Grain Growth, Sintering, and Vitrification Lecture 1 PDF

GRAIN GROWTH, SINTERING, and VITRIFICATION SHEM Q. SALDIA AY 2024-2025 Dept. of Materials and Resources Engineering and Technology MSU-Iligan Institute of Technology A. Bonifacio Ave., Tibanga Iligan City, Philippines CRYSTALLINE SOLIDS -Crystalline solids exist as eithe...

GRAIN GROWTH, SINTERING, and VITRIFICATION SHEM Q. SALDIA AY 2024-2025 Dept. of Materials and Resources Engineering and Technology MSU-Iligan Institute of Technology A. Bonifacio Ave., Tibanga Iligan City, Philippines CRYSTALLINE SOLIDS -Crystalline solids exist as either single crystals or polycrystalline solids. A single crystal is a solid in which the periodic and repeated arrangement of atoms is perfect and extends throughout the entirety of the specimen without interruption. 1 Single Crystals -have a uniform and continuous crystal lattice throughout. Photograph of a garnet single crystal that was found in Tongbei, Fujian -Because of this uniformity, their properties Province, China. (Photograph courtesy of Irocks.com, Megan Foreman photo.) 2 can vary depending on the direction in which they are measured. This directional dependence of properties is known as anisotropy. 3 1. Barsoum, M. (1997). Fundamentals of Ceramics. John Wiley & Sons, NY 2. Callister, W.D., Jr. (2000). Materials Science and Engineering: An Introduction, New York: Wiley 3. https://www.nde-ed.org/Physics/Materials/Structure/anisotropy.xhtml POLYCRYSTALLINE MATERIAL -A polycrystalline material is comprised of a collection of many single crystals, termed grains, separated from one another by areas of disorder known as grain boundaries. 1 -Polycrystalline materials are solids that consist of many small crystals (the (a) Schematic of a polycrystalline sample. A polycrystal is made up of many “grains”). The grains are separated by grains separated from one another by regions of disorder known as grain boundaries. grain boundaries and normally have (b) typical microstructure as seen through an optical microscope. 1 random crystallographic orientations. 2 -In many cases, the random orientation of these grains averages out the directional properties, making the material isotropic, meaning its 1. Barsoum, M. (1997). Fundamentals of Ceramics. John Wiley & Sons, NY properties are the same in all directions. 2. Wang, M., & Duan, B. (2019). Materials and their biomedical applications. Encyclopedia of Biomedical Engineering. 3. https://www.nde-ed.org/Physics/Materials/Structure/anisotropy.xhtml Note that polycrystalline materials can also exhibit anisotropy if the grains have a preferred orientation or texture. EXAMPLES: 1. Rolled Metals When metals like aluminum or steel are rolled during manufacturing, the grains tend to elongate and align in the rolling direction. This creates anisotropy in mechanical properties such as yield strength and ductility 1 2. Textured Ceramics Ceramics that undergo processes like hot pressing or extrusion can develop a preferred grain orientation, leading to anisotropic properties in terms of thermal expansion and electrical conductivity 2 3. Magnetic Materials Certain polycrystalline magnetic materials, such as silicon steel used in 1. https://www.nde-ed.org/Phys transformers, are processed to have grains aligned in a specific direction ics/Materials/Structure/aniso tropy.xhtml to optimize magnetic properties 2 2. https://link.springer.com/cha pter/10.1007/978-1-4471-6314- 5_1 4. Piezoelectric Materials Polycrystalline piezoelectric materials, like lead zirconate titanate (PZT), can exhibit anisotropy in their piezoelectric response due to the alignment of grains during the manufacturing process. 2 GRAINS -The term ‘grain’ refers to a crystal in a polycrystalline material. 1 -Grains are the reason why most objects don’t “look” like a crystal to you. 2 Grain Boundary 2 -is an interface between the grains generated by a difference in crystal orientation during the 1. White, A. A., & Best, S. M. (2009). Properties and characterisation of bone repair materials. In Bone repair sintering process. 3 biomaterials (pp. 121-153). Woodhead Publishing. 2. https://msestudent.com/what-are-crystals-and-grains/ 3. Bai, L., Xue, W., Li, Y., Liu, X., Li, Y., & Sun, J. (2018). The interfacial behaviours of all-solid-state lithium ion batteries. Ceramics International, 44(7), 7319-7328.  