Processing and Properties of zirconia-CNT composites

In the last decades there has been growing interest in developing ceramic materials with high fracture toughness (KIc) and strength for structural applications. In the specific case of 3 mol % yttria-doped tetragonal zirconia (3Y-TZP), K_Ic can be increased by promoting phase transformation from tetragonal (t) to monoclinic (m) phase in front of a propagating crack tip referred to as transformation toughening. However, the stronger the tendency for stress induced transformation, the higher the risk for premature spontaneous t-m transformation in humid atmosphere. This is called hydrothermal degradation or low temperature degradation (LTD) and it results in microcracking and loss of strength. This phenomenon is limits the use of 3Y-TZP. The resistance to LTD can be increased by reducing the grain size into the nanoscale by using Spark Plasma Sintering (SPS). However, the reduction of grain size will reduce the transformation toughening and the fracture toughness will decrease. One way to enhance K_(Ic )is the incorporation of a second phase as a toughening mechanism into zirconia matrix. In the present study, Carbon Nanotubes (CNTs) were used to reinforce zirconia matrix.A novel method was developed in this project in order to calculate the "true" fracture toughness of 3Y-TZP/CNT composites. The method is based on producing a very sharp notch using Ultra-short Laser ablation (UPLA). The influence of transformation toughening in small cracks behaviour was also studied. Therefore, a similar short sharp notch using UPLA was induced in conventionally sintered 12Ce-ZrO2 (300 nm grain size) because it has much higher plateau fracture toughness than SPSed 3Y-TZP with 177 nm grain size. Moreover, the wear behaviour of zirconia/CNT composite was investigated by studying the effect of CNTs on the friction coefficient and the wear rate of the composites. The wear behaviour was investigated with scratch tests and reciprocating sliding. The machinability of zirconia/CNTs using Electrical Discharge Machining (EDM) was evaluated by studying the electrical conductivity, the thermal conductivity and the damage produced after machining. Besides that, the influence of grinding, thermal etching after grinding, and annealing of SPS zirconia with different grain sizes were studied. It was found that by inducing a very sharp notch using UPLA, the "true" K_Ic of SPSed 3Y-TZP and 3Y-TZP/CNT composites is low and independent of the CNT amount induced. Vickers indentation K_Ic is higher and increases with increasing CNT. The increase in indentation K_Ic with the addition of MWCNT is attributed to an increase in the resistance to cracking under sharp contact loading. Therefore, indentation K_Ic is not an appropriate method for analysing the influence of MWCNT on "true" fracture toughness. Moreover, only 10 % of difference in strength was found in 12Ce-ZrO2 and 3Y-TZP using UPLA method indicating that the "true" K_Ic of both materials is almost similar. Thus, the beneficial effect of higher indentation K_Ic in 12Ce-ZrO2 reported in literature has a very small effect on the "true" K_Ic that determines the strength of unshielded small cracks.The incorporation of CNTs into zirconia matrix increases the friction coefficient and drastically decreases the wear rate when the amount of CNT reaches the percolation value (2 wt % CNT) under relatively low loads. However, during scratch test and under high loads, the composites develop chipping and brittle fracture. The addition of CNTs strongly enhances the electrical conductivity of the composite and induces slight changes in the thermal conductivity which results in successful EDM machining of the composites with 1 and 2 wt % CNT. The material removal mechanisms in the composites are melting/evaporation and spalling.The thermal etching of ground SPS zirconia at 1100 °C for 1 hour in air induces a surface nanograin layer with crystallized grains of about 60 nm sizes and a thickness of less than few hundred nanometers, which is independent of the original grain size of the bulk material. The annealing of ground SPS zirconia at higher temperatures 1575 °C results in similar grain sizes as the ones achieved during sintering of carefully polished zirconia.Godkänd; 2016; 20160331 (latmel); Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Avhandlingen är resultatet av ett samarbete mellan Luleå tekniska universitet och Universitat Politècnica de Catalunya som syftar till en dubbel doktorsexamen. Namn: Latifa Melk Ämne: Materialteknik/Engineering Materials Avhandling: Processing and Properties of Zirconia-CNT Composites Opponent: Senior Scientist Pavol Hvizdos, Director Institute of Material Research SAS, Kosice, Slovakia. Ordförande: Professor Marc Anglada, Universitat Politècnica de Catalunya, Barcelona, Spain. Tid: Fredag 20 maj, 2016 kl 11.00 Plats: Aula Capella, ETSEIB, UPC, Barcelona, Spanie