In general, the bearing alloys have two types of microstructure i.e., either a soft matrix with discrete hard particles or a continuous matrix of the harder metal with small amount of the softer metal finely dispersed in it. The aluminium and copper based bearing alloys which are widely studied fall in the second category. However, the bearing materials which have been studied have micron sized dispersoids. In recent times, it is possible to produce nanoscale dispersoids in a hard matrix by the novel processing route of rapid solidification. This offers an opportunity to study the small length scale effect on tribological processes. In this thesis, we deal with aluminium alloys where nanoscaled dispersions of lead, bismuth and indium are produced by rapid solidification processing. Chapter 1 of the thesis is an introduction, followed by Chapter 2, which reviews the literature on nanomaterials. Special attention is given to the monotectic system, followed by a brief description on friction and wear of materials which is necessary for our present investigation. The details of experimental and characterisation techniques are given in Chapter 3. In Chapter 4, we present a brief study of white metal bearings (babbit). Tin-based babbit of composition, Sn-6wt% Cu-llwt% Sb was studied. The study of babbit was mainly carried out with the idea that it could serve as a benchmark for subsequent studies in aluminium alloys, in terms of tribological properties. In particular, we have carried out a detailed electron microscopic investigation on the phases present in the bearing alloy. The friction and wear behaviour of this material confirms the proper calibration of our setup for wear studies. This is followed by a detailed study on the synthesis, microstructure and tribological behaviour of nanodispersed aluminium alloys, Al-6wt% Pb and Al-10wt% Pb in Chapter 5. For comparison, we have also studied melt-spun aluminium without dispersoids. Detailed electron microscopic characterisation indicates that lead has a cube on cube orientation relationship with the aluminium matrix, and the particles exhibit a lognormal distribution with the mode of the particle size distribution being 15 nm. The pin on disc results suggest a distinct lowering of coefficient of friction corresponding to pure aluminium (μ= 0.40) and as cast aluminium-lead alloys (μ= 0.41). Detailed SEM studies indicate a tribolayer consisting primarily of Al, Pb and Fe. The later comes from the counterface material. Our results clearly indicate that at an early stage, little or no oxidation takes place at the sliding interface. TEM observations indicate significant deformation of lead particles in the sub-surface region. The observations suggest spreading of the lead, which acts as a lubricating layer. Wear behaviour is primarily adhesive and follows Archard's wear law. However, the rate of wear is less than that reported by other investigators on micronsized lead dispersions in aluminium. In Chapter 6, we present the results for alloys dispersed with nanosized indium and bismuth. We show that indium particles on melt-spinning exhibit both cubic and tetragonal crystal structure. The indium particles are coarser (with a mode of 25 nm) than the lead and bismuth particles (which have mode of 15nm). The bismuth containing alloys have a lower wear rate and coefficient of friction compared to lead and indium alloys. However, both indium and bismuth particles do not follow Archard's wear law and the wear vs load graph shows a non-linear behaviour. The results are discussed in terms of known mechanisms of the coefficient of friction and wear. Chapter 7 gives the salient conclusions while in Chapter 8 we discuss some of the unanswered questions and the potential for future work in this field.

<p dir="ltr">Many promising energetic materials fall short of their maximum theoretical potential due to high ignition temperatures, molten agglomerates that promote two-phase flow losses, poor combustion stability, and so forth. Additive materials such as lithium in aluminum or hydrocarbons in ammonia can help improve performance, but these additives introduce challenges of their own. For example, lithium in an aluminum-lithium alloy increases the specific impulse of a rocket motor and reduces agglomerate sizes through micro-explosions, but large quantities of lithium make the propellant susceptible to premature oxidation and age poorly over time. Similarly, hydrocarbon additives to ammonia improve combustion performance and militate against flame extinction, but they also decrease some of ammonia’s more attractive properties, such as its high hydrogen density, heat capacity, and low carbon content. This suggests that a balance must be made between improving the energetic properties of a material while mitigating any negative side-effects of the additive itself.</p><p dir="ltr">Therefore, the objective of this research was to investigate the effects that small quantities of gallium, indium, and lithium have on aluminum alloys in solid propellant formulations and the effects that small quantities of lithium, sodium, and calcium have on certain unique liquid propellant formulations.</p><p dir="ltr">The first area of research discusses methods used to fabricate aluminum-gallium and aluminum-indium alloy powders using a ball-mill and then examines their morphology, composition, oxidation behavior, and onset temperature in a slow-oxidation environment. Building on this work, these alloy powders were introduced into an ammonium perchlorate composite propellant formulation where their theoretical performance was calculated, and their experimental burning rate and condensed combustion products were measured.</p><p dir="ltr">This first area of research examined how gallium and indium may make aluminum more reactive through active oxide-disruption mechanisms and how this improvement affects the performance of an energetic formulation. However, aluminum-lithium (Al-Li) alloys suffer from the opposite problem, where their thermal instability and pre-mature oxidation near ambient conditions are a concern for practical viability. Therefore, the second area of research focused on the low-temperature oxidation of Al-Li alloys and then developed a theoretical understanding of their passivation behavior. Combining the results and lessons learned aluminum-gallium/indium/lithium alloys, a more complete understanding of aluminum alloy oxidation can be made. Particularly, that even small quantities of metal additives can have a significant effect on the thermal stability, oxidation behavior, and energetic performance of the alloy in propellant formulations.</p><p dir="ltr">The final area of research deviates from solid propellant formulations. Learning from the significant impact that lithium can have on the stability and performance of aluminum metal in solid propellant formulations, this last area of research expands this application to liquid propellants. Specifically, the energetic effects of small amounts of metals additives dissolved into certain liquid fuels to form highly esoteric solvated metal solutions that are hypergolic with white-fuming nitric acid.</p>