Surface compressive layers

The introduction of surface compressive layers can strengthen ceramics and is a well-established technique for glasses (see Sec. 13.5 for more details). The underlying principle is to introduce a state of compressive surface residual stress, the presence of which would inhibit failure from surface flaws since these compressive stresses would have to be overcome before a surface crack could propagate. These compressive stresses have also been shown to enhance thermal shock resistance and contact damage resistance. 

Clearly all indents made with a pyramidal indenter should have the same shape regardless of their size. Thus, since we take pressure used to make this shape to be a measure of hardness—see equations (1.6) and (1.7)—we would expect hardness to be the same and there to be no load effect. Therefore when hardness increases as the applied load decreases, as shown in Figure 1.3, it must be because the volume of material used to yield is smaller and the mechanism for yielding is dependent on a volume term which becomes more significant as the indent size decreases. The most obvious development of this idea is that the shallow near-surface volume of the deformation zone can become a significant fraction of the total affected volume when a very small load is used to make the indent. Thus, work hardened layers, surface compressed layers, ion-implanted layers, and the possibility of chemical reactions between the atmosphere and the surface can dominate the yielding mechanism to produce nonstandard hardness values. Conversely we can say that these phenomena could be studied by measuring the ISE of a ceramic. 

As equation (5.91) shows, the threshold load can be estimated from a knowledge of Hy, K, and all of which are found by the indentation hardness technique. In the case just considered, with Kc = 0.75 MN Hy = 6 GN m, ( c = 130 MN m , and A = 0.076, an estimate for P is 0.43 N. This small load over the area represented by a, is of course a large local stress, but the estimate does emphasize that small particles can generate critical flaws beneath the surface. The effect of the surface compression layer can be seen by removing the -1.6critical load decreases to 0.068 N for this glass. 

Many ways have been demonstrated for effectively generating surface compressive layers, some of which have been applied commercially. These include thermal tempering, ion exchange at temperatures either above or below the glass transition, surface crystallization, lamination, and glazing. 

Using a method based on the filtration of air through a compressed layer of powder Ezerskii et al determined the visible specifie.surface area of neomycin sulphate to be 2.6 m2g. ... 

Lakshminarayanan, R., Shetty, D.K., Cutler, R.A., Toughening of layered ceramic composites with residual surface compression, J. Am. Ceram. Soc., 79(1), 79-87, 1996. 

Other methods of chemically strengthening glass include Li ions diffused into a sodium-containing glass to create a composition profile in which the resulting surface has a lower expansion than the core region, and thus develops a compressive layer on... 

Compressive layers may be formed on glass ceramics by controlling the crystallization sequence so that the surface crystallizes and becomes rigid before the interior. As the interior subsequently crystallizes, it shrinks, thereby placing the surface in compression. Preferential surface crystallization may occur if the article is heated in a wet atmosphere or if the surface is abraded to provide nucleation sites. 

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