Graded High-Porous Microfilters by Powder Metallurgical Coating Techniques

Powder metallurgical (PM) filters are used for an increasing number of applications in the field of chemical industry or process engineering due to some specific advantages including low-cost fabricability. In general, low pressure drop as well as high permeability are required for the filters, but these requirements are difficult to fulfill for microfilters. One possibility to overcome this difficulty is to produce a graded filter structure. A high-porous material offering good permeability is used as a substrate on which thin layers with lower and finer porosities are applied. By coating the substrate with several layers, a graded transition to a pore size of less than one micron can be realized. Different processes to produce such a graded filter structure are described in this paper. By optimizing the processing, a multi layer structure with a thickness of less than 50 μm for each layer could be achieved. Homogeneous multi layer structures demand special requirements in the sintering process. Properties such as porosity, pore size distribution and permeability of the sintered systems were measured by various testing methods. The influence of the coating parameters, e.g coating deposition rate and grain size distribution of the initial powder on these properties is discussed.