Characterization of modified thick thermal barrier coatings

In gas turbines and diesel engines, there is a demand for thick thermal barrier coatings (TTBCs) due to the increased process combustion temperatures. Unfortunately, the increased thickness of plasma-sprayed thermal barrier coatings (TBCs) normally leads to a reduced coating lifetime. For that reason, the coating structures have to be modified. When modifying the structure of TTBCs, the focus is normally on elastic modulus reduction of the thick coating to improve the coating strain tolerance. On the other hand, coating structural modification procedures, such as sealing treatments, can be performed when increased hot-corrosion resistance or better mechanical properties are needed. In this article, several modified zirconia-based TTBC structures with specific microstructural properties are discussed. Coating surface sealing procedures such as phosphate sealing, laser glazing, and sol-gel impregnation were studied as potential methods for increasing the hot-corrosion and erosion resistance of TTBCs. Some microstructural modifications also were made by introducing segmentation cracks into the coating structures by laser glazing and by using special spraying parameters. These last two methods were studied to increase the strain tolerance of TTBCs. The coating microstructures were characterized by optical microscopy, a scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The effect of sealing procedures on the basic thermal and mechanical properties of the coatings was studied. In addition, some correlations between the coating properties and microstructures are also presented, and the advantages and drawbacks of each modification procedure are discussed.     

激光重熔纳米氧化锆热障涂层的抗热冲击性能

采用纳米氧化锆团聚粉末和等离子喷涂技术制备了纳米氧化锆涂层,试验研究了激光重熔工艺参数(激光比能量)对纳米氧化锆涂层抗热冲击性能的影响.结果表明,激光重熔工艺参数对重熔涂层的抗热冲击性能影响显著,采用合适的工艺参数(激光比能量),可以使重熔涂层获得最佳的抗热冲击性能.不同激光重熔工艺参数处理的涂层形成的组织结构不同,使得涂层的抗热冲击性能不同.合适的激光重熔工艺参数下涂层表现出高的抗热冲击性能,主要是因为重熔后的涂层组织结构有利于热应力的释放以及其相结构在高温冲击下具有良好的稳定性.     

Sealing procedures for thick thermal barrier coatings

Zirconia-based 8Y₂O₃-ZrO₂ and 22MgO-ZrO₂ thick thermal barrier coatings (TTBC, 1000 μm), were studied with different sealing methods for diesel engine applications. The aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of TTBCs was sealed with three different methods: (1) impregnation with phosphate-based sealant, (2) surface melting by laser glazing, and (3) spraying of dense top coating with a detonation gun. The thicknesses of the densified top layers were 50–400 μm, depending on the sealing procedure. X-ray diffraction (XRD) analysis showed some minor phase changes and reaction products caused by phosphate-based sealing treatment and some crystal orientation changes and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values. The hot corrosion resistance of TTBCs against 60Na₂SO₄-40V₂O₅ deposit was determined in isothermal exposure at 650 °C for 200 h. Corrosion products and phase changes were studied with XRD after the test. A short-term engine test was performed for the reference coatings (8Y₂O₃-ZrO₂ and 22MgO-ZrO₂) and for the phosphate-sealed coatings. Engine tests, duration of 3 h, were performed at the maximum load of the engine and were intended to evaluate the thermal cycling resistance of the sealed coatings. All of the coatings passed the engine test, but some vertical cracks were detected in the phosphate-sealed coatings.     

Pre-/During-/Post-Laser Processes to Enhance the Adhesion and Mechanical Properties of Thermal-Sprayed Coatings with a Reduced Environmental Impact

Lasers have been used to improve the ultimate performance of thermal spray coatings for specific applications, but the full potential of additional laser treatments must be further explored. Laser treatments (auxiliary processes) can be applied before, during or after thermal spraying (main process), leading to a wide range of coating improvements (microstructure, adhesion, etc.). The aim of this review is to introduce the most significant laser treatments for thermal spray applications. The potential improvements for thermal spray coatings are illustrated by a selection of representative research cases. Laser pretreatments (ablation and texturing) promote coating/substrate adhesion and are suitable to prepare the surface of sensitive substrates such as aluminum, titanium, or magnesium alloys. The use of these techniques, which leads to several benefits such as surfaces free of grit-particle inclusions, directly improves the quality of coatings. Laser treatments applied simultaneously during the spraying process deeply modify the coatings microstructure. These hybrid technologies allow in situ laser melting of coatings, resulting in improved mechanical properties and enhanced wear and corrosion behaviors. Finally, laser posttreatments can improve coatings density and adhesion, and also induce phase transformations and structure refinement. As a summary, laser treatments seem particularly promising for improving the thermal spray coating microstructure and the coating/substrate adhesion. In addition, they offer a more environmentally friendly alternative to the conventional surface preparation treatments.     

