Densification of boron carbide

Question 1

What is the result of boron carbide being hard to pressurelessly sinter?

Answer 1

~ they are typically hot-pressed at 2100C and 30-40MPa to obtain dense particles. Hot pressing is only applicable to simple shapes

Question 2

What are sintering aids?

Answer 2

~ Al2O3, TiB2, and W2B5; not entirely satisfactory because of their facilitation of exaggerated grain growth

Question 3

What is the best additive for sintering for B4C?

Answer 3

~ carbon; can be incorporated through in-situ pyrolysis of a nonvolaque-type phenolic resin (soluble in spray drying slurry) which serves as a pressing agent and later yields ~50wt% C on decomp, which is well mixed with B4C powder

~ carbon additions can facilitate sintering to 98% relative density (2150C), but in the form of graphite in the microstructure; hardness is degraded

Question 4

What causes poor sintering of B4C?

Answer 4

~ related to the decay of the onset of sintering because of thin B2O3 coatings, which invariably are on B4C particles, and extensive particle coarsening via evaporation/condensation of high vapor pressure B4C at elevated temp

Question 5

How do you remove the B2O3 coatings?

Answer 5

~ using hydrogen or vacuum treatment at 1300C and rapid heating rate to the sintering dwell temp to minimize particle coarsening relative to sintering facilitates relative densities (with no sintering aids) of 97%
–> under this closed porosity stgate, post-HIPING removes all remaining porosity