A Comprehensive Review on the Impact of Porosity Fraction, Size, and Morphology on the Tribological Characteristics of Porous Steels Alloys Produced by Powder Metallurgy (PM) and Related Techniques

Porosity is a physical property of a material that indicates the presence of voids within it, which alters the way the material interacts mechanically with its surroundings. There are many reasons why metals become porous, and these reasons depend primarily on the method by which the metal was manufactured. Porous steel alloys are used in bearings, gears, and other components that experience sliding or rolling contact. This review consolidates recent findings on how porosity fraction, size, and morphology shape the tribological response of porous steels produced by powder metallurgy (PM), metal injection molding (MIM), and laser-based additive routes. Across most studies, increasing porosity elevates surface roughness and reduces hardness, concentrating stresses at pore rims and lowering the real load-bearing area. Consequently, friction coefficients and wear rates generally rise with porosity. Some exceptions occur when pores are fine, closed, or deliberately oil-impregnated, where pores can act as micro-reservoirs that stabilize boundary films, trap debris, and intermittently reduce friction and scuffing—especially under conformal contacts and wet environments at dissimilar tribo-pair. For similar tribo-pairs (porous steel against the same steel), porosity tends to be detrimental even under lubrication. At similar tribo-pair contacts, adhesive wear mechanism dominate, but with dissimilar tribo-pair contacts, abrasive and delamination wear dominate as pores act as crack initiators; under lubricated gears and pins, debris sequestration and self-lubrication can offset part of the hardness penalty. The paper also reviews strategies to improve performance: surface densification (rolling/burnishing), thermochemical treatments (carburizing/nitriding) tailored to porous substrates, solid lubricant or hard coatings (e.g., MoS₂/graphite, high-entropy or hard alloy overlays), ultrasonic nanocrystal surface modification, and purposeful pore design or oil impregnation. Collectively, the evidence supports a design window in which low-to-moderate porosity with controlled pore size and appropriate surface treatment can balance weight reduction and damping with acceptable friction and wear.