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Papers by eric jordan
International Journal of Solids and Structures, 1998
ABSTRACT
Journal of The American Ceramic Society, 2007
Yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) were produced by conventional a... more Yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) were produced by conventional air plasma spray (APS) and solution precursor plasma spray (SPPS) processes. Both TBCs were isothermally heat treated from 1200° to 1500°C for 100 h. Changes in the phase content, microstructure, and hardness were investigated. The nontransformable tetragonal (t′) phase is the predominant phase in both the as-sprayed APS and SPPS TBCs. APS and SPPS coatings exhibit similar thermal stability behavior such as densification rate, hardness increase, and grain coarsening rate. Both the as-received and heat-treated APS and SPPS TBCs show a bimodal pore size distribution with nano- and micro-size pores. After 1400°C/100 h heat treatment, equiaxed grains replace the columnar structure in APS TBCs and the splat structure disappears. Vertical cracks remain after the 1500°C/100 h exposure in SPPS TBCs. The monoclinic phase appears in APS TBCs after a 1400°C/100 h exposure and in SPPS coatings after a 1500°C/100 h exposure.
Surface & Coatings Technology, 2004
The durability and failure mechanisms of solution precursor plasma spray (SPPS) thermal barrier c... more The durability and failure mechanisms of solution precursor plasma spray (SPPS) thermal barrier coatings (TBCs) are investigated. SPPS TBCs exhibit an average life of 1018 cycles in a 1-h 1121 °C cycle furnace test, that is more than 2.5 times of that of a commercial APS TBC in the same test. Failure of the SPPS TBC starts with crack initiation along the unmelted particles in the ceramic top coat and the non-alumina oxides. The cracks propagate and coalesce with thermal cycling. The extensive cracking of the rapidly formed non-alumina oxides, resulting from the depletion of aluminum in the bond coat, leads to the development of a large separation between the TBC and substrate. When a crack of sufficient size emerges, the TBC separates from the metal substrate by large scale buckling. The greatly improved durability of the SPPS TBCs compared to APS TBCs on the same substrate and bond coat is attributed to the reduced stress near the bond coat-SPPS ceramic interface as a result of the vertical microcracks in the SPPS microstructure.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
The phase and microstructural stability of thermal barrier coatings (TBCs) deposited using the so... more The phase and microstructural stability of thermal barrier coatings (TBCs) deposited using the solution precursor plasma spray (SPPS) process is studied as a function of thermal cycles at 1121 °C. In the SPPS process, an aqueous chemical precursor feedstock, that results in ZrO2–7 wt.% Y2O3 (7YSZ) ceramic coating, is injected into the plasma jet and the coating is deposited on a metal substrate. The resulting coating has the desired non-transformable tetragonal phase structure and this phase is stable throughout the thermal cycling test. SPPS TBCs consist of ultra-fine splats and unmelted particles that include some non-pyrolyzed precursor. The non-pyrolyzed precursor in the coating was observed to decompose and crystallize during initial thermal cycling, which results in an increase of coating hardness. No sign of sintering of the ultra-fine splats in the coating was observed after 1090 cycles. The spacing of through-coating-thickness cracks (160–190 μm) does not change with thermal cycling, while the opening of cracks increases from around 0.4 μm at as-sprayed state to 1.7 μm at 40 cycles, and to 3.1 μm at 800 cycles.
Journal of Thermal Spray Technology, 2008
Thermo-physical processes in liquid ceramic precursor droplets in plasma were modeled. Models inc... more Thermo-physical processes in liquid ceramic precursor droplets in plasma were modeled. Models include aerodynamic droplet break-up, droplet transport, as well as heat and mass transfer within individual droplets. Droplet size, solute concentration, and plasma temperature effects are studied. Results are discussed with the perspective of selecting processing conditions and injection parameters to obtain certain types of coating microstructures. Small droplets (5 microns) tend to surface precipitate-forming shells with liquid core. They may be subjected to internal pressurization leading to shattering of shells and secondary atomization of liquid within. They arrive at the substrate as broken shells and unpyrolized material.
