From the pioneering works of McPherson in 1973 who identified nanometre-sized features in thermal spray conventional alumina coatings (using sprayed particles in the tens of micrometres size range) to the most recent and most advanced work aimed at manufacturing nanostructured coatings from nanometre-sized feedstock particles, the thermal spray community has been involved with nanometre-sized features and feedstock for more than 30 years.
Both the development of feedstock (especially through cryo-milling, and processes able to manufacture coatings structured at the sub-micrometre or nanometre sizes, such as micrometre-sized agglomerates made of nanometre-sized particles for feedstock) and the emergence of thermal spray processes such as suspension and liquid precursor thermal spray techniques have been driven by the need to manufacture coatings with enhanced properties. These techniques result in two different types of coatings: on the one hand, those with a so-called bimodal structure having nanometre-sized zones embedded within micrometre ones, for which the spray process is similar to that of conventional coatings and on the other hand, sub-micrometre or nanostructured coatings achieved by suspension or solution spraying. Compared with suspension spraying, solution precursor spraying uses molecularly mixed precursors as liquids, avoiding a separate processing route for the preparation of powders and enabling the synthesis of a wide range of oxide powders and coatings. Such coatings are intended for use in various applications ranging from improved thermal barrier layers and wear-resistant surfaces to thin solid electrolytes for solid oxide fuel cell systems, among other numerous applications.
Meanwhile these processes are more complex to operate since they are more sensitive to parameter variations compared with conventional thermal spray processes. Progress in this area has resulted from the unique combination of modelling activities, the evolution of diagnostic tools and strategies, and experimental advances that have enabled the development of a wide range of coating structures exhibiting in numerous cases unique properties. Several examples are detailed. In this paper the following aspects are presented successively (i) the two spray techniques used for manufacturing such coatings: thermal plasma and HVOF, (ii) sensors developed for in-flight diagnostics of micrometre-sized particles and the interaction of a liquid and hot gas flow, (iii) three spray processes: conventional spraying using micrometre-sized agglomerates of nanometre-sized particles, suspension spraying and solution spraying and (iv) the emerging issues resulting from the specific structures of these materials, particularly the characterization of these coatings and (v) the potential industrial applications.
Further advances require the scientific and industrial communities to undertake new research and development activities to address, understand and control the complex mechanisms occurring, in particular, thermal flow—liquid drops or stream interactions when considering suspension and liquid precursor thermal spray techniques. Work is still needed to develop new measurement devices to diagnose in-flight droplets or particles below 2 µm average diameter and to validate that the assumptions made for liquid–hot gas interactions.
Efforts are also required to further develop some of the characterization protocols suitable to address the specificities of such nanostructured coatings, as some existing ‘conventional’ protocols usually implemented on thermal spray coatings are not suitable anymore, in particular to address the void network architectures from which numerous coatings properties are derived.
From APS to HVOF spraying of conventional and nanostructured titania feedstock powders : A study on the enhancement of the mechanical properties Lima, R. S.; Marple, B. R. Abstract Nanostructured and conventional titania (TiO 2 ) feedstocks were thermal sprayed using air plasma spray (APS) and high-velocity oxy-fuel (HVOF). The HVOF-sprayed coatings made from the nanostructured feedstock exhibited superior abrasion resistance, bond strength and crack propagation resistance when compared to the coatings made from the conventional feedstocks sprayed using HVOF and APS. The enhancement of the mechanical properties was due to (i) the processing (HVOF) and (ii) the nanocharacter of the feedstock. It was found that the HVOF-sprayed coating made from the nanostructured feedstock exhibited isotropic characteristics and microstructure with tiny zones of agglomerated nanostructured particles randomly spread throughout the coating structure. It was observed that these nanostructured zones acted as crack arresters by blunting and branching crack tips, enhancing the crack propagation resistance of the coating. Due to the isotropic characteristics of mechanical properties, the HVOF-sprayed coating made from the nanostructured feedstock exhibited uniform crack propagation under Vickers indentation, i.e., four cracks emanating from the corners of the Vickers indentation impression. D
The oxidation behavior of an air-plasma-sprayed thermal barrier coating (APS-TBC) system was investigated in both air and low-pressure oxygen environments. It was found that mixed oxides, in the form of (Cr,Al) 2 O 3 d Ni(Cr,Al) 2 O 4 d NiO, formed heterogeneously at a very early stage during oxidation in air, and in the meantime, a layer of predominantly Al 2 O 3 grew rather uniformly along the rest of the ceramic/bond coat interface. The mixed oxides were practically absent in the TBC system when exposed in the low-pressure oxygen environment, where the TBC had a longer life. Through comparison of the microstructures of the APS-TBC exposed in air and low-pressure oxygen environment, it was concluded that the mixed oxides played a detrimental role in causing crack nucleation and growth, reducing the life of the TBC in air. The crack nucleation and growth mechanism in the air-plasma-sprayed TBC is further elucidated with emphasis on the Ni(Cr,Al) 2 O 4 and NiO particles embedded in the chromia. D
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