In this study, the influence of temperature on the tribological properties of unreinforced or glass-fiber reinforced PA66 in contact with carbon steel under boundary lubrication with grease was studied when considering the temperature dependence of the mechanical properties on the sliding surface of PA66 and the tribochemical reaction to zinc carboxylate additives in grease. XPS and ToF-SIMS analyses revealed the formation of a carboxylate tribofilm on the steel surface and a zinc sulfide reactive film on the PA66 surface, which are related to the tribochemical reaction of the additives present in the grease applied. The formation of the tribofilm contributed to an improvement of the tribological properties, particularly at 80 degrees C.

BACKGROUND:Acrylic resin is employed for drilling bone biomodels. Since drilling causes temperature rise, the mechanical properties of thermoplastic acrylic resin can be altered, consequently affecting drilling properties. However, it is currently unclear how this temperature increase impacts drilling.OBJECTIVE:This study reports the effects of temperature rise on both mechanical and drilling properties through experiments in which acrylic resin is drilled under machining conditions employed in surgical operations.METHODS:Drilling tests were performed using a surgical drill on medical acrylic resin under dry conditions to observe generated cutting chips and measure drilling properties such as torque, drilling time, and temperature rise. Dynamic mechanical analysis measurements were performed to consider temperature effects.RESULTS:According to the morphological classification of the cutting chips, the drilling process is divided into three phases corresponding with the generation of cylindrical helix, waved, and rounded nubby chips respectively. During drilling, the temperature of the chips can exceed the glass transition temperature (100∘C) resulting in decreased viscoelasticity, which is associated with decreased torque.CONCLUSIONS:While drilling acrylic resin under surgical machining conditions, increasing temperature can decrease torque and morphologically change cutting chips due to the decrease in mechanical properties above the glass transition temperature.

In this paper, the evolution of damage when curving a substrate with an alumina layer has been studied. A sample preparation method has been used, so that substrate is free from any stress due to the adhesive insulation during the anodization of one side. The curvature evolution as a function of alumina layers thickness from 50 to 200 μm on aluminum substrates from 0.5 to 1.4 mm thick has been observed. Crack initiations start around film defects and spread transversely and longitudinally to eventually intersect. SEM examination of the anodized layers has shown that the stresses involved by curving create cracks up to 3 μm wide to provide accommodation of substrate-coating assembly. An approach has been adopted to describe the different stages of defects appearance with respect to stresses evolution. Due to internal stress effects, cracks continue to propagate and secondary ones are created in order to release the stored elastic energy in the interface substrate-alumina.

A tribofilm was formed on the Ti-6Al-4V surface when sliding against a DLC coating under fretting condition in air. The tribofilm is mainly composed of the oxidized wear product of Ti-6Al-4V alloy. The mechanical properties of the tribofilm and the Ti-6Al-4V alloy were characterized using nano-indentation and micro-pillar compression techniques. From nano-indentation testing, the hardness of tribofilm (10 +/- 0.4 GPa) was 2.6 times higher than that of Ti-6Al-4V alloy (3.9 +/- 1.5 GPa). The reduced Young's modulus of tribofilm (170 +/- 3 GPa) was around 1.3 times higher than that of Ti-6Al-4V alloy (130 +/- 30 GPa). From micro-pillar compression testing, the compression modulus of tribofilm (around 160 GPa) was 2.5-3.3 times higher than that of Ti-6Al-4V alloy (47-62 GPa). The yield strength of tribofilm (around 6.4 GPa) measured by micro-pillar compression, was 6.7-7.1 times higher than that of Ti-6Al-4V alloy (0.90-0.95 GPa). When measuring the hardness on the tribofilm, micro-pillar compression tests could obtain more accurate values than the nano-indentation tests.

Low friction between DLC coating and Ti-6Al-4V alloy was investigated under fretting conditions on a fretting-wear testing machine. The results indicated that, during the beginning period, the Ti-6Al-4V surface was damaged as a consequence of adhesion (and abrasion), leading to high friction coefficients of around 0.5. With the test ongoing, a tribofilm was formed on the rubbed Ti-6Al-4V surface. This tribofilm was derived from the wear product of Ti-6Al-4V alloy with oxidization. Its nano-hardness and reduced elastic modulus were greater than the Ti-6Al-4V matrix. Meanwhile, structural transformation occurred on the rubbed DLC surface. The tribofilm and transformed carbonaceous layer prevented the Ti-6Al-4V alloy and the DLC surface from direct contact and led to low friction coefficients (below 0.2).

