Low Pressure Plasma Spraying Research Articles

The Protectiveness of Thermally Grown Oxides on Cold Sprayed CoNiCrAlY Bond Coat in Thermal Barrier Coating

This paper presents the results of an oxidation behavior study for a thermal barrier coating (TBC) with air plasma sprayed yttria-stabilized zirconia top coat and CoNiCrAlY bond coat deposited using low pressure plasma spray (LPPS) and cold spray (CS). The TBC is subjected to isothermal oxidation and creep tests at 900 °C and evaluated using scanning electron microscopy, energy dispersive x-ray spectrometry transmission electron microscopy and electron backscatter diffraction. The thermally grown oxide (TGO) developed in the TBC with the LPPS bond coat was composed of only α-Al2O3 and the TGO developed in the TBC with a CS bond coat is composed of α-Al2O3 and γ-Al2O3. Despite the presence of this metastable γ phase, the TGO in the CS specimens exhibits a dense microstructure and lower amounts of mixed oxides. The correlation between γ-Al2O3 and the formation of mixed oxides was investigated through the measurement of γ-Al2O3 thickness ratio and mixed oxides coverage ratio. It was found that the mixed oxides coverage ratio is inversely proportional to the γ-Al2O3 thickness ratio.

Characteristics of a Plasma Torch Designed for Very Low Pressure Plasma Spraying

Unlike atmosphere plasma spraying (APS), very low pressure plasma spraying (VLPPS) can only weakly heat the feed materials at the plasma-free region exit of the nozzle. Most current VLPPS methods have adopted a high power plasma gun, which operates at high arc currents up to 2500 A to remedy the lower heating ability, causing a series of problems for both the plasma torch and the associated facility. According to the Kundsen number and pressures distribution inside of the nozzle in a low-pressure environment, a plasma torch was designed with a separated anode and nozzle, and with the powder feed to the plasma jets inside the nozzle intake. In this study, the pressures in the plasma gas intake, in the nozzle intake and outside the plasma torch were measured using an enthalpy probe. For practice, SUS 316 stainless steel coatings were prepared at the plasma currents of 500-600 A, an arc voltage of 50 V and a chamber pressure of 1000 Pa; the results indicated that coatings with an equiaxed microstructure could be deposited in proper conditions.

Characterization of Defectiveness of Oxygen Transport Membranes Deposited by Low Pressure Plasma Spraying –thin Film Processes

Characterization of Defectiveness of Oxygen Transport Membranes Deposited by Low Pressure Plasma Spraying –thin Film Processes

Thermal Corrosion Behavior of NiCoCrAlTaY Coating for a Ni Base Superalloy Prepared by Low Pressure Plasma Spraying

The low pressure plasma spraying (LPPS) method was used to prepare NiCoCrAlTaY bond coating on a nickel base superalloy with the composition(wt, %)of C 0.13-0.22, Cr 8.50-9.50, Co 9.0-11.0, Mo 2.50-3.50, Al 4.80-5.70, Ti 4.10-4.70, Fe ≤1.0, V 0.60-0.90, and Ni in balance. The thermal corrosion tests, including Isothermal and cycle oxidation, were conducted for the specimens of the base alloy and the alloy with coatings. The experimental results showed that the thermal spraying NiCoCrAlTaY bond coating improved the thermal corrosion resistance of the alloy significantly. It has been found that the base alloy had high weight gain of 68.91g/m2 at the test condition of 1100°C for 100hrs, while the weightl gain of the specimens of the alloy with NiCoCrAlTaY bond coating, was only -0.76∼5.72g/m2. And the slight weight loss appeared when the specimen with with NiCoCrAlTaY bond coating was treated for 40hrs at1100°C. The results also showed that the addition of tantalum element in the coating was beneficial for the thermal oxidation resistance. It has been found that after oxidation at 1100°C for 100hrs, the continued Al2O3 film nuderneath the outside layer mixture oxides of Cr, Ni, Al and Co, was formed on the substrate surface after both the isothermal oxidation and cycling oxidation. The coating has a good combination with the nickel base superalloy substrate.

