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Boron carbide (B4C) coatings have high reflectivity and are widely used as mirrorsfor free-electron lasers in the x-ray range.However,B4 C coatings fabricated by direct-current magnetron sputtering show a strong compressive stress of about-3 GPa.By changing the argon gas pressure and nitrogen-argon gas mixing ratio,we are able to reduce the intrinsic stress to less than-1 GPa for a 50-nm-thick B4C coating.It is found that the stress in a coating deposited at 10 m Torr is-0.69 GPa,the rms roughness of the coating surface is 0.53 nm,and the coating reflectivity is 88％,which is lower than those of coatings produced at lower working pressures.When the working gas contains 8％ nitrogen and 92％ argon,the B4C coating shows not only-1.19 GPa stress but also a low rms roughness of 0.16 nm,and the measured reflectivity is 93％ at the wavelength of 0.154 nm.