Radio frequency sputtering has been used to deposit α-alumina (α-Al2O3) thin films at substrate temperatures of 280–560 °C. The films are shown to be single phased and hard. Nanoindentation gives values of 306±31 and 27±3 GPa for elastic modulus and hardness, respectively, for a substrate temperature of 280 °C. Growth of the α phase was achieved by in situ predeposition of a chromia template layer. Chromia crystallizes in the same hexagonal structure as α-alumina, with a lattice mismatch of 4.1% in the a- and 4.6% in the c-parameter, and is shown to nucleate readily on the amorphous substrates (silicon with a natural oxide layer). This results in local epitaxy of α-alumina on the chromia layer, as is shown by transmission electron microscopy. The alumina grains are columnar with grain widths increasing from 22±7 to 41±9 nm, as the temperature increases from 280 to 560 °C. This is consistent with a surface diffusion dominated growth mode and suggests that α-alumina deposition at low temperatures is possibl...

Microstructure of α-alumina thin films deposited at low temperatures on chromia template layers

Efficiency of α-alumina as diffusion barrier between bond coat and bulk material of gas turbine blades

Improved cyclic oxidation resistance of electron beam physical vapor deposited nano-oxide dispersed β-NiAl coatings for Hf-containing superalloy

This study systematically analyzes the experimental results of the resistance of various coatings on superalloy IN-738LC to high temperature oxidation and corrosion. Three different pack cementation processes are carried out: pack-aluminization process, two-step aluminization–chromization process and siliconization process (with or without TiB2 in the packing mixture). Rhodium or palladium is deposited on the superalloy IN-738LC specimen by electroplating prior to the pack-aluminization processes. NiAl and Ni2Al3 phases are the main constituents of the aluminized coatings. Single Cr-rich phase forms on the surface of aluminized–chromized coatings. The Ni2Si, Ni3Si and TiSi phases exist after simple siliconization while Ni2Si, Ni3Si, TiSi, TiSi2 and TiB phases form after siliconization with TiB2 in the packing mixture. Among the three different categories of pack cementation coatings, aluminized coatings have the best isothermal and cyclic oxidation resistance at high temperature. When the temperature is over 850 °C, aluminized–chromized coating layers lose their protection against oxidation due to the spalling or cracking of the oxide scales during the test for cyclic oxidation. Siliconized coatings are brittle and crack severely due to thermal shock. Therefore, siliconized coatings are not suitable for superalloy IN-738-LC. Hot corrosion tests were carried out at 900 °C, a temperature above the melting point of Na2SO4 (884 °C). All three categories of pack cementation coatings show degradation of the corrosion scales and internal attack. Nevertheless, the aluminized coatings were still protective under hot corrosion conditions. The ranking of high temperature oxidation and corrosion resistance of all pack cementation coatings, from best to worst, is as follows: Rh–Al, simple Al, Pd–Al, Si+TiB2, simple Si, Pd–Al–Cr, Rh–Al–Cr and simple Al–Cr samples.

Improvement of high temperature oxidation and corrosion resistance of superalloy IN-738LC by pack cementation

Alumina coatings were deposited on glass, silicon and stainless steel substrates by magnetron sputtering of two cylindrical aluminum targets in argon–oxygen plasma using a mid frequency AC power supply operated at 41 kHz and 5 kW. No substrate heating was used during the deposition process. Reactive sputtering was carried out in the poisoned mode of the targets at low deposition rates of about 0.09 nm s − 1 . The coatings were characterized by X-ray diffraction studies and found to be nano-crystalline, γ-phase of aluminum oxide. The coatings thickness was measured using the optical interference effects in the UV–visible spectrum of thin films deposited on glass substrates. Thin film thickness after 4 h of reactive sputtering was found to be 1.3 μm. We further studied the oxidation resistance of the alumina coatings produced on stainless steel substrates at a temperature of 400 °C. Coatings were found to be completely stable at this temperature and prevented the oxidation of underlying stainless steel substrate.

