Sintering: From Empirical Observations to Scientific Principles

By Randall German

As sintering applications march toward a $30 billion global business, the models for sintering have progressed, but generally follow behind observation. Documentation of the steps needed to build to a quantitative and predictive theory are often missed. Sintering: From Empirical Observations to Scientific Principles partitions sintering applications and observations to show critical turning points required to establish modern sintering as a predictive science.

This book, written by the most cited author in his field, is laced with people, organizations, critical steps, and important formulations in a mixture of history, personalities, and applications. Exploring how insights in seemingly unrelated fields sparked progress, it is also a teaching tool to show where there is success, where there are problems, and how to organize teams to leapfrog to new applications or plateaus of use. Randall German's Sintering: From Empirical Observations to Scientific Principles is a platform for directly addressing the critical control parameters in these new research and development efforts.

• Shows how the theories and understanding of sintering were developed and improved over time, and how different products were developed, ultimately leading to important knowledge and lessons for solving real sintering problems
• Covers all the necessary infrastructure of sintering theory and practice, such as atomic theory, surface energy, microstructure, and measurement and observation tools
• Introduces the history and development of such early sintered products as porcelain, tungsten lamp filaments, bronze bearings, steel automotive components, platinum crucibles and more

• Language: English
• Publisher: Elsevier Science
• Release date: Feb 7, 2014
• ISBN: 9780124016774

Randall German

Professor German obtained his PhD from the University of California at Davis (1975), He is a Fellow of the American Society for Metals and Fellow of American Powder Metallurgy Institute. His awards include the Tesla Medal, Nanyang Professorship, Japan Institute for Materials Research Lectureship, Penn State Engineering Society Outstanding Research Award and Premiere Research Award, Distinguished Research Award from the Japan Society for Powder Metallurgy, Kuczynski Prize, and Samsonov Prize. He is listed in several Who's Who and serves as an editor or key reader for more than 20 journals and held several director positions, including two terms with APMI, and served on the Fellows Awards Committee of two professional societies. He has supervised 100 theses, published over 960 articles, 25 patents, and 16 books, including Mathematical Relations in Particulate Materials Processing (2008), Powder Metallurgy and Particulate Materials Processing (2005), Liquid Phase Sintering (1985), Sintering Theory and Practice (1996), and Powder Injection Molding - Design and Applications (2003). He has edited 19 books and co-chaired more than 30 conferences. Professor German's research and teaching deal with the net-shape fabrication of engineering materials via sintering techniques as used in powder metallurgy, cemented carbides, and ceramics.

Book preview

Bose

Preface

Randall M. German

Exciting new designs are enabled by the sintering process. Although practiced for over 28,000 years, recent discoveries are moving sintering into some bold new applications. New energy systems, ranging from solar cells to nuclear reactors, are critically contingent on sintered structures. Another example is the fabrication of porous tissue scaffolds for biomedical implants, in which the device is custom laser sintered to match strength and elastic modulus of the patient. In the same manner, dental crown and bridge constructions are produced overnight using additive computer driven sintering routes. An enormous effort is pushing forward the sintering of thin printed electronic structures, such as small radio frequency identification circuits to be embedded in consumer products, allowing information transfer when activated by near-field cellular telephones. Related efforts are taking place in replacement interconnections for solar cells and a host of capacitor, energy storage, and magnet devices. The field of superabrasives is producing sintered diamond bonded onto sintered cemented carbides substrates to make long-lasting oil and gas drilling tools. Another growth area is that of sintered thermoelectric junctions to convert waste heat into electricity, including waste heat from automobile engines.

This modern era of sintering traces to the early 1800s when the first platinum crucibles were made for melting glass. Significant progress came in the early 1900s with the production of incandescent lamp filaments, but theoretical explanations awaited the development of atomic theory and the atomic motion concepts that emerged in the 1940s. Once atomic theory was melded with sintering observations, quantitative conceptualizations arose. In turn, that effort matured to produce computer simulations. Now those simulations are approaching the accuracy that is demanded by manufacturing. Soon extraterrestrial sintering will use lunar soils and solar energy to construct buildings on the moon. The prospects for expanded applications are outstanding.