Surface & Coatings Technology, 2006
When tailored to make durable thermal barrier coatings (TBCs), the Solution Precursor Plasma Spra... more When tailored to make durable thermal barrier coatings (TBCs), the Solution Precursor Plasma Spray (SPPS) process produces a microstructure containing uniformly vertical cracks. These cracks provide a high degree of strain tolerance to the ceramic top coat. In order to understand the formation of vertical crack in SPPS process, coatings of various thicknesses were deposited on a variety of substrates with vastly different thermal properties. These coatings were characterized in the as-sprayed state and after heat treatment. It has been determined that the tensile stress derived from the pyrolysis of precursor occurring during coating deposition or post heat-treatment is the major driving force for the formation of vertical cracks in SPPS TBCs.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1999
The spallation failure of a commercial thermal barrier coating (TBC), consisting of a single-crys... more The spallation failure of a commercial thermal barrier coating (TBC), consisting of a single-crystal RENE N5 superalloy, a platinum aluminide (Pt-Al) bond coat, and an electron beam-deposited 7 wt pct yttria-stabilized zirconia ceramic layer (7YSZ), was studied following cyclic furnace testing. In the uncycled state and prior to deposition of the ceramic, the Pt-Al bond-coat surface contains a cellular network of ridges corresponding to the underlying bond-coat grain-boundary structure. With thermal cycling, the ridges and associated grain boundaries are the sites of preferential oxidation and cracking, which results in the formation of cavities that are partially filled with oxide. Using a fluorescent penetrant dye in conjunction with a direct-pull test, it is shown that, when specimens are cycled to about 80 pct of life, these grain-boundary regions show extensive debonding. The roles of oxidation and cyclic stress in localized grain boundary region spallation are discussed. The additional factors leading to large-scale TBC spallation are described.
Surface & Coatings Technology, 2006
This work seeks to develop an innovative thermal barrier coating (TBC) that will exhibit low ther... more This work seeks to develop an innovative thermal barrier coating (TBC) that will exhibit low thermal conductivity and high durability compared with that of current TBCs. To achieve this objective, a multiple component co-doped zirconia chemistry was selected for the topcoat of the TBC system, and a new process – Solution Plasma Spray (SPS or SPPS) – was employed to produce desirable microstructural features: the co-doped zirconia TBC contains ultrafine splats, high volume porosity and vertical cracks, for lower thermal conductivity and better durability, respectively. Test results verified a low thermal conductivity of 0.55–0.66 W/K·m over a temperature range of R.T. to 1300 °C, 1/3X that of EB-PVD 8YSZ TBCs, and its thermal cycling durability was 2.5X that of APS 8YSZ TBCs and 1.5X that of EB-PVD 8YSZ TBCs.
Journal of Thermal Spray Technology, 2008
The solution precursor plasma spray (SPPS) process is a relatively new and flexible thermal spray... more The solution precursor plasma spray (SPPS) process is a relatively new and flexible thermal spray process that can produce a wide variety of novel materials, including some with superior properties. The SPPS process involves injecting atomized droplets of a precursor solution into the plasma. The properties of resultant deposits depend on the time-temperature history of the droplets in the plasma, ranging from ultra-fine splats to unmelted crystalline particles to unpyrolized particles. By controlling the volume fraction of these three different constituents, a variety of coatings can be produced, all with a nanograin size. In this article, we will be reviewing research related to thermal barrier coatings, emphasizing the processing conditions necessary to obtain a range of microstructures and associated properties. The SPPS process produces a unique strain-tolerant, low-thermal conductivity microstructure consisting of (i) three-dimensional micrometer and nanometer pores, (ii) through-coating thickness (vertical) cracks, (iii) ultra-fine splats, and (iv) inter-pass boundaries. Both thin (0.12 mm) and thick (4 mm) coatings have been fabricated. The volume fraction of porosity can be varied from 10% to 40% while retaining the characteristic microstructure of vertical cracks and ultra-fine splats. The mechanism of vertical crack formation will be described.
