Continuous Al2O3 Film Research Articles

Effect of continuous gradient Al on high-temperature oxidation of Ni-Cr-Co based superalloys via a high-throughput method

This study focuses on the effect of the main antioxidation element Al on the high-temperature oxidation resistance of Ni-Cr-Co based superalloys. A novel “3D printing-high temperature diffusion” high-throughput method was used to accurately determine the critical Al content to form continuous Al2O3 film. Results show that Al2O3 film significantly affects the element diffusion process during oxidation, and the critical Al content to form continuous Al2O3 film is 0.8 wt%. This conclusion was further confirmed by the corresponding discrete samples. It was also found that Ni-Cr-Co based superalloys exhibit “oxidation weight loss” and “Kp-antioxidation positive correlation” phenomena in high humidity condition.

Improved hot corrosion resistance of TiBw/TA15 composite via pack cementation aluminide coating

An aluminide coating was prepared on the TiBw/TA15 composite by pack cementation to improve the hot corrosion resistance. After 100 h test, the mass gain for uncoated and coated samples was 39.9 and 2.1 mg cm−2, respectively, which showed that the aluminide coating effectively improved the hot corrosion resistance. For the uncoated sample, the circulation of sulfurization/chlorination–oxidation and the basic dissolution with basicity/acidity change in molten salt caused serious hot corrosion. For the coated sample, the continuous and dense Al2O3 film effectively weakened the basicity/acidity change by preventing the diffusion of S and Cl elements. The high Al content in the TiAl3 phase could maintain the continuous Al2O3 film, although parts of it could be dissolved by molten Na2SO4.

Dopamine facilitates Al2O3 film growth on polyethylene terephthalate by low-temperature plasma-enhanced atomic layer deposition

Polymeric materials, including polyethylene terephthalate (PET), are widely used in various fields because of their beneficial properties. Functional films are deposited on these materials through different approaches, such as plasma-enhanced atomic layer deposition (PEALD), to enhance their performance and prolong their life span. However, the inert and thermally fragile nature of most polymers hinders deposition. We developed a strategy for the PEALD of nanoscale Al2O3 films on PET substrates. First, a PET substrate is subjected to alkali treatment, which gives it basic hydrophilicity for the subsequent dopamine modification. After 24 h of dopamine deposition, the substrate shows adequate active sites (phenolic hydroxyl groups), which can chemisorb large amounts of precursor during the initial deposition. The island growth mode was observed during the PEALD processes. We analyzed the detailed chemical components of Al2O3 on alkali-treated PET and dopamine-modified PET. After 100 cycles of deposition, the Al2O3 films on both samples contained much hydrogen. Benefitting from the more active sites, we observed more continuous Al2O3 film on dopamine-modified PET, which exhibited excellent water vapor blocking performance. Our findings suggest that dopamine could act as a ‘bridge’ between polymers and PEALD functional films.

Hot corrosion behavior of a NiCoCrAlY coating fabricated by laser cladding on 17-4PH stainless steel

NiCoCrAlY coatings were prepared using laser cladding technology and subjected to a continuous hot corrosion test in a eutectic Na2SO4/NaCl (75:25, wt. %/wt. %) salt at 800 ℃ in air. The corrosion kinetics curves of the NiCoCrAlY coating and the untreated substrate over 100 h of hot corrosion were compared and analyzed. The results show that in the early stage of hot corrosion, a chemical reaction that produces alkaline fusion near the oxide scale takes place at the molten salt/coating interface, causing Al2O3/Cr2O3 to form and dissolve continuously. In the late stage, a new continuous Al2O3 film is formed.

Effect of Al content on the high-temperature oxidation behavior of 18Cr–Al–Si ferritic heat-resistant stainless steel

The effect of Al content on the high-temperature oxidation (800 and 900 °C) of 18Cr–Al–Si FHSS (ferritic heat-resistant stainless steel) for up to 180 h in air was investigated via the isothermal oxidation method. The oxidation kinetic curves followed a parabolic law. A higher Al content reduced the oxidation rate and improved the oxidation resistance and also promoted the formation of a denser and more continuous Al2O3 film during the early oxidation stages, which provided an effective barrier that hindered the diffusion of oxygen into the matrix. The oxide films that formed on the 0.63Al FHSS and 1.06Al FHSS were composed of spinel MnCr2O4 in the outer layer, MnO and Cr2O3 in the middle layer, and Al2O3 and SiO2 in the inner layer. The middle layer of 0.63Al FHSS also contained a small amount of Fe2O3. The oxidation mechanism was also discussed based on oxidation kinetics, morphological observations, and phase analysis of oxide films.

