Yttria-stabilized Zirconia Layer Research Articles

High temperature corrosion resistant coatings for gas flare systems

Gas flaring systems are used in processing plants to eliminate excess gases while serving as a safe pressure-relieving system. These systems are built using 310 stainless steel (SS310) thanks to its mechanical properties and performance under high temperature operating conditions. However, the findings have revealed that SS310 is susceptible to high temperature sulfidation when exposed to a corrosive environment. In the present work, SEM analysis has been conducted to investigate the causes of this type of failure. To tackle this problem, high velocity oxygen fuel (HVOF) thermal spraying was used to deposit a double-layer thermal barrier coatings, including a top yttria stabilized zirconia layer (TBC) and a bond coating CoNiCrAlY layer (BC) on SS310 substrates. The performance of the coatings was tested by exposing it to a high temperature corrosive environment. As a result, an improved resistance to corrosion was observed. This improved performance can be attributed to the absence of tetragonal to monoclinic phase change transformation, and to a decrease in the volume fraction of the monoclinic phase in yttria stabilized zirconia top coat, as indicated by XRD analysis.

Epitaxial growth of metallic buffer layer structure and c-axis oriented Pb(Mn1/3,Nb2/3)O3–Pb(Zr,Ti)O3 thin film on Si for high performance piezoelectric micromachined ultrasonic transducer

This paper reports on the development of a metallic buffer layer structure, (100) SrRuO3 (SRO)/(100) Pt/(100) Ir/(100) yttria-stabilized zirconia (YSZ) layers for the epitaxial growth of a c-axis oriented Pb(Mn1/3,Nb2/3)O3–Pb(Zr,Ti)O3 (PMnN–PZT) thin film on a (100) Si wafer for piezoelectric micro-electro mechanical systems (MEMS) application. The stacking layers were epitaxially grown on a Si substrate under the optimal deposition condition. A crack-free PMnN–PZT epitaxial thin films was obtained at a thickness up to at least 1.7 µm, which is enough for MEMS applications. The unimorph MEMS cantilevers based on the PMnN–PZT thin film were fabricated and characterized. As a result, the PMnN–PZT thin film exhibited −10 to −12 C/m2 as a piezoelectric coefficient e31,f and ∼250 as a dielectric constants εr. The resultant FOM for piezoelectric micromachined ultrasonic transducer (pMUT) is higher than those of general PZT and AlN thin films. This structure has a potential to provide high-performance pMUTs.

YSZ/Al2O3 multilayer thick films deposited by spin coating using ceramic suspensions on Al2O3 polycrystalline substrate

Yttria Stabilized Zirconia (YSZ) high chemical and thermomechanical stability under Solid Oxide Fuel Cells (SOFCs) standard operational conditions and its high electrolytic domain indicate that an interesting alternative for lowering SOFCs operating temperature would be the development of high conductive YSZ-based electrolytes compared to conventional electrolytes through microstructure engineering. In this work, YSZ/Al2O3 micrometric multilayer films obtained by spin coating presented electrical conductivity 3.5 times higher than traditional YSZ ceramics. The multilayer film was formed by three layers of YSZ with 3.7 µm of thickness each were intercalated by two 7.4 µm thick layers of Al2O3. Therefore, the high electrical conductivity observed in nanometric multilayer films obtained by advanced techniques was achieved, even with lower magnitude, with ceramic suspensions and in the micrometric scale. Computer simulation shows higher stresses on YSZ than on Al2O3 layers, correlating the heterofilms higher conductivity with the stress imposed on the YSZ layer during sintering.

Relationship between cracks and microstructures in APS YSZ coatings at elevated temperatures

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were prepared by plasma spraying. Thermally grown oxide (TGO) was formed between the YSZ layer and the bond coat during heat treatment of the YSZ TBCs. The generation and extension behavior of cracks before and after heat treatment were characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) combined with 3D reconstruction technology. The characterization results give a direct evidence that the generation and extension of cracks is closely related to other microstructures such as columnar grains, TGO morphology, TGO composition, and TGO orientation. For example, large interlaminar cracks in as-sprayed YSZ coatings often appeared along with columnar grains, which can be explained by their similar formation conditions. The formation of new cracks such as vertical cracks, plane cracks and other cracks along the TGO grain boundary after heat treatment often resulted from TGO growth stresses. Therefore, the generation and extension of cracks can be controlled by regulating the formation of columnar grains and TGO.

