Present status and future prospects of plasma sprayed multilayered thermal barrier coating systems

Thermal barrier coatings (TBCs) play a pivotal role in protecting the hot structures of modern turbine engines in aerospace as well as utility applications. To meet the increasing efficiency of gas turbine technology, worldwide research is focused on designing new architecture of TBCs. These TBCs are mainly fabricated by atmospheric plasma spraying (APS) as it is more economical over the electron beam physical vapor deposition (EB-PVD) technology. Notably, bi-layered, multi-layered and functionally graded TBC structures are recognized as favorable designs to obtain adequate coating performance and durability. In this regard, an attempt has been made in this article to highlight the structure, characteristics, limitations and future prospects of bi-layered, multi-layered and functionally graded TBC systems fabricated using plasma spraying and its allied techniques like suspension plasma spray (SPS), solution precursor plasma spray (SPPS) and plasma spray –physical vapor deposition (PS-PVD).     

等离子喷涂羟基磷灰石涂层

羟基磷灰石由于具有优良的生物性能，被广泛应用于生物材料领域，而等离子喷涂制备羟基磷灰石涂层是应用最为广泛的制备方法之一。在综合国内外献的基础上，本从羟基磷灰石的本征性能、喷涂工艺的影响、结合强度和梯度涂层等方面进行综述。     

等离子喷涂羟基磷灰石涂层

羟基磷灰石由于具有优良的生物性能，被广泛应用于生物材料领域，而等离子喷涂制备羟基磷灰石涂层是应用最为广泛的制各方法之一。在综合国内外文献的基础上，本文从羟基磷灰石的本征性能、喷涂工艺的影响、结合强度和梯度涂层等方面进行综述。     

Structure and properties of AlSi/polyester coating

Powder coatings, which are formed by plasma spray technique, are being used in industrial applications. Resistance of plastics and their based composite materials to chemicals, solvents, atmospheric conditions, and high impact strength even at low service temperature increases the importance of plastic and plastic based coating applications. In this study, aluminum silicon based polyester (AlSi/polyester) coating was applied by plasma spraying technique with and without intermediate bond layer coat (NiAl). The effect of coating thickness, intermediate bond layer coat, and plasma spraying parameters on bond strength of coating were studied experimentally. The bond strengths of the coatings were determined according to the ASTM C‐633–79. Microstructures of the coating were examined by optic microscopy and scanning electron microscopy (SEM), respectively. Obtained results indicated that plasma spraying current rate, coating thickness, and spraying distance were important factors on bond strength of coating. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2437–2444, 2004     

Plasma-Sprayed YSZ/Ni–LSGM–LSCo Intermediate-Temperature Solid Oxide Fuel Cells

An intermediate-temperature solid oxide fuel cell based on YSZ/Ni anode, LSGM electrolyte, and lanthanum strontium cobaltite (LSCo) cathode coatings were sequentially deposited onto a porous Ni substrate by atmospheric plasma spraying (APS). The spray parameters for each coating are well selected. The sprayed YSZ/Ni anode having a novel nanostructure with advantageous triple phase boundaries after hydrogen reduction shows a good electrocatalytic activity for hydrogen oxidation reactions. Dense LSGM with a thickness of about 60 μm and a conductivity of about 0.053 S/cm at 800°C shows a good gas tightness and gives an open circuit voltage value >1 V. The sprayed LSCo cathode with a thickness of 10–20 μm and a porosity of about 25% keeps the right phase structure and good porous network microstructure for conducting electrons and negative oxygen ions after plasma spraying and heat treatment at about 1000°C for 1 h. A maximum output power density of the sprayed cell achieved 365 mW/cm² at 800°C, 250 mW/cm² at 750°C, and 180 mW/cm² at 700°C. The results show that the use of APS cell allowed the reduction of the operating temperature to below 750°C.     

Atmospheric plasma sprayed silica–hydroxyapatite coatings on magnesium alloy substrates

Silica-doped hydroxyapatite as a bioactive coating presents some advantages compared to the pure one, such as: increased in vivo bioactivity and early bone ingrowth. The aim of this study is to obtain a deposition of silica-doped hydroxyapatite on magnesium alloy plates by atmospheric plasma spraying. The coating material was prepared by a precipitation method with sodium silicate addition as a source of silica, and various methods were used to characterize it. Spraying conditions including powder feed rate and current values were varied. The coating properties were defined by determining the purity, phase composition, morphology and corrosion protection of the HAP–Si deposits on the magnesium plates.     

