The NICO-BF process (NItrogen, Carbon and Oxygen contained Blue, Black, Bright Finished compound layer surface) The NICO-BF process is a surface modification technology which requires no plating and makes no pollution, thus keeping the environment clean and safe. It requires a temperature range of 400~720˚C for at least 30 minutes and up to 20 hours using the gases of N2, NH3, CO2, Air, C3H8 and O2 selectively for each purpose. An object can be buffed or polished after the NICO-BF process according to their roughness necessity. If necessary, the object can be just cooled down and be wax coated for better corrosion resistance. All gases are introduced into the bottom part of the furnace used in the NICO-BF process. There are fans on both the top and the bottom of the furnace to obtain a smooth gas mixture and a uniform temperature distribution. This makes it possible for a smoother circulation and supply of active gas by bottom-in and top-out when objects are charged in the furnace. This uses the light weight characteristics of NH3. The size of furnace can be vary if necessary. However, we are using the furnace 1200mmø×3000mmH at present and will use the furnace 1600mmø×4000 mmH in near future for saving processing cost. The total gas capacity used in the furnace is changed according to the NICO-BF process application. The amount of gases determines the compound layer thickness, porous layer thickness, corrosion resistance, and other factors based upon the residual ammonia gas volume analyzed from the discharged gas. 15 to 20μm thickness for low carbon steels and 18 to 24μm for medium carbon steels (AISI 1045) are obtained with 2 hours of processing at 570˚C. The rates of Fe3(C,N) phase and Fe4N phase, which consist of compound layers, and the thickness differences are managed by controlling the residual ammonia. A phase of over 95% Fe3(C,N) can be obtained with over 28% residual ammonia volume in the NICO-BF process. Medium carbon steels (AISI 1045) is the best material for corrosion resistance after the NICO-BF process and a better corrosion resistance is obtained with the wax coating process. A beautiful blue-black color can be obtained with the NICO-BF process and the color turns from black to blue according to the surface roughness. The items require that wax coats must be cooled down first with 3~5% polymer quenchant, washed the quenchant with clean water, and then coated with wax. A 30 minutes to 5 hours drying process is necessary after the wax coating process. The wax-coated mild steel after application of the NICO-BF process for 2 to 3 hours at 570 to 580˚C may last more than 500 hours of the salt spray test (5% NSST) without rusting. Only with buffing or polishing with a no wax coating will also have the same result as with wax coating. The characteristic is determined by the gas type and mixture rate applied in the NICO-BF process. In particular, the treatment in an NH3-Air-C3H3 atmosphere which performs the nitriding-oxidation reaction in the furnace does not requires extra post oxidation process. Also, the formed oxide in the furnace can be easily removed during buffing or polishing process. The NICO-BF process application for the replacement of hard Cr-plating does not requires a second oxidation process after buffing or polishing. This makes possible for the NICO-BF treated object to have an improved fatigue strength. No problems have been reported on a test of strut-type shock absorber applications for passenger cars so far. The shock absorbers installed on 40 taxis have had no problems for up to 115000 to 198000 km driving experiments. In conclusion, the NICO-BF process is superior to the regular nitriding process for the replacement of plating in tests of corrosion resistance, endurance, and manufacturing cost. Particularly, the equipment has a good reliability and durability (15 year warranty) as well as a superior productivity and better price.
By figure (1),(5) occurs Nitriding, By figure (5),(6), occurs Carburising, By figure (7), Oxynitrocarburising occurs after making an oxide layer such as Fe3O4 on the most surface of the Compound layer