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CHAPTER ONE
Introduction
CONTEXT
Sintering is a thermal process used to bond contacting particles into a solid
object. Students in school learn to work wet clay to shape a pot, and then heat or
fire that body to create a strong pot. That firing process is sintering. In the same
manner, newly fallen snow bonds, to harden and eventually form ice�this is a colder
version of sintering. For industrial components, sintering is a means to strengthen
shaped particles to form useful objects such as electronic capacitors, automotive trans-
mission gears, metal cutting tools, watch cases, heart pacemaker housings, and oil-less
bearings.
As a thermal treatment, sintering is crucial to the success of several engineering
products; including most ceramics and cemented carbides, several metals, and some
polymers. Powder shaping prior to sintering is done by die compaction for simpler
shapes, such as automotive transmission gears. For complicated three-dimensional
shapes, such as watch cases, injection molding is the favored shaping process. Long,
thin objects, such as catalytic converter substrates, are shaped by extrusion through a
die, in the same manner as graphite is extruded to form refills for mechanical pencils.
There are technologies for shaping flat structures such as ceramic electronic substrates
(tape casting), hollow bodies such as porcelain statuary (slip casting), and one of a
kind metal prototypes (laser forming). Following each of these forming steps is a sin-
tering treatment�defined by a heating cycle to a peak temperature. The hold time at
the sintering temperature ranges from a few minutes to a few hours. Although the
shaped body is weak prior to sintering, after the firing cycle it is very strong, compet-
itive in properties with that attained via other manufacturing routes such as casting,
machining, grinding, or forging.
This book addresses sintering by describing why it occurs, how it is measured, and
the key control parameters. Property changes during sintering are outlined.
The prediction of optimal cycles to generate desired properties is a key goal for sinter-
ing science. In this book, we see how sintering theory evolved from its empirical
base�going to the lab to “see what happens.” New materials and detailed phenomeno-
logical observations emerged long before atomic structure conceptualizations.
Predicative sintering theory awaited an understanding of atoms and atomic motion.
1
Sintering: From Empirical Observations to Scientific Principles
DOI: http://dx.doi.org/10.1016/B978-0-12-401682-8.00001-X
© 2014 Elsevier Inc.
All rights reserved.
http://dx.doi.org/10.1016/B978-0-12-401682-8.00001-X
Diffusion concepts engaged leading scientist in the 1940s. Once atomic motion was
understood, the platform was in place to allow rapid progress on sintering theory.
Accordingly, a burst of applications, literature, patents, and materials emerged and grew
from the late 1940s through the 1960s. The expansion continues today to accommo-
date the increased composition complexity and more complex designs.
This book details the developments that converged to give today’s sintering theory.
Optimism abounds in the sintering community as we continue to push forward with
improved understanding of a complicated process.
PERSPECTIVES
Archeological findings date sintered objects back 26,000 years. Early fired earth-
enware structures are found in China, India, Egypt, Japan, Turkey, Korea, Central
America, and Southern Europe. About 3000 years ago firing to improve strength was
practiced in many locations. By a few hundred years ago sintered products were man-
ufactured under controlled conditions in Spain, China, Korea, Japan, Germany,
England, and Russia.
English geologists used the term “sintering” in 1780 to describe the bonding of
mineral particles and the formation of crusted stones in Iceland. This was in reference
to the way silicates formed hardened crusts around hot geyser vents. The English bor-
rowed the term “cinder” from German to describe the agglomeration or hardening of
mineral particles. By 1854 the concept was used to describe the thermal bonding of
coal particles and in the 1860s to describe the thermal hardening of iron ore, a process
also known as induration.
The United States patent literature shows the first use of the term “sintering” in
1865 with respect to thermal cycles applied to mineral calcination. The agglomeration
of flue dust, iron ores, and other minerals were early sintered products. Subsequently
the term sintering was widely used to describe agglomeration with an emphasis on
sinter plants for iron ore agglomeration.
By the 1880s the term “sintering” was applied to describe gold and silver purifica-
tion, platinum bonding, iron powder consolidation, and the fabrication of platinum
jewelry. In 1913 Coolidge refers to his heating process to form tungsten lamp fila-
ments as involving “. . . filaments are still further treated to free then from all easily
vaporizable components and to sinter together the refractory residue into a coherent
conductor . . ..” [1] In ceramics the term sintering was reserved for describing the
agglomeration of refractory, abrasive, or insulator powders. However, in 1939 White
and Shremp [2] used “sintering” to describe ceramic particle bonding with reference
2 Sintering: From Empirical Observations to Scientific Principles
to properties of beryllia heated under different conditions. By World War II the
importance of sintering jumped due to its military applications. In 1943, the US
Library of Congress published a survey of the field, citing 700 publications and 600
patents [3]. After that time “sintering” was commonly used to describe thermally
induced particle bonding [4�7].
