Thermal Barrier Coating (TBC) systems are frequently used in gas turbine engines to provide thermal insulation to the hot-section metallic
components and also to protect them from oxidation, hot corrosion and erosion. Surface sealing treatments, namely laser-glazing, have been
showing a high potential for extending in-service lifetimes of these systems by improving chemical and thermo-mechanical resistance. In this
investigation, both as-sprayed and laser-glazed TBCs were exposed to hot corrosion in molten salts. The glazed coatings were obtained by
scanning the surface of the plasma-sprayed coatings with either a CO2 or a Nd:YAG laser. The hot corrosion investigation was accomplished by
subjecting the specimens to an isothermal air furnace testing under V2O5 and/or Na2SO4 at a temperature of 1000 °C for 100 h. Spallation has been
observed in coatings in the as-sprayed condition under V2O5 or V2O5+Na2SO4. Na2SO4 itself had no or minimal effect on the degradation of the
laser-glazed or as-sprayed condition coatings, respectively. The degradation in V2O5 was accomplished by destabilization of YSZ as a
consequence of depletion of yttria from the solid solution to form YVO4 and therefore led to the disruptive transformation of the metastable
tetragonal phase to the monoclinic phase. Moreover...
Laser glazing has been revealing a high potential for the improvement of plasma-sprayed (PS) thermal barrier coatings (TBCs) by
reducing surface roughness, eliminating open porosity on the surface and generating a controlled segmented crack network, although the
relationship of the processing parameters with the resultant properties has not yet been completely established. In this investigation, TBCs
consisting of atmospheric plasma-sprayed (APS) ZrO2–8wt.%Y2O3 were subjected to a CO2 continuous wave laser-glazing process in
order to seal its surface porosity, generating an external dense layer. For that purpose, different amounts of radiation resulting from different
scanning speeds were applied to the specimens as well as different track overlapping. Results have shown a significant decrease of the
surface roughness after the laser treatment. All specimens presented a fully dense and porous free external layer with a polyfaceted
columnar microstructure highly adherent to the plasma-sprayed coating. Controlled surface crack networks, extremely dependent on the
laser scanning speed and track overlapping, were achieved for each set of processing parameters. The cracks were found to have a
tendency to be oriented in two perpendicular directions...
Atmospheric plasma-sprayed (APS) ZrO2–8%WtY2O3 thermal barrier coatings (TBCs) were subjected to a CO2 continuous
wave laser-glazing process in order to generate an external dense layer produced by different processing parameters. For that
purpose, different beam scanning speeds and track overlapping were chosen.
Surface roughness has been reduced significantly after laser-glazing. Despite the surface crack network, all laser-glazed
specimens presented a fully dense and porous free external layer with a columnar microstructure. Surface cracks along the
densified layer were found to have tendency to be oriented in two perpendicular directions, one in the direction of the laser beam
travel, the other perpendicular to it. Moreover, the cracks parallel to the beam moving direction are found to be on the
overlapping zone, coinciding with the edge of the subsequent track. The cracks along the densified layer are vertical and tend to
branch and deviate from the vertical direction within the porous PS coating. The largest overlapping allied to the smallest amount
of irradiated energy generated the most uniform layer with the shortest crack branches within the PS coating. For the as-sprayed
coating, the XRD results revealed mainly t0 non-transformable tetragonal zirconia with a small percentage of residual
monoclinic zirconia. All glazed coatings presented only t0 non-transformable tetragonal zirconia with some variations on
preferable crystal orientation.; Fundação para a Ciência e a Tecnologia (FCT) - Project POCTI/CTM44590/2002.; União Europeia (UE). Fundo Europeu de Desenvolvimento Regional (FEDER).
