The fatigue behaviour of fine grained Al2O3 and ZrO2 toughened Al2O3 (ZTA) compositions with 15 vol% ZrO2 (3 mol% Y2O3 stabilized: 3Y-TZP) have been investigated by using three different techniques, Primarily l-point bending load was employed to generate tension-tension fatigue data under both static and cyclic conditions, The results clear-ly showed that the materials were susceptible to both the static and cyclic fatigue and the time to failure under cyclic loading was considerably shorter than the equivalent static loads. The repeated indentations at the same spot with varying loads showed a typical fatigue behaviour. In addition, both the materials were subjected to the repeated impact cycles at varying subcritical loads simulating impact fatigue, In all the cases typical fatigue curves were obtained having a progressive endurance at subcritical loads having an endurance limit, The fatigue behaviour of Al2O3 was much improved by the addition of 15 vol% 3Y-TZP. having micro-plasticity contributing towards the cyclic fatigue phenomena of these materials

Basu, Debabrata

Sarkar, Bijit Kumar

IR@CGCRI - Central Glass and Ceramic Research Institute (CSIR)

Bull. Mater.  Sci., Vol.  24, No. 2, April 2001,  pp. 101–104.  © Indian  Academy  of  Sciences.  101 Effect of zirconia addition on the fatigue behaviour of fine  grained alumina D  BASU  and B  K  SARKAR*  Central Glass and Ceramic Research Institute, Kolkata 700 032, India *Indian Association for the Cultivation of Science, Kolkata 700 032, India  Abstract. The fatigue behaviour of fine grained Al2O3 and ZrO2 toughened Al2O3 (ZTA) compositions with 15 vol% ZrO2 (3 mol% Y2O3 stabilized: 3Y-TZP) have been investigated by using three different techniques.  Primarily 4-point bending load was employed to generate tension–tension fatigue data under both static and cyclic conditions. The results clearly showed that the materials were susceptible to both the static and cyclic  fatigue and the time to failure under cyclic loading was considerably shorter than the equivalent static loads. The repeated indentations at the same spot with varying loads showed a typical fatigue behaviour. In  addition, both the materials were subjected to the repeated impact cycles at varying subcritical loads simulat ngimpact fatigue. Inall the cases typical fatigue curves were obtained having a progressive endurance at subcriti-cal loads having an endurance limit. The fatigue behaviour of Al2O3 was much improved by the  addition of 15 vol% 3Y-TZP, having micro-plasticity contributing towards the cyclic fatigue phenomena of these materials.  Keywords. Fatigue; static/cyclic loading; microplasticity.   1. Introduction  The failure under fatigue conditions, at loads much below the critical failure strength is a common phenomena in all the materials including ceramics. The pioneering observa-tion of Gernet (1899) on the strength dependence of glass on the rate of loading remained unexplained for nearly half of a century till Charles (1961) and others (Hillig 1962; Phillips 1965) explained the stress related chemical reactions between water vapour and glass surface. Pear-son (1950) claimed that the deterioration in strength under static fatigue of Al2O3 too was dependent on the com-bined effect of moisture and stress. Krohn and Hassleman (1972) demonstrated a thermally activated process in the cyclic fatigue of Al2O3. Ko (1986, 1989) correlated the static bending and fatigue strength of sintered Al2O3 by an S–N curve generated from the experimental values. Guiu and co-workers (Guiu and Vaughan 1986; Vaughan and Guiu 1987; Vaughan et al 1987; Reece and Guiu 1990) introduced a novel technique to study the tribological  effect under normal/fluid environment on the fatigue properties of bio-grade Al2O3. Attempting to stimulate the fatigue conditions, they performed repeated indentations with varying subcritical loads on the same position having a pre-indentation with a higher load till chipping