Plasma Spraying Of Powders Research Articles

Effects of Humidity on the Flow Characteristics of a Composite Plasma Spray Powder

The effects of environmental humidity on the flow characteristics of a multicomponent (composite) plasma spray powder have been investigated. Angular and spherical BaF2−CaF2 powder was fabricated by comminution and by atomization, respectively. The fluorides were blended with nichrome, chromia, and silver powders to produce a composite plasma spray feedstock. The tap density, apparent density, and angle of repose were measured at 50% relative humidity (RH). The flow of the powder was studied from 2 to 100% RH. The results suggest that the feedstock flow is only slightly degraded with increasing humidity below 66% RH and is more affected above 66% RH. There was no flow above 90% RH except with narrower particle size distributions of the angular fluorides, which allowed flow up to 95% RH. These results offer guidance that enhances the commercial potential for this material system.

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.

Laser treatment of plasma-sprayed ZrSiO 4 coatings

Self-supported zircon (ZrSiO 4) coatings have been deposited by means of atmospheric pressure plasma spraying, a high rate deposition method. Zircon is one of the technologically important oxide ceramic material known for its refractoriness, its low thermal expansion and low thermal conductivity. However, it is well known that ZrSiO 4 dissociates into ZrO 2 and SiO 2 in the high temperature plasma torch during plasma spraying, the rapid quenching preventing reverse combination of both components into ZrSiO 4. Usually, high temperature annealing (about 1500 °C) is applied in order to combine SiO 2 and ZrO 2 into ZrSiO 4. In this contribution, we investigate an attractive technological alternative to recombine SiO 2 and ZrO 2 into ZrSiO 4 by laser treatment with a scanning continuous wave CO 2 laser. We also investigate the addition of SiO 2 rich glassy particles to the plasma spray powders in order to facilitate the recombination of ZrO 2 and SiO 2 into ZrSiO 4 during laser treatment. By carefully adjusting the CO 2 laser treatment parameters (laser power density, scanning speed) we show that the SiO 2 and ZrO 2 phases indeed recombine into ZrSiO 4. It appears in this contribution that the addition of low melting point SiO 2 rich particles strongly benefits the reaction of zirconia and silica into zircon.

Glass-alumina composite coatings by plasma spraying. Part I: Microstructural and mechanical characterization

Composite coatings on ceramic tiles have been produced by plasma-spraying waste glass powders together with alumina as reinforcing phase. Obtaining a tough coating, in which a high amount of waste glass can be recycled, is the aim of the study. Glass powders with different particle size distributions were tested; coatings with increasing glass volume fractions (0%, 20%, 40%, 60%, 80%, 100%) were sprayed. Furthermore, two different kinds of alumina feedstocks were employed, namely, a commercial plasma-spray powder and a more economical laboratory manufactured spray-dried powder. Vickers microhardness, fracture toughness and abrasion resistance have been measured. The coatings were tested both in as-sprayed condition and after a thermal treatment. An abrasion resistance comparable to common industrial glazes has been obtained with as much as 60% of glass volume fraction, employing fine glass powders (< 45 μm). No significant decrease in mechanical properties has been noticed replacing the commercial alumina powder with the spray-dried one.

Microstructural and mechanical properties of nickel-base plasma sprayed coatings on steel and cast iron substrates

This paper studies the phase composition, microstructure, hardness and wear resistance of nickel-base plasma sprayed coatings, on two types of substrates, steel and cast iron. The processing method of metal and compound powders for plasma spraying is detailed, together with the characteristics of the powders before and after processing. It has been found that in as-coated condition, the surface layers have phase composition of crystalline γ-Ni solid solution and amorphous phases on steel substrate. On cast iron substrate, in addition to these, Ni 3B, CrB, Cr 23C 6 have been identified. The effect of heat treatment of coatings on steel substrate at temperatures up to 1200 °C has been investigated. It has been found that the coatings have high values of hardness and wear resistance. There are changes in the phase compositions on the surface of the Ni-base coatings during wear testing upon the effect of friction heat, and the wear volume after 60 min for the coating on cast iron substrate is much lower than for those on steel substrate.

