Envisioning the use of nanostructured YSZ coatings at high temperatures cause concerns in the scientific community. Questions have been raised about the possibility of accelerated sintering of these ultra-fine materials and the associated changes in properties that could accompany this sintering. In this work, nanostructured YSZ coatings were engineered to counteract sintering effects by tailoring the coatings to exhibit a bimodal microstructure formed by (i) a matrix of dense YSZ zones (produced from molten YSZ particles) and (ii) large porous nanostructured YSZ zones (produced from semi-molten nanostructured YSZ particles) that were embedded in the coating microstructure during thermal spraying. These coatings were subjected to heat treatment in air at 1400°C for 1, 5 and 20 h. The superior driving force for sintering exhibited by the porous nanozones, when compared to that of the dense zones, caused the nanozones to shrink at much faster rates than those exhibited by the denser matrix zones (i.e., differential sintering), thereby creating a significant network of voids in the coating microstructure. Due to these effects, after 20 h exposure at 1400°C, the thermal conductivity and elastic modulus values of the conventional coatings were approximately two times higher than those of the nanostructured ones.

Spark-plasma sintering (SPS) was adopted in this study as a rapid post-spray treatment for yttria stabilized zirconia (YSZ) electrolytes prepared through the direct current (dc) plasma spray process. The lamellar microstructure in the as-sprayed samples was found to significantly reduced the ionic conductivity of the YSZ electrolytes. However, the ionic conductivity (at 1053 °C) increased sharply from 0.065 S/cm for the as-sprayed electrolyte to 0.122 S/cm for electrolyte post-spray treated through SPS at 1400°C for 3 minutes. These phenomena were attributed to the microstructure transformation from lamellar structure of the as-sprayed samples to equi-axial type granular structure of the post-spray samples treated by SPS. Correspondingly, porosity reduced from ~ 10.72 % in as-sprayed coating to ~ 1.38% in the SPS sample treated at 1400°C for 3 min. Majority of the pores in the SPS sample were also found to have contracted to a narrow size range 0.03 – 1 µm. AC impedance spectroscopy demonstrated the effect of the microstructure modification between samples treated in the SPS at temperature 1200, 1300 and 1400°C for 3 minutes with the concomitant reduction of the resistivity, which is consistent with the microstructure changes of the YSZ electrolyte from lamellar structure to granular structure. Overall, the results show that spark-plasma-sintering (SPS) is an effective and rapid post-spray treatment to improve the relative density and electrical properties of plasma sprayed YSZ electrolytes.

This study investigates the effects of spark plasma sintering (SPS) on the microstructure and properties of cold-sprayed metallic coatings. Water-atomized Cu powder was deposited on Al 5052 substrates by high-pressure cold gas spraying, and the resulting coatings were treated by spark plasma sintering and annealing heat treatment (AHT) at 200°C, 300°C, and 400 °C. To assess the effects of diffusion generated by pulsed dc power, a vertical load was not applied in the SPS system. In addition, a short duration time was used to inhibit crystal grain growth. Treated specimens were evaluated by SEM, EBSD, and hardness and tensile testing. The findings show that the microstructure and hardness of SPS specimens treated at 300 °C are close to that of AHT specimens treated at 400 °C. Tensile strength, however, is clearly higher in the SPS300 specimens, indicating that pulsed dc power accelerates particle interdiffusion due to Joule heating and electromigration, thereby increasing adhesion strength between particles in the coating.

Dysprosia stabilized zirconia (DySZ) is a promising candidate to replace yttria stabilized zirconia (YSZ) as a thermal barrier coating due to its lower inherent thermal conductivity. It is also suggested in studies that DySZ may show greater stability to high temperature phase changes compared to YSZ, possibly allowing for coatings with extended lifetimes. Separately, the impurity content of YSZ powders has been shown to influence high-temperature sintering behavior. By lowering the impurity oxides within the spray powder, a coating more resistant to sintering can be produced. This study evaluates high purity and standard purity dysprosia and yttria stabilized zirconia coatings and their performance after extended heat treatment. Coatings were produced using powders with the same morphology and grain size; only the dopant and impurity content were varied. Samples were heat treated for up to 400 hours at 1150 °C and thermal conductivity measurements were plotted to show the evolution of thermal properties with respect to time. Thermal conductivity is compared to coating microstructure and porosity in order to track structural changes due to sintering.