occurred.Thus an S–N curve was constructed which differe  from the similar curves obtained by cyclic loading. Several workers (Sarkar and Glinn 1969; Huffine and Berger 1977; Maity et al 1994) have demonstrated the suscepti-bility of Al2O3 ceramics to an impact fatigue and  explained that the stresses induced by shock were more damaging. It has been widely reported (Pabstet al 1980; Becher 1983; Orange et al 1988) that the static fatigue parameters of Al2O3 was significantly improved by ZrO2 addition but for the cyclic fatigue the effect was not significant. It has been recently claimed (Mencik 1984) that the toughening mechanism operative in ZrO2/Al2 3–ZrO2 systems impart-ing a crack resistance (R) curve often results into a poor resistance to the cumulative damage of cyclic fatigue. A clear understanding of the degradation behaviour however, has ot been developed.  In the present paper the fatigue behaviour of Al2O3  reinforced with 15 vol% 3Y-TZP under the 4-point bend-ing, indentation under both static and cyclic conditions and repeated impact loading were investigated. The  fatigue properties of both the materials w e compared. An attempt was made to correlate the fatigue resistance p rameters arising out of the different techniques adopted to explain the fatigue behaviour of such materials. 2. Experimental 2.1 Specimens Al2O3 (XA-16, USA) powder of 0×7 mm size was mixed with 15 vol% ZrO2 partially stabilized with 3 vol% Y2O3 (3Y-TZP) (Toyosoda Corporation, Japan) having 0×2 mm  *Author for correspondence D  Basu  and  B  K  Sarkar  102and milled in a planetary mill for 4h in isopropanol  medium. The mixture was subsequently dried and  calcined at 700°C and reground with an addition of poly-vinyl alcohol. The mixed powder was uniaxially com-pacted into 55 ´  5 ´  5 mm bars at 100 MPa followed  by isostatic compression at 150 MPa. Green cylindrical samples were prepared for the impact fatigue studies. The samples were presintered at 800°C and finally sintered at 1550°C for 1 h. The sintered grain sizes of Al2O3 and ZrO2 were 1–3 mm and 0×3–0×4 mm respectively. The  sintered density was measured by the water displacement method. The transverse flexural strengths (TRS) were measured on a 4-point bending fixture with a stressing rate of 10 MPa sec–1 using 1185 Instron machine. Hard-ness was measured using a Vicker’s hardness tester. The Young’s modulus was estimated by an ultrasonic velocity measurement using Kraut Kramer USIP 12 tester intr-faced with a Philips 3350, 100 MHz Oscilloscope. The fracture toughness, KIC, was determined from the length of the cracks generated at the corners of the Vicker’s  indentation with different loads from   KIC = 0×016(E/H)1/2 P×C–3/2, (1)  where E is the Young’s modulus, H the hardness, P the indentation load, and C the crack length. The properties of the sintered composite specimens compared with Al2O3 is given in table 1. 2.2 Static and cyclic fatigue The static fatigue behaviour with constant load was  studied on a 4-point bending fixture in an electro-s rvo hydraulic Instron machine of 20 KN capacity. The  corresponding failure time for each subcritical applied load was recorded for an S–T plot. The specimen were loaded at the rate of 2N/sec up to a predetermined level and during the operation the instrument was kept under load control mode with a narrow range. The cyclic  fatigue was studied under a sinusoidal stress wave form within a stress ratio of 0×5 and frequency of 5 Hz. The mean value of the cyclic stress was maintained identical with the static stress. A computer software recorded all the load deflections data along with the number of cycles required for fracture. 