Naturally Occurring vs. Manufactured Abrasives
Common Types of Manufactured Abrasives
Throughout history, humans have used as abrasives everything
from beach sand to walnut shells to paper bags. This section covers some of
the most widely used types of manufactured abrasives. By manufactured abrasives,
we mean primarily those materials that are created through a manufacturing process
as opposed to being mined from the earth. Some naturally occurring materials
are also produced artificially (such as aluminum oxide and diamond), so it is
impossible to draw a hard and fast line between these two categories. Additionally,
naturally occurring abrasives are often used in the same applications as manufactured
abrasives. Thus, some naturally-occurring abrasives are mentioned briefly and
listed in the glossary.
Widely-used naturally occurring abrasives include garnet,
cerium oxide, flint, emery, corundum (aluminum oxide), and diamond. These materials
may have varying characteristics and chemical compositions depending on the
specific geological source. Manufactured versions of these materials are usually
more consistent in chemical composition and other characteristics.
Abrasives can be distinguished in a variety of ways- their
hardness, color, chemical composition, crystal shape, and friability, to name
but a few. Since the chemical composition- that is, the type of material- determines
many of the other characteristics, we use that as the primary means of distinguishing
one type of abrasive from another.
1. Chemical Composition
a. Alumina Based ( Aluminum Oxide, Al2O3) (9.0 on Mohs hardness scale)
White Fused Aluminum Oxide - High chemical purity (>99%
Al2O3); generally used for applications where high purity is important (medical,
dental or other industrial uses); generally softer or more friable than other
abrasives; used in grinding applications where a more friable product is desired.
Granules typically are blocky shaped (see below, Shape), with an aspect ratio
of approximately 1:1 to 3:1.
White Calcined Aluminum Oxide - High purity aluminum oxide (>99.5%
Al2O3), manufactured by growing individual crystals from seed crystals at high
temperature. Crystals are hexagonal platelet shaped with an aspect ratio of
5:1. Used in a variety of lapping, buffing and polishing applications, incorporated
into bars and pads, and used in ceramics.
Aluminum Oxide with Chrome - White Aluminum Oxide fused with Cr2O3 to
enhance grinding; a pink abrasive used in grinding applications requiring slightly
more toughness than White Fused Aluminum Oxide.
Brown Fused Aluminum Oxide - Abrasive with a content of 2-4% TiO2 to further enhance toughness; the "workhorse" of the industry; used in a wide variety of uses including Bonded, Coated, Refractory and Industrial markets; probably the most widely used abrasive.
Low Titania Brown Fused Aluminum Oxide - Abrasive with
a content of 1-2% TiO2 to enhance toughness of the grain; generally used in
Bonded or Coated applications requiring an abrasive slightly tougher than White
Zirconia-Alumina - The toughest of the alumina based
products; used in Bonded, Coated and Sandblasting applications requiring an
extra tough abrasive.
Hydrated Alumina - Aluminum oxide with water chemically
bonded to the alumina. Crystals are small (typically 1 mm or less in size) and
very soft; typically used for fine polishing applications and as a raw material
Ceramic Aluminum Oxide - A high-purity fine-grained
alumina obtained from sintering dispersed colloidal alumina. This provides a
tough product primarily used for precision grinding of steels and hard alloys.
b. Silicon Carbide (SiC) (9.3 on Mohs hardness scale) - Silicon Carbide
is a man-made abrasive material formed by a series of vapor-phase reactions
of carbon and silicon dioxide at high temperature in an Acheson furnace. Alpha
phase silicon carbide, in the form of hexagonally shaped platelets, is the most
common form observed in the abrasives and refractories industries. The character
of alpha silicon carbide is dependent on a number of factors such as purity
of raw materials used in the Acheson furnace, and the reaction time and temperature.
(see below, p. 9, Furnacing Silicon Carbide.)