Superplasticity of zirconia–alumina–spinel nanoceramic composite by spark plasma sintering of plasma sprayed powders

Zirconia 3 mol% yttria–alumina–alumina magnesia spinel nanoceramic composite was synthesized by spark plasma sintering of plasma sprayed particles. For compacts sintered from high energy ball milled powders, superplasticity was observed at temperatures between 1300 and 1450 °C and at strain rates between 10 −4 and 10 −2 s −1, while for those without high energy ball milling, deformation at the same temperature and strain rate range did not show superplastic behavior. Also, the apparent activation energy (945 kJ/mol) of the high energy ball milled batch was much higher than that of the same composite processed from nanopowder mixtures (621 kJ/mol). The flow stresses were also higher at the same temperatures and strain rates. The difference may be related to the unique low angle grain boundaries in the grains that nucleated and grew from the metastable phase inside the plasma sprayed agglomerate at elevated temperatures. Such boundaries were not intrinsically easy to slide.

Effect of Particle Morphology on Flow Characteristics of a Composite Plasma Spray Powder

The effects of BaF2-CaF2 particle morphology on National Aeronautics and Space Administration (NASA) PS304 feedstock powder flowability were investigated, BaF2-CaF2 eutectic powders were fabricated by comminution (producing an angular morphology) and by gas atomization (producing a spherical morphology). The fluoride powders were added incrementally to the other powder constituents of the NASA PS304 feedstock, (Ni-Cr, Cr2O3, and Ag powders). A linear relationship between flow time and concentration of the BaF2-CaF2 powder was found. The flow of the powder blend with spherical BaF2-CaF2 was better than that with angular BaF2-CaF2. The flowability of the powder blend with angular fluorides decreased linearly with increasing fluoride concentration. However, the flow of the powder blend with spherical fluorides was independent of fluoride concentration. The results suggest that for this material blend, particle morphology plays a significant role in flow behavior, offering potential methods to improve powder flowability and enhance the commercial potential. These findings may be applicable to other difficult-to-flow powders such as cohesive ceramics.

Plasma-sprayed human bone-derived hydroxyapatite coatings: effective and reliable

Human bone-derived hydroxyapatite (HHA) was used as plasma-spraying powder in this study. The starting powder was derived with the calcination method as described in recent literature. There have been very few researchers who were seriously interested in calcined HHA as a grafting material. Instead, demineralized HHA (from other humans) was usually preferred among surgeons. This brings some possible danger of transmission of infectious diseases like hepatitis B and C, human immunodeficiency virus (HIV), and almost incurable prion diseases like Creutzfeldt–Jakob disease (CJD) and bovine spongiform encephalopathy (BSE). Due to very high temperature application (850 °C), calcined HA seems to be safe and promising as an alternative product to other HA materials in the biomaterial market. As it is known, HA is very weak at load-bearing applications. To overcome this weakness, it is suggested to use it as a coating on metallic substrates. The HA plasma spraying process is a well-known process for such durable coatings. In addition, in order to gain higher strength, it is recommended to use a very thin bond-coating layer to prevent the mismatch of the thermal coefficients between metal and ceramic. Bond coatings are used very widely in industrial applications. Some biomedical bond-coating research has been conducted, but the reports are not yet commercialized. In this study, tensile strength, SEM and X-ray diffraction studies were performed. Bonding strength was found to be 6.91 MPa for nonbond-coated and 13.15 MPa for bond-coated HHA coatings. These results indicate that the application of bond coating with HHA had strengthened the structure of this material. The X-ray diffraction analysis showed a relatively high crystalline phase. SEM pictures indicate very good adhesion of the bond coating on the metal and on the main coating. The production of HHA with the calcination method is economic, and the resulting material (in combination with plasma spraying) can be used for bone grafting purposes.

