Stainless austenitic steels like the 316L (1.4404) are widely applied in various applications and were also used for surface protection using thermal spraying. The reason for this is the easy processability and the high corrosion resistance. Stainless austenitic steels typically contain the following alloying elements: The formation of an austenitic microstructure is achieved by nickel (Ni). The addition of chromium (Cr) lead to good corrosion resistance due to formation of an oxide layer. For resistance against pitting corrosion, molybdenum (Mo) can be added. Also, stainless austenites usually exhibit very low carbon and nitrogen contents to prevent chromium carbides and nitrides which reduces the corrosion resistance. However, both alloying elements cannot be classified as being detrimental in stainless austenites in general. In contrast high nitrogen contents can also be used to improve the chemical properties, especially the resistance against pitting corrosion. Finally, carbon and nitrogen lead to an increase in hardness of the thermal sprayed layer. Based on this knowledge, a high-strength austenite for thermal spraying was developed. The new high strength austenite was processed by HVAF spraying with different particle distributions and parameter variations. Resulting coatings were investigated regarding the microstructure, elemental composition, hardness and corrosion properties in comparison to the standard coating material 316L.

NiCrBSi alloys are often used in thermal spraying because of their good wear and corrosion resistance even at temperatures over 500°C. Experience has proved these alloys are a good choice for components in the presence of hard particles. The main wear mechanism here is abrasive wear caused by hard particles. Some examples are wear plates exposed to impact sliding; extruders, screw conveyors or mixer parts exposed to grooving; fans, rotor wheel blades or impellors transporting sand/granular material at temperatures over 500°C; or pump parts exposed to fluid containing sand. In spite of such widespread use of NiCrBSi alloys in thermal spraying, their abrasive wear resistance is still not fully understood. In order to better understand, a series of sprayed and fused NiCrBSi coatings with hardness from 36 to 62 HRC were tested for abrasive wear according to ASTM G65–04 norm and the wear volumes achieved are presented. Tribological and metallographic analysis of track wear was done in order to better understand how microstructure and hardness of NiCrBSi coatings influence abrasive wear mechanisms. These results are compared to results previously published.

Iron-based hardfacing alloys are widely used to counteract abrasive and impact wear of industrial components soil in, sand and mineral processing applications. These alloys show a high performance to cost ratio as well as a low environmental impact. The wear resistance of the components hardfaced with these alloys depends on achieved coating microstructure i.e. on the alloys chemical composition, the coating method and process parameters selected. The present work focuses on iron based hardfacing alloys with varying amount of chromium, vanadium, tungsten, molybdenum, boron and carbon deposited by plasma transferred arc (PTA) overlay welding. Weldability, hardness, abrasive and impact wear of the overlays are presented and interpreted through their microstructure. The performance of the iron based overlays is compared with that of nickel-based metal matrix composite coatings with tungsten carbide (MMC) commonly used for hardfacing of parts subjected to severe abrasive wear. The hardness of the iron based overlays investigated ranges between 60 and 65 HRC while abrasive wear is typically below 20 mm 3 (ASTM G65, procedure A). Microstructure consists of different primary precipitated carbides or borides, a martensitic matrix and eutectic structures.

This study compares the corrosion resistance of a wide range of coatings in a chlorine-containing atmosphere at elevated temperatures. Four HVOF-sprayed NiCr coating samples were produced and tested along with an iron-aluminide HVOF layer, a chromium diffusion layer, and a laser-treated HVOF NiCrMo layer. The investigators found that the structure of HVOF coatings has a major effect on corrosion resistance and that chlorine corrosion primarily attacks the substrate through cracks and interconnected networks of pores and oxides at splat boundaries. They also observed that laser melting increases corrosion resistance by homogenizing the coating structure. Paper includes a German-language abstract.

