DEPOSITION AND INVESTIGATION OF TIN PVD COATINGS ON CAST STEEL FORMING TOOLS

PVD hard coatings are frequently applied for tools in forming processe s. Usually heat-treated, ground and polished tool steels (e.g. high-sp eed steels) are used as substrates. Therefore, the deposition of titan ium nitride onto soft steel castings with rough surfaces, as bring use d for sheet forming, needs some additional efforts. A low alloy cast s teel was coated by reactive magnetron sputtering. Intermediate layers of titanium were used to increase coating adhesion. The thickness of t hese layers was about 200 nm. They exhibited a composition gradient wi th a nitrogen content increasing from 0 at the substrate to 50% at the top. The coatings were investigated by glow discharge optical spectro scopy (GDOS) and by metallography. The scratch test was used to estima te adhesion of the coating. in spite of the high substrate roughness, the intermediate layers could be detected by GDOS depth profiles and w ere visible in metallographic preparations. The tools were used for sl ide forming of steel and aluminium sheets in a specially designed fric tion test. Friction force and contact normal load were analysed contin uously. The coefficient of friction is reduced by the coatings and con tact normal force can be increased. Failure of the coatings occurred o nly in local areas and was caused by small defects in the machined sub strate. (C) 1997 Elsevier Science S.A.