HTMat

Gas atomized Ti–48Al–2Cr–2Nb powders have been used as precursor material in order to evaluate additive manufacturing for the production of near-net-shape γ-TiAl specimens to be employed in the field of aero-engines. In particular electron beam melting (EBM) is used to realize a selective densification of metal powder by melting it in a layerwise manner following a CAD design. The microstructure, the residual porosity and the chemical composition of the samples have been investigated both immediately after EBM and after heat treatments.

In present work, SiC multilayer samples for thermal conductivity test in X (along tape casting direction), Y (perpendicular to tape casting direction and thickness direction) and Z (through thickness direction) directions were successively prepared by tape casting and pressureless sintering. Effect of oxidation treatment on the thermal diffusivity and conductivity was investigated. Elongated SiC grains were observed in three directions, especially in Y direction. The specific heat capacity of SiC multilayer increased with temperature, which could be well fitted.

In view of possible application of multilayer SiC as oxidation resistant and self-passivating component of re-usable thermal protection systems for space re-entry vehicles, this material was tested in a re-entry simulation chamber. A multilayer SiC laminate was processed by tape casting and pressure less sintering. Both the as-processed multilayer SiC and a similar passivated material, with a surface silica layer obtained by high temperature oxidation, were investigated. The microstructure and the mechanical features of these two materials were compared before and after the re-entry test.

Silicon carbide multilayer composites containing short carbon fibres (Csf/SiC) were prepared by tape casting and pressureless sintering. The C fibres were dispersed in solvents with dispersant (Triton X-100) firstly and then mixed with the SiC slurry to make green Csf/SiC tapes. Fibres were homogeneously distributed in the tape and tended to align fairly well along the tape casting direction. The addition of short C fibre hindered the shrinkage in the plane containing the fibres as well as the grain growth of SiC during sintering.

The thrust and efficiency of turbine engines are directly related to the operating temperature. In order to increase temperature without damaging the metallic parts of the engine, thermal barrier coatings (TBC) made of thick ceramic layers are currently deposited on the surface of high pressure turbine blades and vanes of aero engines. However, in the hostile engine environment, the TBC is subjected to degradation phenomena that limit its reliability and time of life. Thus, the comprehension of the TBC degradation process in this aggressive environment entails the greatest importance.

Third generation γ-TiAl alloys with a high niobium content, Ti–(47–48)Al–2Cr–8Nb, were processed by electron beam melting (EBM). This near-net-shape additive manufacturing process produces complex parts according to a CAD design. The starting powder is deposited layer by layer on the building table and selectively melted to progressively form the massive part. The EBM parameters such as layer thickness, melting temperature, scanning speed, or building strategy were set up to minimize porosity.

In this work, thermal degradation of commercial polymer mixtures was evaluated as a promising approach to the production of ceramic brake components. Currently, there is a wide interest for the production of cost effective ceramic pads, and carbon based materials are good candidates both in monolithic form and as porous networks to be reinfiltrated with a convenient polymer. The process parameters for obtaining porous glassy carbon, starting from mixtures of commercial polymers, have been evaluated in this work with the goal of producing samples with optimized mechanical properties.

In view of the possible application of SiC-based ceramic laminates as critical components of hydrogen generation plants based on sulfur-iodine or hybrid-sulfur thermochemical cycles, the effect of corrosion on this kind of ceramics has been investigated.

A processing method common to composite ceramics with very different ZrB2/SiC ratios was developed in order to exploit ZrB2–SiC laminates comprising alternate layers with different compositions for thermal protection systems of re-entry vehicles. Ceramic laminates were made using SiC, ZrB2 and composites with a SiC/ZrB2 ratio ranging from 100 vol.-%SiC to 100 vol.-%ZrB2. The preparation was performed by tape casting of a slurry, layer stacking, debinding and pressureless sintering.

Phase transition temperatures of γ-TiAl alloys were successfully identified by Thermo Mechanical Analysis. The derivative curve of the displacement as a function of temperature exhibits well defined peaks at the temperatures 1100 ≤ Teu ≤ 1200 °C and 1250 ≤ Tα ≤ 1350 °C which correspond to the phase transformations (α2 + γ) ↔ α and (α + γ) ↔ α respectively. The well-known effects of Al and Nb content on transition temperatures were studied for validation.