A comparative study on the synthesis and properties of suspension and solution precursor plasma sprayed hydroxyapatite coatings

In the present study, hydroxyapatite (HAp) coatings were deposited on Ti-6Al-4V alloy by solution precursor plasma spray (SPPS) and suspension plasma spray (SPS) processes and the properties of the coatings were compared. The feedstock powder for SPS method was prepared by coprecipitation technique and characterized for phase and morphology. The obtained HAp coatings were characterized by X-ray diffractometry, Raman spectroscopy and FT-IR spectroscopy. The biocompatibility of the coatings was evaluated using osteoblast like cells. Both the SPS and SPPS hydroxyapatite coatings exhibited similar crystallinity. Interestingly, the HAp-SPS coating showed marginally higher biocompatibility compared to HAp-SPPS and control samples. The wear and corrosion behavior of these coatings was also studied in Hanks' medium. The hydroxyapatite coating fabricated from SPS technique exhibited better corrosion and wear resistance compared to SPPS coating.     

Processing of a multi‐layer polyetheretherketone composite for use in acetabular cup prosthesis

The hydroxyapatite polyetheretherketone (HAPEEK) as a non‐degradable bioactive polymer composite material with coating of hydroxyapatite (HA) as a bioactive ceramic material can enhance the osteointegration of carbon fiber reinforced polyetheretherketone (CFRPEEK) as a non‐degradable bioinert polymer composite. This study describes the joining process of CFRPEEK and HAPEEK beam components and coating process of HA on the HAPEEK substrate to achieve the multi‐layer PEEK composite for use in the application of acetabular cup prosthesis. The CFRPEEK and HAPEEK components were ultrasonically welded while the HA was plasma sprayed on the HAPEEK substrate. Ultrasonic welding parameters (length and direction of the energy directors at the interface, welding time, and pressure) were investigated by single cantilever beam and lap shear tests to achieve the optimum bonding strength of CFRPEEK and HAPEEK components. Plasma spraying parameters (e.g., surface speed, powder feed, current, primary gas flow, and system voltage) were altered to achieve the good adhesion of HA coating on the HAPEEK substrate, which was evaluated by scratch test. The results showed that the proposed multi‐layer composite was successfully processed by carrying out the ultrasonic welding and plasma spraying coating processes. The outcomes of this study could be used to develop a non‐metal acetabular cup prosthesis using the proposed multi‐layer composition. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40915.     

Structure and mechanical properties of plasma sprayed coatings of titania and alumina

The paper concerns the use of traditional and depth-sensing indentation (DSI) for investigation of deposits produced from powders based on conventional and nano-sized particles by plasma spray technology.

Plasma sprayed coatings of titania and alumina were studied. Polished cross-section of each coating was prepared and matrices of nano-indents with Berkovich tip were applied onto both materials to explore local elastic behavior. Applied load was in the range of mN to create indents with the same size scale as the thickness of splats—the main building units of the coating. The hardness value as well as the load/unload curve for each indent was stored. Titania coating was sprayed from a novel type of nanoscale-size powder agglomerated to particles useful for plasma spraying, whereas fused and crushed conventional powder was utilized for alumina spraying and for titania coating used as reference. The effect of annealing on elastic properties of titania was studied as well. The values of elastic parameters as well as the character of the coating inhomogeneity seem to reflect: (i) the composition of material and the fabrication technique and (ii) microstructural differences between coatings that are partly inherited from the feedstock powders. The results of DSI tests are discussed also in comparison with common technique used for the investigation of plasma coatings hardness—Vickers microhardness measurement.     

Ablation resistance of HfC-TaC/HfC-SiC alternate coating for SiC-coated carbon/carbon composites under cyclic ablation

HfC-TaC/HfC-SiC alternate coatings with different sublayer thicknesses were fabricated on SiC-coated carbon/carbon composites by supersonic atmosphere plasma spraying. Their ablation resistance was studied under oxyacetylene torch and compared with monolayered HfC-TaC coating. The alternate coating with 6 spray cycles of HfC-TaC and 3 spray cycles of HfC-SiC sublayers exhibited the best ablation performance as confirmed by the integral coating morphology and the lowest ablation rates. A dense oxide layer acting as an oxygen insulator and the release of thermal stress induced by the formation of dendritic cracks are thought to be responsible for its great ablation resistance. For the alternate coating with 4 spray cycles of HfC-TaC and 2 spray cycles of HfC-SiC sublayers, exfoliation occurred at the interface of two adjacent sublayers, leading to violent evaporation of exposed HfC-SiC sublayer.     

