Nanostructured YSZ coatings were deposited by plasma spraying at different pressures and plasma currents. Coatings sprayed at low pressures exhibited fine equiaxed grain structures with consolidated coarser grains and loose-packed nano granules. Especially at the low end of the current range, larger amounts of crystalline grain fell into the nanoscale, significantly reducing the average grain size in the coating. At the high end of the range, the coating exhibited a uniform fine equiaxed grain structure of around 200 nm. Mechanical properties, including average microhardness and elastic modulus, were also measured and compared. LPPS YZS coatings showed slightly higher microhardness, but half the elastic modulus of APS YSZ layers.

This study investigates the influence of substrate preheating on the microstructure, hardness, and adhesion of detonation sprayed WC-CoCr coatings. Using commercially available powders, coating samples are deposited on low carbon steel substrates, some at room temperature and some having been preheated to 300°C. Test results show that the coatings deposited on preheated substrates are harder, more crack resistant, and better adhered, with no significant differences in microstructure. After tensile testing, fracture surfaces and interfaces were investigated, showing how fracture behavior, along with hardness and bonding strength, correlate with phase composition and particle impact conditions.

The aim of this work is to fabricate nanostructured ceramic coatings using unsintered agglomerated powder and to characterize differences in microstructure, especially those at the nanoscale, due to spraying conditions. Feedstock powders were prepared from commercial YSZ nanoparticles that were reconstituted into solid spheres (70-100 μm) by spray drying. The surface morphology of sintered and unsintered agglomerates was examined by FE-SEM prior to deposition by atmospheric plasma spraying using two gun configurations, one with a lengthened barrel and one with water cooling. YSZ coating cross-sections were examined by optical and electron microscopy, revealing details at the micro and nano scale. The results show that the unsintered agglomerates, which were successfully deposited using both spray guns, are favorable for developing bimodal coating structures with fine grains and porous nano zones.

A plasma torch with a converging-diverging nozzle has been developed for low pressure plasma spraying. This study investigates the current-voltage characteristics of the plasma arc for argon and argon-hydrogen plasma gases and the effect of hydrogen volume percentage on the plasma jet. Emission spectroscopy is used to analyze the plasma spectra and electron temperatures in the center of the plasma jet are determined based on a Boltzmann distribution. The results show that an increase in input power considerably increases electron temperature and that gas composition has a significant effect on current-voltage characteristics. The impact of detection distance is also addressed.

In this study, an internal injection plasma torch is used to deposit nano-agglomerated YSZ feedstock powders on superalloy substrates at low ambient pressures ranging from 5000 Pa to 6000 Pa. Coatings with unique fully nano-equiaxed structures were obtained when operating the torch below 300 A. With increasing current, up to 700 A, coatings with mixed-grain and eventually large-grain structures were produced. Experimental results suggest that the equiaxed nanoscale structure derives from the original agglomerated nanoparticles that had undergone melting while inside the nozzle of the plasma torch and were subsequently solidified or sintered in the coating. Coating hardness and elastic modulus were also measured and are shown to correspond with microstructure.

This study investigates the spraying characteristics of low-pressure plasma torches with different nozzle sizes. YSZ feedstock powders were sprayed with each torch at different stand-off distances and gas pressures. The plasma jets created were photographed and measured, showing that low-pressure spraying significantly increases plume length and diameter compared to atmospheric conditions. The coatings obtained were examined and microhardness was determined. It was found that the longer nozzle increases the temperature of the plasma jet, and with a longer dwell time, the particles heat more efficiently and evaporate more fully. At a spraying distance of 300 mm, the coatings were mostly composed of equiaxed grains, which were much larger in the coatings produced with the long anode nozzle. At longer spraying distances, more unmelted particles appeared in the coatings, leading to a reduction in hardness.

In contrast with the atmosphere plasma spraying (APS), the very low pressure plasma spraying (VLPPS) shows lower ability for heating spray materials at the plasma free region exit of nozzle. Most very low pressure plasma spraying at the present usually use a high power plasma gun which operated at great arc currents up to 2500A to meet lower ability for heating materials, thus result in a series of problems not only for the plasma torch but also for associated facility. According to the plasma spray characteristic in the low pressure environment we designed a plasma torch with separating anode and nozzle, and the powder injected to the plasma jets is from inside nozzle intake. In this study, the pressures in the plasma gases intake, nozzle intake and out of the plasma torch were measured respectively. For the practice, SUS 316 stainless steel coatings were prepared at plasma currents of 500-600A and arc voltage of 50V and the chamber pressure of 1000 Pa, the results indicated that the coatings with the equiaxed microstructure could deposited in the appropriate conditions.

