Thermal Shock Resistance of Ceramic Cutting Tools

Abstract

Most ceramic materials are unsuited to conditions involving non-uniform temperature distributions because of their inherent brittleness. Ceramic cutting tools operate in conditions of rapid temperature variation and consequently thermal cracking is frequently observed. Under certain cutting conditions, these cracks are the major cause of tool failure. Thus, establishing the conditions under which thermal cracking occurs and identifying material parameters governing thermal shock resistance should lead to clearer ideas about how to develop thermal shock resistant ceramic cutting tool materials. Two tool materials, CC 620 (a pure ceramic containing Al2O3 and ZrO2) and CC650 (a mixed ceramic containing Al2O3 and Ti(N, C)), were used for evaluating thermal shock resistance, both by quenching from elevated temperature into cold water and by steel machining tests (SS 2541). It is shown that material parameters quantifying thermal shock resistance developed from simple quenching experiments, although frequently used, are not directly applicable when assessing thermal shock resistance in metal cutting conditions. This is mainly because of the presence of a region of superficial plastic deformation, ∼1μm deep, at the worn ceramic tool surface and of creep deformation during cutting under conditions severe enough to cause thermal cracking upon cooling. In addition, maximum cutting edge temperatures for a given set of cutting data differ with thermal conductivity of the tool material.

Failure analysis of ceramic tool in intermittent turning of hardened steel

“…Thermal cracks perpendicular to the cutting edge appeared and the cracks extended nearer to the cutting edge of ceramic tools at relatively higher speeds in the previous research. 20 Although a set of cracks perpendicular to the cutting edge could not be seen in our experiment, thermal damage still existed. The mechanical shock prompted the tool edge to fracture due to performance degradation of tool surface material at relatively higher cutting temperature.…”

“…24,25 While with the increasing cutting speed above 150 m/min, the cutting temperature was very higher, which lead to high adhesions on the tool rake surface; in turn, the excess adhesions introduced the higher cutting temperature, which decreased the tool strength; on the other hand, the high thermal stresses in the cutting edge also reduced the tool life. 20,26 On the whole, the maximum flank wear of the cutting tool increased gradually in the initial stage of cutting. With the increase in the cutting time, the failure mode transformed from wear and microchipping to chipping in a large-scale or catastrophic fracture in the final stage.…”

“…22 Some authors explained that these cracks were promoted by the repeated expansion and contraction of the surface layers of cutting tools when they were cyclically heated in the cutting process and cooled in idle periods of intermittent cutting. 19,20 When relatively high cutting speeds are adopted, retention of the mechanical properties to high cutting temperature and resistance to thermal impact were priorities for the increase in the tools' life. 23 In summary, it should be noted that the prevailing percentage of previous research focused on the wear behavior of Al 2 O 3 -based cutting tools in a continuous cutting process, as well as some researches about failure mechanisms of ceramic tools in an intermittent cutting.…”

“…18 Furthermore, a lot of researches were made on thermal fatigue of ceramic and cemented carbide cutting tools in interrupted cutting operations. 19 –21 Severe temperature gradients and thermal stress gradients in the interior of the cutting tool might lead to thermal cracks in the cutting edge under cyclic thermal shock in an intermittent cutting process. 7 Under high cutting speeds, thermal damage was the major cause of tool failure.…”

Multilayer composite nanoscale coatings as a method to increase reliability and tool life of cutting tools made of mixed ceramic Al2O3-TiC

“…Using reliability theory it is postulated that: If the coefficient of tool failure variation does not exceed 0.5, the variation range of tool failure values, resulting from the experiment, will obey the law of Weibull distribution [19,20]. In the specified type of distribution, it can be noted that the wear process for cutting tool, the wear moves from the stage of a sudden failure at any moment of time to the stage of forecasted (predictable) failure.…”