Powder Metallurgy (PM) is a net- shape and cost effective technology used for the production of steel parts having good mechanical properties and geometrical precision. In the conventional press and sinter process, the voids among the powder particles cannot be completely eliminated, and the as sintered microstructure contains a certain amount of residual porosity. Mechanical properties are consequently lower than those of the corresponding wrought steels [1]. In particular, the fatigue resistance is significantly affected by porosity; crack tends to nucleate in correspondence of clusters of pores, and to propagate along the network of interconnected pores [2, 3]. Fatigue resistance can be improved on increasing the density, reducing pore size and pore clustering and enlarging the sintered ligaments between pore, or, similarly to wrought steels, by thermochemical (carburizing and nitriding) or mechanical treatments (shot peening). Carburizing consists in a surface carbon enrichment, which gradually decreases towards the core. After quenching high carbon martensite is formed at the surface, characterized by high hardness and a compressive residual stresses suitable for wear and fatigue resistance. Low pressure carburizing is a variant of the conventional gas carburizing performed under sub-atmospheric pressure with pressurized gas quenching. It is quite attractive for carburized PM sintered steels, for two main reasons. 1. Porosity increases the surface exchange area, enhancing the risk of oxidation mainly in Cr and Cr-Mn steels. Low pressure carburizing uses propane or acetylene, as carburizing gas, which does not contain oxidizing agents. 2. Quenching oil remains entrapped in the open porosity, and has to be eliminated. The possibility to combine low pressure carburizing with gas quenching results in clean parts as well as lower distortion. However, the combination between the very high carburizing potential of LPC and the large surface area of porous steels results in overcarburizing, with the precipitation of grain boundary carbides in Cr steels, and the formation of retained austenite in the case in Cr free ones [4, 5]. This problem can be solved by either increasing density, to close the residual porosity, or rolling and shot peening, to eliminate the surface porosity. Nitriding is based on the nitrogen enrichment of the surface layers of steel. On the base of nitrogen content the surface microstructure can be divided in two zones: the compound and the diffusion layer. The former is in principle a ceramic layer, whilst the latter consists in the base matrix hardened by solid solution and by the precipitation of nitrides. The nitride precipitation induces a compressive residual stress field which offers a resistance to the nucleation and propagation of the fatigue crack, improving the fatigue resistance. In order to obtain a hardened and deep diffusion layer the steel has to contain alloying elements with a high affinity for nitrogen, as chromium and molybdenum. Nickel and manganese have a negligible interaction with nitrogen. Among the different nitriding processes, plasma nitriding is recommended for sintered steel. Plasma nitriding is less sensitive to porosity than gas nitriding due to the particular mechanism of nitrogen diffusion (volume diffusion) which allows a uniform diffusion front on the steel surface and a homogeneous nitrogen distribution [6, 7]. Therefore, a preliminary surface densification is not necessary. Shot peening is a flexible and cost effective solution to improve the fatigue performances of mechanical parts, as gears and springs, thanks to the compressive residual stress generated below the surface and the surface work hardening. The improvement in fatigue resistance is more effective if shot peening is applied on case hardened steels, because of the more stability of the compressive residual stresses. Since the fatigue strength of sintered steels strongly depends on the material density, shot peening is a useful technique to improve such property, owing to the densification of the surface layer [8, 9]. The fatigue cracks nucleates beneath this layer and since it cannot propagate in a compression field, it moves towards the core. This PhD thesis is part of the an international research project, “Höganäs Chair project- fourth round†, financed by Höganäs AB, world leader in the production of ferrous powders, involving four research institutions: Trento University, Technique University of Wien, Carlos III University of Madrid and Slovak Academy of Science, Institute for Materials Research, Kosice. The aim of the project is to carry out a cooperative study to design highly performing structural steels by the conventional Powd

This PhD work mainly focus on the synthesis and characterization of calcium phosphate powders for plasma spray coating. The preparation of high temperature phase stabilized βTCP and HA/βTCP powders for plasma spray coating applications has been the topic of investigation. Nowadays plasma sprayed coatings are widely used for biomedical applications especially in the dental and orthopaedic implantation field. Previously Ti based alloys were widely used for the orthopaedic and dental implant applications because of its high corrosion and good biocompatibility. Due to the limited osteoconductivity edges of Ti implants with fibrous tissues delays the healing time. To overcome these limitations different types of surface modification processes are employed on the surface of Ti. The coating of HA is a widely used surface modification technique due to its excellent biological properties. HA is a well employed bone graft material due to its similarity with human hard tissues. The plasma spraying of HA on the Ti surface is the most widely used technique mainly due to its process simplicity, low cost and bulk production. The present research focuses on the modification of HA coatings for the improvement of bio-degradation properties of HA. HA/βTCP composite powders are used to overcome the poor biodegradation properties of HA. The issue related to the use of βTCP is the phase transformation (β to α) at high temperature. To overcome this phase transformation, the βTCP powder was doped with MgO. The high temperature phase stabilized MgO doped βTCP and HA/βTCP powders were synthesized by solid state method and granulated using spray granulation. The properties of the granulated powders (100-150μm) were analysed with XRD, FT-IR, SEM, flowabilty, density etc. and are used in plasma spray coating process. The produced coatings were subjected to the thermal treatment and βTCP and HA/βTCP plasma sprayed coatings are obtained. The successively produced coatings were characterized, and the invitro properties like solubility and bioactivity behaviours were studied.

