What role for contact spots and dislocations in friction and wear?

Sixteen years of research on the properties of 'contact spots' of ductile materials in 'adhesive' wear are surveyed. They are the locations (typically occupying less than 1% of the macroscopic area of contact) at which solids ale in load-bearing contact during wear. Their properties determine the coefficient of friction, and friction and Joule heat are generated at them. It was found that the mechanical behavior of contact spots is governed by ordinary dislocation plasticity. They remain elastic if the local pressure falls short of the indentation hardness of the softer side. In that case (almost) no wear occurs. Elastic contact spots are promoted by light loads and fine polishing. Ordinarily, wear particles are detached through tangential shearing-off where, statistically, the two sides at contact spots momentarily interlock. The Holm-Archard wear 'law' is a simple direct result of this mechanism. However, quite typically the bulk of sliding takes place within adsorbed moisture films which are ubiquitous in our daily surroundings. Outside of contact spots the adsorbed moisture behaves much like ordinary water. At the typical contact spot the moisture is squeezed down to but two monomolecular layers; and the relative motion between sliding solids overwhelmingly takes place between these. This causes the prevalence of friction coefficients about μ 0.3 in our surroundings. The complex behavior of the adsorbed water can cause stick-slip. As water is desorbed at 170 °C, the determination of flash temperatures at contact spots is critical. A corresponding theory was developed and verified in connection with experiments on graphite lubrication which depends on the presence of adsorbed moisture. Further, plastic contact spots and solid lubrication have been simulated by means of a Bridgman anvil apparatus. The results show that the same work-hardening behavior applies at contact spots as known from the bulk. They also proved that at contact spots intimate mixing of the materials of the two sides gives rise to a finely mixed layer which can amorphize by much the same dislocation mechanism as is believed to cause melting. The amorphous material promptly recrystallizes at least in the Cu-Ag system. It is proposed to try and inhibit such recrystallization by selected lubricants, so as to promote beneficial wear-resistant tribo-films.     

Role of the strengthening phases in abrasive wear resistance of laser-clad NiCrBSi coatings

The influence of the strengthening phases on the tribological characteristics (wear intensity, specific work of wear, coefficient of friction) and the wear mechanisms in two-body abrasion tests with abrasives of different hardnesses (corundum Al₂O₃, ~2000 HV and silicon carbide SiC, ~3000 HV) has been investigated for PG-SR2 (Cr₂₃C₆, 1000–1150 HV), PG-10N-01 (Cr₇C₃, 1650–1800 HV; CrB, 1950–2400 HV), and 75% PG-SR2 + 25% TiC (TiC, 2500–2900 HV; (Cr,Ni)₂₃(C,B)₆ and (Ti,Cr)(C,B), ~2000 HV) coatings. The dominant role of the strengthening phases (compared with the role of the metal matrix) in the abrasive wear resistance of laser-clad NiCrBSi coatings has been estimated. Different wear mechanisms have been identified and, accordingly, different levels of coatings wear resistance have been achieved depending on the ratio between the hardness of the strengthening phases (carbides, borides, carboborides) and abrasive particles.     

Modeling adhesive wear resistance of Al-Si-Mg-/SiCp PM compacts fabricated by hot pressing process, by means of ANN

In this study, modeling adhesive wear resistance of Al-Si-Mg/SiC_p MMC compacts were performed by ANN, using a back-propagation neural network that uses gradient descent learning algorithm. Powder compacts were fabricated by PM hot pressing process with 5–10–20% SiC_p fractions and contents of specimens (N1, N2, N3 andN4) were given in Table 1. The wear tests were carried out under 10, 20 and 30 N variable loads, while disk rotation speed 90 rpm kept unchanged. Adhesive wear looses were measured and recorded for 250, 500, 1,000 and 1,500 m distances. Microstructure examination at wear surface was investigated by optical microscopy and EDS for metallographic evaluations. In neural networks training module, SiC_p reinforcement fractions (wt), loads and wear distances (m) were used as input, lost mass (g) of specimens were recorded as outputs. Then, the neural network was trained using the prepared training set (also known as learning set). At the end of the training process, the test data were used to check the system accuracy. As a result ANN was found successful in modeling of adhesive wear behavior and lost mass values of Al/SiC_p PM compacts.

