Embed Size (px)
Citation preview
Development of Zirconia Toughened Alumina Cutting Tool Inserts and Study of Their Properties.
A thesis submitted to the Department of Materials and Metallurgical Engineering of Bangladesh University of Engineering and Technology (BUET) as a part of the completion of the degree of Bachelor of Science in Materials and Metallurgical Engineering. January 2008.
Muhammad Chishty Asheque 0211031
Md. Jamal Uddin 0211033
Table of Content Topics
Page
Abstract 1Acknowledgement 21. Introduction 32. Theoretical Background 4
2.1 Literature Review 42.1.1 General Introduction to Cutting Tools 42.1.2 Property Requirements for Cutting Tool Materials 42.1.3 Classification of Cutting Tool Materials 62.1.4 Comparison Between Different Cutting Tool Materials 82.1.5 Ceramic Cutting Materials 102.1.6 Zirconia Toughened Alumina 112.1.7 Causes of Cutting Tool Failure 13
2.2 Summery of Previous Works 182.3 Scope of Present Work 19
3 Experimental Procedures 203.1 Introduction 203.2 Selection of Raw Materials 203.3 Batch Preparation 213.4 Cutting Tool Insert Preparation 22
3.4.1 Slip Casting 223.4.2 Sintering 22
3.5 Determination of Properties 243.5.1 Density Measurement 243.5.2 Determination of Hardness 243.5.3 Machining Performance 24
3.6 Observation of Microstructure 254 Result & Discussion 26
4.1 Optimization of Sintering Time 264.2 Density 284.3 Hardness 314.4 Machining Performance 344.5 Microstructure 38
5 Conclusion 396 Scope of Future Works 407. Bibliography 41Appendix 42
1
Abstract
Zirconia toughened alumina (ZTA) cutting tool inserts were developed by slip casting
method, using nano-sized 15 wt% pure ZrO2 and 85 wt% Al2O3. Moisture content
was varied between 40 and 45 wt% of the slip and the green inserts were sintered at
1250-1475 °C for 2-4 hours. Density, hardness, chip removal from work piece and
weight loss due to machining was studied.
Hardness increased with increasing sintering temperature. The best combination of
properties of inserts was obtained at a sintering temperature of 1475°C, although data
for inserts sintered at 1450°C were very close to those values. A maximum density of
4.26 g/cc (which is 100.714% of theoretical density) was achieved, with a maximum
hardness of 9.89 GPa. The microstructure of sintered inserts showed zirconia grains
dispersed in alumina grains and porosity. The machining performance of inserts
improved with the increase in sintering temperature, with the maximum performance
just bettered those of carbide tool.
Consider the overall properties of the ZTA inserts produced using slip casting route
and comparing these with the performance of ZTA inserts produced using compaction
route, slip cast ZTA ceramics can be an economic alternative to manufacture cutting
tool inserts.
2
Acknowledgement
The Authors express their gratitude to Dr. A. K. M. Bazlur Rashid, Professor of
Department of Materials and Metallurgical Engineering, BUET, under whose
supervision this research has been carried out. His guidance, support and
encouragement have made this task possible.
The Authors are beholden to Dr. Md. Fakhrul Islam, Professor and Head of the
Department of Materials and Metallurgical Engineering for his guidance and
assistance and giving the authors the permission to use the laboratory facilities.
The Authors are grateful to Mr. Shawkat Chowdhury of Bangladesh Glass and
Ceramics Institute for his co-operation in preparing the mould. The Authors also
thank all the staffs of the Department of Materials and Metallurgical Engineering,
BUET who helped them conducting different experiments.
The Authors
3
1. Introduction
In this modern day and age, the significance of cutting tool is limitless. Developing
faster cutting, durable and cheaper cutting tool is imperative to increase the level of
production of a metal working industry.
Among various materials ceramic cutting tool is supposed to give the best
performance, because of its high hardness, wear resistance and hot-hardness. Among
all the ceramics materials alumina is readily available and cheap. Further more
ceramics materials are brittle, but alumina can be toughened by zirconia.
The most common route of making cutting tool inserts are powder metallurgy
method, but slip casting method is a low cost alternate route of ceramic fabrication.
This led us to try to develop cutting tool inserts by slip casting method for the easiness
of the process and low investment & maintenance cost associated with the process.
Besides zirconia toughened alumina (ZTA) has attractive properties and low cost.
4
2. Theoretical Background
2.1 Literature Review
2.1.1 General Introduction to Cutting Tools
A cutting tool is a kind of machine tool that is used to remove metal from the
workpiece by means of shear deformation. Metal-cutting tools are classified as single-
point or multiple-point. A single-point cutting tool can be used for increasing the size
of holes, or boring. Turning and boring are performed on lathes and boring mills.
Multiple-point cutting tools have two or more cutting edges and include milling
cutters, drills, and broaches.
There are two types of operation; either the tool is moving on a straight path against
the stationary workpiece, as on a shaper, or the workpiece is moving against the
stationary tool, as on a lathe.
2.1.2 Property Requirements for Cutting Tool Materials
In general, in order to perform a long time, cutting tools must be made of a material
harder than the material which is to be cut, and the tool must be able to withstand the
heat generated in the metal cutting process. For the selection of cutting tool materials
the following properties are of great importance:
Chemical compatibility
If two materials do not react with one another and remain insoluble in each other, they
are call chemically compatible. During cutting operation, since the temperatures at
and near the cutting edge are very high, chemical compatibility should be the first
selection criterion. For one material to cut another effectively, the former must not
react with or dissolve in the later and vice versa at operating condition.
5
Hardness
Hardness is defined as the resistance of a material to indentation. For successful
cutting operation the cutting tool should be hard enough to produce and remove chips
from the workpiece. Therefore the former should be harder than the later.
