This paper demonstrates the capabilities of a new thermal spray process based on cold gas spraying and detonation gun technology. The method, called high-frequency pulse detonation, uses combustion pulses to heat powders to a temperature that allows good substrate-layer adhesion without the powder being melted. Superalloy, copper, and steel layers so produced are examined and compared with layers deposited by conventional thermal and cold spray processes. Paper includes a German-language abstract.

This paper presents a mathematical model of the in-flight oxidation of spherical particles during thermal spray deposition process. The model includes analysis of the mechanical and thermal behavior of the powder particles. The former accounts for acceleration and deceleration of the particles at the spray distance under different fluid velocities. The thermal behavior takes into account heating, melting, cooling and possible solidification as the particle travel towards the substrate. A finite-difference method is used to solve the thermal energy conservation equation of the particles. The model includes nonequilibrium calculations of the phase change phenomena in the liquid-solid (mushy) zone. The growth of the oxide layer at the particle surface is represented by a modified boundary condition, which includes finite-rate oxidation. The results obtained give the interrelations between various process parameters and the oxidation phenomenon and agree with experimental observation.

Due to high kinetic energy of an impinging droplet a strong elastic deformation of the substrate and the formation of its curvature occur which influence the dynamics of the droplet flattening (particularly the thickness and the diameter of a splat) and a pressure formed upon the droplet impact. This paper involves investigation of these phenomena and the development of the simple analytical formulae needed by the engineering practice which enables to estimate the influence of the substrate deformation on the droplet flattening and the high pressure generation. The analytical results are obtained with regard to the variation of the splash properties and the pressure that arises when the droplet is flattened during thermal spraying. The splash curvature caused by the drop impact is taken into account. The paper also analyzes the role of surface effects. The results obtained show good agreement with the experimental observations. Paper includes a German-language abstract.

Wetting between the spreading droplet and the substrate plays an important role in the droplet flattening because it affects not only the surface effects but also the contact thermal resistance at the splat-substrate interface which is an important parameter for the development of the coating structure. In this paper, analytical formulas are determined which show the influence of wetting phenomena on the thickness or the radius of the splash, on the pressure developed upon drop impact and on the splash porosity. An effect of surface tension on the flattening parameters is investigated. The results obtained show good agreement with the experimental data. Paper includes a German-language abstract.

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°.

Influence of the in-flight behaviour of the stainless steel 316 powder particles during high velocity oxy-fuel (HVOF) spraying on the properties (porosity and level of oxidation) of the coating is studied. Modelling of the in-flight behaviour is undertaken which takes into account the combustion process, gas dynamics, gas-particle interactions, acceleration and deceleration of the gas flow, heat transfer in the particles and full thermal history of the particles including their melting, cooling and the possible solidification. The results obtained are used for the explanation of the experimental data on the coating porosity and level of oxidation.

Different mechanisms of splashing of droplets impacting onto the substrate surface during thermal spraying are considered. It is shown that supercooling formed in the flattening droplet consists of the thermal supercooling and that arisen due to the high pressure developed upon the droplet impact. Solidification starts when the supercooling exceeds some critical value. With a "cold" substrate when its temperature is less than a transition temperature the marked contribution to the supercooling is due to its high pressure part. In this case a regular disc-shaped splat will be formed in the central part of the flattening droplet and splashing will occur in the periphery. With a "hot" substrate when its temperature exceeds the transition temperature the thermal supercooling is high enough, no splashing occurs and a regular disc-shaped splat is formed. Theoretical results agree with the experimental observations.

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.

Different mechanisms of development of the substrate-coating adhesion during thermal spraying are considered. One of the most important is mechanical interlocking formed chiefly due to roughness of the substrate surface, high pressures developed upon the droplet impact and solidification of the lower part of the splat. Possible deformation of the substrate surface and rebounding of the impinging droplets are considered. Thermal mechanisms involving partial or complete melting and subsequent solidification in the substrate interfacial region is shown to be effective in creation of the adhesive bonds. The role of the diffusion processes and the influence of the splat morphology on adhesion is discussed. Mechanisms of splashing of droplets impinging onto the substrate surface during thermal spraying and their influence on the coating-substrate adhesion are considered. Roughness of the substrate surface is shown to be critically important to obtain good adhesion. Transition temperature is shown to exist which determines the splat morphology on the smooth surface of the substrate. With a "cold" substrate when its initial temperature is less than the transition temperature the splashing occurs. When the substrate initial temperature exceeds the transition one a regular splat is formed. The theoretical results agree well with the observed behaviour of the thermal spray coatings and in the case of the thermal mechanisms of adhesion these results are in a good agreement with the experimental data.

Mathematical modelling of the formation of the WC-Co coating structure and adhesion on copper substrate during high velocity oxygen-fuel (HVOF) spraying is provided. Smooth (polished) and rough (grit blasted) substrates are considered. Variations of solidification time, solidification velocity, thermal gradient, and cooling velocity in the coating and substrate interfacial region are studied. Formation of the amorphous and crystalline structures in the coating and of the crystalline structure in the substrate interfacial region is investigated. Behaviour of the crystal size and intercrystalline distance with respect to the thermal spray parameters and morphology of the substrate surface is analysed. Optimal conditions for the development of fine and dense crystalline structure are determined. Mechanical and thermal mechanisms of development of the substrate-coating adhesion are discussed. Results obtained agree well with experimental data.