Modelling of Alumina Splat Solidification on Preheated Steel Substrate Using the Network Simulation Method

A mathematical model, consisting of a set of differential equations, for the simulation of the alumina splat solidification on steel substrate is presented. The network simulation method is used to solve the problem, which provides the temperatures and the cooling rate in the splat and substrate with a high temporal and spatial resolution for different values of the preheated substrate temperature. The results of this calculation provide important information for the design of ceramic coatings. The model design is explained in depth and simulated in open source software. As expected, the temperature evolutions in several points of the splat, an important variable to know the type of phases and the effect of the manufacturing parameters on this process, coincide with the experimental results. The model is also checked by another experimental test with tin and a bigger splat, which enables the temperature to be measured during solidification. It is worth highlighting the study of the cooling rate, a fundamental parameter to determine the phase, whether amorphous, gamma or alpha. Furthermore, a sensitive study of the mesh was included in order to optimize the computational time.

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.

Microstructure Study of Spinel Plasma Coatings

Spinel powders of different compositions were fabricated fir their good properties of chemical resistance. These powders were plasma sprayed on steels and their microstructure was investigated by scanning electron microscopy (SEM), microanalysis, X-ray diffraction and transmission electron microscopy (TEM). Due to the powder fabrication process, coatings were very heterogeneous in composition, but had the spinel structure. TEM observations pointed out that splat solidification occured with a cooling rate gradient leading to different crystallization inside a lamella. Young's moduli by the coatings were measured by the resonant frequency method and the correlation with coating microstructure was discussed.

Splat Solidification of Tin Droplets

An experimental study was done of the impact and solidification of tin droplets falling on a stainless steel surface. The surface temperature was varied from 25°C to 240°C. Measurements were made of droplet diameters and contact angles during droplet spread. At a surface temperature of 240°C there was no solidification, and a simple model of liquid droplet impact successfully predicted the extent of droplet spread. Droplets impacting on surfaces at 25°C and 150°C solidified before spreading was complete.