Features of adhesive technology for silicon nitride ceramic components

Abstract

A method is provided for joining (gluing) individual silicon nitride components used in the preparation of structural objects of different types of silicon nitride ceramic with a broad range of porosity (from zero tõ 23%). The method includes preparation of an adhesive mixture based on silicon nitride, successive application of the mixture to component surfaces being joined, and firing of the joined object at 1000°C with exposure for 2 h. The glued joint is distinguished by high adhesive strength, which makes it possible to prepare objects of complex structure by this method.Keywords: reaction-sintered silicon nitride (RSSN), hot-pressed silicon nitride (HPSN), adhesive mixture, aluminoboron phosphate (ABPB) and aluminochromium phosphate (ACPB) binder, glued joint. Adjustment of manufacturing technology for ceramic objects of complex construction (for example presence of joints in engine components, the length of thermocouple sheaths, chlorine conduits, etc.), operating with thermal cycling loads and in chemically active media, continues to remain important at the present time.A reliable method for manufacturing large objects is gluing of individual components. This method for manufacturing silicon nitride assemblies and components for aero engines, thermocouples and chlorine conduits, operating in molten aluminum, is a subject of much research both in our country and abroad. Particular attention is devoted to methods for gluing silicon nitride components in Japan [1 -7].One method of joining components in the form of tubes and disks is sintering a glue composition under pressure in a connecting joint, containing up to 98% of silicon nitride; here 35% of the particles have a size of 8 -120 mm and 65% are less than 50 mm. A binder used is a mixture of carbomethyl cellulose and oleic acid. Sintering of the adhesive is carried out under a pressure of 70 kgf/cm 2 in an argon atmosphere at 1250°C for 1 h [1]. A disadvantage of this method is use of special equipment for its accomplishment.In [2] in order to obtain a glued joint with high strength on gluing silicon nitride components an adhesive composition has been used of CaF 2 (1 -50%) with kaolin (50 -98%). Heating was performed in a nitrogen atmosphere at 1400 -1500°C. Here gaseous fluorine is liberated under whose action there is significant silicon nitride surface etching and corrosion. Here reaction calcium and kaolin reaction products formed impregnated the surfaces being joined and after cooling provided increased joint strength and thermal shock resistance. The ultimate strength in bending of specimens (3´3´20 mm), cut from a glued preform, was up to 50 MPa. It should be noted that this evaluation method (according to strength in bending) is adopted in all of the Japanese patents considered by us.In research [3] for gluing sintered components of internal combustion engines (ICE) adhesive compositions are used based on powders of Si 3 N 4 , SiO 2 , MgO followed treatment in a nitrogen atmosphere at a temperature close to the mixture melting temperature (1620...