Drilling into Hard Non-Conductive Materials by

Localized Microwave Radiation

E. Jerby and V. Dikhtyar

Faculty of Engineering, and Ramot Ltd. Tel Aviv University, Ramat Aviv 69978
Israel

Abstract

The paper describes a novel method of drilling into hard non-conductive materials
by localized microwave energy (US patent 6,114,676). The Microwave Drill
implementation may utilize a conventional 2.45 GHz magnetron, to form a
portable and relatively simple drilling tool. The drilling head consists of a coaxial
guide and a near-field concentrator. The latter focuses the microwave radiation
into a small volume under the drilled material surface. The concentrator itself
penetrates into the hot spot created in a fast thermal runaway process. The
microwave drill has been tested on concrete, silicon, ceramics (in both slab and
coating forms), rocks, glass, plastic, and wood. The paper describes the method
and its experimental implementations, and presents a theoretical model for the
microwave drill operation. The applicability of the method for industrial processes
is discussed.

Introduction

Drilling holes is a fundamental operation in almost any industrial or construction

work. Advanced drilling technologies are being developed for hard non-metallic
materials (i.e. ceramics, concrete, marble, silicate, etc.) [1]. Mechanical drills
satisfy most of the needs, but their operation causes loud noise, vibrations, and
dust effusion, and is not always effective. Hence, other drilling technologies are
utilizing ablation or thermal effects to produce holes. These include mostly lasers
[2, 3], but also jets, flames, plasmas, and electro-erosion tools. Other drilling
methods use ultrasonic devices [4], water jets, and hydraulic presses.
Microwaves are used for a variety of industrial, scientific, and medical (ISM)
applications [5], but not for drills. Their industrial applications include heating and
drying, as well as advanced material processing such as ceramic sintering [6].
However, Microwaves have been proposed also for destructive applications, such
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as crushing of stones, mining, and concrete demolishing [5, 7]. These apparatus
use 2.45 GHz magnetrons (~12 cm wavelength) to generate volumetric heating in
the material to be crashed. The long wavelength has inhibited more delicate
remote drilling operations by microwaves. This paper introduces a novel method
for drilling into hard non-conductive materials by localized microwave energy [8].

The Microwave-Drill Concept

A key principle of the microwave-drilling concept is the concentration of

microwave energy into a small spot, much smaller than the microwave wavelength
itself. This is done by a near-field microwave concentrator, which is brought to
contact with the material to be drilled, as shown in Fig. 1.

Fig. 1. A simplified principle scheme of the microwave drill.

The microwave energy localized underneath the material surface generates a

small hot spot [9] in which the material becomes soften or even molten. The
concentrator pin itself is then inserted into the molten hot spot and shapes its
boundaries. The hole can be shaped other than circular. Finally, the concentrator is
pulled out from the drilled hole, and the material cools down in its new shape. The
process does not require fast rotating parts, and it makes no dust and no noise.
The microwave drill is effective for drilling and cutting in a variety of hard
non-conductive dielectric materials, but not in metals. The latter reflect the
radiation and therefor are almost not affected by the microwave drill. Hence, the
microwave drill enables a distinction between different materials, and in particular
between dielectrics and metals.
Specifically, the microwave drill can be implemented to make holes and
grooves in dielectric coatings on metallic substrates (thermal barrier coating
(TBC) for instance). Furthermore, it can expose existing holes in the metallic
substrate coated by the ceramic, with no damage to the underlying metallic
substrate.
 Drilling into Hard Non-Conductive Materials by Localized Microwave Radiation 689

The microwave drill can be implemented in relatively simple instruments,

consuming moderate electrical powers. However, safety and RF interference
considerations may limit its free public usage. Hence, the microwave drill concept
is proposed first for embedded tooling in industrial manufacturing processes.

