Formation Of Aluminum Nitride Research Articles

Investigation of the formation of Al, Fe, N intermetallic phases during Al pack cementation followed by plasma nitriding on plain carbon steel

Plain carbon steels are not suitable for nitriding as they form an extremely brittle case that spalls off readily, and the hardness increment of the diffusion zone is small. In this research, the effect of plasma nitriding time and temperature variation on the microstructure of the pack cemented aluminized plain carbon steel is investigated. All samples were aluminized at 900°C for 2h; the aluminized samples were subsequently plasma nitrided at 500°C, 550°C and 600°C for 2.5, 5, 7.5 and 10h. The phases formed on the sample surface were detected by X-ray diffraction (XRD). The cross section and samples surface were investigated by optical and scanning electron microscopy (SEM). Microhardness test was conducted to determine hardness change from the surface to the sample core. Results showed that by aluminizing the steel, Fe3Al phases as well as Fe–Al solid solution were formed on the surface and some aluminum rich precipitates were formed in solid solution grain boundaries. Plasma nitriding of the aluminized layer caused the formation of aluminum and iron nitride (AlN, Fe4N) on the sample surface. Consequently, surface hardness was improved up to about eight times. By increasing the nitriding temperature and time, aluminum-rich precipitates dissociated. Moreover, due to the diffusion of nitrogen through aluminized region during ion nitriding, iron and aluminum nitrides were formed in aluminized grain boundaries. Increasing nitriding time and temperature lead to the growth of these nitrides in the grain boundaries of the substrate. This phenomenon results in the increment of sample hardness depth. Plasma nitriding of aluminized sample in low pressure chamber with nitrogen and hydrogen gas mixture reduced surface aluminum oxides which were formed in aluminizing stage.

Nucleation Criteria for the Formation of Aluminum Nitride in Aluminum Matrix by Nitridation

Synthesis of in situ metal matrix composites using liquid metallurgy technique involves processing the melt to generate ceramic particles and entrap them into the matrix during solidification. The process parameters such as temperature, holding time and buoyancy effect generated in the molten bath are crucial factors governing the size and distribution of ceramic particles into the matrix. Temperature being most important parameter and its role is easy to comprehend for in situ formation of ceramic particles using established nucleation criteria. In the present research paper, thermodynamic study is carried to understand the role of temperature on formation of aluminum nitride by nitridation of aluminum melt using ammonium chloride and calcium oxide. Mechanism of aluminum nitride formation by heterogeneous nucleation and its growth has also been discussed based on the nucleation theory. An attempt is also made to correlate nucleation theory for the formation of AlN in Al matrix at different temperatures. The mechanical behavior under compressive loading is carried out on synthesized composites to understand its capability to withstand static loading. Composite strength is analyzed and attempt is made to correlate nucleation dynamics of AlN particles.

A Novel Method of Synthesis of Nanostructured Aluminum Nitride Through Sol‐Gel Route by In Situ Generation of Nitrogen

Nanostructured Aluminum Nitride (AlN) has been prepared by carbothermal reduction followed by nitridation (CTRN) of alumina gel over a temperature range 1200°C–1350°C and time period of 30 min to 3 h. Before heat treatment the gel is repeatedly evacuated and purged with ammonia. The nanopores of the gel are filled with ammonia which acts as a source of in situ nitrogen at heat‐treatment temperature. Dextrose also decomposes at the reduction temperature and generates ultrafine carbon. The stability diagram of the carbon saturated Al–N–O system is constructed and it shows that extremely low partial pressure of oxygen is required for the stability of AlN. The ultrafine carbon as well as hydrogen from the cracking of ammonia is not sufficient to create the extremely low partial pressure of oxygen required for the stabilization of AlN. So the sample is heat treated in charcoal boat in nitrogen atmosphere to achieve an extremely low partial pressure of oxygen required for the formation of AlN. The material has been characterized through XRD, FESEM, and HRTEM analyses. The spherical particle size of AlN is obtained ∼21 nm.

