Reactive Plasma Spraying of Fine Al2O3/AlN Feedstock Powder

Reactive plasma spraying (RPS) is a promising technology for in situ formation of aluminum nitride (AlN) coatings. Recently, AlN-based coatings were fabricated by RPS of alumina (Al₂O₃) powder in N₂/H₂ thermal plasma. This study investigated the feasibility of RPS of a fine Al₂O₃/AlN mixture and the influence of the plasma gases (N₂, H₂) on the nitriding conversion, and coating microstructure and properties. Thick AlN/Al₂O₃ coatings with high nitride content were successfully fabricated. The coatings consist of h-AlN, c-AlN, Al₅O₆N, γ-Al₂O₃, and a small amount of α-Al₂O₃. Use of fine particles enhanced the nitriding conversion and the melting tendency by increasing the surface area. Furthermore, the AlN additive improved the AlN content in the coatings. Increasing the N₂ gas flow rate improved the nitride content and complete crystal growth to the h-AlN phase, and enhanced the coating thickness. On the other hand, though the H₂ gas is required for plasma nitriding of the Al₂O₃ particles, increasing its flow rate decreased the nitride content and the coating thickness. Remarkable influence of the plasma gases on the coating composition, microstructure, and properties was observed during RPS of the fine particles.     

Effect of nitriding on the electrochemical behaviour of Ti‐Al‐Mn alloy

The effect of technological conditions of nitriding such as process time duration and chemical composition of saturating medium, on the corrosion behaviour of nitrided coatings in 14 M solution of sulphuric acid was analyzed. The investigations were done on the alloy Ti‐5,0 Al‐2,0 Mn. The nitriding was carried out in nitrogen both at atmospheric pressure and rarefied nitrogen pressure (1 Pa) at the temperature 850°C and time processing in the range from 5 to 20 h in nitrogen‐containing gas only, and in powder electrode graphite and nitrogen‐containing gas. It was shown that technological conditions of nitriding determine the protective properties of nitrided coatings. It was indicated that the optimal structure of the nitride layer for best corrosion protection is the thin nitride TiN_x with high surface quality and a gas‐saturated layer. Nitriding in graphite powder effects positively the protective properties of nitride coatings due to reducing the nitride‐forming process.     

Effects of Temperature on Microstructure and Wear of Salt Bath Nitrided 17-4PH Stainless Steel

Salt bath nitriding of 17-4 PH martensitic precipitation hardening stainless steels was conducted at 610, 630, and 650?°C for 2?h using a complex salt bath heat-treatment, and the properties of the nitrided surface were systematically evaluated. Experimental results revealed that the microstructure and phase constituents of the nitrided surface alloy are highly process condition dependent. When 17-4PH stainless steel was subjected to complex salt bathing nitriding, the main phase of the nitrided layer was expanded martensite (????), expanded austenite (??_N), CrN, Fe₄N, and (Fe,Cr) _x O _y . In the sample nitrided above 610?°C, the expanded martensite transformed into expanded austenite. But in the sample nitrided at 650?°C, the expanded austenite decomposed into ??_N and CrN. The decomposed ??_N then disassembled into CrN and alpha again. The nitrided layer depth thickened intensively with the increasing nitriding temperature. The activation energy of nitriding in this salt bath was 125?±?5?kJ/mol.     

Development of Nitrogen-Hydrocarbon Atmospheric Carburizing and Process Control Methods

Atmospheric pressure carburizing and neutral carbon potential annealing in nitrogen containing small additions of hydrocarbon gases can offer cost and steel surface quality alternatives to the comparable, endothermic atmosphere, or vacuum operations. An experimental program was conducted for refining real-time process control methods in carburizing of AISI 8620 steel under N₂-CH₄, N₂-C₃H₈ blends containing <5 vol.% of hydrocarbon gas at 900 and 930 °C. Multiple types of gas analyzers were used to monitor residual concentrations of H₂, CO, CO₂, H₂O, O₂, CH₄, C₃H₈, and other hydrocarbons inside furnace. A modified shim stock technique was additionally evaluated for correlation with gas analysis and diffusional modeling using measured carbon mass flux values (g/cm²/s). Results of this evaluation work are presented.     

