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.

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.

Thermal protection systems represent the key issue for the successful re-entry of a space vehicle. Future concepts for space launchers foresee sharp aerodynamic profiles as conventional aircrafts, which offers several advantages with respect to current blunt shapes. As a drawback, aerodynamic heat flux increases dramatically and state of art hot structures materials cannot withstand them. UHTC (Ultra High Temperature Ceramics) materials are very promising candidate materials for such applications. An innovative, proprietary methodology was developed to produce, by plasma spraying deposition, a ceramic composite containing SiC particles (25 wt %) dispersed in a ZrB2 matrix. With such a technique both coatings and self standing parts were fabricated. The present paper reports the results of a development activity aiming at testing plasma sprayed real scale components in simulated operative conditions by means of a Plasma Wind Tunnel. The results of the tests showed that plasma sprayed UHTC materials can withstand very high heat flux conditions in an oxidizing environment. Abstract only; no full-text paper available.

The present work aimed at demonstrating the effectiveness of graphite containing plasma sprayed coatings to reduce friction coefficient in dry or starving lubrication conditions. Operative, environmental and economic considerations were important driving factors. Graphite particles were mechanical mixed with a cermet powder containing a fine dispersion of chromium carbide particles in a nickel-chrome metallic matrix. The coatings material was deposited by Atmospheric Plasma Spray onto steel substrates. The microstructure of the obtained coating was characterised by XRD and SEM/EDS. A strip draw apparatus, simulating the stamping process of steel sheets, was used to test the coatings. Several operative conditions were reproduced, by modifying the load and the amount of lubricant used. For comparison, uncoated samples were tested in the same conditions. The comparison of the results of strip draw tests carried out on coated and uncoated samples, shown that the use of graphite based self-lubricant coatings, deposited by plasma spray, allowed to decrease drastically the amount of lubricant.

Nose and wing leading edges for future generations space vehicles will withstand very high temperature in an oxidizing environment. UHTC (Ultra High Temperature Ceramics) materials are very promising candidate materials for such applications. An innovative, proprietary methodology was developed to produce, by plasma spraying deposition, a ceramic composite containing SiC particles (25 wt %) dispersed in a ZrB 2 matrix. With such a technique both coatings and self standing parts were fabricated. In the present paper, the results of mechanical characterisations, carried out on self standing samples, are presented. Tensile and bending properties were determined by mechanical tests on as sprayed samples and on samples exposed at high temperature (2173 K) in oxidising conditions. Experimental results clearly evidenced the possibility to use the plasma spraying technology and suggest that the so fabricated ZrB 2 -SiC material is suitable to be adopted as protective coating.

Optical selective surfaces, i.e., surfaces with optical properties varying according to the frequency of the impinging radiation, have been exploited in several technical fields. These surfaces consist generally of doped semiconducting films, such as mixed oxides of Indium and Tin as well as Aluminium and Zinc. Thay are currently obtained by physical vapour deposition or sol-gel techniques. The present work aimed at demonstrating that coatings retaining optical selectivity can be obtained also by plasma spraying. Powders of Indium Tin Oxide (ITO) were prepared by an agglomeration technique and sprayed with a plasma torch under air and inert gas atmospheres. Both powders and coatings were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Optical reflection coefficients of the coatings were determined in the wavelength range 0.3-20 µm, i.e., in the visible and in the infrared regions of the spectrum. The experimental results indicated that it was possible to deposit, by plasma spraying, coatings possessing optically selective properties.

Nanocrystalline WC-Co coatings were deposited by high velocity oxy-fuel from commercial nanostructured composite powders. Processing parameters were optimized for maximal retention of the nanocrystalline size and for minimal decarburation of the ceramic reinforcement. Thermo-chemical and gas-dynamical properties of gas and particles flows within the combustion flame were identified in various operating conditions by CFD simulation. Significant improvements of coatings mechanical properties were evidenced: a decrease of the friction coefficient was measured for the nanostructured coatings, together with an increase of microhardness and fracture toughness.

In this investigation, titanium layers with different titanium nitride phases are deposited via reactive gas plasma spraying using a statistical design-of-experiments approach to determine the effect of process parameters on coating properties. The layers are characterized by various means, showing how nitrogen concentration in the plasma gas, spraying pressure, and substrate temperature correspond with coating hardness, porosity, and nitriding levels. The values of all layer properties were found to increase with increasing nitrogen content in either the plasma gas or spraying atmosphere. Paper includes a German-language abstract.

