Application of microhardness

Applications of microhardness testing greatly extend the conventional indentation hardness test to glass and ceramics, metallographic constituents, and to thin coatings or other surface treatments not otherwise testable. 

Pelc A., 1965, Zastosowanie badan mikrotwardosci dla kontroli jakosci w przemysle sciernym (Application of microhardness tests to quality inspection in the abrasive industry), Biul. Inf. POiN. IOS (Krakow), 3, 8-10. 

Application of microhardness techniques to the characterization of polymer materials... 

This chapter presents selected examples of the application of microhardness measurement for the characterization of polymeric materials after various physical treatments. 

For the structural applications of materials, there is no more useful measurable property than mechanical hardness. It quickly and conveniently probes the strengths of materials at various scales of aggregation. Firstly, it does this at the human scale (Brinell hardness—millimeters to centimeters). Secondly, it does so at a microscopic scale (Vickers microhardness—1 to 100 microns). And thirdly, it does so at a nanoscale (nanoindentation—10 to 1000 nanometers). 

Another motivation for measurement of the microhardness of materials is the correlation of microhardness with other mechanical properties. For example, the microhardness value for a pyramid indenter producing plastic flow is approximately three times the yield stress, i.e. // 3T (Tabor, 1951). This is the basic relation between indentation microhardness and bulk properties. It is, however, only applicable to an ideally plastic solid showing no elastic strains. The correlation between H and Y is given in Fig. 1.1 for linear polyethylene (PE) and poly(ethylene terephthalate) (PET) samples with different morphologies. The lower hardness values of 30-45 MPa obtained for melt-crystallized PE materials fall below the /// T cu 3 value, which may be related to a lower stiff-compliant ratio for these lamellar structures (BaM Calleja, 1985b). PE annealed at ca 130 °C... 

Muller (1970) described the application of the microhardness technique using small loads, employing the Vickers approach. The effect of various factors on the microhardness of a wide range of polymers by means of the same approach was reported by Eyerer Lang (1972). These authors reported that the diagonals of the impression did not change after the removal of the load. 

The interest in multicomponent materials, in the past, has led to many attempts to relate their mechanical behaviour to that of the constituent phases (Hull, 1981). Several theoretical developments have concentrated on the study of the elastic moduli of two-component systems (Arridge, 1975 Peterlin, 1973). Specifically, the application of composite theories to relationships between elastic modulus and microstructure applies for semicrystalline polymers exhibiting distinct crystalline and amorphous phases (Andrews, 1974). Furthermore, as discussed in Chapter 4, the elastic modulus has been shown to be correlated to microhardness for lamellar PE. In addition, H has been shown to be a property that describes a semicrystalline polymer as a composite material consisting of stiff (crystals) and soft, compliant elements. Application of this concept to lamellar PE involves, however, certain difficulties. This material has a microstructure that requires specific methods of analysis involving the calculation of the volume fraction of crystallized material, crystal shape and dimensions, etc. (Balta Calleja et al, 1981). 

Bearing in mind the outlined peculiarities of condensation polymer blends, and particularly when they consist of one component and one phase (this case is more the exception rather than the general rule since block copolymers usually consist of two, three, or more phases), the application of the additivity law for the evaluation of their characteristics does not seem to be completely justified. The observed good agreement between the measured microhardness values and the calculated ones (Fig. 5.7) allows one to make an important conclusion in this respect. 

As mentioned above, application of the additivity law (eq. (1.5)) supposes a knowledge of the number of the components (or phases) with given microhardnesses and weight fractions. What is not explicitly given in this equation is the type and the extent of mutual dispersion of the components as well as the quality of the adhesion on the contact surface boundary between the components (phases). We wish to stress here that this has an influence on the reliability of the H values derived from the additivity law. 

In the previous chapters the main fields of application of the microhardness technique in polymer physics have been highlighted. The emphasis has been mostly on solving structural problems, looking for relationships between the structures of polymers and their properties (initially mechanical ones) or on studying the factors which determine the microhardness behaviour of various polymeric systems. 

The correlation of microhardness and morphology for injection-moulded PET will be highlighted in this section as a second example of the application of the microhardness technique. 

Several nanoscale multilayered materials have been prepared. Techniques of Rutherford backscatteiing, electron microscopy and microanalysis and other metallurgical tools have been used to investigate wear resistant, scratch resistant, microhardness, and spark erosion properties of these nanoscale multilayered materials. Preliminary results indicate that nanoscale multilayered materials with improved thermomechanical, properties can be synthesized for application in the EM gun system. Application of ion beam technology for the synthesis of gradient materials appears to have great potential for design of new materials with improved properties to be used in fabrication of many armament materials. 

The Knoop hardness (HK) of a material is a measure of the residual surface changes after the application of pressure with a test diamond. The standard ISO 9385 describes the measurement procedure for glasses. In accordance with this standard, values for Knoop hardness HK are listed in the data sheets for a test force of 0.9807 N (corresponds to 0.1 kp) and an effective test period of 20 s. The test was performed on polished glass surfaces at room temperature. The data for hardness values are rounded to 10 HK 0.1/20. The microhardness is a function of the magnitude of the test force and decreases with increasing test force. 

---