Formation Of AlN Layer Research Articles

Evolution of corrosion behavior for AA7075 aluminum alloy implanted with nitrogen

This work reports the corrosion behavior of the AA7075 aluminum alloy under two different tempers (T6 and T73) after nitrogen implantation in 0.5 mol·L−1 NaCl solution. A combination of Electrochemical Impedance Spectroscopy and surface microstructure characterization was used to analyze the effects of nitrogen implantation on corrosion resistance.Nitrogen implantation, at 2 × 1017 N+ ions·cm−2 and 50 keV energy, induces the formation of AlN layer improving the corrosion resistance of aluminum alloys as evidenced by initial corrosion tests and a less corroded area.The evolution of the impedance indicates that the corrosion improvement vanishes over time. Implanted and unimplanted alloys show similar electrochemical parameters after 20 h immersion. However, microscopic examination shows less corrosion damage for implanted alloys after 72 h of immersion. These findings suggest that the implantation only delays the onset of corrosion probably due to degradation of AlN layer in aqueous medium. The corrosion resistance at long periods only depends on the aging heat treatment applied to the AA7075 aluminum alloy and the effect of implantation is negligible.

Chemical kinetics and thermodynamics of the AlN crystalline phase formation on sapphire substrate in ammonia MBE

Chemical kinetics of a two-dimensional (2D) AlN layer formation on the (0001) sapphire (Al2O3) surface during nitridation as function of ammonia flux and temperature is investigated by reflection high-energy electron diffraction. The process on the surface is described in framework of a chemical reactions kinetic model including interaction between partially reduced aluminum oxide species (AlO) and chemisorbed NH2 particles for the temperature range 1210 K. In this range, the process is described as a phase transition in frame of lattice gas model. Precision measurement of 2D AlN lattice parameters during the nitridation process detects the value of 0.301 nm. This value strongly differs from bulk value of wurtzite AlN structure 0.311 nm, but it coincides exactly with a characteristic structure parameter of the oxygen-deficient (0001) Al2O3 surface with reconstruction (√31×√31)R ± 9°. We assume that this coincidence is the result of minimizing the elastic stresses at the heterojunction of the 2D AlN and the (0001) Al2O3 layer.

Semipolar AlN on Si(100): Technology and properties

Initial stages of the semipolar AlN layer formation by chloride-hydride vapor phase epitaxy on a misoriented Si(100) substrate with a thin intermediate nano-SiC layer grown by atomic substitution method are investigated. It is found that once the growth rate of AlN layer on a nano-SiC/Si(100) substrate is about 0.02μm/h, the layer is formed in a polar direction. On the other hand, at the growth rate of 1μm/h the layer is formed in a semipolar direction. We propose the model of the semipolar AlN layer growth based on the formation of the special structure on the SiC layer synthesized by atomic substitution and on the possibility of the matching of the quasi-lattices of Al(20−23) and 3CSiC(100) planes assuming bonding of four atoms from the layer to four atoms of the 3CSiC quasi-lattice consisting of three lattice unit cells in one of the directions.

Graphene-like AlN layer formation on (111)Si surface by ammonia molecular beam epitaxy

The formation of a graphene-like AlN (g-AlN) layer on a (111)-oriented silicon substrate in ammonia molecular beam epitaxy (MBE) has been investigated by the RHEED method. A flat AlN layer with a thickness of a few monolayers was formed by the deposition of Al atoms onto a highly ordered SiN-(8×8) surface prepared on the atomically flat (111)Si substrate under an ammonia flux. An unusual (4×4) structure of g-AlN on the Si surface was found. The exact coincidence of the fundamental (0 1) AlN streak and the fractional-order (0 5/4) beam of the (4×4) RHEED structure allows us to measure precisely the in-plane lattice parameters of g-AlN. The g-AlN lattice constant of 3.08Å is found in a good agreement with the ab initio calculations performed recently. The evolution of the g-AlN in-plane lattice constant during the initial stages of the g-AlN formation and transformation to the bulk wurtzite AlN was analyzed.

