A new challenge in the transport systems concerns with improving efficiency. Thermal swing coatings are interesting candidates for internal combustion engines due to their potential to reduce cooling requirements and increase efficiency. K 2 Ti 6 O 13 (KTO) thermal barrier coatings (TBCs) were prepared by atmospheric plasma spraying through powder structure design and optimization of deposition conditions. The thermophysical properties of plasma-sprayed KTO deposits and their effect on the thermal swing have been investigated. Their thermal conductivities were tested by a laser flash method and the thermal performance of the coatings was further examined by thermal swing test. The phases, nominal chemical compositions and microstructure of KTO deposits were characterized by X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive spectrometry (SEM-EDS). The results indicated that the chemical composition change occurs to the coatings resulting in a deviation from nominal stoichiometry due to chemical reactions between the plasma gas and particles. The thermal conductivity of the coating is very sensitive to the coating compositions, and the coating prepared using porous powder under pure argon presents a single K 2 Ti 6 O 13 phase and high porosity, and the lowest thermal conductivity of 0.85 W/m·K.

In this study, Al 2 O 3 -based coatings with varying TiO 2 contents (0, 3, 13, and 40%) were fabricated using atmospheric plasma spraying technique. To compare the superiority of the samples, their thermal properties (thermal conductivity and thermal shock resistance) were characterized. As observed, Al 2 O 3 - 40%TiO 2 (A-40T) coating exhibited relatively superior thermal insulation and thermal shock resistance at 600°C. According to the microstructure and phase analysis, this finding can be attributed to the special phase, Al 2 TiO 5 , and the pre-existing microcracks inside the coating. Thus, A-40T manifested excellent characteristics for thermal insulation application compared with pure Al 2 O 3 and low-TiO 2 content coatings.

Environmental degradation of thermal barrier coatings (TBC) by molten deposits such as calcium magnesium alumino-silicates (CMAS) is one of the most vital factors resulting in the failure of thermal barrier coatings, while turbine engine inlet temperatures are kept increasing for higher fuel efficiency. A new phase composite ceramic had been developed and evaluated for the topcoat of a durable thermal barrier coating (TBC) system with low thermal conductivity property and improved erosion resistance. The present work is to continue the effort to exploring the behavior of CMAS resistance of the phase composite TBC at high temperatures. The effects of CMAS attack and thermal exposure on the TBC degradation were investigated in experimental runs. In addition, a YAG-modified layer over the top of the TBC was applied with the attempt to improve CMAS resistance of the TBC system. The evaluation of CMAS resistance was focused on the most important characteristics of coating microstructure, CMAS penetration, and failure mode and test condition factors. The mechanisms for the CMAS infiltration and the TBC damages were discussed based on the analyses of the CMAS corroded samples in details.

The feasibility of processing various polymers by cold spray has been exemplified by depositions with low porosity and properties comparable to the bulk material. However, cold sprayed polymers are generally deposited with low deposition efficiency compared to more extensively studied metal sprays. Low efficiencies in polymer sprays are attributed to characteristic differences in material properties between metals and polymers. Notably, the thermophysical properties of polymers limit heat transfer and promote intra-particle thermal gradients that develop during cold spray processing. These properties (e.g., thermal conductivity, heat capacity, density) and low deposition efficiencies demand alterations to the cold spray process equipment outside typical metal powder spray conditions. Herein, a modified powder feed tube is used to pre-heat powder to temperatures (~84 °C) below the powder melting point, or cool it (~-55 °C) below room temperature before contacting the high velocity carrier gas in the nozzle of a CSM 108 cold spray system. Numerical simulation demonstrated that pre-heating/cooling the powder feedstock is a viable means of adjusting particle temperature upon impact with the substrate; however, this technique has generally not been deliberately utilized in the cold spray of polymers. In the present work, no significant increase in deposition efficiency (~65% for all sprays) was found by increasing the pre-heat temperature. However, pre-heated particles had a mechanical strength 28% higher than particles injected at room temperature and -55 °C. Despite this, scanning electron microscope images indicated no notable differences between the deposit microstructures. Future works are planned to study the effect of pre-heat at higher particle impact velocities and degrees of pre-heat to improve powder consolidation.

Microstructure and physicochemical properties of a thermally sprayed coating depend on the dynamics of the particles interacting with the spray jet. This is especially the case for electrical properties. In this study, different spraying processes were used to spray p-type and n-type half-Heusler powders. Thermoelectric powders, Hf20Zr75Ti05CoSb80Sn20 (p-type) and Hf60Zr40NiSn98Sb02 (n-type), were selected due to their interesting electrical properties. The spray processes were evaluated based on coating composition and mechanical property measurements. The only coatings of practical interest were those that were plasma sprayed and they were examined in detail to assess the effect of process parameters on coating properties.

