MCrAlY Alloy Research Articles

MCrAlY coating design based on oxidation-diffusion modelling. Part I: Microstructural evolution

To improve the efficiency of modern gas turbines, it is highly desired to develop durable MCrAlY alloys, to be used as protective coatings against oxidation and corrosion for superalloys which are the base materials for some hot components like turbine blades and vanes. In this paper, an oxidation-diffusion model was used to simulate the diffusion of alloying elements and the corresponding microstructural changes in different superalloy-coating systems at high temperature. Two important processes are considered in this model: oxidation of the coating and interdiffusion between the superalloy and the coating. The model showed an accurate predictability of the diffusion and microstructural evolution in real superalloy-coating diffusion couples studied at high-temperature exposure. The model was further applied to investigate the elemental effects of Ni, Co, Cr and Al on the microstructural evolution, considering the development of two important phases in superalloys and coatings, i.e. FCC-γ′ and BCC-β, at different temperatures. The results in this paper deepen the knowledge of the MCrAlY coating design for superalloy-coating systems in high-temperature applications.

MCrAlY coating design based on oxidation–diffusion modelling. Part II: Lifing aspects

Coatings from MCrAlY-type alloys are commonly used for oxidation and corrosion protection in gas turbines. As coated components are exposed to high temperature, the coating provides oxidation protection by the formation of an alumina scale, thus depleting the coating of Al which, eventually, will cause the coating to fail. The present study deals with MCrAlY alloy design from a lifing perspective. A previously developed coupled oxidation–diffusion model was used to study the influence of coating composition, substrate composition and oxidation temperature on the expected life of MCrAlY coatings. Eight model coatings, covering the wide range of MCrAlY compositions used industrially, and two model substrates, corresponding to a blade material and a combustor material, were evaluated by the oxidation–diffusion model. Three life criteria were tried: 1) β-phase-depletion, 2) critical Al content at the coating surface, and 3) a critical TGO thickness. It was shown that the critical TGO thickness was the most conservative life criterion for high-Al coatings on high-Al substrates. For low-Cr and low-Co coatings, the β-depletion criterion was usually the most conservative. For cases where β-stability was high (such as at low temperatures and for coatings high in Cr, Co and Al) the critical-Al criterion was often the most conservative.

Laser Cladding of MCrAlY Coatings on Stainless Steel

Abstract MCrAlY alloys (where M stands for Ni,Co or both) are commonly used as overlay protective coatings in gas turbine engine components against high temperature oxidation and corrosion. The protective effect of these alloys is due to the formation of a continuous thermally stable oxide layer on the coating surface. In this work several types of MCrAlY alloys, differing in their elemental composition, have been deposited on austenitic stainless steel by means of laser cladding. The microstructure of the coatings have been characterized by SEM-EDSand XRD. As expected, elemental composition compatible with γNi/γ’ Ni3Al,γNi/β-NiAl or γCo/β-(Co,Ni)Al phases are observed in hypoeutectic or hypereutectic microstructures depending on the alloy composition. The high temperature oxidation behavior of the coatings was evaluated by air furnace oxidation tests at 1100 °C for 200 h. The oxidized surface of the samples was examined by SEM-EDS and the oxides present identified by theoretical stoichiometric calculations. Results indicate the formation of a uniform Al 3 O 2 protective oxide scale with NiO,CoO, Y 2 O 3 /YAlO 3 and Cr 2 O 3 oxide inclusions.

Size effect of (Al2O3–Y2O3)/YSZ micro-laminated coating on high-temperature oxidation resistance

The size effect of structure plays an important role in the high-temperature oxidation and spallation resistance of micro-laminated coating on alloy substrate. In this study, (Al2O3–Y2O3)/YSZ micro-laminated coatings with different thickness ratios and layer numbers were prepared on MCrAlY alloy by electrolytic deposition and microwave sintering under pressure. The oxidation and spallation resistance of the micro-laminated coatings were investigated by cyclic oxidation tests in air at 1050°C for 200h. Results indicate that both oxidation resistance and spallation resistance of the coating have been improved significantly, by increasing the thickness ratio of YSZ layer to Al2O3–Y2O3 layer and layer number in a specific range. In such micro-laminated coating, increasing the thickness ratio of YSZ layer to Al2O3–Y2O3 layer can improve the apparent thermal expansion coefficient of coating and decrease the thermal stresses. While, increasing the layer number can extend the crack propagation paths and improve the plasticity and fracture toughness. They can improve the mechanical properties of micro-laminated coating and exhibit excellent cracking and spallation resistance. This would give rise to the beneficial effects on suppressing the oxygen inward diffusion and consequently improve the high-temperature oxidation resistance of micro-laminated coating. Such size effect would provide reference values in the structural design of micro-laminated coating in high-temperature application.

