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Long-term Water-aging of Whisker-reinforced Polymer-Matrix Composites

Paffenbarger Research Center, 100 Bureau Dr. Stop 8546, American Dental Association Health Foundation, Building 224, Room A-153, National Institute of Standards and Technology, Gaithersburg, MD 20899-8546; hockin.xu{at}nist.gov

View larger version (20K): [in this window] [in a new window]   Figure 1. Flexural strength vs. water-aging time for whisker composites, unfilled resin, and controls A and C. Results of the 5 materials are shown in two plots because the 3 materials in plot (A) had the same polymer composition. Each value is the mean of 6 measurements, with the error bar showing one standard deviation (mean ± SD; n = 6). After 1 day, Si3N4 composite had the highest strength. However, the strength of Si3N4 composite decreased significantly in aging (two-way ANOVA; p < 0.001), while those of SiC composite, unfilled resin, and control A showed little change (p > 0.05).

View larger version (34K): [in this window] [in a new window]   Figure 2. Results of nano-indentation: (A) and (B) elastic modulus, and (C) and (D) hardness. Each value is the mean of 6 measurements with the error bar showing one standard deviation (mean ± SD; n = 6). The moduli of Si3N4 and SiC composites were significantly higher than those of the controls (Tukey’s multiple-comparison test; family confidence coefficient = 0.95). The unfilled resin showed no significant decrease in modulus; all the other materials showed significant decreases, although the amount of decrease was small. The decrease in hardness with increasing aging time was also slight.

View larger version (96K): [in this window] [in a new window]   Figure 3. SEM of fracture surfaces of specimens: (A) control A, (B) SiC whisker composite, and (C) Si3N4 whisker composite, all after 1 day’s immersion. The fracture surfaces of the controls were relatively flat. In contrast, the whisker composites had much rougher surfaces, with fracture steps (large arrows) and whisker pullout (small arrows).

View larger version (83K): [in this window] [in a new window]   Figure 4. SEM of whisker pullout on fracture surfaces of Si3N4 composite: (A) 1 day, (B) 400 days, and (C) 730 days of water-aging, with shorter whisker pullout at 400 and 730 days. Polymer remnants were observed on the pulled-out whiskers (arrows), indicating good whisker-polymer matrix bonding even after 730 days of water-aging.

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