When compared with conventional thermal spraying processes, thermal spraying of suspensions allows to produce coatings with outstanding properties in terms of microstructure, surface topography, and phase compositions, as well as mechanical, electrical or tribological requirements. The use of suspensions as feedstock results in an almost unlimited flexibility in terms of chemical composition of the sprayed coatings. Moreover, thermal spraying of suspensions is a promising technique for processing expensive raw materials. Zn 2 TiO 4 coatings are only one example where the high costs of blended oxide powders as feedstock material hinders the market introduction, whereas outstanding electrical properties and photocatalytic activity of thermally sprayed Zn 2 TiO 4 coatings are of great interest for various industrial applications. In this work, single oxide ZnO and TiO 2 raw materials as well as a Zn 2 TiO 4 feedstock powder were used to develop tailored aqueous suspensions suitable for thermal spraying. To follow the formation of the compositions in the system ZnO-TiO 2 , differential thermal analysis (DTA) and thermal gravimetry (TG) measurements were performed. Preparation routes of stable suspensions with low sedimentation rates, low viscosity and good flowability are discussed. Exemplary microstructures and phase compositions of sprayed coatings are shown. In all sprayed coatings, the Zn 2 TiO 4 phase has been formed during Suspension High Velocity Oxygen Fuel Spraying (S-HVOF). This work demonstrates the potential to develop appropriate cost-efficient suspension feedstocks from single oxide raw materials to obtain Zn 2 TiO 4 coatings.

In this work, CeO 2 -G d2 O 3 co-stabilized ZrO 2 (CGZ) thermal barrier coatings are deposited by solution precursor plasma spraying and the microstructure, phase stability, thermophysical properties, and thermal cycling behaviors of the resulting coatings are investigated and discussed in comparison to conventional 8YSZ coatings.

Ba(Mg 1/3 Ta 2/3 )O 3 (BMT), a high melting point refractory oxide, is envisioned as a thermal barrier coating material. In this study, six chemical reagents combinations are investigated as BMT coating precursors: one BMT powder suspension and five Ta 2 O 5 suspensions in nitrate solutions or acetate solutions. A hybrid suspension / sol plasma spray process is designed to axially inject these precursors into a RF thermal plasma torch to synthesize BMT and to deposit nanostructured coatings. X-ray photoelectron spectroscopy (XPS) was used to evaluate the element evaporation during plasma spraying. Thermogravimetric analysis and differential thermal analysis (TG/DTA) are applied to investigate the BMT formation. Parameters such as precursor chemistry and proportion, plasma power, spray distance and substrate preheating are studied with regards to the coating phase structure. The results indicate that the combination of twice the Mg stoichiometric amount with a power of 50 kW shows the best results when using nanocrystalline Ta 2 O 5 as Ta precursor. When choosing nitrates as Ba and Mg precursors, predominant crystalized BMT can be obtained at lower plasma power (45 kW) when compared to acetates (50 kW). BaTa 2 O 6 , Ba 3 Ta 5 O 15 , Ba 4 Ta 2 O 9 , Mg 4 Ta 2 O 9 are the main secondary phases during BMT preparation process. Because of the complicated acetate decomposition, the coating deposition rate from nitrate precursors is higher than that from acetate ones.

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.

In this study, Fe-Cr-Al and Fe-Cr-Al-B cored wires were produced and deposited on steel substrates by wire arc spraying. The microstructure, hardness, and high-temperature corrosion behavior of the cored-wire deposits were evaluated in comparison to Fe-Cr and commercial Fe-Cr-Al solid-wire coatings. All coating samples exhibited lamellar microstructures with oxide inclusions, the fewest being in the Fe-Cr-Al-B deposits. Microhardness was measured along coating cross-sections at various distances from the coating-substrate interface. The Fe-Cr coatings were the hardest, followed by the Fe-Cr-Al-B deposits. Thermogravimetric analysis was used to evaluate high-temperature corrosion behavior in a molten salt environment under cyclic conditions, with the Fe-Cr-Al-B cored-wire deposits performing the best.

A series of Ni-based cored wires with different boron contents were designed to prepare corrosion-resistant coatings by two-roll wire-arc spraying. These coatings were evaluated for their potential to provide added protection and reduced maintenance for applications in waste-to-energy (WTE) plants. The as-deposited coatings, which primarily are composed of nanocrystalline particles, exhibit uniform and dense layered structures with porosity of about 3%. The investigators selected thermo-gravimetric techniques to evaluate the high-temperature corrosion behavior of the coatings in molten salt environment (Na 2 SO 4 -10 wt% NaCl) at 800°C. The coated surfaces exhibited significantly reduced corrosion rates in comparison to those of the SA 213-T 2 substrate during all tests. These results were due to the formation in the coatings of composite surface oxide films, including Cr 2 O 3 and NiCr 2 O 4 , which serve to prevent the diffusion or penetration of corrosive species. Furthermore, the boron content appears to have a significant influence on the corrosion behavior of the designed coatings: the coating with the best performance had 16 at. % B added. The wire-arc sprayed Ni-based coatings could be an effective and economical treatment to prevent corrosion and extend the lifetime of super-heater tubes in WTE plants.