Wear resistance
Wear resistance is the resistance to abrasion or resistance to the loss of dimensional
tolerances. Wear resistance might be required on a single cutting edge or over the
total surface of the part. From industrial point of view, sharpness of the cutting tool is
crucial. The longevity of sharpness depends on the wear resistance of the tool
material.
High temperature resistance
Cutting speed is directly related to the rate of production in industries. Higher cutting
speed is required for large scale and higher rate of production. Higher speed produces
higher temperature which is generally undesirable because of certain temperature
dependent properties of materials, such as oxidation corrosion, thermal expansion etc.
Red-hardness
Red-hardness or hot-hardness is related to the resistance of the tool to softening due to
the effect heat. Due to the heat generated during cutting operation the steel cutting to
materials tend to lose their hardness. Therefore a cutting tool material should possess
high red-hardness.
Toughness
The term toughness may be thought as the ability to resist breaking other than the
ability to absorb energy during deformation. During cutting operation stress develops
in the tool and stress concentration occurs where flaws or defects are present. This
accelerates crack extension. So a tool material should have enough fracture toughness
and low brittleness.
6
Strength
A tool material should have enough yield strength so that the cutting tool do not
deform plastically during operation. It should be able to retain its shape and also the
sharpness of the tip.
2.1.3 Classification of Cutting Tool Materials
Some of the common cutting tool materials are discussed below –
Carbon steel
• Comparatively inexpensive
• Carbon content about 0.9 to 1.35% with hardness about RC 62
• Tends to lose hardness and cutting ability at temperatures about 250°C
• Is used for low speed and light cutting operations on relatively soft materials
• Limited tool life. Therefore, not suited to mass production
• Maximum cutting speeds about 26 ft/min. in dry condition
High-speed steel
• An alloyed steel with 14-22% tungsten, as well as cobalt, molybdenum and
chromium, vanadium
• Appropriate heat treating will improve the tool properties significantly
• Can cut materials with tensile strengths up to 75 tons/sq.in. at speeds of 50-60
fpm
• Hardness is in the range of RC 63-65
• The cobalt component give the material a hot hardness value much greater
than Carbon Steels
Stellite
• A family of alloys made of cobalt, chromium, tungsten and carbon
• The material has a hardness of RC 60-62 without heat treating
• They can withstand cutting temperature up to 760°C
• Cutting speed of up to 80-100 fpm can be used on mild steels
• The tools that use this method either use inserts in special holders, or tips
brazed to carbon steel shanks
7
Cemented tungsten carbide
• Produced by sintering grains of tungsten carbide in a cobalt matrix. The matrix
provides toughness
• Other materials are often included to increase hardness, such as titanium,
chrome, molybdenum, etc.
• Hardness approaches that of a diamond
• As the material is expensive and has low rupture strength, it is normally
formed as tips that are often brazed to steel shanks, or used as inserts in
holders
• Speeds up to 300 fpm are common on mild steels
• Hot hardness properties are very good
• Coolants and lubricants can be used to increase tool life, but are not required
Coated carbide
• The cutting system is produced by coating the conventional carbide insert with
high wear resistant titanium carbide
• Has higher wear resistance than conventional carbide tools
• Can be used to cut at a higher speed than conventional carbide tools
Diamond
• A very hard material with high resistance to abrasion
• Very good for turning and boring, producing very good surface finish
• Due to the brittleness of the diamonds operations must minimize vibration to
prolong diamond life
• Generally used at very high cutting speed with low feed and light cuts
• Very expensive
8
Ceramic
• These are generally sintered or cemented ceramic oxides
• Can be used for turning and facing most metals
• Mild steels can be cut at speeds up to 1500 fpm
• These tools are best used in continuous cutting operations
• There is no occurrence of welding, or built up edges
• Coolants are not needed to cool the workpiece
• Very high hot hardness and wear resistance properties
• Often used as inserts in special holders
2.1.4 Comparison Between Different Cutting Tool Materials
From the discussion of section 2.1.5 and from Figure 1 and Figure 2, we can compare
different properties of different cutting tool materials. This comparison is shown in
Table 1.
Figure 1: Comparison of hot-hardness between different cutting materials
9
Figure 2: Comparison of the fracture toughness between key structural materials
Table 1: Comparison of different properties between different materials
Metallic Cemented
Carbides
Coated
Carbides
Diamond Ceramics
Moderate
hardness
Very high
hardness
Very high
hardness
Excellent
hardness
High hardness
Fair to very
good hot
hardness
Very good hot
hardness
Very good hot
hardness
Moderate hot
hardness
Excellent hot
hardness
High toughness Moderate
toughness
Moderate
toughness
Low toughness Low to
moderate
toughness
Low wear
resistance
Moderate wear
resistance
High wear
resistance
Very high wear
resistance
Very high wear
resistance
Fairly
inexpensive
Moderately
expensive
Moderately
expensive
Very expensive Moderately
expensive
From Table 1 we can conclude that ceramics are better candidate for cutting tool
materials. Though generally they do possess much fracture toughness; but some of the
ceramics can be made tougher.
10
2.1.5 Ceramic Cutting Materials
Common ceramics include dense form of aluminum oxide (Al2O3), silicon nitride
(Si3N4), silicon carbide (SiC), zirconium oxide (ZrO2), transformation toughened
zirconia (TTZ), transformation toughened alumina (TTA), and alumina nitride (AlN).
Major progress has been accomplished in the past 20 years to increase the capability
of ceramics for thermal, wear, corrosion, and structural applications. In particular, the
strength and toughness have been dramatically improved to the degree that ceramics
are now available that can compete with metals in applications previously thought
impossible for ceramics. Cutting is such an application where ceramic materials can
be used extensively because of their wear and heat resistance, hardness along with
newly enhanced toughness.
