Molybdenum disilicide (MoSi 2 ) is a suitable material for high temperature applications especially because of its excellent high temperature oxidation resistance. For several high temperature applications MoSi 2 shows high potential to be used as a protective coating. The oxidation behaviour of HVOF sprayed MoSi 2 coatings is studied at 1500 °C. The oxidation tests are carried out in a simultaneous thermogravimetric device and the mass change is measured in dependence on the oxidation time. The microstructure of the coatings before and after oxidation is examined by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXS). The mass of the coating increases according to a parabolic function. During the oxidation test the microstructure changes significantly from a typical thermal spray coating microstructure with lamellae, pores and a phase mixture of MoSi 2 and Mo 5 Si 3 to a two phase system with sharply separated grain boundaries. On the surface of the coating a silicon dioxide layer with a thickness of less than 10 µm is formed.

Improved HVOF spraying with a gas shroud has been developed to fabricate environmental barrier coatings of corrosion resistant alloys such as HastelloyC. For such coatings, control of oxidation of the powder material during spraying is very important and the gas shroud has been effective to lower oxygen content to 0.19mass%. In the present study, further reduction of oxygen content to 0.063mass% was achieved by changing the composition of combustion gas by introducing nitrogen into the combustion chamber. This value is almost comparable to the oxygen content 0.042mass% of the feedstock powder but the porosity of the coating increased. Introduction of nitrogen to the combustion chamber lowered the temperature of the spray particles in flight while maintaining their high velocity. Another coating with 0.14mass% was obtained with open porosity below 0.1vol% by changing the mixing ratio of nitrogen, which exhibited improved environmental barrier property in artificial seawater.

Plasma sprayed thermal barrier coatings (TBCs) have been extensively employed in most aerospace and land based turbine engines, permitting gas temperatures to be raised substantially above those for uncoated systems. Traditional TBC systems are composed of a metallic bond coat and a ceramic top coat both applied by plasma spraying. New deposition techniques have been proposed mainly for the metallic bond coat, like high-velocity oxy-fuel (HVOF) and more recently cold gas spray (CGS). CGS is an attractive technology that generates very dense coatings, without oxidation and maintaining the initial powder microstructure, characteristics that are potentially interesting for bond coat application. In the current study, TBCs with two sets of cold gas sprayed bond coatings were prepared and evaluated in high temperature isothermal oxidation tests. Measurements of sample mass as well as microstructure observations were carried out on the as sprayed and oxidized samples to compare the behavior of different bond coat chemical compositions. As sprayed oxidation degree, structural changes and bonding strength of the samples were also determined.

In the current study, the oxidation of hardmetal coatings (WC-12%Co, WC-17%Co, WC-10%Co4%Cr, WC-20%“CrC”-7%Ni, Cr 3 C 2 -25%NiCr, (Ti,Mo)(C,N)-29%Ni and (Ti,Mo)(C,N)-29%Co) in the temperature range 350- 900°C was studied for test durations ranging from 2 h to 128 h. The formation of oxide scales was investigated by X-ray diffraction, as well as by optical microscopy and SEM (including EDX) of coating cross sections. For coatings obtained by spraying with DJH 2700 and TopGun HVOF systems, the phase composition had only a moderate influence on high-temperature oxidation behavior in atmospheric conditions. The first oxides detectable by X-ray diffraction appeared on the coating surfaces after oxidation at 350°C for 128 h for all coatings. Pronounced oxidation (formation of oxide scales with thicknesses of greater than 10 µm) started at 600°C. Oxide scale growth differed significantly above this temperature among the hardmetal compositions studied here. WC-20%"CrC"-7%Ni and Cr 3 C 2 -NiCr had the highest oxidation resistance, with the oxide scale thicknesses lying below 10 ìm after oxidation at 800°C and 900°C for the two materials, respectively.

