Problems with the mineral insulated, metal sheathed lead wires used to instrument aircraft engines have become more severe at the increased operating temperatures of today's engines. To solve these problems, we have applied high purity A12O3 directly onto platinum wires. Three coating methods have been studied, electrophoretic deposition (EPD) from ethanol suspensions, slurry coating, and EPD from aqueous suspensions. The coatings were sintered to theoretical density at 1500- 1600oC. Highly adherent, crack free, coatings were obtained by the slurry coating process. When bent, the coating cracked in localized areas, but very little of it was dislodged from the wire. The wires coated by EPD from ethanol suspensions were free of cracks after drying but after sintering, regularly spaced cracks perpendicular to the wire axis developed. These cracks allow the wire to be bent around corners while still retaining the sintered coating. The coatings obtained by EPD from aqueous suspensions contained large voids due to the gas bubbles generated by electrolysis of water. After drying, these coatings exhibited two different crack patterns. One pattern was similar to that seen in sintered coatings obtained by EPD from organic systems. The other crack pattern occurred in thin coatings and consisted of an interconnected network of longitudinal and perpendicular cracks. The latter cracking pattern is undesirable. The deposition rates of the coatings were studied as functions of the variables in the processes. Although all three coating methods were sufficiently rapid to serve as the basis for a commercial process, aqueous EPD coatings are not recommended for this application. The electrical resistivity of the sintered coatings was studied as a function of temperature from 800 to 1050oC. The resistivities of the coatings at 800oC were 3.4- 7.5 × 108 Ω-cm, depending upon coating type.

Keywords: coatings, electrophoretic deposition, alumina, slurry coating, resistivity, cracking patterns in coatings, electrical insulation, high temperature wires.