Laser-beam and photon-assisted processed materials and their microstructures

Laser processing is a relatively new technique for modifying the near-surface region of materials without altering the in-bulk characteristics. A single laser can perform several functions by manipulating processing conditions such as laser power, beam diameter, and traverse speed. Lasers have shown attractive applications, such as cutting, welding, glazing, alloying, and cladding. A laser glazing process has demonstrated an improvement in the microstructure of vacuum plasma-coated copper-based alloys containing cavities, unmelted particles, and segregation. Laser glazing has also been shown to restore the degraded microstructure of components and make them equivalent to, or better than, the original wrought alloy. The laser cladding concept was used to develop nickel-based alloys for high-temperature applications that exhibited higher thermal stability than the nickel-based Rene-95 alloy. Rapid melting and quenching occurred during the laser glazing, alloying, and cladding processes resulting in a fine-grained microstructure, metastable phases and extended solid solubility of alloying additions in the matrix.Photon-assisted processing of material is a relatively new technique being explored to synthesize new materials from various substrates (solid, liquid, and gas). This process is successfully used to fabricate high-quality thin films for electronic industries. Thin films of multicomponents can be deposited with stoichiometric composition. Diamond thin films have been synthesized from liquid hydrocarbon (Benzene, C₆H₆) by laser-liquid hydrocarbon-substrate interaction. A laser-assisted physical vapour deposition process was found to be very successful in depositing stoichiometric compositions of multilayered thin films such as superconducting YBa₂Cu₃O₇, ferroelectric Pb_0.52Zr_0.48TiO₃ and other coatings such as TiN and CoSi₂. This review reports some of the major advances in the understanding and engineering of new materials for electronic industries and high-temperature applications in the auto, aerospace, and turbine industries.     

Thermal conductivity investigation of zirconia co-doped with yttria and niobia EB-PVD TBCs

A technique used to improve the life cycle and/or the working temperature of the turbine blades uses ceramic coatings over metallic material applied by electron beam-physical vapor deposition (EB-PVD). The most usual material for this application is yttria doped zirconia. Addition of niobia, as a co-dopant in the Y₂O₃–ZrO₂ system, can reduce thermal conductivity. The purpose of this work is to evaluate the influence of the addition of niobia on the microstructure and thermal properties of the ceramic coatings. This new formulation will, in the future, be able to become an alternative to the composition currently used by the aerospace field in EB-PVD thermal barrier coatings (TBC). A significant reduction of the thermal conductivity, measured by laser flash technique, in the zirconia ceramic coatings co-doped with yttria and niobia when compared with zirconia–yttria coatings was observed.     

Erosion of thermal barrier coatings

Abstract

Thermal barrier coatings have been used within gas turbines for over 30 years to extend the life of hot section components. Thermally sprayed ceramics were the first to be introduced and are widely used to coat combustor cans, ductwork, platforms and more recently turbine aerofoils of large industrial engines. The alternative technology, electron beam physical vapour deposition,(EB-PVD) has a more strain-tolerant columnar microstructure and is the only process that can offer satisfactory levels of spall resistance, erosion resistance and surface finish retention for aero-derivative engines.

Whatever technology is used, the thermal barrier must remain intact throughout the turbine life. Erosion may lead to progressive loss of TBC thickness during operation, raising the metal surface temperatures and thus shortening component life. Ballistic damage can lead to total TBC removal.

This paper reviews the erosion behaviour of both thermally sprayed and EB-PVD TBCs relating the observed behaviour to the coating microstructure. A model for the erosion of EB-PVD ceramics is presented that permits the prediction of erosion rates. The model has been validated using a high velocity erosion gas gun rig, both on test coupons and samples removed from coated components. The implications of erosion on component life are discussed in the light of experimental results and the model predictions.     

Aktuelle Applikationsfelder der Hochleistungs-Elektronenstrahltechnik

High-power electron beams for industrial vacuum processes have been available up to megawatt range since the sixties. They have found their firm place in the metallurgy of reactive and refractive metals. New fields of application are large-area coating of metal strips with optical layer systems and the generation of thermal barrier coatings for gas turbine components with EB-PVD technologies (Electron Beam-Physical Vapor Deposition). Even laser technology could not compete with the dominating position of the electron beam in these fields.     

Effect of laser glazing on the thermo-mechanical properties of plasma-sprayed LaTi2Al9O19 thermal barrier coatings

Conventional duplex (DL) and functionally graded (FG) LaTi₂Al₉O₁₉ (LTA) coatings were deposited over C263 nickel alloy by air plasma spray (APS) and compared with subsequent laser glazing processes. The effect of laser glazing on adhesion strength and thermal barrier performance was investigated. The thermal barrier effect was measured using the temperature difference technique involving infrared (IR) rapid heater and the adhesion strength was measured using the scratch tester. The surface morphology and microstructure were analyzed by optical microscopy (OM), Scanning Electron Microscope (SEM) and 3D profilometer. Based on the experimental results, the laser glazing showed a remarkable temperature drop after IR rapid heating. The changes in porosity and grain refinement make more contributions to the temperature drop of the laser-glazed coatings than that of as-sprayed coatings. The temperature drop is about 110°C for laser-glazed LTA FG coating after 100?s of IR flash, while the drop in DL as-sprayed coating is 60°C compared to the base material.     

