Iron-based coatings are often considered as replacement of hard chromium and WC-Co, as they pose lower health and environmental impact. In many cases the combination of mechanical and chemical properties of ferrous based alloys may be satisfactory and their relatively low cost make these coatings an interesting candidate for many applications. This study is inspired by opportunities to harden the ferrous base materials by strain hardening, solid solution strengthening, dispersion strengthening, and precipitation hardening. Already commercially available Fe-based coating materials with precipitates of mixed carbides and borides in the metastable austenitic matrix achieve a high hardness. In this study the cavitation erosion and abrasion resistance of various Fe-based coatings produced by HVAF and HVOF processes were investigated. Two experimental precipitation containing materials were prepared, and the sprayed coatings were tested for abrasive and cavitation erosion wear. In addition to precipitations, the importance of proportion of ferrite and retained austenite phases were studied by affecting the microstructure by heat treatments as the ability of different phases to affect hardening and ductility may become crucial in generating desired material properties. The properties of experimental and some commercial Fe-based alloys are compared with WC-Co and Cr 3 C 2 -NiCr coatings by property mapping.

This study assesses the sliding wear and impact behavior of thick carbide coatings deposited on hot-rolled steel by high-velocity airfuel (HVAF) spraying. Coating samples are evaluated based on scratch, ball-on-disc sliding, normal impact, and compound sliding impact tests and efforts are made to rank materials according to tribological criteria including coating failure mode, friction response, and wear. The approach is intended to provide insights for product designers specifying thermal spray coatings for steel components and structures from a wear performance perspective.

This work assesses the effects of defects in thermally sprayed chromia via multiscale modeling and imaging techniques. Defect distributions in coating samples are identified by way of image processing and synthetic 3D microstructures are generated from extracted statistical information. The properties of the microstructures are determined by subjecting them to simulated tensile testing, and the significance of different types of defects is evaluated through defect-containing coating models. The approach is able to handle complex defect morphologies, including pore, crack, and splat boundary clusters, making it a versatile tool for assessing the influence of defects on component performance.

In this work, a CFD model is built to investigate gas flow and in-flight particle characteristics in HVOF spraying with hydrogen as the fuel. Gas flow is solved for a particle-free jet, combustion is represented using a simple eddy-dissipation model, and a time-averaged fluid flow (k-ε) model is used to account for turbulence. Acceleration and heating of individual particles are modeled in a "snapshot" of the gas flow. Particle acceleration is shown to be governed by drag and particle heating by conduction and radiation between particulates and gas phases. Modeling results agree well with experimentally obtained data and observations.

Alloys with high amount of chromium are used for protection of power plant boilers against high temperature corrosion, especially heat exchanger surfaces, such as superheaters. Thermal spray coatings can be applied to low alloyed steel in order to enhance the lifetime of the cheap substrate material. In this study NiCr (50Ni49Cr1Fe) powder was sprayed with DJ Hybrid and CJS HVOF techniques, and the spraying process was optimized with process map methodology, including SprayWatch for measuring the temperature and velocity of the spray stream, and ICP (In-situ coating property measurement) for measuring the stress state. Different spraying parameters were applied in order to attain the best suitable coating characteristics for high temperature applications, such as high density and low oxidation stage. The coatings were analyzed e.g. by microscopic means and mechanical testing. The use of process optimization, process-structure- properties methodology, and following coating characteristics are presented.

Complexity in dynamics and mechanism of supersonic flame formation and effects of processing variables has made the understanding of interaction of particles and flame a difficult task. Lack of such understanding limits the possibilities of controlling the process to obtain desired in-flight particles temperature and velocity and consequent particles state. This problem is even more pronounced in TS systems with no dedicated decoupled temperature and velocity controlled regime. Different approaches based on total volume flow, back pressure and fuel to oxygen ratio have been examined to address the robustness of each approach to control the temperature and velocity. WC-CoCr material was used employing DJ-2600 torch. A guideline to control the in-flight particles temperature and velocity based on process variables is provided. A process map was developed to establish a correlation between process, in-flight particles state, microstructure, properties and performance.

