1,721,044 research outputs found
Acciai sottoposti a trattamenti duplex; resistenza agli impatti ciclici e alla fatica
Full text linkL’impiego dei rivestimenti PVD interessa sempre un maggior numero di componenti in acciaio, non solo per garantirne un miglioramento nell’affidabilità in termini di resistenza ad usura, ma anche per assicurare miglioramenti in termini di resistenza locale ai carichi applicati, nonché incrementi di resistenza in presenza di sollecitazioni cicliche. In tale ottica si sono di recente sviluppati trattamenti “Duplex” che uniscono i vantaggi dei più tradizionali trattamenti termochimici di indurimento superficiale alla deposizione di film sottili in materiale ceramico PVD. Nel presente lavoro verranno illustrati i risultati ottenuti con prove di impatto ciclico (Impact test) su campioni di acciaio da bonifica in entrambe le condizioni bonificato o nitrurato, rivestiti con la metodica PVD. I risultati ottenuti in termini di tipo ed entità del danneggiamento hanno fornito un ulteriore parametro di valutazione in aggiunta al limite di fatica, ricavato sperimentalmente da prove di flessione rotante, in grado di suggerire l’impiego delle diverse combinazioni substrato/rivestimento in funzione delle richieste dettate dall’utilizzo. La caratterizzazione è stata completata, inoltre, da analisi microstrutturali (SEM) e di stato tensionale residuo (Rx)
Resistenza all’impatto e alla fatica al variare del trattamento termico e/o del rivestimento superficiale
No full textThermoelectric Materials and Applications: A Review
No full textSolid-state energy conversion has been established as one of the most promising solutions to address the issues related to conventional energy generation. Thermoelectric materials allow direct energy conversion without moving parts and being deprived of greenhouse gases emission, employing lightweight and quiet devices. Current applications, main thermoelectric material classes, and manufacturing methods are the topics of this work; the discussion revolves around the crucial need for highly performing materials in the mid-temperature range, and around the development of more scalable fabrication technologies. The different manufacturing methods for thermoelectric bulk materials and films are also discussed. Small-scale technologies are generating increasing interest in research; the high potential of aerosol jet printing is highlighted, stressing the many advantages of this technology. A promising approach to scale the production of miniaturized thermoelectric devices that combines high energy ball milling and aerosol jet printing is proposed in the conclusion
Innovative coating for Electroplated Hard Chromium replacement in severe environments
No full textThe aim of this paper is the study and the characterization of new solutions for the replacement of Electroplated Hard Chromium (EHC) coatings in severe environments. Starting from the failure analysis of hard chromium at high temperature and subjected to severe wear conditions, the phenomena and mechanisms leading to coating failure were investigated and understood. From this preliminary analysis the coating main properties were identified and innovative solutions were considered and studied to check their suitability for the application.Finally, the characterization of the proposed solution was compared to the properties and the failure mechanism of similar type of coatings used in the same environmental conditions.The innovative solutions considered in this paper are electroplated double layer nickel chromium and ceramic coatings obtained by thermal spraying with different techniques, specifically a tungsten carbide in cobalt or cobalt chromium matrix by High Velocity Oxygen Fuel ( HVOF) and High Velocity Air Fuel techniques (HVAF)
Sintering behaviour of 3D-printed 18K 5N gold alloy by binder jetting: a preliminary study
Full text linkBinder jetting is a versatile additive manufacturing technique suitable to produce alloys that are difficult to obtain by powder bed fusion techniques, such as precious metals, due to their high reflectivity and thermal conductivity. In this study, a 18K 5N gold alloy powder was employed in the printing process. Different heat treatments and densification processes were employed to achieve final-stage sintering and remove residual porosity, whilst controlling the evolution of copper oxides by reduction with hydrogen and graphite. Powder, green and sintered samples were characterised at the microstructural level by X-ray diffraction, microscopy and energy dispersive X-ray spectroscopy to assess phase transitions and secondary-phase formation. Oxide-free components with a final relative density above 90% were achieved by densification at 830 °C combined with carbon- and CO-induced reduction of tenorite and cuprite. The optimal manufacturing route was chosen to produce a bezel, as a case study for the adoption of this technique in the jewellery industry
