Boiler tube failure is the number one source of forced outages in all coal-fired and biomass-fired power generation plants. It is estimated that plants lose approximately 6% of their power generation annually, due to boiler tube leaks. The major causes for premature tube failure are excessive fireside boiler tube erosion and corrosion caused by impact, abrasive wear, oxidation and molten corrosion of low eutectic alloys.

To manufacture a protective coating with low thermal conductivity and good frictional wear performance, a Fe 59 Cr 12 Nb 5 B 20 Si 4 coating was designed and produced by high velocity oxygen fuel (HVOF) spraying; the properties and performance of this coating where then compared with those of a commercially available AISI 316L stainless steel coating. In the as-deposited state, both coatings exhibit dense layered structures with porosity below 1% and slight oxidation. The microstructure of the Fe-based coating has an amorphous matrix and some precipitated nanocrystals. The result is that the designed Fe-based coating has a thermal conductivity (2.66 W/m·K) that is significantly lower than that of the 316L stainless steel coating (5.87 W/m·K). Based on its advantageous structure, the Fe-based coating exhibits higher microhardness, reaching 1258±92 HV. The friction coefficient and wear rate of the Fe-based coating show an increase at 200°C followed by a decrease at 400°C, due to the evolution of the wear mechanism at different temperatures. The dominant wear mechanism of the Fe-based coating at room temperature is fatigue wear accompanied by oxidative wear. At 200°C, due to the existence of “third body” abrasive wear, the wear process was accelerated. The large-area oxide layer is likely responsible for the decrease of friction of the coating at 400°C.

This paper considers the deposition of a commercial steel powder with a chemical composition that allows the coating to obtain an amorphous structure using thermal spray techniques. The processes used are characterized by high cooling speeds of the particles after the impact upon the substrate. The powders were sprayed with two different processes: cold gas spray (CGS) and high velocity oxyfuel (HVOF). A comparison between the samples obtained reveals that only the CGS coatings are completely amorphous; the HVOF samples exhibit nanocrystalline phases, detected with XRD analysis and SEM micrographs. Furthermore, the CGS coatings are more compact and show lower hardness with a comparable Young’s modulus. A hypothesis is that the formation of the amorphous structure is related to plastic deformation at impact (due to the high energy of the particles), rather than to the temperature; the mechanism could resemble that of a severe plastic deformation process. Additional thermal treatments and mechanical tests are in progress to investigate the toughness and other mechanical properties of the coatings.

The lower wear and poor impact resistance of the amorphous coatings has been a great problem in the past few years for their use in industrial applications. Several research methods have been reported recently to overcome this issue. The present paper addresses the main work done recently on iron based amorphous composite coatings by the addition of 0-20% Alumina particles. These particles were homogeneously distributed in the amorphous matrix of the coatings which improved wear and impact resistance as compared to the monolithic coatings without any decrease in corrosion resistance. The hard alumina particles enhanced wear resistance to several times not only in air but also in salt water solution with a decrease in friction coefficient. The combined effect of wear and corrosion were also observed to become better by the alumina addition. Furthermore, the impact resistance was also improved three times by the addition of alumina particles. The hard second phase particles present in the amorphous coating matrix disperses the residual stresses generated during the impact loading. The brittle alumina particles absorb the impact energy by breaking itself which stop the initiation of cracks and also play a vital role in the crack arresting and blocking of the crack propagation.

This study investigates the feasibility of forming amorphous iron-based coatings using the cold spray deposition process. Splat tests of cold-sprayed SAM1651 (Fe48Mo14Cr15Y2C15B6 at.%) particles impacting a mild steel substrate were performed using varying gas temperatures and particle diameters. Specimen inspection by scanning electron microscopy revealed splat morphologies that varied from well-adhered particles to substrate craters formed by rebounded particles. Particle flow was analyzed using a finite element model, and impact conditions were predicted using an experimentally validated analytical model, in empirically generating a temperature/velocity window of successful particle deposition as a framework for ongoing work on the formation of cold-sprayed SAM1651 coatings. The results indicate that the unique characteristics of the cold spray process offer a promising means for the formation of metallic glass coatings that successfully retain the amorphous structure, as well as the superior corrosion and wear resistant properties of the feedstock powder.

A Fe48Cr15Mo14C15B6Y2 alloy with high glass forming ability (GFA) was selected to prepare amorphous metallic coatings by atmospheric plasma spraying (APS) process. The as-deposited coatings present a dense layered structure and low porosity. Microstructural studies show that some nanocrystals and a fraction of yttrium oxides formed during spraying process, which induced the amorphous fraction of the coatings decreasing to 69% compared with fully amorphous alloy ribbons of the same component. High thermal stability employs the amorphous coatings to work below 910K temperature without crystallization. Corrosion behavior of the amorphous coating was investigated by electrochemical measurement. The results show that the coatings exhibit extremely wide passive region and low passive current density in 3.5% NaCl and 1mol/L HCl solutions, which illustrate their superior ability to resist localized corrosion. Moreover, the corrosion behavior of the amorphous coatings in 1mol/L H 2 SO 4 solution is similar to their performance in chlorine ions contained conditions, which manifests their flexible and extensive ability to withstand aggressive environments.

