超音速火焰喷涂合成TiC-Ni涂层滑动磨损性能研究

采用二次正交回归试验方法得到了喷涂工艺参数与反应超音速喷涂合成涂层滑动磨损性能的定量关系,研究了涂层的磨损失效机制,并分析了氧气流量、燃气流量和喷涂距离对超音速火焰喷涂合成TiC-Ni涂层滑动磨损性能的影响。结果表明：涂层的磨损机制以粘结相的优先磨损以及硬质相的剥落引起的磨粒磨损为主。氧气流量、燃气流量和喷涂距离对涂层滑动磨损性能有较大影响。适中的氧气流量、燃气流量有利于获得耐磨性较好的涂层,喷涂距离较小时,涂层的磨损失重量变化不明显．喷涂距离较大时,失重量较高。     

Reactive thermal spray by high velocity ceramic jet and characterization of the coatings

A ceramic jet composed of molten particles in an electrothermally exploded powder spray was identified by the flash, soft x-ray radiography technique. The velocity of the leading edge of the jet was estimated to be 900 m/s. The coating obtained by a ceramic jet of titanium diboride consisted of a mixing layer of the substrate material and sprayed ceramics. A coating, which exhibited no pores or cracks, was formed through the dense deposition and solidification of spray droplets. The successive impacts of the droplets caused melting and stirring of the substrate surface to form a mixing layer. Some of these layers were formed due to capillary movement of the molten substrate material into the fractures of coarse ceramic particles. Thermal spray by chemical reaction between titanium and boron nitride particles resulted in a composite coating of TiN and TiB₂. The character of the mixing layer indicated that the depth profiles depended on the substrate material.     

Oxidation of stainless steel in the high velocity oxy-fuel process

The high velocity oxy-fuel (HVOF) spray process has been primarily used for the application of wear-resistant coatings and, with the introduction of new, more powerful systems, is being increasingly considered for producing corrosion-resistant coatings. In this study, the influence of various spray parameters for the JP-5000 and Diamond Jet (DJ) Hybrid systems on the oxidation of stainless steel 316L is characterized. Experimental results reveal that coating oxygen contents of less than 1 wt.% can be more easily attained with the JP-5000 than the DJ Hybrid systems because of the former’s design. In both cases, however, the low particle temperatures necessary for low oxygen content coatings may impair bond and cohesive strength. Heat treating the coatings after processing reduces hardness, metallurgically enhances bond strength, and enables the spheroidization of oxide layers surrounding unmelted particles. An empirical model describing oxidation in the thermal spray process was expanded to explain the oxidation in the HVOF spraying of stainless steel. It was concluded that for these oxygen-sensitive materials, maintaining a relatively low particle temperature throughout the spray process minimizes oxygen pickup by preventing an autocatalytic oxidation process and particle fragmentation upon impact. For the DJ Hybrid systems, understoichiometric fuel settings are selected, whereas for the JP-5000, oxygen-rich mixtures are preferred.     

超音速火焰喷涂CoCrW涂层的高温抗氧化性能

采用超音速火焰喷涂工艺制备了CoCrW涂层，用增重法测试了涂层的高温抗氧化性能。试验结果表明，在氧化性气氛下，涂层在850℃以下或经过较短时间处理时，涂层氧化增重较少，而经850℃以上处理或经过较长时间处理时，涂层氧化增重明显，而且，处理后涂层的X射线衍射结果也证明在涂层中出现了铬和钴的氧化物。可见，CoCrW涂层的稳定使用温度应控制在850℃以下。     

TiAlNb intermetallic compound coating prepared by high velocity oxy-fuel spraying

Ti_28.35Al_63.4Nb_8.25 (at.%) intermetallic compound coatings were sprayed onto 316 L stainless steel substrates by HVOF processes using various parameters. By varying the grit blasting pressure between 0.11 and 0.55 MPa, the effects of substrate roughness on the adhesion of TiAlNb thermal sprayed coatings were investigated. The microstructure, porosity and microhardness of the coatings were characterized by SEM, XRD, Image Analysis and Vickers hardness analysis. The tensile adhesion test (TAT) specified by ASTM C 633-79 was used to measure the tensile bonding strength of the coating. The results show that the coatings with substrate roughness of 8.33 μm displayed the best combined strength. TiAlNb coatings had a lamellar microstructure with different spraying parameters. The porosity, bonding strength, microhardness of coatings were assessed in relation to the spraying processes. The thickness of bond coat on the bond strength of coatings was also discussed.     

