Preparation of SrZrO<Subscript>3</Subscript> Thermal Barrier Coating by Solution Precursor Plasma Spray

The solution precursor plasma spray (SPPS) process is capable of depositing highly durable thermal barrier coatings (TBCs). In this study, an aqueous chemical precursor feedstock was injected into the plasma jet to deposit SrZrO₃ thermal barrier coating on metal substrate. Taguchi design of experiments was employed to optimize the SPPS process. The thermal characteristics and phase evolution of the SrZrO₃ precursor, as well as the influence of various spray parameters on the coating deposition rate, microhardness, microstructure, and phase stability, were investigated. The experimental results showed that, at given spray distance, feedstock flow rate, and atomization pressure, the optimized spray parameters were arc current of 600 A, argon flow rate of 40 L/min, and hydrogen flow rate of 10 L/min. The SrZrO₃ coating prepared using the optimized spray parameters had single-pass thickness of 6.0 μm, porosity of ~18%, and microhardness of 6.8 ± 0.1 GPa. Phase stability studies indicated that the as-sprayed SrZrO₃ coating had good phase stability in the temperature range from room temperature to 1400 °C, gradually exhibiting a phase transition from t′-ZrO₂ to m-ZrO₂ in the SrZrO₃ coating at 1450 °C with increasing time, while the SrZrO₃ phase did not change.     

溶液前驱体等离子喷涂热障涂层的高温稳定性

以锆盐和钇盐水溶液为原料,采用溶液前驱体等离子喷涂(SPPS)技术制备了氧化钇部分稳定氧化锆(7YSZ)热障涂层,利用X射线衍射仪(XRD)、拉曼光谱(Raman)和扫描电镜(SEM)研究了SPPS涂层在1 200~1 400℃下的相结构和微观结构稳定性。结果表明:沉积态SPPS涂层为亚稳四方相(t′),在1 200℃和1 300℃时未发生亚稳四方相(t′)向单斜相(m)的转变,在1 400℃热处理100h后出现了少量单斜相。在1 200~1 400℃下,SPPS涂层发生了不同程度的烧结现象;随热处理温度升高,SPPS涂层晶粒长大速率增加,在1 200、1 300和1 400℃热处理100h后,最大晶粒尺寸分别约为350、700和1 100nm。同时,在1 400℃下热处理100h后,涂层中仍然存在大量1μm的微孔,表明其具有较好的微孔保持能力。     

Processing Parameter Effects and Thermal Properties of Y<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Nanostructured Environmental Barrier Coatings Synthesized by Solution Precursor Induction Plasma Spraying

The solution precursor plasma spray process, in which a solution of metal salts is axially injected into an induction thermal plasma, is suitable for deposition of nanostructured environmental barrier coatings. The effects of main processing parameters, namely the solution precursor concentration, spraying distance, reactor pressure, and atomization gas flow rate, have been analyzed using D-optimal design of experiments regarding the deposition rate and coating porosity responses. Among these four parameters, the solution precursor concentration had the greatest influent on the coating structure, followed by the spraying distance and reactor pressure, and finally the atomization gas flow rate with a small contribution. It is pointed out that the species that impact on the substrate are agglomerates of nanoparticles. The equivalent thermal conductivity of selected coatings was computed from experimental temperature evolution curves obtained by laser flash thermal diffusivity analysis, using two methods: a multilayer finite-element model with optimization, and a multilayer thermal diffusion model. The results of the two models agree, with coatings exhibiting low thermal conductivity between 0.7 and 1 W/(m K) at 800 °C.     