Synthesis of AlN – presence and absence of additive

Direct nitridation of hexagonal aluminium nitride (AlN), using additive like NH4Cl, was performed from Al metal powder. Different methods of AlN synthesis, viz. a) Al powder, b) Al and NH4Cl, c) Al and NH4Cl with urea, were carried out in continuous flowing of N2 gas at 900 °C for 5 h. The synthesized powder was characterized by employing TGA/DTA, XRD, FTIR, SEM, UV- VIS Spectroscopy and CHNS analysis. The mechanism for the formation of AlN by the influence of additive and role of urea are discussed.

Synthesis of Cubic Aluminum Nitride Coating from Al<sub>2</sub>O<sub>3</sub> Powder in Reactive Plasma Spray Process

Plasma spraying is a versatile technique for producing abradable and protective ceramic coatings. However, it was difficult to fabricate aluminium nitride (AlN) coating by conventional plasma spray processes. It is due to the thermal decomposition of the feedstock AlN powder during spraying without a stable melting phase. Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of AlN thermally sprayed coatings. This study investigated the feasibility of reactive plasma spraying of Al2O3 powder in N2/H2 plasma upon fabrication of AlN coating. It was possible to fabricate a cubic-AlN (c-AlN)/Al2O3 composite coating and the fabricated coating consists of c-AlN, i-Al2O3, Al5O6N and £-Al2O3. Furthermore, the AlN content was improved with increasing the flight time (spray distance), due to increasing the reaction time between the Al2O3 particles and the surrounding N2/H2 plasma. It was possible to fabricate coating contains about 97% of c-AlN. However, it was difficult to clarify the in-flight reaction during the coating process, due to losing the particles shape and features after colliding and flattening on the substrate surface. The sprayed particles were collected into a water bath to maintain its particle features in order to investigate the in-flight reaction. It was clear from the microstructure and the cross section observation of the collected particles that, the nitriding reaction started from the surface. During the coating process, the sprayed particles were melted, spheroidized and reacted in the high temperature N2/H2 plasma and formed aluminum oxynitride (Al5O6N) which have cubic structure. The particles collided, flattened, and rapidly solidified on a substrate surface. The Al5O6N is easily converted to c-AlN phase via continuous nitriding (both have the same cubic symmetry: cubic and closely packed) during the rapid solidification and plasma irradiation on the substrate. The high quenching rate of the plasma flame prevents the AlN crystal growth to form the hexagonal phase. Therefore, it was possible to fabricate c-AlN/Al2O3 composite coatings through reactive plasma nitriding of Al2O3 powder and the nitriding process of the Al2O3 particle as well as the formation process of cAlN phase were investigated. [doi:10.2320/matertrans.T-M2012831]

N2 and H2 plasma gasses' effects in reactive plasma spraying of Al2O3 powder

Reactive plasma spraying (RPS) is a promising technology for in-situ formation of aluminum nitride (AlN) coatings. In this study the alumina (Al2O3) powder was used as feedstock powder material upon fabrication of AlN coating in RPS process. The effect of plasma gasses (N2 and H2) on the fabrication process, the phase composition and the coating microstructure was investigated. Thick AlN/Al2O3 composite coating was successfully fabricated and the coating consists of cubic-AlN (c-AlN), α-Al2O3, Al5O6N and γ-Al2O3. The nitride content was improved with increasing the N2 gas flow rate. It is attributed to increasing the amount of the nitriding species surrounding the particles. Increasing the flow rate enhanced the particle velocity and therefore the thickness firstly increased. However, further increase reduced the particle residence time in the plasma and therefore decreased the deposition efficiency. The N2 gas flow rate did not affect so much on the phase transformation of the α-Al2O3 to the γ-Al2O3 phases. Furthermore, increasing the residence time (spray distance) of the particle under high N2 gas flow rate significantly improved the nitriding conversion and suppressed coating growth.On the other hand, the H2 plasma gas is required for the plasma reduction and nitriding of the Al2O3 particles in RPS process. Thus, in the absence of the H2 plasma gas, the AlN phase did not appear in the fabricated coating. However, increasing the H2 gas flow rate did not affect the nitriding conversion. The H2 gas and its flow rate significantly increased the particles' in-flight temperature gradually without any effect on the particles' in-flight velocity. Therefore, the H2 gas assists the particle melting and its phase transformation from the α-Al2O3 to the γ-Al2O3. Although, increasing the particles' temperature improved the coating thickness at short distance, it assists the particle vaporization and suppressed the thickness with increasing the flight time. Therefore, the H2 gas is required and low flow rate is recommended for RPS of Al2O3 powder.