MoSi 2 -TiB 2 coatings were applied on Ti6Al4V substrates by means of low pressure plasma spraying with and without a surface pre-treatment with plasma-transferred-arc. The coatings were characterized by optical and scanning electron microscopy, X-ray diffraction analysis and hardness measurements. No difference in the microstructure between the coatings was detectable. The microstructure shows the typical lamellar structure of thermal spray coatings with a good embedding of the titanium borides in the matrix. At the interface between matrix and some borides a reaction zone is visible. In comparison to the feedstock powder, the phase composition of the coatings has change, because a great amount of the tetragonal MoSi 2 phase transformed into the hexagonal high temperature modification. The content of titanium diboride is lower in the coating. Coatings on substrates with a pre-treatment show a good adhesion to the substrate, while the adhesion of the coatings on the pre-treated substrates is poor.

Plasma spraying of metals and metallic alloys performed in controlled atmosphere or soft vacuum results in coatings with a low oxidation level and excellent thermomechanical properties. Unfortunately, the spraying cost is drastically increased by one or two orders of magnitude compared to air plasma spraying (APS). Thus the minimisation of oxidation during APS is a key issue for the development of such coatings. Oxygen concentrations sucked into plasma jets have been measured by an enthalpy probe linked to a mass spectrometer. This technique allows to determine simultaneously plasma composition, temperature and velocity distributions within the plasma plume. Results have been compared to those obtained with a two-dimensional turbulent flow model. The obtained results have shown that surrounding air entrainment is reduced when using adequate Ar/H2/He mixtures which viscosity is higher than that of Ar/H, mixtures, limiting the turbulence in the jet fringes and pumping of the surrounding atmosphere.

Plasma spraying is known to be one of the main promising processes for the manufacturing of Ti/SiC long fiber composites. However, some improvements remain to be done for this process to be applied in a routine industrial route. These include: oxygen contamination of the sprayed material through that of Ti particles before and during spraying damaging of fibers due to a high level of thermal stresses induced at the spraying stage adequate deposition of Ti-based powder to achieve a low-porosity matrix and good impregnation of the fiber array. This contribution deals with work in the 3 previously mentioned fields, which resulted in a whole 3-fold study of the process. Oxidation was studied using electron microprobe analysis of elementary particles quenched and trapped into a closed box at various given flight distances. Oxygen diffusion phenomena within the particles are discussed from a preliminary theoretical approach coupled with experimental data. Isothermal and thermo-mechanical calculations were made using ABAQUS code to determine stresses arising from contact of a liquid Ti-6Al-4V particle onto a SiC fiber. On a higher scale, i.e. that of the sprayed powder flow, a 2-dimensional original model simulating the deposition of droplets onto a substrate was developed. This model is based on a lattice-gas automaton which reproduces the hydrodynamical behavior of fluids.

The isothermal oxidation of bond coats composed of vacuum plasma sprayed (VPS) MCrAlY (NiCoCrAlYTa and CoNiCrAlY) or palladium modified nickel aluminides (NiPd + APVS) was studied in several oxygen partial pressures (10 5 , 1 and 10 -5 Pa), with two heating rates (20 and 60K/min) at different temperatures (900, 1000 and 1100°C). For MCrAlY coatings, Arrhenius plots of the parabolic rate constants show a kinetic transition below 1000°C. This could be linked to a transition from Al 2 O 3 to Cr 2 O 3 scale growth. Lower oxygen partial pressures induce lower parabolic rate constants at 900°C. This leads to the assumption that scales grown at low oxygen partial pressures are still formed of alumina at 900°C. Nevertheless, these results could not be confirmed by chemical analysis (EDS, XPS). The two tested heating rates show no influence on the oxidation kinetics of both MCrAlY coatings. In the case of aluminide, for low oxygen partial pressures, the parabolic kinetics are reduced of one order of magnitude (for P O2 = 10 5 to 1 Pa) and correspond to a thinner scale of α-alumina. Also, the heating rate modifies the parabolic kinetics (i.e. after the transient stage) and the total weight gains for all oxidation temperatures, with higher parabolic rate constants after heating at slower rate.

