等离子喷涂纳米氧化锆涂层的热震失效机理研究

采用合理的喷涂工艺参数制备了纳米氧化锆涂层并在1100度下测试了其热震性能,利用XRD,SEM和TEM对涂层的结构及表面形貌进行了分析。实验结果显示孔隙或早期存在的微裂纹附近的纳米颗粒在热震实验过程中会长大。通过对结构分析,我们提出了在循环应力作用下纳米涂层的失效机理。即随着纳米结构涂层中的大多数或者全部的纳米颗粒长大后,纳米结构随之变为准微米结构,其热震失效模式将类似于传统微米涂层的失效方式—裂纹形成,扩展直至最后涂层剥落。     

Thermal cyclic behavior of air plasma sprayed thermal barrier coatings sprayed on stainless steel substrates

Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The coating comprised of 93 wt.% ZrO₂ and 7 wt.% Y₂O₃ (YSZ); CoNiCrAlY bond coat; and AISI 316L stainless steels substrate. Thermal cyclic lives of the TBC were determined as a function of bond coat surface roughness, thickness of the coating and the final deposition temperature. Two types of thermal shock tests were performed over the specimens, firstly holding of specimens at 1020 °C for 5 min and then water quenching. The other test consisted of holding of specimens at the same temperature for 4 min and then forced air quenching. In both of the cases the samples were directly pushed into the furnace at 1020 °C. It was observed that the final deposition temperature has great impact over the thermal shock life. The results were more prominent in forced air quenching tests, where the lives of the TBCs were observed more than 500 cycles (at 10% spalling). It was noticed that with increase of TBC's thickness the thermal shock life of the specimens significantly decreased. Further, the bond coat surface roughness varied by employing intermediate grit blasting just after the bond coat spray. It was observed that with decrease in bond coat roughness, the thermal shock life decreased slightly. The results are discussed in terms of residual stresses, determined by hole drill method.     

Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications

A study has been undertaken of the characteristics exhibited by mullite-rich plasma electrolytic oxide coatings grown on aluminium alloys by using silicate-rich electrolytes. It is found that they can be grown at a higher rate, and to a greater thickness, than alumina PEO coatings on aluminium. The thermal conductivity of these coatings has been measured using a steady-state method. It is shown to be of the order of 0.5 W m^− 1 K^− 1, which may be compared with ∼ 1.5 W m^− 1 K^− 1 for pure alumina PEO coatings and ∼ 10-15 W m^− 1 K^− 1 for dense polycrystalline mullite. Coupled with excellent substrate adhesion and good mechanical properties, this relatively low conductivity makes these coatings attractive for thermal barrier applications. Furthermore, they are shown to exhibit a relatively low global stiffness (∼ 40 GPa), which will reduce the magnitude of thermally-induced stresses and improve the resistance to spallation during temperature changes.     

Influence of gas flow on argon microwave plasma jet at atmospheric pressure

Many kinds of an atmospheric-pressure plasma jet have been developed and used for widespread applications such as a surface treatment and modified. This study focused on the argon atmospheric-pressure microplasma jet generated by discharging of RF power of 2.45 GHz microwave. The plasma jet shows sensitivity to surrounding environment: pressure, temperature and gaseous species. It is therefore absolutely imperative that a nature of atmospheric-pressure plasma jet should be understood from a point of fluid dynamics. This study, therefore, focused on the interrelationship between the plasma jet and the working gas. Motion of the plasma jet and the working gas was evaluated by velocity measurement and fast photography. As a result, the unsteady sinusoidal waving motion in the radial direction of a torch was observed. Advection velocity of the plasma in just downstream region of the torch exit increases with the supplying flow rate, and the velocity ratio is in the range of 0.75-0.87.     

Response to thermal cycling of plasma nitrided hot work tool steel at elevated temperatures

Thermal fatigue performance of plasma nitrided hot work tool steel was investigated under conditions encountered by thixoforging dies in semi-solid processing of steels. Plasma nitriding does not offer any improvement in the thermal fatigue performance of hot work tool steels at elevated temperatures, due essentially to poor resistance to oxidation and to temper softening. Fe₃O₄ and Fe₂O₃ scales produced on the nitrided surface fail to sustain the thermal stresses produced by thermal cycling. They spall off, generating fresh surfaces for further oxidation. This sequence leads to substantial material loss and impairs the integrity of the surface beyond a quality level that would be tolerated in steel thixoforming. The surface hardening provided by plasma nitriding is also completely erased. The tempered martensitic structure is replaced by fine, equiaxed ferritic grains implying a dynamic recrystallization process during thermal cycling.     