Cu 54 Zr 22 Ti 18 Ni 6 amorphous powders were deposited onto aluminum substrates by cold spray process with different powder preheating temperature (below T g : 623K, near T g : 703K, and T x : 773K). The microstructure and macroscopic properties of coating layers were investigated using OM, XRD, DSC and hardness, SUGA test, potentio-dynamic corrosion test. XRD results showed that cold sprayed Cu based amorphous coating layers of 300~350 μm thickness could be well manufactured regardless of powder preheating temperature. Porosity measurements revealed that the coating layers of 623K and 773K preheating temperature conditions had lower porosity contents (0.88%, 0.93%) than that of 623K preheating conditions (4.87%). Hardness was measured as 374.8HV (623K), 436.3HV (703K) and 455.4HV (773K) for the coating layers, respectively. Results from the wear resistance examination via SUGA test and those from the hardness testing showed the same trends. Examination of corrosion resistance of the amorphous coating layer showed that the critical anodic current density (ic) of the coating layer, for which powder preheating was provided at 623K and then cold spray deposition, was 5.6X10-3A/cm 2 . The ic values of the 703K and 773K coating layers were 4.8X10 -4 and 1.2X10 -3 A/cm 2 , respectively. Both temperature conditions were found to offer superior corrosion characteristics to those of the 623K-prehaeted and coated specimen. This was assumed to be attributed to the relatively lower level of porosity.

Amorphous metallic coatings are of high interest because of their good wear and corrosion resistance as well as their high hardness and toughness relative to the crystalline alloys with the same composition. Thermal spray that makes it possible to reach quenching rates in the order of 106-107 K/s, has the ability to deposit coatings with a high content of amorphous phase. However, very few studies dealt with the understanding of the spraying factors that affect the formation of the amorphous phase under thermal spray conditions. In this work, the relationship between temperature and velocity of the spray particles and coating characteristics is investigated. Special attention is given to the degree of amorphisation of the as-sprayed coatings. The latter were produced both by plasma and wire-arc spraying in order to get a larger range of particle parameters at impact and different particle heating history in the gas flow before impingement onto the substrate. A commercial iron-based alloy available both in powder and wire forms was used. Microstructural analyses show that the as-sprayed coatings are partially amorphous and that the proportion of the amorphous phases depends on the sizes of the sprayed particles as they control the heating and acceleration of particles in the gas flow and their cooling rate on the substrate.

The development of amorphous and nanocrystalline materials took a significant part in search on materials these last years. Indeed, the magnetic, chemical and mechanic properties of materials are greatly modified when the size of crystallites becomes nanometric. The absence of crystal structure involves a macroscopic behaviour of the alloy, which is completely different from the same alloy in a polycrystalline state, particularly magnetic and mechanical properties. We have carried out coatings by APS plasma thermal spraying on a copper substrate using three types of powders, FeB (17,5% wt-B), FeSi (6,5% wt-Si) and FeNb (67,2% wt-Nb). Structure of these coatings was characterized by SEM and X-rays. We have also tested the magnetic properties of these deposits. Results obtained showed that the FeNb alloys are amagnetic with a partially amorphous structure, however FeSi and FeB alloys presented a microcrystalline structure with soft magnetic properties.

Ultra-fine hydroxyapatite powders were successfully synthesized using radio frequency (RF) suspension plasma spraying (SPS). This novel technique utilises the inherent characteristics of the RF plasma to axially feed and spheroidise a liquid suspension to produce spherical ultra-fine HA powders. This offers an alternative approach over conventional D.C. and flame spheroidising techniques which are better suited for solid feed stocks. Rietveld analysis was subsequently applied using Rietquan Quantitative Analysis software package to determine the amount of decomposed phases and amorphous content of the as-sprayed powder. This was also compared against quantitative XRD analysis employing internal and external standards. However, pure phases needed for calibration is scarce and amorphous calcium phosphate (ACP) is virtually impossible to isolate. In addition, the long and laborious task of obtaining calibration curves makes this technique unpopular. Nevertheless, conventional quantitative phase analysis (QPA) was carried out, using relative peak height ratios of HA and the phase involved, but the calculated decomposition only shows relative trends for a particular parameter variation. Determining the actual phase content is critical because of possible variations in biological responses when used as coatings and inserts in restorative orthopaedic implants. Varying tissue responses can arise from decomposed phases such as α and β-tricalcium phosphate (TCP) and tetra-calcium phosphate (TTCP) as well as ACP which generally have higher solubility as compared to crystalline. QPA via the Rietveld method provides a powerful tool that offers the user simultaneous quantitative phase determination of multiphase systems containing amorphous content. Unlike XRD QPA, the amorphous content could be indirectly calculated using crystalline alumina standard. XRD QPA results showed that decomposition generally rose with plate power without considering the amorphous content. With Rietveld QPA, the results showed an initial rise in decomposition before decreasing at higher plate powers. The amorphous phase content was calculated at different plate powers and concentration of suspension with the aid of alumina as an external standard. Results showed that the amorphous content increased substantially at higher powers. This study demonstrates the ability of Rietveld analysis to completely quantify all associated amorphous and crystalline phases within a multiphase system for any thermally treated material.

