Characterization of Low-Pressure Cold-Sprayed Aluminum Coatings

Aluminum alloys are widely used as materials for engineering components of automobiles and airplanes because of their light weight and high corrosion resistance. However, cracks may develop sometimes in aluminum components, which have to be repaired by welding. It is difficult to weld aluminum components due to its high specific thermal conductivity and high coefficient of thermal expansion. The low-pressure cold-spray technique can be used instead of welding for repairing cracks. However, the effects of surface conditions on particle deposition and the mechanical properties of cold-sprayed coatings have not been investigated thus far. In this study, the effect of surface conditions focusing on active newly formed surface on aluminum particle deposition is studied and the mechanical properties of low-pressure cold-sprayed aluminum coatings are investigated by four-point bending tests. It is found that for efficient particle deposition it was necessary to obtain active newly formed surface of the substrate and particle surfaces by several impingements because the existence of inactive native oxide films has an adverse effect on the deposition. Furthermore, the strength of a cold-sprayed specimen is found to be higher than that of a cold-rolled specimen under compressive loading.     

Hot Corrosion Behavior of Low-Pressure Cold-Sprayed CoNiCrAlY Coatings

CoNiCrAlY coatings were deposited by low-pressure cold spraying and pre-oxidized in a vacuum environment, and its hot corrosion behavior in pure Na₂SO₄ and 75 wt.% Na₂SO₄ + 25 wt.% NaCl salts was investigated. The pre-oxidation treatment resulted in the formation of a dense and continuous α-Al₂O₃ scale on the coating surface. After being corroded for 150 h at 900 °C, the pre-oxidized coating exhibited better corrosion resistance to both salts than the as-sprayed coating. The presence of preformed Al₂O₃ scale reduced the consumption rate of aluminum, by delaying the formation of internal oxides and sulfides and promoting the formation of a denser and more adherent Al₂O₃ scale. Moreover, we investigated the corrosion mechanism of cold-sprayed CoNiCrAlY coatings in the two salts and discussed the effect of the pre-oxidation treatment.     

Microstructural Characteristics and Oxidation Behavior of Low-Pressure Cold-Sprayed CoNiCrAlY Coatings

CoNiCrAlY coatings were deposited by low-pressure cold spraying and subsequently heat-treated at 1050 °C for 4 h in a vacuum environment. The microstructural characteristics and oxidation behavior of CoNiCrAlY coatings were investigated. The as-sprayed coating exhibited low porosity and oxygen content. The high plastic deformation of the sprayed particles led to significant refinement of γ-matrix and dissolution of β-(Ni,Co)Al phase in the as-sprayed coating. After heat treatment, the single phase (γ) in the as-sprayed coating was converted into a γ/β microstructure, and a continuous single α-Al₂O₃ scale was formed on the coating surface. Vacuum heat treatment can postpone the formation of spinel oxides within 100 h. After being oxidized at 1050 °C for 400 h, the heat-treated coating exhibited better oxidation resistance than the as-sprayed coating. The reduced growth rate of the oxide scale and the suppression of the formation of spinel oxides can be attributed to the vacuum heat treatment, as well as the intrinsic microstructure of the cold-sprayed coating. Finally, the effects of the microstructural changes induced during the cold spraying process on the growth of the thermally grown oxide and the oxidation mechanisms of the CoNiCrAlY coatings were discussed.     

Corrosion Properties of Cold-Sprayed Tantalum Coatings

Cold spraying enables the production of pure and dense metallic coatings. Denseness (impermeability) plays an important role in the corrosion resistance of coatings, and good corrosion resistance is based on the formation of a protective oxide layer in case of passivating metals and metal alloys. The aim of this study was to investigate the microstructural details, denseness, and corrosion resistance of two cold-sprayed tantalum coatings with a scanning electron microscope and corrosion tests. Polarization measurements were taken to gain information on the corrosion properties of the coatings in 3.5 wt.% NaCl and 40 wt.% H₂SO₄ solutions at room temperature and temperature of 80 °C. Standard and improved tantalum powders were tested with different spraying conditions. The cold-sprayed tantalum coating prepared from improved tantalum powder with advanced cold spray system showed excellent corrosion resistance: in microstructural analysis, it showed a uniformly dense microstructure, and, in addition, performed well in all corrosion tests.     