Green Silicon Carbide - The highest purity Silicon
Carbide manufactured; typically containing 99% or greater SiC; generally used
in grinding wheels for particular grinding properties or in industrial applications
requiring a high purity SiC (coatings); generally more friable than black SiC.
Black Silicon Carbide - Lower purity (95-98%), tougher
Silicon Carbide; generally used in Bonded, Coated, Refractory and Industrial
markets for a wide variety of applications.
c. Other Manufactured Abrasives -
Including Boron Carbide (B4C), Cubic Boron Nitride (CBN), and Diamond; used
for special applications requiring very hard materials for grinding and polishing.
CBN and Diamond are also known as "Superabrasives."
Particle shape affects the performance of the abrasive in
a variety of ways, such as the rate of stock removal and level of subsurface
damage. Some specialty applications require unusual shapes. For fused alumina
and many other abrasives (such as silicon carbide), the aspect ratio (length
to width) is a primary descriptor of shape. For others, the aspect ratio is
irrelevant or misleading. Thus, fused aluminum oxide tends to have one long
dimension and two smaller, roughly equal dimensions; i.e., the thickness is
roughly equal to the width. Calcined alumina tends to have two long dimensions
and one much smaller one; i.e. the length and width are roughly equal, but the
thickness is about one-fifth the length. (For calcined alumina, this ratio,
length to thickness, is often referred to as the aspect ratio). For more exotic
shapes, aspect ratio may vary with the size of the particle and thus can only
be given as a range.
a. Blocky Shape (High Bulk density) - The abrasive grain is "rounded" by abrading equipment to remove very sharp, weak grain. Depending on application, the grain shape can vary from "mulled" to blocky with sharp edges. The blocky shape enhances toughness and bulk density of the grain. Applications include tougher grinding or sanding applications, longer life for sandblasting and increased density for refractory or ceramic applications. Aspect ratio is approximately 1:1. (See Figure 1)
b. Blocky Shape (Medium Bulk density) - The abrasive grain is shaped to yield particles which are sharp but do not contain weak, platey or needlelike particles. Uses include general grinding, sanding, sandblasting and refractory applications. Aspect ratio is approximatley up to 1.5:1. (See Figure 2.)
c. Sharp Shape (Low Bulk density) - Abrasive grain that has been specially crushed to yield very sharp grain. This is generally required by the Coated abrasives industry and some grinding wheel applications to produce an aggressive, fast cutting product. Aspect ratio ranges from 1.5:1->3:1. (See Figure 3.)
d. Platelet Shaped - Generally found in calcined aluminas. (See Figure 4.)
e. Extreme Shapes - A variety of highly unusual shapes are possible through different manufacturing processes for specific applications. Figure 5 shows one of these, extruded abrasives.
3. Heat Treatments
Heat treatments are often applied to Brown Aluminum Oxides.
These may be either high heat or low heat; both enhance toughness, high heat
more so than low heat. High heat treatment imparts a bluish color to the grain.
Low heat is used to improve cleanliness and capillarity. Uses of heat-treated
abrasives include grinding wheels, coated abrasives, and specialized industrial
4. Surface Treatments
A. Silane Treatments - Generally applied to enhance bonding of the abrasive grains to organic resins or epoxies. Applications include grain used for grinding wheels and industrial uses.
B. Projectivity Enhancing Treatments - A variety of coatings can be applied to abrasive grain for the Coated abrasives industry. These coatings enhance the electrostatic properties of the grain to improve projection during the making of sandpaper. (Grain on a conveyor belt passes through an electrostatic field. It "jumps up" or projects onto a sandpaper belt containing a resin. The electrostatic field polarizes each grain so that elongated ends stick up and improve cutting ability.)
C. Iron Oxide Coatings - Applied to improve bonding to resins for grinding wheel and sandpaper applications.
D. Others - Other specialized treatments may be applied to improve "wetability", flowability, suspension in water or oil, or to improve other characteristics.
Please Note: This is a very general report which does not include all applications.
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