High amorphous phase formation tendency, a desirable microstructure and phase composition and silicon evaporation are the challenges of spraying Yb 2 Si 2 O 7 environmental barrier coatings (EBCs). This research addresses these issues by depositing as-sprayed high crystalline Yb 2 Si 2 O 7 using atmospheric plasma spray (APS) without any auxiliary heat-treating during spraying, vacuum chamber, or subsequent furnace heat treatment, leading to considerable cost, time, and energy savings. Yb 2 Si 2 O 7 powder was sprayed on SiC substrates with three different plasma powers of (90, 72 and 53 kW) and exceptional high crystallinity levels of up to ~91% and deposition efficiency of up to 85% were achieved. The silicon mass evaporation during spraying was controlled with a short stand-ff distance of 50 mm, and an optimum fraction of Yb 2 SiO 5 secondary phases (<20 wt.%) was evenly distributed in the final deposits. The desirable microstructure, including a dense structure with uniform distribution of small porosities, was observed. The undesirable vertical crack formation and any interconnected discontinuities were prevented. Reducing the plasma power from 90 kW to 53 kW, while conducive for mitigating the silicon mass loss, was detrimental for microstructure by increasing the fraction of porosities and partially melted or unmelted fragments. The gradual decrease of the coating temperature after deposition alleviated microcracking but has an insignificant effect on the crystallinity level. Coatings annealed close to their operating temperature at 1300 °C for 24 hours demonstrated sintering and a crack healing effect, closing the tiny microcracks through the thickness. An improved coating composition was detected after annealing by the transformation of Yb 2 SiO 5 to Yb 2 Si 2 O 7 (up to ~10 wt.%).

When testing the thermal cycling resistance of thermal barrier coatings, the surface temperature of the materials must be controlled so that test results can be used for coating life prediction. In this study, the temperature at the surface of plasma-sprayed TBCs was controlled during thermal shock testing using feedback from a double-color IR thermometer and high-rate cooling. The results are presented and discussed, highlighting the capability of the recently designed thermal shock test.

This study investigates the effect of thermal cycling on cold-spray chromium coatings deposited on steel substrates. First, equilibrium stress states are determined for different coating thicknesses. Next, the potential for crack initiation and growth is simulated based on periodic heating and cooling cycles. The corresponding crack driving forces are characterized using interface stresses and energy release rate as a function of the thermal cycles. The effects of coating thickness, embedded microcracks, and initial residual stress on the driving forces are investigated systematically to demonstrate the risk of coating fracture and delamination.

This study investigates the effect of suspension plasma spraying (SPS) parameters on inner diameter coatings produced from yttria suspensions, one in water and one in ethanol. Thermal spray trials were conducted at different spray distances, transverse speeds, and spray angles, with and without a water shroud. The coatings obtained were then examined in order to assess the influence of each parameter and the effect of water cooling on substrate temperature, porosity, vertical cracking, nodule formation, surface roughness, and deposition rate. Key findings and correlations are presented in the paper along with recommended practices and potential improvement pathways.

This study assesses the applicability of high-speed laser cladding for producing iron-based alloy coatings, in particular, CrNi duplex steel, FeCrV, and FeCrNiB. Process parameters are optimized for 150 µm thick claddings on mild steel using different laser power levels, surface speeds, and preheating temperatures. Claddings are also produced on cylindrical substrates of different diameters to investigate dependency on component geometry. Duplex steel was found to be highly processable by high-speed laser cladding. In contrast, crack-free FeCrV claddings can only be produced on small diameter surfaces, and only with preheating, while FeCrNiB could not be applied at all without cracking.

This study investigates the effects of laser remelting on plasma sprayed YSZ thermal barrier coatings using a pulsed laser with and without induction preheating. It is shown that induction preheating decreases the laser threshold energy required for full remelting, which effectively reduces crack density. Induction preheating also helps in developing a steadier melt temperature and in decreasing thermal gradients between successive remelting passes. XRD analysis shows that it can reduce the amount of monoclinic phase as well.

