University of Surrey

Test tubes in the lab Research in the ATI Dance Research

Tantalum CVD coatings: Process, properties, performance

Heard, Hollie N. (2019) Tantalum CVD coatings: Process, properties, performance Doctoral thesis, University of Surrey.

[img]
Preview
Text
HHeard Tantalum CVD Coatings; Process, Properties, Performance 2019 Corrections Final.pdf - Version of Record
Available under License Creative Commons Attribution Non-commercial Share Alike.

Download (9MB) | Preview

Abstract

Failure of current standard pipeline materials in chloride processing environments has a high environmental and economic impact. Tantalum (Ta) is resistant to attack in HCl environments due to its passive oxide layer. Solid and lined Ta components are, however, prohibitively expensive for large-scale deployment. Ta chemical vapour deposition (CVD) coatings on standard carbon steel pipeline materials offer an economic solution with superior properties but have been demonstrated to exhibit a dual phase nature with a metastable, hard, and brittle beta (β) phase, and a ductile alpha (α) phase, and also detrimental interfacial characteristics. This project describes the deposition of bulk α-Ta CVD coatings on standard 0.3 – 0.35 wt.%Cmax pipelines and components. The potential for tailoring of CVD processes such coatings were investigated using a hybrid statistical design of experiment approach. Deposition rate, phase fraction and Knoop hardness of coatings was measured in response to varying deposition temperature, reagent gas flows, and sample position and finish. Statistical analysis identified deposition temperature, hydrogen (H2) reagent and argon (Ar) carrier gas flows as dominant parameters. Optimal levels for the desired properties were obtained and then applied to newly installed large-scale equipment. Ta coatings on higher carbon content steels demonstrated reduced interfacial defects and a reduction in interdiffusion zone thickness. Optical micrographs using an alternate oxalic acid based etch and EBSD highlight the distribution of the deposited α/β-Ta phase within coatings. β-Ta shows preferential positioning at the surface and in the bulk, and α-Ta at the interface. XRD diffraction data is used to calculate approximate coating phase fractions that ranged from 5 – 96%. Heat treatments identified a β→α transition temperature of >950°C, correlated through exothermic peak detection in DSC analysis. Coating adhesion improved with the reduction of interfacial defects and the application of the heat treatment to minimise β-Ta content. Three-point bend testing investigated the performance of single/dual Ta coatings 20-80µm thick on A105 steel with varying post-deposition heat treatments. Single layer coatings, and those with singular heat treatments, suffered delamination on test. Samples with dual Ta coatings that had a heat treatment after both deposition processes had no visible or acoustic sign of failure after bending through 90° demonstrating ductility of the coating. Coating integrity was proved through HCl immersion testing with no measurable mass loss detected. Initial Ta coated mild steel had a corrosion rate of 11.4 MPY (0.26 mm/yr). Using the preferred two-stage deposition and heat treatment process C-ring corrosion testing of stressed A105 steel samples resulted in a ‘nil’ corrosion rate of 0.06 MPY (0.0014 mm/yr) equivalent to that of solid Ta.

Item Type: Thesis (Doctoral)
Divisions : Theses
Authors :
NameEmailORCID
Heard, Hollie N.http://orcid.org/0000-0001-7630-6856
Date : March 2019
Funders : EPSRC
Contributors :
ContributionNameEmailORCID
http://www.loc.gov/loc.terms/relators/THSWhiting, MarkM.Whiting@surrey.ac.uk
http://www.loc.gov/loc.terms/relators/THSBaker, MarkM.Baker@surrey.ac.uk
http://www.loc.gov/loc.terms/relators/THSPrentice, Calvinc.prentice@cvd.co.uk
Depositing User : Hollie Heard
Date Deposited : 07 Feb 2020 14:05
Last Modified : 07 Feb 2020 14:05
URI: http://epubs.surrey.ac.uk/id/eprint/853271

Actions (login required)

View Item View Item

Downloads

Downloads per month over past year


Information about this web site

© The University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom.
+44 (0)1483 300800