Aluminum oxide nucleation in the early stages of atomic layer deposition on epitaxial graphene

The nucleation and growth mechanism of aluminum oxide (Al2O3) in the early stages of atomic layer deposition (ALD) on monolayer epitaxial graphene (EG) on silicon carbide (4H–SiC) has been investigated by atomic force microscopy (AFM), conductive-atomic force microscopy (C-AFM) and Raman spectroscopy. Differently than for other types of graphene, a large and uniform density of nucleation sites was observed in the case of EG and ascribed to the presence of the buffer layer at EG/SiC interface. The deposition process was characterized by Al2O3 island growth in the very early stages, followed by the formation of a continuous Al2O3 film (∼2.4 nm thick) after only 40 ALD cycles due to the islands coalescence, and subsequent layer-by-layer growth. The electrical insulating properties of the deposited ultrathin Al2O3 films were demonstrated by nanoscale current mapping with C-AFM. Raman spectroscopy analyses showed low impact of the ALD process on the defect’s density of EG. The EG strain was also almost unaffected by the deposition in the regime of island growth and coalescence, whereas a significant increase was observed after the formation of a compact Al2O3 film. The obtained results can have important implications for device applications of epitaxial graphene requiring ultra-thin high-k insulators.

Mechanism of Al on FeCrAl steam oxidation behavior and molecular dynamics simulations

FeCrAl alloys exhibit excellent oxidation resistance in high temperature steam environment for the Al percent in the alloy. However, the Fe–Cr–Al alloy becomes more brittle and the defects generated after irradiation also increase with the increasing Al content. To address this obstacle and elucidate the mechanism for the impact of Al on FeCrAl oxidation behavior, in this study, oxidation experiments and molecular dynamics simulations were used to investigate the oxidation of the Fe–Cr–Al alloy in high temperature steam environment with different contents of Al (2 wt% (weight percent) ∼ 6 wt%). When the Al content is between 2 wt% and 3 wt%, the Fe–Cr–Al alloy will form the thick and porous FeCr2O4 film with Fe2O3, Cr2O3 and Al2O3 in it, when the Al content is between 4 wt% and 6 wt%, the Fe–Cr–Al alloy can form a continuous Al2O3 film observed with SEM. From molecular dynamics simulation, it also found that Al2O3 continuous film is easier to form with increasing Al content. The oxidation thermodynamic analysis indicated that Al activity becomes higher in the alloy matrix with increasing Al, and Al can induce reduction reaction with Cr2O3, Fe2O3 and FeCr2O4 in the high temperature steam environment.

Preparation and characterization of NiAl–TiC–TiB2 intermetallic matrix composite coatings by atmospheric plasma spraying of SHS powders

NiAl–TiC–TiB2 intermetallic matrix composite coatings with various TiC–TiB2 contents were fabricated by depositing self-propagating high-temperature synthesized (SHS) NiAl–TiC–TiB2 powders using atmospheric plasma spraying (APS) technique. The preparation process was carefully investigated and the effects of TiC–TiB2 contents on the physical and chemical properties of the composite coating were studied. Results indicate that SHS synthesized NiAl–TiC–TiB2 intermetallic matrix materials show high purity. The TiC–TiB2 particles are well dispersed in the NiAl matrix. There is no obvious oxidation in the composite coating during the APS process. With an increase in TiC–TiB2 contents, the hardness of NiAl–TiC–TiB2 composite coating is increased, while the thermal expansion coefficient (TEC) of the coating is decreased. NiAl–TiC–TiB2 intermetallic matrix composite coating with moderate TiC–TiB2 content shows the best corrosion resistance. Few TiC–TiB2 particles in NiAl promote the formation of the continuous Al2O3 film without destroying the integrity of the film improving the corrosion resistance of the coating.

Nucleation and growth mechanisms of Al2O3 atomic layerdeposition on synthetic polycrystalline MoS2.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of Al2O3 (using Al(CH3)3/H2O ALD) on MoS2 whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS2, we exclude the impact of surface contamination from a transfer process by direct Al2O3 deposition on synthetic MoS2 layers obtained by a high temperature sulfurization process. It is shown that Al2O3 ALD on the MoS2 surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS2 crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS2 top surface. During further deposition, the as-formed Al2O3 nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous Al2O3 film is obtained by lateral growth over the MoS2 crystal basal plane, with the point of layer closure determined by the grain size at the MoS2 top surface and the lateral growth rate. The created Al2O3/MoS2 interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS2 basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the Al2O3 layers. To conclude, our growth study reveals that the inherent reactivity of the MoS2 basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation.

