Both superficial and internal microcracks make ceramic a fragile material. It's a well-known fact that there is a direct relation between microcracks and mechanical strength of material. These microcracks can be spread through deeper layers due to failures which occur during preparation, modelling and cooking and cooling process of ceramic. These kinds of microcracks are called also "Griffith fractures" in memory of first researcher who has defined them. During the cooling process of material cooked in Owen to get room temperature, volumetrical shrinkage cause microcracks on surface of the material.
For that reason, before making an assessment about oral mechanical features of dental ceramics, one should consider operations during production process of restoration. This is a real handicap in dental ceramic practices Technical flaws like microfractures and porosity which appears during production and inappropriate thermal dilatation parameters cause mechanical failure in porcelain restoration. Glass is sensitive to microcracks that show up on surfaces. In other words, the material is basically made up of glass. Ceramic material used in dentistry is a glassy matrix that is produced via sintering and it contains leucite crystals it can be identified as a sort of ceramic that is not totally transformed into glass phase. Even the pressure strength of dental ceramics are between 350-550 MPa, the draw strength value is 20-60 MPa. Ceramic has a high rigidity and pressure strength and low flexion performance. However, because of those bonds, ceramic has a fragile structure. Crystal structure of ceramic material contains ionic and covalent type of strong bonds that gain stability and determination. In the long run, it can lose its endurance against shear and draw strengths that occur while chewing. Today's ceramic restoration application, despite its popularity gained through esthetical advantages and superior hygienically features, has a fragile structure due to its low tensile strength quality.
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