Properties of silicon nitride include high strength over broad temperature range, high
hardness, and unusually high fracture toughness. Combination of these properties
leads to excellent shock resistance and superior wear resistance. Cutting hard metals
such as cast iron and high-temperature alloys results in high temperature at the tool-
workpiece interface. WC-Co cermet, although tougher than Si3N4, wears rapidly if the
temperature gets too high. In the case of high temperature resistance Si3N4 is better
than WC-Co; Si3N4 can cut 12 to 64 times faster that WC-Co depending on the nature
of workpiece material. But silicon nitride is still too expensive.
Alumina is the most mature high technology ceramic with its high hardness and
durability. It s presently the lowest-cost high performance ceramic because of the
large quantity produced.
Transformation toughening was a breakthrough in achieving high-strength, high-
toughness ceramic materials. For the first time in history a ceramic material was
available with an internal mechanism for actually inhibiting crack propagation. A
crack in a normal ceramic travels all the way through the ceramic with little
inhibition, resulting in immediate brittle fracture. TTZ has fracture toughness
(resistance to crack propagation) 3–6 times higher than normal zirconia and most
other ceramics. It is tougher than cast iron and comparable in toughness to WC-Co
cermet. TTZ is so tough that it can be struck with a hammer or even fabricated into a
hammer for driving nails.
11
Transformation toughening can be achieved in other ceramic materials by additions of
particles of partially stabilized zirconia. Toughening occurs if the particles are small,
if the host ceramic is strong enough to prevent the particles from transforming during
cooling, and if there is no chemical interaction between the materials. Alumina is the
most important ceramic that is a suitable host for zirconia toughening. An addition of
15-25% zirconia to alumina results in toughness and strength nearly equivalent to that
of pure Transformation-Toughened Zirconia, but the alumina is cheaper and much
lighter in weight.
2.1.6 Zirconia Toughened Alumina
Zirconium dioxide (ZrO2), otherwise known as zirconia, is a white crystalline oxide of
zirconium. Pure zirconia shows polymorphic transformation at elevated temperatures.
Its room temperature stable form is monoclinic which transforms to tetragonal
structure at temperature about 950°C. This transformation is accompanied by greater
that 1% shrinkage during heating and equivalent expansion during cooling. At a much
higher temperature zirconia changes from tetragonal to cubic structure. This results in
internal stresses which is large enough to cause fracture or substantial weakening.
Figure 3: Polymorphism in ZrO2
Several different oxides are added to zirconia to stabilize the tetragonal and/or cubic
phases: magnesium oxide (MgO), yttrium oxide, (Y2O3), calcium oxide (CaO), and
cerium oxide (Ce2O3), amongst others. With proper chemical additions and heat-
treatments, a microstructure can be achieved during cooling that consists of lens-
shaped “precipitates” of tetragonal zirconia in cubic grains of zirconia, as shown in
Figure 4.
12
Normally, the tetragonal material would transform to the monoclinic form during
cooling, but it must expand to do so. The high strength of the surrounding cubic
matrix prevents this expansion, so the tetragonal form is retained all the way down to
room temperature. As a result, each tetragonal zirconia precipitate is under stress and
full of energy that wants to be released, sort of like a balloon that has been stuffed into
a box that is too small. As soon as the box is opened, the balloon is allowed to expand
to its equilibrium condition and protrude from the box. The same thing happens for
each tetragonal precipitate if a crack tries to form if someone tries to break the
ceramic. The crack is analogous to opening the box. Tetragonal precipitates next to
the crack are now able to expand and transform back to their stable monoclinic form.
This expansion adjacent to the crack presses against the crack and stops it. This is the
mechanism of transformation toughening.
Figure 4: Microstructure of one form of transformation-toughened zirconia.
TTZ has been developed in a couple of different forms. The one described above is
typically called partially stabilized zirconia (PSZ). The second form consists of nearly
every crystallite or grain in the material being retained in the tetragonal form to room
temperature so that each grain can transform instead of only the precipitates. This
material is referred to as tetragonal zirconia polycrystal (TZP). Both types are
mentioned because they have different properties, and one may be preferable for a
specific application. TTZ typically costs around four times as much as steel and two
times as much as WC-Co.
13
Figure 5: Resistance to cracking in transformation-toughened zirconia.
Zirconia Toughened Alumina (ZTA) shows considerable improvement in strength and
toughness over standard alpha alumina. The increase in strength and toughness in
ZTA is attributable to the stress induced transformation toughening mechanism which
is introduced with the addition of optimized amounts of fine zirconia particles
dispersed throughout the alumina body. Typical zirconia content is between 10% and
20%. As a crack grows through the ceramic, the crystal structure of the zirconia
particles in the region of the crack changes from the metastable tetragonal phase to the
stable monoclinic phase. The change increases the volume of the particles by about
3% and produces compressive stresses in the alumina matrix. These stresses in turn
close the crack and act as an energy barrier to further crack growth. The addition of
zirconia to the alumina matrix increases fracture toughness easily by two times and
can be improved by as high as four times, while strength is more than doubled.
2.1.7 Causes of Cutting Tool Failure
In the violent world of metal-cutting, cutting tools must resist extreme heat, high
pressure, abrasion and shock. Temperatures at the cutting edge can exceed 1000°C.
Extreme heat degrades binders and other tool constituents, and can also trigger
detrimental chemical reactions between the tool and workpiece. Abrasion is always
part of the cutting process. While in the cut, the tool is in constant contact with the
workpiece, under pressures greater than 2,000 psi.
14
Varying levels of thermal and mechanical shock also play a role in tool failure.
Thermal shock – rapid heating and cooling of the tool – is most common in milling
operations, in which the insert heats up while cutting and then cools while away from
the cut. Mechanical shock is also a factor in milling, in machining interrupted
surfaces, and even in turning, depending on the operation involved and the condition
of the workpiece.
