Analysis of metal- ceramic bonding by frettage
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I.
INTRODUCTION
Low-cost sintered parts produced by powder metallurgy are widely used by the automotive industry. The wear properties of these parts could be drastically improved by the use of small ceramic inserts bonded to the metal. Metal/ceramic bonds can be produced using three different processing routes: (1) solid-state diffusion bonding under pressure (uniaxial hot pressing or hot isostatic pressingtl'21), (2) liquid-state diffusion bonding which can be obtained using special brazing alloys t3,4,5~or molten metals, 161and (3) mechanical bonding (frettage, riveting, etc.). A way to produce low cost parts is to bond the ceramic during the sintering process itself. Since no external load is used during the process, bonding by frettage will be used. However, due to the high sintering temperature, solid-state diffusion can also take place. The present study is concerned with the bonding during sintering of steel to silicon nitride. Test parts consisting of a metallic ring inside of which a ceramic cylinder is inserted before sintering, are produced. "Pushout" tests are then carried out to evaluate the shear strength of the parts. Finally, a simple analytical model, which takes into account the plastic deformation of the metal, is derived and used to analyze and derive the strength of the metal/ceramic bonds. II.
MATERIALS AND BONDING PROCEDURE
A. Materials The study was carried out on prealloyed iron powders sintered around silicon nitride cylinders [KERSIT 301" (sintering aid: Y203)]. The composition of the iron powder (DISTALOY** AB) as stated by the supplier is *KERSIT 301 is a trademark of Desmarquest, Evreux, France. **DISTALOY is a trademark of Hrggn~is AB, Hrg~in~is, Sweden.
0.5 pct Mo. 0.3 pct of carbon was also added. Alloying elements are mixed with iron, so that a high density can be achieved during cold compaction. These elements diffuse during sintering so that the resulting product is strengthened by solid solution, t7"8] The powder is cold compacted under 600 MPa using Zn stearate as lubricant (0.6 pct). The preform is then sintered in an endothermic atmosphere (cracked NH3) between 1150 ~ and 1300 ~ for 1 hour. At the end of the process, the samples are air quenched. Final relative density after sintering is given in Table I. B. Microstructure Typical microstructures after sintering are shown in Figure 1. Ferrite (Vickers hardness 142 HV~00g)and bainite (205 HV~00g) are observed. Martensite (340 HV~00g) is present in samples sintered below 1200 ~ Its volume fraction is small, which may account for the low value of the measured Vickers hardness. Pores are also observed. X-ray diffraction analysis exhibited the presence of a small amount of residual austenite. Microprobe analysis (Cameca SX 50) using wavelength dispersive spectrometry (WDS) was carded out on the different sintered samples. Concentration profiles were obtained using a 3-~m beam diameter. Figure 2 exhibits such profiles for a sample sintered at 1250 ~ showing an almost complete depletion of alloying elements in some regions. Tab
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