SYNTHESIS AND CHARACTERIZATION OF MgO-Cr2O3-ZTA CUTTINGudTOOL MATERIAL

摘要

The effect of MgO and Cr2O3 addition on the microstructure and mechanicaludproperties of ZTA ceramic composite were investigated in this study. Variousudamounts of MgO and Cr2O3 were added into ZTA separately. The starting materialsudwas mixed continuously for 8 hours and subsequently hydraulically pressed at 300udMPa. The pressed samples were sintered at 1600 oC for 4 hours. There are three partsudin this study. The first part consists of addition of MgO into ZTA. Second part isudfocused on the effect of MgO particles sizes on ZTA mechanical properties and theudthird part is focused on the effect of adding Cr2O3 on ZTA and ZTA-20 nm MgO.udFor the first part, the results show that an addition of 0.7 wt % of 100 nm MgOudproduces the highest Vickers hardness (1710 HV). The fracture toughness decreasedudgradually from 5.93 MPa.m1/2 to 3.79 MPa.m1/2 with further addition of 1.0 MgO wtud%. MgO solid solubility in ZTA was determined around 203 ppm. No newudmechanism was found with the addition of MgO into ZTA. Microstructuraludobservations show that the NL value for grain size is significantly dependent on theudamount of MgO addition. The NL value increase from 0.68 grains/μm to 2.21udgrains/μm with the addition of 0.7 wt % of 100 nm MgO. The increase of hardness isuddue to the small grain size of Al2O3 which is caused by the microstructure pinningudeffect by MgO. In tool wear measurement, an improvement of 30 % is shown forudZTA sample with 0.3 MgO wt %. For the second part, the particle sizes of MgO wereudvaried from 20 nm to 7000 nm. It was observed that finer size of MgO enhances theudmicrostructure pinning effect; a feature introduce by MgO. ZTA samples withudadditives of 20 nm of MgO were found to have fine Al2O3 grain size (2.50udgrains/μm) compared to ZTA with 100 nm MgO (2.21 grains/μm), 500 nm MgOud(1.27 grains/μm) and 7000 nm MgO (0.81 grains/μm). The fine grain of ZTA withud20 nm MgO leads to a high Vickers hardness (1740 HV) but a fracture toughness ofud3.62 MPa.m1/2. Formation of MgAl2O4 was detected at different composition of eachudMgO particle sizes, whereas 1.1 wt % for 20 nm MgO, 0.7 wt % for 100 nm MgO,ud0.6 wt % for 500 nm MgO and 0.5 wt % for 7000 nm MgO. Tool wear measurementudindicated that ZTA samples with 20 nm MgO shows an increase of 54.8 % compareudto ZTA samples with 100 nm MgO. For the third part of the study, amounts from 0udxxvudwt % - 1.0 wt % of Cr2O3 are introduced into two separate systems: ZTA and ZTAudwith 20 nm 1.1 wt % MgO. ZTA samples with the addition of 0.6 wt % of Cr2O3udproduced Vickers hardness of 1683HV and fracture toughness of 7.05 MPa.m1/2. Theudincrease of Vickers hardness is attributed to the addition of Cr2O3, which has higherudhardness than ZTA. The increase of fracture toughness is due to the large Al2O3 grainudand the loss of monoclinic phase inside YSZ due to the presence of Cr2O3. Accordingudto XRD analysis, no new phase was formed with the addition of Cr2O3 since bothudCr2O3 and Al2O3 are in complete solid solubility with each other. NL values for ZTAudsamples decrease with the addition of 0.6 wt % Cr2O3 from 1.30 grains/μm to 0.78udgrains/μm. Sample of ZTA-20 nm MgO-Cr2O3 has higher Vickers hardnessud(1693HV) and lower tool wear (0.015 mm2) compared to ZTA-Cr2O3 (1683 HV andud0.0190 mm2). Result of tool wear measurement shows that sample of ZTA-20 nmudMgO-Cr2O3 has 35 % of improvement compare to ZTA-Cr2O3. In overall, samples ofudZTA-1.1 wt % 20 nm MgO-0.6 wt % Cr2O3 is the best composition for cutting insertudapplication.