Direct Metal Laser Sintering (DMLS) technique is one of the technologies which is generally used to built prototypes and tooling applications. DMLS uses powder bed fusion to bond particles together by laser energy. A new powder layer is spread on top of the previous layers and the process is repeated up to required shape of part can be produced. This review paper presents development, current status and challenges of the DMLS technique with emphasises on material processed by DMLS and is aimed to understand influence of density, microstructure, micro-hardness, tensile strength and wear behaviour of built-up parts. It also highlights the process through proofs based on classical results in terms of advantages and applications.

Thermal spraying may be a promising approach for the deposition of NiAl-Al 2 O 3 composite coatings. To understand the intrinsic mechanical property of NiAl-Al 2 O 3 composite, dense NiAl-Al 2 O 3 composites were prepared by plasma activated sintering using the ball milled Ni/Al- Al 2 O 3 composite powders contain both micro-sized Al 2 O 3 strengthening particles and submicron-sized Al 2 O 3 dispersoids homogeneously distributed in the NiAl matrix phase. The angular morphology of the Al 2 O 3 particles in composite powder was changed to spherical or near spherical morphology after plasma activated sintering. Compared to the NiAl-Al 2 O 3 composites with low Al 2 O 3 content, NiAl- 60vol.%Al 2 O 3 composite consisted of less fine submicron-sized Al 2 O 3 dispersoids. Melting induced mechanism was proposed to explain the spheroidization and the change of Al 2 O 3 particle size distribution. The hardness of the NiAl-Al 2 O 3 composites with both micro-sized Al 2 O 3 strengthening particles and submicron-sized Al 2 O 3 dispersoids increased with the increase of Al 2 O 3 content. The relation was employed to theoretically estimate the hardness using volume fractions of components and corresponding hardness. The estimated hardness was compared with the observed ones. It was found that bimodal sized particles reinforcement results in a higher hardness than the theoretical value, which could be attributed to the dispersion hardening effect.

Lanthanum gallate doped with strontium and magnesium (LSGM) is a good electrolyte candidate for Intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this study, low-temperature sintering is used to increase the density of LSGM coatings prepared by vacuum cold spraying (VCS). LSGM layers with different thickness were deposited by VCS on NiO-YSZ substrates. In order to suppress chemical reactions between Ni and LSGM, the substrates were coated with gadolinium-doped ceria by tape casting. After sintering at 1200 °C, the coatings were found to be denser in most regions due to grain growth, which appears to be accompanied by cracking, particularly in thicker layers. A second layer was deposited on the annealed coatings to seal the cracks and the two-layer structure was further sintered. Gas permeability test results show that the multilayer films are dense enough to consider their use as electrolyte membranes in IT-SOFCs.

Plasma-sprayed thermal barrier coating (TBC) systems are widely used in gas turbines blades in order to increase turbine entry temperature (TET) with better efficiency. Yttria stabilized zirconia (YSZ) has been usually chosen as the top thermal barrier coating material because of its low thermal conductivity, high thermal expansion coefficient and good corrosion resistance. However as a new candidate commercial TBC material, ceria stabilized zirconia (CSZ) currently looks to be promising. Ceria and ceria based ceramics show an outstanding potential for use at temperatures exceeding 1200 °C. CSZ coatings do not only have high temperature stability, good corrosion resistance and high fracture toughness but also lower thermal conductivity and higher thermal expansion coefficient than YSZ coatings. The sintering and phase transformation characteristics of both ceramic thermal barrier coatings under high temperature conditions are complex phenomena. In this paper, microstructural differences, sintering behaviours (1200 oC, 10h, 25h and 50h) and phase transformations of the plasma sprayed ceria stabilized zirconia (CSZ: ZrO 2 –2.5 wt.%Y 2 O 3 – 25 wt.%CeO 2 ) and conventional yttria stabilized zirconia (YSZ; ZrO 2 –8 wt.%Y 2 O 3 ) coatings and their powder materials have been investigated and compared using thermal analysis techniques, XRD and scanning electron microscope.

High-purity nanocrystallized YSZ powders were used to manufacture thermal barrier coatings by air plasma spraying. After spraying, the coating samples were aged at temperatures of 1200, 1300, and 1400 °C. Coating samples made from ordinary YSZ powders were aged at the same temperatures. XRD analysis shows significant tetragonal-to-monoclinic phase transformation in the reference coatings after 100 h at 1400 °C in contrast to the phase stability exhibited by high-purity YSZ. The sintering behavior of the YSZ coatings was also examined along with the influence of MCrAlY oxidation.

