Developments in direct current plasma spraying

Thermal spray processing is used to confer specific in-service properties to components via the production of a coating between 50 μm (minimum value) to a few millimeters thick. Thermal spray represents a global market of about 4.8 Billion Euros (i.e., ∼ US$5 billion) in 2004; 30% of which is European based. 50% of this activity is devoted to plasma spray processing with about 90% dedicated to direct current (DC) plasma torches. Several developments of new torch architectures, among which three-cathode torches, have evolved recently. However, most of the recent progress has been applied to conventional DC torches. The advances were related to two prime factors: (i) the development of industrial sensors permitting to diagnose the processes during spray operation (especially in-flight particle characteristics in terms of their surface temperature and velocity) and additionally the monitoring of the substrate and coating temperatures with the objective of controlling the operating parameters via a close-loop controller; (ii) the adaptation of plasma spray systems to manufacture nano-structured coatings via the development of suspension plasma spray and solution plasma spray. As well, there has been an enhanced understanding of the mechanisms controlling the coating formation and of the effects of the arc root fluctuations; thereby permitting a more robust process. This paper develops the above points by presenting focused examples.     

等离子喷涂的电弧伏安特性

等离子弧的伏安特性决定电弧功率的大小,从而影响喷涂粒子的熔融状态,进而影响了涂层的质量。等离子喷涂过程电弧伏安特性受多种因素的影响。通过改变气体的种类和流量、喷嘴几何尺寸（孔径和压缩角）来研究这些因素对电弧伏安特性的影响。结果表明,电弧电压在一定范围内随着主气流量的增加而增加;氢气流量对电弧电压影响显著;氮气对电弧电压的影响比氩气大;在喷嘴总长度相同的情况下,电弧电压随着压缩角的增大而减小,随着孔径的减小而减小。     

Fluid dynamic study of direct current plasma jets for plasma spraying applications

Recently, direct current (dc) plasma torches equipped with converging-diverging (Laval) nozzles, instead of standard cylindrical ones, have been shown to present several advantages for both vacuum and atmospheric plasma spraying, such as diminishing the gradients of temperature and velocity and reducing the turbulence intensity in the jet fringes. The present study was concerned with the diagnostics of the plasma jets produced by three nozzles of various contours: a standard cylindrical anode and a Mach 2.5 and Mach 3 Laval nozzle. Emission spectroscopy (absolute intensity) and enthalpy probe techniques were used to measure temperature and velocity fields. Special attention was given to the effects of spray chamber pressure on flow regime inside the nozzles and to the distribution of the temperature and velocity fields in the plasma jet. Results showed that under the chamber pressure conditions used (vacuum), for which Laval nozzles originally were designed, the generated plasma jets had greater centerline velocities and larger high temperature zones compared to standard cylindrical nozzles. The results showed significant improvement in the deposition efficiency by using nozzles with these computed contours.     

Preparation of thermal barrier coatings by ultrasonic plasma spraying

Modulated plasma arc not only can heat the powder, but also can excite ultrasonic of different frequencies and different powers. The principles and characters of the plasma arc-excited ultrasonic were described, and the ultrasonic plasma spraying was compared with normal plasma spraying. Zirconia thermal barrier coatings (TBCs) were fabricated with two kinds of method. The TBCs were studied by the optical microscope observation, SEM observation and bonding strength experiment. The results show that suitable ultrasonic changes the performance and microstructure of TBCs in evidence. And the mechanism of ultrasonic influencing the TBCs was also discussed.     

