A suspension high velocity oxy-fuel thermal spray manufacturing route for silicon carbide – YAG composite coatings
Graphical abstract
Introduction
SiC coating is a promising candidate for low corrosion and high wear resistant applications where low density and high temperature capabilities are required. Thermal spray is a coating deposition technique that comprises full or partial melting of the feedstock particles to produce coatings on a substrate. Thermal spraying of SiC is a major challenge as it tends to decompose into gaseous phases (around 2500 °C) under the thermal spraying conditions. There are several approaches to retard the degradation of SiC during spraying. SiC decomposition in Atmospheric Plasma Spray (APS) was retarded by creating protective atmospheres [1] or by the addition of different metallic phases, such as Al [2] or Cu [3], or ceramics such as ZrB2 [4] or ZrO2 [5] in order to prevent the oxidation of SiC. Suspension plasma spraying (SPS) uses a suspension as feedstock and SPS was also recently explored in the production of SiC coatings, since the suspension evaporation can reduce the carbide exposure to the extreme plasma conditions. In addition, to avoid SiC decomposition the incorporation of Al2O3 and ZrO2 [6], [7] was also tested to facilitate the formation of an eutectic phase or a protective ceramic such as YAG [8], [9], [10].
In order to overcome the significant challenges associated with SiC decomposition during thermal spray, this paper explores the possibility of developing a SiC-YAG coatings using a suspension HVOF thermal spray. A ceramic aqueous suspension of engineered SiC powder sintered with YAG was selected to increase the retention of SiC particles against oxidation during the spray deposition.
Section snippets
Experimental procedure
A commercially available powder (SERAM, Sweden) produced by chemical vapour deposition and presented in the form of agglomerates of SiC and YAG fine particles was used as a feedstock material. The colloidal stability of the initial powders in deionised water was studied as a function of pH and dispersant content, and was evaluated through Zeta potential measurement (Zetasizer NanoZS, Malvern, UK). Diluted SiC/YAG suspensions were prepared to a solid content of 0.1 g/L using 10−2 M KCl solution
Results and discussion
The colloidal stability of the initial powders in water is shown in Fig. 1, where the variation of zeta potential versus pH and dispersant content is represented. The suspension without the addition of any dispersant did not reach the isoelectric point in the pH range between 2 and 12. The addition of 0.1 wt% of dispersant had a small effect in the stabilisation of the suspension, but a substantial effect was observed when 0.2 wt% of dispersant was added, since the Zeta potential increased up
Conclusion
Suspension HVOF thermal spray was used to produce SiC-YAG coatings for the first time avoiding any degradation of the SiC phase during the process. A suspension feedstock was optimised by regulating dispersant content and pH. The results showed that the complete melting of the YAG phase contributes to protect SiC from degradation during the spray process. The quenching of YAG phase in the coating allowed the deposition of dense SiC-YAG coatings with a low degree of porosity (0.6 ± 0.2%) and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the Engineering and Physical Sciences Research Council (Grant Number EP/R511730/1). The authors are grateful to the Nanoscale and Microscale Research Centre (nmRC) at the University of Nottingham for providing access to microscopy facilities. Discussions with Dr. Fedrico Venturi are greatly acknowledged.
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