Nanostructured Al-Based Metal Matrix Composite Coating Production by Pulsed Gas Dynamic Spraying Process

The advantage of combining cryomilling and pulsed gas dynamic spraying (PGDS) processes in order to produce a nanostructured, dense and wear resistant coating was demonstrated. Cryomilling was successfully employed to synthesize particulate B₄C reinforced Al matrix nanocomposite feedstock powders, while the PGDS process shows the ability of preserving the microstructure of the starting material. In this study, nanocrystalline and conventional Al5356?+?20%B₄C composite as well as the unreinforced Al5356 alloy feedstock powders were used. The influence of the nature of the feedstock material on the microstructure and mechanical properties of the coatings was studied. The PGDS process provides an opportunity to preserve the phase of the starting material, to produce hard and dense coatings with good cohesion between deformed particles and good adhesion to the substrate. High dry sliding wear resistance was observed when cryomilled composite material was used.     

Particle loading effect in cold spray

Cold gas dynamic spray is a line-of-sight, high-rate material deposition process that uses a supersonic flow to accelerate small particles (micron-sized) above a material-dependent critical velocity. When the particles impact the substrate, they plastically deform and bond to form a coating. The objective of this research is to investigate the influence of the particle mass flow rate on the properties of coatings sprayed using the cold spray process. Varying the mass flow rate at which the feedstock particles are fed into the carrier gas stream can change the thickness of the coating. It was shown that poor coating quality (peeling) was not a result of flow saturation but, instead, the result of excessive particle bombardment per unit area on the substrate. By increasing the travel speed of the substrate, this can be overcome and well-bonded dense coatings can be achieved. It has also been shown that by heating the carrier gas flow poor coating quality is avoided. The original version of this paper was published in the CD ROM Thermal Spray Comects: Explore Its Surfacing Potential, Interational Thermal Spray Conference, sponsored by DVS, ASM International, and HW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmBH, Düsseldorf. Germany.     

Mechanical Characteristics of Al-Co-Ce Coatings Produced by the Cold Spray Process

Gas atomized feedstock particles of an Al-13Co-26Ce alloy system were sprayed using the Cold Spray deposition technique. The microstructures of the coatings produced are examined and the mechanical characteristics, in particular the bending fatigue and the bond strength, of the Al-Co-Ce coatings are reported. The results show that the Al-Co-Ce coatings improved the fatigue behavior of AA 2024-T3 specimens when compared to uncoated and Alclad specimens. During the bond strength tests, the bonding agent failed and no delamination of the coating from the substrate occurred. The microstructural features of the feedstock powder were also found in the coatings. It is suggested that the increase in the fatigue properties of the specimens can be attributed to the residual compressive stresses induced in the coatings and to the high adhesion strength of the coatings to the substrates. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

基体材料对Cr/CrN多层涂层在海水环境中磨蚀性能的影响

目的 讨论海水环境下不同基体材料对Cr/CrN交替的多层复合涂层磨蚀性能的影响,为海水环境下耐磨蚀材料基体的选择和应用提供参考。方法 采用多弧离子镀技术在316L不锈钢和TC4钛合金基体上沉积Cr/CrN多层复合涂层,通过XRD、SEM等技术对涂层材料的微观结构进行表征,通过硬度测试、结合力测试、电化学分析、摩擦磨损试验等技术对涂层材料的力学性能、电化学性能以及摩擦学性能进行分析,比较不同基体对Cr/CrN多层涂层在海水环境中磨蚀性能的影响。结果 以TC4钛合金为基体的Cr/CrN多层涂层的硬度为1727.2HV0.3,虽略小于以316L不锈钢为基体的涂层硬度（2241.5HV0.3）,但其在膜-基结合力、海水环境下电化学性能和摩擦学性能等方面均优于以316L不锈钢为基体的涂层。结合力测试中,以TC4为基体的多层涂层初始裂纹出现在31 N,扩展裂纹出现在42 N,大于316L基体涂层的22 N和35 N。电化学测试中TC4基体涂层的腐蚀电位为?0.20 V,大于316L基体涂层的腐蚀电位（?0.21 V）。海水环境下TC4基体涂层的平均摩擦系数和磨损率分别为0.35和2.9950×10?5 mm3/(N?m),均小于316 L基体涂层的平均摩擦系数（0.36）和磨损率（4.9895×10?5 mm3/(N?m)）。结论 TC4钛合金更适合作为海水环境用Cr/CrN多层涂层耐磨蚀材料的基体材料。     

