Low Pressure Plasma Spraying Research Articles

Characterization of YSZ Solid Oxide Fuel Cells Electrolyte Deposited by Atmospheric Plasma Spraying and Low Pressure Plasma Spraying

Yttria doped zirconia has been widely used as electrolyte materials for solid oxide fuel cells (SOFC). Plasma spraying is a cost-effective process to deposit YSZ electrolyte. In this study, the 8 mol% Y2O3 stabilized ZrO2 (YSZ) layer was deposited by low pressure plasma spraying (LPPS) and atmospheric plasma spraying (APS) with fused-crushed and agglomerated powders to examine the effect of spray method and particle size on the electrical conductivity and gas permeability of YSZ coating. The microstructure of YSZ coating was characterized by scanning electron microscopy and x-ray diffraction analysis. The results showed that the gas permeability was significantly influenced by powder structure. The gas permeability of YSZ coating deposited by fused-crushed powder is one order lower in magnitude than that by agglomerated powder. Moreover, the gas permeability of YSZ deposited by LPPS is lower than that of APS YSZ. The electrical conductivity of the deposits through thickness direction was measured by potentiostat/galvanostat based on three-electrode assembly approach. The electrical conductivity of YSZ coating deposited by low pressure plasma spraying with fused-crushed powder of small particle size was 0.043 S cm−1 at 100 °C, which is about 20% higher than that of atmospheric plasma spraying YSZ with the same powder.

Nozzle Developments for Thermal Spray at Very Low Pressure

Very low pressure plasma spraying has been the object of numerous studies for the past years. However, numerical simulations and experiments revealed some difficulties such as the presence of successive expansions and constrictions of the plasma jet that have an influence on the deposition efficiency and on the coating structure. Optimization of the plasma gun is thus required and the use of bell-contoured De Laval nozzle extensions seems particularly promising. In this paper, new developments concerning the use of an in-house bell-contoured nozzle extension are presented, and both numerical calculations and experiments were performed.

Hard thick-film and wear resistance of Al–50Si–10M ternary alloys on A6063 aluminum alloy coated by low pressure plasma spraying

Aluminum-based hard thick-films were coated on A6063 aluminum alloy with a binary alloy powder of Al–50Si and three ternary alloy powders of Al–50Si–10Mg, Al–50Si–10Cu and Al–50Si–10Co by low pressure plasma spraying in Ar–H 2 atmosphere. Experimental results show that thick-films were compact without large porosity and cracking in them or at interface between coated films and base metal, and intermetallic compounds of Mg 2Si, CuAl 2 and Co 2Al 9 were dispersedly precipitated in their matrix by X-ray diffraction analysis. Compared with hardness of A6063 base metal (45 HV), hardness of coated films was 250 HV for Al–50Si, 350–400 HV for Al–50Si–10Mg and Al–50Si–10Cu, and 650 HV for Al–50Si–10Co. The hardness was obviously modified by dispersion strengthening mainly from precipitated intermetallic phases and partly from primary Si particles. Wear property was determined by ball-on-disc sliding wear test under Al 2O 3 ball revealed that friction coefficient of coated films was decreased from 0.61 to 0.34 and wear depth was remarkably reduced as one third as that of A6063 since dispersion hardening of those intermetallic phases is beneficial to improvement of wear resistance.

Oxidation Property of CoNiCrAlY Coatings Prepared by Various Thermal Spraying Techniques

To protect various gas turbine components against high temperature in the hot sections of power generation plants and aircraft engines, thermal barrier coatings (TBC’s) have been developed and widely used. Conventional TBC’s consist of a MCrAlY (M: Ni, Co, NiCo, etc,) bond coating for oxidation resistance and a ceramic top coating for thermal insulation. High quality coatings of MCrAlYs have been produced mostly by low pressure plasma spraying but other more economical processes are also used depending on the operating conditions of the component to be coated. In this study, CoNiCrAlY powders were deposited on Inconel 718 substrate with three types spraying system, i.e., low pressure plasma spraying, high velocity oxy-fuel spraying, and atmosphere plasma spraying. Specimens were isothermally tested for up to 100 h in air at 1373 K. Mass gain of the coatings was measured. Microstructure of the coating cross sections and the surface oxides were observed with SEM. To identify the crystal structure of the formed oxides, the specimens were analyzed by XRD from the surface.

