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

Prediction of Friction Coefficients in Mixed Lubrication Regime For Lubricants Containing Anti-Wear and Friction Modifier Additives

Many laboratory tribology test machines are available for evaluating the effect of different lubricants and different operating conditions on friction. For the Mini Traction Machine (MTM) there is much published data that shows how the measured friction coefficient varies with operating conditions and lubricant type. Fully formulated lubricants containing the anti-wear additive ZDDP have often been found to have a significantly higher friction coefficient, which persists to higher speeds, compared to base oils (lubricants with no additives). Recent work has found that the surface roughness of ZDDP tribo-films can evolve to become significantly higher than that of the surfaces they are deposited on. When the measured friction coefficients of lubricants tested in the MTM machine are suitably normalized and plotted against the λ ratio (which is equal to the oil film thickness separating the moving surfaces divided by the combined surface roughness) then the curves for various different lubricants lie on a “master curve” which enables reliable friction estimates to be made for lubricated contacts in the mixed lubrication regime. A simple modification to this approach also allows for the calculation method to be extended to lubricants that contain friction modifier additives

The Environmental and Economic Importance of Mixed and Boundary Lubrication

One route to reducing CO2 emissions is to improve the energy efficiency of machines. For example, conventional combustion engines are being downsized (and also down-speeded), and are now running on lower viscosity lubricants (such as 0W-20 or lower viscosity grade lubricants) and often also have stop-start systems fitted. Some of these changes may result in higher levels of mixed and boundary friction, and so estimating the friction losses due to mixed/boundary friction, and the corresponding wear levels, is becoming of increasing importance. There is recent experimental evidence that traditional approaches (such as the Greenwood & Tripp model [1]) to predicting friction in mixed and boundary friction tend to underestimate these losses [2-5]. A new model is described, based on experimental data, that estimates the proportion of mixed/boundary lubrication, X, as a function of the lambda value (where lambda is the ratio of the oil film thickness separating the surfaces to the combined root mean square roughness of the surfaces). The precise equation that describes the way in which X varies with lambda takes the form of a “reverse S-curve” which makes sense physically since S-curves arise naturally in growth processes and the real area of contact of rough lubricated surfaces grows as 1/lambda increases. Numerical estimates of the amount of mixed/boundary lubrication losses in internal combustion engines are made and compared with recently published experimental data [2, 6]. In addition, these improved calculations are used to estimate both the financial cost of mixed/boundary lubrication for today’s vehicle fleet, and the CO2 emissions associated with these losses

Assessing lubricating film thickness between compression rings and engine cylinders: A comprehensive comparison of theoretical predictions and experimental measurements

The purpose of piston rings in combustion engines is to provide an effective seal between the combustion chamber and the crankcase while allowing rapid linear movement of the piston. In this paper a review of around 50 experimental studies and 30 theoretical studies is presented. Papers describing experimental studies report lubricating film thicknesses between 0 µm to 20 µm, while papers describing theoretical results for fully flooded analyses tend to report smaller values (0 µm to 9 µm). Theoretical studies including starvation phenomena normally give even thinner films, typically between 0 µm and 5 µm. The paper presents a discussion of these discrepancies

30th International Conference on Condition Monitoring and Diagnostic Engineering Management (COMADEM 2017)

Proceedings of COMADEM 201

Development of an artificial olfactory system for lubricant degradation monitoring

Off-line strategies are commonly used to evaluate lubricant aging. These methods are expensive, time consuming and often require skilled personnel. Online detection of lubricant degradation would eliminate some of these issues. Lubricant degradation is principally due to oxidation, additive depletion and contamination by water, acid, fuel, sulphur, and insoluble content which happens gradually through different phases of the lubricant lifetime. The by-products and final products of this chemical process characterise the different evolutive phases of oil aging and are reflected in the volatile compounds emitted by the lubricant while degrading. Hence, the lubricant headspace contains a significant amount of information about oil degradation. This paper reports the development of an artificial olfactory system for real-time oil condition evaluation by headspace analysis. The instrument has been optimised to exhibit high discriminatory power and high sensitivity towards the vapours characterising the oil aging process, while the device costs have been kept low. Preliminary measurements have been carried out on water samples, new engine oil and aged engine oil to evaluate the ability of the system to generate sensor patterns distinctive of the samples under test and to discriminate between new engine oil and relatively aged engine oil. The results of these measurements are presented and discussed in the paper