2D AlN crystal phase formation on (0001) Al2O3 surface by ammonia MBE

AbstractKinetics of a two‐dimensional (2D) AlN layer formation on (0001) sapphire (Al2O3) surface during nitridation at different ammonia fluxes is investigated by reflection high energy electron diffraction (RHEED). The process on the surface is described in the framework of a chemical reactions kinetic model including interaction between partially reduced aluminum oxide species (AlO) and chemisorbed NH2 particles. The experimentally determined AlN formation rates as functions of both the temperature and the ammonia pressure are successfully described by a simple set of kinetic equations. Calculated maximum rate of the process well agrees with the experimental values. Precision measurement of 2D AlN lattice parameters during the nitridation process detects the value of 3.01 Å. This value coincides exactly with a characteristic structure parameter of the oxygen‐deficient Al2O3 surface. We assume this coincidence results from flexibility of the 2D AlN monolayer which provides minimization of elastic stresses at the AlN/(0001)Al2O3 interface. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermochemical nitridation of sapphire substrates of different crystallographic orientations

This paper presents the results of the study of the influence of physicochemical parameters of thermochemical nitridation of sapphire substrates of different crystallographic orientations in an N2 + (CO,H2) reducing atmosphere on structural characteristics, composition, and kinetics of AlN layer formation. The effective diffusion coefficients of nitrogen in sapphire at T = 1473−1773 K have been determined. The temperature dependences of the structural quality, thickness, and roughness of the AlN layer surface have been found; their strong dependence on the composition and reduction potential of the nitridation atmosphere has been shown. The structural models of matching the AlN and sapphire lattices during thermochemical nitridation have been proposed.

Gas injection as a tool for plasma process control during coating deposition

The paper describes the effects of utilizing pulse valves for dosing a plasma forming gas supplied directly to the plasma source. The concept of exploiting such valves as actual controllers of the plasma process replacing the ignitrons–spark switch valves used up to now has been practically verified in the technology of TiN layers manufactured by impulse plasma deposition (IPD) method. The TiN layers manufactured at modified plasma pulse generation conditions on unheated cutting tools made it possible to increase the cutting lifetime by a factor of up to twenty. The interpretation suggests that this increase is likely to result from lower energy dissipation for intermolecular collisions because of an increased mean free path. On the basis of positive results of research related to IPD method, the method of plasma excitation through an injection of plasma-forming gas has been applied in conditions of magnetron sputtering (gas injection magnetron sputtering, GIMS). GIMS applied as a material source with a single magnetron has been used for formation of AlN layers on silicon substrates. Electronic examination of MIS structures with such thin AlN layer demonstrated potentially advantageous structural properties of such layers in the context of their application in the design of electronic devices, e.g. as gate insulator layers.

Stability of nitrogen incorporated Al2O3 surfaces: Formation of AlN layers by oxygen desorption

The incorporation of nitrogen on Al2O3(0001) and (11̅02) surfaces is theoretically investigated on the basis of total-energy electronic-structure calculations within the density functional theory. The calculated surface energies as functions of oxygen and nitrogen chemical potentials reveal that under pregrowth conditions the O atoms of Al2O3 surfaces can be replaced by N atoms. Moreover, we find that the outermost O atoms tend to desorb from the surface and AlN layers are consequently formed on Al2O3 surfaces. This implies that the desorption of oxygen is crucial for the nitridation process. The structures of nitrogen incorporated Al2O3 surfaces obtained in the present study are consistent with those observed in the experiments.

Formation of AlN layer on (1 1 1)Al substrate by ammonia nitridation

AlN layers were formed on (1 1 1)Al substrates at a temperature below the melting point of Al using preheated ammonia in vacuum. The effect of the removal of the surface oxides on the substrate in the process was investigated. It was found that treatment using a buffered HF solution and subsequent annealing in vacuum was effective for obtaining a clean Al substrate surface. It was clarified that the removal of the surface oxides is required for the nitridation of Al substrates at a low temperature below the melting point of Al.

Formation of Aluminum Nitrides by Precipitate-Accommodated Plasma Nitriding

AbstractPrecipitate accommodated nitriding is proposed as a new, non-traditional surface treatment of aluminum alloys. In the case of nitriding the aluminum-copper alloys, Al2Cu plays an important role as a nucleation site to form AlN by the two step reactions of Al2Cu + 2N → 2AlN + Cu and Cu + 2Al → Al2Cu and as a helper to preserve the fast nitrogen diffusion path. In this precipitate accommodation, AlN islands form on Al2Cu in coherent, and, gradually cover the matrix surface by the AlN layer. This Al2Cu accommodation in nucleation reduces the pre-sputtering time and shortens the incubation time prior to formation of AlN layer. Furthermore, the interface between Al2Cu and AlN plays a nitrogen diffusion path across the nitrided layer. This Al2Cu accommodation in growth of AlN accelerates the growth rate of AlN layer in practice. The formation rate of AlN approaches to 7 μm/ks in the present nitriding. This reduction of processing time is much favored for industrial manufacturing of automotive parts and electrical components in practice. Low friction and wear is also attained in dry sliding condition by this AlN-layered specimen: μ = 0.13 and Ws = 1.5 × 10−5 mm3N−1m−1. Since Al2Cu is common precipitate to most of aluminum alloys, this AlN-layer formation is also favored to protective coating for automotive parts and electric components.