Fundamental Coating Development Study to Improve the Isothermal Oxidation Resistance and Thermal Cycle Durability of Thermal Barrier Coatings

Thermal barrier coatings(TBCs) are used in high temperature gas turbines to reduce the surface temperature of cooled metal parts such as turbine blades[1]. TBC consist of a bondcoat (e.g. MCrAlY where M is Co, Ni, CoNi, etc.) and a partially stabilized zirconia ceramic topcoat. Usually, the MCrAlY bondcoat is applied by LPPS (low pressure plasma spray) or HVOF(high velocity oxi-fuel spray). The topcoat is applied by APS (atmospheric plasma splay) or EB-PVD (electron beam-physical vapor deposition). High temperature oxidation properties, thermal barrier properties and durability of TBC are very important to increase the reliability in high temperature service. In this study, new TBC has been investigated. The new TBC consists of a two-layered bondcoat (LPPS-MCrAlY plus dense PVD overlay MCrAlY) and the EB-PVD type YSZ columnar structure topcoat. As a result of evaluation tests, it was confirmed that the new TBC had better oxidation properties and durability than a conventional TBC system.

Hot corrosion behavior of MCrAlY coatings on IN738LC

The hot corrosion behavior of CoNiCrAlY coatings deposited on IN738LC super alloy using low pressure plasma spray (LPPS) was investigated using samples immersed in a solution of Na2SO4–10 wt.% NaCl and dried as to be covered with a 2.5 mg/cm2 costing. Specimens were heat-treated in furnace at 850 °C and after 24 h in the furnace were accurately weighed. Microstructural characterizations were carried out by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) indicated interactions within microstructure of the coating with clearly diminished thickness of MCrAlY coating. Phase transformation and β-NiAl phase depletion in the MCrAlY coatings were shown to be directly related to the thermal cycles experienced by the samples and revealed outward diffusion of Al in the coating and the inward migration of Ni toward the coating causing β→γ′ phase transformation as the main cause of instability of the β-NiAl.

Effects of variations in coating materials and process conditions on the thermal cycle properties of NiCrAlY/YSZ thermal barrier coatings

Thermal cycle tests were conducted on a variety of thermal barrier coating (TBC) specimens with bond coats that had been prepared in different ways. Variables include: (1) different thermal spray processes (high velocity oxy-fuel (HVOF) spray and low pressure plasma spray (LPPS)), (2) different feedstock powder (gas-atomized and cryomilled), (3) the introduction of nano-sized alumina additives (particles and whiskers) and (4) with and without a post-spray vacuum heat treatment. The results show that the cryomilling of the NiCrAlY powder and the post-spray heat treatment in vacuum can both lead to significant improvement in the thermal cycle lifetime of the TBCs. The TBC specimens with LPPS bond coats also generally showed longer lifetimes than those with HVOF bond coats. In contrast, the intentional dispersion of alumina particles or whiskers in the NiCrAlY powders during cryomilling did not result in the further improvement of the lifetime of the TBCs. Microstructural evolution, including the thermally grown oxide (TGO) formation, the distribution of the dispersoids in the bond coat, the internal oxidation of the bond coat, the bond coat shrinkage during the thermal cycle tests and the reduction of the ZrO 2 in the top coat during the heat treatment in vacuum, was investigated.

Wear behavior of low-pressure plasma-sprayed AlCuFe quasicrystalline coating on titanium alloy

The wear-resistant AlCuFe quasicrystalline coating was fabricated on substrate of titanium alloy by low-pressure plasma-spraying (LPPS) method. The LPPS AlCuFe quasicrystalline coating has a rapidly solidified lamellar microstructure consisting of mainly icosahedral phase and small amount of cubic phase peaks. The results showed that AlCuFe quasicrystalline coating improved the wear resistance of titanium-based alloys under dry sliding wear test conditions. The excellent wear resistance may be attributed to the high hardness of AlCuFe quasicrystal and the formation of the tribofilm.

