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Thermal expansion
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Proceedings Papers
ITSC 2022, Thermal Spray 2022: Proceedings from the International Thermal Spray Conference, 364-368, May 4–6, 2022,
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During the thermal cycling process in MCrAlY-YSZ thermal barrier coating system, stresses are produced at bondcoat (BC)-topcoat (TC) interface due to the mismatch of the thermal expansion coefficients of the two coating layers. The stresses at the interface are not a single value and can be affected by the coatings’ microstructure. In this paper, finite element (FE) modeling method was used to study the behavior of the stress distribution at the coatings’ interface. The influence of the pore structure in the ceramic TC and the micro bulge structure at the metal BC surface was investigated. The results showed that both structures can change the stress distribution. The pores played a “stone-in-river” role, which trapped higher stress around them and simultaneously reduced the size of the macro stress zones in TC. The micro bulges at the TC/BC interface also trapped high stresses which could cause more interaction between TC cracks and BC roughness.
Proceedings Papers
ITSC 2022, Thermal Spray 2022: Proceedings from the International Thermal Spray Conference, 736-742, May 4–6, 2022,
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This paper presents the results of two metals coatings, molybdenum and tantalum, prepared by Controlled Atmosphere Plasma Spray (CAPS) onto Al 6061 substrates that were thermal cycled to calculate the effective coating modulus. Traditional uniaxial tensile testing samples were prepared from thicker duplicate coatings for comparison, as well as to measure thermal expansion properties and oxygen and nitrogen content. The molybdenum samples cut from thicker coatings were un-able to be tensile tested due to their fragility. Thermal cycle testing of molybdenum on an Al 6061 substrate was found to have a modulus approximately 18 to 19% of literature values for bulk molybdenum using the bi-layer beam thermal cycling method. Additionally, non-linear modulus behaviour was observed in the molybdenum sample when enough thermal strain was induced to shift the coating from a compressive to tensile stress state. The tantalum coating was found to have a modulus approximately 42 to 46% of literature values for bulk tantalum using the bi-layer thermal cycling method. Traditional tensile testing measured a modulus approximately 44 to 46% of bulk, which shows good agreement between the two methods and supports that the bi-layer thermal cycling method is valid for plasma sprayed refractory metal coatings.
Proceedings Papers
ITSC 2022, Thermal Spray 2022: Proceedings from the International Thermal Spray Conference, 907-912, May 4–6, 2022,
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An efficient temperature control on tool surfaces is essential in processes like injection moulding or die casting. A thermally sprayed heating coating could combine dynamic heating properties with a small assembly space as it is sprayed directly onto the cavity surface. With their intrinsically high electrical resistivity and low thermal expansion as compared with traditional alloys, High Entropy Alloys (HEA) show promising properties for the use as heating elements. Thus, the well-studied HEA Al 0.5 CoCrFeNi was used as a starting material for additional alloying with Zr and Si to force further lattice distortion in the solid solution. HEAs of differing compositions were melted and characterized. In the process, the potential of HEAs was assessed by characterizing their phase composition, thermal stability, and electrical resistivity. The characterized HEAs show a solid solution mainly consisting of fcc and bcc structure. Moreover, the composition Al 0.5 CoCrFeNiZr 0.2 Si 0.2 was determined as stable after heat treatment at 600 °C for 324 h. In addition, the electrical resistivity was raised by over 20 % relative to the starting material. As a result, a hitherto unknown HEA composition was detected to possess superior properties to traditional alloys for the application as heating coating.
Proceedings Papers
ITSC 2021, Thermal Spray 2021: Proceedings from the International Thermal Spray Conference, 66-74, May 24–28, 2021,
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As a critical technology, thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades, however, the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats, three new bond coat alloys have been designed and developed, and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys, such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase, absence of mixed / spinel oxides, and TGO self repair ability, which cannot be achieved by the existed MCrAlY alloys.
