Preparation of TiAl<Subscript>3</Subscript>-Al Composite Coating by Cold Spray and Its High Temperature Oxidation Behavior

A novel TiAl₃-Al coating was prepared by cold spray for high temperature protection of titanium aluminum-based alloy. The substrate alloy was orthorhombic-Ti-22Al-26Nb (at.%). The composite coating was mainly composed of TiAl₃ embedded in the matrix of residual aluminum. An interlayer about 10 μm was formed between the coating and the substrate. The oxidation test indicated that this composite coating was very effective in improving the high-temperature oxidation resistance of the substrate alloy at 950 °C in the tested 150 cycles without any sign of degradation. The microstructure analysis of the oxidized composite coating showed that an Al₂O₃ scale with a complex structure can be formed outside the interlayer during oxidation and no oxides beneath the interlayer were detected, which indicated that the complex continuous Al₂O₃ and the interlayer provide the protection of the substrate at high-temperature oxidation condition.     

Oxidation behaviour of Ti-46.7Al-1.9W-0.5Si in air and Ar-20%O2 between 750 and 950 °C

The oxidation behaviour of an intermetallic alloy, Ti-46.7Al-1.9W-0.5Si, was studied in air and Ar-20%O₂ atmospheres at 750, 850 and 950 °C. Oxidation of the alloy followed a parabolic rate law at low temperature (750 °C) in both environments. The alloy oxidised parabolically in air and at a slower rate in Ar-20%O₂ at 850 °C. Following a parabolic oxidation for a relatively short exposure period (72 h) at 950 °C, the oxidation rate was reduced after prolonged exposure (up to 240 h) in air. The alloy oxidised in a slower manner in the Ar-20%O₂ atmosphere at 950 °C. Higher oxidation rates were observed in air than in Ar-20%O₂ at all three experimental temperatures. Multi-layered scales developed in both environments. The scale formed in air consisted of TiO₂/Al₂O₃/TiO₂/TiN/TiAl₂ layers, ranging from the surface to the substrate—whilst the scale developed in the Ar-20%O₂ atmosphere comprised of the sequence TiO₂/Al₂O₃/TiO₂/Al₂O₃/Ti₃Al/substrate. The two layers of Al₂O₃ in Ar-20%O₂ were more effective in providing protection of the substrate against high temperature corrosion than the single layer of Al₂O₃ formed in air.     

Preparation of TiAl3-Al composite coating by cold spraying

TiAl₃-Al coating was deposited on orthorhombic Ti₂AlNb alloy substrate by cold spraying with the mixture of pure Al and Ti as the feedstock powder at a fixed molar ratio of 3:1 when the spraying distance, gas temperature and gas pressure for the process were 10 mm, 250 °C and 1.8 MPa, respectively. The as-sprayed coating was then subjected to heat treatment at 630 °C in argon atmosphere for 5 h at a heating rate of 3 °C/min and an argon gas flow rate of 40 mL/min. The obtained TiAl₃-Al composite coating is about 212 μm with a density of 3.16 g/cm³ and a porosity of 14.69% in general. The microhardness and bonding strength for the composite coating are HV525 and 27.12 MPa.     

Electrodeposition of sol-enhanced nanostructured Ni-TiO2 composite coatings

A novel electroplating method has been developed to produce nanocrystalline metal-matrix nano-structured composite coatings. A small amount of transparent TiO₂ sol was added into the traditional electroplating Ni solution, leading to the formation of nanocrystalline Ni-TiO₂ composite coatings. These coatings have a smooth surface. The Ni nodules changed from traditional pyramid-like shape to spherical shape. The grain size of Ni was also significantly reduced to the level of 50 nm. It was found that the amorphous anatase TiO₂ nano-particles (∼ 10 nm) were highly dispersed in the coating matrix. The microhardness was significantly increased from 320 HV₁₀₀ of the traditional Ni coating to 430 HV₁₀₀ of the novel composite coating with 3.26 wt.% TiO₂. Correspondingly, the wear resistance of the composite coating was improved by ∼ 50%.     