Microwave Drill Apparatus

The experimental laboratory setup for the microwave drill consists of standard
components, including switched power supply for magnetron (0 – 2 kW
adjustable), a 2.45 GHz magnetron, an isolator, a reflectometer with incident and
reflected power indicators, and an E-H tuner. The laboratory setup includes also a
specific transition from a WR340 waveguide to the coaxial microwave drill, and a
chamber in which the microwave-drill is installed.
The microwave-drill head used in this setup is illustrated schematically in
Fig. 1. This is basically an open-end coaxial waveguide with a movable center
electrode (which sustains high temperatures). In this setup the drilling process is
controlled and operated manually (automatic impedance tuner and remote-
controlled actuators are being installed in an advanced laboratory setup).

Fig. 2. The microwave-drill tool version.

Another, more practical version of the microwave drill is shown in Fig. 2. The
telescopic coaxial concentrator is fed directly by the 600 W, 2.45 GHz-magnetron.
Two actuators provide the impedance matching. This tool is much more compact
than the laboratory setup, but is not less effective.
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Experimental Demonstrations

The creation of a hot spot (undesired in most applications) is essential for the
microwave drill operation. Fig. 3 shows a hot spot generated by the microwave
drill in a glass plate before penetration.

Fig. 3. Hot spot generated by the microwave drill in a glass plate.

The microwave drills have been tested on a variety of materials and hole sizes.
Typically, a 600 W microwave-drill can penetrate easily into a concrete slab to
form hole of ~2 mm diameter and ~2 cm depth within less than a minute. The
debris are densified to the wall, evaporated, or converted to a glossy material. A
widening of this basic hole requires a further microwave radiation to soften or to
melt the remaining volume bound in the required (larger) diameter.

Fig. 4. Microwave drilling in concrete: (a) A cut in an extensively radiated slab, and (b) a
13 mm-diameter 10 cm-depth hole made in a concrete slab by a cyclic microwave-drill
operation.

Fig. 4a shows a cut in a drilled concrete slab, which reveals the glossy material
formed around the concentrator pin in an extensive radiation. This fragile debris
can be easily removed mechanically to enlarge the drilling diameter. The hole can
be deepened in successive cycles of microwave radiation and mechanical removal
 Drilling into Hard Non-Conductive Materials by Localized Microwave Radiation 691

of the molten or soften debris. Fig. 4b shows a 13 mm diameter, 10 cm depth hole

made in four cycles of the microwave drilling in a concrete slab.
In silicon wafers, the microwave drill has performed 1 mm-diameter holes
without cracks. The accuracy of their shapes is not satisfying yet, but these
preliminary results provide a proof of principle for this process. Similar results are
obtained in glass plates, but more careful operation is needed there to prevent
cracks.
The microwave drill penetrates also into low-purity alumina and other
industrial ceramics. The microwave drill was used also to insert nails into an
alumina plate. These nails were originally the concentrator pins, left inside the
ceramic after their insertion, and remained bonded to it.
Ceramic coatings on metals (thermal barrier coating, TBC), have been
penetrated successfully by the microwave drill. The microwave radiation does not
affect the underlying metal, and the ceramic structure around the hole is not
damaged [10].
The microwave drill is found useful for other cutting and marking operations in
addition to drilling and nailing.
The operation of the microwave drill is characterized in general by two useful
features. One is a natural tendency of the microwave radiation to concentrate in a
small spot in the vicinity of the concentrator pin. The dimension of the affected
zone is much smaller than a wavelength, and it hardly exceeds few millimeters.
The other feature is the tendency of the microwave drill to reach an impedance
matching. The power acceptance is typically increased with the temperature, and
the impedance matching becomes easier as the process evolves.

Theoretical Analysis

A simulation of the microwave drill operation requires a simultaneous solution of

the wave equation and the heat equation. This should take into account the non-
uniformity evolved in the medium due to the temperature dependence of its
parameters. The microwave power density is larger near the drill concentrator, and
therefore the temperature tends to be higher in this vicinity. The rapid spatial and
temporal temperature variation affects the dielectric properties of the material, and
forms a distributed cavity around the concentrator. This non-uniform distribution
affects the microwave propagation, and increases the stored radiation energy in
this hot cavity. Consequently, a thermal runaway effect occurs rapidly in front of
the microwave drill concentrator, and a hot spot is generated there.
Numerical FDTD simulations related to the microwave drill operation are
presented in one- and two-dimensions in Refs. [11] and [12], respectively. The
latter includes a simulation of the concentrator inserted into the drilled material,
and it shows the thermal-runaway effect in front of the microwave drill
concentrator.
A simplified analytical model of the microwave drill operation assumes a
coaxial open-ended applicator with an extended inner conductor immersed into a
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lossy dielectric material. The temperature dependence of the dielectric parameters

