1,721,509 research outputs found
Rapporto sullo sviluppo e caratterizzazione di materiali a base di Carburo di Silicio – Attività 2002- Dottorato in Chimica Industriale
No full textDiverse tipologie di materiali densi a base di carburo di silicio sinterizzato in fase liquida sono stati ottenuti da polveri di SiC (di tipo alfa-SiC, beta-SiC e miscele delle due tipologie) utilizzando come additivi (Y2O3+Al2O3). Sono state utilizzate diverse formulazioni di additivi di formatura e diversi trattamenti termici di sinterizzazione allo scopo di individuare le migliori condizioni di processo. Le caratteristiche dei materiali preparati (densità, microstruttura, fasi cristalline e fasi a bordo grano) sono state misurate e correlate alle caratteristiche e al trattamento delle polveri.
Parallelamente sono state valutate la caratteristiche dei materiali ottenuti da polveri commerciali pronte per l’uso, contenenti additivi sia di formatura che di sinterizzazione. Sono state prese in considerazione due diverse tipologie di polveri:
-una polvere con additivi di sinterizzazione Y2O3 and Al2O3 ;
-due polveri con additivi di sinterizzazione B e C che prevedono un processo di sinterizzazione in fase solida.
Le caratteristiche microstrutturali di questi materiali sono state valutate e confrontate che i materiali sviluppati in laboratorio
Spark plasma sintering and hot pressing of ZrB2–MoSi2 ultra-high-temperature ceramics
No full textZrB2 + 15 vol.% MoSi2 composites were densified by spark plasma sintering and conventional hot pressing at the same sintering conditions, i.e. maximum temperature: 1750 ◦C, applied pressure: 30MPa and heating rate: 100–150 K/min. The densification behaviour was investigated
through the analysis of the hot pressing and spark plasma sintering shrinkage curves. The microstructures were analyzed and compared in order to understand the influence of the two sintering techniques. The following mechanical properties were measured:Vickers hardness, fracture toughness, Young’s modulus and 4-pt bending strength. The main outcome of the present work is that when the ZrB2-based composite was densified either by spark plasma sintering or by hot pressing using the same sintering parameters, no differences were observed in terms of microstructural features and mechanical properties
Influence of nanoparticles on the inhibiting efficiency of organosilane coatings on bronze. Part 1: Electrochemical characterization
No full textThe development of effective and environmentally acceptable corrosion inhibitors for the conservation of bronze and gilded bronze artworks is an important and still open issue. In this field, most studies are focused on the use of benzotriazole or its derivatives but, in spite of their high efficiency in the corrosion protection of pure copper, these compounds are not very effective when applied on bronze, besides being toxic for the environment and the living organisms.
In the recent years the “green” silane coatings aroused great attention as a promising alternatives to conventional chromating. This class of organic coatings showed a good protection against corrosion of many different alloys, depending on the moiety of the silane molecules.
This work is part of a wider project concerning the conservation of bronze and gilded bronzes. It aims at evaluating the protective efficiency of a silane (3-mercapto-propyl-trimethoxy-silane), in the absence and in the presence of some oxide nanoparticles (CeO2, TiO2, CeO2ZrO2 and La2O3), towards the corrosion of an alloy reproducing the composition and microstructure of a Renaissance bronze.
The tests on the silane coatings consisted in monitoring the corrosion process by Electrochemical Impedance Spectroscopy during 20 days exposures to 3.5% NaCl solution and in recording the polarization curves at specific immersion times. The results showed that these surface treatments offer an excellent protective properties if a proper curing time at room temperature is waited. The addition of CeO2, La2O3 and TiO2 nanoparticles permits to short down the curing time, while keeping the coating protectiveness
Corrosion and tribocorrosion behaviour of cermet and cermet/nanoscale multilayer CrN/NbN coatings
No full textThe corrosion and tribocorrosion behaviour of thermally sprayed coatings and thermally sprayed/nanoscale multilayer CrN/NbN coatings deposited on steel specimens has been studied in 3.5% NaCl solutions. The thermally sprayed coatings are bulk HVOF deposits with nominal composition WC-12Co and Cr3C2-37WC-18Me, while the nanoscale multilayer deposit, topping the thermally sprayed ones, is a CrN/NbN coating, obtained by PVD
magnetron sputtering technique. Combined wear-corrosion conditions have been achieved by sliding an alumina antagonist on the lateral surface of coated steel cylinders, during their
exposure to the aggressive solution. Polarization curves recorded after 1h of immersion and polarization resistance values
monitored during 3 days exposures showed that under free corrosion conditions Cr3C2-37WC-18Me always exhibits low corrosion rates and efficiently protects the underlying steel substrate from corrosion. Even on WC-12Co-coated specimens, low corrosion rates are measured at long immersion times. Both cermet coatings suffer severe corrosion attack under tribocorrosion conditions. On the contrary, the duplex nanoscale multilayer CrN/NbN /WC-12Co deposit affords very good corrosion resistance both in the absence and in the presence of wear, within the studied exposure period. SEM observations and EDS analyses have also been carried out to characterize the original powder and coating microstructures and to assess
the damage extent at the end of the corrosion and tribocorrosion tests
The corrosion behaviour of alumina/titanium diboride composites in neutral and acidic chloride solutions
No full textThe development of electroconductive ceramic materials to replace conventional metal parts allows the production of high-performance electronic devices, wear parts, electrodes, high temperature heathers, heat exchangers, and so on.
