In this work, we investigated the potential of a dense oxide layer in resisting ammonia corrosion. First, the degradation behavior of Hastelloy X substrate and HVOF sprayed CoNiCrAlY coating (as-sprayed condition) was studied in an ammonia gas flow environment. The coating was then heat-treated in air to pre-oxidize the surface, enabling the formation of a dense and stable oxide layer. Thereafter, the degradation characteristics of the pre-oxidized coating was investigated under the same environment. The mechanisms of degradation and corrosion resistance of the materials are elucidated.

Thermochemical processes are an appropriate way to improve the surface hardening of the material against wear. Thermal spraying is a group of deposition processes that can deposit different classes of materials. The use of thermomechanical process after metallic coatings deposition can result in a unique combination of bulk and surface properties. There are some studies that indicate the defects and stresses caused in the crystal lattice as one of the factors that most influence nitrogen diffusion during the nitriding process. The HVAF (High Velocity Air-Fuel) process can generate different fault conditions and stress-strain in the crystal lattice. The aim of this work is study the effect of the plasma nitriding or, as it is known, Glow Discharge (GD), on FeMnCrSiNi coating deposited with HVAF process. Initially, it was observed the formation of expanded austenite and CrN on the HVAF coating, followed by important increase on the hardness of the coating.

Ti-Al and Al-Cr metallic coatings were deposited on Superfer800H (Fe-based superalloy) through a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma sprayed coated specimens. Characterization and high temperature corrosion behaviour of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDAX and EPMA analysis have been used for characterization of the coatings and to analyze the oxide scale. Both the coatings have successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation at 900°C for 50 cycles in air and molten salt. The coatings subjected to cyclic oxidation in air have shown relatively high weight gains in the early cycles of the exposure. Uncoated Superfer800H (Fe-based superalloy) showed very poor resistance to hot corrosion in molten salt environment.

This study compares the effects of plasma nitriding and nitrocarburizing treatments on HVOF sprayed stainless steel coatings with different crystal structure. The treatments were conducted at 550 °C for 10 h in a gas mixture of N 2 and H 2 for nitriding and N 2 , H 2 , and C 2 H 2 for nitrocarburizing. Optical microscopy, SEM-EDS, and XRD show that the treatments produced thick nitride layers consisting of a compound layer and a nitrogen diffusion layer. The treatments increased not only the surface hardness, but also the load bearing capacity of the coatings due to the formation of CrN, Fe 3 N, and Fe 4 N phases. Plasma nitrocarburized 410 stainless steel had the highest microhardness and load bearing capacity because of the precipitation of Cr 23 C 6 on the surface.

Reactive plasma spraying (RPS) has been considered as a promising technique for in situ formation of aluminum nitride (AlN) based thick coatings. This study investigated the reactive plasma spraying of AlN coating with using Al 2 O 3 powder and N 2 /H 2 plasma. It was possible to fabricate a cubic- AlN (c-AlN) based coating. The phase composition of the coating consists of c-AlN, α-Al 2 O 3 , Al 5 O 6 N and γ-Al 2 O 3 . Understanding the nitriding process during coating deposition is essential to control the process and improve the coating quality. The nitriding process was performed by spraying, collecting the particles into a water bath (to maintain its particle features) and observing their microstructures and cross sections. During the coating process, the sprayed particles were melted, spheroidized and nitrided in the N 2 /H 2 plasma to form the cubic aluminum oxynitride (Al 5 O 6 N). The particles collided, flattened, and rapidly solidified on the substrate surface. The Al 5 O 6 N is easily transformed to c-AlN phase (same cubic symmetry) by continuous reaction through plasma environment. Improving the specific surface area by using smaller particle sizes enhances the surface nitriding reaction and improves the nitriding conversion. Furthermore, using AlN additives enhances the nitride content in the coatings. It was possible to fabricate thick and uniform coatings with high AlN content by spraying fine Al 2 O 3 /AlN mixture. Furthermore, the N 2 gas flow rate improved the nitriding conversion and the coating thickness.

