Surface Modification of Titanium-Titanium Boride Composite

Abstract

Titanium – Titanium boride (Ti-TiB) metal matrix composites are known for their high strength, stiffness and creep resistance, but they exhibit poor tribological properties. This work studies the improvement of wear resistance and corrosion resistance of a Ti-TiB composite by surface treatment using CO2 laser, without any additive materials. The Ti-TiB composite taken up for the study has an elemental composition ( by weight) of Ti-69.86%, B-15.77%, Mo- 8.51% , Fe- 5.09%, Si-0.50% and Al-0.21%. The composite has 34 vol% of TiB whiskers in beta titanium matrix. One important application of laser surface treated Ti-TiB composite is in armoured vehicles which require good ballistic resistance. Earlier works on laser surface treatment of Ti-TiB composite have always involved preplacing or adding of certain materials (additives) like boron, carbon etc. The objective of the present work is to improve wear resistance and corrosion resistance of Ti-TiB composite by structural modification on the surface using a CO2 laser treatment, without introducing any additives. Laser surface treatment was performed using a 10 kW CO2 laser system integrated with a 3-axis work station. The main variables in the laser treatment were laser power and traverse speed. Structural modifications of the surface were studied using characterization tests like X-ray diffraction analysis, optical microscopy, scanning electron microscopy and Vickers hardness tests. The wear resistance and corrosion resistance were characterized by using pin-on-disc wear test and electrochemical polarization test. In the X-ray diffraction technique the strong peaks in the treated surface were found to be due to β-Ti phase and TiB whiskers. It was found that the relative quantity of TiB whiskers increases as the incident energy of laser treatment increases. The morphology of TiB whiskers was studied using scanning electron microscopy (SEM) and optical microscopy. It was found that at higher incident energies of laser treatment, the TiB whiskers appear in more quantity and in fine size. Hardness surveys were conducted on sections transverse to the laser treated surfaces, to determine the variation of hardness from surface to core. It was found that the hardness of the laser treated surface increases with increase in incident laser energy. The surface hardness increased to about 1193 HV for incident energy of 175 kJ/m. This is considered to be a good level of surface hardening when compared to hardness of 513 HV for untreated material. The hardness profile also indicates that there appears to be a critical incident energy at around 70 kJ/m, above which the increase in surface hardness is significant. This is consistent with enrichment of TiB whiskers for specimens treated at energy higher than this level, as revealed by XRD and SEM analysis. The wear resistance of the laser treated material was studied using pin-on-disc wear testing machine. The results indicate better wear resistance for specimens treated at laser energy levels greater than 70 kJ/m. The densely formed TiB whisker particles with higher aspect ratio, formed in the surface of specimens treated with incident energy greater than the critical level (>70 kJ/m), appear to exhibit beneficial effects in improving the wear resistance of laser treated surface. The corrosion resistance of the laser treated surfaces was studied using electro chemical polarization test. The Tafel plot indicated lesser current density and hence higher corrosion resistance for specimens laser treated at incident energy of 117 kJ/m, when compared to the untreated specimen. The scanning electron micrograph corresponding to the corroded surface reveals uniform corrosion in the surface treated with higher incident energy (117 kJ/m). The enrichment of borides in the surface along with strong interface has contributed for reducing pitting corrosion.