Proceedings Papers
ITSC 2019, Thermal Spray 2019: Proceedings from the International Thermal Spray Conference, 125-130, May 26–29, 2019,
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Yttrium aluminum garnet (YAG) has desirable properties for a thermal barrier coating (TBC), although there are two production challenges. One, YAG has a relatively low thermal expansion coefficient which leads to large thermal mismatch stresses, and two, amorphous phases are produced by atmospheric plasma spraying. Solution precursor plasma spraying (SPPS) has to potential to solve both problems. First off, it produces no amorphous phases. Secondly, it can produce a cracked microstructure that mitigates the CTE mismatch issue. To judge the adequacy of the properties of SPPS YAG, a summary of the properties of common TBCs is presented. It is shown that the properties of SPPS YAG fall within desirable or usable ranges. Current efforts described in this paper focus on improving the efficiency and rate of deposition.
Proceedings Papers
ITSC 2019, Thermal Spray 2019: Proceedings from the International Thermal Spray Conference, 131-135, May 26–29, 2019,
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A wide range of properties can be achieved in intermetallic coatings applied by gas detonation spraying (GDS). The properties of Fe-40at%Al GDS layers, however, may change when exposed to temperatures exceeding a threshold level. To characterize such changes, Fe-40at%Al GDS coatings were subjected to systematic dilatometric studies in which temperatures were cycled from room temperature to 1180 °C. The investigation revealed both irreversible and reversible phase transitions as described in the paper. Dilatometry measurements obtained from sintered samples made from the same powder are presented for comparison.
Proceedings Papers
ITSC 2019, Thermal Spray 2019: Proceedings from the International Thermal Spray Conference, 838-845, May 26–29, 2019,
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This study investigates the effect of thermal cycling on cold-spray chromium coatings deposited on steel substrates. First, equilibrium stress states are determined for different coating thicknesses. Next, the potential for crack initiation and growth is simulated based on periodic heating and cooling cycles. The corresponding crack driving forces are characterized using interface stresses and energy release rate as a function of the thermal cycles. The effects of coating thickness, embedded microcracks, and initial residual stress on the driving forces are investigated systematically to demonstrate the risk of coating fracture and delamination.
Proceedings Papers
ITSC 2015, Thermal Spray 2015: Proceedings from the International Thermal Spray Conference, 99-106, May 11–14, 2015,
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Abradable seals are used in aircraft engine compressor and turbine to decrease fuel consumption. Their role is to minimize the clearance between the rotating blades of an engine rotor and the casing to reduce air leakages (compressor) or air-fuel combustion product leakages (turbine). Operating temperatures in turbines (up to 1800°C) can induce a thermal expansion of the blades and give rise to contacts providing damages to the blades or casing. Thus, in case of contact, the blade should remove the abradable seals applied to the casing without being damaged. Besides, the seal must be resistant to the turbine environment. Direct relationships have been observed between plasma spray operating parameters and abradable coating performances. The aim of this study is to determine those relationships for YSZ-Polyester abradable composite coatings. This study is conducted within the frame work of the 7FP European project E-BREAK to reach the environmental objectives of the European Advisory Council for Aviation Research and innovation.
Proceedings Papers
ITSC2014, Thermal Spray 2014: Proceedings from the International Thermal Spray Conference, 184-189, May 21–23, 2014,
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This study compares the dielectric properties of annealed forsterite (Mg 2 SiO 4 ) and alumina coatings deposited on mild steel substrates by atmospheric plasma spraying. As-sprayed coating samples were electrically characterized then submitted to a series of one-hour annealing treatments at temperatures from 300 to 800 °F. After each treatment, impedance measurements were recorded over a frequency range of 30 to 100 kHz. An electrical model was fitted to Nyquist data (Im Z vs. Re Z) using a least-mean-square algorithm with a weighting function. Although impedance spectroscopy measurements were obtained at different temperatures, this paper focuses on the acquisition, modeling, and comparison of room temperature properties, particularly electrical resistivity and dielectric constant. It also compares the microstructure of as-sprayed and annealed forsterite and alumina coatings and discusses coating degradation mechanisms stemming from differences in CTE.
