Process for forming a mullite-zirconia engine part

A process for forming a thermally insulating component for an engine part comprising the steps of:

• providing a solids mixture of zirconium oxide and mullite chemically mixed by sol-gel process, said zirconium oxide being present in the range of about 5% to about 25% by volume of said solids mixture and said mullite being present in the range of about 75% to about 95% by volume of said solids mixture, and said solids mixture being free of any sintering aids;
• a suspending vehicle, said suspending vehicle being water;
• a dispersant, said dispersant being an acid;
• forming a slip of said solids mixture, said suspending vehicle and said dispersant;
• adding sufficient acid to said slip and maintaining the pH of said slip at a value less than about 7;
• pouring said slip into a mold for forming the slip into a desired shape;
• drying said slip inside said mold for a period not less than about 1 week, and forming a slip-cast article having the desired shape;
• removing said slip-cast article from said mold;
• densifying said slip-cast article by pressureless sintering at a temperature no greater than about 1700.degree. C. for a period no greater than about 3.5 hours, and forming said thermally insulating component, said thermally insulating component having a density of at least 99% of theoretical density and a Weibull modulus in the range of about 10 to about 20.