Titanium boride coating by high power diode laser alloying of amorphous boron with titanium and its surface property investigations

Present work deals with fabrication of superhard titanium boride coating on titanium surface with preplaced amorphous boron layer using high power diode laser alloying. Amorphous boron instantaneously converts into crystalline boron by the effect of high-power laser interaction, which rapidly drives the boron species into titanium liquid pool and form titanium boride precipitates by its convective flow. Negative Gibbs free energy (Delta G) and enthalpy (Delta H) of amorphous boron crystallization reaction introduces the concept of laser induced self-propagation synthesis. Gibbs free energy (Delta G) for the chemical reactions between Ti and B was calculated to confirm the spontaneous formation of titanium boride compound phases. The diffusion coefficient and activation energy were estimated to understand the phase formation and microstructural evolution. The structural phase, microstructure, microhardness and wear resistance of the laser borided titanium was investigated by XRD, optical and scanning electron microscopy, EDAX, Vickers microhardness analysis and linear reciprocating wear testing. The laser alloyed coating across the cross section shows formation of dendrites and whisker like microstructures. Average microhardness of 800-1600 HV0.2 was measured at different cross-sectional depths and an extreme hardness of about 3800-4700 HV0.2 was obtained in some regions of boron precipitates. The results showed a high negative value of Gibbs free energy of about -250 kJ/mol and -170 kJ/mol at similar to 2000 degrees C respectively which confirms the spontaneity of TiB2 and TiB phase formation. The activation energy of the titanium boride is estimated to be 140 kJ/mol. The laser borided coating with the formation of TiB and TiB2 phases could be useful for surface engineering applications.

Automated summary

This study fabricates a superhard titanium boride coating on titanium surface using high power diode laser alloying. Amorphous boron is transformed into crystalline boron by high-power laser interaction and rapidly combines with titanium to form titanium boride precipitates. The laser borided coating exhibits dendrites and whisker-like microstructures, with microhardness ranging from 800-1600 HV0.2 and extreme hardness of 3800-4700 HV0.2 in some regions of boron precipitates. The formation of TiB and TiB2 phases shows a high negative Gibbs free energy and an activation energy of 140 kJ/mol, indicating the spontaneity of the reaction. The laser borided coating with TiB and TiB2 phases has potential for surface engineering applications.