Tribological behaviour of thermally sprayed silicon carbide coatings

Highlights

• Thermal spray of SiC by detonation gun using newly patented feedstock powders.

• Tribochemical reaction of SiC in dry sliding generates SiO₂ tribo-layers.

• Hydrodynamic lubrication on PAO lubricant sliding produces lowest CoF and wear-rate.

• Tribocorrosion of the ball counterpart in NaCl sliding leads to largest wear rates.

• SiC coating is superior in dry sliding as compared to bulk LPS–SiC.

Abstract

SiC coatings have been successfully deposited using thermal spray detonation technique with a newly patented feedstock. Their tribological performance was compared to bulk SiC for dry and lubricated conditions (polyalphaolefin and 3.5 wt% NaCl solution). The lowest coefficient of friction (CoF=0.10) and wear-rate were detected with polyalphaolefin lubricant regardless of the test pair due to mixed fluid-film lubrication. Contradicting results were recorded under other test conditions. The coatings show low CoF of 0.20 in comparison to four times higher CoF of bulk SiC under dry sliding. Oppositely, SiC coatings in NaCl solution record five times higher CoF compared to bulk SiC CoF of 0.20. Such behaviour is associated with tribochemical reaction and tribo-corrosion mechanisms occurring in dry and NaCl sliding, respectively.

Introduction

Silicon carbide based ceramics have been widely employed in tribological systems for decades due to their high chemical and structural stability. Coatings of SiC have been extensively used in applications requiring low friction and high wear resistance [1]. Some of the applications include, but are not limited to, aerospace moving components [2], metal working tools [3], and protective coatings against corrosion in steel [4]. Most of these coatings are produced via dynamic ion mixing [2], RF magnetron sputtering [3], [4] or chemical vapour deposition [5]. These methods are only able to produce SiC coatings on limited size components and they are considered costly and unsuitable for large-scale machine components [6]. For larger components such as turbine bearings, highly versatile, low-cost techniques, requiring minimum equipment investment and maintenance, and that are relatively easy to perform (i.e. thermal spray) have until now failed in the production of SiC coatings [7]. In the case of thermal spray, this is due to the lack of melting point of SiC. This material tend to decomposes or sublimates at 2500–2600 °C under atmospheric conditions thus limiting their spray-ability [8]. Thermal spray is a technology that uses torches/flames at high temperatures (above 2500 °C) for melting or partially melting a feedstock material that is finally propelled towards a substrate to create a coating layer by layer [9]. Since many years, several researchers had attempted to deposit SiC coating by thermal-spraying methods with different grades of success [8], [10], [11]. In these studies, the key point is always the preparation of SiC feedstock material, which has to be adjusted by introducing metal or ceramic binders that will hinder the decomposition of SiC during spraying. Unfortunately, only limited studies on their tribological performance are available. Bach et al. [8] had performed wear tests on plasma sprayed SiC coatings and showed that the wear resistance of the coatings was about 30 times lower as compared to thermally sprayed WC–Co coating. The low wear resistance of SiC thermally sprayed coating was mainly attributed to the large porosity in the coating and the brittle silicon phase found in the matrix [8]. On the other hand, the tribological performance of bulk silicon carbide materials under various experimental testing conditions has been widely reported in dry and lubricating conditions [1], [12], [13], [14], [15], [16], [17]. These studies found the good performance of SiC self-mated pairs or rubbing against graphite or other ceramics. In most cases friction is very low, in the hydrodynamic regime, and whether the lubricating mechanisms are purely hydrodynamic or boundary lubrication (via tribochemical reaction forming silicon oxide or hydroxide) is still an open topic for discussion.

In the present work, the tribological performance of silicon carbide coatings produced by thermal spraying will be investigated and compared to bulk silicon carbide obtained by liquid phase sintering. High-frequency pulse detonation (HFPD) thermal-spraying technique has been utilised to deposit a modified SiC feedstock powder developed and patented by the authors of this work [18]. The modified SiC feedstock powder consists of nano-films of yttrium aluminium garnet (YAG) acting as matrix binders and deposited on each single SiC particle by co-precipitation technique. The goal of the oxide binders is to limit the interaction of SiC particles with the thermal spray flame, thereby greatly reducing their decomposition. Since this oxide phase is homogenously distributed, it is expected to supply a liquid phase in the SiC particles vicinities upon their melting in the flame during spraying. The oxide liquid phase will bind the SiC particles and will become a matrix of the coating increasing densification and cohesive strength of the thermally sprayed SiC coating. The prospects of thermally sprayed SiC coating in offshore applications, such as wind turbine bearings, will be assessed through reciprocating sliding tests under three different conditions namely dry, 3.5 wt% NaCl solution and PAO lubricated sliding. As comparison, liquid phase sintering (LPS)–SiC bulk material with YAG binder will be also tested. Their friction and wear behaviour under different reciprocating sliding conditions has been the focus of the investigation.