Surface & Coatings Technology, 2003
Photo-stimulated luminescence Piezo-spectroscopy (PLPS) is being developed as a non-destructive t... more Photo-stimulated luminescence Piezo-spectroscopy (PLPS) is being developed as a non-destructive technique for thermal barrier coatings (TBC). In this study, the evolution of photo-stimulated luminescence with thermal cycling was systematically investigated from the thermally grown oxide (TGO) in a production TBC, which consists of an electron beam physical vapor deposited (EB-PVD) 7 wt.% Y2O3–ZrO2 top coat, a grit blasted (Ni,Pt)Al bond coat and a CMSX-4 superalloy substrate. The change of compressive stress in the TGO layer on the bond coat with thermal cycling was calculated from the wavelength shift of the luminescence spectra. The compressive stress increased from 1.0–2.2 GPa in the as received state to 2.8–3.3 GPa at 10 cycles, then gradually decreased to 1.2–1.9 GPa until 500 cycles and remained at this level until TBC spallation. Other fluorescence spectra characteristics, such as the width of R1 and R2 peaks and their relative intensity, were also evaluated. These PLPS measurements on TBCs with grit blasted bond coats are compared with previous measurements on similar TBC system but with non-grit blasted bond coats. It is concluded that the initial increase in stress is associated with the formation of a continuous oxide layer. The lower stress of the specimens with the grit blasted bond coats compared to that of the as-coated bond coats is associated partly with the greater surface roughness. And the fast decline in compressive stress is the result of bond coat surface rumpling facilitated by the initially rougher surface. The lifetime of the TBCs with grit blasted bond coats varies over a narrow range, 600–750 cycles with an average of 675 cycles, which is related to their consistent bond coat surface roughness.
Scripta Materialia, 2008
MgO nanocrystalline powders were synthesized via a wet precipitation process. X-ray diffraction a... more MgO nanocrystalline powders were synthesized via a wet precipitation process. X-ray diffraction analysis of the heat-treated precursor powders shows that a crystalline MgO phase forms at ∼500 °C. Translucent MgO ceramics were prepared by pressureless sintering the nanocrystalline MgO powders at 1400 °C for 2 h under ambient atmosphere. The as-sintered MgO ceramics have a relative density of 98.1% with an average hardness of 6.8 GPa. Scanning electron microscope characterization revealed that the translucent MgO ceramics have an average grain size of ∼6 μm.
Journal of The American Ceramic Society, 2008
For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) p... more For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al2O3–40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of α-Al2O3 and tetragonal ZrO2 phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1–5 μm) and unmelted spherical particles (<0.5 μm). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al2O3–40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°–1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity.
Acta Biomaterialia, 2008
A dense titania (TiO2) coating was deposited from an ethanol-based solution containing titanium i... more A dense titania (TiO2) coating was deposited from an ethanol-based solution containing titanium isopropoxide using the solution precursor plasma spray (SPPS) process. XRD and Raman spectrum analyses confirmed that the coating is exclusively composed of rutile TiO2. SEM micrographs show the as-sprayed coating is dense with a uniform thickness and there are no coarse splat boundaries. The as-sprayed coating was chemically treated in 5 M NaOH solution at 80 °C for 48 h. The bioactivity of as-sprayed and alkaline-treated coatings was investigated by immersing the coatings in simulated body fluid (SBF) for 14–28 days, respectively. After 28 days immersion, there is a complete layer of carbonate-containing apatite formed on the alkaline-treated TiO2 coating surface, but none formed on the as-sprayed coating.