In this study, different sets of plasma-sprayed YSZ thermal barrier coatings were deposited via Ar/H 2 and N 2 /H 2 plasmas and compared based on deposition efficiency (DE), thermal conductivity (TC), and furnace cycle testing (FCT). The top-performing coatings exhibited equivalent FCT lifetimes with TC values in the range of 1.15-1.25 W/mK at 1200 °C, but the deposition efficiency of those produced with N 2 /H 2 plasma was twice as high, resulting in a 55% reduction in production costs.

Yttrium aluminum garnet (YAG) has desirable properties for a thermal barrier coating (TBC), although there are two production challenges. One, YAG has a relatively low thermal expansion coefficient which leads to large thermal mismatch stresses, and two, amorphous phases are produced by atmospheric plasma spraying. Solution precursor plasma spraying (SPPS) has to potential to solve both problems. First off, it produces no amorphous phases. Secondly, it can produce a cracked microstructure that mitigates the CTE mismatch issue. To judge the adequacy of the properties of SPPS YAG, a summary of the properties of common TBCs is presented. It is shown that the properties of SPPS YAG fall within desirable or usable ranges. Current efforts described in this paper focus on improving the efficiency and rate of deposition.

Cation-deficient perovskite-type oxides have received considerable attention as new thermal barrier coating materials because of their extremely low thermal conductivities. In this study, sintered samples produced from RTa 3 O 9 (R: Y, La or Yb) powders are examined and the mechanisms behind their low thermal conductivity are investigated. Thermal conductivity was found to vary primarily with the ionic radius of the R element. As ionic radius decreases, nanodomains form via tilting of the TaO 6 octahedra. Phonon scattering at the domain boundaries is thus likely responsible for the low thermal conductivity of cation-deficient perovskite oxides.

Strontium zirconate is a candidate material for thermal barrier coatings due to its high melting point, good sintering resistance, and high TCE. One drawback, however, is a phase transition that occurs below 1200 °C , although rare-earth element doping offers a way to suppress it. In this study, SrZrO 3 doped with two rare earth oxides, ytterbia and gadolinia, is deposited by solution precursor plasma spraying and the layers obtained are evaluated before and after heat treatment. The coatings are characterized by two phases, SrZrO 3 and t-ZrO 2 , with interpass boundary structure, nano and microscale porosity, and through-thickness vertical cracks. XRD analysis after heat treatment at 1400 °C for 360 h shows that the two phases are very stable due to the doping of rare-earth elements, which is also shown to reduce thermal conductivity in the as-sprayed deposits by nearly 35%.

The aim of this work is to find a path toward a thermal barrier coating (TBC) that is more thermally stable and less thermally conductive than current 8YZS coatings. The concept of dual-phase composite ceramics is proposed in an effort to combine the desirable attributes of unique phase constitution, low conductivity, and high ceramic fracture toughness. In addition, efforts are made to optimize the spraying process for low-k ceramic topcoats by controlling the effect of key parameters on porosity, deposition rate, and deposition efficiency. Isothermal oxidation and thermal cycling tests are conducted to evaluate the performance of the low-k TBCs with promising results.

A low thermal conductivity in feedstock material and high plasma temperatures generally lead to inhomogeneous heating of particles in plasma spraying. Existing modeling methods can determine heat transfer within idealized spherical particles with homogenous morphology, but in many cases, particles have an agglomerated morphology, consisting of multiple smaller particles that are packed together. The reduced contact area between the individual smaller particles results in a drastic reduction of the effective thermal conductivity of the agglomerate. On the other hand, it enhances heat transfer from the plasma gas due to the increased particle surface area and penetration of the hot plasma into the agglomerate. Moreover, the momentum transfer from the plasma to the agglomerate differs from that of a homogenous spherical particle, which can significantly affect heating dynamics. This paper presents a novel particle modeling approach that accounts for all such phenomena. Differences in kinematics and heating dynamics of the agglomerates are analyzed with regard to their packing densities.