Corrosion Behavior of Low Pressure Plasma Sprayed MoB/CoCr Coatings Exposed to Molten Al-12.07wt.%Si Alloy

MoB/CoCr, a novel material for thermal spraying, is used to resist erosion by molten Al-12.07wt.%Si alloy due to its high durability. MoB/CoCr cermet coatings were deposited by low pressure plasma spraying (LPPS), and the durability of the MoB/CoCr coating has been investigated using a molten-metal immersion tester. The microstructure of coating was characterized by XRD, SEM and EDAX. The results show that the coatings with layer structure are mainly composed of ternary borides of Co2MoB2 and CoMoB, and the coatings have excellent performance of bond strength and thermal shock resistance. After 1080h immersion test in the molten Al-12.07wt%Si, the results show that the mainly chemical composition and crystal structure were little changed, and MoB/CoCr coatings have shown excellent durability in the Al-12.07wt%Si alloy without dissolution.

Phase evolution in an MCrAlY coating during high temperature exposure

MCrAlY (M = Ni and/or Co) coating systems are often applied on gas turbine blades and vanes to withstand the challenges of severe conditions. During service MCrAlY coatings are subjected to microstructural transformations that can be an indication of components service temperatures. The development of indirect methods to measure this parameter is of great concern in the gas turbine ?world? due to the impossibility of direct measurements. In the present work the evolution of an MCrAlY coating applied on Rene80 by LPPS (Low Pressure Plasma Spray) technique has been studied in order to verify if it was possible to identify a microstructural indicator of the service temperature. The specimens were exposed for different lengths of time at test temperatures of 700 - 800 - in order to characterize the phase evolution with time and temperature. Selective etching was employed for optical metallographic investigation. Scanning Electron Microscopy (SEM) observation combined with Electron Backscattered Diffraction (EBSD) and Energy Dispersive Spectroscopy (EDS) showed that the coating is composed of a ?- Co matrix, ?-AlNi, ?-(Cr, Co), Cr carbide and Y-rich phases. Among these phases, the sigma phase resulted in a temperature - composition dependence that can be a useful tool for evaluating the local service temperature and modelling the residual lifetime.

1011 大気圧マイクロ波プラズマの溶射プロセスへの応用について

Conventional plasma spraying systems are difficult to operate less than a few kW at atmospheric pressure condition and fabricate coating on low-melting-point substrate. Atmospheric microwave discharge easily generates high temperature plasma at a microwave power of not more than 1kW. Thus, the atmospheric pressure microwave plasma was applied to low power atmospheric pressure plasma spraying. The atmospheric pressure microwave plasma spraying was successfully fabricated ceramic coatings below 1kW of microwave power.

Very Low Pressure Plasma Spray—A Review of an Emerging Technology in the Thermal Spray Community

A fundamentally new family of thermal spray processes has emerged. These new processes, collectively known as very low pressure plasma spray or VLPPS, differ from traditional thermal spray processes in that coatings are deposited at unusually low chamber pressures, typically less than ~800 Pa (6 Torr). Depending upon the specific process, deposition may be in the form of very fine molten droplets, vapor phase deposition, or a mixture of vapor and droplet deposition. Resulting coatings are similar in quality to coatings produced by alternative coating technologies, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD), but deposition rates can be roughly an order of magnitude higher with VLPPS. With these new process technologies modified low pressure plasma spray (LPPS) systems can now be used to produce dense, high quality coatings in the 1 to 100 micron thickness range with lamellar or columnar microstructures. A history of pioneering work in VLPPS technology is presented, deposition mechanisms are discussed, potential new applications are reviewed, and challenges for the future are outlined.

Abrasive Wear Behaviour of MoB/CoCr Cermet Coatings under Different Parameters <sup></sup>

In this study, abrasive wear behaviour of MoB/CoCr coating deposited by low pressure plasma spraying (LPPS) was investigated. The properties of microhardness and bond strength of the MoB/CoCr coatings were tested. The abrasive wear properties were evaluated by MLS-225 wet sand rubber wheel tester. The results show that MoB/CoCr coatings are dense and have excellent mechanical properties with high microhardness (930HV0.2) and bond strength (71MPa). The MoB/CoCr coatings have high abrasive wear resistance property.

