ION IMPLANTATION can improve the resistance of surfaces of materials to wear [ l , 21, corrosion [3] and fatigue [4]. In the current work plane bending constant maximum stress cantilever fatigue samples of thermomechanically treated (TMT) eutectoid steel [5-71 were subjected to boron ion implantation. In the work of Hu et al.[4] some slight improvement in fatigue life was experienced immediately after N + implantation of AlSl 1018 steel, however1 each of subsequent natural and artificial aging produced much greater improvements. The detailed reasons for those improvements were unknown but the authors stated that nitrogen diffusion to dislocations probably caused dislocation pinning, and that the formation of metastable nitrides also was involved. A finely dispersed metastable nitride of type Fel,N2 was identified in the As-implanted specimens [4]. The present work will compare results between samples immediately after ion implantation and after diffusion annealing and will compare results for samples ion-bombarded both before and after TMT.Consumable electrode vacuum arc remelted eutectoid steel, (0.85 C, 0.77 Mn, 0002 P, 0908 S, 0.17 Si, 0.2 Mo, 0.002 V, 002 Ni, 0.001 Ti, 003 Cu, 0002 Sn and 0-005 Al) was given the TMT's and ion bombardments listed in Table 1. Boron ions were implanted at an energy of 200 keV at fluences of 5 x lo" or 8 x lOI5 ions cmP2. The bombardments generally required 2-3 h at beam currents of 3-4 PA. Specimen temperatures never exceeded ambient. A Knoop hardness indentor with loads ranging from 15 to 500 g was used to get different indentor penetration depths. The depth was calculated from the measurement of the diagonal length of the impression (penetration depth : longer diagonal length of impression = l:30-53) [8]. Fatigue specimens (ASTM D 671-71) were cycled at 30 Hz about a mean stress of zero.Normally the material being measured should be more than ten times as thick as the penetration depth of the indentor [S]. Thus, with a small load, say 1 g, a conventional hardness value cannot be reported for boron implanted steel, since the projected penetration range, calculated by the procedure of Shiott [9], for 200 keV accelerated boron in iron is only 0 2 p m . Figure 1 shows the hardness indentor, hardened layer and base material. If one assumes that in a multilayer material the load applied on the indentor is sustained by the material according to the thickness and strength of each layer, then the observed Knoop hardness, H, which actually is defined as the applied load divided by the projected area of the impression, can be calculated as follows: If one assumes that the hardness H is a summation effect of sublayers, then, n H = X i P i , 27 1 i