Thermal Barrier Coatings Made by the Solution Precursor Plasma Spray Process

The solution precursor plasma spray (SPPS) process is a relatively new and flexible thermal spray process that can produce a wide variety of novel materials, including some with superior properties. The SPPS process involves injecting atomized droplets of a precursor solution into the plasma. The properties of resultant deposits depend on the time-temperature history of the droplets in the plasma, ranging from ultra-fine splats to unmelted crystalline particles to unpyrolized particles. By controlling the volume fraction of these three different constituents, a variety of coatings can be produced, all with a nanograin size. In this article, we will be reviewing research related to thermal barrier coatings, emphasizing the processing conditions necessary to obtain a range of microstructures and associated properties. The SPPS process produces a unique strain-tolerant, low-thermal conductivity microstructure consisting of (i) three-dimensional micrometer and nanometer pores, (ii) through-coating thickness (vertical) cracks, (iii) ultra-fine splats, and (iv) inter-pass boundaries. Both thin (0.12 mm) and thick (4 mm) coatings have been fabricated. The volume fraction of porosity can be varied from 10% to 40% while retaining the characteristic microstructure of vertical cracks and ultra-fine splats. The mechanism of vertical crack formation will be described.     

Microstructure and Thermal Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Coating

In the present work, Yb₂Si₂O₇ powder was synthesized by solid-state reaction using Yb₂O₃ and SiO₂ powders as starting materials. Atmospheric plasma spray technique was applied to fabricate Yb₂Si₂O₇ coating. The phase composition and microstructure of the coating were characterized. The density, open porosity and Vickers hardness of the coating were investigated. Its thermal stability was evaluated by thermogravimetry and differential thermal analysis (TG-DTA). The thermal diffusivity and thermal conductivity of the coating were measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₂Si₂O₇ with amorphous phase. The coating had a dense structure containing defects, such as pores, interfaces and microcracks. The TG-DTA results showed that there was almost no mass change from room temperature to 1200 °C, while a sharp exothermic peak appeared at around 1038 °C in DTA curve, which indicated that the amorphous phase crystallized. The thermal conductivity of the coating decreased with rise in temperature up to 600 °C and then followed by an increase at higher temperatures. The minimum value of the thermal conductivity of the Yb₂Si₂O₇ coating was about 0.68 W/(m K).     

Microstructure and Thermomechanical Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating

In this study, a Yb₂O₃ coating was fabricated by the atmospheric plasma spray technique. The phase composition, microstructure, and thermal stability of the coating were examined. The thermal conductivity and thermal expansion behavior were also investigated. Some of the mechanical properties (elastic modulus, hardness, fracture toughness, and flexural strength) were characterized. The results reveal that the Yb₂O₃ coating is predominantly composed of the cubic Yb₂O₃ phase, and it has a dense lamellar microstructure containing defects. No mass change and exothermic phenomena are observed in the thermogravimetry and differential thermal analysis curves. The high-temperature x-ray diffraction results indicate that no phase transformation occurs from room temperature to 1500 °C, revealing the good phase stability of the Yb₂O₃ coating. The coefficient of thermal expansion of the Yb₂O₃ coating is (7.50-8.67)?×?10^?6 K^?1 in the range of 200-1400 °C. The thermal conductivity is about 1.5 W m^?1 K^?1 at 1200 °C. The Yb₂O₃ coating has excellent mechanical properties and good damage tolerant. The unique combination of these properties implies that the Yb₂O₃ coating might be a promising candidate for T/EBCs applications.     

Microstructure and Adhesion Strength of Ni<Subscript>3</Subscript>Ti Coating Prepared by Mechanical Alloying and HVOF

In the present work we report the development of Ni₃Ti intermetallic compound by high energy ball milling of Ni and Ti powders. The ball milled powders were taken at various intervals (4, 6, 8, 10, and 11 h) to analyze the formation of Ni _x Ti _x intermetallic compounds. The ball milled powders were analyzed using scanning electron microscopy and X-ray diffraction. The layered shaped powder particles of Ni₃Ti phase were formed after 11 h of ball milling, which was confirmed by X-ray peaks. Further High-Velocity Oxy-Fuel (HVOF) process was used to coat Ni₃Ti and Ni₃Ti + (Cr₃C₂ + 20NiCr) on MDN 420 steel. Both the coated materials displayed excellent cohesion with minimal porosity less than 2%. The tensile adhesion strength test was carried out on these coatings to check the bond strength. Out of the two the Ni₃Ti coating showed excellent bond strength of 41.04 MPa compared to that of Ni₃Ti + (Cr₃C₂ + 20NiCr) coating.     

