Gas Tunnel Type Plasma Spraying Research Articles

Thermal conductivity of plasma-sprayed W/SiC composite for high-temperature energy applications

W/SiC metal matrix composites were produced by gas tunnel type plasma spraying (GTPS) using a mixture of 12 wt.% SiC–88 wt.% W feedstock powder. This work aimed at the optimization of the plasma gun current for deposition of a W/SiC composite with fine microstructure on AISI 304 substrate. Characterization of deposits was performed in order to assess microstructure, micro-hardness, thermal diffusivity and thermal conductivity. WO 3 was detected in the composite deposits, which indicated that the tungsten partially oxidized during plasma spraying. Also, the deposit composite was dense and nearly free of pores due to the little mismatch between the coefficient of thermal expansion (CTE) for W and SiC. Microhardness values gradually decreased as a function of input current due to the formation of WO 3 and the decomposition of SiC particles in high temperature flame region. The thermal conductivity as high as ∼ 59 W/mK was obtained at gun current 80 A. It was found that both tungsten oxide and structure imperfections have a significant influence on the thermal conductivity and mechanical properties.

Influence of spray parameters on the microstructure and mechanical properties of gas-tunnel plasma sprayed hydroxyapatite coatings

For biomedical applications, hydroxyapatite (HA) coatings were deposited on 304 stainless steel substrate by using a gas tunnel type plasma spraying process. The influences of spraying distances and plasma arc currents on the microstructure, hardness and adhesion properties of HA coatings were investigated. Microstructure observation by SEM showed that HA coatings sprayed at low plasma power have a porous structure and poor hardness. HA coatings sprayed at high plasma power and short spraying distance are characterized by good adhesion and low porosity with dense structure. Hardness increased for HA coatings sprayed at shorter spraying distance and higher plasma power, mainly due to the formation of dense coatings.

Enhancement of TBC (Thermal Barrier Coatings) Characteristics by Gas Tunnel Type Plasma Spraying

Zirconia (ZrO2) coating formed by plasma spray method is widely used industrially as a thermal barrier coating (TBC). Presently, there are some problems such as spallation and cracks inside the coating. As one solution given by the development of new spaying processing, the gas tunnel type plasma spraying is one of excellent method to enhance the TBC performances. The zirconia-alumina (ZrO2-Al2O3) composite coating formed by this method has a high hardness layer at the surface side of the coating, which shows the graded functionality of hardness, and is superior as a TBC. In this paper, the performance of such high hardness ZrO2-Al2O3 composite coating was investigated and the merit as TBC was clarified. The Vickers hardness of the high hardness layer near the coating surface increased by the thermal process of high energy plasma, which corresponded to the result that the coating became denser. Also, the effect of alumina mixing was discussed about the microstructure of this composite coating. The combination of high hardness of Al2O3 with the low thermal conductivity of ZrO2 resulted to the development of high performance TBC. The transverse thermal conductivity of such ZrO2-Al2O3 composite coatings was proved to be much smaller than that in the longitudinal direction.

Effect of gun current on the microstructure and crystallinity of plasma sprayed hydroxyapatite coatings

Hydroxyapatite (HA) is a bioactive material because its chemical structure is close to the natural bone. Its bioactive properties make it attractive material in biomedical applications. Gas tunnel type plasma spraying (GTPS) technique was employed in the present study to deposit HA coatings on SUS 304 stainless steel substrate. GTPS is composed of two plasma sources: gun which produces internal low power plasma (1.3–8 kW) and vortex which produces the main plasma with high power level (10–40 kW). Controlling the spraying parameters is the key role for spraying high crystalline HA coatings on the metallic implants. In this study, the arc gun current was changed while the vortex arc current was kept constant at 450 A during the spraying process of HA coatings. The objective of this study is to investigate the influence of gun current on the microstructure, phase crystallinity and hardness properties of HA coatings. The surface morphology and microstructure of as-sprayed coatings were examined by scanning electron microscope. The phase structure of HA coatings was investigated by X-ray diffraction analysis. HA coatings sprayed at high gun current (100 A) are dense, and have high hardness. The crystallinity of HA coatings was decreased with the increasing in the gun current. On the other hand, the hardness was slightly decreased and the coatings suffer from some porosity at gun currents 0, 30 and 50 A.