2.3 Indentation fatigue The repeated indentations with 300, 200, 156× 5, 100, 50 and 30 N loads were made on the same spot of the speci-men with a Vicker’s hardness indenter. The experiments were repeated five times for each load maintaining inden-tation for 60 sec and the corresponding number of cycles to chipping was noted for an S–N plot. 2.4 Impact fatigue A modified Charpy impact tester with a swinging pendu-lum was designed and constructed for the repeated impact (fatigue) test similar to Sarkar and Glinn (1969) and Maity et al (1994). A pendulum length of 38×3 cm and a hammer weight of 0×2358 kg were used in the  present investigation. The impact energy, E1 was calcu-lated by using    E1 = mgh (1 – cosq),  (2)  where m is the mass of the hammer, g the gravitational constant, h the height of pendulum and q the angle of  release of the hammer. Energy losses due to friction, windage, and toss losses were calibrated with a broken specimen held between the grips, from the differences between the angle of release and the angle of the final upswing of the hammer. Considering the failure to be  under tension at the bowed and the impact stress s, it was calculated from the modification of the equation proposed by Timoshenko and Young (1968). Thus,    s = P/A + (P2/A2 + 2Ei×E/A×l), (3)  where E is the Young’s modulus, P the load, A the cross-section of the specimen, l the length of the specim n. Table 1. Physical properties of sintered Al2O3 and Al2O3 + 15 vol% 3Y-TZP ZrO2.       Properties Al2O3 Al2O3 + ZrO2 (ZTA)       Sintered density (g/cc) 3×95 4×26 Porosity (%) < 1×0 < 2×0 Transverse rupture strength (MPa) 290 775 Fracture toughness (MPa×m1/2) 3×5 5×7 Hardness (GPa) 17.8 17.0 Young’s modulus (GPa) 380 320       Figure 1. Static and cyclic fatigue of Al2O3 and ZTA with 15 vol% 3Y-TZP.  Zirconia  effect  on fatigue  behaviour  of alumina 103  The critical single blow impact strength was determine  from the release of the hammer from the highest angle at which the sample broke, thereafter the angle was reduced at 5° intervals and the number of impacts endured was noted to plot an S–N curve. The frequency of impacts was set at 8 impacts/min. 3. Results and discussion Figure 1 represents the results of the static and cyclic  fatigue of Al2O3 and 3Y-TZP ZrO2 reinforced Al2O3. The logarithm of the constant stress in case of the static tests or the peak (smax) in the cyclic tests is plotted against  the logarithm of both time, t and number of cycles, N tofailure.  The experimental data was thus fitted to a relationship    log smax = 1/n log N(or t) + C, (4)  where C is a constant and  the fatigue resistance  parameter. It was apparent that n for static and cyclic conditions for Al2O3 increased from 30 to 25 to 39 and 27 respectively by the additions of ZrO2 in agreement with the observations made by Guiu and others (Guiu 1978; Reece et al 1989). The results of the indentation fatigue with 100 and 200 N loads for both the static and cyclic conditions are shown in figure 2.  It was revealed that in the case of the static loading on both the materials the fatigue resistance parameter or the endurance limit to the breaking stress ratio did not change significantly and the moderate deviation was well within the scatter of the data points. The above two parameters varied widely during cyclic loading particularly for the composition with the 3-YTZP although no clear-cut trend was visible. In this material the cyclic fatigue limit was 27, while the specimen with Vicker’s indent of 100 N (c = 131 mm) and 200 N (c = 205 mm) were 20 and 18 respectively. Similarly the endurance limit to the breaking stress ratio for this material without preindentatio  was 0×726 and 0×629 which was reduced to 0×664 and 0×538, 0×697 and 0×451 when indented with the Vicker’s indent of 100 N and 200 N respectively.  The differences in the indentation fatigue behaviour between the two materials is well demonstrated in figure 3. It indicates that although the number of indentation cycles required to produce chipping increased with decrease in indentation load but this dependence was small in com-parison to the conventional fatigue data. Further, the wide scatter in the data restricted any clear interpretations.  