The objective of this work was to examine the microstructure, interdiffusion of elements, and hardness of joining sialon to AISI 420 martensitic stainless steel using diffusion bonding process. These materials were diffusion bonded at 1200oC for one hour under 20 MPa in a vacuum of 2.1x10-6 Torr. The microstructural analyses showed that joining sialon to nitrided steel produced thinner reaction layers and no gap or crack were formed on the sample. Gaps were produced in joining sialon to as-received steel. From the elemental analyses, alumina and iron silicides were formed at the interface layer of sialon/as-received steel joint. Alumina and smaller amount of silicides were detected at the interface layer of sialon/nitrided steel joint. Diffusion layer and parent steel of the sialon/nitrided steel joining contained nitrides. The hardness test across the joints indicated that reaction layers possessed intermediate hardness between sialon and steel. The layers contributed to ductility of the joint that help to attain the joint.

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In the past years, not many works on joining sialon to martensitic stainless steel have been carried out. The effect of the cut off heat upon cooling on the joint has yet to be discussed. In this paper, this effect is studied in terms of the microstructure and hardness of the reaction layer. Diffusion bonding was utilized to join sialon and AISI 420 martensitic stainless steel. The joining processes were conducted at 1150°C and 1200°C for one hour under a uniaxial pressure of 17 MPa in a vacuum (i.e. 1.0x10-5Torr). The heat was cut off upon cooling and the samples were left in the furnace for about 20 hours to cool down. Thicker reaction layer was formed in 1200°C sample because interdiffusion and reaction of elements occurred more rapidly at this temperature. The cut off heat had caused the sialons to crack and it was very severe in 1150°C sample due to the formation of a very thin interface layer. This layer did not have the sufficient strength to bind sialon and steel together because one of the sialons was completely detached from the joint. Diffusion layer and parent steel were segregated due to the difference in properties between the regions and it was more noticeable in 1200°C sample. Iron silicides and aluminium oxide might be formed in the interface layer. Microstructure of the parent steel transformed from globular coarse carbide in ferrite matrix to large grain size with carbide’s precipitation along the grain boundaries. More precipitates were formed in 1200°C sample and they were concentrated near the segregation line of the diffusion layer and the parent steel. The weakest part of the joint was at the border of sialon and interface layer since their hardness were extremely different.

Joining sialon to as-received and nitrided AISI 420 martensitic stainless steels using diffusion bonding is reported in this paper. The samples were joined at 1200°C for one hour under uniaxial pressure of 17 MPa in a vacuum (1x10-5 Torr). After joining process, the microstructure, interdiffusion of elements, and hardness of the joint were studied. The interdiffusion and reactivity of the elements created the reaction layer. It consisted of interface layer on the sialon side whereas thicker diffusion layer was formed on the steel side. Thinner reaction layers were observed in joining sialon to nitrided steels compared to joining sialon to as-received steel due to less reactivity between the joined materials. However, more precipitates such as carbides were formed in the parent steel with longer nitriding time. Gaps were formed between the diffusion layer and the parent steel but the interfacial bonds were strong since no cracking occurred on the samples. Since the reaction layer had intermediate hardness, it contributed to the joint’s ductility that reduced the effect of thermal expansion mismatch between the joined materials by acting as a shock absorbing zone.

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