This book explains the basics of sintering. The concepts are equally applicable to the fabrication of electronic capacitors, automotive transmission gears, high intensity lights, jet engine control linkages, or high speed end mills. The approach starts with historical concepts, mixes history and science, and outlines the theoretical evolution. The scientific underpinnings arose from simple questions that form the key points covered by this book:

What is sintering?

How do we observe sintering?

What are some of the key parameters?

How can we improve sintering?

Where did sintering theory come from?

Where do we stand on modeling sintering?

Included are chapters on emerging topics, such as the role of rapid heating, and introductions to sintering tools.

The effort of writing this book started in the preparation of a Plenary Seminar for Sintering 2011 and a Keynote Lecture for the 2012 Materials Science and Technology Conference. My thanks go to Suk-Joong Kang, Eugene Olevsky, and Khalid Morsi for their early support. Several students helped, notably Wei Li, Timothy Young, Michael Brooks, and Shuang Qiao. Kenneth Brookes provided background information on sintered carbides, while Zak Fang, Animesh Bose, and Donald Heaney organized relevant reviews. Louis Rector and Howard Glicksman provided information on sintered electronic applications. Lanny Pease donated a missing book to provide insights. Other missing information was obtained from the Metal Powder Industries Federation and American Ceramic Society. I am thankful to a host of other individuals for their efforts and kind words, and to San Diego State University for giving semesters without teaching to complete this project.

This book is dedicated to Animesh Bose; he is a testimonial to what can be done by applying sintering. And the scary thing is his children are smarter and more motivated.

Chapter One

Introduction

An overview of sintering is presented, with a first introduction to the history of firing powders to increase bonding strength. The term sintering is related to its early use in English. A definition is given for sintering and related green body properties. An outline is presented of how the book was created and organized, noting the construction of modern sintering concepts as related to the first recognition of critical underpinnings such as surface energy and atomic diffusion. The emergence of sintering theory is traced to the 1940s, but sintering practice started 26,000 years earlier. Thus, a pragmatic trial and error approach dominates sintering even today. Some of the exciting new sintering applications and challenges are noted. The explosive growth in sintering knowledge is documented in terms of publications and patents.

Keywords

sintering; definition; literature; history; induration; iron ore; mineral; geology; particles; bonding; densification; shrinkage; publications; knowledge

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 transmission 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 sintering 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, competitive 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 sintering 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 phenomenological observations emerged long before atomic structure conceptualizations. Predicative sintering theory awaited an understanding of atoms and atomic motion. 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 accommodate 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 earthenware 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 manufactured 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 borrowed 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 purification, platinum bonding, iron powder consolidation, and the fabrication of platinum jewelry. In 1913 Coolidge refers to his heating process to form tungsten lamp filaments 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 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 spherical particles were initially poured into a crucible. During heating, bonds grew at the particle contacts. This occurred through atomic motion.

Figure 1.1 Scanning electron micrograph of bronze sphere sintering.

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.

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 in the 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, predominantly 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 common 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 density 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 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 particle 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 pressure 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 usually 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 usually 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 sintering, 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 stainless steel (Fe-18Cr-8Ni), bronze (Cu-10Sn), or spinel (Al2O3-MgO).

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 pressing, and spark sintering. These produce high fractional densities by applying temperature and pressure simultaneously. Pressures range from 0.1 MPa up to 6 GPa.

Figure 1.2 The taxonomy of sintering, showing process differentiation by various branches, starting with the application of pressure-assisted versus pressureless sintering.

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 sintering. 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.

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 sintering 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, many of which concerned iron ore hardening. By 2013 the total was 600,000 publications. Regression analysis gives the following relation:

(1.1)

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 publications by 2012.