1 1. https://msestudent.com/what-are-crystals-and-grains/  Particle vs. Grain Particle: Grain: ○ A particle is a small portion of matter that ○ A grain is a single crystal within a can be composed of one or more grains. polycrystalline material. ○ Particles can be of various sizes and ○ Grains are bounded by grain boundaries, shapes and are often used to describe which are regions where the crystal powders or dispersed phases in orientation changes. composites. ○ The size and orientation of grains can ○ They can be made up of multiple grains, significantly affect the material’s especially in polycrystalline materials. properties, such as strength, ductility, and conductivity. 1 1. https://gyansanchay.csjmu.ac.in/wp-content/uploads/2022/01/205-L29.pdf GRAIN GROWTH -Grain growth refers to an increase in the size of crystallites (grains) in a material at high temperatures. 1 -Grain growth occurs by the migration of grain boundaries. Not all grains can enlarge, but large ones grow at the expense of small ones that shrink. Thus, the average grain size increases with time, and at any particular instant there will exist a range of grain sizes. 2 2 1. https://www.corrosionpedia.com/definition/6414/grain-growth 2. Callister, W.D., Jr. (2000). Materials Science and Engineering: An Introduction, New York: Wiley 3. Dimokrati, A., & Benyoucef, M. (2016, September). Phase Field Simulation of Normal Grain Growth. In CONAT 2016.  The crystallization of a large amount of material from a single point of nucleation results in a single crystal. Normally when a material begins to solidify, multiple crystals begin to grow in the liquid and a polycrystalline (more than one crystal) solid forms. The moment a crystal begins to grow is know as nucleation and the point where it occurs is the nucleation point. 2 a) Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together, d) grain 1. https://www.nde-ed.org/Physics/Materials/Structure/solidification.xhtml boundaries as seen in a microscope. Grain growth via atomic diffusion -Atoms from the convex side of grain move to the concave side of the grain surface -Driving force for grain growth: reduction in total energy 2 1. Gusak, A. M., Chen, K. J., Tu, K. N., & Chen, C. (2021). Modeling of abnormal grain growth in (111) oriented and nanotwinned copper. Scientific reports, 11(1), 20449.  Scanning electron micrograph of an aluminum oxide powder compact that was sintered at 1700C for 6 min. 5000. (From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2nd edition, p. 483. Copyright © 1976 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) 1 1. Callister, W.D., Jr. (2000). Materials Science and Engineering: An Introduction, New York: Wiley Grain boundary energies in yttria-stabilized zirconia 1 1. Dillon, S. J., Shen, Y. F., & Rohrer, G. S. (2022). Grain boundary energies in yttria‐stabilized zirconia. Journal of the American Ceramic Society, 105(4), 2925-2931. SINTERING -(1) Sintering is a thermal process of converting loose fine particles into a solid coherent mass by heat and/or pressure without fully melting the particles to the point of melting. 1 -The sintering process increases the grain growth and decreases surface energy and porosity of the material. 1 -(2)Sintering is effectively a process where porosity, i.e. open space, is removed from compacted powder particles to form a solid mass. This is a means of densification since you are decreasing the amount of volume that a given amount of mass takes up. 2 1. https://www.sciencedirect.com/topics/engineering/sintering-process 2. https://ceramics.org/wp-content/uploads/2019/05/PCSA-Ceramic-Lessons_Sinte ring1.pdf  Sintering vs. Firing Sintering: Firing: ○ is a process that occurs in ○ is a high-temperature various materials, including treatment used in ceramic metals, ceramics, plastics, and technology. It involves minerals. It involves heating a heating a consolidated material to a temperature below powder compact, also known its melting point, but high enough for the atoms in the as a green body, to transform material to diffuse across it into a rigid ceramic 1 particle boundaries. material. 