Densification behaviour of pure copper processed through cold pressing and binder jetting under different atmospheres
Full text linkPurpose: Binder jetting is a promising route to produce complex copper components for electronic/thermal applications. This paper aims to lay a framework for determining the effects of sintering parameters on the final microstructure of copper parts fabricated through binder jetting. Design/methodology/approach: The knowledge gained from well-established powder metallurgy processes was leveraged to study the densification behaviour of a fine high-purity copper powder (D50 of 3.4 μm) processed via binder jetting, by performing dilatometry and microstructural characterization. The effects of sintering parameters on densification of samples obtained with a commercial water-based binder were also explored. Findings: Sintering started at lower temperature in cold-pressed (∼680 °C) than in binder jetted parts (∼900 °C), because the strain energy introduced by powder compression reduces the sintering activation energy. Vacuum sintering promoted pore closure, resulting in greater and more uniform densification than sintering in argon, as argon pressure stabilizes the residual porosity. About 6.9% residual porosity was obtained with air sintering in the presence of graphite, promoting solid-state diffusion by copper oxide reduction. Originality/value: This paper reports the first systematic characterization of the thermal events occurring during solid-state sintering of high-purity copper under different atmospheres. The results can be used to optimize the sintering parameters for the manufacturing of complex copper components through binder jetting
Microstructure and mechanical behavior of hot-work tool steels processed by Selective Laser Melting
No full textThe present study is aimed at identifying and testing high-strength alloys for tooling applications featuring suitable processability for laser-based additive manufacturing technologies. The microstructure and mechanical properties of the H11 hot-work tool steel and a leaner version of the same alloy (L-H11) processed by Selective Laser Melting were assessed as a function of specific microstructural conditions obtained by performing different heat treatments. Tempering was performed on quenched alloys or simply from as built material. The rapidly solidified microstructures revealed able to respond directly to precipitation hardening treatment without performing any prior solution annealing. The microstructure of the as-built alloys revealed characterized by α-Fe dendritic cells decorated at boundaries by C-rich γ-Fe regions. Air quenching was responsible for the transformation of the solidification cells into lath martensitic structures and for the formation of the M3C phase, which transformed into more complex carbide species on tempering. The hardness of quenched and tempered H11 steel is similar to that obtained by processing the alloy with conventional routes, and the final hardness gap between the two SLM processed H11 and L-H11 alloys treated according to optimal tempered condition was limited to 62 HV
Carburizing and nitriding as surface pre-treatment of PVD coating for gears application
No full textAmong the different treatments that can be carried out to locally improve the mechanical behaviour of gears a combination of case hardening followed by PVD coatings (duplex treatment) seems to give promising results in terms of surface hardness, residual stress profile and fatigue resistance. In particular considering the carburizing and the nitriding treatments they can be both aimed, in the same way than the surface coatings, to introduce a different mechanical behaviour between surface and core in order to improve life, reliability and load capacity of the treated component. This is fundamental for gears whose damage is mainly related to contact fatigue, fatigue at the tooth root and pitting on the tooth flank [1-3]. The need of optimising the surface material in order to delay the progressive deterioration of the components due to wear, fatigue or contact fatigue mechanisms, often worsened by the presence of hostile environments, explains the increasing attention on different coating technologies [5-7], In particular, considering the PVD coatings, chemical composition of the surface deposited film, coating thickness, hardness, adhesion with the substrate material and plastic deformation of the substrate material have an important influence on the damage mechanism affecting the coated component. Although hard PVD coatings are well known for improving friction and resistance to wear and corrosion, their tribological performance is often limited by elastic and plastic deformation of the substrate, which can allow to coating failures [12]. The emergence of the duplex treatments, consisting in the sequential application of two o more established surface technologies, has represented a novel approach