In this study, twin wire arc spraying is used to bond wire mesh to the outside surfaces of stainless steel pipes in order to increase heat transfer surface area. At the optimum spray distance, the oxide content, porosity, and adhesion strength of the coatings are shown to be 6.6%, 2.1%, and 24 MPa, respectively. Pipes with different wire mesh configurations were placed in an oven and heated to temperatures from 300 °C to 900 °C. Water temperatures were measured at the inlet and outlet of the pipe for flow rates between 0.2 and 0.5 gpm. A maximum water temperature rise of 13 °C was achieved, corresponding to a total heat flux of 57 kW/m2. Heat transfer efficiency is shown to depend strongly on the quality of the bonds between the wire mesh and pipe and the spacing of wires in the mesh.

This study investigates isothermal transformation kinetics in Ti-6Al-4V alloys structures produced by vacuum plasma spray forming. As-sprayed samples were homogenized in the β phase followed by fast cooling to the two-phase temperature region, then quenching to suppress further transformation. The microstructure of heat-treated specimens was examined by optical microscopy and equilibrium phases were measured using image analysis. The kinetics of the β → α+β phase transformation are revealed by plotting the amount of α-phase obtained over a 10 to 60 s interval at isothermal temperatures of 800, 850, 900, and 950 °C. Corresponding phase transformation rates are also calculated based on Johnson-Mehl-Avrami (JMA) theory. At temperatures below 900 °C, the main phase transformation mechanism is homogeneous nucleation and growth of α-phase. At higher temperatures, phase transformations are driven by two mechanisms: the formation of α-phase in grain boundaries and α-plate nucleation and growth.

Solution precursor plasma spraying is used to produce finely structured ceramic coatings with nano- and submicron features. The solution is injected into a plasma jet as a liquid stream or gas atomized droplets that break up into a finer mist. The deposition process and coating properties are extremely sensitive to torch operating conditions, injection modes, and substrate temperatures. This study employs numerical methods to investigate the size distribution of injected droplets for liquid stream and gas blast injection. Droplet or particle size, temperature, and substrate position is predicted for different injection modes.

Solution precursor plasma spraying has been used to deposit ceramic coatings with submicron/nanocrystalline structures. Previous studies revealed that the deposition mechanism in the solution precursor plasma spraying differs from that in the conventional plasma spraying. To increase the understanding of the deposition mechanism in the solution precursor plasma spraying, a numerical model is used to predict the particle conditions on the substrate. Five types of particle conditions, melted particles; small sintered particles; dry agglomerates; wet agglomerates; and wet droplet are assumed based on the computed temperature distribution of the particles. An analysis of the deposition mechanism in the solution precursor plasma spraying is performed. Experiment results s are also collected to verify the numerical prediction and the analysis of the deposition mechanisms.

In this paper we proposed a rapid and continuous process for the production of nanoporous coatings for functional applications. Experiments following two statistical designs were implemented to screen and investigate the spraying parameters’ effects on coating crystallinity and porosity to gain a better understanding. The spraying stand-off distance, solution flow rate and arc current were identified as having significant effects on coating porosity and crystallinity. The investigation yielded a microstructure comprised of interpenetrating pores and layered structures with embedded nanopores. A deposition mechanism was postulated to explain this peculiar microstructure. Gas sensors constructed from the coatings had ethanol sensitivities at room temperature comparable to those reported in the literature for conventional thick-film coatings and a maximum sensitivity near 200°C.

In this work we present the numerical simulation results for the molten nickel and zirconia (YZS) droplets impact on different micro-scale patterned surfaces of silicon. The numerical simulation clearly showed the effect of surface roughness and the solidification on the shape of the final splat, as well as the pore creation beneath the material. The simulations were performed using a computational fluid dynamic software, Simulent Drop, The code uses a three-dimensional finite difference algorithm solving full Navier Stokes Equation with heat transfer and phase change. Volume of fluid (VOF) tracking algorithm is used to track the droplet free surface. Thermal contact resistance at the droplet– substrate interface is also included in the model. Specific attention is paid to the simulation of droplet impact under plasma spraying conditions. The droplet sizes ranged from 15 to 60 microns with the initial velocities of 70-250 m/s. The substrate surface was patterned by a regular array of cubes spaced at 1 µm and 5 µm from each other. The peak to valley height of each cube was between 1 to 3 µm. Different splat morphologies will be compared with those obtained from the experimental results under the same impact and surface conditions.

