This work focuses on the processing and deposit by suspension plasma spraying (SPS) of ZrO 2 -based ceramic materials for Thermal Barrier Coatings (TBC's) applications. The system of interest is ZrO 2 -16mol%Y 2 O 3 -16mol%Ta 2 O 5 (16YTZ). This ceramic has been reported to keep a non-transformable tetragonal phase (t'-phase), suitable to overcome the thermodynamic limits of the mostly used conventional 7-8wt.% yttria stabilized zirconia (YSZ). The research consists into evaluate the t'-phase stability and performance of the 16YTZ SPS coating. Synthesis of 16YTZ and, the evolution of the resulting microstructure in the dense ceramic and in the coating are a central part of the study. Sintering behavior in dense ceramics prepared from both precursor derived and milled powders is evaluated. Microstructural characterization by XRD, SEM and RAMAN spectroscopy of the as-deposited ceramic coating is presented and discussed.

Both as bulk material and coatings, cemented carbides currently occupy very well-established market niches and exhibit a promising future thanks to the development of compositions and manufacturing parameters. Direct comparisons of the properties of both are found only very rarely in the literature, very likely because the fields of application are complementary to each other but keep mostly separated. The current work is intended to evaluate similarities and differences in terms of microstructure and properties for two submicron WC-12 wt.%Co coatings obtained by High Velocity Air Fuel (HVAF) and Cold Gas Spray (CGS), together with a conventional sintered part. Microstructural features are discussed according to the inherent characteristics of each processing method. This covers a wide range in terms of the mechanical and thermal stresses acting on the material. While in CGS, the impacting particles do not melt, but experience extremely high plastic strain rates, the cobalt matrix is fully molten in the conventional sintering process, allowing time enough for diffusion processes. HVAF is to be placed in between, since the deposition process is characterized by a moderate heat input, leading to partial and/or full melting of cobalt, followed by rapid cooling. The microstructure and phases of the deposited coatings and bulk are characterized by using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Electron Backscattered Diffraction (EBSD) investigations enable local phase distribution of Co and WC in the samples. The hardness of the alloy processed by the three different routes is investigated as well. Additionally, electrochemical corrosion measurements in NaCl media are presented to evaluate the facility for electrolyte penetration and how the degradation of the material is affected by its inherent microstructure.

Cermets are composite materials consisting of a ceramic reinforcement and a metal matrix. Conventional tungsten carbide cermet parts containing a cobalt matrix phase are mainly produced by powder sintering. Laser Powder Bed Fusion (L-PBF) is an additive manufacturing technology widely applied for direct fabrication of metal functional parts with complex geometry. The present paper deals with the feasibility study of additive manufacturing of cermet parts by L-PBF using WC-17Co powder. The results showed that parametric optimisation of the L-PBF process allowed the production of solid WC-17Co part. Structural analysis revealed the presence of significant porosity (1.41%) and small-scale cracks in the as-built samples. Post-processing, such as HIP (Hot Isostatic Pressure) significantly improved the structure of manufactured parts. The porosity after HIP was very low (0.01%) and phase analysis revealed that the samples after HIP did not contain the fragile W 2 C phase. Abrasive wear tests showed that the wear resistance performance of additively manufactured parts was comparable to a reference produced by powder sintering. High values of hardness (around 1100 HV 30 ) were observed for the as-built and HIP samples. The study successfully demonstrated the possibility of manufacturing wear-resistant cermet parts by L-PBF.

Decomposition of the nitride ceramic particles like aluminium nitride (AlN) during conventional thermal spray process prevents their deposition on the substrate. Reactive plasma spraying (RPS) is a promising solution to fabricate AlN coatings. It is based on reaction and deposition of molten particles in active nitrogen ambient to form the AlN phase. Several thick AlN based coatings were fabricate successfully by reactive plasma spraying of aluminium and/or alumina particles. This study shows our recent achievements of fabrication of AlN coatings with improved conductivity. It was possible to fabricate AlN based coatings through reactive spraying of fine alumina particles mixed with fine AlN additives. Using small particle size powders improved the particles melting, surface area, therefore nitriding conversion and the AlN content. The fabricated AlN based coating contains several of oxide phases, with low density and high porosity, therefore its thermal conductivity was very low (about 2.6 W/m.K). To fabricate AlN coatings with high thermal conductivity, a liquid phase promoting additive (yttria) was added to the feedstock powder. It assists the formation of the yttrium aluminate (Y-Al-O) phase and therefore the sintering of the coatings during heat treatment. Finally, AlN coating with improved thermal conductivity (above 90 W/m.K) was developed.

