Sales Training 2014 - Dentistry

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Section I * Fundamentals of Dental Materials

Chemical Properties Metals tend to lose electrons easily to form cat- ions. They react with oxygen in the air to form oxides; the time taken may vary. For example, iron takes years to rust while potassium burns in seconds. Transition metals such as iron, copper, zinc, and nickel take much longer to oxidize while noble metals do not react with atmo- spheric oxygen at all. Mechanical Properties These properties are also the result of the crystalline structure and metallic bonds of a metal. Most metals are generally ductile (able to draw into a thin wire) and malleable (able to beat to a thin sheet). This is due to the ability of the layers of atoms to slide against each other into new positions within the same crystal structure without breaking the metallic bond. If a small stress is put on the metal, the layers of atoms will start to roll over each other. When they fall back to their original positions on release of this stress, the metal is said to be elas- tic . This rolling of layers of atoms over each other is hindered by grain boundaries, because here the rows of atoms are not aligned properly. Therefore, the smaller the individual grains, the more the grain boundaries, and hence the harder the metal becomes. Since atoms in the grain boundaries are not in good contact with each other, the metal tends to fracture at these grain boundaries. Hence increasing the number of grain boundaries makes the metal not only harder, but also more brittle. It can be concluded that hardness and brittleness of a metal depend on its grain size . Yield strength is the amount of stress required to produce a pre-established amount of permanent strain (i.e., change in length) of the alloy. Ideally, alloys used in the oral cavity should have tensile yield strengths of more than 300 MPa so that a great deal of stress can be applied before they permanently deform. Modulus of elasticity is the measure of rigidity or stiffness .

Since the distance between the atoms in a crystal lattice is different in the horizontal and vertical directions (depending on their lattice structure; Fig. 5.1 ), the properties may also vary if a single crystal is observed. This directional property principle is followed in the manufacture of microchips for computers. However, in general practice it is not possible to evaluate properties in different directions; hence, the properties of a whole metal or alloy are taken into consideration. So a fine-grained structure is desirable for metals and their alloys to have uniform properties in any direction. Nonuniformity of directional properties is termed anisotropy . Dislocations Metallic crystals are not perfect; they have flaws called dislocations . Grain boundary is a type of dislocation. Dislocations are of several types, but generally all alloweasy deformation of themetal. Fracture of metals occurs when the atomic centers cannot slide over one another freely, because impurities block the flow of dislocations. Hence, all methods that increase the strength of the metals act by controlling the movements of these dislocations. Coring During solidification of an alloy, the last liquid to solidify is the metal with lowest solidus tempera- ture. Hence when an alloy is rapidly cooled, the alloy has core structures, with themetal with high solidus temperature forming the dendrites within these cores. Themicrostructure of the rest of the matrix between these cores consists of metals with lower solidus temperatures.

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