Optimal composition of ceramic binder for highly porous abrasive tools

Abstract

1043 The operational characteristics of abrasive tools largely depend on the structure and properties of the materials from which they are made. Machining experience shows that highly porous abrasive tools are the most effective. In comparison with standard tools, their productivity is higher, while the depth of the defect layer in the ground surface is smaller by a factor of 5‐6. The pore volume may be increased by reducing the proportion of grains and/or binder. Increasing the porosity by reducing the grain content is the most promising, since it significantly (by a factor of 3‐5) exceeds the binder content, as a rule. However, simply decreasing the content of grains will now work, for the following reasons. With uniform grain distribution in the abrasive mass, the intergrain contact will include a binder layer. On baking, the binder melts, and the grains begin to move toward one another under the action of their own weight and the surface tension of the binder at contact, to form a more compact rigid framework. The abrasive tool shrinks: its density is increased, and the pore volume declines. Existing tool-manufacturing technology calls for the use of various fillers to replace the withdrawn grains. The most promising fillers—hollow spheres of corundum, glass, ceramic, and other materials—were listed in [1]. In that case, the hollow sphere acts both as a pore and as a structural element of the tool. The benefits of corundum spheres are strength and adhesion to the ceramic binder. However, the tools obtained using this filler have a closed pore system, since the melting point of the spheres is above the conventional baking temperatures. In tool operation, the chip and the coolant fluid are packed in pores at the working surface, which consequently becomes clogged, with increase in the stress and thermal stress of the process as a whole. Glass spheres whose melting point is lower than the baking temperature dissolve in the binder, and a single pore system is formed in the resulting tool. However, glass spheres, although fusible, reduce the refractory properties of the binder and hence increase the risk of extreme tool shrinkage. This may be avoided by increasing the proportion of relatively infusible components in the binder or reducing the baking temperature. Both methods reduce the binder reactivity and hence the tool strength, on account of the chemical inhomogeneity of the binder bridges, and also reduce the proportion of corundum that passes to the binder from the grain. The optimal approach is to use spheres made of the binder material. The benefits are obvious: the chemical composition of the material is unchanged; the melting point of the sphere corresponds to the tool-baking temperature; and the binder reactivity remains unchanged. We have selected the filler that best meets these requirements: the spheres are close to the ceramic binder in composition (no more than 5‐10% difference in the Al 2 O 3 and SiO 2 content), while the refractory properties are 10% higher. In terms of their physicomechanical properties, the spheres are intermediate between glass and corundum spheres (Table 1). In developing the composition of the tool material, we have taken into account that the spheres will contribute to the binder content. For example, in wheels of structure 12 with hardness VM0, 12% of the grains must be replaced by the spheres, with the addition of 5.5% binder. Since the porosity of the spheres is about 80%, they account for around 2.4% of the binder after melting. In other words, we reduce the amount of binder added from 5.5% to 3.1%.