Determining a critical strain for APS thermal barrier coatings under service relevant loading conditions

Despite the huge progress made in recent years in analysing the degradation behavior and the reliability of thermal barrier coating systems, there is still some deficit in the capability to predict damage evolution in terms of crack initiation and crack growth, which ultimately leads to macroscopic delamination and spallation of the coating system. In order to obtain this prediction capability, a fundamental understanding of the damage evolution processes under isothermal, thermo-cyclic and under thermo-mechanical loading conditions has to be developed.The aim of the presented work is to determine the critical strain, i.e. the strain at which cracking initiates, and to analyse the evolution of a network of cracks for widely used atmospheric plasma sprayed (APS) thermal barrier coating (TBC) systems. The TBC system has been exposed in our study to service relevant loading conditions, namely to thermal gradient mechanical fatigue (TGMF). TGMF tests for in-phase as well as out-of-phase loading cycles were performed on hollow cylindrical specimens made of the single crystal super alloy CMSX-4, loaded mechanically in 〈001〉 orientation, and being coated with a duplex system comprised of a CoNiCrAlY bond coat and a 8wt.% Yttria partially stabilized Zirconia (YSZ) TBC. The CoNiCrAlY bond coat was deposited by Low Pressure Plasma Spraying (LPPS), while the ceramic top coat was deposited using the APS process. The loading cycles were chosen to represent an industrial gas turbine engine. Critical strains measured for delamination (within the ceramic coating or at the CoNiCrAlY – TBC interface) and through cracking, i.e. segmentation of the ceramic top coat was determined using a special compression test equipped with in situ acoustic emission technique. The mechanical testing was performed at room temperature after TGMF exposure. In order to study the impact of thermally grown oxide (TGO), specimens have been TGMF tested in the “as received” conditions as well as after isothermal aging (up to 3000h at 1000°C). To correlate the signal obtained by acoustic emission (AE) with the evolution of (micro-) cracks, the specimens have been carefully sectioned and investigated by standard metallographic means.The measured critical strains are used as a data basis for a strain-based lifetime model developed for isothermal and cyclic oxidation as well as thermo-mechanical loading. The lifetime model considers two failure modes, namely delamination and (vertical) through cracking.Metallographically obtained crack patterns within the TBC system have been incorporated into finite element models to quantify stress–relaxation as a consequence of damage evolution in the TBC system.The observations show that thermal gradient fatigue loading under in-phase loading leads to a shorter lifetime compared to out-of-phase loading.For the delamination mode, the critical strain values of the model are in good agreement with the experimental data of the TGMF experiments. The modeled critical strain for through cracking, on the other hand, is consistently lower than the experimentally determined failure strains, implying that the model describes the failure situation in a conservative manner.

Influence of Protective Coatings on Damage and Lifetime of Alloy 247 DS in Thermomechanical Fatigue and Bending Tests

Abstract The present work was focused on the damage mechanisms and lifetime of a directionally solidified, oxidation protection coated Ni-base superalloy under out-of-phase thermomechanical fatigue (OP TMF) loading. The main scope of the work was the analysis of crack formation and propagation in the coating depending on its fracture strain. Two coating systems applied by low pressure plasma spraying were investigated: (i) MCrAlY-coating and (ii) duplex-coating consisting of the same MCrAlY coating and a NiAl top coat. The fracture strains of the coatings were determined at T = 20°C up to 600°C by four-point bending with acoustic emission analysis. The MCrAlY-coating did not influence significantly the TMF lifetime of the superalloy, whereas duplex-coatings reduced it drastically. This correlates with the fracture strains of the coatings that are for the MCrAlY-coating distinctly higher and for the duplex coating lower than the mechanical strain range of the TMF tests. Thus, the damage process was controlled by propagation of cracks formed after few TMF cycles in the duplex coated material and by slower initiation and propagation of fatigue cracks in the MCrAlY coated alloy. Moreover, crack branching at the interface between coating and substrate alloy affects crack penetration into the substrate. In addition to measurement of TMF life, the early stages of crack initiation and propagation in the duplex coating were studied in more detail after TMF tests that were interrupted after defined cycle numbers as well as during in situ bending tests in the scanning electron microscope. Furthermore, the influence of pre-oxidation on crack propagation is discussed.