Plasma-sprayed human bone-derived hydroxyapatite coatings: effective and reliable

Human bone-derived hydroxyapatite (HHA) was used as plasma-spraying powder in this study. The starting powder was derived with the calcination method as described in recent literature. There have been very few researchers who were seriously interested in calcined HHA as a grafting material. Instead, demineralized HHA (from other humans) was usually preferred among surgeons. This brings some possible danger of transmission of infectious diseases like hepatitis B and C, human immunodeficiency virus (HIV), and almost incurable prion diseases like Creutzfeldt–Jakob disease (CJD) and bovine spongiform encephalopathy (BSE). Due to very high temperature application (850 °C), calcined HA seems to be safe and promising as an alternative product to other HA materials in the biomaterial market. As it is known, HA is very weak at load-bearing applications. To overcome this weakness, it is suggested to use it as a coating on metallic substrates. The HA plasma spraying process is a well-known process for such durable coatings. In addition, in order to gain higher strength, it is recommended to use a very thin bond-coating layer to prevent the mismatch of the thermal coefficients between metal and ceramic. Bond coatings are used very widely in industrial applications. Some biomedical bond-coating research has been conducted, but the reports are not yet commercialized. In this study, tensile strength, SEM and X-ray diffraction studies were performed. Bonding strength was found to be 6.91 MPa for nonbond-coated and 13.15 MPa for bond-coated HHA coatings. These results indicate that the application of bond coating with HHA had strengthened the structure of this material. The X-ray diffraction analysis showed a relatively high crystalline phase. SEM pictures indicate very good adhesion of the bond coating on the metal and on the main coating. The production of HHA with the calcination method is economic, and the resulting material (in combination with plasma spraying) can be used for bone grafting purposes.

Spark plasma sintering (SPS) consolidated ceramic composites from plasma-sprayed metastable Al2TiO5 powder and nano-Al2O3, TiO2, and MgO powders

Abstract Spark plasma sintering (SPS) was applied to consolidate the nano-Al 2 O 3 , TiO 2 , MgO powders and plasma-sprayed metastable Al 2 TiO 5 powder. The fully dense ceramics had been prepared successfully. It was found that the starting nano-Al 2 O 3 reacted with nano-TiO 2 powder to form the Al 2 TiO 5 phase after SPS consolidation at 1150 °C under a pressure of 63 MPa. The starting plasma-sprayed Al 2 TiO 5 was stable above 1150 °C after SPS under 63 MPa, though in oxidizing atmospheres under the ambient air pressure, the stable temperature range of Al 2 TiO 5 was above 1280 °C. There was a fast decomposition rate for plasma-sprayed Al 2 TiO 5 at 1000 °C during SPS, whereas in oxidizing atmospheres under the ambient air pressure at 1000 °C its decomposition is very slow. The microstructure of powder, applied pressure, and/or SPS characteristics, etc. obviously had an effect on the stable temperature and/or decomposition rate of Al 2 TiO 5 . The addition of MgO advanced the formation of Mg 0.3 Al 1.4 Ti 1.3 O 5 along with MgAl 2 O 4 in the ceramics prepared both from plasma-sprayed powder and from nano-powder. In comparison with the composition of the ceramics without MgO additive, α-Al 2 O 3 , TiO 2 , and Al 2 TiO 5 , the ceramics with MgO additive consisted of only two phases, Mg 0.3 Al 1.4 Ti 1.3 O 5 and MgAl 2 O 4 spinel. The small equiaxed MgAl 2 O 4 spinel grains were distributed in a matrix of Mg 0.3 Al 1.4 Ti 1.3 O 5 with a large grain size.