Higher fullerene-coupled porous silicon systems with blue emission

Higher fullerenes (C₇₀, C₇₆, C₈₄, C₉₄) were covalently coupled on porous Si (PS) to form fullerene-coupled PS systems. After stored in air for 3–13 months, a blue photoluminescence (PL) was observed in the range of 430–480 nm. PL excitation spectral examinations reveal that the photoexcited carriers are generated in the quantum confined nanocrystalline Si (nc-Si ) cores, whereas radiative recombination occurs at the coupled nc-Si surface. Fourier-transform infrared absorption data disclose that the blue PL energy has a dependence on the oxygen content of nc-Si surface, indicating that the luminescent centers should be some oxygen-related defect states. Based on the annealing behavior of the blue PL peak in O₂ and N₂, we attribute the luminescent center to a pair of an oxygen vacancy and an interstitial oxygen, which also forms a peroxy linkage with an lattice oxygen.     

Effect of Plasma Nitriding and Nitrocarburizing on HVOF-Sprayed Stainless Steel Coatings

In this work, the effects of plasma nitriding (PN) and nitrocarburizing on HVOF-sprayed stainless steel nitride layers were investigated. 316 (austenitic), 17-4PH (precipitation hardening), and 410 (martensitic) stainless steels were plasma-nitrided and nitrocarburized using a N₂ + H₂ gas mixture and the gas mixture containing C₂H₂, respectively, at 550 °C. The results showed that the PN and nitrocarburizing produced a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer depending on the crystal structures of the HVOF-sprayed stainless steel coatings. Also, the diffusion depth of nitrogen increased when a small amount of C₂H₂ (plasma nitrocarburizing process) was added. The PN and nitrocarburizing resulted in not only an increase of the surface hardness, but also improvement of the load bearing capacity of the HVOF-sprayed stainless steel coatings because of the formation of CrN, Fe₃N, and Fe₄N phases. Also, the plasma-nitrocarburized HVOF-sprayed 410 stainless steel had a superior surface microhardness and load bearing capacity due to the formation of Cr₂₃C₆ on the surface.     

The influence of the conditions of synthesis on the composition and mechanical properties of silicon oxycarbonitride nanocomposite films

Dielectric films of hydrogenated silicon oxycarbonitride SiC _x N _y O _z :H were prepared by plasmaenhanced chemical vapor deposition using gas mixtures of 1,1,1,3,3,3-hexamethyldisilazane (HMDS) or 1,1,3,3-tetramethyldisilazane (TMDS) with oxygen and nitrogen in the temperature range of 373–973 K. The effect of the conditions of synthesis on the chemical and phase composition of the films was studied, in the amorphous part of which nanocrystals belonging to the phases of the Si–C–N system α-Si₃N₄, α-Si_3–x C _x N₄, and graphite were distributed. To measure the hardness and Young’s modulus, the nanoindentation method was used. The influence that the synthesis temperature and nitrogen-to-oxygen ratio in the initial gas mixtures HMDS + O₂ + xN₂ and TMDS + O₂ + xN₂ have on the hardness and Young’s modulus of the resulting SiC _x N _y O _z :H films was investigated. The maximum obtained values of these parameters were 20.4 and 201.5 GPa, respectively.     

