Martensitic Relief Observation by Atomic Force Microscopy in Yttria-Stabilized Zirconia

The tetragonal to monoclinic (t–m) phase transformation of zirconia has been the object of extensive investigations of the past 20 years and is now recognized as being of martensitic nature. However, martensitic transformation has only been observed by transmission electron microscopy or indirect methods. Though the benefit on the fracture toughness and crack resistance was the main interest, the transformation is now considered for its consequences on the degradation of the material. The use of atomic force microscopy reported here allowed the observation of the first stages of martensite relief growth and of new martensitic features.     

Effect of Crystal Grain Size and Thermal Stress on Martensitic Transformation of Phosphorus-Bearing Dicalcium Silicates

A series of P₂O₅-bearing Ca₂SiO₄ solid solutions, (Ca_{2- x /2}open square _{x /2})(Si_{1- x} P _x )O₄ with x = 0.02 and 0.06 lessthan equal to 0.11, was prepared. The alpha´_L <–> ß martensitic transformation temperatures were determined by high-temperature XRD for powder specimens with x = 0.02. With decreasing particle size, both M _s and M _f decreased, while both A _s and A _f were almost unchanged. Each transformation temperature was lower than that of pure Ca₂SiO₄. The phase composition at 25°C for the crystals with 0.075 lessthan equal to x lessthan equal to 0.10 was dependent not only on the x -value but also on the particle size and the intensity of the thermal stress upon quenching. The effect of the latter two was most clearly recognized in the phase composition of the crystals with x = 0.09. The phase composition was changed between the two extremes; one was composed of 8% alpha´_L-phase and 92%ß-phase and the other was 96% alpha´_L-phase and 4%ß-phase. For the crystals with 0.06 lessthan equal to x lessthan equal to 0.065 (ß-phase) and 0.105 lessthan equal to x lessthan equal to 0.11 (alpha´_L-phase), only the x -value governed the phase composition.     

Physical Stabilization of the β→γ Transformation in Dicalcium Silicate

It has been shown that the monoclinic β -phase of dicalcium silicate (Ca₂SiO₄) can be stabilized against transformation to the orthorhombic γ -phase by physical rather than chemical factors. Stabilization was studied in different types of microstructures fabricated under various processing conditions such as different powder or grain sizes, chemical additives, cooling kinetics, or high-temperature annealing treatments. The observations can be explained in terms of a critical particle size effect controlling nucleation of the transformation. Rapid quenching through the high-temperature hexagonal ( α ) to orthorhombic ( a' _H) transformation at 1425°C, which is accompanied by a −4.7% volume decrease, causes periodic fracture of β -twins due to accumulated strains. Chemical doping with K₂O or Al₂O₃ promotes the formation of amorphous phases which mold themselves around β -Ca₂SiO₄ grains. Annealing treatments cause crystallization of the glass and subsequent transformation to the γ -phase.     

Effect of Substituent Ions on Martensitic Transformation Temperatures in Dicalcium Silicate Solid Solutions

Five types of Ca₂SiO₄ solid solutions, doped with either P⁵⁺, Ge⁴⁺, Fe³⁺, Mg²⁺, or Ba²⁺, were prepared and examined by high-temperature X-ray diffractometry up to 800°C. The starting and finishing temperatures were determined for the α'_L-to-β martensitic transformation during cooling and the reverse (β-to-α'_L) transformation during heating. These four types of transformation temperatures for the preparations doped with either P⁵⁺ or Ge⁴⁺ steadily decreased with increasing substituted fraction. The effect of the substitution on the decrease for each transformation temperature was quantitatively evaluated by Δ T / x , where Δ T is the difference in the transformation temperatures between the solid solutions and pure Ca₂SiO₄, and x represents the fraction substituted for Si⁴⁺ or Ca²⁺ in the α'_L-phase structure. The evaluated value for the substitution of P⁵⁺ was more than 3 times that of Ge⁴⁺. The effect of the substituent ions mentioned above, together with Na⁺ and Sr²⁺, on the lowering of the starting temperature of the α'_L-to-β transformation was principally determined by the differences in the ionic radius between the interchanging cations.     

