Rapid nitriding of pure iron by thermal plasma jet irradiation

Pure iron samples were exposed to thermal plasma jet and nitrided for 15 min at a temperature ranging from 545 °C to 742 °C. For comparison purposes, conventional ion nitriding was also performed using glow discharge plasma at 580 °C for 8 h. For conventional plasma nitriding, a top compound layer and a bottom diffusion zone are observed. In contrast, an additional “transition zone” is observed between the compound layer and the diffusion zone for the thermal plasma jet treatment when the process temperature is 667 °C or above. This “transition zone” is quite thick (a few tens of micrometers), determined by the treatment temperature, and has unique features in the phase formation and nitrogen distribution. At 706 °C, the content of γ′phase, as well as the content of nitrogen and the hardness, reaches the maximum. Compared with the traditional plasma nitriding process, the reaction speed of thermal plasma irradiation is much higher. A nitrided case depth over a few tens of micrometers is obtained in plasma jet nitriding for only 15 min versus a depth of only a few micrometers in conventional plasma nitriding for 8 h. The formation of the “transition zone” and the mechanism for the high nitriding speed are discussed in the paper. It is believed that this technology can be applied at atmospheric pressure in field without the requirement for a vacuum system. Hence, this technology may be advantageous for practical applications.     

Atmospheric plasma sprayed thermal barrier coatings with high segmentation crack densities: Spraying process, microstructure and thermal cycling behavior

Thermal barrier coatings (TBCs) with high strain tolerance are favorable for application in hot gas sections of aircraft turbines. To improve the strain tolerance of atmospheric plasma sprayed (APS) TBCs, 400 μm-500 μm thick coatings with very high segmentation crack densities produced with fused and crushed yttria stabilized zirconia (YSZ) were developed. Using a Triplex II plasma gun and an optimized spraying process, coatings with segmentation crack densities up to 8.9 cracks mm^− 1, and porosity values lower than 6% were obtained. The density of branching cracks was quite low which is inevitable for a good inter-lamellar bonding.Thermal cycling tests yielded promising strain tolerance behavior for the manufactured coatings. Samples with high segmentation crack densities revealed promising lifetime in burner rig tests at rather high surface (1350 °C) and bondcoat temperatures (up to 1085 °C), while coatings with lower crack densities had a reduced performance. Microstructural investigations on cross-sections and fracture surfaces showed that the segmentation crack network was stable during thermal shock testing for different crack densities. The main failure mechanism was delamination and horizontal cracking within the TBC near the thermal grown oxide layer (TGOs) and the TBC.     

Corrosion properties of surface-modified AZ91D magnesium alloy

Samples of AZ91D magnesium alloy were dipped into AlCl₃–NaCl molten salt at different temperatures between 250 °C and 400 °C for 28800 s. The thickness of the alloying layer is increased with the rise of the treatment temperatures. The coating was mainly composed of Al₁₂Mg₁₇ and Al₃Mg₂ intermetallic compounds. The corrosion resistance of the coating which is obtained at 300 °C for 28800 s is the best. When the treatment temperature is higher than 300 °C, some cracks developed in the alloying layers. The cracks were resulted from the thermal stress due to the different thermal expansion coefficient of the AZ91D substrate and the alloying coating during the rapid cooling process.     

Mechanical Properties of LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript> and Thermal Cycling Behaviors of Plasma-Sprayed LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript>/YSZ Thermal Barrier Coatings

Recently, extensive efforts have been made to develop new thermal barrier coating (TBC) materials which can operate at temperatures above 1523 K over a long term. In this article, LaTi₂Al₉O₁₉ (LTA) was synthesized by solid-state reaction at 1773 K, and the mechanical properties of the LTA bulk were evaluated. The microhardness is about 14 GPa, comparable to that of YSZ bulk, whereas the Young’s modulus is about 44 GPa, lower than the value of YSZ. However, the fracture toughness of 0.8-1 MPa m^1/2 is much lower than that of bulk YSZ. A double-ceramic-layer LTA/YSZ TBC structure was proposed and the TBC sprayed by plasma spraying. Thermal cycling tests of the TBC specimens were performed at 1373 K with a dwell time of 10 min. The LTA remained good stability with ZrO₂ and Al₂O₃. However, the single layer LTA TBC was cracked at the LTA/bond coat interface after about 300 cycles, due to its poor thermal shock resistance, while the YSZ TBC yielded a lifetime of about 1000 cycles. The LTA/YSZ TBC remained intact even after 3000 cycles, exhibiting a promising potential as new TBC materials.     

Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray

7-8 wt.% Yttria-stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used by the gas turbines industry due to its excellent thermal and thermo-mechanical properties up to 1200 °C. The need for improvement in gas turbine efficiency has led to an increase in the turbine inlet gas temperature. However, above 1200 °C, YSZ has issues such as poor sintering resistance, poor phase stability and susceptibility to calcium magnesium alumino silicates (CMAS) degradation. Gadolinium zirconate (GZ) is considered as one of the promising top coat candidates for TBC applications at high temperatures (>1200 °C) due to its low thermal conductivity, good sintering resistance and CMAS attack resistance. Single-layer 8YSZ, double-layer GZ/YSZ and triple-layer GZdense/GZ/YSZ TBCs were deposited by suspension plasma spray (SPS) process. Microstructural analysis was carried out by scanning electron microscopy (SEM). A columnar microstructure was observed in the single-, double- and triple-layer TBCs. Phase analysis of the as-sprayed TBCs was carried out using XRD (x-ray diffraction) where a tetragonal prime phase of zirconia in the single-layer YSZ TBC and a cubic defect fluorite phase of GZ in the double and triple-layer TBCs was observed. Porosity measurements of the as-sprayed TBCs were made by water intrusion method and image analysis method. The as-sprayed GZ-based multi-layered TBCs were subjected to erosion test at room temperature, and their erosion resistance was compared with single-layer 8YSZ. It was shown that the erosion resistance of 8YSZ single-layer TBC was higher than GZ-based multi-layered TBCs. Among the multi-layered TBCs, triple-layer TBC was slightly better than double layer in terms of erosion resistance. The eroded TBCs were cold-mounted and analyzed by SEM.     

Proto-TGO formation in TBC systems fabricated by spark plasma sintering

Thermal barrier coatings (TBC) are commonly used in modern gas turbines for aeronautic and energy production applications. The conventional methods to fabricate such TBCs are EB-PVD or plasma spray deposition. Recently, the spark plasma sintering (SPS) technique was used to prepare new multilayered coatings. In this study, complete thermal barrier systems were fabricated on single crystal Ni-based superalloy (AM1®) substrate in a one-step SPS process. The lifetime of TBC systems is highly dependent on its ability to form during service a dense, continuous, slow-growing alumina layer (TGO) between an underlying bond coating and a ceramic top coat. In the present paper, we show that such kind of layer (called proto-TGO in the following) can be in situ formed during the SPS fabrication of TBC systems. This proto-TGO is continuous, dense and its nature has been determined using TEM-EDS-SAD and Raman spectroscopy. This amorphous oxide layer in the as-fabricated samples transforms to α-Al₂O₃ during thermal treatment under laboratory air at 1100 °C. Oxidation kinetics during annealing are in good agreement with the formation of a protective α-Al₂O₃ layer.     

High temperature tribological behavior of W-DLC against aluminum

Diamond-like carbon (DLC) coatings are well suited for applications that require minimum adhesion and low coefficient of friction (COF) against aluminum alloys. These properties however deteriorate rapidly at elevated temperatures, and coating wear occurs. In this study, tribological behavior of W containing DLC (W-DLC) were studied as a function of testing temperatures up to 500 °C, and the sliding-induced surface and subsurface damage at these temperatures was investigated. Pin-on-disk tests performed on W-DLC run against 319 Al showed a low COF of 0.2 at 25 °C, whereas between 100 °C and 300 °C, a high average steady-state COF of 0.60 was recorded. At 400 °C the COF decreased to 0.18, and this reduction in COF continued with increasing the temperature to 500 °C (0.12). It was observed that the formation of transferred material layers on 319Al was the governing mechanism for the low COF. The Raman analysis revealed that at room temperature these layers were rich in carbon, whereas at 400 °C the transfer layers consisted of tungsten oxide. According to transmission electron microscopy (TEM), and X-Ray photoelectron spectroscopy (XPS), of the coatings tested at 400 °C and 500 °C a thin (20 nm) tungsten oxide layer was formed on their top surface. This in turn led to the formation of tungsten oxide rich transfer layers that is believed to reduce the COF at temperatures above 400 °C.     