Deposition and characterization of flame-sprayed aluminum on cured glass and basalt fiber-reinforced epoxy tubes

An oxy-acetylene flame spray torch was used to deposit thin layers of aluminum onto cured glass and basalt fiber-reinforced epoxy tubes. The composite specimens were fabricated by filament winding. Surface coatings embedded in composite laminates were produced. The composite substrates were grit blasted to promote adhesion of the molten aluminum particles. It was found that adhesion increased significantly when the composite substrate was lightly grit blasted, with no adhesion on smooth composite surfaces. The number of passes of the flame spray torch was varied to change the coating thickness and uniformity over the substrate. The electrical resistance of the coatings was measured to assess the suitability of a coating as a conductor. It was found that uniform, electrically conductive coatings were produced with a minimum of two torch passes. Optical images were captured to characterize the coating microstructure and thickness. This investigation did not reveal any visible evidence of damage to the composite substrate. To assess possible degradation effects from the grit blasting and flame spraying processes, the tube specimens were subjected to mechanical testing by applying internal pressurization with hydraulic oil. The tests indicated that the grit blasting and flame spraying processes must be carefully executed to mitigate degradation of the strength of the composite material substrate.     

Cold spraying of Al-Sn binary alloy: Coating characteristics and particle bonding features

In this study, Al-Sn binary alloy coatings were prepared with Al-10 wt.% Sn (Al-10Sn) and Al-20 wt.%Sn (Al-20Sn) gas atomized alloy powders by low pressure cold spray process. The microstructure and microhardness of the coatings were characterized. The deposition efficiency of Al-Sn and pure aluminum powder were tested on sand blasted substrates of Al6061, copper and SUS304. To investigate the particle bonding features of Al-10Sn powder, the wipe test was carried out on polished substrate surfaces. The average critical velocity of Al-Sn alloy powders was estimated combining with the deposition efficiency. The results proved that Al-Sn coatings with dense and uniform structure can be deposited successfully by low pressure cold spray with helium as the propellant gas. The weight content of tin in Al-Sn coating is 12% and 22% for Al-10Sn and Al-20Sn coating, respectively. With the increase of tin content in feedstock powder, the as-sprayed coatings present similar microhardness of 73 and 74 for Al-10Sn and Al-20Sn while, on the other hand, the deposition efficiency decreased. Bonding features of Al-10Sn particles indicated that the melting of tin phase occurred on the contact interface during particle impacting. This melting of tin phase and low strength of tin may affect the particle bonding process and hence increase the critical velocity for Al-Sn binary alloy powders.     

WC-based composite coatings prepared by the pulsed gas dynamic spraying process: Effect of the feedstock powders

WC-based cermet coatings are typically produced using the HVOF process, due to high particle velocity and the lower heating characteristic of this technique. Despite the effort of optimisation of the coating process, degradation of the feedstock materials such as decarburisation of WC and amorphization of the metallic phase still occurs. It is known that the coating properties do not depend only on the spray process and its parameters, but also on the feedstock powder characteristics such as its chemistry, carbide size, particle morphology and production method. The work presented here is part of a research program aimed at exploring the possible advantages of the Pulsed Gas Dynamic Spray (PGDS) process, as an alternative technique for the preparation WC-based cermet coatings. In this paper, WC-based coatings have been prepared using six different types of cermets powders. In order to study the effects of the feedstock powder on the coatings microstructures and hardness, the selected starting powders differed not only in microstructural features such as size and morphology but also in the chemistry and phases. Using different analysis technique (OM, SEM, XRD, and HV), a detailed comparison of powders and coatings microstructures, phase compositions, and hardness are presented and discussed in detail. It was found that the PGDS process preserves the microstructure of the starting cermet powders in such a way that no significant degradation of the phase composition, even those that show the pre-existence of complex carbides, has been observed. Furthermore, although the same spray parameters were used, the thickness, deposition efficiency, porosity, and micro-cracks within the coatings are different from one type of cermet to another, suggesting that PGDS optimum process parameters are material dependant.     

