8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 1/26

Machining of ceramics

Advanced Material Removal Processes Page 1

Chapter 4

MACHINING OF CERAMICS

Introduction:

A ceramic is an inorganic, non-metallic solid  prepared by the action of

heat  and subsequent cooling. Ceramic materials may have a crystalline or

 partly crystalline structure, or may be amorphous (e.g., a glass). Because most

common ceramics are crystalline, the definition of ceramic is often restricted to

inorganic crystalline materials, as opposed to the noncrystalline glasses, a

distinction followed here.

The word "ceramic" comes from the Greek   word (keramikos)  "of

 pottery" or "for pottery" (keramos) "potter's clay, tile and pottery".

Types of ceramic material: 

A ceramic material is an inorganic, non-metallic, often crystalline oxide,

nitride or carbide material. Some elements, such as carbon or silicon, may be

considered ceramics.

Traditional ceramic raw materials include clay minerals such as

kaolinite,  whereas more recent materials include aluminium oxide, more

commonly known as alumina.  The modern ceramic materials, which are

classified as advanced ceramics, include silicon carbide and tungsten carbide. 

Both are valued for their abrasion resistance, and hence find use in applications

such as the wear plates of crushing equipment in mining operations.

1)  Crystalline ceramics:

Crystalline ceramic materials are not amenable to a great range of

 processing. Methods for dealing with them tend to fall into one of two

categories –  either makes the ceramic in the desired shape, by reaction in

 situ, or by "forming" powders into the desired shape, and then sintering 

to form a solid body. Ceramic forming techniques  include shaping by

hand (sometimes including a rotation process called "throwing"),   slip

casting,  tape casting  (used for making very thin ceramic capacitors,

e.g.), injection moulding, dry pressing, and other variations.. A few

methods use a hybrid between the two approaches.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 2/26

Machining of ceramics

Advanced Material Removal Processes Page 2

2)  Non-crystalline ceramics:

 Non-crystalline ceramics, being glass, tend to be formed from

melts. The glass is shaped when either fully molten, by casting, or when

in a state of toffee-like viscosity, by methods such as blowing into amould. If later heat treatments cause this glass to become partly

crystalline, the resulting material is known as a glass-ceramic, widely

used as cook-top and also as a glass composite material for nuclear waste

disposal.

Types of ceramic product:

For convenience, ceramic products are usually divided into four sectors;

these are shown below with some examples:

  Structural, including bricks, pipes, floor and roof tiles 

  Refractories, such as kiln linings, gas fire radiant, steel and glass making

crucibles

  Whitewares, including tableware, cookware, wall tiles, pottery products

and sanitary ware

  Technical, is also known as engineering, advanced, special, and in Japan,

fine ceramics. Such items include tiles used in the Space Shuttleprogram, gas burner  nozzles, ballistic protection, nuclear fuel uranium

oxide pellets, biomedical implants,  coatings of  jet engine turbine 

 blades, ceramic disk brake,  missile  nose cones, bearing

(mechanical),etc. Frequently, the raw materials do not include clays.

  Classification of technical ceramics:

Technical ceramics can also be classified into three distinct material

categories:

  Oxides: alumina, beryllia, ceria, zirconia

  Nonoxides: carbide, boride, nitride, silicide 

  Composite materials:  particulate reinforced, fibre reinforced, 

combinations of  oxides and non-oxides

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 3/26

Machining of ceramics

Advanced Material Removal Processes Page 3

Properties of Ceramics:

Ceramics have some attractive properties compared to metals and

 polymers, which make them useful for specific applications. Their physical

 properties have been utilized for many applications. In other applications their

mechanical properties are important.

The most important advantageous features of ceramic materials are:

  Low electrical conductivity,

  Low thermal conductivity,

  Low density,

  High strength at high temperatures,

  Wear resistance,

  Corrosion resistance,

  Specific physical properties (optical, electrical, magnetic).

The main disadvantages of ceramics are:

  Low tensile strength at room temperature for some materials,

  Brittleness,

  Large scatter of strengths,  Subcritical crack extension.

1)  Physical properties: 

Physical properties means the behaviour of materials in response to

 physical forces other than mechanical, such as; Volumetric, thermal,

electric and electrochemical properties.

  Most Ceramics are lighter than metals but heavier than polymers.  Most ceramics have a higher melting point than most metals as it is

that some ceramics such as China can with stand high temperatures

to about 1200 degrees centigrade.