Thermocyclic Behavior of Differently Stabilized and structured EB-PVD thermal barrier coatings

The electron-beam physical vapor deposition (EB-PVD) process provides distinctive coatings of a unique columnar microstructure for gas turbine components. Main advantage of this structure is superior tolerance against straining, erosion and thermoshock, thus giving it a major edge in lifetime. This paper outlines the interaction between chemical composition and microstructural evolution EB-PVD zirconia-based thermal barrier coatings (TBCs) and their respective lifetimes in cyclic burner rig and furnace tests. Customizing TBC microstructure by adjusting EB-PVD processing parameters is emphasized. A structural zone diagram for PVD is modified by interconnecting the influence of substrate rotation with microstructural evolutions. Finally, some basic aspects of single source and dual source evaporation are compared.     

Improving thermal barrier coatings by laser remelting

Thermal barrier coatings are extensively used to protect metallic components in applications where the operating conditions include aggressive environment at high temperatures. These coatings are usually processed by thermal spraying techniques and the resulting microstructure includes thin and large splats, associated with the deposition of individual droplets, with porosity between splats. This porosity reduces the oxidation and corrosion resistance favouring the entrance of aggressive species during service. To overcome this limitation, the top coat could be modified by laser glazing reducing surface roughness and sealing open porosity. ZrO2(Y2O3) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The top coat was laser remelted and a densified ceramic layer was induced in the top surface of the ceramic coating. This layer inhibited the ingress of aggressive species and delayed bond coat oxidation.     

Overview on the Development of Nanostructured Thermal Barrier Coatings

Thermal barrier coatings （TBCs） have successfully been used in gas turbine engines for increasing operation temperature and improving engine efficiency. Over the past thirty years, a variety of TBC materials and TBC deposition techniques have been developed. Recently, nanostructured TBCs emerge with the potential of commercial applications in various industries. In this paper, TBC materials and TBC deposition techniques such as air plasma spray （APS）, electron beam physical vapor deposition （EB-PVD）, laser assisted chemical vapor deposition （LACVD） are briefly reviewed. Nanostructured 7-8 wt pct yttria stabilized zirconia （7-8YSZ）TBC by air plasma spraying of powder and new TBC with novel structure deposited by solution precursor plasma spray （SPPS） are compared. Plasma spray conditions, coating forming mechanisms, microstructures,phase compositions, thermal conductivities, and thermal cycling lives of the APS nanostructured TBC and the SPPS nanostructured TBC are discussed. Research opportunities and challenges of nanostructured TBCs deposited by air plasma spray are prospected.     

Review Nano and macro-structured component fabrication by electron beam-physical vapor deposition (EB-PVD)

The objective of this paper is to demonstrate the versatility of electron beam-physical vapor deposition (EB-PVD) technology in engineering new materials with controlled microstructure and microchemistry in the form of coatings. EB-PVD technology is being explored in forming net-shaped components for many applications including space, turbine, optical, biomedical and auto industry. Coatings are often applied on components to extend their performance and life under severe environmental conditions including thermal, corrosion, wear, and oxidation. In addition, coatings have been used in designing and developing sensors. Performance and properties of the coatings depend upon its composition, microstructure and deposition condition. This paper presents recent results of various materials including ceramic, metallic, and functionally graded coatings produced by EB-PVD. Simultaneous co-evaporation of multiple ingots of different compositions in the high energy EB-PVD chamber has brought considerable interest in the architecture of functional graded coatings, nano-laminated coatings and designing of new structural materials that could not be produced economically by conventional methods. In addition, high evaporation and condensate rate allowed fabricating precision net-shaped components with nanograined microstructure for various applications. This paper will also present the results of various metallic and ceramic coatings including chromium, titanium carbide (TiC), hafnium carbide (HfC), tantalum carbide (TaC), hafnium nitride (HfN), titanium-boron-carbonitride (TiBCN), and partially yttria stabilized zirconia (YSZ), and HfO₂-based TBC coatings deposited by EB-PVD for various applications.     