This paper introduces a new way to inject nanoparticles into a plasma flame and demonstrates its use in the deposition of dense ceramic coatings. Instead of suspensions or pastes, nanoparticles are dispersed in micro resin fragments. In this case, zirconia particles with an average diameter of 200 nm were mixed with a thermosetting acrylic liquid resin and the mixture was solidified, crushed, and screened. Micro resin fragments are fed into the plasma flame using a conventional powder feeder. The resin content mostly burns away in the plasma jet, which heats and propels the nanoparticles into the substrate. SEM analysis of the zirconia deposits shows that they are free of microcracks and pores, although carbon contamination was detected by thermogravimetry. Coating hardness tests were also conducted and the results are presented.

Tungsten carbide coatings are often applied to improve surface properties such as wear, high temperature degradation, and corrosion resistance. Zirconia coatings have also been used extensively in various industries due to their excellent tribological and insulation properties combined with high stiffness. It is speculated that adding zirconia to tungsten carbide may result in a coating with combination of excellent thermal and mechanical properties of constituents. In the current study, a powder mixture of 50 wt. % WC-Ni and 50 wt. % ZrO 2 -Y 2 O 3 deposited on a low carbon steel substrate using atmospheric plasma spray technique. The microstructural evolution of deposited sample was investigated. Splat boundaries, micro cracks, pore morphology conversion, and grain growth mechanism were elucidated comprehensively. Results indicated a good adhesion between two different major components. No porosity formed due to mismatch between zirconia and tungsten carbide. This study pays special attention to the dependency of the microstructural characteristics to the phase distribution within the coating.

This study demonstrates an experimental setup in which acoustic emission sensing is used to monitor a twin wire arc spraying (TWAS) process. Emitted acoustic signals were recorded by broadband sensors attached to the spray nozzle and mounted under the substrate. Sensor outputs were converted from the time domain to the frequency domain by fast Fourier analysis. Acoustic emission amplitude plots were produced and are correlated with gas pressure, arc voltage, in-flight particle velocity and temperature, coating thickness, and crack formation due to cooling.

During service damage in terms of small cracks develops in thermal barrier coatings (TBC), composed of partially yttria stabilized zirconia (PYSZ), and applied to gas turbine components made of Ni-base superalloys coated with an aluminide diffusion or MCrAlY overlay coating. Growth and coalescence of these microcracks results in cracks that run parallel to the interface with the substrate leading to failure by delamination of the TBC. A mechanism is proposed to heal the micro-cracks in a TBC by introducing MoSi 2 particles. Upon high temperature expose in air, in the range of 1200 °C, MoSi 2 forms amorphous SiO 2 that can fill micro-cracks, thereby restoring the integrity of the TBC.

Intermediate temperature - solid oxy-fuel cells (IT-SOFCs) include in their design a solid electrolyte layer made of yttria-partially stabilized zirconia (Y-PSZ), an ionic conductor, through which oxygen ions diffuse. This layer needs to fulfill several characteristics among which a low leakage rate corresponding to a non-connected pore network and a low level of stacking defects such as microcracks or globular pores. Moreover, the thickness of this layer needs to be as low as possible (about 20 µm) in order to limit ohmic losses. Suspension plasma spraying (SPS) appears as a potential technological route to manufacture such layers structured at micrometric or sub-micrometric scales. In SPS, a stabilized suspension, made of a liquid, solid particles and a dispersant, is injected within the plasma flow. The liquid is very quickly fragmented and then vaporized and the individual particles, or the particle agglomerates, depending on the average size and morphology of the solid feedstock, are heated and simultaneously accelerated towards the substrate surface where they impact, spread and solidify, analogously in a first approximation to larger particles, to form a layer. The architecture of the layer is very closely related to plasma operating parameters (from which derive plasma flow stability), from the suspension characteristics, in particular the feedstock particle size distribution and from the suspension injection parameters. This work aims at presenting recent developments made to optimize some of these operating parameters to maximize the electrolyte layer characteristics.