Journal of The American Ceramic Society, 2008
A dense titania (TiO2) coating was deposited from an ethanol solution containing titanium isoprop... more A dense titania (TiO2) coating was deposited from an ethanol solution containing titanium isopropoxide using the solution precursor plasma spray process. Thermal and crystallization behaviors of the solution precursor were investigated by thermal gravimetric analysis–differential thermal analysis, Fourier-transform infrared spectrum, and X-ray diffraction (XRD). XRD and Raman spectrum analyses confirmed that the coating is exclusively composed of rutile TiO2. Scanning electron micrographs show the as-sprayed coating is dense with uniform thickness and there are no coarse splat boundaries. The individual ultrafine splats (1–5 μm diameter) are composed of columnar grains. X-ray photoelectron spectroscopy analysis indicates that some Ti–OH groups exist in the as-sprayed coating surface. Deposition mechanisms are described based on model experiments.
Acta Materialia, 2004
Thermal cycling tests were conducted on a commercial yttria-stabilized zirconia electron beam-phy... more Thermal cycling tests were conducted on a commercial yttria-stabilized zirconia electron beam-physical vapor deposited thermal barrier coating (TBC) on a platinum aluminide (β-(Ni, Pt)Al) bond coat. Surprisingly, the longest life sample lasted 10 times longer than the shortest life sample. Two distinct mechanisms have been found responsible for the observed damage initiation and progression at the thermally grown oxide (TGO)/bond coat interface. The first mechanism leads to localized debonding at the TGO/bond coat interface due to increasing out-of-plane tensile stresses at ridges that form along bond coat grain boundaries. The second mechanism is driven by cyclic plasticity of the bond coat that leads to cavity formation at the TGO/bond coat interface. The primary finding of this work is that the first mechanism, involving tensile stress at ridge tops, is life limiting. Based on this mechanism, it is demonstrated that the variation in bond coat ridge aspect ratio can explain the unusual 10× variation in observed sample life. It is proposed that ridge top spallation leads to debonds of sufficient size to result in unstable fracture driven by the strain energy stored in the TGO. The criticality of the flaw created by local debonding is supported by experimental determination of the strain energy available in the TGO through measurement of TGO stress and thickness combined with published fracture mechanics solutions of the relevant flaw geometry.
Surface & Coatings Technology, 2008
Porous titania coatings were deposited from aqueous solution containing titanium isopropoxide usi... more Porous titania coatings were deposited from aqueous solution containing titanium isopropoxide using the solution precursor plasma spray (SPPS) process. Effects of plasma power on coating microstructure and phase composition were investigated. X-ray diffraction and Raman spectra analyses indicated that the amount of anatase and rutile phases in the as-sprayed coatings can be adjusted by simply changing the plasma power. With the increase of plasma power, the coating anatase content decreases and the rutile content increases. Scanning electron microscope characterization shows that all of the as-sprayed coatings are very porous.
Surface & Coatings Technology, 2004
Journal of Materials Science, 2004
The solution-precursor plasma-spray (SPPS) process is capable of producing highly durable thermal... more The solution-precursor plasma-spray (SPPS) process is capable of producing highly durable thermal barrier coatings. In an effort to improve the understanding of the deposition mechanisms in this novel process, a series of specific experiments, where the substrate is held stationary and the plasma torch is programmed to scan a single pass across the substrate, were conducted and the resulting deposits were carefully characterized. In addition to the deposition mechanisms identified previously in the stationary torch experiments, the deposition mechanisms of two other types of deposits, thin film and fine spherical particles, were identified in this study. The melting of inflight formed 7YSZ particles and their rapid solidification to form ultra-fine splats on the substrate was found to be the dominant deposition mechanism. The characterization of actual SPPS coatings confirmed that the various coating-deposition mechanisms identified in the model experiments occur in concert during the actual coating process. Adherent deposits (ultra-fine splats, deposits from gel-like precursor and film formed via chemical vapor deposition), unmelted particles (spherical particles, deposits from non-decomposed precursor) and porosity were estimated to constitute ∼65, ∼19 and ∼16 vol%, of the coating, respectively.