Fretting wear of Ti6Al4V versus DLC coating has been investigated [1,2]. It was shown that a tribofilm is formed during fretting. It is mainly composed of oxidized debris of Ti6Al4V, involving a decrease of wear rate and friction. This study aims at understanding the mechanical properties of the tribofilm responsible of its "lubricating" effect. These mechanical properties have been measured by nanoindentation and by micropillars compression tests. Nanoindentation tests have been performed on the tribofilm, the DLC coating and the titanium alloy. Micropillars have been FIB-machined on the tribofilm and on the titanium substrate. These pillars have been compressed with a diamond flat punch, using an in situ Alemnis indenter, installed in a SEM, in order to extract their mechanical behavior (fig. 1). The results revealed that the tribofilm Young’s modulus, hardness, and Yield stress are higher than the ones from the titanium alloy. Consequently, these high properties could be responsible of the good tribological properties of the tribofilm. Surprisingly, the hardness, calculated from the yield stress obtained by micropillar compression, is higher than the one measured by nanoindentation. It was shown there is no substrate influence on hardness measured using y micropillar compression, which is the case by nanoindentation. Fig.1 Stress–stain (σ–ε) curves measured micro-pillar

Influences of diamond-like carbon (DLC) coatings and roughness on fretting behaviors of Ti–6Al–4V were investigated using a fretting-wear test rig with a cylindrical-on-flat surface contact. The results indicated that, without the DLC coating, the friction coefficient was high; the wear volume was high under large-displacement conditions. Smoother surfaces extended the gross slip regime to smaller-displacement and higher-normal-force conditions. For tests on DLC coating, the coating response wear maps could be divided into three areas: the coating working area (small-displacement and low-normal-force conditions), the coating failure area (large-displacement and high-normal-force conditions), and the transition area. In the coating working area, DLC coatings could protect the substrate with low friction, low wear volume, and mild damage in the coatings; the running condition lied within the gross slip regime. The harder and stiffer DLC A coating presented better fretting resistance than that of DLC B coating. Coatings on smooth surfaces exhibited better tribological performance than on rough surfaces. The increase in the normal force and displacement accelerated the coating failure process.

Research on tungsten carbides, used for drag bits inserts in tunnel boring machine, has been going on through decades for the improvement of mechanical and wear properties. The microstructure, the toughness, the hardness as well as the design of inserts are the main parameters in their functionalities and lifetime. This study emphasizes the behavior of tungsten carbide inserts in impact-sliding conditions. In order to test materials under an impact-sliding motion, a test rig has been developed, with a ball on flat configuration. The ball undergoes a vertical sinusoidal motion derived by an electromagnetic shaker. A piezoelectric force sensor and a laser displacement sensor permit the monitoring of the impact energy. Two vertical foils initiate the sliding when the ball impacts the sample, with a given angle. The impact angle variation dampens or accentuates the impact effect over the sliding component. Three types of tungsten carbides with different chemical compositions, production processes and mechanical properties have been tested: a tungsten carbide used nowadays in tunnel boring machine as inserts in drag bits, and two tungsten carbides under development. Two types of ball materials are considered: steel and silicon carbide. Main damages observed in wear scars are abrasive and adhesive wear combined with fatigue cracking. Damage mechanisms are quantified as a function of impact energy, impact angle, number of cycles and counterbody material, with a view to comparing the different tungsten carbides. Wear volume results show that inserts material currently used in the industry has a wear factor ten times higher than the two others, but is less prone to cracking. A numerical simulation with ABAQUS Explicit gives an insight of the wear process. Compressive and tensile stresses at the contact explain the cracks formation at the top of the wear scar. Tensile stresses may promote cracks formation and their propagation.

To achieve desired porous structure for specific applications, the film growth kinetics and nanostructure can be largely controlled by well monitoring anodization parameters. The pores distribution and dimensions can be significantly affected by the electrolyte nature, the applied voltage and the anodization temperature which seem to be one of the major parameters. In the present work, all specimens anodized with the same starting courant density value of 1.4 A were analyzed. The effects of Low (LT) and room (RT) temperatures on hard anodizing layers were studied qualitatively and quantitatively. At room temperature, the total current is maintained at high levels due to large reduction current that slows the growth of the oxide layer. For low temperature, the current decreases more rapidly against anodizing time which leads to thick alumina layer with high mechanical properties. The best results of anodized layer hardness both, on the surface and at the profile section were observed at 120 min anodizing time; beyond this time, the thickness increases up to 140 µm for 5 h but with lower proprieties due to porosity. DOI: http://dx.doi.org/10.5755/j01.mech.23.6.16309

WC composites are largely used in many engineering and industrial fields such as mining, cutting tools, and underground works. This interest in WC materials is related to their high wear resistance due to their convenient mechanical properties and controllable microstructures. However, the abrasive behavior of WC materials is very complicated because they are formed from several phases and their damage mechanism involves numerous parameters and factors that may be related to the composites themselves and the external environment. Reinforcing particles play a critical role in the achievement of wear-resistant materials. In this study, the impact of the presence of different types of reinforcing particles in the chemical makeup of WC–Ni composites on their resistance to scratch formation is highlighted through experimental abrasion tests by a single diamond tip.

