TRIBOLOGICAL PROPERTIES AND WEAR MECHANISM OF HARD COATINGS

In the modern technology, tribologically suitable components and devices are important to increase the energy efficiency. It is possible when one can reduce the friction coefficient and wear of sliding components. The economic effectiveness can be achieved by better tribological system and therefore research in tribology is aimed at minimizing the energy losses resulting from friction and wear. In this view, hard coatings deposited by physical vapor deposition (PVD) are adequate solutions for increasing the work efficiency, lifetime of tools and components. The present thesis deals with hard tribological materials γ-TiAl and coatings such as TiAlN, CrN/NbN superlattice, diamond like carbon (DLC) and nanocrystalline diamond nanowire (DNW) films. Various characterization techniques were used to study morphology, microstructure and chemical state of the materials. The thesis describes tribological properties of above mentioned hard coatings sliding against 100Cr6 steel, Al2O3 and SiC balls. It also describes friction and wear based on classified mechanisms and outlines material properties that influence the performance of sliding surfaces. Traditionally wear is associated with friction and wear mechanisms are classified as adhesion, abrasion, erosion, fatigue and oxidational. Mechanical and tribological properties of γ-TiAl alloy, TiAlN, CrN, carbon based coatings of DLC and nanocrystalline DNW were reviewed. The importance of such hard coatings and critical application in machine and industries are highlighted. Moreover, tribological properties and evaluation of wear mechanism is introduced in the respect of microstructure and chemical behavior of the sliding interfaces. Various wear mechanism with different combination of sliding surfaces such as hard coating/soft ball and soft ball/hard coating is reviewed in order to understand the wear mechanism. The fundamentals of some of the characterization techniques used to study the mechanical, tribological, morphological, structural, and chemical properties of the coating and wear track is introduced. Instrumented micro-indentation technique have been used to characterize the mechanical properties namely hardness and elastic modulus of γ-TiAl alloy. The recorded indentation curves and the related energetic properties were analysed in order to compare the Attaf energetic approach and Oliver-Pharr method. Moreover, tribological behavior of γ-TiAl alloy was studied by sliding against 100Cr6 steel, SiC and Al2O3 balls as counterbodies for friction pairs. The friction coefficient and wear rate was found to high when γ-TiAl alloy slides against Al2O3 and SiC balls. However, these values were less while sliding against steel ball. The wear mechanism is explained by the sliding combination of harder/harder system such as SiC/γ-TiAl, and Al2O3/γ-TiAl alloy. However, steel/γ-TiAl alloy acts as softer/harder sliding combination. Tribological behavior of TiAlN coating were studied by sliding against 100Cr6 steel, SiC and Al2O3 as counterbodies for friction pairs. Two distinct types of wear modes such as oxidational and plastic deformation are investigated. It is shown that wear of metal debris tribochemically reacted with moisture available in ambient atmosphere and metal oxide formation which leads oxidational wear in TiAlN/steel sliding pair. In TiAlN/SiC sliding pair, low friction coefficient is measured and this is attributed to the formation of lubricious composite tribofilm. In contrast, TiAlN/Al2O3 pair shows high friction coefficient and wear mechanism is governed by plastic deformation. CrN/NbN superlattice coating sliding against 100Cr6 steel, SiC and Al2O3 ball as counterbodies for friction pairs was investigated. The value of friction coefficient and wear rate was lowest ~0.01 and 2.6×10–7 mm3/Nm, respectively, when coating slides against Al2O3 ball. In contrast, friction coefficient and wear rate is increased while sliding with steel and SiC balls. It is observed that the deviation in friction coefficient is described by mechanical and chemical properties of these balls. In this respect, hardness of Al2O3 and SiC ball is comparable but significant deviation in friction coefficient is observed. This is related to oxidation resistance of balls which is high for Al2O3 compared to SiC as evident by Raman analysis of the wear track. However, steel ball shows oxidational wear mechanism against CrN/NbN superlattice coating. The tribological properties of DLC and nanocrystalline DNW films were investigated. A friction mechanism based on surface chemistry and mechanical properties of sliding interfaces such as DLC/100Cr6 steel, DLC/SiC and DLC/Al2O3 is studied. In DLC film, the high friction coefficient is governed by surface roughness of the sliding interfaces during initial sliding passes. However, for longer sliding cycles, the sliding interfaces get smoothened and magnitude of friction coefficient is reduced. Under these experimental conditions, carbonaceous transferlayer forms on the ball sliding surface. Nanocrystalline DNW films was deposited in N2 enriched microwave plasma enhanced chemical vapor deposition (MPECVD) system. As-deposited DNW film was treated in O2 plasma which resulted in chemical and microstructural modification. The sheath of the nanocrystalline DNW is chemically constituted by amorphous carbon (a-C) and graphite (sp2C-C) like bondings. However, nanowires transformed into ultra- small spherical grains after the O2 plasma treatments. In this condition, a-C and sp2C-C fraction get significantly reduced due to plasma etching. Oxidation and formation of functional groups increases on the surface and inside the wear track. The friction coefficient of O2 plasma treated film showed super low value of ~0.002 with exceeding high wear resistance of 2×10–12mm3/Nm. Such an advance tribological properties is explained by passivation of covalent carbon bonding and transformation of sliding surfaces by van der Waals and hydrogen bondings. High surface energy and the consequent superhydrophilic behavior of film attributed to the formation of an adsorbate layer of above mentioned functional groups which acts as a lubricant

