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NiCrAlY layers containing different amounts of Al 2 O 3 (0, 3, 6, 12, 18 wt%) were deposited onto stainless steel substrates by a “hybrid” plasma spray process whereby the NiCrAlY powder was fed in dry form whilst fine Al 2 O 3 powder, dispersed in ethanol, was injected through a suspension feeding system. The Al 2 O 3 reinforcement, consisting of fine, rounded particles interspersed within larger NiCrAlY lamellae, only causes marginal changes in hardness, due to the limited particles-matrix cohesion. Nonetheless, at room temperature, ball-on-disk dry sliding wear rates against sintered Al 2 O 3 counterparts decrease from ≈5×10 −4 mm 3 /(Nm) for pure NiCrAlY to ≈5×10 −6 mm 3 /(Nm) with 18 wt% Al 2 O 3 addition. Pure NiCrAlY indeed suffers adhesive wear, whereas, on the composite coatings, the pull-out of some Al 2 O 3 particles triggers the formation of a tribo-layer of smeared oxide fragments, which mediates the contact with the counterbody. At 400 °C and at 700 °C, all wear rates are levelled to ≈8×10 −5 mm 3 /(Nm) and ≈2×10 −5 mm 3 /(Nm), respectively. An oxide layer grows on the NiCrAlY matrix upon high-temperature exposure, resulting in a tribo-oxidation wear mechanism, which makes the addition of Al 2 O 3 irrelevant. At 700 °C, coatings are further strengthened by partial healing of interlamellar defects and by fine-grained β-NiAl precipitating within the metal matrix. [ABSTRACT FROM AUTHOR]

This study examines the fundamental reactions that occur in-flight during the solution precursor plasma spraying (SPPS) of solutions containing Zr- and Y-based salts in water or ethanol solvent. The effect of plasma jet composition (pure Ar, Ar-H and Ar-He-H mixtures) on the mechanical break-up and thermal treatment of the solution, mechanically injected in the form of a liquid stream, was investigated. Observation of the size evolution of the solution droplets in the plasma flow by means of a laser shadowgraphy technique, showed that droplet break-up was more effective and solvent evaporation was faster when the ethanol-based solution was injected into binary or ternary plasma gas mixtures. In contrast with water-based solutions, residual liquid droplets were always detected at the substrate location. The morphology and structure of the material deposited onto stainless steel substrates during single-scan experiments were characterised by SEM, XRD and micro-Raman spectroscopy and were shown to be closely related to in-flight droplet behaviour. In-flight pyrolysis and melting of the precursor led to well-flattened splats, whereas residual liquid droplets at the substrate location turned into non pyrolysed inclusions. The latter, although subsequently pyrolysed by the plasma heat during the deposition of entire coatings, resulted in porous 'sponge-like' structures in the deposit. [ABSTRACT FROM AUTHOR]

Abstract: Titanium plates were coated by high-velocity suspension flame spraying (HVSFS) technique using a novel bioactive glass composition based on the K2O–CaO–P2O5–SiO2 composition (“Bio-K”). On half of the samples, an atmospheric plasma sprayed (APS) TiO2 bond coat was preliminarily deposited; suspensions of attrition-milled micron-sized glass powders, dispersed in a water+isopropanol mixture, were then sprayed onto both bare and bond-coated plates using five different process parameter sets. The microstructure of the coatings is independent of the presence of the bond coat but is strongly influenced by the deposition parameters. If the latter result in surface temperatures larger than the glass transition temperature of the Bio-K composition, large-scale viscous flow allows the expansion of the air entrained in the porosities, developing large rounded pores. When this phenomenon is avoided, denser layers are obtained. In tensile adhesion tests, porous layers fail cohesively at low loads, whereas adhesive/cohesive failure occurs in denser layers. In this latter case, the adhesion strength is significantly improved by the bond coat, reaching maximum values of 17MPa. When immersed in simulated body fluid (SBF), the coating surface is rapidly converted into a silica gel because of ion leaching. A hydroxyapatite layer starts precipitating on top of it after 3days and grows into a uniform film (of ≈10μm thickness) after 2weeks. [Copyright &y& Elsevier]