Effect of YSZ Coatings as Diffusion Barrier between Glass Sealing and Steel

In solid oxide fuel cell (SOFC) stacks the sealing is a critical element which has to ensure gas tightness between different gas streams as well as electrical insulation in several parts of the system depending on its design. It has therefore to establish stable interfaces with different materials such as the ferritic stainless steel (FSS) used for interconnects frame or gas manifolds and the ceramic materials of the cell electrolytes. Therefore, sealants have to be chemically and mechanically stable in contact with such materials at the SOFC working conditions in terms of temperatures, thermal and stress gradients, and exposition to different gas compositions. Glasses are widely used as sealing material because of their thermal expansion characteristics, mechanical properties, the possibility to obtain gas tight structures and ease of application. However, due to the operating temperatures, reactions and diffusion processes can occur at the interfaces resulting in the formation of undesirable phases that could compromise the glass properties increasing the risk of failures. The application of a barrier layer between the two components is considered as suitable countermeasure avoiding the migration of elements at the interfaces. In this work the barrier properties and compatibility at the interfaces of Yttria-stabilized zirconia (YSZ) has been investigated. YSZ layers have been applied by means of plasma spraying on as-rolled FSS substrates. Several glass compositions, applied on substrates with and without YSZ coating, have been tested at 780°C in static air up to 1000 h to investigate the barrier layer effects, the compatibility of the materials and the stability of the interfaces. Microstructural changes and migration of elements have been characterized post-experiment in cross-section by means of scanning electron microscopy- energy dispersive X-ray analysis.

Investigation of La0.6Sr0.4Co0.2Fe0.8O3- δ Electrode Performance and Microstructural Evolutions Based on Three Dimensional Microstructure Reconstruction and Electrochemical Simulation

The influence of initial microstructure of gadolinium-doped-ceria (GDC) interlayer on La0.6Sr0.4Co0.2Fe0.8O3- d (LSCF) performance was studied with three dimensional microstructure reconstruction by focused ion beam – scanning electron microscopy (FIB-SEM). When the GDC interlayer was fired at 1300 ˚C (GDC1300), discontinuous SrZrO3 phase was formed between GDC interlayer and yttria-stabilized-zirconia (YSZ) electrolyte. In addition, inter-diffused layer of GDC and YSZ was formed at the neighbor of the SrZrO3 phase and along the YSZ electrolyte interface. When the GDC interlayer was fired at 1200 ˚C (GDC1200), continuous SrZrO3 layer was formed. Inter-diffused layer was formed along the YSZ electrolyte. Both cells were operated at 700 ˚C and 800 ˚C . The initial electrode resistance of GDC1200 was more than 20 times larger than that of GDC1300. Slight increase of electrode resistance was found for GDC1300 after the cell operation, whereas the drastic activation was found for GDC1200. The activation was enhanced at operating temperature of 700 ˚C than 800 ˚C . For GDC1300, Sr rich phase was formed after the operation at 800 ˚C. It is considered that the differences of performance and microstructural evolution are highly dependent on initial microstructures and operating temperatures. The different electrode performances are evaluated based on electrochemical simulation using reconstructed microstructures.

Plasma-assisted reactive high-rate vapor deposition of yttria-stabilized zirconia using electron beam evaporation and spotless vacuum arc

Some applications in energy technology will require large-area and dense coatings of yttria-stabilized zirconia (YSZ) while high coating rate is demanded for economic reasons. A combination process of co-evaporation of yttrium and zirconium by EB-PVD from a dual crucible, reactive processing procedure of introducing oxygen, and a spotless arc burning between zirconium as cathode and yttrium as anode were investigated experimentally. The YSZ layers were deposited at relatively high static coating rates (20 to 80nm s−1) in comparison to other PVD processes. The cubic crystal structure that was identified by means of XRD corresponds to the YSZ phase with the highest ionic conductivity and is therefore especially well-suited for use as a solid-state electrolyte.Pores were evidenced in the microstructure of the layers deposited at a coating rate of >50nm s−1. Nevertheless, very dense YSZ layers could be obtained at a coating rate of 30nm s−1 and a spotless arc current of 300A. Specific leakage rates of YSZ layers on porous Ni/NiO-YSZ anode substrates measured using air are in the region of 1Pams−1. The investigations have shown that the intense plasma created with a spotless arc has a considerable influence on growth and microstructure of YSZ layers, even at high coating rates.

