Dr. Uwe Tröger, Matthias John Lang Zeuna Stärker GmbH & Co. KG, Augsburg
1 Introduction
The process of automobile catalytic converter manufacturing comprises production of a mono- lithic substrate for the ceramic coating, the coating itself, and a housing suitable for insertion of the catalytic converter into the exhaust system of the automobile's engine. One important step in this process is the so-called canning of the coated substrate within it's housing. As in most cases the substrate is made from ceramic materials, it cannot sustain high stresses neither dur- ing the manufacturing process nor during subsequent service conditions. The housing is made form heat resistant steel sheet. One way to convey stresses between the substrate and the housing is to insert a supporting material.
2 General Properties of Support Materials
As can be seen from the extreme conditions of operation of a catalytic converter - temperatures up to 950 °C, and a thermal expansion that is an order of magnitude larger for the housing than for the substrate – the supporting material plays a key role for proper and long-term service. In detail, the supporting material has to fulfill the following requirements:
· fixation of substrate within housing
· compensation of CTE mismatch between substrate and housing
· compensation of geometrical tolerances of the substrate
· resistance against corrosion due to acidic gas components
· heat resistance
· resistance against erosion due to high gas velocities and gas pulsation
· low gas leakage
Additionally, low thermal conductivity and capacity are advantageous properties.
When the first cars where equipped with catalytic converters, wire mesh was chosen as sup- port material. As wire mesh does not conform with a number of the before-mentioned criteria, e.g. it has a high gas leakage, support materials made from mineral components came into use.
An example of a mineral-base support material is the intumescent mat. In this material, artifi- cial mineral fibers and natural mineral Vermiculite grains are compounded by means of or- ganic binders and pressed into sheet form. The binder assures the integrity of the support mat during manufacturing and is burned out during service, while the fibers serve to embed the Vermiculite grains during operation. The Vermiculite, owing to it's crystallographic structure, expands upon heating, increasing thereby it's volume by up to 1000 %. From this volume in-
Material Aspects in Automotive Catalytic Converters, Hans Bode Copyright © 2002 Wiley-VCH Verlag GmbH &Co. K aA ISBN: 3-527-30491-6G
crease stems the excellent ability of intumescent support mat materials to provide a required minimum of stress conveyance in manufacturing and a maximum of fixation force in service.
However, as catalytic converters are positioned closer to the engine in order to achieve quicker heat-up to operating temperature, intumescent support materials approach their limits regarding thermal stability and erosion resistance. An alternative are non-intumescent support mats. In these the Vermiculite is replaced by a higher density and modified morphology of the fibers. In the following properties of intumescent and non-intumescent mat materials are com- pared with respect to their stress relaxation and mechanical aging behavior, and their erosion and corrosion resistance.
3 In-Service and Life-Time Related Properties of Support Materials Mineral-base support materials consist of the following microstructural features:
1. fibers
2. Vermiculite grains (intumescent mat only) 3. binder islands
4. bonded fiber junctions 5. open fiber junctions 6. fiber bends 7. droplets 8. shot
During the canning process, the substrate is enveloped in a sheet of supporting mat and in- serted or wrapped into the housing, the supporting mat being thereby compressed. If the re- sulting pressure between mat and housing is plotted against time (see figure 1), it can be seen that initially an exponential increase takes place until a peak pressure is reached. Afterwards the pressure decays towards a relaxed pressure. The peak pressure and the relaxed pressure are closely related to the closing force used during canning, the gap between substrate and hous- ing, and the support mat material's properties, e.g. density, microstructure, and composition. In the initial part of the pressure-time curve the support material's fibers are deflected according to their moments of inertia and the free loop lengths between junctions. As closing force is further increased and the material is being compressed, new open fiber junctions form and the loop lengths are shortened. The material becomes therefore stiffer and pressure increases ex- ponentially. Upon maximum closing force the peak pressure is reached, and relaxation sets in.
Relaxation is due to viscous fiber deformation and fiber rearrangement against frictional forces. With increasing manufacturing speed higher peak pressures are obtained. The resulting reduced relaxed pressures are ascribed to fiber breakage and junction rearrangement. The de- scribed behavior has been observed with both intumescent and non-intumescent support mats, although intumescent materials show a less pronounced relaxation behavior as the Vermiculite grains do not participate in viscous deformation or geometrical rearrangement (see fig. 1).
Figure 1: Pressure between supporting material and housing during the canning process for different closing rates (intumescent mat)
Mechanical aging of the support material is a result of cyclic expansion and compression during engine start-up caused by the differences in thermal expansion between substrate and housing. The measurement of pressure versus time under cycled external compression reveals that in the decompression cycle a relaxation behavior equivalent to that in the compression cycle occurs (see fig. 2).
Figure 2: Pressure between supporting material and housing under cyclic loading (intumescent mat)
0 200 400 600 800 1000 1200
time [s]
pressure 0,25mm/min10mm/min 1mm/min
600mm/min
0 500 1000 1500 2000 2500
time [s]
When comparing relaxation under static conditions with that under cycled conditions it is found that in the latter case lower relaxed pressures are attained. This is assumed to arise from a rearrangement of fibers and open fiber junctions into geometrically favorable sites under the action of cycled pressure, as is the case with hysteretic dislocation movement. Also, fiber breakage may contribute significantly. The reduced relaxed pressures under cycled conditions requires compensation by correspondingly modified manufacturing or material parameters, i.e.
increased closing force, reduced gap, or improved support mat properties.
Under the pulsating pressure of the exhaust gases erosion of exposed edges of the support material may lead to extensive material removal and the formation of gas bypasses or substrate dislocation. In a test simulating gas pressure pulsation the difference in erosion resistance between intumescent and non-intumescent materials is evident (see fig. 3). The erosion, meas- ured as mass loss per time, is about one order of magnitude higher for intumescent material than for non-intumescent material. Main contributors to erosion are the Vermiculite grains which are not effectively bonded to the surrounding fibers, but move relatively freely and can in some cases be observed to act as miniature grinding stones on neighboring fibers.
In order to analyze short and long-term corrosion behavior of different support materials acid resistance and base resistance tests were defined (see table 1). Under short term acid at- tack intumescent and non-intumescent materials similarly show almost no weight loss. How- ever, in the long term test significantly lower acid resistance was observed with intumescent material. It remains to be evaluated which components of the support materials examined are especially susceptible to corrosive attack.
Figure 3: Erosion of differerent support materials versus mount density. The mount density is the density of the support mat after canning.
0,1 1 10 100
0,25 0,5 0,75
mount density (g/m3)
erosion (g/h)
intumescent mat
intumescent mat with rigidizer
non intumescent mat
Table 1: Mass loss in percent during immersion in acids and bases acid resistance
(18,5 % HCl)
base resistance (20 % NaOH)
10 min 10 days 1 day
intumescent material 99,81 70,92 79,45
non-intumescent material 99,95 99,72 94,90
4 Summary
Support materials play a key role in the manufacturing and operation of catalytic converters. In the present work, an attempt was made to correlate macroscopic mechanical and chemical properties to the microstructure of support materials. It was shown that under service condi- tions, which means cyclic compression and decompression, microstructural processes lead to a reduced relaxed pressure. Manufacturing parameters have to be adapted correspondingly.
Significant differences between intumescent and non-intumescent support materials with re- spect to erosion and corrosion resistance were observed. A more detailed examination of the microstructure-property relationship, with a focus on the relaxation phenomena and high tem- perature aging, is in progress.
5 References
[1] R. J. Locker, C. B. Sawyer, G. Eisenstock, SAE 2001-01-0223