Basic failure mechanisms include crater wear, thermal deformation and cracking, nose
wear, depth-of-cut notching, built-up edge, chipping, fracture and flank wear.
Crater Wear
Crater wear occurs on the rake face or top of the insert, typically when machining
steels at elevated cutting speeds. Unlike abrasive wear, this kind of wear is caused by
a chemical interaction between the hot chip and the workpiece material. When a tool
is used to machine steels and other materials at high speeds, the tool material may
dissolve into the chip, or tiny particles of the tool may adhere to the chip and get
carried away. In either case, a crater forms. Excessive cratering weakens the cutting
edge, inhibits proper chip flow, and increases heat and pressure on the tool. Left
unchecked, crater wear can lead to tool fracture.
Figure 6: Crater wear
15
Flank Wear
All tools wear, but some types of wear are more desirable than others. The motion of
the tool’s flank face against the surface of the workpiece causes flank wear. When
wear dulls the cutting edge, tool pressure and stress on the machine and the part
increase, excessive edge wear causes deflection of the part and a change in part size
and forces more heat back into the part. The longer edge wear is left unchecked, the
worse it gets. In the end, it can precipitate tool fracture.
Figure 7: Flank wear
Depth-of-Cut Notching
When machining stainless steels, high-temperature alloys and work-hardening
materials that generate high cutting temperatures, depth-of-cut notching can occur at
the free end of the chip. A depth-of-cut notch can cause a burr to form, leading to tool
fracture.
Figure 8: Notching
Nose Wear
When machining hard alloy steel, rubbing or abrasion and local deformation of the
tool’s nose into the workpiece can occur. As the tool nose wears, part size changes
and surface finish deteriorates. Often, the workpiece material will smear.
16
Thermal Deformation
Heat and pressure generated by machining can cause the cutting tool’s binder to
soften, allowing the carbide grains to move. Little insert material is actually worn
away, as it is in crater wear, but the nose of the insert becomes distorted. As thermal
deformation progresses, heat and cutting pressure increase. Inconsistent part size and
tool breakage can follow.
Figure 9: Deformation
Thermal Cracking
Large differences in temperature between the cutting edge and the bulk of the insert
cause evenly spaced cracks perpendicular to the cutting edge. Interrupted cutting (as
in milling), or machining materials like titanium that generate high heat when cut, can
cause these temperature fluctuations. Cracks will progress slowly, leading to chipping
and eventually to tool fracture.
Figure 10: Cracking
17
Built-Up Edge
When soft materials such as aluminum, brass or soft steels are machined, the
workpiece material can bond to the cutting tool chemically and mechanically. Built-
up edge can increase tool pressure and cause poor surface finish, part size changes
and tool breakage. The build-up is often unstable, and is periodically washed away by
the cutting action. Part size and finish will fluctuate. Tools can chip or break because
of built-up edge, but users may not recognize it as the cause.
Figure 11: Built-up edge
Chipping/Fracturing
Non-rigid setups, with vibration or inconsistent cutting pressures, can cause a tool to
chip. Interrupted cuts can often cause chipping or fracturing. Tool fracture can occur
when one or more failure mechanisms weaken the tool, or when cutting forces rise to
such a level that the insert can no longer bear the load.
Figure 12: Chipping and Fracturing
18
2.2 Summery of Previous Works
• Zirconia toughened alumina for wear resistant engineering and
machinability of steel application: Synthesis of Y2O3 based partially stabilized
zirconia by evaporative decomposition of solution (EDS) and α-alumina by wet
chemical precipitation for making zirconia toughened alumina (ZTA) ceramics by
a powder metallurgy process route. The ZTA sintered product has been
characterized for use in cutting tool inserts as well as for other wear resistant
engineering properties. The stress induced transformation toughening of ZTA
ceramic is experimentally observed during machining of steel, and the effects of
crater and flank wear on machining of steels has been investigated. Hardness and
fracture toughness have been correlated to improve machinability (steel cutting
efficiency) and wear resistance for other engineering applications.
• Erosion Damage and Strength Degradation of Zirconia-Toughened Alumina:
The erosion wear and post erosion strength properties of Al2O3 containing 10
vol% ZrO2 were determined. Samples were prepared with three different surface
treatments to obtain different amounts of tetragonal zirconia on the surface. The
erosive wear was independent of the initial amount of tetragonal phase at the
surface and was not statistically different from that found for commercial alumina.
The post erosion strength was relatively insensitive to increasing kinetic energy of
the eroding particles, indicating a rising fracture resistance behavior for this
zirconia-toughened alumina
• Strength Improvement in Transformation-Toughened Alumina by Selective
Phase Transformation: Al2O3-15 vol% ZrO2 bar-shaped ceramic specimens
were fabricated in the green state in such a way that the near surface regions
consisted of A12O3 and unstabilized ZrO2 while the bulk consisted of A12O3 and
partially stabilized ZrO2. After sintering, specimens had macroscopic residual
compressive surface stresses and balancing interior tensile stresses due to the
tetragonal-to-monoclinic phase transformation in the outer layers which occurs
during cooling. The depth of the surface region was controlled during green
forming. Residual surface compressive stresses at room temperature varied
between 100 and 400 MPa depending on the outer-layer thickness. The increased
strengths of the three-layer specimens were obtained in the as-fired unground
condition, demonstrating that the stresses introduced are the result of
19
transformation of tetragonal zirconia into monoclinic polymorph which occurred
upon cooling from the sintering temperature. Specimens with residual surface
compressive stresses were 200 MPa stronger at 750°C than monolithic specimens,
demonstrating the viability of this approach for improving elevated-temperature
mechanical properties.