Fabrication of Thermal Barrier Coatings (TBCs) with higher lifetime and relatively cheaper processes is of particular interest for gas turbine applications. Suspension Plasma Spray (SPS) is capable of producing coatings with porous columnar structure, and it is also a much cheaper process compared to the conventionally used Electron Beam Physical Vapor Deposition (EB-PVD). Although TBCs fabricated using SPS have lower thermal conductivity as compared to other commonly used processes, they are still not commercialized due to their poor lifetime expectancy. Lifetime of TBCs is highly influenced by the top coat microstructure. The objective of this work was to study the TBCs produced using axial SPS with different process parameters. The bond coat was deposited using High Velocity Air Fuel (HVAF) spray. Influence of the microstructure on lifetime of the coatings was of particular interest and it was determined by thermal cyclic fatigue testing. Thermal conductivity of the coatings was determined by laser flash analysis. The results show that axial SPS could be a promising method of producing TBCs for high temperature gas turbine applications.

While depositing Fe-Al intermetallic powders applying a gas detonation spraying a certain coating structures containing oxide ceramics are created. These structures exhibit both extreme mechanical resistance and unusual thermophysical properties (TP) also. One of such property is relatively low thermal conductivity. A possible application as thermal barrier coatings needs precise determination of TP dependence on temperature and resistance of the coating structure to temperature exposition. At present study TPs were investigated for a coating produced from Fe-Al intermetallic powder in a course of complex measurements including DSC analyses, laser flash thermal diffusivity measurements, dilatometric studies complemented with microstructural analyses. The study resulted in full characterization of the investigated structure TPs: density, thermal expansivity, heat capacity and thermal conductivity. During thermal analyses interesting phenomena concerning thermal resistance to the temperature exposition of the investigated coating were revealed. The obtained results complement rather sparse literature data on TPs in that subject and contribute to better understanding of gas detonation spraying (GDS) process technology and intermetallic/oxide structures property understanding.

The dynamic properties and thermal history of FeAl particles were estimated and assessed in this study. First, parameters of the detonation process for the investigated mixture were calculated using a thermochemical code. Next, the motion parameters and thermal history of the analyzed powder particles were assessed using computational fluid dynamics software and algorithms developed by the investigators. The appropriate models allowed for determination of melted volume (mass) fraction of a certain analyzed single particles that have diameters ranging from 10 to 160 μm. The results show that only the smallest particles melt under the investigated conditions. Moreover, the estimated radial distribution of the temperature inside the particles is almost homogeneous due to the relatively high FeAl thermal conductivity and relatively low thermal conductance of surface heat transfer. The calculated particle terminal velocity was compared to experimental data and to data from previous studies in the literature to determine the accordance of results among the data sets.

The degradation of pump components by corrosion and complex damage mechanisms, e.g. erosion and cavitation leads to high costs through replacement and maintenance of parts. To increase the lifetime of cost-efficient components with superior casting properties, gray cast iron parts are surfaced with duplex stainless steel using an inert shielding gas metal arc welding process. The dilution of the surfacing increases with both increasing heat input and increasing thermal conductivity of the shielding gas. The microstructure is highly affected by the cooling conditions that may enhance diffusion processes and eventually lead to precipitation of deleterious carbides. Higher heat input and prolonged cooling duration during surfacing lead to high dilution and a pronounced carbide network and thus, substantially reduced corrosion resistance in artificial seawater. The corrosion of the surfacings in the potentiodynamic polarization test is driven by selective corrosion of the phase boundary between carbides and chromium-depleted austenite. Passive behavior is observed for coatings with low dilution and higher cooling rates, which showed homogeneous chromium distribution and no interconnected carbide networks. In conclusion, the corrosion behavior of gray cast iron was improved by surfacing with duplex stainless steel.

Suspension plasma spraying (SPS) process has attracted extensive effort and interest as a method to produce fine-structured and functional coatings. In particular, thermal barrier coating (TBC) applied by SPS process has gained increasing interest due to its potential for producing coatings that provide superior thermal protection of gas turbine hot-section components as compared to conventional APS-TBC and even EB-PVD TBC. The unique columnar architecture and nano- and submicron sized grains in a SPS-TBC coatings demonstrate some advantages in thermal shock durability, low thermal conductivity, and high-temperature sintering resistance. This work addresses some practical aspects of using the SPS process for TBC applications before it becomes a reliable industry method. The spray capability and applicability of SPS to achieve uniform thickness and microstructure on curved substrates was evaluated in designed spray trials to simulate industrial parts with complex configurations. The performance of SPS-TBCs in erosion, free falling ballistic impact, and indentation loading tests was evaluated to simulate SPS-TBC performance in turbine service conditions. The behaviors of SPS-TBCs in those tests were correlated to key test factors including grit incident angles, impact object sizes, indentation head shapes, and coating surface curvatures. Finally, a turbine blade was coated and sectioned to verify SPS sprayability in multiple critical sections. The SPS trials and test results demonstrate that SPS is promising for innovative TBCs, but some challenges need to be addressed before it becomes an economical and reliable industrial process, especially for gas turbine components.