to the achievement of enhancing coating properties. Duplex treatments, comprising a nitriding treatment followed by the deposition of a hard PVD coating, have been proven to be successful in increasing wear, thermal fatigue and corrosion resistance and the load carrying capability of different steel substrates [13-16]. By increasing the hardness of the substrate, for instance using a nitriding case, often provides a suitable load support for PVD coatings so that superior wear resistance can be achieved. The high values of hardness related to the thermochemical treatment, further enhanced by the introduction of the ceramic coating characterized by a strong difference in coefficient of thermal expansion with respect to the substrate material, affects the surface level of compression residual stress data [21-23], Therefore the residual stress gradient must be evaluated when a prediction of the gear life is requested: in fact the residual stress distribution affecting the nucleation of the fatigue cracks is a factor able to control the gear performance. Starting from such considerations, this work is focused on the microstructural (fig.2, fig. 4) and mechanical characterization (nanohardness and fatigue behaviour) of a CrN coating, about 5 μm thick, deposed by PVD technique on two different steels: a carburizing 16MnCrS5 steel grade and a nitriding 42CrMo4 steel grade (Table I). CrN films were deposited by means of the standard cathodic. arc using an industrial devices. Before coating the fatigue specimens (Fig. 1) were polished with a 3 μm diamond suspension and then ultrasonically cleaned. On the basis of published works [11] it is known that, in the case of nitrided substrates, the adhesion with the PVD coating is enhanced by the presence of Feα(N) structure while ε-Fe2-3N or γprime;-Fe 4N ones are detrimental. For such a reason a NITREG treatment was executed on the 42NiCrMo4 steel grade with the purpose of producing a low white layer, further reduced, before the coating deposition step, by means of a mechanical samples polishing targeted to remove the superficial brittle and porous layers. A short ion cleaning executed with Ar was carried out before the beginning of the coating deposition phase. The steel temperature was kept constant at 180°C with an initial peak of 210°C acting for about 2 minutes, independently from the type of substrate considered. Microhardness profiles were measured both on uncoated and on coated samples in order to determine both the thickness of the carburized and nitrided layers and the effect of the thin film deposition process (fig. 3). The coating nanohardness data were also measured by the depth sensing technique using a Fisherscope H100 nanoindenter operating by a computer controlled stress limited device and equipped with a Vickers indenter. X-ray diffractometry (XRD) was used to identify the chemical coating composition (fig. 2) and to measure the residual stresses induced from the sample's process route including the coating step. XRD with Bragg Brentano geometry were performed with a Philips PW 1830 instrument with a goniometer PW 3020 and a control unit Philips PW 3710 (Cu K α radiation, scan rate 1° /min). Surface residual stresses were detected using Cu Kα radiation by means of a Italstructure Stress X3000 diffractometer. The stresses (-120±25 MPa after carburizing; -580±40 MPa after nitriding; -1870±87 MPa after carburizing + PVD and -2350±114 MPa after nitriding + PVD) were calculated using the sinj2 method and adapting the elastic modulus value obtained by nanoindentation measurements and assuming a Poisson ratio of 0.2, value usually taken as a reference when ceramic CrN or Cr(C,N) thin films are considered. Using a rotating bending machine fatigue tests were carried out both on case hardened samples and nitrided plus PVD coated specimens (fig. 1). Experiments were executed at room temperature, in air, at a test frequency of 33 Hz using a sinusoidal load wave form and a load ratio (minimum to maximum load) of R=0. The stress level at which specimens can run without occurrence of failure after 3 · 106 stress cycles was chosen as the fatigue limit. Results of the fatigue tests were analysed according to the stair-case up and down method (Table II). The presence of the PVD film is responsible for a light increase in the fatigue resistance both for the carburized samples and for the nitrided ones. Fatigue nucleation sites resulted affected from the presence of PVD coating only in the case of carburized substrate: the high residual stress level characterizing the ceramic coating excludes the surface as nucleation zone and moves it at the interface with the steel material (fig. 5). No change in the nucleation areas were observed in the nitrided specimens or in the nitrided and coated samples (fig. 6) where the weak points resulted the non metallic inclusions inside the substrate material
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