Evaluation of Adhesive Strengths of Plasma-Sprayed CoNiCrAlY Coatings Using an Indentation Method

Thermal barrier coating (TBC) on the element for high temperature service, such as a gas turbine blade, has become an indispensable technology. In this study, adhesive strengths of plasmasprayed CoNiCrAlY coatings were examined using an indentation method. In order to examine the effects of the spraying process on the adhesive strength of a sprayed coating, coatings were deposited by both atmospheric plasma spraying (APS) and low pressure plasma spraying (LPPS). The half number of CoNiCrAlY(LPPS) specimens were thermally aged for diffusion treatment. The load-displacement curves during the indentation were measured, and the delamination energy per unit area of delamination was estimated. The delamination load was high when the distance from an edge of a specimen was long, however the energy per unit delamination area was almost independent of the distance from the edge and was uniquely determined. The delamination energy of CoNiCrAlY(APS) and CoNiCrAlY(LPPS) coatings were found to be approximately the same. The delamination energy of CoNiCrAlY(LPPS) with diffusion thermal treatment was widely scattered as compared with foregoing two coatings, however the energy of the CoNiCrAlY(LPPS) coating with diffusion thermal treatment was found to be about three times higher than those of both CoNiCrAlY(APS) and CoNiCrAlY(LPPS) coatings. It was concluded that diffusion thermal treatment was effective in improving the delamination strength. The CoNiCrAlY(LPPS) coating with diffusion thermal treatment was also found to be effective in improving the fatigue fracture life of a thermal-barrier-coated material.

Comparison of Microstructure and Oxidation Behavior of CoNiCrAlY Bond Coatings Prepared by Different Thermal Spray Processes

To protect various gas turbine components against high temperature in the hot sections of power generation plants and aircraft engines, thermal barrier coatings (TBCs) have been developed and widely used. Conventional TBCs consist of a MCrAlY bond coating for oxidation resistance and a ceramic top coating for thermal insulation. High quality coatings of MCrAlYs have been produced mostly by low pressure plasma spraying but other more economical processes are also used depending on the operating conditions of the component to be coated. In this study, CoNiCrAlY powders were deposited on Inconel 718 substrate with 3 types spraying system, i.e., low pressure plasma spraying, high velocity oxy-fuel spraying, and atmosphere plasma spraying. The specimens without top ceramic coating were isothermally tested for up to 100 hrs in air at 1373 K and mass gain of the coatings was measured. Microstructure of the coating cross sections and the surface oxides were observed with SEM. Moreover, phase changes during the oxidation test were investigated with calculated phase diagrams for the CoNiCrAlY alloy. [doi:10.2320/matertrans.47.1638]

Microstructural Evolution and Tensile Properties of Ti-Al-V Alloys Manufactured by Plasma Spraying and Subsequent Vacuum Hot Pressing

Ti-Al-V preforms were manufactured by low pressure plasma spraying (LPPS) from two different sizes of Ti-6Al-4V feedstock powders, and subsequently consolidated by vacuum hot pressing (VHP). During LPPS, a loss of alloying elements and an incorporation of [H] and [O] occurred, which were much significant for a smaller feedstock powder. Subsequent VHP completely removed [H] content, whereas a considerable amount of [O] remained. The microstructure of LPPS deposits comprised a splat-quenched lamellar structure, incorporating unmelted particles. The each splat contained � 0 martensite plates and Ti hydrides which provided heterogeneous nucleation sites for

Microstructural Investigation of CoNiCrAlY Coated Ni-Based Single Crystal Superalloy Prepared by LPPS

Recent demand in increasing the efficiency of gas turbines has led the component materials to be exposed at much higher operating temperatures, which accordingly accelerates the microstructural deterioration of the coated materials, mainly caused by the interaction between the coated layer and the substrate. Therefore, controlling the chemistry of interfaces by the coating process is one of the most important keys to minimizing the microstructural changes during service. In this study, effects of surface treatments and coating conditions on microstructure changes of Ni-based superalloy substrates were investigated. CoNiCrAlY (AMDRY 9954) powder was coated on Ni-based single crystal superalloy TMS-82+ by low pressure plasma spraying (LPPS). It was found that grit-blasted treatment drastically distorted the coherent γ⁄γ′ microstructure of substrates, which accordingly promoted the uniform and accelerated precipitation of topologically close-packed (TCP) phases by the post heat treatment at 1273 K for 30 min. On the other hand, specimens without the grit-blast treatment had less amount of TCP precipitates, but showed preferred precipitation orientation along 〈011〉{100} direction. Surface preheating and surface sputter cleaning seemed to have less influences on microstructure change compared to the grit-blast treatment.