Relationship between Transformation Temperature and Time-Temperature-Transformation Curves of Tetragonal-to-Monoclinic Martensitic Transformation in Zirconia-Yttria System

The tetragonal → monoclinic ( t → m ) martensitic phase transformation in ZrO₂–0.5–4 mol% Y₂O₃ proceeds during isothermal aging at various temperatures from 350 to 800 K; i.e., the transformation increases sigmoidally with aging time. The time-temperature-transformation ( T - T - T ) curves show a typical C shape, with very high rate for lower Y₂O₃-content specimens, and the rate decreases with higher Y₂O₃ content. The transformation temperature ( M _s) of the t → m transformation obtained from dilatation curves during the cooling stage coincides well with C curves above the nose temperature. The t → m transformation should occur isothermally, as suggested by a nucleation and growth mode.     

Transformation Toughening in Composites Containing Dicalcium Silicate

By taking into account solid-state Compatibility relations in the system CaO-SiO₂-ZrO₂, a series of dense, tough CaZrO₃-β-Ca₂SiO₄ composites were obtained. The increase in K_IC and σ_f values observed is interpreted in terms of the β-Ca₂SiO₄→γ-Ca₂SiO₄ polymorphic transformation.     

Phase Transformation of Nanocrystalline Anatase Powders Induced by Mechanical Activation

The microstructural evolution of nanocrystalline anatase TiO₂ during high-energy planetary ball milling was studied. The results show that mechanical activation induces the transformations of nanocrystalline TiO₂ from anatase to srilankite and rutile at room temperature and under ambient pressure. As the milling time increases, more anatase powder transforms to the srilankite and rutile phase, and the particle size of the powder decreases. There is no indication of the formation of amorphous phase during ball milling.     

Metastability of the Martensitic Transformation in a 12 mol% Ceria-Zirconia Alloy: II, Grinding Studies

Observations of the grinding-induced transformation in singlephase Ce-TZP materials, referred to in an earlier paper, are presented. Two techniques were used to grind the surface: by hand in a slurry of abrasive particles and with a high-speed diamond-impregnated wheel. Significant differences in X-ray diffraction profiles between the two grinding methods was observed. Limited monoclinic ZrO₂ was detected on the machineground surface, along with the reversal of the tetragonal ZrO₂ (200) peak intensities. On the hand-ground surface, considerable monoclinic phase was observed. The disappearance of the monoclinic phase with heating was followed by X-ray diffraction, and the A _f was found to exceed 700°C, while the reversal in tetragonal (200) peak symmetry and intensity remained unaltered up to at least 1000°C. Transmission electron microscope studies at various depths below the ground surface were undertaken to identify the differences between these surfaces and fractured surfaces. A simple explanation is proposed for the reversal of the tetragonal peak intensities. This reversal has previously led to the notion of a ferroelastic toughening mechanism in similar TZP materials.     

On the Thermoelastic Martensitic Transformation in Tetragonal Zirconia

Recent evidence is summarized showing that the tetragonal ( t ) → monoclinic ( m ) martensitic transformation in ZrO₂ can occur thermoelastically in certain ZrO₂-containing ceramics, and that microcracking accompanying the transformation is more common than had previously been recognized. The implications of these new data for the conditions under which the stress-induced transformation is irreversible, and for the particle size dependence of the transformation start ( M _s), temperature, are discussed.     

Thermodynamic Modeling of Phase Diagrams in Binary Alkali Silicate Systems

A thermodynamic modeling of phase diagrams in binary alkali silicate systems is described by using the Gibbs free energy of the phases. Simple models employing regular, quasi-regular, and subregular solution models were applied to describe the liquid phase. The interaction parameters of the liquid phases were obtained by using the liquidus curves of silica (and subliquidus data) through a multiple linear regression method. The present calculated liquidus curves agree very well with Cs₂O—SiO₂ and Rb₂O—SiO₂ systems. In the K₂O—, Na₂O—, and Li₂O—SiO₂ systems, the calculations permitted discrimination between sets of data and, based upon the calculation choices, could be correctly made to select the most appropriate phase diagram. Also, the spinodals were calculated where phase separation can occur by a spinodal decomposition process.     

