ICMCTF2010 Session A2: Coatings for Use in Harsh Thermochemical Environments

It is well-known that the fireside environment in biomass- and waste-fired power boilers is considerably more corrosive towards high temperature alloys compared to plants burning fossil fuels. To cope with these corrosion problems, the plants are designed for relatively low maximum steam temperatures, thereby severely limiting the utilization of the fuel. The high corrosivity is often attributed primarily to the high chlorine content of the fuel. The present paper investigates another aspect of these corrosive fireside environments, namely their high alkali content. Thus, the corrosion effects of alkali salts towards austenitic and ferritic high temperature alloys have been investigated, focusing on the mechanisms of corrosion. The work involves isothermal thermobalance and tube furnace exposures in the range 400-800^oC. The polished sample coupons are exposed to well-controlled and strongly simplified synthetic environments. A comprehensive analysis of the exposed samples is carried out using ESEM/EDX, FIB, and TEM. In-situ ESEM studies of the reaction of KCl on low alloyed steel will also be presented. Additionally, comparisons are made with materials exposed in real plant environments. We find that alkali chlorides and alkali carbonates tends to accelerate the corrosion of chromia-forming steels by reacting with chromia in the protective scale, converting it to alkali chromate (VI), e.g., K₂CrO₄. The resulting iron-rich scale has poor protective properties resulting in a strong increase of oxidation rate. In contrast, the corresponding reaction with alkali sulphate does not occur because it is disallowed thermodynamically. In the case of alkali chloride, the iron-rich oxide scale that results from the reaction is penetrated by chloride ions. The formation of sub-scale chlorides destroys scale adhesion, further accelerating corrosion attack.

Residual stresses along with adhesion and density are central features in the performance of protective coatings against metal dusting. In the present study, the development of residuals stresses during deposition of Cr/Cr oxide layers on HK40 steel substrates by reactive magnetron sputtering was investigated. Two series of coatings were obtained: the first one varying the voltage bias applied to the substrate and the second one varying the rate of oxygen feeding. The coatings were characterized by optical microscopy, scanning electron microscopy + energy dispersive microanalysis, x-ray diffraction and atomic force microscopy. The samples were then subjected to nanoindentation experiments to evaluate their mechanical behavior as well as the presence of residual stresses. The results indicate that the processing variables selected do have a clear effect on the type and amount of residual stresses as well as on the coating microstructure. The results of the present study are of great value in the optimization of the coating process required to produce more effective protective layers against metal dusting.

Aluminum oxide thin films on HK40 steel substrates were produced by reactive magnetron sputtering to investigate their response to carburizing conditions. The structure of the films was analyzed as a function of process parameters; power, bias voltage, and oxygen flow as well as the application of an adhesion layer under various deposition conditions. The films were subjected to carburization experiments in a CH₄ atmosphere at 600°C and 800°C in a thermobalance. From the deposition experiments, Al/ amorphous Al oxide films varying in thickness from 200 nm to 1 μm were obtained. Relating the film microstructure to the processing conditions indicated that power and bias voltage seemed to have a stronger effect on the film density and thus on their response to the carburizing atmosphere. In general, the coated substrates showed a better response to carburizing than uncoated ones. In particular, results of carburizing tests performed at 800 °C on aluminum oxide films deposited without the support of an aluminum adhesion layer, showed the effectiveness of the adhesion layer and its impact on the mechanical and protective properties of the film.

Advanced coating concept like nanocomposite coatings become increasingly important for wear protection in elevated temperatures as well as corrosion and oxidation protection. Recently an increasing amount of attention is focused on the nitrides based coatings deposited by cathodic arc evaporation. The addition of silicon to the widely used aluminium/chromium containing transition metal nitrides is promising for the synthesis of hard and thermally stable films with good oxidation resistance.

In this study the nanocomposite (Cr, Al)_1-xN_x/a-Si₃N₄ coatings deposited by lateral rotating cathode arc technique (LARC^®) on hot work tool steel AISI H11 were studied by scanning electron microscopy, glazing incident X-ray diffraction, Atomic Force Microscopy, GDOES spectroscopy and nanoindentation.

The effect of annealing heat treatment in inert atmosphere on the coating crystal structure, surface morphology, hardness and elastic modulus were also investigated. It was found that nanocomposite (Cr, Al)_1-xN_x/a-Si₃N₄ coatings undergo phase transformation, above 900°C the hexagonal AlN structure start to precipitate.

The deposited CrAlSiN coatings show an fcc-Cr_1-xAl_xN type structure with different aluminium contents that influence mechanical properties and oxidation resistance.