The phenomena of the first order phase transition (two-dimensional melting) of grain boundary at temperatures 0.6 - 0.9 T_S0 (of the solid state melting point), discovered by the author (1971), is a fundamental property of solid crystalline materials. This finding leads to a principal revision of the scientific concepts of the solid state of substance. The phenomenological description and justification of the finding are developed. The generalized equation of Clausius-Clapeyron type for two-dimensional phase transition was obtained by applying the mathematical tools of the film thermodynamics. The equation has been used for calculating the grain boundary phase transition(GBPhT) temperature T_Sf of any metal, which T_Sf value lies within the range of (0.55 - 0.86) T_S0. The investigation outcomes are applied to develop the methodology for more effective hard coating formation by synthesis of nanosize nitrides and carbonitrides in surface layers of steels and nickel alloys using a thermo-chemical processing (TChP). Production of an overall nitrogen concentration gradient from 4% to 0.5% at within surface layers leads to formation of modified coatings with a stepped change in properties. The mechanical behavior of new tools at the industrial tests indicated a higher heat resistance (nickel alloys), high resistance to surface wears and fragile breaks-down (chromium tool steels). A short overview of the results of some graded alloys characterization is presented.