Public Perceptions of Clean Elections in Mexico: An Analysis of the 2000, 2006, and 2012 Elections

This study explores how citizens in a newly democratized country with a legacy of electoral fraud and manipulation evaluate the cleanliness of the elections that have taken place since democratization. I argue that citizens in these contexts are more likely to express confidence in the credibility of elections when their electoral preferences are realized, due to the competitiveness of contemporary elections, but more importantly due to the legacy of electoral malpractice. Using panel data collected during the 2000, 2006, and 2012 Mexican elections, the evidence indicates that support for electoral winners is indeed associated with greater confidence in the cleanliness of election-day proceedings, whereas support for electoral losers is associated with less confidence.Este estudio explora cómo los ciudadanos de un país recientemente democratizado, con antecedentes de fraude electoral y manipulación, evalúan la limpieza de las elecciones desde la democratización. Sostengo que en estos contextos los ciudadanos tienen probabilidades más altas de expresar confianza en la credibilidad de las elecciones cuando sus preferencias electorales se realizan, debido a la competitividad de las elecciones contemporáneas; pero de manera más importante, debido al legado de las malas prácticas electorales. Utilizando datos de panel recopilados durante las elecciones mexicanas de 2000, 2006 y 2012, la evidencia indica que el apoyo a los ganadores electorales se asocia con una mayor confianza en la limpieza de los procedimientos electorales, mientras que el apoyo a los perdedores electorales se asocia con menos confianza

Jurisdiction over a Parent Corporation in Its Subsidiary\u27s State of Incorporation

In this paper we examine how Spanish language marks a political divide within the Latino community. Spanish language use also provides an opportunity for political parties to send slightly different campaign messages. We illustrate this point with reference to California’s Democratic part

Influence of focus offset on the microstructure of an intermetallic gamma-TiAl based alloy produced by electron beam powder bed fusion

It is well established in literature that, when processing intermetallic gamma-TiAl components by electron beam powder bed fusion, a banded microstructure is frequently formed because of an inhomogeneous Al distribution since more pronounced evaporation of Al occurs at the top of the melt pool. This feature is particularly promoted when highly energetic process parameters (high beam currents, slow beam speeds, narrow line offsets) are used. Therefore, an approach already suggested in the literature to reduce the Al loss is to minimize the energy level of the process parameter during production. However, there is a limit to such kind of approach: minimizing the beam current or increasing the beam speed, or increasing the line offset will, at a certain point, results in not being able to achieve a completely dense material and thus some process -induced porosity, the so-called lack-of-fusion defects, starts to occur in the produced parts.In this study, the effect of an additional parameter of the electron beam powder bed fusion process is taken under consideration: the focus offset (FO), i.e. the distance between the focusing plane of the electron beam with respect to the powder bed. The effect of the FO on the residual porosity, microstructure, phase composition, hardness as well as chemical composition is investigated, thus having the possibility to demonstrate that also the FO can affect the Al loss and play a fundamental role in the generation of a homogenous microstructure, contributing to mitigate the appearance of a banded microstructure

Parameters Optimization and Repeatability Study on Low-Weldable Nickel-Based Superalloy René 80 Processed via Laser Powder–Bed Fusion (L-PBF)