HVOF-sprayed WC–10wt%Co–4wt.%Cr coatings were obtained using experimental feedstock powders (manufactured by spray-drying + sintering), containing nanometric carbide particles. Three reference coatings were also deposited using commercially-available powders containing sub-micrometric carbide particles. The coatings obtained from nanostructured powders, although affected by decarburisation phenomena, contained very fine carbide particles (~200 nm size). Those obtained from commercially-available powders simultaneously exhibited sub-micrometric (~400 nm size) and micrometric carbide particles, and were much less decarburised. Sliding wear tests performed at room temperature against sintered Al 2 O 3 balls showed the occurrence of brittle fracture wear (detachment of near-surface material by local brittle cracking) on the nanostructured coatings, which were embrittled by decarburisation. The reference coatings, by contrast, exhibited either ductile wear behaviour (plastic deformation, pull-out of single carbide particles) or a mix of both ductile and brittle wear mechanisms. When the decarburisation of the nanostructured coatings was not too extensive, their wear loss was comparable to that of the reference ones. At 500 °C, the wear behaviour of all coatings was dominated by abrasive grooving, on account of thermal softening. The most decarburised nanostructured coatings, however, still experienced brittle cracking as well.

This study shows that thermal spraying is a viable technique for the deposition of barium hexaferrite layers suitable for microwave absorption applications. More specifically, the study shows that impact quenching of molten BaCoTiFe 10 O 19 hinders the crystallization of the hexaferrite phase. Consequently, when spraying conditions induce near-full melting of the feedstock, the coating mostly consists of spinel and a glassy phase, a state with poor magnetic properties. These phases can be converted to hexaferrite by annealing, but in order to obtain enough crystalline hexaferrite in as-deposited layers, a controlled amount of unmelted material must be preserved. Atmospheric plasma spraying proved to be well suited for this purpose, producing layers with excellent magnetic properties, close to those of pure crystalline BaCoTiFe 10 O 19 . In these coatings, dense regions entrain many unmelted agglomerates of micron-sized particles, effectively preserving the hexaferrite phase.

Superior wear performance combined with excellent friction properties against metals makes chromium oxide (Cr 2 O 3 ) an interesting coating material for many industrial applications. However, Cr 2 O 3 is a challenging material for HVOF spraying due to its high melting temperature. Fracture toughness and lamella cohesion of a coating is limited and may be improved by using ceramic-ceramic –nanocomposite powders, which forms phases with improved properties. In this study Cr 2 O 3 -TiO 2 systems were selected aiming to improve the toughness and lamella cohesion of coating without reducing the excellent wear properties.

Residual stresses in plasma-sprayed Al 2 O 3 and Cr 2 O 3 coatings, deposited using commercial powders, and in HVOF-sprayed ceramic coatings, deposited using conventional Al 2 O 3 and Cr 2 O 3 feedstock and nanostructured- Al 2 O 3 feedstock, were studied by combining X-ray diffraction, substrate chemical removal technique and analytical modelling. The in-situ curvature technique was also employed for HVOF-sprayed Al 2 O 3 coatings, for further verification. Both HVOF-sprayed Al 2 O 3 -based coatings display similar, tensile residual stresses (≈120 MPa) near the top surface and possess moderate through-thickness stress gradients (≈10 - 20 MPa). Plasma-sprayed Al 2 O 3 possesses a smaller through-thickness stress gradient and a larger near-surface stress (≈220 MPa): this latter result seems to be due to higher quenching stresses in APS Al 2 O 3 , as determined by analytical computation The analytical model is validated by its fairly good agreement to the experimental results obtained both by substrate chemical removal and by in-situ curvature. Cr 2 O 3 -based coatings possess a lower near-surface residual stress (≈20 MPa); the HVOF one also exhibits a very large stress gradient (≈80 MPa). Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.