电热爆炸喷涂和等离子喷涂联合法制备热障涂层的研究

采用电热爆炸喷涂和等离子喷涂联合制备热障涂层,以电热爆炸喷涂法在DZ125合金表面制备NiCoCrAlY粘结层,以等离子喷涂技术制备陶瓷顶层。利用扫描电镜(SEM)和X射线衍射(XRD)仪对所制备的粘结层进行分析,结果表明:电热爆炸喷涂的粘结层与基体结合良好,喷涂态的粘结层的相主要由Ni3Al组成。采用联合法制备的热障涂层,在喷涂态的陶瓷层、粘结层、基体3者结合良好,界面清晰。在高温热循环过程中,粘结层/陶瓷层界面间生成了连续、致密的Al2O3膜,阻碍粘结层的氧化。粘结层/TGO界面产生平行于界面的裂纹,是导致热障涂层失效的主要原因。     

Low Thermal Conductivity Yttria-Stabilized Zirconia Thermal Barrier Coatings Using the Solution Precursor Plasma Spray Process

The primary function of thermal barrier coatings (TBCs) is to insulate the underlying metal from high temperature gases in gas turbine engines. As a consequence, low thermal conductivity and high durability are the primary properties of interest. In this work, the solution precursor plasma spray (SPPS) process was used to create layered porosity, called inter-pass boundaries, in yttria-stabilized zirconia (YSZ) TBCs. IPBs have been shown to be effective in reducing thermal conductivity. Optimization of the IPB microstructure by the SPPS process produced YSZ TBCs with a thermal conductivity of 0.6 W/mK, an approximately 50% reduction compared to standard air plasma sprayed (APS) coatings. In preliminary tests, SPPS YSZ with IPBs exhibited equal or greater furnace thermal cycles and erosion resistance compared to regular SPPS and commercially made APS YSZ TBCs.     

Development of a Thermal Transport Database for Air Plasma Sprayed ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Thermal Barrier Coatings

Thermal diffusivities of air plasma sprayed (APS) thermal barrier coatings (TBCs) were measured by the laser flash method. The data were used to calculate thermal conductivity of TBCs when provided with density and specific heat data. Due to the complicated microstructure and other processing-related parameters, thermal diffusivity of TBCs can vary as much as three- to four-fold. Data collected from over 200 free-standing ZrO₂-7-8wt.%Y₂O₃ TBCs are presented. The large database gives a clear picture of the expected “band” of thermal diffusivity values. When this band is used as a reference for thermal diffusivity of a specific TBC, the thermal transport property of the TBC can be more precisely described. This database is intended to serve researchers and manufacturers of TBCs as a valuable resource for the evaluation of TBCs.     

Thermal Spray Deposition,Phase Stability and Mechanical Properties of La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>/LaAlO<Subscript>3</Subscript> Coatings

This paper deals with the deposition of La₂Zr₂O₇ (LZO) and LaAlO₃ (LAO) mixtures by air plasma spray (APS). The raw material for thermal spray, single phase LZO and LAO in a 70:30 mol.% ratio mixture was prepared from commercial metallic oxides by high-energy ball milling (HEBM) and high-temperature solid-state reaction. The HEBM synthesis route, followed by a spray-drying process, successfully produced spherical agglomerates with adequate size distribution and powder-flow properties for feeding an APS system. The as-sprayed coating consisted mainly of a crystalline LZO matrix and partially crystalline LAO, which resulted from the high cooling rate experienced by the molten particles as they impact the substrate. The coatings were annealed at 1100 °C to promote recrystallization of the LAO phase. The reduced elastic modulus and hardness, measured by nanoindentation, increased from 124.1 to 174.7 GPa and from 11.3 to 14.4 GPa, respectively, after the annealing treatment. These values are higher than those reported for YSZ coatings; however, the fracture toughness (K _IC) of the annealed coating was only 1.04 MPa m^0.5.     