An in-situ study of plasma nitriding

Direct in-situ observation of phase generation and growth during heat treatment cycles gives information independent e.g. of effects resulting from cooling and atmospheric changes of properties. In this investigation time resolved in-situ X-ray diffraction (XRD) analysis of growing nitride layers during plasma nitriding was conducted to gain experimental data of growing compound layers for different plasma nitriding parameters. With two gas mixtures of 5% N₂-95% H₂ and 25% N₂-75% H₂. plasma nitriding of an AISI 1045 steel was performed in the temperature range of 450 °C < T < 560 °C. The in-situ XRD-observation consisted of series of 50 to 60 single runs of phase analysis during a 3-h plasma nitriding treatment. Nitriding with the formation of nitride phases starts at different times, depending on the nitriding temperature and the gas composition in the plasma for the given plasma parameters pressure, voltage and current density. The higher the nitriding temperature and the higher the nitrogen content in the process gas the shorter is the time for the first detection of the γ′-Fe₄N-phase. Single phase γ′nitride layers were detected for the 5% N₂-95% H₂ gas mixture in a temperature range 450 °C < T < 560 °C. For the highest temperatures 540 °C and 560 °C and the gas mixture 25% N₂-75% H₂ the ε-Fe_2-3N phase occurred later in the plasma nitriding process. Assuming that nitride layers in plasma nitriding also grow by nucleation of small γ′ particles up to a complete layer, the experimental data fitted in a reasonable way in plots calculated for the incubation time of the γ′-phase during gas nitriding.     

Silicon oxynitride gas barrier coatings on poly(ether sulfone) by plasma-enhanced chemical vapor deposition

Thin silicon oxynitride (SiO_xN_y) has been deposited for a gas barrier layer on the surface of poly(ether sulfone) film using plasma-enhanced chemical vapor deposition (PECVD) of a mixture of hexamethyldisiloxane (HMDSO) and ammonia. The chemical structure of the deposited layer varied from organic to inorganic structures depending on RF plasma input power applied to the reaction system. A silicon-based undercoat layer, which has an organic/inorganic hybrid structure, was used as an interfacial buffer layer between the organic PES and inorganic SiO_xN_y layer. With the help of the undercoat layer, the dense inorganic SiO_xN_y layer gave a superior oxygen barrier property of 0.2 cm³/m² day at a critical coating thickness of ca. 20 nm. In a highly stressed SiO_xN_y film, the effect of the undercoat layer was remarkable in preventing crack formation during bending tests.     

Loading effect at etching of polypropylene films in oxygen-nitrogen plasma

The results of the studies of the loading effect under the action of a low temperature plasma of an oxygen-nitrogen mixture on the surface of a polypropylene (PP) film are presented. A gravimetric method was used to study the kinetics of the material’s etching. The composition of the functional groups on the PP surface was characterized by the method of Fourier Transform Infrared by Attenuated Total Reflectance (FT-IR ATR). The loading effect was observed in the entire range of the gas compositions. It is accompanied by the increasing of the content of the functional groups in the modified layer; namely, of the double bonds for any gas composition and oxygen- or nitrogen-containing species dependent on the mixture composition. In the general case, the loading effect is more pronounced when the initial gas contains less oxygen. The degree of this dependence differs strongly for various O₂: N₂ ratios. This is probably associated with the extreme character of the changes in the fluxes of the active particles from the plasma to the sample. The influence of the gas composition on the loading effect is relatively small when the O₂ fraction in the mixture constitutes 100–10%. The increasing of the load degree results in the increasing of the concentration of the oxygen-containing groups on the surface; the nitrogen-containing groups were not registered. When the O₂ fraction in the mixture is lower than 10%, the reciprocal influence of the volumetric and heterogeneous processes is enhanced considerably. The water vapors enter into the reactions of etching owing to the oxygen deficit. The nitriding of the surface occurs simultaneously with its oxidation. The competition of these processes is more pronounced for larger loads of the reactor.     

The oxidation of ZrB2, TaB2, NbB2, and W2B5 in atomic oxygen and by anodic polarization

The oxidation of ZrB₂, TaB₂, NbB₂, and W₂B₅ in atomic oxygen and by anodic polarization was studied. All the investigated materials, both in the gas medium and in the electrolyte, were highly resistant to corrosion. The composition of the surface compounds was examined during the oxidation. High resistance to oxidation is explained by the B₂O₃ oxide. Oxidation of borides in both atomic oxygen and in 1N H₂SO₄ takes place at comparable rates, the corrosion resistance in the two media being similar.     