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.

This fit is highly significant with a correlation coefficient of 0.9979.

Quite possibly the accelerating knowledge generation will continue, as more materials, 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), platinum, 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.

Figure 1.5 Cumulative US patent issuance versus years since the first patent to mention sintering in 1865.

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

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 temperature systems.

References

1. W.D. Coolidge, Production of Refractory Conductors; U. S. Patent 1,077,674, issued 5 November 1913.

2. White HE, Shremp RM. Beryllium oxide: I. J Am Ceram Soc. 1939;22:185–189.

3. Goetzel CG. Treatise on Powder Metallurgy. vol. III New York, NY: Interscience Publishers; 1952.

4. Klugh BF. The microstructure of sintered iron bearing materials. Trans TMS-AIME. 1913;45:330–345.

5. Vogel FA. Sintering and briquetting of flue dust. Trans TMS-AIME. 1912;43:381–386.

6. Gayley J. The sintering of fine iron bearing material. Trans TMS-AIME. 1912;42:180–190.

7. Burke JE. A history of the development of a science of sintering. In: Columbus, OH: Amer. Ceramic Society; 1985;315–332. Kingery WD, ed. Ceramics and Civilization, Ancient Technology to Modern Science. vol. 1.

8. Jones WD. Principles of Powder Metallurgy with an Account of Industrial Practice London, UK: Edward Arnold; 1937.

9. Ferguson EG. Bergman, Klaproth, Vauquelin, Wollaston. J Chem Edu. 1941;18:3–7.

10. Smith CS. The early development of powder metallurgy. In: Wulff J, ed. Powder Metallurgy. Cleveland, OH: Amer. Society for Metals; 1942;4–17.

11. Wretblad PE, Wulff J. Sintering. In: Wulff J, ed. Powder Metallurgy. Cleveland, OH: Amer. Society for Metals; 1942;36–59.

12. Huttig GF. Die Frittungsvorange innerhalb von Pulvern, weiche aus einer einzigen Komponente bestehen–Ein Beitrag zur Aufklarung der Prozesse der Metall-Kermik und Oxyd-Keramik. Kolloid Z. 1942;98:6–33.

13. Goetzel CG. Treatise on Powder Metallurgy. vol. I New York, NY: Interscience Publishers; 1949; pp. 259–312.

14. Jones WD. Fundamental Principles of Powder Metallurgy London, UK: Edward Arnold Publishers; 1960.

15. Plotkin SY. Development of powder metallurgy in the USSR during 50 years of soviet rule. Powder Metall Metal Ceram. 1967;6:844–853.

16. Rhines FN, DeHoff RT, Rummel RA. Rate of densification in the sintering of uncompacted metal powders. In: Knepper WA, ed. Agglomeration. New York, NY: Interscience; 1962;351–369.

17. Ivensen VA. Densification of Metal Powders during Sintering New York: Consultants Bureau; 1973.

18. Plotkin SY, Fridman GL. History of powder metallurgy and its literature. Powder Metall Metal Ceram. 1974;13:1026–1029.

19. Ristic MM. Science of Sintering and Its Future Beograd, Yugoslavia: International Team for Science of Sintering; 1975.

20. C.G. Johnson, W.R. Weeks, Powder metallurgy, J.G. Anderson, Metallurgy, (revision), fifth ed., Amer. Technical Publishers, Homewood, IL, 1977, pp. 329–346.

21. Exner HE. Physical and chemical nature of cemented carbides. Inter Met Rev. 1979;24:149–173.

22. Lenel FV. Powder Metallurgy Principles and Applications Princeton, NJ: Metal Powder Industries Federation; 1980.

23. Handwerker CA, Blendell JE, Coble RL. Sintering of ceramics. In: Uskokovic DP, Palmour H, Spriggs RM, eds. Science of Sintering. New York, NY: Plenum Press; 1980;3–37.