2.3 Scope of Present Work
ZTA is has high hardness, excellent hot-hardness and it can be made tough.
Conventional cemented carbide cutting tools are made by powder compaction which
makes it expensive. Where as ZTA cutting to inserts can be fabricated by slip casting
method at a lower cost. Development of ZTA cutting tool insert has not yet been done
in out country. The success of this study has a can have a significant impact on our
country’s economy in the long run.
20
3 Experimental Procedures
3.1 Introduction
In this work, production of zirconia toughened alumina (ZTA) cutting tool inserts was
made using slip casting method. To fabricate the inserts batches of slip using the raw
materials were prepared and cast into pre-fabricated plaster of Paris molds. After
releasing the inserts from the molds, they were dried and then sintered.
Microstructures of the tool were observed and mechanical properties such as density,
porosity and hardness were measured. Finally the cutting performances of the
prepared ZTA inserts were compared with that of a standard carbide cutting tool.
3.2 Selection of Raw Materials
The raw materials to produce ZTA are nanosized pure alumina and zirconia. Alpha-
alumina and monoclinic-zirconia manufactured by Inframat® Advanced MaterialsTM,
product numbers 26N-0802A and 40N-0801 respectively, were used. Their
specifications are given in Table 2.
Table 2: Raw material powder specifications
Powder Properties α-Al2O3 Monoclinic-ZrO2
Purity (%) 99.8 99.9
Grain Size (nm) 40 -
Average Particle Size (nm) 150 30-60
Density (gm/cc) 3.97 5.68
Melting Point (°C) 2045 2700
Boiling Point (°C) 2980 ~5000
Specific Surface Area (m2/gm) ~10 15-40
21
3.3 Batch Preparation
To prepare each batch at first powders were weighed separately. Then they were
transferred into a HDPE pot mills containing Y-PSZ grinding ball of 5 mm in
diameter. Then measured amount of water was poured in the pot. The pot was then
put on a pot-mill for milling for about 16 to 18 hours. Details of each batch is given in
Table 3.
Table 3: Details of batch preparation
Batch # Al2O3
(wt% of solid)
ZrO2
(wt% of solid)
H2O
(wt% of slip)
1 85 15 45
2 85 15 45
3 85 15 45
4 85 15 40
5 85 15 40
6 85 15 40
The percentage of moisture was reduced because of the presence of voids in the green
products at higher moisture percentage.
Figure 13: Figure on the left shows less porosity in insert containing 40% moisture
while figure on the right shows large porosity containing 45% moisture
22
3.4 Cutting Tool Insert Preparation
3.4.1 Slip Casting
After milling the slip was separated from the grinding balls and then transferred into a
hypodermic syringe. The opening of the syringe was wrapped with tissue paper and
placed into the opening of the mold. The purpose of tissue paper was to fill the air-gap
between the syringe and the mold. The piston of the syringe was pressed to fill the
mold cavity with slip. A slight pressure on the piston was kept for about 5-7 minutes
after the mold cavity had been filled. The purpose of this was to ensure the supply of
slip, as the plaster of paris mold absorbed the water from the slip in the mold cavity.
3.4.2 Sintering
After the slip casting process the specimens were let to dry in the mold for about 30 to
60 minutes. Then they are released from the mold. The green specimens were dried
naturally at least for a day. After drying they were sintered according to certain
sintering cycles in a Thermolyne 46200 high temperature furnace. Details of heating
cycles for each batch is given in Table 4.
Table 4: Details of sintering
Batch # Sintering
Temperature
(°C)
Dwelling
Time
(Hours)
Rate of
Heating
(°C/min)
Rate of
Cooling
(°C/min)
1 1250 2
2 1350 2
3 1375 4
4 1400 4
5 1450 4
6 1475 4
2
4
23
To optimize the sintering temperature and time, different temperatures and heating
time were used. The dwelling time was increased to 4 hours because of insufficient
sintering for the first two batches. The reason for this is discussed in detail in Article
4.1 Optimization of Sintering Time. A typical firing cycle is shown in Fig. 14, while a
sintered insert is given in Fig. 15.
Figure 14: A typical sintering cycle
Figure 15: A sintered product
2°C/min
600°C1 Hour
1400°C4 Hour
4°C/min
2°C/min
Time
T e m p e r a t u r e
24
3.5 Determination of Properties
3.5.1 Density Measurement
Densities of the sintered products were measure by using Archimedes’s principle.
According to this principle –
1w-ww Density, =ρ (Equation 1)
Where, w is the weights of a object in air and w1 is the apparent weight of the object
immersed in water.
3.5.2 Determination of Hardness
To determine the Vickers indentation hardness, indentations were made on the
specimens with a diamond pyramid indenter using 800 gm load. A Shimadzu 80380
micro-hardness tester was used for this purpose. After the indentations were made, the
lengths of the diagonals were measure using a scanning electron microscope. A
Philips XL-30 scanning electron microscope was used for this purpose. The Vickers
hardness was determined from the following equation –
GPa )(P/d 0.0018544HV 2= (Equation 2)
Where, P is the load applied, N, and d is the average length of the two diagonals of
the indentations, mm.
3.5.3 Machining Performance
To determine the machining performances of the cutting tool inserts the weight loss
after machining on a mild steel bar for a certain period with certain feed rate/rpm and
depth of cut was determined. Also the amount of metal chips removal after machining
for a certain period with some controlled parameters was also determined. The
composition and hardness of the mild steel bar is given in Table 5.
25
Table 5: Composition and hardness of the mild steel bar used
to determine machining performance.
