bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication and Properties of TiO2 Photo-Catalytic Coatings by Thermal Spraying with TiO2-AI Agglomerated Powder
Authors: Matsusaka S. , Ohmori A. , Toki Y.
Published: June 2022

In the present study, anatase TiO2 photo-catalytic coatings were fabricated by plasma and HVOF spraying. In order to prevent the transformation from anatase to rutile phase, lower heat input conditions were adopted. For the improvement of bondability between coatings and substrate, anatase TiO2-Al agglomerated powder was used for the experiment. The microstructures of the coatings were characterized with SEM and XRD analysis, and the photo-catalytic characteristics of coatings were evaluated by the decomposition rate of acetaldehyde gas. Due to the addition of aluminum, thick coatings were formed even at lower heat input conditions in both plasma and HVOF spraying. The results of XRD analysis for the plasma sprayed coatings, however, indicated the disappearance of anatase phase and deoxidization of TiO2 to Ti3O5 or magnetite phase. On the other hand, high anatase/rutile ratio and high photo-catalytic efficiency were achieved for the coatings formed by HVOF spraying. The temperature measurements of flying particles were also carried out using DPV-2000. As results, significant increase in temperature of TiO2-Al particles was observed compared with the case of using pure TiO2 particle. These results suggest that a chemical reaction between TiO2 and Al occurs during thermal spraying.
HVOF Sprayed Hydroxyapatite Coatings: Powders’ Melting State and Mechanical Properties
Authors: Li H. , Khor K.A. , Cheang P.
Published: June 2022

In the present study, hydroxyapatite coatings were deposited on Ti-6Al-4V alloy substrate by high velocity oxy-fuel (HVOF) spray technique. The as sprayed HA powders and coatings were analyzed with the aim to reveal the melting state of HA powders and its influence on coating properties. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were employed for the characterization of the starting powders and as-sprayed coatings. Differential scanning calorimetry (DSC) was performed to determine the recrystallization temperature of the amorphous phase in HVOF HA coating. Results show that different melting state of HA powders can be achieved through altering HA powder size and/or spray parameters. XRD result reveals that the as sprayed HA coating made from large powders with size of ~50 µm is composed of crystalline HA and very small amount of a-tricalcium phosphate (TCP). While the coatings deposited using fine powders around 30 μm demonstrated a lot of amorphous phase besides crystalline HA and small amount of a-TCP. The recrystallization temperature of the amorphous phase in HA coating is ~720°C. The adhesive strength of the HVOF sprayed HA coatings is ~31MPa and is largely dependent on the melting state of HA powders. This suggests that the fully melted state of the feedstock can result in the formation of amorphous phase, and simultaneously decrease the adhesive strength. It also suggests that the melted fraction of the powders is the most critical factor influencing the adhesive strength and phase composition of HVOF HA coatings. The partial melting state of HA powders is beneficial in terms of adhesive strength and crystallinity.
Corrosion Resistant Coatings for Gas Turbine Exhaust Stack Applications
Authors: Mutasim Z.
Published: June 2022

Land-based gas turbine exhaust stacks are typically made of low carbon steels, and when used in very harsh industrial environments, they are subjected to corrosion attack leading to material degradation and loss of structural integrity. To maintain acceptable condition, various types of corrosion resistant paints and coatings are used. These include aluminum-based polysiloxane organic paints, as well as wire arc sprayed stainless steel and aluminum coatings. Aqueous corrosion protection and thermal stability up to 538°C are required. Development and testing of various coating systems was conducted and presented in this paper. Laboratory and engine field-testing demonstrated the best performance for sealed aluminum wire arc sprayed coatings
Effect of Bond Coat Processes and Surface Characteristics On the Failure Mechanism of Thermal Barrier Coatings
Authors: Ding Z. , Knight R. , Smith R.W.
Published: June 2022

This paper explores the effect of bond coat processes and surface characteristics on the failure mechanism of thermal barrier coatings (TBC's). TBC's consist of a 300µm thick air plasma sprayed (APS) top coating of ZrO2-8Y2O3 w% and CoNiCrAlY bond coats which were deposited using low pressure plasma spray (LPPS), shrouded air plasma spray (SPAS) and high velocity oxy-fuel (HVOF) combustion spray and different size powder. Bond coat surface profiles were measured by profilometric techniques and surface roughness was calculated according to the measured results. TBC performance and failure mechanisms were evaluated by adhesive bond strength testing, image analysis measurements of porosity, thermal cycling testing. X-ray diffraction and microstructural analyses. Research results show that bond coat deposition processes and surface characteristics had significant effects on the thermal cycling lifetime and failure mechanism.
Kinetics of Thermally Grown Oxide at Interface Between Thermal Barrier Coatings and MCrAlY Bond Coatings
Authors: Ogawa K. , Shoji T. , Masuda T.
Published: June 2022

