|dc.description.abstract||This study examines the microstructure and mechanical properties of plasma transferred wire arc (PTWA) coating of typical alloyed steel, deposited on diecast aluminum alloy cylinder bores. The coating surface and microstructure were characterized in terms of surface roughness, features (i.e., defects, splats formation mechanisms, distribution of oxides, re-solidified particles, and interfacial metallurgical bonding) using laser scanning confocal microscope, scanning and transmission electron microscope (SEM and TEM). Residual stress through the thickness of the coating was measured using x-ray diffraction (XRD) and hole-drilling method. In post-processed samples, compressive residual stress was measured throughout the coating with a value close to 100 MPa at the interface, resulting from the thermal mismatch between coating and substrate materials. In terms of mechanical properties, coating hardness was estimated at both the micro- and nanoscale and examined the influence of microstructure inhomogeneity on the mechanical performance and failure modes.
The PTWA coating of typical alloyed steel deposited on diecast aluminum alloy cylinder bores is investigated via experimental pull tests for three types of samples of varying interface pattern and/or substrate material and finite element model (FEM) simulations. FEM simulations account for the portion of adhesion attributed to solely mechanical interlocking, whereas the experimental results rely on the adhesion at the interface due to mechanical interlocking, potential metallurgical bonding, and/or other factors at play. Experimental results show that differences in interface pattern and/or substrate material will result in varying degrees of adhesion failure at the interface and cohesion failure within the sprayed coating itself. The average bonding strength across the three types of samples was found to be 37.59, 27.55, and 33.15 MPa (for D319, W319, and W356 sample types).
Monotonic three-point bending tests of curved samples extracted from trial cylinder bores and consequent analysis using the equivalent section method yielded stress-strain properties for both the substrate and coating materials. SEM observation of fracture surfaces showed three modes of failure involving coating delamination and breakage, which is related to the deposition process and the various features within the coating. Further bending tests of flat samples were performed for four combinations of interface pattern orientation and substrate material. Bending tests were conducted at three temperatures within an environmental chamber (room temperature, 100°C, and 250°C) to simulate the relevant thermal conditions within the engine during operation. Results show that differences in interface pattern and/or substrate material will result in differing failure mechanisms and strengths as well as the trends associated with increasing operating temperatures. It was found that the A356 substrate generally performed better than the corresponding A319, and the samples with dovetail rows running along the length were stronger than the opposing orientation.
Cyclic three-point bending tests were performed for four combinations of interface pattern orientation and substrate material. The Basquin parameters were obtained for all twelve combinations of substrate, orientation, and temperature. The fatigue strength predicted by Basquin parameters are more conservative compared to what is seen in experimental data. Results show that differences in the interface pattern and/or substrate material will result in differing failure mechanisms and fatigue properties, as well as trends associated with increasing temperatures. It was found that the A356 substrate generally performed better than the corresponding A319 and the dominating failure mechanism was specific to the sample orientation (i.e., delamination and interlock breakage/separation). An increase in temperature is generally associated with reduced fatigue properties and increased delamination/separation due to the thermal coefficient mismatch during expansion between the coating and substrate materials. Overall, it was found that the combination of A356 substrate in the H-orientation with the interfacial wave pattern surface activation is a candidate with high potential towards the application of cylinder bores.||en