Composition
Elements Percentage
Hardness Number
(Rockwell B)
Fe 98.807
C 0.833
Si 0.231
Mn 0.618
P 0.029
S 0.058
Cu 0.094
Ni 0.037
Cr 0.410
Al 0.002
75
3.6 Observation of Microstructure
To observe the microstructure at first thermal etching was done at 45-50°C below the
sitering temperature. But the attempt was unsuccessful. Microstructure was
successfully obtained by chemical etching. After etching the specimens were viewed
under a Philips XL-30 scanning electron microscope. The etching conditions are
showed in Table 6.
Table 6: Etching condition
Sintering Temperature (°C) Etching Condition
1475 Immersed in 85% Phosphoric acid
solution of 250°C for 3minutes
26
4 Result & Discussion
4.1 Optimization of Sintering Time
The table below shows sintering temperature, sintering time density and hardness of
first three batch of cutting tool inserts.
Table 7: Sintering condition, density and hardness data for sintering temperature
optimization
Batch #
Sintering
Temperature,
T
(°C)
Sintering
Time,
t
(Hours)
Density,
ρ
(gm/cc)
Average Vickers
Hardness
(800 gm load)
HV
(GPa)
1 1250 2 3.387 0.381
2 1350 2 3.802 2.211
3 1375 4 4.160 3.315
Sintering for 2 hours at 1250°C and 1350°C gave soft and less dense product. So
sintering time was increased to 4 hours for all other batches.
3
3.5
4
4.5
1225 1250 1275 1300 1325 1350 1375 1400
Sintering Temperatures (°C)
Den
sity
(gm
/cc)
Actual value of density at 1375°CPropable value of density at 1375°C if sintering time was unchnaged
Figure 16: Extrapolation to obtain density for sintering time 2 hours at 1375°C
27
From the data of Table 7, graphs in Figure 16 and Figure 17 were developed by
extrapolation showing approximate values of density and hardness if the sintering
time at 1375°C was not increased; which would have been a lower value.
00.5
11.5
22.5
33.5
1225 1250 1275 1300 1325 1350 1375 1400
Sintering Temperature (°C)
Ave
rage
Vic
kers
H
ardn
ess
(GPa
)
Actual value of hardness at 1375°CProbable value of hardness at 1375°C if sintering time was unchanged
Figure 17: Extrapolation to obtain hardness for sintering time 2 hours at 1375°C
28
4.2 Density
Theoretical density of ZTA containing 15% ZrO2 is 4.2265 gm/cc
The densities obtained by the experiments are given in Table 8.
Table 8: Density of ZTA containing 15 wt% ZrO2 produced by slip casting method
Sintering Temperature
(°C)
Density
(gm/cc)
Percentage of Theoretical
Density
1375 4.089 96.740
1400 4.257 100.714
1400 4.243 100.388
1400 3.680 87.078
1400 4.174 98.756
1450 4.011 94.899
1450 4.106 97.160
1450 4.160 98.422
1450 3.058 72.342
1450 3.181 75.267
1475 3.816 90.279
1475 3.996 94.540
1475 3.832 90.671
1475 3.984 94.259
1475 3.437 81.326
29
The graphical representation of Table 8 is as follows
0
20
40
60
80
100
120
1350 1375 1400 1425 1450 1475 1500
Sintering Temperature (°C)
% T
eora
tical
Den
sity
Figure 18: Density vs. sintering temperature of ZTA containing 15% ZrO2 by slip
casting method
During sintering the neighboring particles in a ceramic body coalesce reducing the
size of the porosity between particles. As the sintering temperature in creases the
coalescence of particles increases. Thus volume of porosity decreases, hence the
density increases. Density of ceramic increases up to a certain point with increasing
sintering temperature. Beyond that point the density decreases. Because at that high
temperature less dense glassy phase is produce. With increasing sintering temperature
from 1375°C to 1400° density increases and the value of density at 1400°C is more
than 100% of theoretical density. It should be mentioned that the theoretical density
was calculated by considering that all the zirconia present in the ZTA is monoclinic.
But in reality the monoclinic phase transforms to tetragonal while producing ZTA.
Therefore it can be concluded that the ZTA sintered at 1400°C contains more amount
of denser tetragonal zirconia than 1375°C. It is assumes that this is the reason why
density of sintering temperature 1400°C is more than 100% and density of 1375°C is
less that 100% of the theoretical density.
The curve in Figure 18 shows a maximum value at 1400°C. With increasing sintering
temperature beyond that point density decreases. At higher temperature during
30
cooling due to large grain size of zirconia some tetragonal phase spontaneously
transforms to monoclinic phase, which is less dense. This is the reason for decreasing
density with increasing temperature.
Table 9: Comparison of densities between different composition and fabrication
process.
Density (gm/cc) Sintering
Temperature (°C) 15 wt% ZTA by
slip casting
21 wt% ZTA by
slip casting
15 wt% ZTA by
powder compaction
1400 4.174 3.060 3.545
1450 3.181 3.607 3.829
1475 3.437 3.736 4.167
0.00.51.01.52.02.53.03.54.04.5
1400 1450 1475Sintering Temperature (°C)
Den
sity
(gm
/cc)
15% by slip casting 21% by slip casting 15% by powder compaction
Figure 19: Comparison of density between ZTA produced by different methods
From Figure 19 we observe that the density of 15% ZTA produced by power
metallurgy method and 21% ZTA produced by slip casting method increases with
increasing sintering temperature. This is contradictory to our findings of 15% ZTA.
The reason for this is still unknown to us. Further study in this field has to be done
before drawing any conclution.
31
4.3 Hardness
The Vickers hardness values of ZTA containing 15% ZrO2 using 800gm load
obtained by experiments are given in Table 10.