Oxide scale formation of modified FeCrAl coatings exposed to liquid lead

Modified FeCrAl coatings were studied with respect to their capability to form a thin protective oxide scale in liquid lead environment. They were manufactured by low pressure plasma spraying and GESA surface melting, thereby tuning the Al content. The specimens were exposed for 900h to liquid lead containing 10−6 and 10−8wt.% oxygen, respectively, at various temperatures from 400 to 550°C. Threshold values for an Al content that guarantees the formation of thin protective Al-rich oxide scales are determined, dependent on the respective chromium content, on the presence of yttrium in the modified coating, and on the exposure conditions.

Very low pressure plasma sprayed alumina and yttria-stabilized zirconia thin dense coatings using a modified transferred arc plasma torch

As a novel thermal spray process, very low pressure plasma spray (VLPPS) process has been significantly used to deposit thin, dense and homogenous ceramic coating materials for special application needs in recent years. In this study, in order to enhance low-energy plasma jet under very low pressure ambience, a home-made transferred arc nozzle was made and mounted on a low-power F100 plasma torch to fully melt or evaporate powder feedstock. As a result, thin and dense alumina (Al2O3) and yttria-stabilized zirconia (YSZ) ceramic coatings with an average thickness of 30–40μm were successfully elaborated by the VLPPS process below 1mbar. An optical emission spectroscopy (OES) was used to analyze the plasma jet properties. The microstructures of the coatings were observed by means of a scanning electron microscopy (SEM). It was found that the YSZ coatings displayed a bimodal microstructure which was composed of splats formed by melted particles and a little amount of vapor condensation from evaporated particles. However, vapor condensation could not be observed in the Al2O3 coatings, and only lamellar splats were found. The mechanical properties of both coatings were also evaluated.

Microstructure and tribological properties of Fe-Mo alloy-coated steel specimens prepared by low-pressure plasma spraying

Fe7Mo6-based alloy powder was low-pressure plasma sprayed on SS400 steel substrates, and the microstructure, hardness and tribological properties of the prepared substrates were investigated. Metastable BCC-coated and FeMo-coated steel specimens were obtained by low-pressure plasma spraying. The BCC-type Fe-42at% Mo alloy was stable at temperatures below 955 K. In addition, the BCC-type Fe-42at% Mo alloy coating layer exhibited micro-Vickers hardness as high as the FeMo coating layer. The metastable BCC-coated and FeMo-coated steel disk specimens exhibited friction coefficients as low as Fe7Mo6-based alloy specimens prepared by spark plasma sintering. Moreover, the specific wear rates of the FeMo/ASTM 52100 steel tribopairs were almost the same as those of the Fe7Mo6-based alloy/ASTM 52100 steel tribopairs.

Characterization of Thermally Grown Oxide on Cold Sprayed CoNiCrAlY Bond Coat in Thermal Barrier Coating

This paper presents the results of a study of the microstructure and oxidation behavior of thermal barrier coating (TBC) with air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited using two different spraying techniques, low pressure plasma spray (LPPS) and cold spray (CS). The objective is to investigate the thermally grown oxide (TGO) thickness and oxide scale composition of TBC subjected to isothermal oxidation and creep tests at 900 °C by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDX) analyses in order to evaluate the reliability of the CS technique. It was found that the TGO thicknesses for TBC with CS bond coats were smaller and the TGO was composed of mainly alumina with little or no mixed oxides. TGO growth rate was also affected by the applied stress. Smaller TGO thicknesses were observed for the non-creep TBC for both CS and LPPS bond coats. Overall findings indicate that the oxidation behavior of the TBC with CS bond coat is superior compared to that of the TBC with LPPS bond coat.