Surface & Coatings Technology, 2004
International Journal of Solids and Structures, 1998
ABSTRACT
Journal of The American Ceramic Society, 2007
Yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) were produced by conventional a... more Yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) were produced by conventional air plasma spray (APS) and solution precursor plasma spray (SPPS) processes. Both TBCs were isothermally heat treated from 1200° to 1500°C for 100 h. Changes in the phase content, microstructure, and hardness were investigated. The nontransformable tetragonal (t′) phase is the predominant phase in both the as-sprayed APS and SPPS TBCs. APS and SPPS coatings exhibit similar thermal stability behavior such as densification rate, hardness increase, and grain coarsening rate. Both the as-received and heat-treated APS and SPPS TBCs show a bimodal pore size distribution with nano- and micro-size pores. After 1400°C/100 h heat treatment, equiaxed grains replace the columnar structure in APS TBCs and the splat structure disappears. Vertical cracks remain after the 1500°C/100 h exposure in SPPS TBCs. The monoclinic phase appears in APS TBCs after a 1400°C/100 h exposure and in SPPS coatings after a 1500°C/100 h exposure.
Surface & Coatings Technology, 2004
The durability and failure mechanisms of solution precursor plasma spray (SPPS) thermal barrier c... more The durability and failure mechanisms of solution precursor plasma spray (SPPS) thermal barrier coatings (TBCs) are investigated. SPPS TBCs exhibit an average life of 1018 cycles in a 1-h 1121 °C cycle furnace test, that is more than 2.5 times of that of a commercial APS TBC in the same test. Failure of the SPPS TBC starts with crack initiation along the unmelted particles in the ceramic top coat and the non-alumina oxides. The cracks propagate and coalesce with thermal cycling. The extensive cracking of the rapidly formed non-alumina oxides, resulting from the depletion of aluminum in the bond coat, leads to the development of a large separation between the TBC and substrate. When a crack of sufficient size emerges, the TBC separates from the metal substrate by large scale buckling. The greatly improved durability of the SPPS TBCs compared to APS TBCs on the same substrate and bond coat is attributed to the reduced stress near the bond coat-SPPS ceramic interface as a result of the vertical microcracks in the SPPS microstructure.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
The phase and microstructural stability of thermal barrier coatings (TBCs) deposited using the so... more The phase and microstructural stability of thermal barrier coatings (TBCs) deposited using the solution precursor plasma spray (SPPS) process is studied as a function of thermal cycles at 1121 °C. In the SPPS process, an aqueous chemical precursor feedstock, that results in ZrO2–7 wt.% Y2O3 (7YSZ) ceramic coating, is injected into the plasma jet and the coating is deposited on a metal substrate. The resulting coating has the desired non-transformable tetragonal phase structure and this phase is stable throughout the thermal cycling test. SPPS TBCs consist of ultra-fine splats and unmelted particles that include some non-pyrolyzed precursor. The non-pyrolyzed precursor in the coating was observed to decompose and crystallize during initial thermal cycling, which results in an increase of coating hardness. No sign of sintering of the ultra-fine splats in the coating was observed after 1090 cycles. The spacing of through-coating-thickness cracks (160–190 μm) does not change with thermal cycling, while the opening of cracks increases from around 0.4 μm at as-sprayed state to 1.7 μm at 40 cycles, and to 3.1 μm at 800 cycles.
Journal of Thermal Spray Technology, 2008
Thermo-physical processes in liquid ceramic precursor droplets in plasma were modeled. Models inc... more Thermo-physical processes in liquid ceramic precursor droplets in plasma were modeled. Models include aerodynamic droplet break-up, droplet transport, as well as heat and mass transfer within individual droplets. Droplet size, solute concentration, and plasma temperature effects are studied. Results are discussed with the perspective of selecting processing conditions and injection parameters to obtain certain types of coating microstructures. Small droplets (5 microns) tend to surface precipitate-forming shells with liquid core. They may be subjected to internal pressurization leading to shattering of shells and secondary atomization of liquid within. They arrive at the substrate as broken shells and unpyrolized material.