不同助燃条件下超音速火焰喷涂的燃烧特性

运用FLUENT软件,在气体-颗粒两相流基础上,计算了纯氧气、空气、氧-空气联合助燃3种情况下超音速火焰喷涂的燃烧特性.结果表明:纯氧气助燃时燃烧充分,焰流温度高,但氧气质量流量增加对焰流温度影响很小;空气助燃时所用空气流量大,导致燃烧室压力大,故适宜喷涂低熔点、易氧化粉末材料;氧-空气联合助燃时,氧气含量越大,焰流能达到的最高温度越高,但当氧气含量达到一定比例后可完全替代纯氧助燃;助燃气体中氧气含量的不同间接影响燃烧产物成分分布.     

Studies of Fe-40Al coatings obtained by high velocity oxy-fuel

Interesting properties that intermetallics possess have made them to be promising materials to be used either as bulk materials or as coatings, both at medium or elevated temperature environments. This group of materials possesses a long-range order, which can be kept by some intermetallics until their melting point, which is the main reason why they possess a good stability at high temperatures. Some other properties can be summarized as follows: high thermal conductivity; low density; great strength, particularly at high temperatures; good oxidation resistance at high temperatures (because of the formation of oxide films); low ductility, brittle fracture at room temperature.FeAl coatings from powder of nominal composition Fe-40Al-0.05 Zr (at.%) with 50 ppm B and 1 wt.% Y₂O₃ have been prepared using high velocity oxy-fuel (HVOF) technique. Several standard spraying conditions have been assessed; some parameter variations from those standards intend to find optimal spraying conditions. The characterization has been carried out by DRX, EDS and Scanning Electron Microscopy. The results conclude that a major intermetallic FeAl phase has been obtained.Microhardness and wear properties have been evaluated for those coatings obtained with optimal conditions. Compared to room temperature sliding wear behaviour, the friction coefficient is reduced when the test is performed at 400 °C.     

Molybdenum based amorphous and nanocrystalline coatings prepared by high velocity oxy-fuel spraying

The high velocity oxy-fuel(HVOF) based thermal spray process has developed as a potential advantageous approach for fabricating various kinds of functional coatings.In this article,the coatings of Mo-based alloy were synthesized using the HVOF process.The microstructure and the mechanical properties of the HVOF-processed coatings were investigated using SEM,TEM,XRD,and hardness and wear tests.Annealing treatment was applied to the as-sprayed coatings to develop the microstructure and its effect on the microstructure and mechanical properties of the coatings was examined.It is found that the HVOF-processed Mo-based alloy coatings are comprised of an amorphous splat matrix embedded with nano-sized crystalline particles.Annealing at temperatures over 950 ℃ results into crystallization of the amorphous matrix.The mechanical properties of the as-sprayed coatings are enhanced with annealing temperature up to 750 ℃ and from 950 to 1050 ℃,keeps constant between 750 and 950 ℃,and reduce over 1050 ℃.The change of the mechanical property with the microstructure was illustrated in the study.     

Thermo-physical processes in cerium nitrate precursor droplets injected into high temperature plasma

The thermo-physical phenomena inside a droplet during its flight in a thermal plasma flow have been modeled for SPPS processes using cerium nitrate precursor. The different thermo-physical stages encountered during droplets' flight include aerodynamic breakup, rapid vaporization of solvent and precipitation, internal pressurization with shell rupture and rapid heating of volumetrically precipitated ceramic particle. The effects of injection type (transverse versus axial) and initial size of the droplets on the final deposit layer microstructure have been studied. The results show that for axial injection the heating process is very rapid and smaller droplets (< 10 µm) undergo full pyrolization resulting in favorable microstructure. On the contrary for transverse injection the heating process is slower as the droplets are injected in the outer shear layer of plasma where the temperature and velocity both are much lower than the core of the plasma. The larger droplets (> 20 µm) have better chances of getting pyrolized in the case of transverse injection. It was also observed that the angle of trajectory of the droplets after primary (first) precipitation does not have much effect on the final microstructure.     

