I

0 4 6 8 10 12 14 16 18 20 22 24 26 Concentration o( Y203 in Zr02 wt% Fig. 3. Spall resistance of zirconia coatings vs yttria content.

Continuing to grossly summarize for brevity, the processing effect is that coating performance has been conclusively shown to depend strongly on process-related variables such as quality of the starting powder and porosity and residual stress distribution in the coating. Thus it follows almost axoimatically that process control and reproducibility of processing conditions are essential to the technical and economic success of thermal barrier coatings.

The combination of materials and process development over the past decade has produced overall advances of the order of magnitude diagrammed in Figure 4. Baseline 1977 technology was air sprayed magnesia-stabilized zirconia over NiAl bondcoats, and evolution through compositional and process optimization has resulted in a greater than 10-fold increase in coating performance. The data base which brought about this improvement is now available for application to advanced heat engines.

2Q25*F bumr rig cycflc testing

1977 stato»of-tto-art

•APS MCoCrAlY • APS MCoCrAlY »LPPS MCoCrAlY •MgO-Zrt), • YjOfZiOg »Y ,OrZiO,

Fig. 4. Advances in performance and durability of plasma sprayed thermal barrier coatings.

Thermal Barrier Coatings in Diesel Engines. In contrast to the cooled airfoil previously discussed in Figure 1, many of the proposed applications of thermal barrier coatings in diesel engines are on flat surfaces of pistons, valves, and heads. Such components do not always provide the internal cooling which contributes to the thermal barrier effect in cooled gas turbine airfoils, but an offsetting advantage is that coating thicknesses sufficient to provide the needed insulative capacity can easily be built up on flat surfaces. Also the simple geometry of the coating tends to minimize the sharp thermal gradients which would cause spallation of thick layers of more complex shapes.

Most noteworthy in the area of ceramic coatings for diesel engines is the development work and operating experience compiled by Kvernes et al at the Central Institute for Industrial Research in Norway. Kvernes in 1981 (8) described partial success in application of magnesia-stabilized zirconia coatings to exhaust valves tested under various engine conditions, but summarized the limitations of the then-current coatings as high residual stresses and poor reproduction of microstructure (consequently, low corrosion resistance and insufficient fracture toughness of the coating). Note that both of these problems are largely process related, hence amenable to improvements through process control.

A more recent assessment of ceramic coating development for advanced diesel engines was presented by Fairbanks and Hecht (9). Emphasized in this discussion was the concept of ceramic coatings as thermal insulators to reduce heat rejection in adiabatic diesels. Such technology, coupled with development of high temperature lubricants, would permit elimination of the engine block water cooling system. Since almost 30% of fuel energy is lost as heat to water cooled engine blocks, the potential fuel savings —as well as elimination of major maintenance problems traditionally associated with water pumps, radiators, etc.—is indeed a formidable incentive for use of thermal barrier coatings.

Potential thermal barrier usage in diesel engines is somewhere between 8 and 20 million parts a year, assuming routine application of thermal barrier coatings in light- and heavy-duty truck engines. This estimate projects to coating of approximately 20,000 to 40,000 parts per day. Hence, to make thermal barrier coatings viable in diesel engines, it is important not only to achieve high volume production but also a significantly reduced cost per part. Paramount to both of these achievements—as well as obtaining the desired performace and durability of the coating—is control of coating structure, composition, and thickness via control of the plasma spray process.

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