Surface & Coatings Technology, 2006
When tailored to make durable thermal barrier coatings (TBCs), the Solution Precursor Plasma Spra... more When tailored to make durable thermal barrier coatings (TBCs), the Solution Precursor Plasma Spray (SPPS) process produces a microstructure containing uniformly vertical cracks. These cracks provide a high degree of strain tolerance to the ceramic top coat. In order to understand the formation of vertical crack in SPPS process, coatings of various thicknesses were deposited on a variety of substrates with vastly different thermal properties. These coatings were characterized in the as-sprayed state and after heat treatment. It has been determined that the tensile stress derived from the pyrolysis of precursor occurring during coating deposition or post heat-treatment is the major driving force for the formation of vertical cracks in SPPS TBCs.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1999
The spallation failure of a commercial thermal barrier coating (TBC), consisting of a single-crys... more The spallation failure of a commercial thermal barrier coating (TBC), consisting of a single-crystal RENE N5 superalloy, a platinum aluminide (Pt-Al) bond coat, and an electron beam-deposited 7 wt pct yttria-stabilized zirconia ceramic layer (7YSZ), was studied following cyclic furnace testing. In the uncycled state and prior to deposition of the ceramic, the Pt-Al bond-coat surface contains a cellular network of ridges corresponding to the underlying bond-coat grain-boundary structure. With thermal cycling, the ridges and associated grain boundaries are the sites of preferential oxidation and cracking, which results in the formation of cavities that are partially filled with oxide. Using a fluorescent penetrant dye in conjunction with a direct-pull test, it is shown that, when specimens are cycled to about 80 pct of life, these grain-boundary regions show extensive debonding. The roles of oxidation and cyclic stress in localized grain boundary region spallation are discussed. The additional factors leading to large-scale TBC spallation are described.
Surface & Coatings Technology, 2006
This work seeks to develop an innovative thermal barrier coating (TBC) that will exhibit low ther... more This work seeks to develop an innovative thermal barrier coating (TBC) that will exhibit low thermal conductivity and high durability compared with that of current TBCs. To achieve this objective, a multiple component co-doped zirconia chemistry was selected for the topcoat of the TBC system, and a new process – Solution Plasma Spray (SPS or SPPS) – was employed to produce desirable microstructural features: the co-doped zirconia TBC contains ultrafine splats, high volume porosity and vertical cracks, for lower thermal conductivity and better durability, respectively. Test results verified a low thermal conductivity of 0.55–0.66 W/K·m over a temperature range of R.T. to 1300 °C, 1/3X that of EB-PVD 8YSZ TBCs, and its thermal cycling durability was 2.5X that of APS 8YSZ TBCs and 1.5X that of EB-PVD 8YSZ TBCs.
Journal of Thermal Spray Technology, 2008
The solution precursor plasma spray (SPPS) process is a relatively new and flexible thermal spray... more The solution precursor plasma spray (SPPS) process is a relatively new and flexible thermal spray process that can produce a wide variety of novel materials, including some with superior properties. The SPPS process involves injecting atomized droplets of a precursor solution into the plasma. The properties of resultant deposits depend on the time-temperature history of the droplets in the plasma, ranging from ultra-fine splats to unmelted crystalline particles to unpyrolized particles. By controlling the volume fraction of these three different constituents, a variety of coatings can be produced, all with a nanograin size. In this article, we will be reviewing research related to thermal barrier coatings, emphasizing the processing conditions necessary to obtain a range of microstructures and associated properties. The SPPS process produces a unique strain-tolerant, low-thermal conductivity microstructure consisting of (i) three-dimensional micrometer and nanometer pores, (ii) through-coating thickness (vertical) cracks, (iii) ultra-fine splats, and (iv) inter-pass boundaries. Both thin (0.12 mm) and thick (4 mm) coatings have been fabricated. The volume fraction of porosity can be varied from 10% to 40% while retaining the characteristic microstructure of vertical cracks and ultra-fine splats. The mechanism of vertical crack formation will be described.
