Li Xinhai - Academia.edu (original) (raw)

Papers by Li Xinhai

MRS Proceedings, 2006

Toughening effect of ZrO2-particles on MoSi2-based materials is one of the important toughening m... more Toughening effect of ZrO2-particles on MoSi2-based materials is one of the important toughening mechanism. In this work, the influence of particle size and volume percentage of unstabilized ZrO2-addition on toughening in MoSi2-matrix composites was studied. The measured data revealed that the fine particle size, < 1 ìm, with certain volume percentages around 20% have given more effective toughening results. The tested materials were prepared by two different sintering processes: pressure-less sintering (PLS) and PLS + Hipping sintering. The results on sintered density, RT-hardness and RT-toughness indicated that the PLS process could be a practical and economical method for producing MOSi2-ZrO2 composites in industry. Oxidation behavior of MoSi2-ZrO2 composites was also studied in the work. Clearly, the ZrO2-addition made the composites having a worse oxidation resistance than the monolithic MoSi2, which means that a low ZrO2-addition should be used in the composites, as far as a...

Fracture of Nano and Engineering Materials and Structures

Ceramic thermal barrier coatings are commonly used in gas turbine hot components (e.g., combustor... more Ceramic thermal barrier coatings are commonly used in gas turbine hot components (e.g., combustor liners/buckets and guide vane platforms). In components that are only partially coated or have cooling-air outlets, coating-end stress singularities may lead to the spallation of the coating. Depending on the geometry of the transition from coated to uncoated material, the severity of the stress singularity will vary. One way of decreasing the severity of the stress singularity is by introducing a chamfer angle φ < 90° at the coating end. In the present study, a thin thermal barrier coating system has been studied. Bondand top coats have been sprayed to a thickness of 150µm and 350µm, respectively. Vacuum-plasma-spraying technology was used, and the test specimens were rectangular (30x50x5mm) coupons of a nickel-based superalloy, Haynes 230. A NiCrAlSiY bond coat and an YB 2 B OB 3 B partially stabilised ZrOB 2 B top coat were used. In order to achieve well-defined chamfers, sprayed coupons were ground on the edges with SiC grinding paper to desired geometry. By inspections of cross-sections that had not undergone thermal fatigue cycling, it was ensured that no damage was introduced into the system. Mechanical testing was done in a thermal cyclic test rig where specimens are heated in a furnace and cooled with compressed air. FE modelling of the system has been done, aiming to support the findings from thermal fatigue tests. A parametric study including variation of the chamfer angle φ has been made and the stress state near the chamfer evaluated. Evaluation of fatigue damage can be done visually for observation of coating failure (macroscopic observation on coating surface). 20% area with complete spallation was considered as thermal barrier coating failure. For evaluation of damage development, additional light microscopy investigations of cross-sections have been carried out. Results show that the fatigue life benefits from introduction of a chamfer angle at the coating end during thermal fatigue cycling.

MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been widely used to protect hot com... more MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been widely used to protect hot components in gas turbines against oxidation and hot corrosion at high temperatures. Understanding the influence of the X-elements on oxidation behaviour is important in the design of durable MCrAlX coatings. In this study, NiCoCrAlX coatings doped with Y+Ru and Ce, respectively, were deposited on Inconel-792 substrate by HVOF. The samples were subjected to isothermal oxidation test in laboratory air at 9000, 1000, 1100 ºC and cyclic oxidation test between 100 ºC and 1100…

MATEC Web of Conferences, 2014

In turbine engines, high temperature components made of superalloys may crack in a creep process ... more In turbine engines, high temperature components made of superalloys may crack in a creep process during service. With the inward flux of the gases, e.g. oxygen and nitrogen, along those cracks, the microstructure of the superalloy substrate nearby the cracks may degrade by internal oxidation and nitridation. The aim of this study is to investigate and simulate the oxidation-nitridation-induced microstructural degradation in superalloys by taking a variant of Ni-based superalloy IN-792 as a sample. After the creep testing of the superalloy in air, the microstructures on the cross section of the superalloy were analysed in a scanning electron microscope, equipped with energy/wavelength dispersive systems. Internal oxidation and nitridation, presenting by Al/Ti oxides and nitrides, were observed under a porous and even cracked Cr-oxide scale which was formed on the superalloy surface or along the creep cracks connecting the superalloy surface. Meanwhile, the reinforcing γ precipitates were depleted. Such oxidation-nitridation-induced microstructural degradation was simulated by using an oxidation-diffusion model, focusing the diffusion of the alloying elements in metallic phases of the superalloy.