Effect of Heat Treatment on Mechanical Properties and Corrosion Performance of Cold-Sprayed Tantalum Coatings

The cold-spray technique is of significant interest to deposit refractory metals with relatively high melting point for a variety of demanding applications. In the present study, mechanical properties of cold-sprayed tantalum coatings heat treated at different temperatures were investigated using microtensile testing, scratch testing, and nanoindentation. The corrosion performance of heat-treated coatings was also evaluated in 1 M KOH solution, and potentiodynamic polarization as well as impedance spectroscopy studies were carried out. Assessment of structure–property correlations was attempted based on microstructure, porosity, and intersplat bonding state, together with mechanical and corrosion properties of the heat-treated cold-sprayed tantalum coatings. Coatings annealed at 1500 °C, which is very close to the recrystallization temperature of tantalum, were found to perform almost as bulk tantalum, with exciting implications for various applications.     

反应溅射Ti(0，N)涂层的微结构与力学性能

采用反应磁控溅射方法在Ar、N2和O2混合气氛中制备了一系列Ti(O,N)涂层,并采用EDS、XRD、SEM、AFM和微力学探针研究了氧分压对涂层的化学成分、微结构和力学性能的影响.结果表明:随混合气氛中氧分压的提高,涂层中的氧含量逐步增加,氮含量相应减少,但涂层始终保持与TiN相同的NaCl结构.少量氧的加入,可以改善涂层的结晶状态,涂层的硬度也相应升高,明显高于未含氧的TiN涂层的硬度.氧含量为8.0%(原子数分数)时,涂层硬度达到最大值26.2 GPa.进一步增加氧含量,涂层的硬度基本保持不变.     

Heat Treatment of Cold-Sprayed C355 Al for Repair: Microstructure and Mechanical Properties

Cold gas dynamic spraying of commercially pure aluminum is widely used for dimensional repair in the aerospace sector as it is capable of producing oxide-free deposits of hundreds of micrometer thickness with strong bonding to the substrate, based on adhesive pull-off tests, and often with enhanced hardness compared to the powder prior to spraying. There is significant interest in extending this application to structural, load-bearing repairs. Particularly, in the case of high-strength aluminum alloys, cold spray deposits can exhibit high levels of porosity and microcracks, leading to mechanical properties that are inadequate for most load-bearing applications. Here, heat treatment was investigated as a potential means of improving the properties of cold-sprayed coatings from Al alloy C355. Coatings produced with process conditions of 500 °C and 60 bar were heat-treated at 175, 200, 225, 250 °C for 4 h in air, and the evolution of the microstructure and microhardness was analyzed. Heat treatment at 225 and 250 °C revealed a decreased porosity (~ 0.14% and 0.02%, respectively) with the former yielding slightly reduced hardness (105 versus 130 HV_0.05 as-sprayed). Compressive residual stress levels were approximately halved at all depths into the coating after heat treatment, and tensile testing showed an improvement in ductility.     

Effects of Traverse Scanning Speed of Spray Nozzle on the Microstructure and Mechanical Properties of Cold-Sprayed Ti6Al4V Coatings

Cold spray has the potential to restore damaged aerospace components made from titanium alloy, Ti6Al4V at low temperature (200-400 °C). Traverse scanning speed during deposition is one of the key factors that affect the quality of the Ti6Al4V coatings as it influences the thermal build-up and coating thickness per pass. As there are fewer reported studies on this, this work investigated the effects of different traverse scanning speeds (100, 300 and 500 mm/s) of cold spray nozzle on the microstructure and mechanical properties of cold-sprayed Ti6Al4V coatings. The cross-sectional analysis showed coating porosities reduces with slower traverse speed, from 3.2 to 0.5%. In addition, the microhardness of the coatings increased from about 361-385 HV due to strain hardening. However, the adhesion strength of the coatings to the substrates significantly decreased with reduced traverse speed from about 60 MPa (glue failure) at 500 mm/s to 2.5 MPa (interface failure) at 100 mm/s. Therefore, this study revealed that the control of heat build-up and thickness per pass during the cold spray deposition of the Ti6Al4V coatings is crucial to attain the desirable properties of the coatings.     

Comparison of Oxidation and Microstructure of Warm-Sprayed and Cold-Sprayed Titanium Coatings

Thick titanium coatings were prepared by the warm spraying (WS) and cold spraying (CS) processes to investigate the oxidation and microstructure of the coating layers. Prior to the coating formations, the temperature and velocity of in-flight titanium powder particles were numerically calculated. Significant oxidation occurred in the WS process using higher gas temperature conditions with low nitrogen flow rate, which is mixed to the flame jet of a high velocity oxy-fuel (HVOF) spray gun in order to control the temperature of the propellant gas. Oxidation, however, decreased strikingly as the nitrogen flow rate increased. In the CS process using nitrogen or helium as a propellant gas, little oxidation was observed. Even when scanning electron microscopy or an x-ray diffraction method did not detect oxides in the coating layers produced by WS using a high nitrogen flow rate or by CS using helium, the inert gas fusion method revealed minor increases of oxygen content from 0.01 to 0.2?wt.%. Most of the cross-sections of the coating layers prepared by conventional mechanical polishing looked dense. However, the cross-sections prepared by an ion-milling method revealed the actual microstructures containing small pores and unbounded interfaces between deposited particles.     