Wear of wheel-rail surfaces

The interactions between the tribological stress parameters and the measurable wear quantities during rolling with slip are examined with the help of experimental results already published and the results of some new investigations by the authors of this paper.It is from these that the following model for wear behaviour is derived.

• 1.(1) The wear quantities (wear volume and mass loss due to wear) are first of all proportional to the frictional work.
• 2.(2) The factor of proportionality between frictional work and wear volume depends on (a) the structure of the tribological system (material pairing and environment) and (b) the temperature of the contacting surfaces. This temperature is a function of the stress collective, the influence of which may be approximated as the relation of frictional power to contact area, the surface temperature thus governing the transitions between mild and severe wear.

     

Manipulators: Execution of prescribed trajectories special link positions and versions of assembly

A new technique for the kinematical analysis and design of manipulators based on the following principles is proposed:

• 1.(1) Link positions are determined by subsystems of equations which contain a reduced number of unknowns and may be solved separately. The derivation of such systems of equations may be based: (a) on the modeling of the manipulator by two open chains or (b) on modified closure matrix equations.
• 2.(2) Special 4 × 4 matrices which describe separately the link geometry and the relative motion of links are applied.
• 3.(3) All potential configurations which are formed by the manipulator links may be determined using the information on special link positions.
• 4.(4) Angles of rotation in the joints are determined as functions of the trajectory parameters using a system of six nonlinear equations represented in echelon shape.

The above techniques are applied for the kinematical analysis of a Cincinnati Milacron T3 manipulator with six degrees-of-freedom. A computer program for the control of rotations in the joints has been developed based on these techniques, and the results are presented at the end of this paper.     

Plastic deformations of wheel-rail surfaces

Experimental investigations into the interactions between surface pressure, tangential tractions and plastic deformations during rolling gave the following results.

• 1.(1) The most intensive plastic deformations are caused by lateral frictional forces, followed by longitudinal frictional forces. The material flow is smallest with pure rolling.
• 2.(2) In agreement with Johnson's theory for a steel with a carbon content of 0.6% (similar to those used for wheels and rails) the shakedown limit was found at a maximum hertzian surface pressure of about 1600 N mm⁻².

     

Use of the laser Doppler vibrometer to measure the surface topography of magnetic disks

Run-out and surface roughness of magnetic disks are of increasing concern as the spacing between the recording head and the magnetic medium decreases and the areal density of stored information continues to increase. In this paper we investigate the use of the laser Doppler vibrometer as a means for measuring the surface topography of reflective metal film and oxide particulate magnetic media disks at three different scales:

• 1.(i) largescale disk run-out that results from warpage or clamping distortions,
• 2.(ii) intermediate-scale hills and depressions (of the order of 1 mm in lateral dimension and 1 μm in height) and
• 3.(iii) surface roughness associated with the manufacturing processes (of the order of 10 nm in height). The run-out measurements made with the laser Doppler vibrometer are compared with similar measurements made using a capacitance probe. The intermediate- and smallscale laser Doppler vibrometer measurements are compared with results obtained by the use of a surface profilometer. The agreement is sufficiently close to indicate that the laser Doppler vibrometer provides a reliable noncontact technique for making such surface profile measurements.

     

Velocity fields within sewers: An experimental study

Measuring the flow rate through sewer networks is necessary for a number of reasons. To achieve such measurements, precise knowledge of the hydrodynamic conditions of each potential measurement location proves critical. A research project is currently being conducted to improve the representativeness of flow rate measurements recorded in man-entry sewers. Three objectives are being pursued as part of this project:

• ∘assisting with the selection of a suitable measurement section;
• ∘designing for proper sensor implementation in a given section; and
• ∘setting up an appropriate system for processing the raw data provided by sensors.