Optimization of In-Situ Shot-Peening-Assisted Cold Spraying Parameters for Full Corrosion Protection of Mg Alloy by Fully Dense Al-Based Alloy Coating

Magnesium-based alloys have excellent physical and mechanical properties for a lot of applications. However, due to high chemical reactivity, magnesium and its alloys are highly susceptible to corrosion. In this study, Al6061 coating was deposited on AZ31B magnesium by cold spray with a commercial Al6061 powder blended with large-sized stainless steel particles (in-situ shot-peening particles) using nitrogen gas. Microstructure and corrosion behavior of the sprayed coating was investigated as a function of shot-peening particle content in the feedstock. It is found that by introducing the in-situ tamping effect using shot-peening (SP) particles, the plastic deformation of deposited particles is significantly enhanced, thereby resulting in a fully dense Al6061 coating. SEM observations reveal that no SP particle is deposited into Al6061 coating at the optimization spraying parameters. Porosity of the coating significantly decreases from 10.7 to 0.4% as the SP particle content increases from 20 to 60 vol.%. The electrochemical corrosion experiments reveal that this novel in-situ SP-assisted cold spraying is effective to deposit fully dense Al6061 coating through which aqueous solution is not permeable and thus can provide exceptional protection of the magnesium-based materials from corrosion.     

From Powders to Thermally Sprayed Coatings

Since the early stages of thermal spray, it has been recognized that the powder composition, size distribution, shape, mass density, mechanical resistance, components distribution for composite particles play a key role in coating microstructure and thermo mechanical properties. The principal characteristics of particles are strongly linked to the manufacturing process. Coatings also depend on the process used to spray particles and spray parameters. Many papers have been devoted to the relationships existing between coating properties and structures at different scales and manufacturing processes. In many conventional spray conditions resulting in micrometric structures, among the different parameters, good powder flow ability, and dense particles are important features. Thermal plasma treatment, especially by RF plasma, of particles, prepared by different manufacturing processes, allows achieving such properties and it is now developed at an industrial scale. Advantages and drawbacks of this process will be discussed. Another point, which will be approached, is the self-propagating high-temperature synthesis, depending very strongly upon the starting composite particle manufacturing. However, as everybody knows, “small is beautiful” and nano- or finely structured coatings are now extensively studied with spraying of: (i) very complex alloys containing multiple elements which exhibit a glass forming capability when cooled-down, their under-cooling temperature being below the glass transition temperature; (ii) conventional micrometer-sized particles (in the 30-90 μm range) made of agglomerated nanometer-sized particles; (iii) sub-micrometer- or nanometer-sized particles via a suspension in which also, instead of particles, stable sol of nanometer-sized particles can be introduced; and (iv) spray solutions of final material precursor. These different processes using plasma, HVOF or sometimes flame and also cold-gas spray will be discussed together with the production of nanometer-sized particles via the chemical reaction method or by a special type of milling: the cryogenic milling process often referred to as “cryomilling.”     

