NOVELTY - The substrate carries a refractory layer. This is or contains a porous composite of spherical, spheroidal or polyhedral metal oxide particles. Some or all of the metal oxide particles are hollow. A diffusion layer intervenes between the substrate and the porous composite. The diffusion layer contains the metal form of an oxide contained in the porous composite. It is present in greater proportion, than in the boundary layer of the substrate.    USE - A refractory coating. Applications for the composite layer exist in gas and steam turbine components, in power station boilers and furnaces. It may be used as a heat-resistant coating on fuel cell components, especially on electrodes and interconnects.    ADVANTAGE - As a result of its porosity, the coating has very low thermal conductivity and mass, making it ideal for turbine coatings. Its pores can be open or closed. Starting from metal particles, it is economically produced using known, controlled processes. Sintering forms a coherent network, bonded with the substrate. A very significant feature for design, is increased freedom of ceramic selection. It is no longer necessary to choose a low thermal conductivity ceramic to achieve good insulation. In the past, this could limit the choice to lower-melting ceramics. Instead, highly-conductive ceramics with very high melting points can be used. This increases the upper limits of temperature and thermodynamic efficiency in e.g. a gas turbine, whilst retaining the benefits of high thermal insulation. The intermediate layer protects against corrosion and is a bonding agent. Coating can be envisaged retrospectively and in situ, for refurbishment or local repair. Further benefits are discussed.    DETAILED DESCRIPTION - The metal oxide particle size is preferably 2-20 mu m. The metal oxide particles contain Al, Fe, Ni, Co, Cr, Cu, Ti, Zr, Y, La, Hf and/or Nb as the oxide-forming metal. The metal oxide particles contain or consist of Al2O3 and contain or consist of a thermodynamically-stable oxide phase. The metal oxide particles contain or consist of alpha -Al2O3. Small linear channels pass through the refractory layer. No molybdenum or tungsten are contained. There are two or more refractory layers, differentiated in the type of oxide-forming metal contained, and/or the size of the metal oxide particles. To make the layer, spherical, spheroidal or polyhedral metal particles, 0.1-50 mu m in size, are applied to a substrate and bonded with each other and the substrate. Bonding is achieved by heating and resultant sinter-bridging. Thermal oxidation then produces the porous composite, in which some or all of the particles are hollow. An INDEPENDENT CLAIM is included for the corresponding method of refractory layer formation    DESCRIPTION OF DRAWING(S) - The drawing shows an electron micrograph of an example of the porous sintered structure produced.