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Significant advances in composition and the manufacturing process have led to thin wall cordierite ceramic substrates with low thermal mass, high surface area, and large open frontal area-properties that are critical for fast light-off, high conversion efficiency and low back pressure. Indeed, such substrates are ideal catalyst supports for meeting the ever-stringent emissions regulations, ala SULEV and ULEV, as demonstrated by recent performance data1. This paper focuses on the physical durability of 400/4 and 600/4 cordierite ceramic substrates. In particular, it presents strength, fatigue, and modulus data which influence the mechanical durability. In addition, it presents thermal expansion data which impact the thermal durability. Both of these durabilities are examined as a function of operating temperature.

The stringent emissions standards in the late 1990's like NLEV, ULEV and SULEV have led to major modifications in the composition and design of ceramic substrates. These changes have been necessitated to reduce cold start emissions, meet OBD-II requirements, and to ensure 100,000 mile durability requirement in a cost-effective manner. This paper presents the key advances in ceramic substrates which include lower thermal expansion, lighter weight, higher surface area and improved manufacturing process all of which help meet performance requirements. In addition to above benefits, the compressive and tensile strengths of lightweight substrates, as well as their thermal shock resistance, are found to be adequate following the application of high surface area alumina washcoat. The strength properties are crucial for ensuring safe handling of the substrate during coating and canning and for its long term mechanical durability in service.

Stringent emissions legislation for diesel-powered vehicles, soon to go into effect, has led to new advances in both Diesel Oxidation Catalysts (DOC) and Diesel Particulate Filters (DPF). This paper reviews some of the new developments in DOC support design which lead to improved light-off behavior and higher overall emissions performance through lower thermal mass, higher Geometric Surface Area (GSA) and larger Open Frontal Area (OFA) than those afforded by the standard cordierite 400/6.5 cell configuration. The four different DOC supports examined in this paper include 400/4, 200/8, 300/8 and 400/6.5 - the last one serving as baseline.

Tempered float glass is commonly used for both side windows and backlites in the automotive industry. The success of such products is primarily attributed to high level of residual compression, following tempering, which provides abrasion resistance as well as 3X higher functional strength to sustain mechanical, vibrational and thermal stresses during the vehicle's lifetime. Certain applications of tempered glass, however, require mounting holes whose surface-finish must be controlled carefully to withstand transient tensile stresses during tempering. Simultaneously, the nature and magnitude of residual compression at the hole must provide sufficient robustness to bear mounting, vibrational and thermal stresses throughout the life of the vehicle. This paper presents (i) analysis of residual compression at the hole, (ii) measurement of biaxial strength of annealed glass with hole at center, and (iii) measurement of biaxial strength of tempered glass with hole at center.

The dual requirement of high conversion efficiency and 50K mile durability for cordierite ceramic converters is achievable through optimization of washcoat and catalyst formulation. This paper presents new data for high temperature physical properties, light-off performance, conversion efficiency and pressure drop through an oval cordierite ceramic converter with triangular cell structure and two different washcoat formulations; namely standard vs high-tech. Both of the washcoat systems have a beneficial effect on strength properties with nominal impact on thermal shock resistance. Both the standard and high-tech catalysts provide identical light-off performance for CO, HC and NOx conversion. The high-tech washcoat and catalyst system, in particular, provides consistently superior conversion efficiency for CO, HC and NOx. The pressure drop across the catalyst depends on hydraulic diameter and is only 8% higher for high-tech washcoat than for standard washcoat.

The dynamic fatigue data for two different cordierite ceramic wall-flow diesel filter compositions, EX-54 and EX-66, are obtained at 200° and 400°C using the 4-point bend test. These compositions offer larger mean pore size and experience lower pressure drop than the EX-47 composition, and hence are more desirable for certain applications. Their fatigue behavior in the operating temperature range is found to be equivalent or superior to that of EX-47 composition which helps promote filter durability. The fatigue data are used to arrive at a safe allowable stress, which would ensure the required 290K vehicle mile durability. The paper also discusses the impact of mean pore size on high temperature strength and fatigue properties and their effect on filter durability.

The high temperature creep data for radial specimens, cut from metal and ceramic substrates and subjected to compressive loads representative of mounting and thermal pressure are presented as function of load and temperature. These data show that the creep resistance of metallic specimens under sustained loading varies with temperature and is orders of magnitude lower than that of ceramic specimens. The observed creep deformation in metallic specimens reduces their open frontal area and hydraulic diameter with potentially adverse impact on pressure drop across the metallic substrate.

