EFFECT OF MARTENSITE CONTENT ON THE SLIDING BEHAVIOR OF BORON-ION-IMPLANTED 304-STAINLESS-STEEL

We have investigated the effects of boron ion implantation on the wear behaviour of 304 stainless steel. The initial phase composition of th e specimens was varied by using two different kinds of polishing techn ique. Electropolishing resulted in purely f.c.c. (austenitic) specimen s. Subsequent mechanical polishing resulted in a phase transformation that introduced a b.c.c. (martensite) phase. The specimens studied wer e either electropolished or fully polished (electropolished and mechan ically polished). In general, we observed that boron implantation impr oves the wear performance of the steel. The result of boron implantati on is to inhibit hardening of the surface during wear, as was seen fro m measurements of the microhardness inside the wear tracks. (Such hard ening has previously been shown to occur in studies on unimplanted mat erial, through martensitic transformation, and leads to the formation of a brittle surface sheet that is prone to cracking.) Interestingly, the improvement on implantation and the associated inhibition of harde ning in the wear tracks were seen to be much more pronounced when full polishing was employed rather than just electropolishing, i.e. when t he specimen contained a small amount of martensite prior to implantati on. The measurement of the microhardness inside the wear tracks provid ed a quick method of assessing the transformation during the wear proc ess, which we then correlated with the friction and wear behaviour in each case. Measurements of the microhardness on as-implanted surfaces showed that a fully polished surface strengthens to a greater extent t han an electropolished surface. This appears to result in reduced plas tic deformation, leading to the observed reduction in transformation d uring wear and, thus, could explain the greatly improved behaviour.