等离子喷涂Ag-SnO2涂层的微观结构和性能

采用等离子喷涂技术在铜基体表面制备了Ag-SnO2涂层,应用XRD和SEM对涂层的相结构和微观组织进行了表征,通过拉伸和硬度实验测定了涂层的力学性能,并采用电弧侵蚀实验测试了涂层的耐电弧侵蚀性能。结果表明,所得涂层结构均匀致密,涂层内纳米级SnO2颗粒均匀分布在银基体中;涂层的力学性能和电弧侵蚀性能与块体合金接近;电弧侵蚀试验后,涂层表面阴极斑点分散,烧蚀轻微,表明所制备的Ag-SnO2涂层具有良好的耐电弧侵蚀特性。     

Effect of Spray Distance on Microstructure and Tribological Performance of Suspension Plasma-Sprayed Hydroxyapatite–Titania Composite Coatings

Hydroxyapatite (HA)–titania (TiO₂) composite coatings prepared on Ti6Al4V alloy surface can combine the excellent mechanical property of the alloy substrate and the good biocompatibility of the coating material. In this paper, HA–TiO₂ composite coatings were deposited on Ti6Al4V substrates using suspension plasma spray (SPS). X-ray diffraction, scanning electron microscopy, Fourier infrared absorption spectrometry and friction tests were used to analyze the microstructure and tribological properties of the obtained coatings. The results showed that the spray distance had an important influence on coating microstructure and tribological performance. The amount of decomposition phases decreased as the spray distance increased. The increase in spray distance from 80 to 110 mm improved the crystalline HA content and decreased the wear performance of the SPS coatings. In addition, the spray distance had a big effect on the coating morphology due to different substrate temperature resulting from different spray distance. Furthermore, a significant presence of OH^? and CO₃ ^2? was observed, which was favorable for the biomedical applications.     

TiN-TiB2 composite coatings reactively produced by electrothermally exploded powder spray

Composite coatings composed of titanium nitride, TiN, and diboride, TiB₂, were reactively produced by the electrothermally exploded powder spray technique, in which feedstock powder was prepared from titanium and boron nitride particles. The microstructure of the coating was composed of titanium-ceramic particles the size of which were on the order of several nanometers to a few hundred nanometers. Such reactive thermal spraying brought base-metal saturation into a coating layer at the early stages of coating formation. The ceramic composite spray using feedstock of TiN and TiB₂ particles preferentially brought a new phase of cubic titanium boronitride together with TiN and TiB₂ into a coating. On comparing such a coating to one produced by the conventional method, the reactive thermal spray coating was richer in TiN and TiB₂ due to the excess nitrogen in the feedstock.     

Mechanical Properties of Double-Layer and Graded Composite Coatings of YSZ Obtained by Atmospheric Plasma Spraying

Double-layer and graded composite coatings of yttria-stabilized zirconia were sprayed on metallic substrates by atmospheric plasma spray. The coating architecture was built up by combining two different feedstocks: one micro- and one nanostructured. Microstructural features and mechanical properties (hardness and elastic modulus) of the coatings were determined by FE-SEM microscopy and nanoindentation technique, respectively. Additional adherence and scratch tests were carried out in order to assess the failure mechanisms occurring between the layers comprising the composites. Microstructural inspection of the coatings confirms the two-zone microstructure. This bimodal microstructure which is exclusive of the layer obtained from the nanostructured feedstock negatively affects the mechanical properties of the whole composite. Nanoindentation tests suitably reproduce the evolution of mechanical properties through coatings thickness on the basis of the position and/or amount of nanostructured feedstock used in the depositing layer. Adhesion and scratch tests show the negative effect on the coating adhesion of layer obtained from the nanostructured feedstock when this layer is deposited on the bond coat. Thus, the poor integrity of this layer results in lower normal stresses required to delaminate the coating in the adhesion test as well as minor critical load registered by using the scratch test.     

氧化物/不锈钢梯度复合材料初步设计与评价

采用粉末层叠和整体热压烧结的方法,制备了Y₂Ti₂O₇/316L不锈钢功能梯度复合材料。经金相显微镜、X射线衍射仪和电子探针分析确定,该材料在金属和氧化物界面处实现了成分和微观组织的梯度过渡。对不同梯度含量复合材料的力学性能测试表明,界面处的力学性能存在相应的梯度变化,即随各层金属相含量的逐渐增加,过渡区材料的致密度、抗弯强度不断提升,而硬度则不断下降。     

Corrosion performance of HVOF and APS thermally sprayed NiTi intermetallic coatings in 3.5% NaCl solution