Thermal cycling behavior of EBPVD TBC systems deposited on doped Pt-rich γ–γ′ bond coatings made by Spark Plasma Sintering (SPS)

In the last decade, an increasing interest was given to Pt-rich γ–γ′ alloys and coatings as they have shown good oxidation and corrosion properties. In our previous work, Spark Plasma Sintering (SPS) has been proved to be a fast and efficient tool to fabricate coatings on superalloys including entire thermal barrier coating systems (TBC). In the present study, this technique was used to fabricate doped Pt-rich γ–γ′ bond coatings on AM1® superalloy substrate. The doping elements were reactive elements such as Hf, Y or Zr, Si and metallic additions of Ag. These samples were then coated by electron beam physical vapour deposition (EBPVD) with an yttria partially stabilized zirconia (YPSZ) thermal barrier coating. Such TBC systems with SPS Pt rich γ–γ′ bond coatings were compared to conventional TBC system composed of a β-(Ni,Pt)Al bond coating. Thermal cycling tests were performed during 1000-1 h cycles at 1100 °C under laboratory air. Spalling areas were monitored during this oxidation test. Most of the Pt rich γ–γ′ samples exhibited a better adherence of the ceramic layer than the β-samples. After the whole cyclic oxidation test, cross sections were prepared to characterize the thickness and the composition of the oxide scales by using scanning-electron microscopy. In particular, the influence of the doping elements on the oxide scale formation, the metal/oxide roughness, the TBC adherence and the remaining Al and Pt under the oxide scale were monitored. It was shown that RE-doping did not improve the oxidation kinetics of the studied Pt rich γ–γ′ bond coatings, nevertheless most of the compositions were superior to “classic” β-(Ni,Pt)Al bond coatings in terms of ceramic top coat adherence, due to lower rumpling kinetics and better oxide scale adherence of the γ–γ′-based systems.     

On the electrografting of stainless steel from para-substituted aryldiazonium salts and the thermal stability of the grafted layer

In this study we explore the thermal stability of an organic layer electrografted onto stainless steel (ASTM 316) from four different aryldiazonium salts R-C₆H₄N₂⁺ (R = NO₂, F, H, or OCH₃). The coverage of the surfaces was analysed electrochemically by employing redox probes and cyclic voltammetry. The results obtained clearly show that the steel surface after grafting is electrochemically passivated by the presence of a surface coating. Polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS) was used to characterize the organic films on the surfaces and to monitor the thermal stability of these films from ambient temperature to 400 °C with 50 °C intervals. The PM-IRRAS spectra show a decrease in band intensities at 250 °C for nitrophenyl, independent of layer thickness and atmosphere (air or argon), 300 °C for methoxyphenyl, and 350 °C for phenyl and fluorophenyl films. All the characteristic IR bands were simultaneously and completely lost at 300, 350, 350, 400 and 400 °C for thin-layer nitrophenyl, thick-layer nitrophenyl, methoxyphenyl, fluorophenyl, and phenyl, respectively, which strongly indicates that the entire organic film is lost at these temperatures. The results show that it is mainly the substituent and the layer thickness that are responsible for the difference in thermal stability of the organic films and that all films withstood temperatures up to 200 °C.This study shows that electrochemical grafting from aryldiazonium salts is simple, fast, and has a low energy consumption which makes the procedure suitable for industrial applications.     