Surface alloying of AISI 1045 steel in a nitrogen environment using a gas tungsten arc process

The effects of varying the addition of nitrogen to the shielding gas during GTA (gas tungsten arc) surface alloying of an AISI 1045 steel substrate with a preplaced layer of ferrotitanium (FeTi) powder were investigated. The penetration and cross-sectional area of the alloyed layers increased with the nitrogen content in the shielding gas. Different nitrogen contents in the shielding gas also caused the formation of two main microstructures: (1) TiN dendrites distributed in a ferrite(α)–Fe₃C matrix at a high nitrogen content and (2) Ti(C_xN_y) in a matrix of ferrite(α) and eutectic structure of ferrite(α) and Fe₂Ti at a low nitrogen content. Specimen melted under pure argon (0 vol% N₂) was comprised of TiC in a matrix of ferrite(α) and eutectic structure of ferrite(α) and Fe₂Ti. The latter also showed the highest hardness, which could be attributed to the presence of the fine eutectic structure and low dilution of the layer.     

The growth mechanism of carbon nanotubes from thermal cracking of acetylene over nickel catalyst supported on alumina

Carbon nanotubes (CNTs) were grown on Ni/Al₂O₃ catalyst by thermal cracking of C₂H₂/H₂(N₂) at 600°C for 30 min with a gas flow rate of 10/100 sccm. The Ni/Al₂O₃ catalyst was prepared by mechanically mixing fine Ni and α-Al₂O₃ powders. The growth of CNTs in C₂H₂/H₂ medium is higher than that in C₂H₂/N₂ medium. The grown CNTs is a mixture of single-wall carbon nanotubes (SWNTs) and multi-wall carbon nanotubes (MWNTs). The gas-phase reaction was studied by analyzing gas species generated from the cracked C₂H₂/H₂ (N₂) media during the CNTs growth. C₂H₂ is not decomposed in gas-phase even at the growth temperature 600°C, but is catalytically dissociated on the Ni surface to produce a reactive carbonic species. The growth mechanism of CNTs was intensively discussed in detail in this paper based on the experimental observations.     

Plasma nitriding of plastic mold steel to increase wear- and corrosion properties

In this study, the wear- and corrosion resistance of the layers formed on the surface of a precipitation hardenable plastic mold steel (NAK55) by plasma nitriding were investigated. Plasma nitriding experiments were carried out at an industrial nitriding facility in an atmosphere of 25% N₂ + 75% H₂ at 475 °C, 500 °C, and 525 °C for 10 h. The microstructures of the nitrided layers were examined, and various phases present were determined by X-ray diffraction. Wear tests were carried out on a block-on-ring wear tester under unlubricated conditions. The corrosion behaviors were evaluated using anodic polarization tests in 3.5% NaCl solution.The findings had shown that plasma nitriding does not cause the core to soften by overaging. Nitriding and aging could be achieved simultaneously in the same treatment cycle. Plasma nitriding of NAK55 mold steel produced a nitrided layer consisted of a compound layer rich in ε-nitride and an adjacent nitrogen diffusion layer on the steel surface. Increasing the nitriding temperature could bring about increase in the thickness of the nitrided layer and the nitride volume fraction. Plasma nitriding improved not only surface hardness but also wear resistance. The anti-wear property of the steel was found to relate to the increase in the thickness of the diffusion layer. Corrosion study revealed that plasma nitriding significantly improved corrosion resistance in terms of corrosion potential and corrosion rate. Improvement in corrosion resistance was found to be directly related to the increase in the nitride volume fraction at the steel surface.     