24. Prince A, Jones J. Tungsten and high density alloys. Historical Metall. 1985;19:72–84.

25. Kingery WD. Sintering from prehistoric times to the present. In: Chaklader ACD, Lund JA, eds. Sintering ’91. Brookfield, VT: Trans. Tech. Publications; 1992;1–10.

26. Kolaska H. The dawn of the hardmetal age. Powder Metall Inter. 1992;24(5):311–314.

27. Brookes KJA. Half a century of hardmetals. Metal Powder Rept. 1995;50(12):22–28.

28. Ristic MM. Frenkel’s theory of sintering (1945–1995). Sci Sintering. 1996;28:1–4.

29. German RM. Sintering Theory and Practice New York, NY: Wiley-Interscience; 1996.

30. Haertling GH. Ferroelectric ceramics: history and technology. J Amer Ceram Soc. 1999;82:797–818.

31. Riley FL. Silicon nitride and related materials. J Amer Ceram Soc. 2000;83:245–265.

32. Konstanty J. Powder Metallurgy Diamond Tools Amsterdam, Netherlands: Elsevier; 2005.

33. Peret CM, Gregolin JA, Faria LIL, Pandolfelli VC. Patent generation and the technological development of refractories and steelmaking. Refractories Applic News. 2007;12(1):10–14.

34. Noguez M, Garcia R, Salas G, Robert T, Ramirez J. About the Pre-Hispanic Au-Pt ‘Sintering’ technique. Inter J Powder Metall. 2007;43(1):27–33.

35. Kang SJL. Sintering Densification, Grain Growth, and Microstructure Oxford, United Kingdom: Elsevier Butterworth-Heinemann; 2005.

36. Johnson PK. Tungsten filaments–the first modern PM product. Inter J Powder Metall. 2008;44(4):43–48.

37. German RM, Suri P, Park SJ. Review: liquid phase sintering. J Mater Sci. 2009;44:1–39.

38. Schade P. 100 years of doped tungsten wire. In: Reutte, Austria: Plansee Group; 2009;RM49.1–RM49.12. Rodhammer P, ed. Proceedings of the Seventeenth Plansee Seminar. vol. 1.

39. German RM. Coarsening in sintering: grain shape distribution, grain size distribution, and grain growth kinetics in solid-pore systems. Crit Rev Solid State Mater Sci. 2010;35:263–305.

40. Garay JF. Current activated, pressure assisted densification of materials. Ann Rev Mater Res. 2010;40:445–468.

41. Morsi K. The diversity of combustion synthesis processing: a review. J Mater Sci. 2012;47:68–92.

42. Munir ZA, Quach DV, Ohyanagi M. Electric current activation of sintering: a review of the pulsed electric current sintering process. J Am Ceram Soc. 2011;94:1–19.

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44. Deeley GG, Herbert JM, Moore NC. Dense silicon nitride. Powder Metall. 1961;4:145–151.

45. Terwilliger GR, Lange FF. Pressureless Sintering of Si3N4. J Mater Sci. 1975;10:1169–1174.

46. Walker RF. Mechanism of material transport during sintering. J Amer Ceram Soc. 1955;38:187–197.

47. Hausner HH. Discussion on the definition of the term ‘Sintering’. In: Ristic MM, ed. Sintering-New Developments. New York, NY: Elsevier Scientific; 1979;3–7.

48. Bernard RG. Processes involved in sintering. Powder Metall. 1959;2:86–103.

49. Tikkanen MH. The application of the sintering theory in practice. Phys Sintering. 1973;5(2):441–453.

50. Mohan A, Soni NC, Moorthy VK. Definition of the term sintering. Sci Sintering. 1983;15:139–140.

51. German RM. Sintering. Encyclopedia of Materials Science and Technology London, UK: Elsevier Scientific; 2002; pp. 8640–8643.

52. T. Macfarlane, Improved Process of Preparing Chlorine, Bleaching Powder, Carbonate of Soda, and Other Products; U. S. Patent 49,597, issued 22 August 1865.