Detonation spraying provides the opportunity to produce superabrasive diamond grinding tools under atmospheric conditions. In this study, several methods are used to assess the effects of the spraying process on diamond particles, including SEM analysis, energy dispersive X-ray spectroscopy, differential thermal analysis, thermogravimetric analysis, X-ray diffraction, Raman spectroscopy, and friability and fracture force testing. It was found that under optimized conditions, the thermal and mechanical impact of the detonation can remain low enough to ensure the reliability of the diamonds with no adverse effects.

Detonation-gun spray technology is a novel coating deposition process which is capable of achieving very high gas and particle velocities approaching 4-5 times the speed of sound. This process provides the possibility of producing high hardness coatings with significant adherence strength. In the present study, this technique has been used to deposit WC-Co coatings on T22 boiler steel. Investigations on the oxidation performance of detonation-sprayed WC-Co coatings in air and in molten salt Na 2 SO 4 -82Fe 2 (SO 4 ) 3 at 700°C under cyclic conditions have been carried out. The thermogravimetric technique was used to establish the kinetics of corrosion. The uncoated boiler steel suffered a catastrophic corrosion in the form of intense spalling of its oxide scale during air, as well as, molten salt induced oxidation. The WC-Co coated specimens showed lesser overall weight gains in comparison to their uncoated counterparts in both the environments. The oxidation kinetics for the coated specimens followed nearly the parabolic rate law. The overall weight gain has been found to be higher in the case of air oxidation as compared to that in salt environment for all the cases. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) techniques were used to analyse the corrosion products, which indicated the formation of W and O as main elements in the oxide scales of the coated steel in both the environments.

This paper examines plasma-sprayed alumina and chromia coatings, with and without aluminum phosphate sealing, in order to assess the stability of the as-sprayed layers and phosphate reaction as a function of temperature during the sealing process. Thermogravimetric analysis showed no changes in mass in the alumina layers, an increase of 1.7% in the mass of the chromia layers due to oxidation, and a 14% loss of mass in the sealed layers, which agrees well with the evaporation behavior of a pure seal. Paper includes a German-language abstract.

The isothermal and cyclic oxidation of freestanding Ni-20Cr-10Al-lY thick coatings has been investigated at 1200°C using TGA, SEM, XRD and XPS techniques. Coatings produced by HVOF are dense and remain crack free after thermal treatments. The protective oxide layer formed did not flake off upon cyclic oxidation as confirmed by SEM analysis. In addition, three oxidation regimes were identified after analyzing TGA data: two below 1000 °C and a third one at approximately 1200°C. The regimes below 1000°C correspond to the selective oxidation of elements on the surface and at the subsurface of the coatings whereas the third regime involves element diffusion from the bulk of the coating to the surface. The oxidation regime became asymptotic at 1200 °C as stable oxides formed. The presence of water vapor affects neither the thickness nor the orientation of oxide crystals formed on the surface as confirmed by the X-ray analysis. The XPS and X-ray results show an inter-diffusion between the coating and substrate with a slight increase in chromium concentration at the interface. Element distribution within the oxide layer was found to follow the order: Al-(oxide)Y-(oxide)/Cr-(oxide)/Ni-(oxide)/NiCrAlY from the outermost oxide layer to the bulk of the coating. These results show that HVOF dense Ni-20Cr-10Al-lY sprayed coatings can be used as anti-oxidant barriers in both isothermal and cyclic oxidation at 1200°C.

Kinetic and heat transfer analysis have been undertaken in order to predict the decomposition of polymer feedstock particles during thermal spraying. Thermogravimetric measurements indicated that the decomposition of PMMA had an order of reaction of unity and an activation energy of 135 kJ mol -1 . The polymer decomposition temperature is shown to be a function of the particle residence time in the flame and is much higher than in conventional polymer processing. This has an important influence on process modelling, since the choice of decomposition temperature used in the heat transfer analysis has a major effect on the calculated temperature profiles. The work shows that realistic predictive data can only be obtained by using the dynamic decomposition temperature. Application of the model indicates that only the surface layers of the polymer feedstock particles undergo significant decomposition during plasma spraying and that the feedstock injection position is an important control parameter.

Thermal barrier coating systems have been heat treated in order to study the oxidation kinetics of the bond coat. All the surfaces of Ni superalloy substrates were sprayed with ~100 μm of a NiCrAlY bond coat, with or without ~250 μm of a ZrO 2 top coat. Thermogravimetric analysis (TGA) was used to monitor continuously the mass change as a result of oxidation of the bond coat during heating at 1000°C for 100 hours in flowing air. In addition, some specimens were heated to 1000°C in static air, cooled to room temperature, weighed and re-heated cyclically. The total exposure time was 1000 hours. Rates of weight gain were found to be higher for the cycled specimens, despite the absence of air flow. This is attributed to damage to the oxide film, which was predominantly α-Al 2 O 3 , as a consequence of differential thermal contraction stresses. The changing residual stress state during heat treatment was predicted using a previously-developed numerical model. A thin (1 mm) substrate with ~100 μm bond coat and ~250 μm ZrO 2 top coat was used in these simulations, which incorporated creep of the bond coat and the lateral strain associated with oxidation. It is concluded from these computations that, while high stresses develop in the oxide layer, the associated driving forces for interfacial debonding remain relatively low, as do specimen curvature changes. It seems likely that coating spallation after extensive oxide layer formation arises because the interface is strongly embrittled as the layer thickens.