In this paper, yttria-stabilized zirconia (YSZ) coatings were prepared by plasma spraying of ready-to-spray suspensions provided by three different manufacturers. High-enthalpy hybrid water-argon plasma torch WSPH 500 was successfully used for deposition of coatings with porous and columnar microstructure consisting of tetragonal non-transformable phase. Sensitivity of the deposition process to variation of deposition conditions was also evaluated by the change of suspension injection point position. Slight differences in the microstructures of the deposited coatings (in particular character of porosity and mutual bonding of the microsplats) were reflected in slight but measurable differences in hardness and wear resistance of the coatings indicating changes in the coating cohesion. Tensile adhesion/cohesion strength of the coatings was found to be in the range of 9 to 15 MPa. High coating porosity desirable for low thermal conductivity combined with high suspension feed rate (from about 100 to 120 ml/min in this study) makes the WSP-H coatings promising for further development for example in thermal barrier coatings applications.

A method measuring the thermal conductivity and the interfacial thermal resistance of thermal barrier coatings (TBCs) which consist of metallic bond-coats (BCs) and ceramics top-coats (TCs) on superalloys was newly developed. It was based on the areal heat diffusion time method analysing the heat diffusion across multilayers. The developed method was experimentally verified using the BC and the TBC specimens coated by APS. It was found that there were the interfacial thermal resistance not only between the TC and the BC but also between the BC and the substrate. Furthermore, the thermal conductivities of the BC and the TC obtained from the BC and the TBC specimens by this method considering the interfacial thermal resistance were in good agreement with those measured from the free-standing specimen of each coating. Thus, it was confirmed that the newly developed method is effective to evaluate the thermal conductivity and the interfacial thermal resistance of the TBC.

ZrO 2 -Y 2 O 3 (YSZ) thermal barrier coatings (TBCs) were manufactured via conventional Air Plasma Spray (APS), Suspension Plasma Spray (SPS) and an additional technology hereby termed Finely-dispersed-particle Air Plasma Spray (FAPS). The FAPS processing employs the exact same classification of finely dispersed particles as used in SPS; however, whereas SPS uses a liquid medium, in the case of FAPS the particles are fed conventionally via a carrier gas into the plasma spray torch by using a newly developed powder feeder for fine (suspension-like) particles (NRC patented technology). These finely dispersed YSZ particles consist of irregularly shaped (fluffy-like) agglomerates made from individual nano-sized particles. The conventional APS YSZ TBC was sprayed via a Metco 3MB torch, whereas, both SPS and FAPS YSZ TBCs were sprayed using the Mettech Axial III torch (using the same set of spray parameters). Both SPS and FAPS YSZ TBCs exhibited porous and vertically-cracked microstructures. The conventional APS YSZ TBC microstructure exhibited the traditional lamellar morphology. Elastic modulus, hardness and thermal conductivity values were evaluated for all YSZ TBCs. Microstructures and phase analysis were investigated via SEM and XRD.

To manufacture a protective coating with low thermal conductivity and good frictional wear performance, a Fe 59 Cr 12 Nb 5 B 20 Si 4 coating was designed and produced by high velocity oxygen fuel (HVOF) spraying; the properties and performance of this coating where then compared with those of a commercially available AISI 316L stainless steel coating. In the as-deposited state, both coatings exhibit dense layered structures with porosity below 1% and slight oxidation. The microstructure of the Fe-based coating has an amorphous matrix and some precipitated nanocrystals. The result is that the designed Fe-based coating has a thermal conductivity (2.66 W/m·K) that is significantly lower than that of the 316L stainless steel coating (5.87 W/m·K). Based on its advantageous structure, the Fe-based coating exhibits higher microhardness, reaching 1258±92 HV. The friction coefficient and wear rate of the Fe-based coating show an increase at 200°C followed by a decrease at 400°C, due to the evolution of the wear mechanism at different temperatures. The dominant wear mechanism of the Fe-based coating at room temperature is fatigue wear accompanied by oxidative wear. At 200°C, due to the existence of “third body” abrasive wear, the wear process was accelerated. The large-area oxide layer is likely responsible for the decrease of friction of the coating at 400°C.