The Effect of Microstructure on the Type II Hot Corrosion of Ni-Base MCrAlY Alloys

Alloys of compositions (in wt%) Ni–31.5Cr–11.5Al–0.6Y and Ni–20.9Co–19.2Cr–12.5Al–0.5Y–0.4Si–0.3Hf were prepared using three different processing routes and exposed to Type II (700 °C) hot corrosion conditions in order to evaluate the influence of microstructural scale on degradation resistance. The alloys were prepared in bulk form by drop casting (DC) and injection casting (IC), and as a coating by low-pressure plasma spraying (LPPS) on a Rene N5 superalloy substrate. The resulting microstructures became coarser in the order LPPS < IC < DC for the NiCoCrAlY, and IC < LPPS < DC for the NiCrAlY. The DC and LPPS NiCrAlY displayed very good resistance to Type II hot corrosion; however, rapid Type II attack was observed when the alloy was prepared by the IC process. The LPPS and IC NiCoCrAlY formed more protective scales than the DC NiCoCrAlY, i.e., Type II hot corrosion resistance increased with microstructural refinement. Pre-oxidation of a cast NiCoCrAlY alloy greatly reduced the amount of Type II attack for the exposure times studied. The localized attack of the pre-oxidized specimens was found to initiate at reactive-element-rich regions in the pre-formed scale.

Influence of Al2O3/YSZ micro-laminated coatings on high temperature oxidation and spallation resistance of MCrAlY alloys

Al2O3/YSZ micro-laminated coatings with different layers were prepared on MCrAlY alloys by magnetron sputtering and characterized by high-resolution field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). Results indicated that the laminated structures of Al2O3 and YSZ layers were observed with compact microstructure and the thickness at sub-micron level each layer. High-temperature cyclic oxidation test at 1000°C in air was performed to investigate the oxidation and spallation resistance of the coatings on MCrAlY substrates. Result shows that the coatings exhibit more excellent oxidation and spallation resistance with the increase of the layers, which can be attributed to the increase of stress tolerance and fracture toughness in the laminated coatings by the thinner layers and crack deflection toughening.

INFLUENCE OF Re ON THE PHASE CONSTITUENT OF A NiCoCrAlY COATING ALLOY

MCrAlY alloy has served as overlay coatings or bond coats in thermal barrier coating systems in gas turbine engines,and its phase constituents play a vital role in determining the performance of these coating systems.In order to further understand the influence of Re on the phase constituents of MCrAlY coatings,the phase evolution of a Ni-20Cr-10Al-20Co coating alloy with 0- 9%(mass fraction) Re addition was predicted the Thermo-Calc thermodynamic software and TTNi7 Ni-based superalloy database.Based on the calculation results,the four NiCoCrA1Y alloys with different levels of Re(0,3%,6%and 9%) were prepared,and their phase constituents were experimentally investigated in the temperature range of 800—1250℃.Both the calculation result and the experimental microstructural observation indicated that the addition of Re slightly increased the fraction ofβ-NiAl phase,but dramatically enhanced the amounts ofσandα-Cr phases.Based on the combined analysis of microstructural observation and EPMA composition identification,it was proposed that the increased amounts ofσandα-Cr phases should result from the changed partition of Cr in all phases caused by Re addition.Theσphase is a low temperature stable phase,and would change toα-Cr phase as temperature rises.The transformation temperature ofσ→α-Cr phase increases as the addition of Re increases. The experimental results also showed that theγ'-Ni_3Al→γ-Ni transformation temperature decreases with the addition of Re.In addition,the calculation results were compared with the experimental results and were found to be in reasonable agreement with the experimental observations from the aspect of the MCrAlY phase evolution.Some deviations of the calculation and experiment results about the precise phase-change temperatures,such asγ'-Ni_3Al→γ-Ni andσ→α-Cr phase transformation temperatures,were observed,and the reason were discussed in the light of both limited availability of thermodynamic database and experimental problems.