Volume 4: Ceramics; Concentrating Solar Power Plants; Controls, Diagnostics and Instrumentation; Education; Electric Power; Fans and Blowers, 2013

Gas turbine efficiency can be improved with tighter turbine tip clearances. An approach being dev... more Gas turbine efficiency can be improved with tighter turbine tip clearances. An approach being developed by engine manufacturers deploys active tip clearance monitoring where the turbine casing diameter is actively controlled in-service either mechanically or thermally. Typically current engines operate at about 1% clearance of blade span. With active control this could potentially be reduced significantly. Ideally active tip clearance control requires closed loop feedback measurements to maintain very small clearances without the risk of blade tip contact with the casing liner. Therefore reliable and robust sensors systems are required that can operate at the elevated temperatures found in modern gas turbines. Currently there are limited sensor systems available that can operate at these temperatures and survive typical sensor life requirements of many thousands of hours. This study details development of a high temperature eddy current sensor system for hot section applications. Th...

Diffusion coatings are widely used to increase oxidation and corrosion resistance of hot superall... more Diffusion coatings are widely used to increase oxidation and corrosion resistance of hot superalloy components for gas turbines. The aim of this study is to investigate the effect of coatings (NiAl and PtAl) on the creep fracture mechanism of samples with a substrate of IN792. The samples have been creep tested at two temperatures (850 °C and 950 °C) and different applied tensile stresses, until failure between 205 and 21000 hrs. The observation of cross-sections by SEM shows that the microstructural evolution in the coating is dependent on the diffusion of alloying elements in the sample. Furthermore the time and temperature induced growth of the coating is found to be controlled only by inward diffusion of Al. Grain-boundary cracking is the basic fracture mode in the substrate in all samples irrespective if the crack is initiated from coating or substrate. The analysis of microstructure shows that the diffusion coatings display two types of mechanical behavior-being easily plasticized or cracked-dependent on temperature and type of coating, and therefore can be considered as non-load carrying regions. After recalculating the creep stress to exclude the final effective coating thickness from the total sample thickness, the coated samples showed similar creep rupture behavior as the uncoated samples in the Larson-Miller diagram.

Thermal barrier coatings (TBCs) are used in gas turbines to provide insulation against high tempe... more Thermal barrier coatings (TBCs) are used in gas turbines to provide insulation against high temperature and to provide oxidation and corrosion resistance for the superalloys on which they are deposited. TBCs are deposited on hot parts in the combustor and on the turbine blades, and must consequently be compatible with the various superalloys used there. The influence of substrate material on the durability of TBCs has therefore been studied. Air plasma sprayed TBCs have been deposited on Hastelloy X and Haynes 230, which are alloys used in the combustor. The TBC systems have been thermally cycled until failure and their fracture surfaces have been studied. The thermally grown oxides and the substrate/coating interdiffusion have also been analysed by energy dispersive spectroscopy. The fatigue life, fracture mechanism and the oxide composition and kinetics were similar for the two TBC systems; however, one of the TBC systems is thought to have failed prematurely.

The present paper describes the work done in order to establish a correlation between observation... more The present paper describes the work done in order to establish a correlation between observations of thermal barrier coating (TBC) damage in an air-plasma sprayed TBC system and modelling of the delamination and spallation process that over time will degrade the TBC system. By inspections of cross-sections of TBC's on ex-serviced gas turbine components, it will be possible to establish preferred crack paths, and, verify that assumptions made during the modelling and experimental verification stages will be valid for different load / temperature combinations. Previously, a crack growth approach has been adopted for TBC delamination and spallation. This approach assumes that the top coat (TC) / bond coat (BC) interface is the region of the TBC that will develop a delamination crack network. In the present work it has been shown that the assumption of crack growth along the TC / TGO interface is valid unless the TGO growth rate is disturbed by formation of large mixed oxide clusters. The same is true for in-plane stresses, where a typical component load situation will cause interface spallation at the same oxide/oxide interface as was observed in the case of cyclic temperature load. In areas where high tensile strains are introduced, the coating will not delaminate and spall. Instead through-thickness cracks are introduced, i.e. TGO upper limit and maximum tensile strain criteria need to be adopted in addition to the Paris law approach.

