Comparative XRD Analysis of the Stress State of a Thin Tungsten Ribbon and Magnetron-Sputtered Tungsten Coatings

The stress state of a thin tungsten ribbon and magnetron-sputtered tungsten coatings on a ferrite steel substrate were investigated by X-ray tensometry (sin2ψ method). In-plane biaxial compressive stresses were revealed in the samples of the tungsten ribbon: σy = −0.40 GPa and − 0.45 GPa in the rolling direction and σx = − 0.28 GPa and − 0.25 GPa in the direction perpendicular to the rolling direction from the front and back sides, respectively. The magnetron-sputtered tungsten coatings have in-plane equiaxial stresses, rotationally symmetric with respect to the surface normal (σx =σy = σφ). The stress magnitudes in the tungsten coatings are several times higher than those in the tungsten ribbon. The highest compressive stress (– 3.7 GPa) was found in a tungsten coating with a nominal thickness of 250 nm, in a coating with a thickness of 460 nm, the stress level was 1.5 times lower. The peculiarity of the analysis was that in the case of the tungsten ribbon, the line (310) was used, which certainly belongs to the precision region of the diffraction angles, while in the case of the tungsten coatings, due to the overlap of (310) α-W and (220) α-Fe lines, the lines (220) and (211) were used, which satisfy this condition to a lesser extent. The use of relatively soft Co-radiation (compared to Cu-radiation) somewhat mitigated this discrepancy. The stress-free lattice parameter (a0), corresponding to the undeformed cross section of the strain ellipsoid, was lower in the tungsten ribbon and higher in the coatings than the reference value for α-W; as the coating thickness increases, this difference increases. The reasons for these differences are discussed. Considering the broadening of the tungsten diffraction peaks, it was found that both the small size of the crystallites and the microstrain of the crystal lattice are the cause of this broadening in the samples of both groups, although the microstructure of the coatings is much more defective.