  Ceramics also has lower Electrical and Thermal Conductivity than

most metals but the range of value is greater in ceramics permitting

some ceramics to be used as insulators, for example Porcelain

insulators and others as conductors like Lithium-ion conducting

glass-ceramics and oxide ceramics.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 4/26

Machining of ceramics

Advanced Material Removal Processes Page 4

  Thermal expansion is another physical property of ceramics,

Ceramic thermal expansion coefficients are less than those of

metals but effects are more damaging in ceramics bringing about

cracks and other failures (Thermal shock and thermal cracking) as

for ceramic materials with relatively high thermal expansion and

low thermal conductivity however there is glass ceramics that has

low thermal expansion thus resisting thermal shock and thermal

cracking, for example Pyrex glass ceramics. 

2)  Mechanical properties:

Mechanical properties are important in structural and buildingmaterials as well as textile fabrics. They include the properties used to

describe the strength of materials such as: elasticity / plasticity, tensile

strength, compressive strength, shear strength, fracture toughness &

ductility (low in brittle materials), and indentation hardness. 

Ceramic materials are usually ionic or covalent  bonded materials,

and can be crystalline or  amorphous. A material held together by either

type of bond will tend to fracture  before any plastic deformation takes place, which results in poor toughness  in these materials. Additionally,

 because these materials tend to be porous, the pores  and other

microscopic imperfections act as stress concentrators,  decreasing the

toughness further, and reducing the tensile strength.  These combine to

give catastrophic failures, as opposed to the normally much more gentle

failure modes of metals.

Brittleness  means that failure occurs without prior measurable

 plastic deformation. This is due to the strong atomic bonding of ceramics,which lead to high stresses for the motion of dislocations. Thus, failure

can start from small flaws before plastic deformation is possible. This fact

can also be expressed in low resistance against crack extension, which is

characterised by the fracture toughness. The absence of local plastic

deformation leads to failure at locations of high local stresses, e.g. at

notches, at contacts between different material or during thermal shock.

In metals these strain controlled local stresses lead to small plastic strains.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 5/26

Machining of ceramics

Advanced Material Removal Processes Page 5

The large scatter of strength  is caused by the statistical

distribution of the flaw size and the flaw location. This requires a

statistical description of the strength and a relation between failure

 probability and the stress distribution in a component.

Subcritical crack extension  can cause failure under constant or

cyclic loading during the operation of a component and will lead,

consequently, to a limited lifetime.

To overcome the brittle behaviour, ceramic material development

has introduced the class of ceramic matrix composite  materials, in

which ceramic fibres are embedded and with specific coatings are

forming fibre bridges across any crack. This mechanism substantially

increases the fracture toughness of such ceramics. The ceramic disc

 brakes are, for example using a ceramic matrix composite material

manufactured with a specific process.

3)  Electrical properties:

  Semiconductors:-

Some ceramics are semiconductors.  Most of these are

transition metal oxides  that are II-VI semiconductors, such as zinc

oxide. 

While there are prospects of mass-producing blue LEDs  from

zinc oxide, ceramicists are most interested in the electrical properties

that show grain boundary effects.

One of the most widely used of these is the varistor. These are

devices that exhibit the property that resistance drops sharply at a

certain threshold voltage.  This makes them ideal for surge-

protection applications; as there is control over the threshold voltage

and energy tolerance, they find use in all sorts of applications. The

 best demonstration of their ability can be found in electrical

substations,  where they are employed to protect the infrastructure

from lightning  strikes. They have rapid response, are low

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 6/26

Machining of ceramics

Advanced Material Removal Processes Page 6

maintenance, and do not appreciably degrade from use, making them

virtually ideal devices for this application.

Semiconducting ceramics are also employed as gas sensors. 

When various gases are passed over a polycrystalline ceramic, its

electrical resistance changes. With tuning to the possible gas mixtures,

very inexpensive devices can be produced.

  Superconductivity:-

Under some conditions, such as extremely low temperature,

some ceramics exhibit high temperature superconductivity.  The

exact reason for this is not known, but there are two major families ofsuperconducting ceramics.

  Ferro-electricity and supersets -

Piezoelectricity,  a link between electrical and mechanical

response, is exhibited by a large number of ceramic materials,

including the quartz used to measure time  in watches and other

electronics. Such devices use both properties of piezoelectric, using

electricity to produce a mechanical motion (powering the device)

and then using this mechanical motion to produce electricity

(generating a signal). The unit of time measured is the natural

interval required for electricity to be converted into mechanical

energy and back again.