In twin wire arc spraying process it is possible to use feedstock wires of two different compositions at the same time. As a result of this procedure it can be achieved composite coatings called also as pseudo alloys with modified physical properties. In this study nickel and cobalt based super alloy materials were arc sprayed with pure molybdenum wire to tailor corrosion and wear resistance of the coatings. Coatings for the tests were sprayed using two different twin wire Sulzer Metco arc-spraying units, Smart Arc and OSU 300, operating with suitable spray parameters to produce coatings of good quality. It was already known that these twin wire configurations are producing coatings with differing microstructures. Coating sprayed with the OSU system was clearly finer in structure and one purpose of this study was to measure the effect of the micro structural size on the corrosion and wear properties of the final coatings. Microstructures of the coating materials were studied and analyzed from cross-sectional specimens. Volume fraction of pure molybdenum in the coating matrices was evaluated with simple line method and according to the results volume fraction of pure molybdenum metal is over 50 volume-% in all of these tested composite coatings and higher in materials sprayed with OSU unit. Also the microstructure of the coatings was seen to be finer when OSU was used as was expected. Wear resistance was measured with modified ASTM G65 rubber wheel sand abrasion wear test and corrosion resistance was tested in low pH values and chlorine containing environment according to the ASTM G48 corrosion testing standard. Corrosion testing was carried out at room temperature 22°C and also at higher 50°C temperature. Molybdenum addition is clearly improving the abrasion wear resistance of the tested coating systems. At room temperature also the corrosion resistance is getting better with molybdenum addition but at higher temperature this effect is not so clear.

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.

Thermal spray processes are widely used to deposit high-chromium nickel-chromium coatings to improve high temperature oxidation and corrosion behaviour. However, in spite of the efforts made to improve the present spraying techniques, such as HVOF and plasma spraying, these coatings may still exhibit certain defects such as unmelted particles, oxide layers at splat boundaries, porosity and cracks, which are detrimental to corrosion performance in severe operation conditions. Due to low process temperature only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3%Cr- 42.5%Ni - 2.5%Mo - 0.5%B). The coatings were prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power fiber coupled continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of scanning speed and beam width (10 mm and 20 mm). Coating remelted with the highest traverse speed tended to suffer cracking during rapid solidification inherent to laser processing. However, choosing appropriate laser parameters, non-porous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy (OPM, SEM). Dilution between coating and substrate was studied with EDS. The properties of the laser remelted coatings were directly compared with properties of as-sprayed HVOF coatings.

In order to expand the fields of application and to improve the performance of graphite (Cg), it is necessary to reduce its permeability towards of oxygen and to limit its reactivity and especially its oxidation. It is, therefore, essential to protect it from the environment through the use of ceramic coatings. Adhesion between ceramic coatings and graphite is controlled by the mechanical stresses in the coatings and the thermodynamic work of adhesion. Different metal-graphite systems were examined which showed that the adhesion particularly depended on the thermal expansion coefficient mismatch between the two materials and on metal carbide stability. Thus, the role of the addition on the graphite surface of elements such as Cr, Mo, Al, Si, O on the adhesion of metals or ceramics to graphite has been identified.

Sintered protective coatings are a proven technical solution against corrosion and erosion of tubes in biomass- and waste-fired boilers. The main field of application are tubes like superheater tubes or heat exchanger tubes in fluidized-bed installations corrosion endangered by the flue-gas stream. These sintered solutions complete other thermally sprayed coatings in those critical cases, where gas-dense overlays are needed due to corrosive damage. In those cases, where oxidation and erosion dominates, thermal spraying without sintering is remaining as primary solution. The development of Twin coatings out of sintered and non-sintered thermally sprayed alloys is showing a new and interesting alternative for tubes exposed to erosive-corrosive stresses under high temperature conditions.

The application of metallic foam core sandwich structures in engineering components has been of particular interest in recent years because of their unique mechanical and thermal properties. Thermal spraying of the skin on the foam structure has recently been employed as a novel cost-efficient method for fabrication of these structures from refractory materials with complex shapes that could not otherwise be easily fabricated. The mechanical behavior of these structures under flexural loading is important in most applications. Previous studies have suggested that heat treatment of the thermally sprayed sandwich structures could improve the ductility of the skins and so affect the failure mode. In the present study the mechanical behavior of sandwich beams prepared from arc sprayed alloy 625 skin on 40 ppi nickel foam was characterized under four point bending. The ductility of the arc sprayed alloy 625 coatings was improved after heat treatment at 1100ºC and 900ºC while the yield point was reduced. Heat treatment of the sandwich beams reduced the danger of catastrophic failure.

