The Wear-resistant Composite Coatingson The Surface Of Extrusion Screw

The screw is the key part of plastic recycling machine, and its surface treatment has gained scrambling attention and comprehensive study because of the great application potential. The common surface treatment techniques of screw include nitriding, electroplating, thermal spraying, laser cladding, surface welding and surface quenching. After surface treatment, service life of the screw is increased exponentially.There were three methods applied by this experiment to enhance the properties of screw, including gas nitridding, HVAF spraying WC-10Co-4Cr coating, laser cladding Fe-base coating. Phases and microstructures of the substrate and all of three alloy hardening layers were observed by XRD, SEM and Optical Microscopy, while their hardness were measured by microvickers hardness tester. These alloy hardening layers were prepared with the best corresponding experiment parameters and run on the machinery. The results of experiments and the failure mechanisms of alloy hardening layers were analyzed, and the conclusions are come as follows:Among the three alloy hardening layers, the microstructure of nitriding layer was most similar to the substrate, which was composed of tempered sorbite and retained austenite that contain many miniature hard particles include γ′and ζ phases. The phases of laser cladding coat was most similar to the substrate, which were composed of the Fe-base solid solution and alloy carbides.The great difference among the phases of three alloy hardening layers and substrate was the thermal spraying coating, whose bonding strength was the lowest, due to the mechanical bond between coating and substrate as well as the coating inside.In all of the three alloy hardening layers, the friction coefficient and the wear volume of WC coating were the minimum, whose wear resistance was the best. The wear resistance of the nitrided layer was second. The laser cladded Fe-based coating has the worst wear resistance, but the wearing loss rate is still lower two orders of magnitude than the matrix.In the three alloy hardening layers, the hardness of nitriding layer declined slowly with the decrease of nitrogen content in the thickness direction; the hardness of the thermal spraying coat was the highest, but there was a sharp change between the coating and substrate; there were two turning points in the laser cladding coating, which was the starting point and the ending point of dilution area.Although there were certain strengthened by all of the three alloy hardening layers, their failure modes were different because of their own characteristics. The main failure modes of nitriding layer, thermal spraying coat and laser cladding coating were scratched and abrasive wears, spalling and deformation of substrate, deformation and partly spalling of coating, respectively.