Thermal spraying allows the production of overlay protective coatings of a great variety of materials, almost without limitations as to its components, phases and constituents on a range of substrates. Wear and corrosion resistant coatings account for significant utilization of thermal spray processes. Besides being a means to evaluate the coating tribological performance, erosion testing allows also an assessment of the coating toughness and adhesion. Nevertheless, the relationship between the erosion behavior of thermal sprayed coatings and its microstructural features is not satisfactorily understood yet. This paper examines room temperature solid particle erosion of zirconia and alumina-based ceramic coatings, with different levels of porosity and varying microstrucutre and mechanical properties. The erosion tests were carried out by a stream of alumina particles with an average size of 50 µm at 70 m/s, carried by an air jet with impingement angle 90°. The results indicate that current erosion models based on hardness alone cannot account for experimental results, and, that there is a strong relationship between the erosion rate and the porosity.
Implant related infection is one of the key concerns in total joint hip arthroplasties. In order to reduce bacterial adhesion, silver (Ag) / silver oxide (Ag2O) doping was used in plasma sprayed hydroxyapatite (HA) coating on titanium substrate. HA powder was doped with 2.0, 4.0 and 6.0 wt% Ag, heat treated at 800 °C and used for plasma spray coating using a 30 kW plasma spray system, equipped with supersonic nozzle. Application of supersonic plasma nozzle significantly reduced phase decomposition and amorphous phase formation in the HA coatings as evident by X-ray diffraction (XRD) study and Fourier transformed infrared spectroscopic (FTIR) analysis. Adhesive bond strength of more than 15 MPa ensured the mechanical integrity of the coatings. Resistance against bacterial adhesion of the coatings was determined by challenging them against Pseudomonas Aeruginosa (PAO1). Live/Dead staining of the adherent bacteria on the coating surfaces indicated a significant reduction in bacterial adhesion due to the presence of Ag. In vitro cell-material interactions and alkaline phosphatase (ALP) protein expressions were evaluated by culturing human fetal osteoblast cells (hFOB). Present results suggest that the plasma sprayed HA coatings doped with an optimum amount of Ag can have excellent antimicrobial property without altering mechanical property of the Ag doped HA coatings.
Infection in primary total joint prostheses is estimated to occur in up to 3% of all surgeries. As a measure to improve the antimicrobial properties of implant materials, silver (Ag) was incorporated into plasma sprayed hydroxyapatite (HA) coatings. To offset potential cytotoxic effects of Ag in the coatings, strontium (Sr) was also added as a binary dopant. HA powder were doped with 2.0 wt% Ag2O, 1.0 wt% SrO and the powder was then heat treated at 800° C. Titanium substrates were coated using a 30 kW plasma spray system equipped with a supersonic nozzle. X-ray diffraction (XRD) confirmed the phase purity and high crystallinity of the coatings. Samples were evaluated for mechanical stability by adhesive bond strength testing. Results show that the addition of dopants did not affect the overall bond strength of the coatings. The antibacterial efficacies of the coatings were tested against Pseudomonas aeruginosa. Samples that contained the Ag2O dopant were found to be highly effective against the bacterial colonization. In vitro cell-material interactions using human fetal osteoblast (hFOB) cells were characterized by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for cell viability, field emission scanning electron microscopy (FESEM) for cell morphology and confocal imaging for the important differentiation marker alkaline phosphatase (ALP). Our results showed evidence of cytotoxic effects in the Ag-HA coatings...
Chlorapatite can be considered a potential biomaterial for orthopaedic applications. Its use as plasma-sprayed coating could be of interest considering its thermal properties and particularly its ability to melt without decomposition unlike hydroxyapatite. Chlorapatite (ClA) was synthesized by a high-temperature ion exchange reaction starting from commercial stoichiometric hydroxyapatites (HA). The ClA powder showed similar characteristics as the original industrial HA powder, and was obtained in the monoclinic form. The HA and ClA powders were plasma-sprayed using a low-energy plasma spraying system with identical processing parameters. The coatings were characterized by physical-chemical methods, i.e. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, including distribution mapping of the main phases detected such as amorphous calcium phosphate (ACP), oxyapatite (OA), and HA or ClA. The unexpected formation of oxyapatite in ClA coatings was assigned to a side reaction with contaminating oxygenated species (O2, H2O). ClA coatings exhibited characteristics different from HA, showing a lower content of oxyapatite and amorphous phase. Although their adhesion strength was found to be lower than that of HA coatings...
