An expanding microwave plasma is used for surface treatments. Such a plasma allows both to select the impinging species by means of the distance between the discharge center and the substrate and to process substrates of larger dimensions. At distances close to the center of the discharge, the impinging energetic species as simple radicals and ions improve the surface reactivity. The deposition of carbon diamond layers, steel carburizing and thin metal layers nitriding treatments are performed in (Ar-CH₄-H₂), (Ar-CH₄), (Ar-N₂) and (Ar-N₂-H₂) gas mixtures. The composition and morphology of metal layers are investigated by X-ray photoelectron spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry and atomic force microscopy. The first carbon layers build on a Si substrate exposed to the plasma without any surface pretreatment consists of a high density of SiC grains of 1 10⁹ to 5 10⁹ cm^-2 where the carbon diamond particles nucleate. The carbon layers contain almost entirely Csp³ bonding when a large amount of simple radicals as CH_x<3 are selected in the plasma. High dilution of CH₄ in Ar results in an efficient steel carburizing. So, plasma species of high reactivity as CH_x<3 radicals and C_xH_y⁺ ions play a great role in the increase of the infusion of carbon at the steel surface and the storage of carbon in a 2μm thick zone with a diffusion coefficient of 10^-11 cm²s^-1. The addition of H₂ in (Ar-N₂) gas mixtures strongly increases the nitrogen diffusion into the Mo layers even at low temperature. These mechanisms are partly correlated to the reduction of oxides in the metal surface layers.