Alumina-zirconia ceramic material has been plasma sprayed using a water stabilized plasma torch (WSP) to produce free standing coatings. The as-sprayed coatings have very low porosity and are mostly amorphous. The amorphous material crystallizes at temperatures above 900 °C. A spark plasma sintering apparatus has been used to heat the as-sprayed samples to temperatures above 900 °C to induce crystallization while at the same time a uniaxial pressure of 80 GPa has been applied to the their surface. After such post-treatment, the ceramic samples are crystalline and exhibit very low open porosity. The as-sprayed amorphous materials also exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat-treated samples whose microstructure is best described as nanocomposite with the very small crystallites embedded in an amorphous matrix.

Plasma-sprayed thermal barrier coatings are highly insulating due to intersplat pores. High-temperature exposure, however, leads to pore healing and a loss of thermal insulating capability. Previous sintering models based on the healing of iso-thick pores cannot explain the ultrafast sintering kinetics observed during the initial sintering stage. To better understand the sintering process and the kinetics of pore healing, a study was conducted on the changes that occur at the interfaces between lamellae at high temperatures. It was found that splat surfaces develop multiscale convexes, forming bridges between adjacent splats that significantly accelerate pore healing. This model explains the fast sintering kinetics of the initial sintering stage and presents new opportunities to further tailor the sintering kinetics of YSZ coatings and thereby improve TBC performance.

In surface science of functional oxides, Titanium dioxide (TiO 2 ) is one of the most investigated crystalline systems either in rutile or anatase phases. Rutile phase is widely used in microelectronic, tribological applications and in the conversion of solar energy. Anatase phase is used in self-cleaning, antifogging, photo-catalytic and biomedical technologies. This work focuses on studying the required process conditions to obtain TiO 2 targets by APS onto metallic substrates using commercial TiO 2 powders (Oerlikon Metco in Switzerland) with suitable physical and chemical properties for technological and medical applications as PAPVD coatings. APS targets were compared to sintered ones. The raw powders were characterized by laser diffraction, SEM, XRF and XRD while the characteristics of the APS-deposited targets as well as the sintered ones were determined by SEM and XRD to identify the constituent phases. This work allowed confirming the advantages and limitations of both processes in terms of grain size, chemical composition, microstructural homogeneity and density in order to choose the best option to manufacture targets for PAPVD coatings for technological and medical applications.

Recently a Ti–TiAl 3 metal–intermetallic laminate (MIL) composite attracts growing attention because they have potential application in honeycomb or sandwich components of airplanes and as biomaterial with good bio-compatibility. Of the available processing techniques, diffusion bonding of elemental titanium and aluminum foils is an effective low-temperature method to synthesize the composite, allowing growth of the intermetallic layer. However, application of assembling and multi-pass cold rolling operations leads to fact that this technology is complex and expensive. The use of Cold Spray technology instead of aluminum foils utilization and multi-pass cold rolling to produce the Ti–TiAl 3 MIL composites is believed to be more effective. However, reaction diffusion kinetics of Ti-Al particulate composite differs from that of classical MIL composite and needs to be studied. The task of this paper is to define microstructural changes of Tl-TiAl 3 composite coating during cold spraying and reaction sintering. The optical microscopy, SEM, EDS, X-ray and microhardness examinations are presented and discussed.