Aluminum oxide (Al2O3, or alumina) is a conventional ceramic known for applications such as wear resistant coatings, thermal liners, heaters, crucibles, dielectric systems, etc. However applications of Al 2O3 are limited owing to its inherent brittleness. Due to its excellent mechanical properties and bending strength, carbon nanotubes (CNT) is an ideal reinforcement for Al2O3 matrix to improve its fracture toughness. ^ The role of CNT dispersion in the fracture toughening of the plasma sprayed Al2O3-CNT nanocomposite coating is discussed in the current work. Pretreatment of powder feedstock is required for dispersing CNTs in the matrix. Four coatings namely spray dried Al2O 3 (A-SD), Al2O3 blended with 4wt.% CNT (A4C-B), composite spray dried Al2O3-4wt.% CNT (A4C-SD) and composite spray dried A1203-8wt.% CNT (A8C-SD), are synthesized by plasma spraying. Owing to extreme temperatures and velocities involved in the plasma spraying of ceramics, retention of CNTs in the resulting coatings necessitates optimizing plasma processing parameters using an inflight particle diagnostic sensor. A bimodal microstructure was obtained in the matrix that consists of fully melted and resolidified structure and solid state sintered structure. CNTs are retained both in the fully melted region and solid-state sintered regions of processed coatings. ^ Fracture toughness of A-SD...
Plasma sprayed aluminum oxide ceramic coating is widely used due to its outstanding wear, corrosion, and thermal shock resistance. But porosity is the integral feature in the plasma sprayed coating which exponentially degrades its properties. In this study, process maps were developed to obtain Al2O3-CNT composite coatings with the highest density (i.e. lowest porosity) and improved mechanical and wear properties. Process map is defined as a set of relationships that correlates large number of plasma processing parameters to the coating properties. Carbon nanotubes (CNTs) were added as reinforcement to Al2O3 coating to improve the fracture toughness and wear resistance. Two novel powder processing approaches viz spray drying and chemical vapor growth were adopted to disperse CNTs in Al2O3 powder. The degree of CNT dispersion via chemical vapor deposition (CVD) was superior to spray drying but CVD could not synthesize powder in large amount. Hence optimization of plasma processing parameters and process map development was limited to spray dried Al2O3 powder containing 0, 4 and 8 wt. % CNTs. An empirical model using Pareto diagram was developed to link plasma processing parameters with the porosity of coating. Splat morphology as a function of plasma processing parameter was also studied to understand its effect on mechanical properties. Addition of a mere 1.5 wt. % CNTs via CVD technique showed ~27% and ~24% increase in the elastic modulus and fracture toughness respectively. Improved toughness was attributed to combined effect of lower porosity and uniform dispersion of CNTs which promoted the toughening by CNT bridging...
Aluminum oxide (A1203, or alumina) is a conventional ceramic known for applications such as wear resistant coatings, thermal liners, heaters, crucibles, dielectric systems, etc. However applications of A1203 are limited owing to its inherent brittleness. Due to its excellent mechanical properties and bending strength, carbon nanotubes (CNT) is an ideal reinforcement for A1203 matrix to improve its fracture toughness.
The role of CNT dispersion in the fracture toughening of the plasma sprayed A1203-CNT nanocomposite coating is discussed in the current work. Pretreatment of powder feedstock is required for dispersing CNTs in the matrix. Four coatings namely spray dried A1203 (A-SD), A1203 blended with 4wt.% CNT (A4C-B), composite spray dried A1203-4wt.% CNT (A4C-SD) and composite spray dried A1203-8wt.% CNT (A8CSD), are synthesized by plasma spraying. Owing to extreme temperatures and velocities involved in the plasma spraying of ceramics, retention of CNTs in the resulting coatings necessitates optimizing plasma processing parameters using an inflight particle diagnostic sensor. A bimodal microstructure was obtained in the matrix that consists of fully melted and resolidified structure and solid state sintered structure. CNTs are retained both in the fully melted region and solid-state sintered regions of processed coatings.
