The near-surface hardness and wear resistance of AISI 52100, a widely used bearing steel, have been enhanced by energetic ion bombardment using a plasma immersion process. Test flat samples of the steel were immersed in a plasma of methane or acetylene and pulse biased to a negative voltage so as to result in either ion implantation of carbon into the steel surface or deposition of diamond-like carbon film onto its surface. The nanometer size scale surface roughness of the initial sample flats was generally retained after the plasma surface treatments. The surface treatments were carried out with the samples at near-room temperature, which is an important consideration for this type of steel because of its relatively low softening temperature. However, by intentionally using the energy of the bombarding ions to marginally heat the surface of the samples, the depth of the carbon ion implanted layer could be increased by over a factor of two while maintaining the bulk hardness of the steel at acceptable levels. (C) 2003 Elsevier B.V. All rights reserved.