One measure of work of fracture is related to the critical stress intensity factor KIc also denoted to as fracture toughness. The critical stress intensity factor describes a particular stress intensity at a tip of a crack which is required to make a crack propagate. The ultimate fractures strength σc is thus a very important mechanical parameter which describes the critical tensile or bending stress which is required to initiate the crack. For brittle materials like glasses and most of the non-reinforced ceramics, fracture strength and fracture toughness are linked by the so-called Griffith-equation:
equation(1.1)
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This equation Y means a geometry factor which describes the shape and the position of a microstructural inhomogeneity, e.g., a crack or a pore, and a the maximum elongation of this particular inhomogeneity, e.g., the crack length or the pore diameter. As it will be shown later, this fracture mechanical equation does not only correlate the basic mechanical parameters but also shows the direction of a further improvement of properties and thus inherently contains the basic understanding of fracture statistics. Due to the fact that geometrical parameters are involved in the description of fracture initiation, the materials strength cannot be described as a single constant being valid for a certain product but is, instead, a function of the probability of the spatial and size distribution of supercritical microstructural defects.
Since the Young’s modulus is given by the stress-strain relationship, another mechanical property is still missing which is very important for cutting tools and grinding materials: the hardness. The hardness is defined as the resistance of a material against the penetration by a testing device. From the viewpoint of physics, the hardness is related to the lattice properties of crystals and can be therefore derived solely from interatomic forces. In practice however, the hardness is a combined property which involves microstructural characteristics such as porosity, grain size, grain boundaries, dislocation movement, cleavage fracture, and other geometry- and temperature dependent bulk properties.
In the case of cutting tools and grinding grits, these mechanical properties cannot be discussed at room temperature alone. Due to the very small surface area being in contact with the material machined, very high temperatures may develop at the interface between the work material and the cutting material. Accordingly, the temperature dependence of strength, fracture toughness, hardness, and Young’s modulus have to be discussed as well as other thermo-physical properties such as thermal expansion and thermal shock behavior. Additionally, at the contact between ceramics and metals, chemical reaction may be initiated under the high contact pressure and the high temperatures.
Although many theories in fracture mechanics have been developed to describe the service behavior of brittle materials, the prediction of the wear properties from the static mechanical properties is not easy since the interaction between wear couples is manifold. Usually, tool and work material is not simply in contact with each other but a third medium such as cooling agents, lubricants, abrasive additives, chips of the work piece, hard material, and certain atmospheres may form an environment which contributes strongly to the particular wear mechanisms. Taking this third medium into account, one can distinguished four basic wearing effects:
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surface fatigue,
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abrasion,
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adhesion, and
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tribochemical reactions.
Figure 1.4 summarizes schematically the basic interactions, mechanisms, and effects that can be observed in wear couples. Material removal by formation of adhesive bridges between tool and work material, crack formation by delamination, and opening of grain boundaries are visualized for the case of sliding wear inFigure 1.5. From both figures, it becomes evident that chemical interactions contribute to the wear behavior in addition to the mechanical interaction. In the following paragraphs, the particular wear mechanisms are described in detail.
Figure 1.4.
Principal mechanisms and effects of wear
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Figure 1.5.
Surface effects of sliding wear (after Zum Gahr)
(a-b)
Material; removal by adhesive bridges and their chip-off
(c)
Crack formation by delamination (grain boundary sliding and cleaving)
(d)
Crack formation by grain boundary opening
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Abrasion
The term “abrasion” comprises all groove-forming mechanisms on the surface of a material by micro chipping and micro ploughing. This mechanism is a consequence of a high ratio of the hardness of the tool material and the work material. An estimation of this hardness ratio must, however, consider the dramatic decrease with temperature in ceramic materials while metals may reveal an increasing hardness by work hardening effects. Additionally, the dynamic hardness of a metal may be considered higher than the hardness measured by indentation techniques due to incorporated carbide particles. Generally, the ratio of tool material hardness to work material hardness should not be less than 1.5 to 1.7.
Although grooving is an evidence for plastic deformation, the pull-out of chips and particles from the microstructure of both cutting tool and work material must also be considered. Accordingly, the local fracture toughness must be taken into account. Model wear tests performed on a large variety of material couplings indicate that a correlation of the wear amount, or wear rate, to both hardness and fracture toughness is generally possible. Several empirical formulae have been developed from pin-on-disc tests relating the wear resistance to fracture toughness times hardness of several exponents. Table 1.2 shows some more important empirical formulae that have been proved to fit well with experimental results. The Evans-Wilshaw-Equation is accepted most for ceramic-ceramic pairs. It is evident from this expression that the high hardness must always be combined with a high fracture toughness to yield suitable wear properties. Surprisingly, the infl-uence of fracture toughness is more important than the hardness as can be concluded from the particular exponents.
Table 1.2.
Empirical Relations Between Wear Resistance Factor R and Mechanical Properties
R = W-1∼K2ICH3/2 Hombogen
R = W-1∼K3/4ICH1/2 Evans & Wilshaw
R = W-1∼K2ICH-1/2 Zum Gahr
R = W-1∼K4/3H-1/9 Ruff & Wiederhorn
R = inverse volume loss W.