Tomáš Chráska

On the size-dependent phase transformation in nanoparticulate zirconia

Abstract We have studied the structures of zirconia nanoparticles formed by plasma-spraying an organo-metallic precursor. Inspection of the particles in the TEM reveals that they adopt one of two distinct crystal structures, depending upon their size. The smallest particles have the tetragonal structure, while larger ones are monoclinic. Interpolation of the data reveals a critical size above which the monoclinic structure is stable. Upon annealing, the zirconia particles coarsen and undergo a phase transformation when the particle size is of the order of 18 nm, for reasons associated with the surface energy, and the occurrence of this phase transformation produces a sudden change in the driving force for coarsening. Grain size distributions below the critical size for the transforrnation are log-normal, but as the transformation occurs, the size distribution changes to a markedly less skewed form. The development of this distribution is followed to establish whether it grows self-similarly, or returns to log-normality once normal driving forces are restored after the phase transformation is complete.

Transmission electron microscopy study of rapid solidification of plasma sprayed zirconia. Part I. First splat solidification

Although plasma spray processing has been used for several decades, understanding of all the physical and chemical processes involved is rather limited often due to the lack of detailed enough experimental and diagnostic techniques supplying the necessary data. All coatings produced by plasma spraying have in common their fine microstructure, with features often of nanometer scale, as a result of rapid solidification. This work reports on transmission electron microscopy (TEM) studies of plasma sprayed yttria stabilized zirconia (YSZ) ceramic. A modified wedge polishing technique of TEM cross-sectional sample preparation was engaged to prepare high quality samples without the need for extensive ion milling. This paper explains observed microstructural features (e.g. grain size, cracks) and phase composition of the very first layer (splat) on a smooth hot substrate. A rapid solidification model is proposed for the observed narrow columnar grain microstructure of the first YSZ splat. Based on the model, an estimate of liquid undercooling is calculated from the grain size. The solidification of a subsequent splat and more detailed description of splat/substrate and splat/splat interfaces will be discussed in a second paper.

Effect of different substrate conditions upon interface with plasma sprayed zirconia—a TEM study

Transmission electron microscopy (TEM) was used to examine the effect of different substrate conditions on the interface between a stainless steel substrate and plasma-sprayed yttria-stabilized zirconia (YSZ). The paper focuses on the first-to-form isolated single splats, for which interface with the substrate plays a critical role in determining adhesion of the whole coating. Preheated mirror-polished stainless steel substrates served as a reference point for further comparison with rough, cold or bond-coated substrates. A modified wedge-polishing technique enabled the production of high-quality cross-sectional TEM samples with minimum ion-induced damage and large observable areas. The substrate chemistry, roughness and temperature all play an important role in the splat formation process; nevertheless, the substrate temperature effect is the most prominent of these. It was proposed that adsorbed air molecules on the surface of cold substrates cause turbulence in the spreading of molten ceramic droplets, which ultimately leads to poor splat adhesion. At a certain elevated substrate temperature, air molecules desorb from the substrate surface, thereby allowing substantially better contact and adhesion between splats and the substrate.

Transmission electron microscopy study of rapid solidification of plasma sprayed zirconia – part II. Interfaces and subsequent splat solidification

Transmission electron microscopy (TEM) is used to investigate the first two splat layers of a plasma sprayed yttria stabilized zirconia (YSZ). Sample preparation by modified wedge polishing technique and results for the first splat were presented in the first part of this paper. The second part focuses on two different interfaces present in the microstructure, rough substrate effects on first splat solidification, and finally on solidification of the second splat layer. The interface between plasma sprayed ceramic and metal substrate as well as the interface between two subsequent splats were examined by conventional and high resolution TEM. The presence of a thin layer of native oxide on polished and preheated stainless steel substrates was established. Such an interface appears to promote very good adhesion of YSZ. The effect of liquid flow on the solidified microstructure of the second and subsequent splats was clearly demonstrated. The top surface of the first solidified splat acts as an excellent nucleation site for a subsequent splat and causes epitaxial growth of columnar grains in subsequent splats.

The effect of high-flux H plasma exposure with simultaneous transient heat loads on tungsten surface damage and power handling

The performance of the full-W ITER divertor may be significantly affected by the interplay between steady-state plasma exposure and transient events. To address this issue, the effect of a high-flux H plasma on the thermal shock response of W to ELM-like transients has been investigated. Transient heating of W targets is performed by means of a high-power Nd:YAG laser with simultaneous exposure to H plasma in the linear device Magnum-PSI. The effects of simultaneous exposure to laser and plasma have been compared to those sequentially and to laser only. Transient melting is found to be aggravated during plasma exposure and to occur at lower heat flux parameters. Roughness and grain growth are observed to be driven by peak temperature, rather than by the loading conditions. The temperature evolution of the W surface under a series of transients is recorded by fast infrared thermography. By accounting for changes in the reflectivity at the damaged surface as measured by ellipsometry, a reduction in power handling capabilities of the laser/plasma affected W is concluded. The evidence of reduced power handling of the W surface under conditions as described here is of great concern with respect to the durability of W PFCs for application in fusion devices.

