The emergence of powder bed fusion in recent years has made it one of the most demanded additive manufacturing technologies for Ti6Al4V alloys in the biomedical and aerospace industries due to its ...ease of part fabrication with complex geometry. However, Ti6Al4V components require a post-processing treatment to optimize their mechanical properties for engineering applications. The present study offers an analysis of the effects of different industrial heat treatments (704 °C for 120 min, 740 °C for 130 min) on the microstructure and tensile properties of Ti6Al4V samples manufactured by laser powder bed fusion in different orientations (Z, 45°, XY and XZ). These heat treatments were selected to improve the mechanical properties of the as-built material and to obtain samples representative of real industrial applications. SEM observations illustrated that the α’ martensite grown in the columnar parent β-grains was converted into an α+β mixture after both heat treatments. EBSD showed that the newly formed α phase maintained, inside the parent β grains, the same orientation relationship as the as-built α′ martensite. However, on a macroscopic scale, the α phase exhibited no preferential orientation. The heat-treated samples, which exhibited approximately 10% lower ultimate tensile strength and yield strength than the as built samples but a 10–13% higher elongation, were practically isotropic in their mechanical response. What little anisotropy was left was mainly attributed to few elongated interlamellar pores. The sandblasting process was also investigated; it did not affect the static mechanical properties of the samples but it reduced the Ra values by 25%. Finally, Vickers microhardness vs. strength relationships were studied by considering the α-phase orientation.
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•Heat treatments below the β-transus decomposed α′ martensite into α+β.•The newly formed α maintained the orientational relation of α′ with the parent β.•However, at the macroscale, the α phase had no preferential orientation.•Thus, there was almost no anisotropy in mechanical response of heat-treated samples.•Sandblasting did not affect the static mechanical properties.
Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al2O3-SiO2 (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. ...The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.
Simulations of laser-based directed energy deposition of metals have received increasing interest aimed at reducing the experimental effort to select the proper processing condition for the repair or ...overhaul of actual components. One of the main issues to be addressed is the evaluation of the residual stress, which may lead to part failure under nominal loading. In this frame and specifically relating to aluminum alloys, few works have been developed and validated. This lack of knowledge is addressed in this paper: namely, the proper approach to simulate the activation of the deposited metal is discussed in case of single deposition and is shifted to a case of multiple depositions over a substrate. The validation of the predicted residual stress is made by comparison with the actual stress resulting from X-ray diffraction.
This paper studies how the nano-mechanical properties of thermal barrier coatings (TBCs) vary during thermal cycling, as a way to shed new light on their failure mechanisms. In particular, ...high-throughput nanoindentation revealed the evolution of hardness and elastic modulus distributions of plasma-sprayed yttria-stabilized zirconia (YSZ) top layers. The evolution of fracture toughness of the YSZ layers and the thermally grown oxide (TGO) formed onto the vacuum plasma-sprayed NiCoCrAlY bond coat were investigated by nanoindentation micro-pillar splitting.
The TGO fracture toughness increases up to ≈2.5–3.5 MPa√m at the early stages of thermal cycling, followed by a rapid decrease to ≈2.0 MPa√m after a critical TGO thickness of ≈5 μm is reached. Consequently, interface damage is initially limited to short cracks within the YSZ material. As TGO thickness exceeds the critical threshold, multiple cracks originate within the TGO and join through the YSZ to form long delamination cracks. Joining is favoured by a simultaneous loss in YSZ strength, testified by a decrease in the nanomechanical properties (hardness, elastic modulus) of both high- and low-porosity top coats. This is due to microstructural changes occurring because of the continuous interplay between sintering and thermal shock cracking in the YSZ layers.
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•Nanomechanical properties of thermal barrier coatings were measured by high-speed nanoindentation and pillar splitting•During thermal cycling, delamination damage progresses slowly as long as the bond coat forms a dense, tough oxide scale•The fracture toughness of the thermally grown oxide scale drops when its thickness exceeds a 5-μm threshold•Severe delamination damage starts from the embrittled thermally grown oxide and propagates across the zirconia top coat•Such propagation is favoured by a simultaneous drop in top coat strength, due to accumulated microstructural alterations
This paper assesses the diverse potentialities of two different suspension spraying processes, namely High Velocity Suspension Flame Spraying (HVSFS) and Suspension Plasma Spraying (SPS), for the ...deposition of bioactive coatings based on hydroxyapatite and on a new, custom-made K2O–Na2O–CaO–P2O5–SiO2 bioactive glass.
