Solid-State Sintering

Like traditional ceramics, advanced ceramics are densified from powders by applying heat—a process known as sintering. Unlike traditional ceramics, however, advanced powders are not bonded by the particle-dissolving action of glassy liquids that appear at high temperatures. Instead, solid-state sintering predominates. In this process, matter from adjacent particles, under the influence of heat and pressure, diffuses to “neck” regions that grow between the particles and ultimately bond the particles together. As the boundaries between grains grow, porosity progressively decreases until, in a final stage, pores close off and are no longer interconnected.

source : www.britannica.com

Selective Laser Sintering

Selective laser sintering (SLS) is an additive manufacturing technique that uses a high power laser (for example, a carbon dioxide laser) to fuse small particles of plastic, metal (Direct Metal Laser Sintering), ceramic, or glass powders into a mass representing a desired 3-dimensional object. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.

Compared to other methods of additive manufacturing, SLS can produce parts from a relatively wide range of commercially available powder materials. These include polymers such as nylon, (neat, glass-filled or with other fillers) or polystyrene, metals including steel, titanium, alloy mixtures, and composites and green sand. The physical process can be full melting, partial melting, or liquid-phase sintering. And, depending on the material, up to 100% density can be achieved with material properties comparable to those from conventional manufacturing methods. In many cases large numbers of parts can be packed within the powder bed, allowing very high productivity.

SLS is performed by machines called SLS systems. SLS technology is in wide use around the world due to its ability to easily make very complex geometries directly from digital CAD data. While it began as a way to build prototype parts early in the design cycle, it is increasingly being used in limited-run manufacturing to produce end-use parts. One less expected and rapidly growing application of SLS is its use in art.

SLS was developed and patented by Dr. Carl Deckard at the University of Texas at Austin in the mid-1980s, under sponsorship of DARPA. A similar process was patented without being commercialized by R.F. Housholder in 1979.

Unlike some other additive manufacturing processes, such as Stereolithography (SLA) and Fused Deposition Modeling (FDM), SLS does not require support structures due to the fact that the part being constructed is surrounded by unsintered powder at all times.

source : www.wikipedia.org

Selective Laser Sintering

Selective laser sintering (SLS) is an additive manufacturing technique that uses a high power laser (for example, a carbon dioxide laser) to fuse small particles of plastic, metal (Direct Metal Laser Sintering), ceramic, or glass powders into a mass representing a desired 3-dimensional object. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.

Compared to other methods of additive manufacturing, SLS can produce parts from a relatively wide range of commercially available powder materials. These include polymers such as nylon, (neat, glass-filled or with other fillers) or polystyrene, metals including steel, titanium, alloy mixtures, and composites and green sand. The physical process can be full melting, partial melting, or liquid-phase sintering. And, depending on the material, up to 100% density can be achieved with material properties comparable to those from conventional manufacturing methods. In many cases large numbers of parts can be packed within the powder bed, allowing very high productivity.

SLS is performed by machines called SLS systems. SLS technology is in wide use around the world due to its ability to easily make very complex geometries directly from digital CAD data. While it began as a way to build prototype parts early in the design cycle, it is increasingly being used in limited-run manufacturing to produce end-use parts. One less expected and rapidly growing application of SLS is its use in art.

SLS was developed and patented by Dr. Carl Deckard at the University of Texas at Austin in the mid-1980s, under sponsorship of DARPA. A similar process was patented without being commercialized by R.F. Housholder in 1979.

