Stress reduction in nanocomposite coatings consisting of hexagonal and cubic boron nitride

Cubic boron nitride (c-BN) can be produced by PVD and CVD techniques by intensive ion bombardment leading to highly stressed films. To overcome the problem of highly stressed films, nanocomposite coatings consisting of hexagonal and cubic boron nitride (h-/c-BN) phases have been successfully deposited by reactive rf magnetron sputtering from an h-BN target at a substrate temperature of 350 °C and a dc substrate bias of − 700 V close to the resputtering limit. The morphology of the films was characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction/reflectivity (XRD/XRR) and high-resolution transmission electron microscopy (HRTEM). At low ion energy levels corresponding to low substrate bias values of up to ≈ 200 V pure h-BN was deposited. Higher ion energies led to a strong increase of the c-BN content as well as the residual stress in the films reaching their maximum at ≈ − 300 V substrate bias before decreasing again. Both the c-BN content and the residual stress showed qualitatively the same behaviour with a steeper drop of the residual stress between the maximum and the resputter limit. HRTEM investigation of the films with the maximum densification deposited at a substrate bias of ≈ − 300 V showed an h-BN interlayer growing on the Si substrate with the basal planes perpendicular to the substrate surface on which then almost pure c-BN was identified. The density of these coatings was 3.5 g/cm3 and the residual stress was determined to ≈ − 29 GPa. Depositing boron nitride at a substrate bias of − 700 V, these films comprised a hexagonal interlayer and c-BN in an h-BN matrix on top showing a lower density of 3.2 g/cm3 and a significantly reduced residual stress of − 9 GPa. A further reduction of the residual stress can be achieved by a following heat treatment at 900 °C for 2 h resulting in a value of ≈ − 3 GPa.