This project relies upon numerical simulations; moreover, experimental devices have been developed and utilized to collect measurement data for purposes of model validation. This paper will describe such experimental devices and presents the results obtained after several months of investigation.     

An Investigation into Lubrication and Oxide Breakdown During Load-Carrying Capacity Testing

Changes occurring in the friction and wear mechanisms during a load-carrying capacity test, lubricated with cetane containing a carboxylic acid, were investigated. The changes in wear scar/track appearance and oxide coverage/composition were analyzed during every load stage.

The main conclusions were as follows:

• The breakdown in the protective oxide layer formed on the opposing steel surfaces was found to be the prerequisite for initiation of seizure.

• The seizure load achieved during load-carrying capacity testing quantifies the ability of the test fluid to prevent transition to the adhesive wear regime.

• The most severe surface damage was found to occur during the first few seconds after test initiation. Desorption of the adsorbed lubricant film and the subsequent removal of the naturally occurring thin oxide layer results in the initial period of adhesive wear.

• Partial recovery to a state of acceptable friction occurs after the period of initial seizure. During this period, the surface coverage by the adsorption lubricant molecules and the oxide coverage are sufficient to prevent severe adhesive wear from occurring. Wear is primarily a combination of oxidative, abrasive, and fatigue wear (all possible in the regions of mixed friction and boundary lubrication).

• Final lubricant breakdown and eventual seizure are initiated when the oxide removal rate exceeds the oxide formation rate resulting in severe adhesive wear followed by seizure.

     

An impact and erosion study of polyetheretherketone

Single-particle impacts by 4 and 5 mm diameter steel spheres on polyetheretherketone (PEEK) at various angles and speeds were studied using high speed photography and scanning electron microscopy. The following two material removal mechanisms were identified:

• 1.(1) the drawing out of filaments in oblique impact;
• 2.(2) material jetting in normal impact for speeds of approximately 480 m s⁻¹. A “deformation map” was constructed giving the types of crater formed by these spheres at given angles and speeds. Low speed (about 30 m s⁻¹) damage by quartz sand grains, sieved to give the size range 300–600 μm, was also studied. Various types of damage site were identified and the proportion of each for angles of impact ranging from 20° to 90° determined. The compressive stress-strain curve at a strain rate of 10⁴ s⁻¹ has been measured and a high speed photographic sequence of the deformation of a PEEK disc at a strain rate of approximately 2 × 10³ s⁻¹ is presented.

     

Global energy consumption due to friction in passenger cars

This study presents calculations on the global fuel energy consumption used to overcome friction in passenger cars in terms of friction in the engine, transmission, tires, and brakes. Friction in tribocontacts was estimated according to prevailing contact mechanisms such as elastohydrodynamic, hydrodynamic, mixed, and boundary lubrication. Coefficients of friction in the tribocontacts were estimated based on available information in the literature on the average passenger car in use today, a car with today’s advanced commercial tribological technology, a car with today’s best advanced technology based upon recent research and development, and a car with the best technology forecasted in the next 10 years. The following conclusions were reached:

• •In passenger cars, one-third of the fuel energy is used to overcome friction in the engine, transmission, tires, and brakes. The direct frictional losses, with braking friction excluded, are 28% of the fuel energy. In total, 21.5% of the fuel energy is used to move the car.
• •Worldwide, 208,000 million liters of fuel (gasoline and diesel) was used in 2009 to overcome friction in passenger cars. This equals 360 million tonne oil equivalent per year (Mtoe/a) or 7.3 million TJ/a. Reductions in frictional losses will lead to a threefold improvement in fuel economy as it will reduce both the exhaust and cooling losses also at the same ratio.
• •Globally, one passenger car uses on average of 340 l of fuel per year to overcome friction, which would cost 510 euros according to the average European gas price in 2011 and corresponds to an average driving distance of 13,000 km/a.
• •By taking advantage of new technology for friction reduction in passenger cars, friction losses could be reduced by 18% in the short term (5–10 years) and by 61% in the long term (15–25 years). This would equal worldwide economic savings of 174,000 million euros and 576,000 million euros, respectively; fuel savings of 117,000 million and 385,000 million liters, respectively; and CO₂ emission reduction of 290 million and 960 million tonnes, respectively.
• •The friction-related energy losses in an electric car are estimated to be only about half those of an internal combustion passenger car.

