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Gil L.E.,National University of Costa Rica | Liscano S.,National University of Costa Rica | Goudeau P.,University of Poitiers | Le Bourhis E.,University of Poitiers | And 3 more authors.
Surface Engineering | Year: 2010

The present study has been carried out to evaluate the corrosion performance of three different (Ti1-xAlx)N/TiN multilayers coatings commercially deposited on WC-Co inserts. Two of these coatings were nanostructured, and all of them presented different Ti/Al atomic ratios varying from 2 : 3 up to 7 : 3 and were deposited by employing a commercial physical vapour deposition cathodic arc process. X-ray diffraction analysis indicated that the grain size for both the TiAlN and TiN layers in the coatings architecture, varied between approximately 9 to 14 nm. The corrosion performance was evaluated in a 3 wt-%NaCl solution using both open circuit potential and potentiodynamic polarisation measurements. Scanning electron microscopy and energy dispersive spectroscopy surface characterisation before and after corrosion testing allowed the determination that all the coatings under study show a corrosion current density of approximately one order of magnitude smaller than the uncoated substrate, indicating thus their beneficial effect from the corrosion protection viewpoint. © 2010 Maney Publishing.


Santana Y.Y.,Central University of Venezuela | La Barbera-Sosa J.G.,Central University of Venezuela | Bencomo A.,Central University of Venezuela | Lesage J.,University of Lille Nord de France | And 5 more authors.
Surface Engineering | Year: 2012

The present investigation has been carried out in order to study the erosion wear behaviour of WC-Co base thermal spray coatings. WC-12Co and WC-10Co-4Cr coatings were deposited by means of high velocity oxygen fuel (HVOF) thermal spraying. The erosion tests were conducted at impact angles of 30 and 90u using SiC particles of ~50 mm in diameter as erodent, at a velocity of 83.4 m s -1. It has been found that the erosion rate for both coated systems was higher when the test was carried out at an angle of 90°. The through-thickness residual stresses of the coatings, as well as the microstructural characterisation, allowed an explanation of the results and the erosion mechanisms in each case. It has been found that, under the experimental conditions carried out in the present study, the WC-10Co-4Cr coating exhibited a higher erosive wear resistance as compared to the WC-12Co coating. © 2012 Institute of Materials, Minerals and Mining.


Puchi-Cabrera E.S.,Central University of Venezuela | Puchi-Cabrera E.S.,Venezuelan National Academy for Engineering and Habitat | Puchi-Cabrera E.S.,University of Lille Nord de France | Staia M.H.,Central University of Venezuela | And 7 more authors.
Surface and Coatings Technology | Year: 2013

Rotating bending fatigue experiments, both in air and in a 3% NaCl solution, have been carried out in order to study the fatigue behavior of a 7075-T6 aluminum alloy coated with a WC-10Co-4Cr cermet by high velocity oxygen fuel (HVOF) thermal spray, without any grit blasting prior to the coating deposition. The results indicate that the presence of the coating gives rise to a significant increase in the fatigue strength of the substrate and therefore, that from the fatigue behavior point of view this coating could be a feasible replacement of electrolytic hard chromium plating in aircraft applications. Such an increase in fatigue and corrosion-fatigue strength is believed to be associated with the intrinsic microstructural characteristics, corrosion resistance and mechanical properties of the coating, its compressive residual stresses and the possible compressive residual stress field induced in the substrate during coating deposition. The fractographic analysis of the specimens indicates that the final fatigue fracture could be due either to the joint action of a large number of cracks, which propagate from the substrate-coating interface, or to a single dominant fatigue crack, depending on the maximum alternating stress applied to the coated system. It is shown that the lack of grit blasting prior to HVOF deposition does not seem to compromise the functionality of the coated system, while it avoids the introduction of further sharp stress concentrators for the nucleation of fatigue cracks. © 2012 Elsevier B.V.


Chicot D.,University of Lille Nord de France | Puchi-Cabrera E.S.,Central University of Venezuela | Puchi-Cabrera E.S.,Venezuelan National Academy for Engineering and Habitat | Decoopman X.,University of Lille Nord de France | And 3 more authors.
Diamond and Related Materials | Year: 2011

Adhesion and hardness of Diamond-Like Carbon films are improved by nitriding of the steel substrate prior to PVD deposition. Since the mechanical properties of the nitrided steel layer are not homogeneous, i.e. a significant hardness decrease is observed in the upper nitrided layer close to the surface, an outer surface layer of ~ 15 μm is removed prior to the film deposition. In the present work, a 316L stainless steel substrate is nitrided in a cyanide-cyanate solution at 570 °C during 3 h. The coated system involved the deposition of a hydrogenated, amorphous carbon (a-C:H) solid lubricant of ~ 2 μm including a chromium carbide interlayer. The comparison between the hardness behavior of the DLC/steel and the DLC/nitrided steel systems reveals the existence of a very important hardness gap, which highlights the benefit of the nitriding treatment prior to coating deposition. In addition, the microhardness-depth profile is determined from a load-depth curve, by applying a simple hardness model. The predicted change in hardness is found to be in a very good agreement with the experimental profile, which allows the hardness determination both in the white layer and in the diffusion zone over ~ 30 μm in total depth. However, only the composite hardness modeling allows the accurate determination of the intrinsic hardness of the film. © 2011 Elsevier B.V. All Rights Reserved.


