New Directions for Non-Oxide Ceramic Powders
Non-oxide ceramic powder producers are improving chemistry, processing and crystal structure, while also employing new synthesis methods for their products.
The ceramic powder market is facing the same old challenges as it directly follows the demand for ceramic parts. Market pressures include the increasing use of plastics, glass, intermetallics and newer alloys. This has led to an oversupply in the production of powders traditionally used for abrasives, refractories and cutting tools. Ceramic part manufacturers are responding in two ways: they’ve moved to lower cost raw materials, or they’ve turned to high-quality, specific materials.
The non-oxide ceramic powder market is evolving in a different direction than the oxide ceramic powder market. Both markets have been pushed toward higher purity, application-specific chemistries and nanoparticle sizes. Both markets are also limited by the cost of high-purity raw materials. However, unlike the oxide market, the non-oxide market faces limitations on the use of nanomaterials due to the increasing tendency of these materials to become pyrophoric and flammable. These limitations have pushed non-oxide ceramic powder producers to improve chemistry, processing and crystal structure, while also employing new synthesis methods for their products.
Improvements in the chemistry of non-oxide powders have progressed in three different directions: purity or specific doping, co-synthesis, and the development of application-specific materials. Many improvements have been made in the purity or specific doping of ceramic non-oxide powders. It is known that changing the time-temperature profile can alter the oxygen content or free carbon content of ceramic powders to meet specific needs. This information has been used to control the synthesis of aluminum nitride, for example. However, the market is now starting to realize that certain impurities are beneficial to the microstructure. For example, silicon nitride with specified iron and oxygen content is now being specified to a ppm range. This iron level provides nucleation sites for the growth of the beta-elongated crystals, and the oxygen works with the sintering aids during sintering.
An example of the co-synthesis of materials used to make complex systems can be found in the carbide tooling market, which has traditionally relied on blending tungsten carbide, tantalum carbide, titanium carbide, chrome carbide, and other carbides with cobalt and nickel to press and sinter its products. It has been found that the co-synthesis of complex carbide, containing the chemistry needed for that grade of carbide, has increased strength, impact resistance and oxidation resistance compared to conventionally produced carbide. More than two carbide part manufacturers are now using co-synthesized powders.
The development of application-specific complex materials was traditionally accomplished by blending and sintering certain raw materials to make specific chemistries. This has been done for years in the carbide market and in the production of sialon. Most of these were either composite powders, carbides, or limited to four elements. However, as applications have become more exacting, there has been a push to much more complex six-element powders. These powders are of the Tmu, Tmv, Alw, Six, Nmy, Nmz family, where Tm is a transition metal and Nm is boron, nitrogen, carbon, or oxygen. These six element powders are targeted to be amorphous during production. However, a mixture of phases can normally be identified after production. The user’s thermal processing method generally determines the final phases and properties.
Ceramic powder processing improvements are being driven by the requirements of ceramic component manufacturers. In the past most, powders could be produced using conventional furnaces, crushing, milling and classification equipment. Today, advanced ceramic part producers want powders made to their own specifications, and they are asking for very exact particle size specifications.
The specifications provided by a user of silicon carbide have changed over time, from 98% pure, 100% -325 mesh d50 15 um to 99.8% pure, 100% -635 mesh d50 15um. In 2016, the specification changed to spherical powder, d95 -20 um, d5+10 um, 99.8% purity. In the past, it was easy to meet these specifications through a simple ball mill and screening operation; this progressed to a ball mill and air classifier as specifications became more exacting. To meet the current specifications, ceramic powder producers have had to move away from conventional powder production equipment.
Crystal structure becomes very important when producing chalcogenide powders for applications in electronics, solar energy and lubricants. The properties of these materials are dependent on the crystal structure and grain boundaries. It has been found that with optimal synthesis conditions, crystals of over 1,200 um, +16 mesh can be grown. The use of these crystals in solar panel production has led to a significant increase in the electricity generated, as well as the life of the panel.
The process and processing used to synthesize non-oxide ceramic powders is critical but often overlooked. In the case of tungsten carbide, three production methods are normally used to produce the powder: carbo-thermal reduction (most common), direct reaction and gas phase synthesis. The chosen process directly relates to the cost to produce these powders, with carbo-thermal reduction being the least expensive and least reactive.
Through a new nontraditional process to make non-oxide powders, it is possible to manufacture borides, carbides, boro-carbides and complex carbides. The product of this process is large crystals of -2,400 um, -8 mesh that require crushing and milling. The availability of large crystal borides, carbides, boro-carbides, and complex carbides opens new markets for these materials as refractories and abrasives.
Many processing benefits occur from the milling of large crystalline non-oxide powders. For example, particle size can be customized for the intended purpose. Abrasive and thermal spray powders can be directly screened from the large crystals, avoiding the previously required processes of spray drying, pressing, sintering, crushing, and screening. The particle size can also be made wide for slip casting crucibles and parts. Finally, the crystals can be milled to fine powders in order to replace conventionally produced powders.
Milling large crystals can also lead to a noticeable increase in the sinterability of the powders produced. This effect can be explained by the increase in crystal strain caused by milling, as well as a higher specific surface area for a dense particle at the same particle size. Ceramic component manufacturers using these crystals are reporting a reduction in the sintering temperature of 20-50°C.
Particle size distribution can also be graded for maximum particle packing, either through classical discreet particle or Funk-Dinger particle size distribution models. When hot pressing or using spark plasma sintering, the densification rate is greatly increased through a modification of the close particle packing. Currently, parts formed by hot pressing and spark plasma sintering are being tested for rocket nozzles and electrically conductive aluminum refractory plates.
It is clear that non-oxide ceramic powder producers are currently facing many challenges. Most are addressing these challenges by making changes to their powders’ chemistry, processing and crystal structure. The more progressive manufacturers are developing new synthesis methods to meet current and future demands for non-oxide ceramic powders.
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