Manufacturing of Composite Coatings by Atmospheric Plasma Spraying Using Different Feed-Stock Materials as YSZ and MoSi2

Yttria-stabilized zirconia (YSZ) is the state-of-the-art material for the top coat of thermal barrier coatings. To increase the efficiency and lifetime of gas turbines, the integration of MoSi2 as a healing material was proposed. A new method of manufacture was explored in order to enable the spraying of a homogeneous mixed layer of YSZ and MoSi2. As the chemical and physical properties of these powders are very different, they require contrasting process conditions. Due to the evaporation of Si from MoSi2 at spraying conditions suitable for YSZ, more moderate conditions and a shorter time of flight are required for depositing MoSi2. At the same time, the spraying conditions still need to be sufficient for melting the YSZ particles in order to produce a coating. To obtain a homogeneous mixture, both conditions can be matched using an injection system that allows powder injection at two different locations of the plasma jet. Two-color pyrometry during flight (DPV-2000, Tecnar) was used to monitor the actual particle temperature. By optimizing the injection point for the MoSi2, a mixed coating was obtained without decomposition of the MoSi2, which has been analyzed by means of XRD and SEM.

Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray

7–8wt.% yttria stabilized zirconia (YSZ) is the standard ceramic top coat material used in gas turbines to insulate the underlying metallic substrate. However, at higher temperatures (>1200°C), phase stability and sintering becomes an issue for YSZ. At these temperatures, YSZ is also susceptible to CMAS (calcium magnesium alumino silicates) infiltration. New ceramic materials such as pyrochlores have thus been proposed due to their excellent properties such as lower thermal conductivity and better CMAS attack resistance compared to YSZ. However, pyrochlores have inferior thermo mechanical properties compared to YSZ. Therefore, double-layered TBCs with YSZ as the intermediate layer and pyrochlore as the top ceramic layer have been proposed. In this study, double layer TBC comprising gadolinium zirconate (GZ)/YSZ and triple layer TBC (GZdense/GZ/YSZ) comprising relatively denser GZ top layer on GZ/YSZ were deposited by suspension plasma spray. Also, single layer 8YSZ TBC was suspension plasma sprayed to compare its functional performance with the multi-layered TBCs. Cross sections and top surface morphology of as sprayed TBCs were analyzed by scanning electron microscopy (SEM). XRD analysis was done to identify phases formed in the top surface of as sprayed TBCs. Porosity measurements were made using water intrusion and image analysis methods. Thermal diffusivity of the as sprayed TBCs was measured using laser flash analysis and thermal conductivity of the TBCs was calculated. The multi-layered GZ/YSZ TBCs were shown to have lower thermal conductivity than the single layer YSZ. The as sprayed TBCs were also subjected to thermal cyclic testing at 1300°C. The double and triple layer TBCs had a longer thermal cyclic life compared to YSZ. The thermo cycled samples were analyzed by SEM.

Stabilizing Ir(001) Epitaxy on Yttria-Stabilized Zirconia Using a Thin Ir Seed Layer Grown by Pulsed Laser Deposition

We demonstrate the reproducible epitaxial growth of 100 nm thick Ir(001) films on a heteroepitaxial stack consisting of 5 nm Ir and 100 nm yttria-stabilized zirconia (YSZ) grown on Si(001) substrat...

