Tanja Lube

Indentation crack profiles in silicon nitride

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.

Chairside CAD/CAM materials. Part 2: Flexural strength testing

OBJECTIVE Strength is one of the preferred parameters used in dentistry for determining clinical indication of dental restoratives. However, small dimensions of CAD/CAM blocks limit reliable measurements with standardized uniaxial bending tests. The objective of this study was to introduce the ball-on-three-ball (B3B) biaxial strength test for dental for small CAD/CAM block in the context of the size effect on strength predicted by the Weibull theory. METHODS Eight representative chairside CAD/CAM materials ranging from polycrystalline zirconia (e.max ZirCAD, Ivoclar-Vivadent), reinforced glasses (Vitablocs Mark II, VITA; Empress CAD, Ivoclar-Vivadent) and glass-ceramics (e.max CAD, Ivoclar-Vivadent; Suprinity, VITA; Celtra Duo, Dentsply) to hybrid materials (Enamic, VITA; Lava Ultimate, 3M ESPE) have been selected. Specimens were prepared with highly polished surfaces in rectangular plate (12×12×1.2mm3) or round disc (Ø=12mm, thickness=1.2mm) geometries. Specimens were tested using the B3B assembly and the biaxial strength was determined using calculations derived from finite element analyses of the respective stress fields. Size effects on strength were determined based on results from 4-point-bending specimens. RESULTS A good agreement was found between the biaxial strength results for the different geometries (plates vs. discs) using the B3B test. Strength values ranged from 110.9MPa (Vitablocs Mark II) to 1303.21MPa (e.max ZirCAD). The strength dependency on specimen size was demonstrated through the calculated effective volume/surface. SIGNIFICANCE The B3B test has shown to be a reliable and simple method for determining the biaxial strength restorative materials supplied as small CAD/CAM blocks. A flexible solution was made available for the B3B test in the rectangular plate geometry.

Thermal Shock Behavior of an Al2O3/ZrO2 Multilayered Ceramic with Residual Stresses due to Phase Transformations

In this work, the thermal shock behavior of an Al2O3-5%tZrO2/Al2O3-30%mZrO2 multilayer ceramic is studied. On these materials, a tetragonal to monoclinic phase transformation within the Al2O3-30%mZrO2 layers takes place when cooling down from sintering. The latter induces an increase in volume and therefore compressive residual stresses arise in these layers. The residual stress distribution profile in the laminate influences the thermal shock response of the material. A finite element model has been developed to estimate both the thermal strain effects during the sintering process as well as the temperature distribution and stress profile within the laminate during thermal shock testing. Experimental tests on the monoliths and laminates were carried out and compared to the model. It is observed that the presence of the compressive layers within the laminate inhibits the penetration of thermal shock cracks into the body at even more severe conditions than in the monolithic material.

Strength Distributions in Ceramic Laminates

In this work the strength of ceramic laminates is analyzed. Two different architectures with the same surface are studied. One of them, a Al2O3 / Zr(3Y)O2 laminate, contains compressive residual stresses, the other one, a monolithic Al2O3 laminate, has no residual stresses. Residual stresses are estimated analytically on the basis of the materials properties and by an indentation technique. The influence of the residual stress on the strength distribution is investigated. Strength distributions for laminates with compressive stresses at the surface follow a Weibull distribution only under certain conditions.

R-Curves in Al2O3 - Al2O3/ZrO2 Laminates

In order to better understand the fracture behavior of Al2O3-Al2O3/ZrO2 laminates, controlled crack growth experiments were carried out. The method proposed consists of a threepoint bending test of a sandwich beam in which a notched specimen is located in between two metallic bars. Under certain conditions that will be commented, stable crack growth can be obtained. The fracture pattern is related to the residual stresses present in the laminate. R-curves for the laminate were also calculated using the weight function method, including the influence of the free surface. Toughness values are presented in the R-curve after friction considerations.

Development of a Bending-Test Device for Small Samples

Flexural testing of rectangular beams in 4-point geometry is a simple and widely used standardised method [1] to gain uniaxial strength data of ceramic materials. To evaluate the strength of components samples have to be cut out of them, which is not always possible in the prescribed standardised size. Therefore the miniaturisation of the 4-point bending-test is analysed. The errors in determined flexural strength arising from geometry and measuring inaccuracies are calculated and expressed depending on the outer span length. The possibilities of adequate specimen preparation are investigated. The limitation of the resulting relative error in flexural strength to 10% determines minimal dimensions of samples and fixtures. The applicability of such a device is tested by comparing two sets of strength values measured on small and DIN-fixtures with the help of Weibull statistics.

Mechanical properties of ceramics

Advanced ceramic materials have unique properties that make them key elements of current and future technologies. Not only do some forms of ceramic have advantageous refractoriness, hardness, wear resistance, and resistance to corrosion but others also have what is often described as ‘functional’ properties. For example, the high oxygen ion conductivity of zirconia is exploited for high-temperature fuel cells. Some ceramics with perovskite crystal structure can be switched between electrical conducting and nonconducting conditions, triggered by a phase transformation, a feature commonly used in electric circuit protectors. In other perovskite structure ceramics, a strong piezoelectric effect can be developed, and this is exploited in force measurement, for example, load cells, and for displacement control, such as in positioning devices and even as fast-acting valves for diesel injection in vehicle engines.

Mechanical Properties of Ceramic Laminates

The progress of research efforts on the mechanical properties of ceramic laminates is reviewed. Laminates with weak interface are described with respect to their failure mode and the criterions to achieve graceful failure. For laminates with strong interfaces basic principles concerning the residual stresses and their influence on crack propagation are introduced. The implications for strength, indentation strength, strength distributions, macroscopic R-curves and threshold strength are discussed.

Biaxial Strength Testing on Mini Specimens

Biaxial strength testing of brittle materials using the ball on three balls (B3B) test is a new method for strength testing of disc or plate specimens [1] (see Fig. 1). The analysis of the stress fields and the calculation of the effective volumes and surfaces for several types of specimens and testing geometries has been performed recently. The ball on three balls testing method has several advantages compared to the common three- or four point bending tests: the results are very stable against small geometrical inaccuracies of the specimen or the test assembly, edge defects are not relevant and there exists only a very low influence of friction [2]. This makes this type of experiment ideal for testing very small specimens [3].

Influence of the Sample Size on the Results of B3B-Tests

The ball-on-three-balls (B3B)-test is a biaxial strength test for brittle materials. The results of B3B-tests are very stable against small geometrical inaccuracies of the specimens or the test support. In contrast to conventional bending tests there exists only a small influence of friction and edge defects are not relevant. These advantages, compared to beam tests, make the testing of mini-specimens with volumes of a few mm3 feasible. For this investigation silicon nitride specimens of different sizes were tested by use of the B3B-test. The maximum tensile stresses and the effective volumes and effective surfaces of the specimens were determined. The obtained results are compared directly and with the results of conventional 4-point-bending tests and are discussed in the framework of the Weibull-Theory. Additionally fracture surfaces of B3B-specimens and bending specimens were investigated fractographically to identify possible fracture origins.