Air plasma sprayed thermal barrier coatings, which reduce the temperature in the underlying substrate material, are an essential requirement for the hot section components of an industrial gas turbine. For TBC systems, the adherence of the top coating is one of the most important parameter for the durability of TBC system. In this work, the thermal fatigue behaviour of an air plasma sprayed thermal barrier coating was investigated. In addition, the residual interfacial strength was also evaluated by means of the 4-point bending test. From the measurement of the AE signals during the thermal fatigue tests, micro-cracking occurred in each cooling stage of the thermal fatigue cycles and then such damage depends on the number of thermal cycle. In addition, TGO grew at the interface with the exposed time at elevated temperature (the time dependent damage). Thermal barrier coating undergoes both time dependent damage and cycle dependent one under thermal fatigue condition. The life of thermal cycle with high temperature dwell time is shorter than not only that of isothermal exposure but also that of thermal cycle without dwell time.

A copper electroplating method revealed that recently developed commercial Al 2 O 3 thermal spray coatings did not have the well-known lamellar structure but instead had a structure that was made up of tightly bonded particles, with many vertical microcracks in the coating. The coating in the study was prepared by the atmospheric plasma spray system. The copper electroplating method was applied to observation of the Al 2 O 3 thermal spray coating structure. Spray coating specimens were electroplated in an aqueous solution containing CuSO 4 , H 2 SO 4 , CuCl 2 , and additives so that copper was deposited in small pores and narrow cracks in the coatings. Copper in the Al 2 O 3 coatings was observed clearly by scanning electron microscopy, which also revealed the coating’s structure (boundaries of flattened particles and vertical cracks). The coating did not have a lamellar structure (horizontal apertures between flattened particles) and it was dense in spite of the many microcracks.

The microstructure of thermally sprayed ceramic coatings is characterized by the existence of various pores and microcracks. The porous microstructure makes coating desirable for thermal insulation, but this unique microstructural feature also gives rise to anelastic response under tension and compression loads. Detail investigations of curvature measurements of ceramic coated substrate indicate the coatings to exhibit anelastic behavior composed of nonlinear and hysteresis characteristics. In this paper, the mechanisms of such behaviors were studied from curvature-temperature measurements and finite element analysis through modeling the microstructure of yttria stabilized zirconia (YSZ) coating. Computational models contain numerous randomly distributed pores and microcracks with various sizes, aspect ratios, locations and orientations. The effects of such attributes of pores and microcracks on coating anelastic behavior were studied by simulations of curvature change during thermal cycles.

Plasma sprayed Thermal Barrier Coatings (TBCs) exhibit many interlamellar pores, voids and microcracks. These microstructural features are primarily responsible for the low global stiffnesses and the low thermal conductivities commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400°C, for times of up to 10 hours) on the pore architecture in detached YSZ top coats has been characterised by Mercury Intrusion Porosimetry (MIP) and gas-sorption techniques. While the overall porosity level remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (inter-connected) specific surface area. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high treatment temperatures (~1400°C). These changes are thought to be associated with intra-splat microcrack healing, improved inter-splat bonding and increased contact area, leading to disappearance of much of the fine porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Measured thermal conductivity data are compared with predictions from a recently-developed analytical model, using the deduced inter-splat contact area results as input parameters. Good agreement is obtained, suggesting that the model captures the main geometrical effects and the pore size distribution measurements reflect the most significant microstructural changes.

Thermal barrier coatings (TBCs) with different types of microstructures were produced with the atmospheric plasma-spraying (APS) process. The investigation includes a variation of the micro-crack density and of the porosity level. In addition, also segmented TBCs were produced. Finally, also PVD-TBC systems have added to the investigation. The different TBC systems were cycled in a natural gas/oxygen burner rig with a surface temperature of about 1250°C and a bond coat temperature of about 1100°C or below. The use of relatively low surface temperature guarantees a failure mode close to the bond coat promoted by the growth of the thermally grown oxide (TGO). After failure, metallographic inspection was made to determine the thickness of the TGO layer and the β-phase depleted zone. In addition, the crack path was analyzed and compared for the different microstructures.

