Elsevier

Materials Letters

Volume 281, 15 December 2020, 128601
Materials Letters

A suspension high velocity oxy-fuel thermal spray manufacturing route for silicon carbide – YAG composite coatings

https://doi.org/10.1016/j.matlet.2020.128601Get rights and content

Highlights

  • Suspension high velocity oxy-fuel was used to produce SiC-YAG composite coatings.

  • A SiC powder modified with yttrium aluminium garnet (YAG) was used as feedstock.

  • The feedstock stability was optimised to obtain suspensions suitable for spraying.

  • The coatings present no evidence of SiC decomposition after spraying.

  • A liquid YAG phase avoids the decomposition of the SiC during spraying.

Abstract

Silicon carbide (SiC) coatings obtained through thermal spraying represent a major engineering challenge due to the high tendency of SiC to decompose during the spraying process. In order to avoid this decomposition, suspension high velocity oxy-fuel (SHVOF) thermal spray was used in combination with a novel SiC powder modified by addition of yttrium aluminium garnet (YAG) as the feedstock. The colloidal stability of the feedstock in water was optimised in order to obtain stable and well-dispersed suspensions suitable for SHVOF spraying. Dense SiC-YAG coatings with ~80 μm thickness were obtained and the coatings showed promising results in terms of porosity, microstructure, phase distribution and mechanical properties. Energy dispersive X-ray spectroscopy and X-ray diffraction showed a lack of degradation of the SiC phase in the coatings, thanks to the formation of a molten YAG phase during the spray process, which prevents SiC decomposition.

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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