Thermally Sprayed Silicon Carbide Coating

Abstract

Thermal spraying of silicon carbide (SiC) material is a challenging task since SiC tends to decompose during elevated temperature atmospheric spraying process. The addition of metal or ceramic binders as a matrix phase is necessary to facilitate the bonding of SiC particles, allowing SiC coatings to be deposited. In the conventional procedure, the matrix phase is added through mechanical mixing or mechanical alloying of the powder constituents, making it difficult to achieve homogeneous distribution. In the case of thermal spraying, the melted matrix phase will not distribute uniformly and this will lead to porous coating and hence poor performances.

In order to hinder SiC decomposition during thermal spraying, in this work, the matrix phase is delivered as nano-films onto SiC particles surfaces through co-precipitation treatment from their metal salt precursor. The nano-films are aimed to limit SiC particles interaction with the plasma or flame during spraying while providing homogenous distribution of the matrix phase in the SiC particles vicinities. The selected matrix phase is yttrium aluminium garnet (YAG, ) with weight content up to 30 wt.%. The YAG co-precipitated SiC powders in this work is prepared as suspension feedstock suitable for suspension plasma spray (SPS) system and powder feedstock for high frequency pulse detonation (HFPD) and atmospheric plasma spray (APS) systems. The SPS system was chosen with the expectation to retain sub-micron structures in the coating and to avoid the needs of feedstock agglomeration, while the HFPD and APS systems were utilized to test the feasibility of thermal spray grade powders feedstock.

Three different SiC suspension slurries were prepared for the SPS deposition. The first one was prepared from mechanical mixing of powder constituents (SAY feedstock), while the other two were prepared via co-precipitation treatment without calcination (AYS feedstock) and co-precipitated calcined SiC particles (YGS feedstock). The coating x-ray analyses detect identical peaks as those found in the suspension feedstock, indicating no phase transformations occurs during the SPS process. The reduced plasma enthalpy due to evaporation of liquid carrier and the short residence time were not sufficient to favour stoichiometric mixture of  reaction into YAG compound in SAY feedstock and also inadequate to transform amorphous phase of YAG precursor into crystalline phase in the AYS coating. The SiC coatings produced through SPS system show loosely packed powder structures and poor mechanical properties regardless of their feedstock preparation.

On the other hand, HFPD system was able to deliver high quality SiC coatings from calcined and crushed SiC feedstock prepared via co-precipitation treatment and so did the APS systems when sintered and crushed SiC feedstock was used. Interestingly, crystalline phase of YAG in the feedstock was transformed fully into amorphous phase during the HFPD process and partially transformed in the APS process. Rapid solidification of the splats arriving onto colder substrate had led to the formation of the amorphous phase. This affect the hardness as the highest hardness of 484 ± 47 HV0.1 and 1253 ± 313 HV0.3 were measured in HFPD and APS SiC coatings, respectively.

The HFPD-SiC coatings tribological performance was investigated against SiC ball and AISI 440C ball under dry sliding, 3.5 wt.% NaCl solution immersion sliding and polyalphaolefin (PAO) lubricated sliding. The results indicate negligible influence of initial feedstock powder size during thermal spraying on the coefficient of friction (CoF) and wear rate. Similar conclusion also deducted when as-sprayed coating was compared with polished coating. Tribological tests of self-mated SiC demonstrate lowest CoF value in PAO lubricated sliding followed by 3.5 wt.% NaCl immersion sliding thanks to good lubrication conditions. The highest CoF was found in dry sliding in this test setting.  Different results were observed when the counterpart material is AISI 440C as the 3.5 wt.% NaCl immersion sliding gives the erratic fluctuation of CoF exceeding those of dry sliding because of severe tribo-corrosion of the AISI 440C. In contrast, PAO sliding test with AISI 440C gives the lowest CoF value since hydrodynamic or mixed fluid-film perfectly lubricates the surfaces in contact.

The APS-SiC coatings were also investigated their tribological behaviour under dry sliding against AISI 440C ball using similar testing parameters as HFPD-SiC coatings. The results indicate similar CoF values in between 0.40 and 0.50 as those recorded in the HFPD SiC coatings