO are much fewer in number. The purpose of this paper is to draw together details of tribology experiments of this type, to try to clarify their purpose and value. This review presents these studies according to a thematic categorization of the mechanisms involved

Ab initio study of bilateral doping within the MoS2-NbS2 system

We present a systematic study on the stability and the structural and electronic properties of mixed molybdenum-niobium disulphides. Using density functional theory we investigate bilateral doping with up to 25 % of MoS2 (NbS2) by Nb (Mo) atoms, focusing on the precise arrangement of dopants within the host lattices. We find that over the whole range of considered concentrations, Nb doping of MoS2 occurs through a substitutional mechanism. For Mo in NbS2 both interstitial and substitutional doping can co-exist, depending upon the particular synthesis conditions. The analysis of the structural and electronic modifications of the perfect bulk systems due to the doping is presented. We show that substitutional Nb atoms introduce electron holes to the MoS2, leading to a semiconductor-metal transition. On the other hand, the Mo doping of Nb2, does not alter the metallic behavior of the initial system. The results of the present study are compared with available experimental data on mixed MoS2-NbS2 (bulk and nanoparticles).Comment: 7 pages, 6 figure

Correlation between processing parameters and mechanical properties as a function of substrate polarisation and depth in a nitrided 316 L stainless steel using nanoindentation and scanning force microscopy

The effects of substrate polarisation in a nitrided 316L stainless steel have been investigated in an attempt to accurately correlate processing parameters with surface mechanical properties. Nanoindentation allows the Vickers hardness to be measured at precise depths, meaning that the variation in properties with nitriding depth can be evaluated and correlated with the process parameters. By combining such measurements with surface imaging techniques (scanning force microscopy and scanning electron microscopy) and electron probe micro-analysis, it is possible to explain both the mechanical property and microstructural variations of such layers, having been produced in a low pressure arc plasma discharge at 680 K with a mixed Ar-N2 gas. In this study the nanoindentation technique is presented as a new and valid method for the characterisation of nitrided layers, shown by hardness measurements on four nitrided layers produced with different substrate polarisation potentials. The net advantages of such an approach over conventional methods (e.g. microhardness testing) and the possibility of analysing microstructural phases previously not well detected by X-ray diffraction, make nanoindentation an attractive tool for a more complete understanding of the nitriding process

Mechanical Properties of Advanced Gas-Cooled Reactor Stainless Steel Cladding After Irradiation

The production of helium bubbles in advanced gas-cooled reactor (AGR) cladding could represent a significant hazard for both the mechanical stability and long-term storage of such materials. However, the high radioactivity of AGR cladding after operation presents a significant barrier to the scientific study of the mechanical properties of helium incorporation, said cladding typically being analyzed in industrial hot cells. An alternative non-active approach is to implant He2+ into unused AGR cladding material via an accelerator. Here, a feasibility study of such a process, using sequential implantations of helium in AGR cladding steel with decreasing energy is carried out to mimic the buildup of He (e.g., 50 appm) that would occur for in-reactor AGR clad in layers of the order of 10 lm in depth, is described. The implanted sample is subsequently analyzed by scanning electron microscopy, nanoindentation, atomic force and ultrasonic force microscopies. As expected, the irradiated zones were affected by implantation damage (<1 dpa). Nonetheless, such zones undergo only nanoscopic swelling and a small hardness increase (10%), with no appreciable decrease in fracture strength. Thus, for this fluence and applied conditions, the integrity of the steel cladding is retained despite He2+ implantation

Étude d'un réacteur de nitruration à basse pression assistée par un plasma de décharge d'arc : application au traitement de surface des aciers, de l'aluminium et du titane

