Tribochemical Studies of Hard Carbon Films as a Function of Load and Environment

Hydrogen-free, hard carbon thin films are exciting material coatings candidates as solid lubricants. Two examples, ultrananocrystalline diamond (UNCD) and tetrahedral amorphous carbon (ta-C), are particularly promising, because their exceptional mechanical and tribological properties are combined with extremely smooth surfaces. However, their tribological performance can be seriously affected by variations in humidity. These materials do not perform well in vacuum or inert environments. The mechanisms controlling the friction and wear of UNCD and ta-C are not well understood because of a fundamental lack of physical understanding of the surface interactions. The aim of this thesis is to elucidate the fundamental mechanisms of friction and wear in UNCD and ta-C films. An experimental protocol is defined to examine the relationship between the sliding environment, tribological performance, and mechanical and chemical changes to the films. Self-mated reciprocating tribometry in controlled environments measure UNCD and ta-C friction as a function of load and relative humidity (RH). Scanning white light interferometry measures the post-mortem height profile. Finally, chemical changes inside the wear track are characterized by x-ray photoelectron emission microscopy combined with near-edge x-ray absorption fine structure (X-PEEM-NEXAFS) spectromicroscopy. Results for ta-C and UNCD show that both films, like single crystal diamond, perform better at lower loads or with higher amounts of RH in the environment. Previous hypotheses for this suggested that lubrication for these films either comes in the form of graphitization (converting carbon from diamond-type bonding to graphite-like bonding) or by passivation (the termination of broken carbon bonds by species in the environment, such as water). All spectroscopic evidence shows no evidence of graphitization, but support the passivation hypothesis. Furthermore, the spectroscopy shows that the passivation is in the form of hydroxyl groups, most likely from water. This affects the run-in (period at the start of sliding of high friction as asperities are being smoothed) behavior of these films. The level of passivation also controls whether the films have high or low friction

Determination of stress state in rock mass using strain gauge probes CCBO

The strain gauge probes of different construction are typically used for determination of stress state rock mass. The modified overcoring method known as the Compact Conical ended Borehole Overcoring method (CCBO) for stress state determination in rock mass was designed in Institute of Geonics of the CAS (IGN) in cooperation with Kumamoto University in Japan. The implemented adjustment of the overcoring method consists mainly in omitting the overcoring phase (stress relief phase). The probe is glued directly to the conically shaped end of a borehole. The data logger located within the conical probe enables continual strain monitoring directly in the conically shaped end of the borehohole during the overcoring procedure. The conical probe used to monitor stress changes, named Compact Conical ended Borehole Monitoring (CCBM), can continuously monitor rock strain changes in key locations due to mining. Many stress measurements using both strain gauge probes CCBO and CCBM were carried out in the last decade. These measurements were performed in varied rock mass adjacent to mine excavations. Most of the stress measurements were carried out in Carboniferous sedimentary rocks as part of the experimental work in the Czech part of the Upper Silesian Coal Basin (USCB). Several stress measurements were carried out during the mine development operations and associated geotechnical exploration work while constructing the Milasín - Bukov underground gas storage (BUGS) [1], as well as the Bukov Underground Research Laboratory (BURL) [2]. Both underground facilities were designed within Rožná and Olší uranium deposits situated on the north-eastern margins of the Strážek Unit consisting of the metamorphic rock formations. Several measurements were carried out in granitic environments (igneous rocks) as part of the international Large-Scale Monitoring (LASMO) project in Grimsel (Switzerland) and in Josef underground laboratory (Bohemian massif). The article presents the basic principles and the methodology of stress measurements in rock mass using strain gauge probes and the data analysis from the variable rock environments

Research Note:<br>Use of a distributed erosion model (AGNPS) for planning small reservoirs in the Upper Torysa basin

International audienceThis paper assesses the vulnerability to erosion of the soils in the upper Torysa catchment down to the planned drinking water reservoir at Tichy Potok (Slovakia). Experimental observations in the Torysa catchment, together with the outputs of the AGricultural Non-Point Source pollution model (AGNPS), have been used to select optimal sites for small sediment-trapping dams on the Torysa tributaries. The rainfall scenario for the upper Torysa was determined from records at the Plavec climate station from 1980?1998. In addition, an extreme rainfall event scenario was envisaged (100 mm rainfall in three hours); AGNPS model simulations suggest that such an event might produce as much as half or even the total amount of sediment generated by all the rainfalls over the whole period of simulation, 1980?1998. Keywords: erosion, sediment transport, simulation, AGNPS model, Torysa basi

Application of seismic parameters for estimation of destress blasting effectiveness

Coal seams in the Upper Silesian Coal Basin are currently extracted under more and more disadvantageous geological and mining conditions. Mining depth, geological dislocations and mining remnants are factors which affect the rockburst hazard during underground mining to the greatest extent. This hazard can be minimized by employment of active rockburst prevention, where long-hole destress blasts in roof rocks (torpedo blasts) have an important role. The main goal of these blastings is to either destress local stress concentrations in rock mass and to fracture the thick layers of strong roof rocks to prevent or minimize the impact of high energy tremors on the excavations. Sometimes, these blastings are performed to make the roof rocks caving behind the longwall face easier. The efficiency of blasting is typically evaluated from the seismic effect, which is calculated based on seismic monitoring data (seismic energy) and the weight of the charged explosive. This method, as used previously in the Czech Republic, was adopted in a selected Polish hard coal mine in the Upper Silesian Coal Basin. This method enables rapid and easy estimation of destress blasting effectiveness, adjusted to conditions occurring in the designed colliery. Destress blasts effectiveness may be evaluated via the seismic source parameters analysis as well, as was carried out in the selected colliery in the Polish part of the Upper Silesian Coal Basin. These parameters provide information, for example, on its size, state of stress and occurrence of slip mechanism in the source of provoked tremors. Long-hole destress blasting effectiveness in selected colliery has been evaluated using the seismic effect method and seismic source parameters analysis. The results were compared with each other and conditions were observed in situ