Unlike some other additive manufacturing processes, such as Stereolithography (SLA) and Fused Deposition Modeling (FDM), SLS does not require support structures due to the fact that the part being constructed is surrounded by unsintered powder at all times.

source : www.wikipedia.org

Liquid phase sintering

For materials which are hard to sinter a process called liquid phase sintering is commonly used. Materials for which liquid phase sintering is common are Si3N4, WC, SiC, and more. Liquid phase sintering is the process of adding an additive to the powder which will melt before the matrix phase. The process of liquid phase sintering has three stages:

1. Rearrangement - As the liquid melts capillary action will pull the liquid into pores and also cause grains to rearrange into a more favorable packing arrangement.
2. Solution-Precipitation - In areas where capillary pressures are high(particles are close togeather) atoms will preferentially go into solution and then precipitate in areas of lower chemical potential where particles are non close or in contact. This is called "contact flattening" This densifies the system in a way similar to grain boundary diffusion in solid state sintering. Oswald ripening will also occur where smaller particles will go into solution preferentially and precipitate on larger particles leading to densification.
3. Final Densification - densification of solid skeletal network, liquid movement from efficiently packed regions into pores.

For liquid phase sintering to be practical the major phase should be at least slightly soluble in the liquid phase and the additive should melt before any major sintering of the solid particulate network occurs, otherwise rearrangement of grains will not occur. From : www.wikipedia.org

History of Sintering

English engineer A. G. Bloxam registered in 1906 the first patent on sintering powders using direct current in vacuum. The primary purpose of his inventions was the industrial scale production of filaments for incandescent lamps by compacting tungsten or molybdenum particles. The applied current was particularly effective in reducing surface oxides that increased the emissivity of the filaments.

In 1913, Weintraub and Rush patented a modified sintering method which combined electric current with pressure. The benefits of this method were proved for the sintering of refractory metals as well as conductive carbide or nitride powders. The starting boron–carbon or silicon–carbon powders were placed in an electrically insulating tube and compressed by two rods which also served as electrodes for the current. The estimated sintering temperature was 2000 °C.

In the US, sintering was first patented by Duval d’Adrian in 1922. His three-step process aimed at producing heat-resistant blocks from such oxide materials as zirconia, thoria or tantalia. The steps were: (i) molding the powder (ii) annealing it at about 2500 °C to make it conducting (iii) applying current-pressure sintering as in the method by Weintraub and Rush.

Sintering using an arc produced via a capacitance discharge was patented by G. F. Taylor in 1932. This originated sintering methods employing pulsed or alternating current, eventually superimposed to a direct current. Those techniques have been developed over many decades and summarized in more than 640 patents

source : www.wikipedia.org

Sintering of metallic powders

Most, if not all, metals can be sintered. This applies especially to pure metals produced in vacuum which suffer no surface contamination. Sintering under atmospheric pressure requires the usage of a protective gas, quite often endo gas.

Many nonmetallic substances also sinter, such as glass, alumina, zirconia, silica, magnesia, lime, ice, beryllium oxide, ferric oxide, and various organic polymers. Sintering, with subsequent reworking, can produce a great range of material properties. Changes in density, alloying, or heat treatments can alter the physical characteristics of various products. For instance, the Young's Modulus En of sintered iron powders remains insensitive to sintering time, alloying, or particle size in the original powder, but depends upon the density of the final product:

En / E = (D / d)3.4

where D is the density, E is Young's modulus and d is the maximum density of iron.

Sintering is static when a metal powder under certain external conditions may exhibit coalescence, and yet reverts to its normal behavior when such conditions are removed. In most cases, the density of a collection of grains increases as material flows into voids, causing a decrease in overall volume. Mass movements that occur during sintering consist of the reduction of total porosity by repacking, followed by material transport due to evaporation and condensation from diffusion. In the final stages, metal atoms move along crystal boundaries to the walls of internal pores, redistributing mass from the internal bulk of the object and smoothing pore walls. Surface tension is the driving force for this movement.

A special form of sintering, still considered part of powder metallurgy, is liquid state sintering. In liquid state sintering, at least one but not all elements are in a liquid state. Liquid state sintering is required for making cemented carbides or tungsten carbide.

Sintered bronze in particular is frequently used as a material for bearings, since its porosity allows lubricants to flow through it or remain captured within it. For materials that have relatively high melting points, by comparison to other materials of the same type, such as PTFE and tungsten, sintering is one of the few viable manufacturing processes. In these cases very low porosity is desirable and can often be achieved.