Plasma spraying ZrB 2 ceramic coating is considered as potential candidate method of Thermal Protective System. While the application of plasma sprayed ZrB 2 coating is restricted due to its oxidation. Therefore, it is important to study the oxidation behavior of the ZrB 2 material. In this research, oxidation behavior of the ZrB 2 ceramic, which achieve different density by using SPS and pressureless sintering, is studied to explain the oxidation behavior of plasma spraying ZrB 2 ceramic coating. The oxidation behavior of ZrB 2 ceramics is investigated using SEM, XRD and EDS. The ZrB 2 –based ceramic coating is gravely oxidized at 600°C, but the block ZrB 2 –based ceramics also possess excellent oxidation resistance above 1000°C. The density of ZrB 2 ceramics significantly increase when changing the sintering method from pressureless sintering to SPS. The high density has beneficial effect to improve the oxidation resistance of ZrB 2 ceramic, for there are few open pores channel in high density ceramics. The oxygen cannot diffuse to the inner through pores, as a result, the high density ceramics can only be oxidized from outside to inside progressively, unlike low density ceramics, whose surface and inner is oxidized simultaneously.

A commercial abradable coating AlSi-hBN was fabricated by atmospheric plasma spraying and was thermal aged at 450 °C in the atmospheric environment for 1000h. Thermal aging effect on coating abradability, hardness, bond strength and microstructure were evaluated. It was found that coating abradability increased obviously after 2h thermal aging and slightly decreased for extended thermal aging time. A sharp decrease in both coating hardness and bond strength were found after 2h thermal aging and a slow increase occurred with the extended aging time. Relationships between coating properties and microstructure were studied. Decomposition of organic binder and sintering of AlSi matrix metal generated by thermal aging were found to be reasons for coating properties changes.

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.

This work introduces a hybrid spray-and-fuse process and a modified CoCrMoC (Stellite) alloy that significantly expand the manufacturing window for wear-resistant coatings. The Co-based alloy was produced by adding Ni, B, and Si to Stellite 720 to lower its melting points and expand its melting range thereby improving the sprayability and fusibility of the material. The modified alloy was deposited on Inconel 718 balls and 1 in. diameter coupons by HVOF spraying and coating samples were sinter fused at high temperatures followed by furnace cooling. The processes used are described and test results are presented, showing that thick, metallurgically bonded coatings were achieved with high hardness and excellent wear and corrosion resistance.

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.

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.

The stiffness and thermal conductivity of thermal barrier coatings (TBCs) are inevitably changed by healing up of intersplat pores and intra-splat cracks during high temperature exposure, which results in less compliance and thermal insulating performance. However most publications on sintering of plasma sprayed TBCs are based on free-standing coatings, which ignore residual stress and the stress produced by the mismatch of thermal expansion coefficient between substrate and ceramic top coatings. In this paper, individual splat of YSZ and YSZ coatings have been sprayed on substrate of YSZ and Ni-based superalloy. Evolution of healing and morphology of 2D cracks and some properties, such as hardness and thermal conductivity, have been revealed during thermal exposure. Results showed that, during heating stage, the shear stress coming from substrate caused some tearing of bonding area tips and narrowing of inter-splat pores. Some recoverable and unrecoverable widening on intra-splat cracks occurred also due to shear stress. During annealing stage, compared with free-standing coating, the surface hardness of the coating deposited onto the Ni-based superalloy showed enhanced increasing due to the faster healing of inter-splat pores by narrowing down, and the hardness in cross-section presented retarding increasing due to the widening of cracks in out-plane direction leading to slower healing. The case of YSZ substrate fell between free-standing case and Ni-based superalloy. This would benefit the further in-depth understanding of the thermal cycling failure mechanism of plasma sprayed TBCs.

Dense sintered technical ceramics demand special surface preparation in order to be coated by thermal spraying. Sandblasting results in the damage of the interface region and leads to bonding defects. On the contrary, by varying the laser conditions, different laser structured surfaces were generated, which at Rz roughness values in the range of 40 μm, allowed to achieve thick and well bonding coatings. Therefore, laser ablation is proved to be the best method for surface preparation of ceramic substrates. In the case of porous ceramics (including pre-sintered ceramics) substrate preparation can be omitted, depending on their porosity level. Ceramics with porosity content up to 60 % can be coated by APS, whereas HVOF can be used on ceramics with porosity up to 30 %. On ready-to-get-coated pre-sintered ceramic parts (without substrate preparation), followed by co-firing of the substrates and coatings, the development of new ceramic components is possible.