Chapter Two

History of Sintering

Some of the most significant sintered products predate modern times, including fired clay pots, porcelain, platinum, and other metals. This chapter reviews the historical evolution of several sintered structures and finds that many interdependent aspects exist–the early developments in electric furnaces resulted in spark sintering. Those electric discharge techniques then were important to sintering tungsten, and in turn the drawing of tungsten lamp filaments required development of sintered cemented carbides. Subsequently, cemented carbides enabled the production of artificial diamonds. Early sintered products evolved long before any scientific principles. Yet the need to optimize processes, specify raw materials, improve productivity, and establish quality standards spurred scientific understanding to help grow the applications for sintering.

Keywords

history; induration; porcelain; metals; ceramics; platinum crucibles; tungsten lamp filaments; cemented carbide drawing dies; polytetrafluroethylene; abrasives; diamond; steel; bronze bearings; tungsten heavy alloy radiation shields; automotive components

Historical Milestones

Sintering probably started with the observation that clay and ceramic pottery improved in strength as a result of firing in a wood or charcoal fire. There is no record of this discovery, but archeological artifacts show that the early use of sintering probably dates to about 24,000 BC. Only piecemeal remains tell the story. However, between 1700 and 1800 written records emerged on sintering experiments, and starting in the early 1800s several empirical developments are well documented.

Based on reconstructions from various records, a chronological survey of the key segments are as follows [1,2]:

• archeological artifacts, generally before 1700

• retrieved evidence of early sintering successes, most detail is lost

• examples include early earthenware and simple metals

• trial and error sintering, starting about 1700

• records exist for the key actors with some quantitative process details

• examples are porcelain, iron, platinum, iron ore

• qualitative sintering models, starting about 1900

• many discoveries, observations, early conjectures, and publications

• examples are copper, tungsten, cemented carbides, oxide ceramics, and bronze bearings

• quantitative sintering theory, starting about 1945

• mathematical models for neck size, shrinkage, densification, surface area, density, and properties; models emerged to include temperature, time, particle size, heating rate, and atmosphere effects.

Many significant discoveries took place via trial and error efforts without any scientific underpinning. Note the theoretical concepts are relatively recent and occurred in just the last 0.3% of sintering’s history. This chapter focuses on the developments up to about 1945 before there was a science of sintering.

Early Sintered Products

Several examples of sintering practice predate written records. Archeological studies on pottery, casting ceramics, early iron, copper, and precious metal structures provide a sense of the early applications. There are surprising process similarities in spite of the very different materials and applications.

Clay Ceramics

Archeological findings show shaped clay ceramic bodies were fired in open fire pits as early as 24,000 BC, in what is now the Czech Republic. This was followed by periodic advances as outlined in Figure 2.1.

Figure 2.1 An approximate archeological timeline for sintered ceramics, starting with early fired pots found in the Czech Republic and progressing to significant advances in porcelain production.

As seen in Figure 2.2, early sintered pots were not sintered at high temperatures, so they were weak and rarely survived. They leaked liquids because of their porosity. By approximately 10,000 BC, fired clay vessels were used for water storage, indicating techniques for sealing surface pores had been mastered. Recoveries in China, Egypt, and throughout the Middle East document several subsequent examples of fired beads, amulets, figurines, pots, and earthenware vessels by 6000 BC [1–3].

Figure 2.2 Examples of early sintered pottery, where the material was neither strong nor fluid-tight.

Glazes were developed about 3500 BC in the Eastern Mediterranean regions, often imitating the blue of lapis lazuli. These are glassy phases which crystallized on cooling. Often the glaze contained lead, especially in Babylon. Impervious coatings often required multiple layers. The lead glazes were supplemented by tin oxide glazes by 700 BC in Persia. Glazed tiles emerged from this base and remain in production today.

