Future perspectives on sustainable tribology

AbstractThis paper highlights the future perspectives of sustainable tribology by examining the economic, environmental and social impact of three tribological case studies. One case study examines the sustainability and durability of micro-CHP systems looking the tribological phenomena generated within a scroll expander system. The scroll is the main part of a specific micro-CHP system and experiences wear and cavitation damage. The tribological optimization of the scroll expander improves the sustainability of the micro-CHP unit while it has a serious economic and environmental impact to the consumers and to the society in general. Another case study is focused on friction and wear performance of lifeboat launch slipways. The causes of high friction and wear during the RNLI's lifeboat launches along an inclined slipway are investigated with a view to reducing the environmental impact due to slipway panel wear and lubricant release into the marine environment. The project encompasses the sustainable design of slipway panels using design modifications based on tribological investigations to double their lifespan, while environmental and economic impact was significantly reduced by the use of biodegradable greases and water as lubricants. The final case study involves an investigation of recycled plastic materials to replace polyurethane used on skateboard wheels, scooters and similar applications. Polyurethane (PU) is difficult to recycle. With the dwindling resources and environmental problems facing the world today, recycling for both waste reduction and resource preservation has become an increasingly important aspect of sustainability. The tribological results showed that recycled polycarbonate plastic can effectively act as a substitute to polyurethane wheels. Moreover, sustainability considerations showing the environmental benefits of the use of recycled plastics over PU include reducing the CO2 footprint by 50% and the energy consumed by 60%, among other benefits. These case studies emphasise the importance of sustainable tribology in our epoch showing that increased sustainability performance can be achieved through tribology to a significant extent in many cases, providing stability to our world and more viable long term growth to our societies

Evaluation of Cavitation Erosion Behavior of Commercial Steel Grades Used in the Design of Fluid Machinery

The erosion response under cavitation of different steel grades was assessed by studying the erosion rate, the volume removal, the roughness evolution, and the accumulated strain energy. A 20 kHz ltrasonic transducer with a probe diameter of 5 mm and peak-to-peak amplitude of 50 lm was deployed in distilled water to induce damage on the surface of commercial chromium and carbon steel samples. After a relatively short incubation period, cavitation induced the formation of pits, cracks, and craters whose features strongly depended on the hardness and composition of the tested steel. AISI 52100 chromium steel showed the best performance and is, therefore, a promising design candidate for replacing the existing fluid machinery materials that operate within potential cavitating environments

Characterizing the cavitation development and acoustic spectrum in various liquids

A bespoke cavitometer that measures acoustic spectrum and is capable of operating in a range of temperatures (up to 750 degC) was used to study the cavitation behaviour in three transparent liquids and in molten aluminium. To relate these acoustic measurements to cavitation development, the dynamics of the cavitation bubble structures was observed in three Newtonian, optically transparent liquids with significantly different physical properties: water, ethanol, and glycerine. Each liquid was treated at 20 kHz with a piezoelectric ultrasonic transducer coupled to a titanium sonotrode with a tip diameter of 40 mm. Two different transducer power levels were deployed: 50% and 100%, with the maximum power corresponding to a peak-to-peak amplitude of 17 lm. The cavitation structures and the flow patterns were filmed with a digital camera. To investigate the effect of distance from the ultrasound source on the cavitation intensity, acoustic emissions were measured with the cavitometer at two points: below the sonotrode and near the edge of the experimental vessel. The behaviour of the three tested liquids was very different, implying that their physical parameters played a decisive role in the establishment of the cavitation regime. Non dimensional analysis revealed that water shares the closest cavitation behaviour with liquid aluminium and can therefore be used as its physical analogue in cavitation studies; this similarity was also confirmed when comparing the measured acoustic spectra of water and liquid aluminium

Experimental and analytical thermal study of PTFE composite sliding against high carbon steel as a function of the surface roughness, sliding velocity and applied load

This study investigates the contact temperatures caused by frictional heating of sliding parallel pairs. In this case the materials studied are a PTFE composite in contact with a high carbon steel plate. These materials are commonly used for industrial applications, in particular as the main contacting components within a scroll expander system. The expected contact temperature values are important to be quantified in order to predict failure mechanisms associated with excessive thermal effects caused by sliding friction. A rational and coherent interpretation of the thermal effects on the actual tribological contact is presented.Contact temperatures are monitored continuously using a high-precision infrared thermal imaging technique with a systematic variation in surface roughness of the high carbon steel material. These surface temperatures are investigated as a function of the friction coefficient, the sliding velocity and the applied load while the most influential parameter for the temperature rise is determined. Analytical results using conventional mathematical methodology are also produced. The analytical and experimental findings are then compared indicating interesting correlations within the macro- and micro-surface temperature regimes and the experimental conditions. Microscopic observations show that thermal effects can seriously affect fibers durability while transfer films formed across the steel counterpart can be beneficial for the operation of scroll systems under specific roughness and test conditions