Surface & Coatings Technology, 2003
Photo-stimulated luminescence Piezo-spectroscopy (PLPS) is being developed as a non-destructive t... more Photo-stimulated luminescence Piezo-spectroscopy (PLPS) is being developed as a non-destructive technique for thermal barrier coatings (TBC). In this study, the evolution of photo-stimulated luminescence with thermal cycling was systematically investigated from the thermally grown oxide (TGO) in a production TBC, which consists of an electron beam physical vapor deposited (EB-PVD) 7 wt.% Y2O3–ZrO2 top coat, a grit blasted (Ni,Pt)Al bond coat and a CMSX-4 superalloy substrate. The change of compressive stress in the TGO layer on the bond coat with thermal cycling was calculated from the wavelength shift of the luminescence spectra. The compressive stress increased from 1.0–2.2 GPa in the as received state to 2.8–3.3 GPa at 10 cycles, then gradually decreased to 1.2–1.9 GPa until 500 cycles and remained at this level until TBC spallation. Other fluorescence spectra characteristics, such as the width of R1 and R2 peaks and their relative intensity, were also evaluated. These PLPS measurements on TBCs with grit blasted bond coats are compared with previous measurements on similar TBC system but with non-grit blasted bond coats. It is concluded that the initial increase in stress is associated with the formation of a continuous oxide layer. The lower stress of the specimens with the grit blasted bond coats compared to that of the as-coated bond coats is associated partly with the greater surface roughness. And the fast decline in compressive stress is the result of bond coat surface rumpling facilitated by the initially rougher surface. The lifetime of the TBCs with grit blasted bond coats varies over a narrow range, 600–750 cycles with an average of 675 cycles, which is related to their consistent bond coat surface roughness.
Scripta Materialia, 2008
MgO nanocrystalline powders were synthesized via a wet precipitation process. X-ray diffraction a... more MgO nanocrystalline powders were synthesized via a wet precipitation process. X-ray diffraction analysis of the heat-treated precursor powders shows that a crystalline MgO phase forms at ∼500 °C. Translucent MgO ceramics were prepared by pressureless sintering the nanocrystalline MgO powders at 1400 °C for 2 h under ambient atmosphere. The as-sintered MgO ceramics have a relative density of 98.1% with an average hardness of 6.8 GPa. Scanning electron microscope characterization revealed that the translucent MgO ceramics have an average grain size of ∼6 μm.
Journal of The American Ceramic Society, 2008
For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) p... more For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al2O3–40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of α-Al2O3 and tetragonal ZrO2 phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1–5 μm) and unmelted spherical particles (<0.5 μm). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al2O3–40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°–1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity.
Acta Biomaterialia, 2008
A dense titania (TiO2) coating was deposited from an ethanol-based solution containing titanium i... more A dense titania (TiO2) coating was deposited from an ethanol-based solution containing titanium isopropoxide using the solution precursor plasma spray (SPPS) process. XRD and Raman spectrum analyses confirmed that the coating is exclusively composed of rutile TiO2. SEM micrographs show the as-sprayed coating is dense with a uniform thickness and there are no coarse splat boundaries. The as-sprayed coating was chemically treated in 5 M NaOH solution at 80 °C for 48 h. The bioactivity of as-sprayed and alkaline-treated coatings was investigated by immersing the coatings in simulated body fluid (SBF) for 14–28 days, respectively. After 28 days immersion, there is a complete layer of carbonate-containing apatite formed on the alkaline-treated TiO2 coating surface, but none formed on the as-sprayed coating.