Durability of Low Pressure Plasma Sprayed MoB/CoCr Coating on 310S Stainless Steel in Molten Al-12.07wt.%Si Alloy

A sprayed MoB/CoCr cermet coating was formed on 310S stainless steel by low pressure plasma spraying (LPPS), and its durability in the molten Al-12.07wt.%Si has been investigated by SEM、EDS and XRD. The immersion test revealed that the MoB/CoCr coating has much higher durability without dissolution in the molten Al-12.07wt.%Si alloy. Little change of crystal structure, mainly composed of ternary borides of Co2MoB2 and CoMoB, suggesting that the ternary borides have much higher durability.

Very low pressure plasma sprayed yttria-stabilized zirconia coating using a low-energy plasma gun

In the present study, a more economical low-energy plasma source was used to perform a very low pressure plasma-spray (VLPPS) process. The plasma-jet properties were analyzed by means of optical emission spectroscopy (OES). Moreover, yttria-stabilized zirconia coating (YSZ) was elaborated by a F100 low-power plasma gun under working pressure of 1 mbar, and the substrate specimens were partially shadowed by a baffle-plate during plasma spraying for obtaining different coating microstructures. Based on the SEM observation, a column-like grain coating was deposited by pure vapor deposition at the shadowed region, whereas, in the unshadowed region, the coating exhibited a binary microstructure which was formed by a mixed deposition of melted particles and evaporated particles. The mechanical properties of the coating were also well under investigation.

MoB/CoCr Coating by Low Pressure Plasma Spraying against Erosion by Molten Al-12.07wt.%Si Alloy

MoB/CoCr, a novel material for thermal spraying, with high durability is used to resist erosion by molten Al-12.07wt.%Si alloy. The durability of the MoB/CoCr coating prepared by low pressure plasma spraying (LPPS) has been investigated using a molten-metal immersion tester. The test revealed that the MoB/CoCr coating has much higher durability without dissolution in the molten Al-12.07wt.%Si alloy. Little change of crystal structure, mainly composed of ternary borides of Co2MoB2 and CoMoB, is observed after the immersion test, suggesting that the ternary borides have much higher durability.

Thermal Corrosion Behavior of NiCoCrAlY Coatings Prepared by Different Thermal Spraying Methods

The protection of thermal parts has become one of the most important techniques with the development of high capability and high thrust-weight ratio for advanced aeroengine. In this paper, using the nickel base superalloy as the substrate, the thermal corrosion performances of NiCoCrAlY coatings using different thermal spraying methods, which consisted of high velocity oxy-fuel (HVOF), detonation spraying (DS), atmospheric plasma praying (APS) and low pressure plasma spraying (LPPS), were studied. The thermal oxidation experiments, which consisted of isothermal oxidation and cycling oxidation experiments were performed to character the thermal performances. From the results, it was shown that the NiCoCrAlY coatings prepared by high velocity oxy-fuel method were slight for 100 hours at 1100°C. The surface of the NiCoCrAlY coatings prepared by HVOF had a dense, continued Al2O3 oxide layer. Among all the methods to prepare NiCoCrAlY coatings, the method of high velocity oxy-fuel showed excellent performance.

Thin Yttria-Stabilized Zirconia Coatings Deposited by Low-Energy Plasma Spraying Under Very Low Pressure Condition

In recent decades, very low pressure plasma spraying (VLPPS) technology (less than 10 mbar), as a next-generation coating process, has been extensively studied, because it can fully evaporate the materials to deposit dense, thin, and columnar grain coatings. This research aims at applying VLPPS with low-energy plasma source to melt or evaporate ceramic materials to develop high-quality thermal barrier coatings. Thin and homogeneous yttria-stabilized zirconia coatings were deposited successfully on a stainless steel substrate using low-power plasma spraying torch F100 (23 kW maximal) under very low pressure (1 mbar). The optical emission spectroscopy was used to analyze the properties of the plasma jet. The phase composition and the microstructure of the coatings were characterized by x-ray diffraction and scanning electron microscopy. The results showed that the YSZ powder was fully melted and partially evaporated, and the coatings had a hybrid microstructure that was combined with the condensation of the YSZ vapor and the melted particles. In addition, the porosity and microhardness of the coatings were evaluated.