1. https://kindle-tech.com/faqs/what-is-the-difference-between-firing-and-sintering  Starting Powders PROCESSING OF Milling CERAMICS Mix with binder/additives Slurry/Slip/Paste Dry/Granulate Shape forming: Power Powder Slip casting compaction/shaping: Compact Tape casting Dry pressing green Extrusion Injection moulding Dry Binder Burnout Binder Burnout Conventional Dense Hot-pressing Polycrystall Hot isostatic pressing SINTERING SINTERING ine body: Spark Plasma Sintering ceramic Finishing Machining & Finishing 12 SINTERING a process by which a powder compact is transformed to a strongly bonded monolithic mass with the removal of inter-particulate pores. removal of pores between particles accompanied by shrinkage (densification) and grain growth. is “…understood to mean any changes in shape which a small particle or a cluster of particles of uniform composition undergoes when held at high temperature.” 12 During sintering, high temperatures allow material to transport to the open space between powder particles. Eventually, the pores disappear resulting in a dense pore free object. 1. https://ceramics.org/wp-content/uploads/2019/05/PCSA-Ceramic-Lessons_Sintering1.pdf  Sintering Sequence Changes: (1) grain size and shape (2) pore shape (3) pore size 12 Driving Force of Sintering is the lowering of excess energy associated with the free surfaces of the fine powders. This may occur in different ways: (a) Coarsening (b) Densification where the large followed by grain grains grow at growth. In this the expense of case, shrinkage of the smaller the compact has to ones. occur. Schematic of two possible paths by which a collection of particles can lower its energy. 12 ASSIGNMENT NO. 1 1. Define vitrification. 2. Why is vitrification crucial for ceramics? 3. Give some examples of vitrified ceramic products. -Due: Next meeting -1 yellow pad paper -Submit in-person Types of Sintering 1.) Solid State Sintering (SSS) Direct particle to particle contact 2.) Liquid Phase Sintering (LPS) Presence of a liquid film particularly at high temperature ensures adherence of the solid particles together. 3.) Reactive Sintering Particles react with each other to “Fundamentals of Ceramics” By M W Barsoum form new product phases. ISBN 0 7503 0902 4 12 ASSIGNMENT NO. 2 Provide at least 2 examples of ceramic materials which are synthesized/ manufactured through a) solid state sintering, b) liquid phase sintering, and c) reactive sintering -Due: Next meeting -1/2 yellow pad paper -Submit in-person What Happens During Sintering? o Atomic Diffusion takes place o Recrystallization and Grain Growth At elevated temperature, the atoms At the sintering temperature new can move more easily and quickly crystallites form at the points of contact so migrate along the particle surfaces. that the original inter-particle boundaries disappear, or become recognizable merely as grain boundaries. (Recrystallization) 12 What Happens During Sintering? o The total internal surface o Neck-line junctions are area of the body is reduced formed between adjacent by sintering. particles. Neck Pore Grain Boundary Small Grains Sintered Body Before Sintering After Sintering 12 Pictorial Representation of Different Stages of Sintering (a, b) Initial stage sintering Formation of strong bonds and necks between particles at the contact points. Moderate decrease of porosity (initial 40-50%) from particle rearrangement. neck growth (a) (b) 12 Pictorial Representation of Different Stages of Sintering sinter body (c) Intermediate stage sintering pore space The size of the necks increases and the amount of porosity decreases. The sample shrinks (the centers of the grains move towards each other). The grains transforms from spheres to truncated octahedra (tetrakaidecahedra). This stage continues until pores are closed (≤ 90%). (c) (d) Final stage sintering Pores are slowly eliminated and major grain growth can occur. isolated (d) pores 12 Stages of Sintering (a, b) Initial stage sintering Formation of strong bonds and necks between particles at the contact points. Moderate decrease of porosity (initial 40-50%) from particle rearrangement. (c) Intermediate stage sintering The size of the necks increases and the amount of porosity decreases. The sample shrinks (the centers of the grains move towards each other). The grains transforms from spheres to truncated octahedra (tetrakaidecahedra). This stage continues until pores are closed (≤ 90%). (d) Final stage sintering Pores are slowly eliminated and major grain growth can occur. 