Structural properties of Al2O3/AlN thin film prepared by magnetron sputtering of Al in HF-activated nitrogen plasma

A process for ion-plasma formation of aluminum nitride (AlN) nanolayers on nitrided sapphire (Al2O3) substrates is presented. The method is based on the direct current magnetron sputtering of a high-purity aluminum target in the presence of an argon–nitrogen gas mix and high frequency-activated nitrogen plasma. The method, combined with ion etching, produced matched layers by nitration of Al2O3 in the (0001) plane, and formation of high quality AlN epilayers on this surface was observed. The processing characteristics and morphology dependence on synthesis parameters were studied using atomic force microscopy.

Effects of Surface Nitrification on Thermal Conductivity of Modified Aluminum Oxide Nanofibers-Reinforced Epoxy Matrix Nanocomposites

Aluminum oxide () nanofibers were treated thermally under an ammonia () gas stream balanced by nitrogen to form a thin aluminum nitride (AlN) layer on the nanofibers, resulting in the enhancement of thermal conductivity of /epoxy nanocomposites. The micro-structural and morphological properties of the -assisted thermally-treated nanofibers were characterized by X-ray diffraction (XRD) and atomic force microscopy (AEM), respectively. The surface characteristics and pore structures were observed by X-ray photoelectron spectroscopy (XPS), Zeta-potential and /77 K isothermal adsorptions. From the results, the formation of AlN on nanofibers was confirmed by XRD and XPS. The thermal conductivity (TC) of the modified nanofibers/epoxy composites increased with increasing treated temperatures. On the other hand, the severely treated /epoxy composites showed a decrease in TC, resulting from a decrease in the probability of heat-transfer networks between the filler and matrix in this system due to the aggregation of nanofiber fillers.

Solid-Phase Mechanical Alloying of BCC Iron Alloys by Nitrogen in Ball Mills

The methods of Mössbauer spectroscopy and X-ray diffraction analysis have been used to study the processes of a solid-phase alloying of the iron alloys with a bcc lattice by nitrogen that occur upon ball-mill mechanical activation in the presence of chromium nitrides. It is shown that a deformation-induced dissolution of chromium nitrides in the matrix of pure iron and in that of the alloys Fe–3Al and Fe–6V results in the formation of the substitutional chromium and interstitial nitrogen bcc solid solutions. An additional alloying of iron with aluminum or vanadium under the deformation dissolution of nitrides leads to the escape of aluminum and vanadium from the matrix and to a decrease in the nitrogen content characteristic of the interstitial solid solution proper due to the strong chemical bonding of alloying elements with nitrogen. The subsequent annealing leads to the decomposition of already formed solid solutions with the formation of aluminum, vanadium, and chromium nitrides of extreme dispersion.

Study of the aluminum surface characteristics by using the radiation-enhanced diffusion effect during high-flux ion-beam irradiation

This paper presents an experimental investigation of the radiation-enhanced diffusion effect for a pure aluminum (Al 1050) surface by using a high-flux ion beam. When a nitrogen-ion beam with a 24 to ∼40 µA/cm2 flux irradiated Al surfaces, the depth of the AlN (aluminum-nitride) layer on the surface, which was formed by N2+-ion-beam irradiation, was identified through XPS (X-ray photoelectron spectroscopy), and the AlN layer related to the radiation-enhanced diffusion effect strongly depended on the flux and fluence of ion-beam. The atomic composition and the atom ratios were identified through AES (Auger electron spectroscopy). Also, the hardness of the high-flux ionbeam-irradiated surface was decreased by 30 ∼ 40% compared with that of the as-received sample due to material softening by grain growth. We measured the contact angle of the surface to identify the relation between radiation-enhanced diffusion and changes in the surface characteristics. The radiation enhanced diffusion due to high-flux ion-beam irradiation was shown to affect the depth of AlN formation, and the formed AlN layer was shown to give a hydrophilic property to the Al surface.