Potential actions to reduce friction in passenger cars include the use of advanced coatings and surface texturing technology on engine and transmission components, new low-viscosity and low-shear lubricants and additives, and tire designs that reduce rolling friction.     

Summary of achievements of the European Metrology Research Programme Project “Implementing the new Kelvin” (InK 1)

We report a summary of the technical achievements of the European Metrology Research Programme Project (EMRP) “Implementing the new Kelvin” (InK 1). In short these are:

• •The first determination of definitive thermodynamic temperatures for the point of inflection of the high temperature fixed points of Re-C, Pt-C and Co-C as well as a new evaluation of the Cu freezing point.
• •The first trial of the new dissemination mechanisms for thermodynamic temperature at high temperatures, as described in the mise en pratique for the definition of the kelvin (MeP-K).
• •A new ultra-low uncertainty thermodynamic evaluation of T − T₉₀ from about 30 K to 303 K, with particular emphasis on temperatures around the water triple point (273.16 K).
• •The first re-evaluation of T − T₂₀₀₀ from 0.02 K to about 1 K with an uncertainty of <1%.

Taken together these results represent a significant advance in primary thermometry. We also give a brief introduction to the successor project (InK 2) and discuss the impact of this work on the kelvin redefinition and next version of the MeP-K (i.e. the MeP-K-19).     

Global energy consumption due to friction in paper machines

Calculations on the global energy consumption used to overcome friction in paper machines in terms of friction in motors, transmissions, pumps, blowers, agitators, pipes and the roll systems are presented. The following was concluded:

• –The energy consumed to overcome friction in a paper mill is in the range 15–25%.
• –Globally there were 8525 paper and paperboard machines in operation in 2012. One paper machine uses on an average 140 TJ of electrical energy per year. Of this 32% is consumed to overcome friction, 36% is used for the paper production and mass transportation and 32% is other losses.
• –The friction losses in an average paper machine are in total 44.8 TJ per year, and they are distributed as 32% due to friction in water-lubricated sliding in seals, doctor blades and fabric/support contacts, 23% due to friction in elastohydrodynamic rolling contacts, 22% due to friction in elastohydrodynamic rolling–sliding contacts, 15% due to friction in oil-lubricated seals and 8% due to friction in hydrodynamically lubricated contacts.
• –Worldwide 105,000 GWh electrical power was used in 2009 to overcome friction in paper machines. This equals to 381,000 TJ of annual energy consumption.
• –By taking advantage of new technology for friction reduction in paper machines, friction losses could be reduced by 11% in the short term (about 10 years), and by 23.6% in the long term (20–25 years). This would equal to annual worldwide economic savings of 2000 million euros and 4200 million euros; electricity savings of 36,000 and 78,000 GWh; and CO₂ emission reduction of 10.6 million and 22.7 million tonnes.

Potential mechanisms to reduce friction in paper machines include the use of low-friction and highly durable coatings, surface engineering including texturing, low-viscosity and low-shear lubricants and fluids, novel additives, new materials in seals, doctorblades and fabrics, as well as new designs.     