12 Stages of Sintering Sintering Microstructural Relative Idealized Model Stage Features Density Interparticle neck Initial Up to 0.65 Spheres in contact growth Equilibrium pore Tetrakaidecahedron shape with with cylindrical pores Intermediate 0.65 - 0.9 continuous of the same radius porosity along edges Equilibrium pore Tetrakaidecahedron Figure: Schematic of a sintering curve Final shape with isolated ≥ 0.9 with spherical pores porosity at grain corners 12 Mechanisms of Sintering Table: Alternate Paths for Matter Transport During the Initial Stages of Sintering Figure. Alternate Paths for Matter Transport During the Initial Stages of Sintering Courtesy M. A. Ashby. 12 Mechanisms of Sintering ▪ Coarsening -- Surface diffusion -- Lattice diffusion -- Vapor transport Table: Alternate Paths for Matter Transport During the Initial Stages of Sintering Mechanisms of Sintering ▪ Densification -- Grain boundary diffusion -- Lattice diffusion from grain boundary -- Plastic flow Table: Alternate Paths for Matter Transport During the Initial Stages of Sintering https://www.youtube.com/watch?v=O8zYcxTlxo8&pp=ygUWbGlxdWlkIHBoYXNlIHNpbnRlcmluZw%3D%3D factors: Coarsening Densification ○ Higher temperatures enhance atomic ○ Higher temperatures diffusion ○ Longer sintering times ○ External pressure during sintering (e.g., hot pressing) ○ Smaller initial particles ○ Narrow particle size distribution can ○ Atmosphere (Example: improve packing density and enhance an oxidizing densification. atmosphere) ○ Sintering Atmosphere (example, a reducing atmosphere) ○ Impurities and ○ Green Density (packing density) - Higher Additives green density 1,2,3,4,5 ○ Additives 1. https://ntrs.nasa.gov/citations/19900028320 2. https://link.springer.com/article/10.1557/JMR.1999.0188 3. https://ntrs.nasa.gov/citations/19900028320 4. https://www.uobabylon.edu.iq/eprints/publication_4_18408_258.pdf 5. https://link.springer.com/article/10.1007/s10854-023-11460-0 Abnormal Grain Growth ▪ Abnormal grain growth (Secondary Recrystallization) refers to a process whereby a small number of grains grow much faster than the others such that their size becomes an order of magnitude larger than the average. 12 Shrinkage reduction in the size of something, or the process of becoming smaller. 12 12 Vitrification The transformation of a material into a glass-like state, often involving the formation of a glass phase that fills pores and creates a dense, amorphous structure. To vitrify is to make glasslike vitrification process - densification with the aid of a viscous liquid phase--is the major firing process for the great majority of silicate systems 12  Vitrification can be obvious by simple visual inspection The unglazed surface on the right looks more like plaster, The unglazed surface of the left and it is absorbent, about 5% piece has a sheen, it is a product porosity. It is a mix of the of glass development during same stoneware but with 50% firing to cone 6. That body is a ball clay. The refractory ball 50:50 mix of a cone 8 stoneware clay assures that the and a low fire earthenware red stoneware, which was (a material that would normally already inadequately be melted by this temperature). vitreous, is even more so. As Together they produce this you can imagine, the left dense, almost zero-porosity piece is far stronger. ceramic. 12 https://digitalfire.com/ ‘Vitreous’ The term "vitreous" refers to a state (whereas vitrification is a process). Glass is homogeneous, totally dense without a single visible air bubble or pore. But glass is not "vitreous", this term is normally reserved for ceramics, objects that can be fired in a kiln yet maintain their shape without supports. Stonewares and porcelains are most commonly called vitreous. Each body has a temperature at which it is fired to the point of maximum practical density and strength. Fired higher than that pieces begin to warp excessively, surfaces blister and bubbles grow from within (called bloating). The word vitreous means “Glass-Like’’ https://digitalfire.com/ 12 https://yuubath.com/blogs/news/vitreous-china-v s-porcelain-vs-ceramic-what-s-the-difference How much