On Non-Oxide Ceramic Powders

There are a variety of non-oxide ceramic powders, but this article will discuss the following kinds in more detail: alpha and beta silicon carbide and boron carbide. Superior Graphite has been supplying these powders to the advanced ceramic industry for years, and the company wants to share more about how it manufactures them and what makes them unique for the industry.

Alpha silicon carbide

The material is formed in an Acheson furnace and varies in purity according to its distance from the graphite resistor heat source. Colorless, pale yellow and green crystals have the highest purity and are found closest to the resistor. The color changes to blue and black at greater distance from the resistor, and these darker crystals are less pure. Nitrogen and aluminum are common impurities, and they affect the electrical conductivity of SiC.

The material is segregated by purity and then processed down in size to various commonly accepted sizes for a variety of applications. The two commonly used types for ceramic grade applications are black 98% pure SiC, and green 99% pure SiC. For sintered ceramic parts, green 99% pure SiC is the preferred purity.

Superior Graphite's product line focuses on green silicon carbide

Superior Graphite focuses on the most value-added portion of the particle size range, which is from 3μm D50 particle size to 0.5μm D50 particle size. It is well established that 0.6-0.7μm D50 green alpha silicon carbide is the best choice for optimum sintered density of a ceramic part.

The material is wet attrition milled to size then spray dried to yield a highly de-agglomerated ceramic powder.

  • ASIC2500N is Superior Graphite’s ‘naked’ version and the company’s highest-volume silicon carbide powder at 0.7μm D50 PSD
  • ASIC2500NDP is a ready-to-press version based on ASIC2500N with binder and lubricants added for processing and sintering

Beta silicon carbide 

Beta silicon carbide has a cubic structure as opposed to the hexagonal crystal structure of alpha silicon carbide. Superior Graphite has been producing beta silicon carbide since 1989, using a unique continuous furnace operation in Hopkinsville, KY. The synthesized beta silicon carbide is processed down to size using wet ball milling and wet attrition milling techniques. 20μm D50 PSD down to 0.5μm D50 PSD sizes are produced to satisfy market demand for the various applications.

Figure 1: Beta silicon carbide structure

Superior Graphite image 1

Boron Carbide 

Superior Graphite focuses on producing submicron boron carbide of the size 0.6-0.7μm D50 PSD. The primary applications are as a sintering aid in sintered silicon carbide parts, ceramic armor plating and wear parts. Superior Graphite has the ability to produce a very low oxygen impurity product, which is very advantageous for the market.

Applications for alpha silicon carbide powder

  1. Diesel particulate filters
  2. Mechanical seals
  3. Ceramic armor

Applications for beta silicon carbide

  1. Ball-milled beta SiC (0.7μm D50 PSD)
  2. Ceramic composites and pre-ceramic polymers
  3. Ceramic lapping discs
  4. Ceramic brakes

Attrition-milled Beta SiC (0.7μm D50 PSD).

  1. Flow agent for dry toner
  2. Ceramic parts
  3. Ceramic matrix composites

Attrition-milled beta SiC Grit (2-20μm D50 PSD)

  1. Wear parts
  2. Abrasive fillers
  3. Additive manufacturing processes

Applications for boron carbide

  1. Body plate
  2. Structural armor
  3. Blasting nozzle

Figure 2
: Beta SiC process applications for boron carbide

Superior Graphite image 2

Superior Graphite process:

Superior Graphite image 3

Superior Graphite Co will be exhibiting at Ceramics Expo 2019 in Booth 419