Fracture toughness of A-SD...
This paper reports the use of X-ray photoelectron spectroscopy (XPS) to investigate bisphosphonate (BP) adsorption onto plasma sprayed hydroxyapatite (HA) coatings commonly used for orthopaedic implants. BPs exhibit high binding affinity for the calcium present in HA and hence can be adsorbed onto HA-coated implants to exploit their beneficial properties for improved bone growth at the implant interface. A rigorous XPS analysis of pamidronate, a commonly used nitrogenous BP, adsorbed onto plasma sprayed HA-coated cobalt–chromium substrates has been carried out, aimed at: (a) confirming the adsorption of this BP onto HA; (b) studying the BP diffusion profile in the HA coating by employing the technique of XPS depth profiling; (c) confirming the bioactivity of the adsorbed BP. XPS spectra of plasma sprayed HA-coated discs exposed to a 10 mM aqueous BP solution (pamidronate) for periods of 1, 2 and 24 h showed nitrogen and phosphorous photoelectron signals corresponding to the BP, confirming its adsorption onto the HA substrate. XPS depth profiling of the 2 h BP-exposed HA discs showed penetration of the BP into the HA matrix to depths of at least 260 nm. The bioactivity of the adsorbed BP was confirmed by the observed inhibition of osteoclast (bone resorbing) cell activity. In comparison to the HA sample...
Thick (~1.2 mm) thermal barrier coatings (TBCs) consisting of YSZ were deposited by plasma spraying. Spraying parameters were varied in a controlled manner to produce different microstructures. The effect of substrate temperature on the microstructural features and subsequently on the Young's modulus was investigated. In addition, the residual stresses in the coatings were estimated using a numerical model and they were related to the microstructural features observed. Results showed that crack segmentation density, residual stresses in the coatings and thus coating properties are strongly affected not only by the average substrate temperature during spraying but also the variations between the minimum and maximum substrate temperature.; The author acknowledges the financial support received from Sulzer Metco, the Cambridge European Trust and from EPSRC for the realization of this work.
This paper concentrates on the degradation characteristics of hydroxyapatite (HA) coatings on orthopaedic Ti-6Al-4V alloy while immersed in Ringer's salt solution, which were investigated by electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy measurements were used to in situ characterize the electrochemical behavior of the passivated alloy covered with HA during aging in Ringer's solution. Comparison of the electrochemical data for the coated material with that for the uncoated metal substrate was also performed. The characteristic feature that describes the electrochemical behavior of the coated material is the coexistence of large areas of the coating itself with pores where the substrate is exposed to the aggressive media. The interpretation of results was thus performed in terms of a two-layer model of the film, in which the precipitation of hydrated oxide or phosphate compounds seals the pores left by the ceramic coating. The blocking effect due to salt precipitation inside the pores produces an enhancement of the resistance values, thus effectively diminishing the metal ion release in the system.; Collaborative Research Programme (Acción Integrada No. HP1995-0092 and HP1996-0109) between Spain and Portugal.
The corrosion behavior of four different preparations of plasma-sprayed hydroxyapatite (HA) coatings on Ti6Al4V substrates in static Hank's balanced salt solution was investigated using dc potentiodynamic and ac impedance techniques. Two different nominal thicknesses, 50 m and 200 m, and two different spraying conditions, were considered. The electrochemical impedance experiments proved this technique to be very suitable for the investigation of the electrochemical behavior of surgical implant alloys when they are coated with HA, which is characterized by the dissolution and passivation characteristics of the underlying metal substrate. Because the coatings are porous, ionic paths between the electrolytic medium and the base material can eventually be produced, resulting in the corrosion of the coated metal. Differences in the corrosion resistance of the coated materials were detected, and a relevant model for the description of the coating degradation in the biosimulating solution was proposed. The model consisted of the description of the coated system in terms of a two-layer model of the surface film. Significant differences in electrochemical behavior for similar nominal thicknesses of HA coatings obtained under different spraying conditions were found; Ministerio de Ciencia y Tecnología (Madrid...