Puchi-Cabrera E.S.,Central University of Venezuela | Puchi-Cabrera E.S.,Venezuelan National Academy for Engineering and Habitat | Staia M.H.,Central University of Venezuela
Materials Science and Engineering A | Year: 2011

The present investigation has been conducted in order to develop a constitutive description for a low C-Mn steel employed in structural applications, when this material is deformed under hot-working conditions. The set of equations that encompass the constitutive description of the steel are based on a simplified form of the Mechanical Threshold Stress Model. Accordingly, it has been shown that the flow stress of the material can be considered to arise from two different contributions: solid solution and precipitation hardening, which do not evolve in the course of plastic deformation and work-hardening, the only evolving component of the flow stress. The evolution of work-hardening has been introduced in the model by means of the work-hardening law proposed by Estrin and Mecking, in differential form. Also, the temperature and strain rate dependence of the contributions of solid solution and precipitation hardening to the flow stress, as well as that of the saturation stress in the work-hardening law, are described by means of the model earlier advanced by Kocks. The constitutive formulation encompasses seven material parameters which have been determined employing the experimental data corresponding to the strain interval where the material only exhibits positive work-hardening. It is able to predict accurately both work-hardening and work-softening transients when changes in deformation conditions occur. Furthermore, it can be easily implemented in any advanced computational tool employed for the analysis of hot-working processes of this material. © 2010 Elsevier B.V.


Puchi-Cabrera E.S.,Central University of Venezuela | Puchi-Cabrera E.S.,Venezuelan National Academy for Engineering and Habitat | Puchi-Cabrera E.S.,University of Lille Nord de France | Puchi-Cabrera E.S.,Lille Laboratory of Mechanics | And 4 more authors.
Materials Science and Engineering A | Year: 2013

The present study has been conducted in order to formulate a rational description of the flow stress of structural steels, when these are deformed in tension under hot-working conditions. The simplified formulation that has been developed only involves the analysis of the positive work-hardening region of the stress-strain curve and is based on the concepts behind the Mechanical Threshold Stress Model. Accordingly, it is assumed that the flow stress of the material arises from three different contributions: athermal hardening and solid solution, which do not evolve in the course of plastic deformation and work-hardening, the only evolutive component of the flow stress. The evolution of work-hardening is introduced in the formulation by means of the phenomenological work-hardening law earlier proposed by Sah and co-workers, expressed in differential form. The temperature and strain rate dependence of the contributions of solid solution hardening to the flow stress, as well as that of the saturation stress, within the work-hardening law, are described by means of the kinetic model earlier advanced by Kocks. The formulations derived are able to reproduce satisfactorily both the flow stress and work-hardening rate of the material employing a relatively small number of material parameters and therefore, these could be a valuable tool for the analysis of hot-working processes. © 2012 Elsevier B.V.


Puchi-Cabrera E.S.,Central University of Venezuela | Puchi-Cabrera E.S.,Venezuelan National Academy for Engineering and Habitat | Puchi-Cabrera E.S.,University of Lille Nord de France | Staia M.H.,Central University of Venezuela | And 4 more authors.
International Journal of Plasticity | Year: 2014

The flow softening brought about by dynamic recrystallization (DRX) during the plastic deformation of metals and alloys under hot-working conditions is of a great practical significance for the computation of the loads, torques and power consumption required in industrial hot forming operations. The present communication reports the main results of an investigation that was carried out in order to analyze in detail the work-softening transient present on the flow curves of a C-Mn steel deformed in a wide range of temperatures and strain rates. The analysis of the experimental stress-strain curves allowed the description of the temperature and strain rate dependence of four important stress parameters: yield, critical, saturation and steady-state, by means of the Sellars-Tegart-Garofalo model (STG). Also, it has been possible to derive an expression for the time required to achieve 50% recrystallization as a function of the deformation conditions, as well as the computation of the Avrami exponent of the material. All this information has been subsequently employed in the description of the flow stress of the material as a function of deformation conditions. For this purpose, an original constitutive description in differential form, which combines a work-hardening and dynamic recovery term, described by the phenomenological equation earlier advanced by Sah et al. and an additional softening term, which involves the Avrami relationship, is proposed. The evolution equation that has been advanced is independent of strain. Therefore, it is shown that, in principle, it is possible to describe satisfactorily the evolution of the flow stress during transient loading conditions as a consequence of changes in strain rate or deformation temperature, regardless if the material undergoes DRX during such a transient. Contrary to many different models reported in the literature, the approach here proposed is independent of the peak parameters exhibited on the flow curves. © 2013 Elsevier Ltd. All rights reserved.


