Process-Microstructure-Property Correlation in Vacuum Sintered Al-Al2O3Functionally Graded Pellets

Abstract

Aluminum-alumina (Al-Al2O3) functionally graded materials are attractive for components requiring a compliant metallic base and a hard ceramic rich surface. However, their performance is strongly limited by transition layer porosity and incomplete densification during low temperature powder metallurgy. In this study, a five-layer Al-Al2O3graded pellet was fabricated by sequential powder stacking, planetary ball milling of mixed layers, low pressure uniaxial compaction at 20 MPa, and vacuum solid-state sintering at 580 °C. The designed solid phase volume fractions from bottom to top were 100/0, 75/25, 50/50, 25/75, and 0/100 Al/Al2O3, with a nominal pellet diameter of 20 mm and layer thickness of 2 mm. Particle size distribution, X-ray diffraction (XRD), scanning electron microscopy, image-based porosity, Archimedes bulk density, Vickers microhardness, compression, flexural, and sliding wear tests were used to establish the process-microstructure-property relationship. XRD confirmed face centered cubic aluminum and α-Al2O3as the dominant crystalline phases, with no obvious crystalline reaction products after sintering. The pellet retained macroscopic integrity, but porosity varied non-linearly across the gradient, reaching 9.12%, 15.26%, 10.26%, 16.23%, and 15.36% from Layers 1 to 5. The bulk density measured was 2.88 g·cm-3. The microhardness of the Al layer was raised significantly with the Al2O3layer, from 44.6 HV0.1 to 703.0 HV0.1. In addition, the specimen exhibited Compressive strength of 138.24 MPa, Flexural strength of 34.03 MPa and Mean Coefficient of friction of 0.278 ± 0.030. The results indicate that low pressure powder stacking can produce a stable Al-Al2O3hardness gradient but the main limitation, which needs improvement, is the porosity in the transition layer.