Improvement of corrosion and wear resistances of AISI 420 martensitic stainless steel using plasma nitriding at low temperature

The influence of low temperature plasma nitriding on the wear and corrosion resistance of AISI 420 martensitic stainless steel was investigated. Plasma nitriding experiments were carried out with DC-pulsed plasma in 25% N₂ + 75% H₂ atmosphere at 350 °C, 450 °C and 550 °C for 15 h. The composition, microstructure and hardness of the nitrided samples were examined. The wear resistances of plasma nitrided samples were determined with a ball-on-disc wear tester. The corrosion behaviors of plasma nitrided AISI420 stainless steel were evaluated using anodic polarization tests and salt fog spray tests in the simulated industrial environment.The results show that plasma nitriding produces a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer on the AISI 420 stainless steel surface. Plasma nitriding not only increases the surface hardness but also improves the wear resistance of the martensitic stainless steel. Furthermore, the anti-wear property of the steel nitrided at 350 °C is much more excellent than that at 550 °C. In addition, the corrosion resistance of AISI420 martensitic stainless steel is considerably improved by 350 °C low temperature plasma nitriding. The improved corrosion resistance is considered to be related to the combined effect of the solid solution of Cr and the high chemical stable phases of ?-Fe₃N and α_N formed on the martensitic stainless steel surface during 350 °C low temperature plasma nitriding. However, plasma nitriding carried out at 450 °C or 550 °C reduces the corrosion resistance of samples, because of the formation of CrN and leading to the depletion of Cr in the solid solution phase of the nitrided layer.     

Effect of ultrasonic surface treatment of steel 40Kh13 on the microstructure of nitrided layer formed by high-intensity low-energy implantation with nitrogen ions

X-ray diffraction and transmission electron microscopy were used to study the microstructure and phase composition of surface layers of steel 40Kh13 subjected to irradiation with high-intensity low-energy ion beams, ultrasonic surface modification, and combined treatment including ultrasonic surface modification and ion implantation. It was found that the ultrasonic modification of steel surface leads to changes in the structure of tempered martensite and the formation of grain structure with a grain size of 0.3 μm and nanosized special carbides Cr₂₃C₆ in the martensite lamellae. The ion implantation into this steel results in the formation of a nitrided layer consisting of a nitride region, which represents a mixture of several phases (α-, γ′-, ?, and ultrafine chromium nitrides), and a zone of internal nitriding (α″ and nitrogen-containing martensite α_N). The preliminary ultrasonic modification causes an increase in the nanohardness and in the thickness of the nitrided layer, which is due to the more intense penetration of nitrogen atoms into the surface layer and an increase in the volume fraction of iron nitrides and density of ultrafine chromium nitrides in this layer.     

The Effects of KCl, K2SO4 and K2CO3 on the High Temperature Corrosion of a 304-Type Austenitic Stainless Steel

The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600 °C in 5% O₂ + 40% H₂O. Prior to exposure the samples were sprayed with KCl, K₂CO₃ or K₂SO₄, the amount of salt corresponding to 1.35 ??mol K⁺/cm². For reference, salt-free samples were exposed in 5% O₂ + 40% H₂O and in 5% O₂ (N₂ was used as carrier gas). The oxidized samples were analyzed with SEM/EDX, XRD, IC and FIB. KCl and K₂CO₃ strongly accelerate the corrosion of 304L while K₂SO₄ has little influence on the corrosion rate and on the morphology of the corroded surface. KCl and K₂CO₃ react with the chromium-rich oxide on the sample surface, forming K₂CrO₄. The resulting chromium depletion of the protective oxide causes rapid oxidation and the formation of a thick duplex scale consisting of an outer hematite layer and a inner layer made up of FeCrNi spinel-type oxide. The differences in the corrosivity of the three salts are directly connected to their ability to form chromate on the surface and, hence, to the relative stability of the corresponding leaving groups (HCl, CO₂ and SO₃).     