Table 10: Vickers hardness of ZTA containing 15% ZrO2 produced by slip casting
method
Sintering Temperature
(°C)
Average Vickers Hardness
Using 800gm Load (GPa)
1400 4.719
1450 8.297
1475 9.888
The graphical representation of Table 10 is as follows
0
2
4
6
8
10
12
1375 1400 1425 1450 1475 1500
Sintering Temperature (°C)
Har
dnes
s (G
Pa)
Figure 20: Hardness vs. sintering temperature of ZTA containing 15% ZrO2 by slip
casting method
As the sintering temperature increases, the hardness of the inserts increases. The curve
in Figure 20 complies with that. Increased sintering temperature gives denser products
which in turns gives higher hardness.
32
Table 11: Vickers hardness of ZTA containing 15% ZrO2 produced by powder
compaction method
Sintering Temperature
(°C)
Average Vickers Hardness
Using 800gm Load (GPa)
1400 12.95
1450 13.09
1475 15.91
Table 12: Vickers hardness of ZTA containing 21 wt% ZrO2 produced by slip casting
method
Sintering Temperature
(°C)
Average Vickers Hardness
Using 800gm Load (GPa)
1400 4.64
1440 7.9
1475 12.19
0
2
4
6
8
10
12
14
16
18
1400 1440 1450 1475
15% by slip casting 21% by slip casting 15% by powder compation
Figure 21: Comparison of hardness of ZTA produced by different methods
The hardness of ZTA containing 15% ZrO2 and 21% ZrO2 do not differ much at
sintering temperature 1400°C. But at 1475°C it shows a substantial difference in
33
hardness. Compared to powder compaction method, slip casting method give products
of lower hardness. Data for hardness for sintering temperature 1450°C could not be
gathered; but if the data is generated by interpolation it would give a as shown in
Figure 22. In that case hardness of ZTA containing 21% ZrO2 would have a higher
hardness value.
0
2
4
6
8
10
12
14
16
18
1400 1440 1450 1475
15% by slip casting 21% by slip casting 15% by powder compation
Figure 22: Estimation of approximate hardness of 21% ZTA produces by slip casting
method
34
4.4 Machining Performance The machining performance was determined with a constant depth of cut of 0.5 and a
constant rpm of 160. At 160 rpm the average feed rate was 1.25 mm/s. The amount of
chip removal from workpiece and weight loss of the inserts sintered at different
temperature given in Table 13. Detailed calculations are shown in Appendix.
Table 13: Amount of weight loss, chip removal with respect to sintering temperature
Sintering Temperature
(°C)
RPM
Weight Loss of
Insert Per Meter
Distance of Cut
(µg/m)
Chips Removal from
Workpiece Per Meter
Distance of Cut
(gm/m)
1400 160 794.572 0.0630
1450 160 49.275 0.5475
1475 160 39.285 0.5998
Carbide 160 6.830 0.6073
With increasing sintering temperature weight loss after machining operation
decreases. This can be explained by the increasing hardness (i.e. wear resistance) with
increasing sintering temperature. The insert sintered at 1450°C and at 1475°C showed
more or less the same weight loss. The insert sintered at 1400°C showed a very high
weight loss because of severe flank wear due to very low hardness. It had almost half
of the hardness of the insert sintered at 1475°C (Figure 20 & Table 10). The weight
loss of slip cast ZTA cutting tool inserts sintered at higher temperatures was not as
low as the carbide.
35
0.01
0.10
1.00
1400 1450 1475 Carbide
Cutting Tool Inserts
Chip
Rem
oval
Fro
m W
orkp
iece
(g
m/m
)
1
10
100
1000
Wei
ght L
oss
of th
e In
sert
(µg/
m)
Chip Removal (gm/m) Weight Loss (µg/m)
Figure 23: Chip removal from work piece and weight loss of insert after machining
operation on a bar of mild steel
With increasing sintering temperature the (i.e. with increasing hardness) the amount
of chip removal increases. For sintering temperatures of 1450°C and 1475°C the
amount of chip removal is very close to each other and to that of a carbide cutting tool
insert. For sintering temperature of 1400°C severe flank wear occurred (Table 14) due
to low hardness, so it could not retain its sharpness and remove chips from the
workpiece.
Machining performance at 70 rpm was also preformed but it did not give any
encouraging result.
According to Table 14, all the cutting tool inserts showed some amount of flank wear
and also small amount of crater wear. There was very little build-up edge on the
inserts.
36
Table 14: SEM micrograph of insert tips before & after machining operation and their mode of failures.
Before After Condition & Comment Condition: Sintered at 1400°C machining at 160 rpm Comment: Severe flank Wear
Condition: Sintered at 1450°C machining at 160 rpm Comment: Flank Wear & slight crater wear
37
Condition: Sintered at 1475°C machining at 160 rpm Comment: Flank Wear & slight crater wear
38
4.5 Microstructure
SEM micrograph of a chemically etched sample is show in Figure 21.
Figure 24: SEM micrograph of ZTA cutting tool insert sintered at 1475°C and etched
in 85% phosphoric acid of 250°C for 3 minutes
In this micro structure the lighter phases are zirconia grains while the darker phases
are alumina grains. We can see most of the zirconia grains are of small size (862 nm)
and few of the larger particles are around 3.5µm (approximately 5 times the smaller
particles). Alumina particles are not also uniformly sized. A moderate amount of
porosity can also be seen.
39
5 Conclusions Considering hardness, chip removal and weight loss ZTA sintered at 1475°C shows a
promising possibility of serving as a cutting tool insert. Form the research we can
draw the flowing conclusions -
• Sintering for 4 hours gives a better sintered product.
• 45% moisture in the slip gives large blowholes inside the products but 40%
moisture in the slip eliminates that.
• Of the three sintering temperatures 1400°C, 1450°C and 1475°C, 1475°C
gives the highest hardness.
• Among those three sintering temperatures 1475°C give the best cutting
performance. Sintering temperature 1450°C shows a cutting performance very
close to that.
• Considering chips removal from work piece ZTA cutting inserts matched the
cutting performance of carbide inserts although weight loss of the inserts after
machining operation was higher than that of the carbide ones.