This work aims to investigate the processability of René 80 via laser powder–bed fusion (L-PBF). René 80 is a poorly weldable Ni-superalloy, currently processed via investment casting to fabricate turbine blades working at an operating temperature of about 850 °C. The L-PBF parameters optimization aims to increase part integrity and enhance processing repeatability. This part was tackled by creating a complete design of experiments (DOE) in which laser power, scan speed and hatching distance were varied accordingly. Optimizing the abovementioned parameters minimized the crack density and pore area fraction. Hence, five parameter sets leading to a crack density lower than 100 µm/mm2 and a pore fraction between 0.045% and 0.085% were selected. Furthermore, the intra-print repeatability was studied by producing three specimens’ repetitions for each optimal set of parameters in the same build. The porosity value obtained was constant among repetitions, and the crack density (around 75 µm/mm2) had a slight standard deviation. The third step of the research assessed the inter-prints repeatability by producing a replica of the five selected parameter sets in a different build and by comparing the results with those studied previously. According to this latter study, the porosity fraction (ca. 0.06%) was constant in intra- and inter-print conditions. Conversely, crack density was lower than 100 µm/mm2 only in three sets of parameters, regardless of the intra- or inter-build cross-check. Finally, the best parameter set was chosen, emphasizing the average flaw fraction (least possible value) and repeatability. Once the optimal densification of the samples was achieved, the alloy’s microstructural features were also investigated

Effect of Cold Rolling on Microstructural and Mechanical Properties of a Dual-Phase Steel for Automotive Field

A new advanced dual-phase (DP) steel characterized by ferrite and bainite presence in equal fractions has been studied within this paper. The anisotropy change of this steel was assessed as a progressively more severe cold rolling process was introduced. Specifically, tensile tests were used to build a strain-hardening curve, which describes the evolution of this DP steel's mechanical properties as the thinning level increases from 20 to 70% with 10% step increments. As expected, the cold rolling process increases mechanical properties, profoundly altering the material's microstructure, which was assessed in depth using Electron Backscatter Diffraction (EBSD) analysis coupled with the Kernel Average Misorientation (KAM) maps. At the same time, the process strongly modifies the material planar anisotropy. Microstructural and mechanical assessment and the Kocks-Mecking model applied to this steel evidenced that a 50% strain hardening makes the DP steel isotropic. The material retains or resumes anisotropic behavior for a lower or higher degree of deformation. Furthermore, the paper evaluated the forming limit of this DP steel and introduced geometric limitations to testing the thin steel plates' mechanical properties

A new test device for the study of metal wear in conditioned granular soil used in EPB shield tunneling

The wear phenomenon evaluation in EPB shield tunneling machines is not a simple issue, as a large number of parameters are involved, such as soil and tool material properties, soil conditioning and pressure in the bulk chamber. The evaluation of the influence of these parameters and predicting this influence is a complex task and the research has proposed different test procedures and approaches. In this paper a new procedure for testing wear of tools with an innovative concept and design is presented. The experimental results obtained using conventional steel and hard material tools, tested with natural and conditioned soils, are discussed. The outcomes show the feasibility of the proposed procedure and the quality of the measurements that can be obtained using the proposed wear tool shape

Failure mode analysis on compression of lattice structures with internal cooling channels produced by laser powder bed fusion

Conformal cooling coils have been developed during the last decades through the use of additive manufacturing (AM) technologies. The main goal of this study was to analyze how the presence of an internal channel that could act as a conformal cooling coil could affect compressive strength and quasi-elastic gradient of AlSi10Mg lattice structures produced by laser powder bed fusion (LPBF). Three different configurations of samples were tested in compression at 25 °C and 200 °C. The reference structures were body centered cubic (BBC) in the core of the samples with vertical struts along Z (BCCZ) lattices in the outer perimeter, labelled as NC samples. The main novelty consisted in inserting a straight elliptical channel and a 45° elliptical channel inside the BCCZ lattice structures, labelled as SC and 45C samples respectively. All the samples were then tested in as-built (AB) condition, and after two post process heat treatments, commonly used for AlSi10Mg LPBF industrial components, a stress relieving (SR) and a T6 treatment. NC lattice structures AB exhibited an overall fragile fracture and therefore the SC and 45C configuration samples were tested only after thermal treatments. The test at 25 °C showed that all types of samples were characterized by negligible variations in their quasi-elastic gradients and yield strength. On the contrary, the general trend of stress-strain curves was influenced by the presence of the channel and its position. The test at 200 °C showed that NC, SC and 45C samples after SR and T6 treatments exhibited a metal-foam like deformation