In recent years, thermal barrier coatings (TBC) have been used in advanced gas turbine plants for improved durability and performance. Typically, TBCs consist of an inner layer of metallic bond coating (MCrAlY) and an outer layer of ceramic top coating (8wt% yttria stabilized zirconia (YSZ)). According to several studies, the failure of coating is induced by thermal stress due to formation of oxides at the interface between YSZ and MCrAlY. Therefore, it is important to investigate kinetics of oxidation at the interface. In this work, the interface between YSZ and MCrAlY is studied and characterized. TBC specimens are thermally aged at 1000 deg. C to simulate the surface temperature of first rotating blades. After aging, thermally grown oxide (TGO) is formed at the interface. The TGO has two layers of different contrasts. One layer is black, and is closer to MCrAlY; the other layer is gray, and is closer to YSZ. The black layer is identified as Al2O3 by energy dispersive X-ray spectroscopy (EDX) and electron probe micro analyzer (EPMA). The EDX and EPMA spectra of the gray layer contain various peaks of Al, Cr, Co, Ni, and O, which suggests that the layer can be mixed oxide which is a combination of NiO, CoO, Cr2O3, and Ni(Cr, Al)2O4. However, the outer layer of mixed oxide contains only Cr and O. Accordingly, Cr2O3 forms at the outer layer of mixed oxide, and the other oxides are distributed at the inner layer of mixed oxide. The thickness of the two oxide layers increases with aging time. While the formation of mixed oxide layer obeys a parabolic law, the formation of an alumina layer cannot be expressed in terms of a parabolic law. Due to the formation of protective mixed oxide on the alumina layer, the oxidation rate of alumina decreases as the thickness of mixed oxide increases.
Reaction Diffusion Behavior of Thermally Sprayed MCrAlY Coatings On Ni-Based Super Alloys
Authors: Teratani T. , Tani K. , Harada Y.
Published: June 2022

The gas turbine blades in a severe environment are overlaid with MCrAlY coatings by Low Pressure Plasma Spray (LPS) process for protection against hot corrosion. However, the service life of each blade is limited by damage due to embrittlement layer, which is formed by reaction diffusion at the interface between the coating and the substrate. Reaction diffusion behavior between the CoNiCrAlY coatings and substrates was investigated. In addition, high-temperature oxidation behavior of the CoNiCrAlY coating by LPS was evaluated. The CoNiCrAlY coatings for reaction diffusion test were carried out by 3 kinds of spray process (LPS, High Velocity Oxygen Fuel Spray: HVOF, Atmospheric Plasma Spray: APS) on 2 kinds of substrate (Directional Solidification and Single Crystal Ni-based super alloys). It has been found that the CoNiCrAlY coating by APS inhibited the reaction diffusion at the boundary of the coating and the base material as compared with LPS coating. It was also confirmed that the protective dense layer of aluminum oxide against hot corrosion was formed in the surface of the CoNiCrAlY coatings by LPS.
Removal of Thermal Barrier Coatings From Turbine Blades: A Comparison of Mechanical and Chemical Methods
Authors: Scrivani A. , Groppetti R. , Ianelli S. , Bardi U. , Ballerini G. , Bonacchi D. , Fantini M. , Rizzi G.
Published: June 2022

Thermal barrier coatings are gaining considerable importance for the improvement the energetic efficiency of turbines. These materials are often applied on the surface of blades and are based on a layer of antioxidation material (mainly MCrAlY alloys) and a top layer that acts as proper thermal barrier (normally Yttria partially stabilized Zirconia). Coating removal is an important aspect in the production of these blades. "Decoating" or "stripping" is needed for the production of new components as well as for the reconditioning of existing ones. The present paper is dedicated to the comparison of different stripping methods and to the characterization of the blades surface after removal of thermal spray coatings both of Zirconia and of MCrAlY. The results reported here show that chemical stripping is particularly suitable for MCrAlY coating removal and does not affect the substrate. Water jet stripping can successfully be used for Zirconia-MCrAlY system removal although care is needed to avoid substrate damage. Salt bath technologies have been formed to be effective for TBC removal but not for MCrAlY removal.
Induction Plasma Synthesis of MoSi2 Ultrafine Powders
Authors: Gutierrez S. , Gitzhofer F.
Published: June 2022

Ultrafine MoSi2 powders have been synthesized from commercial MoSi2 powders by using an Ar-H2 induction plasma. Reactor pressure and plate power were taken as the experimental parameters to optimize the phase as well as the size distribution of ultrafine MoSi2 powders. The powders were collected from porous metal fibers. They were composed of both metastable hexagonal structure (β-MoSi2) and stable tetragonal structure (α-MoSi2) with small levels of Mo5Si3 and free silicon.
Plasma Spray Dissociation of ZrSiO4 As the Basis of Forming ZrO2-Mullite Composites From a ZrSiO4 + AI2O3 Mixture
Authors: Li Y. , Khor K.A.
Published: June 2022