La Barbera-Sosa J.G.,Central University of Venezuela | Santana Y.Y.,Central University of Venezuela | Villalobos-Gutierrez C.,Central University of Venezuela | Cabello-Sequera S.,Central University of Venezuela | And 3 more authors.
Surface and Coatings Technology | Year: 2010

The present work has been conducted in order to determine the influence of the spray distance, on the corrosion-fatigue behavior of a SAE 1045 steel substrate coated with a Ni base coating deposited by high velocity oxygen fuel (HVOF) thermal spray. The spray distances employed in the present investigation were of 380, 425 and 470. mm. The mechanical properties of the coated systems were evaluated by means of tensile and corrosion-fatigue tests conducted with cylindrical samples. Corrosion-fatigue tests were carried out under rotating bending conditions (R=- 1), at a frequency of 50. Hz and maximum alternating stresses in the range of 250-420. MPa, employing a 3. wt.% NaCl solution. The results indicate that varying the spray distance in the range of 380-470. mm has apparently no significant influence on the corrosion-fatigue behavior of the coated systems. However, the presence of the Ni base coating gives rise to a significant increase in the corrosion-fatigue life of the coated substrate, in comparison with the uncoated steel. Such an increase varies between ~. 90 and 440% in the interval of maximum alternating stresses investigated in the present work. Also, the coated systems exhibited a better corrosion-fatigue performance in comparison with the same steel substrate coated with an electrolytic hard chromium (EHC) deposit. © 2010 Elsevier B.V.


Puchi-Cabrera E.S.,Central University of Venezuela | Puchi-Cabrera E.S.,Venezuelan National Academy for Engineering and Habitat | Puchi-Cabrera E.S.,University of Lille Nord de France | Staia M.H.,Central University of Venezuela | And 4 more authors.
International Journal of Plasticity | Year: 2013

The computation of loads, torques and power consumption of hot rolling processes conducted at industrial scale requires a detailed analysis of the work-hardening rate of the material, as well as a precise description of the changes in flow stress with microstructure and deformation conditions. The present work describes two rational experimental methodologies that can be applied in order to accomplish this analysis, which encompasses the determination of the athermal stress, as well as the temperature and strain rate dependence of both the yield and saturation stresses of the material. The evolution of the flow stress in the course of plastic deformation is determined by means of the numerical integration of the phenomenological exponential-saturation work-hardening law advanced by Sah, J.P.; Richardson, G.; Sellars, C.M.; 1969. Recrystallization during hot deformation of nickel. J. Aust. Inst. Met. 14, 292-297 expressed in differential form. In this way, it is possible to compute the current value of the flow stress in terms of its previous value and to update the changes in deformation temperature and strain rate that occur after each strain interval during the course of plastic deformation, as expected in industrial hot working processes. In the present work, these methodologies are applied to the analysis of the work-hardening transients of a number of stress-strain curves obtained for a C-Mn steel deformed under plane strain compression conditions, in the temperature range of 1123-1373 K, at strain rates in the range of 0.4-24 s-1. The results indicate that both the work-hardening rate and flow stress of the material can be satisfactorily described for most deformation conditions. It is shown that by employing the Sellars-Tegart-Garofalo model and the Zener-Hollomon parameter, in order to account for the temperature and strain rate dependencies of the stress parameters, the accuracy in the description of the experimental flow stress can be improved, but at the expense of an increase in the number of material parameters involved in the analysis. The limitations of employing a single internal state variable for the computation of the work-hardening rate and flow stress have also been discussed. © 2013 Elsevier Ltd. All rights reserved.


Chicot D.,Lille Laboratory of Mechanics | N'Jock M.Y.,Lille Laboratory of Mechanics | Roudet F.,Lille Laboratory of Mechanics | Decoopman X.,Lille Laboratory of Mechanics | And 5 more authors.
International Journal of Mechanical Sciences | Year: 2016

In this work, the total and plastic hardness of a variety of materials were determined from the work-of-indentation. We show that by taking into account the influence of the tip defect, both the total and plastic hardness have the same values and that the hardness-load dependence is considerably reduced, thus explaining that part of the indentation size effect can be related to an incorrect estimation of the tip defect. To consider the influence of the indenter tip defect, we propose a calculation of the impression volume involving the length of the truncated indenter tip. For the residual impression volume, we have demonstrated that it can be simply calculated from the total volume using a proportionality constant equal to the residual to maximum depth ratio. Moreover, the hardness derived from work-of-indentation is found to be equal to the Meyer hardness. Finally, the ratios of Meyer hardness to the reduced modulus and elastic work to total work of indentation are found to be very close to the proportionality factor (π tanα) of Stilwell and Tabor. © 2015 Elsevier Ltd. All rights reserved.

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