Abstract: In this study a ceramic glaze composed by relevant amounts (>80 wt.%) of a frit obtained from vitrified Municipal Solid Waste Incineration (MSWI) bottom ash was produced. This glaze was applied by a conventional wet technique (already in use in the ceramic industry) and by an innovative one, i.e. the plasma-spraying technique. Potential advantages of plasma spraying over conventional wet glazing techniques are lower raw material wastage, no production of glazing wastewater and wet sludge and therefore a simpler process management. The technical characteristics (microstructure, aesthetic appearance, dry particle abrasion resistance and acid resistance) of glazes applied by both methods were compared. The wet-deposited glaze, after firing at 1200 °C, was dense and completely amorphous. In the as-deposited condition, the plasma-sprayed glaze was also completely amorphous, but its technical usefulness was limited by the presence of pores and defects. Through heat treatments at different temperatures, it was possible to induce crystallisation of the plasma-sprayed glaze and to modify its colour; in all cases, however, its aesthetic appearance (quantitatively determined by colorometry) was significantly different from that of the wet-deposited glaze. Complete densification and extensive crystallisation could be achieved after heat treatment at 1200 °C. The wet-deposited glaze and the plasma-sprayed glaze heat-treated at 1200 °C had similar abrasion resistance; however, this latter exhibited significantly better resistance to acids. [Copyright &y& Elsevier]

In this study, hydroxyapatite (HAp) coatings were deposited on Ti plates by the high-velocity suspension flame spraying (HVSFS) technique. The process characteristic, the microstructure and phase composition of the coatings are significantly influenced by the solvent and by the design of the combustion chamber (CC) of the HVSFS torch. Water-based suspensions always lead to fairly low surface temperatures (≈350 °C), deposition efficiencies <40%, and produce coatings with low amount of crystalline HAp, which tend to dissolve very rapidly in simulated body fluid (SBF) solutions. DEG-based suspensions, when sprayed with properly-designed CCs, produce deposition efficiencies of 45-55% and high surface temperatures (550-600 °C). In these coatings, the degree of crystallinity increases from the bottom layer to the top layer, probably because the increasingly large surface temperature can eventually favour re-crystallisation of individual lamellae during cooling. These coatings are much more stable in SBF solutions. [ABSTRACT FROM AUTHOR]

Abstract: SrFe12O19 coatings, intended as electromagnetic wave absorbers, were produced by atmospheric plasma spraying (APS) using two different kinds of feedstock powders: spray-dried agglomerates of micrometric SrFe12O19 particles (type-A) or spray-dried agglomerates of raw materials (SrCO3, Fe2O3), reactively sintered at 1100°C (type-B). During spraying, type-A agglomerates either remain unmelted, producing porous coating regions where crystalline hexaferrite is retained, or are disrupted into smaller granules which melt completely, resulting in dense coating regions with no crystalline hexaferrite. The sintered type-B agglomerates possess higher cohesive strength and do not fall apart: the finer ones melt completely, whereas, in the larger ones, the outer region melts and infiltrates the porous unmelted core which retains crystalline hexaferrite. Dense coatings can therefore be obtained while preserving high amounts of crystalline hexaferrite even inside the dense areas. Such coatings show magnetic properties that are promising for electromagnetic wave absorption applications. [Copyright &y& Elsevier]

Abstract: This paper deals with the splat–substrate and intersplat interfaces of splats produced from Ni–5wt.% Al and alumina based ceramic powders. The cross section of the splat–substrate interface was studied using a focused ion beam (FIB) assisted Field Emission Scanning Electron Microscope (FE-SEM). The metallic splat showed good interfacial integrity apart from a few interfacial pores resulting from entrapped gases and edge curl resulting from quenching stresses. The ceramics showed generally well adherent interfaces for both single and multilayer splats. Good adherence of both alumina splats was attributed to the improved wettability provided by the bond coat. One kind of alumina splats, namely, those obtained from agglomerated nanostructured powder was found to be free from crack and edge curl. This was attributed to the presence of a glassy phase in this splat, which can absorb quenching stresses. [Copyright &y& Elsevier]

Abstract: Three Al2O3-based suspensions were prepared using two nano-sized Al2O3 powders (having analogous size distribution and chemical composition but different agglomeration behaviour) and a micron-sized one. High velocity suspension flame sprayed (HVSFS) coatings were produced using these suspensions as feedstock and adopting two different sets of deposition parameters. The coatings produced by the micrometric powder suspension are denser and more wear resistant (as revealed by ball-on-disk testing) than those deposited using the two nanopowder suspensions, irrespective of the deposition parameters. The tribological behaviour of all coatings is dictated by the formation and subsequent removal of surface tribofilms: these tribofilms are more stable in the coatings obtained by the micrometric powder suspension, although they exhibit the largest tensile residual stress (as determined by X-ray diffraction and hole drilling techniques). Even though the as-deposited coatings are quite smooth (Ra∼1.3–2μm), polishing to Ra∼0.1μm further improves the wear resistance in all cases, as it delays the onset of tribofilm delamination phenomena. [Copyright &y& Elsevier]

