PROPERTIES OF TIN COATINGS DEPOSITED ONTO HOT WORK STEEL SUBSTRATES PLASMA NITRIDED AT LOW-PRESSURE

The properties of the surface structure of a plasma nitrided layer beneath a TiN overcoating were investigated. The substrates were AISI H11 grade steel quenched and tempered according to the manufacturer's recommendation. Prior to deposition of the coating the substrates were plasma nitrided at a pressure of 5 Pa in a specially designed sputter ion plating unit with an auxiliary anode. The same unit was used for deposition of the TiN coating and plasma nitriding of the substrates. Several substrate induced effects were found to be caused by the previous plasma nitriding of substrates. An excellent coating to substrate adhesion was found by measuring the critical load as defined in the scratch test method. Two modes of coating failure, cohesive and adhesive, were detected and the corresponding critical loads measured. Adhesive failure of the coatings starts at loads in excess of 100 N. The substrate induced preferred orientation of the TiN coating was analyzed by X-ray diffraction of non-nitrided, conventionally plasma nitrided and low pressure plasma nitrided substrates. It is shown that the preferred orientation of coating growth changes from (111) to (220). The conventionally plasma nitrided/TiN coating composite may be considered as intermediate between non-nitrided and low pressure plasma nitrided layers with deposited TiN coating. Compressive stresses were found in both the plasma nitrided layer and the TiN coating.