Boron try-nitrite and Tungsten carbide are chemical compounds, they are not composites.
The composite is a man made material which utilize some substances or materials with different proprieties and are bond together to be usually light and tough.
In my experience of mechanical engineer and of the teachers which tout me, the hardest composites are metallic powder made pieces, which include B3N and Tungsten Carbides, TiC Ti3NC, Vanadium nitro-carbides,Rhenium and other very strong chemical compounds.
There are also new discoveries with combination between natural Diamond and Nanotubes. Check the link below.