should be available, but the heat equation is simplified to include only the
blackbody radiation, the dominant effect at high temperatures. The complex
impedance of the microwave drill (i.e. of a monopole antenna in a lossy dielectric)
is found vs. temperature assuming steady-state conditions. Unlike [12], this
simplified analytical model neglects the spatial non-uniformity evolved in the
dielectric material.
The simplified microwave-drill model utilizes analytical expressions for the
spatial radiation distribution of a monopole antenna in a lossy dielectric medium,
derived in Ref. [13]. These result in a slightly off-axis power distribution profiles
near the antenna. The drilled medium is characterized by a temperature-dependent
complex dielectric parameter. Using the dielectric parameters given in Ref. [14]
for pottery clay, Fig. 5a shows the relative absorbed power in small spherical
volumes around the antenna, vs. temperature. At high temperatures, the lossy
material near the antenna absorbs most of the microwave energy. Fig. 5b presents
the corresponding normalized admitance (Y=G+jB) of the monopole-antenna vs.
temperature. At the beginning of the drilling process the antenna responses mostly
as a reactive load, but as the temperature increases it becomes more resistive. This
semi-analytical description coincides with the experimental observations of the
improved impedance matching during the temperature increases.

Fig. 5. Analytical calculations of the temperature dependence of (a) the relative absorbed
power in spherical volumes of radius R around the antenna of h=5 mm, and (b) the real and
imaginary components of the monopole-antenna admitance (G and B, respectively) . The
material is pottery clay, and the monopole length is h=3 mm.

Discussion

The microwave drill presented in this paper has shown capabilities to create holes
in concrete, ceramics, silicon, basalt, and glass, as well as plastics and wood. As
compared to mechanical drills, the microwave-drill has a quiet and clean
 Drilling into Hard Non-Conductive Materials by Localized Microwave Radiation 693

operation. It does not contain any fast rotating part, and its operation is dust-free.
Laser drills, however, are essentially more accurate and they can produce much
smaller holes, but they must evaporate the removed material, whereas the
microwave drill only melt or even just soften it (letting the penetrating
concentrator to shape the hole). The latter is therefore much cheaper, in both
equipment and operation costs.
Concerns of safety and RF interference are real difficulties that impede the
promotion of the microwave-drilling technology. These difficulties could be
alleviated by proper screening and appropriate operating procedures.
The microwave drill can be operated not only as a stand-alone tool, but also in
combinations with other instruments, for instance mechanical machining tools.
This may lead to a new concept of microwave-assisted machining.
The microwave drill concept can be extended to other operations [8], such as
cutting, nailing, milling, and jointing. Furthermore, the microwave drill enables a
distinction between different materials, and certainly between ceramics and
metals. Specifically, the microwave drill can be implemented to make holes in
ceramic or plastic coating on metallic substrates (including in thermal-barrier
coating). And, in principle, one may conceive that the advanced microwave drill
will have a ”radar” feature, enabling to ”sense” the underlying material conditions
in self-controlled processes.

Conclusion

The basic microwave-drill is a relatively simple apparatus and it is expected to be

a low-cost tool for specific industrial applications. In view of the above mentioned
materials and experimental results, various schemes of the device can be
considered for several identified applications. These include industrial drilling and
cutting machines for electronics, ceramics, and wood industries; drills for
construction works (mainly drilling, nailing, and insertion tools for concrete), and
high-power microwave drills for geological surveys, oil and gas productions.
However, the microwave-drill concept presented in this paper is yet in a premature
stage of development, and it requires now extensive interdisciplinary - scientific,
technological, and commercial- efforts in order to become a valid and useful
technology.

References

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694 Jerby

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