Department of mechanical engineering, NIT Warangal

Department of mechanical engineering, NIT Warangal

HARDNESS OF METAL MATRIX COMPOSITESHardness tests are routinely used as a simple and effective means of quantifying the tensile strength of metallic materials. The correlation between various hardness scales and tensile strength has been compiled for a variety of metals and alloys. When a metal is reinforced with ceramic particles or short fibers, higher stiffness, higher strength and lower ductility for the composite can be observed. The overall elastic-plastic nature of the composite, however, bears a qualitative resemblance to that of monolithic metals. This implies that traditional macro hardness may be useful in characterizing the mechanical properties of the composite, as in the case of many engineering alloys.

The reinforcement fraction and matrix strength appear to play an important role in influencing the behavior of the composite under hardness and tensile loading conditions. As reinforcement fraction increases, so does hardness. The different loading modes of the tensile test compared to the hardness test, along with the local increase in particle concentration directly underneath the indenter during indentation, result in a significant overestimation of the tensile strength by the hardness test, especially when the matrix strength is relatively low.

The correlation between hardness and tensile strength is not as straightforward as it in the case of metals. This is particularly true for composites with large reinforcement particles that are more prone to fracture during deformation processing. While fractured particles in tension do not bear any applied load, they do not significantly alter the resistance to local compressive loading in a macrohardness test. As a result, the hardness test can significantly overestimate the overall tensile strength of the composite.

Optical micrograph of the 2080:SiC:20p-T8 composite near theindentation site.

The above figure shows the microstructure of the depressed region caused by indentation of a hardness-tested 2080: SiC:20p-T8 composite. Note that the indented region is large compared to the size and spacing between the particles. Thus, the indentation measures the overall response of the material and is relatively insensitive to localized effects. Very few cracked particles were observed directly below or away from the indentation.

When the strength of the matrix is relatively low a wide range of hardness exists even when all materials were heat-treated to the same tensile strength. This can be explained by the fact that as the matrix strength decreases, load transfer to the high modulus particles is diminished. A lower-strength matrix also renders a more prominent effect of reinforcement under indentation. If the matrix strength is high, the effect is diminished.

Another explanation to rationalize the experimental findings (discrepancy between macro hardness and tensile strength) is related to the localized nature of indentation loading.

Higher particle densitybelow the indenter.

During a tensile or compressive test, the material within the gauge section undergoes nominally uniform deformation. In a hardness test, however, severe plastic flow is concentrated in the localized region directly below the indentation, outside of which the material still behaves elastically. Directly below the indentation the density of particles is forced to increase, compared to regions away from the depression.

Following tests are generally used to determine the hardness values.

1) Vickers bulk hardness test

2) Brinell hardness test

3) Rockwell hardness testHardness of metal matrix composites BINNET B SAM(143501)Page 1