Development of wear resistant and dry self-lubricating coatings on sintered steels from polymer-derived ceramics (PDCs)
Glass, Ceramics and Derived Composites
Materials in Sintering Processes and Generative Manufacturing Processes
Mechanical Properties of Metallic Materials and their Microstructural Origins
Final Report Abstract
The collaborative research between UFSC in Brazil and UBT in Germany led to the development of the novel plasma assisted pyrolysis (PAP), whereby an increased active-filler conversion and coating matrix densification are possible due to a higher degree of interaction between atmosphere, substrate and coating, when compared to the conventional pyrolysis (CP). The steel substrate was processed by sintering the composition Fe1.5Mo2.0Ni0.6C (Fe wt.%) at 1150 °C in Ar/H2, which resulted on the formation of oxidation stable phases like bainite and marstenite. The coating development was based on mixtures of a liquid commercial silazane Durazane 1800 (high ceramic yield, oxidation and thermal stability and good adhesion to metals) and ceramic filler powders with wear resistant and self-lubricating properties. The first coating system consisted of a mixture of Durazane 1800 as binding agent, ZrO2, hBN and Si3N4 as passive fillers and G018-311 glass powder as a sealing agent. The deposition of the coatings occurred by spraying of the substrates. After pyrolysis at 1000 °C in N2- atmosphere, SEM images showed a porous microstructure, which emphasized the need of active fillers, capable to react and expand, decreasing porosity and increasing the mechanical stability of the coating. The use of Ti powder enabled the formation of phases like TiCN and TiN, which embedded in a SiCN matrix, possess low friction coefficient and wear resistance properties. However, the shrinkage of the silazane during pyrolysis was not totally compensated, leading to defects. Thus, its substitution by TiSi2 led to the formation of the same phases after pyrolysis at 1000 °C, although in a lower amount, confirmed by XRD analysis. Therefore, N2-plasma atmosphere (PAP) was used to increase the filler conversion. The behavior of the TiSi2-based coating system during pyrolysis in plasma was studied by pressing and pyrolyzing the coating components and therefore excluding the influence of the steel substrate. At 900 °C, pyrolysis in plasma already yielded comparable results to the CP at 1150 °C. Additionally, by employing plasma, it was possible to reduce the porosity from 27.7 % (CP) to only 2 % at 1150 °C. At this temperature, the reaction between TiSi2 and Durazane 1800 enabled the formation of TiC, α-Ti and Si3N4, whereas not observed in the conventional process. Finally, the coating feasibility was tested and SEM and XRD results demonstrated the formation of a dense microstructure with a correspondent crystalline system as detected in the pressed specimens at 1150 °C after PAP. In order to better understand the effects of the plasma parameters on the coated samples, the bias voltage, pulse time (TON) and pressure were varied within the stability range of the process (400-600 V, 70-130 µs, 1.5-3 Torr, respectively). In this step, the filler was changed to TiB2, due to the expected formation of phases like hBN. The temperature was limited to 1000 °C due to the uncontrolled phase formation observed at 1150 °C. Whereas the bias voltage and pressure did not have any influence on the filler conversion, the relative amount of TiB2 decreased from 94 to 40 wt% when TON increased from 70 to 130 µs, due to the longer time of interaction between plasma atmosphere and sample. In this case, the formation of hard phases due to Fe-diffusion was restrained to the interface coating/substrate and a rather porous and brittle microstructure was obtained. However, the increase of the pyrolysis temperature to 1150 °C resulted in a dense and well adherent coating layer. Additionally, the formation of a TiCxNy top-layer was evidenced, which is believed to have contributed to the reduction of the friction coefficient from 0.37 to 0.31 in comparison to the uncoated substrate, although further analyses are required to better understand the mechanism of sliding and wear of this system. In further attempt to decrease the friction coefficient, 15 vol.% of the TiB2 filler was substituted by hBN and pyrolyzed at the same conditions. However, no or only slight changes in the microstructure, phase formation and friction coefficient were noticed. For future work, a more detailed study on the influence of the amount of hBN-filler on the sliding behavior and wear mechanism is suggested as well as the addition of other self-lubricating fillers.
Publications
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“A novel approach to develop composite ceramics based on active filler loaded precursor employing plasma assisted pyrolysis”, Materials & Design. 89 (2016) 893–900
M. Seifert, P. Gonçalves, T. Justus, N. Martins, A. N. Klein, G. Motz
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“Formation of Mg-silicates by reaction of perhydropolysilazane with MgO during pyrolysis in air”, Journal of the Ceramic Society of Japan. 124 (2016) 1003-1005
M. Seifert, M. L. Leite, G. Motz
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“Synthesis and high-temperature oxidation of a polymer-derived Mo-SiN based ceramic composite”, Journal of the European Ceramic Society. 36 (2016) 3601-3606
M. Seifert, G. Motz
