Influence of laser‐glazing on hot corrosion resistance of yttria‐stabilized zirconia TBC in molten salt mixture of V2O5 and Na2SO4

Plasma‐sprayed 8YSZ (zirconia stabilized with 8 wt% yttria)/NiCoCrAlYTa thermal barrier coatings (TBCs) were laser‐glazed using a continuous‐wave CO₂ laser. Open pores within the coating surface were eliminated and an external densified layer was generated by laser‐glazing. The hot corrosion resistances of the plasma‐sprayed and laser‐glazed coatings were investigated. The two specimens were exposed for the same period of 100 h at 900 °C to a salt mixture of vanadium pentoxide (V₂O₅) and sodium sulfate (Na₂SO₄). Serious crack and spallation occurred in the as‐sprayed coating, while the as‐glazed coating exhibited good hot corrosion behavior and consequently achieved a prolonged lifetime. The results showed that the as‐sprayed 8YSZ coating achieved remarkably improved hot corrosion resistance by laser‐glazing. Changes in the coatings were studied by scanning electron microscopy (SEM) to observe the microstructure and X‐ray diffraction (XRD) technique to analyze the phase composition. XRD results showed that the reaction between yttria (Y₂O₃) and V₂O₅ produced yttrium vanadate (YVO₄), leaching Y₂O₃ from YSZ and causing the progressive destabilization transformation from the tetragonal (t) to monoclinic (m) phase. The external dense layer produced by laser‐glazing restrained the penetration of the molten salt, to a certain extent, into the coating, which led to a relatively low m‐ZrO₂ content in the coating after the hot corrosion test. Additionally, the segmented cracks in the coating surface induced by laser‐glazing were helpful to the improvement of strain tolerance of the coating. The two factors were important contributions to the significant enhancement of hot corrosion resistance of the as‐glazed YSZ coating.     

The Characteristics of Alumina Scales Formed on HVOF-Sprayed MCrAlY Coatings

HVOF MCrAlY (M=Ni, Co) coatings were isothermally oxidized in synthetic airbetween 850 and 1050°C for times up to 167 hr. During thermal spraying,aluminum and yttrium oxidized to form a fine oxide dispersion. The HVOFMCrAlY coatings exhibited a microstructure similar to ODS alloys. The finedispersion consisted of Al₂O₃ and aluminum–yttriumoxides. The oxidation experiments showed that the oxidation rate of HVOFcoatings was two times slower than the oxidation rate of VPS MCrAlYcoatings. The oxidation mechanism changed mainly in the transient-stage(no metastable modification of Al₂O₃ formed) and it wasassumed that the oxide dispersion hindered diffusion of various elementsfrom the bulk material during oxidation. The formation of the fine oxidedispersion also influenced the adherence of the oxide scale. Themicrostructures of the transient oxide scales were examined by X-raydiffraction (XRD) scanning electron microscopy (SEM), and transmissionelectron microscopy (TEM).     

Isothermal Oxidation Behavior of NiCoCrAlTaY Coating Deposited by High Velocity Air-Fuel Spraying

The performance of thermal barrier coatings is influenced by the high temperature oxidation behavior of the bond coat. In this paper, NiCoCrAlTaY bond coat was deposited by high velocity air-fuel (HVAF) spraying, and the microstructure and surface morphology of the bond coat before and after oxidation were examined to aim at developing high performance thermal barrier coatings. Results showed that the HVAF sprayed NiCoCrAlTaY coating presented a dense microstructure and some partially melted particles with a near spherical morphology were deposited on the coating surface. A uniform ??-Al₂O₃ scale was formed on the HVAF sprayed MCrAlY coating surface after the pre-oxidation treatment in an argon atmosphere. A small fraction of nodular-shaped mixed oxides was formed when the MCrAlY coating was oxidized for 100?h at 1000?°C. The amount of the mixed oxides increased less significantly after 200?h oxidation. A homogeneous ??-Al₂O₃ oxide scale was maintained over the large particles on the bond coat surface after 200?h oxidation at 1000?°C in air. A model is proposed to explain the formation of nodular-shaped mixed oxides.     

