A Model for Rapid Solidification for Plasma Spraying

A two-dimensional heat transfer model, extended phase change, is developed with an enthalpy formulation in order to predict the splat/substrate thermal story and its effects on plasma surface interaction. The model validated with respect to experimental results is used to estimate the early solidification of yttria stabilized zirconia splat deposited on a cooled substrate. Simulations quantify transfers in terms of some working parameters such that the splat thickness, the splat/substrate contact quality, the latent heat of solidification and the initial temperatures.

Effect of Surface Roughness on Remelting and Stresses During Splat Solidification [PowerPoint Submission]

In order to understand the microstructural evolution in plasma sprayed coatings, the solidification process was modeled using a 2-D FEM model based on enthalpy formation. Studies of the surface of the coatings showed surface roughnesses across multiple length scales. The model was used to examine the effects of the substrate and splat temperatures and the surface roughness features on the onset of remelting of the underlying surface on which the splat solidifies. The surface roughness was found to promote remelting, indicating that it was an important parameter that determines splat solidification. The temperatures of the splat and substrate were consolidated into one non-dimensional parameter that captured the onset of remelting with a non-dimensional remelting point.A fully coupled thermo-mechanical finite element model was also run for a single splat case, to provide more insight stress buildup during solidification. An important result was that the relative size of the surface roughness features, as compared to the splat thickness, is very important. Very large wavelengths compared to splat thickness lead to smaller stresses, since the solidification and the interface are essentially 1-D. Very small wavelengths compared to splat thickness also leads to reduced stresses, since the solidification front quickly becomes 1-D. Only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration, which are locations of microcrack formation.

Effect of Surface Geometry on Stress Generation in Thermal Barrier Coatings During Plasma Spray Deposition

ABSTRACTA fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification, and to understand the effect of deposition conditions on crack formation during plasma spray deposition. Through the simulation, the locations and magnitudes of maximum stresses were identified, where crack formation would presumably initiate. The model showed that the stresses scaled with the temperature difference between the superheated splat and the substrate. The simulation further showed that the stresses scale with the three geometric parameters, and two independent geometric ratios were defined; ζ (defined as t/λ) and ψ (defined as A/λ). 2D maps of maximum S11 and S22 under different combinations of ζ and ψ were constructed. The mappings showed that only roughness features on the scale of splat thickness were important in providing locations of maximum stress concentration.

Effect of Substrate Temperature on Splashing of Molten Metal Droplets

The effect of substrate temperature on the splashing of molten metal droplets was studied experimentally. Uniform-size molten tin droplets (550 μm diameter) were produced using a drop-on-demand generator. To achieve high impact velocities substrates were mounted on the rim of a rotating flywheel and heated using cartridge heaters to vary substrate temperature. Droplets hitting a smooth cold substrate splashed extensively producing many small satellite droplets and leaving on the surface a small, irregular splat with many fingers projecting from its periphery. Droplets hitting a hot substrate did not splash but spread out to form a smooth disc. A new splashing criterion was developed to calculate the substrate temperature at which this transition occurred. It assumes splashing to occur when the solid layer produced as a result of droplet solidification grows equal to the splat thickness and obstructs the spreading liquid.

Droplet Flattening During Thermal Spraying at Off-Normal Angles

Analytical correlations between the flattening characteristics and the Reynolds number and the spraying angle are obtained. The final splat thickness is shown to decrease and the final splat radius is found to vary nonuniformly with a decrease in the spraying angle. Analytical formulae are obtained describing the pressure distribution in a flattening droplet along the droplet-substrate interface during thermal spraying at off-normal angles. The magnitudes of droplet-substrate microadhesion, deformation of the substrate surface and the coating porosity depend upon the spraying angle. The spraying angle 45° can be recommended as a reasonable limit for off-normal thermal spraying for achieving the quality coatings. The theoretical results obtained on flattening characteristics agree with those of the experimentally observed tendencies of thermal spraying at off-normal angles. The theoretical results for % relative porosity are in a reasonable agreement with experimental observations at off-normal angles between 30° and 90°.

Engineering Formulas for Flattening of Composite Powder Particles during Thermal Spraying

Engineering analytical formulas describing variations of the final values of the splat thickness and radius during flattening of composite particles in thermal spraying are obtained. The effective values of the droplet parameters (impact velocity and viscosity) and the Reynolds number are introduced taking into account a composition of the composite particles. Analytical results obtained agree well with the experimental data available.

Generation of Alumina Plasma Sprayed Coatings on Alumina Substances

The influence of alumina substrate temperature and phase structure : columnar gamma phase, columnar alpha phase and granular alpha phase on splat formation and crystal growth has been studied by SEM and Atomic Force Microscopy. X Ray Diffraction at low angle has allowed to obtain informations on phase structure of layered splats according to substrate phase structure and coating thickness. Column sizes of splats are correlated to a ID model of splat cooling showing the influence of substrate thermal properties and splat thickness on crystal growth kinetic. Finally, coatings adhesion-cohesion values function of spraying parameters are in good agreement with splat morphology and microstructural evolution.