Microcontact printing: new mastering and transfer techniques for high throughput, resolution and depth of focus

Abstract We describe techniques for high resolution, high throughput, high depth of focus microcontact printing. Printhead masters are fabricated by focused ion beam (FIB) micro-machining. We describe the instrumental limitations of these techniques, and show how high-resolution features ( 100 μm). We also demonstrate scaling of feature writing rates to >10 4 /s through topographic mastering in PMMA. Microcontact printing resolution is assessed for different elastomer mold materials, and it is shown how rigid polymer composites allow pattern transfer of features finer than 100 nm, such that the current limit in pattern transfer resolution is defined by the grain size in the target Ag film. Finally, we describe an all-polymer microcontact printing process employing elastomer molding from topographically patterned PMMA printheads.

Phase transformations in nickel rich NiAl using electrical resistivity measurements

The intermetallic phase NiAl is a perspective material for high-temperature and shape memory effect applications. This phase is stable in a wide composition range and shows thermoelastic martensitic transformations in a region supersaturated in nickel. It is known that formation of Ni{sub 5}Al{sub 3}, Ni{sub 2}Al, Ni{sub 3}Al phases influences the martensitic transformation and, consequently, the shape memory effect. The aim of this paper is to throw more light upon structure changes of the mentioned phases using resistivity measurements during heating and cooling of the nickel rich NiAl phase specimens within a relatively broad temperature region ({minus}180 C to 900 C). Identification of the phases was done using the Ni-Al phase diagram, their experience in SEM investigation of the structural changes in this system, and electrical properties of the phases.

Resistometric investigation of phase transformations in NiAl alloys

Abstract The paper represents a contribution to the martensitic transformation and to the existence regions of Ni2Al and Ni5Al3 phases in the Ni–Al phase diagram. Resistivity, optical microscopy and differential thermal analysis (DTA) investigations were performed on 63.4, 64.8 and 66.2 at.% Ni alloys in the temperature region between −190 and 850°C. The martensite–austenite transformation (reversible for the alloy with the lowest Ni content) takes place between 60 and 300°C according to the nickel content of the alloy. In the 63.4 at.% Ni alloy, the Ni2Al phase originates above 350°C and exists up to 580°C. After an incubation period, the Ni5Al3 phase starts to nucleate from austenite above 640°C. The Ni5Al3 phase then dissolves above 740°C forming Ni3Al+NiAl phases in a peritectoid reaction. In Ni-richer alloys, martensite rapidly changes into the Ni5Al3 phase which does not transform martensitically. Only very rapid heating to 600°C retains the possibility of martensitic transformation in these alloys.

Optimization of High Porosity Thermal Barrier Coatings Generated with a Porosity Former

Yttria-stabilized zirconia thermal barrier coatings are extensively used in turbine industry; however, increasing performance requirements have begun to make conventional air plasma sprayed coatings insufficient for future needs. Since the thermal conductivity of bulk material cannot be lowered easily; the design of highly porous coatings may be the most efficient way to achieve coatings with low thermal conductivity. Thus the approach of fabrication of coatings with a high porosity level based on plasma spraying of ceramic particles of dysprosia-stabilized zirconia mixed with polymer particles, has been tested. Both polymer and ceramic particles melt in plasma and after impact onto a substrate they form a coating. When the coating is subjected to heat treatment, polymer burns out and a complex structure of pores and cracks is formed. In order to obtain desired porosity level and microstructural features in coatings; a design of experiments, based on changes in spray distance, powder feeding rate, and plasma-forming atmosphere, was performed. Acquired coatings were evaluated for thermal conductivity and thermo-cyclic fatigue, and their morphology was assessed using scanning electron microscopy. It was shown that porosity level can be controlled by appropriate changes in spraying parameters.

Spark Plasma Sintering of a Gas Atomized Al7075 Alloy: Microstructure and Properties

The powder of an Al7075 alloy was prepared by gas atomization. A combination of cellular, columnar, and equiaxed dendritic-like morphology was observed in individual powder particles with continuous layers of intermetallic phases along boundaries. The cells are separated predominantly by high-angle boundaries, the areas with dendritic-like morphology usually have a similar crystallographic orientation. Spark plasma sintering resulted in a fully dense material with a microstructure similar to that of the powder material. The continuous layers of intermetallic phases are replaced by individual particles located along internal boundaries, coarse particles are formed at the surface of original powder particles. Microhardness measurements revealed both artificial and natural ageing behavior similar to that observed in ingot metallurgy material. The minimum microhardness of 81 HV, observed in the sample annealed at 300 °C, reflects the presence of coarse particles. The peak microhardness of 160 HV was observed in the sample annealed at 500 °C and then aged at room temperature. Compression tests confirmed high strength combined with sufficient plasticity. Annealing even at 500 °C does not significantly influence the distribution of grain sizes—about 45% of the area is occupied by grains with the size below 10 µm.