Microstructure and properties of laser-borided composite layers formed on commercially pure titanium

• Laser boriding was proposed to formation of composite boride layers on pure titanium.
• The microstructure of laser-alloyed layer consisted of TiB, TiB2 and Tiα′ phases.
• The increased hardness and wear resistance of composite boride layers were obtained.
• The wear tests of the layers showed the catastrophic wear of the counter-samples.
• The adhesion of counter-sample particles on the laser-borided samples was observed.

Laser-boriding was proposed in order to produce composite boride layers on commercially pure titanium. Three zones were observed in the microstructure: laser-borided re-melted zone (TiB, TiB2 and Tiα′-phase), heat affected zone (Tiα′-phase) and the substrate without heat treatment (Tiα-phase). The stick-like titanium borides occurred in the re-melted zone. In some areas, the tubular nature of titanium borides was visible. Among the sticks of titanium borides the needles of Tiα′-phase appeared. The high overlapping of multiple laser tracks (86%) caused the formation of uniform laser-alloyed layer in respect of the thickness. The microcracks and pores were not detected in the laser-borided composite layer. The high hardness of the re-melted zone (1250–1650 HV) was obtained. The hardness gradually decreased up to 250–300 HV in heat affected zone and up to about 200 HV in the substrate. In case of higher laser beam power used (1.95 kW), the re-melted zone was thicker and more homogeneous in respect of the microstructure and hardness. The craters obtained at the surface after the Rockwell C indentation test evidently revealed ideal cohesion of the laser-borided layer (HF1 standard). The significant increase in wear resistance of laser-borided composite layers was observed in comparison with commercially pure titanium. The lower mass wear intensity factors were obtained for laser-alloyed layers. The measurements of relative mass loss were also used in order to evaluate wear behavior of the investigated materials. The tests of laser-borided layers showed the catastrophic wear of the counter-specimens. The separated particles of counter-sample caused the accelerated wear of the laser-alloyed specimen. The longer duration of the tests, carried out without the change in a counter-specimen, caused the adhesion of counter-sample particles on the laser-borided specimen. The increased contact surface was the reason for the higher temperature and created the favourable conditions for this adhesive wear.