NiCrAl/ZrO 2 -8Y 2 O 3 coatings deposited on SUS304 stainless steel and 45 carbon steel substrates were prepared by APS at different preheating temperatures, of which thickness exceeded 1mm. This study analyzed the coatings’ separation from different preheated substrates in the cooling process after spraying due to residual thermal stress. The Young’s modulus of the porous YSZ coatings was calculated and also measured by Knoop indentation methods for comparison purposes. The result indicated that the failure of porous thick YSZ coatings is mainly caused by the cracks nucleation, propagation and coalescence, which is related to the thermal-expansion coefficient difference between substrate and coatings, preheating temperature, porosity of coatings and so on. Due to their increased porosity, the porous and thick YSZ coatings had much lower calculated and measured Young’s modulus values than the sintered YSZ coatings.

The FeAl intermetallic compound coatings were sprayed by low pressure plasma spraying (LPPS), air plasma spraying (APS) and high velocity oxy-fuel spraying (HVOF). The influence of three kinds of thermal spraying processes on the microstructure, microhardness, elastic modulus and fracture toughness of coatings were investigated. The results showed that the APS and HVOF coatings exhibited similar microhardness, about 540HV 0.3 , which is much lower than that of LPPS coatings, 860HV 0.3 . The elastic modulus measured for APS, HVOF and LPPS coatings using Knoop indentation were 96, 84, 176GPa, respectively. The APS coatings were also observed to have lower elastic modulus values in the in-plane direction than those in the perpendicular direction, as a result of microcracks scattered within the coatings. In fracture toughness tests, the LPPS coatings revealed the lowest fracture toughness, as compared with other two spraying processes, indicating low porosity and crack levels are related to low fracture toughness. From these results, it appeared that potential improvements to certain mechanical properties can be achieved using low pressure plasma spraying process.

Plasma spraying with axial powder injection inside the anode nozzle is a method considered to markedly enhance deposition efficiency and reduce plasma power compared to the most general method of injecting powder at the nozzle exit. However, powder injecting inside the nozzle will also likely cause problems from powder deposition on the nozzle wall and clogging of the particle feed channels. In this study, spherical stainless steel 316 and angular alumina powders with a mean size of 20-40 μm are used to deposit coatings via axial powder injection. The effect of powder feed rate, gas flow rate, and plasma power on deposition efficiency and particle clogging are investigated. The results show that particle clogging can be avoided by reducing powder feed rate and increasing the velocity of the plasma jet.

To improve the mechanical properties of 316L stainless steel coatings prepared by plasma spraying, post-spray heat treatments were conducted at 600-800 °C for 1-2 hours. The effect on microstructure and hardness was assessed via XRD and SEM analysis and microhardness measurements. The results show that heat treatment at various temperatures improved coating hardness as well as fracture behavior.

This paper discusses the double-arc phenomenon that occurs in plasma arc spraying and its prevention through the use of a bi-anode torch. It also presents the results of a study that compares voltage-current characteristics of the arc for different plasma gases and arc root attachment points. It is shown that the arc has different electrical characteristics when changing from the first to the second anode, which is explained using a simplified arc model.

In this paper, tin-bronze/TiN and tin-bronze/quasicrystal (AlCuFeB) composite coatings were fabricated by cold spray process. Microstructures and microhardness of coatings were characterized. Ball-on-disc dry sliding wear tests were conducted in an ambient condition to examine the tribological performance of the composite coatings. The results show that the microhardness and densities of composite coatings increase significantly compared to those of the pure tin-bronze coating. The friction coefficients of coatings decrease with the introduction of reinforces. Furthermore, the tin-bronze/quasicrystal composite coating yields a lower friction coefficient and wear rate compared to the bronze/TiN coating. The tribological mechanisms were discussed.

Strontium aluminates activated by Eu, Dy 3+ ions has recently attracted more attention due to not only their bright luminescence and super long afterglow without radioactivity but also the extensive application. SrAl 2 O 4 :Eu, Dy coatings were prepared by low power plasma thermal spraying (2.5-4W). The optimum spraying conditions have been established with spray parameters. The effects of the plasma conditions on the porosity rate of the coatings were investigated by many techniques, respectively X-ray diffraction (XRD) and scanning electron microscopy (SEM). To better qualify the characteristics of these coatings prepared from spray-dried powders, luminescence characteristics were also studied.

