Mechanical and tribological properties of Sn-Cu-O films prepared by reactive magnetron sputtering

Tin-copper-oxide (Sn-Cu-O) thin films were deposited by reactive magnetron sputtering in an argon-oxygen atmosphere. The effect of the substrate temperature T_s and the amount of Cu in the Sn-Cu-O film on its mechanical and tribological properties was investigated in detail. It was found out that (i) the Sn-Cu-O films prepared at T_s = 500 C are more crystalline and exhibit a higher hardness H (up to 19.5 GPa) compared with the Sn-Cu-O films sputtered on unheated substrates at room temperature (H ≈ 11 GPa), (ii) the coefficient of friction μ of the Sn-Cu-O film is relatively high and decreases from ∼0.9 to ∼0.5 with a decreasing ratio H/E* of the hardness H and the effective Young's modulus E; here E* = E/(1 - ν²), E is the Young's modulus and ν is the Poisson's ratio, (iii) the coefficient of friction μ strongly depends on the relative humidity RH and decreases with an increasing RH of air used in the tribological test, and (iv) the sputtered Sn-Cu-O films exhibit a very low wear rate k, less than ∼2 × 10^-6 mm³/Nm. The most important result of the investigation reported is the finding that the knowledge of interrelationships between the mechanical and tribological properties of films makes it possible to optimize their tribological properties and to prepare Sn-Cu-O films with minimum values of the coefficient of friction and wear rate k.