Ceramic flat sliding valve
Contact
Proof of the functionality of a flat sliding valve with ceramic components for the hydraulics
Funding Period: 06/2017 – 06/2020
Motivation
For the control of fluid flows, traditional piston sliding valves with metallic cylindrical spools are primarily used in hydraulic systems. However, this kind of valve has typical corrosion and wear problems at the spool edges, which lead to the rounding of edges. Moreover, the principal gap between the spool and sleeve leads to an annular gap with a height of about 3 to 10 microns. The edge rounding and the annular gap result in an unavoidable leakage between the chambers when a pressure differential is applied. This causes a loss of performance and a reduction of efficiency of valves.
Compared to metals, high-performance ceramics have significantly higher corrosion- and wear resistance. Additionally, the high possible parallelism of the ceramic plates allows almost closed gaps. Therefore, the edge rounding and the annular gap of the piston sliding valves can be significantly reduced when the flat sliding valve with high-performance ceramic plates are applied.
Figure 1: Wear at the spool edges and principal gap between spool and sleeve lead to the application of ceramic com-ponents in flat sliding valve.
Goals
- Selection of the most suitable ceramic pair for the application of flat sliding valves
- Reduction of fracture probability by optimizing the geometry of ceramic plates
Content
- Characterization of different high-performance ceramics
- Investigation on the friction performance of ceramic pairings
- Determination of flexural strength and Weibull parameters via bending tests
- Prediction of short-term fracture probability of ceramic components
Results
- All three involved material candidates Al2O3, SiC and Si3N4 can be evaluated as fine-grained, densely-sintered ceramics with negligible porosity (example of Al2O3 see Figure 1).
Figure 1 : a) Light microscope image of Al2O3 b) Grain size distributions c) XRD phase analysis
- Figure 2 shows the average coefficients of static friction with standard deviations of all ceramic pairings in the different measurement situations. The lowest coefficient of static friction with a value of only 0.042 was measured in the dissimilar pairing of Al2O3 (LS137) with Si3N4 (polished) using HLP 46 lubricating oil.
Figure 2: a) Coefficients of static friction of Al2O3-Al2O3 under different con-tact pressures; b) Average coefficients of static friction of similar ce-ramic pairings; c) Average coefficients of static friction of dissimilar ceramic pairings.
- Laser structuring increases the flexural strength of ceramics and reduces their strength scatter, the 'economic' LS68 is a potential method in the future (see Figure 3).
Figure 3 : a) Si3N4 sample with three different laser textured surface in the middle region prepared for B3B test; b) Weibull diagram of Al2O3; c) Weibull diagram of Si3N4.
- Cracking starts at the high stress position on the slider, as predicted by FE simulation. The short-time fracture probability was calculated based on Weibull theory, Principle of Independent Action (PIA) and Weakest Link Theory (WLT). (see Figure 4).
Figure 4: a) Stress distributions at the Al2O3 slide and control plate; b) Fracture of the Al2O3 slide plate after 5 min under 225 bar.
Publications
Chao Liu, Stefan Aengenheister, Simone Herzog, Yuanbin Deng, Anke Kaletsch, Katharina Schmitz, Christoph Broeckmann: Application of Weibull theory to laser surface textured Al2O3, Journal of the European Ceramic Society, Volume 41, Issue 2, 2021,1415-1426,https://doi.org/10.1016/j.jeurceramsoc.2020.10.003.
Chao Liu, Simone Herzog, Yuanbin Deng, Anke Kaletsch, Philip Oster, Christoph Broeckmann: Influence of laser surface texturing on the flexural strength of Al2O3 and Si3N4, Journal of the European Ceramic Society, Volume 42, Issue 10, 2022, 4286-4295, https://doi.org/10.1016/j.jeurceramsoc.2022.04.013.
Funding: IGF – AiF: 19576 N/2