Joining (bonding) of ceramic components based on reaction-bonded silicon nitride

Abstract

666.762.93:621.792 In recent years, ceramic materials are being widely used in aeroengines. Most of the foreign engine-manufacturing companies are carrying out testing of the ceramic structures and the engines incorporating the elements and assemblies based on silicon nitride and silicon carbide. In our country, testing of a number of structural components based on reaction-bonded silicon nitride has been carried out successfully under the conditions simulating the operational conditions [1].When producing ceramic structures based on silicon nitride, it is usually difficult to obtain intricate products or to develop reliable bonds between individual ceramic elements and create assemblies that can withstand cyclic thermal loads at a temperature of approximately 1400~ without failing. Thus, in order to avoid overflow of the gases in the nozzle box consisting of the blades made from reaction-bonded silicon nitride, it is necessary to bond them along the upper racks to form a continuous ring or separate sections consisting of three or more blades. In this context, we note that producing a monolithic nozzle box using the methods of casting or pressing is rather difficult.In a number of cases, when making ceramic assemblies using silicon nitride, different mixtures consisting of refractory oxides are applied in advance on the surfaces of the elements being bonded. Thereafter, the assemblies are heat treated at temperatures close to the temperatures used during the production of the ceramic materials; the intermediate binder interacts with the surface of the elements based on silicon nitride [2].The adhesive bond obtained between the elements exhibits high strength from room temperature up to 1000-1200~ At higher temperatures, the strength of the assemblies decreases in the region of bonding because of the formation of lowmelting glassy phases during the course of heat treatment. The loss of strength combined with the insufficient resistance to cyclic thermal loads (due to the difference between the coefficients of thermal expansion of the material of the joint and the ceramic elements) restricts the possibility of using such assemblies. During heat treatment, a change in the linear and the volumetric dimensions of the bonding layer occurs due to its shrinkage and, consequently, it is difficult to join more than two components ensuring retention of the desired geometry.We studied the possibility of bonding the specimens and the intricate products obtained from reaction-bonded silicon nitride. Preforms were obtained using thermoplastic bodies consisting of 82-83 wt. % semiconductor-purity silicon having a particle size of less than 10/zm, 17-18 wt. % paraffin wax binder, and a small quantity of the additives accelerating the nitriding process. A benzene suspension containing a dispersed phase having a composition similar to that of the bodies of the molded preforms (dispersed phase : benzene = 1 : 1) was applied to the surface of the preforms of the specimens or the products as a thin layer; in lieu of this, the surf...

Features of adhesive technology for silicon nitride ceramic components

“…A method is presented in [16] for preparing joints of silicon nitride components, molded by hot casting from a thermoplastic slip. A paraffin wax binder is used in order to prepare the slip.…”