Hexagonal Boron Nitride Is Graphite That Does Not Conduct and Does Not Burn Off at 600°C

People call hexagonal boron nitride white graphite because the crystal is the same idea: stacked plates that shear. That is why a dry h-BN film feels slippery. The comparison stops being useful the first time the job is hot, oxidizing, or electrically live. Graphite is a conductor and it starts oxidizing in air around 500–600°C. Hexagonal BN stays insulating and, in air, we treat 900–1000°C as the practical oxidation window. In nitrogen or vacuum it goes much higher, toward 2000°C and beyond. That is the reason we specify BN instead of carbon when the coating has to survive the heat and stay dielectric.

The powder we sell for coatings is usually plate-like h-BN, often in the few-micron range, sometimes finer for a smooth spray. Mixed into a binder it becomes a release paint for molds, dies, ladles, and thermocouple tubes. The useful trick with molten aluminum is wetting. Aluminum sits on boron nitride with a high contact angle — around 160° up toward 900°C in published sessile-drop work — so the metal does not grab the ceramic the way it grabs alumina or a dirty steel shoe. Skim, pour, knock the skull off. Graphite can release too, until it burns and leaves a carbon stain or a pit. BN leaves a white film. Shops that coat launders and dip tubes notice the difference after a week, not after a lab talk.

Thermal conductivity is decent and strongly anisotropic in a single crystal. In a hot-pressed part or a coating you get a more averaged number, still high enough that boron nitride ceramic is used where heat has to move and current cannot: heat-spreaders, setter coatings, high-temp electrical insulation. That combination — conduct heat, block current — is not a graphite property. Graphite does both in the wrong direction for an insulator.

Cubic BN is a different material. Same elements, diamond-like lattice, cutting-tool hardness. Do not order c-BN when you meant a release coating, and do not expect a paint-grade h-BN powder to machine steel. Pyrolytic BN is another grade again: CVD plates and crucibles for semiconductor work, very pure, very expensive. Most of our industrial calls are still hexagonal powder and hot-pressed shapes.

h-BN is not magic at low temperature. Below about 600°C graphite often has the lower friction coefficient. BN pulls ahead once graphite starts to oxidize. Moisture, oil, and a thick, flaking coat ruin a good powder. Thin, even, dry, reapplied when the white disappears. If the melt is aluminum or magnesium and the tool has to stay clean and electrically quiet, hexagonal boron nitride is the lubricant that is still there after graphite has become CO₂.