Surface and Coatings Technology, 2015

MCrAlY coatings are widely used to protect superalloys against oxidation and corrosion at high te... more MCrAlY coatings are widely used to protect superalloys against oxidation and corrosion at high temperature in gas turbine engines. To design a durable MCrAlY coating, the behaviour of alloying elements in coating-superalloy system needs to be better understood. After oxidation tests in temperature range between 900 °C to 1100 °C, the development of the microstructures in the samples was analysed at the coating surface or in areas near the coatingsuperalloy interface. Some interdiffusion simulations were also done to model the diffusion behaviour of alloying elements in different coating-superalloy couples. The results show that both oxidation at the coatings' surface and the elements' diffusion inside of the materials were temperature-and chemical-composition dependent. The behaviour of some minor elements like Y, Hf, Ru and Ir in the oxidation processes was particularly studied by tracking their position and composition in the materials. Abbreviations high-velocity oxy-fuel (HVOF) coating-substrate interface (CSI) inner-β-depletion zone (IBDZ) γ'-depletion zone (GPDZ) secondary β phase (SBP)

Surface and Coatings Technology, 2015

MCrAlY coatings can be corroded due to the basic fluxing (type-I hot corrosion) of the supposeto-... more MCrAlY coatings can be corroded due to the basic fluxing (type-I hot corrosion) of the supposeto-be protective alumina scale in a molten sulphate environment. In this study, two MCrAlY coatings, coating A (10 wt.% Al, 20 wt.% Cr) and coating B (7 wt.% Al, 28 wt.% Cr), were tested in 48-hour cycles at 900 °C in a sodium-potassium sulphate environment with and without SO 2 (500 ppm). The aim was to study the effect of SO 2 at the typical type-I-hot-corrosion temperature-900 °C. The results showed that the corrosion behavior of the MCrAlY coatings depended not only on the coating composition but also on the corrosion environment. It was found that in coating A alumina scale was more resistant in the sulphate-plus-SO 2 condition than that in the sulphate-only condition. Such phenomenon indicated a beneficial effect of SO 2. On Coating B, however, mixed oxide layers, consisting of alumina and other oxides and sulphides, formed after a certain number of cycles in the sulphate environments with or without SO 2 gas. In this coating, the addition of SO 2 in the sulphate environment promoted the formation of non-dense chromia oxides which may form from oxidation of Cr-sulfides.

Surface and Coatings Technology, 2013

Thermal barrier coatings (TBCs) are used in gas turbines to prolong the life of the underlying su... more Thermal barrier coatings (TBCs) are used in gas turbines to prolong the life of the underlying substrates and to increase the efficiency of the turbines by enabling higher combustion temperatures. TBCs may fail during service due to thermal fatigue or through the formation of non-protective thermally grown oxides (TGOs). This study compares two atmospheric plasma sprayed (APS) TBC systems comprising of two identical TBCs deposited on two different substrates (Haynes 230 and Hastelloy X). The thermal fatigue life was found to differ between the two TBC systems. The interdiffusion of substrate elements into the coating was more pronounced in the TBC system with shorter life, however, very few of the substrate elements (only Mn and to some extent Fe) formed oxides in the bond coat/top coat interface. Fractography revealed no differences in the fracture behaviour of the TBCs; the fracture occurred, in both cases, to about 60 % in the top coat close to the interface and the remainder in the interface. Nanoindentation revealed only small differences in mechanical properties between the TBC systems and a finite element crack growth analysis showed that such small differences did not cause any significant change in the crack driving force. The oxidation kinetics was found to be similar for both TBC systems for the formation

Surface and Coatings Technology, 2014

ABSTRACT The fracture surfaces from adhesion tested thermal barrier coatings (TBC) have been stud... more ABSTRACT The fracture surfaces from adhesion tested thermal barrier coatings (TBC) have been studied by scanning electron microscopy. The adhesion test have been made using the standard method described in ASTM 633, which makes use of a tensile test machine to measure the adhesion. The studied specimens consist of air plasma sprayed (APS) TBC deposited on disc-shaped substrates of Hastelloy X. The bond coat (BC) is of NiCoCrAlY type and the top coat (TC) consists of yttria partially-stabilised zirconia. Before the adhesion test, the specimens were subjected to three different heat treatments: 1) isothermal oxidation 2) thermal cycling fatigue (TCF) and 3) burner rig test (BRT). The fracture surfaces of the adhesion tested specimens were characterised. A difference in fracture mechanism was found for the different heat treatments. Isothermal oxidation gave fracture mainly in the top coat while the two cyclic heat treatments gave an increasing amount of BC/TC interface fracture with increasing number of cycles. Some differences could also be seen between the specimens subjected to burner rig test and furnace cycling.