Light water reactors (LWR) use zirconium-alloy fuel claddings, the tubes that hold the uranium-dioxide fuel pellets. Zr-alloys have very good neutron transparency, but during a loss of coolant accident or beyond design basis accident (BDBA) they can undergo excessive oxidation in reaction with the surrounding steam environment. Relatively thin oxidation-resistant coatings on Zr-alloy fuel cladding tubes can potentially buy coping time in these off-normal scenarios. In this study, cold spraying, solid-state powder-based materials deposition technology has been developed for deposition of oxidation-resistant Cr coatings on Zr-alloy cladding tubes, and the ensuing microstructure and properties of the coatings have been investigated. The coatings when deposited under optimum conditions have very good hydrothermal corrosion resistance as well as oxidation resistance in air and steam environments at temperatures in excess of 1100 °C, while maintaining excellent adhesion to the substrate. These and other results of this study, including mechanical property evaluations, will be presented.

As steam power plants continue to move towards higher operating temperatures in order to improve efficiency, materials exposed to the working fluid are subjected to accelerated degradations in the forms of surface oxidation and reduced mechanical properties. In this study, the oxidation behavior of two cobalt base alloys, CoCrMoSi (T14) and CoCrNiMoSi (T19), was evaluated in superheated steam (SHS, 0.1MPa) at 800 °C for up to 500 hours. After the exposure, both T14 and T19 alloys experienced weight gain caused by oxidation. Visual observation and SEM surface analysis revealed that T19 had greater extent of surface oxide spallation than that seen on T14. From the cross-sectional evaluation, however, a thin, adherent oxide layer was found to have formed on T19. T14 in fact had suffered from excessive internal oxidation and the surface oxide was uneven. Based on the results obtained so far, it is believed that the finer Laves phase combined with greater amount of Cr in alloy T19 have enabled the formation of a protective oxide layer and thus reduced the extent of internal oxidation. Due to the extensive oxidation ingress along the large Laves phase, it is concluded that T14 is not suitable for applications in SHS at 800 °C.

Ni-Al intermetallics have excellent corrosion and oxidation resistance, but their use in thermal spraying has been limited due to issues with in-flight oxidation. In this study, a novel approach is proposed to remove oxide from Ni-Al droplets in-flight by adding a deoxidizer (diamond) to the feedstock powder. A mixture of nickel, aluminum, and diamond powders was mechanically alloyed using a combination of cryogenic and planetary ball milling. The resulting Ni/Al/diamond composite powder was then plasma sprayed via the APS process, forming Ni-Al coatings on Inconel 738 substrates. Phase composition, microstructure, porosity, and microhardness of the coatings were characterized by X-ray diffraction, scanning electron microscopy, image analysis, and hardness testing, respectively. Oxygen content measurements showed that the coatings contained significantly less oxygen than coatings made from ordinary Ni/Al powders. In-flight particle temperatures were also measured and found to be higher than 2300 °C. The low oxygen content in the coatings is attributed to the in-situ deoxidizing effect of ultrahigh temperature droplets which are also oxide-free.

HVOF coatings of 80Ni-20Cr and 50Ni-50Cr powders were carried out on 9Cr-1Mo steel substrate. The coating thickness was around 60 µm. The coated specimens were steam oxidized in four different temperatures, ranging from 600 to 750°C. The steam oxidized specimens were taken out from the chamber at 10, 100 and 1000 hours to examine their protectiveness against scale growth. X-ray diffraction, scanning electron microscope and electron probe microanalysis were carried out on the steam-oxidized specimens in reference with as-coated conditions. Both 80Ni- 20Cr and 50Ni-50Cr coatings show neither scale growth at the interface nor de-lamination in the coating structure. Fe, and Ni diffusion was found in the case of 80Ni-20Cr coatings. The diffusion increased with increase in the temperature and test duration. On the other hand, 50Ni-50Cr coatings, showed an excellent performance in all tested temperatures. The effect of Cr content in the coating, change in phase and compound formation during oxidation, and their influence on the diffusion process will be discussed in detail.

Thermal spray of Al was carried out on the modified 9Cr-1Mo steel to evaluate the steam oxidation resistance of the sprayed Al coating. Atmospheric plasma spray process (APS) was used to coat aluminum on sandblasted 9Cr-1Mo steel substrate. The coating thickness was around 40 µm. The coated specimens were steam oxidized in four different temperatures, ranging from 600 to 750°C. The results show that the scale growth occurred in the interface between coating and substrate subsequently it penetrated into the coating structure. Al diffused into the alloy substrate with high solubility. The diffusion increased with increase in the steam temperature and test duration. Diffused aluminum formed the high hardness intermetallic compound in the substrate near the coating/substrate interface. With increase in the test duration, the intermetallic compound moved towards the bulk and at prolonged aging, it became dissolved. This was identified from the decrease in the micro hardness values at coating/substrate interface at prolonged duration. The scale growth at the substrate surface of Al sprayed steel was much controlled compared to the uncoated specimens.