The growth characteristics of thermally grown oxides (TGO) and their influence on microcracking in an air-plasma sprayed (APS) thermal barrier coating (TBC) were studied. The TBC samples were prepared in either as-received condition or with a pre-heat treatment. In the as-received TBC, TGO that formed upon thermal exposure predominantly consisted of layered and clustered chromia, spinels and nickel oxide, whereas in pre-heat treated samples the TGO was predominantly alumina. The growth characteristics of TGO was found to exhibit a three-stage behavior that was most pronounced in the as-received TBC. Micro-cracks were found to nucleate in clustered oxides, these cracks would grow in association with thickening of the TGO layer. Eventually, oxide-induced cracking and cracking along pre-existing discontinuities near the ceramic/bond coat interface led to spallation of the topcoat. A relationship between the maximum crack size and TGO thickness was established based on fracture mechanics considerations. This relationship is shown to be useful for TBC life prediction.

Plasma sprayed ceramic coatings have much lower stiffness in comparison to sintered ceramics. The reason for that is their characteristic microstructure with porosity and microcracks. Microcracks decrease the interlamellar cohesion in vertical direction, but also affect the individual splat properties in the horizontal direction. For that reason sealing treatments are often applied with plasma sprayed ceramic coatings in order to improve their corrosion resistance and mechanical properties. In this paper the effect of aluminum phosphate sealing treatment on the elastic properties of plasma sprayed Al 2 O 3 and Cr 2 O 3 coatings were studied. Residual stresses in the plane of coating surfaces were compared using the X-ray diffraction analysis (XRD). A special four point bending device, designed for the X-ray diffractometer, was used in determining the effect of additional load on coating elastic behavior. In as-sprayed alumina coatings tensile stresses of about 400 MPa were detected while only about 40 MPa of compressive stresses were measured in the as-sprayed chromia coatings. Microstructural characterization revealed that sealing treatment had apparently affected the coating microstructure and filled some microcracks and interlamellar spacings. As a result, in both sealed coatings, compressive stresses of about 100 up to 150 MPa were observed. Also a better stiffness of both materials was detected during the bending of specimens. In addition, the sealing treatment increased nearly ten times the Young’s modulus, determined by XRD analysis under various tensile loads.

The effects from thermal shock loading on pre-existing microcracks within thermal barrier coatings (TBCs) have been investigated through a finite element based fracture mechanical analysis. The TBC system consists of a metallic bond coat and a ceramic top coat. The rough interface between the top and bond coats holds an alumina oxide layer. Stress concentrations at the interface due to the interface roughness as well as the effect of residual stresses were accounted for. At eventual closure between the crack surfaces, Coulomb friction was assumed. To judge the risk of fracture from edge cracks and centrally placed cracks, the stress intensity factors were continuously monitored during simulation of thermal shock loading of the TBC. It was found that fracture from edge cracks is more likely than from centrally placed cracks. It was also concluded that propagation of an edge crack is initiated already during the first load cycle whereas the crack tip position of a central crack determines whether or not propagation will occur.

Quantitative characterization of microstructures of plasma sprayed coatings, that accounts for their anisotropic and “irregular” character, is developed. An important finding is that “islands” of partial contacts along microcrack faces –even if they are very small - produce a strong effect on both elasticity and conductivity, thus reducing the “effective” microcrack density. Theoretical results are twofold: (1) conductive/elastic properties of the coatings in terms of the microstructure, and (2) conductive–elastic cross-property connections, that interrelate the anisotropic overall conductive and elastic constants. They can be utilized for “mapping” possible combinations of the two properties and, thus, for optimization of the microstructure for the combined conductive/elastic performance. Testing of the model against experimental data on YSZ coatings produces a good agreement.