Laser glazing of plasma-sprayed zirconia coatings

A CO₂ laser with cylindrical focal lens has been used to glaze the surface layer of plasma-sprayed ZrO₂-20wt% Y₂O₃/MCrAlY coatings. Both a continuous-wave laser and a pulsed laser were used in this study. Different parameter settings for power, travel speed, and pulse frequency were used, and their effects on the melting width, melting depth, coupling efficiency, microstructure, surface roughness, and process defects have been evaluated. Results show that the melting width of the glazed track was slightly smaller than the diameter of the raw beam. The melting depth increased with increasing energy density for both a continuous-wave laser and a pulsed laser. The coupling efficiency as about 40 to 65% for a continuous-wave laser, which increased with increasing laser travel speed, but decreased with an increase in energy density. The power density has no significant effect on coupling efficiency. Defects, such as bubbles or depressions, occur easily with a continuous wave laser. A high-quality glazed layer is successfully produced using a pulsed laser. The surface roughness of the plasma-sprayed ceramic coatings was significantly improved by laser glazing. Surface roughness decreased slightly as the pulse frequency increased for the glazed surface. Based on this study, proper processing parameters have been suggested.     

高速火焰喷涂高质量NiCoCrAlTaReSiY抗氧化涂层

研究了用高速火焰喷涂(HVOF)替代低压等离子喷涂(LPPS)沉积高质量的MCrAlY涂层。试验用粉料为NiCoCrAlTaReSiY,采用以煤油为燃料的K2型HVOF系统沉积涂层,研究喷嘴长度、喷涂工艺参数对粉末沉积工艺过程以及涂层性能的影响;测量涂层的孔隙率及氧含量,观察涂层经真空热处理以及高温空气氧化后的显微结构,测量了Al、O等元素在氧化涂层中的分布。结果表明,所沉积的NiCoCrAlTaReSiY涂层具有优越的抗氧化性。     

Influence of the mode of introduction of a reactive element on the high temperature oxidation behavior of an alumina‐forming alloy. Part I: Isothermal oxidation tests

Different modes of introduction of yttrium have been tested with regard to the influence on the high temperature oxidation behavior of a FeCral alloy. Y₂O₃ sol‐gel coatings, Y₂O₃ metal‐organic chemical vapor deposition (MOCVD) coatings, implanted yttrium ions and yttrium as alloying element (0.1 wt.%) in the same Fe‐20Cr‐5Al alloy were oxidized at 1100°C in air under atmospheric pressure. Whatever the mode of introduction of the reactive element, the oxidation rates were not decreased compared to the oxidation rate of the blank specimen. The observation of the oxidized surface indicated that the alumina scale largely spalled from the blank alloy. Spallation was reduced for the Y₂O₃ sol‐gel coated, the Y₂O₃ MOCVD coated alloys and the yttrium ion implanted steels. The Y‐containing alloy did not exhibit any detachment of the oxide scale, indicating the best high temperature oxidation behavior, at least from the viewpoint of scale adherence.     

Effect of manufacturing related parameters on oxidation properties of MCrAlY‐bondcoats

The effect of MCrAlY‐bondcoat manufacturing parameters prior to TBC deposition on the bondcoat oxidation behavior and TBC lifetime was studied. The studied material was a NiCoCrAl(Y/Hf) free‐standing coating. It was found that variation of oxygen partial pressure during vacuum plasma spraying and the vacuum heat treatment procedure significantly affects the yttrium and hafnium distribution in the coating. In coatings sprayed at high pO₂, yttrium and hafnium were mainly tied up into oxide precipitates. This effect is apparently responsible for an early alumina scale spallation and failure of the initially studied TBC system during cyclic oxidation. In contrast, the coating sprayed at low pO₂ revealed an overdoping effect, i.e. extensive yttrium and hafnium incorporation into the scale resulting in an accelerated scale growth rate and internal oxidation. It was shown that by variation of the vacuum heat treatment parameters the yttrium and hafnium distribution in the near‐surface regions of the low oxygen coating can be modified. The latter result demonstrates the potential of minimizing the negative overdoping effect on the scale growth in the thermal‐sprayed MCrAlY coatings with low oxygen and/or high reactive element contents by optimization of the vacuum heat treatment procedure.     