Mechanical properties and oxidation behaviour of plasma sprayed functionally graded zirconia–alumina thermal barrier coatings

The microstructures of thermally sprayed coatings usually incorporate process-dependent defects such as globular pores, interlamellar pores, cracks (in case of ceramics), etc. Porosity is a prevalent feature in the microstructure and affects various coating properties such as elastic modulus, thermal conductivity and dielectric behaviour. This study is conducted to improve the image analysis (IA) as a reliable method for characterization of porosity in thermally sprayed coatings. The versatility of IA methods for microstructural quantification has been investigated for TBCs deposited with partially stabilized zirconia (PSZ), alumina and zirconia–alumina composite coatings by gas tunnel type plasma spraying. This study confirms the applicability of image analysis as a straightforward, versatile, reliable and inexpensive method for porosity analysis agreed with coating qualities and the influences of the Vickers hardness. The paper also discusses the thermal behaviour and high temperature oxidation resistance of the Al 2O 3 coatings as compared to composite coatings at the interface. This interlayer is preferred to minimize the detrimental effect of phase transformation of γ-Al 2O 3 to α-Al 2O 3.

Comprehensive investigations on the influence of gun current of plasma spraying on the properties of silicon carbide films

Polycrystalline silicon carbide films have been prepared by the gas tunnel type plasma spraying method (GTPS). The effect of gun current on microstructure and mechanical properties was investigated. Scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy, nanoindentation and abrasive wear were used to characterize the structure, thickness, composition and the mechanical properties of SiC films. Microstructural studies revealed that the formation of cubic silicon carbide (C-SiC) at higher gun currents from 120 to 140 A. The SiC films have good-adhesion, dense, smooth and compact morphology. Hardness of SiC films strongly improved from 23 to 31.5 GPa as the gun current increased from 0 to 140 A. SiC films formed at higher gun current exhibits better wear resistance than that deposited at low gun current, mainly due to SiC films become more hard and more dense. The crystalline cubic silicon carbide films with good morphology and mechanical properties have been obtained from the GTPS method, which makes it a suitable material for high-temperature thermoelectric and mechanical applications.

Abrasive wear behavior of sprayed hydroxyapitite coatings by gas tunnel type plasma spraying

Hydroxyapatite (HA) coatings were sprayed using gas tunnel type plasma spraying at different arc currents. Abrasive wear test was carried out for the coatings sprayed at different arc currents under unlubricated conditions in air atmosphere. The abrasive wear rate was measured at different coatings thickness to study the effect of coating thickness on the anti-abrasion resistance of HA coatings. The results showed that the abrasive wear resistance of HA coatings increases as the operating arc current of the plasma torch increases. On the other hand, the abrasive wear rate reaches a minimum value near the substrate with coating thickness less than 50 μm. The results showed that the coating hardness increases in the region near the substrate and increases as the arc current increases. The experimental results indicated that there is a relation between the abrasion resistance and hardness properties of HA coatings.

Influence of gas flow rate on the microstructure and mechanical properties of hydroxyapatite coatings fabricated by gas tunnel type plasma spraying

Gas tunnel type plasma torch was used to spray hydroxyapatite (HA) coatings on 304 stainless steel substrate at different plasma gas (Ar) flow rates (100–170 l/min). Microstructure was observed by scanning electron microscope and phase analysis by X-ray diffraction. The mechanical properties such as hardness and abrasive wear resistance of HA coatings sprayed at different gas flow rates were investigated. The results showed that the gas flow rates affect greatly the microstructure and mechanical properties of HA coatings. Crystallinity increased and porosity decreased as the gas flow rate increased. Hardness and abrasion resistance increased as the gas flow rates increased.