This paper reviews the development and successful application of ceramic catalytic converters for controlling automotive exhaust emissions. It presents the scientific rationale for designing the high surface area substrate to meet both performance and durability requirements. This is followed by a step-by-step design process for each of the converter components. The initial design stage focuses on understanding automaker's requirements and optimizing component design commensurate with them. The intermediate stage involves laboratory testing of converter components in simulated environment and ensuring component compatibility from durability point of view. The final design stage addresses the critical tests on converter assembly to ensure performance and field durability. In addition, it examines the necessary trade-offs and associated design modifications and evaluates their impact on warranty cost for system failure.

This paper addresses the packaging design for monolithic cordierite ceramic converters to meet the new, stringent durability requirements of the 1990's, while minimizing warranty cost for the automaker. These objectives are best met by using a systems approach during the early phases of packaging design, i.e. by examining design interactions between the ceramic monolith, alumina coating, ceramic mat or wiremesh mounting material with seals, stainless steel can, heatshields, and associated peripheral components. Failure of any one of these components can prove detrimental to converter durability. In this paper we take advantage of overall understanding of the observed failure modes and individual component behavior, and present new data for optimizing the total converter durability through initial design. In particular, the impact of symmetric gas entry, monolith contour, clamshell anisotropy, mount density, stiffener ribs, and heatshield insulation on total durability is highlighted.

The typical ceramic catalyst support for automotive application has a total volume of 1640 cm3. Approximately 10% of this volume is subjected to tensile thermal stresses due to a radial temperature gradient in service [1]*. These stresses are kept below 50% of the substrate strength to minimize fatigue degradation and to ensure long-term durability [2]. However, the tensile strength measurements are carried out in 4-point bending using 2.5 cm wide x 1.2 cm thick x 10 cm long modulus of rupture bars in which the specimen volume subjected to tensile stress is merely 3.2 cm3 or 0.2% of the total substrate volume [3]. Thus, a large specimen population is often necessary (50 specimens or more) to obtain the strength distribution representative of full substrate. This is particularly true for large frontal area substrates for diesel catalyst supports with an order of magnitude larger stressed volume. In this paper, the modulus of rupture data are obtained as function of specimen size.

High temperature modulus of rupture (MOR) data, published previously, show that the ceramic catalyst supports get stronger with temperature due to the absence of water vapor and closure of microcracks which would otherwise act as stress concentrators [1, 2 and 3]*. The increased MOR value is partially responsible for the excellent durability of ceramic catalyst supports at high temperature. In this paper, we will present the compressive strength data of ceramic substrates at high temperature, namely the crush strength along B-axis and biaxial compressive strength of the whole substrate. Since the honeycomb strength is directly related to that of the individual cell wall, the compressive strength should also increase with temperature similar to the modulus of rupture. Accordingly, the ceramic substrates are capable of supporting higher mounting pressures exerted by the intumescent mat at high temperature [4].

The converter assembly consists of a ceramic monolith with racetrack cross-section, a suitable “springy” mat wrapped around it and a clam-shell steel can to contain and guard these components against road hazards. The process to effect this assembly is rather dynamic and introduces directional loads onto the monolith in view of the anisotropic stiffness of the can. If these loads exceed certain values, they may cause failure of the monolith either by crushing it or by shearing it. In this paper we analyze the stiffness of various components of converter assembly, determine the load distribution around the monolith, and modify the design of can and monolith to make the load distribution more favorable. It is concluded that the converter assembly can be optimized and the failure of monoliths, if any, eliminated during closure. The present monoliths do not suffer from such failure.

Under certain operating conditions when the combined stresses in a ceramic wall-flow diesel filter from mechanical, thermal, and vibrational loads exceed its threshold strength, the fatigue effects become important. This paper reviews the theory of static and dynamic fatigue, and presents fatigue data for Coming's high efficiency filter composition (EX-47, 100/17) in the temperature range 25° - 400°C which is representative of the stressed peripheral region during regeneration. The measurement and analysis of fatigue data, together with the implication on long-term durability of cordierite ceramic filters, is discussed.

