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Despite developments and advances in dental materials which allow for greater restorative performance, there are still challenges and questions regarding the formulation of new compositions and chemical reactions of materials used in restorative dentistry. The aim of this study was to assess and compare the mechanical and tribological characteristics of a bioactive resin, a composite resin, and a glass ionomer. Twenty specimens of each material were divided into two groups: one control group (n = 10), not subjected to thermocycling, and one test group (n = 10) submitted to thermocycling. The Vickers microhardness test was carried out and surface roughness was evaluated. The tribological sliding indentation test was chosen. The bioactive resin had the lowest hardness, followed by the composite resin, and the glass ionomer. The bioactive resin also showed greater resistance to fracture. For the tribological test, the wear rate was lower for the bioactive resin, followed by the composite resin, and the glass ionomer. The bioactive resin presented a smooth surface without visible cracks, while the other materials presented a brittle peeling of great portions of material. Thus, the bioactive resin performs better in relation to fracture toughness, wear rate and impact absorption than the composite resin and much better than the glass ionomer. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Understanding friction behavior of human skin is indispensable in order to optimize surfaces and materials in contact with the skin. The coefficient of friction (COF) for different materials contac...

Tungsten carbide (WC) based composites are highly wear resistant, exceptionally durable and used in numerous cases as a variety of machining tools, punch and dies and drill bits. These materials are generally obtained by sintering a tungsten carbide powder and a metal binder, being cobalt the most common binder. The binder fraction and the carbide grain size of powders determine the mechanical resistance of the final product and, consequently, its tribological behaviour. However, in order to impart higher wear and corrosion resistance to the product, other binders may be used. In the present research work, several composites with different binder compositions were prepared, resulting in seven different specimens: WC-Co, WC-Ni, WC-CoNi, WC-CoCr, WC-NiCr, WC-CoNiCr and WC-NiCrMo. The mechanical properties of the different composites were evaluated and correlated with the microstructure and tribological behaviour. Concerning the abrasion characterization, from each test, the wear volume was assessed and the details of the wear surfaces were analyzed by Optical Microscopy, Scanning Electron Microscopy and profilometry. It was found that the presence of chromium in the binder increased the abrasion resistance, comparing to the other composites under study.

The aim of the present research work was to investigate the effectiveness of grooved surface texturing with a rhombic geometry under different lubrication regimes. Tribological investigation under unidirectional sliding was focused on the effect of texturing parameters including pattern area density on the coefficient of friction under different lubrication regimes, achieved by varying sliding speed and lubricant viscosity. Grooved patterns with different textured area densities were produced on steel samples by electrical discharge machining. Results of this investigation showed that under boundary lubrication, textures resist sliding thus resulting in increased friction. The largest improvement of friction reduction was observed under hydrodynamic lubrication, for low-viscosity oil when using the textured disc with 21% pattern area density. The reduction of the coefficient of friction if compared with the untextured surface was of approximately 24%. Examination of the sliding surfaces has not shown any quantifiable wear for the contact conditions studied.

Additively produced Ti6Al4V implants display mechanical properties that are economically infeasible to achieve with conventional subtractive methods. The aim of the present research work was to characterize the tribocorrosion behaviour of the newly produced Ti6Al4V, also known as titanium grade 5, by a selective laser melting (SLM) technique and compare it with another specimen produced by a conventional method. It was found that the tribological properties were of the same order, with the wear rate being k= 6.3 &times, 10&minus, 4 mm3/N&middot, m and k = 8.3 &times, m for respectively, SLM and conventional method. Regarding the friction behaviour, both methods exhibited similar COF in the order of 0.41&ndash, 0.51. However, electrochemically, the potentiodynamic polarization curves presented some differences mainly in the potential range of the passive films and passive current density formed, with the passive current density being lower for the SLM method.

Additive manufactured (AM) parts made in TiAl6V4 alloy are increasingly used in medical prostheses and dental implants, because of its high strength, low weight and excellent biocompatibility. These components work under environmentally assisted cyclic loading, i.e. under corrosion-fatigue, and/or subject to wear conditions. Fatigue performance of additive manufactured alloys is significantly influenced by the porosities, residual stresses, which can reduce its strength when compared with traditional materials. This paper presents the results of a fatigue crack propagation study in titanium TiAl6V4 specimens produced by selective laser melting (SLM) under artificial saliva ambient. Tests were performed using standard 6 mm thick compact specimens (CT) tested at R=0.05 and with frequencies of 1 and 10 Hz. The main objective of the current research work was studying the corrosion effect on the fatigue crack propagation of Selective Laser Melting (SLM) specimens, and to compare the tribocorrosion behaviour of two different specimens: one produced by SLM and the other by the conventional/traditional method. The study concluded that: AM Titanium Ti6Al4V alloy exhibits a moderate effect of saliva ambient on fatigue crack nucleation and on fatigue crack propagation, the wear rate coefficients for SLM and conventional manufactured specimens is of the same order, and the mechanism of abrasive wear is mainly with grooves aligned with the direction of sliding.

