378 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju 1.5 1.4 Scan 1 Scan 2 Scan 3 sninpoW pazileuoN 1.3 12 PMR-15 1.1 Unaged modulus: 0.9 0 50 100 150200 250 Distance from specimen edge (um) Fig.9.10.Normalized modulus of PMR-15 resin after 196 h of aging at 343C Based on optical observations,the three distinct specimen regions are identified in Fig.9.10 as the higher modulus oxidized surface layer,the lower modulus unoxidized interior,and the reaction zone or transition region in which the modulus reduces from the oxidized to unoxidized values.The average thickness measurements of oxidized zones and the transition zone, as obtained from optical measurements,are plotted as dotted vertical lines in Fig.9.10.It is observed that the average thickness of the oxidation layer and active "reactive"zone measured by optical methods is in good agree- ment with the boundaries of three regions suggested by the nanoindentation data.Similar observations were made by Johnson et al.[50]using atomic force microscopy (AFM).They summarized that the outer"plateau"region is a homogeneous oxidized layer,which is a result of a zero-order reaction. The transition reaction zone is a diffusion-controlled oxidation zone,which is a result of a first-order reaction,and the third region in the specimen interior is the unoxidized PMR-15. Figure 9.11 shows the average elastic moduli of the oxidized region and the unoxidized interior of PMR-15 specimens aged at 343C as a function of aging time [78].In addition,the average elastic moduli of a PMR-15 specimen thermally aged in a nonoxidizing environment is shown in the figure.The reported average values of all the moduli measurements made within their respective regions include data from multiple scans. There is only a marginal increase in the modulus of the oxidized layer with aging time,as seen in Fig.9.11.Thus,once the material oxidizes,little change in material modulus occurs with aging time.Fig. 9.10. Normalized modulus of PMR-15 resin after 196 h of aging at 343°C Based on optical observations, the three distinct specimen regions are identified in Fig. 9.10 as the higher modulus oxidized surface layer, the lower modulus unoxidized interior, and the reaction zone or transition region in which the modulus reduces from the oxidized to unoxidized values. The average thickness measurements of oxidized zones and the transition zone, as obtained from optical measurements, are plotted as dotted vertical lines in Fig. 9.10. It is observed that the average thickness of the oxidation layer and active “reactive” zone measured by optical methods is in good agree￾ment with the boundaries of three regions suggested by the nanoindentation data. Similar observations were made by Johnson et al. [50] using atomic force microscopy (AFM). They summarized that the outer “plateau” region is a homogeneous oxidized layer, which is a result of a zero-order reaction. The transition reaction zone is a diffusion-controlled oxidation zone, which is a result of a first-order reaction, and the third region in the specimen interior is the unoxidized PMR-15. Figure 9.11 shows the average elastic moduli of the oxidized region and the unoxidized interior of PMR-15 specimens aged at 343°C as a function of aging time [78]. In addition, the average elastic moduli of a PMR-15 specimen thermally aged in a nonoxidizing environment is shown in the figure. The reported average values of all the moduli measurements made within their respective regions include data from multiple scans. There is only a marginal increase in the modulus of the oxidized layer with aging time, as seen in Fig. 9.11. Thus, once the material oxidizes, little change in material modulus occurs with aging time. 0.9 1 1.1 1.2 1.3 1.4 1.5 0 50 100 150 200 250 Scan 1 Scan 2 Scan 3 Normalized Modulus Distance from specimen edge ( µm) Unaged modulus PMR-15 378 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju