Cavitation and corrosion on hydrodynamic components and systems reduces the operational efficiency. The use of wear resistant coatings have been studied as a solution to the problem of corrosion and cavitation in the industrial environment. Thermal spray processes are recognized as excellent technique to deposit coatings. The high velocity oxy-fuel process (HVOF) can produce high density and bond strength coatings. High velocity air-fuel process (HVAF) is an alternative process, shown to be superior regarding corrosion protection and production costs. HVAF can deposit coating with shorter dwell time and lower temperature, producing coating with lower oxide content. This paper presents the use of HVOF and HVAF process to deposit FeCrMnSiNi and FeCrMnSiB coatings, studying the resistance against corrosion and cavitation in comparison to 316L HVOF coating. Microstructure was analyzed by XRD, microscopic means and mechanical testing. Cavitation and corrosion behavior of the coatings were also studied comparatively. HVAF coatings presented lower porosity and oxide levels, as well as higher hardness values, compared with the coatings deposited by HVOF process. The HVAF process presented better cavitation resistance than HVOF coatings. The FeCrMnSiNi HVAF coating had the best corrosion protection performance between the developed alloys.

In a variety of engineering applications, components are exposed to corrosive/erosive environment. Protective coatings are essential to improve the functional performances and/or extend the lifetime of the components. Thermal spraying as a cost-effective coating deposition technique offers high flexibility in coatings’ chemistry/morphology/microstructure design. However, the pores formed during spraying inherently restrict the use of coatings for corrosion protection. In view of the above gap to have a high quality coating, bi-layer coatings have been developed to boost the corrosion performance of the coatings. In a bi-layer coating, an intermediate layer is deposited on the substrate before spraying the coating. The electrochemical behavior of each layer is critical to ensure a good corrosion protection. The corrosion behavior of the layers strongly depends on coating composition and microstructure, which are affected by feedstock material and spraying process. In the present work, Cr 3 C 2 -NiCr top layer with different intermediate layers (i. e., Fe-, and Ni-based) were sprayed by high-velocity air fuel (HVAF) process. Microstructure analysis, as well as electrochemical tests, e.g., open-circuit potential (OCP) and polarization were performed. The results showed a direct link between the OCP of each layer in a bi-layer coating and corrosion mechanisms. It was found that the higher corrosion resistance of Ni-based intermediate layers than Fe-based coatings was due to higher OCP of the coating in the galvanic couple with top layers. Splat boundaries and interconnected pores reduced the corrosion resistance of the intermediate layers, however a sufficient reservoir of protective scale-forming elements (such as Cr or Al) improved the corrosion behavior.

This study investigates the corrosion resistance Gd 2 Zr 2 O 7 /YSZ coatings and a YSZ layer of similar thickness. All coatings were produced by suspension plasma spraying, resulting in a columnar structure. Corrosion tests conducted at 900 °C for 8 h in a molten salt bath show that Gd 2 Zr 2 O 7 is not as corrosion resistant as YSZ. Molten salts react with Gd 2 Zr 2 O 7 producing GdVO 4 along the surface as well as between the columns of the coating. The formation of GdVO 4 between the columns, in combination with the low fracture toughness of Gd 2 Zr 2 O 7 , is likely responsible for the lower corrosion resistance. Furthermore, the presence of another layer of Gd 2 Zr 2 O 7 on top of the Gd 2 Zr 2 O 7 /YSZ coating, to prevent salt infiltration, did not improve corrosion resistance.

In this study, the current industry standard topcoat for thermal barrier coatings, 8YSZ, is deposited by suspension plasma spraying and its room-temperature erosion resistance is compared with that of SPS sprayed gadolinium zirconate/YSZ and triple-layered GZ dense /GZ/YSZ. A columnar microstructure was observed in both the single- and multi-layered TBCs. Single-layer 8YSZ had a higher erosion resistance than multi-layered GZ/YSZ despite of its higher porosity among the as-sprayed coatings. In the case of the triple-layer coating, the denser top layer helped to slightly improve erosion resistance over that of the GZ/YSZ double-layer TBC.

The objective of this work is to analyze the thermal conductivity of suspension plasma sprayed thermal barrier coatings using experimental techniques and finite element modeling. The results indicate that the scale of the porosity in the coating has a significant influence on thermal conductivity. Smaller grains, higher overall porosity content, and lower columnar density correspond to lower thermal conductivity. It is shown that FEA can be a powerful tool to predict the thermal conductivity of SPS thermal barrier coatings.

This paper examines the microstructure and morphology of zirconia coatings and demonstrates the calculation of elastic modulus and Martens hardness based on instrumented indentation test results. Coatings samples varying in microstructure, phase content, and chemical composition were deposited by suspension plasma spraying using different torches and different suspension formulations. Coatings produced from low-concentration suspensions with submicron-size powders had a columnar structure with long vertical pores between the columns and fine spherical pores within the columns. Coatings made from suspensions with high concentrations of solids and coarser, more irregular powders, on the other hand, were more uniform and their surfaces smoother. They are also shown to be harder and have higher elastic modulus based on indentation test results.