Preparation of Al2O3–ZrO2–Y2O3 Composite Coatings by a Modified Sol–Gel Technique for Thermal Barrier Application

Spatial-network Al2O3–ZrO2–Y2O3 composite coatings were prepared by a modified sol–gel technique, so-called thermal pressure and filtration of sol–gel paint. The composite coatings were derived from a composite paint of yttria partially stabilized zirconia (YSZ) particles, Al2O3 particles and Al2O3–Y2O3 sol. Their microstructure showed that YSZ particles were covered with spatial-network Al2O3–Y2O3 blanket. Cyclic oxidation at 1,050 °C in air for 200 h demonstrates that the oxygen diffusion rate in the coatings could be effectively inhibited. Meanwhile, suitable coefficients of thermal expansion (CTE) gave the composite coatings better spallation resistance than that of Al2O3–Y2O3 or ZrO2–Y2O3 coatings. The positive results of cyclic oxidation indicated that the composite coating can be used as an interlayer between the bond coat and the top ceramic layer in traditional TBCs. Not only the depletion rate of aluminum-rich phase in MCrAlY alloy could be slowed down by spatial-network Al2O3–Y2O3, but also different thermal expansion between thermally grown oxides layer and top layer could be relieved by suitable CTE. In this paper, the mechanisms of the inhibition of oxygen diffusion and thermal match between ceramic coating and alloy are also discussed.

Influence of Co addition on constituent phases and performance of a NiCrAlYRe alloy system

MCrAlY alloys can serve as stand-alone overlay coatings or bond coats in thermal barrier coating systems, and their phase constituents play a vital role in determining their performance. In the present investigation, the influence of Co addition on the constituent phases of a NiCrAlY alloy system containing 3 wt.% Re in the temperature range of 800 °C–1200 °C was predicted through thermodynamic modeling. Based on the modeling results, four NiCrAlY alloy systems with different levels of Co (0, 10 wt.%, 20 wt.% and 30 wt.%) were prepared and evaluated experimentally. Scanning electron microscopy and transmission electron microscopy results show that the additions of Co increase the fractions of γ-Ni and β-NiAl at the expense of γ′-Ni 3Al and α-Cr, and thus change the depletion scheme from β → γ′ → γ to β → γ during 1100 °C oxidation.

Self-organized nanostructuring of composite coatings at high temperatures for drag reduction and self-cleaning

Coated and nanostructured surfaces gain much importance for improving the efficiency of high temperature applications (e.g. in turbines). By embedding ceramic particles with a negative thermal expansion coefficient (NTEC) into a metallic matrix, a reversible thermal activation of a nanostructured surface can be established. At high temperatures a defined drag reducing surface microstructure (“shark-skin”) is formed in the coating surface, while at low temperatures a self-cleaning effect (in idle periods) is achieved by the reversal of the thermal deformation. A feedstock powder produced by high energy milling and consisting of nanocrystalline yttrium oxide and tungsten oxide particles embedded into a conventional MCrAlY alloy was used for the investigations. By using different thermal spray and cladding techniques the powder is deposited on steel- substrates. In the next step, the coating is implanted with yttrium or xenon in order to induce the formation of Y 2W 3O 12. The latter is a ceramic with a strong negative thermal expansion coefficient and is stable up to temperatures above 1373 K. The effects during annealing and the morphology changes are analyzed in detail. Results of the phase transformation, the surface micro-morphology and the microstructured properties of these high-temperature coatings are presented.