Oxide scale growth on MCrAlY bond coatings after pulsed electron beam treatment and deposition of EBPVD-TBC

Thin layers restructured by surface melting to approximately 30-μm depth on MCrAlY coatings were produced using the large-area pulsed electron-beam GESA facility. With a beam diameter of up to 10 cm, it is possible to treat large surface areas, with just a few overlapping electron beam pulses. The high cooling rates lead to nanocrystalline structures at the sample surface. Their oxidation properties were studied at 950°C in air. As the surface treatment leads to smooth surfaces, with an R_A<1.5 μm, even on formerly rough low-pressure plasma spray (LPPS) samples, the samples can subsequently be coated directly with electron beam physical vapor deposition (EBPVD) thermal barrier coatings (TBC). The growth of the thermally grown oxide (TGO) on such samples was also studied at 950°C in air for up to 5000 h and compared to that of samples without the TBC. The treated samples appeared to have a strongly enhanced oxidation resistance without spinel formation in the α-Al₂O₃ oxide scale.     

Sealing procedures for thick thermal barrier coatings

Zirconia-based 8Y₂O₃-ZrO₂ and 22MgO-ZrO₂ thick thermal barrier coatings (TTBC, 1000 μm), were studied with different sealing methods for diesel engine applications. The aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of TTBCs was sealed with three different methods: (1) impregnation with phosphate-based sealant, (2) surface melting by laser glazing, and (3) spraying of dense top coating with a detonation gun. The thicknesses of the densified top layers were 50–400 μm, depending on the sealing procedure. X-ray diffraction (XRD) analysis showed some minor phase changes and reaction products caused by phosphate-based sealing treatment and some crystal orientation changes and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values. The hot corrosion resistance of TTBCs against 60Na₂SO₄-40V₂O₅ deposit was determined in isothermal exposure at 650 °C for 200 h. Corrosion products and phase changes were studied with XRD after the test. A short-term engine test was performed for the reference coatings (8Y₂O₃-ZrO₂ and 22MgO-ZrO₂) and for the phosphate-sealed coatings. Engine tests, duration of 3 h, were performed at the maximum load of the engine and were intended to evaluate the thermal cycling resistance of the sealed coatings. All of the coatings passed the engine test, but some vertical cracks were detected in the phosphate-sealed coatings.     

Optimization of spraying process and laser treatment of CoNiCrAlY

This article describes the investigation of a new generation of vacuum plasma sprayed CoNiCrAlY coatings on NiCr20Ti in the sprayed as well as the laser-treated condition. The aim of the study was the improvement of MCrAlY coating properties by modifying the microstructure through laser remelting with 5-kW Co₂: lasers. Parameter analysis and optimization was carried out for the vacuum plasma spraying process, as well as for the laser remelting technique. The effect of the laser treatment on microstructure, quality of the coatings, and oxidation as well as hot gas corrosion behavior are reported.     

Effect of Thermal Treatment on High-Temperature Mechanical Properties Enhancement in LPPS, HVOF, and APS CoNiCrAlY Coatings

The mechanical properties of a MCrAlY coating significantly influence the initiation of cracks in the superalloy substrate under thermomechanical-fatigue conditions. Previous studies have developed a convenient method for evaluating the mechanical properties of sprayed coatings by lateral compression of a circular tube coating. This method does not need chucking, and manufacturing the free-standing coating is quite straightforward. In this study, the mechanical properties of the free-standing CoNiCrAlY coatings prepared using low-pressure plasma spraying (LPPS), high-velocity oxyfuel (HVOF) spraying, and atmospheric plasma spraying (APS) were systematically measured with the lateral compression method at room temperature through to 920 °C. The effect of postspray thermal treatments, in vacuum and in air, on the mechanical properties was investigated in the 400 to 1100 °C temperature range. It was found that high-temperature thermal treatment in air was effective in increasing the bending strength and Young’s modulus. It was especially effective on the APS coatings, which were produced using powders with average size 60 μm, and on HVOF coating, whose bending strengths increased by approximately three times. On the contrary, the enhancement in the LPPS and APS coatings produced with powders 21 μm in size was found to be approximately 1.6 times.     

Thermal fatigue behavior of thermal barrier coatings with the MCrAlY bond coats by cold spraying and low-pressure plasma spraying

The thermal fatigue behavior of thermal barrier coatings (TBCs) with the NiCoCrAlTaY bond coats deposited by cold spraying and low-pressure plasma spraying (LPPS) was examined through thermal cyclic test. The TBCs were subjected to the pre-oxidation before the test in an Ar atmosphere. The results show that a more uniform TGO in both thickness and composition forms on the cold-sprayed bond coat than that deposited by LPPS. The TBCs with the cold-sprayed bond coat exhibit a longer thermal cyclic lifetime than that with the LPPS bond coat. The differences in oxidation behavior and thermal cyclic behavior between two TBCs were discussed based on the evident difference in the surface morphology of two MCrAlY bond coats deposited by cold spraying and LPPS.     

