L Ceseracciu et al International Journal of Refractory Metals& Hard Materials 23(2005)375-381 applying the equation proposed by Antsis et al.[17] yielding the values of KMA=3.5+0.8 MPa m /2for the 6.0±0.8MPam1/2 AAZ for the outer alumina layer in the laminated material. If this difference in apparent fracture toughness values is solely attributed to the existence of a homogeneous MA residual stress, the mean value of this residual stres in the zone of the surface interested by the crack can be evaluated to be 183 MPa [15]. Hertzian contact loads were applied on the laminated composites and in the reference MA material. This was carried out at the top layer with a wC-Co spherical in- denter of 2.5 mm. As expected due to the relative small grain size of the alumina, the main mechanism for dam- 10000 age showed to be cone cracking, and, for that particular Number of cycles indenter size, the critical load for cone cracking under Fig. 2. Applied indenter load against time for cyclic loading tests for monotonic loading rate was Pa=607+ 50N for the alumina(MA)and laminate(A/AZ), after[15] Ranges indicate alumina and PA/Az=675+50N for the alumina outer development of ring crack damage. Lines are best fits layer in A/AZ composite As before mentioned, the appearance of ring cracking in the sphere. Cyclic loading was carried out between [18, 19] in the surface of the samples was previously 50N and 500n by using a sinusoidal wave shape at determined as a function of number of cycles [15]. The a frequency of 10 Hz for different number of cycles experiment tal results of this study are summarized in (N= 10, 10, 10). The damaged surface of both ag.2. After these results, a load of 500N was selected laminated composite and monolithic alumina were sub- for studying the evolution of damage with increasing sequently observed, noting the different damage pro- number of cycles duced under repetitive contact loading. Fig 3 presents he typical surface damage on the samples after the cyc lic loading 23. Contact fatigue tests Several samples were also polished in the damaged surface after testing, in order to observe the microstruc Load was applied with a wC-Co sphere of 2.5 mm ture damage as a function of depth. This was done with diameter in a universal servohydraulic test machine a 30 um diamond suspension applying a low load. The (Instron 8500). Care was taken to properly hold the depth of material removal was measured with a microm pecimen in order to avoid small displacements and, eter of 10 um precision. Some samples were ground and consequently, fretting fatigue. The wC-Co sphere was finally polished in the perpendicular plane to the in- periodically inspected for damage, and rotated or re- dented surface in order to observe under Sem, the dam placed when some damage or deformation was observed age produced 10 cycles 10 cycles 10 cycles AAZ Fig. 3. Damage produced in A/AZ and MA by Hertzian fatigue tests under load of 500N and different numbers of cycles. Although A/AZ has a etter resistance to appearance of ring crack, it presents an apparent higher surface degradation(chipping) for severe conditions(N=10 cycles)applying the equation proposed by Antsis et al. [17], yielding the values of KMA I c ¼ 3.5
0.8 MPa m1/2 for the monolithic alumina and KA=AZ I c ¼ 6.0
0.8 MPa m1/2 for the outer alumina layer in the laminated material. If this difference in apparent fracture toughness values is solely attributed to the existence of a homogeneous residual stress, the mean value of this residual stress, in the zone of the surface interested by the crack can be evaluated to be 183 MPa [15]. Hertzian contact loads were applied on the laminated composites and in the reference MA material. This was carried out at the top layer with a WC–Co spherical in￾denter of 2.5 mm. As expected, due to the relative small grain size of the alumina, the main mechanism for dam￾age showed to be cone cracking, and, for that particular indenter size, the critical load for cone cracking under monotonic loading rate was P A c ¼ 607
50 N for the alumina and P A=AZ c ¼ 675
50 N for the alumina outer layer in A/AZ composite. As before mentioned, the appearance of ring cracking [18,19] in the surface of the samples was previously determined as a function of number of cycles [15]. The experimental results of this study are summarized in Fig. 2. After these results, a load of 500 N was selected for studying the evolution of damage with increasing number of cycles. 2.3. Contact fatigue tests Load was applied with a WC–Co sphere of 2.5 mm diameter in a universal servohydraulic test machine (Instron 8500). Care was taken to properly hold the specimen in order to avoid small displacements and, consequently, fretting fatigue. The WC–Co sphere was periodically inspected for damage, and rotated or re￾placed when some damage or deformation was observed in the sphere. Cyclic loading was carried out between 50 N and 500 N by using a sinusoidal wave shape at a frequency of 10 Hz for different number of cycles (N = 103 , 104 , 105 ). The damaged surface of both laminated composite and monolithic alumina were sub￾sequently observed, noting the different damage pro￾duced under repetitive contact loading. Fig. 3 presents the typical surface damage on the samples after the cyc￾lic loading. Several samples were also polished in the damaged surface after testing, in order to observe the microstruc￾ture damage as a function of depth. This was done with a 30 lm diamond suspension applying a low load. The depth of material removal was measured with a microm￾eter of 10 lm precision. Some samples were ground and finally polished in the perpendicular plane to the in￾dented surface in order to observe under SEM, the dam￾age produced. Fig. 2. Applied indenter load against time for cyclic loading tests for alumina (MA) and laminate (A/AZ), after [15]. Ranges indicate development of ring crack damage. Lines are best fits. Fig. 3. Damage produced in A/AZ and MA by Hertzian fatigue tests under load of 500 N and different numbers of cycles. Although A/AZ has a better resistance to appearance of ring crack, it presents an apparent higher surface degradation (chipping) for severe conditions (N = 105 cycles). 378 L. Ceseracciu et al. / International Journal of Refractory Metals & Hard Materials 23 (2005) 375–381