Diffusion bonding of structural ceramics to superalloys by HIP

Field: Materials Science (Physical Sciences)

Diffusion bonding of structural ceramics to superalloys has been investigated. Emphasis was placed on methods resulting in joints durable at elevated temperatures (500 - 700ºC) between these two classes of high - temperature materials. Hot isostatic pressing (HIP) was used to enhance void elimination at relatively low diffusion bonding temperatures. This results in lower residual stresses, thinner reaction layers and a retained microstructure in the superalloy. The investigation centred on ceramic materials based on silicon nitride (Si3N4). This is the main structural ceramic intended for use in components facing very high temperatures and/or aggressive environment at a certain level of thermomechanical stress. These ceramics were densified by HIP, with or without a few percent of yttria (Y2O3) as a sintering additive.A silicon nitride composition developed for the turbine wheel in a vehicular gas turbine was the main ceramic, HIPed with 2.5 wt% Y2O3 to give a material with good mechanical properties up to about 1400ºC.The increased oxidation resistance of silicon oxynitride (Si2N2O), as well as the formation of a layer consisting mainly of silicon oxynitride on the surface of silicon nitride components during glass-encapsulated HIP, made it valuable to compare the joining behaviour of these two materials to superalloys. Reaction sinterings of Si2N2O without sintering additives using HIP, as well as simultaneous joining and sintering of powder bodies of Si2N2O to Si3N4 were therefore conducted. Diffusion bonding of these ceramics to the superalloy Incoloy 909 was performed by HIP at 200 MPa and 900ºC or 1000ºC for two hours. The thin reaction layers (apparently about 2 micrometer in thickness) were examined by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). Enrichments of titanium and niobium were detected at the interface with silicon nitride, while no such enrichments were observed when the silicon oxynitride was bonded. The main physical limitation for durable joints (intended for use up to about 700ºC) between silicon nitride materials and superalloys is the large mismatch in coefficient of thermal expansion (CTE). This leads to extreme residual stresses and cracking of the ceramic. Two possible solutions to this problem were investigated: The use of a low-expansion superalloy, Incoloy 909, could reduce the residual stresses in the joint. The thermal expansion of this alloy was measured to higher temperatures than the maximum recommended temperature during use (65OºC) since the contraction from higher joining temperatures at about 900-1000ºC has a significant influence on the residual stresses in the joint after cooling. A temperature range suitable for joining without impairment of the microstructure and properties of the superalloy was determined. The relatively high average CTE of Incoloy 909 during a joining cycle, together with the serious limitations in methods suggested in the literature such as as interlayers of refractory metals such as tungsten, low-expansion alloys like Kovar or ductile metals such as nickel) lead to the conclusion that the CTE mismatch could not be sufficiently reduced by modifying only the metal part. The composition in the ceramic part of the joint should therefore be graded to increase the thermal expansion behaviour from the low CTE value in monolithic silicon nitride up to a significantly higher value at the ceramic/metal interface.Titanium nitride (TiN) and titanium diboride (TiB2) were chosen as supplementary phases to the silicon nitride due to their suitable properties and the availability of a variety of such composites. A gradation up to 80vol% TiN could reduce the CTE mismatch against Incoloy 909 to less than half from delta-alpha=8.8 micrometer/(mxK) to delta-alpha=3.5 micrometer/mxK).The two composites used in diffusion couples were densified by HIP from powder mixtures Of Si3N4 and either 60 vol% TiN or 50 vol% TiB2. Monolithic silicon nitride HIP ed with 2.5 wt% Y2O3 was also included in this study of interfacial reactions with Incoloy 909. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 9270C (1200K) and 200 MPa for four hours.The formation of reaction layers was very limited, being less than one micron in total layer thickness for all the Si3N4-based ceramics used. Two reaction products were found by AEM; a continuous, very thin, (less than or equal 100 micrometer)layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100-500 micrometer into the superalloy and forming a semi-continuous layer of a Gphase silicide containing mainly nickel, silicon and niobium. A ceramic composite, in which both the continuous fibres and the matrix consist of silicon carbide (S W), was diffusion bonded to two superalloys, Incoloy 909 and Hastelloy X, by HIP at 200MPa and 9000C or 10000C for one hour.Using SEM/EDS, the reaction zones were found to consist of a thin line of carbide formers from the superalloys (Cr+Mo and Nb+Ti, respectively), bounded by several layers containing silicides and free carbon, depending on the superalloy involved. The width and composition of the reaction zones formed were found to depend more on the different compositions of the superalloys (Hastelloy X or Incoloy 909) than on a difference of 100 K in joining temperatures (1000ºC or 900ºC). At 1000ºC, the reaction zones were typically about 40 micrometer thick in joints with Hastelloy X and about 100 micrometer in joints with Incoloy 909.The SiC/SiC composite was considerably more prone to reactions with superalloys compared to the behaviour Of Si3N4 under similar conditions. The higher reactivity of SiC requires efficient diffusion barriers to be developed since the reaction zones formed will otherwise grow more than two orders of magnitude thicker (about 100 micrometer) than for silicon nitride ceramics under similar joining conditions.