Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
Fabrication Methods for the Manufacture of
Sapphire Microparts
David M. Allen, Roxana Redondo and Maximilien Dany
Precision Engineering Centre, Cranfield University, Bedford MK43 0AL, UK
Abstract- There is an increasing demand for microparts
to be fabricated from an extremely hard-wearing,
durable material such as sapphire but machining it to
demanding specifications and tolerances poses
considerable challenges. This paper describes
experimental results obtained from laser machining and
diamond machining of sapphire and concludes that, for
optimum machining, a combination of these two
techniques is required.
I. INTRODUCTION
The properties of sapphire, a form of alumina
(Al 2 O 3 ), are attracting the attention of manufacturers
for many different reasons. The material is the
second hardest material known (9 on the Mohs
hardness scale); second only to diamond (10 Mohs)
and as such is “scratch-proof” and extremely durable
with an exceptionally long service-life if used as a
component. It is chemically inert and therefore
resistant to attack by acidic and alkaline etchants. It
also has exceptional optical properties being
transparent in the infra-red, visible and ultra-violet
regions of the electromagnetic spectrum from 170nm
to 5500nm (see Fig. 1).
It is worth noticing that various properties of sapphire
such as hardness, dielectric constant, and thermal
coefficient vary depending on the crystal orientation.
For example, as shown in Table I, if the crystal’s
orientation is perpendicular to the c-axis ( ┴ c-axis),
the material is harder and a better insulator than if it
is oriented parallel to the c-axis (║c-axis).
However, the hardness of sapphire makes it a
“difficult-to-machine” material. There is very little
open literature, or even patents, on the methods of
manufacturing sapphire parts from single crystal
boules (ingots), although commercial companies
obviously process single crystal sapphire by methods
that are kept in-house as closely-guarded secrets. A
paper published in 2010 shows a 500µm thick single
crystal disk of sapphire that has been cut out by fine
abrasive water jet machining [1]. However, it is
acknowledged that the cut edge definition is not
ideal, resulting in chipping as (quote) “small flakes
were cut out from the edge”.
Sapphire disks are frequently used in optical
applications such as lenses or windows. To obtain the
best optical quality, the most common crystalgrowing
method is the Kyropolis method. The disks
must then be cut and polished, which are two
mechanical machining processes. A conventional
approach to cut sapphire blocks involves the use of
diamond abrasives bonded onto saw blades.
Fig. 1. Transmission spectrum of a 2 mm thick sapphire window
[2]
However, one of the most recent tools developed to
cut ceramic ingots or blocks, involves the use of
wires coated with diamond abrasives. This technique
is commonly applied in the form of multi-wire
slicing, to cut very thin wafers of sapphire for the
light emitting diodes (LED) industry. It is a fast
process, though the loose grains may reduce the rate
of material removal [3].
The diamond abrasives used in wire cutting are
bonded to a steel wire by nickel electroplating (Fig.
2). According to the nomenclature of abrasives, the
grit type is described using a letter and number
system. The letter refers to the type of material,
whilst the number refers to the average grain size of
the abrasives, expressed as average diameter, or
grains per unit area or volume. The grit type
commonly used to wire-cut sapphire ranges between
D 07 and D 91 grits, where D stands for diamond [3].
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