Journal of the American Ceramic Sociery-Toschi et al. Vol, 86, No. 9 A AZ (MA) after sintering after machining Fig. 1. Diagram of sample preparation sintering. Because heating and cooling rates are crucial parameters face material. The pins were cylinders with a diameter of 5 mm determining residual stresses in the structure, the sintering and spherical caps with a surface roughness R=0. 13 um. To cycle was carefully controlled, Details on sintering procedures avoid the effects of the contact temperature (i.e, relaxation of have been reported already. Final dense samples were -2 mm residual stresses and deterioration of the mechanical properties) thick, with layers of -180 um. To obtain a full, dense, stress-free previously observed. 3.25-27 loads and sliding speeds were chosen material for use as a reference, pure monolithic alumina(MA) was in accordance with the values obtained from an analysis of the prepared by cold isostatic pressing (pressure, 150 MPa)and Ashby map, with the aim of keeping the contact temperature sintered at 1600 C for I h. The density of the various samples was measured using the Archimedes method. Theoretical density of the calculated in the worst testing conditions. This was done by ing the theoretical density of the different layers and their number. foreseen tests' in the mathematical model. The additional data The porosity value was then deduced from the relative density required for this calculation were taken from direct measurements values. The grain sizes of the different alumina surfaces were on the samples and from the literature, according to the database standard prEN 623-3 For mechanical and wear tests, the shown in Fig. 2 with the points identifying the experiment surfaces of the specimens were machined to a roughness Ra 0.16±0.03 conditions used in the study, Notwithstanding some approxima (2) Mechanical Properties he experimental parameters selected, the contact temperatures involved in the tests should not exceed 100%C Vickers hardness was measured using a load of 9.81 N, Three different loads(P= 50, 100, and 150 N) and three ccording to European standard prEN 843-4. The theoretical value of 410 GPa o was taken as Youngs modulus of dense selected while the sliding distance was kept constant (15 km). The alumina. For the laminates it was calculated from this value using the polynomial equation proposed by De Portu et al., taking the temperature(22 C)and humidity(70%)of the laboratory atmo- porosity (-3%)present in these structures into account. The sphere were also kept constant. fracture toughness (K,) of the stress-free material (MA)was Before testing, the samples and pins were cleaned in an measured on bars, which were cut and machined from monolithic ultrasonic bath with acetone for 15 min and then dried at 80C for 30 min. After I h of natural cooling, the samples and pins were slice model. The apparent surface toughness of the three differ. weighed using a balance with an accuracy of 10g. After testing of indentation techniques. 23 the samples were cleaned using the same procedure described sing the same loads used to calculate residual stresses. Residual above and weighed again. The specific wear was calculated as stresses at the surface were calculated using the procedure de- weight loss divided by sliding distance and applied load. The dept scribed by Widjaja et al., by measuring the length of the cracks of the wear tracks was measured using a profilometer (Talysurf generated at the corners of the indentation impressions as a Plus, Taylor Hobson, Leicester, U. K ) The torque value Mg, used function of applied loads( 29.4, 49. 1, 98. 1, 147.2, and 196.2 N)on to calculate the friction coefficient(), was continuously moni both the stressed and stress-free material. The maximum load was tored during the tests. hosen, considering that the maximum crack length had to be less Disk surface features before and after the wear tests were han the layer thickness to ensure that all cracks were located in the nalyzed using a scanning electron microscope(SEM). extermal layer. Details on this technique can be found in the literature. (3) Wear Tests Sliding wear tests were conducted on disks using g an inverted pin-on-disk apparatus in unlubricated conditions. Pins of pure dense alumina, obtained from MA samples, were used as counter from Ref. 29