Hydrothermal carbonization (in acronym, HTC) is a thermochemical conversion process through which it is possible to directly transform wet organic substrates into a carbonaceous material, referred as hydrochar. Hydrochar has chemical and physical characteristics that make it similar to fossil peats and lignite. Depending on the process conditions, mostly temperature and residence time, this material can be enriched in its carbon content, modifying its structure and providing it interesting characteristics that make it possible to be used for several applications, such as for energy production, as a soil conditioner and improver, for carbon dioxide sorption and sequestration, and some others reported in literature. HTC is a different process, if compared to other common thermochemical processes, such as pyrolysis, torrefaction, gasification, etc., because it works in wet conditions (humidity content higher than 60%). As a matter of fact, biomass is transformed into hydrochar because of the properties of hot pressurized water, that acts both as a reactant and as a catalyst. The HTC process has been studied from many years, although at present not all the chemical reactions that occur during the process are completely known. Moreover, the application of this quite new process to different substrates can bring to different results. Even though HTC can be applied to any kind of organic material (of both animal and vegetable derivation), the possible uses of hydrochar can strongly be influenced by the characteristics of the feedstock. This, for example, can be due to legislative constraints. In Chapter 1, an overview of the existing literature is presented. To get insights on this process, a small bench scale batch reactor has been designed and built at the Department of Civil, Environmental and Mechanical engineering of the University of Trento, Italy. This reactor has been tested, prior to be used with real substrates. In Chapter 2 the reactor and the preliminary tests done are described. In this work, the HTC process applied to three different substrates have been studied: grape marc, the EWC 19.05.03 residue and the EWC 19.12.12 residue. In Chapter 3 the three raw substrates are described. Grape marc is produced by the winery industries or by distilleries. This feedstock is composed by woody seeds and holocellulosic skins and it presents an average humidity content of about 60%. At present, it is used for the production of animal food or it is landfilled. In this case, the application of HTC can be an interesting alternative to these end uses because, through this process, grape marc can be recovered, for example, for energy production. The hydrochar produced from this feedstock could be even used as a soil conditioner. In Chapter 4 several analyses on the hydrochar, on the process water and on the gaseous phase obtained during the carbonization tests are presented. The EWC 19.05.03 residue is a by-product of the composting treatment applied to the organic fraction of municipal solid waste (MSW). In collaboration with Contarina S.p.A., a company that collects and treats MSW in the province of Treviso, in the North-East of Italy, this by-product was carbonized and tested both as a soil conditioner and for energy production. Results of the analyses on the solid, liquid and gaseous phases produced by the HTC process are reported in Chapter 5. The EWC 19.12.12 residue is a by-product of the refuse derived fuel (RDF) production, from the residual fraction of the MSW. This substrate was provided by Contarina S.p.A. and preliminary tests on the exploitability of the hydrochar for energy production are reported in Chapter 6, together with analyses on both the liquid and gaseous phases. A rigorous energy balance has been proposed in Chapter 8, based on the experimental data obtained for grape seeds. In this chapter, all the hypotheses and the assumptions taken to evaluate the enthalpy of the HTC reaction at different process conditions (namely, three different temperatures and three residence times) are described. In Chapter 8 a kinetic model is proposed, based on a two-step reaction mechanism. The activation energy and pre-exponential factor of the various degradation reactions were determined by means of least square optimization versus the experimental data of grape marc. A thermo fluid model is even proposed in this chapter. The model integrates mass, momentum and heat equations within the reactor domain by means of the finite volumes method (f.v.m.) approach. Convective and radiative exchange between the reactor and the fluid within the reactor have been implemented in the f.v.m. model. Under two strong assumptions (mono-component and mono-phase fluid, which fulfils the reactor), it was possible to estimate the behaviour of an equivalent fluid (eq_fluid), in terms of thermal properties of the fluid (thermal capacity, thermal conductivity and thermal diffusivity). Moreover, a simplified dynamic analytic model is also presented – based on lumped capacitance method – in order to simulate the thermal behaviour of the system, using the actual temperature profile imposed by the reactor external heater. A resistance-capacitance network was used to describe the system. Finally, the Henry’s law has been applied to assess the amount of gas really produced during the HTC process. In Chapter 9, the main conclusions of this work are reported.