Sliding Wear Performance of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings Against Graphite under Severe Conditions

Al₂O₃, Cr₂O₃, and Al₂O₃-Cr₂O₃ composite coatings were produced by plasma spraying. Their tribological properties were evaluated at high load conditions. The average friction coefficients, wear rates, and worn surface temperatures of the coating/graphite pairs were measured. Compared with the single coating/graphite pairs, the friction coefficients of composite coating/graphite pairs are more stable. The corresponding wear rates and worn surface temperatures are lower, which may be conducive to the formation of more effective and stable graphite transfer film on the surface of the coating subjected to abrasion. Especially, 10wt.%Al₂O₃-90wt.%Cr₂O₃ (AC₉₀) composite coating shows better anti-wear performance, which may be attributed to its higher thermal conduction.     

Thermal spraying and adhesion of oxides onto graphite

Surface treatment of graphite (Cg), which is based on thermal spraying of oxides (Al₂O₃, ZrO₂), is proposed to reduce graphite reactivity and permeability toward oxygen. The study of coating adhesion allows recognition of the parameters that favor mechanical and chemical bonds. It also indicates the pretreatment of graphite, which leads to Cr₃C₂ or SiC bond coats. Different graphite-oxide systems were examined and showed that adhesion particularly depends on the thermal expansion coefficient mismatch between Cg, oxide, and the bond coat. Therefore, a post-treatment based on an impregnation with enamel of the porous oxide is essential to obtain effective protection of the graphite against the external environment.     

Dual-Layer Oxidation-Protective Plasma-Sprayed SiC-ZrB<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Carbon Nanotube Coating on Graphite

Graphite is used in high-temperature gas-cooled reactors because of its outstanding irradiation performance and corrosion resistance. To restrict its high-temperature (>873 K) oxidation, atmospheric-plasma-sprayed SiC-ZrB₂-Al₂O₃-carbon nanotube (CNT) dual-layer coating was deposited on graphite substrate in this work. The effect of each layer was isolated by processing each component of the coating via spark plasma sintering followed by isothermal kinetic studies. Based on isothermal analysis and the presence of high residual thermal stress in the oxide scale, degradation appeared to be more severe in composites reinforced with CNTs. To avoid the complexity of analysis of composites, the high-temperature activation energy for oxidation was calculated for the single-phase materials only, yielding values of 11.8, 20.5, 43.5, and 4.5 kJ/mol for graphite, SiC, ZrB₂, and CNT, respectively, with increased thermal stability for ZrB₂ and SiC. These results were then used to evaluate the oxidation rate for the composites analytically. This study has broad implications for wider use of dual-layer (SiC-ZrB₂/Al₂O₃) coatings for protecting graphite crucibles even at temperatures above 1073 K.     

Corrosion protection of magnesium alloys by cerium, zirconium and niobium-based conversion coatings

A new Ce, Zr and Nb-based conversion coating was designed for AZ91 and AM50 magnesium alloys. The corrosion protection provided by this coating was evaluated by electrochemical measurements (polarization curves, electrochemical impedance spectroscopy) in Na₂SO₄ electrolyte, and accelerated atmospheric corrosion tests (humid, SO₂ polluted air, and salt spray). Its chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements showed that Mg alloys treated during 24 h in the Ce-Zr-Nb conversion bath exhibit: (i) increased corrosion potential, (ii) decreased corrosion and anodic dissolution current densities, and (iii) increased polarization and charge transfer resistances. The accelerated corrosion tests revealed excellent atmospheric corrosion resistance for all Ce-Zr-Nb-treated samples, with or without an additional layer of epoxy-polyamide resin lacquer or paint. XPS analysis showed that the coating includes CeO₂, Ce₂O₃, ZrO₂, Nb₂O₅, MgO, and MgF₂ as main components. No significant modification of the chemical composition was observed after cathodic and anodic polarization in Na₂SO₄. This new coating provides improved corrosion resistance, and excellent paint adhesion. It offers an alternative to the chromate conversion coating for magnesium alloys.     

