Editing Sintering (section)

== Sintering of metallic powders ==
[[File:Iron powder.JPG|thumb|[[Iron powder]]]]

Most metals can be sintered, although some with greater difficulty (i.e., [[aluminium alloy]] and [[titanium alloys]]<ref>German, R. M. (2020). Titanium sintering science: A review of atomic events during densification. International Journal of Refractory Metals and Hard Materials, 89, 105214. https://doi.org/10.1016/j.ijrmhm.2020.105214</ref>). This applies especially to pure metals produced in vacuum which suffer no surface contamination. Sintering under atmospheric pressure requires the use of a protective gas, quite often [[endothermic gas]]. Sintering, with subsequent reworking, can produce a great range of material properties. Changes in density, [[alloy]]ing, and heat treatments can alter the physical characteristics of various products. For instance, the [[Young's modulus]] ''E<sub>n</sub>'' of sintered [[iron]] powders remains somewhat insensitive to sintering time, alloying, or particle size in the original powder for lower sintering temperatures, but depends upon the density of the final product:

<math display="block">E_n/E = (D/d)^{3.4}</math>
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 (which is still considered part of powder metallurgy) is [[Sintering#Liquid phase sintering|liquid-state sintering]] in which at least one but not all elements are in a liquid state. Liquid-state sintering is required for making [[cemented carbide]] and [[tungsten carbide]].

Sintered [[bronze]] in particular is frequently used as a material for [[bearing (mechanical)|bearings]], since its porosity allows lubricants to flow through it or remain captured within it. Sintered copper may be used as a wicking structure in certain types of [[heat pipe]] construction, where the porosity allows a liquid agent to move through the porous material via [[capillary action]]. For materials that have high melting points such as [[molybdenum]], [[tungsten]], [[rhenium]], [[tantalum]], [[osmium]] and [[carbon]], sintering is one of the few viable manufacturing processes. In these cases, very low porosity is desirable and can often be achieved.

Sintered metal powder is used to make [[frangibility|frangible]] shotgun shells called [[breaching round]]s, as used by military and SWAT teams to quickly force entry into a locked room. These shotgun shells are designed to destroy door deadbolts, locks and hinges without risking lives by ricocheting or by flying on at lethal speed through the door. They work by destroying the object they hit and then dispersing into a relatively harmless powder.

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 employed for filtering steam in food and pharmaceutical applications, and sintered bronze in aircraft hydraulic systems.

Sintering of powders containing precious metals such as [[silver]] and [[gold]] is used to make small jewelry items. Evaporative self-assembly of colloidal silver nanocubes into supercrystals has been shown to allow the sintering of electrical joints at temperatures lower than 200&nbsp;°C.<ref>{{cite journal | last1 = Bronchy | first1 = M. | last2 = Roach | first2 = L. | last3 = Mendizabal | first3 = L. | last4 = Feautrier | first4 = C. | last5 = Durand | first5 = E. | last6 = Heintz | first6 = J.-M. | last7 = Duguet | first7 = E. | last8 = Tréguer-Delapierre | first8 = M. | title = Improved Low Temperature Sinter Bonding Using Silver Nanocube Superlattices | journal = J. Phys. Chem. C | date = 18 January 2022 | volume = 126 | issue = 3 | pages = 1644–1650 | issn = 1932-7447 | eissn = 1932-7455 | doi = 10.1021/acs.jpcc.1c09125| url = https://hal.archives-ouvertes.fr/hal-03558577 }}</ref>

===Advantages===
Particular advantages of the powder technology include:

# Very high levels of [[purity (disambiguation)|purity]] and uniformity in starting materials
# Preservation of purity, due to the simpler subsequent [[Manufacturing|fabrication]] process (fewer steps) that it makes possible
# Stabilization of the details of repetitive operations, by control of [[Crystallite|grain]] size during the input stages
# Absence of binding contact between segregated powder particles – or "inclusions" (called stringering) – as often occurs in melting processes
# No [[Deformation (engineering)|deformation]] needed to produce directional elongation of grains
# Capability to produce materials of controlled, uniform porosity.
# Capability to produce nearly net-shaped objects.
# Capability to produce materials which cannot be produced by any other technology.
# Capability to fabricate high-strength material like turbine blades.
# After sintering the mechanical strength to handling becomes higher.

The literature contains many references on sintering dissimilar materials to produce solid/solid-phase compounds or solid/melt mixtures at the processing stage. Almost any substance can be obtained in powder form, through either chemical, mechanical or physical processes, so basically any material can be obtained through sintering. When pure elements are sintered, the leftover powder is still pure, so it can be recycled.

=== Disadvantages ===
Particular disadvantages of the powder technology include:{{original research inline|date=September 2024}}
# sintering cannot create uniform sizes
# micro- and nanostructures produced before sintering are often destroyed.