Properties of Reactive Plasma Sprayed TiB2-TiC0.3N0.7 Based Composite Coatings with Cr Addition and Laser Surface Treatment

Thick TiB₂-TiC_0.3N_0.7 based composite coatings were deposited by reactive plasma spraying (RPS) successfully in air. The influence to the coating properties (morphology, Vickers microhardness and corrosion resistant property) with Cr addition in the thermal spray powder and TiB₂-TiC_0.3N_0.7 based coatings treated by laser were investigated. The phase composition, structure and properties of composite coatings were studied using XRD, SEM, EDS, Vickers microhardness and electrochemical testers. The results show that the Vickers microhardness values and the density of laser surface treated coatings are improved significantly. The Cr addition in the thermal spray powder can increase the density, improve the wettability of ceramic phases, uniform the phase distribution and enhance the corrosion-resistant property of coatings. However, due to lower microhardness of metal Cr than ceramic phases in coatings, the Vickers microhardness values of plasma sprayed coatings and plasma sprayed coatings with laser surface treatment are a little lower than that of each coating without Cr addition in the thermal spray powder.     

Characterization of thermal sprayed hydroxyapatite powders and coatings

Calcium phosphate materials such as hydroxyapatite (HA) have biocompatible properties that can promote osteogenesis or new bone formation. Thermal spraying is an economical and effective process for coating the hydroxyapatite onto metal. It has been reported that plasma spraying changes the degree of crystallinity as well as the phase composition of the HA. This article reports the preparation and characterization of HA powders and coatings by two thermal spray processes (plasma and combustion flame) and suggests that the state of the starting powder adversely affects the coating characteristics. The raw HA powders are synthesized through a chemical reaction involving calcium hydroxide and orthophosphoric acid. Phase analysis using an X- ray diffractometer revealed that the synthesized powder consists of predominantly the HA phase. Calcined and crushed HA powders of various size ranges were fed into the plasma jet to produce HA coatings on metallic substrates. In addition, some HA powders were sprayed into distilled water by plasma spraying and combustion flame spraying to study powder melting characteristics. Other samples were plasma sprayed onto a solid rotating target to study atomization and impact behavior. The morphology of the rapidly solidified powders and thermal sprayed coatings were examined by scanning electron microscopy (SEM). An X- ray sedimentation particle size analyzer, laser diffraction particle size analyzer, and image analyzer performed the particle size analysis. Preliminary results indicate that particle cohesion, size range, and thermal treatment in the plasma affect the phase and structure of the as- sprayed coating, and some post- spray treatment may be necessary to produce a dense and adherent coating with the desired biocompatible properties.     

Post-treatment of thermal spray coatings on magnesium

Magnesium alloys have a beneficial combination of high strength to weight ratio, good machinability and high recycling potential. Despite this, the application of magnesium still is behind that of other constructive materials mainly due to low wear and corrosion resistance. For more demanding applications, a large amount of surface treatment methods are developed to overcome this problem. Thermal spraying is an efficient and flexible method of coating deposition and is widely used for protection of different materials against corrosion and wear. Nevertheless, the bonding of thermal spray coatings on magnesium alloys is not sufficient, so the following post-treatment processes are needed. One of such possibilities is high energy beam treatment of thermally sprayed coatings. During the heat treatment of magnesium substrates with coating the remelting of coating and a thin surface layer of substrate occurs. Depending on the combination of applied coating system and treatment method, different processes can be realised in modified layers: the alloying of magnesium substrate with other elements to improve corrosion properties, redistribution of hard particles from composite coating and new phases formation during the processing to improve the wear resistance of magnesium alloys. In the present work some examples concerning the laser and electron beam treatment of aluminium based composite coatings as well as infra red irradiation of zinc based coatings are described. Coatings are deposited on magnesium substrates (AM20, AZ31, AZ91) by arc spraying with Zn, ZnAl4 and ZnAl15 solid wires and cored wires in aluminium core with powder filling containing different hard particles, such as boron, silicon and tungsten carbide or titanium oxide. Remelting of thermal spray coatings is carried out by means of continuous irradiation of СО₂-laser in nitrogen or argon atmosphere, electron beam in vacuum and focused tungsten halogen lamp line heater in atmosphere. Microstructure of sprayed coatings as well as that of modified surface layers is investigated by metallographic methods. Corrosion properties are estimated by electrochemical measurements. Abrasion wear resistance of the modified layers is determined by scratch test, corundum grinding disk test and Rubber wheel test. It is shown that all methods applied for processing of thermal spray coatings lead to formation of modified surface layers in magnesium substrate with improved wear and corrosion properties. Different mechanisms of microstructure formation such as redistribution of chemical composition of composite coating components, partial remelting of hard phase particles, and new phases formation are discussed. Electrochemical behaviour of modified surface layers is mostly improved due to alloying, homogenization of element distribution and strong decrease of as-sprayed coating porosity. Abrasion wear resistance of processed magnesium substrates strongly depends on the microstructure and usually is 5 to 20 times higher compared with base material.     

