NiCrAlY BOND COAT 100 - 200/i.m (0.0039"-0.0078")

Figure 2. Plasma sprayed, yttria stabilized zirconia coating

Figure 3. Ceramic coated valve face - original design

*Numbers in parentheses designate references at end of paper.

Figure 3. Ceramic coated valve face - original design where the % yttria varied from 8 to 13. The base metal was prepared for coating by grit blasting and cleaning prior to applying the bond coat.

For selected coating application, where minimum surface porosity or a harder, smoother surface was desired, a vendor proprietary chrome oxide seal coating was applied. This process required a relatively high temperature soak and therefore had limited application to cast iron engine components.

Ceramic Coated Components

The various coated components evaluated in the Phase 1 investigation were: (1) a cast iron cylinder liner with a millimeter thick coating the full length of the bore ID; this coating was given a chrome oxide surface diversification treatment; (2) the cast iron cylinder head with a millimeter of coating applied to the portion of the head face exposed to combustion gases; (3) the steel valves, intake and exhaust with six tenths of a millimeter applied across the full valve face (Fig. 3); and (4) the full combustion face of the articulated piston- top including the reentrant bowl surface; coating thickness was nominally one millimeter although some variation occurred due to constraints placed on the plasma spraying by the bowl geometry.

A variety of results were obtained. The full length coated and treated cylinder was not tested extensively. For performance reasons (excessive intake charge

Figure 4. Ceramic coated valve coating failure heating), this concept was abandoned.

The original coated valve configuration failed around the valve face OD particularly on the larger diameter, more flexible intake valve (Fig. 4).

The redesigned coated valve incorporated an OD rim with tapered recess (Fig. 5).

This redesign was considered successful. The loss of coating was eliminated and the evaluation of all concepts which included coated valves was completed. A coating problem, however, is present (Fig. 6). The extensive crazing represents a long term durability concern.

The second coated component to be discussed is the piston. In the uncooled engine concepts the base aluminum piston is replaced with an articulated piston configuration made up of a steel or cast iron upper portion and an aluminum skirt joined by the piston pin. The original coated piston consisted of a nominal one millimeter thick plasma sprayed zirconia coating on the combustion face of the steel top and included the surface of the reentrant bowl. Under engine r\

Figure 5. Ceramic coated valve face - redesign with OD rim

Figure 6. Ceramic coaled valve with extensive coating crazing testing, the coating failed (spalled or flaked off around the periphery of the reentrant bowl lip.)

The redesigned insulated piston top_ incorporated the valve coating redesign recommendations and eliminated the coating problems associated with the reentrant lip region by the insertion of a PSZ ring (Fig. 7).

Like the redesigned valves, these changes allowed successful completion of concept evaluations using an insulated piston. The same extensive crazing seen on the valve is present in the piston top coating (Fig. 8) which again represents a long term durability problem.

The coated cylinder head experience paralleled the redesigned valve and piston top developments. The coating was applied to a recessed surface machined in the cylinder head. The area coated was that surface

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