Electrochemical Corrosion of HVOF-Sprayed NiCoCrAlY Coatings in CO<Subscript>2</Subscript>-Saturated Brine

The effect of pre-oxidation treatment and surface preparation of optimized NiCoCrAlY coatings deposited by high-velocity oxygen fuel (HVOF) spraying and exposed to a low-temperature corrosive environment is reported herein. Coatings with two surface finish conditions (as-sprayed and ground) were heat treated under two different oxygen partial pressures (air and argon). The electrochemical corrosion behavior was evaluated in CO₂-saturated brine via potentiodynamic polarization, polarization resistance, and electrochemical impedance measurements. The results show that the grinding process and pre-oxidation treatment in argon enhanced growth and formation of α-Al₂O₃ scale. The potentiodynamic polarization results show that both pre-oxidation and surface treatment had a positive influence on the corrosion resistance of the coating. The reduction of the porosity and the formation of a dense, uniform, and adherent oxide scale through pre-oxidation treatment led to an increase of the corrosion resistance due to a decrease in active sites and blocking of diffusion of reactive species into the coating. However, according to the results, complete transformation from metastable alumina phases to α-Al₂O₃ in addition to formation and growth of dense α-Al₂O₃ is required to ensure full protection of the coating and base material over long periods.     

The Prospect of Y2SiO5-Based Materials as Protective Layer in Environmental Barrier Coatings

Bulk yttrium monosilicate (Y₂SiO₅) possesses interesting properties, such as low thermal expansion coefficient and stability in water vapor atmospheres, which make it a promising protective layer for SiC-based composites, intended for the hottest parts in the future gas turbines. Because protective layers are commonly applied by thermal spraying techniques, it is important to analyze the changes in structure and properties that these methods may produce in yttrium silicate coatings. In this work, two SiO₂-Y₂O₃ compositions were flame sprayed in the form of coatings and beads. In parallel, the beads were spark plasma sintered at relatively low temperature to obtain partially amorphous bulk specimens that are used as model bulk material. The thermal aging—air and water vapor atmosphere—caused extensive nucleation of Y₂SiO₅ and Y₂Si₂O₇ in both the bulk and coating. The rich water vapor condition caused the selective volatilization of SiO₂ from Y₂Si₂O₇ at the specimen surface leaving a very characteristic micro-ridged Y₂SiO₅ zones—either in coatings or sintered bodies. An important increase in the thermal conductivity of the aged materials was measured. The results of this work may be used as a reference body for the production of Y₂SiO₅ coatings using thermal spraying techniques.     

TiAl合金表面激光重熔MCrAlY涂层热腐蚀性能

采用等离子喷涂技术在TiAl合金表面制备了MCrAlY涂层,并用激光重熔工艺对涂层进行处理,研究了TiAl合金、等离子喷涂MCrAlY涂层及激光重熔MCrAlY涂层850℃下75%Na₂SO₄+25%NaCl(质量分数)混合盐浸泡热腐蚀性能,分析了不同试样的热腐蚀破坏机理,并讨论了激光重熔处理对涂层热腐蚀性能的影响.结果表明,等离子喷涂MCrAlY涂层能显著提高TiAl合金的耐热腐蚀性能,经过激光重熔后可进一步提高其耐热腐蚀性能.MCrAlY涂层在高温熔盐中的热腐蚀发生的是表面氧化反应和内部硫化反应,主要生成Al₂O₃,Cr₂O₃,NiO,NiCr₂O₄,Ni₃S₂及CrS等腐蚀产物.     