Identification of Desirable Precursor Properties for Solution Precursor Plasma Spray

In solution precursor plasma spray chemical precursor solutions are injected into a standard plasma torch and the final material is formed and deposited in a single step. This process has several attractive features, including the ability to rapidly explore new compositions and to form amorphous and metastable phases from molecularly mixed precursors. Challenges include: (a) moderate deposition rates due to the need to evaporate the precursor solvent, (b) dealing on a case by case basis with precursor characteristics that influence the spray process (viscosity, endothermic and exothermic reactions, the sequence of physical states through which the precursor passes before attaining the final state, etc.). Desirable precursor properties were identified by comparing an effective precursor for yttria-stabilized zirconia with four less effective candidate precursors for MgO:Y₂O₃. The critical parameters identified were a lack of major endothermic events during precursor decomposition and highly dense resultant particles.     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiC Composite Prepared by Spark Plasma Sintering Process: Evaluation of Mechanical and Tribological Properties

Al₂O₃-10TiC composites were synthesized by spark plasma sintering (SPS) process. Microstructural and mechanical properties of the composite reveal homogeneous distribution of the fine TiC particles in the matrix. The samples were produced with different sintering temperature, and it shows that the hardness and density gradually increases with increasing sintering temperature. Abrasion wear test result reveals that the composite sintered at 1500 °C shows high abrasion resistance (wt. loss ~ 0.016 g) and the lowest abrasion resistance was observed for the composite sample sintered at 1100 °C (wt. loss ~ 1.459 g). The profilometry surface roughness study shows that sample sintered at 1100 °C shows maximum roughness (R_a = 6.53 µm) compared to the sample sintered at 1500 °C (R_a = 0.66 µm) corroborating the abrasion wear test results.     

Resistance switching of heteroepitaxial Cr-doped SrZrO<Subscript>3</Subscript> thin films

Heteroepitaxial Cr-doped SrZrO₃ thin films were grown on 200 nm-thick SrRuO₃ films deposited on SrTiO₃ (100) substrates by pulsed laser deposition. The Cr-doped SrZrO₃ films on the SrRuO₃ bottom electrode exhibited an XRD peak for the (hh0/00l) planes of SrZrO₃ and SrRuO₃ thin films, showing a good epitaxial relationship. The I–V characteristics of the Au/Cr-doped SrZrO₃/SrRuO₃ MIM structures revealed resistance switching behavior with an ON/OFF resistance ratio of 20.     

Mechanical Properties of Layered La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Thermal Barrier Coatings

Lanthanum zirconate (La₂Zr₂O₇) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La₂Zr₂O₇ and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La₂Zr₂O₇ + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La₂Zr₂O₇ TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La₂Zr₂O₇. The erosion rate of La₂Zr₂O₇ TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La₂Zr₂O₇. DCL porous 8YSZ + La₂Zr₂O₇ had a lower erosion rate than other SCL and DCL La₂Zr₂O₇ coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.     

Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Environmental Barrier Coatings Deposited by Various Thermal Spray Techniques: A Preliminary Comparative Study

Dense, crack-free, uniform, and well-adhered environmental barrier coatings (EBCs) are required to enhance the environmental durability of silicon (Si)-based ceramic matrix composites in high pressure, high gas velocity combustion atmospheres. This paper represents an assessment of different thermal spray techniques for the deposition of Yb₂Si₂O₇ EBCs. The Yb₂Si₂O₇ coatings were deposited by means of atmospheric plasma spraying (APS), high-velocity oxygen fuel spraying (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) techniques. The initial feedstock, as well as the deposited coatings, were characterized and compared in terms of their phase composition. The as-sprayed amorphous content, microstructure, and porosity of the coatings were further analyzed. Based on this preliminary investigation, the HVOF process stood out from the other techniques as it enabled the production of vertical crack-free coatings with higher crystallinity in comparison with the APS and SPS techniques in atmospheric conditions. Nevertheless, VLPPS was found to be the preferred process for the deposition of Yb₂Si₂O₇ coatings with desired characteristics in a controlled-atmosphere chamber.     

