Boron nitride in powder or other forms classifies under heading 2850 as a boride/nitride compound, regardless of its end use as a lubricant or ceramic precursor. Prepared abrasive articles or wheels made from boron nitride would classify elsewhere, but the chemical substance itself belongs here.

Yes, metal hydrides as chemical substances classify under heading 2850. Finished battery cells or electrode assemblies incorporating those hydrides would classify under the chapter covering electrical accumulators. The distinction turns on whether the article is the chemical compound itself or a manufactured component.