Adiabatic Engine Material Requirements

In order to allow the adiabatic engine to reach its full potential, the need for new, low-cost, and reliable insulation materials continues to be critically important. The high pressures and. temperatures of military diesel engines place very stringent reliability and durability constraints upon the advanced materials to be used within adiabatic engines. Typical material options include: superalloys, composites, structural ceramics, and ceramic coatings. Structural ceramic materials have an excellent potential for meeting required engine characteristics, while '

maintaining acceptable costs; however, much work remains if successful production is to be achieved. Superalloys tend to inject unacceptable cost into the ultimate application of adiabatic engines for most uses. In addition, their strategic material status for most military applications rejects their use as a first choice. Composites offer great promise for such advanced engine applications; however, actual high-output engine durability is yet to be proven. Likewise, ceramic coatings present potential low cost materials for innovative purposes within an adiabatic engine; however, problems continue to plague this approach from durability and low cost standpoints, particularly as coating thickness exceeds 1.0 mm (0.040 inch).

Specific material requirements and selection methodology for adiabatic type engines are somewhat constrained by specific design and application approaches; however, the following properties are representative of important characteristics: (1) insulative properties, (2) expansion coefficients, (3) high temperature capability, (4) high strength, (5) fracture toughness, (6) high thermal shock resistance, (7) low friction and wear characteristics, and (8) low cost.

Regarding structural ceramics, latest laboratory silicon nitride displays an exceptionally good strength-temperature relationship. However, a major problem concerning silicon nitride continues to be its poor insulating property. Some key ceramic properties that play important roles in the engine's instantaneous heat flux are thermal conductivity, density, and specific heat; and thus, a fundamental deficiency of silicon nitride is evident for adiabatic engine application. Partially stabilized zirconia (PSZ) with its excellent toughness, higher expansion coefficient, high insulation factor, and other desirable adiabatic engine features appears to fall short in high temperature strength. In addition, the rapid phase change of PSZ from the tetragonal to monoclinic phase is undesirable and its long term durability is not acceptable in the current state-of-the-art form. Silicon carbide displays high strength capability at the very high temperatures, but is generally not as strong as silicon nitride at te engine use temperature of 600-1000C. Advances continue to present themselves with respect to custom tailored ceramics using compounds, such as AI2O3, Cr203/Zr02, and Hf 02. It should be noted that in some instances such mixtures are made at the sacrifice of flexural strength or other key properties. As a final note regarding structural ceramics, the perennial problem of machining hard ceramic materials is being reduced by improved processing techniques, such as with near net shaping techniques. Non-destructive evaluation for quality control has al so improved, and cost of ceramic powder is dropping rapidly as the volume increases. Prospects for these advanced ceramics for advanced adiabatic engines looks promising; however, much remains to be proven by actual demonstration prior to final acceptance.

As listed above, an alternative to the use of monolithic ceramics is the ceramic coating. There are many ceramic coating techniques which are quite attractive substitutions to the monolithic ceramic approach.. Some key coating techniques include: (1) post densified C^Oo coating,' (2) plasma spray, (3) flame spray, (4) water plasma, (5) physical vapor deposition (PVD), and (6) chemical vapor deposition (CVD). The most popular ceramic coating for heat engine use is plasma spray zirconia on a metal substrate with a suitable bond coat. However, the coating of aluminum pistons with zirconia is very difficult and usually ends in failure due to the large thermal expansion coefficient mismatch. Recent work with graded coatings have appeared to alleviate this mismatch, even with thick coatings (i.e., greater than 1mm thickness). Despite advances within the plasma spray coating area, the difficulty of plasma spraying a thick thermal barrier coating persists. To achieve significant insulation effectiveness which results in maximum improvement in engine efficiency, a 5mm (0.200 inch) coating is needed. Thus far, plasma spray coating technology has been consistently successful only in the 1.0mm or less (0.040 inch) thickness range. Beyond a 1mm thickness, spalling occurs due to thermal stress concentrations. By increasing the power level and arc gas composition used during plasma spraying, improvements in plasma coating haVe been shown on the life of two layer thertnal barrier systems. The insulating properties of coatings are quite effective when compared to their monolithic counterparts because of the high degree of porosity in most coatings. A final note concerning coatings deals with the transient heat transfer differences between a coated piston and a solid metallic piston. Due to the low thermal diffusivity of Sprayed zirconia pistons, the cyclic transients penetrate into the wall structure for onlv a limited distance relative to the solid metallic piston. Thus the combination of large temperature Swings and short penetration depth's in the zitconia-coated piston results in high thermal gradients, and thus high stress levels. However, the coatings allow more appropriately phased heat flow during the heat release period of the engine in concert with the coating's low thermal inertia characteristics, and thus permit maximized engine efficiency payoffs.

Composites are a promising area of materials research whose characteristics are desirable for adiabatic engine application. Examples of composite material systems being investigated for adiabatic engine applications are: (1) metal or ceramic matrix reinforced with fibers and/or particulates, (2) laminated coatings of ceramic and/or metal materials, (3) monolithic (isotropic or orthotropic) ceramics attached to metals, and (4) fiber reinforced compliant attachment layers cast into and joining a monolithic ceramic and metal. Desirable composite material properties for adiabatic engines include: (1) low thermal conductivity, (2) low thermal stress, (3) greater compliance than monolithic ceramics, (4) non-brittle failure modes, (5) good attachment techniques, and (6) orthotropic stiffness and material properties. One difficulty which arises in the design of adiabatic engines is that properties 1 and 2 cited above oppose each other. As the thermal conductivity is decreased, the thermal gradients become larger and produce higher thermal stresses. Typically, the highest thermal stress in a highly insulated adiabatic engine represents 90% of the total stress occurring on the combustion chamber surfaces. Composite material systems offer an engine designer the opportunity to manage thermal stresses by varying the material's thermal conductivity throughout the bpt region. One laminated coating approach is to apply coatings of varying thermal conductivities. A second design opportunity afforded by (fiber) composite material systems ).s compliant attachment techniques. Monolithic ceramics are very stiff and therefore sensitive to impact loads. Compliant layers and compliant attachment techniques can be employed by the engine designer to decrease the apparent stiffness of the composite material and therefore create designs which are more tolerant to impact loads and less likely to fail by brittle fracture. Accurate theoretical predictions of the structural behavior of composites have proven critical for the continued commercialization (primarily military and aircraft to date) of composites. Within the last decade, the increased use of composites has paralleled advances in analytical techniques.

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