Merrilea J. Mayo

Processing nanocrystalline ceramics for applications in superplasticity

Abstract The production of nanocrystalline ceramics for subsequent use in superplastic forming operations requires that the ceramics be made in large quantities, with high densities, and under stringent grain growth control. To make large amounts of nanocrystalline starting powders, two popular wet chemical techniques (precipitation from salt solutions and alkoxide hydrolysis) can be used and are described in this paper. Unfortunately, pressureless sintering of these powders does not typically lead to the high densities and ultrafine grain sizes desired in the final product. Sintering data suggest that pore shrinkage occurs only when grains reach a critical size with respect to the pore size; thus, if the ceramic contains large pores, densification can require significant grain growth. Separation of large pores from grain boundaries may also occur and lead to incomplete densification, even at extremely large grain sizes. In all cases the pressureless sintering behavior of the nanocrystalline ceramics appears to adhere to well established theories used to explain the sintering of conventional, larger-grained ceramics. During both pressureless sintering and sinter-forging experiments, the grain size of a nanocrystalline ceramic is identical to the average spacing between open pores in the sample. Pressureless sintering results in the closure of these pinning pores by about 90% density and thus3leads to a substantial grain growth at densities greater than 90%. Sinter-forging, however, often allows one to maintain a stable population of small open pores (for pinning purposes) throughtout sintering, while preferentially eliminating the large pores which detract from the sample density. The deformation regime in which sinter-forging is performed has a decided effect on whether large pores or small pores are eliminated preferentially and, consequently, on whether a high density and fine grain size combination is achieved or not.

Fracture toughness of nanocrystalline tetragonal zirconia with low yttria content

Abstract Nanocrystalline tetragonal zirconia samples are shown to reach toughnesses between 16 and 17 MPa·m 1/2 , provided the conventional levels of 3 mol% yttria additive are lowered to 1 or 1.5 mol%. For 1 and 1.5 mol% compositions, maximum toughness occurs just below the critical grain sizes of 90 and 110 nm, respectively—i.e. just before spontaneous transformation to the monoclinic phase occurs. Moving away from the critical grain size, towards smaller grains, toughness decreases monotonically up to a factor of 5. These results indicate a critical parameter for maximizing toughness in nanocrystalline zirconia ceramics is proximity to the phase transformation boundary (critical grain size). Combining the present results with data from the literature, a linear relationship between yttria content and inverse critical grain size is found. A power law relationship between grain size and toughness increment is also obeyed by the present data.

THERMAL PROPERTIES OF ZIRCONIA CO-DOPED WITH TRIVALENT AND PENTAVALENT OXIDES

Zirconia doped with 6-8 wt% (3.2-4.2 mol%) yttria (6-8YSZ), the most common thermal barrier coating material, relies mostly on oxygen vacancies to provide the phonon scattering necessary for low thermal conductivity. The present study examines whether specific substitutional defects—in addition to, or instead of, oxygen vacancies—can provide similar or greater reductions in conductivity. To this end a series of zirconia samples co-doped with varying levels of yttrium (trivalent) and tantalum/niobium (pentavalent) oxides were synthesized, thereby allowing oxygen vacancy and substitutional atom concentration to be varied independently. The results show that Nb-Y and Ta-Y co-doped zirconia samples containing only substi- tutional defects produce stable single-phase tetragonal materials with thermal conductivities very close to that of the conventional 6-8YSZ. In these samples, Nb 51 and Td 51 are similarly effective in lowering thermal conductivity, in contradiction to phonon scattering theories that consider primarily mass effects and thereby predict significantly greater conductivity reduction due to Ta 51 doping than Nb 51 doping. Finally, Nb 51 /Ta 51 - Y 31 doped samples, which contain both oxygen vacancies and substitutional defects, are found not to be stable in single-phase form; however, the thermal conductivities of the two-phase tetragonal 1 cubic mixtures are again as low as that of the conventional 6-8YSZ.

Fracture toughness of nanocrystalline ZrO2-3mol% Y2O3 determined by vickers indentation

Nanocrystalline 3Y-ZrO{sub 2} with densities ranging from 90.4% to 98.7% of theoretical and grain sizes from 55 nm to 160 nm were tested by Vickers indentation. The hardness of nanocrystalline and submicron (Tosoh) 3Y-ZrO{sub 2} was a strong function of density but independent of grain size and could be expressed as Hardness (GPa) = 0.549*Density (in %) + 40.449 with a correlation coefficient, R, of 0.919. The fracture toughness of both submicron and nanocrystalline 3Y-ZrO{sub 2} was low, about 2.5--4.5 MPa{center_dot}m{sup 1/2}, in contrast with the 8.4 MPa{center_dot}m{sup 1/2} for a micron-grained 3Y-ZrO{sub 2} sample. In addition, the fracture toughness was relatively constant over all density and grain size ranges studied (55--400 nm; 90--100% density). The low fracture toughness, combined with no perceptible grain size dependence, implies that there is not a significant room temperature ductility mechanism peculiar to nanocrystalline materials. Minor effects may exist, but they are not sufficient to noticeably increase the fracture toughness. Because the data fit with existing trends of decreasing transformation toughening with decreasing grain size, it is presumed the poor fracture toughness of the nanocrystalline samples is due to a diminished capacity to transformation toughen via the tetragonal-to-monoclinic transformation.

