In this study, three types of WC based coating were characterised in terms of both microstructure and performance in abrasive wear, corrosion and a combination of both. The coating types were high velocity oxy-fuel (HVOF) sprayed WC-Co and WC-CoCr and a tungsten carbide-nickel alloy weld-overlay plasma transferred arc welded (PTAW) coating . For the PTAW deposit, the nominal carbide content was varied. All coatings were characterised using x-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) analysis. The abrasive wear behaviour of the coatings was examined using micro-scale abrasion and dry sand rubber wheel (DSRW) abrasive testing. Corrosion behaviour of the coatings in sulphuric acid and alkaline drilling fluid were assessed through potentiodynamic testing. In addition, the combined effect of abrasion and corrosion was assessed through the use of micro-scale abrasion. Both HVOF coatings contained angular carbides with size of N 0.5 to 5 ~lm. The main carbide is WC in both HVOF sprayed coatings. In contrast, the carbides in the PTAW coating are a combination of WC and W2C, are spherical in shape and are much larger than those in the HVOF-sprayed coatings, being N 40 to 150 f.lm in diameter. Due to variation of carbide distribution through the coating resulting from carbide sinking during solidification in PTAW coatings, the local carbide content is considered when discussing the coating behaviour. It has been shown that the wear behaviour is strongly dependent on the relative size of the carbide and the abrasive employed in the test. The wear rate was shown to decrease with increasing local carbide content in the PTAW coatings. The precipitates in the matrix phase of the PTAW coating actively contribute to increasing the wear resistance of these materials. Both HVOF coatings have similar wear resistances to each other when abraded with either alumina or silica abrasive in both the micro-scale abrasion and DSRW test. In micro-scale abrasion, the wear rate of PTAW with the largest fraction of carbides (65 wt%) was approximately 1.5 times that of the HVOF-sprayed coatings when abraded with alumina, but was more than ten times that of the HVOF-sprayed coatings when abraded with silica. In contrast, in DSRW abrasion, the wear rate of PTAW with the largest fraction of carbides (65 wt%) was approximately twice that of the HVOF-sprayed coatings when abraded with alumina, but was only around half of that of the HVOFsprayed coatings when abraded with silica.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:659208 |
| Date | January 2012 |
| Creators | Kamdi, Zakiah |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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