The most common such materials are lead zirconate

titanate and barium titanate.  Aside from the uses mentioned

above, their strong piezoelectric response is exploited in the designof high-frequency loudspeakers,  transducers for sonar,  and

actuators for  atomic force and scanning tunnelling microscopes. 

4)  Optical properties:

Optically transparent materials focus on the response of a

material to incoming light waves of a range of wavelengths. Frequency

selective optical filters can be utilized to alter or enhance the brightness

and contrast of a digital image.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 7/26

Machining of ceramics

Advanced Material Removal Processes Page 7

Thus, there is an increasing need in the military  sector for high-

strength, robust materials which have the capability to transmit light 

(electromagnetic waves) in the visible (0.4 –  0.7 micrometres) and mid-

infrared (1 –  5 micrometres) regions of the spectrum. These materials are

needed for applications requiring transparent  armour, including next-

generation high-speed missiles  and pods, as well as protection against

improvised explosive devices (IED).

In the 1960s, scientists at General Electric (GE) discovered that

under the right manufacturing conditions, some ceramics, especially

aluminium oxide (alumina), could be made translucent. During the past

two decades, additional types of transparent ceramics have been

developed for applications such as nose cones for  heat-seeking missiles, 

windows  for fighter   aircraft,  and scintillation counters  for computed

tomography scanners. 

Applications of Ceramics:

  Knife blades: the blade of a ceramic knife  will stay sharp for much

longer than that of a steel knife, although it is more brittle and can snap

from a fall onto a hard surface.  Ceramic brake disks  for vehicles are resistant to abrasion at high

temperatures.

  Advanced composite ceramic and metal matrices have been designed

for most modern armoured fighting vehicles  because they offer superior

 penetrating resistance against shaped charges  (such as HEAT  rounds)

and kinetic energy penetrators. 

  Ceramics such as alumina and boron carbide have been used in ballistic 

armoured vests  to repel large-calibre rifle  fire. Such plates are knowncommonly as small arms protective inserts, or SAPIs. Similar material

is used to protect the cockpits of some military airplanes, because of the

low weight of the material.

  Ceramics can be used in place of steel for ball bearings.  Their higher

hardness means they are much less susceptible to wear and typically last

for triple the lifetime of a steel part. Two drawbacks to ceramic bearings

are a significantly higher cost and susceptibility to damage under shock

loads.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 8/26

Machining of ceramics

Advanced Material Removal Processes Page 8

  Recent advances have been made in ceramics which include bio-

ceramics, such as dental implants and synthetic bones. Hydroxyapatite, 

the natural mineral component of bone, has been made synthetically from

a number of biological and chemical sources and can be formed into

ceramic materials. Orthopaedic implants coated with these materials bond

readily to bone and other tissues in the body without rejection or

inflammatory reactions

  High-tech ceramic is used in watchmaking for producing watch cases.

The material is valued by watchmakers for its light weight, scratch

resistance, durability and smooth touch. 

Machining Techniques:

Ceramics are manufactured by compacting powder to a body which is

then sintered at high temperatures. The geometry, production volume and

characteristic requirements for the component govern the choice of

manufacturing process.

 Ultrasonic machining:

1. Introduction:

Ultrasonic machining is a non-traditional machining process. USM

is grouped under the mechanical group NTM processes. Fig. 4.5 briefly

depicts the USM process.

Fig 4.5: The USM process

Slurry of abrasive

and water

Horn

Force, F

Vibration frequency f~ 19 - 25 kHz

Amplitude, a ~ 10 –  50 μm

Tool

Work

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 9/26

Machining of ceramics

Advanced Material Removal Processes Page 9

In ultrasonic machining, a tool of desired shape vibrates at an

ultrasonic frequency (19 ~ 25 kHz) with an amplitude of around 15  –  50

μm over the workpiece. Generally the tool is pressed downward with a

feed force, F. Between the tool and workpiece, the machining zone is

flooded with hard abrasive particles generally in the form of water based

slurry. As the tool vibrates over the workpiece, the abrasive particles act

as the indenters and indent both the work material and the tool. The

abrasive particles, as they indent, the work material, would remove the

same, particularly if the work material is brittle, due to crack initiation,

 propagation and brittle fracture of the material. Hence, USM is mainly

used for machining brittle materials {which are poor conductors of

electricity and thus cannot be processed by Electrochemical and Electro-

discharge machining (ECM and ED)}.