Proceedings Papers
ITSC2014, Thermal Spray 2014: Proceedings from the International Thermal Spray Conference, 273-278, May 21–23, 2014,
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Thermal barrier coatings typically incorporate a YSZ topcoat and a metallic bond coat. During service, a reaction zone consisting of different thermally grown oxides forms at the interface. Although most such oxides are detrimental, one (α-Al 2 O 3 ) improves service life due to its barrier effect on oxygen diffusion. In this study, Al and AlOx films are deposited on metallic bond coats by dc magnetron sputtering prior to topcoat deposition. The resulting TBCs were thermally cycled to determine the effect of the interlayer films on service life and TGO formation. It is shown that the Al films transform in situ into dense Al 2 O 3 layers that act as oxygen diffusion barriers. TBCs with interlayer alumina, whether deposited directly or formed in situ, showed less cracking and were more mechanically stable during thermal cycle tests.
Proceedings Papers
ITSC2014, Thermal Spray 2014: Proceedings from the International Thermal Spray Conference, 745-750, May 21–23, 2014,
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This investigation evaluates the microstructure and properties of thermal barrier coatings produced by suspension plasma spraying. A YSZ suspension was injected axially into a plasma jet and deposited on a superalloy substrate with a CoNiCrAlY bond coat. SEM examination revealed a columnar microstructure with a network of vertical segmentation cracks and horizontal branching cracks. In furnace cycle testing, the TBCs withstood 166 thermal shock cycles with failure attributed to partial spallation of the columnar segments initiating at the edge and center of the coatings. The TBCs were also subjected to burner rig tests to assess thermal insulation properties and to heat treatments up to 1600 °C to evaluate thermal stability based on phase composition, grain size, and microhardness.
Proceedings Papers
ITSC 2013, Thermal Spray 2013: Proceedings from the International Thermal Spray Conference, 35-40, May 13–15, 2013,
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This study investigates the phase stability and thermophysical properties of Y 2 O 3 and Yb 2 O 3 co-doped SrHfO 3 (SHYY) powder and bulk material along with the phase stability and microstructure evolution of as-sprayed SHYY coatings during annealing. The powder was synthesized by a solid-state reaction at 1450 °C, showing good phase stability up to 1400 °C. Dilatometry measurements revealed no abnormal changes in the coefficient of thermal expansion over a temperature range of 200-1300 °C. The thermal conductivity of the bulk material was found to be 16% lower than that of SrHfO 3 . Free-standing SHYY coatings deposited by air plasma spraying were also tested. The coatings consisted of SHYY and a minor amount of secondary phase Yb 2 O 3 and exhibited good phase stability during heat treatment at 1400 °C for 288 h. Coating samples examined after 216 h still exhibited a columnar microstructure.
Proceedings Papers
ITSC2012, Thermal Spray 2012: Proceedings from the International Thermal Spray Conference, 58-63, May 21–24, 2012,
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Advanced ceramic materials with perovskite structure have been developed for potential applications in thermal barrier coating (TBC) systems in an effort to overcome the properties of the pre-existing ones like 8wt% yttria stabilized zirconia (8YSZ). Y 2 O 3 and Yb 2 O 3 co-doped strontium zirconate with chemistry of Sr(Zr 0.9 Y 0.05 Yb 0.05 )O 2.95 (SZYY) was synthesized using ball milling prior to solid-state sintering, and had a minor second phase of Yb 2 O 3 . The SZYY showed good phase stability not only from room temperature to 1400°C, but also at high temperature of 1450°C for a long period, analyzed by thermogravimetry-differential scanning calorimetry (TG-DSC) and X-ray diffraction (XRD), respectively. The thermal expansion coefficients (TECs) of the sintered bulk SZYY were recorded by a high-temperature dilatometer and revealed a positive influence on phase transitions of SrZrO 3 by codoping Y 2 O 3 and Yb 2 O 3 . The thermal conductivities of SZYY were at least ~30% lower in contrast to that of SrZrO 3 and 8YSZ in the whole tested temperature range. The good chemical compatibility was observed for SZYY with 8YSZ or Al 2 O 3 powders after 24 h heat treatment at 1250°C. The phase stability and the microstructure evolution of the as-sprayed SZYY coating during annealing at 1400°C were also investigated in this work.
