Impact of deposition pressure on the structural, nanomechanical, and tribological properties of δ-TaN coatings deposited via magnetron sputtering on Ti6Al7Nb alloy

In the present study, δ-TaN thin films were deposited by reactive magnetron sputtering (physical vapour deposition) on Ti6Al7Nb alloy by varying the deposition pressure (0.8–0.2 Pa). Their crystalline structure, chemical composition, surface roughness, and surface morphology were investigated by using grazing incidence X-ray diffraction, energy dispersive spectroscopy, scanning probe microscope, and field emission scanning electron microscopy (FESEM), respectively. Structural analysis results confirmed the deposition of cubic δ-TaN thin films along the (111) basal plane; moreover, spherical dome-like surface morphology was observed by FESEM. Further, to analyze the nanomechanical properties of the deposited δ-TaN coatings, such as hardness (H) and modulus (E), scratch tests were performed utilizing the nanomechanical system. Moreover, friction and wear properties of the coating and bare sample (substrate) were investigated on nano-tribometer equipment using a rotary ball-on-disk type configuration. The stainless steel (SS-316) and silicon nitride (Si3N4) balls were used as the counter materials for the tribological tests. The observations of nanomechanical tests revealed that H (GPa), E (GPa), and H/E ratio values increased from 11.83 to 28.30 GPa, 176.02 to 248.45 GPa, and 0.06 to 0.11, respectively, with the decrease of the deposition pressure. In the scratch test, the highest critical load (cohesion failure) was found for δ-TaN coating deposited at the lowest deposition pressure (0.2 Pa). Tribological results of δ-TaN coatings demonstrated average coefficient of friction (COF) value ranges between 0.066–0.092 and 0.072–0.029 against steel and Si3N4 balls, respectively. The wear rate values were observed to vary from 8.15 × 10−5 to 7.56 × 10−6 and from 1.77 × 10−3 to 8.99 × 10−6 against steel and Si3N4 balls, respectively. Generally, the average COF and wear rate decreased against 316 SS and Si3N4 balls as the deposition pressure of coatings decreased. However, the coating deposited at 0.8 and 0.6 Pa against Si3N4 ball showed early delamination of the coating, resulting in the sudden fluctuation in COF plots and higher wear rate in the range of 10−3 mm3/N.m.