Microstructural evolution in AlSn-based gas atomised powder and thermally sprayed coatings
This thesis reports on the microstructure of Al-Sn based powders and the development of Al-Sn based coatings for automotive shell bearing applications deposited using the high velocity oxy-liquid fuel (HVOLF) thermal spray technique. The microstructure of the coating and its associated physical and chemical properties, such as microhardness and corrosion resistance, are investigated as a function of the HVOLF thermal spraying parameters. In particular, a detailed microstructural understanding of the thermal sprayed coatings is developed to explain the coating properties. Two alloy systems, Al-12wt. %Sn-1 wt. %Cu and Al-20wt. %Sn-3wt. %Si have been investigated in detail using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. The high resolution transmission electron microscope (HRTEM), electron energy loss spectroscopy (EELS) and energy filtered TEM have also been used to examine nanoscale precipitates as supplementary methods. The statistical image analysis of fine scale particulate dispersions has also been used to study the second phase evolution with annealing. The microstructure of the large gas atomised powder particles used in the HVOLF thermal spray process comprise dendritic Al and interdendritic Sn, whilst the small powder particles exhibit fine scale Sn particles distributed within an Al matrix. The as-sprayed coatings comprise a mixture of melted and partially melted splats due to the full and partial melting of the deposited powder. Nanoscale Sn particles distributed in the Al matrix are present in fully melted regions, whilst micron / sub-micron Sn particle distributions and Sn-particle free Al regions delineate partially melted regions. Cu remains in solid solution within the Al matrix of the Al-12wt. %Sn-1 wt. %Cu as-sprayed coatings, whilst Si formed nanoscale particles in the Al-20wt.%Sn-3wt.%Si as-sprayed coatings. The critical cooling rate to form the metastable liquid phase separationh within Al-12wt%Sn alloys is put forward according to calculation. If the cooling rate is lower than the critical cooling rate, dendritic Al and interdendritic Sn are formed, thereby explaining the structure of large gas atomised powder particles. If the cooling rate is higher than the critical cooling rate, a liquid phase separation reaction occurs to form fine scale Sn dispersion. The calculated critical Al-12wt.%Sn powder diameter for liquid phase separation is close to the experimentally observed Al-12wt. %Sn-l wt. %Cu powder diameter. The discrepancy between experiment result and theoretical calculation is attributed to the additional element Cu promoting the liquid phase separation. The nano and sub-micron scale Sn distribution in small gas atomised powder particles and the as-sprayed coatings is attributed to the cooling rate being higher than the critical cooling rate. The dendritic structure of the large Al-Sn-Cu gas atomised powder is due to the cooling rate being lower than the critical value. Heat treatments are applied to the as-sprayed coatings to alter the mechanical and chemical properties, such as, microhardness and corrosion resistance, of the bearing material coatings. Annealing causes the nanoscale and sub-micron Sn particles to coarsen within both Al-12wt.%Sn-1 wt.%Cu and Al-20wt.%Sn-wt.3%Si coatings according to the analysis of SEM and TEM images. The Sn particles coarsen greatly within the Al-12wt.%Sn-1wt.%Cu coatings annealed at 300°C for 5 hours, as compared with coatings annealed for 1 hour. The Ɵ’-phase (CuAl2) also precipitates in the Al-12wt.%Sn- 1 wt.%Cu coatings after annealing at 300°C. Annealing also causes fine scale Si particles to coarsen greatly in the Si containing alloy. The microhardness decreases in the annealed coatings for both alloys and is attributed to a coarsening of Sn particles and the release of residual strain within the as-sprayed coatings. As compared with the as-sprayed coatings and the coatings annealed at 300°C for 1 hour, the corrosion rate in 0.1M NaCI solution of Al-12wt.%Sn-1wt.%Cu coatings annealed at 450°C for 1 hour is very greatly reduced. However, an annealing temperature of ~450°C is not appropriate for these coatings because of the introduction of interlayer cracks and a coating / substrate reaction which might degrade the mechanical properties of the bearing.