Figure 4 represents the result of the repeated impact tests on Al2O3 and ZTA.  Both the materials demonstrated a clear S–N curve which was in agreement with the findings of Sarkar and Glinn (1969). The mechanism of such a fatigue failure had been discussed (Maity et al 1994). The n value of 5×  for Al2O3 obtained by them was less than the value 14 obtained from the static and cyclic loading in the present Figure 2. Static and cyclic fatigue of ZTA with 3Y-TZP revealing the effect of indentation cracks generated in the system.  Figure 3. Dependence of chipping on indentation load for Al2O3 and ZTP with 15 vol% 3Y-TZP.  D  Basu  and  B  K  Sarkar  104investigation. The reason could be attributed to the purity of the material. Our material had a purity of 99×7% against 96% of Sarkar and Glinn (1969). The results  further indicated that although 3Y-TZP additions  improved the endurance limit of the composite samples from 270×5 to 395×  MPa with an improved strength  but the fatigue resistance parameter did not change. The  fatigue ratio (the ratio between the endurance limit and single impact strength) however, decreased from 0×71 or 3Y-TZP to 0×52 for Al2O3 imperative of the former being more prone to failure by a sudden shock. 4. Conclusions The addition of 15 vol% 3Y-TZP in Al2O3 had increased the static fatigue resistance parameter ‘n’ from 30 to 39, and so did the endurance limit. The improvement from 25 to 27 in cyclic fatigue was, however, marginal. Indenta-tion fatigue revealed an S–N type curve having a progre-ssive endurance with decreasing subcritical loads having an endurance limit, the magnitude of which depended on the pre-applied indentation load. Strengtheni  Al2O3 with TAP improved the fatigue behaviour.  Impact fatigue studies also demonstrated a typical S–N behaviour having  = 14 for both the materials. The  endurance limit of the composite was improved from 270×5 to 395× . The fatigue ratio was lower than that of pure Al2O3. The static fatigue behaviour of these materials was superior to the cyclic fatigue. While moisture has been attributed for the static fatigue phenomena the  susceptibility to the cyclic fatigue is a clear evidence of the existence of micro-plasticity, over and above the role played by moisture, in contributing to the failures under fatigue. Acknowledgements The authors are grateful to CSIR, New Delhi, for  providing the necessary funds. Dr R W Steinbrach,  Energietechnik Forschungszentrum, Julich, Germany, is acknowledged for providing facilities to conduct  mechanical and indentation fatigue studies.References Becher P F 1983 J. Am. Ceram. Soc. 66 483 Charles R J 1961 Progress in ceramic science (ed.) J E Burke (New York: Pergamon Press) vol. I Davidson D L, Campbell J B and Lankford J 2001 Acta Metall. (in press) Evans A G and Fuler E R 1974 Met. Trans. 5 27 Green D J 1982 J. Am. Ceram. Soc. 65 610 Grenet L 1899 Bull. Soc. Enc. Industr. Nat. Paris 4 838 Guiu F 1978 J. Mater. 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Ceram. 10  603 Pearson S 1950 Royal Aircraft Establishment Report p. 51 Phillips C J 1965 A M Scientist 53 20 Reece M J and Guiu F 1990 J. Am. Ceram. Soc. 73 1004 Reece M J, Guiu F and Sammur M F R 1989 J. Mater. Sci. 72 348 Sarkar B K and Glinn T G J 1969 J. Mater. Sci. 4 951 Timoshenko S P and Young D H 1968 Elements of strength of materials (New York: Littleton Educational Publishing Inc) Vaughan D A J and Guiu F 1987 Engineering with ceramics (eds) R Freer, S Newman and G Syers (Shelton Stoke- -Trent, UK: Institute of Ceramics) 2 Vaughan D A J, Guiu F and Dalmau M R 1987 J. Mater. Sci. Lett. 689  Figure 4. Impact fatigue of Al2O3 and ZTA with 15 vol% 3Y-TZP.  

Effect of zirconia addition on the fatigue behaviour of fine grained alumina 

Author(s): Basu D; Sarkar BK

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Effect of zirconia addition on the fatigue behaviour of fine grained alumina Author(s): Basu D; Sarkar BK

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