VAlSiN 硬质涂层的结构与机械性能

目的研究Si的掺入对VAlN硬质涂层机械性能的影响行为。方法利用双靶(V50Al50靶与Si靶)磁控溅射沉积技术,保持V50Al50靶的功率为一定值,通过控制Si靶的功率,获得Si含量不同的VAlSiN硬质涂层,分析涂层的成分、相组成、硬度及耐磨损性能。结果 Si在涂层中以Si3N4非晶相存在,Si3N4起到细化晶粒的作用,少量Si的掺入可使涂层的硬度与耐磨性能得到提升。但当Si大量掺入后,Si3N4非晶相成为涂层中的主相,导致涂层的硬度与耐磨性能出现下降。结论少量Si的掺入有益于VAlN涂层机械性能的提升。     

Mechanical and Tribological Properties of Cold-Sprayed Ti Coatings on Ti-6Al-4V Substrates

A cold spray process was used to deposit titanium (Ti) coatings of different thicknesses on commercial Ti-6Al-4V (Ti64) substrates. The hardness of the Ti coatings was measured using a Vickers micro-indenter. It was found that the thicker Ti coatings had higher hardness probably due to the better uniformity and higher density of the coatings. The tribological results showed that the friction and wear of the Ti coatings tested against a steel ball under dry condition became lower with higher thickness probably due to the higher wear resistance of the thicker coatings associated with their higher hardness. The specific wear rates of all the Ti coatings were significantly lower than that of the Ti64 substrate as a result of the higher wear resistance of the Ti coatings associated with their cold-worked microstructures and the formation of high wear resistant oxide layers on their wear tracks during the wear testing.     

Microstructures and Thermal Properties of Cold-Sprayed Cu-Cr Composite Coatings

Copper-based composites for thermal conductive components were prepared via the cold spray process, and the deposition efficiency and adhesion morphology of feedstock powders on Cu substrate were evaluated. Cu-based composites were fabricated using Cu-Cr mixed powders with their mixture ratio of 20, 35, 50, and 65 mass% Cr onto oxygen-free copper substrate with N₂ carrier gas. Cu-Cr composite coatings were investigated for their Cr content ratio, microstructures, and thermal conductivity. The Cr content ratio in the coating was approximately 50-60% of feedstock mixture ratio due to the low formability of the hard particles. Transmission electron microscopy characterizations revealed that an oxygen-rich layer exists at the Cr particle/Cu substrate interface, which contributes to the deposition of the Cr particles. After the heat treatment at 1093 K, the coatings showed denser cross-sectional structures than those before the heat treatment, and the thermal conductivity was improved as a result of the recrystallization of Cu matrix.     

Mechanical Properties and Microstructure of VPS and HVOF CoNiCrAlY Coatings

In this study, high velocity oxy-fuel (HVOF) and vacuum plasma spraying (VPS) coatings were sprayed using a Praxair (CO-210-24) CoNiCrAlY powder. Free-standing coatings underwent vacuum annealing at different temperatures for times of up to 840 h. Feedstock powder, and as-sprayed and annealed coatings, were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The hardness and Young’s modulus of the as-sprayed and the annealed HVOF and VPS coatings were measured, including the determination of Young’s moduli of the individual phases via nanoindentation and measurements of Young’s moduli of coatings at temperatures up to 500 °C. The Eshelby inclusion model was employed to investigate the effect of microstructure on the coatings’ mechanical properties. The sensitivity of the mechanical properties to microstructural details was confirmed. Young’s modulus was constant up to ~200 °C, and then decreased with increasing measurement temperature. The annealing process increased Young’s modulus because of a combination of decreased porosity and β volume fraction. Oxide stringers in the HVOF coating maintained its higher hardness than the VPS coating, even after annealing.     

