M. Erkin Cura

Pulsed Electric Current Sintering of the Al2O3 - 15wt% ZrO2 Nanocomposites with 3wt% of Different Solid Lubricants

High temperature low friction materials are sought for use in engines in order to reduce energy consumption of the machines. Due to the high service temperatures solid lubricating materials are necessary. This study is designed to find the optimal processing conditions for preparing these materials by pulsed electric current sintering. In this study, the Al2O3 - 15wt% ZrO2 (AZ) nanocomposite was modified with 3 wt% of self-lubricating component (CaF2, BaF2, MoS2, WS2, h-BN, or graphite). After the preparation of the alumina-zirconia powder mixture solid lubricant powder was added. Powders were then mixed in ethanol for 24 h, dried in a rotary evaporator, and in oven at 80°C for 24 h. The particle size distribution of the powders was established with the laser method. Powders were compacted by using pulsed electric current sintering technique at 1300 °C with 50 MPa for 5 min in vacuum. The structure of the materials was studied with XRD and SEM. Density of the compacts was measured with the Archimedes method and their hardness was evaluated by applying HV1 hardness with the instrumented indentation techniques. Their mechanical behavior was further studied with the instrumented scratch testing.

Mechanical properties of alumina based nanocomposites

To study the microstructure and mechanical properties of alumina nanocomposites, Al2O3/2.5 vol.% Ni and Al2O3/10 vol.% ZrO2 nanocomposites were consolidated by pulsed electric current sintering (PECS). Fracture toughness was found to increase by 13 % and 16 % respectively compared to reference alumina. Hardness increased slightly in Al2O3/Ni because of a fraction of nickel particles under the critical size (2 following the rule of mixtures. By investigating the results, causes of improved mechanical properties were critically evaluated.

Densification and Microstructure Development in Zirconia Toughened Hardmetals

Different process methods and parameters together with different amount of additives were used to fabricate WC-Ni-ZrO2 hardmetals with mechanical properties aiming at improved performance under erosive wear. XRD observation showed the presence of tetragonal zirconia in the cermet matrix after processing. The best erosion resistance with erosion rate of about 0.7 mm3/kg was demonstrated by the specimen produced either by vacuum sintering or SPS and added by 0.2 wt% of free carbon. This cermet has also demonstrated the highest hardness of 17.7 GPa.

Effects of Fabricated Method on the Coefficient of Friction of Al2O3-15 wt% ZrO2-3 wt% Solid Lubricant Composites

The coefficient of friction was very important factor for the applications of high temperature parts. In vehicles, the coefficient of friction was decreased due to lubricants as like engine oil etc. Lubricant such as oils is difficult to apply at high temperature. To apply high temperature parts, lubricants were demanded for high temperature stability. This work is to use the pulsed electric current sintering (PECS) technique and the atmospheric plasma spraying (APS) method in order to make self-lubricating Al2O3-15wt% ZrO2-solid lubricant composites. We focused on the coefficient of friction for the fabrication method of self-lubricating Al2O3-15wt% ZrO2-solid lubricant composites. We compared with the coefficient of friction of PECSed and APSed composites. The surface roughness of PECSed Al2O3-15wt% ZrO2-solid lubricant composites were 0.06 ~ 0.31 μm of Ra and 10.16 ~ 33.12 μm of Ry. In the case of APSed Al2O3-15wt% ZrO2-solid lubricant composites, as-coated samples were 6.56 ~ 11.42 μm of Ra and 59.68 ~ 81.79 μm of Ry, and polished samples were 1.12 ~ 3.70 μm of Ra and 11.66 ~ 32.22 μm of Ry. The coefficient of friction of PECSed and APSed Al2O3-15wt% ZrO2-solid lubricant composites were 0.19 ~ 0.49 and 0.41 ~ 0.61, respectively.

Pulsed Electric Current Sintered Cr2O3-rGO Composites

Chromium oxide is a promising material for applications where excellent corrosion resistance, high hardness, and high wear resistance are needed. However, its use is limited because of low fracture toughness. Improvement of fracture toughness of chromium oxide while maintaining its afore mentioned key properties is therefore of high interest. In this communication we study the possibility of increasing the toughness of pulsed electric current sintered (PECS) chromium oxide by the addition of graphene oxide (GO). The indentation fracture toughness was improved markedly with the addition of graphene oxide. Materials prepared by direct chemical homogenization had better fracture toughness. In composites with 10 vol.% GO piling of thin graphene oxide layers resulted in the formation of graphite layers between Cr2O3 and in carbide formation, which were observed to be the main reasons for the degradation of the mechanical properties. The distribution of graphene oxide was more homogeneous, when the GO amount was 0.1 vol.% and the formation of graphitic layers were avoided due to lesser amount of GO as well as ultrasonic treatment following the ball milling.