Surface and Coatings Technology, 2011

The adhesion of thermal barrier coatings (TBC) has been studied using the standard method describ... more The adhesion of thermal barrier coatings (TBC) has been studied using the standard method described in ASTM C633, which makes use of a tensile test machine to measure the adhesion. The studied specimens consist of air plasma sprayed (APS) TBC deposited on disc-shaped substrate coupons of Ni-base alloy Hastelloy X. The bond coat (BC) is of a NiCoCrAlY type and the top coat (TC) consists of yttria-stabilised-zirconia. Before the adhesion test, the specimens were subjected to three different heat treatments: 1) isothermal oxidation at 1100 up to 290 h, 2) thermal cycling fatigue (TCF) at 1100 up to 300 cycles and 3) thermal shock at ∼ 1140 BC/TC interface temperature up to 1150 cycles. The adhesion of the specimens is reported and accompanied by a microstructural study of the BC and the thermally grown oxides (TGO), as well as a discussion on the influence of BC/TC interfacial damage on adhesion properties of TBC. The adhesion was

Procedia Engineering, 2011

Thermal barrier coatings (TBC) are used in gas turbines to protect metallic components from high ... more Thermal barrier coatings (TBC) are used in gas turbines to protect metallic components from high temperature. In the present study adhesion tests have been conducted on APS TBC coated specimens subjected to different heat treatments. Isothermal and cyclic heat treatments have been conducted at temperatures around 1100 °C and the adhesion have been tested using the method described in ASTM C633. The fracture surfaces resulting from the adhesion test have been investigated and the fracture behavior has been characterized. A difference in fracture mechanism between the three heat treatments has been found. The two cyclic heat treatments give fracture in the top coat/bond coat interface while isothermal heat treatment gives fracture in the top coat.

Surface and Coatings Technology, 2013

Thermal barrier coatings (TBCs), when used in gas turbines, may fail through thermal fatigue, cau... more Thermal barrier coatings (TBCs), when used in gas turbines, may fail through thermal fatigue, causing the ceramic top coat to spall off the metallic bond coat. The life prediction of TBCs often involves finite element modelling of the stress field close to the bond coat/top coat interface and thus relies on accurate modelling of the interface. The present research studies the influence of bond coat/top coat interface roughness on the thermal fatigue life of plasma sprayed TBCs. By using different spraying parameters, specimens with varying interface roughness were obtained. During thermal cycling it was found that higher interface roughness promoted longer thermal fatigue life. The interfaces were characterised by roughness parameters, such as Ra, Rq and R∆q, as well as by autocorrelation, material ratio curves and slope distribution. The variation of spray parameters was found to affect amplitude parameters, such as Ra, but not spacing parameters, such as RSm. Three different interface geometries were tried for finite element crack growth simulation: cosine, ellipse and triangular shape. The cosine model was found to be an appropriate interface model and a procedure for obtaining the necessary parameters, amplitude and wavelength, was suggested. The positive effect of high roughness on life was suggested to be due to a shift from predominantly interface failure, for low roughness, to predominantly top coat failure, for high roughness.

Thermal barrier coatings (TBC) are coating system comprising a heat insulating ceramic coating de... more Thermal barrier coatings (TBC) are coating system comprising a heat insulating ceramic coating deposited on top of a oxidation resistant metallic coating. During thermal cycling, cracks grow in the metal/ceramic interface which causes the coating system to fail. In order to model such crack growth by finite element (FE) modelling, accurate interface models must be developed. The presented research studies the influence of interface roughness on the thermal fatigue life of TBCs, and suggests how interface models can be derived from interface roughness parameters. High interface roughness was found to promote longer fatigue lives. Interface models were derived from the roughness parameters Rc, Ra, RSm and Rdq and used for FE modelling of crack growth in the interface. The calculated stress intensities, KI and KII, increased with increasing interface roughness and thereby did not predict a slower crack growth with higher interface roughness as was observed experimentally.

Journal of Inorganic Materials, 2009

ABSTRACT

Surface and Coatings Technology, 2013

ABSTRACT MCrAlY coatings are deposited onto superalloys to provide oxidation and corrosion protec... more ABSTRACT MCrAlY coatings are deposited onto superalloys to provide oxidation and corrosion protection at high temperature by the formation of a thermally grown oxide scale. In this project, the oxidation behavior of a HVOF CoNiCrAlYSi coating on IN792 was studied for both isothermal oxidation (900, 1000 and 1100 degrees C) and thermal cycling (100-1100 degrees C). The microstructural evolution of the CoNiCrAlYSi coatings after oxidation was investigated. It was found that the Al-rich beta phase is gradually consumed due to two effects: surface oxidation and coating-substrate interdiffusion. Some voids and oxides along the coating-substrate interface, or inside the coating, were considered to play a role in blocking the diffusion of alloying elements. Based on the microstructural observation, an oxidation-diffusion model, considering both surface oxidation and coating-substrate interdiffusion, was developed by using Matlab and DICTRA software to predict the useful life of MCrAlY coatings. A new concept of diffusion blocking is also introduced into the model to give more accurate description on the microstructural evolution. The results from modeling and the experimentally established composition profiles showed good agreement.