Amorphous/nanocrystalline Ni-Ti powders produced by low energy mechanical alloying were used as feedstock to deposit NiTi intermetallic coatings on 316L stainless steel substrate using high velocity oxy-fuel (HVOF) and air plasma spraying (APS) processes. Electrochemical impedance spectroscopy (EIS) and polarization tests indicated that the corrosion performance and passive behaviour of HVOF coating were far better than those of APS coating. The study also showed that the solution had penetrated through the coating microcracks and caused interior corrosion of APS coating, while the HVOF coating was immune from interior corrosion attack and consequently exhibited a good passive behaviour during long-term immersion.     

Cold Spray Deposition of Ni and WC-Reinforced Ni Matrix Composite Coatings

Ni-WC composites are ideal protective coatings against wear and are often fabricated using laser cladding and thermal spray processes, but the high temperatures of these processes result in decarburization, which deteriorates the performance of the coating. Cold spray has the potential to deposit Ni-WC composite coatings and retain the composition of the initial WC feedstock. However, the insignificant plastic deformation of hard WC particles makes it difficult to build up a high WC content coating by cold spray. By using three different WC powder sizes, the effect of feedstock powder size on WC retention was tested. To improve WC retention, a WC/Ni composite powder in mixture with Ni was also sprayed. Microstructural characterization, including the deformed structure of Ni splats, retention, distribution, and fragmentation of WC, was performed by scanning electron microscopy. An improvement in WC retention was achieved using finer WC particles. Significant improvement in WC particles retention was achieved using WC/Ni composite powder, with the WC content in the coating being close to that of the feedstock.     

Air gas dynamic spraying of powder mixtures: Theory and application

The radial injection gas dynamic spray (RIGDS) technology of powder coatings deposition was considered for this work. A coating was created by injecting powders with variable compositions into a supersonic air jet and depositing powder on the substrate. This study describes the preliminary analysis of an air gas dynamic spray method realized by a portable RIGDS apparatus with a radial injection of powder. Attention was given to shock compaction processes during the coating structure formation and examples of powder mixtures utilization in RIGDS. It was shown that the operational parameters of supersonic powder-gas jet have a significant influence on the coating's microstructure, thus defining the high performance of the coating. Compaction and bonding of particles were analyzed. The original version of this paper was published in the CD ROM Thermal Spray Connects: Explore Its Surfacing Potential, International Thermal Spray Conference, sponsored by DVS, ASM International, and HW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmbH, Düsseldorf, Germany.     

In-situ plasma spraying: Alumina formation and in-flight particle diagnostic

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al-12Si and SiO₂ powder was deposited onto an aluminum substrate by atmospheric plasma spraying (APS) to obtain a composite coating consisting of in-situ formed alumina reinforced hypereutectic Al-18Si matrix alloy. The effects of spray parameters (arc current and spray distance) and in-flight particle characteristics (temperature and velocity) on in-situ reaction intensity (alumina and silicon) have been investigated. The results show that, in-situ alumina formation and silicon intensity strongly depend on in-flight particle characteristics, spray distance and substrate temperature.     

Structural evolution of mechanically alloyed Al-12Si/TiB2/h-BN composite powder coating by atmospheric plasma spraying

In the present study, mechanically alloyed Al-12Si/TiB₂/h-BN composite powder was deposited onto an aluminum substrate by atmospheric plasma spraying. The effect of mechanical alloying (MA) and plasma spray parameters on composite powder and coating structure were investigated. It has been observed that the MA process has a significant effect on the composite powder morphology and reactivity between the selective powders. Results also demonstrate that, at relatively high milling time h-BN decomposes into B and N and forms a solid solution. Also, it has been found that, the relative amount of the in-situ formed AlN through the reaction between h-BN and Al and/or the decomposition of Al-B-N solid solution is independent from the plasma parameters (arc current and secondary gas flow rate). However, spray parameters remarkably affects the coating hardness due to coarsening of Si during the solidification of the coating.