Effect of Silicate Anion Structure on Fixation of Chloride Ions by Calcium Silicate Hydrates

Calcium silicate hydrates, CaO–SiO₂-H₂O (C-S-H), were studied as a chloride fixation material. C-S-H of two different CaO/SiO₂ ratios were synthesized and burned with calcium chloride in a temperature range from 600° to 1000°C. Minerals with a chemical composition of CaO·SiO₂·CaCl₂ and 9CaO·6SiO₂·CaCl₂ were identified by X-ray diffraction analysis. Comparing the diffraction intensity, it was found that the most efficient chloride fixation was attained when burned at 800°C. Changes in the morphology of silicate anion associated with burning and fixation of the chloride were studied in terms of chloride fixation capability using the trimethylsililation technique. It was confirmed that some silicate anions formed a glassy infinite chain where the chloride ions were fixed as a solid solution.     

Anatase-to-Rutile Transformation in Titania Powders

Titania (TiO₂) is an important electronic ceramic material for use in diverse applications such as gas sensors, catalysts, dielectrics, and ceramic membranes. TiO₂ exists as several polymorphic phases, most commonly as rutile or anatase. This paper investigates the microstructural evolution of anatase-based commercial TiO₂ powders, with an average size of 100 nm, at high temperatures. These powders transform to the rutile structure at 1000°C. The characteristics of the anatase-to-rutile transformation have been studied using transmission electron microscopy analysis, and new information regarding the nature and mechanisms of this polymorphic reaction has been revealed.     

Transitional Phase of Ca2SiO4 Solid Solution with Incommensurate Superstructure

Crystals of Ca₂SiO₄ doped with K, P, Al, and Fe were grown and examined by means of both X-ray and electronbeam diffraction. They are made up of domains 120° different in orientation around the c axis of the previous α phase. Each of the domains gives an orthorhombic superstructure whose cell is incommensurate with the underlying orthorhombic subcell with a=0.540, b=0.936, and c=0.681 nm. This orthorhombic phase, characterized by the cell edge length of 3.8a, is termed 3.8aC₂S(ss). Both the domain and the incommensurate structure strongly suggest that this is the precursor to a phase thermodynamically more advantageous at lower temperatures.     

Growth and Phase Transformation of Nanometer-Sized Titanium Oxide Powders Produced by the Precipitation Method

We report an in situ TEM investigation of the growth and transformation in nanometer-sized titania powders. The powders were produced through precipitation of titanium tetrachloride under different pH conditions. The initial phase of the produced powders was amorphous or was a mixture of anatase and brookite according to the pH conditions. During calcination, the anatase particles grew and transformed into rutile. The transformation temperature increased with increasing pH value. In situ TEM observations showed that the anatase particles were absorbed into rutile, and then rutile particles grew by coalescence. Furthermore, small pores were observed to form in samples prepared with high pH from the effects of hydroxyl ions and zeta potential. Pore formation increased the surface area, which delayed the transformation and nucleation of rutile. This explains the difference of growth and transformation of titania powders produced under different pH conditions during calcination.     

Kinetics of Tricalcium Silicate Hydration in Diluted Suspensions by Microcalorimetric Measurements

Investigations of tricalcium silicate (C₃S) suspensions using different techniques have shown the initial hydration kinetics of C₃S are strongly dependent on the water/C₃S ratio. In order to provide new experimental data on the hydration, we have adapted a Tian-Calvet heat flow isothermal microcalorimeter to study thermal flow variation released by C₃S stirred suspensions in water and saturated lime water. We observed three exothemic peaks and one endothermic peak. Their relative magnitude and duration depend on the W/C ration and the lime solution concentration. The protective layer theory appears to be consistent with our results for high W/C ratio. Moreover, portlandite precipitation seems to be a consequence and not the cause of the acceleratory period.     

Surface Characteristics of Kaolinite and Other Selected Two Layer Silicate Minerals

The electrokinetic features and interfacial water structure, as revealed from molecular dynamics simulations (MDS) are considered with respect to the anisotropic features of selected two layer silicate minerals. Planar structures of kaolinite and antigorite are compared to the, compositionally equivalent, tubular structures of halloysite and chrysotile with respect to the pH dependency of zeta potential as determined from the electrophoretic mobility measurements. The importance of the atomic mismatch between the tetrahedral and octahedral sheets in a particular bilayer is discussed in order to explain the electrokinetic behaviour of these two layer silicate minerals. Interfacial water structure from MDS suggests that the silica tetrahedral surface is not wetted by water. In the case of planar silicates structural imperfections are considered to explain wetting characteristics. The possibility of polarity reversal (domain inversion) within a tetrahedral/octahedral layer and an out‐of‐order layer (inserted layer) within the tetrahedral/octahedral stack are considered in order to explain electrokinetic behaviour and wetting characteristics.     