大气等离子喷涂热障涂层CMAS防护层成分及厚度优化

目的优化热障涂层(TBCs)CMAS(CaO-MgO-Al_2O_3-SiO_2)阻抗层的成分和厚度,使其能有效阻抗CMAS沉积物的腐蚀,并同时与热障涂层有较高的结合力。方法首先利用多孔无压烧结陶瓷块体研究了不同含量Al_2O_3和8YSZ(8wt.%氧化钇稳定氧化锆)均匀混合后在高温(1250℃)条件下对CMAS沉积物的防护作用。采用扫描电子显微镜(SEM)、能谱仪(EDS)以及X射线衍射(XRD)仪,分析研究了CMAS腐蚀层的显微结构、腐蚀深度及反应产物。其次,基于最优成分,利用大气等离子喷涂(APS)制备了具有8YSZ/Al_2O_3陶瓷层的热障涂层。对CMAS腐蚀厚度进行分析测量,提出CMAS阻抗层的厚度。结果 Al_2O_3的添加可以有效地阻碍CMAS的渗入,并且Al_2O_3含量越多,防护效果越好。但是CMAS的渗入深度和氧化铝的添加量呈非线性关系。结合TBC陶瓷层的热学性能和力学性能的要求,本实验中最佳的TBCs复合陶瓷层组分为70wt%8YSZ+30wt%Al_2O_3。基于实验结果,提出YSZ/Al_2O_3复合陶瓷层(50μm)-YSZ陶瓷层(150μm)的双层TBC陶瓷层结构,并综合计算出复合陶瓷层的热膨胀系数为9.93×10-6℃-1以及双层TBC陶瓷层的热导率为2.4 W/(m·K)。最后对Al_2O_3减缓CMAS腐蚀的机理进行了量化分析。结论 YSZ/Al_2O_3复合阻抗层的最优成分为70wt%8YSZ+30wt%Al_2O_3,厚度为50μm,能有效阻碍高温下CMAS腐蚀。     

Miniaturized bend tests on partially stabilized EB-PVD ZrO2 thermal barrier coatings

The elastic properties of thermal barrier coatings (TBCs) are important for modelling the lifetime of these coatings. A new test setup has been developed to measure the system modulus of electron-beam enhanced physical vapour deposited (EB-PVD) TBC coatings by miniaturized bend tests.Due to the brittleness, low stiffness and small thickness of the top coat and its complex microstructure, it is difficult to measure its Young's modulus by standard mechanical testing. For this reason, a special sample material has been prepared which consists of a 1 mm thick layer of EB-PVD TBC. This material was isothermally heat treated for different times at 950 °C, 1100 °C and 1200 °C and then tested in a specially developed miniaturized bend test. The bend test setup permits mechanical tests with a high resolution in stress and strain, where the strain is measured by digital image correlation. So the stiffness of the free-standing TBC samples could be measured with a high accuracy and the sintering behaviour of the EB-PVD TBC and the consequent rise of Young's modulus could be determined. The results show a significant increase of the system modulus with heat treatment time and temperature caused by sintering of the coating. An activation energy of 220 kJ/mol for the process has been determined.In addition, the material was tested by nanoindentation in order to measure Young's modulus on a local scale, and the porosity of the samples was determined by quantitative image analysis.     

Tensile fracture behavior of thermal barrier coatings on superalloy

Tensile fracture behavior of thermal barrier coatings (TBCs) on superalloy was investigated in air at room temperature (RT), 650 °C and 850 °C. The bond coat NiCrAlY was fabricated by either high velocity oxygen fuel (HVOF) or air plasma spraying (APS), and the top coat 7%Y₂O₃-ZrO₂ was deposited by APS. Thus two kinds of the TBC system were formed. It was shown that the coating had little effect on tensile stress-strain curves of the substrate and similar tensile strength was obtained in two kinds of the TBC system. However, the cracking behavior in the two kinds of TBC system at RT was different, which was also different from that at 650 °C and 850 °C by scanning electron microscopy. The interface fracture toughness of the two kinds of TBC system was evaluated by the Suo-Hutchinson model and the stress distribution in the coating and substrate was analyzed by the shear lag model.     

Low Temperature Plasma Nitriding of H13 Steel for Improved Surface Hardness

AISI H13 steel samples were plasma nitrided to improve their surface hardness using a locally developed combined reactor.Pre-ionized RFICP plasma was employed in combination with DC glow discharge and thermal emission source to achieve the nitride precipitates in iron-matrix under low sample temperature.Thick nitride layers over 150 microns could be realized with low RF power of 100 W under the processing time between 1-20 h and low sample temperature of 300℃.The gas mixtures of H2 and N2 were utilized while the processing pressure and the DC bias to the sample were maintained at 0.5 torr and 300 V,respectively.Scanning electron microscope(SEM),energy dispersive X-ray spectroscopy(EDS),glancing incident angle X-ray diffractometer(GIXD)and Vickers hardness test were employed to characterize the properties of sample surfaces.Significant increases of surface hardness to over 1,000 HV were observed after treatment.     