The effect of powder injecting location of the plasma spraying on spraying properties was studied. Three different powder injecting methods were applied in the experiment. In the first method, the particles were axially injected into the plasma flow from the cathode tip. In the second method, the particles were radially injected into the plasma flow just downstream of the anode arc root inside the anode nozzle. In the third method, the particles were radially injected into the plasma jet at the nozzle exit. The alumina particles with a mean diameter of 20µm were used to deposit coatings. Spraying properties such as the deposition efficiency, the melting rate of the powder particles, and the coating quality were investigated. The results show that the spraying with axial particle injecting can heat and melt the powder particles more effectively, produce coatings with better quality, and have higher deposition efficiency.

Al-Cu-Cr quasicrystalline (QC) coatings were deposited onto mild steel Q235A substrate by low power plasma spraying. The plasma torch was designed such that the powders can be injected axially (parallel to the axis of the plasma arc) into the region between the anode and cathode, and the carrier gases were transferred to the plasma flame, so the heating efficiency of the powders is greatly increased and the velocity of particles is much higher than an externally-fed powder system. The Al-Cu-Cr QC coatings were sprayed via this new technique at different plasma power (3~7kW) by controlling the secondary plasma gases (H 2 and N 2 ) flow or changing the arc current. The results show that, the original powder was mainly composed of a dominant icosahedral quasicrystalline phase I-Al 65 Cu 24 Cr 11 and a minor monoclinic θ-Al 13 Cr 2 . However, besides these two phases, body-centered cubic α- Al 69 Cu 18 Cr 13 , the approximant to quasicrystal I-Al 65 Cu 24 Cr 11 , has been detected in as-sprayed coatings. On the other hand, peak intensity of QC phase I-Al 65 Cu 24 Cr 11 decreased and that of crystalline phases (θ-Al 13 Cr 2 or α-Al 69 Cu 18 Cr 13 ) increased as the plasma power was increased.

The effects of substrate temperature during the low-power plasma spraying (2.5~8.0kW) and subsequent annealing on the phase transformation of Al-Cu-Cr quasicrystalline coatings were investigated. Compared with the original powder mainly composed of icosahedral I-Al 65 Cu 24 Cr 11 quasicrystalline phase and monoclinic θ-Al 13 Cr 4 , the coatings show an icosahedral quasicrystal I-Al 65 Cu 24 Cr 11 and its body-centered cubic approximate phase α-Al 69 Cu 18 Cr 13 . XRD patterns indicated that, for identical spraying condition, a rise in substrate temperature resulted in an increase of the I/α peak intensity ratio in the coatings, and the crystalline phases of AlCu 4 and Al 86 Cr 14 were observed in the coatings after annealing at 1113K for half an hour.

A special bi-anode plasma torch has been developed for elongating the arc length and obtaining the arc with a fixed length. The relations between the arc voltage and the arc current were investigated for different gas compositions, and the effect of the gas flow rate on the arc voltage was also studied for the pure Ar plasma. The results show that for the pure Ar plasma, the arc voltage had a dropping characteristic for an arc with instable length and a rising characteristic arc voltage for an arc with fixed length, but for Ar-H 2 plasma or Ar-N 2 plasma, the arc voltage always had dropping characteristic. Increasing the gas flow rate will enhance the arc voltage both for an instable and fixed length arc.

Compound coatings of MCrAlY and alumina shows a sandwich structure with good wear-resistance and intensity at elevated temperature, the coating are usually applied to the furnace roll in modern continuously annealing line or continuously galvanizing line, to prevent the pickup forming on the roll surface. Coatings were prepared by the detonation spraying and the effect of spraying parameter of the denotation gas composition on the microstructure of coatings was investigated. The percentages of MCrAlY and alumina in the coating were determined by the composition of detonation gas mixture. Alumina content in the coating increased with the reduction of the nitrogen dilution in the gas mixture, which resulted in the hardness enhancement of the coating. The microstructure of coatings is different from that prepared by thermal plasma spraying or HVOF.

The phase transformation and reaction of ZrO 2 -CaO- ZrSiO 4 and ZrO 2 -Y 2 O 3 -ZrSiO 4 coatings with manganese oxide at 1273 K were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). SiO 2 phase formed in the coatings, which was from the ZrSiO 4 decomposed and easy react with manganese oxide or CaO. SiO 2 has precedence over react with CaO than manganese oxides for ZrO 2 -CaO-ZrSiO 4 coatings, and which result in to promote t-m phase transformation. On the contrary, the reaction between SiO 2 and MnO is primary for the ZrO 2 -Y 2 O 3 -ZrSiO 4 coatings and result in the damage or exfoliation on the surface of the coatings.