Preparation of YSZ/Al 2O 3 micro-laminated coatings and their influence on the oxidation and spallation resistance of MCrAlY alloys

YSZ/Al 2O 3 micro-laminated coatings were successfully prepared on the surface of MCrAlY substrates by means of electrolytic deposition and microwave sintering. The as-prepared YSZ/Al 2O 3 coatings were characterized by high-resolution field emission SEM and XRD. Laminated structures of alternate YSZ and Al 2O 3 layers were observed in the coating with the phase composition of Y 2O 3 stabilized t-ZrO 2, α-Al 2O 3 and θ-Al 2O 3. High-temperature cyclic oxidation test at 1000 °C in air was also performed to investigate the oxidation and spallation resistance of such coatings on MCrAlY substrates. The results indicate that such coatings exhibit not only excellent oxidation resistance but also good spallation resistance under thermal cycling due to the structure of multi-sealed Al 2O 3 layers and the preferable high-temperature mechanical properties induced by the designed laminated composite structures, respectively.

Oxidation Control with Chromate Pretreatment of MCrAlY Unmelted Particle and Bond Coat in Thermal Barrier Systems

MCrAlY alloy bond coat is widely used in thermal barrier coating (TBC) systems to protect substrates from high-temperature oxidizing environments. However, failure of the ceramic topcoat can occur due to a thermally grown oxide (TGO) that grows at the interface between the bond coat and the topcoat. In this study, the effect of chromate treatment was investigated. Prior to topcoat deposition, a thin film of Cr2O3 was formed on the bond coat surface. High-temperature oxidation tests were carried out, and the oxidation rates were determined by inspection of cross sections. Similar oxidation tests were carried out using MCrAlY powder material assumed to be unmelted particles. As a result, the chromate-treated bond coat showed outstanding oxidation resistance. Calculations that take into account the oxidation of particles in the topcoat indicated the generation of internal stress to cause local fracture of the topcoat.

Effect of Bond Coat Materials on Thermal Fatigue Failure of EB-PVD Thermal Barrier Coatings

Effect of MCrAlY bond coat alloy systems on thermal fatigue failure of thermal barrier coatings (TBCs) was investigated, where the TBC specimen consisted of Ni-based superalloy IN738LC substrate, bond coat, and 8 wt.% Y2O3-stabilized ZrO2 (YSZ) top coat. The top coat was fabricated by EB-PVD method with 250 μm in thickness. Three kinds of MCrAlY alloys were studied as the bond coat material. Employing the originally developed test equipment, thermal fatigue tests were carried out, by applying thermal cycles between 400 and 950°C in air. Special attention was paid not only to the failure life of the TBC specimen, but also the underlying failure mechanisms. The experimental results clearly demonstrated that the effect of MCrAlY bond coat alloys on the thermal fatigue life was very significant. Some discussions were made on the experimental results based on the measurements of mechanical and metallurgical properties of the bond coat alloys: i.e., elastic stiffness, thermal expansion coefficient and high temperature oxidation resistance.

遮熱コーティングの酸化挙動に及ぼすボンドコート表面処理およびボンドコート中のイットリウムの影響

This paper focuses on the oxide layer formed at bondcoat/topcoat interface of thermal barrier coatings (TBCs), which grows faster in comparison with the oxide layer formed on overlay coatings, although both bondcoat and overlay coating are made of MCrAlY alloy (M is Ni or Co, or combination of Ni and Co). The authors clarify the influence of ① the surface treatment (shot peening) of bondcoat and ② yttrium in bondcoat on the oxidation behavior of the thermal barrier coatings. Oxidation tests were carried out at 950°C in air by means of TBC specimens. The specimen consists of Ni-base superalloy (Inconel738LC), CoNiCrAlY bondcoat and ZrO2-Y2O3 topcoat. In the TBC specimen with as-sprayed bondcoat, the oxide layer consisted of “wart-like oxide” and “continuous alumina layer”. The wart-like oxide is caused due to un-melted particles of CoNiCrAlY alloy that remained at the bondcoat/topcoat interface in spraying process. The surface treatment of bondcoat prevented the formation of wart-like oxide because the unmelted particles were removed. In the TBC specimen with as-sprayed bondcoat, yttrium-rich oxide particles were included in the continuous alumina layer. The surface treatment of bondcoat and/or the removal of yttrium from the chemical composition of bondcoat in TBC specimen prevented the formation of the yttrium-rich oxide particles in the continuous oxide layer, and as a result, the growth of continuous alumina layer was decreased. There is a possibility that the application of the bondcoat without yttrium restrains the growth of the continuous oxide layer in TBC.