ZrB 2 -SiC composites are considered a class of promising materials for aerospace applications such as nose and leading edges of re-entry vehicles. Results on such materials obtained by hot isostatic pressing have confirmed their high resistance to the oxidation at temperature up to 2000°C. Ongoing work has shown that such materials can be obtained in the form of coatings by means of Plasma Spraying techniques. On this regard, the most critical aspect was correlated to the decomposition of the SiC phase at a temperature quite lower than the melting point of ZrB 2 . Experimental evidence indicated that such decomposition can be avoided when a proper methodology of preparation of the starting powders is adopted, and if suitable thermal spraying parameters are selected. In any case, high temperature oxidation testing (up to 1800°C) confirmed the outstanding behaviour of this materials obtained by plasma spraying. This paper is focussed on preliminary studies of oxidation behaviour for plasma sprayed ZrB 2 -SiC composites suitable for thermal protection shields.

Thermal sprayed ceramic coatings play an important role in those industrial applications where exceptional erosion and wear resistance are required. In particular, nickel-chromium based coatings containing chromium carbide particles dispersion are widely used when environment temperature rises up to 800°C. Thick Cr 3 C 2 -NiCr coatings were produced with two thermal spray processes: Air Plasma Spray (APS) and High Velocity Oxy-Fuel (HVOF). Two different powders have been selected as starting materials. Their dimensional and morphological properties were assessed to verify their sprayability, both in terms of flowability and deposition efficiency. For both APS and HVOF processes, most deposition parameters were selected, after preliminary spraying tests, on the basis of statistical analysis of results, in terms of coating density, hardness and substrate-coating interface quality. The tribological properties of the coatings were evaluated in order to investigate the influence of the deposition process on the behavior of coatings under wear conditions

Reusable space vehicles, which must withstand re-entry into the Earth's atmosphere, require external protection systems (TPS) which are usually in the forms of rigid surface in areas of high or moderate working temperature. High heat fluxes and temperatures related to high performance hypervelocity flights also require the use of TPS materials having good oxidation and thermal shock resistance, dimensional stability, and ablation resistance. Components by these materials are usually fabricated, starting from either billets or plate stocks, by uniaxial hot pressing, and complex parts, such as low radius edges, are then obtained by electrical discharge machining technique. This article investigates an alternative fabrication technology, based on plasma spraying, to produce near net shape components. Results of experimental activities, such as optimization of plasma spraying parameters based on a DOE approach, are reported and discussed.

An innovative methodology to deposit, by plasma spraying, ceramic thermal barrier coatings on gas turbine blades and vanes was developed. Such a methodology produces a pattern of microcracks in the coating, thus improving its thermal shock resistance. After a laboratory campaign of process optimization and coating characterization, real components were coated with a 150µm thick layer of NiCoCrAlY as a bond coat and a 300µm thick layer of ZrO2, partially stabilised with 8%of Y2O3, as a top coat. In particular, four vanes, taken from the first stage of a land based gas turbine (V64.3, produced by Ansaldo), were coated on the whole airfoil. The four vanes were submitted to a cyclic oxidation test in a burner rig simulating the operative conditions of a gas turbine. In particular, they were exposed to a gas flow with the same composition, temperature and speed of the inlet gas of a real gas turbine; moreover, they were cooled by an internal stream of compressed air for obtaining the same temperature profile of a vane in operation. The surface temperature of the vanes was monitored during the test by an optical pyrometer and the internal temperature by a thermocouple. After 550 hours of test, corresponding to 550 cycles, the four vanes did not show any sign of damage.

Pressure inside the spray chamber plays a key role during coatings manufacturing by thermal spraying and coating properties can be strongly affected by the selected pressure value. Spraying at low pressure results in a longer plasma jet length, higher particle velocity, lower coating porosity and higher purity and phase stability. For what concerns plasma-particle interactions, a reduction of pressure value drastically decreases heat transfer towards particles, therefore high power plasma equipment must be used to achieve a suitable melting degree of sprayed powders. Effects of low pressure values are well known, but few investigation have been carried out on effects of pressure for values higher than 1,000 mbar. In this paper a preliminary evaluation of pressure effects on plasma jet modifications, particle velocity and coatings microstructure is presented. By using the very innovative CAPS (Controlled Atmosphere Plasma Spraying) system, Ni-20%Al powders were sprayed at different pressure values, up to 3,600 mbar. The length and width of the visible part of the plasma jet was measured and controlled. Average particle velocity was also evaluated as a function of pressure. Coatings, manufactured on stainless steel substrates, were characterized by means of scanning electron microscopy and energy dispersive spectroscopy, x-ray diffraction and Vickers microhardness measurements. Results indicate that the higher the spraying pressure the lower the plasma jet length and particle velocity; but also a lower selective evaporation of aluminum and higher microhardness values were observed.