Zirconia is effective in improving the fracture toughness to a number of ceramics when introduced as a reinforcement either in the form of particulate, dispersed phase, or whiskers because of its unique tetragonal-monoclinic (t → m) transformation. In this paper, the authors attempt to prepare ZrO2 reinforced Mullite by plasma spraying mixtures of zircon and alumina. Pre-mixed powders of zircon (ZrSiO4) and alumina are injected into a dc plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The dissociation of zircon in thermal plasma is a well-established fact. The present investigation aims to utilize the plasma dissociation of zircon to produced ZrO2-toughened mullite from a zircon + alumina mixture. The results indicate that the plasma sprayed powders consist of zirconia, amorphous SiO2, zircon, and alumina. It was found that ball milling and plasma spraying could yield fine grained, even amorphous and chemically homogeneous, composite powders. Recrystallization of amorphous phases and formation of mullite occurred at about 1000 °C in plasma sprayed powders. This value is more than 500 °C lower than the formation of mullite in as-milled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders. Preheat treatment of the spheroidized powder promoted dissociation of zircon. Zirconia remained as tetragonal under 1000 °C in the sprayed coatings.
HVOF Sprayed Hydroxyapatite/Titania Composite Coatings
Authors: Khor K.A. , Li H. , Cheang P.
Published: June 2022

As ceramic materials, the use of hydroxyapatite (HA) in clinical applications is severely limited by its intrinsic poor mechanical properties. The incorporation of some bioinert ceramics is believed to be a way to improve the mechanical reliability of HA matrix. HA coatings with titania addition were produced by using high velocity oxy-fuel (HVOF) spraying process in the present study. The mechanical properties of the as-sprayed coatings in terms of adhesive strength, shear strength and fracture toughness were investigated aiming to reveal the reinforcing effect of the titania addition in HA coatings. Qualitative phase analysis through X-ray diffraction (XRD) showed that mutual chemical reaction between TiO2 and HA occurred during coating formation, from which CaTiO3 was resulted. Totally unmelted titania powders were observed which suggests that the mutual reaction locates at HA/TiO2 splats' interface. Significant influence of coating microstructure on mechanical properties was revealed. As the content of titania in HA coatings reached 20vol%, the adhesive strength decreased largely. As the content of titania reached 30vol%, the further augmentation of the adhesive strength of is possibly resulted from the improved coating microstructure. The fracture toughness exhibited the values of 0.48 Mpa ⋅ m½ 0.60 Mpa ⋅ m½ and 0.67 Mpa ⋅ m½ for pure HA coating, 10vol% TiO2 blended HA coating and 20vol% TiO2 blended HA coating, respectively.
Abradable Coatings for Low-Temperature Applications
Authors: Nava Y. , Mutasim Z. , Coe M.
Published: June 2022

Various low-temperature abradable coatings for applications reaching 482°C were evaluated with the purpose of being applied onto the compressor spacers of an 11 MW land-based gas turbine engine. The coating is intended to reduce the compressor rotor tip-clearance, and is expected to increase the compressor efficiency by a range of 0.5 to 1%. This study evaluated physical and metallurgical characteristics, abradable and oxidation-resistance properties of plasma-sprayed AlSi-BN, NiCrFeAl-BN and quasicrystal AlCuFeCr, and flame-and plasma-sprayed NiCrFeAl-BN.
Developments of Plasma Sprayed Ceramic Coatings
Authors: Ding C.X. , Liu X.Y. , Zheng X.B.
Published: June 2022

Recent developments in the field of plasma sprayed ceramic coatings at Shanghai Institute of Ceramics (SIC) are presented. Nano-titania and nano-tungsten carbide coatings were prepared. Their structure and properties were detected. The super hard B4C coating was deposited by APS. The physical and mechanical and anti-irradiation properties of B4C were measured. Wollastonite coating was deposited and its bioactivity has been tested. The results obtained indicated that (1) nano-titania coating possessed porous structure and unique electric properties; (2) nano-WC-Co coating exhibited notable wear resistance; (3) B4C coating was excellent irradiation resistance and (4) the carbonate-containing hydroxyapatite was formed on the surface of wollastonite coating, which indicated that this coating has excellent bioactivity.
On the Tailoring of Spray Dried Thermal Spray Powders
Authors: Keller N. , Bertrand G. , Comas B. , Coddet C.
Published: June 2022

In thermal spray, it is well established that tailoring the powder characteristics is of major importance to achieve reproducible coatings on a microstructural and chemical point of view. Among the techniques developed to produce thermal spray powders, spray drying has proved to be the most versatile process. The spray drying method consists in spraying a slurry containing finely dispersed particles of the materials to be agglomerated. However, in order to prepare specific thermal spray powders, two steps have to be mastered: the slurry stability and the spray drying operating conditions. The present study was focused on the relationships that exist between the slurry rheology, the powder morphology and the coating properties. This work was performed on a model material namely Al2O3. In a first part, the effects of the surfactant percentage and pH on the stability of the suspensions were determined. The evolution of the viscosity of the slurries versus the amount of binder was measured. In a second part, these slurries were used to prepare spray-dried powders. The effects of some process parameters such as atomizing air pressure and slurry feed rate on the granule characteristics (morphology, density, particle size distribution, and powder flow ability) were investigated. Finally, some coatings were deposited using the APS technique on steel substrates from the non-sintered spray-dried powder previously realized. The coating morphology and the crystallographic structure were evaluated as a function of the spraying conditions.
A Comparative Study of the Deposition of Chromium Carbide Based Metal Matrix Composites by Axial Plasma Spray, Air Plasma Spray and High Velocity Oxygen Fuel
Authors: Scrivani A. , Groppetti R. , Ianelli S. , Bardi U. , Ballerini G. , Bertini S.
Published: June 2022