Perovskites are considered as potential materials in solid oxide fuel cells (SOFC) for different reasons at different parts of the fuel cells. Perovskites such as La 0.8 Sr 0.2 MnO 3 (LSM) and other compositions are electrically conductive which is necessary for SOFC applications. One possible application is protection coating for interconnect plates (bipolar plate) to avoid chromium oxide evaporation from the surface of ferritic stainless steel. Different commercial and experimental perovskite powders were sprayed by plasma and HVOF spraying under different spray conditions. Spraying of pervoskites was found to be challenging and required careful parameter optimization in both spray methods. Microstructure and phase structure of the coatings were investigated. A very fine crack structure, possibly caused by low mechanical strength and low ductility of the compounds, was easily formed in coatings prepared by plasma and HVOF spraying. Spraying method, parameters and spraying atmospheres were found to affect the stability of the perovskite compounds due to low chemical stability at high spray temperatures. Oxygen deficiency or oxygen surplus was concluded to cause distortion of the compounds crystal structure, causing thus shifting of XRD-peaks due to change of lattice parameters. Electric conductivity was affected by the crystal structure.

The potential of the high velocity oxy-fuel (HVOF) thermal spray process to produce coatings with reduced porosity is well known. The ability to produce high density ceramic coatings offers potential in high performance applications in the fields of wear, corrosion resistance and dielectric coatings. It has been, however, demonstrated that benefits from HVOF ceramic coatings can be obtained only if particles are melted enough and good lamella adhesion is produced. Therefore, due to the operational limits of the HVOF process, the process-structure- relationship must be well optimized. One strategy to improve melting of ceramic particles in the relatively low flame temperatures of the HVOF process is to modify particle crystal structure and composition. In this paper, the effects of the powder structure and the composition on coating microstructure and deposition efficiency of the HVOF spray process are studied. The effect of fuel gas, hydrogen vs. propane, was also demonstrated. The studied materials were agglomerated alumina- and titania-based pure and composite powders. Coating properties such as microstructure, hardness, and abrasive wear resistance, were compared to the coating manufactured by using conventional fused and crushed powders.

Plasma sprayed oxides are effective coatings against wear and corrosion. Low particle velocity in the plasma jet causes a limited interlamellar cohesion. HVOF-sprayed ceramic coatings emerged as an improved alternative. In this paper, microstructural characteristics and tribological performances of HVOF sprayed Al 2 O 3 , nanostructured Al 2 O 3 and Cr 2 O 3 coatings are compared to reference plasma-sprayed Al 2 O 3 and Cr 2 O 3 . The microstructure is analysed by SEM, EDS and XRD. Hardness and fracture toughness are investigated by instrumented indentation and elastic modulus by 3-point bending. Steel wheel and rubber wheel tests have been used to assess dry particle abrasion resistance. Sliding wear resistance is tested by pin-on-disk at room temperature and at 400°C, against SiC and 100Cr6 steel balls. HVOF-sprayed coatings are denser and have better interlamellar cohesion thanks to increased particle velocity. They are harder, tougher, possess a higher elastic modulus and lower porosity. Dry particles abrasion resistance is definitely superior to plasma-sprayed ceramics due to higher toughness; sliding wear resistance is higher, particularly at 400°C.

HVOF thermal spraying has been developed to deposit dense ceramic coatings with improved protective properties for various applications. Even though HVOF coatings are much denser as compared to ordinary plasma sprayed coatings, the coating properties are inferior as compared to bulk ceramics because of pores and microcracks, which influence adversely the coating properties, i.e. toughness, hardness and wear resistance. One strategy to improve the properties of the coatings is to decrease the grain size of the ceramic phase and to add toughening elements to the microstructure. Nanocrystalline coatings have been found to offer better thermal shock resistance, lower thermal conductivity and better wear resistance than their conventional counterparts. In this paper we describe the development of nanocrystalline ceramic composite coatings, where the grain size of ceramic has been decreased and a few percents of alloying element has been added in order to toughen the coating. Dense nanostructured alumina coatings alloyed with Ni and ZrO 2 nanosized particles were manufactured by HVOF spraying by using HV2000 spray gun. Mechanical properties, especially elastic modulus and relative fracture toughness were studied. Used techniques were instrumented nanoindentation and KIC evaluation. As a result coatings with nearly 100% improvements in relative fracture toughness were produced for nanoreinforced alumina composite coating. Results are compared with the microstructure, hardness and abrasive wear resistance of the coatings.

