On the tribological behaviour of metal/ceramic and metal/metal couplings

Corrosion, wear or the combined effects of these failure modes cost the industrial economies hundreds of billions of euros each year. One of the more effective ways to reduce damage due to corrosion and wear is to treat, or “engineer” the surface, so that it can perform functions that are distinct from those required by most materials. For example, steel is a very popular material, because it is inexpensive, strong and easily manufactured. Unfortunately, it is highly susceptible to corrosion in many environments and therefore needs to be coated in order to achieve a satisfactory life service. Small components can be painted, aluminized, electroplated, or clad with more corrosion resistant materials. For parts that also require wear strength, they can be thermally sprayed or coated with thin-films. Finally, when dealing with large components, the size, weight, and handling problems may limit the type of surface treatment to be considered. The aim of the research work presented in this PhD thesis was to study the interactions between the metal/ceramic and metal/metal coupling surfaces to improve the wear resistance of the materials in sliding motion. Most of my research work is included here as a collection of three papers submitted in the same format they were published or in a preprint version. Each paper is an individual piece of work with separate sections including: abstract, introduction, materials and experimental details, results and discussion, conclusions, and references. A failure analysis study was carried out on some specific mechanical components of diesel engines for industrial application. In order to increase the wear resistance of rocker arms, pushrods and valves, the deposition of ceramic coatings thermally sprayed was considered. Wear tests were performed on a plasma-sprayed ceramic coating in sliding motion against steel in order to understand the effect of a normal load applied, as well as the environmental conditions on the metal/ceramic coupling. Moreover, the influence of relative humidity on the tribological behaviour of two types of plasma-sprayed ceramic coatings and two types of HVOF-sprayed cermets in sliding motion against alumina, was evaluated through pin-on-disk testing. Different techniques for the microstructural examination were employed in this research study: Optical Microscopes (OM) and Scanning Electron Microscopes (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) microprobe were used and also X-ray Diffraction (XRD) was carried out to describe the morphology of worn surfaces and to identify the main wear mechanisms involved. In addition, surface roughness, Vickers microhardness and fracture toughness measurements were performed in order to characterise the materials subjected to wear testing.