Structure and mechanical properties of arc evaporated Ti-Al-O-N thin films

The structure, mechanical properties, and machining performance of arc evaporated Ti-Al-O-N coatings have been investigated for an Al_0.66Ti_0.34 target composition and O₂/(O₂+N₂) gas flow-ratio varied between 0 to 24%. The coating structure was analysed using SEM, EDX, XRD, XPS, TEM, and STEM. Mechanical properties were analysed using nanoindentation and the deformation behaviour was analysed by probing the nanoindentation craters. The coatings performances in cutting tests were evaluated in a turning application in low carbon steel (DIN Ck45). It is shown that the addition of oxygen into the arc deposition process leads to the formation of a dual layer structure. It consists of an initial cubic NaCl-structure solid solution phase formed closest to the substrate, containing up to 35 at.% oxygen (O/O+N), followed by steady-state growth of a nanocomposite compound layer comprised of Al₂O₃, AlN, TiN, and Ti(O,N). The addition of oxygen increases the ductility of the coatings, which improves the performances in cutting tests. At high levels of oxygen, (> 13 at.%), however, the performance is dramatically reduced as a result of increased crater wear.     

High-coercivity samarium-iron-nitrogen from nitriding melt-spun ribbons

Melt spinning has proven to be an excellent technique for magnetic hardening of a variety of permanent magnet materials, especially Nd-Fe-B. Recently, a new permanent magnet material has been discovered by nitriding the compound Sm₂Fe₁₇ to obtain Sm₂Fe₁₇N_x. The authors have obtained magnetically hard Sm-Fe-N ribbons with a room-temperature intrinsic coercivityH _ci = 22 kOe (1.8 MA/m) by nitriding melt-spun Sm-Fe precursor ribbons. Best results were obtained by grinding the ribbons to a <25 µm powder, then heat treating the powder in vacuum for 1 h at 700 °C prior to nitriding in N₂ gas at 450 to 480 °C. X-ray diffraction shows that the primary phase is TbCu₇-type Sn₂Fe₁₇N_x, a disordered hexagonal modification of the rhombohedral Snu₂Fe₁₇ phase.     

Mechanical and tribological properties of duplex treated TiN, nc-TiN/a-SiNx and nc-TiCN/a-SiCN coatings deposited on 410 low alloy stainless steel

The use of hard and superhard nanocomposite (nc) coatings with tailored functional properties is limited when applied to low alloy steel substrates due to their low load carrying capacity. Specifically in this work, in order to enhance the performance of martensitic SS410 substrates, we applied a duplex process which consisted of surface nitriding by radio-frequency plasma followed by the deposition of single layer (TiN, nc-TiN/a-SiN_x or nc-TiCN/a-SiCN) or multilayer (TiN/nc-TiN/a-SiN_x, TiN/nc-TiCN/a-SiCN) coating systems prepared by plasma enhanced chemical vapor deposition (PECVD). We show that plasma nitriding gives rise to a diffusion layer at the surface due to diffusion of nitrogen and formation of the α-Fe and ε-Fe₂N phases, respectively, leading to a surface hardness, H, of 11.7 GPa, compared to H = 5 GPa for the untreated steel. Among the TiN, nc-TiN/a-SiN_x and nc-TiCN/a-SiCN coatings, the latter one possesses the highest H value of 42 GPa and the highest H³/E_r² ratio of 0.83 GPa. Particularly, the TiN/nc-TiCN/a-SiCN multilayer coating system exhibits superior tribological properties compared to single layer TiN and multilayer TiN/nc-TiN/a-SiN_x coatings: this includes excellent adhesion, low friction (C_f = 0.17) and low wear rate (K = 1.6 × 10^− 7 mm³/N m). The latter one represents an improvement by a factor of 600 compared to the bare SS410 substrate. The significance of the relationship between the H/E and H³/E_r² ratios and the tribological performance of the nano-composite coatings is discussed.