With both feedstock types, the HVSFS process imparts high in-flight velocities to the particles and aggregates released after solvent vaporisation, resulting in well flattened, tightly bound lamellae. The coatings, <50μm thick and very dense, have hardness and elastic modulus values close to those of the corresponding bulk materials. They can be employed as high-quality bioactive layers on metallic implantable devices. Few days of soaking in simulated body fluid (SBF) results in the re-precipitation of a surface hydroxyapatite layer, albeit through different mechanisms. In HVSFS bioactive glass coatings, ion leaching turns the surface into a silica gel, onto which hydroxyapatite subsequently deposits. In HVSFS hydroxyapatite, the amorphous fraction is progressively dissolved and microcrystalline hydroxyapatite precipitates onto the remaining coating layer.
The SPS technique, due to the lower in-flight velocity of particles and agglomerates, always produces more porous, rougher layers with columnar-like growth. They are not mechanically strong, but their peculiar structure can be useful for specific, functional applications. The high surface area of porous SPS bioactive glass coatings favours ion leaching and fast dissolution in simulated body fluid (SBF); hence, it is suggested that SPS bioglass could be useful as a rapidly resorbable layer. SPS hydroxyapatite, by contrast, is more stable than the corresponding HVSFS layer, despite its porosity, because of the higher crystallinity. After the amorphous fraction is dissolved in SBF, newly formed hydroxyapatite does not constitute a surface layer but precipitates inside the pores, suggesting that a sealing pre-treatment in SBF could be a means to tune porosity and phase composition.
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•Bioglass and hydroxyapatite layers deposited by SPS and HVSFS processes are compared.•SPS coatings have porous columnar structure, with low particle flattening degrees.•SPS bioglass layers are rapidly resorbable in simulated body fluid solution.•HVSFS coatings are very dense, thin (<50μm), with tightly bound, flattened lamellae.•HVSFS bioglass layers react slower when immersed in simulated body fluid solution.
Ti6Al4V-ELI is the most-used lightweight alloy in the aerospace industrial sector thanks to its high mechanical strength and corrosion resistance. The present paper aims, firstly, to evaluate the ...effects induced by different heat treatments, which were performed above and below the β-transus temperature on Ti6Al4V-ELI samples manufactured via Laser Powder-Bed Fusion in different orientations (XZ, XY, Z and 45°). The first set of tensile samples and bars were heat-treated at 1050 °C × 1 h, while the second and third set were heat-treated at 704 °C × 120′ following the AMS2801 standard specification, and at 740 °C × 130′. These heat treatments were chosen to improve the as-built mechanical properties according to the ASTM F3001 and also ASTM F2924-14 standard specifications. Optical and SEM measurements reveal primary, secondary and tertiary α-laths below the β-transus, while above this temperature, the microstructure varies in relation to the sample’s thickness. Secondly, this work analyzed the results obtained after a sandblasting process, which was performed on half of all the available heat-treated tensile samples, through XRD and Vickers microhardness measurements. XRD analysis also highlighted the presence of α2-Ti3Al and TiAl3 precipitates and the microstructural change in terms of the α-phase.
Interconnects employed in solid oxide fuel cells require electrically conductive protective coatings such as those based on manganese cobalt oxide spinels in order to prevent evaporation of volatile ...Cr(VI)-compounds and to minimize high temperature corrosion. MnCo2−xFexO4 based (where x = 0.1 and 0.3) oxide spinel protective coatings were manufactured by the atmospheric plasma spraying process on Crofer 22 APU substrates. The coated substrates were oxidized at 700 °C in air for 1000 h and post-mortem analyses were conducted to study the performance of the thermal sprayed coatings. During the high temperature oxidation, a four-point on-line measurement technique was used for area specific resistance studies. The MnCo1.7Fe0.3O4 coating was tested together with the La0.85Sr0.15Mn1.1O3-spacer.
The atmospheric plasma sprayed MnCo2−xFexO4 showed excellent Cr-barrier properties and decreased oxidation of the substrate. In addition, resistance as low as 9.20 mΩ cm2 values was obtained after the 1000 h oxidation cycle. The MnCo1.9Fe0.1O4 coating manufactured by the atmospheric plasma spraying process is a promising candidate for SOFC interconnect applications.
