Y.D. He

Oxidation behaviour of nanocrystalline Fe-Ni-Cr-Al alloy coatings

AbstractNanocrystalline Fe–Ni–Cr–Al alloy coatings with ∼4 wt-%Al were produced using the unbalanced magnetron sputter deposition technique with a composite 310S stainless steel target embedded with aluminium plugs. The oxidation behaviour of the coatings was studied, during which complete external α-Al2O3 scales were formed. During isothermal oxidation tests at 950, 1000, and 1050°C, the oxidation kinetics followed an essentially parabolic rate law, and the oxidation constants were measured to be 2·06 × 10-3, 4·23 × 10-3, and 1·14 × 10-2 mg2 cm-4 h-1 respectively. During a cyclic oxidation test at 1000°C the α-Al2O3 scale showed good scale spallation resistance. The surface hardness of the coatings was measured with a Knoop indentor before and after oxidation. After oxidation, the coating surface hardness was still significantly higher than that of the uncoated specimen, demonstrating the potential this coating has in the improvement of high temperature erosion resistance.

Micro-crystalline Fe–Cr–Ni–Al–Y2O3 ODS alloy coatings produced by high frequency electric-spark deposition

Fe–Cr–Ni–Al–Y2O3 oxide dispersion strengthened (ODS) alloy coatings were deposited on the surface of 1Cr18Ni9Ti stainless steel by using high-frequency electric-spark technique. The composition of these ODS coatings was determined by the interaction between the electrode material MA956 ODS alloy and the substrate alloy. These ODS coatings possess micro- or nano-crystalline structure with metallurgical bonding to the substrates. The microstructures of the dispersed oxide particles were observed by TEM. High-temperature oxidation was performed in air at 1000 °C. Results indicated that the selective oxidation of Cr was greatly promoted and the scale spallation resistance was significantly improved. The synergistic effects of micro-crystallization and dispersed Y2O3 particles on the oxidation behaviors have been discussed.

Synergistic effects of surface micro-crystallisation and Ce addition on selective oxidation of Ni-20Cr alloy

Surface micro-crystalline coatings were prepared on Ni-20Cr and Ni-20Cr-Ce alloys by high-energy pulse plasma treatment (HEPP). After oxidation at 1000°C in air for 200h, it was shown that both oxidation resistance and the Cr content in the oxide scales on these four alloys increased in the order of Ni-20Cr < Ni-20Cr-Ce < Ni-20Cr (HEPP) < Ni-20Cr-Ce (HEPP). The results indicated that the selective oxidation of Cr in Ni-20Cr alloys can be improved either by adding Ce or surface micro-crystallisation. The synergistic effects have been discussed based on Wagners selective oxidation theory, together with the reactive element effects, short circuit diffusion, oxide nucleation and growth processes, and stress release in the oxide scales.

Aluminized coating and AlY and AlCe codeposited coatings obtained by metalliding from molten salt films

Abstract Metalliding from molten salt films was studied. The salt film is a dispersed system composed of solid and liquid phases. The mass transport in the salt film is through both diffusion and electric migration, but not convection. Because of this, the critical current density required to deposit aluminium is decreased, and the cathodic reactions are controlled by diffusion, which makes AlY and AlCe codeposition easier. there is a critical current density required for yttrium or cerium to be codeposited with aluminium. The AlY and AlCe codeposited coatings have better resistance to high temperature corrosion.

OXIDATION RESISTANCE OF NANOCRYSTAL ODS ALUMINIDE COATINGS PRODUCED BY PACK ALUMINIZING PROCESS ASSISTED BY BALL PEENING

Nanocrystal ODS (oxide dispersion strengthening) aluminide coatings were produced on a stainless steel and nickel-based superalloy by the pack aluminizing process assisted by ball peening. Pure Al powders and 1% of ultra-fine Y 2 O 3 powders were mixed by ball milling. The ultra-fine Y 2 O 3 powders were dispersed in Al particles. Ball peening welded the Al particles onto the substrate and accelerated the formation of aluminide coating. Nanocrystal ODS aluminide coatings were produced by the outward growth at a much low temperature (below 600°C) in a short treatment time. The effects of the operation temperature and treatment time on the formation of the coatings were analyzed. SEM (scanning electron microscope), AFM (atomic force microscope), EDS (energy dispersive X-ray spectroscopy), XRF (X-ray fluorescence spectrometer) and XRD (X-ray diffraction) methods were applied to investigate the microstructure of the coatings. High-temperature oxidation tests were carried out to evaluate the oxidation resistance of the ODS aluminide coatings.

High temperature oxidation protection using nanocrystalline coatings

Abstract: This is an overview of the research work at The University of Auckland in collaboration with the University of Science and Technology Beijing on the development of nanostructured coating systems for protection of alloys from high temperature oxidation. The first section will cover the preparation of nanocrystalline metallic coatings by unbalanced magnetron sputtering and electro-spark deposition. The effect of grain size reduction on the selective oxidation of alloys, and the growth of protective oxide scales such as Al2O3 and Cr2O3 on the external surfaces at elevated temperatures, will be discussed. The second part will be focused on our recent progress on fabrication and characterization of ceramic coatings with nanostructural features; in particular, multilayered and composite oxide coatings for applications in high temperature oxidation protection.

Oxidation behaviour of Cr25-Ni20 stainless steel with nano-crystalline surface coatings

Unbalanced magnetron sputtering depositions have been performed on 310S stainless steel specimens. Different Ar partial pressures were applied to produce nanocrystalline coatings with different grain size. A scanning electron microscope (SEM) and an atomic force microscope (AFM) were used to study the detailed morphology of the coatings. The cyclic oxidation behaviours of the coated and uncoated specimens at 1000°C for up to 200 hours have been studied. It was found that the coated and uncoated specimens showed similar oxidation mass gains, but the spallation resistance of the coated specimens was improved significantly. The uncoated 310S showed scale spallation much earlier than the coated specimens, and the quantity of the spalled scales was reduced by ∼80% from the uncoated specimens. The oxidation kinetics of the coated specimens were analysed by considering both the grain boundary and lattice diffusion effects. The spallation behaviours were analysed in terms of fracture energy.

Oxidation Resistant Coatings of ZrO2 and ZrO2-Y2O3 Applied by High Energy Pulse Plasma Deposition

A novel surface treatment technique has been developed for producing coatings of ZrO 2 and ZrO 2 -Y 2 O 3 on the surface of metals, ceramics or polymers by using high-energy pulse plasma. The present study was concentrated on the basic mechanism and the effects of operation conditions on the coating qualities. It was shown that the oxidation resistance of 18-8 stainless steel was markedly improved by applying ZrO 2 and ZrO 2 -Y 2 O 3 coatings. The oxidation rates were reduced by a factor of 20, and the amounts of the oxide scale spallation were reduced by a factor of 100 - 300. It is believed that the improvements of the oxidation resistance were caused by the promotion of selective oxidation of Cr in the steel.