Calcium phosphate (Ca-P) coatings have been used as surface coatings on porous metallic implants in dentistry and orthopaedics for the last twenty years. These Ca-P coatings, nominally hydroxyapatite (HA), have been shown to promote bone fixation and osteconductivity on Ti and Ti alloy substrates used for those purposes. Such coatings can be formed by different methods including plasma spray. In addition to the well known advantages of the plasma spray technique to deposit coatings, a new version of this technique, i.e. solution precursor plasma spray (SPPS), has been reported to produce submicron/nanocrystalline structured coatings. Nanocrystalline HA coatings may improve the resorption of the coating in the body, avoiding the irritant effect of large particles which may be seen in current thermal sprayed HA coatings. The main purpose of this work was to study the suitability of a sol-gel Ca-P solution precursor (calcium nitrate tetrahydrate and ammonium dihydrogen phosphate) as feedstock for the air plasma spray (APS) coating technique. We report on the formation and the characteristics of the coatings so formed on Ti6Al4V substrates. The presence of different Ca-P crystalline and amorphous phases was assessed by X-ray diffraction analysis. The X-ray photoelectron spectroscopy technique was used to characterize the surface chemical composition of the Ca-P coatings. The microstructural features of the coatings were characterized by scanning/transmission electron microscopy combined with image analysis in order to evidence the presence of submicron/nanocrystalline Ca-P features. Final results are discussed in terms of the spraying parameters. Abstract only; no full-text paper available.

One of the tubular cell designs for solid oxide fuel cells is based on a closed-end tube made from the cathode material, with an electrolyte layer coating the outside of the cathode tube, and the anode coating the electrolyte layer. The outer anode layer of one tubular cell is connected to the inner cathode layer of the next tube by the interconnect material. High density is required in the interconnects to prevent mixing of the air and fuel gases. The fabrication of interconnect strips in tubular fuel cell stacks by DC-arc plasma spray deposition has been demonstrated in the past, both in air (APS) and in low pressure (vacuum) conditions (VPS). The High Velocity Oxy Fuel (HVOF) spray deposition technique typically yields among the highest density coatings of all common thermal spray techniques due to the high gas and particle velocities achieved, and therefore would appear to be an excellent method for depositing the interconnects if the powder could be sufficiently melted during spraying. The most common material choice is a doped LaCrO3, a ceramic material with a good thermal expansion coefficient match with the other components of the cell and an acceptable electrical conductivity. The microstructure, phase and chemical composition, and electrical properties of doped LaCrO3 deposited on (La,Sr,Mn)2O3 cathode tubes by HVOF was examined as a function of deposition conditions. Abstract only; no full-text paper available.

Increasing the combustion temperature in diesel engines is an idea which has been pursued for over 20 years. Increased combusting temperature can increase the power and efficiency of the engine, decrease the specific fuel consumption, CO and (possibly) the NOx emission rate. At the same time, TBCs should protect the metallic substrate against the corrosive attack of fuel contaminants (Na, V, and S). The most common system used is Yittria Partially Stabilized Zirconia (Y-PSZ). However, in diesel engines Y-PSZ TBCs have not met with wide success. To reach the desirable temperature of 850-900°C in the combustion chamber from the current temperature of 350- 400°C, a coating with a thickness of at least 1mm is required. This introduces different considerations than in the case of turbine blade coatings, which are on the order of 100µm thick. The design of a multilayer coating employing relatively low cost materials with complementary thermal properties is described. Numerical models were used to optimize the thickness for the different layers to yield the minimum stress at the operating conditions while achieving the desired temperature gradient. Abstract only; no full-text paper available.

Properties of MCrAlY coatings obtained by High Velocity Oxy-Fuel (HVOF) thermal spray process operated in a standard configuration were compared with those obtained using a gas shroud attachment to the HVOF gun. Our measurements show that the attached gas (nitrogen) shroud nozzle considerably reduces the oxygen content in the coating without an appreciable change in the microstructure. The particle temperatures were decreased by an average of 100 °C at a standoff distance of 0.275 m (11 inches). There was also a large reduction in the particle velocity at this distance. Both these effects were related to the excessive amount of nitrogen used for shrouding.