Biodiversity Science, 2014

MRS Proceedings, 2006

Toughening effect of ZrO2-particles on MoSi2-based materials is one of the important toughening m... more Toughening effect of ZrO2-particles on MoSi2-based materials is one of the important toughening mechanism. In this work, the influence of particle size and volume percentage of unstabilized ZrO2-addition on toughening in MoSi2-matrix composites was studied. The measured data revealed that the fine particle size, < 1 ìm, with certain volume percentages around 20% have given more effective toughening results. The tested materials were prepared by two different sintering processes: pressure-less sintering (PLS) and PLS + Hipping sintering. The results on sintered density, RT-hardness and RT-toughness indicated that the PLS process could be a practical and economical method for producing MOSi2-ZrO2 composites in industry. Oxidation behavior of MoSi2-ZrO2 composites was also studied in the work. Clearly, the ZrO2-addition made the composites having a worse oxidation resistance than the monolithic MoSi2, which means that a low ZrO2-addition should be used in the composites, as far as a...

Fracture of Nano and Engineering Materials and Structures

Ceramic thermal barrier coatings are commonly used in gas turbine hot components (e.g., combustor... more Ceramic thermal barrier coatings are commonly used in gas turbine hot components (e.g., combustor liners/buckets and guide vane platforms). In components that are only partially coated or have cooling-air outlets, coating-end stress singularities may lead to the spallation of the coating. Depending on the geometry of the transition from coated to uncoated material, the severity of the stress singularity will vary. One way of decreasing the severity of the stress singularity is by introducing a chamfer angle φ < 90° at the coating end. In the present study, a thin thermal barrier coating system has been studied. Bondand top coats have been sprayed to a thickness of 150µm and 350µm, respectively. Vacuum-plasma-spraying technology was used, and the test specimens were rectangular (30x50x5mm) coupons of a nickel-based superalloy, Haynes 230. A NiCrAlSiY bond coat and an YB 2 B OB 3 B partially stabilised ZrOB 2 B top coat were used. In order to achieve well-defined chamfers, sprayed coupons were ground on the edges with SiC grinding paper to desired geometry. By inspections of cross-sections that had not undergone thermal fatigue cycling, it was ensured that no damage was introduced into the system. Mechanical testing was done in a thermal cyclic test rig where specimens are heated in a furnace and cooled with compressed air. FE modelling of the system has been done, aiming to support the findings from thermal fatigue tests. A parametric study including variation of the chamfer angle φ has been made and the stress state near the chamfer evaluated. Evaluation of fatigue damage can be done visually for observation of coating failure (macroscopic observation on coating surface). 20% area with complete spallation was considered as thermal barrier coating failure. For evaluation of damage development, additional light microscopy investigations of cross-sections have been carried out. Results show that the fatigue life benefits from introduction of a chamfer angle at the coating end during thermal fatigue cycling.

MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been widely used to protect hot com... more MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been widely used to protect hot components in gas turbines against oxidation and hot corrosion at high temperatures. Understanding the influence of the X-elements on oxidation behaviour is important in the design of durable MCrAlX coatings. In this study, NiCoCrAlX coatings doped with Y+Ru and Ce, respectively, were deposited on Inconel-792 substrate by HVOF. The samples were subjected to isothermal oxidation test in laboratory air at 9000, 1000, 1100 ºC and cyclic oxidation test between 100 ºC and 1100…

MATEC Web of Conferences, 2014

In turbine engines, high temperature components made of superalloys may crack in a creep process ... more In turbine engines, high temperature components made of superalloys may crack in a creep process during service. With the inward flux of the gases, e.g. oxygen and nitrogen, along those cracks, the microstructure of the superalloy substrate nearby the cracks may degrade by internal oxidation and nitridation. The aim of this study is to investigate and simulate the oxidation-nitridation-induced microstructural degradation in superalloys by taking a variant of Ni-based superalloy IN-792 as a sample. After the creep testing of the superalloy in air, the microstructures on the cross section of the superalloy were analysed in a scanning electron microscope, equipped with energy/wavelength dispersive systems. Internal oxidation and nitridation, presenting by Al/Ti oxides and nitrides, were observed under a porous and even cracked Cr-oxide scale which was formed on the superalloy surface or along the creep cracks connecting the superalloy surface. Meanwhile, the reinforcing γ precipitates were depleted. Such oxidation-nitridation-induced microstructural degradation was simulated by using an oxidation-diffusion model, focusing the diffusion of the alloying elements in metallic phases of the superalloy.

