G.S.A.M. Theunissen

Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

The surface- and grain boundary composition of Y, Ce and Ti doped zirconia were studied by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy/Scanning Auger Microscopy. The grain boundaries and free surfaces showed the same enrichment levels. After heat treatment ≥ 1000 ‡C all yttria doped samples showed yttrium enrichment. In the ZrO2-Y2O3 system the yttrium enrichment did not depend on the bulk concentration and amounted 30–34 mol% YO1.5 in all cases. As a consequence the segregation factor increases with decreasing solute concentration in the bulk. The thickness of the segregation layer was about 2–4 nm. In the ternary Y doped systems yttrium is the main segregant. In ceria-doped tetragonal zirconia polycrystals (Ce-TZP) systems significant segregation of cerium starts atT≥1300‡C and is mainly attributed to Ce3+. In Y,Ti-TZP systems also strong segregation of Ti4+ occurs. The absolute value of the increase of the surface concentration in fine grained material is smaller than in coarse grained material. This is mainly due to depletion of the bulk.

Sintering kinetics and microstructure development of nanoscale Y-TZP ceramics

Zirconia samples doped with 3 mol% yttria were prepared by gel precipitation from a metal chloride solution and their sintering behaviour compared with that of a commercial powder. Dense (relative density 97%) nanoscale ceramics with a mean grain size of 60 nm are obtained after sintering at 1050°C for 7h. Important densification mechanisms in the initial sintering stage are grain boundary sliding and grain boundary diffusion. Grain growth in the final sintering stage seems to be impurity drag controlled. Extremely low activation energies are obtained for both densification and grain growth in the initial sintering stages. Special attention has been paid to the effect of aggregate size of the precursor powder on the final grain size.

Powder preparation and compaction behaviour of fine-grained Y-TZP

Two wet chemical preparation methods are described for yttria-doped tetragonal zirconia powders. Both methods yield powders with an extremely small crystallite size (8 nm) and a narrow size distribution. The agglomerate and aggregate structure of these powders have been investigated by several techniques. Gel precipitation from an alkoxide solution in water (“alkoxide” synthesis) results in a ceramic powder with irregular-shaped weak and porous agglomerates, which are built up from dense aggregates with a size of 18 nm. Gel precipitates formed from a metal-chloride solution in ammonia (“chloride” synthesis) do not contain aggregates. Both types of agglomerate are fractured during isostatic compaction. Hydrolysis and washing under (strong) basic conditions probably decrease the degree of aggregation. The aggregate morphology and structure are key parameters in the microstructure development during sintering of a ceramic. Several characteristics of these powders are compared with those of a commerical one (Toyo Soda TZ3Y).

Mechanical properties of ultra-fine grained zirconia ceramics

The mechanical properties of tetragonal zirconia (TZP) materials doped with Y, Ce or Ti were studied as a function of temperature and grain size. Fine grained Y-TZP (grain size < 0.3 μm) shows values for fracture toughness and strength at room temperature, which are comparable with the coarse grained transformation toughened materials, despite lacking transformation toughening. The morphology of the fracture surface points to crack deflection as the most important toughening mechanism. At 800 °C fracture toughness and strength are higher than in coarse grained Y-TZP materials. Doping Y-TZP with Ce or Ti results in a similar trend in mechanical properties, for fine grained material, as for the Y-TZP materials.

Effect of Dopants on the Sintering Behaviour and Stability of Tetragonal Zirconia Ceramics

The microstrueture development during non-isothermal and isothermal sintering has been studied for tetragonal zireonia ceramics (TZP) containing carious amounts of Y, Ce and Ti. Smaller grain sizes were obtained when Ce-TZP was doped with yttrium. This could he attributed to segregation of yttrium to the grain boundaries, thus causing an impurity drag. With increasing temperature the grain growth in the Ce-TZP samples became faster, which could be attributed to the absence of a dragging force. The slow grain growth at low temperature in the Ce-TZP samples could he attributed to the slow diffusion kinetics of the diffusing species (trivalent and tetravalent cerium). The critical grain size for retainment of the tetragonal phase at room temperature is larger in the ECe-TZP systems compared to the Y-TZP amt Ce-TZP systems. The chemical stability increased by doping Y-TZP with cerium or titanium.

Microstructure characteristics of ultra-fine ZrO2---Y2O3 ceramic powdersstar, open

Abstract Yttria-doped zirconia powders with an extremely small crystallite size (8 nm) and a narrow size distribution have been prepared by gel precipitation methods. Precipitation of an alkoxide solution in water (“alkoxide” synthesis) results in weaker agglomerates when compared with a precipitation of a metal chloride solution in ammonia (“chloride” synthesis). Both types of agglomerate are fractured during isostatic compaction. The “alkoxide” agglomerates consist of strong small aggregates (15–20 nm) while within the “chloride” agglomerates no, or only a few, aggregates are present.

Grain Growth During Sintering of Nanocrystalline Y and/or CE-Doped Tetragonal Zirconia

A non-isothermal analysis of grain growth in tetragonal zirconia ceramics doped with various amounts of yttria and/or ceria in the temperature range 700–1150°C is presented. In these nanocrystalline ceramics, prepared by the gel precipitation technique, two grain growth regimes are recognized. At temperatures up to 900–1000°C grain growth proceeds slowly at high values of porosity (55%→35%); probably by means of a surface diffusion mechanism. At higher temperatures grain growth is much faster and occurs in locally dense regions by a normal grain growth or an impurity drag mechanism. Apparent activation energies are given for five compositions in both regimes. The isothermal sintering behavior of 3 mol% Y 2 O 3 -containing TZP at 1050°C is analysed. A dense ceramic with a crystallite size in the nanometer regime (

Microstructure Development During Sintering of Ultra Fine Grained Y-TZP

Ultra-fine grained, weakly agglomerated Y2O3 doped tetragonal ZrO2 powders with a primary particle size of about 8 nm were prepared by means of two different hydrous-gel precipitation techniques. These methods are respectively the hydrolysis of a metal alkoxide (“alkoxide” method) or a metal chloride (“chloride” method) solution. The sintering behaviour of these powders is compared with a commercial powder (Tosoh TZ3Y). After isostatic compaction at 400 MPa the chloride, alkoxide and TZ3Y compacts densify to 97% relative density after 10 hours sintering at 1050, 1200 and 1100°C respectively. The change in crystallite size and pore morphology has been studied as function of time. The sintering kinetics are probably determined by the aggregate structure within the green compact which is different for the investigated powders. A nanoscale ceramic can be obtained (grain size 53 nm) by sintering a chloride compact during 6.5 hr at 1044°C.

Segregation in ZrO2-Y2O3 Ceramics

The composition of grain boundaries and surfaces often control phenomena like sintering, grain growth, wear resistance and corrosion behaviour. In the present study the surface of zirconia ceramics doped with various amounts of Yttria was examined in order to find an explanation for observed differences in grain growth. This was done by using AES (Auger Electron Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy). Specimen heat treated at 873K had a surface composition similar to their bulk composition. Specimen heat treated at 1273K showed a surface composition containing 30-34 at% Yttria independent of the bulkcomposition. This results in a larger solute drag for zirconia with low amounts of yttria which probably causes a slower grain growth by means of an impurity drag mechanism. It could be shown that the relative increase of the 76eV Augerline was not due to Silicon segregation but solely to the segragation of Yttrium.