This paper presents the wear characteristics of the composite ceramic coating made with Al2O3-40wt%8YSZ on the biomedical grade Ti-6Al-4V alloy (grade 5) used for total joint prosthetic components, with the aim of improving their tribological behavior. The coatings were deposited using a plasma spraying technique, and optimization of plasma parameters was performed using response surface methodology to obtain dense coating. The tribological behaviors of the coated and uncoated substrates were evaluated using a ball-on-plate sliding wear tester at 37°C in simulated body-fluid conditions. The microstructure of both the titanium alloy and coated specimen were examined using an optical microscope and scanning electron microscope. The hardness of the plasma-sprayed alumina–zirconia composite coatings was 2.5 times higher than that of the Ti-6Al-4V alloy, while the wear rate of Ti-6Al-4V alloy was 253 times higher than that of the composite-coated Ti-6Al-4V alloy. The superior wear resistance of the alumina–zirconia coated alloy is attributed to its enhanced hardness and intersplat bonding strength. Wear-track examination showed that the predominant wear mechanism of Ti-6Al-4V alloy was abrasive and adhesive wear, whereas, in the case of alumina–zirconia composite coated alloy...
The increase of the petroleum cost in the last decades revitalized the interest for lighter and more economic vehicles. Simultaneously, the demand for safe and unpolluted transports grows. The application of thermal barriers coatings (TBC) on combustion chamber and on flat surface of pistons reduces the thermal losses of the engines, resulting in higher temperatures in the combustion chamber. This fact contributes to the improvement of the thermal efficiency (performance) and for the reduction of incomplete combustion. Supported on these initial ideas, thermal barriers coatings constituted by CaO partially stabilized zirconia were produced and their microstructure examined. This coating still presents some drawbacks associated with thermal stresses and permeability to oxidizing gases, which will, eventually, lead to failure of the TBC by spallation. The failure may, in general, be associated to one of three factors: oxide growth at the ceramic-metal interface, formed during thermal cycling; stress build-up due to thermal cycling; and metal-oxide interface segregation, mainly of S. However, it is also relevant to understand the behavior of TBC's under isothermal oxidation. Therefore, this paper investigates the effect of oxidation on the adherence of thermal sprayed coatings. The adherence was measured by linear scratching tests...
Journal home page:
http://www.elsevier.com/wps/find/journaldescription.cws_home/504101/description#description; Three different projection system are used to prepare NiCrAlY bond coats over metallic substrates:
atmospheric plasma spray (APS), high velocity oxyfuel (HVOF) and high frequency pulse detonation
(HFPD). These coatings were tested in hot corrosion experiments with sprayed Na2SO4 at 1000ºC for 20
and 100 hours experiments in air. Coatings surface composition after thermal treatment was
characterised by XRD and SEM. Cross section of coatings were analysed by SEM-EDX. A relationship
between microstructural characteristics of initial coatings and final performance in hot corrosion was
found in terms of porosity percentage: plasma sprayed coatings present higher percentage of porosity
compared to HVOF and HFPD projection systems for the same composition and Al is heavily consumed
in interparticle oxidation. This Al depletion in turn involves intrinsic chemical failure and surface layer
is comprised by a porous spinel of mixed oxides. On the other hand, high energy projection systems
produce dense coatings allowing the Al migration to external alumina layer, particularly in the case of
HVOF coating.; The authors want to express their gratitude to the Spanish Ministry of Education and Science for the
financial support of the work...