A novel electroless plating of Ni-P-TiO2 nano-composite coatings

A new processing concept has been developed to produce nano-structured metal-matrix composite coatings. This method combines sol-gel and electroless plating techniques to prepare highly dispersive oxide nano-particle reinforced composite coatings. Transparent TiO₂ sol was added into the standard electroless plated Ni-P solution at a controlled rate to produce Ni-P-TiO₂ nano-composite coatings on Mg alloys. The coating was found to have a crystalline structure. The nano-sized TiO₂ particles (∼ 15 nm) were well dispersed into the Ni-P coating matrix during the co-deposition process. This technique can effectively avoid the agglomeration of nano-particles in the coating matrix. As a result, the microhardness of the composite coatings were significantly increased to ∼ 1025 HV₂₀₀ compared to ∼ 710 HV₂₀₀ of the conventional composite coatings produced with solid particle mixing methods. Correspondingly, the wear resistance of the new composite coatings was also greatly improved.     

Characteristic of AlON-Al2O3 coatings on Al6061 alloy by electrolytic plasma processing in aluminate and nitride electrolytes

In this work, 6061 series aluminum alloy is used as the matrix material for its wide application in engineering to make AlON coating layers by the electrolytic plasma processing (EPP) method. The experimental electrolytes include: 10 g/L NaAlO₂ as alumina formative agent, 2 g/L NaOH as the electrolytic conductive agent, 0.4 g/L NaNO₃ as a nitride supply agent. A combined composition and structure analysis of the coating layer was carried out by X-ray diffractometer (XRD) and scanning electron microscopy (SEM) for the specimens EPP-treated at 25 °C in 5 to 30 min under a hybrid voltage of 260 V DC plus AC 50 Hz power supplies (200 V). In addition, microhardness values were measured to correlate the evolution of microstructure and resulting mechanical properties. A composite of AlON-Al₂O₃ coating was formed as a result of a reactive process between Al in the alloy itself and O-N supplied by the electrolyte, which presents high hardness and anti-abrasion behaviors.     

Cyclic oxidation behavior and thermal barrier effect of YSZ-(Al2O3/YAG) double-layer TBCs prepared by the composite sol-gel method

Novel YSZ (6 wt.% yttria partially stabilized zirconia)-(Al₂O₃/YAG) (alumina-yttrium aluminum garnet, Y₃Al₅O₁₂) double-layer ceramic coatings were fabricated using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies. The thin Al₂O₃/YAG layer had good adherence with substrate and thick YSZ top layer, which presented the structure of micro-sized YAG particles embedded in nano-sized α-Al₂O₃ film. Cyclic oxidation tests at 1000 °C indicated that they possessed superior properties to resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1000 °C and 1100 °C indicate that the 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. These beneficial effects should be mainly attributed to that, the oxidation rate of thermal grown oxides (TGO) scale is decreased by the “sealing effect” of α-Al₂O₃, the “reactive element effect”, and the reduced thermal stresses by means of nano/micro composite structure. This double-layer coating can be considered as a promising TBC.     

Fabrication and oxidation behavior of Cr2AlC coating on Ti6242 alloy

An ∼ 5 µm Cr₂AlC coating was synthesized on near-α titanium alloy Ti6242 using an industrially sized magnetron sputtering coater. Isothermal oxidation at 700 °C and 800 °C, and cyclic oxidation at 700 °C of the bare alloys and coated specimens were investigated in air. The results indicated that the Ti6242 alloy faced serious oxidation problems at 700 °C and 800 °C. Repeated formation and spallation of the multilayered oxide scale on the Ti6242 alloy occurred during oxidation testing. The coated specimens exhibited much better oxidation behaviour as compared to the bare alloy. A continuous Al-rich oxide scale formed on the coating surface during the initial oxidation stages. The oxide scale and coating itself acted as diffusion barriers blocking the further ingress of oxygen and protected the substrate alloy from oxidation. The oxidation mechanisms of the bare alloy and the coated specimens were investigated based on the experimental results.     