The fretting phenomenon was investigated experimentally in contacts between nitrided, coated and nitrided-coated Ti-6-4 rods against uncoated M50 rods generating a circular Hertzian contact. A fretting wear test rig was designed and developed to facilitate mounting of rod specimens of different coating thicknesses. Fretting wear tests were performed on low temperature and high temperature nitrided Ti-6-4 rods as well as on T-800 (CoCrMoSi) thermal spray coated Ti-6-4 and T-800 coated M50 rods. Finally, tests were carried out on Ti-6-4 rods nitrided at low and high temperatures and T-800 thermal spray coated on the top. The results obtained from fretting tests of each surface against uncoated M50 are studied and compared. Fretting wear volumes and surface profiles are presented for the contacts studied. The fretting wear resistance of each surface is quantified and compared with Archard’s wear equation. The role of amplitude of motion and number of cycles on the fretting wear of coatings is discussed. It was observed that increase in fretting wear resistance of uncoated Ti-6-4 rods by nitriding is greater than thermal spray coating. The fretting wear resistance was found to be higher for high temperature nitrided-coated rods than for low temperature nitrided-coated rods.

This paper presents a way of processing cold-sprayed Ti-Al to produce titanium aluminum nitride coatings. These coatings are meant to serve as a durable protective layer on tools exposed to molten aluminum alloys. A Ti-Al powder mixture with a weight ratio of 70/30 was cold sprayed onto specially prepared substrates using nitrogen as a process and powder delivery gas. The resulting coatings were alloyed at different temperatures to obtain a stabilized Ti-Al intermetallic phase for further nitriding treatment. The nitriding process was carried out in an ammonia-nitrogen atmosphere at 900 °C. The final product had a web-shaped microstructure with the same thickness as the cold-sprayed Ti-Al. Test samples were placed in molten aluminum for 1200 hours without notable chemical reaction.

In this paper, spectroscopic and electrostatic probe measurements are made to examine the characteristics of a supersonic dc plasma jet near the surface of titanium plate during a nitriding treatment. The low-pressure nitriding process is done using a mixture of ammonia, nitrogen, and hydrogen gasses. Heating effects from the plasma are evaluated with nickel slug and thermocouple attached to the plate. The authors present the results of their study along with observations, insights, and suggestions on how to improve plasma nitriding processes. Paper includes a German-language abstract.

Most of the work published to date on thermally sprayed titanium has been carried out in controlled atmospheres, yielding little information about the reaction of titanium with nitrogen and oxygen. The aim of this study is to investigate the influence of atomization gas on the formation of titanium nitrides and oxides during wire arc spraying. In the experiments, three types of gases (air, nitrogen, and argon) are used to deposit Ti on steel substrates and the microstructure and composition of the coatings, as well as the wire feedstock, are assessed by means of SEM and XRD analysis. The effect of spraying distance on crystal structure and nitrogen content is also investigated in the case of the argon-atomized coating. Paper includes a German-language abstract.

Chemical reactions between melted materials and their gaseous environment are generally inherent to the thermal spray process. Measures to promote and control these reactions are the distinguishing characteristic of reactive plasma spraying. In this paper, Ti-6Al-4V nitrided coatings are produced by high-pressure reactive plasma spraying. The coatings are deposited at different pressures up to 250 kPa in a reactive nitrogen atmosphere as well as air in order to study the influence of spraying pressure and atmosphere. The microstructure and phase composition of the Ti-6Al-4V layers are examined with the aid of X-ray analysis, microprobe measurements, and electron imaging. The investigations show that the pressure-supported nitrogen application during spraying led to the formation of fine and coarse TiN in the Ti matrix. Paper includes a German-language abstract.

Spectroscopic and electrostatic probe measurements were carried out to understand the plasma feature inside and outside a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle. Ammonia and a mixture of nitrogen and hydrogen were used as the working gas. The NH3 and N2+3H2 plasmas in the throat were expected to be nearly in a temperature-equilibrium condition, although the plasmas in the expansion nozzle and in the downstream plume without substrate plates were in thermodynamical nonequilibrium states. As a result, the H-atom excitation temperature and the N2 rotational excitation temperature decreased from 7000-11000 K in the throat to about 4000 K and to 1000-1500 K, respectively, on the nozzle exit at 0.1-0.2 g/s, although the NH rotational temperature did not show an axial decrease even in the nozzle. On the other hand, each temperature was almost kept a small range in the downstream plume without substrate plates under an ambient pressure of 130 Pa except for the NH rotational temperature for NH, working gas, although in the case with a titanium plate the nonequilibrium plasma came to a temperature-equilibrium one as approaching the plate.

