Mechanical and Microstructural Analysis of Friction Surfaced Aluminum Coatings on Silicon Nitride Ceramic Substrates

The lack of suitable techniques for joining Si ₃N ₄ ceramics with metals has limited the usage of this otherwise outstanding material for composite applications. In this study, aluminum AlMgSi0.5 (EN AW-6060) was coated onto silicon nitride Si ₃N ₄ ceramic substrates using friction surfacing technology. Experimental work revealed that the harmful effects of thermal shock (e.g., substrate cracking, coating delamination) observed with other material combinations can be avoided by selecting materials with a low coefficient of thermal expansion, low Young’s modulus and high thermal conductivity. Design of experiments derived models for coating thickness and bonding strength fit the data well (i.e., the regression model accounts for most of the variation in the response variable). Whereas the coating thickness is predominately dependent on the rotational speed used, the bonding strength is also affected by the traverse speed. Coating thicknesses upto 2.03 mm and bonding strengths of 42.5 MPa were achieved. Deposition rates exceed those of physical vapor deposition by a magnitude of ×1000 and bonding strength is on-par with thin-film metallization. Scanning transmission electron microscope analysis revealed formation of a glassy phase at the interface. Using energy-dispersive X-ray spectroscopy analysis high silicon and oxygen content with smaller percentages of aluminum and nitrogen were detected. High-resolution transmission electron microscope imaging revealed no distinct lattice structure leading to the assumption that the composition is predominantly amorphous and consists of SiAlON.