[ES] Los recubrimientos de circona parcialmente estabilizada con itria se usan habitualmente como barreras térmicas en turbinas
y motores. Estos recubrimientos se suelen realizar mediante proyección térmica plasma (APS), una técnica relativamente
económica y bien conocida en el sector industrial. Las barreras consisten de una capa de 0.5 a 1 mm de cerámica muy porosa
unida a la aleación metálica mediante una capa de enlace. La elevada porosidad permite incrementar la resistencia al choque
térmico pero provoca que el oxigeno u otros gases perniciosos para el substrato lleguen con relativa facilidad, produciéndose
ataques que pueden acelerar el proceso de delaminación. Asimismo la superficie es rugosa y por tanto expuesta a la erosión
por partículas. En este trabajo se presentan las condiciones óptimas que permiten la resolidificación mediante láser de la
superficie de las capas de Y-PSZ (circona parcialmente estabilizada con itria) depositadas por proyección térmica de plasma
en aleaciones de Ni. Se presenta un sencillo modelo que permite explicar la energía umbral de fusión y su dependencia
con la velocidad de tratamiento. Se observa cómo la capa externa tratada se densifica, disminuye su rugosidad y queda
íntimamente ligada con la capa porosa de Y-PSZ.; [EN] Coatings of partially stabilised zirconia are currently used as thermal barrier coatings in gas turbines and diesel engines.
Zirconia coatings are usually processed via plasma spraying a well-established and relatively cheap technique. Thermal
barrier coatings consist of 0.5 to 1 mm of porous ceramic bonded to the metallic substrate by a bond-coat. High porosity in
the ceramic coating assure a good thermal behaviour but increases the chances for corrosion or chemical attack of the bondcoat
and coating detach by delamination processes. The as sprayed surface is rough and sensitive to erosion. In the present
work we obtained the processing conditions for laser remelting of Y-PSZ coatings deposited by atmospheric plasma spraying
technique on Ni- alloys. A simple model accounts for the onset melting energy and its dependence with the processing speed.
The remelted film is dense and with smooth surface and well bonded to the underlying porous coat.; Este proyecto ha sido financiado por el MCyT a través del proyecto MAT2003-06085-C03-01.; Peer reviewed
State of the art thermal barrier coatings (TBCs) for gas turbine applications comprise (7 wt.%) yttria partially stabilized zirconia (7YSZ). 7YSZ offers a range of attractive functional properties ? low thermal conductivity, high thermal expansion coefficient and high in-plane strain tolerance. However, as turbine entry temperatures are raised, the performance of 7YSZ coatings will be increasingly affected by sintering and environmental contamination, by calcia-magnesia-alumina-silica (CMAS) deposits.
The effect of sintering-induced stiffening on the driving force for spallation of plasma-sprayed (PS) TBCs was investigated. Spallation lifetimes of TBC specimens sprayed onto alumina substrates were measured. A simple fracture mechanics approach was employed in order to deduce a value for the strain energy release rate. The critical strain energy release rate was found to be constant, and if this value had been known beforehand, then the rationale presented here could be used for prediction of coating lifetime.
The effect of vermiculite (VM) and volcanic ash (VA) contamination on the sintering-induced spallation lifetime of PS TBCs was also investigated. The presence of both VM and VA was found to accelerate the rise in their Young?s modulus with sintering. Spallation results show that coating lifetime may be significantly reduced...
Phase transformations in air plasma-sprayed thermal barrier coatings composed of ZrO2 - 8 wt.% Y2O3 (zirconia - 8 wt.% yttria) are studied using X-Ray diffraction and Rietveld refinement measurements. Samples of TBC deposited onto Inconel 625 substrate were fabricated and heat treated at two different conditions: exposition to 1100ºC up to 1000 hours and exposition to temperatures between 700ºC and 1100ºC during 50 hours. According to Rietveld refinement measurements, the content of the cubic phase in the top coat increases with time and temperature; it starts at 7.3 wt.% and reaches 15.7 wt.% after 1000 hours at 1100ºC. The presence of a cubic phase in high amounts is undesirable due its lower mechanical properties compared with the tetragonal phase. After 800 hours of exposure to high temperature, the amount of Y²O³ in the tetragonal phase reduces to 6.6 wt.% and a fraction of this phase transforms to a monoclinic structure during cooling. The monoclinic phase reached 18.0 wt.% after 1000 hours. This phase is also undesirable, not only due to its higher thermal conductivity, but also because the tetragonal-to-monoclinic transformation implies a volume change of circa 5%, which favors crack formation and propagation and compromises the coating integrity.