Porcelain

Porcelain was a valuable sintered product that arose first in China. A key to porcelain production is in attaining a high firing temperature. Progressive advances in kiln design generated higher firing temperatures and greater strength. Accordingly, porcelain firing advanced to give leak-free structures by about 1600 BC.

Firing temperatures of 1300°C (1573 K) allowed quartz to partially dissolve into silicate glass which then gained strength by crystal precipitation on cooling. The success of Chinese porcelain was widely recognized, and became a target for considerable trade. An example of the highly valued Tsing dynasty porcelain is shown in Figure 2.3.

Figure 2.3 A Tsing dish fabricated with a high temperature sintering process.

About the same time, fired casting molds for bronze were fabricated from sintered ceramics. The idea of firing ceramic molds for additional strength had also emerged by 1000 BC and spread along the trade routes; for example, Figure 2.4 is an example bronze casting made by the Hittites in Turkey. Trade routes not only carried the products, but also spread the technologies.

Figure 2.4 This bronze headdress was cast by the Hittites using a sintered ceramic mold.

By 900 AD porcelain production was an important industry in China, Korea, and Japan. The Italian explorer Marco Polo brought porcelain back from China in about 1295 AD, generating much interest throughout Europe. By 1580 AD an inferior porcelain sintering practice existed in Florence, but it was not competitive with porcelain from China. The secret of the Chinese porcelain was in the furnace design. A novel dragon kiln sketched in Figure 2.5 produced the needed high firing temperatures.

Figure 2.5 Early Chinese porcelain production excelled in reaching high sintering temperatures by use of sloped dragon kiln design, sketched in this early illustration.

Porcelain is based on mixtures that include quartz (SiO2), feldspar (KAlSi3O8-NaAlSi3O8-CaAl2Si2O8), and kaolinite (Al2Si2O5(OH)4), the latter being a platelet-shaped clay particle. This mixture was heated in a multiple step cycle reaching about 1400°C (1673 K), giving a final product consisting of quartz, mullite (Al6Si2O13), and glass. Bulk compositions show 60% SiO2, 32% Al2O3, 4% K2O, 2% Na2O, and oxides of iron, titanium, calcium, and magnesium. German porcelain gained traction based on the empirical work by Tschimhaus and Böttger. In the absence of phase diagrams, Böttger’s success came from his apothecary training, where he examined a broad range of local minerals, in a systematic array of compositions relying on special sintering enclosures to reach higher firing temperatures.

Böttger became a prisoner because he pretended to be an alchemist. He made the unfortunate mistake of demonstrating his alchemy, most likely by secretly swapping gold for silver overnight. His alchemist trick resulted in his imprisonment to supposedly protect his nonexistent secret. While held in prison, Böttger had outside success with porcelain. Along with von Tschimhaus, their porcelain resulted in a factory that ramped up production between 1708 and 1725. An example of fired porcelain is shown in Figure 2.6. The fabrication of such objects required the isolation of many variables, and today those familiar with sintering would likewise isolate and study the same variables.

Figure 2.6 European porcelain eventually found composition formulations and sintering techniques capable of forming high value products, initially near Dresden.

Iron, Copper, Silver

As outlined in Figure 2.7, the sintering of metallic objects was developed through discoveries made around the world. Early examples included gold, silver, copper, iron, and bronze. The first of these are from approximately 3000 BC [4,5]. Most of these easily reduced metals were sintered long ago.

Figure 2.7 An approximate timeline for the evolution of sintered metals, with early examples arising in the Middle East, with apparently independent discoveries in India and Ecuador, and finally via systematic study the Wollaston process was developed in 1805.

The first sintered ferrous shapes were derived from meteors, which was softer since meteors contain nickel, and then subsequently from smelted iron. Sinter-forged iron artifacts were discovered in Tutankhamen’s tomb–one of the few Egyptian tombs not raided in ancient times. Most likely the artifacts were fabricated by the Hittites in Turkey using a recarburization process. In this approach, iron oxide is heated in a reducing charcoal fire, and while hot the sintered agglomerates were hammered to densify the sponge into a relatively simple shape. When reheated, the iron absorbed carbon to become steel as required for swords and shields. Variants of this process were discovered in other parts of the world, including Bulgaria, China, Greece, and India. Chemical analysis indicates these were independent activities.