Non-destructive evaluation of plasma sprayed functionally graded thermal barrier coatings

Acoustic emission (AE) as a non-destructive evaluation technique has recently been used in a number of studies to investigate the performance and failure behavior of plasma sprayed thermal barrier coatings. The mechanism of coating failure is complex, especially when considering the composite nature of the coating. In the present paper, the thermal shock tests with in situ acoustic emission are used to study the cracking behavior of plasma sprayed functionally graded thermal barrier coatings. Each thermal cycle consists of 8 min heating in the furnace at 1000°C and 8 min cooling from 1000°C to the room temperature by a compressed air jet. The AE signals are recorded during the quench stage. Three, four and five layer functionally graded coatings have been tested. The results show that the five layer functionally graded coatings appear to have the best thermal shock resistance in the specimens tested, because of the gradual changes in material properties. Higher AE energy counts and cumulative counts recorded by the tests are associated with the macro-crack initiation and growth. On the other hand, micro cracking and phase transformation only give rise to lower AE signals.     

Microcrack formation in plasma sprayed thermal barrier coatings

Air plasma sprayed ZrO₂–8wt%Y₂O₃ thermal barrier coatings were deposited under tightly controlled conditions. The lengths and orientations of the horizontal cracks and vertical cracks in these coatings were characterized in detail, and process/structure maps of the crack distribution as a function of particle and substrate states were constructed. A fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification, and to understand the effect of deposition conditions on crack formation during plasma spray deposition. The model also showed that surface roughness plays a key role in determining the magnitude of maximum stresses, and that only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration.     

Structural modifications of M35 steel submitted to thermal gradients in plasma reactor

Heat transfer between plasma and a solid material occurs mainly due to the radiation and collision of particles onto the material surface. In this process, the heat is transferred from the surface to interior of the material. In a solid material, depending on the particles collision rate and the thermal conductivity, there will be a thermal gradient along and across surface of the samples, caused by thermal peak, with consequent local microstructural modifications. In this work, quenched samples of AISI M35 steel (diameter: 9.5 mm and thickness: 3.0 mm) were tempered in plasma of H₂, using hollow cathode configuration and work pressure of 4 mbar. Microstructural characterization of the samples was carried out by SEM and optical microscopy. Analyzing the microstructure and hardness (surface and profile), the thermal gradient in the samples was evaluated.     

Atmospheric pressure pulsed-periodic source of high energy plasma flows and its applications

A new electromagnetic plasmadynamic system, pulsed-periodic plasmatron, operating at standard atmospheric pressure with high frequency has been developed. In this system plasma flow velocity reaches 3–5 ? 10³ m/s with temperature up to 15·10³ K. The results are presented on the use of the pulsed-periodic plasmatron for carbon steel (0, 45% C) surface layer hardening and boronized layer modification.     

Analysis of the unsteadiness of a plasma jet and the related turbulence

Plasma spraying using liquid feedstock allows realizing thin coatings (< 100 μm). In this process, the plasma jet fluctuations play an important role in the behavior of the liquid fragmentation and then in the final deposit characteristics. In this study, numerical simulations of two different plasma jets (argon or argon hydrogen mixture) issuing into air are investigated. The computations are based on a compressible model and a Large Eddy Simulation (LES) modeling for the turbulence. The aim of this work is to analyze the unsteady effects of the plasma jet regarding those of turbulence. In particular, the characterization of the turbulent zone inside the plasma jet is provided. A turbulence analysis is carried out in order to estimate the levels of turbulence and modeling effects.     

大气常压等离子弧清洗的能量耦合作用有限元分析

大气压等离子弧清洗是一种新型的表面预处理清洗技术,了解等离子弧与工件间的能量耦合作用是研究大气压等离子弧清洗技术的关键。通过建立有限元模型,并对分析结果进行回归计算,得到了界面清洗力和界面清洗温度的经验公式,进一步揭示了等离子弧功率,基板厚度和清洗速度等主要参数对能量耦合的影响,并对回归分析结果进行了验证。结果表明:等离子弧输出功率与清洗速度的相互作用对界面清洗力和界面温度影响最大,其中,等离子弧输出功率对界面清洁力和界面温度的影响最大,清洗速度次之,基板厚度对它们的影响最小。     