Proceedings Papers
Microstructure and Properties of Porous Ni50Cr50- Al 2 O 3 Cermet Support for Solid Oxide Fuel Cells
ITSC2012, Thermal Spray 2012: Proceedings from the International Thermal Spray Conference, 639-645, May 21–24, 2012,
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Microstructure of cermet support influences significantly the performance and stability of solid oxide fuel cells (SOFCs). The properties required for the support include high electrical conductivity, necessary permeability, good match of thermal expansion with other layers and high temperature strength. In this study, a porous Ni50Cr50-Al 2 O 3 cermet was designed as the support of SOFC. The porous cermet was deposited by flame spraying with a powder mixture of 30%vol Al 2 O 3 and 35%vol Ni50Cr50 and 35%vol polyester. The effect of cermet microstructure on its gas permeability was investigated. The electrical conductivity, thermal expansion coefficient and bending strength of cermet support were also studied. The results showed that the gas leakage rate of the cermet support increased with the increase of polyester content in the starting powder. The thermal expansion coefficient of the composite cermet decreased with the increase of the volume fraction of Al 2 O 3 . Moreover, the electric conductivity of the cermet increased significantly after high temperature sintering, and reached 1015 S/cm after sintering at 1000°C for 15 hours. The three point bending strength of the Ni50Cr50-based cermet support reached 171 MPa. The cermet stability at high temperatures and SOFCs performance were discussed.
Proceedings Papers
ITSC 2011, Thermal Spray 2011: Proceedings from the International Thermal Spray Conference, 571-575, September 27–29, 2011,
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Ceramic Thermal Barrier Coatings (TBCs) on superalloy components are being used successfully in land-based gas turbine and aircraft engines. These coatings are generally made by either air plasma spraying (APS) or electron beam physical vapour deposition (EB-PVD). In general, EB-PVD TBCs have superior durability due to the columnar structure, but they are very expensive compared to APS TBCs. EB-PVD TBCs are used primarily in the most severe applications such as turbine blades and vanes in aircraft engines. This paper presents an economical process to make durable TBCs, called Axial Suspension Plasma Spray (ASPS). This technology combines Mettech’s axial injection plasma process and automatic suspension feed system. The resulting TBCs exhibit columnar structures with vertical cracks, similar to EB-PVD coatings. Such structures allow the TBC to compensate for thermal expansion differences between it and the base material. The ASPS process presents an economical alternative to EB-PVD to produce durable columnar TBCs.
Proceedings Papers
ITSC 2011, Thermal Spray 2011: Proceedings from the International Thermal Spray Conference, 821-822, September 27–29, 2011,
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The dense refractory is applied for the high-temperature glass melting equipment. Furthermore, platinum or its alloy-clad refractory bricks are utilized to melt high quality glass in fields of optical and display glasses. From the viewpoint of the resource saving of rare metal, the decreasing of Pt-consumption is very serious problem for glass manufacturing. The platinum thermal spray coating is effective alternative technology to solve this problem for platinum cladding. A Pt-spray coated ceramics is difficult to get reliability due to the large difference of thermal expansion between the ceramic and the metal. In this study, we have investigated the bond strength of the platinum coat on the dense refractories which was textured by the mechanical process. As a result, the bond strength between the ceramic and the metal is 3-5 MPa. It is almost the same as the thermal spray ceramics coating on the metal. Furthermore, in case of the ceramics substrate that contains the glass phase, the bond strength between the ceramic and the metal increased to 14-17 MPa after 1773 K heat treatment.