The last few decades in the automotive industry have been marked by a heavy concern with the environment, saving energy and reducing material wastage, while aiming to maintain good mechanical properties, essential in the components usage. Additive manufacturing (AM) techniques present themselves as a viable option in the matter, with Laser Metal Deposition (LMD), rising as one of the most promising techniques within this category, capable of producing near-net shape components, with a layer upon layer construction of three-dimensional solid parts from a 3D CAD model, with good mechanical properties and acceptable surface finishing. Laser Metal Deposition is a relatively recent technique, which is made noticeable by the lack of clarification about the influence of several parameters in the final components characteristics, ultimately leading to a scarce availability of the process in the market. This paper aims to clarify and evaluate, how LMD produced parts can suit the automotive industry, by measuring and analysing their behaviour under several mechanical tests. These mechanical tests have specific focus on wear and abrasion behaviour, as well as elastic properties determination, as these are the characteristics that allow a better overview over the expected performance of LMD components for automotive applications.

Understanding the Biotribology involved between contact lenses and the eye, namely the friction between the eyelid and the surface of the contact lens during the blink movement, is a crucial factor in avoiding irritation, discomfort and other eye infections. The aim of the present research work is to investigate the tribological behavior of different soft contact lenses under different contact conditions. Since there is no commercially available equipment, a tribometer has been developed for this purpose, which operates with vibration horizontal movement. The friction assessment method is based on the evaluation of the free damping vibration movement, of the mass - spring system. This technique proved to have adequate sensitivity for the biologically relevant conditions, light normal load, high speed and hydrodynamic lubrication.

Galling or severe adhesive wear is a well-known failure mechanism caused by adhesion between sliding surfaces and is frequently found in sheet metal forming (SMF) operations. In SMF operations of materials such as aluminium, transfer and accumulation of sheet material to the stainless steel forming tool surface happens for certain conditions and can result in severe scratching of produced parts and toll steel damage. The aim of the present research work was to assess if load-scanning test rig can be used to evaluate and compare the galling resistance of different engineered surfaces, namely the material couple aluminium—M2 steel under dry and lubricated sliding conditions, for single- and multi-passing tests with increasing normal loads. In order to determine the galling resistance, a new test method resorting to a Computer Numerical Control (CNC) machine was used. It was found that the galling resistance increases through the use of the proper lubrication, as well as for low applied loads.

In order to understand the effect of surface texturing parameters on the frictional behavior of textured surfaces and to correlate results of different lubrication regimes, Computational Fluid Dynamics (CFD) numerical analysis of the fluid flow was performed for four different textured surface geometries. The aim of the present research paper is to get theoretical background for the frictional behavior of textured surfaces under hydrodynamic lubrication. Since it is unrealistic to make a direct analysis of a real problem that can possess more than several thousand micro-dimples, the purpose is then to investigate the flow in single cells of periodical micro-dimple patterns and to extract useful conclusions for the lubrication’s framework. Among all geometries studied, optimum geometry shapes in terms of hydrodynamic performance were reported. It was found that the best hydrodynamic performance was achieved with the rectangular geometry (lowest shear force).

Knowledge of the tribology of human skin is essential to improve and optimize surfaces and materials in contact with the skin. Besides that, friction between the human skin and textiles is a critical factor in the formation of skin injuries, which are caused if the loads and shear forces are high enough and/or over long periods of time. This factor is of particular importance in bedridden patients, since they are not moving about or are confined to wheelchairs. Decubitus ulcers are one of the most frequently-reported iatrogenic injuries in developed countries. The risk of developing decubitus ulcers can be predicted by using the “Braden Scale for Predicting Pressure Ulcer Risk” that was developed in 1987 and contains six areas of risk (cognitive-perceptual, immobility, inactivity, moisture, nutrition, friction/shear), although there are limitations to the use of such tools. The coefficient of friction of textiles against skin is mainly influenced by: the nature of the textile, skin moisture content and ambient humidity. This study will investigate how skin friction (different anatomical regions) varies, rubbing against different types of contacting materials (i.e., fabrics for medical use) under different contact conditions and their relationship in the formation and prevention of decubitus ulcers.

The aim of the present research work was to investigate the possibilities of designing surface texturing for different lubrication regimes and to evaluate its effectiveness, especially under starved, boundary and mixed lubrication regimes. This was achieved by combining an experimental tribological investigation with a surface-roughness analysis, a 2D FEM simulation and fluid dynamic modelling. The tribological investigation under unidirectional and reciprocating sliding was focused on the effect of the laser-texturing parameters—including the dimple depth and size, the dimple area density and the contact size—on the coefficient of friction under different lubrication regimes, achieved by varying the sliding speed, the normal load and the lubricant viscosity. The results of this investigation show that under starved lubrication conditions the textures resist sliding, resulting in increased friction, as was also indicated by the FEM simulation. Only when a very low dimple density was used could the oil-pocket effect be observed. In boundary lubrication the tribological behaviour of textured surfaces can be related to the surface-roughness parameters (R sk and R ku), with smaller dimples and lower dimple densities leading to reduced levels of friction. The largest gain in terms of friction reduction was observed when approaching full-film lubrication, where fluid dynamic modelling was used to correlate the effect of the dimple depth, size and shape on friction.