High ionic conductivity in confined bismuth oxide-based heterostructures

Bismuth trioxide in the cubic fluorite phase (δ-Bi2O3) exhibits the highest oxygen ionic conductivity. In this study, we were able to stabilize the pure δ-Bi2O3 at low temperature with no addition of stabilizer but only by engineering the interface, using highly coherent heterostructures made of alternative layers of δ-Bi2O3 and Yttria Stabilized Zirconia (YSZ), deposited by pulsed laser deposition. The resulting [δ-Bi2O3/YSZ] heterostructures are found to be stable over a wide temperature range (500-750 °C) and exhibits stable high ionic conductivity over a long time comparable to the value of the pure δ-Bi2O3, which is approximately two orders of magnitude higher than the conductivity of YSZ bulk.

Thermal behaviour of multilayer and functionally-graded YSZ/Gd2Zr2O7 coatings

Zirconates with pyrochlore structure, such as Gd2Zr2O7, are new promising thermal barrier coatings because of their very low thermal conductivity and good chemical resistance against molten salts. However, their coefficient of thermal expansion is low, therefore their thermal fatigue resistance is compromised. As a solution, the combination of yttria-stabilised zirconia (YSZ) and Gd2Zr2O7 can reduce the thermal contraction mismatch between the thermal barrier coating parts.In the present study, two possible designs have been performed to combine YSZ/Gd2Zr2O7. On the one hand, a multilayer coating was obtained where YSZ layer was deposited between a Gd2Zr2O7 layer and a bond coat. On the other hand, a functionally-graded coating was designed where different layers with variable ratios of YSZ/Gd2Zr2O7 were deposited such that the composition gradually changed along the coating thickness.Multilayer and functionally-graded coatings underwent isothermal and thermally-cycled treatments in order to evaluate the oxidation, sintering effects and thermal fatigue resistance of the coatings. The YSZ/Gd2Zr2O7 multilayer coating displayed better thermal behaviour than the Gd2Zr2O7 monolayer coating but quite less thermal fatigue resistance compared to the conventional YSZ coating. However, the functionally-graded coating displays a good thermal fatigue resistance. Hence, it can be concluded that this kind of design is ideal to optimise the behaviour of thermal barrier coatings.

Phosphor-Doped Thermal Barrier Coatings Deposited by Air Plasma Spray for In-Depth Temperature Sensing.

Yttria-stabilized zirconia (YSZ)-based thermal barrier coating (TBC) has been integrated with thermographic phosphors through air plasma spray (APS) for in-depth; non-contact temperature sensing. This coating consisted of a thin layer of Dy-doped YSZ (about 40 µm) on the bottom and a regular YSZ layer with a thickness up to 300 µm on top. A measurement system has been established; which included a portable; low-cost diode laser (405 nm); a photo-multiplier tube (PMT) and the related optics. Coating samples with different topcoat thickness were calibrated in a high-temperature furnace from room temperature to around 900 °C. The results convincingly showed that the current sensor and the measurement system was capable of in-depth temperature sensing over 800 °C with a YSZ top layer up to 300 µm. The topcoat thickness was found to have a strong effect on the luminescent signal level. Therefore; the measurement accuracy at high temperatures was reduced for samples with thick topcoats due to strong light attenuation. However; it seemed that the light transmissivity of YSZ topcoat increased with temperature; which would improve the sensor’s performance at high temperatures. The current sensor and the measurement technology have shown great potential in on-line monitoring of TBC interface temperature.

Improvement in Response/Recovery Characteristics of Mixed-Potential-Type Zirconia-Based CO Sensor Using ZnCr2O4 Added with Au Particles-Sensing Electrode