Volume 4: Ceramics; Concentrating Solar Power Plants; Controls, Diagnostics and Instrumentation; Education; Electric Power; Fans and Blowers, 2013

Gas turbine efficiency can be improved with tighter turbine tip clearances. An approach being dev... more Gas turbine efficiency can be improved with tighter turbine tip clearances. An approach being developed by engine manufacturers deploys active tip clearance monitoring where the turbine casing diameter is actively controlled in-service either mechanically or thermally. Typically current engines operate at about 1% clearance of blade span. With active control this could potentially be reduced significantly. Ideally active tip clearance control requires closed loop feedback measurements to maintain very small clearances without the risk of blade tip contact with the casing liner. Therefore reliable and robust sensors systems are required that can operate at the elevated temperatures found in modern gas turbines. Currently there are limited sensor systems available that can operate at these temperatures and survive typical sensor life requirements of many thousands of hours. This study details development of a high temperature eddy current sensor system for hot section applications. Th...

Diffusion coatings are widely used to increase oxidation and corrosion resistance of hot superall... more Diffusion coatings are widely used to increase oxidation and corrosion resistance of hot superalloy components for gas turbines. The aim of this study is to investigate the effect of coatings (NiAl and PtAl) on the creep fracture mechanism of samples with a substrate of IN792. The samples have been creep tested at two temperatures (850 °C and 950 °C) and different applied tensile stresses, until failure between 205 and 21000 hrs. The observation of cross-sections by SEM shows that the microstructural evolution in the coating is dependent on the diffusion of alloying elements in the sample. Furthermore the time and temperature induced growth of the coating is found to be controlled only by inward diffusion of Al. Grain-boundary cracking is the basic fracture mode in the substrate in all samples irrespective if the crack is initiated from coating or substrate. The analysis of microstructure shows that the diffusion coatings display two types of mechanical behavior-being easily plasticized or cracked-dependent on temperature and type of coating, and therefore can be considered as non-load carrying regions. After recalculating the creep stress to exclude the final effective coating thickness from the total sample thickness, the coated samples showed similar creep rupture behavior as the uncoated samples in the Larson-Miller diagram.

Thermal barrier coatings (TBCs) are used in gas turbines to provide insulation against high tempe... more Thermal barrier coatings (TBCs) are used in gas turbines to provide insulation against high temperature and to provide oxidation and corrosion resistance for the superalloys on which they are deposited. TBCs are deposited on hot parts in the combustor and on the turbine blades, and must consequently be compatible with the various superalloys used there. The influence of substrate material on the durability of TBCs has therefore been studied. Air plasma sprayed TBCs have been deposited on Hastelloy X and Haynes 230, which are alloys used in the combustor. The TBC systems have been thermally cycled until failure and their fracture surfaces have been studied. The thermally grown oxides and the substrate/coating interdiffusion have also been analysed by energy dispersive spectroscopy. The fatigue life, fracture mechanism and the oxide composition and kinetics were similar for the two TBC systems; however, one of the TBC systems is thought to have failed prematurely.

The present paper describes the work done in order to establish a correlation between observation... more The present paper describes the work done in order to establish a correlation between observations of thermal barrier coating (TBC) damage in an air-plasma sprayed TBC system and modelling of the delamination and spallation process that over time will degrade the TBC system. By inspections of cross-sections of TBC's on ex-serviced gas turbine components, it will be possible to establish preferred crack paths, and, verify that assumptions made during the modelling and experimental verification stages will be valid for different load / temperature combinations. Previously, a crack growth approach has been adopted for TBC delamination and spallation. This approach assumes that the top coat (TC) / bond coat (BC) interface is the region of the TBC that will develop a delamination crack network. In the present work it has been shown that the assumption of crack growth along the TC / TGO interface is valid unless the TGO growth rate is disturbed by formation of large mixed oxide clusters. The same is true for in-plane stresses, where a typical component load situation will cause interface spallation at the same oxide/oxide interface as was observed in the case of cyclic temperature load. In areas where high tensile strains are introduced, the coating will not delaminate and spall. Instead through-thickness cracks are introduced, i.e. TGO upper limit and maximum tensile strain criteria need to be adopted in addition to the Paris law approach.