HVOF thermal spraying has been developed to deposit dense Al 2 O 3 -coatings with improved protective properties for various applications. Nanocrystalline coatings have been found to offer better thermal shock resistance, lower thermal conductivity and better wear resistance than their conventional counterparts. In this paper we describe the development of nanocrystalline Al 2 O 3 and Al 2 O 3 -Ni -coatings, where the grain size of Al 2 O 3 has been decreased and a few percents of nickel has been added in order to toughen the coating. Coatings were manufactured by HV- 2000 HVOF using spray parameters determined based on the on-line spray diagnostics. Parameters were selected aiming at different melting stages of the powder. The resulting microstructure of the coatings and its influence on the coating properties is discussed

In this paper the point of view for the industrial coating manufacturing is presented. The perspective for industrial manufacturing of thermal spray coatings and facts that effect on the processibility of coatings in real life is often requested but seldom published. The perspective is from Pikoteknik Oy, which has a wide and long-term experience in the improvement and repair of machine parts in-situ, containing measurements, machining, coating and balancing of rolls and cylinders. Pikoteknik Oy is able to perform maintenance of paper cylinders on-site including pretreatments, thermal spraying and balancing quickly and skillfully. Due to their versatile state-of-the-art equipment, they are able to work flexibly on-site. To be able to operate on-site there are special demands for equipment, personnel and scheduling. The quality of the final product is in the hands of operator. Destructive testing is not possible to carry out and large amount of variables, such as moisture and positioning of equipment's exist. Grinding and grit blasting must carry out without dust formation due to the surrounding bearings and sensors. Large amount of experience and special knowledge is needed to be able to produce high quality coatings over paper cylinders.

Use of high-chlorine and alkali containing fuels such as biomass and refuse is increasing in production of heat and electricity. Fluidized bed boilers experience severe and harsh conditions, in which high temperatures combined with inhomogenous fuel cause severe material wastage to metallic parts in boilers. Thermally sprayed coatings have been reported to provide protection to the boiler tubes. However, thermally sprayed coatings have encountered serious problems, when corrosion has proceeded to the substrate material through voids and oxides in lamella boundaries. Sealing of the coatings can solve these problems. Sealing of coatings with commercial sealants and laser fusion treatment was investigated. Thermally sprayed HVOF metallic coatings were sealed with different commercial sealing agents and diode laser. The coatings were tested in molten salt, simulating condition of fluidized bed boiler superheaters. Optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray analysator (EDX) were used for examination of the tested and non-tested specimens. Some of the tested sealants protected the coatings adequately in the short time alkali chloride - alkali sulphate exposure test. Best sealant contained aluminium oxide and aluminium phosphate. Laser treated coatings had good corrosion resistance in a short-term test.

Functionally graded (FG) coatings were manufactured by High Velocity Oxy-Fuel (HVOF) thermal spraying and tested aiming for the high temperature applications. Single layers were manufactured and their elastic modulus measured by using Impulse Excitation Technique (IET). Obtained data was used for modeling of optimal gradient structure. Dual feeding hose for HVOF gun was developed. Calibration procedure for the concurrent use of two powder feeders was performed. NiCr-Al 2 O 3 coatings with coating thickness of 600 µm and 1000 µm were manufactured and tested. Promising results were obtained from high temperature corrosion tests.

This paper evaluates various methods for adjusting the degree of oxidation in arc-sprayed layers. Oxidation control is primarily achieved through the use of a nonoxidizing gas, such as argon or nitrogen, especially in combination with a fine nozzle. In the case of Ni18Cr6Al2Mn deposits, the measured oxide content varied from 0.85 to 3.41 wt% based on the atomizing gas, nozzle, and stand-off distance used. Paper includes a German-language abstract.

HVOF-sprayed alumina appears to be well suited for applications in semiconductor devices. This paper investigates the influence of HVOF spraying parameters on the electrical properties of alumina layers. Diagnostic tests show that small changes in gas ratios and flow rates can significantly alter particle and splat characteristics as well as the dielectric breakdown strength of the coatings. A large number of parameters are changed in order to assess the extent to which electrical properties can be controlled. Paper includes a German-language abstract.