2. Mechanisms of Material Removal:

As has been mentioned earlier, USM is generally used for

machining brittle work material. Material removal primarily occurs due to

the indentation of the hard abrasive grits on the brittle work material. As

the tool vibrates, it leads to indentation of the abrasive grits. Duringindentation, due to Hertzian contact stresses, cracks would develop just

 below the contact site, then as indentation progresses the cracks would

 propagate due to increase in stress and ultimately lead to brittle fracture

of the work material under each individual interaction site between the

abrasive grits and the workpiece. The tool material should be such that

indentation by the abrasive grits does not lead to brittle failure. Thus the

tools are made of tough, strong and ductile materials like steel, stainless

steel and other ductile metallic alloys.

Other than this brittle failure of the work material due to

indentation some material removal may occur due to free flowing impact

of the abrasives against the work material and related solid-solid impact

erosion, but it is estimated to be rather insignificant.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 10/26

Machining of ceramics

Advanced Material Removal Processes Page 10

3. Process Parameters and their Effects:

During discussion and analysis as presented in the previous

section, the process parameters which govern the ultrasonic machining

 process have been identified and the same are listed below along withmaterial parameters.

• Amplitude of vibration (ao) –  15 –  50 μm

• Frequency of vibration (f) –  19 –  25 kHz

• Feed force (F) –  related to tool dimensions

• Feed pressure (p)

• Abrasive size –  15 μm –  150 μm

• Abrasive material –  Al2O

3

- SiC

- B4C

- Boronsilicarbide

- Diamond

• Flow strength of work material

• Flow strength of the tool material

• Contact area of the tool –  A

• Volume concentration of abrasive in water slurry –  C

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 11/26

Machining of ceramics

Advanced Material Removal Processes Page 11

Fig 4.6 Effect of machining parameters on MRR  

  The vibration frequency is 19 ~ 25 kHz.

  The amplitude of vibration 13 –  50 μm.

  Conventional ultrasonic machining (UM) is characterized by

low material removal rates: up to 49 mm3/min. 

MRR, Q

F

a0

fMRR

a0

MRR, Q

dgf

MRR

MRR, Q MRR

B C

Al2O3 

cλ=

 

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 12/26

Machining of ceramics

Advanced Material Removal Processes Page 12

  4. Machine:

The basic mechanical structure of an USM is very similar to a drill

 press. However, it has additional features to carry out USM of brittle

work material. The workpiece is mounted on a vice, which can be locatedat the desired position under the tool using a 2 axis table. The table can

further be lowered or raised to accommodate work of different thickness.

The typical elements of an USM are (Fig. 4.7)

•  Slurry delivery and return system

•  Feed mechanism to provide a downward feed force on the tool

during machining

•  The transducer, which generates the ultrasonic vibration.

•  The horn or concentrator which mechanically amplifies the

vibration to the required amplitude of 15  –   50 μm and

accommodates the tool at its tip.

Fig 4.7 Schematic view of an Ultrasonic Machine 

Feed motion

Transducer

Horn

Workpiece

Slurry pump

Slurry tank

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 13/26

Machining of ceramics

Advanced Material Removal Processes Page 13

The ultrasonic vibrations are produced by the transducer. The

transducer is driven by suitable signal generator followed by power

amplifier. The transducer for USM works on the following principle

• Piezoelectric effect

• Magnetostrictive effect

• Electrostrictive effect

Magnetostrictive transducers are most popular and robust amongst

all. Fig. 4.8 shows a typical magnetostrictive transducer along with horn.

The horn or concentrator is a wave-guide, which amplifies and

concentrates the vibration to the tool from the transducer.

Fig 4.8 Working of horn as mechanical amplifier of amplitude of vibration

The horn or concentrator can be of different shape like

• Tapered or conical

• Exponential

• Stepped

Machining of tapered or stepped horn is much easier as compared

to the exponential one. Fig.4.9 shows different horns used in USM

Coolant in

Power

am lifier

Signal

generator

Horn

Tool

Vibration

 propagation

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 14/26

Machining of ceramics

Advanced Material Removal Processes Page 14

Exponential Tapered Stepped

Fig 4.9 Different Horns used in USM

 Machining Ceramics with Rotary Ultrasonic Machining:-

History of RUM:

Rotary ultrasonic machining was invented by P. Legge in 1964. In

the first rotary ultrasonic machining device, the slurry was abandoned,

and a vibrating diamond-impregnated tool was used against a rotatingworkpiece. However, because the workpieces were held in a rotating

four-jaw chuck, only circular holes could be machined, and only

comparatively small workpieces could be drilled with this device.