A new temperature stable microwave dielectric ceramics: ZnTiNb2O8 sintered at low temperatures

The phases, microstructure and microwave dielectric properties of ZnTiNb₂O₈-xTiO₂ composite ceramics with different weight percentages of BaCu(B₂O₅) additive prepared by solid-state reaction method have been investigated using the X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results showed that the microwave dielectric properties were strongly dependent on densification, grain sizes and crystalline phases. The sintering temperature of ZnTiNb₂O₈ ceramics was reduced from 1250 °C to 950 °C by doping BaCu(B₂O₅) additive and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative value of −52 ppm/°C to 0 ppm/°C by incorporating TiO₂. Addition of 2 wt% BaCu(B₂O₅) in ZnTiNb₂O₈-xTiO₂ (x = 0.8) ceramics sintered at 950 °C showed excellent dielectric properties of ?_r = 38.89, Q × f = 14,500 GHz (f = 4.715 GHz) and τ_f = 0 ppm/°C, which represented very promising candidates as LTCC dielectrics for LTCC applications.     

Sinterability and dielectric properties of Ba0.55Sr0.4Ca0.05TiO3-CaTiSiO5-Mg2TiO4 composite ceramics

The composite ceramics of Ba_0.55Sr_0.4Ca_0.05TiO₃-CaTiSiO₅-Mg₂TiO₄ (BSCT-CTS-MT) were prepared by the conventional solid-state route. The sintering performance, phase structures, morphologies, and dielectric properties of the composite ceramics were investigated. The BSCT-CTS-MT ceramics were sintered at 1100 °C and possessed dense microstructure. The dielectric constant was tailored from 1196 to 141 as the amount of Mg₂TiO₄ increased from 0 to 50 wt%. The dielectric constant and dielectric loss of 40 wt% Ba_0.55Sr_0.4Ca_0.05TiO₃-10 wt% CaTiSiO₅-50 wt% Mg₂TiO₄ was 141 and 0.0020, respectively, and the tunability was 8.64% under a DC electric field of 8.0 kV/cm. The Curie peaks were broadened and depressed after the addition of CaTiSiO₅. The optimistic dielectric properties made it a promising candidate for the application of tunable capacitors and phase shifters.     

Microwave dielectric properties of (1 − x)ZnMoO4-xTiO2 composite ceramics

(1 − x)ZnMoO₄-xTiO₂ (x = 0.0, 0.05, 0.158, 0.25, and 0.35) composite ceramics were synthesized by the conventional solid state reaction process. The sintering behavior, phase composition, chemical compatibility with silver, and microwave dielectric properties were investigated. All the specimens can be well densified below 950 °C. From the X-ray diffraction analysis, it indicates that the triclinic wolframite ZnMoO₄ phase coexists with the tetragonal rutile TiO₂ phase, and it is easy for silver to react with ZnMoO₄ to form Ag₂Zn₂(MoO₄)₃ phase and hard to react with TiO₂. When the volume fraction of TiO₂ (x value) increasing from 0 to 0.35, the microwave dielectric permittivity of the (1 − x)ZnMoO₄-xTiO₂ composite ceramics increases from 8.0 to 25.2, the Q_f value changes in the range of 32,300-43,300 GHz, and the temperature coefficient τ_f value varies from −128.9 to 157.4 ppm/°C. At x = 0.158, the mixture exhibits good microwave dielectric properties with a ?_r = 13.9, a Q_f = 40,400 GHz, and a τ_f = +2.0 ppm/°C.     