Stainless Steels are required for many applications for ship building as well as for offshore structures such as oil exploration. AISI type 304 stainless steel is not very suitable for such applications as it has a strong tendency for pitting and crevice corrosion. Even type 316 and 317 stainless steels which have respectively 2.5 and 3.5% Mo are not very effective in these environments. Commercially available stainless steels, viz., Avesta 254 SMO is being employed for such applications because of its strong resistance to pitting and crevice corrosion. This is mainly because of high Mo concentration (6.5%). Such steels are not only costly but are prone to form deleterious phases such as delta ferrite and sigma during welding or other heat treatment operations. Hence, an alternative technique to restrict Mo at the surface is needed. In the present work, surface alloys consisting of an austenitic stainless steel with Mo content as high as 10-12% have been formed on stainless steel type 304 substrates. These steels show enhanced passivity and strong resistance to pitting corrosion.

In this study, several thermal spray coatings and reference materials were evaluated for potential use in biomass co-fired boilers. The coatings were applied to T92, A263, and X10Cr13 substrates by HVOF and wire-arc spraying using powder (IN625, FeCr, NiCr) and wire (NiCrTi) feedstocks. Coating samples were examined then tested for 5900 h in the superheater area of a fluidized bed boiler burning a mixture of wood, peat, and coal. The corrosion behavior of the coating and reference materials is reported in the paper and the underlying corrosion mechanisms are discussed.

Twin wire arc spray (TWAS) coatings were produced under varying spray conditions (spray angle, traverse rate, and spray distance) to simulate on-site hand spraying operations typically used to coat existing refinery vessels. Two materials, Alloy C276 (commonly used for corrosion protection of refinery vessels) and the newly developed Nicko-Shield 200 chemistry (designed to reduce porosity and oxide content under arc spray conditions) were compared in the testing. Alloy C276 coatings showed good coating performance (>40 MPa adhesion) when sprayed under ideal conditions, but showed a sharp drop off in coating integrity (<20 MPa adhesion) when sprayed at lower traverse rates, sharper angles, and closer spray distances. Deviating from non-ideal conditions resulted in increased porosity and oxide content leading to increased permeability. It was concluded that non-ideal conditions, which intermittently occur in hand spraying operations on large surface areas, can lead to coating patches with unacceptably low adhesion, potential spalling, and high permeability when spraying Alloy C276. Patches of low coating quality require additional maintenance or result in coating failure, creating a lack of confidence in thermal spray technology as a protective solution in the industry. This study shows the results of an effort to develop an alloy solution which is more reliable in spraying large surface areas by hand for corrosion protection. The developed Ni-based material showed improved adhesion (70+ MPa) and greatly reduced permeability (as measured by ferroxyl exposure). This performance was stable across the wide range of spray conditions used in this study. This suggests that alloy design can be used to increase the reliability for twin wire arc spray coatings, and enable confidence for expanded use in this industry.

Thermal spray processes using wires as feedstock are widely used to produce wear and corrosion protective coatings of nickel, cobalt or iron based alloys. In general, these coatings are processed by flame or arc spraying. In view of using massive wires as spraying material, the hardness and wear resistance of layers is limited by the possibility to produce the corresponding wires of such materials. In addition, the performance of wire sprayed coatings can be restrained by the amount of defects in the microstructure, like pores, oxides and cracks, which are particularly evident in the cases of flame and arc spraying. New High Velocity Combustion Wire (HVCW) systems open the opportunity to reduce the amount and size of the defects by an increased particle velocity. Also, improvements on wear resistance may be achieved by using cored wires. The paper gives an overview on recent developments in HVCW spraying using massive and cored wires.

Atmospheric plasma spraying of alloys often results in their composition changes. The main source of the changes is usually preferential oxidation of some elements composing the alloy. As a rule, these are the alloying elements whose affinity to oxygen is high. Changes due to this effect are well known from metallurgy; however, they were scarcely studied from the point of view of plasma spraying. Preferential evaporation of some elements may also contribute to the alloy composition changes. The aim of the present paper is to give quantitative data on composition changes of selected alloys sprayed by a water-stabilized plasma gun. Two Ni-base alloys and one high-alloy Cr-Ni-steel were studied. The main tool for determining the sample composition was electron probe X-ray microanalysis. To quantify the results and to eliminate the systematic errors, the data obtained by this method were calibrated by repeated chemical analysis of feedstock powders. The alloy composition was determined after both stages of plasma spraying, i.e. after the inflight stage of molten particles and after the stage comprising particle impact, solidification, coating formation and cooling. To study the situation after the former stage, the flying particles were trapped and quenched in liquid nitrogen. In the Ni-Cr alloy containing 20%Cr, strong Cr depletion was observed. The Fe depletion in the Ni-Fe alloy (47%Fe), though unambiguous, was less significant. The high-alloy steel (Czech equivalent of AISI 316) was also Cr depleted, whereas the concentrations of other alloying elements (Mo, Ni) remained effectively unchanged. In all cases, the depletion occurred at the first spraying stage and became more pronounced during the second stage. Strong air entrainment occurs not only in a plasma jet produced by a water-stabilized plasma gun, but also if gas-stabilized plasma guns are used in atmospheric plasma spraying. It follows that the dominant mechanisms of composition changes during plasma spraying by both techniques are similar.