An investigation of the effect of processing conditions on the microstructure of vacuum plasma-sprayed Ti–Zr–Ni quasicrystal coatings

Ti_41.5Zr_41.5Ni₁₇ (at.%) powders from two batches having different size fractions have been vacuum plasma sprayed to form coatings using different sets of spray parameters. The powders are composed mainly of the HCP alpha and Laves phases which transform to i-phase owing to rapid quenching during plasma spraying. The coatings are examined using image analysis, hardness tester, XRD, SEM, and TEM. TEM studies revealed that the coating has a microstructure constituted by extremely fine grains. It has been observed that the polycrystalline to i-phase transformation occurs in both coatings irrespective of the differences in starting powder size and other spray parameters.     

无气喷涂机在钢结构涂装中的应用

介绍了无气喷涂法在钢结构涂装中的优点,说明了无气喷涂机的现状,详细论述了无气喷涂机在钢结构涂装中的应用及技术参数控制。     

Effects of different nano-agglomerated powders on the microstructures of PS-PVD YSZ coatings

Plasma spray physical vapor deposition (PS-PVD) is a technology that combines the advantages of traditional atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD). As the feedstock of the PS-PVD, nano-agglomerated powder is critical on determining the microstructure of the obtained coating. In this study, a method to characterize the cohesion of nano-agglomerated powders was investigated. The nano-agglomerated powders fractured into smaller particles under ultrasonic waves. Their particle size distributions were measured to quantitatively compare their cohesiveness. The change rate in the percentage of powders with particle size less than 5 μm was selected as the value for the cohesion comparison. A high change rate corresponded to a faster fracture and lower powder cohesion. Furthermore, the fracture behavior and heat and mass transfer process of nano-agglomerated powders in the plasma torch were studied by combining 3-D simulation and observation of the microstructures of PS-PVD coatings sprayed with different powders. To obtain a quasi-columnar coating, the nano-agglomerated powder required high cohesion. Finally, a suitable powder was selected and quasi-columnar structure coatings were obtained by optimizing the PS-PVD parameters.     

挪威Hardanger大桥钢箱梁外侧电弧喷锌工艺设计

在承接挪威Hardanger大桥钢结构制作中,根据业主的要求,以电弧喷锌作为配套涂层底漆。为了适应钢结构涂装流水线以及佐敦公司的重防腐配套涂料,从钢结构缺陷处理等级、磨料拼配方案、喷砂技术条件、电弧喷锌设备及锌丝线材要求、电弧喷锌工艺参数以及喷锌层质量要求等方面对钢箱梁外侧电弧喷锌工艺进行了设计。     

Structural Evolution of Thermal-Sprayed Yttria-Stabilized ZrO2 Thermal Barrier Coatings with Annealing—A Neutron Diffraction Study

We studied the influence of the annealing temperature on the atomic structure of yttria-stabilized tetragonal zirconia (YSZ) which was deposited as a 300 μm thick thermal barrier coating (TBC) on nickel superalloy substrates by plasma spraying. To obtain neutron powder diffraction patterns of the barrier coatings we used an experimental technique where the sample is randomly rotated in the neutron beam. The time-averaged neutron diffraction pattern was then analyzed using the Rietveld refinement technique without any need for corrections. This allowed the comparison of the average crystals structures from bulk tetragonal samples obtained via common ceramic routes and those of micrometer thick films deposited on substrates using plasma spray or other nonequilibrium techniques.     

Functionally graded calcium zirconate molds with alginate-based spray coating for titanium investment casting

Surface quality and dimensional accuracy of titanium components produced by investment casting are limited by the conventional dip-coating procedure of the lost-wax process. This study presents a novel approach with spraying and centrifugation as alternative coating technologies. Aiming at improved surface quality, calcium zirconate slips and coatings based on alginate gelation were developed. Scanning electron microscopy and computed tomography revealed a homogeneous spray coating. The alginate-based spray coating was used for producing functionally graded calcium zirconate shell molds. These molds exhibited a fine-grained microstructure with pores in the lower μm range. Applying an intermediate layer between the fine-grained spray coating and the coarse-grained dip-coating layers as well as the alternating application of dip-coating layers prevented cracking. The characteristic microstructure showed a bridging zone and a homogeneous distribution of coarse grain across the diameter. Ti6Al4V cast parts with no presence of an alpha case layer were obtained.     