Improvements led to the development of a machine comprising a

rotating ultrasonic transducer. The rotating transducer head made it

 possible to precisely machine stationary workpieces to close tolerances.

With different shaped tools, the range of operations could be extended to

end milling, tee slotting, dovetail cutting, screw threading, and internal

and external grinding.

RUM Equipment:

RUM devices contain a uniquely designed spindle that is coupled

to an ultrasonic transducer. The ultrasonic power supply converts

conventional line voltage into 20 kHz of electrical energy. This output is

fed to the piezoelectric transducer located in the spindle, and the

transducer converts electrical input into mechanical vibrations. By

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 15/26

Machining of ceramics

Advanced Material Removal Processes Page 15

changing the setting of the output control of the power supply, the

amplitude of the ultrasonic vibration can be adjusted. The spindle speed

(measured in revolutions per minute [rpm]) is programmable using the

CNC controller for speeds up to 8000 rpm.

A variety of tool shapes are used for rotary ultrasonic machining,

and ceramic and technical glass machining applications typically use

either a diamond-impregnated or electroplated tool. Diamond-

impregnated tools are more durable, but electroplated tools are less

expensive, so the selection depends on the particular application.

One of the major differences between USM and RUM equipment

is that USM uses a soft tool, such as stainless steel, brass or mild steel,

and slurry loaded with hard abrasive particles, while in RUM the hard

abrasive particles are diamond and are bonded on the tools. Another

major difference is that the RUM tool rotates and vibrates

simultaneously, while the USM tool only vibrates. These differences

enable RUM to provide both speed and accuracy advantages in ceramic

and glass machining operations.

Rotary ultrasonic machining provides a fast, high-quality

machining method for many ceramic and glass applications. Rotary

ultrasonic machining (RUM) is a hybrid machining process that combines

the material removal mechanisms of diamond grinding with ultrasonic

machining (USM), resulting in higher material removal rates (MRR) than

those obtained by either diamond grinding or USM alone. Experiments

with calcium aluminium silicate and magnesia-stabilized zirconia have

shown that the MRR obtained with RUM is six to 10 times higher than

that of a conventional grinding process under similar conditions, and it is

about 10 times faster than USM. It is also easier to drill deep holes with

RUM than with USM, and the hole accuracy is improved. Other

advantages of this process include a superior surface finish and low tool

 pressure.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 16/26

Machining of ceramics

Advanced Material Removal Processes Page 16

Figure-4.10: In ultrasonic machining, the tool, which is shaped conversely to the desired hole

or cavity, oscillates at high frequency, typically 20 kHz, and is fed into the workpiece by a

constant force.

In ultrasonic machining, the tool, shaped conversely to the

desired hole or cavity, oscillates at high frequency, typically 20kHz, and is fed into the workpiece by a constant force (see

Figure 4.10). Abrasive slurry composed of water and small

abrasive particles is supplied between the tool tip and the

workpiece. Material removal occurs when the abrasive particles,

suspended in the slurry between the tool and workpiece, impact

the workpiece due to the down stroke of the vibrating tool.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 17/26

Machining of ceramics

Advanced Material Removal Processes Page 17

Figure-4.11: In rotary ultrasonic machining, a rotating core drill with metal-bonded diamond

abrasives is ultrasonically vibrated in the axial direction while the spindle is fed toward the

workpiece at a constant pressure.

In rotary ultrasonic machining, a rotating core drill with metal

 bonded diamond abrasives is ultrasonically vibrated in the axial direction

while the spindle is fed toward the workpiece at a constant pressure (see

Figure 4.11). Coolant pumped through the core of the drill washes away

the swarf, prevents jamming of the drill and keeps it cool. By using

abrasives bonded directly on the tools and combining simultaneous

rotation and vibration, RUM provides a fast, high-quality machining

method for a variety of glass and ceramic applications.

The RUM material removal rate is up to 492 mm3/min.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 18/26

Machining of ceramics

Advanced Material Removal Processes Page 18

5. Applications of Ultrasonic machining:

1.  Machining of cavities in electrically non-conductive ceramics.

2.  Used to machine fragile components in which otherwise the scrap rate is

high.3.  Used for multistep processing for fabricating silicon nitride (Si3N4)

turbine blades.