Sonoprocessing of materials (Special issue)

Editoria

Calibration and performance assessment of an innovative high-temperature cavitometer

This paper describes a series of systematic experimental studies to evaluate the performance of a high-temperature cavitometer under well-controlled conditions. The cavitometer was specifically designed for measurements in liquid metals: it operates through a long tungsten waveguide (probe), providing thermal protection to the piezo sensing elements placed outside the hot area, and with sufficient bandwidth to enable the monitoring of broadband acoustic emissions associated with cavitation activity. It was calibrated electrically, and acoustically, at kHz and MHz frequencies, and so can be used to estimate acoustic pressures (in Pa), providing physical, and consequently practical, meaning to cavitation measurements within liquid metals. Results obtained from ultrasonic sources in a cylindrical vessel using water showed that the cavitometer is a reliable and robust device for characterizing direct field acoustic pressures and broadband emissions from the resulting cavitation. Additionally, preliminary characterization of the real-time acoustic pressures during ultrasonic processing of liquid aluminium (Al) in a standard clay-graphite crucible were performed for the first time. The use of the calibrated cavitometer will establish a more generalized approach for measuring the actual acoustic pressures over a broad range of liquid temperatures within a sonicated medium, demonstrating its potential use as a tool for optimizing, controlling, and scaling-up processes

In situ synchrotron radiography and spectrum analysis of transient cavitation bubbles in molten aluminium alloy

The melt processing of conventional and advanced metallic materials with high-intensity ultrasonic vibrations significantly improves the quality and properties of molten metals during their solidification. These improvements are primarily attributed to ultrasonic cavitation: the creation, growth, pulsation, and collapse of bubbles in the liquid. However, the development of practical applications is limited by the lack of fundamental knowledge on the dynamics of the cavitation bubbles; it is very difficult to directly observe ultrasonic cavitation using conventional techniques in molten metals due their high temperature and opaqueness. In this study, an in situ synchrotron radiography experiment was performed to investigate bubble dynamics in an Al-10 wt.% Cu alloy under an external ultrasound field at 30 kHz. Radiographs with an exposure time of 78 ms were collected continuously during the sonication of molten alloys at temperatures of 660±10 °C. To the best of our knowledge, this is the first time that transient cavitation bubbles have been observed in liquid aluminium. Quantification of bubble parameters such as average size and time of collapse were evaluated from radiographs using advanced image analysis. Additionally, broadband noise associated with the acoustic emissions from shock waves of transient cavitation bubbles and estimation of the real-time acoustic pressure at the driving frequency were assessed using an advanced high-temperature cavitometer in separate bulk experiments.The ExoMet Project (FP7-NMP3-LA-2012-280421), the UK Engineering and Physical Sciences Research Council (EPSRC) (EP/K005804/1 and EP/I02249X/1), and provision of beamtime on the Diamond Manchester Branchline at Diamond Light Source (expt. MT9082-1)

Smoke-free hospitals in Greece: Personnel perceptions, compliance and smoking habit

Smoke-free environments in Greece are scarce. Despite existent legislation that forbids smoking in all health care service centers, smoking is still evident. Using a random sample of hospital personnel from a large university hospital in Greece, we evaluated their smoking habits, perceptions and compliance towards hospital smoking regulations. 57.8% of the nursing personnel and 34.5% of medical/research staff were found to be current smokers (p < 0.05). Although 66% of the staff does not oppose the complete hospital smoking ban, 95% responded that they would prefer it to be partial. The above findings warrant the necessity for nurturing efforts to reduce smoking and increase the health professionals' awareness of their position as a role model to both patients and the society

Comparison of cavitation intensity in water and in molten aluminium using a high-temperature cavitometer

The application of ultrasound to industrial casting processes has attracted research interest during the last 50 years. However, the transfer and scale-up of this advanced and promising technology to the industry have been hindered by difficulties in treating large volumes of liquid metal due to the lack of understanding of certain fundamentals. In the current study, experimental results on ultrasonic processing in deionised water and in liquid aluminium (Al) are reported. Cavitation activity was determined in both liquid environments using an advanced high-temperature cavitometer sensor. In water, the highest cavitation activity is obtained for the lowest sonotrode tip amplitudes. Below the sonotrode, the cavitation intensity in liquid aluminium is found to be four times higher than in water.Ultramelt Project financially supported by the EPSRC grants EP/K005804/1 and EP/K00588X/