Thermal Performance

The LP-30SC fiber produced at FFF by R-LCVD has had its high temperature response and performance evaluated in several ways, in a range from 1315oC to 1700oC.  In each and every evaluation, the FFF SiC fiber has demonstrated excellent material properties and performance that exceeds that of other commercially available SiC fibers.  

The FFF SiC fiber has been exposed to high temperature furnace environments for up to 200 hours in temperature range from 1500oC to 1650oC.  The gas environments evaluated were open air (oxygen) and argon, fed into the furnace at ambient conditions.  A thin silica layer forms on the fiber surface, less than 1 micron in thickness, or about 3% of a fiber diameter. This compares very favorably with the oxide layers that form on polymer-precursor-derived fibers, which grow to thickness as high as 25% in the same conditions.

Grain growth of the laser printed LP-30SC silicon carbide fibers has been studied as well at various temperatures, exposure times and gas environments using X-Ray diffraction (XRD) techniques as seen below. Curiously, the grain size seems to achieve a limit of sorts in the mid-30 micron range. This augurs well for high temperature creep performance of this SiC fiber.  Sample grain growth data can be found in the table below.

LP-30SC SiC fiber grain growth as a function of various temperatures, exposure times, and gas environments

LP-30SC SiC fiber grain growth as a function of various temperatures, exposure times, and gas environments.

The high temperature capability and endurance of FFF’s LP-30SC SiC fiber and commercial SiC fiber products were evaluated by placing test fibers in the flame tip of an oxy-acetylene torch, reaching an exposure temperature of approximately 1625oC.  The samples were tautly mounted onto a metal frame that held the fibers securely across a gap.  Real-time video captured the performance of the various SiC fiber products in the flame tip, with FFF’s LP-30SC fiber far and away outperforming the polymer-precursor based materials, Sylramic and Hi-Nicalon Type S.

Comparative Burn Test Video

FFF LP30-SC 2 ½ Hour Burn Test Video

FFF designed, built, and tested a new approach to determining the creep behavior of single fibers (important note:  NOT single tow of fiber).  This creep rig, seen below, exposes a 1 inch gage section of fiber to elevated temperatures in the slit of a compact MHI FibHeat heater, with Keyence sensors tracking the movement of positional flags placed on the fiber sample to micron resolution.  The creep strain is determined from the permanent ‘set’ change in the flag positions upon cool down of the heater and fiber.  The creep behavior can be evaluated in a range of environments, from open air (extreme oxidation attack) to inert gas (argon/nitrogen) as the heater/fiber region can be fully enclosed and filled with a controlled gas flow.  Using a residual gas analyzer (RGA), FFF has performed argon environment creep runs in which the gas composition was determined to have approximately 0.1% oxygen remaining, which is several times lower than the residual oxygen found in the polymer precursor-based SiC fiber.

One of the most aggressive tests performed examined the cyclical creep behavior of the LP-30SC under test conditions of 1500oC, air environment, 550 MPa applied stress, and successive 24 hour hold cycles at the peak temperature.  Through 12 heat-cool down cycles, for a total of 364 hours of cumulative exposure to air at 1500oC, the FFF SiC fiber revealed a 0.66% total creep strain.

The impact of oxidation on the various SiC fibers tested in the FFF creep rig shows a stark contrast in resistance to oxygen attack.  Hi-Nicalon Type S shows significant oxide scale building and fiber morphology change while the LP-30SC presents minimal impact from the creep testing in air environment (images for both seen below).

24 hour exposure of Hi-Nicalon Type S at 1500oC in air at 550 MPa applied stress (virgin fiber to the left, exposed fiber to the right)

36 hours exposure of LP-30SC at 1500oC in air at 550 MPa stress (virgin fiber to the left, exposed fiber to the right)