A composite coating for corrosion protection of AM60B magnesium alloy

Oxide films have been produced on AM60B magnesium alloy by micro-arc anodic oxidation in an environmentally friendly alkaline solution, with and without addition of nanoparticles (TiO₂, ZrO₂ and Al₂O₃). Because of the anodic oxide porosity, inherent in the sparking process, organo-functional silanes top coat has been applied to seal pores and cracks, and achieve an efficient protective coating system. The surface and cross-section morphology of samples were analyzed by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS). Scratch tests were performed for evaluating the adhesion strength and scratch hardness of the anodic oxides to the AM60B substrate. The corrosion resistance of both anodic oxides and oxide/silane composite coatings was evaluated in 0.6 M NaCl solution using potentiodynamic polarization tests. The addition of nanoparticles to the anodizing solution doesn't affect significantly the corrosion resistance in comparison with anodic oxides produced in nanoparticles free solutions. Conversely, the adhesion strength and scratch hardness of the anodic oxides to the substrate is quite scattered, and it is higher for the samples produced in ZrO₂ and in Al₂O₃ rich solutions. For this reason specimens anodized in ZrO₂ and Al₂O₃ containing solutions were chosen for silane deposition. Two silanes were used, namely octyltrimethoxysilane (OSi) and 1, 2-bis [triethoxysilyl] ethane (BTSE). The anodizing treatment carried out in oxides nanoparticles containing solutions (ZrO₂ or Al₂O₃), followed by a silane top coat treatment performed using OSi precursor, is an interesting way, suitable for industrial applications, to synthesize adherent corrosion resistant coatings on magnesium alloy AM60B in a short process time.     

Synthesis and oxidation behavior of nanocrystalline MCrAlY bond coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO₂/Y₂O₃). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al₂O₃ layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Effect of bond-coat pre-aluminization and pre-oxidation duplex pretreatment on the performance of ZrO2-8 wt.% Y2O3/Co-29Cr-6Al-1Y thermal-barrier coatings

A modification to the conventional thermal-barrier coating (TBC) system was made. In this study, the low-pressure, plasma-sprayed Co-29Cr-6Al-1Y bond coat received a duplex pretreatment of bond-coat pre-aluminizing and pre-oxidation prior to overlaying the air-plasma sprayed ZrO₂-8 wt.% Y₂O₃ top coat. The effects of this treatment on the properties of the TBC system were evaluated by thermal-cyclic test at 1050° C in air. The results of cyclic-oxidation tests showed that the proposed processes could remarkably improve the performance of ZrO₂-8 wt.% Y₂O₃/Co-29Cr-6Al-1Y TBC. The improvement was attributed to a modification of bond-coat oxides and the associated reduction of the oxidation rate of the MCrAlY bond coat.     

Wear and corrosion properties of plasma sprayed AI2O3 and Cr2O3 coatings sealed by aluminum phosphates

Plasma-sprayed aluminum oxide (AI₂O₃) and chromium oxide (Cr₂O₃) coatings were sealed by aluminum phosphates. Phosphates were formed throughout the coating, down to the substrate, and were verified by scanning electron microscopy and hardness measurements. The sealing increased the hardness of the coatings by 200 to 300 Vickers hardness (HV) units. Abrasion and erosion wear resistances were increased by the sealing treatment. Sealing also substantially closed the open porosity, as shown in electro-chemical corrosion tests. The sealed structures had good resistance against corrosion during 30 days of immersion in both acidic and alkaline solutions with pH values from 0 to 10. No decrease in abrasion wear resistance was observed after immersion.     

Characterization of oxide scales formed on high-velocity oxyfuel-sprayed Ni-Co-Cr-Al-Y + ReTa coatings

A high-velocity oxyfuel-sprayed 30 wt.% Ni-20 wt.% Co-30 wt.% Cr-10 wt.% Al-2 wt.% Y-4 wt.% Re-4 wt.% Ta coating was oxidized between 1000 and 1200 °C for up to 200 h in air, and the oxide scales were examined. The dense, sprayed coating consisted mainly of Cr₃Ni₂, Ni₃Al, Ni₃Ta, Ni, NiO, Al₅Y₃O₁₂, and Cr₂O₃. Intermetallics and some oxides formed during spraying. During oxidation, mainly αAl₂O₃, along with some Al₅Y₃O₁₂, CoAl₂O₄, CoCr₂O₄, Ta₂O₅, and Ta₂O_2.2 formed on the coating. The preferential oxidation of Al to form the Al-rich scales resulted in the formation of an Al-depleted region beneath the scales. Rhenium, being the most noble element, was distributed throughout the oxide scale and the coating, without forming any independent oxides.     

Corrosion behaviors of a new titanium alloy TZNT for surgical implant application in Ringer’s solution

A new near α-titanium alloy Ti12.5Zr2.5Nb2.5Ta (TZNT) for surgical implants was designed. The potentiodynamic technique was performed to investigate the corrosion behaviors of TZNT in Ringer's solution, and Ti6Al4V, Ti6Al7Nb, and TA2 were taken as comparison. The structure of the passive film was analyzed using an X-ray photoelectron spectrometer (XPS). The results indicate that TZNT possesses better corrosion resistance, when compared with Ti6Al4V, Ti6Al7Nb, and TA2. The passive film formed on the TZNT surface is composed of oxides, such as TiO2, ZrO2, Nb2O5, and Ta2O5. The elements Zr and Ta are rich, whereas Ti and Nb are poor in the passive film. The addi-tion of Zr, Nb, and Ta with relatively low electrochemical reaction potentials can reduce the anode activity and improve passive properties. Other than that, oxides such as ZrO2, Nb2O5, and Ta2O5 with the nobler equilibrium constants make the passive film more stable.     

