The unique biocompatible and wear resistant nature of ultrahigh molecular weight polyethylene (UHMWPE) makes it a suitable material for load bearing applications in total joint replacements (TJR). However, oxidation induced wear is a common cause limiting the life span of implants. Hence, Vitamin-E, a common antioxidant, is added to prevent the wear loss due to oxidation of UHMWPE. In addition, Hydroxyapatite does improve the toughening mechanisms occurring in the polymer matrices. Hence, in this study, a novel hydroxyapatite (HA) nanoparticles reinforced Vitamin-E doped UHMWPE nanocomposites (UHMWPE-E/HA) were developed here in this work. The HA concentration was varied between 0.5 - 3 wt.% and optimum wt% is reported. The mechanical and thermal properties were investigated thoroughly using an array of characterizations. Particularly, a considerable improvement in fracture toughness (K_IC) was obtained. Detailed examination of fractured surfaces was performed to understand the effect of HA reinforcement on fracture toughness of UHMWPE-E/HA nanocomposites.

Ultrahigh molecular weight polyethylene (UHMWPE) is a known to be the material of choice for bearing components in joint arthroplasty. However, oxidation wear of UHMWPE components is considered to be a major drawback limiting the lifespan of implants. Vitamin E was considered as a promising antioxidant to prevent long-term oxidation and reduce the wear degradation of UHMWPE material. Nevertheless, there are limited results on the improvements of vitamin E on the mechanical and thermal properties of UHMWPE. In this study, we investigated the incorporation of 0.5–3 wt.% carbon nanoparticles: Multiwalled Carbon Nanotubes (MWCNTs), Graphene (GO) and Nanodiamonds (ND) on the mechanical and thermal properties of UHMWPE blended vitamin E (UHMWPE-E). Surface analysis of the composite powders showed well-dispersed carbon nanoparticles within the UHMWPE-E matrix. Thermogravimetric (TGA) and Differential Scanning Calorimetry (DSC) were used to study the thermal behavior of the nanocomposites. It was found that the addition of GO, MWCNTs and ND improved the thermal stability of the nanocomposites compared to neat UHMWPE-E. However, the addition of carbon nanoparticles had no significant effect on the crystallization parameters of the composites. In addition, the incorporation of MWCNT and ND improved significantly the fracture toughness of the composites. The addition of 0.7 wt.% ND and 1 wt.% MWCNT increased the fracture toughness from 5.93 MPa m^1/2 for neat UHMWPE-E to 7.38 and 9.19 MPa m^1/2 respectively. The enhanced fracture toughness and thermal stability of the nanocomposites could be due to the successful powder processing technique where an optimized mixing and ball milling parameters were used.

Several composites of tetragonal zirconia polycrystals doped with 3 mol% yttria (3Y-TZP) and multiwalled carbon nanotubes (MWCNT) with concentrations from 0.5 to 4 wt% CNT were processed, spark plasma sintered, and characterised for a wide range of mechanical, electrical and thermal properties. In particular, a strong increase in electrical conductivity at room temperature was found with only 0.5 wt% CNT. However, the thermal conductivity was decreasing with increasing CNT content. Electrical discharge machining (EDM) using die sinking was carried out using the composites of 1 and 2 wt% CNT as workpieces. It was shown that both compositions could be successfully machined by EDM. The surface integrity and the subsurface were studied by SEM/FIB in order to determine the material removal mechanisms, which were found to be associated to spalling and melting/evaporation. Raman Spectroscopy was used to evaluate the damage of CNTs after EDM.

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.

Spark plasma sintered zirconia (3Y-TZP) specimens have been produced of 140 nm 372 nm and 753 nm grain sizes by sintering at 1250 C, 1450 C and 1600 C, respectively. The sintered zirconia specimens were grinded using a diamond grinding disc with an average diamond particle size of about 60 µm, under a pressure of 0.9 MPa. The influence of grinding and annealing on the grain size has been analysed. It was shown that thermal etching after of ruff grinding of specimens at 1100 C for one hour induced an irregular surface layer of about a few hundred nanometres in thickness of recrystallized nano-grains, independently of the initial grain size. However, if the ground specimens were exposed to higher temperature, e.g. annealing at 1575 °C for one hour, the nano-grain layer was not observed and the final grain size was similar to that achieved by the same heat treatments on carefully polished specimens. Therefore, by appropriate grinding and thermal etching treatments, nanograined surface layer can be obtained which increases the resistance to low temperature degradation.

The fracture toughness, KIC of two 3Y–ZrO2 with different grain size (177 and 330 nm) and 12Ce–ZrO2 were determined from a sharp micro-machined notch by Ultra-Short Pulsed Laser Ablation (UPLA) where a micro-cracked zone and non-transformed is generated in front of the notch. The notch plus the damage behaved as an unshielded edge surface crack. The fracture stress, σf of both 330 nm-3Y–ZrO2 and 12Ce–ZrO2 with similar short crack sizes were found to be comparable in despite of their different published R-curves. The results of KIC were discussed in terms of the type of cracks induced and by using a simple R-curve model. It was concluded that for the development of high strength composites with 12Ce–ZrO2 as the matrix, the relevant KIC that controls the σf with surface unshielded short cracks is much closer to the intrinsic KIC than to the indentation KIC or to the plateau KIC of long cracks.

Composites of 3 mol.% yttria-doped tetragonal zirconia (3Y-TZP) reinforced with multiwalled carbon nanotubes (MWCNT) up to 2 wt. % content have been produced using spark plasma sintering (SPS). The theoretical densities of the studied composites were found to be between 99.4 and 97.4%. The addition of MWCNT content resulted in reduction of 3Y-TZP grain size from 174 to 148 nm. The effect of MWCNT on the friction coefficient (COF) was studied by performing nano- and macro-scratches using diamond Berkovich and Rockwell indenters, respectively. Moreover, the COF and the wear rate were also investigated in reciprocating sliding against a zirconia ball under a load of 5 N. The results showed that the COF decreased upon the increase in MWCNT content. However, in macro-scratch testing, there was a critical load over which brittle fracture sets in and its value decreases as the MWCNT content increases. The wear resistance was found to be decreasing very slightly for less than 1 wt. % MWCNT, while it increases strongly for the addition of 2 wt. % MWCNT under the conditions studied. The results were discussed in terms of material properties, scanning electron microscopy observations of the wear track and nanoindentation tests.

Multi-walled carbon nanotubes (MWCNTs)/3 mol% yttria-doped tetragonal zirconia (3Y-TZP) composites were produced using spark plasma sintering (SPS) with MWCNT content ranging within 0-2 wt%. In the present paper, it was shown that the addition of MWCNTs results in a refinement of the composites microstructure. Moreover, nanoindentation tests were performed in order to monitor the change in elastic modulus and hardness with MWCNT content and it was found that both properties decrease with the addition of MWCNT content. A novel method was used to measure the true fracture toughness of the composites by producing a shallow surface sharp notch machined by ultra-short pulsed laser ablation on the surface of beam specimens. The true fracture toughness obtained on this laser machined single edge V-notch beam (SEVNB) specimens tested in four point bending was compared to the indentation fracture toughness measured using a Vickers indenter. It was found that the indentation fracture toughness increases with increasing MWCNT content, while the true fracture toughness determined with SEVNB was practically independent of the composition. Finally, it was concluded that the increase in the resistance to indentation cracking of the composites with respect to 3Y-TZP matrix cannot be associated to higher true fracture toughness. The results were discussed in terms of transformation toughening, damage induced in front of the notch tip, microstructure of the composites, and fracture toughness of 3Y-TZP.