REACTIVE PLASMA NITRIDING OF AL2O3 POWDER IN THERMAL SPRAY

Among advanced ceramics, aluminum nitride ( AlN ) had attracted much attention in the field of electrical and structural applications due to its outstanding properties. However, it is difficult to fabricate AlN coating by conventional thermal spray processes directly. Due to the thermal decomposition of feedstock AlN powder during spraying without a stable melting phase (which is required for deposition in thermal spray). Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of AlN thermally sprayed coatings. In this study the possibility of fabrication of AlN coating by reactive plasma nitriding of alumina ( Al 2 O 3) powder using N 2/ H 2 plasma was investigated. It was possible to fabricate a cubic- AlN (c- AlN ) based coating and the fabricated coating consists of c- AlN , α- Al 2 O 3, Al 5 O 6 N and γ- Al 2 O 3. It was difficult to understand the nitriding process from the fabricated coatings. Therefore, the Al 2 O 3 powders were sprayed and collected in water. The microstructure observation of the collected powder and its cross section indicate that the reaction started from the surface. Thus, the sprayed particles were melted and reacted in high temperature reactive plasma and formed aluminum oxynitride which has cubic structure and easily nitride to c- AlN . During the coatings process the particles collide, flatten, and rapidly solidified on a substrate surface. The rapid solidification on the substrate surface due to the high quenching rate of the plasma flame prevents AlN crystal growth to form the hexagonal phase. Therefore, it was possible to fabricate c- AlN / Al 2 O 3 based coatings through reactive plasma nitriding reaction of Al 2 O 3 powder in thermal spray.

Haber process for ammonia synthesis

Fixed nitrogen from the air is the major ingredient of fertilizers which makes intensive food production possible. During the development of inexpensive nitrogen fixation processes, many principles of chemical and high-pressure processes were clarified and the field of chemical engineering emerged. Before synthetic nitrogen fixation, wastes and manures of various types or their decomposition products, and ammonium sulfate, which is a by-product from the coking of coal, were the primary sources of agricultural nitrogen. Chilean saltpetre, saltpetre from human and animal urine, and later ammonia recovered from coke manufacture were some of the important sources of fixed nitrogen [1]. During the first decade of the twentieth century, the worldwide demand for nitrogen-based fertilizers far exceeded the existent supply. The largest source of the chemicals necessary for fertilizer production was found in a huge guano deposit (essentially sea bird droppings) that was 220 miles in length and five feet thick, located along the coast of Chile. Scientists had long desired to solve the problem of the world’s dependence on this fast disappearing natural source of ammonia and nitrogenous compounds. Priestly and Cavendish passed electric sparks through air and produced nitrates by dissolving the oxides of nitrogen thus formed in alkalis. Commercial development of this process had proved elusive, for much electrical energy was consumed at low efficiency. Nitrogen had been fixed as calcium cyanamide, but the process was too expensive except for producing chemicals requiring the cyanamide configuration. Other processes, such as thermal processing to mixed oxides of nitrogen (NOX), cyanide formation, aluminum nitride formation and decomposition to ammonia, etc., showed little commercial promise although they were technically possible. It was Fritz Haber, along with Carl Bosch, who finally solved this problem. Haber invented a large-scale catalytic synthesis of ammonia from elemental hydrogen and nitrogen gas, reactants Haber Process for Ammonia Synthesis

