V.V. Sobolev

Investigation of coating porosity formation during high velocity oxy-fuel (HVOF) spraying

Abstract The analytical formulae describing the formation of large pores arisen between the flattened particles (particles and substrate) due to gas porosity and small pores within the flattened particles produced with shrinkage porosity are obtained. The gas porosity is shown to decrease with the increase in particle density, its temperature and velocity, particle material viscosity and surface tension. The enhancement of spraying distance leads to porosity increase. The gas porosity grows along the flattened droplet attaining the maximal value at its periphery. The shrinkage porosity is established to be formed in the surface layers of the flatened droplets as well as in the coating surface layer. The conditions of this porosity formation become more favourable with the increase of the substrate surface curvature. The theoretical results agree well with the experimentally observed tendencies of the porosity behaviour.

Flattening of droplets and formation of splats in thermal spraying: A review of recent work—Part 2

This is the second part of a review article that deals with an analysis of the influence of wetting at the substrate-coating interface and surface phenomena, substrate deformation, dynamics of splashing, splatsubstrate interaction and spraying at off-normal angles on droplet flattening, and the formation of splats in thermal spraying, which affects the coating quality. The results agree well with experimental data and improve understanding of the thermal spray processes to make them more effective.

Modelling of particle movement and thermal behaviour during high velocity oxy-fuel spraying

Abstract A theoretical model is presented to investigate the particle mechanical and thermal behaviour in the process of high velocity oxy-fuel (HVOF) spraying. The model accounts for the various phenomena which influence the momentum and heat transfer during particle movement and heating. The mathematical formulation takes into account the fluid velocity dependence on the particle density, internal heat conduction in particles and their composite structure, steep temperature gradients near the particle surface and fluid parameter variations. Comparison with experimental data shows that the present model enables accurate predictions to be made and optimal conditions of HVOF spraying to be established.

Effect of oxidation on droplet flattening and splat-substrate interaction in thermal spraying

The processes of oxidation that occur during particle inflight motion and during splat solidification in an oxygen-rich atmosphere were considered for the thermal spray process. The effect of oxidation on droplet flattening, splat-substrate mechanical and thermal interaction, splat morphology, and development of coating porosity and adhesion was studied. The influence of wetting and oxygen dissolution on flattening and splat-substrate adhesion was also investigated. The results from theoretical treatment agree with experimental observations.

Influence of surface roughness on the flattening of powder particles during thermal spraying

The time evolution of the splat thickness, radius, and rate characteristics in the process of flattening of droplets during thermal spraying is investigated taking into account the surface roughness, splat solidification, and mass loss of the droplet liquid phase. Analytical formulas describing the final values of the splat thickness, radius, and rate characteristics are found. Results agree well with the experimental data. They can be used to predict the splat flattening parameters.

Analysis of coating gas porosity development during thermal spraying

Abstract The formation of gas porosity during thermal spraying is investigated. The development of pores is studied, taking into account the parameters of spraying, powder particle material and substrate, droplet flattening and solidification kinetics, heat and mass transfer processes and pore interaction. The results obtained agree well with thermal spraying practice.

Oxidation of coatings in thermal spraying

Abstract Mechanisms of the coating oxidation during thermal spraying are discussed. They are associated with the in-flight oxidation of the powder particles and oxidation of the solidifying splat on the substrate surface. Influence of oxidation on the splat morphology, porosity and the splat–substrate adhesion is outlined. Theoretical results agree well with the observed tendencies of the development of oxidation and its effect on the coating formation.

Droplet-substrate impact interaction in thermal spraying

The analytical formulae describing the pressure distribution in a flattening droplet along the droplet-substrate interface during thermal spraying is obtained. The droplet-substrate microadhesion and the coating porosity are shown to depend significantly on the ratio of the pressure developed upon impact to the capillary pressure. The theoretical results agree with the experimental data. They can be used for prediction purposes.

Flattening of thermally sprayed particles

Abstract The analytical formulae describing the time evolution as well as the final values of the splat thickness and radius in the process of flattening of droplets at a substrate surface during thermal spraying are obtained. The theoretical results agree well with the experimental data and the results of the other authors. They can be used for the prediction of the splat flattening parameters.

Formation of splats during thermal spraying of composite powder particles

Abstract Development of splats during thermal spraying of the composite powder particles is studied. The influence of the solid volume fraction and the size of the solid phase on the splat–substrate mechanical interaction, splat–substrate adhesion, porosity and splashing is analysed. Effect of the wetting phenomena is considered. The results obtained agreed well with the experimental observations.