In this study, zirconia coatings were fabricated by vacuum plasma spraying using hollow spherical and fused and crushed YSZ powders. Relationships between spray parameters and in-flight particle velocities and temperatures were investigated in real time and correlated with coating microstructure and density obtained under vacuum as well as atmospheric spraying conditions. The results indicate that plasma sprayed particles reach higher velocities under vacuum and slightly higher temperatures in atmospheric conditions. Powder morphology and structure play a major role in determining coating microstructure and porosity, especially in vacuum spraying. The fused and crushed powder yielded the densest coatings under the vacuum process conditions employed.

This work introduces a hybrid spray-and-fuse process and a modified CoCrMoC (Stellite) alloy that significantly expand the manufacturing window for wear-resistant coatings. The Co-based alloy was produced by adding Ni, B, and Si to Stellite 720 to lower its melting points and expand its melting range thereby improving the sprayability and fusibility of the material. The modified alloy was deposited on Inconel 718 balls and 1 in. diameter coupons by HVOF spraying and coating samples were sinter fused at high temperatures followed by furnace cooling. The processes used are described and test results are presented, showing that thick, metallurgically bonded coatings were achieved with high hardness and excellent wear and corrosion resistance.

The mechanical, tribological, and corrosion properties of two hybrid coating systems were assessed: 1) a tungsten–tungsten carbide (W-WC) top layer and a laser cladded cobalt– chromium (Co-Cr) interlayer (Stellite 6 superalloy) applied to a 316 stainless steel substrate; and 2) the same W-WC top layer and an HVOF spray-and-fused Ni-W-Cr-B interlayer (Colmonoy 88 superalloy) applied to an Inconel 718 substrate. X-ray diffraction, energy dispersive spectroscopy, and scanning electron microscopy were used to analyze the microstructure of the coating layers. Microindentation was used to measure surface hardness and the hardness profile of the coating systems. Rockwell indentation was used to assess coating adhesion according to CEN/TS 1071-8. Surface load-carrying capacity was also assessed by measuring micro- and macrohardness at high loads. Tribological properties were assessed with a linear reciprocating ball-on-flat sliding wear test, and corrosion resistance was measured by potentiodynamic polarization and electrochemical impedance spectroscopy. W-WC layers showed class I adhesion to both the SS 316 and Inconel 718 substrates, with and without an interlayer. Hardness profile measurements on cross section showed hardness of 13.6 GPa and 7.0 GPa for W-WC and Co-Cr, respectively, with average hardness of 9.7 GPa for Ni-W-Cr-B. Furthermore, hardness measurements at different high loads revealed that the addition of an interlayer increases surface hardness by up to 200% compared to the same coating system provided without an interlayer, quantifying the additional load-carrying capacity provided by the supplementary interlayer. The tribological measurements show that, except for the Inconel 718 / Ni-W-Cr-B / W-WC system, the hardest interlayer or substrate leads to the highest wear rates. In addition, the W-WC layer showed excellent corrosion protection, with no pitting observed after potentiodynamic polarization testing.

This work examines the corrosion behavior of cobalt and nickel base coatings produced by APS and HVOF spraying. Laser fusing and sealing are also assessed for comparison as are the corrosion properties of tungsten carbide, chromium carbide, and chromium oxide. All coatings and processes are analyzed by cyclic polarization testing and optical and scanning electron microscopy. Test results are presented and discussed along with the relative merits of each process.

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.

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.

Thermal conductivity of plasma-sprayed YSZ thermal barrier coatings depends on microstructure and significantly influences the effectiveness of the thermal barrier. In this study, YSZ coatings are deposited by plasma spraying using fused and crushed Y 2 O 3 -ZrO 2 powder. Coating microstructure was modified by substrate temperature, which was varied from room temperature to 880 °C during deposition. The coating microstructure was examined from the fractured cross-section of the deposits to reveal changes in interlamellar bonding due to substrate temperature. The thermal conductivities of the YSZ coating were tested by laser flashing from room temperature to 1300 °C. The results showed that the coating deposited at room temperature exhibited a typical lamellar structure and a thermal conductivity of 1.26 W·m -1 ·K -1 measured at room temperature. With the increase of substrate temperature, the thermal conductivity was increased. The coating deposited at a substrate temperature of 840-880 °C consisted of trans-lamellae long columnar grains and exhibited a value of 1.95 W·m -1 ·K -1 at room temperature. The influence of substrate temperature on thermal conductivity is discussed based on the changes in microstructure observed.

