The emerging methods of coating deposition by suspension and solution thermal spraying are described. The liquid suspensions of fine powders and liquid precursors are injected into flames and/or jets generated in the torches. The formulation and stability of suspensions as well as the methods of fine powders synthesis are briefly described. Typical solutions, being often the liquid organo–metallics are also briefly described. An important problem of injection of liquids into jets and flames is then presented. Two principal modes of injection, used at present, are outlined, i.e.: (i) atomization; and, (ii) injection of a continuous jet. Subsequently, the phenomena occurring in flames and plasma jets are discussed and the major differences to these occurring during conventional spraying are stressed up. The build up of coatings starting from the impact of fine particles on the substrate is described and typical microstructures of suspension and solution sprayed coatings are shown. Some properties of the sprayed coatings, including mechanical, electrical, chemical, and thermophysical ones are collected and presented. Finally, the emerging applications of coatings are shown and the possible future applications are discussed.

Suspension and solution thermal spray coatings

Novel thermal barrier coatings that are resistant to high-temperature attack by glassy deposits

This article presents what is our present knowledge in plasma spraying of suspension, sol, and solution in order to achieve finely or nano-structured coatings. First, it describes the different plasma torches used, the way liquid jet is injected, and the different measurements techniques. Then, drops or jet fragmentation is discussed with especially the influence of arc root fluctuations for direct current plasma jets. The heat treatment of drops and droplets is described successively for suspensions, sols, and solutions both in direct current or radio-frequency plasmas, with a special emphasize on the heat treatment, during spraying, of beads and passes deposited. The resulting coating morphologies are commented and finally examples of applications presented: Solid Oxide Fuel Cells, Thermal Barrier coatings, photocatalytic titania, hydroxyapatite, WC-Co, complex oxides or metastable phases, and functional materials coatings.

Parameters Controlling Liquid Plasma Spraying: Solutions, Sols, or Suspensions

A solid oxide fuel cell (10) comprises a solid electrolyte layer (30), a barrier layer (40), and a cathode (50) The cathode (50) includes a cathode current collecting layer (51) and a cathode active layer (52) The cathode active layer (52) includes a plurality of micro-cracks (SL) in an interface region (52a) within a predetermined distance (Da) from the interface (P2) between the cathode current collecting layer (51) and the cathode active layer (52)

Solid-oxide fuel cell

Nanostructured Co3O4 electrodes for supercapacitor applications from plasma spray technique

A thermal spray process comprises injecting precursor solution droplets into the hot zone of the thermal spray flame. Also described are materials resulting from the process.

Coatings, materials, articles, and methods of making thereof

In an effort to understand the deposition mechanisms in solution-precursor plasma-spray (SPPS)—a promising method for the deposition of highly-durable thermal barrier coatings (TBCs)—some model spray experiments were performed. In the SPPS process an aqueous chemical precursor feedstock, that results in ZrO2–7 wt.% Y2O3 (7YSZ) ceramic solid solution coating, is injected into the plasma jet and the coating is deposited on a metal substrate. The model experiments performed here entailed holding the plasma-torch stationary and arranging the substrates in two different ways: (i) substrate placed at an offset below the plasma-torch center-axis and (ii) window-shielded substrate placed along the plasma-torch center-axis. These experiments allowed us to collect deposits from different regions of the plasma jet, which were then characterized, in an effort to identify the deposition mechanisms. The operative deposition mechanisms were found to depend critically on the trajectories of the precursor droplets in the plasma jet. Actual coatings deposited using scanning plasma-torch experiments were also characterized. Striking resemblance was found between the morphologies of the microstructural features in the deposits from the stationary and the scanning plasma-torch experiments. This is used to elucidate the coatings deposition mechanisms in the scanning plasma-torch SPPS process.

Identification of coating deposition mechanisms in the solution-precursor plasma-spray process using model spray experiments

Solution precursor plasma spray (SPPS) is a promising process for making thermal barrier coatings (TBCs) with improved durability. The improved durability of SPPS TBCs results from the novel microstructural features, (i) absence of splat boundaries, (ii) generation of through-thickness vertical cracks (iii) existence of uniformly distributed porosity. In this process, the coating is built up by the overlapping and stacking of layers deposited in each pass of the plasma torch across the substrate. Information concerning the lamella is generally referred to as the spray pattern. The spray patterns from SPPS of 7wt.%Y 2 O 3 –ZrO 2 precursor were studied via a fixed scan spray method, where the plasma torch repeatedly scans on a single line across the substrate. The surface of the resulting spray patterns can be divided into adherent deposits and powdery deposits that correspond to the hot and cold regions of the plasma jet, respectively. The penetration depth of the liquid into the plasma jet has a significant effect on the position of the adherent deposits and the deposition efficiency. The structure of the adherent deposits across the section normal to the motion of plasma torch is symmetric about its center. It becomes more porous and the thickness of the deposits is reduced towards the edges. Implications of these results for understanding and improving the SPPS process are discussed.

Processing parameter effects on solution precursor plasma spray process spray patterns

The solution-precursor plasma-spray (SPPS) process is capable of producing highly durable thermal barrier coatings. In an effort to improve the understanding of the deposition mechanisms in this novel process, a series of specific experiments, where the substrate is held stationary and the plasma torch is programmed to scan a single pass across the substrate, were conducted and the resulting deposits were carefully characterized. In addition to the deposition mechanisms identified previously in the stationary torch experiments, the deposition mechanisms of two other types of deposits, thin film and fine spherical particles, were identified in this study. The melting of inflight formed 7YSZ particles and their rapid solidification to form ultra-fine splats on the substrate was found to be the dominant deposition mechanism. The characterization of actual SPPS coatings confirmed that the various coating-deposition mechanisms identified in the model experiments occur in concert during the actual coating process. Adherent deposits (ultra-fine splats, deposits from gel-like precursor and film formed via chemical vapor deposition), unmelted particles (spherical particles, deposits from non-decomposed precursor) and porosity were estimated to constitute ∼65, ∼19 and ∼16 vol%, of the coating, respectively.

Deposition of thermal barrier coatings using the solution precursor plasma spray process

In an effort to understand the deposition mechanisms in the solution precursor plasma spray (SPPS)—a promising method for the deposition of highly-durable thermal barrier coatings—some model spray experiments were performed. In the SPPS process, an aqueous chemical precursor feedstock, which results in a ZrO2–7wt.%Y2O3 ceramic solid solution coating, is injected into the plasma jet and the coating is deposited on a metal substrate. The model experiments performed here entailed holding the plasma torch stationary. The deposits collected in these experiments were then characterized to identify the deposition mechanisms. In addition, actual coatings deposited by multiple-pass scanning plasma-torch experiments were also characterized to verify the identified deposition mechanisms. Striking resemblances were found between the morphologies of the microstructural features in the actual coating and in the deposits from the model experiments. Melting and solidification, resulting in ultra-fine ‘splats’, were identified for the first time in the SPPS process.

Danny T. Xiao

Papers

Coatings, materials, articles, and methods of making thereof

Identification of coating deposition mechanisms in the solution-precursor plasma-spray process using model spray experiments

Processing parameter effects on solution precursor plasma spray process spray patterns

Deposition of thermal barrier coatings using the solution precursor plasma spray process

Deposition mechanisms of thermal barrier coatings in the solution precursor plasma spray process