Surface and Coatings Technology, 2015

MCrAlY coatings are widely used to protect superalloys against oxidation and corrosion at high te... more MCrAlY coatings are widely used to protect superalloys against oxidation and corrosion at high temperature in gas turbine engines. To design a durable MCrAlY coating, the behaviour of alloying elements in coating-superalloy system needs to be better understood. After oxidation tests in temperature range between 900 °C to 1100 °C, the development of the microstructures in the samples was analysed at the coating surface or in areas near the coatingsuperalloy interface. Some interdiffusion simulations were also done to model the diffusion behaviour of alloying elements in different coating-superalloy couples. The results show that both oxidation at the coatings' surface and the elements' diffusion inside of the materials were temperature-and chemical-composition dependent. The behaviour of some minor elements like Y, Hf, Ru and Ir in the oxidation processes was particularly studied by tracking their position and composition in the materials. Abbreviations high-velocity oxy-fuel (HVOF) coating-substrate interface (CSI) inner-β-depletion zone (IBDZ) γ'-depletion zone (GPDZ) secondary β phase (SBP)

Surface and Coatings Technology, 2015

MCrAlY coatings can be corroded due to the basic fluxing (type-I hot corrosion) of the supposeto-... more MCrAlY coatings can be corroded due to the basic fluxing (type-I hot corrosion) of the supposeto-be protective alumina scale in a molten sulphate environment. In this study, two MCrAlY coatings, coating A (10 wt.% Al, 20 wt.% Cr) and coating B (7 wt.% Al, 28 wt.% Cr), were tested in 48-hour cycles at 900 °C in a sodium-potassium sulphate environment with and without SO 2 (500 ppm). The aim was to study the effect of SO 2 at the typical type-I-hot-corrosion temperature-900 °C. The results showed that the corrosion behavior of the MCrAlY coatings depended not only on the coating composition but also on the corrosion environment. It was found that in coating A alumina scale was more resistant in the sulphate-plus-SO 2 condition than that in the sulphate-only condition. Such phenomenon indicated a beneficial effect of SO 2. On Coating B, however, mixed oxide layers, consisting of alumina and other oxides and sulphides, formed after a certain number of cycles in the sulphate environments with or without SO 2 gas. In this coating, the addition of SO 2 in the sulphate environment promoted the formation of non-dense chromia oxides which may form from oxidation of Cr-sulfides.

Surface and Coatings Technology, 2013

Thermal barrier coatings (TBCs) are used in gas turbines to prolong the life of the underlying su... more Thermal barrier coatings (TBCs) are used in gas turbines to prolong the life of the underlying substrates and to increase the efficiency of the turbines by enabling higher combustion temperatures. TBCs may fail during service due to thermal fatigue or through the formation of non-protective thermally grown oxides (TGOs). This study compares two atmospheric plasma sprayed (APS) TBC systems comprising of two identical TBCs deposited on two different substrates (Haynes 230 and Hastelloy X). The thermal fatigue life was found to differ between the two TBC systems. The interdiffusion of substrate elements into the coating was more pronounced in the TBC system with shorter life, however, very few of the substrate elements (only Mn and to some extent Fe) formed oxides in the bond coat/top coat interface. Fractography revealed no differences in the fracture behaviour of the TBCs; the fracture occurred, in both cases, to about 60 % in the top coat close to the interface and the remainder in the interface. Nanoindentation revealed only small differences in mechanical properties between the TBC systems and a finite element crack growth analysis showed that such small differences did not cause any significant change in the crack driving force. The oxidation kinetics was found to be similar for both TBC systems for the formation

Surface and Coatings Technology, 2014

ABSTRACT The fracture surfaces from adhesion tested thermal barrier coatings (TBC) have been stud... more ABSTRACT The fracture surfaces from adhesion tested thermal barrier coatings (TBC) have been studied by scanning electron microscopy. The adhesion test have been made using the standard method described in ASTM 633, which makes use of a tensile test machine to measure the adhesion. The studied specimens consist of air plasma sprayed (APS) TBC deposited on disc-shaped substrates of Hastelloy X. The bond coat (BC) is of NiCoCrAlY type and the top coat (TC) consists of yttria partially-stabilised zirconia. Before the adhesion test, the specimens were subjected to three different heat treatments: 1) isothermal oxidation 2) thermal cycling fatigue (TCF) and 3) burner rig test (BRT). The fracture surfaces of the adhesion tested specimens were characterised. A difference in fracture mechanism was found for the different heat treatments. Isothermal oxidation gave fracture mainly in the top coat while the two cyclic heat treatments gave an increasing amount of BC/TC interface fracture with increasing number of cycles. Some differences could also be seen between the specimens subjected to burner rig test and furnace cycling.