TiAl合金表面激光重熔热障涂层组织及抗高温氧化性能

为了进一步提高TiAl合金表面等离子喷涂ZrO2-7%(质量分数)Y203热障陶瓷涂层的性能,采用激光重熔工艺对涂层进行处理,研究了激光重熔对涂层微观组织和抗高温氧化性能的影响.用扫描电镜(SEM)分析了涂层形貌和微观结构,同时对其抗高温(850℃)氧化性能进行了考察.结果表明,等离子喷涂热障陶瓷涂层呈典型的层状堆积特征,有一定的孔隙且分布有微裂纹;经过激光重熔处理后,陶瓷涂层片层状组织得以消失,致密性提高,获得了基本没有裂纹等缺陷的重熔层;整个重熔层包括界面没有明显特征的平面晶和上部沿热流方向生长的柱状晶组织.等离子喷涂热障涂层具有较好的抗高温氧化性能,经过激光重熔后可进一步提高其抗高温氧化能力.     

Effect of laser remelting on the tribological performance of thermal barrier coatings

Gas turbine's efficiency improves as operating temperature is increased. For this reason, metallic components used in turbine engines, for propulsion and power generation, are protected by thermal barrier coatings (TBC). Laser glazing has been used to enhance the oxidation and corrosion resistance of thermally sprayed TBC, but there is no information about the effect of this treatment on the tribological performance. ZrO2(CaO) top coat and NiAIMo bond coating were flame sprayed onto an AlSI 1045 carbon steel. The top coat was laser remelted and a densified ceramic layer was induced in the top surface of the ceramic coating. Both, the as sprayed and the laser remelted top coatings, were formed by cubic ZrO2 with some tetragonal precipitates. The grain size was reduced by the laser treatment. The mechanical properties and the local wear rate were evaluated by depth sensing indentation and scratch tests respectively. The nanoscale wear behaviour was always improved by the laser treatment.     

TiAl合金表面涂层技术研究现状

TiAl合金由于其密度低,比强度和比刚度高,是航空航天工业理想的新型高温结构材料.室温塑性差的问题已通过添加合金元素和显微组织调控等手段基本得到解决,进一步提高其高温抗氧化和耐磨性能已成为需要重点研究的问题.表面涂层技术为这一问题的解决提供了一条有效的途径,为此,综述了国内外TiAl合金表面涂层技术的研究现状,重点介绍了激光技术和热喷涂技术及其应用,并展望了TiAl合金表面涂层技术的发展趋势.     

Tailored microstructure of zirconia and hafnia-based thermal barrier coatings with low thermal conductivity and high hemispherical reflectance by EB-PVD

Zirconia and hafnia based thermal barrier coating materials were produced by industrial prototype electron beam-physical vapor deposition (EB-PVD). Columnar microstructure of the thermal barrier coatings were modified with controlled microporosity and diffuse sub-interfaces resulting in lower thermal conductivity (20–30% depending up on microporosity volume fraction), higher thermal reflectance (15–20%) and more strain tolerance as compared with standard thermal barrier coatings (TBC). The novel processed coating systems were examined by various techniques including scanning electron microscopy (SEM), X-ray diffraction, thermal conductivity by laser technique, and hemispherical reflectance.     

Architecture of thermal barrier coatings produced by electron beam-physical vapor deposition (EB-PVD)

Extremely high temperatures and severe atmospheric conditions in the hot section of aircraft engines during operation result in degradation and structural failures of turbine components. Replacing these components is very expensive. Thermal barrier coatings (TBC) composed of ZrO₂-8wt%Y₂O₃(8YSZ) applied by Electron Beam-Physical Vapor Deposition (EB-PVD) to turbine components offer excellent properties for thermal protection and resistance against oxidation - induced erosion and corrosion. However, the life of turbine components is still limited due to premature failure of the TBC. It is hypothesized that the life of the coated components can be extended by lowering the thermal conductivity of the TBC by creating multiple non-distinct or distinct interfaces and alloy additions such as Nb-oxide which will result in a reduction in the thermal conductivity and oxygen transport through the coating. This paper presents the microstructural results of standard 8YSZ, layered 8YSZ, Nb-oxide alloyed 8YSZ and functionally graded 8YSZ with Nb-oxide deposited by EB-PVD. TBC samples were examined by various methods including scanning electron microscopy (SEM), high-resolution optical microscopy (OM), X-ray diffraction (XRD), and thermal cycling tests. The preliminary results strongly suggest that multiple interfaced TBC exhibits better oxidation resistant properties as compared to standard and alloyed TBC.     