In order to fulfill strict environmental regulations regarding air pollutions such as carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2), high performance gas sensors have been widely investigated for the last decade. Among air pollutions, CO is well known as colorless and toxic pollutant. Currently, the sensitive and selective detection of CO at high operating temperatures remains a challenging task. Recently, we have reported that the CO response of a mixed-potential-type yttria-stabilized zirconia (YSZ)-based sensor using a ZnCr2O4-sensing electrode (SE) was augmented by the dispersion of nano-sized Au (nano-Au) particles into the SE layer. However, the 90% response and recovery times against 800 ppm CO at 550°C for the sensor using ZnCr2O4 (+nano-Au)-SE were more than 40 and 160 s, respectively. From a practical point of view, it is very important to improve such poor response and recovery characteristics to CO and to achieve fast response and recovery behavior. In this study, therefore, to improve the response and recovery characteristics against CO for the sensor using ZnCr2O4 (+ nano-Au)-SE, Au particles with three different sizes (nano-Au, subμ-Au, and μ-Au) were separately added to ZnCr2O4-SE, and then the CO sensing performances of the resulting sensors were examined. The SE materials were prepared by mixing commercial ZnCr2O4 powder (Kojundo Chemical Lab. Co., Ltd., Japan) and one of the following Au particles (nano-Au: 1-4 nm; subμ-Au: 0.3-0.6 μm; μ-Au: 2-5 μm) in an agate mortar with ethanol, which was subsequently dried to evaporate the dispersant solution. The sensors were assembled in a tubular configuration using a commercial hemispherically-terminated YSZ tube (8 mol.% Y2O3-doped ZrO2, Nikkato Corp., Japan). A YSZ layer was firstly formed on the outer surface of the YSZ tube using a YSZ paste, which was made by mixing a commercial YSZ powder (Tosoh Corp., Japan) and α-terpineol, to improve the mechanical and electrochemical stability of the interface between the SE layer and the YSZ tube. After drying the YSZ layer, the SE material pastes, which were also made by mixing with α-terpineol, were applied on the YSZ layer to form SE. A commercial Pt paste was applied on the inner surface of the closed-end YSZ tube to form a Pt/air-reference electrode (RE). Finally, the painted and assembled YSZ tube was sintered at 1100°C for 2 h in air to fabricate the final sensing device. Gas sensing measurements were carried out in a conventional gas-flow apparatus equipped with a furnace operating at 550°C. The SE was exposed to the humidified base gas (synthetic air + 5 vol.% H2O) or sample gases with a flow rate of 100 cm3/min. The Pt/air-RE was always exposed to an atmospheric air. The difference in electromotive force (emf) between SE and RE was measured as the sensing signal by means of a digital electrometer. The effect of the addition of Au particles with different sizes into the ZnCr2O4−SE on the CO sensing characteristics of the mixed-potential-type YSZ-based sensors was investigated by measuring the emf response against the inner Pt/air−RE after exposure to 20 - 800 ppm CO. The response transients indicate that the sensitivity toward CO decreases with increasing Au particle size. Among the three SEs tested, the ZnCr2O4(+subμ-Au)-SE gave relatively high sensitivity and good response/recovery against CO. Although CO sensitivity of the ZnCr2O4(+subμ-Au)-SE became lower than that of the ZnCr2O4(+nano-Au)-SE, the 90% recovery time to 800 ppm CO was improved and it was approximately 80 s. Additionally, to examine the influence of Au additive on the sensitivity and response/recovery behavior to CO, the content of subμ-Au additive to ZnCr2O4 was varied from 1 to 10 wt.%. As a result, the CO sensitivity increased with increasing content of subμ-Au, as expected. Examinations of the detailed CO sensing characteristics for the present sensor are now in progress.

Material properties of pulsed-laser crystallized Si thin films grown on yttria-stabilized zirconia crystallization-induction layers by two-step irradiation method

Amorphous Si thin films on yttria-stabilized zirconia (YSZ) layers were crystallized widely in solid phase by the two-step method with a pulsed laser, moving the sample stage. The crystalline quality, impurity diffusion, and electrical properties of the crystallized Si films were investigated. It was found that the crystallinity of the Si thin films was improved and their surface was smooth without an incubation layer at the interface, indicating the uniform crystallinity of Si on YSZ. The diffusion of Zr and Y into the Si thin films was as small as or smaller than the order of 1017 atoms/cm3. We evaluated the electrical properties of carrier concentration and Hall mobility of the Si thin films with/without YSZ layers by using the resistivity and AC Hall effect measurements. The temperature and doping concentration dependences were measured for both undoped and P-doped films. It was found that both the undoped and P-doped Si/YSZ/glass films showed higher mobilities and carrier concentrations (and therefore higher conductivities), which indicate a smaller number of defects, than the Si/glass films. This suggested that the Si film crystallized on the YSZ layer is more suitable for application to electronic devices than the Si film on glass.