4.  Large number of holes of small diameter. 930 holes with 0.32mm has

 been reported (Benedict, 1973) using hypodermic needle.

5.  Used for machining hard, brittle metallic-alloys, semiconductors, glass,

ceramics, carbides etc.

6.  Used for machining round, square, irregular shaped holes and surface

impressions.7.  Used in machining of dies for wire drawing, punching and blanking

operations.

8.  USM can perform machining operations like drilling, grinding and

milling operations on all materials which can be treated suitably with

abrasives.

9.  USM has been used for piercing of dies and for parting off and blanking

operations.

10. USM enables a dentist to drill a hole of any shape on teeth without any pain.

11. Ferrites and steel parts, precision mineral stones can be machined using

USM.

12. USM can be used to cut industrial diamonds.

13. USM is used for grinding Quartz, Glass, and ceramics.

14. Cutting holes with curved or spiral centre lines and cutting threads in

glass and mineral or metallo-ceramics.

6. Advantages of ultrasonic machining include:

1.  The process is non-thermal, non-chemical, and non-electrical, leaving the

chemical and physical properties of the workpiece unchanged. This low-

stress process translates into high reliability for your critical applications.

2.  Multiple features can be machined at the wafer or substrate level

simultaneously, and the process is scalable. Our process is often the

highest quality and lowest cost solution.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 19/26

Machining of ceramics

Advanced Material Removal Processes Page 19

3.  Ultrasonic machined features have vertical side walls, enabling you to

 preserve valuable space for your designs that translate into higher

 productivity.

4.  The process integrates well with semiconductor and MEMS processes.

Machined features can be aligned to previously patterned, machined, or

etched substrates.

 Grinding of ceramics:

Grinding is used in machining of Ceramics  in the sintered  state.

Grinding operation involves a rotating abrasive wheel removing thematerial from the surface of the workpiece.

The grinding zone is continuously flushed with a fluid coolant, 

which cools the grinding zone, lubricates the contact between the wheel

and the part surfaces, removes the micro-chips (debris) produced in the

grinding process.

Resin-bond wheels with either synthetic or natural diamond of

different grit size pressed at different concentrations in polymer (resin)

matrices are commonly used for grinding ceramics.

Electrolytic in-process dressing (ELID) technique of dressing

metal-bonded grinding wheels is used for fine (Nano) finishes grinding.

The Material Removal Rate (MRR) of grinding ceramics is

maximum 9.832 mm3/min. 

 Laser assisted machining of ceramics:

Laser assisted machining (LAM)  is the method of machining

ceramics using a laser beam directed to the workpiece area located

directly in front of the conventional cutting tool.

The laser beam heats and softens (not melts) the ceramic material

at the surface just prior the cutting action.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 20/26

Machining of ceramics

Advanced Material Removal Processes Page 20

As a result the cut material becomes ductile and it may be removed

much faster than in conventional cutting operation without laser

assistance. The LAM material removal rate is up to 983 mm3/min.

Titanium nitride coated tools are used for the laser assisted

machining of ceramics.

Traditional machining operations (milling, turning) may be

 performed by the method of the laser assisted machining.

  Laser machining of ceramics

Laser machining  of ceramics is the machining operation

 performed by a high power laser melting the material, which is blown

away by a supersonic gas jet. The laser energy density required for

melting alumina ranges from 750 J/cm2  to 1000 J/cm

2.

The following machining operation may be performed by laser: 

  Drilling

  Cutting

  Scribing and marking

Residual stresses and micro-cracks may form at the cut edge as a

result of the shrinkage of the solidified  molten material.

Preheating of the ceramic workpiece to 2550ºF (1399ºC) prior to the laser

machining allows to minimize micro-cracking due to reduction of the

temperature gradients and thermal stresses.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 21/26

Machining of ceramics

Advanced Material Removal Processes Page 21

A note on Fabrication of ceramics:

 Sintering:

Sintering  is a method for creating objects from powders, including metal  and ceramic  powders. It is based on atomic diffusion. 

Diffusion occurs in any material above absolute zero, but it occurs much

faster at higher temperatures. In most sintering processes, the powdered

material is held in a mould and then heated to a temperature below the

melting point. The atoms in the powder particles diffuse across the

 boundaries of the particles, fusing the particles together and creating one

solid piece. Because the sintering temperature does not have to reach the

melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as

tungsten and molybdenum. 