The present work has the purpose of comparing different thermal spraying techniques, namely axial plasma spray, standard air plasma spray and high velocity oxygen flame (HVOF), for depositing metal matrix composites, in this case chromium carbide nickel-chromium based. The quality of the coatings deposited by these three techniques has been assessed in terms of structural characteristics (porosity, oxide concentration, unmelted particles presence, etc.) and of mechanical characteristics (hardness, adhesion, etc.) as well as surface morphology. A specific efficiency test has been carried out to compare the three examined technologies. The results of the present study indicate that, against a slightly decrease in the quality of the film in terms of structural and mechanical properties, axial plasma sprayed coatings can be sprayed with a higher efficiency in comparison to the traditional technologies.
Heating Effect of Flame Impingement on Polymer Coatings
Authors: Gawne D.T. , Zhang T. , Bao Y.
Published: June 2022

A computational model of the effect of the tail end of the flame on the temperature of polymer coatings during thermal spraying is presented. The low thermal conductivity of polymers results in a substantial build up of temperature at the surface of the coating and large temperature gradients are developed throughout its thickness. This is particularly problematic for polymer deposition owing to their low decomposition temperatures. The model quantifies the heat transfer from the impinging flame and the in-coming feedstock particles to the coating and the subsequent heat flow into the substrate and surroundings. The work shows that the heat input from the in-coming particles can be neglected in first-order computations. The scanning action of the flame across the substrate is simulated and the temperature profiles within the coating and substrate are calculated. The predictions are consistent with the experimental measurements. The model shows that overheating of polymer coatings can readily occur during combustion flame spraying and indicates remedial measures.
Mechanical Properties of Thermal Spray PEEK Coatings
Authors: Liao H. , Coddet C. , Simonin L.
Published: June 2022

The influences of process parameters on the microstructure of PEEK coatings deposited by flame spraying were analyzed by different physical and thermal methods, considering the density, the hardness and the Young's modulus. The cooling rate of the coating after spraying leads to large variations of the microstructure which evolves between the crystalline and amorphous states. Amorphous coatings exhibit less residual stress than semi-crystalline ones, which has an influence on the friction behavior.
Structure and Thermal Conductivity of Flame Sprayed Polyamide 11 Coatings Filled With Al2O3, AIN and BN Ceramics
Authors: Leivo E.M. , Kellberg T.H. , Vuoristo P.M.J. , Mäntylä T.A. , Kolari M.J.
Published: June 2022

Polymer ceramic composite coatings were applied by flame spraying on preheated steel substrates. Polyamide (PA 11, nylon) powder was blended with alumina (Al2O3), aluminium nitride (AIN) or boron nitride (BN) powder with two different filler content to increase thermal conductivities of polymer coatings. Morphologies and particle sizes of polyamide and filler powders were examined by scanning electron microscope and laser scattering methods. Coating structures and thicknesses of composite coatings were studied by the polished cross sections. Thermal conductivities were measured by hot disc method. Thermal conductivities of polyamide coatings increased from 30 % up to few times by blending ceramic filler material on PA 11 powder. Structures of composite coatings were dense and filler materials were clearly observed in polyamide matrix.
Sprayed Composite Coating of High Performance Thermoplastics
Authors: Kawase R. , Tanaka Y.
Published: June 2022

Polyetherether-ketone (PEEK) and Polyphenylene-sulfide (PPS) are high performance thermoplastics having high heat resistance and high corrosion resistance. Composite powders of PEEK / Alumina (Al2O3) with different Al2O3 content have been produced by four processes, which are Mixing (MX) process. Mechanical granulation (MG) process. Melting-crush (MC) process and Heating-granulation (HG) process. PEEK / Al2O3 composite powders have been sprayed with High Velocity Air Fuel (HVAF) spray system. The microstructures of these composite coatings have been evaluated and the properties of these composite coatings have been investigated by the corrosion test, the adhesion test, the ACT-JP test and the abrasive wear test. It was recognized that all the composite coatings containing Al2O3 powder have a good wear resistance. PPS / Al2O3 composite powders have been sprayed with HVAF spray system onto substrate of various pre-heat temperature. It was recognized that the adhesion strength of the composite coatings increases with increasing pre-heat temperature of substrate.
Effects of Shock Waves on Impact Velocity of Cold Spray Particles
Authors: Jodoin B.
Published: June 2022

The modeling of the cold gas dynamics spray process is conducted. The Navier-Stokes equations are solved for the gas flow, including turbulence model equations. The model is validated with experimental measurements for flows with and without shock waves. It predicts accurately the gas flow for both cases. A one-dimensional model is used to track the particles injected in the flow. It is shown that the effects of the shock wave present in front of the substrate are not negligible for small diameter particles and have a direct influence on their impact velocity. The spray stand-off distance is shown to have a direct effect on the flow structure and on the impact velocity of the particles. It is shown that the one-dimensional approach is not sufficient for the particle analysis.
Improvement in Wire Arc Spray by Super Sonic Cold Gas Jet
Authors: Sakaki K. , Shimizu Y.
Published: June 2022