CrAlMoN梯度薄膜的微观结构及力学性能研究

采用磁控溅射技术,在单晶Si片和M2工具钢表面沉积CrAlMoN梯度薄膜,利用SEM、EDS、XRD、划痕仪、纳米压痕仪研究了CrAlMoN梯度薄膜的微观结构、膜/基结合力、硬度和弹性模量。结果表明,CrAlMoN梯度薄膜呈面心立方结构,薄膜的表面和截面结构紧凑致密。梯度结构的CrAlMoN薄膜具有高的膜/基结合强度,薄膜硬度和弹性模量分别达到24.13GPa和342.33GPa,力学性能明显优于CrAlN薄膜     

Effects of Dissimilar Alumina Particulates on Microstructure and Properties of Cold-Sprayed Alumina/A380 Composite Coatings

In this study,alumina/A380 composite coatings were fabricated by cold spray.The influence of alumina particulates' morphology(spherical and irregular) and content on the deposition behavior of the coatings(including surface roughness,surface residual stress,cross-sectional microstructure and microhardness) was investigated.Results revealed that the spherical alumina mainly shows micro-tamping effect during deposition,which result in remarkable low surface roughness and porosity of the coatings.In addition,very low deposition efficiency and good interfacial bonding between the coating and the substrate were achieved.For irregular alumina particles,the embedding of ceramic particulates in the coating was dominant during deposition process,resulting in high retention in the final deposit.However,it showed limited influence on porosity,surface roughness and interfacial bonding of the deposit.The coatings containing irregular alumina particulates exhibited much higher microhardness than those containing spherical alumina due to the higher load-bearing capacity of deposited alumina.     

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采用MEVVA离子源技术对由磁过滤阴极真空电弧沉积的TiN薄膜注入不同剂量的Si元素,利用XPS和纳米硬度仪表征Si离子注入后化学成分、元素键合状态以及硬度的变化。结果表明,Si离子注入后,薄膜表面硬度得到提高,5×1016 ions/cm2的样品硬度峰值从27.18GPa增加到39.85GPa,随着注入剂量的增加,纳米硬度峰值有下降的趋势,1×1017 ions/cm2的样品硬度峰值为33.27GPa,但表面改性层的深度增加,纳米硬度在一定的深度范围内得到了整体的提高。离子注入使薄膜表面层的弹性模量显著提高,表层弹性模量随注入剂量的增加而提高。并且由于Si元素的注入,形成了新的微结构相Si3N4,新相的含量与注入剂量有关。     

溅射技术对TiN涂层结构和力学性能的影响

目的 沉积条件对Ti N涂层的组织结构和力学性能有着至关重要的影响,而溅射技术又决定了涂层的沉积条件,探究不同溅射技术对Ti N涂层的微观组织结构和性能的影响,提高Ti N涂层的力学性能和高温摩擦磨损性能。方法 采用不同的溅射技术(dcMS、Hi PMS、Hybrid)在M2高速钢表面沉积Ti N涂层,利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)、sin²ψ法、纳米压痕仪、洛氏压痕法、划痕法和CSM球盘式摩擦试验机分别测试了Ti N涂层的组织结构特征、沉积速率、残余应力、纳米硬度、膜基结合力和高温摩擦磨损性能。结果 不同溅射技术制备的Ti N涂层均为柱状晶结构和Ti N (111)择优取向。HiPIMS-TiN涂层具有最高的纳米硬度(29.7GPa)和最低的膜基结合力(HF2),而Hybrid-TiN涂层呈现出最小的残余应力、高沉积速率和高膜基结合力,其膜基结合力达到HF1级,临界载荷(Lc2)达到82.5 N。不同溅射技术制备的Ti N涂层的摩擦因数均随着温度的升高而降低,在500℃时,Ti N涂层的摩擦因数约为0.53。Ti N涂层的磨损率随着温度的升...     

High-Pressure Cold-Sprayed Ni and Ni-Cu Coatings: Improved Structures and Corrosion Properties

Cold spraying is a promising technique for the production of dense metallic coatings. In cold spraying, coating formation is through high velocity impacts of solid particles with high kinetic energy. During impact, particles deform plastically and adhere to the substrate, gradually building-up the coating. This makes it possible to form pure and dense coating structures. These impermeable coatings are advantageous in many applications such as those where corrosion protection is required. Nickel and nickel-copper alloys have good corrosion resistance and therefore, as dense coatings, have high potential for employment as corrosion barrier layers. In this study, the structural and corrosion properties of high-pressure cold-sprayed (HPCS) Ni and NiCu coatings are characterized. NiCu alloys are known to have good corrosion resistance in sulphuric and hydrochloric acids, whereas Ni is resistant to caustic soda and alkaline salt solutions. This study also shows the effect of heat treatments on coating properties. FESEM studies of cross-sectional samples reveal structural details of the HPCS coatings while corrosion properties are evaluated with polarization measurements. The corrosion behavior of both the bulk and substrate material is determined in order to assess the real corrosion protection potential of the coatings.