High temperature oxidation behavior of MCrAlY coatings was studied at several temperatures in the range from 800 to 1100°C. In this study the MCrAlY coatings were obtained by plating using CrAlY as precursor powders in an electrolytic bath containing nickel and cobalt salt in solution. The size of the precursor CrAlY powders used was generally below 10 um. As-plated coatings consisted of a random distribution of CrAlY particles in the Ni-Co matrix. The heat-treatment of the as-plated coatings at elevated temperature resulted in the development of a gamma and beta structure. Both as-deposited and oxidized coatings were characterized by optical, scanning electron microscope and electron beam microprobe. During oxidation the coatings formed alumina scale with a negligible amount of transient nickel and chromium oxides. The spallation resistance of the oxide scale was investigated by thermal shock testing. The test consisted of a rapid cooling from 1000°C to 100°C with a two- minute dwell time at the maximum temperature. The thermal shock test was conducted in a) as–deposited and heat-treated condition and b) after preoxidation at 800°C and 1050°C, respectively. The coatings retained the alumina scale during thermal shock cycling.

Air engulfment by the plasma jet in Air Plasma Spraying (APS) causes in-flight oxidation of metallic particles. This oxidation, often complex and difficult to explain by classical diffusion-controlled oxidation, is governed by several mechanisms. This paper highlights the possible in-flight oxidation mechanisms in metallic particles with a focus on convective oxidation. Two different types of austenitic stainless steel particles, Metco 41C (-106+45 µm) and Techphy (-63+50 µm) were air plasma sprayed using a dc plasma gun (PTF4 type) and were collected in an argon atmosphere. Preliminary experiments indicated that different mechanisms are likely to occur during the in-flight oxidation of particles. Mass transfer from surface to interior of particle occurred forming oxide islands in particles. The mass transfer is governed by convective movements inside liquid particles within plasma jet core due to higher plasma-particle kinematic viscosities ratio and particles Reynolds number higher than 20. The islands were composed of metastable phases consisting of mixed oxide of Fe and Cr, likely in a nonstoichiometric form of FeCr 2 O 4 . Convective movements within particles cease roughly outside of the plasma jet core and classical surface oxidation was found to be the dominating phenomenon forming the surface oxide layer. Moreover, the molten surface oxide outside the jet core is entrained to the tail of the particle if plasma conditions promote higher particle temperature, velocity and Re number.

During air plasma spraying, molten metal particles flying in the plasma jet are oxidized. As a result, a part of the metal melt is converted into oxide melt. After the particle impact and solidification, oxidation continues as a gas – solid reaction. The present paper deals with oxidation of two binary Ni-based alloys. One of them, Ni-20%Cr, is frequently used in thermal spray applications. Another one was a Ni-Fe alloy with an approximate proportion of both components 1:1. The feedstock powders were plasma sprayed by a water-stabilized gun WSP. PAL 160. To analyze the reaction products of the in-flight oxidation stage, the flying particles were trapped and quenched in liquid nitrogen. Oxides resulting from both oxidation stages were studied in the as-sprayed deposits after their cooling down to room temperature. The oxide amounts in the samples were determined indirectly by oxygen level measurement using "extractive fusion" (LECO-method). Structure of the oxides, separated by dissolution of the metallic phase, was investigated by X-ray diffraction. Iron-containing oxides were also characterized by Mössbauer spectroscopy. From the point of view of reaction kinetics, both alloys behaved in a similar way. The particles quenched in liquid nitrogen contained less than 2% of oxygen, whereas in the deposits these values were higher, up to 4.5 %. Two oxide phases were found in all plasma sprayed Ni-Cr samples: a rhombohedral phase similar to (Ni,Cr) 2 O 3 and a tetragonally distorted spinel phase (Ni,Cr) 3 O 4 , both of them very rich in chromium. Another oxide, NiO, was present mainly in the deposits. In the oxidation products of Ni-Fe alloy, the dominant phase was similar to nonstoichiometric wüstite FeO. The results of thermodynamic calculations are in sound agreement with the experiments except for the presence of the tetragonal phase, the composition of which is near to Cr 3 O 4 , in oxidation products of Ni-Cr alloy.

