J. She et al. Ceramics International 26 (2000)801-805 strengths are insensitive to the increases in the indenta- MPa. Again, this predicted value from Eq.(2)is con- ion load, indicative of excellent damage resistance. In sistent with the measured strengths of 376.5, 417.3 and fact, the indentation strength, i, is related to the 376. 7 MPa for an indentation load of 200, 300 and 500 indentation crack depth, a, by N, respectively. These results have clearly shown that multilayer Al2O3/SiC ceramics can retain a significant =b(1 (2) fraction of their strength even under contact damage conditions. This should allow them to exhibit superior resistance to impact- or abrasion- induced damage in where oo is the fracture strength of the unindented sp service cimens Substituting the measured value of 497 MPa for To evaluate the R-curve behavior, the fracture resis- go and taking the indentation crack depth as the thick ance, Kr, was computed from the results of the inden ness of the individual Al,O3 layers, one can obtain a tation strengt calculated strength of 450 MPa. This suggests that as equation [16] long as the indentation crack does not penetrate specime the y rta is srlength tick vase th bnodtn4sd K, =0.S9(6)(o,- play MPa. Based on the experiments of Lawn and co-work- ers [14, 15], the indentation crack depth, a, can be esti mated fro where ai is the fracture strength of the indented spec mens at a load P. The computed fracture resistance is illustrated in Fig. 5 as a function of the estimated a=0016 (3) indentation-crack depth from Eq (3). It can be seen in Fig. 5 that the fracture resistance ing increasing indentation-crack depth, suggesting a strong where E is the Youngs modulus, H is the Vickers R-curve effect of multilayer Al,O3/SiC ceramics hardness, P is the indentation load, and Kic is the frac Also, the rising R-curve behavior was observed in the ture toughness. Using the experimental values of E= fracture toughness measurements using the SENB(sin 343.2 GPa, H=16.1 GPa, and KIC =5.32 MPa m gle-edge notched beam)method. Fig. 6 presents the the indentation crack depths at different loads were cal- measured Kic values at different notch depths, in which culated and are listed in Table 1. when the indentation the Kic was calculated from the load maximum in the load is below 200 N, the estimated indentation-crack load-displacement curve of a notched specimen. As can depth is smaller than the Al2O3-layer thickness of 182.7 be seen in Fig. 6, the measured fracture toughness Hm, indicating that the indentation crack is completely increases from 8.0 to 15.5 MPa m 2 as the notch depth contained within the outer Al,O3 layer. In this case, the increases from 0.5 to 2.0 mm indentation strengths should be kept at M450 MPa. A On the other hand, the load-displacement curves of can be seen in Fig. 4, this strength value is in good notched specimens were observed to be essentially the agreement with the experimental data. When the inden- same, except for the decreased peak load with increasing tation load is between 200 and 500 N, the indentation- notch depth. By assuming that the net section stress in induced crack may penetrate through the surface layer and into the second Al2O3 layer. Due to the deflection of such a through-thickness crack along the interface with the second SiC layer, however, it is possible for the indented specimens to retain a fracture strength of -405 Table l Estimated indentation-crack depths at different loads for multilayer Al2O3/SiC ceramics Indentation load (N) 200 30040 258.9 363.9 Fig. 5. Dependence of fracture resistance on crack depth for multi-strengths are insensitive to the increases in the indenta￾tion load, indicative of excellent damage resistance. In fact, the indentation strength, i, is related to the indentation crack depth, a, by i ˆ 0 1 ÿ a t  2 …2† where 0 is the fracture strength of the unindented spe￾cimens. Substituting the measured value of 497 MPa for 0 and taking the indentation crack depth as the thick￾ness of the individual Al2O3 layers, one can obtain a calculated strength of 450 MPa. This suggests that as long as the indentation crack does not penetrate through the outer Al2O3-layer thickness, the indented specimen may retain its strength at a value of about 450 MPa. Based on the experiments of Lawn and co-work￾ers [14,15], the indentation crack depth, a, can be esti￾mated from a ˆ 0:016 E H  1=2
P KIC " #2=3 …3† where E is the Young's modulus, H is the Vickers hardness, P is the indentation load, and KIC is the frac￾ture toughness. Using the experimental values of E ˆ 343:2 GPa, H ˆ 16:1 GPa, and KIC ˆ 5:32 MPa m1/2, the indentation crack depths at di€erent loads were cal￾culated and are listed in Table 1. When the indentation load is below 200 N, the estimated indentation-crack depth is smaller than the Al2O3-layer thickness of 182.7 mm, indicating that the indentation crack is completely contained within the outer Al2O3 layer. In this case, the indentation strengths should be kept at 450 MPa. As can be seen in Fig. 4, this strength value is in good agreement with the experimental data. When the inden￾tation load is between 200 and 500 N, the indentation￾induced crack may penetrate through the surface layer and into the second Al2O3 layer. Due to the de¯ection of such a through-thickness crack along the interface with the second SiC layer, however, it is possible for the indented specimens to retain a fracture strength of 405 MPa. Again, this predicted value from Eq. (2) is con￾sistent with the measured strengths of 376.5, 417.3 and 376.7 MPa for an indentation load of 200, 300 and 500 N, respectively. These results have clearly shown that multilayer Al2O3/SiC ceramics can retain a signi®cant fraction of their strength even under contact damage conditions. This should allow them to exhibit superior resistance to impact- or abrasion-induced damage in service. To evaluate the R-curve behavior, the fracture resis￾tance, Kr, was computed from the results of the inden￾tation strength measurements using the following equation [16] Kr ˆ 0:59 E H  1=8 i
P1=3 ÿ
3=4 …4† where i is the fracture strength of the indented speci￾mens at a load P. The computed fracture resistance is illustrated in Fig. 5 as a function of the estimated indentation-crack depth from Eq. (3). It can be seen in Fig. 5 that the fracture resistance increases with increasing indentation-crack depth, suggesting a strong R-curve e€ect of multilayer Al2O3/SiC ceramics. Also, the rising R-curve behavior was observed in the fracture toughness measurements using the SENB (sin￾gle-edge notched beam) method. Fig. 6 presents the measured KIC values at di€erent notch depths, in which the KIC was calculated from the load maximum in the load±displacement curve of a notched specimen. As can be seen in Fig. 6, the measured fracture toughness increases from 8.0 to 15.5 MPa m1/2 as the notch depth increases from 0.5 to 2.0 mm. On the other hand, the load±displacement curves of notched specimens were observed to be essentially the same, except for the decreased peak load with increasing notch depth. By assuming that the net section stress in Table 1 Estimated indentation-crack depths at di€erent loads for multilayer Al2O3/SiC ceramics Indentation load (N) Crack depth (mm) 3 12.0 10 26.8 50 78.4 100 124.5 200 197.6 300 258.9 500 363.9 Fig. 5. Dependence of fracture resistance on crack depth for multi￾layer Al2O3/SiC ceramics. J. She et al. / Ceramics International 26 (2000) 801±805 803