Geophysical Evaluation of Effectiveness of Blasting for Roof Caving During Longwall Mining of Coal Seam

Deep longwall mining of coal seams is made in the Upper Silesian Coal Basin (USCB) under complicated and mostly unfavourable geological and mining conditions. Usually, it is correlated with rockburst hazard mostly at a high level. One of the geological factors affecting the state of rockburst hazard is the presence of competent rocks in the roof of extracted coal seams, so rock falling behind the longwall face does not occur, and hangingup of roof rocks remains. The long-lasting absence of caving may lead to an occurrence of high-energy tremor in the vicinity of the longwall face. Roof caving behind the longwall face may be forced by blasting. The column of explosives is then located in blastholes drilled in layers of roof rocks, e.g. sandstones behind the longwall face. In this article, a characterization of tremors initiated by blasts for roof caving during underground extraction of coal seam no. 507 in one of the collieries in the USCB has been made using three independent methods. By the basic seismic effect method, the effectiveness of blasting is evaluated according to the seismic energy of incited tremors and mass of explosives used. According to this method, selected blasts gave extremely good or excellent effect. An inversion of the seismic moment tensor enables determining the processes happening in the source of tremors. In the foci of provoked tremors the slip mechanism dominated or was clearly distinguished. The expected explosion had lesser significance or was not present. By the seismic source parameters analysis, among other things, an estimation of the stress drop in the focus or its size may be determined. The stress drop in the foci of provoked tremors was in the order of 105 Pa and the source radius, according to the Brune’s model, varied from 44.3 to 64.5 m. The results of the three mentioned methods were compared with each other and observations in situ. In all cases the roof falling was forced

Phase field simulation of liquid filling on grooved surfaces for complete, partial, and pseudo-partial wetting cases

We develop and harness a phase field simulation method to study liquid filling on grooved surfaces. We consider both short-range and long-range liquid–solid interactions, with the latter including purely attractive and repulsive interactions as well as those with short-range attraction and long-range repulsion. This allows us to capture complete, partial, and pseudo-partial wetting states, demonstrating complex disjoining pressure profiles over the full range of possible contact angles as previously proposed in the literature. Applying the simulation method to study liquid filling on grooved surfaces, we compare the filling transition for the three different classes of wetting states as we vary the pressure difference between the liquid and gas phases. The filling and emptying transitions are reversible for the complete wetting case, while significant hysteresis is observed for the partial and pseudo-partial cases. In agreement with previous studies, we also show that the critical pressure for the filling transition follows the Kelvin equation for the complete and partial wetting scenarios. Finally, we find the filling transition can display a number of distinct morphological pathways for the pseudo-partial wetting cases, as we demonstrate here for varying groove dimensions

MOSEM2 project: Integration of data acquisition, modelling, simulation and animation for learning electromagnetism and superconductivity

The MOSEM2 project, funded by European Commission, seeks to extend the minds-on experiments and materials from the twin project MOSEM by adding a set of computer aided activities covering a series of topics in Electromagnetism and Superconductivity. The new activities will integrate different ICT technologies: data logging, data video, modelling, simulation and animation. The MOSEM2 primarily targets physics teachers in upper secondary schools and trainee physics teachers. Teacher training departments at universities will implement the teacher seminars and new materials developed in the project. During theWS4 workshop held at MPTL 14 examples of teaching and learning activities from MOSEM2 were demonstrated

Thermal Stability and Rehybridization of Carbon Bonding in Tetrahedral Amorphous Carbon

We preform a quantitative investigation of the energetics of thermally induced sp3 → sp2 conversion of carbon-carbon bonds in tetrahedral amorphous carbon (ta-C) films by using near edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy. We investigate the evolution of the bonding configuration in ta-C thin films subjected to high temperature annealing in flowing Argon gas using a rapid thermal annealing furnace over the range of 200-1000 ºC. We observe no substantial change in bonding structure below 600 ºC, and by 1000 ºC a significant increase in the sp2 bonding in the film is observed. No oxygen bonding is detected in the NEXAFS spectra, but we do observe an isosbestic point, demonstrating that the thermally driven sp3 → sp2 conversion reaction occurs without passing through an intermediate transition state. This allows us to use NEAFS spectra of thermally annealed ta-C films to quantitatively determine that the activation energy for directly converting the sp3-bonded carbon to the s

Origin of Ultralow Friction andWear in Ultrananocrystalline Diamond

The impressively low friction and wear of diamond in humid environments is debated to originate from either the stability of the passivated diamond surface or sliding-induced graphitization/rehybridization of carbon. We find ultralow friction and wear for ultrananocrystalline diamond surfaces even in dry environments, and observe negligible rehybridization except for a modest, submonolayer amount under the most severe conditions (high load, low humidity). This supports the passivation hypothesis, and establishes a new regime of exceptionally low friction and wear for diamond