Gold, silver, and iron sintering were established in India by about 400 AD. The most notable example is the 7.2 m tall Delhi Iron Pillar weighing about 6000 kg [6,7]. A photograph is shown in Figure 2.8. This pillar contains 0.25% phosphorous with 0.15% carbon and traces of nickel, copper, silicon, and manganese. Such a composition forms a passive film to provide excellent corrosion resistance.

Figure 2.8 A photograph of the Delhi Iron Pillar fabricated about 400 AD using sintered lumps of iron.

To fabricate the pillar, iron granules or lumps were formed using clay crucibles charged with iron oxide, bamboo charcoal, and plant leaves. The reduced iron lump was reheated in charcoal. While hot, the charcoal was swept away and the sintered lumps were hot-hammered together to form ingots. A combination of hot forging, as an additive process, and cold chiseling, as a subtractive process, produced the desired shape.

Subsequent developments in England turned to coke for oxide reduction. Carbon additions to the reduced iron plus quenching and tempering produced exceptional strength. Layers of high and low carbon sponge resulted in a hard but tough laminate structure. Widely heralded armorers formed sword blades using this approach.

Platinum Crucibles

Platinum has a melting point of 1769°C (2042 K), much higher than the flame temperature attained by wood, charcoal, and other common combustibles. Since platinum powder is found in nature, early efforts relied on compaction and sintering of collected powder to fabricate platinum objects.

The Inca sintered gold-platinum jewelry as early as 300 BC [8]. The peak sintering temperature was in the 1100°C (1373 K) range, sufficient to melt gold. Indeed, a liquid phase sintering process with gold-silver additives provided a variety of platinum alloy objects–needles, spoons, fish hooks, forceps, nose rings, and safety pins. Such fabrication involved compacting, sintering, hammering, and annealing, with the latter steps being repeated until the desired geometry was attained. Both yellow and platinum colors were formed with these formulations; either high gold (12% platinum) or high platinum (60 to 85%) with traces of copper and silver. An examination of the Au-Pt binary phase diagram, shown in Figure 2.9 reveals the two compositions are on the sides of the two phase field. Figure 2.10 is a photograph of an archeological find which shows the two colors formed in a sintered creation.

Figure 2.9 The gold-platinum binary phase diagram exhibits a lower melting region in the gold-rich side and a two phase region near the center. The empirical findings out of Ecuador resulted in sintered gold-platinum objects either rich in gold or rich in platinum to avoid the miscibility gap.

Figure 2.10 Photograph of an early sintered gold-platinum decorative medallion.

After the discovery voyages of Columbus, sintered articles arrived in Spain, resulting in jewelry formed from mixed platinum and gold powders [9]. A rash of platinum sintering efforts followed across Europe, some relying on lead, arsenic, or mercury additives [10–14]. Various forms of sintered platinum appeared throughout Europe by 1750. A dense product required repeated heating and deformation cycles, up to 30 times. Charcoal fires provided sufficient heat if arsenic was added to induce liquid phase sintering. After densification the arsenic was evaporated. Later, mercury served a similar role and was taken in to production. However, the most successful route avoided toxic additions, possibly because those using toxic additions did not survive.

In 1805 Wollaston developed a process where platinum powder was precipitated into discrete small particles, pressed, heated, and hot worked to full density, without mercury or arsenic additions [5,14]. His process was kept confidential until his death in 1828 [15–18]. Between 1805 and 1828 Wollaston became quite wealthy by selling custom-fabricated platinum crucibles, highly regarded for making glass windows. This was the first significant case where public records document the individuals and their contributions.

