Surface finish

Résumé

Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness. It comprises the small, local deviations of a surface from the perfectly flat ideal (a true plane). Surface texture is one of the important factors that control friction and transfer layer formation during sliding. Considerable efforts have been made to study the influence of surface texture on friction and wear during sliding conditions. Surface textures can be isotropic or anisotropic. Sometimes, stick-slip friction phenomena can be observed during sliding, depending on surface texture. Each manufacturing process (such as the many kinds of machining) produces a surface texture. The process is usually optimized to ensure that the resulting texture is usable. If necessary, an additional process will be added to modify the initial texture. The latter process may be grinding (abrasive cutting), polishing, lappin

Source officielle

https://en.wikipedia.org/wiki/Surface_finish

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Characterization of Grey Cast Iron-Steel Tribo-system under Starved Lubrication Conditions

Fatemeh Saeidi

Reducing friction and wear is essential for building efficient systems with low energy consumption and a long lifetime. Surface texturing is one of the methods to reduce friction and wear, especially in oil-lubricated systems. However, there is still a lack of knowledge about the fundamental aspects involved in the improvement of tribological performance, especially for the mechanical parts sliding under starved lubrication conditions. Moreover, failure in such lubricated contacts is poorly understood and there are little agreements regarding the mechanisms leading to scuffing. The present research focuses on three main objectives. The first is to characterize a cast iron-steel tribo-system under starved lubrication conditions, and to improve its tribological performance by laser surface texturing. All tribo-tests are performed using a flat-on-flat set-up with reciprocal movements. This set-up simulates specific industrial operating conditions of a semi-journal bearing used in a cutting machine. A Design of Experiments (DoE) approach is selected to investigate the effect of the different geometrical micro-texture parameters on the coefficient of friction (COF) and the lifetime of the cast iron samples. An optimum micro-texture geometry leading to a low COF and a long lifetime is determined based on a fractional factorial design. The second goal is to investigate the failure mechanisms of the selected tribo-pair. Failure of the tribo-system is characterized for both textured and un-textured surfaces. Scuffing is found to be the failure mechanism for the un-textured cast iron samples. For the textured cast iron samples, in contrast, two different failure mechanisms are observed depending on the distance between the micro-textures in direction of sliding (DMS). The textured surfaces with a DMS > 3.5 mm fail via the scuffing mechanism, whereas the surfaces with a DMS < 3 mm fail by a different mechanism named the oxidation mechanism. In contrary to the scuffing mechanism, the oxidation failure is found to be more a gradual failure. Based on the experimental observations, two hypotheses are suggested for the scuffing mechanism, and one for the oxidation mechanism. Finally, predicting the lifetime of a tribo-system is of the utmost importance to save costs. The DoE model developed for the lifetime was found to be a weak predictive model due to the sudden nature of scuffing. The accuracy of this model could only be improved by performing large statistics, which is not time and cost effective. Consequently, the last objective of this work is to employ an acoustic emission (AE) technique to detect precisely the surface state of our tribo-system. The application of wavelet packet decomposition, as an advanced signal processing method used for AE signals, is found to be promising for early detection of scuffing. The extension of this work using random forest regression shows the possibility of predicting scuffing 5 min before it occurs.

