FIB-SEM Sectioning Study of Decarburization Products in the Microstructure of HVOF-Sprayed WC-Co Coatings

Significance 

Industrial applications for thermal spray coatings that are corrosion and wear resistant have been on the rise over the years. These coatings have proven beneficial and are replacing the toxic hard chrome plating. The new thermal spray coatings are typically carbides composed of tungsten in a cobalt or cobalt/nickel/Chromium-alloyed binder. During fabrication, their deposition demands the use of high velocity combustion thermal spray techniques, such as: HVAF and HVOF, for their relatively low deposition temperatures. However, thermal dissolution and decarburization of tungsten carbide (WC) still remains a significant obstacle in obtaining high-performance wear resistant coatings. As a result, mechanisms trying to elucidate on the aforementioned shortcoming have been proposed. Unfortunately, the exact formation mechanism of decarburization products such as metallic tungsten is yet to be established. In fact, the morphology of the observed metallic tungsten crystalline precipitates, so far, has been arbitrary.

Recently, a team of researchers at University of Surrey: Dr. Vasileios Katranidis, Professor Sai Gu, Dr. David C. Cox, Dr. Mark J. Whiting and Dr. Spyros Kamnis investigated circumstances surrounding metallic tungsten nucleation by presenting a closer examination on the three-dimensional morphological features of the tungsten carbide decarburization products. Particularly, they focused on the structure of metallic tungsten. Their research work is currently published in Journal of Thermal Spray Technology.

In brief, their experimental work commenced with the utilization of commercial agglomerate sintered powder of tungsten carbide to coat the selected substrates. They then employed HVOF coating deposition technique to deposit the coating. The coating was sprayed intentionally at an exceedingly long spray distance to exaggerate the decarburization effects. The researchers then used energy-dispersive X-ray spectroscopy (EDS) analysis to quantify and spatially discriminate against the decarburization products. Finally, plasma FIB-SEM was employed to probe the three-dimensional features of the microstructure and decarburization products.

Progressive xenon plasma ion milling of the examined surface revealed microstructural features that would have been smeared away by conventional polishing. In addition, the authors observed that the metallic tungsten formed a shell around small splats that did not deform significantly upon impact. This observation suggested that its crystallization occurred during the in-flight stage of the particles.

In summary, Vasileios Katranidis and colleagues, identified new insights on the crystallization process of metallic tungsten during the thermal spray of the WC-Co particles by studying the three-dimensional morphology of WC decarburization products in the coating. Specifically, research work of University of Surrey scientists revealed a pathway that explains the occurrence of the metallic tungsten that is commonly observed in arbitrary shapes in such coatings. Tungsten-shells that have formed in-flight, break and deform into creased flakes upon the impact of adequately large particles. In turn, those flakes appear as long aspect ratio stringers in the two-dimensional SEM images that are typically used to study these coatings.

FIB-SEM Sectioning Study of Decarburization Products in the Microstructure of HVOF-Sprayed WC-Co Coatings - Advances in Engineering

About the author

Dr. Vasileios Katranidis holds a Ph.D. in Chemical Engineering from the University of Surrey in UK, an M.Sc. in Manufacturing Technology and Management from the University of Cranfield in UK and an M.Sc. in Product and Systems Design Engineering from the University of the Aegean in Greece. He is currently a research fellow in the Chemistry Department in the University of Surrey, working on materials compatibility for space applications.

His research interests concern the applicability of Thermal Spray Coatings on complex geometries, Digital Manufacturing, EB-PVD Thermal Barrier Coatings and Industrial automation.

About the author

Dr. David Cox is a senior research fellow in the Advanced Technology Institute, university of Surrey and visiting research fellow at the National Physical Laboratory. Following his PhD in materials Science in 2000 from the University of Cambridge he has worked primarily with focused ion beam systems in a wide range of disciplines, with circa 130 publications in the fields of metallurgy, metrology, nanotechnology, optics, and device fabrication.

He currently manages the focused ion beam activities in the university of Surrey where his current main research interests are the metrology of focused ion beam systems and development of focused ion beams for deterministic ion implantation.

About the author

Dr Mark J. Whiting is Senior Lecturer in Metallic Materials in the Department of Mechanical Engineering Sciences at the University of Surrey. He has been researching metallic engineering materials for around thirty years. His earliest work was on the mechanisms of diffusional phase transformations. Such questions have remained a strong thread throughout his research. His current research portfolio is in two areas: (i) additive manufacture of metallic materials, and (ii) understanding the diverse physical phenomena that occur at metal/metal and metal/inorganic interfaces in engineering alloys. These two areas underpin various aerospace, manufacturing, automotive and defence applications.

As an advocate of metallic additive manufacture (AM), and other advanced manufacturing methods he works on understanding the diverse materials issues that are slowing the adoption of such technologies. Such issues are especially acute in medium and large scale additive manufacture where reproducibility is a key challenge. In his current research advanced microscopy plays a part in exploring the key microstructural issues in AM. Plasma focused ion beam methods and electron backscatter diffraction are the two key tools in this research.

About the author

Dr Spyros Kamnis (CEng, FIMMM) is the R&D manager at Castolin Eutectic- Monitor Coatings. Holds a BEng in Mechanical Engineering and an MSc in Thermal Power and Fluids Engineering from the University of Manchester (UMIST). He obtained his PhD in modelling of thermal spraying processes from Aston University (UK) and further developed his computational expertise in material modelling as a Post-Doctoral Research Associate at Aston University and later within the Energy Technology group in the University of Southampton (UK). He worked for Xi’an Jiaotong-Liverpool University as a lecturer for 4 years and has published 40 papers in international journals. He has long experience in R&D and he is the project lead manager in several UK government funded projects.

About the author

Professor Sai Gu joined the University of Surrey as Head of The Department of Chemical and Process Engineering in 2015 after holding various academic posts at Aston University, University of Southampton and Cranfield University. He obtained a PhD in Material Modelling from the University of Nottingham and also did his post-doc research at the University of Cambridge.

Professor Gu has an international reputation for clean energy and material research, currently leading a number of EPSRC-funded projects – worth around £2.5 million – focused on the development of advanced bio-energy and and carbon capture technologies. He has a long track-record of coordinating large collaborative projects with international partners and has successfully won over £10 million in grants from EPSRC, EU, Innovate UK and industry.

Reference

Vasileios Katranidis, Sai Gu, David C. Cox, Mark J. Whiting, Spyros Kamnis. FIB-SEM Sectioning Study of Decarburization Products in the Microstructure of HVOF-Sprayed WC-Co Coatings. Journal of Thermal Spray Technology (2018) volume 27: page 898–908

Go To Journal of Thermal Spray Technology (2018)

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