Sintering is a thermal treatment to bond particles, leading to improved strength.
This is evident in microscopic images, such as that shown in Figure 1.1. These spheri-
cal particles were initially poured into a crucible. During heating, bonds grew at the
particle contacts. This occurred through atomic motion.
To explain the changes induced by heat, sintering theory emerged to provide a
mathematical collection of key parameters such as particle size, heating rate, hold
temperature, and hold time. The material is also important since it determines the
surface energy, atomic size, activation energy for diffusion, and crystal structure.
Consequently, several parameters enter into sintering models, so much background
knowledge needed to be developed as a foundation for the models. For example,
although liquid surface energy has long been an accepted concept, solid-vapor surface
energy only was accepted in the 1940s. Solid surface energy is a necessary concept to
explain the stress acting at particle contacts to produce sintering shrinkage.
Thus, the approach used here is as follows:
Assemble an outline of sintering concepts and models
- trace back to find the important building blocks
- determine how the building block concepts intersected with sintering
- isolate early critical events via first publications
- identify pivotal people and concepts.
Figure 1.1 Scanning electron micrograph of bronze sphere sintering.
3Introduction
This backward tracing is built from many prior assessments [7�43]. These reports
were complimented by patent searches and on-line databases. Conflicts were vetted to
correct errors in spellings, references, years, and incorrect citations. An example was
some early work on spark sintering, which was attributed to the patent agent Arthur
Bloxam instead of the inventor Johann Lux.
A master spreadsheet was created to understand the evolution of sintering concepts
and the enabling infrastructure. It identified critical steps and individuals. Sometimes
the priority was unclear. For example, silicon nitride was formed in 1896, reaction
bonding was developed in the 1930s, hot pressing was developed in the 1960s, and
pressureless liquid phase sintering emerged inthe 1970s [31,44,45]. Since the first
densification was by hot pressing, this date was used to tag the emergence of sintered
silicon nitride.
By early 2013 over one million articles were indexed under the terms “sinter” and
“sintering.” This literature is the basis for this book, condensing the sintering concept
from a large, constantly expanding body of knowledge.
DEFINITIONS
Sintering as a term arose in the 1800s and became more common in the middle
1900s. Although variants exist, the following definition captures both the historical
and modern usage [19,29,46�51]:
Sintering is a thermal treatment for bonding particles into a coherent, predomi-
nantly solid structure via mass transport events that often occur on the atomic scale.
The bonding leads to improved strength and lower system energy.
A few other terms are important to understanding the overt impact of sintering.
Density is the mass per unit volume so it has units of g/cm3 or kg/m3. Density
depends on the material and changes during most sintering treatments, so it is a com-
mon measure of the degree of sintering. Theoretical density corresponds to the pore-
free solid density. Fractional or percentage density is useful for comparing the behavior
of powder systems without the confusion over differing theoretical densities. For this
book the preferred expression for sintered density will be fractional or percentage den-
sity based on the ratio of the measured density to the theoretical density. Green density
is the density prior to sintering and green strength corresponds to the strength prior to
sintering.
Porosity is the unfilled space in a powder compact. Prior to sintering it is called
the green porosity. Since there is no mass associated with porosity, it is simply treated
as a fraction or percentage of the body. Thus, a sintered component that is 80% dense
4 Sintering: From Empirical Observations to Scientific Principles
has 20% porosity. The fractional density and fractional porosity sum to unity. In cases
involving liquid phases during sintering, three phases are present�solid, liquid, and
porosity. The fractional density in those cases is the sum of the solid and liquid
portions.
Particles are discrete solids, generally smaller than 1 mm in size, but larger than an
atom. Powders are collections of particles, usually with a range of sizes and shapes.
Powders do not fill space efficiently. For example, monosized spheres pour to fill a
container at approximately 60% density. After vibration these same spheres will reach
a maximum packing density of 64%. Higher densities come from changes to the parti-
cle shape, particle size distribution, or by the application of pressure.
Green bodies prior to sintering are termed compacts. They are usually prepared by
mixing a binder or lubricant with the powder (wax-type molecule) and applying pres-
sure to the powder to increase density and shape the powder. The pressure ranges
from gravity to thousands of atmospheres of applied pressure. A green compact is usu-
ally weak; vitamin pills are examples of pressed powders. Hot consolidation relies on
the application of pressure during sintering.