Sintering is traditionally used for manufacturing ceramic  objects

 but finds applications in almost all fields of industry. The study of

sintering and of powder-related processes is known as powder

metallurgy.  A simple, intuitive example of sintering can be observed

when ice cubes in a glass of water adhere to each other.

In certain moulding processes a relatively large amount of

temporary organic binder is used, which needs to be burnt out in a

separate step. Sintering is then carried out in air. We have furnaces for

sintering in air up to 1800°C, or inert graphite furnaces with a maximum

temperature of 2300°C.

Ceramic sintering:

Sintering is part of the firing process used in the manufacture of

pottery  and other ceramic objects. These objects are made from

substances such as glass,  alumina, zirconia,  silica,  magnesia,  lime, 

beryllium oxide  and ferric oxide.  Some ceramic raw materials have a

lower  affinity for water and a lower  plasticity index than clay, requiring

organic additives in the stages before sintering. The general procedure of

creating ceramic objects via sintering of powders includes:

  Mixing water, binder,  deflocculant,  and unfired ceramic powder toform a slurry; 

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 22/26

Machining of ceramics

Advanced Material Removal Processes Page 22

  Spray-drying the slurry;

  Putting the spray dried powder into a mould and pressing it to form a

green body (an unsintered ceramic item);

  Heating the green body at low temperature to burn off the binder;  Sintering at a high temperature to fuse the ceramic particles together.

All the characteristic temperatures associated to phase

transformation, glass transitions and melting points, occurring during a

sinterisation cycle of a particular ceramics formulation (i.e., tails and

frits) can be easily obtained by observing the expansion-temperature

curves during optical dilatometer thermal analysis. In fact, sinterisation

is associated to a remarkable shrinkage of the material because glass

 phases flow, once their transition temperature is reached, and start

consolidating the powdery structure and considerably reducing the

 porosity of the material.

There are two types of sintering: with pressure (also known as hot

pressing), and without pressure. Pressure less sintering is possible with

graded metal-ceramic composites, with a nanoparticle sintering aid and

 bulk moulding technology. A variant used for 3D shapes is called hot

isostatic pressing. 

To allow efficient stacking of product in the furnace during

sintering and prevent parts sticking together, many manufacturers

separate ware using Ceramic Powder Separator Sheets. These sheets

are available in various materials such as alumina, zirconia and magnesia.

They are additionally categorized by fine, medium and coarse particle

sizes. By matching the material and particle size to the ware being

sintered, surface damage and contamination can be reduced while

maximizing furnace loading.

 Hot isostatic pressing:-

Hot isostatic pressing (HIP) is a manufacturing  process used to

reduce the porosity of  metals and increase the density of many ceramic 

materials. This improves the material's mechanical properties and

workability.

The HIP process subjects a component to both elevatedtemperature and isostatic  gas pressure in a high pressure containment

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 23/26

Machining of ceramics

Advanced Material Removal Processes Page 23

vessel. The pressurizing gas most widely used is argon. An inert gas is

used, so that the material does not chemically react. The chamber is

heated, causing the pressure inside the vessel to increase. Many systems

use associated gas pumping to achieve the necessary pressure level.

Pressure is applied to the material from all directions (hence the term

"isostatic").

For processing castings,  metal powders can also be turned to

compact solids by this method; the inert gas is applied between 50.7 MPa

and 310 MPa, with 100 MPa being most common. Process soak

temperatures range from 482 °C for aluminium castings to 1,320 °C for

nickel- based superalloys.  When castings are treated with HIP, the

simultaneous application of heat and pressure eliminates internal voidsand micro-porosity  through a combination of plastic deformation, 

creep, and diffusion bonding; this process improves fatigue resistance of

the component. Primary applications are the reduction of micro-

shrinkage, the consolidation of powder metals, ceramic composites and

metal cladding. Hot isostatic pressing is also used as part of a sintering

(powder metallurgy)  process and for fabrication of metal matrix

composites. 

Enhanced product properties:- 

The ability to manufacture HIP products with irregular shapes and

complex geometry offers several advantages over castings, forgings and

fabricated materials, both in terms of design flexibility and material

 properties.

The fine microstructure and isostatic pressure with which the HIP

 products are processed result in isotropic mechanical properties, in other

words, properties that are equal in all directions. The isotropic properties

can contribute to, for example, lighter constructions.