Thermal-sprayed Fe-10CM3P-7C amorphous coatings possessing excellent corrosion resistance

An alloy of Fe-10Cr-13P-7C was thermally sprayed by three different processes: (1) 80 kW low-pressure plasma spraying (LPPS), (2) high-velocity oxyfuel (HVOF) spraying, and (3) 250 kW high-energy plasma spraying (HPS). The as-sprayed coating obtained by the LPPS process was composed of an amorphous phase. In contrast, the as-sprayed coatings obtained by the HVOF and HPS processes were a mixture of amorphous and crystalline phases. The as-sprayed coatings showed a high hardness of 700 DPN. A very fine structure composed of ferrite, carbide, and phosphide was formed, producing a maximum hardness of greater than 1000 DPN in the LPPS coating just after crystallization on tempering. The corrosion re-sistance of the amorphous coating was superior to a SUS316L stainless steel coating in 1N H₂SO₄ solution and 1N HC1 solution. Furthermore, the amorphous coating underwent neither general nor pitting corro sion in1NUCI solution and 6% FeCl₃ 6H₂O solution containing 0.05N HCl, whereas the SUS316L stain less steel coating was attacked aggressively.     

MCrAlY 材料成分改性研究进展

MCrAlY(M=Ni,Co,NiCo)合金材料由于其优异的抗氧化性能和抗腐蚀性能,作为热障涂层的粘结层材料或单层涂层得到广泛应用。简要介绍了MCrAlY涂层的组成,并系统地阐述了为提高MCrAlY粘结层材料性能,对粘结层成分进行改性的研究进展,主要包括掺杂合金元素和掺杂硬质颗粒。合金元素主要包含Re,Hf,Pt,Ta等稀土元素;硬质颗粒主要是CeO2,Al2O3等氧化物颗粒。通过掺杂合金元素和硬质颗粒,可以使MCrAlY涂层中形成扩散障碍,改变MCrAlY的相组成,降低粘结层中Al,Cr等元素的扩散速率,从而有效地降低TGO的生长速率,延缓TGO中尖晶石相的形成,提高MCrAlY涂层的抗氧化性能。     

High-temperature corrosion performance of plasma-sprayed CrNiMoSiB coatings

This study examined the effects of application parameters for plasma spraying and CCh- laser glazing of two types of chromium-base coatings. Coatings were deposited by low-pressure and atmospheric plasma spraying. The high-temperature corrosion resistance of Cr-Ni-2.5Mo-lSi-0.5B (55 and 58% Cr) coatings was evaluated with respect to structural and compositional changes both in the as- sprayed condition and after CCb- laser glazing. Coatings that were deposited by atmospheric plasma spraying and subsequently laser glazed showed excellent resistance to oxidation and sulfate- vanadate attack at 900 °C due to the formation of a protective chromia film and a high silica concentration on the top layers of the oxide.     

Effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound

The effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound was investigated in static air. The bare TiAl alloy exhibited poor isothermal and cyclic-oxidation resistance at 800–1000°C due to the formation of TiO₂-base scales which tend to spall during cooling. A sputtered Ti-50Al-10Cr coating remarkably improved the oxidation resistance of TiAl, due to the formation of an adherent Al₂O₃ scale at 800–1000°C. After long-term oxidation (at 900°C for 1000 hr), TiAlCr coating still provided excellent protection for the TiAl alloy. Minor interdiffusion occurred due to the inward diffusion of Cr, while no Kirkendall voids were found at the coating/ substrate interface. In contrast, NiCrAlY and CoCrAlY coatings reacted extensively with the TiAl alloys. Moreover, the TiAlCr coating alloy is based on -TiAl and TiAlCr Laves phases, which may offer improved mechanical properties. The TiAlCr coating exhibited a better combination of oxidation resistance and substrate compatibility than conventional aluminide and MCrAlY coatings.     

Properties of alumina-titania coatings prepared by laser-assisted air plasma spraying

Studies have shown that microstructures formed by post-laser remelting of air plasma sprayed coatings exhibit densification but also numerous macrocracks due to the rapid cooling and thermal stresses. In laser-assisted air plasma spraying (LAAPS) process, the laser beam interacts simultaneously with the plasma torch in order to increase the temperature of the coating and possibly remelt the coating at the surface. As a result, the microstructure is partially densified and macrocracks, which are generally produced in the post-laser irradiation treatment, may be inhibited. In this paper, LAAPS was performed to improve the hardness and wear resistance of Al₂O₃-13%TiO₂ coatings. These coatings prepared by air plasma spraying (APS) are widely used to protect components against abrasive wear at low temperatures. The coating microstructure was characterized by SEM and X-ray diffraction. The mechanical characterization was done by hardness measurements, erosive wear tests and abrasion wear tests. Results showed that laser assistance may improve the microstructural and mechanical properties. Phenomena involved in LAAPS of alumina-titania coatings are discussed in this paper.