Several monitors for sintering appear in this book�surface area, neck size ratio,
shrinkage, swelling, and densification. Surface area is the solid-vapor area and is usu-
ally captured in terms of area per unit mass, such as m2/g. Neck size is the diameter
of the sinter bond between two particles, and the neck size ratio is the ratio of the
neck size divided by the particle size (dimensionless). Shrinkage refers to the decrease
in linear dimensions, while swelling refers to an increase in dimensions. They are both
linear dimensional changes, where the change is size is divided by the size prior to
sintering. Measures such as density and shrinkage are easy to perform, and provide
insight into the changes during sintering. Densification is the change in porosity with
sintering divided by the starting porosity. If all pore space is eliminated during sinter-
ing, then the densification is 100%. It is a useful concept when comparing systems of
differing theoretical densities or initial porosities. Densification, final density, neck
size, surface area, and shrinkage are related measures of sintering.
Mixed powders of differing compositions are a common basis for sintering. The
convention is to list the major component first. Thus, the term WC-Co implies that
the bulk of the material is composed of tungsten carbide (WC), with cobalt (Co)
being the minor component. When a number is embedded in the formula, such as
Fe-8Ni, this designates 8 wt.% nickel powder has been added to iron powder.
Composition is given on a mass basis unless explicitly stated otherwise. The atomic
composition is given by a chemical formula showing the stoichiometry�for example
MoSi2 indicates two silicon atoms for each molybdenum atom. When the powder is
pre-compounded, it is designated by the common name where possible, such as stain-
less steel (Fe-18Cr-8Ni), bronze (Cu-10Sn), or spinel (Al2O3-MgO).
5Introduction
SINTERING TECHNIQUES
Sintering theory is most accurate for the case of single phase powders sintered
by solid-state diffusion. Unfortunately, this is a small portion of sintering practice.
More common are sintering techniques involving multiple phases and liquids.
Figure 1.2 organizes the sintering techniques into general categorizations. Pressure is
the first differentiation. Most industrial sintering is performed without an external
pressure. Pressure-assisted sintering techniques include hot isostatic pressing, hot press-
ing, and spark sintering. These produce high fractional densities by applying tempera-
ture and pressure simultaneously. Pressures range from 0.1 MPa up to 6 GPa.
For pressureless sintering, one major distinction is between solid-state and liquid
phase processes. Single phase, solid-state sintering is the best understood form of sin-
tering. Among the solid-state processes, there are options involving mixed phases,
such as those to form composites or alloys. Compact homogenization occurs when
sintering mixed powders that are soluble in each other and produce an alloy.
Activated sintering is a special treatment involving small quantities of insoluble species
that segregate to the grain boundaries to accelerate sintering. Mixed phase sintering is
often employed to form composites, where one phase is dispersed in a matrix phase.
Another variant occurs when a material is intentionally sintered in a two phase field,
such as when steel is sintered at a temperature where both body-centered cubic and
face-centered cubic phases coexist.
Pressureless Pressure-assisted
Sintering processes
Liquide phaseSolid-state
Mixed
phase
Single
phase
Transient
liquid
Persistent
liquid
Low stress High stress
Creep
flow
Viscous
flow
Plastic
flow
Mixed
phase
Supsersolidus
Viscous
phase
ReactiveComposites
Activated
Homogenization
Solid
solution
Figure 1.2 The taxonomy of sintering, showing process differentiation by various branches, start-
ing with the application of pressure-assisted versus pressureless sintering.
6 Sintering: From Empirical Observations to Scientific Principles
Commonly, sintering involves a liquid phase that improves the sintering rate.
Most industrial sintering involves forming a liquid phase, accounting for nearly 90%
of the value of all sintered products. The two forms involve persistent or transient
liquids. Persistent liquid phases exist throughout the high temperature portion of the
sintering cycle and can be formed using prealloyed powder (supersolidus liquid phase
sintering) or from a mixture of powders. Transient liquid phase sintering produces a
liquid during heating, but that liquid subsequently dissolves into the solid. In some
cases an exothermic heat release occurs, leading to reactive liquid phase sintering
when a compound forms.
Although the roadmap shown in Figure 1.2 is schematic,it helps to tie the various
chapters together into an overall technological landscape.