For deposition of protective coatings different coating techniques are available. Usually, detailed evaluation of various deposit types and materials is necessary for selection of the best suited coating for specific application fields and demands. Subject of this work are thermally sprayed functional coatings applied as wear (and corrosion) protective layers. Examination of different optimized thermal spray coatings, i.e. HVOF sprayed WC/Co(Cr) and Cr 3 C 2 /NiCr coatings, conventional flame sprayed and fused self fluxing alloy coatings reinforced by hardmetal and APS sprayed oxide Al 2 O 3 /TiO 2 and Cr 2 O 3 coatings, is done in comparison to thick hard chromium platings. Two abrasive wear tests featuring wear by lose abrasive particles are carried out. These impart dry wear conditions according to ASTM G65 (Rubber Wheel test) and wear by abrasive suspensions according to ASTM G75 (Miller test). The work also contains evaluation of newly developed HVOF torch components permitting increased combustion gas, and therefore also particle, velocities concerning the benefit in terms of coating properties. Exemplary evaluation of the new components influence on velocity and temperature of spray particles is carried out by comparative SprayWatch analyses. Both the influence on the coatings microstructure and the wear performance are studied. Coating microstructure is evaluated qualitatively by optical and scanning electron microscopy and the micro hardness HV0.3 is measured. Worn surfaces are studied by SEM in order to deduce wear mechanisms.

Calcium zirconate and magnesium zirconate prepared by synthesis and agglomeration were sprayed using a water stabilized plasma gun under varied spraying conditions. The same set of conditions was maintained during spraying of fused lime stabilized zirconia and magnesia stabilized zirconia. The intent of the study was to compare spraying behaviour and deposit properties of pairs of feedstock material that have just about the same chemical composition but differ considerably in the way of preparation. Free-flight particles as well as deposits were characterised by standard techniques, such as light microscopy, scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, differential thermal analysis, and pycnometry. The differences between relevant coatings were evaluated namely in terms of morphology, chemical and phase composition, density and porosity. Sources of observed differences are discussed.

This paper reports on a structural characterisation of the nickel alloy coatings, before and after the fuse process is done and the main phases in each case, showing important differences between the nickel alloy coatings. Spray and fuse process involves thermal spraying to apply a coating of special self-fluxing alloys and a post thermal treatment at temperature between the solidus and liquidus of the alloy, when important diffusion processes take place. An improvement of the hardness and tribological properties is observed with the addition of tungsten carbide-cobalt to the nickel alloy powder. The results show the excellent tribological properties of the spray and fused coatings, better than the obtained for the as-sprayed coatings. The adhesion strength of the nickel alloy coating after the thermal treatment achieve a value above 80 MPa. Paper includes a German-language abstract.

NiCrBSi spray and fuse types of coatings are routinely applied on various jobs where metallurgical bond of the coating to the substrate is desired. These coatings require a subsequent fusing operation at temperature of about 1050°C. In the present study NiCrBSi coatings were formed by the HVOF process and by the conventional spray and fuse route. The coatings were characterized for their microstructure, hardness and porosity. The aim of this study was to see whether HVOF spraying NiCrBSi spray and fuse powder could cause instant fusing of the coating without the need for additional fusing operation. XRD study of the coatings was also carried out to find out the difference between the spray and fused NiCrBSi coatings and the HVOF sprayed coatings. The HVOF spraying of NiCrBSi was carried under different set of parameters to determine the effect of spraying parameters on the properties of coating.

