An article in the June 2009 issue of Pumps & Systems detailed a new technology to treat the surface of silicon carbides. The treatment is not a coating and not homogenous with depth. Since the publication of this article, independent parties have tested the treatment and returned the results from several applications. This article summarizes some of those tests.
The technology involves treating a finished silicon carbide (SiC) component with a specific chemistry that etches the silicon from the surface of the SiC and leaves behind the carbon.
This process occurs at the nanoscale. The silicon is removed, and the carbon—which was initially reacted with the silicon and sintered into the SiC substrate—remains in its original, covalently-bonded crystal lattice. During the reaction, the carbon further reorganizes into various nanospecies and creates a treatment zone as shown in Image 1.
Image 1. Transmission electron microscope images taken at different depths within the treatment zone (Article images and graphics courtesy of Carbide Derivative Technologies.)
These transmission electron microscope images are taken at different depths within this treatment zone. Its outer zone is mostly planar graphite and disordered carbon, giving the surface a controllable, low coefficient of friction in the range of 0.08 to 0.12. Nanocrystalline diamond combined with the carbon and graphite emerges further down into the surface. Eventually, all these constituents merge with the virgin SiC, where SiC and all carbon nanospecies are present.
As the surface runs in, it operates at the robust zones in which graphite, carbon and nanocrystalline diamond provide a tough,low-friction running surface. Because it is not a coating, and the carbon is covalently bonded to itself and the supporting SiC substrate, this technology cannot delaminate, spall or peel off. Compared with current sealing surface solutions, the resulting treated surface runs cooler, longer and withstands significant periods of dry running. It has never delaminated in 25 years of rigorous testing.
The 2009 article showed results from flashing hot water tests. Specimens in that series were run for 24 to 100 hours at 250 F and 140 pounds per square inch (psi).
The specially treated specimens exhibited minimal wear scars on the mating rings between 1.7 and 4.4 micrometers (µm), but the untreated SiC specimens wear scars were between 47 and 173 µm. Furthermore, the specially treated surfaces were still smooth, but the untreated surfaces were heavily grooved.
All the recent tests, detailed in this section, had the hard/hard running combination of a treated SiC surface against another SiC-treated surface.
A seal pair was run in rigorous water test in conditions that typically damage sealing faces, as shown in Figure 1. The water test lasted for 350 hours. Next, the pump was drained, and the seal was run for another 75 minutes in dry nitrogen before a temperature spike stopped the test. This test proved extensive wet- and dry-running capabilities, even after running under rigorous wet conditions. No leakage occurred throughout the combined test.
Figure 1. Results of a mechanical seal tested for 350 hours in water followed by 75 minutes of running in dry nitrogen
This mechanical seal test included stressful water conditions followed by dry nitrogen conditions. Most demonstrations in stressful water conditions, including deionized water, caused the water to flash or simulate other intermittent dry and high-temperature running conditions. These tests, which typically destroy plain SiC sealing faces, have ranged in duration from 24 to 1,000 hours. Treated sealing faces survive such conditions with smooth faces even after the rigors of the test.
Figure 2. Results of multiple 15-minute dry-running tests on the same seal
Dry nitrogen is difficult to seal with plain, fully contacting seal faces. The 2009 article showed that treated surfaces actually ran with lower friction in pin-on-disc tests than in ambient air. Several mechanical seal tests in dry nitrogen have been conducted. Some were fully dry, without the pump being wet before the dry running. Specialized treatment for silicon carbide surfaces has survived tests in dry air or nitrogen for durations ranging from 45 minutes to four hours at aggressive pressure velocity conditions of between 6,000 and 80,000 psi-feet/minute (ft/min). The surfaces typically did not groove. They remained smooth and continued to provide a good sealing surface. These tests demonstrate that treated seal faces run well in dry air or nitrogen environments. This performance characteristic can provide long-running life for pure dry mixer seals and lightly loaded safety seals.