Improving the Wear-Resistance of BT22 Titanium Alloy by Forming Nano-Cellular Topography via Laser-Thermochemical Processing.

This paper studies the microstructure, phase composition and tribological response of BT22 bimodal titanium alloy samples, which were selectively laser-processed before nitriding. Laser power was selected to obtain a maximum temperature just a little above the αâ†”ß transus point. This allows for the formation of a nano-fine cell-type microstructure. The average grain size of the nitrided layer obtained in this study was 300-400 nm, and 30-100 nm for some smaller cells. The width of the "microchannels" between some of them was 2-5 nm. This microstructure was detected on both the intact surface and the wear track. XRD tests proved the prevailing formation of Ti2N. The thickness of the nitride layer was 15-20 µm between the laser spots, and 50 µm below them, with a maximum surface hardness of 1190 HV0.01. Microstructure analyses revealed nitrogen diffusion along the grain boundaries. Tribological studies were performed using a PoD tribometer in dry sliding conditions, with a counterpart fabricated from untreated titanium alloy BT22. The comparative wear test indicates the superiority of the laser+nitrided alloy over the one that was only nitrided: the weight loss was 28% lower, with a 16% decrease in the coefficient of friction. The predominant wear mechanism of the nitrided sample was determined to be micro-abrasive wear accompanied by delamination, while that of the laser+nitrided sample was micro-abrasive wear. The cellular microstructure of the nitrided layer obtained after the combined laser-thermochemical processing helps to withstand substrate deformations and provide better wear-resistance.

Microstructure and Friction Response of a Novel Eutectic Alloy Based on the Fe-C-Mn-B System.

This paper focuses on the microstructure and tribological properties of novel hardfacing alloy based on Fe-C-Mn-B doped with Ni, Cr, and Si. The 4 mm-thick coating was deposited on the AISI 1045 carbon steel by the MIG-welding method using flux-cored wires in three passes. The transition zone thickness between the weld layers was ~80 µm, and the width of the substrate-coating interface was 5-10 µm. The following coating constituents were detected: coarser elongated M2B borides, finer particles of Cr7C3 carbides, and an Fe-based matrix consisting of ferrite and austenite. The nanohardness of the matrix was ~5-6 GPa, carbides ~16-19 GPa, and borides 22-23 GPa. A high cooling rate during coating fabrication leads to the formation of a fine mesh of M7C3 carbides; borides grow in the direction of heat removal, from the substrate to the friction surface, while in the transition zone, carbides become coarser. The dry sliding friction tests using a tribometer in PoD configuration were carried out at contact pressure 4, 7, 10, and 15 MPa against the AISI 1045 carbon steel (water-quenched and low-tempered, 50-52 HRC). The leading wear phenomenon at 4 and 7 MPa is fatigue, and at 10 and 15 MPa it is oxidation and delamination.