Influence of injection mode on properties of DC plasma jets for thermal plasma spraying

In plasma spraying powder particles are transported to the plasma jet with the help of a carrier gas. The influence of this gas was investigated by means of an enthalpy probe system with a mass spectrometer for measuring plasma temperature, velocity and plasma gas composition, and visualized by means of Schlieren optics. The enthalpy probe system does not allow measurements of the plasma flow containing solid particles. Therefore, to establish changes of the jet characteristics the carrier gas was supplied through different ports without addition of any powders. Nitrogen and helium were used as carrier gases. They were supplied into the jet with flow rates from 5 to 20 slpm either directly into the plasma beam through a hole in the nozzle or with help of an external injector positioned at a distance of several millimeters from the exit nozzle. Injection of the carrier gas led to high jet perturbations. Values of the centerline temperature and the velocity were reduced. The higher the carrier gas flow rate the stronger the changes of the jet properties. Nitrogen carrier gas perturbed plasma jet flow more than helium.

Microstructural characterization of plasma-sprayed nanostructured zirconia powders and coatings

Abstract The objective of this paper was to study the melting state of nanoscale zirconia powders in plasma jet, examine the crystalline phase and microstructural characterization of the plasma sprayed nanostructured zirconia powders and coatings. The cross-section morphology of water-quenched powders revealed that most of the starting powders were molten in the plasma jet. Phase analysis using XRD spectra proved that the monoclinic phase zirconia of the starting powders transferred into tetragonal phase zirconia after the plasma spraying. SEM analysis indicated that the as-sprayed coatings exhibited lamellar structure, about 0.5–5 μm in thickness of a single lamella, which was found to be a characteristic feature of the plasma-sprayed nanostructured zirconia coating. Inside of the lamellae, columnar grains were observed. The mean grain size of the as-sprayed zirconia coating is about 80 nm.

Characterization of plasma sprayed wollastonite powder and coatings

Spheroidized wollastonite powder was prepared by plasma spraying natural wollastonite TC powder into water. The natural and spheroidized wollastonite powders were deposited onto Ti–6Al–4V substrates using an atmospheric plasma spraying system. The phase composition and microstructure of the powders and coatings were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The spheroidized powder prepared by plasma spray presented a sphere-like morphology and was composed of crystalline wollastonite and a little glassy phase. The SEM view of the surface and the cross-section of the coatings revealed that the coating produced with the spheroidized wollastonite powder possesses a denser structure with fewer microcracks and pores than the coating generated from the natural powder. The tensile adhesion test (TAT) specified by ASTM C-633 was used to measure the tensile bonding strength of the coating. The results indicated that the coating produced with the spheroidized powder is superior to the coating prepared from the natural powder.

In Situ Characterization of Small-Particle Plasma Sprayed Powders

The effect of various small-particle plasma spray powder injection parameters on the in situ particle position, velocity, and temperature is measured for yttria-stabilized zirconia and yttrium-aluminum-garnet powder. Using full-factorial experiments and multiple regression analysis, carrier gas flow, injector angle, and powder feeder disc speed were found to significantly affect the particle properties. Temperature and velocity were inversely related; on average, the cooler particles traveled faster. These properties also correlated to the particle position in the flame, where particles above the centerline of the flame traveled faster. The trends are discussed on the basis of residence time in the flame, as well as in terms of particle size segregation effects. Coating density and splat geometry reflect the temperature and velocity differences between the runs. Slower, hotter particles possessed more intrasplat and intersplat porosity and less splat-substrate contact area, leading to lower overall coating density.

Modeling of Oxidation During Plasma Spraying of Iron Particles

The plasma-sprayed metallic powder deposits contain usually some amounts of oxides originating from a gas–liquid reaction after melting the particles flying in the plasma plume, and a gas–solid reaction after the particle impact and solidification. In the present paper, a model of the in-flight oxidation of molten metallic paricles is described. Only the growth of a continuous oxide layer on the molten particle surface is assumed, whereas melt convection and oxide nodule formation inside the particles are not taken into consideration. The model is applied to spraying of pure iron powder by a water-stabilized plasma gun. After the presumed flight time, the calculated and measured values of the oxide thickness are of the same order of magnitude.

Characterization of hydroxyapatite: before and after plasma spraying.

