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Producing Cu-TiB2 Nano Composite Using Powder Metallurgy Route and Study on its Mechanical and Physical Behavior

Milani, Vanda | 2014

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 45870 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pourazrang, Kazem; Abachi, Parvin
  7. Abstract:
  8. In applications such as high duty sliding contacts and other electrical components requiring good electrical- and thermal-conductivity, proper mechanical wear resistance in combination with arc erosion resistance are required.The Cu/TiB2 nano-composite seems to be a good candidate for such cases. At present work TiB2 nano-particles has been selected as reinforcement, due to its valuable properties, i.e., high melting point (2850 - 2900 °C), low linear CTE (8.10 µmm-1 °C-1), thermal conductivity (60.0 - 120 W/m-K) as well as low specific electrical resistivity (10 - 30 μΩ.cm) and high hardness (25-35 GPa HV). Additionally, there are limited articles which have focused on evaluation of physical and mechanical properties of Cu-TiB2 nano-composites. The TiB2 nano-particles (dP<90nm), which were used as reinforcement in this study, were prepared by self-propagating high temperature synthesis, where reactants were titanium oxide, boron oxide, and magnesium.Reactants mixed through low energy ball-mill and then pressed under 150MPa pressure. SHS synthesis performed in tube furnace with argon atmosphere and at the temperature of 800ᵒC. Samples prepared by SHS were leached by nitric acid in order to eliminating the side products of the reaction, which mostly were MgO. To assure acceptable purity of powder, XRD analyses werewas used in each step.
    To obtain fully dense and high quality specimens in short periods, the spark plasma sintering (SPS) was chosen as consolidation process. Cu–TiB2 composite (2, 3, 5% vol. TiB2) have been prepared by the process powder mixing followed by consolidation of mixed powders by spark plasma sintering process with the initial pressure of 10 MPa, holding pressure of 20 MPa, sintering temperature of 800ᵒC, holding time of 6 min, and heating rate of 50ᵒC/min. Moreover, one Copper composite containing 5 vol.% TiB2 prepared by milling of the mixed powders before SPS process in order to investigating the effect of milling on distribution of TiB2 particles through copper matrix. The milling process was performed at high energy ball mill under speed of 240 rpm and BPR of 22:1 and 60 hour’s milling time. The density of specimens was determined based on Archimedes principle using a proper kit equipped an accurate balance with an accuracy of 0.1mg. The porosity was below 1.5% when volume percent of TiB2 were lower than 3% and it increased by increasing TiB2 content. The size and aggregation level of TiB2 particles tend to increase as increasing the volume percent of TiB2 particles. Ball-milled sample exhibited a homogeneous distribution and lower amount of aggregation of TiB2 particles among composite compared with same composite prepared only by mixing process.Referring to microstructures, the nano-particles distribution is almost uniform, especially at that one having lower particles content and the milled one. To measure specimens’ hardness in HV and HB scale, the load of 10kg and 31.25Kg was applied respectively to the polished surfaces with the holding time of 30 second for each sample. Generally, composites made by higher volume percent of TiB2 exhibited higher electrical resistivity, lower thermal conductivity, and greater hardness, composites with 3 vol. % titanium diboride exhibits a lower hardness due to its lower density. To evaluate the dry sliding wear resistance of specimens, the tests were conducted using S.S disk at constant speed of at 0.4 m/s under different loads of 20, 50 and 80 N and sliding distances of 200, 400, 600 and 1000 m. Considering the wear test results, the nano-composite with 5 vol.% particles offer substantially better wear resistance,the weight and vol. losses were generally lowered compared tocomposites with lower volume percent of reinforcement. Ball milled samples shows exceptional hardness in comparison with others which only were mixed before sintering process. However, its electrical conductivity is degraded drastically. Wear test results shows that ball milled composite has higher wear resistance as the wear load is lower than 50 N and the wear distance is below 600 meter. In above mentioned load and distance Copper composite with 5% TiB2 which were not ball milled in wear distances above 400m and in all applied loads shows the highest wear resistance in comparison to other fabricated composite. It also shows better electrical conductivity in comparison with electrical conductivity of ball-milled samples. All samples show a decrease in coefficient of friction in 50N normal load.Wear resistance of fabricated composite make that a potential candidate for sliding electrical contacts
  9. Keywords:
  10. Mechanical Milling ; Wear Properties ; Powder Metallurgy ; Copper-Titaniume Diboride Nanocomposite ; Spark Plasma Sintering ; Self-Propagating High Temperature Synthesis

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