Dilatometer studies of NiCrAlY coatings

The purpose of this work was to determine if lower thermal expansion bondcoat MCrAlY alloys could be developed for thermal barrier coating (TBC) systems. NiCrAlY coatings applied by plasma spray deposition were studied in the dilatometer for first-cycle sintering and in the thermally stabilized state for thermal expansion. Several different compositions were created and tested. A new composition was developed that has both low thermal expansion and avoids the expansion upsweep at 950 °C, common in current NiCrAlY coatings. Phase analysis of selected coatings was done to explain the control of thermal expansion. Comparisons were made to previous thermal expansion values of other MCrAlY coatings.

Study of phase states and oxidation of B2-structure based Al–Ni–Ru–M alloys

Phase states and oxidation resistance of B2-structure based Al–Ni–Ru–M alloys have been investigated on equilibrium alloys and by the diffusion couple approach. The isothermal cross-sections of the Al–Ni–Ru phase diagram have been partially defined in a range of Al<70 at.% at 900 and 1100 °C. The existence of a miscibility gap between AlRu (β1) and AlNi (β2) phases in the Al–Ni–Ru ternary system has been clearly established. The following phase equilibria have been experimentally identified: β1-AlNi 3, β1–β2, β2-Al 2Ru, Al 2Ru–AlNi 3–Al 13Ru 4 at 900 °C and β1–β2, β2-Al 2Ru at 1100 °C. The high temperature properties of B2-structure based Al–Ni–Ru–M alloys are presented. All the studied samples display high melting points up to more than 2100 °C. Oxidation of Al–Ni–Ru–M B2 alloys in laboratory air has been found to follow parabolic law at 950 and 1050 °C and it is slower than that of a commercial MCrAlY alloy. There is no remarkable difference of oxidation mechanisms of the B2 structure based Al–Ni–Ru–M alloys at 950 and 1050 °C. Protective α-Al 2O 3 scale develops already after 100–150 h of oxidation. The protective α-Al 2O 3 scale (i) is mono-phased (α-Al 2O 3), (ii) is dense, (iii) does not contain remarkable amount of Ni and Ru and (iv) has good adhesion to bulk alloy. A dense continuous Ru underlayer forms between the α-Al 2O 3 film and the bulk alloy for compositions with Ru>25 at.%. The Ru layer can contain considerable amounts of Cr or Ir and exhibits good adhesion to both the scale and the bulk alloy. Physical properties of the B2 structure based Al–Ni–Ru–M alloys make them good candidates for oxidation resistant coatings in different high temperature industrial applications.

Comparison of thermal expansion and oxidation behavior of various high-temperature coating materials and superalloys

The thermal expansion mismatch between a metallic substrate and its external oxide scale generates a strain on cooling that is a primary cause of spallation of protective oxide scales. This study compares thermal expansion behavior and cyclic oxidation performance of the two major composition classes of high-temperature commercial coatings for protection of single-crystal superalloys. The thermal expansion of cast MCrAlY (M = Ni and/or Co) alloys and cast aluminides (NiAl, (Ni,Pt)Al and Ni3Al) was measured at temperatures up to 1300°C and compared to that of a single-crystal Ni-base superalloy. The tendency for scale spallation from each alloy was evaluated by cyclic oxidation testing at 1150°C. The coefficients of thermal expansion for the aluminides were lower than those of the MCrAlY-based alloys at all temperatures and scale adherence to the Hf-doped aluminides was generally superior. Scale adherence to the various compositions of MCrAlY-type alloys did not directly correlate to their thermal expansion behavior or substrate strength. For both types of materials, the presence of a reactive element (Y,Hf, etc.) had no detectable effect on thermal expansion but a major effect on scale adherence. There was no obvious influence of Al content on the thermal expansion of β phase Ni–Al compositions. The addition of Pt resulted in a lower average thermal expansion for hyperstoichiometric (Ni,Pt)Al at temperatures above 930°C, but this effect was not observed in hypostoichiometric (Ni,Pt)Al.