Heat transfer analysis of atomized droplets during high velocity arc spraying

The heat transfer problem of the atomized droplets during high velocity arc spraying (HVAS) was modeled and solved by a numerical method using a Fe-Al alloy, and the influences of several important process parameters on the heat transfer behaviors of the atomized droplets were analyzed. The results show that the initial cooling rates of different size droplets range from 105 to 107 K/s, thus producing the coating microstructure with the features of rapid solidification. The droplet size, atomization gas pressure and droplet superheat have great influences on the heat transfer behavior of the droplet. The droplet temperature and cooling rate are much sensitive to the droplet sizes, but insensitive to the atomization gas pressure and droplet superheat. It can be predicted that the properties of HVAS coatings will be improved by decreasing droplet size as well as increasing atomization gas pressure and droplet superheat in certain extents.     

Formation of amorphous Fe-Cr-Mo-8P-2C coatings by the high velocity oxy-fuel process

Alloy powders of Fe-10%Cr-8%P-2%C(10Cr), Fe-20%Cr-8%P-2%C(20Cr), and Fe-10%Cr-10%Mo-8%P-2%C(10Mo) compositions (in mass%) were sprayed by the high velocity oxy-fuel (HVOF) process under different conditions. The as-sprayed coatings of 10Mo alloy were composed of only an amorphous phase under all the spray conditions, while the as-sprayed coatings of the 10Cr and 20Cr alloys consisted of an amorphous phase with a small amount of crystalline material. The volume fraction of the crystalline material increased slightly with the rise of the flame temperature. The hardnesses of the as-sprayed coatings of the 10Cr and 20Cr alloys were 600 to 700 DPN, respectively, while the 10Mo coating composed of an amorphous phase revealed 560 DPN. The corrosion resistance of the as-sprayed coating of the 10Mo alloy was the best among three amorphous coatings and also superior to the nickel-base self-fluxing alloy and SUS316L stainless steel coatings in 1N H₂SO₄ and 1N HCl solutions.     

Mechanisms and kinetics of WC-Co−Cr high velocity oxy-fuel thermal spray coating degradation in corrosive environments

In this work, aspects of the corrosion behavior of WC-Co−Cr high velocity oxy-fuel (HVOF) thermal spray coatings have been assessed using a combination of x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to understand the corrosion mechanisms and, in particular, the electrochemical interactions between phases. Direct curent electrochemical accelerated corrosion techniques (potentiodynamic and potentiostatic tests) were performed to evaluate the corrosion kinetics of the coating. After the corrosion tests, the solution was analyzed using the inductively coupled plasma (ICP) technique, and a considerable amount of dissolved tungsten was detected. By combining information from XPS, SEM, ICP, and anodic polarization results, it is possible to propose a number of key reactions that can take place during WC-Co−Cr coating degradation, thus enabling the susceptible components of the coating to be identified. The implications of these findings for coating durability are discussed.     

Properties of Cr3C2-NiCr cermet coating sprayed by high power plasma and high velocity oxy-fuel processes

The structure, hardness, and shear adhesion strength have been investigated for Cr₃C₂-NiCr cermet coatings sprayed onto a mild steel substrate by 200 kW high power plasma spraying (HPS) and high velocity oxy-fuel (HVOF) processes. Amorphous and supersaturated nickel phases form in both as-sprayed coatings. The hardness of the HVOF coating is higher than that of the HPS coating, because the HVOF coating contains more nonmelted Cr₃C₂ carbide particles. On heat treating at 873 K, the amorphous phase decomposes and the supersaturated nickel phase precipitates Cr₃C₂ carbides so that the hardness increases in the HPS coating. The hardness measured under a great load exhibits lower values compared with that measured with a small load because of cracks generated from the indentation. The ratio of the hardnesses measured with different loads can be regarded as an index indicating the coating ductility. The ductility of the HVOF coating is higher than that of the HPS coating. Adhesion strength of the HVOF coating was high compared with the HPS coating. The adhesion of the coatings is enhanced by heat treating at 1073 K, and that of the HVOF coating is over 350 MPa.     

不同燃料超音速火焰喷涂NiCr-Cr3C2涂层

采用超音速火焰喷涂工艺在20CrMo钢圆片上成功制备25%NiCr-Cr3C2金属陶瓷涂层,研究了不同燃料对喷涂层组织性能的影响。结果表明,以丙烷为主要燃料所得喷涂层的组织不均匀,存在典型的层状结构,孔隙率约为3.2%,显微硬度仅为836 HV,涂层断裂韧性KIC为2.08 MPa.m3/2;以煤油为主要燃料所得喷涂层的组织分布均匀,细小致密,孔隙率约为2.4%,显微硬度可达1045 HV,涂层断裂韧性KIC为2.56 MPa.m3/2。前者的组成相为Ni(Cr)固溶体、Cr3C2和微量NiCrO4;后者的组成相除Ni(Cr)固溶体和Cr3C2外,还有Cr7C3、Cr23C6和Ni(Cr)相出现。     