Since plasma jets, which have been used as heat sources of thermal plasma spraying process, expand adiabatically under a low pressure environment, the plasma temperatures drastically fell down to 2000K at the nozzle out let at 30Pa chamber pressure. However, the plasma jets still had enough reactivity to form hard nitride layer on the surface of the titanium samples by only a few minutes treatment. In this study, in order to obtain useful information for the practical applications of this plasma as low temperature and high rate surface modification processes, nitriding of nitriding steel and carbon steel using supersonic expanding hydrogen/ nitrogen mixture plasma jets were carried out. Consequently, though surface hardening was occurred slightly in the case of carbon steel, surface hardening was obviously promoted in the case of nitriding steel. In both cases, surface hardening was promoted with increasing hydrogen flow rate and thermal damages of the samples due to heat transfer from plasma jets weren't observed. Besides, according to the results of wear testing, wear mass loss of nitrided samples were much lower than that of non-nitrided samples. From these results, this process was found to have a high potential even in the case of surface modification of steel materials.

This paper contributes to the broadening of knowledge about process combinations in connection with thermally sprayed coatings. It describes the influence of a later annealing, nitriding, and carburizing on different thermal sprayed coatings. The connection between the processes is determined and optimal process parameters is adjusted for both processes. The paper analyzes the coating characteristics and the alterations of the substrate. The results of the metallographic and X-ray analysis as well as of the wear and corrosion tests are discussed. Prospects for possible further applications are given. Paper includes a German-language abstract.

In this paper, as a basic study of the plasma jet process under a low pressure, the nitriding of titanium plates and atmospheric plasma sprayed titanium coatings are carried out using nitrogen and hydrogen added nitrogen plasma jets at 30Pa. The plasma torch used in this paper is equipped with a supersonic expansion nozzle to improve the acceleration efficiency of plasma jets. The effects of ambient pressures and supersonic expansion nozzles on transformation of plasma particles are examined from gas-dynamical viewpoints. It was observed that from the results, the supersonic plasma jet process under a low pressure was proved to have a high potential for nitriding. Paper includes a German-language abstract.

This paper presents a novel process that uses RF plasma spraying and premixed elemental powders to produce intermetallic matrix composites in situ without having to add reinforcement fibers or particles. Splats were collected on a stainless steel substrate and were analyzed to determine if nitrides had formed in metal droplets during flight and how it affected splat morphology. The typical splat morphology of impinged Ti droplets is disk-type with an outer peripheral fringe. Aluminum splats, on the other hand, are classified into two categories: a disk-type with an irregular outer periphery and a semi-massive-type. Other composites produced and examined include TiAl, AlN, and Ti2AlN.

At the present, components which require both nitriding and locally a thermal sprayed coating or nitrided components which should l)e reworked are usually nitrided before spraying and the area to be coated is masked during nitriding or is prepared before spraying by locally removing the nitrided layer by grinding. Seen technically, advantages are to be expected if the nitriding process can be carried out after spraying. Moreover a post-nitriding of thermal sprayed coatings is of interest for improving coating characteristics, mainly wear resistance. Understanding the behaviour of sprayed coatings during nitriding in comparison to bulk materials will help to understand generally the behaviour of such coatings in gas atmospheres at increased temperatures. The objectives of the project are the investigation of the interaction between thermal spraying and nitriding, and the optimisation of both processes to achieve improved bonding, wear and corrosion characteristics respectively to get nitriding of the substrate through the coating without spalling or cracking. Furthermore the behaviour and structural changes of different coatings at increased temperatures are determined. The metallographic, X-ray, wear and corrosion results of the resulting compound coatings and parts are presented. Possible new applications are discussed. The project is funded by the German Research Ministry.