Hydroxyapatite (HA) powder was characterized before and after plasma spraying using infrared spectroscopy and thermoanalytical techniques. Structural changes were observed from the infrared spectra for the plasma-sprayed powder. These changes involved dehydroxylation and misoriented hydroxyl ions with different configurations. Dehydroxylation was accompanied by decreased intensities for the O-H stretching mode at 3571 cm(-1) and the O-H librational mode at 633 cm(-1). A broad infrared band near 3400 cm(-1) that was observed after spraying was attributed to the misoriented hydroxyl ions. The combined results from thermoanalytical techniques indicated that adsorbed water evolved in three stages below 500 degrees C, and that the dehydroxylation of HA started as low as 700 degrees C. A weight gain of the plasma sprayed HA was observed above 500 degrees C in inert atmospheres. This result indicates a strong tendency of dehydroxylated HA to restore hydroxyls. The decrease in the decomposition temperature of HA after spraying was attributed to largely perturbed structures.

Sliding and Abrasive Wear Resistance of Thermal-Sprayed WC-CO Coatings

We studied the resistance of the coatings to abrasive and unlubricated sliding wear of 40 WC/Co coatings applied by high velocity oxygen fuel (HVOF), high-energy plasma spray (HEPS), and high velocity plasma spray (HVPS), using commercial and nanostructured experimental powders. The hardness of the coatings varies from 3 to 13 GPA, which is much lower than that of sintered samples (10 to 23 GPA) because of the porosity of the coatings. Phase analysis by x-ray diffraction revealed various amounts of decarburization in the coatings, some of which contain WC, W2C, W, and η phase. The abrasive and sliding wear resistance is limited by the hardness of the samples. For a given hardness, the wear resistance is lowered by decarburization, which produces a hard but brittle phase. Nanocarb powders have the shape of thin-walled hollow spheres that heat up rapidly in the gun and are more prone to decarburization than commercial materials. The work shows that, in order to obtain the performance of nanostructured coatings, the powder and spray techniques must be modified.

Processing of aluminium-silicon carbide powders for plasma spraying : K.Ghosh et al. (University of British Columbia, Vancouver, Canada.) Int J. Powder Metall., vol 25, no 2, 1999, 27–35

Processing of aluminium-silicon carbide powders for plasma spraying : K.Ghosh et al. (University of British Columbia, Vancouver, Canada.) Int J. Powder Metall., vol 25, no 2, 1999, 27–35

Application of mechanically alloyed titanium-aluminium powders for plasma spraying

Application of mechanically alloyed titanium-aluminium powders for plasma spraying

Zirconates as New Materials for Thermal Barrier Coatings

Zirconates with high melting points were investigated for application as materials for thermal barrier coatings at operating temperatures &gt;1300°C. SrZrO3, BaZrO3, and La2Zr2O7 powders were synthesized and sintered to compacts with various levels of porosity. The sintering results indicated a promising low‐sintering activity of the investigated materials. Thermal properties of these dense materials were determined. Thermal expansion coefficients were slightly lower than those of Y2O3‐stabilized ZrO2 (YSZ); thermal conductivities of SrZrO3 and BaZrO3 were comparable or slightly higher than those of YSZ. La2Zr2O7 had a lower thermal conductivity. SrZrO3 was not suitable for application as a thermal barrier coating because of a phase transition at temperatures between 700° and 800°C. Mechanical properties (hardness, fracture toughness, and Young's modulus) of dense BaZrO3 and La2Zr2O7 samples were determined by indentation techniques and showed lower hardness and Young's modulus compared to YSZ. BaZrO3 and La2Zr2O7 powders were optimized for application as powders for plasma spraying. Plasma‐sprayed coatings were produced and characterized. Thermal cycling with a gas burner at 1200°C showed a massive attack of the BaZrO3 coating, with loss of BaO. On the other hand, the La2Zr2O7 coating showed excellent thermal stability and thermal shock behavior.