Thick thermal barrier coatings (TBCs), consisting of a CoNiCrAlY bond coat and yttria-partially stabilized zirconia top coat with different porosity values, were produced by air plasma spray (APS). The thermal fatigue resistance limit of the TBCs was tested by furnace cycling tests (FCT) according to the specifications of an original equipment manufacturer (OEM). The morphology, residual stresses, and micromechanical properties (microhardness, indentation fracture toughness) of the TBC systems before and after FCT were analyzed. The thermal fatigue resistance increases with the amount of porosity in the top coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling; nevertheless the increasing rate has a trend contrary to the porosity level of top coat. The data suggest that the spallation happens at the TGO/top coat interface. The failure mechanism of thick TBCs was found to be similar to that of conventional thin TBC systems made by APS. [ABSTRACT FROM AUTHOR]

Abstract: Glass and glass-ceramic coatings on ceramic tiles have been manufactured by plasma-spraying high-performance CAS (in wt%—SiO2, 60%; Al2O3, 15%; CaO, 23%; others, traces) and CZS (in wt%—SiO2, 50%; CaO, 31%; ZrO2, 16.5%; Al2O3, 2%; others, traces) glass frits. The CZS system has a surface crystallization at about 1050°C. Such behaviour would not easily allow to obtain a fully crystalline bulk glass-ceramic, but the defectiveness of the plasma-sprayed coating supplies many nucleation sites. Thus, it becomes completely crystalline and well sintered after a 850°C for 30min+1050°C for 15min treatment. The CAS frit, designed not to produce significant crystallization, is well sintered after a 850°C for 30min+950°C for 30min thermal treatment, but remains too brittle due to its glassy nature. A 1050°C treatment allows a few pseudowollastonite crystals to form in a glassy matrix; their formation also hinders sintering. Thus, mechanical properties are inferior to heat-treated plasma-sprayed CZS. [Copyright &y& Elsevier]

Abstract: Plasma-sprayed ceramic coatings were manufactured on sintered alumina–mullite refractory bricks to improve their chemical resistance to molten glass. Mullite and alumina powders were employed. Graded layered coatings were designed and produced, to reduce the thermal expansion mismatch with the substrate: in all cases, the upper layer consisted in pure alumina (very resistant to chemical attack); alumina–mullite intermediate layers were added to match the low thermal expansion of the porous substrates. Plasma-sprayed coatings definitely improved both the abrasion resistance and the chemical resistance to long-time (8h at 1400°C) contact with molten glass, since the coating preserved its original microstructure. However, in thermal shock tests, some transverse cracks appeared; thus, thermal cycling tests in presence of molten glass indicated that, after a few cycles, the glass can penetrate down to these cracks. The tested samples were studied by means of scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). [Copyright &y& Elsevier]

Abstract: Co,Ti-substituted Ba-hexaferrite (BaCoTiFe10O19) coatings for applications as microwave absorbers were deposited by the APS and HVOF techniques, using BaCoTiFe10O19 powders manufactured by solid-state reaction followed by spray-drying. The SEM+EDX, XRD and micro-Raman analyses indicated that impact quenching of molten agglomerates on the substrate hindered the crystallisation of the hexaferrite structure, resulting in poor magnetic properties of the coatings. Adjusted processing conditions enabled the deposition of a coating retaining enough hexaferrite phase, whose magnetic properties, close to bulk BaCoTiFe10O19, are very promising for electromagnetic wave absorption. [Copyright &y& Elsevier]

Abstract: Bioactive glass coatings deposited via suspension plasma spraying were studied to improve the adhesion between orthopaedic implants and bone. Fine powders of a bioactive glass, named BG_Ca, having composition (in wt.%): 4.7 Na2O, 42.3 CaO, 6.1 P2O5, and 46.9 SiO2, were produced and dispersed in ethanol to form a suspension used as a feedstock. Various sets of spray parameters were applied in order to define the influence of the deposition process on the final coating properties. Consequently, the coatings were characterized in as-sprayed state and after soaking in a simulated body fluid (SBF) for different periods ranging from 1 to 14days. The microstructural investigations were carried out using environmental scanning electron microscope (ESEM) and X-ray diffraction (XRD). The coatings'' adhesion to the substrate was evaluated by means of scratch tests. Finally, hardness and elastic modulus were determined by means of depth-sensing indentation methods. [Copyright &y& Elsevier]