Proceedings Papers
ITSC 2011, Thermal Spray 2011: Proceedings from the International Thermal Spray Conference, 944-948, September 27–29, 2011,
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La 2 (Zr 0.7 Ce 0.3 ) 2 O 7 (LZ7C3) ceramic was prepared by solid state reaction method at 1400 °C for 12h using La 2 O 3 , ZrO 2 , and CeO 2 as starting reactants. The phase composition and microstructures were studied by XRD and SEM technology. The thermal conductivity and linear thermal expansion coefficient were investigated by laser-flash method and pushing-rod method respectively. XRD results revealed that LZ7C3 is a mixture of pyrochlore and fluorite, and the pyrochlore is the main phase which is a small solid solution of La 2 Ce 2 O 7 (LC) in La 2 Zr 2 O 7 (LZ). The thermal conductivity of LZ7C3 decrease gradually with the increase in temperature until 1200°C, and the value is 0.79 W⋅m -1 ⋅K -1 , which is almost 50% lower than that of LZ. The linear thermal expansion coefficient which is 11.6 × 10 -6 K -1 at 1200°C is larger than that of LZ. These results show that LZ7C3 ceramic material can be explored as a novel prospective candidate material for use in new thermal barrier coating systems in the future.
Proceedings Papers
ITSC 2011, Thermal Spray 2011: Proceedings from the International Thermal Spray Conference, 1203-1206, September 27–29, 2011,
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NiCrAl/ZrO 2 -8Y 2 O 3 coatings deposited on SUS304 stainless steel and 45 carbon steel substrates were prepared by APS at different preheating temperatures, of which thickness exceeded 1mm. This study analyzed the coatings’ separation from different preheated substrates in the cooling process after spraying due to residual thermal stress. The Young’s modulus of the porous YSZ coatings was calculated and also measured by Knoop indentation methods for comparison purposes. The result indicated that the failure of porous thick YSZ coatings is mainly caused by the cracks nucleation, propagation and coalescence, which is related to the thermal-expansion coefficient difference between substrate and coatings, preheating temperature, porosity of coatings and so on. Due to their increased porosity, the porous and thick YSZ coatings had much lower calculated and measured Young’s modulus values than the sintered YSZ coatings.
Proceedings Papers
ITSC 2011, Thermal Spray 2011: Proceedings from the International Thermal Spray Conference, 1339-1344, September 27–29, 2011,
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FeAl intermetallics matrix composites reinforced by ceramics particles such as titanium carbide have attracted much attention in recent years. In this study, shrouded plasma spraying with nitrogen as a protective gas was employed to deposit TiC particles dispersed FeAl/TiC composite coatings. Fe-35Al powder and Fe-35Al/TiC composite powders containing 35 vol.% and 45 vol.% TiC prepared by mechanical alloying were used as feedstock powders. The microstructure of the ball-milled powders and the as-sprayed coatings was characterized by scanning electron microscopy and X-ray diffraction. The mean coefficient of thermal expansion (CTE) of FeAl and FeAl/TiC was measured. The results showed that dense FeAl and FeAl/TiC coatings with low oxide inclusions were deposited by shrouded plasma spraying. The mean CTEs calculated based on the Reuss formula are reasonably consistent with those measured in the present study. As a result, the CTE of FeAl-based composite coating can be properly controlled by adjusting TiC content in the composite coating to match with that of the substrate.
Proceedings Papers
ITSC 2011, Thermal Spray 2011: Proceedings from the International Thermal Spray Conference, 115-120, September 27–29, 2011,
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Cr 3 C 2 -NiCr coatings are commonly used to provide abrasion and erosion wear resistance on the surface of components, in particular for corrosive and atmospheric high-temperature environments. For these classical and new applications the knowledge of the thermophysical properties is highly important. In the present work the dependence of the heat conductivity on temperature of two HVOF-sprayed Cr 3 C 2 -25NiCr-coatings prepared by a liquid-fuelled HVOF-process from two different feedstock powders from room temperature up to 700 °C was determined. Thermal diffusivities, density functions, specific heat capacities and coefficient of thermal expansion (CTE) were measured in order to compute the heat conductivity for the coatings. All measurements were performed twice (as-sprayed and after a first thermal cycle) in order to take into account the structural and compositional changes. XRD and FESEM studies were performed in order to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Heat conductivities (average of the two coatings) ranging from about 11 W/(mK) at 50°C up to about 20 W/(mK) at 700°C were determined. Differences between the two coatings were clearly detectable. The heat conductivity of the Cr 3 C 2 -NiCr coatings is significantly lower than determined previously for a WC-17%Co coating.
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