Preparation and Evaluation of Ordinary Attritor Milled Ti-Al Powders and Corresponding Thermal Sprayed Coatings

Ordinary attritor milling of elemental metallic powders under atmospheric condition was utilized to prepare desirable amount of powders for thermal spraying. The effect of different BPR (Ball to Powder weight Ratio) has been investigated in terms of nitridation during milling. To investigate the effect of heat treatment on the formation of dispersed phases, heat treatment to the powder was performed as well. Titanium aluminide coatings with carbonitride dispersed phases were successfully fabricated by low pressure plasma spraying. The hardness and specific wear of the coatings prepared by the powders with different milling conditions was measured so as to investigate the effect of the content of dispersed titanium based carbonitride phases. Experimental results show that the formation of dispersed carbonitride phases depends strongly on milling condition, irrespective of heat treated powders or thermal sprayed coatings, and directly affects the mechanical properties of the coatings. Compared with the phase composition of heated powders and corresponding thermal sprayed coatings, it seems that the temperature of processing the MA powders is also a decisive factor on the phase formation, especially carbonitride phases and oxide phase. [doi:10.2320/matertrans.47.1717]

ダイヤモンド研磨用TiAl溶射皮膜の創製およびその特性評価

TiAl intermatallic compound coatings for diamond polish were fabricated by mechanical alloying (MA) and thermal spraying in this study. Titanium powder and aluminum powder were mixed by MA method. Then the MA powder was used as the feedstock powder for low pressure plasma spraying (LPPS). It was possible to fabricate thick coatings with dense microstructure by controlling of MA time, spraying distance and chamber pressure. Furthermore, almost completely TiAl constituent coating was fabricated by controlling of Ti and Al powder mixing ratio upon the raw material of MA. The TiAl coating had good diamond polishing property. Therefore, thermal sprayed TiAl coating is suitable for the diamond polisher.

Evaluating Strength of Thermally Sprayed Al2O3 on Aluminum

We evaluated the strength of thermally sprayed Al2O3 on aluminum. The thermally sprayed Al2O3 films were processed using low-pressure plasma spraying. The thickness of the thermally sprayed Al2O3 was 0.3 mm and 0.7 mm. We used a 4-point bending test and a heating test to evaluate the strength of the thermally sprayed Al2O3. We also investigated the effect of residual stress on the strength by measuring deformation of the thermally sprayed Al2O3 after removing the aluminum substrates. The bending strength was 120 MPa, regardless of thickness. We assumed that the bending strength equaled tensile strength because the thermally sprayed Al2O3 films were very thin. A crack was generated at 433 K, regardless of thickness. The thermal stress was 160 MPa when the crack was generated. It was 40 MPa higher than we estimated. We found that the residual stress was compression stress that measured 40 MPa, which contributed to the prevention of the crack generation. We presume that the tensile strength was lower than the thermal stress because the residual stress was reduced by the relaxing of the stress of the aluminum near the interface in the bending test.

Inclusion of Aerodynamic Non-Equilibrium Effects in Supersonic Plasma Jet Enthalpy Probe Measurements

Low pressure plasma spraying (LPPS) is a thermal spraying technique that has found a niche for low oxidation products. It uses a low pressure environment (i.e., chamber pressure between 2 and 90 kPa) and yields supersonic plasma jets. The enthalpy probe technique is a common measurement method in plasmas. However LPPS jets are difficult to diagnose as their supersonic nature forces the apparition of a shock wave in front of any measuring device inserted in the jet. Incomplete or erroneous assumptions are usually invoked to overcome the difficulties associated with this shock wave and carry out the LPPS jet diagnosis from enthalpy probe measurements. In this work, a new device is designed to gain access to an additional physical quantity, which is needed to assess the aerodynamic non-equilibrium state of the jet. It is combined with enthalpy probe measurements, and the resulting set of experimental data is used with a numerical procedure based on gas dynamics theory, yielding free-stream supersonic plasma jet values from the measurements behind the induced shock wave. The results agree well with the phenomenology of supersonic jets in aerodynamic nonequilibrium. However this new method is restricted by the local thermodynamic equilibrium assumption, which is directly linked with the pressure and temperature conditions of the plasma jet.