Effect of absorbed moisture on the atmospheric plasma etching of polyamide fibers

A potential problem for the atmospheric pressure plasma treatment is that the moisture absorbed by the substrate may influence plasma surface modification processes. This study evaluated the effect of moisture regain on the surface morphology change of polyamide fibers by plasma etching. Polyamide 6, poly(p-phenylene terephthalamide) (PPTA, aromatic polyamide), wool (polyamide 2), and ultrahigh modulus polyethylene (UHMPE, polyamide infinity) fibers, were selected to represent various polyamide molecular structures. The fibers were plasma treated at three moisture regains corresponding to three different relative humidity levels (10, 65, and 100%). Scanning electron microscope (SEM) showed that no apparent morphology change was observed on the surface of UHMPE and PPTA fibers. Under the nano-scale surface analysis of atomic force microscopy (AFM), however, rougher surface of UHMPE and PPTA fibers appeared with elevated relative humidity or higher moisture regain. In terms of polyamide 6 and wool, SEM images revealed that compared to the slight plasma etching effect of fibers with the lowest moisture regain, a thin surface layer of the treated fibers with higher moisture regain was partially or completely peeled off. It may be concluded that fiber moisture regain plays an important role in atmospheric pressure plasma etching of polyamide fibers, which may be mainly due to the interaction between the absorbed water and the polymer molecules. It can be concluded that the etching rate of atmospheric pressure plasma for a polymer depends on its moisture regain, intermolecular forces, crystallinity, and molecular structure.     

Characterization of pulsed discharge plasma at atmospheric pressure

The production of hydrogen and carbon nanotubes (CNTs) by the decomposition of methane inside the reactor using pulsed discharge plasma under conditions of room temperature and atmospheric pressure was investigated and characterized. The development of dissociation of methane with corona discharge under several conditions had been observed. We have noticed that the key radicals such as CH₃ and H produced by pulse corona plasma from source gas, will undergo chain reaction leading to efficient formation of hydrogen gas as well. We have found the external diameter and resistivity of CNTs around 50 nm and 0.15 Ω cm respectively. The main results obtained from the present experiments could be quite useful for the production of hydrogen, CNTs, and future industrial applications.     

An experimental investigation on thermo-mechanical buckling delamination failure characteristic of air plasma sprayed thermal barrier coatings

The primary intention of this work is to investigate the thermo-mechanical buckling delamination failure characteristic of air plasma sprayed thermal barrier coatings (TBCs) under compression tests at high temperature. The TBCs samples with a pre-delamination were firstly designed and they had been successfully prepared by air plasma sprayed technique. The main novelty of this paper is that the first work to validate and obtain three kinds of the interface failure forms in TBCs system during compression tests, i.e. buckling delamination, edge delamination and global buckling failure. The effects of the initial delamination length, temperature gradient and applied mechanical load on the delamination resistance of the TBCs system were discussed in detail. It is difficult to observe buckling delamination or edge delamination failure phenomena until the initial delamination length in TBCs reaches or exceeds 4 mm or more. For edge delamination failure, the interface fracture toughness (Γ_i^II), energy release rate (G_ss^edge) and stress intensity factor (K_II) between the TBC/TGO interface were 35 J m^− 2, 38.8 J m^− 2 and 0.97 MPa at high temperature gradient, respectively. Using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX), it was inferred that the delamination fracture located within the ceramic coating close to the TBC/TGO interface. The results agree well with other experimental and theoretical results.     

Investigation of novel low temperature atmospheric pressure plasma system for deposition photo-catalytic TiO2 thin film

The purpose of this study was to develop a novel low-temperature atmospheric pressure (AP) plasma system and to use the system to deposit photo-catalytic titanium dioxide (TiO₂) thin film. In this study, titanium tetraisopropoxide (TTIP) was used as a precursor for TiO₂ thin film deposition. The precursor was vaporized by ultrasonic oscillator and introduced into an atmospheric plasma system by argon (Ar) carrier gas. The main plasma working gas was Ar mixed with O₂. Microstructure evolutions of TiO₂ thin film were investigated by low-angle grazing-incidence x-ray diffraction (GID), x-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The photo-catalytic properties were determined by contact angle and methylene orange de-coloration testing. In this study, the substrate temperature, the precursor flow rate and the O₂ flow rate were varied. TiO₂ thin film grown at a temperature of 350 °C, with precursor and O₂ flow rates of 20 sccm and 200 sccm, respectively, revealed the optimum photo-catalytic properties. It was also found that titanium dioxide thin films synthesized by the AP plasma method possess reasonable photo-catalytic characteristics like other deposition techniques.     

