Process Study Of Hot-Dipping Aluminium Coating On Ceramics

Aluminum is extremely easily oxidized and a layer of oxide film alwayscovers the surface of aluminum at temperatures below1000℃. The oxide filminhibits the wetting and bonding of aluminum with ceramics. An improvedhot-dipping process for metalizing ceramics with aluminum was explored.Systematical research has been done on the influence of flow rate of nitrogengas (oxygen content), temperature, holding time of ceramics in the aluminummelt and alloy element on hot-dipping process. The mechanism of joiningaluminum film to the ceramics was also studied.Using the improved hot-dipping method, a layer of homogenousaluminum or Al-Si alloy film with a thickness of5～10μm can be tightlycoated on the surface of ceramics under certain temperature and atmospherecondictions.The hot-dipping process was conducted in the nitrogen atmosphere, andthe results demonstrate that oxide film can be avoided at the interface ofaluminum melt and the ceramics. Thus molten aluminum can directly reactwith the ceramics or oxygen that diffusing along the interface. An interfaciallayer can be formed at the interface, which keeps coherent relationship withaluminum and ceramics grain. Obviously the formation of the interfacial layercan reduce interfacial energy between ceramics and aluminum, and increasesthe bond strength. Moreover, the surface energy of aluminum is reduced whenaluminum is slightly oxidized.The flow rate of nitrogen gas has a great influence on the hot-dipping.Under low flow rate conditions, oxide film formed at the interface of ceramicsand aluminum inhibits the directly contacting and bonding of aluminum andceramics. However, under high flow rate conditions, the aluminum meltadhered on the ceramics plate agglomerates and slides down since the surfacetension of aluminum is relatively large, which is harmful to hot-dipping. Only under appreciate flow rate of nitrogen, the surface of aluminum melt wouldnot be heavily oxidized, nor would the aluminum melt slides down theceramic plate since oxygen absorbed on the surface of the melt can lower thesurface tension of aluminum., which is critical to hot-dipping process.High temperature favors the reaction of aluminum with ceramics or theoxygen in the interface and increases the adhesion strength between ceramicsand aluminum. However, high temperature impedes oxygen atoms absorbingon the surface of aluminum. Under high temperature conditions, the flow rateof nitrogen should be lower in order to avoid the aluminum sliding down theceramics plate.Increasing holding time of ceramics in the molten aluminum promotesthe reaction of aluminum with ceramics or oxygen at the interface, benefitsthe formation of coherent interfacial layer, and also improves the hot-dippingquality.