Hollow corundum microspheres in abrasives

An important parameter of abrasive tools is the existence of pores (hollow spaces between neighbouring abrasive particles)
of a specified size. The specified size of pores develops the structure of abrasive tools and their cutting ability.
Open porosity in abrasive tools can be obtained in several ways: in abrasives all open pores are created by mutual
arrangement of abrasive grain particles relative to one another and by burning additives, i.e. crashed fruit pits (apricot pits,
walnuts, etc.), tar camphor, and similar materials. Closed pores are formed by non-burning additives, i.e. glass and
aluminosilicate microspheres.
The use of hollow corundum microspheres (HCM) of a specified size in abrasive tools enables us to deal with several tasks at a time:

Let us consider the basic effects of using hollow corundum microspheres in more detail.

Mechanical strength ans sphericality of HCM

The spherical shape and relatively high strength of corundum microspheres (40-120 MPa) ensures wonderful mobilityof raw
ceramic mixture in the process of jar-moulding; and in the process of moulding in a hydraulic press, the pressing force
reduces 1.5 or 2 times. The pressing force is reduced due to the reduced internal friction of ceramic mixture components when
redistributing and binding in the mould. The reduced energy intensity of this operation enables us to save a pretty large amount
of electricity at the manufacturing stage of abrasive tools.

Volumetric deformation and coefficient of thermal expansion of HCM

In the process of annealing abrasive tools with HCM, the volumetric deformation reduces twice or thrice. This happens due to
the low coefficient of thermal expansion of corundum microspheres when heated or cooled. The reduced volumetric deformation
of the end grinding product enables us to reduce the amount of raw ceramic mixture which is taken a little more than enough for
further machining of annealed abrasive tools to the size provided in the drawing. In the case of extensive manufacturing of
abrasive tools, the annual amount of saved raw mixture will be tens of tons.

The effect of self-sharpening of a HCMThe effect of self-sharpening of a HCM

A hollow corundum microsphere has an internal closed cavity of a certain size. The size of this cavity depends on the size of
the microsphere and thickness of its walls. When added to the ceramic mixture, HCM finds itself among sharp faceted abrasive
particles and creates a pore around itself. A particle of an abrasive grain (molten electrocorundum, silicone carbide) is much
stronger than HCM. Therefore, metal is mainly cut by the abrasive grain, while the opened microsphere "mildly" trims small burrs
and disintegrates forming new sharp edges. Thus, the opened microsphere is self-sharpened. As a result of this effect, the
surface of the processed part:

Size and chemical stability of HCM

Whereas HCM are made from aluminium oxide, they are therefore chemically homogenous with main abrasive grains (normally, it is molten aluminium oxide) and are, unlike other
fillers like glass or aluminosilicate microspheres which dissolve in a chemically active bonding, stable to chemically aggressive additives (bonding) in abrasive tools, which makes
abrasive tools harder.
The extended surface of hollow corundum microspheres, as well as the possibility of mixing of microspheres of various diameters (5 to 125 microns) with standard grains of abrasive
materials (electrocorundum, silicon carbide (SiC), or cubic boron nitride (CBN)) enables us to achieve unique properties to produce highly-porous abrasive tools. The combination
of F180 grain of e.g. chromium electrocorundum and hollow corundum microspheres in volumetric proportion 70/30%, or chromium electrocorundum grain F80 and hollow corundum
microspheres in volumetric proportion 85/15%, enables us to achieve the prescribed porosity and excellent mechanical properties in highly porous grinding wheels.
Pink alumina grain and HCM, big and small size Grain and HCM_normal_normal
Distribution of chromium electrocorundum particles and hollow corundum microspheres: on the left:
electrocorundum F80 (85%) of microsphere F180 (15%); on the right, electrocorundum F180 (70%) of microsphere F180 (30%).

High annealing temperature and environmental friendliness of HCM in use

Fused alumina and HCM in grinding wheel

In the process of high-temperature processing – annealing and baking – of abrasive tools
with HCM additive, no harmful substances evolve in the environment, and the microsphere does
not disintegrate due to its spherical shape and high softening point (1600-1800°C).
Kit-Stroy SPb

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