Journal of The American Ceramic Society, 2008
A dense titania (TiO2) coating was deposited from an ethanol solution containing titanium isoprop... more A dense titania (TiO2) coating was deposited from an ethanol solution containing titanium isopropoxide using the solution precursor plasma spray process. Thermal and crystallization behaviors of the solution precursor were investigated by thermal gravimetric analysis–differential thermal analysis, Fourier-transform infrared spectrum, and X-ray diffraction (XRD). XRD and Raman spectrum analyses confirmed that the coating is exclusively composed of rutile TiO2. Scanning electron micrographs show the as-sprayed coating is dense with uniform thickness and there are no coarse splat boundaries. The individual ultrafine splats (1–5 μm diameter) are composed of columnar grains. X-ray photoelectron spectroscopy analysis indicates that some Ti–OH groups exist in the as-sprayed coating surface. Deposition mechanisms are described based on model experiments.
Acta Materialia, 2004
Thermal cycling tests were conducted on a commercial yttria-stabilized zirconia electron beam-phy... more Thermal cycling tests were conducted on a commercial yttria-stabilized zirconia electron beam-physical vapor deposited thermal barrier coating (TBC) on a platinum aluminide (β-(Ni, Pt)Al) bond coat. Surprisingly, the longest life sample lasted 10 times longer than the shortest life sample. Two distinct mechanisms have been found responsible for the observed damage initiation and progression at the thermally grown oxide (TGO)/bond coat interface. The first mechanism leads to localized debonding at the TGO/bond coat interface due to increasing out-of-plane tensile stresses at ridges that form along bond coat grain boundaries. The second mechanism is driven by cyclic plasticity of the bond coat that leads to cavity formation at the TGO/bond coat interface. The primary finding of this work is that the first mechanism, involving tensile stress at ridge tops, is life limiting. Based on this mechanism, it is demonstrated that the variation in bond coat ridge aspect ratio can explain the unusual 10× variation in observed sample life. It is proposed that ridge top spallation leads to debonds of sufficient size to result in unstable fracture driven by the strain energy stored in the TGO. The criticality of the flaw created by local debonding is supported by experimental determination of the strain energy available in the TGO through measurement of TGO stress and thickness combined with published fracture mechanics solutions of the relevant flaw geometry.
Surface & Coatings Technology, 2008
Porous titania coatings were deposited from aqueous solution containing titanium isopropoxide usi... more Porous titania coatings were deposited from aqueous solution containing titanium isopropoxide using the solution precursor plasma spray (SPPS) process. Effects of plasma power on coating microstructure and phase composition were investigated. X-ray diffraction and Raman spectra analyses indicated that the amount of anatase and rutile phases in the as-sprayed coatings can be adjusted by simply changing the plasma power. With the increase of plasma power, the coating anatase content decreases and the rutile content increases. Scanning electron microscope characterization shows that all of the as-sprayed coatings are very porous.
Surface & Coatings Technology, 2004
Journal of Materials Science, 2004
The solution-precursor plasma-spray (SPPS) process is capable of producing highly durable thermal... more The solution-precursor plasma-spray (SPPS) process is capable of producing highly durable thermal barrier coatings. In an effort to improve the understanding of the deposition mechanisms in this novel process, a series of specific experiments, where the substrate is held stationary and the plasma torch is programmed to scan a single pass across the substrate, were conducted and the resulting deposits were carefully characterized. In addition to the deposition mechanisms identified previously in the stationary torch experiments, the deposition mechanisms of two other types of deposits, thin film and fine spherical particles, were identified in this study. The melting of inflight formed 7YSZ particles and their rapid solidification to form ultra-fine splats on the substrate was found to be the dominant deposition mechanism. The characterization of actual SPPS coatings confirmed that the various coating-deposition mechanisms identified in the model experiments occur in concert during the actual coating process. Adherent deposits (ultra-fine splats, deposits from gel-like precursor and film formed via chemical vapor deposition), unmelted particles (spherical particles, deposits from non-decomposed precursor) and porosity were estimated to constitute ∼65, ∼19 and ∼16 vol%, of the coating, respectively.
Surface & Coatings Technology, 2004