Improvement of Oxidation Resistance of Remelted Zone in an Al2O3-Forming Austenitic Stainless Steel by Annealing

To improve the oxidation resistance of a remelted zone in Al2O3-forming austenitic stainless steels, effects of annealing temperature on microstructural evolution and Al2O3 formation in remelted zone of Fe–25Ni–10Cr–5Al–1Nb–Y steel (wt%) were investigated. The results showed that a continuous Al2O3 film could not form in the remelted zone before annealing. After annealing at 1,100, 1,150 and 1,180 °C, a continuous Al2O3 film formed. The existence of a second phase containing Al is the key for the formation of a continuous Al2O3 film. The thickness of Al2O3 film increased with the rise in Al content in the second phase.

Improved nucleation of TiN atomic layer deposition films on SiLK low-k polymer dielectric using an Al2O3 atomic layer deposition adhesion layer

Diffusion barriers are required to prevent copper from diffusing into low-k polymer dielectrics in backend interconnects. The ability to deposit conformal diffusion barriers onto high aspect ratio, low-k polymer features requires atomic layer deposition (ALD) techniques. This study examined TiN ALD on SiLK (a trademark of the Dow Chemical Company) low-k polymer dielectric using tetrakis-dimethylamino titanium and NH3. X-ray fluorescence spectroscopy (XRFS), optical microscopy, and surface profiling of the TiN ALD films deposited on SiLK revealed discontinuous films displaying distinct patchy regions. The patches corresponded to a thinner TiN coating and were attributed to difficulties for TiN ALD nucleation on SiLK. To study TiN ALD nucleation, in situ quartz-crystal microbalance (QCM) measurements were performed by spincoating SiLK onto the QCM sensor. Subsequent QCM measurements during TiN ALD revealed very low initial TiN ALD growth rates indicating poor nucleation. Al2O3 ALD was then performed on the SiLK film using trimethyl aluminum and H2O. Surface profiling, XRFS, QCM, and transmission electron microscopy measurements revealed that the Al2O3 ALD film nucleates immediately on SiLK producing a continuous Al2O3 film. In addition, QCM measurements showed that TiN ALD nucleates readily on the Al2O3 surface. The Al2O3 ALD adhesion layer facilitated the growth of a continuous TiN ALD film on SiLK. Examination of TiN ALD films prepared on SiLK with progressively thinner Al2O3 ALD adhesion layers revealed that 10 Al2O3 ALD cycles were sufficient to promote the nucleation of the TiN ALD film.

Effect of chromium on the oxidation resistance of TiAl intermetallics

The effect of 10 at.%Cr on the oxidation resistance of TiAl intermetallic compound at 800-1100 degrees C in air was investigated The results indicated that 10 at.%Cr equally substituting for Ti and Al in TiAl alloy had duplex effects on the isothermal kinetics of TiAl. At lower temperatures (800-900 degrees C), Cr increased the oxidation rates as a result of the doping effect of Cr in the scale and at higher temperatures (1000-1100 degrees C), especially at 1100 degrees C, Cr significantly reduced the oxidation rates as a result of the formation of a continuous Al2O3 film on the surface. 10 at.%Cr only substituting for Ti in TiAl alloy remarkably reduced the oxidation rates at all temperatures by about two orders of magnitude. Moreover, 10 at%Cr significantly improved the cyclic-oxidation rsistance of TiAl alloy.

Synthesis of Al2O3 matrix composites by reactive infiltration

Reactive infiltration of a NiO-base blended powder with molten aluminium was attempted at 1673 K in order to obtain Al2O3 matrix composites containing a dispersion of Al3Ni, AlNi and/or AlNi3. The NiO powder was barely infiltrated by the molten aluminium after a 3600 s holding time at 1673 K. A continuous layer of Al2O3 was observed to exist at the infiltration front, which prevented any further infiltration. TiB2 particles were added to the NiO powder in order to absorb the heat of reaction between NiO and aluminium. When the TiB2 particle content in the [NiO+TiB2] powder blend was greater than 20 vol%, spontaneous infiltration occurred completely. Thus, it was shown that the addition of the TiB2 particles assisted in the spontaneous infiltration. The specimens produced by the in situ reaction consisted of Al2O3, TiB2 and Al3Ni. Al3Ni was mainly located between the TiB2 and Al2O3. The effect of the TiB2 addition on the infiltration kinetics was to decrease the maximum attainable temperature caused by the exothermic reaction. This in turn prevented the formation of a continuous Al2O3 film at the infiltration front. This resulted in the production of pathways for the infiltration of the molten aluminium and made possible the complete infiltration.