Tribological characterization of zirconia coatings deposited on Ti6Al4V components for orthopedic applications

One of the most important issues leading to the failure of total joint arthroplasty is related to the wear of the plastic components, which are generally made of ultra high molecular weight polyethylene (UHMWPE). Therefore, the reduction of joint wear represents one of the main challenges the research in orthopedics is called to address nowadays. Surface treatments and coatings have been recognized as innovative methods to improve tribological properties, also in the orthopedic field.This work investigated the possibility to realize hard ceramic coatings on the metal component of a prosthesis, by means of Pulsed Plasma Deposition, in order to reduce friction and wear in the standard coupling against UHMWPE.Ti6Al4V substrates were coated with a 2μm thick yttria-stabilized zirconia (YSZ) layer. The mechanical properties of the YSZ coatings were assessed by nanoindentation tests performed on flat Ti6Al4V substrates. Tribological performance was evaluated using a ball-on-disk tribometer in dry and lubricated (i.e. with fetal bovine serum) highly-stressing conditions, up to an overall distance of 10km. Tribology was characterized in terms of coefficient of friction (CoF) and wear rate of the UHMWPE disk. After testing, specimens were analyzed through optical microscopy and SEM images, in order to check the wear degradation mechanisms. Progressive loading scratch tests were also performed in dry and wet conditions to determine the effects of the environment on the adhesion of the coating.Our results supported the beneficial effect of YSZ coating on metal components. In particular, the proposed solution significantly reduced UHMWPE wear rate and friction. At 10km of sliding distance, a wear rate reduction of about 18% in dry configuration and of 4% in presence of serum, was obtained by the coated group compared to the uncoated group. As far as friction in dry condition is concerned, the coating allowed to maintain low CoF values until the end of the tests, with an overall difference of about 40% compared to the uncoated balls. In wet conditions, the friction values were found to be comparable between coated and uncoated materials, mainly due to a premature delamination of the coating. Scratch tests in wet showed in fact a reduction of the critical load required to a complete delamination due to a formation of blister, although no change or damage occurred at the coating during the soaking period.Although conditions of high values of contact pressure were considered, further analyses are however required to fully understand the behavior of YSZ coatings in wet environment and additional research on the deposition process will be mandatory in order to improve the coating tribological performance at long distances addressing orthopedic applications.

Deposition and properties of a multilayered thermal barrier coating

Abstract The widely used air plasma sprayed thermal barrier coatings (TBCs) in industrial gas turbine engines are limited by its durability. The failure of plasma sprayed TBCs in thermal cycling is usually induced by the spallation of ceramic coating within the ceramic coating near the top coat/bond coat interface. In this research, a multilayered ceramic coating architecture is introduced comprised of the layers of segmentation cracks, porous and dense. The segmented yttria stabilized zirconia (YSZ) layer was deposited at the substrate temperature of 750 °C without any artificial cooling, which resulted in the formation of YSZ layer with obvious columnar crystal structure and segmentation cracks. And a dense top YSZ layer was also included on the top to restrain the penetration of oxygen and the molten calcium–magnesium–alumina-silicate (CMAS) glass deposits. Additionally, the conventional porous YSZ coating was preserved in order to get the low thermal conductivity. The thermal cycling tests show that the lifetime of the multilayered TBCs increases by a factor of 2 compared with that of the conventional one, which could attribute to the high strain tolerance and fracture toughness of the segmented and columnar microstructure. Oxidation resistance and CMAS corrosion resistance are also better for the multilayered TBCs compared with that of the conventional one, due to the suppression function of dense top YSZ layer. It is potential to obtain the high performance TBCs by adjusting plasma spraying.

Deposition, Heat Treatment And Characterization of Two Layer Bioactive Coatings on Cylindrical PEEK.