In wire arc spray, atomizing gas is one of the most important parameters. The atomizing gas in wire arc spray is improved by using super sonic cold gas (non combustion gas) jet for better coating characteristics. To generate the super sonic cold gas jet, one is an optimization of nozzle shape with conventional compressed gas and another is that the pressed gas heated at 500-800K is supply to converging-diverging nozzle. Namely, coating deposition mechanism of cold spray, which is high particle impact velocity to substrate, is applied to wire arc spray. In this study, gas dynamics investigated the effect of pressure and temperature of supplied nitrogen gas, nozzle geometry on wire arc spray process (thermodynamical behavior of atomizing gas and coating properties).
HVOF Wire (HIJET) Sprayed Coatings of 0.8% C Steel—Its Structure & Applications
Authors: Modi S.C. , Calla E.
Published: June 2022

The high velocity combustion wire (HVCW) sprayed coatings have unique structures and properties which are different from the conventional wire / powder HVOF coatings. This paper studies the coatings of 0.8% C Steel formed by the HVCW system. 0.8% C Steel coatings formed by the HVCW system were studied for their wear resistance (pin on desk wear test) and phase composition. Methods like SEM with Wavelength Dispersion Spectroscopy (WDS) attachment were utilized for determining the composition of the coatings. Microhardness and tensile bond strength of the coatings were also ascertained. A set of conventional oxy-acetylene wire flame spray coatings of 0.8% C Steel were also prepared and these coatings were then compared with the HVCW coatings. Possible applications of the HVCW coatings are discussed based upon the properties of the coatings.
High Velocity Combustion Wire Spraying—Systems and Coatings
Authors: Kreye H. , Kirsten A. , Gärtner F. , Qi X. , Krõmmer W.
Published: June 2022

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.
Process-Microstructure-Property Relationships in Controlled Atmosphere Plasma Spraying (CAPS) of Ceramics
Authors: Beauvais S. , Guipont V. , Borit F. , Jeandin M. , Español Pons M. , Khor K.A. , Robisson A. , Saenger R.
Published: June 2022

Thermal plasma spray processes with their various operating parameters can be considered as flexible technique to carry out appropriate ceramics coatings. This work deals with plasma spraying of several ceramics powders (hydroxyapatite (HA), Al2O3-TiO2, Al2O3, ZrO2-Y2O3 (YSZ) and Cr2O3) with suitable parameters using a CAPS system ("Controlled Atmosphere Plasma Spraying"). The HPPS (High Pressure Plasma Spraying), APS (Air Plasma Spraying) and IPS (Inert Plasma Spraying) modes were applied in order to obtain the suitable microstructure. The microstructures and phase compositions allowed to establish that surrounding high-pressure in the CAPS chamber is leading to a good heating of the powder and a good quality for the coatings.
Impact of Self Propagating High Temperature Synthesis of Spraying Materials on Coatings Based on Aluminium and Metal-Oxides
Authors: Bach Fr.-W. , Babiak Z. , Duda T. , Rothgardt T. , Tegeder G.
Published: June 2022

The properties of thermal sprayed coatings depend mainly on the thermal and kinetic energy of the spray particles. Increase of thermal energy of sprayed particles can be realized using exothermic reactions between components in sprayed particles. Self propagating high temperature synthesis (SHS) is especially suitable to benefit from released energy in the spraying process. At present most commonly used spray material with exothermal reaction is Ni+Al. However, the highest amount of heat is produced in the reactions of aluminium and metal oxides. Of special interest are Cr2O3, NiO, CuO and V2O5 because they obtain high reaction energies. Furthermore products of the reaction are of special, functional interest like NiAl as bonding agent or alumina as a wear resistant coating. To assure good contact between reacting substances (Al/Oxides) powders for plasma spraying were prepared by mechanical alloying. Calorimetric investigations of plasma sprayed coatings prove that during spraying Al reacts exothermically with oxides. Increase of oxide contents improves coating adhesion/ cohesion properties, hardness, and reduction of porosity. Results are discussed on the base of light microscopy, scanning electron microscopy (SEM) and X-ray structure analysis (XRD).
Reactive HVOF Thermal Spraying of Nanocrystalline Ni Powders
Authors: Lee J. , Ajdelsztajn L. , Lavernia E.J. , Kim N.J.
Published: June 2022

Ni powders prepared by mechanical milling under liquid nitrogen for 15 hr were sprayed using two stoichiometric ratios of the oxygen-fuel mixture in an effort to promote the formation of fine oxide phases. The oxide phases were introduced in an effort to improve mechanical properties and thermal stability of the coatings, via chemical reaction between oxygen and milled powders during flight and after impingement. The microstructure and properties of the milled powders and as-sprayed coatings were characterized by scanning electron microscopy, transmission electron microscopy and nanoindentation. The average grain size of the milled powders was 15.7 ± 5.1 run and ultrafine NiO and Ni3N particles with a size less than 5 run were distributed in the milled powders. These fine oxide and particles distributed in the powders were formed as a result of interaction between Ni, N from the milling slurry, and O from the surrounding environment under the energetic milling conditions. The coating microstructure was composed of nanocrystalline grains with an average grain size of 92.5 + 41.6 nm and extremely fine NiO particles of ~5 nm distributed homogeneously inside the grains. Ni3N phase was not found in the coating as it appears to have decomposed during HVOF thermal spraying. The coating sprayed with higher oxygen fraction in a hydrogen-oxygen mixture showed no significant increase in hardness and elastic modulus when compared to those of the coating sprayed with lower oxygen fraction in hydrogen-oxygen mixture. This was attributed to the small difference in the volume fraction of NiO particles between the coatings. These results indicate that new techniques of ultrafine dispersoid introduction in nanocrystalline coatings are potentially attractive as a means to improve the mechanical properties of the coating through reactive HVOF spraying.
Cryogenic Cooling in High Power Plasma Spraying
Authors: Dhers J. , Goubot N. , David T. , Baudoin J.
Published: June 2022