In particular the addition of 30% TiB2 on Al2O3 matrix can lower its electrical resistivity down 10-3 Ohm.cm and at the same time can increase its hardness strength and fracture toughness.
Since applications require a suitable corrosion resistance, it is important to evaluate the corrosion behaviour of these electroconductive ceramic materials towards common aggressive environments, such neutral or acidic chloride solutions.
In this work, two different composites (50%Al2O3-50%TiB2 and 70%Al2O3-30%TiB2) were exposed to 3.5%NaCl solutions at pH 7 and 3; the corrosion was studied by electrochemical tests and chemical analyses and was compared to the behaviour of monolithic TiB2 materia
Corrosion behaviour of ZrN/Ti ceramic solid solution in fluids simulating biological inflammatory conditions
No full textWhen an implant is inserted into a living tissue, the response of the organism is al-ways associated with an inflammatory process caused by the surgical intervention and/or the foreign materials. Therefore, for the evaluation of the biocompatibility, it is important to investigate the behaviour of materials under such conditions.
In the current study the corrosion behaviour of a ZrN/Ti (ZNT) ceramic solid solution under inflammatory conditions was investigated and compared with CP Ti.
The ZNT ceramic material was prepared by hot pressing and the presence of tita-nium in the solid solution showed some beneficial effect on sinterability and on me-chanical properties, mainly hardness, if compared to monolithic ZrN.
A phosphate buffer saline (PBS) solution (pH 7.4) and a PBS/0.1M H2O2 (pH=4) solution were used to simulate the body fluids in the absence and in the presence of inflammatory conditions respectively. The corrosion behaviour of the materials was monitored by linear polarization resistance measurements and polarization curves for two weeks. After the first week the PBS/H2O2 solution was replaced by fresh PBS solution in order to simulate the normalization process after the acute inflammatory phase. The behaviour of the tested materials under inflammatory conditions was compared with that in the absence of an inflammatory process.
For ZNT solid solution exposed to inflammatory conditions the results showed a bet-ter corrosion behaviour than that observed on CP Ti, as an effect of the formation of a more protective surface oxide layer.
The electrochemical results and the mechanical properties of ZNT ceramic material suggest that it is a promising candidate for biomedical applications where high corrosion and wear resistance are required
Oxidation behaviour of a pressureless sintered HfB2–MoSi2 composite
No full textThe thermal stability of a 80-vol.% HfB2 + 20 vol.% MoSi2 Composite is tested under oxidizing environment. Oxidation tests are carried out in flowing synthetic air in a TG equipment from 1000 to 1400 ◦C with exposure time of 30 h. At temperatures ≥1200 ◦C the silica resulting from oxidation of molybdenum disilicide seals the sample surface, preventing hafnium diboride from fast degradation. Analysis of the kinetics is carried
out through fitting of the thermogravimetric curves. Between 1200 and 1400 ◦C, the kinetic curves deviate from a parabolic behaviour, being more close to a logarithmic–parabolic behaviou
Improvement offered by coprecipitation of sintering additives on ultra-fine SiC materials
No full textA commercial SiC powder is processed through either conventional ultrasonication or coprecipitation of sintering additives, to obtain dense materials. The coprecipitation process involves a change of the surface properties of SiC particles which concerns the formation of amorphous precipitates (see the figure) deriving from the sintering aids and partial dissolution of the surface silica. These factors lead to microstructural refinement and reduction of the intergranular phase amount in dense samples
Sinterizzazione di carburo di silicio in fase liquida
No full textMateriali densi a base di carburo di silicio sinterizzato in fase liquida sono stati ottenuti da due polveri (di tipo α-SiC e β-SiC) utilizzando come additivi (Y2O3+Al2O3) Sono state analizzate le densità e la microstruttura dei materiali, in relazione alle caratteristiche e trattamento delle polveri
High strength and toughness electroconductive SiC-based composites
No full textThe development of silicon carbide (SiC) based composites using MoSi 2 as the secondary electroconductive phase for the matrix was investigated. A mixture of SiC and 10 vol % MoSi2 was hot pressed at 1900°C, without any addition of sintering aids. An improvement of the final; relative density from 60 to 86% was obtained with the addition of 2 wt% of Y2O3. In the reinforced composite containing 30 vol% of MoSi2, the fracture toughness increased of about 27 % in comparison with the reference SiC material. It was observed that the addition of MoSi 2 particles to the Sic matrix caused a gradual decrease of the electrical resistivity of the composites. The hardness of the composites decreased with increasing the MoSi2 content, while the Young's Modulus existed near the SiC reference value
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