Mechanical evaluation of unbiased W–O–N coatings deposited by d.c. reactive magnetron sputtering

Tungsten nitride (WNy) and tungsten oxynitride films (WOxNy) were prepared with chemical compositions in the range 0 < x < 1 and 0 < y < 1. Besides the detection of amorphous features, the following crystalline phases were identified and related with the amount of the non-metal elements: α-W, β-W and β-W2N. All coatings exhibited compressive residual stresses between 1.5 and 5 GPa. Generally, the lower values were concerned with either O-containing coatings or the presence of amorphous phase. The values of the hardness varied from 20 to 40 and 15 to 33 GPa in W–N and W–O–N coatings, respectively. Very good correlation was detected between the residual stress and the hardness values. Moreover, a linear relationship between the hardness and the Young's modulus of the coatings was found. The scratch-test values for W–N coatings decreased from 70 to 15 N with nitrogen increase, whereas W–O–N coatings showed very low values (< 15 N) in all range of compositions.