Main advantages with products produced by hot isostatic pressing:

  Increased design flexibility

  Reduction of costly operations like machining and welding

  Improved process safety  Enhanced material properties

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 24/26

Machining of ceramics

Advanced Material Removal Processes Page 24

HIP Applications:-

  Aerospace Casting Densification:

HIP is used to remove porosity from a wide range of nickel-based

super alloy and titanium precision castings for aircraft engines and

structural components.

  Biomedical Casting Densification:

HIP is used in the medical industry to improve the properties of

cast cobalt chrome, titanium and stainless steel implants.

  Industrial Casting Densification:

Industrial applications for hot isostatic pressing cover a wide range

of industries and products such as electrical and electronic casings,

machine tools and pump impellers.

  Power Generation Casting Densification:

In the power generation industry, hot isostatic pressing is used to

densify turbine blades, shafts, discs and other components to remove

 porosity and enhance performance.

  Powder Metallurgy:

Powder metal consolidation via HIP is used to produce near net

shape components faster, with reduced weight and less machining than

traditional castings and forgings.

  Cladding & Diffusion Bonding:

HIP is used in a range of applications to clad, weld and join

dissimilar materials.

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 25/26

Machining of ceramics

Advanced Material Removal Processes Page 25

Finishing Process:

Ceramic glaze:

Glaze is a layer or coating of a vitreous substance which has beenfused to a ceramic  object through firing. Glaze can serve to color,

decorate, strengthen or waterproof an item.

Use:-

Glazing is important for earthenware  vessels as otherwise they

would be unsuitable for holding liquids due to porosity.  Glaze is also

used on stoneware and porcelain. In addition to the functional aspect of

glazes, they can form a variety of surface finishes, including degrees ofgloss and matte and color. Glazes may also enhance an underlying design

or texture which may be either the unmodified texture of the underlying

 body, or an inscribed, carved or painted design.

Glaze is used on building materials. The Iron Pagoda,  built in

1049 in Kaifeng, China, of glazed bricks is an example.

Composition:-

Ceramic glaze raw materials generally include silica, which will be

the main glass former. Various metal oxides, such as sodium, potassium

and calcium,  act as a flux  to lower the melting temperature. Alumina, 

often derived from clay,  stiffens the molten glaze to prevent it from

running off the piece. Colorants, such as iron oxide, copper carbonate 

or cobalt carbonate,  and sometimes opacifiers such as tin oxide  or

zirconium oxide,  are used to modify the visual appearance of the fired

glaze.

Process:-

Glaze may be applied by dry dusting a dry mixture over the surface

of the clay body or by inserting salt or soda into the kiln at high

temperatures to create an atmosphere rich in sodium vapor that interacts

with the aluminium and silica oxides in the body to form and deposit

glass, producing what is known as salt glaze pottery. Most commonly,

glazes in aqueous suspension of various powdered minerals  and metaloxides are applied by dipping pieces directly into the glaze. Other

8/10/2019 Chapter 4-Machining of Ceramics

http://slidepdf.com/reader/full/chapter-4-machining-of-ceramics 26/26

Machining of ceramics

d d l l

techniques include pouring the glaze over the piece, spraying it onto the

 piece with an airbrush or similar tool, or applying it directly with a brush

or other tool.

To prevent the glazed article from sticking to the kiln during firingeither a small part of the item is left unglazed or supported on small

refractory supports called kiln spurs  which are removed and discarded

after the firing. Small marks left by these spurs are sometimes visible on

finished ware.

Decoration applied under the glaze on pottery is generally referred

to as underglaze. Underglazes are applied to the surface of the pottery,

which can be either raw, "greenware", or "biscuit" fired (an initial firing

of some articles before the glazing and re-firing). A wet glaze — usually

transparent — is applied over the decoration. The pigment fuses with the

glaze, and appears to be underneath a layer of clear glaze. An example of

underglaze decoration is the well-known "blue and white"  porcelain

famously produced in England,  The Netherlands,  China and Japan. 

The striking blue color is achieved by using cobalt in the form of either

cobalt oxide or cobalt carbonate, both of which are still commonly used.

Decoration applied on top of a layer of glaze is referred to asoverglaze.  Overglaze methods include applying one or more layers or

coats of glaze on a piece of  pottery or by applying a non-glaze substance

such as enamel or metals (i.e., gold leaf ) over the glaze.

Overglaze colours are low-temperature glazes that give ceramics a

more decorative, glassy look. A piece is fired first, overglaze is applied,

and it is fired again. Once the piece is fired and comes out of the kiln, its

texture becomes smoother because of the glaze.