Hardness and abrasive behaviour of chrome-doped TiO2 obtained by electrochemical anodization in an HCl-based solution

One-step chromium incorporation into TiO2 films was performed by an electrochemical anodizing process, using HCl as anodizing base solution and K2Cr2O7 as chromium precursor salt. Due to the corrosion process that arises from the use of high concentrations of HCl, the preparation of two different electrolytes was considered in order to carry out the anodizing and doping: one with ethylene glycol and the other with glycerol. Energy dispersive spectroscopy confirmed the presence of chromium in the elemental composition of the synthesized films, which reached a maximum value of 1.42%. The FE-SEM analysis revealed that the presence of chromium modified the characteristic nanotubular morphology into compact nanometric grains. X-ray diffraction patterns demonstrated that chromium integration into the structure facilitated the transition from anatase to rutile after annealing at 700°C. The presence of chromium improved hardness, since the values obtained from the doped samples exceeded those obtained from titanium and TiO2 without doping. After annealing at 700°C, the sample with the highest amount of chromium reached a hardness value of 7.6 GPa. The microtribological analysis highlighted the improvement of the coefficient of friction for samples with chromium addition, as well as of film resistance to fracture during the tests.