In Situ Fabrication of AlN Coating by Reactive Plasma Spraying of Al/AlN Powder

Reactive plasma spraying is a promising technology for the in situ formation of aluminum nitride (AlN) coatings. Recently, it became possible to fabricate cubic-AlN-(c-AlN) based coatings through reactive plasma spraying of Al powder in an ambient atmosphere. However, it was difficult to fabricate a coating with high AlN content and suitable thickness due to the coalescence of the Al particles. In this study, the influence of using AlN additive (h-AlN) to increase the AlN content of the coating and improve the reaction process was investigated. The simple mixing of Al and AlN powders was not suitable for fabricating AlN coatings through reactive plasma spraying. However, it was possible to prepare a homogenously mixed, agglomerated and dispersed Al/AlN mixture (which enabled in-flight interaction between the powder and the surrounding plasma) by wet-mixing in a planetary mill. Increasing the AlN content in the mixture prevented coalescence and increased the nitride content gradually. Using 30 to 40 wt% AlN was sufficient to fabricate a thick (more than 200 µm) AlN coating with high hardness (approximately 1000 Hv). The AlN additive prevented the coalescence of Al metal and enhanced post-deposition nitriding through N2 plasma irradiation by allowing the nitriding species in the plasma to impinge on a larger Al surface area. Using AlN as a feedstock additive was found to be a suitable method for fabricating AlN coatings by reactive plasma spraying. Moreover, the fabricated coatings consist of hexagonal (h-AlN), c-AlN (rock-salt and zinc-blend phases) and certain oxides: aluminum oxynitride (Al5O6N), cubic sphalerite Al23O27N5 (ALON) and Al2O3. The zinc-blend c-AlN and ALON phases were attributed to the transformation of the h-AlN feedstock during the reactive plasma spraying. Thus, the zinc-blend c-AlN and ALON phases were not included in the feedstock and were not formed through nitriding of the Al.

Controlling of Nitriding Process on Reactive Plasma Spraying of Al Particles

Reactive plasma spraying (RPS) has been considered as a promising technology for in-situ formation of aluminum nitride (AlN) thermally sprayed coatings. To fabricate thick A lN coatings in RPS process, controlling and improving the in-flight nitriding reaction of Al particles is required. In this study, it was possible to control the nitriding reaction by using ammonium chloride (NH4Cl) powders. Thick and dense AlN coating (more than 300 μm thickness) was successfully fabricated with small addition of NH4Cl powders. Thus, addition of NH4Cl prevented the Al aggregation by changing the reaction pathway to a mild way with no explosive mode (relatively low heating rates) and it acts as a catalyst, nitrogen source and diluent agent.

Cluster model of the formation of aluminum nitride in alumina nanoreactors

The non-traditional method of obtainingclusters of heterogenous and homogenous nanooxides, nitrides of aluminum by substitution ofoxygen atoms with low-energy nitrogen ions inthe solid phase inside the nanopores of aluminum oxide Al2O3 (nanosapfire) is proposed and implemented. The AlN, AlNxO1-x, Al2O3 nanolayers was experimentally obtained

BN-アルミニウム間反応を利用したAlN基複合材料の合成におけるプロセス条件の影響

The reactive infiltration of molten aluminum into a powder preform was investigated as a processing route for aluminum nitride (AlN) matrix composites. As the powder preform, titanium and boron nitride (BN) powders were blended and compacted, the infiltration occurred spontaneously at temperatures ranging from 1073 K to 1673 K. The conversion ratio of BN to TiB2 and AlN turned out to be insufficient (<0.8) at temperatures below 1273 K. The conversion ratio increased by increasing the processing temperatures, and it exceeded 0.95 at 1673 K. The conversion ratio at 1473 K was almost saturated with a 3.6 ks holding and the effect of extending the holding time was not confirmed. Decreasing the BN powder size was effective to promote the reaction between aluminum, titanium and BN. The effectiveness was particularly prominent at 1473 K in comparison with the specimen fabricated at 1673 K. The titanium powder size did not show a significant effect on the formation of AlN and TiB2. The volume fraction of AlN increased by changing the atmospheric gas from argon to nitrogen.

Evaluation of Nitrogen Conversion Ratio on Reaction between BN and Aluminum

The fundamental reaction behavior between aluminum powder and boron nitride (BN) powder was investigated, and a quantitative evaluation of aluminum nitride (AlN) formation was carried out. As a result of differential scanning calorimeter (DSC) analysis, an exothermic reaction between BN and aluminum was detected at temperatures between 1073 and 1173K. The conversion ratio of BN to AlN increased with increasing heating temperature. However, the nitrogen conversion ratio appeared to saturate with a holding time of 3.6 ks at temperatures above 1273K. The nitrogen conversion ratio increased by using 1mm BN powder instead of 10mm BN powder. The effect of using the fine BN powder was significant especially when the heating time was short (<300 s). [doi:10.2320/matertrans.M2011184]

