Deformation Microstructure of Cold Gas Sprayed Coatings
- PDF / 4,553,464 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 223 Views
Deformation Microstructure of Cold Gas Sprayed Coatings C.Borchers, T.Stoltenhoff, F.G¨ artner, H.Kreye, H.Assadi1 University of the Federal Armed Forces, Dept. Maschinenbau, Hamburg, Germany 1 Dept. of Materials Science Engineering, Tarbiat Modarres University, Tehran, Iran ABSTRACT Cold Gas Spraying is a new coating technique, in which the formation of dense, tightly bonded coatings occurs only due to the kinetic energy of high velocity particles of the spray powder. These particles are still in the solid state as they impinge on the substrate. Adiabatic heating after impingement can cause local shear instabilities and jet formation. The local microstructure is strongly dependant on local stress state and temperature rise. A variety of different microstructures is observed by TEM. The results are compared with modelling of the spray process. INTRODUCTION In conventional thermal spray techniques like Flame Spraying, High Velocity Oxy-Fuel Spraying or Plasma Spraying, the spray process is accompanied by a partial melting of the powder feedstock material, which can result in high residual stresses of the coatings due to the shrinkage during rapid solidification on the substrate, and oxidization of the material where the process takes place in ambient atmosphere. The former can detract the adhesion and contiguity of the coating, whereas the latter has direct influence on phase formation because of oxides at particle-particle interfaces and therefore determines the physical properties of the coating, e.g. its resistivity. In the new coating technique of cold gas spraying (CGS), bonding of particles is a result of extensive plastic deformation and related phenomena at the interface [1,2,3]. In the process, solid copper particles in size range between 5 and 25 µm are introduced into the high pressure chamber of a converging-diverging Laval type nozzle and are accelerated in a supersonic stream by a propellant inert gas, which has been preheated to temperatures below 600o C. The particles reach velocities of 500 to 1000 m/s. Because of the low thermal influence, the method can open up new applications for metastable, nanocrystalline or oxidation sensitive materials. On one hand emphasis of this work is given to the modelling of particle impact where special attention is given to the criteria for bonding and here mainly to a critical particle velocity that the particles must not fall below. In addition the influence of impact angles, and the possible formation of regions where viscous flow of the material occurs are investigated. The modelling presented here is done for the example of copper. On the other hand, the deformation microstructure and binding mechanism of copper coatings are examined. The investigations presented in this work are performed by transmission electron microscopy (TEM) as well as resistivity measurements.
P7.10.1
EXPERIMENTAL Cold sprayed copper coatings are prepared by using nitrogen as process and carrier gas. To obtain a maximum deposition efficiency and dense coatings, preheating of the proces
Data Loading...
