Fracture Toughness of Ti/TiB2 Bionic Multilayers on the Surface of Titanium Alloy

To enhance the fracture toughness of TiB2 ceramic film, Ti/TiB2 bionic multilayers were formed by period intervention of Ti layer which was abstracted into a soft layer inspired by the structural of nacre. An improved indentation method was proposed to characterize the fracture toughness (K-IC) of the Ti/TiB2 bionic multilayers. The results show that the K-IC of the multilayers strongly depends on the modulation ratios (Lambda, the ratio of TiB2 to Ti thickness is denoted as t(TiB2):t(Ti)). With the increase of modulation ratio, the K-IC of the multilayers increases first, gets a maximum of 2.68 MPa.m(1/2) at Lambda=5, and decreases subsequently. For the same thickness of TiB2 monolayer, the K-IC is only 0.76 MPa.m(1/2); that is, the fracture toughness of the ceramic film can be enhanced effectively via formation of Ti/TiB2 multilayers. The fracture energy is released mainly in the form of radial cracks, and partly in the form of annular cracks of the multilayers deposited on the substrate of Ti6Al4V alloy. The stress of the TiB2 ceramic film can be effectively alleviated due to the periodic insertion of Ti sublayer in the multilayers. Passivation effect may occur at the crack tip, which leads to a path deflection of the crack propagation, as a result, improving the fracture toughness of the multilayers. Furthermore, the extra interface in the multilayers increases the resistance of crack growth and thus enhances the fracture toughness.