The fatigue performance of conventional structural steel with an applied thermal barrier coating (TBC) was evaluated via cyclic bending. Tests were carried out for as-received and grit-blasted substrates as well as for samples with thermally sprayed bond coats and topcoats. Failure mechanisms were identified and changes in fatigue resistance were assessed based on results obtained for different loading amplitudes supplemented by fractographic analysis.

In previous work, it was observed that atmospheric plasma sprayed bond coats perform better than their HVOF counterparts, which is contrary to current literature data. The objective of this work is to understand the observed difference with the aid of finite-element modeling. Different thermally grown oxide layer thicknesses and surface topographies are evaluated and the modeling results are compared with current theories based on simplified sinusoidal profiles. It is shown that modeling can be used as an effective tool to understand the stress behavior in TBCs with different roughness profiles.

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 coating microstructure and thermal conductivity is important to be able to understand the influence of coating defects, such as delaminations and pores, on heat insulation in thermal barrier coatings. Object-Oriented Finite element analysis (OOF) has recently been shown as an effective tool for evaluating thermo-mechanical material behaviour, because of this method’s capability to incorporate the inherent material microstructure as an input to the model. In this work, this method was combined with multi-variate statistical modelling. The statistical model was used for screening and tentative relationship building and the finite element model was thereafter used for verification of the statistical modelling results. Characterisation of the coatings included microstructure, porosity and crack content and thermal conductivity measurements. A range of coating architectures was investigated including High purity Yttria stabilised Zirconia, Dysprosia stabilised Zirconia and Dysprosia stabilised Zirconia with porosity former. Evaluation of the thermal conductivity was conducted using the Laser Flash Technique. The microstructures were examined both on as-sprayed samples as well as on heat treated samples. The feasibility of the combined two modelling approaches, including their capability to establish relationships between coating microstructure and thermal conductivity, is discussed.

The thermo-mechanical properties of a thermal barrier bond coat (BC) play an important role in governing the life-time of a coating system. The presented work aims to determine these properties for NiCoCrAlY coatings sprayed on Hastelloy X substrates sprayed under different process conditions. Temperature dependent Young’s modulus values are determined for both Atmospheric Plasma Sprayed (APS) and HVOF sprayed coatings using the four-point bending test. Particular attention is paid to microstructure-property relationships during heating. Young´s modulus was determined up to 950°C and evaluated for coatings loaded in both tension and compression. Results are discussed in the context of the effect of feedstock material, process conditions and microstructure characteristics. The methods and results presented are attractive, particularly for the thermal spray industry, since these properties are a prerequisite when the BC is to be considered in component design.

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.

This paper provides an overview of thermal spray activities in Finland, Sweden, Norway, and Denmark. In Finland, thermal spray technology has widest use in pulp and paper processing, protecting various types of rolls and cylinders. In Sweden, the technology is of great importance in the manufacture of aero engines and industrial gas turbines. In Norway, thermal spraying is widely used in offshore applications, including subsea oil drilling enterprises, and in Denmark, thermal spraying is used in boilers, maritime diesel engines, and for wear resistance of machine tool components. Development and innovation in thermal spray technology in northern Europe is dominated by research conducted in Finland and Sweden, followed by Denmark and Norway.

Residual stress build up in thick thermal spray coatings is a property of concern. The adhesion of these coatings to the substrate is strongly influenced by the residual stress generation during the coating deposition process. In the HVOF spray process, due to lower processing temperature and higher particle velocity as compared to plasma spraying, significant peening stresses are generated during the impact of semi molten particles on the substrate. The combination of these peening stresses together with quenching and cooling stresses that arise after deposition can be of significant importance. In this paper both a numerical finite element analysis (FEA) method, to calculate peening, quenching and cooling residual stresses, and experimental methods, as Modified Layer Removal Method (MLRM) and Neutron Diffraction analysis, are applied. The investigation is performed for thick Inconel 718 coatings on Inconel 718 substrates. Combined, these numerical and experimental techniques yield a deeper understanding of residual stress formation and a tool for process optimisation. The relationship between the stress state and deposit/substrate thickness ratio is given particular interest.

Thermally sprayed Inconel 718 coatings have been deposited by high velocity oxy-fuel (HVOF) spraying on Inconel 718 substrates. The aim of the on-going study is to understand and control the adhesion mechanisms and the residual stress state of the deposit/substrate system, in order to build up thick coatings for maintenance purposes. The coating adhesion strength was evaluated by the standard ASTM C633 tensile test. Coating shear strength was evaluated by the recently developed prEN15340 Shear Test. A modified Layer Removal Method (MLRM) test was carried out to measure residual stresses. The work is a part of an ongoing study for evaluation of relationships between process parameters, residual stress distribution and adhesion strength.

