Additive Manufacturing (AM) processes offer geometrical freedom to design complex shaped parts that cannot be manufactured with conventional processes. This leads to new applications including aerospace propulsion systems where the Ni-superalloy based material has to withstand high operating temperatures. In this contribution, the influence of heat treatment and surface conditioning of the additively manufactured Inconel 718 substrates on the thermocycling performance of suspension sprayed YSZ coatings was investigated. The different surface conditions included as-built, sandblasted and milled substrate surfaces with and without heat treatment. YSZ coatings were applied using suspension plasma spraying (SPS) with commercial available suspensions. Thermal cycling tests (FCT) at 1100°C, 1300 °C, and 1500 °C were applied to coating systems until failure occurred. The microstructures of the samples were characterized before and after thermal cycling. The performance of the coatings was mainly influenced by the coating morphology and FCT test conditions and less by the state of the AM substrates. Columnar-like YSZ SPS sprayed coatings on AM Inconel 718 substrates seemed to be a promising candidate for rocket engine applications.

Different surface protection technologies were investigated in a waste-wood fired fluidized bed boiler. This biomass fuel environment is more aggressive than those firing virgin wood due to the elevated presence of sodium, potassium, lead, and zinc, leading to the deposit of alkali metal chlorides in conjunction with ash on boiler tube surfaces. As laboratory tests are seldom representative of the complex firing, chemistry, temperature, and local heat flux encountered in actual operating conditions, five different commercial, near commercial, and development coatings were applied to a 1 m length of plain carbon steel tubing used in the furnace walls. The coatings were fully characterized and measured prior to installation and after exposure. Iron and nickel-based weld overlays, two high velocity thermal spray coatings, and a laser-clad nanosteel coating were tested. After exposure, the tube was extracted from the boiler and corrosion scales and material losses were evaluated in comparison to unprotected tube material.

Ceramic coatings are often applied to metallic substrates using a bond coat to promote adhesion. The corrosion environment between the substrate and ceramic layer can be very harsh due to the absence of dissolved oxygen, high concentrations of corrosive electrolytes, and galvanic and crevice corrosion mechanisms within the bond coat itself. This study assesses the performance of several bond coats in sulfuric acid, including plasma sprayed tantalum and HVOF sprayed NiCr, Hastelloy C, and Ultimet. The bond coats were deposited on Hastelloy substrates and covered with a Cr 2 O 3 topcoat. Electrochemical polarization and open circuit potential measurements were recorded and coating microstructures were examined before and after the corrosion tests. Plasma sprayed tantalum exhibited the best corrosion resistance at room temperature, followed by HVOF sprayed Hastelloy C.

NiCr-Mo composite coating was prepared by plasma spraying of shell-core-structured NiCr-Mo powders. The morphologies of the NiCr-Mo powders and microstructure of the corresponding NiCr-Mo coating were characterized by SEM. Furthermore, the erosion behavior of the NiCr-Mo coating at impact angles of both 30° and 90° was investigated, and was further compared with that of the Ni20Cr coating and the In-738 alloy bulk. Results showed that fully-dense and homogenous NiCr-Mo coating with excellent interface bonding and no pure Mo inclusions was obtained. Furthermore, the erosion test results showed that the erosion rate of the optimized NiCr-Mo coating is lower than that of NiCr coating at both impact angles. Moreover, the NiCr-Mo coating presented excellent erosion resistance which was comparable as that of In-738 alloy bulk, attributing to the fully-dense microstructure and metallurgical interface bonding within coating.

The Cold spray method of material deposition is widely used for surface enhancement, to improve properties such as corrosion and wear. A detailed microstructural analysis of cold sprayed Ni based coatings (IN625 (Inconel 625 is a Trademark of Huntington alloys corporation) using the transmission electron microscope, revealed the occurrence of three distinct types of microstructures in the as sprayed condition, adjacent to each other. These include the occurrence of large (> 1 μm) grains having a high dislocation density, along with regions comprising shear bands (20-30 nm wide) and twins with a large aspect ratio (> 1000), along with locations having a very fine grained structure (20-30 nm). The crystallite size, using a Hall-Williamson plot measures an average 50 nm. The substrate deformation indicates a bilayer dynamic recrystallization, as a means to accommodate the strain. The microscopy studies will serve to correlate the bonding mechanism of cold sprayed IN625 on 4130 steel.

The bonding between flattened particles in plasma-sprayed metal coatings dominates their corrosion behavior by influencing the porosity in coatings, especially the porosity connected to the substrate for coatings used in a corrosive environment. Therefore, how to efficiently enhance the lamellar interface bonding in metallic coatings has been an important issue which has not been settled effectively. In this study, a shell-core structured powder particle designing with cladding spherical Ni20Cr powders with refractory molybdenum as alloying element is proposed to limit the evaporation of low melting point elements and subsequently raise particle temperature significantly high enough to cause impact melting. Results show that a dense coating with much low porosity was obtained due to the improved lamellar interface bonding by gas shrouded plasma spraying of the composite NiCr -Mo particles. Electrochemical method was employed to evaluate the polarization behavior of the NiCr - Mo coating to estimate its connected porosity. It was revealed that NiCr-Mo coating of excellent corrosion resistance with low connected porosity can be obtained by designing the shell-core-structured powder.