Higher Temperature Thermal Barrier Coatings with the Combined Use of Yttrium Aluminum Garnet and the Solution Precursor Plasma Spray Process

Gas-turbine engines are widely used in transportation, energy and defense industries. The increasing demand for more efficient gas turbines requires higher turbine operating temperatures. For more than 40 years, yttria-stabilized zirconia (YSZ) has been the dominant thermal barrier coating (TBC) due to its outstanding material properties. However, the practical use of YSZ-based TBCs is limited to approximately 1200 °C. Developing new, higher temperature TBCs has proven challenging to satisfy the multiple property requirements of a durable TBC. In this study, an advanced TBC has been developed by using the solution precursor plasma spray (SPPS) process that generates unique engineered microstructures with the higher temperature yttrium aluminum garnet (YAG) to produce a TBC that can meet and exceed the major performance standards of state-of-the-art air plasma sprayed YSZ, including: phase stability, sintering resistance, CMAS resistance, thermal cycle durability, thermal conductivity and erosion resistance. The temperature improvement for hot section gas turbine materials (superalloys & TBCs) has been at the rate of about 50 °C per decade over the last 50 years. In contrast, SPPS YAG TBCs offer the near-term potential of a > 200 °C improvement in temperature capability.     

Nano silicon top-layer for composite-induced multiphasic enhancement of thermal stability of hardness of diamond-like carbon nanofilm at 900 °C

The effect of 25-nm silicon top-layer on the hardness and thermal stability of 100-nm diamond-like carbon (DLC) film annealed at 750–900 °C has been investigated. The evolution of surface morphology, microstructure and reaction between C and Si was examined by high resolution scanning/transmission electron microscope, Raman and FTIR spectroscopy. The hardness of films was investigated using nano-indentation. After 750–900 °C annealing, the hardness of single carbon layer greatly decreased at 750 °C and then slightly increased at 900 °C due to the formation of SiC at the interface between the single C film and the Si substrate. In contrast, no significant variation occurred on the hardness of two-layer Si/C film under RTA at 750–900 °C. Although the higher annealing temperature resulted in higher sp²/sp³ bonding ratio as well as more sp² bonding formation in the carbon layer to soften the structure, the added Si top-layer can protect DLC from reaction with environmental oxygen and sustain the hardness of the composite film because of the multiphasic formation with extra SiC on the surface and at the interface between the C layer and Si substrate through great interdiffusion between Si and C for extending DLC high-temperature application.     

Investigation of interfacial properties of atmospheric plasma sprayed thermal barrier coatings with four-point bending and computed tomography technique

A modified four-point bending test has been employed to investigate the interfacial toughness of atmospheric plasma sprayed (APS) yttria stabilised zirconia (YSZ) thermal barrier coatings (TBCs) after isothermal heat treatments at 1150 °C. The delamination of the TBCs occurred mainly within the TBC, several to tens of microns above the interface between the TBC and bond coat. X-ray diffraction analysis revealed that the TBC was mainly tetragonal in structure with a small amount of the monoclinic phase. The calculated energy release rate increased from ~ 50 J/m^− 2 for as-sprayed TBCs to ~ 120 J/m^− 2 for the TBCs exposed at 1150 °C for 200 h with a loading phase angle about 42°. This may be attributed to the sintering of the TBC. X-ray micro-tomography was used to track in 3D the evolution of the TBC microstructure non-destructively at a single location as a function of thermal exposure time. This revealed how various types of imperfections develop near the interface after exposure. The 3D interface was reconstructed and showed no significant change in the interfacial roughness after thermal exposure.     