The hot corrosion resistance of 20 mol% YTaO4 stabilized tetragonal zirconia and 14 mol% Ta2O5 stabilized orthorhombic zirconia for thermal barrier coating applications

Abstract Zirconia stabilized with 3.2–4.2 mol% (6–8 wt.%) yttria (3–4YSZ), the current material of choice for thermal barrier coating applications, is susceptible to hot corrosion by acidic oxides such as vanadia in the 700–900 °C range. The current study is a preliminary examination of the hot corrosion resistance to NaVO 3 –V 2 O 5 mixtures in the above temperature range of two alternative materials: a tetragonal zirconia co-doped with 10 mol% yttria+10 mol% tantala (20YTaO 4 Z) and an orthorhombic zirconia doped with 14 mol% tantala (14TZ). Results show that the 20YTaO 4 SZ is resistant to destabilization by NaVO 3, but is attacked at higher V 2 O 5 activities, resulting in the formation of YVO 4 and orthorhombic zirconia. Studies on the 14TZ itself then indicated that it is substantially more resistant than the YSZ to attack by environments more acidic (specifically V 2 O 5 rich) than pure NaVO 3 . However, it is less suitable than either 20YTaO 4 SZ or 3–4YSZ for environments that are more basic. A comparison of the resistance of the 14TZ, the 3–4YSZ and the 20YTaO 4 Z shows that the 20YTaO 4 SZ is more resistant to acidic oxides than the YSZ and more resistant to the basic oxides than the 14TZ.

Thermodynamics of the tetragonal-to-monoclinic phase transformation in fine and nanocrystalline yttria-stabilized zirconia powders

The current study uses high-temperature differential scanning calorimetry to document the shift in phase-transformation temperature with particle size throughout a series of alloys in the zirconia–yttria system (0–1.5 mol% yttria). The tetragonal-to-monoclinic (T→M) phase-transformation temperature is seen to vary inversely with particle size. It is shown that a simple thermodynamic approach first proposed by Garvie predicts this inverse linear relationship. Subsequent determination of the key thermodynamic parameters therein (e.g., the surface and volume free energy, enthalpy, and entropy changes involved in the phase transformation) allows a complete predictive equation for the T→M phase transformation in the yttria–zirconia system to be developed as a function of particle size and yttria dopant level. The yttria–zirconia phase diagram is then redrawn with grain size as a third variable. It should be stressed that the current analysis is valid for particulate systems only; a parallel paper tackles the problem for fine-grained yttria–zirconia solids, where the approach is similar, but additional strain energy terms come into play.

Surface chemistry effects on the processing and superplastic properties of nanocrystalline oxide ceramics

Abstract The unusual bulk behavior of nanoparticle and nanograined systems often originates in surface chemistry effects. Three examples are used to illustrate this point. In the first, newly precipitated nanocrystalline titania is washed with ethanol, and the mixture of these two supposedly inert substances causes the titania to lose its anatase crystal structure and become amorphous. This phenomenon is attributed to a reverse hydrolysis reaction at the particle surface. In the second example, nanocrystalline ZrO 2 -3mol%Y 2 O 3 is observed to partially dissolve on exposure to pH-adjusted water, due to the formation of soluble hydroxides at the particle surface. A major consequence of the dissolution is the formation of large, hard, multiparticle agglomerates on subsequent drying. In the final example, ZrO 2 -3mol%Y 2 O 3 particles are intentionally surface-doped with submonolayer levels of Cu-containing ions from ammoniacal solutions. The ceramics fabricated from such powders exhibit superplastic strain rates 100 or so times faster than in comparable undoped systems, due to the dopant’s role in lowering of the activation energy for diffusion along grain boundaries.

HVOF thermal spray deposited Y2O3-stabilized ZrO2 coatings for thermal barrier applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (Po) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures Tp that are high enough for partial melting of small (∼10 µm) zirconia particles, Tp=(1.10−0.95)Tm. Further analysis shows that for larger particles (38 µm), adherent coatings are produced when the particle temperature, Tp=0.59−0.60 Tm, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.

A topological rationale for the dependence of grain growth on strain during superplastic deformation

Abstract A model which relates the grain growth which occurs during superplastic deformation to the amount of strain has been developed. This model relies on three-dimensional grain topology, and is based on the premise that as grains grow in a constant volume system, the number of grains must diminish accordingly. During normal grain growth, surrounding grains will occupy—in a generally isotropic manner—the space occupied by a grain which is shrinking and eventually disappears. An applied uniaxial stress will act to bias the movement of surrounding grains into such spaces in a way which results in macroscopic strain. By following the evolution of a single column of grains within a three-dimensional sample loaded in uniaxial compression, the relationship between grain growth and macroscopic strain is predicted to be linear, with a ratio of 0.29. This ratio is a factor of two greater than that which has been observed experimentally, which can be explained by the inability of the present model to account for grains leaving the column in a manner other than by shrinking to zero volume.

Dynamic and Static Grain Growth During the Superplastic Deformation of 3Y-TZP

The present study has sought to elucidate the mechanism of dynamic grain growth, using 3Y-TZP as a model material due to its slow static grain growth. In an attempt to show that dynamic grain growth is a phenomenon intimately related to the mechanism of superplasticity, the results of the present study have been compared with results from a number of other studies on a variety of different materials, both metal and ceramic. A companion paper will subsequently describe a model -- derived from the present work -- which can suitably explain the empirical connection between dynamic grain growth and superplasticity.