In this paper, the morphology, the microstructure, the thermal stability ranges, the devitrification kinetics, and the hardness of amorphous detonation sprayed FeCrPC coatings are examined and compared with those of amorphous tapes of similar composition. Nanocoatings were made by heat treating the amorphous coatings. It is observed that thermal spraying (using the detonation gun method) produced coatings with an improved thermal stability of the amorphous component. Paper includes a German-language abstract.

Using a DC-plasmajet amorphous and nanocrystalline Si-C-layers are synthesized from chloromethylsilanes on various substrate materials. Though most of the layers show granular morphologies with cluster diameters between 25 and 400 nm depending on the process parameters, coatings with a dense or columnar morphology and with a smooth surface can be synthesized as well. XRD analyses verify β-SiC crystals with an average diameter of 5 nm. In some samples produced from carbon rich precursors also graphite is detected. Depending on the substrate material and the process parameters deposition rates up to 1,300 µm/h are obtained. Apart from silicon and carbon the coatings convey oxygen and chlorine verified by EDX. Coatings removed from the substrate can withstand several bending cycles (45°) without any visible indication of failure. Paper text in German.

Fe-Cr(-Mo) alloy coatings were thermal sprayed by different processes of LPPS, HVOF and HPS. The as-sprayed coating by LPPS is perfectly amorphous and coatings by other processes contain partly crystalline phases. The amorphous phases crystallize at 773 K or more and shows a high hardness of about 1000 to 1400 DPN just after crystallization. The anodic polarization curves of the coatings shift from active to passive state in 1N H2SO4 and 1N HCl solutions. The coatings obtained by LPPS indicate the lowest active and passive current densities and possess the best corrosion resistance. The corrosion resistance of the coatings obtained by other processes are better than a SUS316L stainless steel coating. The LPPS coating of Fe-Cr-C-P alloy is not attacked on immersion test in 6% FeCl3·6H2O solution containing 0.05N HCl at the corrosion potential, while large pit corrosion is developed in a SUS316L stainless steel sheet.

Physical properties of coatings based on Fe-B, Fe-Ni-B, Fe-Cr-P-C, Fe-Ni-Si-B, Ni-P, Ni-Nb and Co-Fe-B-Si, deposited by the methods of flame, plasma-arc, and detonation spraying were investigated. The coatings have mostly the amorphous structure with the volume content of the amorphous phase equal to 75-95 %. Values of the distribution and temperature coefficients of electric resistance of the coatings, depending upon a method and conditions of spraying, as well as upon their treatment parameters, were determined. Comparative studies of these coatings and thin amorphous strips produced by the melt spinning method were conducted. The amorphous coatings of ferromagnetic iron and cobalt alloys are shown to be magnetically soft materials and are characterized by a high magnetic induction combined with a high magnetic permeability. As compared with the amorphous strips, Curie temperature of the amorphous ferromagnetic coatings is by 50-140 K higher and their anisotropy of magnetic properties is lower.

This paper examines the stress state of plasma-sprayed amorphous coatings of Fe-B with additions of Ni, Cr, and Mo. Internal stresses depend on the type of plasma gas used, the thickness and composition of the coating, and the material and temperature of the substrate. In this study, additional cooling of the substrate was found to be the most efficient way to reduce internal stresses. Amorphous coatings were also found to improve fatigue strength by as much as 25-30%, which is attributed to the formation of compressive stresses in the coating layers adjoining the substrate.

An integrated approach was developed for investigation of thermal spray coatings with the amorphous-crystalline structure. The new approach combines methods of metallography, differential thermal and X-ray phase analysis, scanning electron microscopy and X-ray microanalysis. This makes it possible to reveal structural, phase and chemical heterogeneity, determine the degree of amorphization of coatings, temperature and heat of crystallization of the amorphous phase during heating. The new integrated approach was used to study amorphous-crystalline coatings of the Ni-P, Fe-Ni-B and Fe-B systems produced by thermal spraying.