According to the present invention these carbon-carbon composites are diffusion coated with hafnium, titanium, boron or other material that forms a carbide resistant to temperatures as high as 3000?? C. Ordinary pack diffusion in a pack containing about 5% to about 20% of the hafnium or other diffusing material, the balance powdered inert diluent such as Al2 O3, yields an effective diffusion at 18