Heat Transfer Through Ceramic Barrier Coatings Used in Internal Combustion Engines

The paper presents the results of research concerned with heat exchange through surfaces covered with a thin ceramic layer. Theoretical analysis has been carried out to show that the presence of a ceramic layer reduces the density of heat flux flowing through the tested surface, provided the convection heat transfer coefficient is constant - the same as for surfaces without any ceramic layer. Heat exchange for three kinds of thin heat-insulting layers has been examined at a specially designed test stand. The ceramic layers were made of ZrO₂ partially stabilized with Y₂O₃ or Ln₂O₃ and a plasma coated onto the base of the Al alloy. The density of heat flux, heat transfer coefficient equivalent, and heat resistance of the ceramic layer have been established. It has been proven that in the case of thin heat-insulating coatings of metal surfaces, the establishment of conductive heat resistance is not the only or the most decisive factor affecting heat exchange through such systems. Heat exchange is greatly affected by porosity, roughness and emissivity. Taking into consideration the above, it follows that knowing only the layer thickness and thermal conductivity value is not enough to make calculations of heat flux applied to surfaces with tested layer. In such cases heat exchange can be examined only by means of experimentation.