Sintered bronze and stainless steel are used as filter materials in applications requiring high temperature resistance while retaining the ability to regenerate the filter element. For example, sintered stainless steel elements are used for filtering steam in food and pharmaceutical applications.

Separation of items within the furnace is achieved using sheets similar to those described in the ceramic process above.

from : www.wikipedia.org

Ceramic Sintering

Sintering is a method for making objects from powder, by heating the material in a sintering furnace below its melting point (solid state sintering) until its particles adhere to each other. Sintering is traditionally used for manufacturing ceramic objects, and has also found uses in such fields as powder metallurgy.

The word "sinter" comes from the Middle High German Sinter, a cognate of English "cinder".

Sintering is part of the firing process used in the manufacture of pottery and other ceramic objects. Some ceramic raw materials have a lower affinity for water and a lower plasticity index than clay, requiring organic additives in the stages before sintering. The general procedure of creating ceramic objects via sintering of powders includes:

* Mixing water, binder, deflocculant, and unfired ceramic powder to form a slurry
* Spray-drying the slurry
* Putting the spray dried powder into a mold and pressing it to form a green body (an unsintered ceramic item)
* Heating the green body at low temperature to burn off the binder
* Sintering at a high temperature to fuse the ceramic particles together

All the characteristic temperatures associated to phases transformation, glass transitions and melting points, occurring during a sinterisation cycle of a particular ceramics formulation (i.e. tails and frits) can be easily obtained by observing the expansion-temperature curves during optical dilatometer thermal analysis. In fact, sinterisation is associated to a remarkable shrinkage of the material because glass phases flow, once their transition temperature is reached, and start consolidating the powdery structure and considerably reducing the porosity of the material.

There are two types of sintering: with pressure (also known as hot pressing), and without pressure. Pressureless sintering is possible with graded metal-ceramic composites, with a nanoparticle sintering aid and bulk molding technology. A variant used for 3D shapes is called hot isostatic pressing.

To allow efficient stacking of product in the furnace during sintering and prevent parts sticking together, many manufacturers separate ware using Ceramic Powder Separator Sheets. These sheets are available in various materials such as alumina, zirconia and magnesia. They are also available in fine medium and coarse particle sizes. By matching the material and particle size to the ware being sintered, surface damage and contamination can be reduced while maximizing furnace loading.

source : www.wikipedia.org

Definition of Ceramic

Defining what the term “ceramic” means is not simple, as there is no single definition on which everyone agrees; there are in fact various definitions depending on the point of view adopted. We can thus consider the points of view of a historian, a scientist (physicist, chemist, etc.), an engineer or a manufacturer.

Historically, ceramics is related to terracotta, pottery, fired clays and arts. Ceramic is traditionally defined as high temperature firing of clay that causes an
irreversible physicochemical transformation.

Physicists – electronic structure Ceramics is any material related to oxides (conductors, semi-conductors, isolators)

Chemists – inter-molecular bonding forces Ceramics is material having strong polycrystalline molecular bond type.

Material science – organic,metalic,inorganic Ceramics is material related to inorganic and non-metallic subtances Examples: compounds of carbides, nitrides, silicides, borides

Dictionary of Ceramic Science and Eng.: Any inorganic and non-metallic product prepared by treatment at temperatures higher than 540°C including metallic oxides and borides, carbides, nitrides and mixtures of these compounds

Concise Encyclopedia of Advanced Ceramic Matl: Ceramics are inorganic non-metallic compounds, primarily oxides, also nitrides,carbides, silicides; contain at least 30% of crystallized phases in volume; exhibit a fragile behavior, with a stress-strain curve obeys Hooke’s law of linear elasticity

“Ceramic materials are synthetic materials, mainly composed of iono-covalent inorganic phases, not fully amorphous, and generally consolidated by the sintering at high temperatures of a powdery “compact” formed into the shape of the desired object,the starting powders being frequently prepared from crushed rocks”