INVESTIGATION OF THE PROCESSING-STRUCTURE-PROPERTY RELATIONSHIP IN Ta-Si-N NANOCOMPOSITE COATINGS

Abstract

The present-day industry demands development of coatings for harsh environmental applications which can resist impact and oxidation at high temperatures. It is vitally important to develop new hard (>30 GPa) protective coatings which will be thermally stable at temperatures, T > 800 °C and simultaneously will ensure a good protection of the substrate against oxidation from an external atmosphere. The limitations of conventional coatings due to inferior hardness or poor oxidation stability can be overcome by nanocomposite coatings as they exhibit enhanced and completely new properties. To address this requirement of new enhanced protective systems, there is a need to get a better understanding about the effects of the elemental composition, phase composition and microstructure on the mechanical and thermal properties of the coatings. In this body of work, two transition metal nitride (TmN) systems, (Ta-N and Ta-Si-N) were explored in detail to develop an overall understanding of the relationship between the processing conditions, the microstructure, and the mechanical properties of the as-deposited thin film nanocomposite coatings. Reactive magnetron sputtering was used to deposit tantalum nitride (Ta-N) thin films on Si substrate. The effect of varying the N₂ percentage in the N₂/Ar gas mixture, varying substrate bias and varying temperature on the Ta-N film characteristics was investigated. The highest hardness of ~ 33 GPa was shown by the Ta-N films containing the hexagonal Ta₂N phase (films deposited with 5% and 3% N₂ in the gas mixture). Decreasing the N₂ content in gas mixture below 7% was found to result in microstructural refinement with grain size ~5-15 nm. The film deposited with 3% N₂ in the gas mixture exhibited nano-needle like morphology and besides displaying highest hardness, it exhibited a high hardness/modulus ratio (1.33), elastic recovery (68%) and very low wear rate (3.1×10⁻⁶ mm³/Nm). This film (deposited with 3% N₂ content in the gas mixture) remained thermally stable upto 780 °C. The knowledge gained from the study of the Ta-N system was used as a base to explore ternary Ta-Si-N coating. Relationships between the crystal structure, microstructure and mechanical properties of this coating system were discovered and explored in detail. The film crystal structure displayed the same transition as that observed in the Ta-N system, changing from face centered cubic (fcc) TaN (at 20% N₂ in the gas mixture) to a mixture of fcc TaN₁.₁₃ and hexagonal Ta₂N (at 15% N₂) to non-textured hexagonal Ta₂N (at 13% and 10% N₂) and finally to textured hexagonal Ta₂N (at 7% N₂). XPS revealed Ta-N and Si₃N₄ binding states in the films. The microstructure changed from a columnar morphology (20 - 30 nm wide columns) with visible amorphous boundaries (5-10 nm thick) at 13% N₂ content in the gas mixture, to columnar without presence of any amorphous boundaries (at 15% N₂). The films deposited with 13% - 15% N₂ content in the gas mixture, displayed high hardness of ~40±2 GPa and remained thermally stable upto 800 °C. Besides high hardness, the 13% N₂ film had high H/E* ratio of 0.11, elastic recovery of ~60 % and relatively low friction coefficient of 0.6 and low wear rate (7.09x10⁻⁶ mm³/Nm). The high hardness of the films was attributed to the dense nanocolumnar structure, with nanocrystals of ~5-10 nm in size with different crystallographic orientations within these columns.