Changes in friction of zinc flake coated threaded fasteners due to humidity, temperature and storage duration

The friction coefficient of a specific material combination is often assumed to be invariant, even in manufacturing processes, such as during the tightening of threaded fastener joints. This paper considers the impact of storage conditions on threaded fastener friction. Fasteners were stored in hot humid and sub-zero temperature conditions to study friction in the thread and underhead contacts. Four Zn-flake coatings, commonly used in the automotive industry were considered. Consequent tightening of these fasteners at room temperature revealed that storage history had a significant impact on their friction coefficients, halving under-head friction in some cases. This varied behaviour was considered to be a response to coating nano-hardness and structure and differences in adsorption/absorption of water and zinc-oxide formation during storage

Advances in Sensing for Real-Time Monitoring of Tribological Parameters

The wider availability of low-cost sensing and data acquisition technologies means that real-time sensing of tribological parameters is becoming increasingly viable. Consequently, the potential to use these technologies to monitor in-service tribological components has increased significantly. This paper presents a review of a number of state-of-the-art in sensors for measuring friction, wear and lubricant properties. It also elaborates on the use of sensor coatings as an emerging area for directly probing the tribological interface. It is concluded that sensors will find ever increasing uses in condition monitoring” applications. However, sensing and tribology is beginning to evolve towards “Tribotronics” where combining the sensing of machine elements that have conventionally been passive with computational capability, or even embedded intelligence, along with actuation can create active machine elements, optimised to operate with say minimum power loss in all situations of duty. Additionally, it is noted that by incorporating sensing and responsive capabilities, functional surfaces can also become part of a bigger connected systems particularly in association with Industry 4.0. Increased use of sensors in tribological components alongside machine learning and artificial intelligence, will also support the shift in industrial tribological analytics

Lubricant Degradation Monitoring with AI-Assisted Sensors

The value of machine lubrication is well understood, but all lubricants must be periodically tested to verify their condition. This has driven intense research towards the development of efficient, low cost and timely degradation monitoring solutions. However, the periodic testing currently used results in a difficult decision between the labour and downtime costs of testing more frequently and the risk of inter-inspection faults if testing is delayed. A series of six metal oxide semiconductor gas sensors has been used within an artificial olfactory system (e-nose) to monitor the volatile compounds released by samples of mineral oil at different levels of thermal degradation. Data collected from the sensors has been used to train an artificial intelligence pattern recognition system based on principal component analysis and a support vector machine for both classification and regression predictions. The classifier achieved a 95.5% accuracy and the regression was accurate within a root-mean-square error of 2.47 showing the effective performance of an e-nose when applied to oil condition monitoring

Piston-ring film thickness: theory and experiment compared

A review of the published literature has demonstrated a large variability and discrepancies in the measured and predicted values of piston-ring lubricating film thickness in internal combustion engines. Only 2 papers have been found that compare experiments in firing engines directly with outputs from sophisticated ring-pack lubrication models. The agreement between theory and experiment in these comparisons was limited, possibly because of inadequacies in the models and/ or inaccuracies of measurement. This paper seeks to contribute to the literature by comparing accurately calibrated experimental measurements of piston-ring film thickness in a firing engine with predictions from an advanced, commercial software package alongside details of the systematic analysis of the measurement errors in this process. Suggestions on how measurement accuracy could be further improved are also given. Measurements of oil film thickness with an error (standard deviation) of +/-15% have been achieved. It is shown that this error can be reduced further, by changes in the design and installation of the sensors. Detailed experimental measurements of film thickness under the top compression ring in a firing petrol engine have been made and compared with the predictions from a commercial, state-of-the art modelling package. The agreement between theory and experiment is excellent throughout the stroke in most cases, but some significant differences are observed at the lower load conditions. These differences are as yet unexplained, but may be due to the sensor topography influencing the hydrodynamic lubrication, lubricant availability, out-of-roundness in the cylinder, or squeeze effects. This a topic that requires further stud