Polyether ether ketone (PEEK) rods were coated via ion beam asssited deposition (IBAD) at room temperature. The coating consists of a two-layer design of yttria-stabilized zirconia (YSZ) as a heat-protection layer, and hydroxyapatite (HA) as a top layer to increase bioactivity. A rotating substrate holder was designed to deposit an even coating on the cylindrical surface of PEEK rods; the uniformity is verified by cross-sectional measurements using scanning electron microscopy (SEM). Deposition is followed by heat treatment of the coating using microwave annealing and autoclaving. Transmission electron microscopy (TEM) showed a dense, uniform columnar grain structure in the YSZ layer that is well bonded to the PEEK substrate, while the calcium phosphate layer was amorphous and pore-free in its as-deposited state. Subsequent heat treatment via microwave energy introduced HA crystallization in the calcium phosphate layer and additional autoclaving further expanded the crystallization of the HA layer. Chemical composition evaluation of the coating indicated the Ca/P ratios of the HA layer to be near that of stoichiometric HA, with minor variations through the HA layer thickness. The adhesion strength of as-deposited HA/YSZ coatings on smooth, polished PEEK surfaces was mostly unaffected by microwave heat treatment, but decreased with additional autoclave treatment. Increasing surface roughness showed improvement of bond strength.

Preparation, characterization and in vitro response of bioactive coatings on polyether ether ketone.

Polyether ether ketone (PEEK) is a highly heat-resistant thermoplastic with excellent strength and elastic modulus similar to human bone, making it an attractive material for orthopedic implants. However, the hydrophobic surface of PEEK implants induces fibrous encapsulation which is unfavorable for stable implant anchorage. In this study, PEEK was coated via ion-beam-assisted deposition (IBAD) using a two-layer design of yttria-stabilized zirconia (YSZ) as a heat-protection layer, and hydroxyapatite (HA) as a top layer to improve osseointegration. Microstructural analysis of the coatings showed a dense, uniform columnar grain structure in the YSZ layer and no delamination from the substrate. The HA layer was found to be amorphous and free of porosities in its as-deposited state. Subsequent heat treatment via microwave energy followed by autoclaving crystallized the HA layer, confirmed by SEM and XRD analysis. An in vitro study using MC3T3 preosteoblast cells showed improved bioactivity in heat-treated sample groups. Cell proliferation, differentiation, and mineralization were analyzed by MTT assay and DNA content, osteocalcin expression, and Alizarin Red S (AR-S) content, respectively. Initial cell growth was increased, and osteogenic maturation and mineralization were accelerated most on coatings that underwent a combined microwave and autoclave heat treatment process as compared to uncoated PEEK and amorphous HA surfaces. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 560-567, 2017.

Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray

Rare earth zirconates have lower thermal conductivity, better phase stability, improved sintering resistance and CMAS (calcium magnesium alumino silicates) infiltration resistance than yttria stabilized zirconia (YSZ) at temperatures above 1200 °C. However, their lower fracture toughness and lower coefficient of thermal expansion (CTE) compared to YSZ lead to premature coating failure. In order to overcome these drawbacks at higher temperatures, a multilayered coating approach is attempted in this study and compared with the single layer YSZ. Suspension plasma spray of single layer YSZ, single layer gadolinium zirconate (GZ) and double layer GZ/YSZ was carried out. Additionally, a triple layer coating system, with denser gadolinium zirconate on top of the GZ/YSZ system was sprayed to impart an added functionality of sealing the TBC from CMAS infiltration. Microstructural analysis was done using scanning electron microscopy and optical microscopy. Columnar microstructure with vertical cracks was observed. XRD analysis was used to identify phases formed in the as sprayed TBC samples. Porosity measurements were done using water impregnation method. Thermal diffusivity of single and multi-layered coatings was obtained by laser flash analysis and thermal conductivity of the coating systems was determined. It was found that the thermal conductivity of single layer gadolinium zirconate was lower than YSZ and that the thermal conductivity of multilayered systems were between their respective single layers. The single (YSZ), double (GZ/YSZ) and triple (GZ dense/GZ/YSZ) layer TBCs were subjected to thermal cyclic fatigue (TCF) test at 1100 °C and 1200 °C. It was observed that the single layer YSZ had lowest TCF life whereas the triple layer TBC had highest TCF life irrespective of test temperature.