Nanocrystalline gamma alumina coatings by inverted cylindrical magnetron sputtering

With rising operating temperatures of gas turbines, interdiffusion may occur between the structural material of the blades (superalloy) and the corrosion-resistant layer (MCrAlY) leading to premature coating failure. Depositing a diffusion barrier between the base material and the layer is a conceivable means to suppress this diffusion. α-Al 2 O 3 appears to be well suited as a diffusion barrier since it passes through no phase transition at high temperatures and possesses a hcp crystal structure with a very small number of defects. However, chemical vapor deposition of α-Al 2 O 3 is known to be difficult because of preferred whisker formation due to the high contents of Ni and Co in the substrate. In this paper two ways to suppress the growth of whiskers and to deposit smooth α-Al 2 O 3 films on nickel base superalloy substrates are presented. Furthermore, the efficiency of the diffusion barrier was tested by annealing experiments at 1100°C for 100 h. The α-Al 2 O 3 films remained dense and very effective as a diffusion barrier.

Chemical vapor deposition of smooth α-Al2O3 films on nickel base superalloys as diffusion barriers

The deposition rate of α-Al 2 O 3 was measured at 0.1 and 1 bar and variable partial pressures of AlCl 3 , CO 2 , H 2 and HC by means of thermobalance. HCl additions proved to have a retarding effect on the deposition. From the temperature and the total pressure dependence of the deposition rate it was concluded that the process was determined by chemical reactions rather than gas diffusion. The rate increased with AlCl 3 , CO 2 and H 2 partial pressures, decreased with rising HCl contents, and was independent of the CO pressure. The partial pressure dependences did not lead to simple reaction orders indicating the overlap of at least two parallel reactions. Two rate equations were deduced front the measurements to describe two parallel reaction paths, one equation being based on an assumed reaction mechanism involving the homogeneous watergas shift reaction and the subsequent heterogeneous formation of AlOCl(ad) as rate-determining intermediate. The second equation is purely empirical. Results of the numerical simulation based on these equations and heat and mass transport calculations are compared with the experimental observation.

Deposition kinetics of Al2O3 from AlCl3-CO2-H2-HCl gas mixtures by thermal CVD in a hot-wall reactor

The kinetics of CVD of boron nitride from a gas mixture of 1,3,5-tri(N-methyl)borazine (TMB), ammonia, and hydrogen were studied at 900 to 1050°C and a total pressure of 1 bar. TMB, a liquid between -3 and 136°C, is assumed to be a single source precursor for CVD of BN. However, to suppress gas phase nucleation, one has to add sufficiently large amounts of ammonia. Below 900°C no deposition was observed within 10 hours. Between 900 and 980°C the deposition rate was controlled by a surface reaction with an apparent activation energy of 145 kJ mol -1 . The reaction was found to be first-order with respect to TMB and zero-order in both NH 3 and H 2 . Above 1000°C the activation energy decreased to 26 kJmol -1 which corresponds to gas diffusion as the rate-determining step. Gas phase nucleation makes it difficult to produce smooth films above 1000°C. The reaction product was turbostratic hexagonal boron nitride with about 52 at.-% nitrogen, 46 at.-% boron, and 1-2 at.-% codeposited carbon.

/pdf/kinetics-of-chemical-vapour-deposition-of-boron-nitride-from-k7b3gin0wy.pdf

Kinetics of Chemical Vapour Deposition of Boron Nitride from a Gas Mixture of Trimethylborazine, Ammonia, and Hydrogen at 900 to 1050 °C and 1 Bar Total Pressure

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Chemical vapor deposition of smooth α-Al2O3 films on nickel base superalloys as diffusion barriers

Deposition kinetics of Al2O3 from AlCl3-CO2-H2-HCl gas mixtures by thermal CVD in a hot-wall reactor

Kinetics of Chemical Vapour Deposition of Boron Nitride from a Gas Mixture of Trimethylborazine, Ammonia, and Hydrogen at 900 to 1050 °C and 1 Bar Total Pressure