Low temperature sintering and microwave dielectric properties of (Mg0.7Zn0.3)0.95Co0.05TiO3 ceramics with BaCu(B2O5) additions

The effects of BaCu(B₂O₅) additives on the sintering temperature and microwave dielectric properties of (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics were investigated. The (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics were not able to be sintered below 1000 °C. However, when BaCu(B₂O₅) were added, they were sintered below 1000 °C and had the good microwave dielectric properties. It was suggested that a liquid phase with the composition of BaCu(B₂O₅) was formed during the sintering and assisted the densification of the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics at low temperature. BaCu(B₂O₅) powders were produced and used to reduce the sintering temperature of the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics. Good microwave dielectric properties of Q × f = 35,000 GHz, ?_r = 18.5.0 and τ_f = −51 ppm/°C were obtained for the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics containing 7 wt.% mol% BaCu(B₂O₅) sintered at 950 °C for 4 h.     

Development and oxidation resistance of air plasma sprayed Mo-Si-Al coating on an Nbss/Nb5Si3 in situ composite

A Mo-Si-Al coating, which is mainly composed of Mo(Si,Al)₂ and Mo₅(Si,Al)₃, was developed to protect a Nb_ss/Nb₅Si₃ in situ composite by air plasma spraying. After oxidation at 1250 °C, the oxidation curve followed parabolic law and even after oxidation for 100 h, the weight gain of Mo-Si-Al coating was 8.24 mg/cm². The surface of the oxidized samples became flatter and smoother as time increased due to the formation of SiO₂ glass. Moreover, the microstructure of Mo-Si-Al coating changed and a layer structured interdiffusion zone was formed at the substrate-coating interface after oxidation.     

Investigations on synthesis of HfB2 and development of a new composite with TiSi2

This paper presents the results of investigation carried out on synthesis and densification of monolithic HfB₂ and the effect of TiSi₂ as sinter additive. Pure phase HfB₂ was prepared by boron carbide reduction of HfO₂ and hot pressed to full density with the addition of TiSi₂. Isothermal oxidation study of this composite was carried out at 850 °C up to 64 h. Formation of HfB₂ was seen at 1200 °C but pure HfB₂ was formed at a much higher temperature of 1875 °C in vacuum. Hot pressing of HfB₂ at 1850 °C and 35 MPa pressure gave a compact of 80% TD. Addition of TiSi₂ helped in achieving a much higher density at a lower temperature of 1600 °C and a pressure of 20 MPa. A fully dense composite of HfB₂ and TiSi₂ was obtained with 15% TiSi₂. Hardness and fracture toughness of this composite were 27.4 ± 1.9 GPa and 6.6 ± 0.2 MPa m^1/2, respectively. Considerable deflection was observed in the crack propagation in composites. Oxidation studies indicated the formation of HfO₂, SiO₂, TiO₂ and HfSiO₄ with some glassy phase and the composite with 15% TiSi₂ was seen to be completely covered with a protective glassy layer.     

Oxidation behavior of Ti3AlC2 at 1000-1400 °C in air

The isothermal oxidation behavior of bulk Ti₃AlC₂ has been investigated at 1000-1400 °C in air for exposure times up to 20 h by means of TGA, XRD, SEM and EDS. It has been demonstrated that Ti₃AlC₂ has excellent oxidation resistance. The oxidation of Ti₃AlC₂ generally followed a parabolic rate law with parabolic rate constants, k_p that increased from 4.1×10⁻¹¹ to 1.7×10⁻⁸ kg² m⁻⁴ s⁻¹ as the temperature increased from 1000 to 1400 °C. The scales formed at temperatures below 1300 °C were dense, adherent, resistant to cyclic oxidation and layered. The inner layer of these scales formed at temperatures below 1300 °C was continuous α-Al₂O₃. The outer layer changed from rutile TiO₂ at temperatures below 1200 °C to a mixture of Al₂TiO₅ and TiO₂ at 1300 °C. In the samples oxidized at 1400 °C, the scale consisted of a mixture of Al₂TiO₅ and, predominantly, α-Al₂O₃, while the adhesion of the scales to the substrates was less than that at the lower temperatures. Effect of carbon monoxide at scale/substrate was involved in the formation of the continuous Al₂O₃ layers.     