Plasma Spray-PVD: A New Thermal Spray Process to Deposit Out of the Vapor Phase

Plasma spray-physical vapor deposition (PS-PVD) is a low pressure plasma spray technology recently developed by Sulzer Metco AG (Switzerland). Even though it is a thermal spray process, it can deposit coatings out of the vapor phase. The basis of PS-PVD is the low pressure plasma spraying (LPPS) technology that has been well established in industry for several years. In comparison to conventional vacuum plasma spraying (VPS) or low pressure plasma spraying (LPPS), the new proposed process uses a high energy plasma gun operated at a reduced work pressure of 0.1 kPa (1 mbar). Owing to the high energy plasma and further reduced work pressure, PS-PVD is able to deposit a coating not only by melting the feed stock material which builds up a layer from liquid splats but also by vaporizing the injected material. Therefore, the PS-PVD process fills the gap between the conventional physical vapor deposition (PVD) technologies and standard thermal spray processes. The possibility to vaporize feedstock material and to produce layers out of the vapor phase results in new and unique coating microstructures. The properties of such coatings are superior to those of thermal spray and electron beam-physical vapor deposition (EB-PVD) coatings. In contrast to EB-PVD, PS-PVD incorporates the vaporized coating material into a supersonic plasma plume. Owing to the forced gas stream of the plasma jet, complex shaped parts such as multi-airfoil turbine vanes can be coated with columnar thermal barrier coatings using PS-PVD. Even shadowed areas and areas which are not in the line of sight of the coating source can be coated homogeneously. This article reports on the progress made by Sulzer Metco in developing a thermal spray process to produce coatings out of the vapor phase. Columnar thermal barrier coatings made of Yttria-stabilized Zircona (YSZ) are optimized to serve in a turbine engine. This process includes not only preferable coating properties such as strain tolerance and erosion resistance but also the simultaneous coverage of multiple air foils.     

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。     

Investigation on surface figuration and microstructure of laser glazed nanostructure zirconia thermal barrier coatings

CO2 continuous wave laser beam had been applied to the laser glazing of plasma sprayed nanostructure zirconia thermal barrier coatings. The effects of luser glazing processing parameters on the surface figuration and microstructure change had been carried out, the microstructure and phase composition of the coatings had been evaluated by the scanning electron microscope (SEM) and the X-ray diffraction (XRD). SEM observation indicates that the microstructure of the as-glazed coating could be altered from single columnar structure to a combination of the columnar grain and fine equiaxed grain with the different laser glazing conditions. XRD analysis illustrates that the predominance phase of the as-glazed coating is the metastable tetragonal phase, and the glazed coating with the single columnar structure has shown the clear orientation in (220) and (400) peaks while the other coatings do not show that.     

Black anodic coatings for space applications: Study of the process parameters,characteristics and mechanical properties

Black inorganic anodized aluminium alloys are used for managing passive thermal control on spacecraft and for avoiding stray light in optical equipment. Spalling of these coatings has sometimes been observed after thermal cycling on 2XXX and 7XXX aluminium alloys. This phenomenon could generate particulate contamination in satellites and may affect mission lifetime. In this work, the influences of the four main steps of the process (pretreatments, sulphuric anodizing, colouring and sealing) on the coating characteristics have been studied for a 7175T7351 aluminium alloy. The chemical heterogeneity of the coating has been underlined, and its mechanical behaviour observed through crazing. Scratch-testing, used to evaluate coating adhesion to its substrate, revealed the negative impact of thermal cycling.     

Influence of Laser Glazing on the Characterization of Plasma-Sprayed YSZ Coatings

In this study, 8 wt.% yttria-stabilized zirconia powder was deposited on the substrates by atmospheric plasma spray. The coatings were post-treated by laser glazing under different parameters in order to densify them. The characterization of the laser molten pools under different laser treatment conditions was studied. Preheating processes were also employed. Scanning electron microscopy observations of the surface and cross section of as-sprayed and laser-glazed coatings were carried out to investigate the influence of laser glazing on the microstructure on laser-glazed coatings. The results show that preheating processes improve the coating in terms of deepening the laser-glazed layer, reducing the number of vertical cracks and surface density of cracks and widening the molten pool. Finally, the influences of linear energy density on the characterization of the glazed layer are discussed.     

