Tribocorrosion behavior of surface-modified AISI D2 steel

Abstract

Three different thermochemical coatings were obtained on the surface of an AISI D2 cold work tool steel under different process conditions through the use of bonding, titanizing, and borotitanizing (titanizing followed by bonding) treatments. The phases formed on the surface of the steel were identified using scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD). The thicknesses of the layers were characterized and surface hardness profiles were constructed. Wear tests were carried out using a ball-on-disk type tribometer by applying 10 N load for a total sliding distance of 250 m against an abrasive WC ball in both ambient air and 3.5% NaCl. Microstructural studies revealed a boride layer composed of FeB + Fe2B with typical sawtooth morphology in the bonded sample, a compact TiC layer on the surface of the titanized sample, and a complex polyphase layer consisting of TiB2, TiO2, Fe2B, and FeB in the borotitanized sample. Layer hardnesses ranged between 1647 and 2176 HV for the titanized sample and between 1720 and 2232 HV for the borotitanized sample. The TiC layer formed on the surface of the titanized sample was very thin compared to the layers grown on the bonded and titanized samples and had an average hardness of 2447 HV. All coatings displayed reduced wear losses compared to the untreated AISI D2 steel, both in air and 3.5% NaCl. The coated samples exhibited better wear performance in 3.5% NaCl compared to ambient air due to the lubricating effect of the NaCl solution. The enhanced wear resistance of the coated samples was attributed mainly to their superior surface hardness.