Not availableLes développements technologiques récents dans le domaine des décharges dites intensifiées ont permis de diminuer les pressions utilisées en nitruration assistée par des décharges diodes. L’objet de cette étude est de caractériser un nouveau procédé de nitruration assistée par un arc transféré. Celui-ci est basé sur une décharge d'arc à filament thermoémissif générée dans une chambre d'ionisation placée au dessus du réacteur de nitruration. Des anodes segmentées distribuées dans le réacteur permettent de créer un plasma basse tension (20-40 V) à fort courant (100-300A) dans des mélanges gazeux Ar-N2 et Ar-N2-H2. Un diagnostic du plasma mène par spectroscopie d'émission optique, a permis de déterminer les mécanismes d'excitation. En particulier, des résultats concernant la chimie en phase gazeuse des ions Ar+ et N2+ ont été mis en évidence. Une partie de cette étude est consacrée à l'analyse du plasma d'argon issus de la chambre d'ionisation, une seconde partie est consacrée à l'étude de la décharge d'arc dans différents mélanges gazeux (Ar-H2, Ar-N2, Ar-N2-H2 et Ar-N2-CH4). Ces résultats sont confrontés à ceux de l'analyse métallurgique d'échantillons en fer armco nitrurés au potentiel flottant et une corrélation entre l'intensité lumineuse des espèces excitées et la réactivité du fer a pu être mis en évidence. L’élaboration de couches nitrurées sur des substrats en acier AISI 316L, en aluminium et en titane a été abordée pour évaluer les potentialités du procédé. La nitruration de pièces en 316L pour différents temps de traitements à deux températures (310 et 410°C) et de pièces en titane (480°C-750°C) a permis de déterminer les cinétiques de croissance et les structures des couches de diffusion. La concentration superficielle en azote diminue avec la température même lorsque les caractéristiques du plasma ne sont pas modifiées ; ceci est probablement du à la recombinaison de l'azote à l'interface plasma-solide. L’influence de la polarisation des pièces en 316L et en aluminium a été étudiée afin de corréler les paramètres du procédé aux propriétés de surfaces ; en particulier, un compromis entre un décapage ionique efficace et une polarisation des pièces permet la formation d'une couche continue de nitrure d'aluminium sur l'aluminiu

Method for measuring the mechanical properties of materials using depth sensing indentation

Apparatus for use in determining a property of a sample material tested by a depth- sensing indentation system comprises a memory storing data representative of points on a load-displacement curve for a test of the sample generated by the depth-sensing indentation system. The data includes an unloading curve of the measured displacement, h, of the probe of the depth-sensing indentation system as the previously applied indentation load, P, thereon is released. The apparatus further comprises a full elastic point, FEP, determination module configured to determine, based on the unloading curve data, a point on the unloading curve having the highest load at which the measured acceleration of the indenter displacement (optionally using (I) instead of (II), as (III) is constant), becomes a constant indicating that the indenter and sample thereafter move together at the same speed, said point taken to be the full elastic point, FEP, at which any delayed elasticity in the sample in response to the indentation ceases. The apparatus further comprises a material property determination module configured to determine, based on the unloading curve data at the full elastic point, FEP, one or more material properties of the sample

Properties of rubbed and Unworn bulk MoS2 Material, MoS2 and MoS2/Titanium Composite Coatings Deposited by Closed Field Unbalanced Magnetron Sputter Ion Plating

Magnetron sputtered MoS2 coatings have been widely studied over many decades for tribological space applications, their performance can be greatly improved with the intercalation of elements such as Ti. While MoS2 retains some of the crystalline characteristics found in the original MoS2 sputtering targets, as deposited MoS2/Ti composite coatings are almost completely amorphous or “quasi-amorphous”. Tribological bidirectional wear tests have been carried out on the MoS2/Ti coatings. A surface sensitive structural analysis performed before and after wear test using Reflection High Energy Electron diffraction (RHEED). Additional analytical techniques including optical microscopy, scanning electron microscopy and transmission electron microscopy have been used. The development of a preferred orientation within the coatings was observed dependent on the number of wear cycles and the applied load

A Novel System for Analysis of Surface Profiles from 3-D Components Using the Dickinson Rotating Ring Contact Profiler

In this paper the authors present a novel system, the Dickinson Rotating Ring Contact Profiler (DRRCP) for measuring the surface profiles of components with ring profiles. Race car piston rings have been used as a test case to prove the simplicity and validity of the new system. The results obtained from the DRRCP were compared with the standard system and similar values of the mean and standard deviation were obtained, Ra = 0.3±0.05 and Rz = 0.6 ± 0.09 respectively. The system is readily adaptable to other cylindrical components and is predicted to have further applications in other fields/systems