High-Temperature Oxidation Behavior of Sputtered IN 738 Nanocrystalline Coating

A sputtered nanocrystalline coating of IN 738 alloy was obtained by means of magnetron sputtering. The isothermal oxidation behavior at 800, 900, and 1000°C and the cyclic oxidation behavior at 950°C of the coating were studied in comparison with IN 738 cast alloy. The results indicated that a double external oxide scale was formed on the nanocrystalline coating at 900, 950, and 1000°C without internal oxide and nitride. The scale consisted in an outer mixture of Cr₂O₃, TiO₂, and NiCr₂O₄ and an inner, continuous Al₂O₃ layer, which offered good adhesive and protectiveness. However, at 800°C a continuous Al₂O₃ scale could not be formed during oxidation of nanocrystalline coating and aluminum was still oxidized internally.     

Microstructure and high-temperature friction and wear behavior of WC-(W,Cr)2C-Ni coating prepared by high velocity oxy-fuel spraying

WC-(W,Cr)₂C-Ni coating was prepared by high velocity oxy-fuel spraying (HVOF). The microstructure and phase composition of the as-sprayed coating and that after oxidation at high temperature were analyzed by means of scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The oxidation behavior of as-sprayed coating and starting powders was evaluated by thermogravimetry. Dry sliding friction and wear behavior of the WC-(W,Cr)₂C-Ni coating sliding against Si₃N₄ ball at different temperatures (room temperature 20 °C and elevated temperature of 700 °C and 800 °C) was evaluated using an oscillating friction and wear tester. Besides, the microhardness and fracture toughness of the coating was also measured. Results show that sintering agglomerated WC-20 wt.%Cr-7 wt.%Ni powder is an effective method to prepare agglomerated and sintered WC-(W,Cr)₂C-Ni composite powder. The excellent oxidation resistance of WC-(W,Cr)₂C-Ni coating is mainly resulted from a double-decker shell-core microstructure formed in the coating. The composition of the outer shell is (W,Cr)₂C phase and that of the inner shell is Cr₃C₂. During high-temperature friction and wear test, well remained hard WC phase in the WC-(W,Cr)₂C-Ni coating can guarantee its good mechanical properties and wear resistance, and newly generated nano NiWO₄, CrWO₄ and Cr₂WO₆ particles can further improve these properties significantly.     

反应等离子喷涂 Al2O3-TiB2-Al 复相涂层的反应机理

目的在铝合金表面制备Al2O3-TiB2-Al复合涂层,研究Al,TiO2,B2O3在等离子喷涂中的反应机理。方法采用反应等离子喷涂技术在铝合金表面制备复合涂层,应用扫描电镜与X射线衍射技术测试复合涂层的物相组成和显微组织,并通过燃烧波淬熄试验分析等离子喷涂产物。结果机械球磨可以有效降低粉末发生反应的活化能,等离子喷涂最佳飞行距离范围为150~200 mm。结论喷涂粉末在飞行过程中发生反应,经历了预热、熔化、分解、团聚等过程,验证了最终引燃发生燃烧化学反应的机理。     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

Ion-plated Al-Al2O3 films as diffusion barriers between NiCrAlY coating and orthorhombic-Ti2AlNb alloy

Ion-plated Al-Al₂O₃ cermet films were fabricated as diffusion barriers between NiCrAlY coating and orthhombic-Ti₂AlNb alloy. The oxidation and interdiffusion behaviour of coatings with and without diffusion barrier were investigated in isothermal and cyclic oxidation tests at 800 °C. The results indicated that substantial interdiffusion and rapid oxidation degradation occurred in the coated specimens without diffusion barrier. With Al-Al₂O₃ diffusion barriers, deferred interdiffusion and improved oxidation resistance was observed. Among them, duplex coating containing 1Al-Al₂O₃ interlayer exhibited the best performance. Coefficient of diffusion hindering and factor of reaction hindering were proposed to compare and quantify the efficiency of the diffusion barriers.