F.J. Belzunce

High temperature oxidation of HFPD thermal-sprayed MCrAlY coatings

NiCrAlY and CoNiCrAlY powders were thermal-sprayed using the high frequency pulse detonation method (HFPD) onto AISI 310 austenitic stainless steel samples in order to obtain dense, adherent high temperature oxidation resistant coatings. The cyclic oxidation behaviour in air of both types of coatings was determined at a maximum temperature of 1273 K. The porosity, internal oxidation (after the first 8-h oxidation cycle), hardness, coating thickness and substrate-coating adherence were not significantly modified by the high temperature oxidation cycles. The surface phase composition was evaluated throughout the tests using X-ray diffraction and scanning electron microscope techniques revealing the formation of a continuous and highly protective alumina layer. The oxidation kinetics of both coatings can be characterized by parabolic rate constants, which are very similar to those of pure alumina.

Influence of the thermal-spray procedure on the properties of a nickel-chromium coating

A modified NiCr coating was thermal-spray projected using different procedures (flame, plasma, HVOF and HFPD) onto stainless steel specimens. This type of coating is normally used as protection against heat, corrosion and erosion actions encountered in superheater and reheater tubes in power plant boilers. The microstructures, porosities, oxide contents and microhardnesses of the coatings were determined. Thermal fatigue tests under an atmosphere similar to power plant service conditions were conducted in an experimental combustion chamber and, finally, the adhesion between the substrate and the coating layer was evaluated by means of tensile tests. The results obtained are discussed, with special attention being paid to the specific characteristics of the different spraying procedures.

High temperature oxidation of HFPD thermal-sprayed MCrAlY coatings in simulated gas turbine environments

NiCrAlY and CoNiCrAlY powders were thermal-sprayed using the high frequency pulse detonation method (HFPD) onto AISI 310 austenitic stainless steel samples to obtain dense, adherent, high temperature oxidation resistant coatings. The oxidation behavior of both types of coatings in a 1000°C simulated gas turbine environment was experimentally determined. The porosity, hardness, coating thickness, and microstructure were not significantly modified by the high temperature oxidation cycles, but the internal oxidation increases significantly after a very low oxidation time. Surface phase composition was evaluated using x-ray diffraction (XRD) and scanning electron microscope (SEM) techniques, revealing the formation of a continuous and highly protective alumina layer. The oxidation kinetics of both coatings can be characterized by parabolic rate constants, which are very close to those for the formation of aluminum oxide on nickel or cobalt based alloys at similar conditions.

Use of small punch test to estimate the mechanical properties of powder metallurgy products employed in the automotive industry

Abstract Powder metallurgy is a manufacturing technology widely used, especially in the automotive sector. In order to assure the quality of these components different conventional tests are employed directly on the final product, but in order to mechanically characterise the sintered material, standard mechanical tests can only be performed onto samples compacted and sintered in controlled conditions similar to the ones used in the real production. The final mechanical properties of these products after compacting and sintering are the input needed to define and analyze new geometries and products. The use of the small punch test (SPT) to mechanically characterise in a direct way the final sintered products was explained in this research work and the regressions to determine the tensile mechanical properties of the aforementioned products were also developed.