Dominant microstructural feature over photocatalytic activity of high velocity oxy-fuel sprayed TiO2 coating

TiO₂ photocatalytic coatings were deposited through high velocity oxy-fuel spray using anatase powder and rutile powder as feedstock. The as-sprayed TiO₂ coating was composed of anatase phase and rutile phase. The anatase content in the coating was significantly influenced by fuel gas flow and melting condition of spray powder. A high anatase content of 35% was achieved for the coating deposited using rutile powder. The anatase content in the coating deposited using anatase powder reached 55-65%. The as-sprayed TiO₂ coating was photocatalytically reactive for degradation of acetaldehyde in air. The photocatalytic activity was influenced by spray conditions. The surface morphology and phase structure of coatings deposited at different spray conditions were investigated to clarify the relationship between the coating microstructure and activity. It is found that the photocatalytic activity is significantly influenced by anatase content and surface area.     

Electrical structure of the jet of a gas mixture flame

The combustion of the open flame of an air-domestic gas mixture in air is considered. The concept of the stoichiometric ratio for such a system is specified. The combustion of an enriched mixture, a stoichiometric mixture, and a depleted mixture are singled out. For each case, the structure of the flame and its variations under the condition of the transition of the combustion into the pulsating mode are found by electric and optic measurements. A significant role of the secondary (ambient, atmospheric) air for the stoichiometric flame combustion is noted.     

Modeling the first instant of the interaction between a liquid and a plasma jet with a compressible approach

In this work, the numerical simulation of the interaction between a liquid jet and a plasma flow is investigated for understanding and predicting the physical parameters involved in the liquid plasma spraying process. This process is used for the production of high performance coatings to obtain thinner deposits (< 100 μm) than in conventional plasma spraying. The up-to-date goal is to describe the dispersion of liquid in order to understand the effect of injection conditions on the surface coating quality. This work proposes an original model for dealing with three-dimensional interactions between a plasma flow and a liquid phase with Volume Of Fluid (VOF) methods. A compressible model, capable of managing incompressible two-phase flows as well as compressible motions, is used. First comparisons with experimental photography show rather good agreement during the first moments of the injection.     

HVOF喷涂Fe-Cr基涂层中非晶与纳米晶形成的研究

采用多元Fe基合金（含Cr、Si、Mn、B等）作为喷涂粉末,用超音速火焰（HVOF）喷涂法在不锈钢基体上制备厚度约200μm的Fe-Cr基涂层.用X射线衍射、扫描电镜、透射电镜、差示扫描量热仪（DSC）对涂层的组织结构特征进行了研究.涂层由于熔融和半熔化状态的液滴的连续堆积而成层状,由Fe基非晶、纳米晶及硼化物组成,纳米晶尺寸约为10-30nm.DSC分析表明非晶的晶化温度约为605℃.非晶的形成是由于喷涂液滴快的冷却速度及合适的粉末成分;非晶由于后续熔融液滴的堆积对前涂层产生退火效应,以非均匀形核的方式分别在非晶内部和非晶与硼化物的界面形成.     

超音速火焰喷涂碳化钨-钴涂层磨粒磨损行为

采用超音速火焰喷涂工艺在16Mn钢上制备了WC-12Co涂层,并测试了该涂层的力学性能特别是其抗磨粒磨损性能。结果表明:WC-12Co涂层的主相为碳化钨,显微硬度为(1341.0±134.3)HV,孔隙率为0.21%±0.04%。该涂层的磨损率随着磨粒硬度、磨粒粒度和加载载荷的增加而增加。当磨粒的硬度低于涂层硬度时,涂层的磨损机制以磨耗磨损为主,磨损率低;当磨粒的硬度超过涂层的硬度时,涂层的磨损以微切削为主,磨损率高。另外,WC-12Co涂层的耐磨性相对于16Mn钢也是随着磨粒的硬度变化而变化。     

LHVOF超音速喷涂技术在工业关键零部件上的新应用及思考

热喷涂技术是中国“十一五”计划和2020年远景规划中要大力发展的先进制造技术。文中着重介绍利用LHVOF（hquate high velocity oxygen flame）超音速喷涂技术在工业关键零部件上的新应用。同时对比国外热喷涂技术的现状，提出对中国热喷涂应用今后发展趋势的一些思考。