This paper investigates the influence of metal bond coat (MBC) thickness on the adhesion strength of air plasma spray (APS) ceramic coatings for extreme service ball valve applications. The study was conducted using three different ceramic coatings (Cr 2 O 3 , Cr 2 O 3 -Al 2 O 3 , and TiO 2 ) applied on three different substrates (Titanium G12, Inconel 625, and 2507 Duplex stainless steel) with varying thicknesses of MBC. Tests were performed for adhesion strength (ASTM C633), metallography (ASTM E1920), and percent porosity (ASTM E2109 Method B). The results showed that the adhesion strength of the ceramic coatings was not reduced until the MBC thickness exceeded 0.060 in. The failures observed were either cohesive in the ceramic, cohesive in the adhesive, or a combination of both. It was concluded that a recommended MBC thickness limit of 0.030 in. can be established based on the experimental data.

In this study, porous and dense layers of alloy 625 are deposited on nickel foam sheets using a modified twin wire arc spraying process. Sandwich panels with arc-sprayed alloy 625 skins on nickel foam cores were fabricated then subjected to four-point bend testing. The effects of skin porosity on flexural rigidity and overall mechanical behavior are investigated. The ductility of porous alloy 625 skins was improved after heat treatment at 1100 °C for 3 h.

This study evaluates the base mechanical properties of Hastelloy C-276, WC-Co, and Cr 3 C 2 -NiCr coatings produced by HVOF spraying at angles ranging from 60° to 90°. Examinations and tests were conducted to characterize changes in microstructure, density, hardness, wear resistance, surface roughness, and deposition efficiency based on spraying angle. Only minor spray-angle dependencies were found for base mechanical properties, although thickness, roughness, and deposition efficiency varied greatly.

In this study, twin wire arc spraying is used to deposit dense Inconel skins on 40 PPI nickel foam sheets. A design-of-experiments approach is used to investigate the effects of grit blasting on the surface characteristics of paste-filled foam. In-flight behavior of molten droplets and its effect on coating properties is assessed at three spray distances. The Inconel coatings are evaluated based on SEM and EDS analysis, roughness measurements, and adhesion testing. The results indicate that acceptable adhesion, porosity, and oxide content can be achieved over a small range of grit blasting parameters.

An important problem for the petrochemical industry is the behaviour of materials in aggressive environments, when hydrogen sulphide, carbon dioxide and sand, which contribute to corrosion erosion of the surface, are present. Generally, the use of hard materials such as thermal sprayed tungsten carbide and chromium carbide reduces this problem. Cemented carbides are quite suitable for this purpose: they are composite materials of pure carbides with binder metal alloys of low melting point and high ductility; the selection of the binder metals depends mostly on its ability to wet the surface of the carbide particles to ensure secure coating adhesion. Among the cemented carbides, namely tungsten carbide cobalt-chromium based (WC/CoCr) is considered as the standard for application to ball valve bodies and seats in the petrochemical field, while chromium carbide nickel-chromium based (Cr 3 C 2 /NiCr) is suitable for particular applications. Inconel 625 is also used in this field and usually applied by welding. This paper addresses the characterization of corrosion behavior of HVOF coated samples of WC/CoCr, Cr 3 C 2 /NiCr and Inconel 625 in aggressive environments, and in particular ferric chloride test according to standard ASTM G48-92 and H 2 S/CO 2 test based on NACE standards has been carried out. According to the test results, WC/CoCr based coatings show the best behavior both in terms of corrosion, thus confirming to be very versatile and useful for the application in petrochemical field.

Within Surface and Coating Technologies, the High Velocity Oxy-Fuel (HVOF) thermal spray process generates significant peening stresses due to the impact at high velocity of semi molten particles onto the substrate. The level of high kinetic and thermal energy of impinging particles is a key-parameter to understand how residual stresses build up through the whole system during spraying, and to which extend these stresses influence the resulting coating adhesion strength. While an appropriate combination of thermal and peening stresses is beneficial to the deposit bonding, no systematic study has been carried out to determine their respective amplitudes. A numerical Finite Element Analysis (FEA) has been developed to isolate peening stresses from thermal stresses developed into the substrate target, after successive impacts of single particle. The investigation is performed on Inconel 718 feedstock material HVOF sprayed on Inconel 718 substrate. The relationship between the developed stress state at the substrate interface and the impinging particle temperature and velocity is given a particular interest.