The La 2 Zr 2 O 7 /SrZrO 3 composite with a mol ratio of 1:2 named (La 0.5 Sr 0.5 )ZrO 3.25 (LSZ) was in-situ synthesized by co-precipitation method using ammonia and ammonium oxalate as precipitants. The synthesized LSZ powder showed good phase stability not only from room temperature to 1400°C but also at higher temperature of 1450°C for a long period, as analyzed by thermogravimetry, differential scanning calorimetry and X-ray diffraction, respectively. The bulk LSZ with relative density >95% was prepared by pressureless sintering at 1500°C for 2 h and spark plasma sintering (SPS) at 1300°C for 5 min, respectively. The fracture toughness of the bulk LSZ prepared by both pressureless sintering and SPS were 1.80±0.20 MPa·m 1/2 and 1.95±0.09 MPa·m 1/2 , respectively, which are higher than that of both bulk SrZrO 3 and La 2 Zr 2 O 7 . The coefficients of thermal expansion (CTEs) of the bulk LSZ were 8.4-9.5×10 -6 K -1 in a temperature range of 200-1200°C, which are higher than that of La 2 Zr 2 O 7 but lower than that of SrZrO 3 . The thermal conductivity of the bulk LSZ prepared by pressureless sintering was ~1.1 W·m -1 ·K -1 at 1000°C, which is lower than that of both bulk SrZrO 3 and La 2 Zr 2 O 7 . The LSZ composite is considered as a promising thermal barrier coating material.

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 this study, La 2 Zr 2 O 7 coatings produced by atmospheric plasma spraying were heated to 1300°C for up to 100 h. Residual pores in the coatings were characterized by SEM. Porosity was found to decrease with increasing sintering duration, although large pores still existed after 100 h. The relationship between the opening of residual pores and sintering time was closely analyzed and is described in detail. The results presented in the paper can inform the design of pores that endow TBCs with higher sintering resistance.

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.

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.

Microstructure of cermet support influences significantly the performance and stability of solid oxide fuel cells (SOFCs). The properties required for the support include high electrical conductivity, necessary permeability, good match of thermal expansion with other layers and high temperature strength. In this study, a porous Ni50Cr50-Al 2 O 3 cermet was designed as the support of SOFC. The porous cermet was deposited by flame spraying with a powder mixture of 30%vol Al 2 O 3 and 35%vol Ni50Cr50 and 35%vol polyester. The effect of cermet microstructure on its gas permeability was investigated. The electrical conductivity, thermal expansion coefficient and bending strength of cermet support were also studied. The results showed that the gas leakage rate of the cermet support increased with the increase of polyester content in the starting powder. The thermal expansion coefficient of the composite cermet decreased with the increase of the volume fraction of Al 2 O 3 . Moreover, the electric conductivity of the cermet increased significantly after high temperature sintering, and reached 1015 S/cm after sintering at 1000°C for 15 hours. The three point bending strength of the Ni50Cr50-based cermet support reached 171 MPa. The cermet stability at high temperatures and SOFCs performance were discussed.

The FeAl intermetallic compound offers a combination of attractive properties such as thermal barrier, good strength at intermediate temperatures and an excellent corrosion resistance at elevated temperatures under oxidizing, carburizing and sulfidizing atmospheres. So they have attracted considerable attention as potential candidates for structural and coatings applications at elevated temperatures. However, the application of these intermetallics has been limited due to lack of deposition techniques and their low ductility at room temperature. To overcome the drawbacks we apply Low Pressure Cold Spray (LPCS) with following sintering for improving coating ductility and structure. The aim of this paper is to present the first results of FeAl intermetallic compound synthesis with this technique. A CS deposit is built up by the successive impact of individual powder particles that are the ‘‘building blocks’’ of the deposit. Sintering is applied to utilize reactions between the particles and obtain complex intermetallic compound. The microstructures and properties of the coatings were characterized by SEM, EDX and thermal diffusivity tests to define the structure formation mechanisms.

Suspension plasma spraying is a process which enables production of finely grained nanometric or submicrometric coatings. The suspensions were formulated with the use of fine powder of ceramic particles of yttria stabilized zirconia in water with alcohol. The present paper focuses on the theoretical analysis of the formation process of sintering of fine solids impacting the growing coating’s surface. The heat flux input to the coatings was estimated and their surface temperature at spraying was measured. The theoretical analysis of sintering during the coating’s growth was carried out. The different models of sintering were analyzed and adapted to the suspension plasma spraying conditions. The model of surface diffusion was found to be the most appropriate to describe the sintering during suspension plasma spraying. The formation of the necks having the relative size equal to 10 % of the particle diameter was found to be possible during the coatings deposition.