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Development of TiC/Ni-P-hBN composite coating for high temperature triboligical applications
International Islamic University Malaysia
Kulliyyah of Engineering, International Islamic University Malaysia
Low alloy steel (LAS) is extensively used in many structural applications due to its attractive properties and low cost. However, this alloy steel suffers from low hardness and poor wear resistance that limits its usage in many tribological applications without surface treatment. Titanium carbide (TiC) composite coating has demonstrated a high potential for developing hard and wear-resistant surface layers on LAS working under mild sliding conditions. However, such surfaces would need further improvement to meet the severe condition of an advanced mechanical system which requires high working temperature and long life. Since h-BN ceramic is a promising solid lubricant at an elevated temperature, the possibility of enhancing the sliding interface of conventional TiC composite coating with the addition of hBN is therefore explored in this study. The surface of hBN lubricant powder is first modified by electroless Ni-P co-deposition process, with the goal of increasing its wettability property and inhibiting its decomposition at high temperature. The overall investigation consists of electroless Ni-P-hBN lubricant development; development of TiC-based composite coating with three schemes (viz; TiC, TiC/hBN and TiC/Ni-P-hBN) using powder preplacement and tungsten inert gas (TIG) torch melting technique; parametric investigation of the composite coatings with Taguchi S/N ratio and GRA approach to optimize surface coating characteristics (surface hardness, wear rate, and friction coefficient); and finally, evaluation of the performance integrity of the optimized surfaces at various temperatures from room temperature to 800 oC. The Surface morphology of the as-deposited Ni-P-hBN powder indicates successful deposition of Ni-P composite film on the surface layer of hBN. Validation of the Taguchi's predictive model and optimal parameters for surface responses of the three composite coatings show that their prediction accuracy error is within accepted limit of 2.71 to 4.7 %. Within these process parameters, the result shows that peak hardness of the coatings was up to 770 HV which is three times that of LAS substrate. The dendrites of TiC and reprecipitated particles in the resolidified melt pool are responsible for the development of uniform hardness in TiC/Ni-P-hBN coating scheme. XRD analyses of the worn surface of the composite coating produced with TiC and TiC/hBN powders revealed that compound like Ti2O3, TiO2, Ti3O5, Mo2B, Fe3B, B4C etc) are formed on the coating surface when tested at the temperature up to 800 oC. This indicates reaction and decomposition of coating structure at high temperature which was almost absent in TiC/Ni-P-hBN composite coating. Furthermore, evaluation of the coating performance shows that about 55.19 % and 93.75 % reduction in the wear rate is achieved with TiC/hBN and TiC/Ni-P-hBN coating schemes respectively over that of TiC at the maximum dry sliding temperature of 800 oC. Under the same condition, the lowest friction coefficient of 0.325 obtained with TiC/Ni-P-hBN hybrid coating is equivalent to 58 % and 67 % improvement over those of TiC/hBN and TiC coating schemes respectively. The level of improvement achieved on the worn surfaces of these coating schemes is correlated with the mild abrasive wear, surface softening, lubricant smearing and formation of tribo-oxide layers. Thus, the work developed a new route for TIG process modelling and compositional control of composite coating that could be well applied for the wear protection of LAS during sliding contact up to elevated temperature of 800 oC.
Thesis (Ph.D)--International Islamic University Malaysia, 2017.
Includes bibliographical references (leaves 234-251).