The development of thermal barrier coatings (TBCs) for diesel engines has been driven by the potential improvements in engine power and fuel efficiency that TBCs represent. TBCs have been employed for many years to reduce corrosion of valves and pistons because of their high temperature durability and thermal insulative properties. There are research programs to improve TBCs wear resistance to allow for its use in tribologically intensive areas of the engine. This paper will present results from tribological tests of ceria stabilized zirconia (CeSZ). The CeSZ was applied by atmospheric plasma spray process. Various mechanical and thermal properties were measured including wear, coefficient of thermal expansion, thermal conductivity, and microhardness. The results show the potential use of CeSZ in wear sensitive applications in diesel applications. Keywords: Thermal Barrier Coating, Diesel Engine, Wear, Thermal Conductivity, and Thermal Expansion

Phase stability of the thermal barrier deposits made from yttria-partially-stabilized zirconia (Y-PSZ) is a requirement for extended service lifetime. The response of Y-PSZ plasma-sprayed deposits to annealing at 1000 °C, 1200 °C, and 1400 °C with times from 1 to 1000 hours has been evaluated using Rietveld analysis of neutron diffraction data. Results show that yttria concentration of the as-sprayed tetragonal zirconia component generally decreased with increasing annealing temperature and time. As the yttria content in the tetragonal phase approached a limiting concentration, the tetragonal phase transformed into monoclinic phase on cooling. An increase in monoclinic phase content was clearly observable after annealing 24 hours at 1400 °C and was nearly 35 % after 100 hours at 1400 °C. A similar trend was observed at 1200 °C for longer annealing times, with monoclinic phase formation beginning after 400 hours. At 1000 °C experimental times were not sufficient for monoclinic phase to form although a decrease in the yttria concentration in the tetragonal phase was observed. Keywords: neutron scattering, yttria-stabilized zirconia, phase composition, Rietveld analysis

Over the past 30 years, there has been considerable interest in the development of thermally sprayed thermal barrier coatings (TBCs) for aerospace and land based turbine applications. The use of TBCs enables higher operating temperatures, resulting in significant fuel efficiency savings. This paper reports on the development of dense Yttria Stabilised Zirconia (YSZ) thermal barrier coatings produced by High Velocity Oxygen Fuel (HVOF) spraying using acetylene as the fuel gas. The use of a high temperature gas erosion rig allowed the controlled evaluation of erodent size, velocity, impact angle, and temperature on coating performance. The work also covers the optimization of process parameters, including powder morphology, stand-off distance, oxygen to fuel ratio, gas pressures, and flowrates, and their effect on coating characteristics such as deposition efficiency, microhardness, and surface roughness.

Results concerning the relationship between deposition efficiency and mechanical properties with coating roughness are presented. A commercial partially stabilized zirconia powder was plasma sprayed at different power levels, H2/Ar ratio and spray distance. Coatings sprayed at high deposition efficiencies demonstrated improved mechanical properties. The deposition efficiency is also proportional to the coating roughness. When prepared with a high plasma power and a high H2/Ar ratio, and at short spray distances, these coatings exhibited the highest deposition efficiency and coating roughness. The high coating roughness associated with a high deposition efficiency arises because coarse powder particles are likely to be melted at higher plasma power. However, opposite trends were also observed within different regimes of powder size distribution and torch power. Another objective of this work was to demonstrate that a surface profilometer can be used to understand the deposition efficiency and mechanical properties of thermal spray coatings. Key words: Roughness, Mechanical Properties, DE.

Thermal spray deposited Zirconium oxide (or Zirconia, ZrO2) is used in several industrial sectors for various purposes: as thermal barrier for turbine blades, as wear and corrosion resistant coating in industrial applications as well as biomedical applications. This paper reports the results of a study aimed at determining the relation between the plasma spray process conditions and the coating properties. A 24-1 fractional factorial Design of Experiments has been used and the coatings have been characterised in terms of chemical composition, crystal lattice structure and mechanical properties (morphology, porosity, roughness, hardness). Coatings with different characteristics have been obtained depending on the spray parameters combination. We found that it is possible to vary the spraying parameters in such a way to obtain dense, compact Yttria Partially Stabilised ZrO2 deposits which can be useful as wear and corrosion resistant coatings. In other conditions less dense and more porous layers can be obtained, useful for thermal barrier applications.

Experimental designs have been used by the thermal spray community to improve and optimize spray parameters to produce coatings with desired properties. The influence of four spray parameters including top and bond coat thicknesses, substrate temperature, and spray distance on the mechanical properties of plasma sprayed thermal barrier coatings has been examined. Two experimental matrices; (i) a four by nine according to a Taguchi experimental design, and (ii) a four by seventeen according to a full factorial design of the experiment, were developed. Six samples from each group were tested using a four point bend arrangement. Yield strength and elastic modulus were calculated from the four point bend test. A multi-linear regression analysis on yield strength and elastic modulus values from each experimental matrix was carried out to determine the influence of each spray parameter on these properties. The multi-linear regression analysis results for these two experimental matrixes are compared.

One of the main application fields of the thermal spraying process are thermal barrier coatings (TBC). Today partially stabilised zirconia (YSZ or MSZ) is mainly used as TBC material. At temperatures above 1000°C, zirconia layers ages distinctively including shrinkage and microcrack formation. Therefore there is a considerable interest in TBCs for higher temperature application. In this paper lanthanum aluminate, a newly developed TBC material with long term stability up to 1400 °C, is presented. It ages significantly slower at these high temperatures than commercial zirconia based TBCs. Its composition favors the formation of platelets, which prevent a densification of the coating by postsintering. It consists of La2O3, AI2O3 and MgO. Its crystal structure corresponds to lanthanum aluminate powders were produced using two different fabrication routes, one based on salts, the other one based on oxides. To optimise the granulate various raw materials and additives were tested. The slurry was spray dried in a laboratory spray drier and calcined at 1650°C. Using these two powders, coatings were produced by atmospheric plasma spraying (APS). The residual stresses of the coatings were measured by the hole drilling method and the deposition process was optimised with respect to the residual stresses of the TBC. The coatings were extensively analysed regarding phase composition, thermal expansion, long term stability as well as microstructural properties.