Numerical study on modification of ceramic coatings by high-intensity pulsed ion beam

ZrO₂ ceramic coatings, which often call thermal barrier coatings (TBCs), fabricated by electron beam physical vapor deposition (EB-PVD), are widely used in high-temperature environment of aircraft and industry gas-turbine engines, because of the excellent strain tolerance imparted by the columnar structure. However, channels separating the columnar grains in EB-PVD TBCs provide paths for oxygen or other aggressive species from ambient atmosphere into the bond coat, resulting in the premature spallation-failure during high-temperature service. In our previous study, high-intensity pulsed ion beam (HIPIB) technique has been proposed to modify the EB-PVD TBCs, where a melted, densified top layer can be produced as a result of extremely thermal effect induced by the HIPIB irradiation. In this paper, HIPIB melting process is investigated numerically using a physical model based on experimental data, taking into account the surface morphology of HIPIB-melted TBCs to explore the mechanism of interaction between HIPIB and the coatings. Deposition process of the beam energy in TBCs was simulated by Monte Carlo method, and the non-linear equations describing the thermal conducting process were solved numerically based on the deposited energy to obtain the evolution of the temperature field of TBCs. The calculated melting depth of irradiated EB-PVD TBCs is consistent with results obtained in the HIPIB irradiation experiments.     

Substrate Effects on the High-Temperature Oxidation Behavior of Thermal Barrier Coatings

The high-temperature oxidation behaviors of the NiCrAlYSi/P-YSZ thermal barrier coatings (TBCs) produced by electron beam-physical vapor deposition (EB-PVD) on directionally solidified (DS) and single crystalline (SC) Ni-based superalloy substrates were investigated. The cross-sectional microstructure investigation, isothermal and cyclic oxidation tests were conducted for the comparison of oxidation behaviors of TBCs on different substrates. Although TBC on DS substrate has a relatively higher oxidation rat...     

镁合金表面耐磨涂层研究进展

镁及其合金具有优异的物理及机械性能,从而在许多领域得到应用.但其不耐磨性限制了镁合金在汽车和航空工业中的应用.最简单且有效的解决办法是在镁合金表面施加耐磨涂层.综述了几种新的表面耐磨涂层技术,即激光表面改性涂层(激光表面重熔/激光表面合金化/激光表面熔敷)、表面纳米陶瓷涂层和微弧氧化陶瓷基涂层,并分析了它们的研究和应用前景.     

合金元素对激光熔覆高熵合金涂层硬度影响的研究进展

激光熔覆技术采用高能量密度的激光作为工艺的能量来源,能够对工件表面进行改性和修复,显著地改善了基体的表面力学性能,从而有效地延长了产品的生命周期。激光熔覆是制备高熵合金的典型工艺之一,采用该技术并且添加合适的合金元素可以制备具备卓越性能的高熵合金涂层。为清晰地阐明加入元素后增强激光熔覆高熵合金涂层硬度的作用机制,首先综述了目前国内外在激光熔覆过程中加入常见元素所制备的高熵合金涂层硬度性能的研究现状,其中高熵合金有特殊的“4种效应”,对金属间化合物有促进作用,其内部微观结构一般为FCC、BCC或者HCP等固溶相,通常通过固溶强化、沉淀强化和分散强化来强化,并且激光熔覆法会使高熵合金涂层快速冷却,从而显著改善合金的力学性能。其次,分析了金属与非金属两大类元素对激光熔覆制备高熵合金涂层硬度强化的机理,总结了金属元素与非金属元素的添加对高熵合金涂层硬度的影响规律。最后,针对激光熔覆制备高熵合金涂层硬度性能的改进,总结出了有效的方法,并对其未来发展进行了展望。研究结果揭示了激光熔覆高熵合金涂层硬度强化的理论基础,为该领域的进一步发展提供了理论依据。     

表面改性中稀土提高材料抗高温氧化及耐蚀性能的作用

稀土元素具有特殊的电子结构,对材料表面有着优异的改性潜力,在表面工程技术领域具有很好的应用前景.综述了稀土元素在化学热处理、激光熔覆、离子注入、真空等离子体镀膜、热喷涂等技术中对工艺过程的影响和提高材料表面改性层抗高温氧化和耐蚀性能的效果.稀土元素在改性层中的作用机理为:稀土元素的微合金化作用,改变了改性层中第二相或夹杂物的形态与性能,改性层组织细化和结构因素,是材料表面改性层抗高温氧化和耐蚀性能得到改善的主要原因.     

Design and Properties of Thermal Barrier Coatings for advanced turbine engines

The efficiency and performance of advanced aircraft turbines can be markedly increased if higher gas temperatures are used. Although the highly loaded blades and vanes in the high pressure turbine are heavily cooled, today's substrate materials are unable to provide sufficient strength in the temperature range up to 1500°C and above. If thermal barrier coatings (TBCs) are applied on superalloy turbine blades a substantial temperature drop of the parts can be achieved. The resulting increase in efficiency comes from reduced cooling and/or increased gas turbine inlet temperatures of up to 150°C. TBCs are either processed by plasma spraying (PS) or electron beam physical vapour deposition (EB-PVD). While PS is lower in cost EB-PVD leads to superior strain and thermoshock tolerant coatings. Furthermore, cooling hole closure of turbine blades and vanes is prevented and aerodynamic design maintained. Finally, future research and development needs in TBC technology are stressed.