Thick Physical Vapour Deposited TiB 2 Coatings

Direct current magnetron sputtering combined with electron assisted growth has been used to grow thick TiB₂ coatings. The coatings were evaluated with respect to structure, using X-ray diffraction (XRD), and morphology, using scanning electron microscopy (SEM). Fundamental mechanical properties were examined, such as hardness (52 GPa) and Young's modulus (590 GPa), using nanoindentation testing. The fracture toughness K_c was determined to be 4·1 MPa m^1/2 based on the length of radial cracks in Vickers indentation (the Anstis method). This deposition process, which yields low stressed magnetron sputtered TiB₂ coatings makes it possible to obtain 60 μm thick coatings with excellent practical adhesion to the substrate. Scratch testing revealed that no critical failure (i.e. cohesive spalling) has occurred below 70 N. The improved scratch test behaviour is attributed to the increase in load carrying capacity by depositing a thick coating. However, small cohesive cracks were generated above 35 N. This study shows that very thick, hard and stiff coatings, with good adhesion, can be grown using the physical vapour deposition (PVD) technique if the residual stress state is controlled. One technology for that is magnetron sputtering with electron assistance.