L Ceseracciu et al. International Journal of Refractory Metals Hard Materials 23(2005)375-381 3. Discussion According to the model proposed by rhee et al. [20] it is possible to predict the tendency to brittle or quasi Fig 3 presents the surface damage obtained for dif- plastic behaviour of a given material, as the ratio of the ferent number of cycles(N=103, 104, 105)on both critical loads for the appearance of the two kinds of materials for a fixed maximum load of 500N. At a damage( PY, critical load for quasi-plasticity; Pc, critical load for ring/cone cracking ), as crack appears in the alumina, while there is little appre. Py/Pe=(D/)(H/)(H/KI)r ciable damage in the A/AZ material. For an intermedi- ate number of cycles (N= 10), the cracking in the where D=(1. 11/3)[3(1-2)/4], C=C(v)from([21].H alumina becomes more severe, while there is only minor is the hardness, KIc is the fracture toughness of the ring cracking in the A/AZ. These results are consistent material (which, in our case is taken as the apparent with the previous ones [15], where it was shown that fracture toughness), r is the radius of the indenter and the laminate showed better resistance to contact E is the effective modulus of the system indenter- damage bitrate However, for large number of cycles(N=10), the type of damage is essentially different in the A/AZ E laminated composite than in the monolithic alumina: while in the alumina, secondary ring cracks appear at where E and v are Young modulus of the indented mate- the surface, together with radial cracking, in the A/Az rial (no subscript) and indenter(subscript i) material there is a off surface around ring Eq (1)implies that, when the ratio PY/Pc is high, the cracking, that is, there is a mild chipping in the contact damage produced in the material will be mainly ring and 2 This may be due to the fact that there is more quasi- dominant damage will be quasi-plasticity cone cracking, while when the ratio PY/Pc is low the pre- plastic damage under the contact loading than in In our case, as has been shown before both materials monolithic alumina, due to the higher apparent tough- present similar values of hardness and elastic properties less of the laminated material. The presence of a (E, v), which imply similar values of parameters D and juasi-plasticity can suggest the formation of a shear C. Moreover, the indenter size and material used are driven microcracking volume under the contact area he same for both materials. The only difference is in which will provoke an inelastic deformation. the apparent toughness, which equals to approximately Fig 4 presents the microstructure observed in the A/ 3.5 MPa m /2 in the monolithic alumina and 6.0 AZ material beneath the indentation site for 10 cycles, MPa m /2 in the laminated composite together with the microstructure of the same material This difference in apparent fracture toughness for the away from the indentation. It can be seen that beneath two materials implies that the ratio Px/pe for the lami the indentation site, a large amount of microcracking nated material is approximately three times smaller can be found, in comparison with the relative crack-free than the ratio for monolithic alumina. That means microstructure. This microcracking is produced by the that the A/AZ material has a more pronounced quasi strong shear stresses generated in that volume during plastic behaviour than the MA. This behaviour, as said the indentation, which results in a macroscopic inelastic before is a consequence of the more extensive micro- deformation, that is, in quasi-plasticity cracking produced in the A/AZ material due to the μm Fig 4. Scanning electron microscope pictures of the alumina layer in A/AZ composite showing: (a)damage zone underneath the indenter, where the evere microcracking produced by shear loading can be appreciated (b) undamaged microstructure of the same material away from the indentation site. Both pictures have been taken at a depth of 50 um from the surface.3. Discussion Fig. 3 presents the surface damage obtained for dif￾ferent number of cycles (N = 103 , 104 , 105 ) on both materials for a fixed maximum load of 500 N. At a low number of cycles (N = 103 ), it is seen that a ring crack appears in the alumina, while there is little appre￾ciable damage in the A/AZ material. For an intermedi￾ate number of cycles (N = 104 ), the cracking in the alumina becomes more severe, while there is only minor ring cracking in the A/AZ. These results are consistent with the previous ones [15], where it was shown that the laminate showed better resistance to contact damage. However, for large number of cycles (N = 106 ), the type of damage is essentially different in the A/AZ laminated composite than in the monolithic alumina: while in the alumina, secondary ring cracks appear at the surface, together with radial cracking, in the A/AZ material there is a spalled off surface around ring cracking, that is, there is a mild chipping in the contact area. This may be due to the fact that there is more quasi￾plastic damage under the contact loading than in the monolithic alumina, due to the higher apparent tough￾ness of the laminated material. The presence of a quasi-plasticity can suggest the formation of a shear driven microcracking volume under the contact area, which will provoke an inelastic deformation. Fig. 4 presents the microstructure observed in the A/ AZ material beneath the indentation site for 105 cycles, together with the microstructure of the same material away from the indentation. It can be seen that beneath the indentation site, a large amount of microcracking can be found, in comparison with the relative crack-free microstructure. This microcracking is produced by the strong shear stresses generated in that volume during the indentation, which results in a macroscopic inelastic deformation, that is, in quasi-plasticity. According to the model proposed by Rhee et al. [20], it is possible to predict the tendency to brittle or quasi￾plastic behaviour of a given material, as the ratio of the critical loads for the appearance of the two kinds of damage (PY, critical load for quasi-plasticity; Pc, critical load for ring/cone cracking), as: P Y=Pc ¼ ðD=CÞðH=E0 ÞðH=KI c Þ 2 r ð1Þ where D = (1.1p/3)3 [3(1 m 2 )/4]2 , C = C(m) from [21], H is the hardness, KIC is the fracture toughness of the material (which, in our case is taken as the apparent fracture toughness), r is the radius of the indenter and E0 is the effective modulus of the system indenter– substrate: E0 ¼ 1 m2 E þ 1 m2 i Ei
ð2Þ where E and m are Young modulus of the indented mate￾rial (no subscript) and indenter (subscript i). Eq. (1) implies that, when the ratio PY/Pc is high, the damage produced in the material will be mainly ring and cone cracking, while when the ratio PY/Pc is low the pre￾dominant damage will be quasi-plasticity. In our case, as has been shown before, both materials present similar values of hardness and elastic properties (E, m), which imply similar values of parameters D and C. Moreover, the indenter size and material used are the same for both materials. The only difference is in the apparent toughness, which equals to approximately 3.5 MPa m1/2 in the monolithic alumina and 6.0 MPa m1/2 in the laminated composite. This difference in apparent fracture toughness for the two materials implies that the ratio PY/Pc for the lami￾nated material is approximately three times smaller than the ratio for monolithic alumina. That means that the A/AZ material has a more pronounced quasi￾plastic behaviour than the MA. This behaviour, as said before, is a consequence of the more extensive micro￾cracking produced in the A/AZ material due to the Fig. 4. Scanning electron microscope pictures of the alumina layer in A/AZ composite showing: (a) damage zone underneath the indenter, where the severe microcracking produced by shear loading can be appreciated (b) undamaged microstructure of the same material away from the indentation site. Both pictures have been taken at a depth of 50 lm from the surface. L. Ceseracciu et al. / International Journal of Refractory Metals & Hard Materials 23 (2005) 375–381 379