Design of the synthesis of fine HA powder for suspension plasma spraying

Hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) is a bioactive material being frequently used as a coating onto implants. The typical coating technology is air plasma spraying with the use of coarse powder. The resulting coatings are relatively thick (about 200-400 µm) and porous. Much thinner coatings being 5-40 µm thick, can be obtained by emerging technology of suspension plasma spraying with the use of powder particles having the diameters ranging from a few submicrometers to a few micrometers. The paper describes the way of synthesizing and preparing such fine powder starting from an aqueous solution of ammonium phosphate (H₂(PO₄)NH₄) and calcium nitrate (Ca(NO₃)·4H₂O) using statistical design of experiments (DOE). The design was made using composite matrix including a full factorial plan, star points and 3 experiments in the centre. The crystal phases purity and the mass of powder batch were the optimized responses of the powder synthesis and the concentration of calcium ions and volume of ammonium hydroxide were the experimental variables. The synthesized material was characterized by X-ray diffraction (XRD). The powder was calcined and crushed using a milling machine with zirconia balls and resulting morphology and size of fine particles was characterized using scanning electron microscope (SEM) and laser sizer correspondingly. The powder was then formulated into water and alcohol based suspension and the zeta potential was determined to understand its capacity of agglomeration. It was found out that the formulation of the suspension with the use of ethanol slightly favours dispersion of solid particles in the suspension. The initial tests of water based suspension plasma spraying onto titanium substrate were also carried out and the XRD phase analysis of obtained coatings was carried out the presence of HA and its phases of decomposition.     

高速燃气喷涂枪用燃油雾化喷嘴结构参数优化

为开发出以航空煤油为燃料的新型高速燃气喷涂枪,需设计出高效的煤油雾化喷嘴.文中提出了一种双气流空气助力雾化喷嘴,并运用计算流体动力学（CFD）模拟技术计算了该喷嘴的气流场,分析了喷嘴主、辅气出口截面积比对雾化气流场分布的影响规律,发现主、辅气出口截面积比增大,气流的喷射锥角相应增大,最大速度减小.综合考虑喷射锥角和气流速度对雾化效果的影响,确定主、辅气出口截面积比在1.01~1.34范围内喷嘴雾化效果较好.利用高速摄像系统对优化喷嘴的喷雾形态进行了试验分析,发现拍摄到的喷雾形态和计算机模拟结果一致,具有良好的雾化效果.     

基于工控组态平台的机器人等离子喷涂控制系统设计

分析了机器人等离子喷涂工艺特点，开发了一种基于工控组态软件的机器人等离子喷涂控制系统。该系统能满足喷涂工艺控制要求，并对电流、电压、机器人状态等信息进行实时监控，具有开发周期短、用户界面友好、易于操作和控制效果优良等特点。     

等离子喷涂梯度热障涂层的抗热震性能

对比研究了等离子喷涂梯度热障涂层与双层热障涂层,试验中梯度热障涂层选用不同比例的NiCoCrAlY与ZrO₂-8%Y₂O₃复合粉末作为梯度过渡层材料,并对两种结构的热障涂层进行了抗热震性能试验。抗热震试验结果表明,梯度热障涂层的抗热震寿命明显高于双层热障涂层的抗热震寿命。     

等离子体喷涂纳米结构热障涂层微观组织及性能

以纳米结构氧化钇部分稳定的氧化锆热喷涂粉末为原料,采用大气热等离子体喷涂法制备了纳米结构热障涂层。利用扫描电子显微镜和X射线衍射仪对粉末原料及涂层的微观组织和结构进行分析,并对涂层的结合强度及热导率进行测定。结果表明,纳米结构热障涂层具有优异的性能,热导率为1.1 W/(m.K),界面结合强度为47 MPa。并分析了涂层纳米结构组织对涂层性能的影响,明确了优化涂层微观组织结构和提高涂层性能的具体方法。     

High-power hybrid plasma spraying of large yttria-stabilized zirconia powder

To testify to the advantage of large ceramic powder spraying, numerical simulations and experimental studies on the behavior of large yttria-stabilized zirconia (YSZ) powder in a high-power hybrid plasma spraying process have been carried out. Numeric predictions and experimental results showed that, with the high radio frequency (RF) input power of 100 kW, the most refractory YSZ powder with particle sizes as large as 88 μm could be fully melted and well-flattened splats could be formed. A large degree of flattening (ξ) of 4.7 has been achieved. The improved adhesive strength between the large splat and the substrate was confirmed based on the measurement of the crack density inside of the splats. A thick YSZ coating >300 μm was successfully deposited on a large CoNiCrAlY-coated Inconel substrate (50×50×4 mm in size). The ultradense microstructure without clear boundaries between the splats and the clean and crack-free interface between the top-coat and the bond-coat also indicate the good adhesion. These results showed that highpower hybrid plasma spraying of large ceramic powder is a very promising process for deposition of highquality coatings, especially in the application of thermal barrier coatings (TBCs).     

