The refractory tungsten (W) coating due to its low sputtering yield is not easy to be obtained, and the bonds between the W coating and its substrate are weak and easily broken. A novel gradual tungstenized layer as the plasma facing armor material is prepared by using the double glow plasma surface alloying technology (DGPA). The effects of process factors on the microstructure and micro-hardness of the alloyed layers are investigated. It is found that TixW1-x gradual layers are formed in the alloyed layer. Due to the fact that the tungsten content in the gradual tungstenized layer gradually decreases from surface to the substrate, the layer is composed of both pure tungsten coating and transitional coatings (e.g. TixW1-x (x > 0.5) and TixW1-x (x < 0.5) coatings). Hence, the gradual layer can enhance the bond strength between the layer and substrate, and the improved mechanical properties such as high hardness can also be obtained. The fact is that vacancies, dislocation and crystal defects could form on the surface of the substrate due to the effect of the double glow plasma bombardment. Therefore, a particular attention is paid to the diffusion mechanisms of W atoms in the W gradual layers and a diffusion model is proposed to explain the forming process of the gradual layers according to the Fick's first law. The findings reported here demonstrate the potential of an effective surface modification method for improving the diffusivity of W atoms. The present work provides a better understanding of the gradual diffusion process of tungsten element. (C) 2017 Elsevier B.V. All rights reserved.