Effect of substrate surface temperature on the microstructure and ionic conductivity of lanthanum silicate coatings deposited by plasma spraying

Apatite-type lanthanum silicate shows a high ionic conductivity and low activation energy at intermediate temperature (600-800 °C) compared with Yttria stabilized Zirconia (YSZ). In addition, its excellent stability in a wide oxygen partial pressure range meets the requirement for the electrolyte. Thus it is a potential candidate for intermediate temperature solid oxide fuel cell (IT-SOFC) electrolytes. Moreover, atmospheric plasma spraying is expected to be a promising alternative to other costly or low-deposition rate electrolyte processing methods. However, the amorphous phase and pores existing in the plasma sprayed coatings might impair the stability and the higher ionic conductivity of lanthanum silicate. The present work investigated the effect of substrate surface temperature from 545 °C to 900 °C on the microstructure and ionic conductivity of the lanthanum silicate coating. The crystallinity of as-deposited coatings increased with the increase of substrate surface temperature, whereas the porosity showed a contrary tendency. Ionic conductivity increased about four times with increasing substrate surface temperature, which is related to the low porosity and high crystallinity of coatings. Increasing the substrate surface temperature during plasma spraying is a feasible method to increase ionic conductivity of the lanthanum silicate coatings by reducing the porosity and enhancing the crystallinity.     

Nanosized polycrystalline diamond cladding for surface protection of zirconium nuclear fuel tubes

A 300 nm thick polycrystalline diamond layer has been used for protection of zirconium alloy nuclear fuel cladding against undesirable oxidation with no loss of chemical stability and preservation of its functionality. Deposition of polycrystalline diamond layer was carried out using microwave plasma enhanced chemical vapor deposition apparatus with linear antenna delivery (which enables deposition of PCD layers over large areas). Polycrystalline diamond coated zirconium alloy fuel tubes were subjected to corrosion tests to replicate nuclear reactor conditions, namely irradiation and hot steam oxidation. Stable radiation tolerance of the polycrystalline diamond layer and its protective capabilities against hot steam oxidation of the zirconium alloy were confirmed. Finally, the use of polycrystalline diamond layers as a sensor of specific conditions (temperature/pressure dependent phase transition) in nuclear reactors is suggested.     

Simultaneous synthesis by spark plasma sintering of a thermal barrier coating system with a NiCrAlY bond coat

As-fabricated thermal barrier coating (TBC) systems generally consist of a superalloy substrate, a MCrAlY bond coat (M = Ni, Co, Fe), and a ceramic (usually partially stabilized zirconia) top coat. The conventional methods for producing the two coating layers generally derive from thermal spray and physical vapor deposition techniques. Thermal exposure leads to the formation of an additional layer: the thermally grown oxide (TGO) between the bond coat and top coat. In the present work, a TBC system is synthesized through the application of spark plasma sintering (SPS), which provides not only the opportunity to synthesize all three layers at once, but the process is quite rapid and can produce dense layers. More specifically, this paper describes the application of this method to an yttria-stabilized ZrO₂ (YSZ) top coat and a NiCrAlY bond coat on a Ni-base Hastelloy X substrate. A one-micron thick Al₂O₃ TGO layer is also created from the reaction between an Al foil layer inserted in the stack prior to sintering and the ZrO₂ in the top coat. The effects of select process conditions are considered. The resulting multi-layer system is characterized with optical microscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDAX) and X-ray diffraction (XRD). Differential thermal analysis (DTA) is used to investigate the reaction between the Al foil and the YSZ top coat.     

Damage mechanisms and lifetime behavior of plasma sprayed thermal barrier coating systems for gas turbines—Part I: Experiments

Detailed damage analyses of a plasma sprayed ZrO₂/8 wt.-% Y₂O₃-MCrAlY-CMSX-4 TBC system during isothermal and cyclic oxidation tests with different dwell times at high temperature have been performed. The resulting failure mode, i.e. the particular delamination crack path, is strongly dependent on the temperature cycle applied. Isothermal exposure promotes crack propagation within the TGO, whereas thermal cycling shifts the crack path towards the TBC. Thermal cycling with dwell time at high temperature leads to a mixed delamination crack path (partly within TBC and TGO). The respective correlation between TBC lifetimes and duration of high temperature dwell time per cycle (cycle frequency) is shown and discussed.