Non-equilibrium Microstructure and Thermal Stability of Plasma-sprayed Al–Si Coatings

A splat-quenched, thick Al–Si deposit was manufactured by low-pressure plasma spraying (LPPS) and investigated in terms of microstructural inhomogeneity, Si solid solubility in α–Al, formation of metastable phases, and thermal stability. The LPPS Al–Si deposit had an inhomogeneous, layered microstructure consisting of splat-quenched lamellae and the incorporation of unmelted or partially melted particles. The splat-quenched Al–Si lamellae were formed by deposition of a fully liquid droplet and had an almost featureless microstructure at relatively low magnifications. There was a significant reduction in the α–Al lattice parameter in the LPPS Al–Si deposit because of extended Si solubility in the α–Al matrix. Transmission electron microscopy investigations showed that the splat quenching of liquid Al–Si droplet led to (i) columnar grain growth of α–Al(Si), (ii) formation of nano-sized Si precipitates in the Al matrix which was supersaturated with Si; and (iii) formation of amorphous Si phase embedded in the crystalline Al matrix. On reheating, the amorphous Si transformed into fine crystalline Si by interdiffusion of Al and Si atoms. Simultaneously, Si precipitation occurred in the supersaturated α–Al matrix. The overall activation energy for the Si crystallization/precipitation was estimated as ∼81 kJ/mol from a modified Kissinger analysis.

Electrochemical behavior of low-pressure plasma-sprayed Al–Cu–Fe–Cr quasicrystalline coating

Al–Cu–Fe–Cr quasicrystalline coating was deposited on a substrate of stainless steel by low-pressure plasma spraying (LPPS) method. The corrosion behavior of such coating was studied by polarization in 1 mol/l H 2SO 4 and 0.1 mol/l NaOH solutions at room temperature. The polarization curve shows that LPPS Al–Cu–Fe–Cr quasicrystalline coating can turn to passive state both in 1 mol/l H 2SO 4 solution and in 0.1 mol/l NaOH solution. The corrosion resistance of the coating is poorer than that of bulk quasicrystal in 0.1 mol/l NaOH solution. Also in strong alkaline solution the corrosion resistance of the coating is much poorer than single-phase austenitic stainless-steel 1Cr18Ni9Ti. In the later period of polarization, the Al 2O 3 layer formed on surface of the coating is destroyed by localized corrosion.

Corrosion of Al-Cu-Fe-Cr Quasicrystalline Coating

Al-Cu-Fe-Cr quasicrystalline coating was deposited on a substrate of stainless steel by low-pressure plasma spraying (LPPS) method. The corrosion behavior of such coating was studied by polarization in 1mol/l H2SO4 and 0.1mol/l NaOH solutions at room temperature. The polarization curve shows that LPPS Al-Cu-Fe-Cr quasicrystalline coating can turn to passive state both in 1mol/l H2SO4 solution and in 0.1mol/l NaOH solution. The corrosion resistance of the coating is poorer than that of bulk quasicrystal in 0.1mol/l NaOH solution. Moreover, in strong acid solution LPPS Al-Cu-Fe-Cr quasicrystalline coating has more corrosion resistance than 1Cr18Ni9Ti in some potential range ranging from -200mVSCE to -35mVSCE, but in strong alkaline solution the corrosion resistance of the coating is poorer than 1Cr18Ni9Ti.

Investigation of an as-sprayed NiCoCrAlY overlay coating - microstructure and evolution of the coating

A low pressure plasma sprayed (LPPS) NiCoCrAlY aircraft turbine blade overlay coating was investigated in the as-sprayed condition by X-ray diffraction (XRD) and analytical transmission electron microscopic (TEM) techniques. γ-Ni, β-NiAl and γ′-Ni3Al phases were identified by XRD showing predominant γ phase at the expense of β phase if compared to the fully processed state with γ and β at about equal portions. Besides grains of γ-Ni and β-NiAl, amorphous metallic grains and Cr-rich oxides were found by TEM. Close to the surface to the atmosphere γ′-Ni3Al phase was localized in composite grains with an off-plane oriented γ-γ′ fibrous eutectic structure. They were neighbored by amorphous metallic grains. The evolution of these structures is discussed within the scope of crystal growth behavior. The dominant occurrence of face-centered cubic at the expense of body-centered phases observed in as-sprayed coatings is attributed to their higher growth rates on quenching. Also benefiting from rapid growth the γ-γ′ fibrous eutectic grains will form, hereby relying on essentially binary phase compositions of highly extended solubility ranges. Accumulation of alloy constituents like Cr and Ti by segregation to the growth front is considered to pave the way for the evolution of amorphous grains at the coating surface on top of γ-γ′ grains. The potentials of the microstructures in service are addressed.