Analysis of corrosion protection behavior of Al2O3-TiO2 oxide ceramic coating on carbon steel pipes for petroleum industry

Corrosion is the deterioration of materials by chemical interaction with their environment. In the oil and gas industry, corrosion of the pipelines and other equipment is one of the leading causes of failure and the corrosion-related costs are very high. Hence, corrosion protection is an essential requirement. In this study, the objective is to analysis of the corrosion protection behavior of spray Alumina-Titania (Al₂O₃-TiO₂) oxide ceramic coating on carbon steel pipes C45 using two different thermal spray coatings processes. These two different thermal spraying coating, High velocity oxy-fuel (HVOF) and plasma thermal spraying techniques can be used instead of extensive treatment by expensive chemical formation of coatings on pipelines and equipment to improve or restore a component's surface properties or dimensions and to protect them from corrosion. Molten or semi-molten ceramic composite powders are sprayed on the surface in order to produce a dense coating layer. FESEM of coated samples showed that a high temperature of plasma coating method end in melting the ceramic powders and creation of completely melted regions on the coated samples’ surface compared to HVOF coating techniques. Corrosion testing of coated samples in seawater (3.5% NaCl) was conducted within 30 days. Electrochemical impedance spectroscopy (EIS) as well as potentiodynamic polarization outcomes represented that the corrosion resistivity of plasma coating technique for this type of ceramic composite is better than HVOF coating technique. However, both types of coating techniques are protecting the substrate against seawater.     

电弧喷涂工艺参数对铝涂层孔隙率的影响

研究了电弧喷涂工艺参数对铝涂层孔隙率的影响,结果表明：在其它工艺参数不变的情况下,随着空气雾化压力的增加,铝涂层的孔隙率呈下降趋势;喷距在150～200 mm变化时,对孔隙率的影响不太明显,但喷距大于200 mm时,涂层的孔隙率明显增大;随着工作电压和工作电流的增加,涂层的孔隙率都是先减小后增大,并且工作电压在35～36 V之间,工作电流在170 A左右时,涂层的孔隙率达到最低。     

双组分硅橡胶防热涂料防热涂料自动喷涂工艺研究

为了提高双组分硅橡胶防热涂料（ TR-37G）自动喷涂厚度均匀性,对其自动喷涂工艺进行了研究。研究了喷涂参数对该涂料自动喷涂状态和涂层厚度的影响,分析了涂层厚度均匀性影响因素,之后对自动喷涂单道理论厚度进行了推导,分析了涂层厚度影响因素,最后通过模拟件自动喷涂,验证该涂料自动喷涂厚度均匀性。结果表明：黏度、喷涂扇幅、泵压、自动调节阀气流大小、喷涂距离等参数均会影响涂料自动喷涂,对于锥柱型产品外壁喷涂,横向厚度均匀性主要受偏轴距离影响,纵向厚度均匀性主要受喷枪高度影响。自动喷涂时,涂层厚度影响因素包括涂料利用率、喷枪单位时间出料量、涂料密度、产品直径、喷枪纵向运行速度,锥柱型产品竖直状态下自动喷涂时,不同高度喷涂速度需进行修正。     

Deposition of hydroxyapatite coatings by axial plasma spraying: Influence of feedstock characteristics on coating microstructure,phase content and mechanical properties

Axial plasma spray is one of the thermal spray techniques to deposit multifunctional advanced coatings. The present work explores the use of this process to deposit thin, continuous, and adherent Ca₅ (PO₄)₃OH (hydroxyapatite, HAp) coatings and characterize its microstructure, phases, hardness and adhesion strength. Three different suspension-deposited HAp coatings were investigated and compared with powder-deposited HAp coating on a Ti6Al4V substrate. The effect of mean solute particle size and solid-loading in the suspension has been explored on the evolution of microstructure, phase content and mechanical properties of axial suspension plasma sprayed (ASPS) coatings. Phase-characterization has shown retention of hydroxyapatite phase and coating crystallinity in the deposited coatings, whereas the adhesion strength of the HAp coating decreased from ～40 MPa to ～13 MPa when bioglass was added to the feedstock material. The lower solid load content and lower mean solute particle size in the suspension were found to be beneficial in achieving porous, rougher, and well-adhering coatings. This work concludes that ASPS can potentially deposit thin HAp coatings (< 50 μm) with high adhesion strength.