This paper considers the deposition of a commercial steel powder with a chemical composition that allows the coating to obtain an amorphous structure using thermal spray techniques. The processes used are characterized by high cooling speeds of the particles after the impact upon the substrate. The powders were sprayed with two different processes: cold gas spray (CGS) and high velocity oxyfuel (HVOF). A comparison between the samples obtained reveals that only the CGS coatings are completely amorphous; the HVOF samples exhibit nanocrystalline phases, detected with XRD analysis and SEM micrographs. Furthermore, the CGS coatings are more compact and show lower hardness with a comparable Young’s modulus. A hypothesis is that the formation of the amorphous structure is related to plastic deformation at impact (due to the high energy of the particles), rather than to the temperature; the mechanism could resemble that of a severe plastic deformation process. Additional thermal treatments and mechanical tests are in progress to investigate the toughness and other mechanical properties of the coatings.

Wear-resistant cobalt–based alloy (Stellite 12) coatings deposited by plasma transferred arc (PTA), commonly used to protect critical mechanical components in harsh environments, were modified by addition of hard ceramic particles (TiC) and solid lubricant compounds (MoS 2 and CaF 2 ) to improve the overall tribological performance. In this preliminary study, microstructural, microhardness and tribological analyses were carried out to assess: a) the feasibility of PTA deposition of thermally sensitive phases characterised by very low density; b) the effect of the addition of a mixture of soft and hard phases on the coating hardness; c) the effect of the modified composition in terms of wear resistance; d) the effect of the addition in terms of lubrication (friction coefficient and produced heat). Results showed that: a) an appropriate pre-consolidation of feedstock materials can be effective in preserving the heat-sensitive phases within the microstructure of PTA deposits; b) the addition of a total amount of 5% wt. of solid lubricants and reinforcing carbides produced a limited decrease in the coating hardness (about 13%) and an evident improvement in terms of friction coefficient but, on the other hand, a remarkable reduction (about 30%) in wear resistance. Further investigation will be addressed to optimize the composition of modified feedstock to counteract the softening effect of lubricant phases without depressing the self-lubrication behaviour.

Coatings containing up to 65 volume % of silicon carbide were deposited by plasma spray. Potential applications can be found in the protection of CMC (Ceramic Matrix Composite) against wear and high temperature oxidation. It is well known that SiC can not be deposited by thermal spray because it decomposes before melting. To face this problem, a mixture of SiC and ZrB 2 was deposited, since those two compounds form an eutectic phase, at a temperature lower to the one of SiC decomposition. Coatings microstructure was characterised by XRD, SEM, and EDS, confirming the presence of SiC in the deposited layer and the formation of the eutectic phase during spraying. Samples of the coatings were exposed in air at high temperature, in the range between 1373 and 1873 K. The oxide scale was investigated by means of SEM and EDS. It was constituted by a SiO 2 layer, which includes islands of ZrO 2 . Test results showed the good potentiality of the material investigated to be used as a protection against the high temperature oxidation.

In this work the high temperature mechanical properties of UHTC coatings deposited by plasma spraying have been investigated; particularly the stress-strain relationship of ZrB2 based thick films has been evaluated by means of 4-point bending tests up to 1500 °C in air. Results show that at each investigated temperature (500, 1000, 1500 °C) Modulus of Rupture (MOR) values are higher than the ones obtained at room temperature; moreover at 1500°C the UHTC coatings exhibit a marked plastic behaviour, maintaining a flexural strength 25 % higher compared to RT tested samples. The coefficient of linear thermal expansion (CTE) has been evaluated up to 1500 °C: obtained data are of primary importance for substrate selection, interface design and to analyze the thermo-mechanical behaviour of coating-substrate coupled system. Finally SEM-EDS analyses have been carried out on as sprayed and tested materials in order to understand the mechanisms of reinforcement activated by high temperature exposure and to identify the microstructural modifications induced by the combination of mechanical loads and temperature in an oxidizing environment.

In this study, plasma sprayed ceramic coatings with different amounts of SiC and MoSi 2 particles dispersed in a ZrB 2 matrix were produced and tested. The results show that MoSi 2 addition in ZrB 2 -SiC samples improves high-temperature oxidation resistance without affecting mechanical properties. Demonstrative components were manufactured and tested in simulated operating conditions by means of a plasma wind tunnel. Preliminary results indicate that thermal sprayed ceramics are well suited for use in space re-entry vehicles.