As is shown in Fig. 2(d), plasma-nitriding using high power of 200 W drastically decreased the sharpness of protrusions and the aspect ratio of height to diameter. In addition, the whole of the protrusions is composed of ε-(Fe, Cr)3N nitride layer (Figs. 4(b), 5(a)), where the thickness of nitride layer that AM095 formed on the matrix around the protrusions is much smaller than the height of the protrusions. It is possible that high speed impingement of nitrogen atoms to the protrusion surface under high nitriding power has resulted in a much higher temperature of the protrusions than the matrix surface. The high temperature has not only accelerated the formation of nitrides on the protrusions but also facilitated nitrogen ion sputtering of the protrusions rather than the matrix around the protrusions, which has reduced the height of the protrusions. On the other hand, when the nitriding power is 50 W (Figs. 2(b), 4(d)), the impingement of nitrogen ions with moderate speed seems to have prevented the surface from large increase in temperature and formation of thick nitride layer keeping the original sharpness of protrusions with small sputtering effect.