Development of Cu-bearing bake-hardenable steel sheets for automotive exposed panels

Recently, newly developed bake-hardenable (BH) steel sheets strengthened by copper sulfide (CuS) have been successfully employed in commercial production lines that supply automotive outer panels. The metallurgical concepts governing fabrication of these new BH steel sheets require keeping carbon content as low as 0.0015 wt.% without any additional amount of titanium and/or niobium for solute carbon scavenging. The role of CuS precipitates has turned out to raise the yield strength acting as a barrier against dislocation movement. In this paper, we studied the effects of chemical compositions and manufacturing process variables on the microstructure and mechanical properties of newly developed BH steel sheets. We found that the control of carbon and nitrogen showed a good balance between bake-hardenability (BH) and yield point elongation (YP-El). We identified the crystallographic relationship between the nano-size CuS precipitates and the ferrite matrix of (001)sulfide//(001)α-Fe and [001]sulfide//[001]α-Fe. We also found that the BH and YP-El were affected by the formation of aluminium nitride (AlN) and the annealing temperature.

Influence of NH4Cl Powder Addition for Fabrication of Aluminum Nitride Coating in Reactive Atmospheric Plasma Spray Process

Reactive plasma spray is the key to fabricating aluminum nitride (AlN) thermally sprayed coatings. It was possible to fabricate AlN/Al composite coatings using atmospheric plasma spray process through plasma nitriding of Al powders (Al 30 μm). The nitriding reaction and the AlN content could be improved by controlling the spray distance and the feedstock powder particle size. Increasing the spray distance and/or using smaller particle size of Al powders improved the in-flight nitriding reaction. However, it was difficult to fabricate thick and dense AlN coatings with an increase in the spray distance and/or when using fine particles. Thus, the coatings thickness was suppressed because of the complete nitriding of some particles (formation of AlN particles) during flight, which prevents the particle deposition. Furthermore, the excessive vaporization of Al fine particles (due to increased particle temperature) decreased the deposition efficiency. To fabricate thick AlN coatings in the reactive plasma spray process, improving the nitriding reaction of the large Al particles at short spray distance is required to decrease the vaporization of Al particles during flight. This study investigated the influence of adding ammonium chloride (NH4Cl) powders on the nitriding process of large Al powders and on the microstructure of the fabricated coatings. It was possible to fabricate thick AlN coatings at 100 mm spray distance with small addition of NH4Cl powders to the Al feedstock powders (30 μm). Addition of NH4Cl to the starting Al powders promoted the formation of AlN through changing the reaction path to vapor-phase nitridation chlorination-nitridation sequences as confirmed by the thermodynamic analysis of possible intermediate reactions. This changes the nitriding reaction to a mild way, so it is more controlled with no explosive mode and with relatively low heating rates. Thus, NH4Cl acts as a catalyst, nitrogen source, and diluent agent. Furthermore, the evolved gases from the sublimation or decomposition of NH4Cl can prevent the Al particles coalescing after melting.

Effect of process conditions on the microstructural formation of dc reactively sputter deposited AlN

Thin film aluminum nitride (AlN), because of its attractive properties, is a material with many applications. Its microstructure and hence properties are greatly influenced by the deposition process conditions. In this work, AlN was reactively deposited in a dc magnetron sputtering system at different proportions of nitrogen in the process gas mixture and at different process conditions. The microstructure and composition of the films were analyzed using x-ray diffraction data, energy dispersive spectroscopy, and scanning electron microscopy. Results show that for a process gas pressure of 0.67 Pa, a magnetron power of 100 W, and a substrate-target distance of 10 cm, a near stoichiometeric AlN can be prepared at nitrogen proportions as low as 20%. At these process conditions, (002) was the preferred crystal orientation. Dense fibrous structures were obtained, especially at low deposition rates with high proportions of nitrogen. Increase in magnetron power and decrease in distance result in a more porous structure. High kinetic energies (average) of the sputtered Al particles and high deposition rates tend to favor AlN(101) formation, while low kinetic energies of the Al particles and low deposition rates generally favor more of the AlN(100) formation.