Thermal stresses constitute a major portion of the total stress which the ceramic wall flow filter experiences in service. The primary source of these stresses is the temperature gradients, both in radial and axial directions, which attain their maximum values during regeneration. The level of particulate loading, the flow rate, the filter size and the mounting design govern the severity of temperature gradients which, together with physical properties and aspect ratio of the filter, dictate the magnitude and distribution of thermal stresses. The filter, the mounting, and the regeneration conditions should be so designed as to minimize these stresses to insure reliable and fracture free performance of the filter throughout the lifetime of the vehicle. In this paper we present a thermal stress model, based on finite element method, which computes stresses in the axisymmetric filter subjected to linear or step temperature gradients in radial and axial directions.

An important element of the diesel filter assembly is a resilient ceramic mat placed between the ceramic filter and the stainless steel can. It has four key functions: i) to provide adequate gripping pressure, ii) to permit free axial expansion of can, iii) to act as a seal for gases, and iv) to minimize temperature gradients in the filter, which require certain mat properties, namely low-to-medium compression modulus, low shear modulus, and low friction coefficient between mat and filter. This paper compares the properties and performance of two different mats, Interam® I and III, in “hot shake” and “exhaust gas simulator” tests. The results indicate that Interam® III is a superior material for diesel filter application and that a complete coverage by this mat will prolong the durability of the filter.

An approximate analytical solution for stress distribution in the rectangular lens of a glass headlamp due to static and impact loading is presented. Both low mass/high velocity and high mass/low velocity impact data and the resulting failure modes are discussed. Generally, glass headlamp lenses break either due to Hertz stress (front surface under high localized tension), or due to flexural stress (back surface under tension due to bending), or the combination of two. Failure due to flexural stress is illustrated by a star-crack, while that due to Hertz stress is illustrated by a Hertzian cone or “bullet hole” in the lens. The failure mode during low mass/high velocity impact is predominantly Hertzian while that during high mass/low velocity impact is flexural for lenses 0.120″ to 0.150′ thick. No significant differences are observed in the impact resistance of standard and light-weight lenses in this thickness range.

The high temperature dynamic fatigue data for the catalyst support composition, EX-20, 400/6.8, are presented. These data indicate that the fatigue effects are more severe when the substrate temperature in the peripheral region is near 200°C. The major impact of high temperature fatigue is the slow degradation of substrate’s initial strength while in service. Such a degradation must be taken into account in designing the total converter package to meet life requirements. For the EX-20, 400/6.8 substrate, approximately 50% of its initial strength is available to withstand the combined stresses from mechanical, thermal, and vibrational loads in service. At temperatures well above 200°C, the available design strength can be as high as 65% of substrate’s initial strength. The fatigue theory, the measurement technique, and the application of fatigue data to long term durability of cordierite substrates are discussed.

Ceramic honeycomb structures have been used successfully as catalyst supports in gasoline-powered vehicles for the past ten years. They are currently the leading candidate for trapping and oxidizing the carbonaceous particulate emissions in diesel-powered vehicles. In both of these applications the long term durability of the ceramic substrate is of prime importance. This, in turn, depends on the physical properties of cellular structure, cyclic nature of service loads and design of the mounting assembly. This paper examines the nature and dependence of both the mechanical and thermal stresses in the substrate on its geometry, properties, mounting parameters, and the operating conditions. It also compares the observed failure modes with those predicted by the theory. The paper concludes with a set of recommendations for optimal systems design and acceptable operating conditions which will promote the long term durability of the ceramic substrate.

The long term durability of a heavy duty gasoline truck converter is addressed by examining thermal stresses due to radial temperature gradients under three different driving schedules. The pertinent physical properties of a catalyzed cordierite ceramic converter, with triangular cell structure, are first measured as function of temperature. These are followed by thermal mapping of mid-bed temperatures with the aid of thermocouples under various driving cycles on the truck dynamometer. Both the physical properties and the temperature distribution are then used as input parameters in the finite element thermal stress model to compute stresses in the oval converter.

The physical properties, including modulus of rupture, structural modulus and thermal expansion coefficient of segmented, large frontal area, ceramic wall-flow diesel filters are presented. The effect of cement composition, its coverage and segmentation pattern on these properties and on the failure modes during strength testing is discussed. Using these properties the mechanical and thermal integrities of LFA filter are computed and compared with those of monolithic filter,. The paper discusses both the high efficiency (EX-47, 100/17) and low efficiency (EX-66, 100/25) filter compositions.