Thermal spray usually involves high temperatures in short times. This phenomenon is enhanced with the use of an HPPS system (Plazjet from Tafa). To spray a Zirconia coating, high powers are necessary such as 200 kW providing for the substrate an important increase in the surface temperature. Furthermore, the good quality of a thermal spray coating results in the low surface roughness, a satisfactory microstructure cohesion, a minimal distortion of the part when spraying on large sheets with a thick thermal barrier coating. Cooling with compressed air is widely used but other solutions are studied concurrently; CO2, nitrogen, and air; these gas are tested in different conditions of pressure and flow. Some improvements in terms of temperature in the coating and roughness have been found.
On the Adhesion Mechanisms of Thermal Spray Deposits Manufactured While Implementing the PROTAL Process
Authors: Verdier M. , Montavon G. , Costil S. , Coddet C.
Published: June 2022

Excepted in a few cases where metallurgical bonding occurs between deposit and substrate, thermal spray deposit adhesion generally results from a mechanical anchoring. In this case, the very first impinging particles forming the first deposited layer spread and solidify into and around the cavities of the grit-blasted surface. A palliative process to degreasing and grit-blasting prior to thermal spraying is simultaneous laser ablation; i.e., the PROTAL process. In such a case, little topographic change results from the laser-matter interaction: deposit adhesion does not derive anymore mainly from mechanical anchoring but from other types of bonding such as chemical bonding. This paper aims to clarify the bonding mechanisms of thermal spray coatings manufactured implementing the PROTAL process. The case of metallic coatings deposited on metallic substrates is especially discussed. At first, laser ablation effects on various metallic substrates are presented, from the topographic and energetic points of view. Then, the induced effects on impinged particle morphologies are discussed. The results are correlated to thick deposit adhesion.
Influence of Laser Glazing on the Characteristics and Tribological Performance of Plasma and Detonation Sprayed WC-Co Coatings
Authors: Joshi S.V. , Thawari G. , Kumar L.S. , Sen D. , Sundararajan G.
Published: June 2022

This paper deals with a comprehensive evaluation of the laser glazing or re-melting route as a possible means of specifically enhancing the performance of thermal sprayed WC-Co coatings. In the present study, a high-power continuous-wave 9kW CO2 laser was utilized for laser treatment of plasma sprayed as well as detonation sprayed WC-Co coatings. The influence of the two most important laser-related variables, namely laser power and scan speed, on the properties of the laser-treated layers was investigated. Both mere surface densification by melting a thin top layer of the coating as well as melting of the entire portion of the coated layer were targeted during laser treatment. In each case, the laser treated coatings were fully characterized by optical microscopy, scanning electron microscopy, and microhardness measurements. In addition, the influence of laser processing on the elemental distribution, phase constitution and extent of defects in the treated layers was investigated. The tribological performance of the laser-glazed coatings was also evaluated and compared against the performance of their as-sprayed counterparts. The study has revealed significant differences between the response of plasma and detonation sprayed WC-Co layers when subjected to laser treatment. The potential of plasma-sprayed coatings to match the performance of the inherently superior detonation sprayed coatings by adopting laser glazing as a post-processing step has also been assessed.
Preparation of Bioactive and Porous Titanium Coating by Vacuum Plasma Spraying and Chemical Treatment
Authors: Shi J.M. , Ding C.X. , Wu X.Q.
Published: June 2022

This article describes a method of preparing bioactive and porous titanium coatings on titanium-based substrate using vacuum plasma spraying and chemical treatment in alkali solution. The porous titanium coating was fabricated in two-layer structure. The bond strength, average porosity and roughness (Ra) of the porous titanium coating are 55MPa, 30% and 21µm, respectively. The chemical treatment of as-sprayed titanium coatings was carried out in 5.0M NaOH solutions at 40 °C for 24h. The surface morphology and structure of the porous titanium coating before and after chemical treatment were examined by SEM and laser Raman spectroscopy. The treated titanium coating was immersed into SBF to evaluate its bioactivity by examining apatite formation on its surfaces. It was observed by SEM and TF-XRD that apatite was formed on the surface of the treated titanium coating after immersion in SBF. The spherical aggregates gradually grew large by consuming calcium and phosphate ions in SBF and covered the surface with increase in soaking time. Incorporation of CO32- ions into apatite crystal lattice was revealed by FT-IR. The results obtained indicated that net-like sodium titanate formed due to NaOH attack was responsible for apatite nucleation and growth. It is concluded that vacuum plasma spraying and subsequent chemical treatment is an effective way to produce bioactive and porous titanium coatings.
Evaluation of Sealants on Al-Zn Alloy Sprayed Coating by Galvanostatic Technique
Authors: Tanaka M. , Takatani Y.
Published: June 2022