Gas tunnel type plasma spraying deposition and microstructure characterization of silicon carbide films for thermoelectric applications

Gas tunnel type plasma spraying deposition has been applied successfully to the deposition of the SiC films on stainless-steel substrates. The microstructure and the surface morphology of the SiC films were characterized by means of X-ray diffraction (XRD) and scanning electron microscope (SEM). The control of the processing parameters such as powder feeding rate, composition of plasma working gases, spraying distance, and carrier gas flow rate allowed the deposition of dense, uniform, continuous, and high purity crystalline SiC films. The thickness of the SiC films varied from 3 to 10 μm. EDS analysis confirmed the presence of SiO 2 in the deposited SiC films.

Microstructural characterization and properties of ZrO 2/Al 2O 3 thermal barrier coatings by gas tunnel-type plasma spraying

Spraying condition plays an important role in the plasma-sprayed coating process and affects the final properties of the coatings. Zirconia, alumina and zirconia/alumina composite coatings were prepared on a stainless-steel substrate (SUS304) by the gas tunnel-type plasma spraying. Effects of different alumina mixing ratios on the coating properties were investigated. The results indicated that the mixing ratio of powders and the traverse number of substrate had an influence on the hardness, porosity and wear weight loss of composite coatings. The hardness increased while the porosity decreased with the increase in alumina mixing ratio. The porosity that was less than 10% and a hardness about Hv = 1400 was obtained for the alumina coating. The adhesive strength and wear weight loss of the composite coatings were also clarified at different alumina mixing ratios.

Formation of tungsten coatings by gas tunnel type plasma spraying

Tungsten is a material that has the highest melting point of 3422 °C among metals. Therefore, when deposited as a coating, it can protect the substrate surface from high heat flux. In this study, tungsten (W) sprayed coatings were formed on stainless steel substrates by gas tunnel type plasma spraying at a short spraying distance. The Vickers micro-hardness of the W coating was measured as a guide to its mechanical properties. The tungsten coating has a hardness of Hv=260–300 along its cross section, which is a little lower than the hardness usually quoted for pure tungsten. Regarding the microstructure, the coating contains some pores. The results of X-ray diffraction and energy dispersive X-ray spectroscopy show that the coating consists of pure tungsten.

Relationship between Corrosion Resistance and Microstructure of Al2O3-ZrO2 Composite Coatings Obtained by Gas Tunnel Type Plasma Spraying

Relationship between Corrosion Resistance and Microstructure of Al2O3-ZrO2 Composite Coatings Obtained by Gas Tunnel Type Plasma Spraying

Adhesion characteristics of zirconia–alumina composite coatings by gas tunnel type plasma spraying

Zirconia–alumina composite coatings on the Type 304 stainless-steel substrate were performed by using the gas tunnel type plasma spraying apparatus without the metallic bond coat. The adhesive characteristics of the coating to substrate were investigated by using 3D analysis of the fracture surface. The alumina–zirconia composite coatings exhibited lamellar structure consisting alumina and zirconia layers, below 2 μm in thickness of a single lamella each other. Inside the lamellae, columnar grains were observed. Numerous microcracks occurred in the alumina splats or zirconia splats deposited during plasma spraying, and then fractured there by the tensile testing. The loose microstructure appeared near the interface between the coating and the stainless-steel substrate. The fracture morphology of the alumina–zirconia composite coating showed the round type pattern, which was related to the microstructure.

Adherence of zirconia composite coatings produced by gas tunnel-type plasma spraying

The sprayed coating is formed by the mechanical bonding to the substrate. Zirconia coating produced by gas tunnel-type plasma spraying at a short spraying distance has a high hardness layer at the surface side of the coating, which shows the graded functionality of hardness. However, the adhesive strength between the coating and the substrate has not been investigated for these coatings. In this study, the adhesive strength of such high hardness zirconia composite coatings in the case of different mixtures of alumina was investigated as well as its mechanical properties. The alumina mixture of spraying powder weakened the adherence of such high hardness coatings to the substrate. The hardness of the coating was also much influenced by the alumina mixing rate.