Thermal conductivity and phase stability of plasma sprayed ZrO2–Y2O3–La2O3 coatings

Thermal conductivity and phase stability of La₂O₃,Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. La₂O₃ was selected as a stabilizer because it had a significant effect on reducing densification of Y₂O₃ stabilized ZrO₂ (YSZ). The developed coating showed low thermal conductivity even after high temperature exposure due to its high resistance to sintering. However, phase stability and thermal cycle life of the coating decreased as the amount of La₂O₃ was increased. It was concluded that optimum amount of La₂O₃ addition was about 1 mol% to suppress sintering with little degradation of other properties.     

涂层材料对钛合金机加工石墨型铸造工艺成形质量的影响

通过在石墨铸型内表面涂敷Y₂O₃、Al₂O₃、CaZrO₃耐火氧化物涂料,研究涂层成分对纯钛铸件界面反应和表面组织性能的影响规律。结果表明：涂敷涂料能有效克服石墨铸型对铸件的激冷作用。Y₂O₃涂料与钛液无化学反应发生,稳定性最佳,仅物理粘附于铸件表面。Al₂O₃和CaZrO₃涂料均与钛液发生了界面反应,分别生成Ti-Al-O和 Ca-Ti-O的化合物,Al₂O₃反应程度尤为剧烈。涂刷涂料后钛铸件表面晶粒尺寸、粗晶层厚度和显微硬度明显增大,变化趋势均为未涂刷< Y₂O₃< CaZrO₃< Al₂O₃。     

Corrosion and wear behavior of thermally sprayed nano ceramic coatings on commercially pure Titanium and Ti–13Nb–13Zr substrates

Surface treatments and coatings are the practical approaches used to extend the lifetime of components and structures especially when the surface is the most solicited part of the considered engineering component. Hard thermally sprayed coating is one of the most wear resistance coating widely used in many practical mechanical applications. In the construction of articulating parts of medical devices, titanium and its alloys have to be surface coated to improve their tribocorrosion behavior. In this way, the use of porous thermal coatings is known to be a strategy for better binding bone or tissue on femoral stem for example. It is, thus, important to evaluate the corrosion and the wear behaviors of such materials for biosecurity considerations in the human body. In this study, we investigate the behavior of new nano ZrO₂ and Al₂O₃-13 wt.% TiO₂ thermal sprayed coatings on commercially pure (cp)-Ti (grade 4) and titanium alloy substrates. Friction and wear tests against Al₂O₃ balls showed that the wear resistance of Al₂O₃-13 wt.% TiO₂ is better than that ZrO₂ coating. Both plasma sprayings have similar abrasive wear behavior; however, the average friction coefficient is higher for alumina–titania coating. Electrochemical tests, open circuit potential monitoring and potentiodynamic polarization, were performed in simulated body conditions (Hank’s solution, 37 °C). Results showed that corrosion resistance was appreciably higher for alumina–titania coating.     

Microstructure of Suspension Plasma Spray and Air Plasma Spray Al2O3-ZrO2 Composite Coatings

Al₂O₃-ZrO₂ coatings were deposited by the suspension plasma spray (SPS) molecularly mixed amorphous powder and the conventional air plasma spray (APS) Al₂O₃-ZrO₂ crystalline powder. The amorphous powder was produced by heat treatment of molecularly mixed chemical solution precursors below their crystallization temperatures. Phase composition and microstructure of the as-synthesized and heat-treated SPS and APS coatings were characterized by XRD and SEM. XRD analysis shows that the as-sprayed SPS coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases, while the as-sprayed APS coating consists of tetragonal ZrO₂, α-Al₂O₃, and γ-Al₂O₃ phases. Microstructure characterization revealed that the Al₂O₃ and ZrO₂ phase distribution in SPS coatings is much more homogeneous than that of APS coatings.     

Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings

High-temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl₂O₄ spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl₂O₄/α-Al₂O₃ interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl₂O₄, θ-Al₂O₃, Y₃Al₅O₁₂ at the TGO surface after pre-oxidation was avoided. The α-Al₂O₃ layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of θ-Al₂O₃ instead of α-Al₂O₃. Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.     