Strengthening of Ceria-Doped Tetragonal Zirconia Polycrystals by Reduction-Induced Phase Transformation

Phase-transformation-induced compressive surface stresses were introduced into ceria-doped tetragonal zirconia polycrystals by reduction of CeO₂. Four-point-bending strength of sintered ZrO₂ containing 12 mol% CeO₂ increased from 240 to 545 MPa after it was annealed at 1400°C for 2 h in nitrogen. The strength of the same material hot isostatically pressed in oxygen increased after it was annealed in nitrogen for 2 h at 1500°C from 430 to 595 MPa.     

Phase Transformation and Texture in Hot-Forged or Annealed Liquid-Phase-Sintered Silicon Carbide Ceramics

Starting from three powder mixtures of 80 vol% SiC (100α, 50α/50β, 100β) and 20 vol% YAG, liquid-phase-sintered silicon carbide ceramics were prepared by hot pressing at 1800°C for 1 h under 25 MPa, and then by hot forging or annealing at 1900°C for 4 h under an applied stress of 25 MPa in argon. The phase transformation and texture development in the as-hot-pressed, hot-forged, and annealed SiC ceramics were investigated via X-ray diffraction (XRD) and the pole figure measurements. The 6H → 4H polytypic transformation was observed in samples consisting of both α- and β-SiC phases when subjected to compressive deformation but absent in the case of annealing, suggesting the deformation-enhanced solubility of aluminum in SiC. Deformation was also found to enhance the 3C → 4H transformation in the sample containing entirely β-phase, which is due to the accelerated solution-precipitation process assisted by grain boundary sliding. The current study showed that the β- →α-phase transformation had little effect on texture development in SiC. Hot forging generally produced the strongest texture, with the calculated maximum of 2.2 times random in samples started with pure α-SiC phase. The mechanism for texture development was explained based on the microstructural observations.     

Direct Amorphous-to-Cubic Perovskite Phase Transformation for Lead Titanate

The phase evolution of lead titanate processed by the polymeric precursor method was investigated by thermal analysis, X-ray diffraction, and high-resolution transmission electron microscopy. The results showed that the cubic perovskite PbTiO₃ (PT) phase is formed from an inorganic amorphous precursor at a temperature of 444°C. A gradual transition from cubic to tetragonal perovskite PT was observed with the increase of calcination time at this temperature. HRTEM results showed that the cubic PT particles have a size of around 5 nm. The identification of cubic PT as an intermediate phase supports the hypothesis that the chemical homogeneity was kept at the molecular level during the synthesis process, with no cation segregation.     

HRTEM and EELS Studies on the Amorphization of Hexagonal Boron Nitride Induced by Ball Milling

We present a new approach, namely ball milling, to synthesize amorphous boron nitride ( a -BN). The amorphization kinetics are revealed by X-ray diffractometry (XRD), high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS). HRTEM investigations indicate that, in the early stage of milling, the thick sp ² layers are sliced into many thinner sheets because of cleavage along the basal planes. In the intermediate stage of milling, deformation is accommodated primarily by simultaneous shearing along the basal planes. As a result of sustained shearing, a number of defects, such as stacking faults, (0002) [11¯20] twinning, simultaneous shearing of lattice planes, and half Frank loops with Burgers vectors of 1/2[0001], are introduced in the hexagonal BN ( h -BN) grains. Simultaneous shearing also causes significant change in the lattice symmetry of most grains. In the final stage of milling, the fiberlike tightly bonded sp ² sheets are broken and refined further, until a nanocrystalline and amorphous mixture is formed. XRD of the sample milled for 180 h exhibits an amorphous halo pattern; nevertheless, HRTEM demonstrates that the end product is essentially a nanocrystalline and amorphous mixture. The grain sizes of the nano-crystals are <3 nm, and their stacking is turbostratic. EELS investigations of the a -BN indicate that bonding is primarily sp ², but a small fraction of sp ³ a -BN is also found, which is assumed to be the nuclei of the cubic phase ( c -BN) in the high-pressure and high-temperature experiments and thus facilitates the hexagonal to cubic transition. The present a -BN fabrication method can provide an effective way to facilitate the synthesis of sintered bulk c -BN materials.