Atmospheric plasma sprayed (APS) thermal barrier coatings (TBCs) with lamellar structure exhibit low thermal conductivity and low stiffness. However, high temperature exposure for certain long duration causes the sintering which heals two-dimensional (2D) inter-lamellar pores and intrasplat pores. Such sintering effect increases the stiffness and thermal conductivity of the coatings and consequently reduces the stability and durability of TBCs. It can be expected that large 2D pores with a wide opening is difficult to be eliminated. In this study, inter-lamellar 2D pores with large opening width were fabricated in the La 2 Zr 2 O 7 (LZO) coatings through spraying LZO+SrO coatings and removing the SrO splats in the water. Then, the conventional LZO coating and the porous LZO coating were subjected to high temperature exposure in the air at 1300 °C for different durations. The microstructure evolution especially in terms of the shape and density of inter-lamellar 2D pores was examined. In addition, the change of thermo-physic properties and the mechanical properties of the coatings with increasing exposure duration were studied. Results show that the 2D pores in LZO coating created by those SrO splats inherit primarily large opening width from 200nm to about 1 µm which endows the LZO coating with high sustainability at high temperature environment. Under thermal exposure at 1300°C, it was found that 2D pores resulting from SrO splats are free from healing while conventional 2D inter-lamellar pores with small opening width formed during splat cooling became healed rapidly. Thus, thermal conductivity and Young's modulus of the conventional LZO coating increased rapidly, while unhealed 2D pores in the highly porous LZO coatings contributed to the low Young's modulus and low thermal conductivity of LZO coating with remarkably high stability. With addition of 30% SrO in spray powder, a LZO coating with a thermal conductivity of about 0.39 W.m -1 .K -1 in the as-prepared state was obtained. The coating maintained a thermal conductivity of 0.57 W.m -1 .K -1 even after 100 hours exposure at 1300°C. The present results indicated that high sintering-resistant thermal barrier coating can be fabricated though designing inter-lamellar 2D pores with large opening width in the coating by the present novel approach.

Multilayer thermal barrier coatings are being investigated for high temperature applications by employing a strain accommodating interlayer. Plasma sprayed coatings of nanostructured feedstock have shown promise in this direction. Layers of nanostructured yttria stabilized zirconia (nano–YSZ) and conventional YSZ were deposited on mullite substrates using the triple torch plasma reactor (TTPR), and on NiCrAlY coated steel substrates using the Praxair SG-100 plasma torch. The coatings were heat treated and the microstructure evaluated. Heat treating the samples lead to the formation of larger pores with a significant proportion of partially molten particles. The porosity evolved from the partial sintering of the nano-agglomerates. Porosity change during the sintering process was measured and the microstructure observed using electron microscopy. The nanostructured coatings were compared to conventional YSZ coatings.

This paper examines the effects of spark plasma sintering (SPS) on flame-sprayed zirconia coatings. It describes how the zirconia layers were produced, treated, and tested. The combination of heating and loading increased coating hardness and adhesion strength by a factor of three and caused a significant reduction in porosity. It also led to phase transformations which, in some cases, had an offsetting effect on coating properties. Paper includes a German-language abstract.

Plasma spraying is a fast and inexpensive process for fabricating YSZ electrolyte for SOFCs. In this investigation, free-standing plasma sprayed YSZ disks are treated by spark plasma sintering at different temperatures, soak times, and loading cycles. SEM examination shows that the lamellar microstructure of the as-sprayed zirconia is converted to a predominantly granular structure with no significant phase changes as per XRD analysis. Microhardness and laser flash diffusivity measurements show that the SPS treatments also improve YSZ layer density, tensile modulus, and thermal conductivity. Paper includes a German-language abstract.

This paper investigates the influence of microstructure on the sintering behavior of thermal barrier coatings. A significant difference is observed between coatings with typical microcracks and those with segmentation cracks. Segmented coatings showed a lower sintering rate and little change in pore size distribution after heat treatment. Conventional coatings, on the other hand, showed an increase in pore diameter and in more porous areas, sintering was more pronounced. All coatings had an elastic modulus in the range of 15-20 GPa, which more than doubled after a 10-h heat treatment at 1200 °C. Paper includes a German-language abstract.

This paper examines the influence of spark plasma sintering (SPS) on plasma-sprayed hydroxyapatite (HA) coatings. For comparison purposes, a conventional heat treatment is also carried out. The surface microstructure as well as the crystallinity of each layer is determined by means of SEM and XRD analysis. Test results show that the crystallinity of the layers increases with increasing SPS temperature up to 800 °C and a large amount of β tricalcium phosphate is formed. Paper includes a German-language abstract.

Superalloy substrates coated with plasma sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats have been subjected to a high heat flux in a controlled atmosphere chamber. The sintering exhibited by the top coat under these conditions has been studied and compared with the behavior observed during isothermal heating, both when attached to the substrate and when detached. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical heat flow model has been used to explore the stress state under isothermal and thermal gradient conditions. Sintering proceeds faster at higher temperature, but is retarded by the presence of tensile stresses (from differential thermal expansion between coating and substrate) within the top coat. Sintering occurs preferentially near the free surface of the top coat under gradient conditions, not only because of the higher temperature, but also because the in-plane stress is more compressive in that region.