Until now the use of thermal sprayed metallic coatings for wet corrosion protection is limited to applications where the coating (Al-alloys, Zn-alloys and their mixtures) acts as an anode to protect the substrate or special cases, where thick cold sprayed metallic layers give good results. Other atmospheric cold sprayed layers made of corrosion resistant Ni, Co, Cu or Fe base alloys have their limitations due to the process related discontinuities like pores and oxide films. In more aggressive environments thermal sprayed and fused layers made of so called self-fluxing Ni and Co based alloys are commonly applied. Also in some applications the use of specially designed gas shrouds or of spraying techniques running in inert gas atmospheres or vacuum can yield protective coating solutions. However, these techniques have high investment or service costs or the size of the parts to be coated is restricted. Recent developments in HVOF-spraying open new possibilities in applying cold sprayed coatings on site with good corrosion resistance. The aim of this paper is on one hand to give a comparative overview about the wet corrosion behaviour of thermally sprayed metallic coatings using different spraying techniques related to the performance of the corresponding bulk materials and on the other hand to demonstrate the potential of a new generation of coatings to protect or repair structures exposed to aggressive environments. Keywords: corrosion behavior, thermal sprayed metallic coatings, cermet coatings HVOF, HVIF, PTAW.

A simplified 1D model has been developed to calculate the temperature-time history of alumina layering splats. The splats were obtained by plasma spraying alumina fused-and-crushed particles onto steel substrates. The model correlates solidification time with splat layer thickness and cooling rate and helps to explain the process of columnar growth, the development of residual stresses, and the effect of quenching and expansion mismatch.

The demand for highly continuous thermal spray deposition processing to gain manufacturing efficiency and enhanced control of deposition microstructure has driven the search for powder feedstocks that exhibit uniform flow behavior in pneumatic feeding devices even at low carrier gas flow ratios. This paper compares the continuous powder feeding characteristics of one type of feeding device for two different powder alloys fabricated by a representative set of powder processing methods, including spray dried, fused (cast) and crushed, and inert gas atomized. Powders were fabricated by both commercial vendors and Ames Laboratory processing equipment. The powder mass flow rate was found to depend directly on the apparent density of a specific powder type instead of the dry powder flowability, as initially expected.

Fusible Ni-B-Si alloys with a variety of alloy additions (Cr, Mo, Cu etc.) have been in service for many years as fused coatings with moderate corrosion resistance. Both gas- and water-atomised powders have been used with the spray and fuse and with the plasma transferred arc process to produce coatings. As the severity of corrosive industrial environments has increased, for example in waste burning boilers, existing alloys have not provided the desired service performance. This study was undertaken to develop a new family of alloys with improved corrosion resistance without sacrificing usability, wear resistance or cost effectiveness. A range of compositions was prepared and evaluated for deposition characteristic, microstructure, hardness, wear resistance and corrosion resistance in various media. The resulting alloy has an exceptional combination of wear and corrosion resistance in comparison to conventional alloys, when tested under comparable conditions.

Fusible coatings of Nickel-Chromium alloys with various amounts of Boron and Silicon commonly used for severe load applications. The coating is normally sprayed, then fused by heating to the point of liquation. The fusing process causes powder coalescence and increases density. At the same time, the high fusing temperatures creates a “brazed” bond which gives these coatings extremely high adhesive bond strengths. The improved bond strength is the result of the metallurgical bond as compared to the majority of thermal spray coatings which rely only on mechanical bonding mechanisms. The fusing operation is very sensitive, especially when a hand torch fuse is required. To circumvent these problems, a study was conducted to see if high density HVOF sprayed coatings might achieve fused quality by furnace heating to temperatures well below the liquation point. Various times and temperatures were surveyed. Bond strength tests of coatings sprayed to heavy thicknesses, hardness and impact tests, and metallography were used for evaluation. It was determined that heating as low as 1500° F for three hours could improve the properties of an as-sprayed HVOF coating to where it developed characteristics very similar to that of a fused coating.

Spray and Fuse coatings are ideally suited to boiler environments. The fusing step gives to optimized coatings a uniform chemistry and microstructure, a metallurgical bonding, no through porosity, a low oxide content and no cracking. The relative simplicity of the traditional processing equipment is suited to on-site and automated application including coating repair. Although enjoying considerable success in traditional boilers, their advance into new applications appears restrained by the issue of substrate property modifications during fusing. This paper studies the modification in substrate boiler tube materials (microstructure and mechanical properties) associated with the coating of advanced NiCrBSi alloys. Charpy impact, tensile strength, yield strength, elongation and thermal expansion coefficient testing between 22°C and 450°C are measured. By optimal coating design to a specific substrate, the effect on mechanical property modifications by fusing could be dramatically reduced to allow new and reliable coatings systems to be demonstrated for advanced applications. The future for Spray and Fuse coatings in boilers is discussed.