Physical vapour deposition of aluminium-rich nanostructured/amorphous metallic coatings for wear and corrosion protection

• Main Author: Lawal, Josephine
• Other Authors: Leyland, Adrian ; Matthews, Allan
• Published: University of Sheffield 2017
• Subjects: 620.1
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.724410

This research is aimed at designing and depositing multi-functional aluminium-based Physical Vapour Deposition (PVD) coatings that can serve as a protection for light alloys, non-ferrous alloys and low alloy steels against wear and corrosion. Coatings deposition was achieved by exploring the capabilities an environmentally-friendly plasma-assisted PVD process (i.e. closed-field unbalanced magnetron sputtering method). Three deposition sequences (series 1, 2 and 3) were undertaken, using two different target configurations, in an argon or argon/nitrogen atmosphere. A wide range of coatings compositions was deposited on commonly used engineering alloy substrates (i.e. austenitic stainless steel, AISI 304; low alloy steel, AISI 4145 and high speed steel, AISI M2), as well as silicon wafer. A variety of coatings characterisations were carried out; these include: Scanning and Transmission Electron Microscopy (SEM/TEM), Focussed Ion Beam microscopy (FIB), optical microscopy, X-Ray Diffraction (XRD), Energy-Dispersive X-ray analysis (EDX) and Glow Discharge Optical Emission Spectrometry (GDOES). The International Centre for Diffraction Data (ICDD) PDF-4+/SIeve+ phase identification database and digital pattern simulation tool was demonstrated to be useful for phase identification and was also extended to crystallite size estimation. Nanoindentation and micro-hole drilling tests were carried out to assess coating mechanical properties. Coating tribological properties were evaluated by reciprocating-sliding and slurry micro-abrasion wear tests. The responses (in terms of open circuit potential (OCP) and potentiodynamic polarisation behaviour) of coatings to a neutral salt corrosive environment were also recorded. In addition, annealing was used to establish coatings thermal stability, as well as the possibility of producing nanostructured coatings by promoting devitrification in completely amorphous coatings (as deposited). A mathematical approach for analysing experimental results and predicting coating properties has also been proposed. Lightly-stressed, partially nanocrystalline and completely amorphous coatings (with a wide range of chemical compositions) were deposited. The thickness of coatings in the main deposition sequence (series 2) ranged from approximately 7 to 14 μm. Coatings showed relatively high hardness (ranging from 8 to 17 GPa) and low modulus (< 200 GPa) – matching closely the modulus of commonly used steel substrates (e.g. stainless steel). Excellent tribological properties, especially in terms of resistance to abrasion (comparable to those of common PVD ceramic coatings like CrN) were achieved. Coatings also possessed a wide range of corrosion properties, with OCP values between -0.3 and -0.6 V. Most of the coatings remained thermodynamically stable up to 600ᵒC. Results of various correlation analyses (and structural similarities of corresponding coatings in different deposition sequences) illustrate the reproducibility of the AlNiTiSiB(N) coatings deposited. The AlNiTiSiB(N) coatings presented in this Thesis are environmentally-benign and can be used in many engineering applications especially where there are concerns about simultaneous wear and corrosion attacks of the underlying metal substrate. The coatings can also be utilised for oxidation resistance in moderately high-temperature applications.