The scratch behaviour of three kind of steels witch one was work-hardened by uniaxial tensile tests until a three different values of deformation. In order to get rid of the indenter geometry effect, the representative deformation and the rheological factor X were used. To compare the results with analytical models, the hardness ratio was presented in function of the attack angle. In the aim of studying the effect of work hardening on the scratch behaviour of C45 steel, numerical simulations using ABAQUS/Explicit code were carried out for two cases of work-hardening. Furthermore, an examination by scanning electronic microscopy and optical interferometer of the ridges were performed. We show that, for steels slightly work-hardened, the shape ratio versus the factor X increase with a logarithmic law before being stabilized, towards a value of about 2.2 for X > 60. Lower values are observed for C45 steel with the annealed state. For attack angles between 0 and 30° the ratio of the normal hardness, Hn, with the indentation hardness, H, of these steels remains close to 1, except for C45 steel in the annealed state where for the weak ones β, Hn/H ~1.6. The ratio of the tangential hardness to the normal hardness is largely greater than 1 and decreases for the steels studied to reach 1,6 – 1,9. The apparent friction coefficient will be decomposed to the sum of three times the representative deformation plus the adhesive friction coefficient. The model developed made it possible to interpret the phenomena of microcutting observed on the scratch of strongly work-hardened C45 steel.

The planetary roller screw mechanism is used in the aeronautics industry for electro-mechanical actuators application. It transforms a rotational movement into a translation movement, and it is designed for heavy loads. The main components are made of martensitic stainless steel, and lubricated with grease. Like most usual rolling mechanisms, smearing and jamming can occur before the theoretical fatigue lifetime, especially in defective lubrication conditions. The actuated load is carried by small contacts between the threads of the screw, the rollers and the nut.The static single contact cari be described as an ellipsoid on flat contact; motion consists of rolling with sliding perpendicular to the rolling direction. A calculation method based on elastic theories (Hertz, Carter, Johnson) has been implemented. It calculates the normal and tangential stresses distributions, generated in the micro-slip and stick zones of the contact area, using several input parameters such as material properties, normal force, and creep ratio.A specific apparatus has been developed to support these calculations and to experimentally study the damage of the contacts in this mechanism. It consists of a freely rolling wheel loaded on a rotating disc with a component of sliding that simulates the roller screw contact. The tribometer inputs are the normal load, the speed, the creep ratio, and the lubrication. The wheel rolling speed and the tangential force generated in the direction perpendicular to rolling are measured.The experiments reveal a quick adhesive wear in dry or bad lubricated conditions, while a low friction coefficient remains if the contact is well lubricated. The influence of the input parameters concurs with the theoretical calculation. The evolution of grease lubrication during duty lifetime and the influence of the tribo-chemical films on this lifetime are also studied. (C) 2015 Elsevier B.V. All rights reserved.

Coatings are used to protect surfaces subject to fretting wear in many contacts under oscillating motions. In all these contacts, the failure of the coating is a concern. Accordingly, the durability and wear properties of coatings are widely investigated for various coatings sliding against different metal counterfaces. Most studies focus only on the coating properties. In contrast, this study is devoted to the research of substrate effect (including porous substrate, materials and shape) on the tribological performance of a given coating by various statistical analyses.In this study, six factors (including the porous substrate, contact configuration, coating position, coating thickness, contact force and displacement amplitude) are selected. The factorial analysis is used to give a quantitative description for the contribution of each factor on the tribological performance of the coating. The porous substrate can effectively improve the coating lifetime under cylinder-on-flat configuration, while it has no significant effect on the friction coefficient Friction coefficient is always dependent on the contact pressure. With the increase of contact pressure, the friction coefficient is decreased. When the coating is only 10 mu m, coating on spherical surface can effectively improve the coating lifetime for non-porous surface. Besides, it proves that the Weibull distribution can be used as an effective way to distinguish the different wear mode. (C) 2015 Elsevier B.V. All rights reserved.