40
6 Scope of Future Works Slip preparation
Rheology of the slip is an important factor of slip casting. Some means of controlling
the fluidity of the slip with out having to add too much water can be employed. This
would give a less porous, hence denser product.
Mold preparation
Tool geometry plays an important role in cutting performance. Special mold can be
prepared for specific tool geometry. A well designed mold can help prevent premature
consolidation.
Casting process
Cutting tool inserts are small in dimension. Preparing such a small object by slip
casting method has a disadvantage of premature consolidation if simply solid casting
is employed. Pressure casting with appreciable amount of pressure can be used to
make a sounder, denser product.
Sintering temperature
Sintering temperature can be optimized to achieve the highest possible hardness, wear
resistance for a given composition.
Machining performance and microstructure
Further study in machining performance can be done with different operating
conditions on different materials. Also further study on attaining microstructure needs
to be done.
41
7. Bibliography 1. Avner, Sydney H: “Introduction to Physical Metallurgy”, Second Edition, Tata
Mcgraw-Hill Publishing Company Limited, New Delhi, 1997
2. Freitag, D.W. and D.W. Richerson: “Opportunities for Advanced Ceramics to
Meet the Needs of the Industries of the Future”, Office of Industrial
Technologies, U.S. Department of Energy, Washington, D.C., 1998
3. Biest, O. Van der and J. Vleugels, “Perspectives on the Development of
Ceramic Composites for Cutting Tool Applications”, Key Engineering
Materials, Vols. 206-213, Trans Tech Publications, Switzerland, pp. 955-960,
2002
4. Bengisu, Murat: “Engineering Ceramics”, Springer-Verlag, Berlin, 2001
5. Kingery, W.D, H. K. Bowen, and D. R. Uhlmann, “Introduction to Ceramis”,
John Wiley & Sons (Asia) Pte. Ltd., Singapore, 2004
6. Richerson, David W.: “Modern Ceramics Engineering”, Second Edition,
Marcel Dekker, Inc., New york, 1992
7. King, Alan G. and W. M. Wheildon: “Ceramics in Machining Process”,
Academic Press Inc., New york, 1966
8. Goold, V. C., Cryll Donaldson, and George H. LeCain: “Tool Design”,
McGraw-Hill, Inc., 1983
9. Encyclopædia Britannica 2007 Ultimate Reference Suite, Encyclopædia
Britannica, Inc.
10. Wikipedia, the free encyclopedia, http://en.wikipedia.org
11. The A to Z of Materials Online, http://www.azom.com
12. Engineering Server of Grand Valley State University,
http://claymore.engineer.gvsu.edu
13. Desktop CNC, http://www.desktopcnc.com
14. American National Carbide, http://www.anconline.com
15. Tools & Machines, http://machine-tools.netfirms.com
16. Blackwell Synergy; http://www.blackwell-synergy.com
17. INIST-CNRS Website; http://cat.inist.fr
18. SpringerLink; http://www.springerlink.com
19. Inframat® Advanced MaterialsTM, http://www.advancedmaterials.us
42
Appendix Calculations of Machining Performance
Table for calculation of periferial distance traveled
Sintering Temperature
(°C)
RPM,
R
Length of
Cylinder, L (mm)
Time
Elapsed, t
(sec)
Feed Rate, L/t
(mm/sec)
Revolutions, n = R*(t/60)
Initial
Dia, d1 (mm)
Final
Dia, d2 (mm)
Peripherial Distance,
1000D 2
)21( ndd ××=
+π
(m)
Distance Traveled, d = ΣD
(m)
160 75 60 1.25 160.000 25.25 24.92 12.609 160 75 60 1.25 160.000 24.92 24.6 12.446 160 75 65 1.153846154 173.333 24.6 24.08 13.254
1475 160 75 63 1.19047619 168.000 24.08 23.67 12.601
50.910
160 75 60 1.25 160.000 23.67 23.5 11.855 160 75 63 1.19047619 168.000 23.5 23.28 12.345 160 75 65 1.153846154 173.333 23.28 22.85 12.560
1450 160 75 63 1.19047619 168.000 22.85 22.42 11.946
48.706
160 75 63 1.19047619 168.000 22.42 22.3 11.801 160 75 65 1.153846154 173.333 22.3 22.28 12.138 160 75 62 1.209677419 165.333 22.28 22.09 11.523
1400 160 75 60 1.25 160.000 22.09 22.09 11.104
46.566
160 75 63 1.19047619 168.000 21.55 21.25 11.295 160 75 65 1.153846154 173.333 21.25 20.9 11.476 160 75 61 1.229508197 162.667 20.9 20.5 10.578
Carbide
160 75 62 1.209677419 165.333 20.5 20.23 10.578
43.927
43
Table for calculation of weight loss and chip removal
Sintering Temperature
(°C)
RPM, R
Distance Traveled, d
(m)
Initial Weight, w1
(gm)
Final Weight, w2
(gm)
Weight Loss Per
Unit Distance Traveled,
61021×
−=
dwwwloss
(µg/m)
Chip Removal From Work
Piece, w
(gm)
Chip Removal Per Unit Distance Traveled,
dwwremoval =
(gm/m) 1400 70 53.910 2.5917 2.5845 133.5567221 - - 1450 70 13.666 2.1104 2.0202 6600.084878 - - 1475 70 55.583 2.7678 2.7521 282.4611907 - -
1400 160 46.566 2.3569 2.3199 794.5721198 2.935 0.063029326 1450 160 48.706 2.1968 2.1944 49.27482371 26.665 0.547464849 1475 160 50.910 2.2773 2.2753 39.28506846 30.538 0.59984371
Carbide 160 43.927 - - - 26.675 0.607256846