An Experimental Study of Microstructure-Property Relationships in Thermal Barrier Coatings

The thermal-mechanical properties of thermal barrier coatings are highly influenced by the defects present in coating microstructure. The aim of this study was to meet the future needs of the gas turbine industry by further development of zirconia coatings through the assessment of microstructure-property relationships. A design of experiments was conducted for this purpose with current, spray distance, and powder feed rate as the varied parameters. Microstructure was assessed with SEM and image analysis was used to characterize porosity content. Evaluations were carried out using laser flash technique to measure thermal properties. A bi-layer beam curvature technique in conjunction with controlled thermal cycling was used to assess the mechanical properties, in particular their nonlinear elastic response. Coating lifetime was evaluated by thermo-cyclic fatigue testing. Relationships between microstructure and coating properties are discussed. Dense vertically cracked microstructure and highly porous microstructure with large globular pores were also fabricated. Correlations between parameters obtained from nonlinear measurements and lifetime based on a priori established microstructural analysis were attempted in an effort to develop and identify a simplified strategy to assess coating durability following sustained long-term exposure to high temperature thermal cycling.     

Plasma spraying of functionally graded yttria stabilized zirconia/NiCoCrAlY coating system using composite powders

Pre-alloyed and plasma spheroidized composite powders were used as the feedstock in the plasma spraying of functionally graded yttria stabilized zirconia (YSZ)/NiCoCrAlY coatings. The ball milling parameters of the composite powders and the plasma spraying parameters for preparing functionally graded materials (FMGs) coatings were optimized to obtain the best performance for the thermal barrier coatings (TBCs). Microstructure, physical, mechanical, and thermal properties of YSZ/NiCoCrAlY FGMs coatings were investigated and compared with those of traditional duplex coatings. Results showed that the advantages of using pre-alloyed composite powders in plasma spraying were to ensure chemical homogeneity and promote uniform density along the graded layers. Microstructure observation showed the gradient distribution of YSZ and NiCoCrAlY phases in the coating, and no clear interface was found between two adjacent different layers. Oxidation occurred during plasma spray and the resultant aluminum oxide combines with YSZ in a wide range of proportions. The bond strength of functionally graded coatings was about twice as high as that of the duplex coatings because of the significant reduction of the residual stresses in the coatings. The thermal cycling resistance of functionally graded coating was much better than that of duplex coating.     

黏结相对YSZ基高温封严涂层沉积效率和热循环性能的影响∗

为解决 YSZ(Y₂O₃ 部分稳定的 ZrO₂ ) 陶瓷基高温封严涂层沉积效率较低和易于过早剥落失效的问题,选择氧化铝 (Al ₂O₃ )、钇铝石榴石(Y₃Al ₅O₁₂ ,YAG)及镁铝尖晶石(MgAl ₂O₄ )为三种高温黏结相,通过喷雾造粒的方法分别制备出 B0(无黏结相)、B1(含 Al ₂O₃ )、B2(含 YAG)和 B3(含 MgAl ₂O₄ )等四种 YSZ 基团聚颗粒,研究黏结相对涂层沉积效率和热循环性能的影响。 结果表明,YAG 的黏附和包裹作用最强,与 B0 型粉末相比,B2 型粉末的沉积效率提高了 181. 49%;热循环过程中不断增长的裂纹扩展驱动力和陶瓷材料的脆性属性是导致陶瓷涂层剥落失效的主要原因,由于 YAG 相的弥散增韧作用,B2 型涂层的断口呈现出类韧窝形貌,与 B0 型涂层相比,B2 型涂层的断裂韧性提高了 39. 76%,热循环寿命提高了 43. 68%。 YAG 黏结相的添加显著地提高了 YSZ 陶瓷基高温封严涂层的沉积效率,同时较好地提高了涂层的热循环寿命。     

Cyclic oxidation resistance of Ni–Al alloy coatings deposited on steel by a cathodic arc plasma process

The cyclic oxidation resistance of nickel-aluminide coatings deposited on steel using a cathodic arc plasma (CAP) process has been investigated. Our results show that nickel-aluminide films can be successfully deposited on carbon steel and stainless steel substrates by this process; NiAl₃ is the major phase in the deposited films. The thermal cycling behaviour suggests that such coatings can resist oxidation through physical blocking of oxygen, either by the coating itself or by the aluminium oxide scale subsequently formed in-service. Aluminium diffusion inwards to the substrate may also be beneficial to the thermal oxidation resistance. The coating protects stainless steel substrate materials at 500°C by transforming the NiAl₃ phase into NiAl, producing aluminium oxide on the open substrate surface. At 800°C, oxide flaking is suppressed by the trace amounts of nickel or aluminium which have partially diffused into the substrate.     