does a porcelain piece shrink on firing? Left: Dry mug. Right: Glazed and fired to cone 6. This is Polar Ice porcelain from Plainsman Clays. It is very vitreous and has the highest fired shrinkage of any body they make (14-15% total). This is the highest firing shrinkage you should ever normally encounter with a pottery clay. 12 https://digitalfire.com/ Manufacture of Porcelain Insulators A typical formulation for porcelain mass contains varying proportions of ball clay, kaolin (for strength and plasticity), feldspar (a flux that helps sintering in the kiln) and fillers such as quartz, alumina or calcined bauxite (intended to impart additional mechanical strength). A variety of secondary materials are also used to facilitate processing, including water and additives such as binders which are burned off in the kiln during firing at up to 1300°C. https://www.inmr.com/manufacture-of-porcelain-insulators/ Manufacture of Porcelain Insulators These various minerals are mined from sites the world over and, since no two locations are alike in all aspects, some variation can be expected in raw material quality. This must be considered and controlled during the production process to ensure consistent quality of the final product. https://www.inmr.com/manufacture-of-porcelain-insulators/ Manufacture of Porcelain Insulators Growing attention has been placed on the microstructure of these raw materials to avoid development of undesirable interfaces at the microscopic level. For example, experts believe that quartz crystallites found in some ceramic aggregates can feature critically over-sized particles that modify and shrink during firing. Resultant micro-cracks can then propagate and become areas of inherent weakness in the porcelain body during dynamic mechanical loading or even under constant change in ambient service temperatures. The larger these quartz crystallites, the more they are at risk of cracking and the sooner the insulator becomes at risk of failing. Indeed, research has demonstrated that punctured porcelain insulators typically have some or all of the following micro-defects: agglomerations of corundum crystals; long running micro-cracks; large quartz crystals; and numerous agglomerates of pores. By contrast, porcelain insulators that have well dispersed and relatively few pores are less likely to fail mechanically. https://www.inmr.com/manufacture-of-porcelain-insulators/ Manufacture of Porcelain Insulators Unwanted quartz crystallite separated from vitreous phase in C-130 alumina porcelain body. https://www.inmr.com/manufacture-of-porcelain-insulators/ Flexural Strength of Vitreous Ceramics Based on Lithium Disilicate and Lithium Silicate Reinforced with Zirconia for CAD/CAM Hazel P. R. Corado, Pedro H. P. M. da Silveira, Vagner L. Ortega, Guilherme G. Ramos, Carlos N. Elias First published: 02 February 2022 https://doi.org/10.1155/2022/5896511Citations: 12 Academic Editor: Yingchao Su Currently, the manufacture of dental prostheses has been carried out with CAD-CAM (computer-aided design and computer-aided manufacture) systems, which present excellent results and ease of execution. https://onlinelibrary.wiley.com/doi/10.1155/2022/5896511 Scanning Electron Microscopy (SEM) After bending testing, the microstructure of the fractured ceramics was analyzed by scanning electron microscopy (Quanta FEG 250, Thermo Fisher Scientific, Massachusetts, USA). The MEV is equipped with a field-emission electron gun, operating at 5 and 10 kV in low-vacuum mode, with 10000x magnification for all samples. The objective of the analysis was to identify the microstructure of the material and evaluate the morphology of the crystals. Samples were received and cut without additional heat treatment and after heat treatment. All samples were treated with 10% hydrofluoric acid for 15 seconds, then coated with a thin layer of gold using a sputter (ACE600, Leica, Germany) for 30 min. The materials with crystallization after machinability and lower zirconia content, e-Max CAD, and Rosetta, G3 (Figure 1(d)) and G5 (Figure 1(h)) respectively, revealed that the microstructures of lithium disilicates were elongated crystals. https://onlinelibrary.wiley.com/doi/10.1155/2022/5896511 -The End- Lecture 1 Some images used in this lecture are Google images.