KNOWLEDGE
More than a million publications exist on sintering. Of those, almost half are
conference proceedings and the other half are a mixture of archival publications and
patents. Figure 1.3 plots the cumulative number of archival articles dealing with sin-
tering from 1900 to 2013. To appreciate the acceleration in knowledge, Figure 1.4
plots the data on a log-log basis. By 1900 there were 135 publications on sintering,
500000
Cumulative publications
400000
300000
100000
0
200000
1900 1920 19601940 1980 2000 2020
Year
Figure 1.3 Cumulative number of archival journal publications on sintering, showing a surge in
recent years. Almost as many conference publications exist, giving over one million total publica-
tions by 2012.
7Introduction
many of which concerned iron ore hardening. By 2013 the total was 600,000 publica-
tions. Regression analysis gives the following relation:
log10 publication sumð Þ52 4611 141 log10ðyearÞ (1.1)
This fit is highly significant with a correlation coefficient of 0.9979.
Quite possibly the accelerating knowledge generation will continue, as more mate-
rials, applications, and techniques emerge. Five nations lead the publication
activity�China, Japan, USA, Korea, and Germany, in that order, followed by India,
France, United Kingdom, Taiwan, and Spain. In terms of citations, sintering papers
from the USA, UK, and Germany are the most frequently cited. With regard to the
material treated, the highest impact papers deal with compositions based on alumina,
iron, copper, and tungsten. These are followed by cemented carbides (WC-Co), plati-
num, silica, glass, aluminum, silver, and silicon carbide.
Generally, scientific developments lead patent activity. The first US patent
to mention sintering was granted to MacFarlane of Canada in 1865 [52].
Subsequently the US patent literature grew to a position where about seven patents
were issued each day in 2012. To appreciate the rise in activity, Figure 1.5 plots the
cumulative US patent history, passing 100,000 issued patents in 2011. The rate of
patent activity remains high and this indicates much future commercial activity
involving sintering.
Log(year)
3.28
2
3
4
5
6
Log(sum)
3.29 3.30 3.31
Figure 1.4 A log-log plot of the cumulative sum of the journal publications versus the publication
year, producing a very significant regression line.
8 Sintering: From Empirical Observations to Scientific Principles
KEY RESOURCES
Information on sintering and sintered products is available in a host of journals.
A few journals are very popular, including ceramic and powder metallurgy journals
such as these:
Acta Materialia (Acta Metallurgica, Acta Metalluirgica et Materialia)
Ceramic Bulletin (Bulletin of the American Ceramic Society)
Ceramics International
International Journal of Powder Metallurgy
International Journal of Refractory Metals and Hard Materials (formerly
Planseeberichte fuer Pulvermetallurgie)
Journal of Applied Physics
Journal of the Korean Powder Metallurgy Institute
Journal of Materials Research
Journal of Materials Science
Journal of the American Ceramic Society
Journal of the Ceramic Society of Japan
Journal of the European Ceramic Society
Journal of the Japan Society of Powder and Powder Metallurgy
Materials Science and Engineering
0
20
40
60
80
100
Cumulative years
Cumulative percent patents
0 20 40 60 80 100 120 140 160
Figure 1.5 Cumulative US patent issuance versus years since the first patent to mention sintering
in 1865.
9Introduction
Materials Transactions
Metallurgical and Materials Transactions (formerly Metallurgical Transactions,
Transactions TMS-AIME)
Powder Metallurgy
Powder Technology
Science of Sintering (formerly Physics of Sintering)
Conferences with high sintering content include the following:
International Conference on Sintering�held every four years in South Bend, Indiana;
Vancouver, British Columbia; Tokyo, Japan; State College, Pennsylvania;
Grenoble, France; JeJu, Korea; Dresden, Germany.
Materials Science and Technology Conference�organized by several societies and, held every
fall in cities such as Columbus, Ohio; Cincinnati, Ohio; Pittsburgh, Pennsylvania.
World Congress on Powder Metallurgy�held every two years, rotating between
Europe, North America, and Asia; recent meetings were held in Yokohama, Japan;
Florence, Italy; Washington, DC; Vienna, Austria; Busan, Korea; Orlando,
Florida; Grenada, Spain; Kyoto, Japan.
Plansee Seminar�held every four years in Reutte, Austria�premier conference on
sintered hard materials, refractory metals, particulate composites, and high temper-
ature systems.
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11Introduction
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12 Sintering: From Empirical Observations to Scientific Principles
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	One Introduction
	Context
	Perspectives
	Definitions
	Sintering Techniques
	Knowledge
	Key Resources
	References