•Dense spinel coatings were produced with the atmospheric plasma spraying process.•MnCo1.9Fe0.1O4 and MnCo1.7Fe0.3O4 coatings provided good Cr-barrier properties.•9.20 mΩ cm2 values were obtained by using MnCo1.9Fe0.1O4 oxide spinel coatings.•50 mΩ cm2 values were obtained with MnCo1.7Fe0.3O4-LSM contact.
Post‐surgery microbial infections are still one of the main reasons for implant failure, which results in very high physical and psychological pain for the patient and an increased cost for the ...healthcare system. A polymer nanocomposite antibacterial coating on titanium implants represents a valuable alternative to the more expensive and energy‐consuming technological solutions nowadays used. In this regard, a sprayable thermoset nanocomposite composed of silanized‐terminals polyethylene glycol (PEG)/ZnO nanoparticle is herein proposed. Initially, PEG's terminals' solvent‐free silanization and curing are studied by Fourier Transform Infrared and µRaman spectroscopies. Scanning Electron Microscope investigations and scratch tests have shown that the spraying procedure optimization and the oxidation treatment of the titanium substrate lead to a homogeneous coverage and improved adhesion of the coatings. The antibacterial activity is tested against not only both S. aureus and P. aeruginosa bacterial American Type Culture Collection strains, but also using very aggressive antibiotic‐resistant clinical strains. Interestingly, antibacterial activity, evaluated by time‐killing tests, is observed for all tested bacterial strains. Live/dead tests further confirm that 5 wt% of ZnO allows obtaining a bacteriostatic activity within 24 h, whereas a complete growth inhibition (bactericidal activity) of both tested strains is observed for coatings with 20 wt% of ZnO nanoparticles.
Sprayable silanized‐polyethylene glycol (PEG)/ZnO nanocomposite coatings are developed for preventing post‐surgery microbial infections of titanium implants. The proposed coatings have a simple and reproducible application procedure and show very high adhesion to the titanium substrate. Moreover, the combination of PEG and ZnO results in a material with versatile antibacterial properties against several different bacterial strains and biofilm formation.
In this study, two polymeric materials were tested in a dry rotating “pin-on-disc” configuration against differently coated surfaces, to evaluate their tribological response under conditions, such as ...those of rotary lip seals, and to identify the wear mechanism of each coupling. A PTFE based material, reinforced with glass fibers and a solid lubricant, and unreinforced polyketone were tested against a chromium oxide coating deposited by plasma thermal spraying, a CrN/NbN superlattice coating deposited by Physical Vapor Deposition (PVD), and a Diamond-Like Carbon (DLC) coating obtained through a hybrid PVD/PECVD (Plasma-Enhanced Chemical Vapor Deposition) process. The PTFE matrix composite offers better overall performance, in terms of specific wear rates and friction coefficients than polyketone. Although the tribological behavior of this material is generally worse than that of the PTFE matrix composite, it can be used without reinforcing fillers. Our analysis demonstrates the importance of transfer-film formation on the counter-surfaces, which can prevent further wear of the polymer if it adheres well to the counterpart. However, the tribofilm has opposing effects on the friction coefficient for the two materials: its formation leads to lower friction for PTFE and higher friction for polyketone.
The microstructure, mechanical properties (Vickers microhardness, fracture toughness, elastic modulus) and tribological behaviour (abrasion and unlubricated sliding wear resistance) of various kinds ...of electrolitic hard chrome (EHC) coatings and of metallic and cermet HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) have been studied. EHC coatings are very tough, in fact no cracks could be produced with 10 N Vickers microindentations. However, their superficial morphology and Vickers microhardness are significantly affected by the substrate preparation technique, the deposition process characteristics, and by post-deposition treatments. This causes the coatings to show different behaviors in different wear conditions; in particular, harder coatings are more abrasion-resistant, but undergo a more severe mass loss under unlubricated sliding conditions. HVOF-sprayed cermet coatings are harder but less tough than EHC ones. Therefore, they undergo a comparable or even higher mass loss when subjected to three-body abrasion conditions. However, their two-body sliding resistance definitely overcomes that of EHC coatings, because they form a tough and uniform surface film protecting them from further damage.
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