Microstructure Characterization of the FeAl<Subscript>2</Subscript>O<Subscript>4</Subscript>-Based Nanostructured Composite Coating Synthesized by Plasma Spraying Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al Powders

The microstructure of the coating prepared by reactive plasma spraying Fe₂O₃/Al composite powders was characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicated that the coating exhibited nanostructured microstructure which consisted of FeAl₂O₄, Fe or Fe solid solution, Al₂O₃ and a little FeAl. In the composite coating, spherical Fe particles (tens of nanometers to hundreds of nanometers) were distributed uniformly within the equiaxed and columnar nanograins FeAl₂O₄ matrix. There were two kinds of Al₂O₃ phases present in the composite coating. One kind was nano-sized Al₂O₃ particles uniformly dispersed within the matrix, forming eutectic structure of (FeAl₂O₄ + γ-Al₂O₃); the other was 1-1.5 μm Al₂O₃ particles embedded individually within the matrix. The composite coating had higher toughness than the conventional microstructured Al₂O₃ coating.     

Effect of Critical Plasma Spray Parameters on Microstructure and Microwave Absorption Property of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript>/Cordierite Coatings

Ti₃SiC₂/cordierite coatings with different critical plasma spray parameters (CPSP) were fabricated via atmospheric plasma spraying method. The microstructure and phase constitution of the as-sprayed Ti₃SiC₂/cordierite coatings were characterized. The effects of CPSP conditions on the electromagnetic shielding, and dielectric and microwave absorption properties of coatings in the frequency of 8.2-12.4 GHz were also measured and investigated. The results showed that both real and imaginary part of the complex permittivity decrease with increasing CPSP values, which can be ascribed to the decomposition of some Ti₃SiC₂ into TiC. The calculated reflection loss of the as-sprayed Ti₃SiC₂/cordierite coatings with different CPSP conditions and thicknesses indicates that coatings with CPSP 0.3, 0.35, and 0.425 exhibit excellent microwave absorption property in the thickness of 1.5 mm. In order to broaden the bandwidth of the coatings, a double-layer coating system was designed. The calculated reflection loss results show that when the thickness of matching layer is 0.3 mm and the thickness of absorbing layer is 1.5 mm, the double-layer coating system shows a proper microwave absorption property with a minimum absorption value of ?17.37 dB at 9.67 GHz and a absorption bandwidth (RL less than ?5 dB) of 4.16 GHz in the investigated frequency.     

Microstructure and Optical Properties of SS/Mo/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Spectrally Selective Solar Absorber Coating

Surface-textured Mo thin film is fabricated by magnetron sputtering through the adjustment of deposition parameters, which exhibits a high absorptance of 0.80 and a low emittance of 0.09. The single-layer Mo deposited on stainless steel (SS) is characterized by x-ray diffraction (XRD), ultra-high resolution scanning electron microscope, atomic force microscope and optical measurement. The controlled surface roughness combined with larger aspect ratio contributes much to the high absorptance and low emittance. Based on the SS/Mo coating, a spectrally selective coating (SS/Mo/Al₂O₃) is designed and fabricated. The coating shows an amorphous structure and exhibits an absorptance of 0.90 and an emittance of 0.08. Tauc-Lorentz and Drude free-electron models are used to modeling the optical properties of Al₂O₃ and Mo layers by phase-modulated spectroscopic ellipsometry.     

Preparation of TiO<Subscript>2</Subscript> Nanopowders by Plasma Spray and Characterizations

TiO₂ powders with the range of 10-60 nm were prepared successfully by plasma spray in the self-developed plasma spray equipment. The prepared nanopowders were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The results showed that the prepared TiO₂ nanopowders were the mixture of anatase phase and rutile phase, the main phase was anatase. There were O, Ti, and C elements in powders; Ti element still existed in tetravalent. The photocatalytic degradation of methyl orange indicated that all methyl orange (20 mg/L) can be degraded fully when the addition of prepared TiO₂ nanopowders and illumination time were 1 g/L and 150 min, respectively.