Process–microstructure–properties relationship during formation of AlN layers by physical vapour deposition

The relationship of processing, microstructure and properties during deposition of AlN layers on glass substrates by reactive magnetron sputtering was investigated. The AlN films were prepared with two working gas mixtures: 3 : 1 and 1 : 1 Ar/N2 volume ratios; three working pressure values: 0·2, 0·5 and 0·8 Pa and two target to substrate distances: 2·5 and 3 cm. The resulting films were analysed by scanning electron microscopy and energy dispersive X-ray microanalysis and X-ray diffractometry. In the ranges of processing variables studied, film growth morphology and orientation were mainly affected by working pressure, where deviations from expected behaviour were attributed to process instabilities. No significant influence of gas mixture composition or substrate to target distance was detected in the ranges studied. The effect of pressure was rationalised in terms of the associated energy of the Ar ion film bombardment, which was expected to cause only surface film modifications owing to its low estimated values, but enough to produce structural changes. The development of residual stresses and their effect on hardness and fracture behaviour of the films were mostly influenced by the type of columnar growth and film thickness.

A model of AlN layer formation during ion nitriding of Al

A diffusion model of AlN layer formation by ion nitriding of Al is proposed based on the analysis of atomic transport during the process. This model is reduced to the following. Implantation of N ions to the surface of the specimen, named the reaction zone; extraction of Al from the substrate; diffusion transport of Al to the reaction zone through an AlN layer formed during the process; formation and growth of AlN in the reaction zone; sputtering of the AlN layer. Equations controlling the growth process have been obtained.

AlN formation and enhancement of high-temperature oxidation resistance by plasma-based ion implantation

Nitrogen ion implantation by plasma-based ion implantation (PBII) with a negative pulse voltage of 10 kV amplitude led to formation of AlN layer on an aluminum casting alloy (Al–7Si). The structural analysis by super-glancing-angle X-ray diffraction using synchrotron radiation indicated that the crystalline AlN phase was formed in the nitrogen-ion-implanted AlN layer. The AlN layer on the sample surface prevented oxygen from penetrating into the sample matrix at the temperature of 500 °C, suggesting the enhancement of oxidation resistance by PBII nitrogen ion implantation. The remarkable enhancement of oxidation resistance was ascribed to the production of AlN layer with no microscopic defect such as pinhole.

Metalorganic molecular beam epitaxy and etching of AlGaSb using trisdimethylaminoantimony

Metalorganic molecular beam epitaxy and etching of AlGaSb using trisdimethylaminoantimony (TDMASb) are investigated. Etching rate of Al x Ga 1− x Sb by TDMASb rapidly decreases with an increase of x, and no etching is observed for x>0.3. Reflection high-energy electron diffraction (RHEED) intensity oscillation for the etching of AlGaSb is only observed during a few oscillations, and it attenuates rapidly showing a diffused RHEED pattern. This indicates that the etching of AlGaSb ( x≠0) stops after a few monolayers of etching due to the formation of AlN layer on the surface by the adsorption of nitrogen related species. No growth of AlSb is observed for the simultaneous supply of trimethylamine alane (TMAAl) and TDMASb. Successful growth of AlSb is achieved only by the supply of TMAAl and thermally precracked TDMASb.

Plasma nitriding of Al 99.5

Aluminium nitride (AIN) is a very interesting ceramic because of its combination of properties such as high thermal stability, high hardness and an unusual combination of high thermal and low electrical conductivity. But it is the difficulty to obtain an AIN layer on the aluminium substrates by thermochemical nitriding process. Since a thin film of aluminium oxide existing on the surface of every aluminium substrate prevents the nitrogen atoms from diffusing into the aluminium lattice. However, it is possible to sputter the oxide film away from the aluminium surface in a low discharge with the use of plasma nitriding technik and to allow the formation of AlN layer on the aluminium bulk. In the present work specimen of aluminium Al 99.5 has been plasma nitrided in a modified plasma nitriding unit, in which a diffusion pump was used to obtain an especially flow partial pressure of oxygen in the vacuum chamber. The sputter-cleaning prior to the nitriding was conducted with the variation of the applied voltage and the sputtering time. The plasma nitriding after sputtering was carried out in a glow discharge of pure nitrogen. The metallurgical properties of the AlN-Al-composites were characterized in detail. The results show that shiny black AlN layers with thickness up to a few micrometers have been created on the surface of aluminium and so the enhancement of wear and corrosion resistances of the substrate can be obtained