The effect of grinding-induced damage, on the strength of a yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and a zirconia-tough-ened alumina (ZTA) was investigated. The four-point flexure test was used to measure fracture strength as a function of grinding conditions. Flexure bar specimens were prepared by surface grinding transverse to the tensile stress direction in flexure testing. Two series of grinding experiments were performed. In the first series, four wheels with diamond particle sizes ranging from 25 to 180 μ,m were used to grind specimens at a fixed depth of cut. The strength of the Y-TZP decreased slightly with increasing diamond particle size. For ZTA the strength loss was increased as diamond particle size was increased. In the second series, one grinding wheel with a diamond particle size of 180 μm was used to grind specimens at a depth of cut per pass ranging from 2.5 to 100 μm. The Y-TZP showed a slight decrease in strength when the depth of cut was increased. However,...

Effect of grinding on strength of tetragonal zirconia and zirconia-toughened alumina

Evaluating dental zirconia.

The strength degrading effects of sharp-contact damage are examined for Y-TZP ceramic plates bonded to a plastic substrate. Contacts are made with Vickers and Berkovich diamond indenters at low loads (0.1 N to 100 N) in the ceramic lower surfaces prior to bonding to the substrates. The indentations remain in the subthreshold region, that is, without visible corner radial cracks, over the lower region (< 10 N) of the load range. A concentrated load is then applied sinusoidally to the ceramic upper surface, with the loading axis centered on the subsurface indentation flaw, thereby subjecting the flaw to cyclic tension. Relative to polished surfaces, the indentations diminish the single-cycle strength by an amount that increases with increasing indentation load. The critical number of cycles required to cause failure from the indentation flaws is then measured at specified maximum lower surface tensile stresses. At each indentation load, the strength of the ceramic plates diminishes with increased cycling. The scale of degradation is compared with that from previous studies on Y-TZP surfaces containing larger-scale surface damage: sandblast damage, as used in dental crown preparation; and blunt-contact trauma from a spherical indenter at 3000 N. These other damage modes are shown to be equivalent in their strength-degrading capacity to diamond pyramid indentations in the microscale load range 0.1 N to 1 N, attesting to the highly deleterious nature of sharp particulate contacts. The mechanism of fatigue is considered in terms of microcrack evolution within the indentation damage zone.

/pdf/fatigue-sensitivity-of-y-tzp-to-microscale-sharp-contact-arf08t4jq0.pdf

Fatigue sensitivity of Y-TZP to microscale sharp-contact flaws.

Indentation crack profiles that form under Vickers and Knoop indentations in silicon nitride were investigated. Two different experimental techniques were used to obtain a complete representation of the radial/median as well as the lateral crack system and the plastic deformation zone as a function of indentation load. Vickers indentation cracks exhibit a load dependent shape. At low loads two seperate radial cracks form per median plane, at high loads these cracks merge beneath the hardness impression to form an annular crack. Lateral cracks appear much closer to the surface than frequently assumed. Knoop cracks are nearly semielliptical. The crack aspect ratio and the aspect ratio of the elongated wedge-shaped plastic deformation zone are load dependent.

Indentation crack profiles in silicon nitride

The utility of indentation testing for characterizing a wide range of mechanical properties of brittle materials is highlighted in light of recent articles questioning its validity, specifically in relation to the measurement of toughness. Contrary to assertion by some critics, indentation fracture theory is fundamentally founded in Griffith–Irwin fracture mechanics, based on model crack systems evolving within inhomogeneous but well-documented elastic and elastic–plastic contact stress fields. Notwithstanding some numerical uncertainty in associated stress intensity factor relations, the technique remains an unrivalled quick, convenient and economical means for comparative, site-specific toughness evaluation. Most importantly, indentation patterns are unique fingerprints of mechanical behavior and thereby afford a powerful functional tool for exploring the richness of material diversity. At the same time, it is cautioned that unconditional usage without due attention to the conformation of the indentation patterns can lead to overstated toughness values. Limitations of an alternative, more engineering approach to fracture evaluation, that of propagating a precrack through a “standard” machined specimen, are also outlined. Misconceptions in the critical literature concerning the fundamental nature of crack equilibrium and stability within contact and other inhomogeneous stress fields are discussed.

The compelling case for indentation as a functional exploratory and characterization tool

Indentation radial cracking has been examined in a series of yttria-tetragonal zirconia polycrystals (Y-TZP) with a range of grain sizes. For all materials, two distinct types of cracks were found to exist. Above a threshold indentation load, short cracks of variable length were observed, “trapped” in a compressive, contact-induced transformation zone. Above a second threshold, longer, “well-developed” cracks of more consistent length formed, extending beyond the zone boundary. The stochastic nature of the formation of both types of cracks, and the existence of the transformation zone in general, generate many artefacts in toughness estimates made using unmodified indentation fracture mechanics analyses.

Trapped cracks at indentations

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Trapped cracks at indentations