Ceramics fracture

This presentation shows fractographic analyses of seven fractured ceramic bridges and one crown made with modem dental restorative materials. The overall breakage patterns were evaluated and stereoptical and scanning electron microscopy used to find the causes of fracture. There were multiple causes of fracture, but faulty restoration design or fabrication laboratory faults accounted for most fractures. Fracture stresses were relatively low. 

This subject has been discussed widely in the past mainly in regard to metallic materials, in which plastic deformation before failure is a key factor leading to fracture. Ceramics, which are mostly brittle at RT, fail almost completely without deformation, but are, nevertheless, exposed to fatigue fracture. It is useful to first consider this concept in metals in general before focusing our attention on ceramics. 

R.W., Rice, Ceramic fracture mode Intergranular vs. Transgranular Fracture. Ceramic Transec tions, 64 Fractography of glasses and Ceramics III (JR Varner, VD Frechette, and GD Quinn, eds). Am. Cer. Soc., Westerville, OH, 1996, p.l 53. 

It must be emphasized that differential hardness has been verified in hip simulator studies only for combinations in which a ceramic head articulates against a CoCr Uner. Conversely, it is not recommended that metal heads be used in combination with a ceramic finer. Two case studies of metal-on-ceramic combination bearings have been associated with severe wear and, for one revision case in which a metallic head was used to replace a fractured ceramic component, the head wear that ensued resulted in cobalt pwison-ing and blindness [144,145]. 

Figure C2.11.5. Scanning electron micrographs showing the microstmcture of an alumina ceramic spark-plug body (a) fracture surface and (b) polished and thennally etched cross section.

Melt flow rate Melt-formed ceramics Melt fracture Melting temperature... 

Finally, the nature of the crystalline microstmcture, ie, crystal size and morphology and the textural relationship among the crystals and glass, is the key to many mechanical and optical properties, including transparency/opacity, strength and fracture toughness, and machinabiUty. These microstmctures can be quite complex and often are distinct from conventional ceramic microstmctures (6). 

Fig. 2. RepHca electron micrograph of the fracture surface of enstatite—P-spodumene—zirconia glass-ceramic, showing twinning in the enstatite grains...

Fig. 4. Fracture surface (repHca micrograph) of fluorcanasite glass-ceramic showing interlocking blade-shaped crystals and effects of cleavage splintering...

Fig. 11. Micrographs of (a) a hot-pressed alumina—TiC ceramic showing a white TiC phase and a dark alumina phase (3) and (b) a fracture surface of an...

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