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Up to ~ 3 µm thick alumina coatings on corundum ceramic, sodalimesilica glass and Inconel718 were produced from mixtures of boehmite sols and corundum suspensions.
Transmission electron micrographs in combination with electron diffraction and energy dispersive X-ray spectroscopy served to identify crystallographic phases and to characterize the microstructure of the coatings. Using corundum ceramic as substrate the initially deposited boehmite transforms via transition aluminas to corundum while heating to 1200 °C. In the cases of glass and Inconel718 thermal treatments up to 520 °C and 1100 °C, respectively, cause diffusion of ions from the substrates into the coatings. Thus additional oxide phases were formed.
All coatings are free of cracks or delaminations and do not show any directed crystal growth.
Aluminas resulting from sols prepared via a modified Yoldas procedure were studied with differential thermal analysis (DTA), differential thermal gravimetrie (DTG), 27Al nuclear magnetic resonance (27Al MAS NMR) and X-ray diffraction (XRD) concerning their thermal properties, aluminum speciation and phase content.
Hydrolysis of aluminum-sec-butoxide in aluminum nitrate solutions allowed to prepare stable sols with varying NO3-/Al molar ratios, solids contents and pH values. Resulting sols contained different aluminum species including also Al13 polycations. Sol preparation conditions also determined aluminum speciation in solid products obtained after thermal treatments of gels obtained from these sols. Al13 polycations and AlO5 species were found to play an important role for thermally induced transformation from amorphous products via eta-Al2O3 to alpha-Al2O3. Intermediately formed eta-Al2O3 promotes the phase transformation to alpha-Al2O3.
Alumosols in combination with well dispersed corundum were successfully used to form up to 2.5 ìm thick coatings on the Ni-base alloy SC16. These coatings withstood heat treatments at 700 °C in a water vapour containing atmosphere. The heat treatment caused formation of delta-alumina in the coating and diffusion of chromium and titanium into the coating.
The need for a more efficient coal power plant generation (e.g. oxyfuel technology) results in modified process parameters and enhanced corrosion. To reach the necessary service life of high temperature parts protective coatings may be a sufficient technical solution. A modified Yoldas sol (Al2O3 based) was used to coat X20CrMoV12-1 by spin coating. After appropriate heat treatments transition alumina coatings being about 400 nm thick were obtained. Oxidation studies were carried out in laboratory air at temperatures up to 650 °C for up to 500 h exposure time. In case of the uncoated sample a rough oxide layer formed on the surface and a remarkable weight gain (2.62 mg/cm²) were detected. The sol–gel alumina layer (mainly δ-Al2O3) demonstrated a high protection, i.e. a very low weight gain (0.05 mg/cm²). Diffusion of alloying elements into the coating was observed. No indication of spallation of the coating occurred. Local defects (2 µm–30 µm) in the coating led to the formation of iron-oxide islands.
Alumina thin films
(2016)
This chapter gives an overview over preparation, properties and applications of alumina thin films. The preparation of polymeric and colloidal alumina sols and their deposition on diverse substrate materials with different geometries are described first. Consideration of properties (surface tension, particle size, viscosity, aging of the sols and their optimization by using certain additives) is included, too. A description of post-treatments of the coatings, which initiate phase transformations and also determine their microstructure, completes the part on coating preparation. Data on adhesion, thickness, surface roughness and mechanical, textural as well as optical properties of alumina thin films are provided in a further paragraph. In a third main part experience concerning possible applications, such as (i) membranes for catalysis, separation and filtration, (ii) barrier layers against electrochemical and high temperature corrosion or inter-diffusion of ions between different phases, (iii) mechanical protection, (iv) sensor construction, (v) improvement of optical properties, as well as (vi) achieving water repellency is summarized.
This work compares thin layers (films) and monoliths prepared from alumina sols with respect to their microstructure, thermal evolution, porosity and specific surface area. After heat treatment at similar temperatures, films and monoliths showed the same qualitative changes in porosity and specific surface area. However, some marked quantitative differences were detected. Film fragments had a lower open porosity, a lower specific surface area and a narrower pore size distribution. Furthermore, the thermal evolution showed a markedly different burnout of organic components between films and monoliths. The observed differences between films and monolith can be explained by the ageing history of the sols during sample preparation.
In order to reduce CO2 emissions fossil fuelled power stations with high combustion efficiency are being developed. The increase of the operating temperature, a common way to improve combustion efficiency, leads to enhanced corrosion of heat exchange steel tubes in the power plants. Within the framework of the European project “Production of Coatings for New Efficient and Clean Coal Power Plant Materials” (POEMA)", high temperature corrosion protection coatings are now under investigation.
Thin porous sol-gel alumina films are promising candidate coating materials. Coatings were prepared by applying boehmite sols on grinded steel P92 and subsequent heat treatments at temperatures up to 650 °C. Thus a porous layer of worm-like particles was formed consisting of nano-crystallites and amorphous alumina. A dense interface with satisfying adhesion resulted from diffusion of chromium and iron ions out of the steel into the porous coating. However, the film locally exhibited some cracks caused by steep edges in the grinded steel surface. An appropriate substrate pretreatment should avoid this problem.
The protection of the steel relies on the barrier function of the alumina coating and the formation of a dense chromia layer at the steel surface.