Microindentation instruments

The basic principles of microindentation have been presented in Sections 2.1, 2.2 and 2.3. There is a wide variety of traditional commercial instruments which give an accurate measurement of the microhardness of a polymer surface. In what follows we will just summarize the main features of a typical microindentation instrument ... 

The indentation test is one of the simplest ways to measure mechanical properties of a material. The micromechanical behavior of polymers and the correlation with microstrnctnre and morphology have been widely investigated over the past two decades (23). Conventional microindentation instruments are based on the optical measnrement of the residual impression produced by a sharp indenter penetrating the specimen surface under a given load at a known rate. Microhardness is obtained by dividing the peak load by the contact area of impression. From a macroscopic point of view, hardness is directly correlated to the yield stress of the material, ie, the minimnm stress at which permanent strain is produced when the stress is snbseqnently removed. 

It should be noted that hardness values obtained from instrumented nanoindentation and microindentation may differ significantly and this phenomenon is invoked as the indentation size effect (ISE). 

When the material to be tested is very thin, the indentation should be shallow and the applied load small. This is called microindentation hardness or nanoindentation and the indentation load can be as low as 0.05 milligrams. One commercial instrument is capable of performing indentation tests with a load of 2.5 millinewtons and depth resolutions of 0.4 nanometers. It detects penetration movement by changes in capacitance between stationary and moving plates. 

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