Traditional fused and crushed (F&C) titania feedstock powders have relatively poor flow and are limited in the feed-rates that can be achieved before deposition efficiencies (DEs) start to decline. In addition, the coatings made with F&C powders generally have high stiffness and limited fracture toughness. Such coatings bear the risk of cracking/delaminating particularly in the case of thick coatings. To address these issues, a new agglomerated and sintered (A&S) titania feedstock powder has been developed and compared in side-by-side spray tests to the traditional F&C titania powder of comparable particle size. The spray behavior, achievable feed-rates and DEs, as well as the resulting coating characteristics were evaluated from the view point of their application as thick, electrically conductive coatings. The new A&S powder yields up to approximately 200% improvement in DE while producing coatings with more suitable microstructure, lower electrical resistivity and higher thickness. Furthermore, for a given set of process parameters, the DEs obtained with this new powder show little sensitivity to the powder feed-rates, thus allowing spraying at higher feed-rates without compromising coating DEs. This feature of the powder has significant commercial advantages for thick coatings when combined with high throughput guns such as TriplexPro-200. Preliminary results of particle diagnostics towards understanding of the fundamental principles behind these improvements are also discussed.

This paper provides an overview of the high velocity suspension flame spraying (HVSFS) process that covers spray gun design, suspension and substrate preparation, and process optimization. Examples are given showing how the process is used to produce tribofunctional coatings for engine applications, electrolyte layers for SOFCs, and netshape chromia for high-end scissor blades. The substrates in the three examples are AlSi 9 Cu 3 , Crofer nickel cermet, and low carbon steel.

In this study, a TiO 2 (anatase) nanopowder suspension was processed by high velocity suspension flame spraying (HVSFS). The resulting coatings were characterized and compared to conventional HVOF and atmospheric plasma sprayed layers. It is shown that the HVSFS operating parameters can be adjusted to achieve dense titania with a near nanostructure and homogeneous distribution of anatase and rutile phases. These coatings have lower pore interconnectivity and higher wear resistance than the APS and HVOF layers. Alternatively, large unmelted agglomerates of anatase nanoparticles can be embedded in the coating, increasing the porosity and anatase content for enhanced photocatalytic efficiency.

This study compares the tribological performance of HVOF and HVSFS coatings applied to gray cast iron and aluminum cylinder liners. Five different materials, including Fe alloy, FeCrMo, CrC-NiCr, NiCrBSi, and WC-Co, were sprayed using a conventional HVOF torch operated by a six-axis robot while the liners were manipulated by means of a rotary table. A similar setup was used to spray TiO 2 -TiC coatings, but the gun was modified for nano-sized particles in a suspension fed axially into the combustion chamber. Coating microstructures were examined using optical and SEM imaging and friction and wear properties were determined through oscillating friction wear tests. The results obtained are compared to state-of-the-art cylinder liners.

High Velocity Suspension Flame Spraying (HVSFS) has been developed to thermally spray suspensions containing micron, sub micron and nanoparticles with hypersonic speed. For this purpose, the suspension is introduced directly into the combustion chamber of a modified HVOF torch. The aim in mind is to achieve dense coatings with a refined microstructure. Especially from nanostructured coatings superior physical properties are expected for many potential applications. Direct spraying of suspensions offers flexibility in combining and processing different materials. It is a cost saving process and allows the allocation of entirely new application fields. The paper gives an overview of the HVSFS spray method and will present some actual results that have been achieved by spraying the nanooxide ceramic materials Al 2 O 3 , TiO 2 , 3YSZ and Cr 2 O 3 .

The High-Velocity Suspension Flame Spraying (HVSFS) technique, a recently-developed modification to the standard HVOF process enabling the use of suspension feedstock, was employed in order to deposit Al 2 O 3 coatings from a nanopowder suspension. These coatings were compared to conventional APS and HVOF-sprayed ones. HVSFS coatings possess lower overall porosity and lower pore interconnectivity degree. Indeed, most of the nanoparticles were fully melted by the gas jet, thus forming very thin, well-flattened lamellae, having smaller columnar crystals than conventional coatings. Accordingly, HVSFS coatings possess higher hardness and elastic modulus, as determined from nanoindentation tests. Ball-on-disk tribological tests also indicate that HVSFS coatings possess much better sliding wear resistance than conventional ones, because they are capable of forming denser and more protective surface tribofilms during dry sliding.