Study on the interface strength of zirconia coatings by a laser spallation technique

Tensile strength of interfaces between zirconia coatings and stainless steel substrates were measured using a laser spallation technique. In this technique, a laser-generated compression stress wave reflects into a tensile wave from the free surface of the coating and pries off its interface at a critical amplitude. Zirconia coatings were deposited using the gas tunnel-type plasma spraying process which was previously shown to result in denser and harder deposits compared with those produced by conventional spray methods. The interface strength was found to increase with a decrease in the spray distance and traverse speed, and with an increase in the traverse number, and coating and substrate thickness. The variation in the measured strength with these variables could be explained through their effect on controlling the temperature of the particles and that in the interfacial region. Higher temperature in the interfacial region during coating deposition is expected to increase adhesion by enhancing chemical and physical bonding mechanisms. Depending upon the processing conditions used, the tensile strength for the zirconia/steel interface system was found to vary anywhere between 206 and 501 MPa.

Plasma spraying of micro-composite thermal barrier coatings

The thermal barrier coatings (TBCs) by gas tunnel-type plasma spraying exhibited ceramic-composite features consisting of a host oxide matrix ceramic with an embedded second phase material. The densities of the composite TBC were found to be higher than those sprayed with 100 wt% ZrO 2 or Al 2O 3. In the coatings produced with powder mixtures of 50 wt%, the embedded splats are found to have a relatively uniform thickness between 1 and 10 μm and they exhibited clear and pore-free interfaces with the host material. The micro-composite coatings also exhibited thickness-dependent functionally gradient Vickers hardness values by the hardness measurements across the coating thickness. A one-dimensional series heat transfer model was developed to estimate upper and lower bounds of the transverse thermal resistance as a function of alumina–zirconia weight ratio. The model shows that the addition of higher thermally conducting Al 2O 3 can result in an increase in the transverse thermal resistivity of YSZ.

Effect of heat treatment on high-hardness zirconia coatings formed by gas tunnel type plasma spraying

By means of gas tunnel-type plasma spraying, a high-hardness zirconia coating could be obtained at a short spraying distance. For example, Vickers hardness of zirconia coating was H v=1050 at the spraying distance of L=30 mm, when the power input was P=20 kW. This high-hardness coating had a hardness layer near the surface. In this paper, the effect of heat treatment on the characteristic of such a high-hardness zirconia coating is investigated, and the influence on the Vickers hardness and structure of the coating is discussed. The hardness of this high-hardness layer became lower after heat treatment. The crystal form of the cubic ZrO 2 of powder was transformed into a tetragonal phase through this plasma spraying. After heat treatment, the angular separation between the two tetragonal peaks was broader and clearer.

Formation of high hardness zirconia coatings by gas tunnel type plasma spraying

The high hardness zirconia (ZrO 2) coatings could be obtained at an atmospheric pressure by using a gas tunnel type plasma spraying. The characteristics of these high hardness ZrO 2 coatings were investigated. The Vickers hardness of the ZrO 2 coating at a short spraying distance was very high; a high hardness of more than H v=1200 was achieved at the surface side of the coating. The microstructure of the obtained high hardness ZrO 2 coating was also investigated by the microscopic method. And the characteristics of the high hardness ZrO 2 coating was discussed in comparison with that of the coating formed by the conventional type plasma spraying. It was clarified that the ZrO 2 coating of the gas tunnel type was not only much harder but also less porous than that of the conventional type.

ガストンネル型プラズマ溶射装置による溶射粒子の付着特性とセラミックス皮膜特性

The deposit characteristics of ceramic powder sprayed onto a substrate by the gas tunnel type plasma spraying apparatus were studied, and it was found that the special spraying distance L* can be determined by the deposit characteristics. The relation between the deposit characteristics and the coating quality such as the Vickers hardness and porosity was discussed and it was clarified that the special spraying distance L* governing deposit characteristics corresponded to the critical spraying distance Lc which indicated the largest distance in order to obtain the good coating quality.