The Wollaston approach scaled to large, defect-free pieces. Sponge was precipitated from ammonium platinum chloride. The precipitate was heated to remove water and pulverized and sieved to form discrete particles with a high packing density. Compaction was done in a horizontal press, as illustrated in Figure 2.11. The green compact was pre-sintered to increase handling strength and heated to a white heat, after which it was hot-forged to shape, probably at around 1000°C (≈1300 K). Note that there was no means to measure high temperatures, so the descriptions were subjective.

Figure 2.11 Wollaston relied on a lever press to compact his chemically precipitated platinum powder prior to sintering.

Competitive offerings of the time suffered impurity effects, most evident as blisters. By 1809 Wollaston’s efforts were producing 13 kg crucibles in his London laboratory, a process subsequently licensed by Johnson, Matthey and Company. Then, as early as 1820 sintered platinum was adapted for incandescent lamps [19].

The Wollaston approach was subsequently adopted for copper in 1830, by von Welsbach for osmium in 1870, and by Coolidge for tungsten in 1910. It is no surprise then that the idea of compaction, sintering, and hot working, initially applied to iron and then platinum, spread to higher temperature materials over the next century.

In Russia, platinum studies were started by Musin-Puskin at the Mining Cadet Corps of St. Petersburg [11]. Like earlier approaches, he used mercury amalgams to form the powder with subsequent distillation to remove the mercury. The resulting sponge powder sintered to give a malleable product, but it failed to move into production.

In about 1825, efforts were restarted in St. Petersburg because of the discovery of new platinum deposits in the Urals. Conversion of platinum powder into coins provided the monarch with a significant source of revenue. Sobolevskii was hired at the same Mining Cadet Corps and quickly succeeded in 1826 to form platinum in a manner similar to Wollaston [5,10,12,13]. The duplication was not initially recognized, since Wollaston waited until 1828 to disclose his process. Between 1828 and 1845, Russian coinage production using sintered platinum totaled about 14,000 kg.

Many important discoveries took place in the quest to produce platinum by sintering. Early recipes were qualitative and imprecise. For example, red hotness is a subjective temperature specification; high temperature gas thermometers did not emerge until 1828. Although not alchemy, platinum sintering in the 1800s showed no appreciation for the underlying atomic events. This was evident as late as 1923, when Smith [20] conjectured that platinum sintering was caused by a melting point depression or a crystallization event. Consideration of diffusion events came in the 1940s.

Other metals were produced by related approaches, including copper, silver, and lead. In the Osann sintered copper approach, powder was precipitated from copper carbonate and reduced using charcoal heating [5,14]. Coins were fabricated by sintering compressed powder in containers sealed to avoid oxidation. By 1841, articles of copper, silver, and lead were in production using an approach similar to modern powder metallurgy. The sintered metals developed in a sintering technology resulted in a diversity of approaches as outlined in Figure 2.12. Up to this time the thermal bonding of powder was still not yet termed sintering.

Figure 2.12 Much early effort in sintered metals tended to converge to a few process variants that fundamentally relied on powder shaping, sintering, and hot densification. Modern powder metallurgy likewise follows a similar combination of steps.

Iron Ore Induration

The term induration describes the hardening of a powdery substance. For example, in steel production, iron ore pellets are fed into melt furnaces. To avoid dusting and loss of ore, small oxide particles are agglomerated by sintering. Although initially applied to iron ore, soon the briquette agglomeration concept spread to a variety of materials [21].

Percy [22] describes iron ore agglomeration in 1864 and notes how oxide inclusions are detrimental. By the early 1900s large scale sintering agglomeration systems were in use [23–26]. Figure 2.13 shows one such plant that helps demonstrate the large scale application of sintering by 1912. In the 1930s and 1940s, ore sintering included zinc, lead, lead sulfide, carbonates, chlorides, and precious metals.

Figure 2.13 This picture is of an early iron ore sintering plant used to agglomerate powder prior to melting to make