Comparison of thermal expansion and oxidation behavior of various high-temperature coating materials and superalloys

The thermal expansion mismatch between a metallic substrate and its external oxide scale generates a strain on cooling that is a primary cause of spallation of protective oxide scales. This study compares thermal expansion behavior and cyclic oxidation performance of the two major composition classes of high-temperature commercial coatings for protection of single-crystal superalloys. The thermal expansion of cast MCrAlY (M = Ni and/or Co) alloys and cast aluminides (NiAl, (Ni,Pt)Al and Ni3Al) was measured at temperatures up to 1300°C and compared to that of a single-crystal Ni-base superalloy. The tendency for scale spallation from each alloy was evaluated by cyclic oxidation testing at 1150°C. The coefficients of thermal expansion for the aluminides were lower than those of the MCrAlY-based alloys at all temperatures and scale adherence to the Hf-doped aluminides was generally superior. Scale adherence to the various compositions of MCrAlY-type alloys did not directly correlate to their thermal expansion behavior or substrate strength. For both types of materials, the presence of a reactive element (Y,Hf, etc.) had no detectable effect on thermal expansion but a major effect on scale adherence. There was no obvious influence of Al content on the thermal expansion of β phase Ni–Al compositions. The addition of Pt resulted in a lower average thermal expansion for hyperstoichiometric (Ni,Pt)Al at temperatures above 930°C, but this effect was not observed in hypostoichiometric (Ni,Pt)Al.

MCrAlY (M: Ni, Co) ボンドコート上の未溶融粒子の酸化現象とトップコートの力学的挙動に対する影響

A MCrAlY alloy bond coat is widely used in thermal barrier coating (TBC) systems to protect substrates from high-temperature oxidizing environments. However, failure of the ceramic top coat can occur due to a thermally grown oxide (TGO) layer that grows at the interface between the bond coat and the top coat. Local stresses are produced by multiple oxides that grow separately at the interface. It seems that the multiple oxides may originate in the rapid oxidation of unmelted MCrAlY particles attached to the bond coat during spraying. Thus, it has become an important issue to control the growth behavior of TGOs.In the present study, the effect of chromate treatment was investigated. Prior to top coat deposition, a thin film of Cr2O3 was formed on the bond coat surface. High-temperature oxidation tests were carried out using samples coated both with and without chromate pretreatment, and the oxidation rates were determined by inspection of cross sections. Similar oxidation tests were carried out using MCrAlY powder material assumed to be unmelted particles. Chromate-treated bond coat showed outstanding oxidation resistance in comparison with bond coat without chromate pretreatment. In the case of pretreated MCrAlY powder, only the surfaces of particles were oxidized, even under conditions in which most particles without pretreatment were fully oxidized. This outcome is thought to result from the preferential formation of an oxidation-resistant Al2O3 layer in the early stage of oxidation. Calculations that take into account the oxidation of particles in the top coat indicate the generation of sufficient internal tensile stress to cause local fracture of the top coat. Because of its simplicity and applicability to complicated shapes, the chromate treatment is quite useful as a countermeasure against bond-coat oxidation and local fracture of the top coat near the interface.

A Comparative Study of High Velocity Oxygen Fuel, Vacuum Plasma Spray, and Axial Plasma Spray for the Deposition of CoNiCrAlY Bond Coat Alloy

In the aerospace field as well as in the stationary gas turbine field, thermally sprayed coatings are used to improve the surface properties of nickel-super-alloys materials. Coatings are commonly used as bond coat and antioxidation materials (mainly MCrAlY alloys) and as thermal barrier coatings (mainly yttria partially stabilized zirconia). The purpose of the current study was to assess the properties of thermally sprayed bond coat CoNiCrAlY alloys comparing the performance of three different techniques: vacuum plasma spray (VPS), high velocity oxygen fuel (HVOF), and axial plasma spray (AxPS). The quality of the deposited films has been assessed and compared from the point of view of microstructural (porosity, oxide concentration, unmelted particles presence) and mechanical (hardness) characteristics. The surface composition and morphology of the coatings were also determined. Specific efficiency tests were performed for the three examined technologies. The highest quality coatings are obtained by VPS, but also high velocity oxygen fuel and AxPS sprayed films have interesting properties, which can make their use interesting for some applications.