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as HVOF and plasma spraying, these coatings may in certain operation conditions show inadequate performance, e.g. due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermal sprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal sprayed coating material. In order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coating was studied in the present work. The coating material was nickel based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy. Laser remelting resulted in full homogenization of the sprayed structure. This strongly influenced positively the performance of the laser remelted coatings in adhesion, wet corrosion and high temperature oxidations test. The properties of the laser remelted coatings were compared directly with the properties of as-sprayed HVOF coatings, and with PTA overlay coatings and wrought Inconel 625.

The wear of piston rings in large marine two-stroke diesel engines is a major maintenance cost. Applying coatings with good oxidation, corrosion resistance and high temperature strength, can lower the total maintenance cost. In the past nickel aluminide with chromium carbide have been applied to pistons by thermal spraying. Using laser cladding a suitable microstructure can be formed while at the same time avoiding cracks and bonding issues. In this report powders and coatings were manufactured in order to be able to investigate the dry-sliding wear behavior. Material with three levels of carbides was atomized. Wear test samples were manufactured by laser cladding. The dry sliding wear-mechanism maps are generated by using block on ring test setup where coated blocks slide against cast iron rings. All alloys exhibited regions of plasticity-dominated wear and oxidational wear with a transition region in-between. The carbide-containing alloys showed lower friction and wear in comparison to the carbide free nickel aluminide alloy.

Electroplated chromium improves corrosion resistance while providing resistance to wear, fatigue and impact. However, hard chromium plating uses chromic acid, which releases fumes containing carcinogenic chromium +6 ions into air during the process. Therefore, numerous efforts have been carried out worldwide in the last decade to develop alternatives and several applications and processes were validated, among which trivalent chromium plating, electroless nickel and nickel alloy coatings, micro-welding, PVD, CVD, and thermal spraying. Nevertheless, these finishing processes have impacts on human health, ecosystems and resources. In this work, a Life-cycle assessment (LCA) methodology based on Eco-indicators 99 was used to compare the environmental impacts and benefits of thermal spraying (including APS- and HVOF-sprayed WC-Co coatings and TWEA- and APS-sprayed hard steel coatings) to conventional chromium plating.

Computational metallurgy is a technique being used and developed in the field of bulk alloys to design and develop novel amorphous and nanocrystalline materials. This technology can be transitioned to develop chemistries for both wear and corrosion resistant thermal spray coatings. Using computational metallurgy and small scale laboratory experiments, nanostructured and amorphous chemistries can be designed to specifically accommodate one of the many environmental conditions challenging the oil and gas industry. This study reviews the design procedures behind developing three unique chemistries intended to function in different environments: 1) an Fe-based chemistry designed for metal to metal sliding wear resistance, 2) an Fe-based chemistry containing elevated refractory content intended specifically for spray and fuse applications to resist sulfurous corrosion, and 3) a Ni-based chemistry similar to Alloy C276 for high temperature corrosion resistance. All three alloys were designed using computational techniques and eventually manufactured into cored wires for use within the twin wire arc spray (TWAS) process. The fine grained structure provides unique benefits to each application including 1) high hardness, 2) ability to rapidly form protective scale, 3) low melting temperature and creep resistance.

This paper presents the results of studies on powders used in wear-resistant thermal spray coatings. The work focuses on the influence of the shape, size, and structure of composite powders containing nickel, iron, and chromium mixed with TiC and dry lubricants such as MoS 2 and CaF 2 . The powders are analyzed using SEM and metallographic methods along with X-ray diffraction. Paper includes a German-language abstract.