A new application field of plasma spraying technique to environmental depollution

To expand the application of plasma spraying technique, TiO2 coatings were prepared using agglomerated anatase TiO2 powder to solve the environmental problems. The composition and photocatalytic activity of plasma sprayed TiO2 coatings were investigated systematically. The content of anatase TiO2 in the sprayed coatings was approximate to 7%- 15%, which was influenced by the melting state of TiO2 powder in plasma spraying process. The surface of sprayed coating was very rough and the arithmetical mean deviation of the surface profiles (Ra) was in the range of 5.7 - 8.8 μm. Under lower arc current, the surface of the coating became rougher. The anatase to rutile phase transformation temperature of agglomerated anatase TiO2 powder was approximate to 1 173 K. The TiO2 coating sprayed under the arc current of 400 A had good photocatalytic activity for the relative high content of anatase phase in it. It is concluded that the application of plasma spraying technique to environmental field has been developed.     

Nanostructured yttria stabilized zirconia coatings deposited by air plasma spraying

Nanostructured yttria partially stabilized zirconia coatings were deposited by air plasma spraying with reconstituted nanosized powder. The microstructures and phase compositions of the powder and the as-sprayed nanostructured coatings were characterized by transmission electron microscopy（TEM）, scanning electron microscopy（SEM） and X-ray diffxaction（XRD）. The results demonstrate that the microstructure of as-sprayed nanostructured zirconia coating exhibits a unique tri-modal distribution including the initial nanostructure of the powder, equiaxed grains and columnar grains. Air plasma sprayed nanostructured zirconia coatings consist of only the nontransformable tetragonal phase, though the reconstituted nanostructured powder shows the presence of the monoclinic, the tetragonal and the cubic phases. The mean grain size of the coating is about 42 nm.     

等离子喷涂陶瓷涂层的应用

简述等离子喷涂陶瓷涂层的研究进展与现状,依据陶瓷涂层应用的分类,从等离子喷涂陶瓷涂层的耐磨涂层、热障涂层、耐蚀涂层、超导涂层、压电陶瓷涂层、生物活性涂层方面介绍了目前常用的等离子喷涂陶瓷材料的性能及其应用的研究现状。     

等离子喷涂YPSZ陶瓷梯度涂层的组织与抗热震性能

利用等离子喷涂工艺在汽车铝活塞表面制备了氧化钇部分稳定的氧化锆(YPSZ)陶瓷梯度涂层,用电子扫描显微镜、金相显微镜等手段对涂层从组织形貌、物相、显微硬度和热震性能进行了分析.结果表明:基体与涂层结合紧密;涂层在冷却过程中发生了t-ZrO2→m-ZrO2的相变过程;涂层的热震性能良好,可满足活塞的使用要求.     

Mold manufacture with plasma spraying

A process has been developed to produce molds or tooling using a steel or chrome- plated steel model. The investigation examined the effect of coating and model materials, model temperature and spray angle on the coating separation from the model surface, coating delamination, and surface quality. A polished model disk was heated and then plasma sprayed with iron, nickel, Ni- Al, or Ni- Cr- B- Si. It was found that the minimum temperature to facilitate entire coating removal was lower for steel models and varied be tween 200 and 450 °C depending on the material. However, at higher temperatures the higher bond strength produced by oxidation on the steel resulted in significant coating pullout. A chrome- plated model, heated to 600 to 700 °C, is required to produce a defect- free coating. The effect of substrate angle on open porosity is most critical for the Ni- Cr- B- Si alloy and least important for Ni- Al coatings. The sur face roughness of the plasma- sprayed molds is comparable to the corresponding models, permitting good surface detail reproducibility. Several molds and tools were produced for use in the glass, rubber, and plastics industries.     

