High Velocity Oxy-Fuel (HVOF) Suspension Spraying of Mullite Coatings

This article is an invited paper selected from presentations at the 2008 International Thermal Spray Conference and has been expanded from the original presentationPeer reviewed: YesNRC publication: Ye

Suspension plasma spraying of nanostructured WC-12Co coatings

Nanostructured WC-12% Co coatings were deposited by suspension plasma spraying of submicron feedstock powders, using an internal injection plasma torch. The liquid carrier used in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a fine surface finish.Apolyethylene-imine (PEI) dispersant was used to stabilize the colloidal suspension in an ethanol carrier. In-flight particle states were measured for a number of operating conditions of varying plasma gas flow rates, feed rates, and standoff distances and were related to the resulting microstructure, phase composition (EDS, SEM, XRD), and Vickers hardness. High in-flight particle velocities (>800 m/s) were generated, leading to dense coatings. It was observed that the coating quality was generally compromised by the high temperature and reactivity of the small particles. To compensate for this shortcoming, the suspension feed rate was adjusted, thereby varying the thermal load on the plasma. Results showed that a slightly larger agglomerate size, in conjunction with low particle jet temperatures, could somewhat limit the decomposition of WC into brittle W\u2082C/W\u2083C and amorphous cobalt containing binder phases.Peer reviewed: YesNRC publication: Ye

SUSPENSION PLASMA SPRAYING OF INTERMEDIATE TEMPERATURE SOFC COMPONENTS USING AN AXIAL INJECTION DC TORCH

Intermediate temperature SOFC components, such as dense, nanostructured SDC electrolytes (samarium doped ceria) and porous anode sublayers were fabricated by suspension plasma spraying using an axial feed dc plasma torch. The liquid carrier employed in this approach allowed for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a refined microstructure. Dense, thin (<10(m) and non-fractured electrolytes were created. Various processing routes for SOFC half-cells, using tape-cased, plasmasprayed and suspension-sprayed anodes, were explored. Loss of integrity and non-continuous coverage of the anode constituted the principal difficulties in the subsequent electrolyte deposition. The role of suspension feedstock particle size is discussed. Amongst various schemes investigated, a processing route that employs sequential suspension plasma spraying steps for both the electrolyte and the anode, using relatively large primary particles in the feedstock, constituted the most promising approach.Peer reviewed: NoNRC publication: Ye

Suspension HVOF Spraying of Reduced Temperature Solid Oxide Fuel Cell Electrolytes

Peer reviewed: YesNRC publication: Ye

Structure property relationship of suspension thermally sprayed WC-Co nanocomposite coatings.

Tribomechanical properties of nanostructured coatings deposited by suspension high velocity oxy-fuel (S-HVOF) and conventional HVOF (Jet Kote) spraying were evaluated. Nanostructured S-HVOF coatings were obtained via ball milling of the agglomerated and sintered WC-12Co feedstock powder, which were deposited via an aqueous-based suspension using modified HVOF (TopGun) process. Microstructural evaluations of these hardmetal coatings included transmission electron microscopy, x-ray diffraction, and scanning electron microscopy equipped with energy dispersive x-ray spectroscopy. The nanohardness and modulus of the coated specimens were investigated using a diamond Berkovich nanoindenter. Sliding wear tests were conducted using a ball-on-flat test rig. Results indicated that low porosity coatings with nanostructured features were obtained. High carbon loss was observed, but coatings showed a high hardness up to 1000 HV2.9N. S-HVOF coatings also showed improved sliding wear and friction behavior, which were attributed to nanosized particles reducing ball wear in three-body abrasion and support of metal matrix due to uniform distribution of nanoparticles in the coating microstructure

Structural considerations in plasma spraying of the alumina zirconia composite

The focus of this study is the amorphous and crystalline phase formation in air plasma sprayed alumina yttria stabilized zirconia coatings. In this multi-component system at compositions close to its eutectic, amorphous structures can arise by virtue of the high cooling rates of melted particles. Two avenues for amorphous phase formation have been identified: in-flight and upon-impact mixing. While the crystalline structure is largely retained by unmelted or partly melted feed particles embedded in the coating, it can also be created in the solidification process. The formation of a supersaturated crystalline phase is proposed. It was found that the formation of the crystalline phases with supersaturated composition in alumina YSZ composite is possible, in spite of the high cooling rates during spray process.Peer reviewed: YesNRC publication: Ye

Plasma spraying of a novel material with amorphous and nanocrystalline microstructure

A novel material has been used for plasma spraying by WSP\uae. The material is composed of three main phases, namely corundum (aluminum oxide), baddeleyite (zirconium oxide), and glassy phase (silicon oxide). The material is a refractory and exhibits very high hardness, extremely high abrasion resistance, and chemical resistance. Conventionally, the material is fabricated by melt casting and machining. Cast tiles of the material were ground and sieved to obtain the right powder cut size for plasma spraying by water stabilized plasma torch (WSP\uae). Both dense coatings and free standing parts were successfully produced from the new material by WSP\uae. Spraying parameters were varied and molten particles were monitored in flight by DPV 2000. The coatings exhibit very low porosity and high hardness. The as-sprayed material is mostly amorphous with some nanocrystalline grains of aluminum and zirconium oxide present. The phase composition of the as-sprayed material is thus different from that of the feedstock material, which is mostly crystalline with a small faction of amorphous silica glass. The micro-structure of the newly sprayed material was studied by electron microscopy (SEM, TEM) and is very complex. Upon annealing, the as-sprayed material crystallizes around 950\ub0C. This result and other thermal properties were obtained by TMA and DTA measurement. The ease of plasma spraying and the coating properties make this material a suitable candidate for many industrial applications.Peer reviewed: NoNRC publication: Ye