Combined Effects of Exposure to Salt (NaCl) Water and Oxidation on the Strength of Uncoated and BN‐Coated Nicalon™ Fibers

Commercially available SiC fibers were coated with monazite (LaPO4) using a continuous vertical coater at 1100°C. Coated fibers were heat treated in dry air, argon, and laboratory air at 1200°C for 1–20 h. The tensile strengths of uncoated and coated fibers were measured and evaluated before and after heat treatment. Fiber coating did not degrade SiC fiber strength, but heat treatment afterwards caused significant degradation that correlated with silica scale thickness. Possible strength degradation mechanisms for the coated fibers are discussed. Coating morphology, microstructure, and SiC oxidation were observed with scanning electron microscopy and transmission electron microscopy. Monazite reacted with SiC to form lanthanum silicate (La2Si2O7) in argon, but was stable with SiC in air. Despite the large coefficient of thermal expansion difference between monazite and SiC, micron thick monazite coatings did not debond from most types of SiC fibers. Possible explanations for the thermomechanical stability of the monazite fiber coatings are discussed.

“…This method needs verification for SiC fibers. SiC fibers also suffer from strength loss after oxidation 9 . An additional benefit of oxide coatings on SiC is oxidation retardation.…”

Section: Introductionmentioning

confidence: 99%

Monazite Coatings on SiC Fibers I: Fiber Strength and Thermal Stability

Boakye

Mogilevsky

Parthasarathy

et al. 2006

“…Moisture in the oxidizing atmosphere accelerates the degradation of SiC through the loss of Si(OH) 4 and the incorporation of hydroxyl groups into SiO 2 that enhance O 2 diffusion 4 . Flaws in the silica scale may be strength limiting for SiC fibers 5 . Minimization of the SiO 2 scale is therefore desirable for SiC fiber‐reinforced ceramic matrix composites (CMCs).…”

Section: Introductionmentioning

confidence: 99%

Monazite Coatings on SiC Fibers II: Oxidation Protection

Mogilevsky¹,

Boakye²,

Hay³

et al. 2006

The effect of monazite on the oxidation of SiC was studied in laboratory and dry air. Monazite inhibited the oxidation of Tyranno‐SA™, Tyranno‐ZX™, Sylramic, and Hi‐Nicalon SiC fibers. Oxidation of pure chemical vapor deposition SiC and undoped SiC single crystals was not inhibited by monazite. Dry oxidation of both uncoated and monazite‐coated Tyranno‐SA™ fibers initially displayed parabolic kinetics with an activation energy of 180–190 kJ/mol. Subsequently, the oxidation rate of monazite‐coated fibers increased and the oxide‐scale growth rate approached that of the uncoated fibers. No differences in the composition or structure of the silica oxidation product could be detected between uncoated and coated fibers using transmission electron microscopy and energy‐dispersive spectroscopy. Possible mechanisms of the inhibition of SiC oxidation by monazite are discussed.

“…These vary significantly from one application to another. In the application of nozzles for Navy aircraft engines, exposure to salt water during takeoff and landing and the resultant effects during operation is a serious concern 3–7 . In such applications, the material is often exposed to salt‐containing sea water at ambient conditions, immediately followed by high‐temperature excursions typical of the use.…”

Section: Introductionmentioning

confidence: 99%

“…The oxide composites rely on matrix porosity for their toughness, 8 and the matrix densification from reaction or impurity‐induced faster kinetics is considered the key mechanism for degradation in mechanical properties 9 . In addition, the porosity levels in the matrix are such that the fibers are also exposed to environment and fiber strengths are now known to be very sensitive to environmental attack 5,6,10 . Thus, the composite degradation could arise from either fiber degradation, or matrix degradation.…”

Section: Introductionmentioning

confidence: 99%

Combined Effect of Salt Water and High‐Temperature Exposure on the Strength Retention of Nextel^™720 Fibers and Nextel^™720‐Aluminosilicate Composites

Parthasarathy

Cinibulk

Zawada

2006

Self Cite

The relative contribution of fiber strength loss to reported degradation in the mechanical behavior of Nextel™720‐aluminosilicate composites after exposure to salt fog (ASTM B117) was explored. Single filament tension tests were performed on Nextel™720 (3M, Inc., Minneapolis, MN) fibers after immersion in NaCl solutions followed by high‐temperature exposure in air. The results were compared with the behavior of control specimens which received high‐temperature exposure but were not immersed in NaCl solution. There was no degradation in fiber strengths for NaCl solutions below 1 wt%. However, significant degradation was observed at 5 wt% NaCl upon exposure to temperatures between 900° and 1150°C, while no degradation was observed upon an exposure to 1200°C. The relative contribution of fiber strength loss to composite degradation was estimated as nearly 50%, indicating that both fibers and matrix/interface degrade from exposure to salt water. X‐ray diffraction and transmission electron microscopy of the exposed fibers and composites were conducted to help rationalize the observations. Microstructure of degraded fibers showed presence of Na at grain boundaries near the surface, without any evidence of a crystalline phase, indicating weakening from segregation or formation of an amorphous phase. The degraded composites showed that matrix and fiber/matrix interfaces had Na rich regions/phases.