Fabrication of finely structured silicon-supported SOFC with anode deposited by multi-phase plasma spraying

The mechanically mixed NiO/YSZ powder was usually used as the anode material of atmospheric plasma sprayed (APS) solid oxide fuel cells (SOFC). Big particles and the non-uniform distribution of the pores were observed in the resultant anode layer. To overcome the limitations, a method of fabricating anode layer by multi-phase plasma spraying (MPS) was proposed in this paper. The NiO and YSZ powders were delivered into plasma jet by a separate injection, where nitrogen carrier was employed to feed micrometer-sized NiO powder and liquid carrier was to feed submicrometer-sized YSZ powder. Suspension plasma spraying (SPS) was applied to fabricate dense electrolyte layer. The microstructure and composition of coatings were characterized by SEM and EDS. The results showed that finely structured anode layer with small particle size (d ∼ 2 μm) was achieved by the MPS method. The MPS anode layer was porous with the porosity of 32.1% while the APS anode layer was 22.6%. Three kinds of elements (Ni, Y, Zr) were observed in the MPS anode layer and the NiO content was calculated to be 49.6 wt%. In the SPS process, the suspension flow rate was matched to the plasma gas flow rate to obtain proper injection condition.     

大方坯连铸喷嘴的喷雾覆盖与喷嘴堵塞对铸坯表面温度的影响

对连铸喷嘴的喷雾覆盖计算做了探讨,介绍了喷雾覆盖与支撑导向辊直径和间距的关系,喷雾覆盖与喷嘴堵塞对铸坯表面温度的影响。     

等离子弧喷涂法制备铁基非晶合金的研究

非晶态合金具有强度、韧性以及耐磨性能优于普通晶态金属材料,耐蚀性好,磁抗小等优点,倍受材料科学和工程技术人员的关注,但是非晶态材料只有在非平衡条件下才能形成,加工难度大,阻碍了非晶态材料的发展和应用.对此,采用等离子弧喷涂方法制备铁基非晶合金,制备了Fe79Si10B11非晶态合金.对制备的非晶合金进行了XRD和TEM分析,并研究了喷涂过程中粒子的飞行温度和速度,通过传热学方法计算了喷涂过程中的粉末冷却速度.结果表明,所制备的非晶合金组织均匀,喷涂过程冷却速度达4.16×107 K/s.     

Graded plasma spraying of premixed metalceramic powders on metallic substrates

The mismatch between the thermal expansion coefficients of ceramics and metals and the differential stresses it causes at the interface create problems in metal to ceramic joining. Research has been con-ducted to solve this problem in thermal barrier coating technology. Previous studies have considered met-al-ceramic multilayers or graded-coatings, which include a metallic bond coat. In this study, a graded plasma-sprayed metal-ceramic coating is developed using the deposition of premixed metal and ceramic powders without the conventional metallic bond coat. Influences of thickness variations, number, and composition of the layers are investigated. Coatings are prepared by atmospheric plasma-spraying on In-conel 718 superalloy substrates. Ni-Cr-Al and ZrO₂ -8 % Y₂O₃ powders are used for plasma spraying. Ad-hesive and cohesive strength of the coatings are determined. The concentration profile of the elements is determined by x-ray energy-dispersive analysis. The microstructure and morphology of the coatings are investigated by optical and scanning electron microscopy (SEM). Results show that the mixed metal-ce-ramic coating obtained with the deposition of premixed powders is homogeneous. The morphology and microstructure of the coatings are considered satisfactory.     

等离子喷涂设备的PLC自动控制

介绍了等离子喷涂设备的组成及技术要求,分析并设计了等离子喷涂的电路图及工作程序流程图。以西门子公司S7—200可编程控制器为中心元件,设计了等离子喷涂设备的PLC自动控制系统。该系统具有操作方便、可靠性高、工作电流稳定性好的特点,控制功能和精度优良,可喷涂各种合金及陶瓷粉末,喷涂效果理想。