The effects of substrate polarisation in a nitrided 316L stainless steel have been investigated in an attempt to correlate processing parameters with surface mechanical properties. Nanoindentation allows the hardness to be measured at precise depths, meaning that variations in properties with nitriding depth can be evaluated. Fig. 1 shows a cross-sectional SEM micrograph of the diffusion layers of a typical nitrided stainless steel biased at -20 V. Secondary electron analysis confirmed the presence of a surface layer having a composition different to that of the saturated austenite phase (γΝ) beneath it. This MN-like CrN phase had a thickness of approximately 300-600nm. The surface topography of this sample was investigated with scanning force microscopy (SFM) and the characteristic step-like grain structure is shown in Fig. 3.
Nano Hardness Tester
Indentations were performed using the Nano Hardness Tester (NHT) from CSM Instruments at depths of 100, 200, 300, 400, 500, 750, 1000, 1250, 1500 and 1750nm in order to evaluate the evolution of hardness with depth into the MN-like surface layer. The loading rate was kept constant and 5 indentations were made at each depth, with each measurement being made on fresh surface material. The results are summarised in Fig. 2 for four samples nitrided at different polarisation potentials and the virgin 316L substrate as a reference.
Two separate regions can be distinguished as a function of penetration depth, dP; In the first region (dP<300nm), the presence of a different phase (MN-like CrN) is confirmed by a sharp transition in hardness.
This phase, created during the nitriding process, can be attributed to the decomposition of the γΝ phase during treatment. In the second region (dP>300nm), the hardness gradually decreases with the -50V sample decreasing more strongly than the others due to its lower nitrogen concentration at the surface.
Figure 1. Cross-sectional SEM micrograph of diffusion layers for an AISI 316L steel sample nitrided in Ar-60%N2 during 6 h at 680 K and at 0.8 Pa on each anode at a polarisation potential of -20V.
Figure 2. Variation of nanohardness with depth below surface for the four samples nitrided at different polarisation potentials and the virgin AISI 316L substrate: (◊) +20 V; (×) 0 V; (Δ) -20 V; () -50 V; (*) 316L substrate.
Regarding the nanoindentation profile of the 316L substrate, the hardness value stabilised to around 3.8GPa at depths greater than about 700 nm, this correlating well with values obtained by conventional microindentation. The relatively sharp increase in hardness over the first 500 nm was attributed to a mixture of mechanical and chemical surface artefacts. The former relates to the mechanical polishing of the surface prior to nanoindentation (in this case down to a finish of 0.25μm using alumina paste) which is known to induce a plastically deformed layer on the surface which is significantly harder than the base material. The latter refers to the oxide and other chemical films that inevitably form on normal experimental surfaces, even if only as a result of oxidation.
Figure 3. Three-dimensional SFM image of nitrided surface topography (sample polarisation = -20V) showing the step-like grain boundaries between three grains.