Interfacial Reactions of CaZrO3 -Y2O3 Composite Ceramic Shells for Titanium Alloy Investment Casting

Imagine trying to pour molten metal into a mold, but the mold itself begins to melt or react chemically with the liquid metal. This is a constant struggle when casting titanium, a metal prized for its strength and lightness but notorious for being chemically aggressive at high temperatures. When molten titanium touches a ceramic shell, it often steals oxygen and other elements from the ceramic, creating a ruined surface layer on the final part. This contamination makes the metal brittle and unusable for critical applications like aircraft engines or medical implants. For decades, scientists have searched for a ceramic material that can withstand this heat without breaking down or reacting, hoping to find a perfect barrier that keeps the metal pure.

Researchers at the Shenyang Research Institute of Foundry Co., Ltd. tackled this problem by mixing two different ceramic materials to create a stronger shield. They started with calcium zirconate, a ceramic known for its stability but which still reacts too much with titanium. To fix this, they added varying amounts of yttria, a rare-earth oxide that is excellent at resisting heat but is difficult and expensive to use on its own. By blending these two materials, the team aimed to create a composite shell that offered the best of both worlds: the low cost and availability of calcium zirconate with the superior heat resistance of yttria. Their goal was simple yet vital: to see if adding more of the heat-resistant material could stop the chemical attack on the molten titanium and produce a cleaner, stronger casting.

The team prepared four different types of ceramic shells, each containing a different percentage of yttria mixed into the calcium zirconate. They ranged from a shell with only twenty percent yttria to one with eighty percent. Using a special liquid binder to hold the powder together, they built up layers of this mixture to form the molds. Once the shells were dried and fired in a furnace, they were used to cast a specific titanium alloy known as ZTA15. The process involved pouring the molten metal into the preheated ceramic shells and letting it cool. After the metal solidified, the researchers broke open the shells to examine the surface of the castings, looking closely at how much the ceramic had reacted with the metal.

What they found was a clear and direct relationship between the amount of yttria added and the quality of the final product. In the shell with the lowest amount of yttria, the reaction between the ceramic and the metal was severe. A thick layer of damaged metal formed on the surface, measuring nearly seventy-one micrometers in thickness. This damaged zone, where the metal's internal structure had been altered by the chemical reaction, stretched deep into the casting, reaching a depth of two hundred twenty-nine micrometers. As the researchers increased the amount of yttria in the mix, this damage zone shrank dramatically. In the shell with the highest concentration of yttria, the contaminated layer was reduced to just twenty-one micrometers, and the altered structure of the metal was confined to a much thinner strip of only one hundred seven micrometers.

The improvement was not just about thickness; the nature of the reaction changed as well. In the shells with less yttria, the calcium zirconate broke down at high temperatures, releasing elements that reacted with the titanium to form unwanted compounds and a mix of different metal structures. However, as the yttria content increased, it acted as a stabilizer, preventing the ceramic from breaking down. In the most successful shells, the surface of the titanium remained almost entirely pure, retaining its original structure without the formation of these new, brittle phases. The researchers also observed that the ceramic shells themselves became stronger and less likely to crack or flake off when the yttria content was higher, which further protected the metal from contamination.

To understand exactly what was happening inside the shell, the team analyzed the chemical composition of the layers using advanced imaging and X-ray techniques. They discovered that in the shells with low yttria, the ceramic material decomposed, creating a chaotic mix of oxides that reacted aggressively with the molten metal. But when the yttria content was high, the ceramic remained stable, forming a protective barrier that kept the titanium pure. The team also measured the temperature at which the reaction between the ceramic and the metal began to occur. They found that the shells with more yttria could withstand higher temperatures before any reaction started, confirming that the material had become thermodynamically more stable.

The study concluded that there is a tipping point for how much yttria is needed to make this work effectively. When the amount of yttria was reduced below forty percent, the material stopped acting as a primary protective component and instead became a minor additive that failed to prevent the ceramic from breaking down. This shift caused the reaction layer to thicken significantly, undoing the benefits of the composite design. Therefore, the researchers determined that to achieve the best results, the shell must contain at least forty percent yttria. This finding provides a practical guideline for manufacturers, showing that while adding more of the expensive material improves performance, a specific minimum threshold is required to make the ceramic shell truly effective against the aggressive nature of molten titanium.