Improving corrosion properties of high-velocity oxy-fuel sprayed inconel 625 by using a high-power continuous wave neodymium-doped yttrium aluminum garnet laser

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as high-velocity oxy-fuel (HVOF) and plasma spraying, these coatings may in certain service conditions show inadequate performance,e.g., due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermalsprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal-sprayed coating material. To overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coatings was studied in the present work. The coating material was nickel-based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by HVOF spraying onto mild steel substrates. High-power continuous wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical microscopy and scanning electron microscopy (SEM). Laser remelting resulted in homogenization of the sprayed structure. This strongly improved the performance of the laser-remelted coatings in adhesion, wet corrosion, and high-temperature oxidation testing. The properties of the laser-remelted coatings were compared directly with the properties of as-sprayed HVOF coatings and with plasma-transferred arc (PTA) overlay coatings and wrought Inconel 625 alloy.     

Effect of Particle Size on the Micro-cracking of Plasma-Sprayed YSZ Coatings During Thermal Cycle Testing

The failure of plasma-sprayed thermal barrier coatings (TBCs) during service or thermal cycle testing usually results from internal cracking in the top coat, erosion and CMAS (calcium-magnesium-alumina-silicate)-induced damage, etc. The microstructure of ceramic coatings affects their durability and other properties of TBCs. In the present study, yttria-stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. In addition, the effect of particle size on damage evolution in the top coat was investigated. It is found that the coatings deposited using coarse particles show the higher thermal cycle life. Crack length grew with increasing numbers of thermal cycles. The faster crack growth rate can be found for the coatings deposited from fine particles. The porosity of the coating made from the coarse powder is larger than the porosity of the coating made from fine powder both in the as-sprayed condition and after thermal cycling. The changes in crack growth rate and the porosity are related to the effect of sintering and stress evolution in coatings during the thermal cyclic tests.     

Microstructures and Properties of Laser-Glazed Plasma-Sprayed ZrO2-YO1.5/Ni-22Cr-10AI-1Y Thermal Barrier Coatings

Thermal barrier coatings (TBCs) consisting of two layers with various yttria contents (ZrO ₂- YO_1.5/Ni-22Cr-10Al- lY) were plasma sprayed, and parts of the various specimens were glazed by using a pulsed CO₂ laser. All the specimens were then subjected to furnace thermal cycling tests at 1100 °C; the effect of laser glazing on the durability and failure mechanism of the TBCs was then evaluated. From these results, two models were developed to show the failure mechanism of as- sprayed and laser- glazed TBCs: model A, which is thermal-stress dominant, and model V, which is oxidation-stress dominant. For top coats containing cubic phase, cubic and monoclinic phases, or tetragonal and a relatively larger amount of monoclinic phases, whose degradation is thermal- stress dominant, laser glazing improved the durability of TBCs by a factor of about two to six. Segmented cracks that occurred during glazing proved beneficial for accommodating thermal stress and raising the tolerance to oxidation, which resulted in a higher durability. Thermal barrier coatings with top coats containing tetragonal phase had the highest durability. Degradation of such TBCs resulted mainly from oxidation of the bond coats. For top coats with a greater amount of monoclinic phase, thermal mismatch stress occurred during cooling and detrimentally affected durability.     

Low conductivity plasma sprayed thermal barrier coating using hollow psz spheres: Correlation between thermophysical properties and microstructure

Life and thermal properties of plasma sprayed TBCs - widely used in gas turbine engines - are closely related to the microstructure of the ceramic top coating. Especially, the thermal behaviour of this coating is induced by the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material.A specific hollow yttria partially stabilised zirconia powder was produced in a one-step process by spray drying and an experimental statistical design study was conducted to investigate the influence of spraying variables (primary and secondary gas flow rates, arc current, spraying distance, spraying angle and traverse speed) on structure and properties of resulting plasma sprayed coatings. The coatings were characterized with respect to deposition efficiency, roughness, porosity and thermal conductivity. A reduction of 25% of the thermal conductivity was achieved by improving the spray and powder parameters. A quantitative characterization of the porous structure using image analysis of polished cross-sections was implemented. The parameters that have relevant influence on the coating porous structure were identified, and their relative importance was determined. An attempt was made to identify morphological criteria of the porous network (coarse/fine porosity ratio, cracks total length, cracks orientation) correlating with the thermal conductivity values.