Abstract This paper aims at characterizing the frictional behaviour at the cutting tool-workmaterial interface during the dry machining of a Inconel 718 in its aged state with various coated carbide tools and c-BN tools. A specially designed open tribometer has been used to characterize friction coefficient, heat partition coefficient under extreme conditions corresponding to the ones occurring in cutting. The tribometer provides the evolution of the apparent friction coefficient and of the heat partition coefficient for a large range of sliding velocity and contact pressure. It has been shown that friction coefficient as well as heat partition coefficient decrease with sliding velocity or contact pressure. A threshold effect of the contact pressure has been highlighted. On the contrary, any sensitivity to coatings deposited on carbide has been observed, whereas c-BN leads to very low friction coefficients.

ABSTRACTBurnishing is a cold working surface treatment process in which plastic deformation of surface irregularities occurs by exerting pressure through a very hard and a very smooth roller or ball on a surface to generate a uniform and work‐hardened surface. This treatment occurs generally after the machining process. In this study, a new combined machining/burnishing tool is designed and is fabricated. This tool allows for generating simultaneously the machining (turning) and the burnishing of the cylindrical surface using a turning machine.First, turned surfaces at different conditions, sketches, finishing and half finishing were performed using only the cutting tool. The evolutions of a surface roughness parameter and the technological time relative to every test condition have been investigated. Second, using the combined machining/burnishing tool at coarse conditions, the evolutions of the surface roughness and the technological time have been also investigated. A comparison among the parameters obtained under different machining conditions and those obtained using the combined machining/burnishing tool has been carried out. Moreover, the analyses of the layers obtained on the combined machined/burnished surface have shown that the burnishing process induces compressive residual stresses on the subsurface treated specimens. Copyright © 2013 John Wiley & Sons, Ltd.

Coatings are applied to protect surfaces subjected to fretting wear in many contacts under oscillating motions. Among these contacts, failure of the coating is a major concern. In most studies, durability and wear properties of coatings have been investigated on the coatings sliding against their metal counterfaces. However, few studies have been made on situations such that coating sliding against coating, which is common in industrial applications.In this study, research has been carried out on the fretting wear of the coating on flat or on counterbody (ball or cylinder) as well as the coating on both contacting bodies to quantify effects of coating positions (on the flat, on the ball or cylinder and on both contacting bodies) and effects of contact configuration, normal force and displacement amplitude on coating lifetime and friction coefficient. New statistical methods such as factorial analysis, regression and survival analysis have been introduced. Application of these methods allows us to take into account simultaneously effects of many experimental parameters (and their interactions) for determining friction coefficient and coating lifetime related wear mode of the coating. (C) 2013 Elsevier B.V. All rights reserved.

Fretting wear is a common failure at contact surfaces of tight assemblies in industries, such as those of transport, power transmission and nuclear power station. Using friction reduction coatings is one of the most effective methods to palliate fretting failure. However, in view of numerous available coatings, it is still a tough task to evaluate them and to select the optimum one for a given application. In this paper, based on the investigation of fretting behaviors of three bonded solid lubricant coatings, an initial maximal dissipated energy density (Ed0maxini) approach and a local Archard factor (KA0) approach were suggested to evaluate and predict coating durability. The lifetime of each coating under different values of test parameters can be fitted by one master curve. The master curves of a coating may be used to predict the coating lifetime only by running a new test for few cycles under relevant test conditions. For a given test condition, the durability of coatings can be easily evaluated by comparing their KA0 master curves. Ed0maxini master curves include comprehensive tribological performance, which is helpful for coating selection.

Coatings are more and more used to reduce friction or resist wear of contacting surfaces. However, the selection of the optimal coating from many possibilities is still difficult for a given tribological application. In the present work, a systematic approach is suggested to help the selection of tribological coatings. Firstly, a pre-selection tool based on database is developed to pick out several coatings as candidates for performing further tests. In the pre-selection tool, many industrial tribological coatings are included, and they can be ranked according to the extent that they meet the requirements of a given tribological application. Next, some simple evaluation techniques (nanoindentation test, scratch test, ball cratering test, etc.) are used to assess the properties of the candidate coatings to screen out some poor ones. Then, the remaining coatings are tested under the relevant conditions to investigate their tribological behaviors. According to the test results, a dissipated energy density approach can be easily used to evaluate and compare their tribological performance, because the relationship between coating lifetime and dissipated energy density can be fitted as one master curve whatever the values of test parameters. Finally, in view of non-tribological and non-functional requirements, the coatings are synthetically compared using a flexible polar diagram, and a weight point method can assist the comparison.