Somewhat unconventional plasma sprayed TBC systems were produced and evaluated by interrupted or cyclic furnace oxidation life testing. Approximately 250 µm thick 8YSZ coatings were directly sprayed onto grit blasted surfaces of PWA 1484, without a bond coat, in order to take advantage of the excellent oxidation resistance of this superalloy. For nominal sulfur contents of 1 ppmw, total coating separation took place at relatively short times (200 hr at 1100oC). Reductions in the sulfur content, by melt desulfurization commercially (0.3 ppmw) or by hydrogen annealing in the lab (0.01 ppmw), improved scale adhesion and extended life appreciably, by factors of 5-10. However, edge-initiated failure persisted, producing massive delamination as one sheet of coating. To subvert this mechanism, samples surfaces of melt desulfurized PWA 1484 were EDM’ed with a grid of grooves or ribs (~250 µm wide and high), resulting in a segmented TBC surface macrostructure. Now failure only occurred as independent single segments events. For grooved samples, 1100 C segment life was extended to ~1000 hr for 5 mm wide segments, with no failure observed out to 2000 hr for segments ≤ 2.5 mm wide. Ribbed samples were even more durable, and segments ≤ 6 mm remained intact for 2000 hr. Larger segments failed by buckling at times inversely related to the segment width and decreased by oxidative effects at higher temperatures. This critical buckling size was consistent with that predicted for elastic buckling of a TBC plate subject to thermal expansion mismatch stresses. Thus, low sulfur substrates demonstrate appreciable coating lives without a bond coat, while rib segmenting extends life considerably.

Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO 2 /Y 2 O 3 ). In this work the oxidation behavior of conventional and nanostructured HVOF NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Free-standing bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at a 1000°C for different time periods in order to form the thermally grown oxide (TGO) layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared to that of the conventional one. This behavior is a result of the formation of a continuous Al 2 O 3 layer on the top surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.

The goal of this paper is to evaluate high temperature ageing properties of a new high temperature titanium blade compatible abradable material DurabradeTM 2614. Coatings were tested in as-sprayed condition and after ageing at 550°C and 655°C for up to 8,030 hours. Coating properties such as coating hardness, erosion resistance, and cohesive strength were evaluated at regular time intervals. Abradability was tested in as-sprayed condition and after ageing. The results show that coating hardness, GE erosion resistance, and cohesive strength of the new material change most in the first 200 hours and SMC90 erosion resistance, and oxidation weight gain change most in the first 1,000 hours and then they stabilize at values that guarantee good seal performance. The good performance of the new seal after 8,030 hours ageing has been demonstrated by abradability and erosion testing.

This paper examines the in-flight oxidation of molten aluminum sprayed in air using the twin-wire electric arc (TWEA) thermal spray process. The oxidation reaction of aluminum in air is highly exothermic and is represented by a heat generation term in the energy balance. Aerodynamic shear at the droplet surface: (1) enhances the amount of in-flight oxidation by promoting entrainment and mixing of the surface oxides within the droplet, and (2) causes a continuous heat generation effect due to the exothermic oxidation reaction that sustains droplet temperature as compared to a droplet without internal circulation. This continual source of heat input keeps the droplets in a liquid state during flight. A linear rate law based on the Mott-Cabrera theory was used to estimate the growth of the surface oxide layer formed during droplet flight. An explanation is provided for the elevated, nearly constant surface temperature (~ 2000 °C) of the droplets during flight to the substrate and it is shown that the majority of oxide content in the coating is produced during flight, rather than after deposition. The calculated oxide volume fraction of an average droplet at impact agrees well with the experimentally determined oxide content for a typical TWEA-sprayed aluminum coating, which ranges from 3.3 to 12.7%.