Development of multimaterial coatings by cold spray and gas detonation spraying

The basic objective is the development of multifunctional multimaterial protective coatings using cold spraying (CS) and computer controlled detonation spraying (CCDS).As far as CS is concerned, the separate injection of each powder into different zones of the carrier gas stream is applied. Cu-Al, Cu-SiC, Al-Al₂O₃, Cu-Al₂O₃, Al-SiC, Al-Ti and Ti-SiC coatings are successfully sprayed. As to CCDS, powders are sprayed with a recently developed apparatus that is characterized by a high-precision gas supply system and a fine-dosed twin powder feeding system. Computer control provides a flexible programmed readjustment of the detonation gases energy impact on powder thus allowing selecting the optimal for each component spraying parameters to form composite and multilayered coatings. Several powders are sprayed to obtain composite coatings, specifically, among others, WC-Co-Cr + Al₂O₃, Cu + Al₂O₃, and Al₂O₃ + ZrO₂.     

γ-TiAl合金定向凝固过程中与氧化钇陶瓷材料的界面反应

研究液态金属冷却定向凝固条件下,Ti-47%Al合金与Y2O3/Al2O3双层结构陶瓷管内层Y2O3的相互作用。分析熔体温度和相互作用时间对界面反应、合金组织和成分的影响。结果表明：虽然Y2O3层受到一定程度的侵蚀和溶解,但Y2O3层有效地阻隔化学稳定性较差的Al2O3外层与TiAl合金熔体接触,避免Al2O3与TiAl之间的化学反应。合金熔体受到一定程度的污染且含有少量夹杂物。在一定保温时间内,合金中的杂质含量（Y和O）和夹杂物（Y2O3）体积分数均随着过热温度的提高而增加,但随着保温时间的进一步延长,杂质含量和夹杂物体积分数趋于恒定值,不再显著变化。     

Ta2O5-Y2O3 掺杂剂对氧化锆相结构、断裂韧性和热物理性能的影响

合成制备了一系列的Ta₂O₅-Y₂O₃掺杂的ZrO₂（TYSZ）。研究了掺杂剂含量对处理相结构、相稳定性、断裂韧性、热膨胀系数和导热系数的影响,同时探讨了应力条件下TYSZ的响应机制。结果表明,掺杂Ta₂O₅和Y₂O₃引起的晶格变形有利于t相稳定。但随着掺杂含量的增加,由于应力诱导的t-m相变被抑制和高温下稳定剂析出倾向增加,高温相稳定性和断裂韧性下降。热膨胀系数和热导率的变化则是由晶体结构的混乱程度和晶格膨胀导致的。16TYSZ具有良好的相稳定性、应力敏感性和热物理性能,是先进热障涂层材料的潜在候选材料。     

Neodymium-cerium oxide as new thermal barrier coating material

Neodymium-cerium oxide (Nd₂Ce₂O₇) was proposed as a new thermal barrier coating material in this work. Monolithic Nd₂Ce₂O₇ powder was prepared by the solid-state reaction at 1400 °C. The phase composition, thermal stability and thermophysical properties of Nd₂Ce₂O₇ were investigated. Nd₂Ce₂O₇ with fluorite structure was thermally stable in the temperature range of interest for TBC applications. The results indicated that the thermal expansion coefficient (TEC) of Nd₂Ce₂O₇ was higher than that of YSZ (6-8 wt.% Y₂O₃ + ZrO₂) and even more interesting was the TEC change as a function of temperature paralleling that of the superalloy bond coat. Moreover, the thermal conductivity of Nd₂Ce₂O₇ is 30% lower than that of YSZ, which was discussed based on the theory of heat conduction. Thermal barrier coating of Nd₂Ce₂O₇ was produced by atmospheric plasma spraying (APS) using the spray-dried powder. The thermal cycling was performed with a gas burner test facility to examine the thermal stability of the as-prepared coating.     

Thermal barrier coating bonded by (Al2O3–Y2O3)/(Y2O3-stabilized ZrO2) laminated composite coating prepared by two-step cyclic spray pyrolysis

Thermal barrier coating bonded by (Al₂O₃–Y₂O₃)/(Y₂O₃-stabilized ZrO₂) (YSZ) laminated coating has been developed on Ni-based superalloy by two-step cyclic pyrolysis. It is demonstrated, from cyclic oxidation tests at 1100 °C, that YSZ top coat and alloy substrate can be bonded together effectively by the (Al₂O₃–Y₂O₃)/YSZ laminated coating, showing good resistance to oxidation, cracking, spallation and buckling. These beneficial effects can be attributed to the sealing effect of the designed multi-sealed compact bond coat with α-Al₂O₃ layers, the decrease of thermal stresses, the increase of fracture toughness in such bond coat and no interdiffusion between the substrate and bond coat.