Introducing specific textures on a tribological surface can contribute to friction reduction in sliding contacts. Well defined micro-dimples can act as lubricant reservoirs capable of feeding lubricant directly into the contact and initiating micro elasto-hydrodynamic lubrication, as well as traps for wear debris. A lot of experimental work has been already carried out, but it is mostly based on trial and error approach when defining optimum textures for a given application.

Introducing specific textures on a tribological surface can contribute to friction reduction in sliding contacts. In the present paper, a pulsed Nd:YAG laser emitting at 1064 nm, was used against 100Cr6 steel samples in order to produce well-defined surface micro-pores, which can act as lubricant reservoirs, micro-hydrodynamic bearings as well as traps for wear debris. Due to the high flexibility of the laser system, structural features such as shape, size, density and depth can be varied easily by changing the laser parameters. To optimize the parameters of the laser surface texturing process, an investigation was performed using different pulse numbers, various pulse energies and two different modes (single- and multi-mode). The microtextures were characterized with optical microscopy, scanning electron microscopy (SEM) and by topography techniques. The relationship between the laser processing parameters and qualitative and quantitative profile of the micro-pores was studied. Tribological testing of laser textured surfaces was performed in a low frequency–long displacement reciprocating sliding wear tester under boundary lubrication and results compared to un-textured case. Tribological comparison of textured, textured and lapped, and untextured surfaces shows only minimal influence of texturing for contact conditions investigated.

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Growing demands for new composites showing high performance and enlarging fields of application have drawn a great interest on hard metal industry. In this work, new composites of WC stainless steel AISI 304 (SS) binder, containing Fe, Cr and Ni as main elements have been developed and their mechanical behavior was investigated. Powder mixtures of WC-10 wt% SS without and with carbon addition were prepared, test specimens pressed, sintered and characterized in terms of their phase composition, detailed microstructural features, and mechanical properties. In this study, an enlarged evaluation of the mechanical properties was undertaken which included Young's modulus and transverse rupture strength characterization, besides the common hardness and fracture toughness determinations. A better compromise between hardness and toughness, together with similar Young's modulus values, were obtained for these composites when compared with literature values for equivalent industrial hard metal composites with Co binder. However, the transverse rupture strength presents lower average values, which can be very significantly increased by carbon addition. These results were further analyzed, taking into account the phase composition and the microstructural features. (C) 2014 Elsevier B.V. All rights reserved.

The aim of the present research was to investigate the possibility to design contact surfaces with reduced friction using surface roughness and topography analysis. For this purpose, different 100Cr6 plate samples with different surface topography were prepared. Using different grades and combinations of grinding and polishing samples with similar Ra values, but different Rku and Rsk values were prepared. To evaluate influence of roughness parameters on friction and wear, dry and lubricated pin-on-disc tests were carried out under different contact conditions. Test results show that surfaces with high Rku and negative Rsk values results in reduced friction. To investigate the effect of surface topography on surface roughness parameters and consequently on friction, real roughness profiles were virtually altered to achieve virtually textured surfaces. Using NIST SMATS softgauge for calculation of surface roughness parameters, virtually altered roughness profiles were investigated in terms of texture size, shape and spacing, and their influence on surface roughness parameters, especially on skewness and kurtosis. Lower diameter, higher spacing and wedgeshaped dimples were found to reflect in higher Rku and more negative Rsk parameters, which should lead to lower friction.

Increased requirements for improved performance and reliability of mechanical systems dictate the use of new advanced materials and surface engineering techniques, which would increase efficiency and reduce energy losses. Already with a small change in surface roughness or topography lubrication mechanism can be change, thus leading to substantial improvement in tribological behaviour of contact surfaces. One of the ways of altering surface topography, known as surface texturing is by introducing small dimples or channels at the contact surface. Surface texturing is already successfully used in different applications, however, they are mainly based on trial and error approach. The aim of this investigation was to identify surface roughness parameters which could be used as a design parameter when preparing textured surfaces and to determine the effect of texturing parameters on roughness and tribological properties of boundary lubricated surfaces. Results of this investigation show that textured surfaces can be treated as an ordered roughness, with skewness and kurtosis parameters showing good correlation with the change in tribological behaviour of contact surfaces. Negative skewness and high kurtosis values, which can be achieved through wedge-shaped dimples, reduced dimple size and increased dimple spacing, were found to result in lower friction.