The impact of plasma sprayed sieved Ni-5%Al particles on a titanium substrate was investigated. The particles had a narrow diameter range (-65 + 75 µm), and a speed and temperature just prior to impact of about 100m/s and 2400°C, respectively. These powder particles were sprayed on two sets of polished titanium alloy surfaces. One set was a non-oxidized surface and the other one was a previously oxidized surface at 600°C for two hours. Resulting splats were characterised experimentally by infrared pyrometry and scanning electron microscopy. The effects of the substrate’s oxide layer on the shape of the splat, also on the cooling rate and flattening speed during impact were studied and discussed. This problem is also being investigated numerically. A first step was devoted to the selection of relevant simulation parameters. Test cases to study qualitatively the effect of surface oxidation through parameters such as the contact angle and thermal contact resistance are in progress. They will be compared to the experimental results.

Plasma spraying of thermal barrier coatings (TBCs) on gas turbine parts is widely used today either to enable higher turbine inlet temperatures with consequent improvement of combustion efficiency or to reduce the requirements for the cooling system and increase components life-time. Development of low conductivity TBCs, which allows us to further increase gas turbine efficiency and availability, is an ongoing challenge. In order to get low thermal conductivity values an experimental program was conducted. Two zirconia powders were used for coating deposition: yttria partial stabilised zirconia (YPSZ) and dysprosia partial stabilised zirconia (DyPSZ). Microstructure evaluations were performed to evaluate the influence of the spraying parameters on the coating morphology and porosity level. Two methods were utilised to evaluate the coatings thermal conductivity: Laser Flash (LF) and Transient Plane Source (TPS). A comparison between the two methods was made as well as a correlation study between coating microstructure/composition and thermal conductivity (TC).

To achieve sufficient adhesion strength within thermal spraying, the surface to be coated has to be modified. Grit blasting is the most common way to generate a clean and roughened surface. The bonding mechanism between the grit-blasted substrate and the coating is assumed to be due to mechanical anchoring, why an optimal surface roughness is essential. The surface roughness is usually evaluated using Ra which cannot fully characterize the complex nature of the chaotic substrate topography. This study was performed in order to evaluate if Ra can be replaced by other surface characteristic parameters such us RΔq, Rpk, Rpv, Rk…with higher correlation to adhesion strength. Average roughness was measured by a perthometer and with white light interferometry to get 3D images of the surface topography. Disc shaped substrate samples of Ti6Al4V (AMS 4928) were grit blasted with aluminium oxide grit and plasma sprayed with a Ni5%Al coating. Adhesion strength was determined according to the ASTM C633 standard. The correlation between a number of different surface-parameters and adhesion strength were evaluated and compared with Ra.

This paper presents a method to increase deposition rate of thermal plasma spray operations through the use of multiple injection ports. Numerical simulations indeed revealed a major energy loss in the process when using only one port. The influence of the carrier gas and the particle stream on the heat flow coming from the plasma torch was found very local and small compared to the total amount of energy. To take as much as possible advantage of the energy available in the plume, we thus propose to use a multiple number of injectors around the flame. Computational simulations are carried out to estimate the feasibility. They are based on the 3D Navier-Stokes equations coupled with a turbulence model. The gases (plasma gas, surrounding air and carrier gas) are supposed to be in local thermal and chemical equilibrium and loading effects are accounted for. The numerical results are supplemented by experimental results showing that multiple injectors can significantly increase deposition rate while preserving or even slightly improving the deposition efficiency. Characterisation of the microstructure, evaluated for all tests, is similar and no obvious differences can be detected apart from the porosity. This method thus results in a substantial reduction of the production cost.

The Protal process combines surface preparation using a laser and thermal spraying in one production step. The laser preparation is based on a photomechanical reaction induced by the interaction between a laser of high instantaneous power and a polluted surface. The mechanism of bonding and the coating-substrate interface are then changed in comparison with grit blasting resulting in a significantly reduced substrate roughness. This study is aimed at finding the optimal Protal process parameters for the coating adhesion of a Ni5%Al sprayed on Ti6Al4V and IN718 alloys. The parameters investigated are laser beam intensity, the time delay between the laser impact and the spray impact, powder feed rate, substrate roughness and temperature. A test plan including these parameters is analysed by means of a fractional factorial design of experiment method. The adhesions of the coatings are measured using the ASTM C633 standard test. Data are analysed by a multiple linear regression model using a least squares fit. In addition, the coating/substrate interface is examined by optical and electron scanning microscopy (SEM) techniques as well as by Auger electron spectroscopy. Substrate roughness, substrate temperature and laser intensity are all shown to have a negative correlation with adhesion strength within the investigated range. Areas of diffusion are noticed at the coating/substrate interface.