Fundamental understanding of relationships between process parameters, particle in-flight characteristics and adhesion strength of HVOF sprayed coatings is important to achieve the high coating adhesion that is needed in aeronautic repair applications. In this study statistical Design of Experiments (DoE) was utilized to identify the most important process parameters that influence adhesion strength of IN718 coatings sprayed on IN718 substrates. Special attention was given to the parameters combustion ratio, total gas mass flow, spray distance and external cooling, since these parameters were assumed to have a significant influence on particle temperature and velocity. Relationships between these parameters and coating microstructure were evaluated to fundamentally understand the relationships between process parameters and adhesion strength.

Corrosion is a very important problem in the Municipal Solid Waste Incinerator (MSWI) superheaters. This problem causes the plant stops and tube replacements, promoting the loss of energy generation rate. The main corrosion agent is the chlorine deposits. HVOF coatings have been sprayed to improve corrosion resistance of the superheaters inside the MSWI boilers. Inconel 625 and Hastelloy C22 alloys have been sprayed as a powder feedstock material. The spray process has been analyzed by a Spray Watch system that allows carrying out the temperature and velocity measurements of the particles in flight in order to optimize the spraying process. The produced coatings have been characterized by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) to evaluate the porosity and oxidation produced in the spray process. Laboratory oxidation test have revealed the formation of Cr 2 O 3 , NiO and Fe 2 O 3 as corrosion products as well as Microanalysis by Energy dispersion Spectroscopy (EDS) composition profiles have been used to evaluate the penetration of the corrosion front inside the coating. The Nickel alloys show a good resistance in aggressive environment test and can be a good solution for the corrosion problems in the superheater tubes in MSWI.

Coatings of HastelloyC fabricated by HVOF spraying with a gas shroud (GS) have shown the superior barrier characteristic and corrosion resistance in seawater environment. During immersion of these coatings in artificial seawater, however, vibrational behaviour of the corrosion potential was observed. Some types of surface modification of the sprayed coatings and changing of the spray condition were carried out and their effect on the corrosion potential was investigated. It was revealed that the vibration is related to surface oxides formed during the spray process. Surface modification of the sprayed coatings and changing of the spray condition could reduce the vibration effectively.

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.

In this paper, Hastelloy C-276 layers are produced using the Axial III plasma spray method and assessed based on oxide content and microhardness. Test results show that Hastelloy coatings with low hardness can be achieved in different atmospheres with appropriate adjustments in process parameters. Paper includes a German-language abstract.

This paper investigates the microstructure and corrosion resistance of Hastelloy C-276 and 316L coatings produced by various thermal spray methods, including arc spraying, flame spraying, HVOF spraying, and a recently developed method called high-velocity combustion wire spraying. The microstructures of the coatings were examined before and after corrosion testing in order to gain information on corrosion mechanisms. Several corrosion tests were performed on each sample and various coating properties were measured including thickness, hardness, oxygen content, porosity, and adhesion strength. Test results for sealed coatings and detached layers are also presented in the paper, giving additional insight into corrosion behavior. Paper includes a German-language abstract.

316L stainless steel and Hastelloy C alloy powders were sprayed by an HVOF apparatus onto mild steel substrates. The microstructure, pore size distribution, composition and corrosion resistance of thus obtained coatings were evaluated experimentally. Corrosion resistance in sea-water was examined by monitoring the impedance and corrosion potential of samples immersed in artificial sea-water at 300 K over a period of more than 3 months and also by polarization measurement. It was found that the stainless coatings composed mainly of plastically deformed particles and some splats which were molten at the impact. By increasing the combustion pressure, the porosity as measured by mercury porosimeter could be reduced to below 1%. In comparison, Hastelloy C deposits sprayed under the standard condition were so dense that its porosity could not be measured by the porosimeter. The polarization curve and the results of impedance monitoring both exemplified that the Hastelloy C coatings possess much superior corrosion resistance to the stainless coatings in sea-water, which was attributed to the higher density and better adhesion of the Ni-base alloy coatings.

The corrosion characteristics of two bespoke Ni-Cr-Mo-B alloy powders sprayed by the high velocity oxy-fuel (HVOF) process have been studied using potentiodynamic and potentiostatic corrosion analysis in 0.5M H2SO4. The deposits have also been microstructurally characterised using X-ray diffraction (XRD), scanning electron microscopy (utilising both secondary electron (SE) and backscattered electron (BE) modes), and transmission electron microscopy (TEM). Results from the microstructural examination of the two alloys have revealed a predominantly amorphous/nanocrystalline (fcc) matrix containing submicron boride precipitates as well as regions of martensitically transformed laths. Apparent recrystallisation of the amorphous matrix has also been observed in the form of cellular crystals with an fcc structure. The oxide stringers observed at splat boundaries were found to be columnar grained α-Cr 2 O 3 , though regions of the spinel oxide NiCr 2 O 4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistance to corrosion in 0.5M H 2 SO 4 , as revealed by potentiodynamic tests. They both had rest potentials approximately equal to -300mV(SCE) and passive region current densities of around 1mAcm-2. Microstructural examination of samples tested potentiostatically revealed the prevalence of degradation at splat boundaries, especially those where significant oxidation of the deposit had occurred.