Hydroxyapatite coatings for biomedical applications deposited by different thermal spray techniques

Thermally sprayed hydroxyapatite (HAp) coatings are widely used for various biomedical applications due to the fact that HAp is a bioactive, osteoconductive material capable of forming a direct and firm biological fixation with surrounding bone tissue.Bioceramic coatings based on nanoscale HAp suspension and microscale HAp powder were thermally sprayed on Ti plates by high-velocity suspension flame spraying (HVSFS) technique and atmospheric plasma spraying (APS) as well as high velocity oxy fuel spraying (HVOF) technique. HVSFS is a novel thermal spray process developed at IMTCCC, for direct processing of submicron and nano-sized particles dispersed in a liquid feedstock.The deposited coatings were mechanically characterized including surface roughness, micro hardness and coating porosity. The bond strength of the layer composites were analyzed by the pull-off method and compared for the different spray techniques. Phase content and crystallinity of the coatings were evaluated using X-ray diffraction (XRD). The coating composite specimen and initial feedstock were further analysed by scanning electron microscope (SEM) and rheology analysis.     

Improvement of thermal shock resistance for thermal barrier coating on aluminum alloy with various electroless interlayers

NiCoCrAlY/8YSZ coating was firstly directly deposited on aluminum alloy 5A06 by atmospheric plasma spray to make it applicable to short-time high temperature condition. The failure after thermal shock test was mainly due to the stress caused by thermal expansion mismatch between the bond coat and the substrate as well as the galvanic corrosion of the aluminum alloy. Ni-P, Ni-W-P and Ni-Cu-P as interlayers were electrolessly deposited on the substrate in order to mitigate the thermal stress. The composition and thermal transformation of the interlayers were investigated. Thermal shock resistance and bonding strength of multilayer coatings (interlayer/NiCoCrAlY/8YSZ) were tested. Diffusion layers mainly composed of AlNi, Al₃Ni₂ and Al₃Ni were observed between the interlayers and the substrate after thermal shock test. The oxidation of the substrate was effectively inhibited. Ni-P interlayer obtained at lower pH value was superior to the other two interlayers and enhanced the thermal shock life from 38 to more than 200 cycles. With the application of the Ni-P and Ni-Cu-P interlayers, the bonding strength examined by pull-off test was also largely improved from 11.7 MPa to 18.8 and 19.0 MPa, respectively.     

Microstructure and thermal cycle resistance of plasma sprayed mullite coatings made from secondary mullitized natural andalusite powder

Natural andalusite powder was calcinated at a high temperature in air to realize secondary mullitization. The resultant secondary mullitized powder was spray-dried and heat-treated to improve sprayable capability. The heat-treated spherical powder was then plasma sprayed onto Ni-based high-temperature alloy (Hastelloy C-276) to form mullite coatings. The chemical composition and phase structure of the as-sprayed and thermally cycled mullite coatings were determined by means of energy dispersive X-ray fluorescence (ED-XRF) and X-ray diffraction. The microstructure of the as-sprayed coatings was analyzed by using a scanning electron microscope; and their porosity, microhardness and bonding strength were measured. Moreover, the phase transition temperature and enthalpy of the coatings were determined by means of differential scanning calorimetry; and their thermal shock resistance was evaluated as well. Results show that the spray-dried and heat-treated powder consists of mullite and a small amount of Al₂O₃; while the as-sprayed mullite coatings are composed of crystalline mullite as the major phase and a small amount of amorphous glass phase. During thermal cycle test, the amorphous glass phase is partially transformed to crystalline mullite, finally leading to failure of the coatings. Whether before or after thermal cycle, the mullite coatings experience phase transition around 980 °C, and the enthalpy of crystallization is determined to be − 141.9 × 10^− 3 J/kg and − 95.48 × 10^− 3 J/kg, respectively. The as-sprayed mullite coatings have a porosity of about 6.0 ± 0.2% and possess good thermal cycle resistance, showing promising prospect in a high-temperature application.     

等离子体渗氮的气压参数动态特性研究

等离子体渗氮的气压工艺参数具有多变量、非线性、大迟延和耦合的动态特性。