Master of Engineering – Research
School of Mechanical, Materials and Mechatronic Engineering
Zhang, Qi, Study on the sliding wear response of cast iron, Master of Engineering – Research thesis, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, 2011. https://ro.uow.edu.au/theses/3435
The oxide film formed on metallic substrate during un-lubricated dry sliding is a critical factor affecting the wear resistance of the sliding components and the coefficient of friction. A healthy and well-maintained tribo-oxide layer kept with proper thickness and good surface conditions provides a solid sheet of lubrication between the rubbing pair by separating them from direct metallic contact and, thus preventing the sliding surfaces from severe damage.
The purpose of this study is to further examine the relationship between the so-called protective tribo-glaze and the substrate underneath, such as the adhesion between the two components with different thermal expansion coefficients, the influence of growth of these oxide plateaux on the coefficient of friction and so forth.
The adherence of an oxide layer onto the metallic substrate, mainly steels and cast irons, is reviewed. Hot compression and heat treatment were conducted on the samples tested using the Gleeble 3500 in this work. The sample is a composite material consisting of three layers, where the formability-poor cast iron is the core layer and low carbon steels are the outer layers. This sandwich-structural material is produced by hot rolling process through which the interface strengths between the cast iron and steels were greatly enhanced with a metallurgical bonding condition, after they were cast together, by pouring the molten steel which has a higher melting point into a casting device, followed by molten cast iron. The sliding wear test was carried out using CETR wear testing apparatus for three sample conditions of cast iron layer before they were cut from the cast iron layers. The three sample conditions are hot rolled sample that was hot rolled into composite with metallurgical bonding at a low carbon steel and cast iron interfaces, heat treated sample that was destabilised in the Gleeble 3500 at 1050 °C for 8 min in a vacuum after hot rolling, and hot compressed sample that was hot compressed in a vacuum (40% reduction at a strain rate of 1/s) in the Gleeble 3500 at 1050 °C for 8 min. The wear test on these three samples was conducted under dry sliding conditions at room temperature, after sliding parameters of 20, 40N, and 45 and 78mm/s were pre-set for different sliding tests in the CETR setup. Sliding durations of 7.5 min and 2 min were selected for various testing conditions in this study. Optical microscope (OM) and scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) were used to observe the worn surfaces of these samples.
After misalignment and running-in, it was found that the wear was always mild for all the testing conditions, due to the formation of oxide plateaux on the disc specimen, cast rubbed against the upper high chrome steel ball shaped specimen. When the load was increased to 40N, a so-called negative wear response occurred on the heat treated sample and the hot compressed sample. However, the hot rolled sample failed to respond in a similar manner under the same sliding condition. The negative wear from these samples generally lasted for some 2 min, followed by a reverse trend where the displacement of the pin became positive, which is what happened to the hot rolled sample during the whole sliding period. A 2 -minute test was carried out on these samples. The results revealed that the worn surfaces of the heat treated and hot compressed samples were coated with a protective oxide plateau which contributed to a negative displacement of pins, which was ready to be break off due to their approach to the critical thickness determined by surface fatigue and their brittle nature. However, the worn surface of hot rolled sample was not covered with such a healthy protective film that was replaced by discontinuous oxide plateaux which had been spalled off during rubbing and oxidised wear debris dispersed ready to be compacted into glaze on the worn track. The differences of stabilities and maintenance of oxide layer formed on these samples were discussed. The relationship between the formation of an oxide layer on the heat treated and hot compressed samples and the coefficient of friction as a function of sliding time was also discussed. It was concluded that the lowering coefficient of friction caused by the formation and development of a protective oxide layer only operates when the surface is kept well before approaching its critical thickness at which the surface severely cracks and is shown as scales instead of a whole oxide continent. This shows that the development of wear resistance offered by this layer is not synchronised with its growth in height.