The sealing effect of various sealants applied to sprayed coatings in sodium chloride aqueous solution was investigated with the galvanostatic technique in which the same quantity of electricity is fed to each test material coated with sealant film cut crossly. The amount of dissolved sprayed coating sealed with sealant can be obtained as the difference between the dissolved material volume at the crosscut part and that in the solution. From the result it was found that the higher the sealing effect of sealant the more aluminum is dissolved from the sprayed coating at the crosscut part, which is considered to mean in case of the sealant poor in sealability, sprayed coating is dissolved even where it is coated with sealant and electrolytic current is dispersed. This proves that the galvanostatic technique is effective in evaluating the sealing effect of sealants.
On the Microstructure of Vacuum Plasma Sprayed Cu-3%Ag Alloy
Authors: Dembinski L. , Grosdidier T. , Coddet C.
Published: June 2022

This study was undertaken to investigate the influence of the initial powder particle size on the microstructure development in a Cu-3wt% Ag alloy obtained by vacuum plasma spraying of a mixture of copper and silver powders. Various mixtures of powder particles were used where the copper powder particle size was kept as –80/+40 μm, while the silver powder particle size fraction was varied. Vacuum plasma spraying (VPS) in inert gas atmosphere was used to elaborate thick coatings (2 mm) with limited oxygen contamination. Significant variations in microstructure were obtained depending on the Ag powder size fraction that was used. These differences in behavior are discussed taking into account differences in the solid state precipitation mechanisms in the alloy.
The Effect of Off-Angle Spraying on the Structure and Properties of HVOF WC/CoCr Coatings
Authors: Ruggiero P.F. , Strock E. , Reynolds D.
Published: June 2022

The aerospace industry currently applies high velocity oxy-fuel (HVOF) coatings to turbine engine, structural, and landing gear components. An increasing demand for HVOF wear resistant coatings to replace electrolytic hard chrome (EHC) on landing gear components has renewed focus on the spray limitations of HVOF WC/CoCr. One such limitation resulting from the line-of-sight HVOF process is the spray angle. In this study, HVOF WC/CoCr coatings were sprayed at several angles while maintaining consistent combustion characterisitics and standoff distance. The measured responses included tensile bond strength, microhardness, residual stress, coating surface roughness, and dry fretting wear resistance. Fatigue response was also of interest, but no results were available at the time this paper was written. The microstructure of each coating was examined, both normal to the surface and in cross section. Coatings sprayed at 90° exhibited the highest microhardness and most compressive residual stress, both considered favorable for good wear response. But these coatings also exhibited the highest as-sprayed roughness, least homogeneous microstructure normal to the surface, and lower wear resistance compared to the off-angle coatings; however, the off-angle coatings apear to cause greater wear of the contacting surface. The microstructural differences among the coatings are related to the measured responses.
Effect of Spraying Distance on Plasma Deposit’s Behavior
Authors: Ctibor P. , Rohan P. , Chráska P.
Published: June 2022

Spraying distance (SD) is one of the main parameters that can affect the spraying process - its efficiency as well as deposit's character. The reason lies in a different thermal history of particles corresponding to different spraying distances. Variation in the structure, preferred orientation, variation of the phase contents and/or in the chemical composition of deposits can be then found for different SDs. Consequently, properties of coatings can greatly vary, not only in the resistance state but also on deposit's annealing. Some materials are, however, more and some less sensitive to that effect. The goal of this work is to compare CaTiO3 samples produced by plasma spraying with WSP at SD = 350 and 450mm with sintered samples. The following properties were compared: microhardness, thermal expansion coefficient, permittivity and reflectivity. Porosity and the differential thermal analysis of resistance and annealed deposits were also compared. Perovskite CaTiO3 belongs among materials very stable during spraying - neither chemical nor phase differences were found between the feedstock powder, free-flight particles, as-sprayed deposit and annealed deposit. Despite that there are significant differences in behavior of deposits and freestanding parts sprayed from different SDs. All the recorded differences for CaTiO3 plasma sprayed deposits with varying SDs must be therefore accounted to the deposit's structural differences, such as pore and splat sizes and shapes and preferred orientation. Additional differences on annealing can be probably attributed to the different amount of "stored" thermal energy in deposits due to the varying SD. However, there is a lower limit for SD assuring a quality deposit on spraying.
Atmospheric Plasma Spraying of Ti Particles — In-Flight Particle Reactivity — Coating Properties
Authors: Ponticaud C. , Grimaud A. , Denoirjean A. , Lefort P. , Fauchais P.
Published: June 2022