Global energy consumption due to friction in trucks and buses

In this paper, we report the global fuel energy consumption in heavy-duty road vehicles due to friction in engines, transmissions, tires, auxiliary equipment, and brakes. Four categories of vehicle, representing an average of the global fleet of heavy vehicles, were studied: single-unit trucks, truck and trailer combinations, city buses, and coaches. Friction losses in tribocontacts were estimated by drawing upon the literature on prevailing contact mechanics and lubrication mechanisms. Coefficients of friction in the tribocontacts were estimated based on available information in the literature for four cases: (1) the average vehicle in use today, (2) a vehicle with today׳s best commercial tribological technology, (3) a vehicle with today׳s most advanced technology based upon recent research and development, and (4) a vehicle with the best futuristic technology forecasted in the next 12 years. The following conclusions were reached:

• •In heavy duty vehicles, 33% of the fuel energy is used to overcome friction in the engine, transmission, tires, auxiliary equipment, and brakes. The parasitic frictional losses, with braking friction excluded, are 26% of the fuel energy. In total, 34% of the fuel energy is used to move the vehicle.
• •Worldwide, 180,000 million liters of fuel was used in 2012 to overcome friction in heavy duty vehicles. This equals 6.5 million TJ/a; hence, reduction in frictional losses can provide significant benefits in fuel economy. A reduction in friction results in a 2.5 times improvement in fuel economy, as exhaust and cooling losses are reduced as well.
• •Globally a single-unit truck uses on average 1500 l of diesel fuel per year to overcome friction losses; a truck and trailer combination, 12,500 l; a city bus, 12,700 l; and a coach, 7100 l.
• •By taking advantage of new technology for friction reduction in heavy duty vehicles, friction losses could be reduced by 14% in the short term (4 to 8 years) and by 37% in the long term (8 to 12 years). In the short term, this would annually equal worldwide savings of 105,000 million euros, 75,000 million liters of diesel fuel, and a CO₂ emission reduction of 200 million tones. In the long term, the annual benefit would be 280,000 million euros, 200,000 million liters of fuel, and a CO₂ emission reduction of 530 million tonnes.
• •Hybridization and electrification are expected to penetrate only certain niches of the heavy-duty vehicle sector. In the case of city buses and delivery trucks, hybridization can cut fuel consumption by 25% to 30%, but there is little to gain in the case of coaches and long-haul trucks. Downsizing the internal combustion engine and using recuperative braking energy can also reduce friction losses.
• •Electrification is best suited for city buses and delivery trucks. The energy used to overcome friction in electric vehicles is estimated to be less than half of that of conventional diesel vehicles.

Potential new remedies to reduce friction in heavy duty vehicles include the use of advanced low-friction coatings and surface texturing technology on sliding, rolling, and reciprocating engine and transmission components, new low-viscosity and low-shear lubricants and additives, and new tire designs that reduce rolling friction.     

A fuzzy-nets tool-breakage detection system for end-milling operations

This paper describes a new approach, the fuzzy-nets system, for monitoring tool breakage in end-milling operations. The fuzzy-nets tool-breakage detection (FNTBD) system has a self-learning capability to generate rule bases and to fine tune the term sets of each linguistic variable to the appropriate level of granularity. A self-learning algorithm for developing the FNTBD system consists of five steps:

1. Divide the input space into fuzzy regions.
2. Generate fuzzy rules from given data pairs through experimentation.
3. Avoid conflicting rules based on top-down or bottom-up methodologies.
4. Develop a combined fuzzy rule base.
5. Determine a mapping system based on the fuzzy rule base.

Learning is accomplished by fine-tuning the parameters in the fuzzy-nets system within the on-line learning capability. Following establishment of the rule base, the performance of the FNTBD system is examined for an end-milling operation. It was observed and verified experimentally that this new FNTBD approach can successfully detect tool breakage in end-milling operations.     

Fretting fatigue strength estimation considering the fretting wear process

In fretting fatigue process the wear of contact surfaces near contact edges occur in accordance with the reciprocal micro-slippages on these contact surfaces. These fretting wear change the contact pressure near the contact edges. To estimate the fretting fatigue strength and life it is indispensable to analyze the accurate contact pressure distributions near the contact edges in each fretting fatigue process.So, in this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage as follows:

W=K×P×S,