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
The use of hollow corundum microspheres (HCM) of a specified size in abrasive tools enables us to deal with several tasks at a time:
- Reduce the mixing time of ceramic components in mixing machines due to the spherical form of microspheres;
- Reduce the pressing force when moulding the compositions which contain corundum microspheres;
- Hollow corundum microspheres do not burn out in the process of annealing, nor do they contaminate the environment (ascompared to tar camphor or stearin);
- Reduce blank volumetric deformation of the product after annealing twice or thrice (as compared to traditional materials);
- In the process of annealing, hollow corundum microspheres do not react with ceramic bonding and disintegrate due to their high chemical purity;
- The low conductivity of corundum microspheres reduces the risk of burns;
- Achieve the desired properties of the grinding wheel as to the degree of hardness (compared to aluminosilicate and glass microspheres);
- Reduce the time of grinding of parts 1.3 times due to the self-sharpening effect (compared to glass microspheres);
- Reduce the abrasive consumption per part 1.25 times due to the reduced smearing and therefore reduced regrinding of the abrasive tool;
- Reduce defects (burns) in the process of grinding due to better cooling and reduced smearing of the abrasive tool surface.
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 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:
- has less burrs once processed by the abrasive tools;
- is less smeared because the cavity of corundum microspheres and neighbouring pores are flushed with coolant stream and self-sharpened, which enables us to regrind the abrasive tools less frequently and increases the number of parts processed by each abrasive tool;
- as a result of better cooling of abrasive tools due to the prescribed porosity, overheating of the abrasive grains and processed material reduces, which enables us to reduce the amount of defective articles (e.g. burns on vital parts).
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.
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
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).