Surface and Coatings Technology, 2011

The adhesion of thermal barrier coatings (TBC) has been studied using the standard method describ... more The adhesion of thermal barrier coatings (TBC) has been studied using the standard method described in ASTM C633, which makes use of a tensile test machine to measure the adhesion. The studied specimens consist of air plasma sprayed (APS) TBC deposited on disc-shaped substrate coupons of Ni-base alloy Hastelloy X. The bond coat (BC) is of a NiCoCrAlY type and the top coat (TC) consists of yttria-stabilised-zirconia. Before the adhesion test, the specimens were subjected to three different heat treatments: 1) isothermal oxidation at 1100 up to 290 h, 2) thermal cycling fatigue (TCF) at 1100 up to 300 cycles and 3) thermal shock at ∼ 1140 BC/TC interface temperature up to 1150 cycles. The adhesion of the specimens is reported and accompanied by a microstructural study of the BC and the thermally grown oxides (TGO), as well as a discussion on the influence of BC/TC interfacial damage on adhesion properties of TBC. The adhesion was

Procedia Engineering, 2011

Thermal barrier coatings (TBC) are used in gas turbines to protect metallic components from high ... more Thermal barrier coatings (TBC) are used in gas turbines to protect metallic components from high temperature. In the present study adhesion tests have been conducted on APS TBC coated specimens subjected to different heat treatments. Isothermal and cyclic heat treatments have been conducted at temperatures around 1100 °C and the adhesion have been tested using the method described in ASTM C633. The fracture surfaces resulting from the adhesion test have been investigated and the fracture behavior has been characterized. A difference in fracture mechanism between the three heat treatments has been found. The two cyclic heat treatments give fracture in the top coat/bond coat interface while isothermal heat treatment gives fracture in the top coat.

Surface and Coatings Technology, 2013

Thermal barrier coatings (TBCs), when used in gas turbines, may fail through thermal fatigue, cau... more Thermal barrier coatings (TBCs), when used in gas turbines, may fail through thermal fatigue, causing the ceramic top coat to spall off the metallic bond coat. The life prediction of TBCs often involves finite element modelling of the stress field close to the bond coat/top coat interface and thus relies on accurate modelling of the interface. The present research studies the influence of bond coat/top coat interface roughness on the thermal fatigue life of plasma sprayed TBCs. By using different spraying parameters, specimens with varying interface roughness were obtained. During thermal cycling it was found that higher interface roughness promoted longer thermal fatigue life. The interfaces were characterised by roughness parameters, such as Ra, Rq and R∆q, as well as by autocorrelation, material ratio curves and slope distribution. The variation of spray parameters was found to affect amplitude parameters, such as Ra, but not spacing parameters, such as RSm. Three different interface geometries were tried for finite element crack growth simulation: cosine, ellipse and triangular shape. The cosine model was found to be an appropriate interface model and a procedure for obtaining the necessary parameters, amplitude and wavelength, was suggested. The positive effect of high roughness on life was suggested to be due to a shift from predominantly interface failure, for low roughness, to predominantly top coat failure, for high roughness.

Thermal barrier coatings (TBC) are coating system comprising a heat insulating ceramic coating de... more Thermal barrier coatings (TBC) are coating system comprising a heat insulating ceramic coating deposited on top of a oxidation resistant metallic coating. During thermal cycling, cracks grow in the metal/ceramic interface which causes the coating system to fail. In order to model such crack growth by finite element (FE) modelling, accurate interface models must be developed. The presented research studies the influence of interface roughness on the thermal fatigue life of TBCs, and suggests how interface models can be derived from interface roughness parameters. High interface roughness was found to promote longer fatigue lives. Interface models were derived from the roughness parameters Rc, Ra, RSm and Rdq and used for FE modelling of crack growth in the interface. The calculated stress intensities, KI and KII, increased with increasing interface roughness and thereby did not predict a slower crack growth with higher interface roughness as was observed experimentally.

Journal of Inorganic Materials, 2009

ABSTRACT

Surface and Coatings Technology, 2013

ABSTRACT MCrAlY coatings are deposited onto superalloys to provide oxidation and corrosion protec... more ABSTRACT MCrAlY coatings are deposited onto superalloys to provide oxidation and corrosion protection at high temperature by the formation of a thermally grown oxide scale. In this project, the oxidation behavior of a HVOF CoNiCrAlYSi coating on IN792 was studied for both isothermal oxidation (900, 1000 and 1100 degrees C) and thermal cycling (100-1100 degrees C). The microstructural evolution of the CoNiCrAlYSi coatings after oxidation was investigated. It was found that the Al-rich beta phase is gradually consumed due to two effects: surface oxidation and coating-substrate interdiffusion. Some voids and oxides along the coating-substrate interface, or inside the coating, were considered to play a role in blocking the diffusion of alloying elements. Based on the microstructural observation, an oxidation-diffusion model, considering both surface oxidation and coating-substrate interdiffusion, was developed by using Matlab and DICTRA software to predict the useful life of MCrAlY coatings. A new concept of diffusion blocking is also introduced into the model to give more accurate description on the microstructural evolution. The results from modeling and the experimentally established composition profiles showed good agreement.

Biodiversity Science, 2014