Titanium powder has been sprayed with nitrogen or Ar/H2 d.c. plasma jets flowing in air. Particles have been collected at several distances downstream of the nozzle exit. In the first 40- 60 mm, convective movements created within the liquid droplets entrain homogeneously nitrogen and oxygen in the particle cores. Farther downstream, convection is less important and absorption of nitrogen and oxygen is controlled by diffusion from the particles surface. After solidification induced by high quenching rates (in the order of 10 K/s) due to different cooling means, particles are composed by a superficial layer which is an oxi-nitride of titanium and in their core by a solid solution α-Ti containing both nitrogen and oxygen.
Ensemble In-Flight Particle Diagnostics Under Thermal Spray Conditions
Authors: Bisson J.F. , Lamontagne M. , Moreau C. , Pouliot L. , Blain J. , Nadeau F.
Published: June 2022

The Accuraspray is a new in-flight particle sensor that provides information on the average in-flight particle temperature, using two-color pyrometry, and velocity, using a cross-correlation calculation. Various aspects influencing the reliability of the sensor estimates are studied. First, the sensitivity of the temperature and velocity estimates to the positioning of the sensor with respect to the particle jet, such as the angular orientation of the fibers and the working distance to the spray plume, is evaluated. Then, the influence of the plasma radiation on the temperature measurement is estimated. This influence can be reduced significantly by filtering out the low frequency components of the pyrometric signals, which contain most of the plasma fluctuations. Finally, a lower limit in the signal-to-noise ratio (SNR), for which an acceptable temperature estimate is obtained, is evaluated. A valid velocity estimate can still be obtained with a lower SNR. All these studies were performed under various spraying conditions, including plasma spraying and HVOF, using various feedstock materials (YSZ, Al-Si, cermets).
Control of Plasma Sprayed Particles Temperature and Velocity by Chamber Pressure
Authors: Sodeoka S. , Suzuki M. , Inoue T.
Published: June 2022

The effect of chamber pressure on the temperature and the velocity of the plasma sprayed particles was measured quantitatively through the chamber window. As the chamber pressure increases, the plasma flame shortens. At the same time, the particle slows down and the particle temperature increases. The multivariate analysis among the spray parameters and the measured values reveal that the particle temperature and the particle velocity can be controlled individually as a function of the chamber pressure and the spray distance. The zirconia coatings were fabricated by controlling the particle temperature and velocity at the impinging upon the substrate individually. Increase of the particle temperature increased the deposition efficiency, the density and the hardness of the coating. On the other hand, increase of the particle velocity up to a certain value also increased them, but the excessive velocity decreased them. Because the shortened dwelling time prevented the powder core from melting, even if the surface was sufficiently heated.
The Benefit of an On-Line Diagnostic System for the Optimization of Plasma Spray Devices and Parameters
Authors: Refke A. , Barbezat G. , Loch M.
Published: June 2022

The coating characteristics of plasma sprayed layers are strongly related to the spray parameters and the gun design which also determine the state of the sprayed powder particles upon impact. On-line optical measurements of particle velocity and temperature can be used for the optimization of the spray parameters as they provide information of both, characteristic of the coating as well as process stability and performance of the plasma process. With the DPV-2000 diagnostic system the correlation between particle and coating characteristics have been investigated, especially for the new Triplex technology. Differences observed in the distribution of the particle properties between the Triplex and F4 gun are due to different standard spray parameters and correlated to the inherent difference of the operation principle of both plasma guns. The typical three fold symmetry of the Triplex plasma and powder injection is observed within the particle distribution pattern which also depends on the type of powder and spray parameter used. With the help of DPV-2000 measurements it is also possible to demonstrate the excellent process stability of the Triplex gun and to define and adjust parameter windows for different applications in order to obtain the desired range of coating properties. The advantages of the DPV-2000 diagnostic should also contribute to an enhanced basic understanding of new spray processes such as the LPPS Thin Film technology. Experiments have been performed to prove the feasibility of particle diagnostics under these special spray conditions. First results are presented.
Verification of Particle Flux Imaging (PFI), an In-Situ Diagnostic Method
Authors: Zierhut J. , Pellkofer W. , Sagel A. , Haug T. , Landes K.D.
Published: June 2022

The innovative diagnostic system, PFI, records the radiation of the plasma jet as well as of the luminous particle flux. A desktop computer immediately converts these brightness distributions to a set of ellipses. The simple set-up of the system and the fast algorithm enables the utilization of the system in serial production. A statistical design of experiments (DOE) was applied for various process parameters to correlate the determined characteristic ellipses to the process parameters. Measurements of the process by DPV2000 and properties of the resultant coating were used to validate the established correlations. The results demonstrate the suitability of the diagnostic method PFI for quality control and quality assurance in serial production.
HVOF Process Control Using Almen and Temperature Measurement
Authors: Sauer J.P. , Sahoo P.
Published: June 2022

The HVOF process with reduced heat effect on the substrate and therefore minimal degradation of fatigue properties is now finding wide application in fatigue critical applications. The critical parameters for process control are residual stress in the deposit and maximum substrate temperature. Quality control tools for these parameters are deflection of Almen Strips (similar to shot peening) for simulating residual stress and the use of infared pyrometry for temperature measurement. Both of these methods are technically sensitive particularly in spraying of coupons to evaluate the effect of coating on material properties. Lessons learned will be presented and recommendations made for applications of these tools in controlling the HVOF process.