Mechanical properties of superhard TiB 2 coatings prepared by DC magnetron sputtering

Abstract

Superhard titanium diboride (TiB 2) coatings ( H v> 40 GPa) were deposited in Ar atmosphere from stoichiometric TiB 2 target using an unbalanced direct current (d. c.) magnetron. Polished Si (0 0 1), stainless steel, high-speed steel (HSS) and tungsten carbide (WC) substrates were used for deposition. The influence of negative substrate bias, U s, and substrate temperature, T s, on mechanical properties of TiB 2 coatings was studied. X-ray diffraction (XRD) analysis showed hexagonal TiB 2 structure with (0 0 01) preferred orientation. The texture of TiB 2 coatings was dependent upon the ion bombardment ( U s increased from 0 to −300 V) and the substrate heating ( T s increased from room temperature (RT) to 700 °C). All TiB 2 coatings were measured using microhardness tester Fischerscope H100 equipped with Vickers and Berkovich diamond indenters and exhibited high values of hardness H v up to 34 GPa, effective Young's modulus E*=E/(1 –ν) ranging from 450 to 600 GPa; here E and ν are the Young's modulus and Poisson's ratio, respectively, and elastic recovery W e≈80%. TiB 2 coating with a maximum hardness H v≈73 GPa and E*≈580 GPa was sputtered at U s=−200 V and T s=RT. Macrostresses of coatings σ were measured by an optical wafer curvature technique and evaluated by Stoney equation. All TiB 2 coatings exhibited compressive macrostresses.