EPFL2016

Chargement

Austenitic stainless steels is used in many components of nuclear power plants, particularly in the pipes of cooling systems. Owing to power transients and to start-ups and shutdowns, these components are subjected to thermo-mechanical loadings (low-cycle fatigue LCF & high cycle fatigue HCF) and flow-induced vibration (HCF). The corresponding fatigue design curves were established with solid smooth specimens tested in air at room temperature under strain-control. However, these curves do not consider the influence of LWR water environments that were shown to reduce fatigue life. US NRC Regulatory Guide 1.207 or other national equivalents (JSME Code in Japan) then were established taking strain rate, dissolved oxygen and temperature into consideration. Besides, a significant number of issues have been recently identified, which may have an impact on fatigue life but have not been sufficiently investigated, e.g., the potential negative effects of mean stress, mean strain, surface finish, long-term static hold, specimen geometry, multiaxial stress state were sensitivities not explicitly addressed. This study was launched to assess the effect of mean stress on fatigue behavior of a 316L austenitic steel in boiling water reactor environment with hydrogen water chemistry (BWR/HWC) at 288°C. Load-controlled fatigue tests were selected as the easiest experimental technique to impose a pre-defined mean stress. The tests were carried out on hollow specimens at different stress amplitudes and mean stresses in BWR/HWC environment and in air at 288°C, the latter serving as reference environment to evaluate the life reduction induced by the BWR/HWC medium. Few tests in strain-controlled were performed to derive a consistent way to represent the data obtained with the two control modes together. The test results reported in the form of stress-life curves showed that, in LCF regime (< 1e5 cycles), positive and negative mean stresses increase and BWR/HWC environment decreases fatigue life. In the HCF regime, negative mean stress is always beneficial for fatigue life but positive mean stress decreases fatigue life in BWR/HWC. The beneficial effect of mean stress is attributed to its enhanced cyclic hardening on material, which leads to smaller strain amplitude at given stress amplitude. The load-controlled data were converted into strain-life presentation by considering the average strain amplitude over the whole loading cycles. Additionally, a modified Smith-Watson-Topper mean stress correction method was successfully used to correlate the results with and without mean stress. Finally we showed that strain energy density criteria are good alternatives to correlate all data. Crack growth rates were determined from the striation spacing on the fracture surfaces with high resolution scanning electron microscopy (HRSEM); crack initiation sites were also studied with HRSEM, and the microstructures were observed with transmission electron microscopy and electron channeling contrast imaging. From the striation spacing measurement, we concluded that BWR/HWC environment essentially reduces the number cycles needed to initiate a physical crack (crack depth < 50 microns) but modifies slightly the crack growth rate, which correlates well with a strain intensity factor and J-integral method. Life predictions were also done by a well-trained artificial neuron network that considers mechanical, environmental and material properties factors.

EPFL2020

Chargement

Tribological Approach to the Watch Movements

Valentine Agnès Madeleine Magnin

Mechanical watches are complex and sensitive systems that have to maintain a faultless precision over their lifetime. However, this precision can be altered by disturbances occurring during the functioning time, such as friction, lubricant drying, or wear, leading to surface modification. Due to the significant number of contacts, and diversity of materials, geometries, motions, and lubricants, these disturbances and their consequences over time are difficult to control. To deeply understand these phenomena and limit their effect on watches precision over time, a tribological study of watch movements is necessary. The goal of this Ph.D. research is to contribute to the development of a scientific methodology to appraise friction and wear evolution over time in watch movements. To achieve this, a system approach combined with a modelling of the phenomena was used to anticipate the evolution of two critical watch components: the barrel and the escapement. Based on existing theories of lubrication, the performance of the Swiss lever escapement was appraised as a function of well defined geometrical, kinematic, and dynamic parameters. The developed formalism allowed to highlight the relevance of the radii of curvature of the escapement tooth and the anchor pallet on the effectiveness of the fluid lubrication of the Swiss lever escapement. The tribology of the barrel, a brass piece coated with a nickel sublayer and a flash of gold, was then investigated. Wear characterization of real pieces allowed to identify the relevant wear mechanisms at the barrel drum/spring contact. Friction tests were performed to reproduce the observed phenomena in controlled conditions. A predictive law was proposed to estimate the loss of power at the escapement wheel and showed that the wear of the barrel drum can significantly affect the energy efficiency of the watch. Model surfaces composed of a nickel substrate of a specific waviness profile coated with different gold thicknesses were used for further investigation on the wear mechanisms of the gold coating. A simple mechanistic model considering contact pressure, geometrical, and frictional properties was developed to predict the minimum gold thickness necessary to lubricate the system. Two different relations were proposed according to the sliding direction. These relations were used to evaluate possible improvement strategies in terms of materials properties and surface finishing. With the intention to perform further tribological studies of any contacts of a mechanical watch, a methodology was proposed. This methodology stands on three phases: analysis, modelling and application to the watch. This thesis emphasizes the need of a tribological study to better understand the tribological response of a mechanical watch. By combining reviews of literature, experimental observations, and theoretical laws, analytical models were developed to identify critical parameters, materials and design strategies for improving the tribological response of watch components.

EPFL2020

Chargement

Characterization of Grey Cast Iron-Steel Tribo-system under Starved Lubrication Conditions

Fatemeh Saeidi

EPFL2016

Tribological Approach to the Watch Movements

Valentine Agnès Madeleine Magnin

EPFL2020