NiCrAlY-PSZ graded materials with two different gradients were synthesized using the plasma spray technique. Their microstructure, thermal and frictional response were evaluated, and the response was compared to that of single layered composite materials. The hardness and frictional properties of PSZ could be successfully optimized by an addition of small amount of NiCrAlY. The results of testing revealed the potential of ceramic/metallic materials systems and can be used to further optimize the spraying process of composite/graded structures subjected to contact damage at high temperatures. The study also revealed new tribo-effects that can be encountered in potential applications of graded structures.

The damaging of the electrodes during spraying can affect the reproducibility of the plasma spray process. Indeed, this may influence the plasma characteristics and the energy transfer to the sprayed particles resulting in significant changes in the coating attributes. In this paper, results from a detailed investigation on the stability of plasma spraying are presented. Specifically designed diagnosis tools were used to study the evolution of key parameters of a plasma spray process during a long-term experiment. A comprehensive analysis is carried out on the collected set of data, with an emphasis on the correlation that may exist among them. Results show significant variations in the particle state and gun characteristics with the spraying time. These variations are reflected in the microstructure of the sprayed coatings. The investigation also gives some indication on how the spray process could be controlled.

Plasma sprayed thermal barrier coatings often face the problems of spallation and cracking in service owing to their poor bond strength and high residual stresses. Functionally graded thermal barrier coatings with a gradual compositional variation from heat resistant ceramics to fracture resistant metals are proposed to mitigate these problems. In this paper, functionally graded Y2O3 stabilized ZrO2 (YSZ) / NiCoCrAlY composite coatings were prepared using pre-alloyed and spheroidized composite powders. The mechanical and thermal properties of graded YSZ/NiCoCrAlY composite coatings, such as elastic modulus, bond strength, coefficient of thermal expansion, thermal cycling and oxidation resistance were investigated. Results showed that the bond strength and thermal cycling resistance of FGM coatings were much better than that of the duplex coatings. The coefficient of thermal expansion and elastic modulus changed gradually through the 5-layer functionally graded coating.

Trends of turbine blades of advanced aircraft gas turbine engines are to increase output power of the engines, to increase engine efficiency, and to reduce environmental emission, and thus, higher operating temperatures of the engines are required. One of the technologies for increasing the operating temperature is a thermal barrier splayed coating. A research paper claims that molybdenum silicide in a splayed coating has a self-healing capability for cracks formed in the coating by embedding the cracks with silicon dioxide formed from molybdenum silicide at high temperatures. This article discusses the methods for the extension of life of thermal barrier sprayed coatings by incorporating molybdenum silicide. It discusses monitoring method for detecting cracking conditions in heating and cooling cycles by signals of acoustic emission. A possibility of estimating fatigue life by utilizing an X-ray method for measuring residual stress is also considered.

A thermal cycling test was designed in order to predict thermal barrier coatings (TBC) life of combustion chambers. The thermal cycle is produced by two 2kW halogen lamps highly focused on the TBC. A 25X100 mm TBC is plasma sprayed centered onto a substrate 25X300 mm. The thermal cycling can be done either under argon atmosphere or air in order to be able to discriminate the oxidation induced acoustic emissions from the expansion mismatch. Two transducers located at each end of the substrate monitor the acoustic signals emitted by crack initiation and/or propagation. The advantage of using two transducers is that with a time of flight approach cracking phenomena can be located along the TBC. This process allows the study of welded substrates coated with TBC, TBC coated around holes and to check for the presence of cracks by using metallography preparation. The challenge of the test is to use the early cycles emission signatures in order to predict the long term durability of the TBC.

With a view to the development of thermal barrier coatings on turbine blades, such coatings were developed on the basis of Y2O3-stabilized ZrO2 and optimized with respect to their reproducibility. Particular attention was given to the spray efficiency and, in particular, the coating porosity. The porosity was measured by Hg porosimetry, which proved to be a reliable method of characterization. The plasma gas flows play an important role for a high reproducibility of the coating properties. High reproducibility of the porosity and spray efficiency was achieved for low argon flows, for which the influences of all the other parameters, such as electric arc current, meandering and sample cooling during plasma spraying or the cathode operating time of the burner, were of minor relevance for the coating properties. The spray efficiency is clearly influenced by the given powder rate and the carrier gas flow for the type of burner used. The change of the grain fraction in the plasma jet was examined for two different powders. Finally, thermal cycling tests were carried out and evaluated for pre-optimized thermal barrier coating samples.

The isothermal oxidation behaviors of plasma sprayed NiCrAlY bond coatings were evaluated. Two unique microstructures, characterized by oxide stringers and improperly flattened zones can be observed in the APS bond coatings. Structure and chemical composition of the oxide stringers were examined by TEM and EDS. Improperly flattened zones had a high density of open porosities. These features affected the oxidation behaviors at both transient and steady state stages. Under transient oxidation, NiO, Cr2O3, and α-Al2O3 were present together on the surface. Oxide stringers and improperly flattened zones had an especially high density of NiO and Cr2O3, which implied that these regions had inherently lower contents of Al. During steady state oxidation, continuous α-Al2O3 reduced the diffusion rate of oxygen and, in turn, the rate of isothermal oxidation. Over longer oxidation times, the depletion of Al led to the formation of NiAl2O4 and Cr2O3 layers along with hemispherical protrusions.