Abstract
In this study, graphite reinforced binderless TiCxNy ceramic composites were successfully consolidated using spark plasma sintering technology. The morphology, chemical composition and phase analysis of the as received and admixed powders were characterized by FESEM/EDS and XRD prior consolidation. The consolidation process was maintained and conducted at the sintering temperature of 2100 ℃, heating rate of 100 ℃/min, 50 MPa pressure and 10 min holding time. Subsequently, post sintering analysis was carried out to determine the influence of graphite reinforcement on the microstructural characteristics, mechanical properties and wear resistance of the sintered composites. Enhanced relative densities were noticeable on the composites with graphite content as compared to composites without reinforcement. With 1.5 wt. % graphite addition, a higher density of 99.56 % was achieved for TiC0.9N0.1 composite. Microstructural observation from FESEM analysis demonstrated distinct morphology with dissimilar levels of microporosity and the specimens without graphite addition exhibited pronounced porosity for all the TiCN compositions. Ceramic composite with 1.0 wt. % graphite reinforcement displayed homogenous microstructure with graphite particles uniformly dispersed within the matrix. Nanoindentation techniques were extensively employed to evaluate the nano-mechanical properties of the composites and sintered pure TiC0.5N0.5 demonstrated lower resistance to plastic deformation by showing a wide plasticity index as compared to TiC0.7N0.3 and TiC0.9N0.1 ceramic based composites. A noticeable improvement in the phase morphology, microstructure refinement and elastic modulus observed in 0.5 wt. % Gr-TiC0.5N0.5 was responsible for the highest nano hardness value of 34.5 GPa achieved. Furthermore, the electrochemical behavior of the sintered compacts was investigated by potentiodynamic polarization and EIS techniques to evaluate the corrosion resistance of the ceramic composites in 3.5 wt. % NaCl and three acidic electrolytes (0.5 mol/L H2SO4, 0.5 mol/L HCl and 0.5 mol/L HNO3). The composites reinforced with adequate amount of graphite showed enhanced resistance to corrosion in the HCl and HNO3 acidic electrolytes due to the inability of aggressive ions to adequately attack the specimen surface owing to the formation of protective oxide layers. In H2SO4 electrolyte, ceramic composite without graphite reinforcement showed enhanced corrosion resistance as compared to other specimens. Excess graphite content did not improve corrosion resistance of the composites due to the large pores caused by undissolved graphite particles. The lower quantity of graphite reinforcement (0.5
vii
and 1.0 wt. %) refined the microstructure and as a result delayed the corrosion course of the substrate. The tribo-tests were performed on a ball on flat geometry tribometers, using alumina ball and ruby ball as counterpart acting in sliding mode to TiCxNy at ambient temperature. The SEM micrographs of the worn ceramic composites without graphite content were characterized by severe abrasion and oxidative wear mechanisms for composite with 50 wt. % TiC. Composite with 70 wt. % TiC content showed minimal grain pull out without oxidation whereas for TiC0.9N0.1 composition, the dominating mechanisms were oxidative and adhesive wear. The average COF for the composites without graphite ranged between 0.11 and 0.67 for unreinforced composites slid against alumina counterpart. Using ruby ball counterpart, the COF of graphite reinforced TiC0.5N0.5 varied between 0.164 – 0.27 for graphite reinforced TiC0.5N0.5 under 5, 10 and 20 N applied loads, and specimens without graphite content demonstrated higher COF values. Incorporating graphite into the matrix drastically reduced the COF values indicating the solid lubricating property of graphite on the composites during wear tests. User defined design (UDD) approach under RSM was utilized to optimize the parameters for dry sliding wear properties of graphite reinforced binderless TiC0.5N0.5 ceramic composites. Based on the statistical analysis, ANOVA results for wear rate indicated that the predictability of the model was at 95 % confidence level. Moreover, wear rate demonstrated a correlation coefficient of R2 = 0.9762 which depicts that only less than 3 % of the total variations are not accounted for by the model and the value of the adjusted determination coefficient (adjusted R2 = 0.9366) was high indicating that the model was significant.