X-Ray spectroscopic investigation of the Zr-site in thin film sol-gel surface preparations

Abstract

X-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) measurements were made on thin film (∼1000 A) sol-gel adhesion promoting surface treatments. These silicon/zirconium-containing sol-gel coatings are possible replacement processes for traditional surface preparations that use environmentally undesirable and potentially toxic materials. The sol-gels were waterborne mixtures formulated with tetra-n-propoxyzirconium (TPOZ) and a silane, either 3-glycidoxypropyl-trimethoxysilane (GTMS) or 3-aminopropyl-triethoxysilane (APS). Results show that dried sol-gel powders formulated with TPOZ or TPOZ + GTMS have longer Zr—O bond lengths (∼2.18 A, CN 7 or 8) than the TPOZ + silane + metal substrate samples (∼2.10 A, CN 6). The fraction (+/− 0.10) of Zr in a short bonded 6-fold site is highest (0.80) for TPOZ + (APS or GTMS) on (Ti or Al), at an intermediate value (0.40) for TPOZ on Ti and low (0.10) for the powders. For sol-gels deposited on a metal substrate, there are indications of a chemical bond between the thin film sol-gel and the metal substrate. The TPOZ + APS coatings on Ti data suggest that this Zr—O—Ti bond is present with a Zr—Ti separation of ∼3.5 A. Only subtle differences were observed in the near-neighbor bonding due to curing treatment variations from room temperature to ∼125°C, alloy substrate variations (Ti-6Al-4V/Al 2024), and age of the deposited sol-gel coating (up to 1 year).