Balijepalli, SK; Colantoni, I; Donnini, R; Kaciulis, S; Lucci, M; Montanari, R; Ucciardello, N; Varone, A
Austenitic stainless steels are interesting materials for several industrial applications where corrosion resistance is a fundamental requirement [1]. However, their use is limited by low tribological behavior, which involves high friction and low wear resistance [2]. Conventional thermo-chemical surface treatments, such as carburizing and nitriding, improve surface hardness but drastically decrease corrosion resistance because treatment temperatures (generally higher than 550 degrees C) induce the precipitation of Cr compounds depleting the surrounding matrix of Cr. To avoid the precipitation of Cr compounds it is necessary to operate at low temperature (< 470 degrees C). Low temperature heat treatments produce a surface layer of austenite supersaturated of interstitial elements (S phase), thermodynamically metastable [3].
An industrial carburizing process at low temperature is kolstering [4]. Surface hardness is strongly enhanced and in addition the different concentration of C in austenitic phase between surface and bulk material induces a strong compression state which hinders the propagation of fatigue cracks [5] remarkably improving the fatigue resistance. The fcc structure guarantees some ductility of the hardened layer so it can be deformed without crack formation [6-7]. Kolstering improves wear resistance of AISI 316L [8] and its corrosion resistance in concentrated H2SO4 and NaOH.
Present paper presents the results of a study on the Young's modulus of S phase in a kolsterized AISI 316L steel carried out by means of Mechanical Spectroscopy (MS) and nanoindentation tests. The nominal composition of the examined material is shown in Table 1. The thickness of the layer hardened by kolstering was 33 +/- 1 mu m.
MS experiments have been carried out by the VRA 1604 apparatus [9-10] and from the resonant frequency fit has been determined the elastic modulus (Eq. 1). Nanoindentation has been performed by using a Berkovitch punch. Three maximum loads (5 mN, 15 mN and 30 mN) have been applied in different tests (360 tests for each load) with a load rate of 2 mN/s. From the slope S of unloading curve the reduced modulus E-r (Eq. 2) and the elastic modulus E (Eq. 3) can be obtained. XPS and AES were employed for micro-chemical analyses of the hardened layer.
A first attempt to measure E by MS was made by considering the S phase as a coating deposited on a substrate, i.e. the bulk material (Fig. 1). A rigorous' treatment of the problem due to Berry et al.[12] permits to determine the elastic modulus of the coating (see Eqs. 4-5-6); some applications of the method are reported in [13-15]. However, such approach did not work for kolsterized steel because it requires the measurement of the resonant frequency of the same sample with and without coating and it is extremely difficult to remove the S phase from the substrate without affecting the substrate itself. According to the mixture's rule (Eq. 7) successive modulus measurements have been carried out on the same sample progressively thinned by mechanical grinding of the untreated surface. It was not possible to perform experiments with samples of thickness below 300 pm because residual stresses in the kolsterized layer induced a remarkable reed bending. The results are shown in Fig. 2 together with those obtained from nanoindentation tests (e.g. see Fig. 3). For a thickness of similar to 1 mm the measured E value is very close to that of not treated steel (190 GPa). As thickness decreases Young's modulus increases because the contribution of the hardened layer becomes more and More important. From the extrapolation of experimental data a value of 202 -GPa at 33 mu m was obtained, that can be considered the mean value of the kolsterized layer. To find a correlation between Young's modulus measured by MS and C concentration in S phase, the kolsterized steel has been investigated by X-ray diffraction (XRD). From the lattice parameter a, that expands of 2.7% owing to C supersaturation, was estimated a mean C content in S phase of 2.2 wt % [16]. Under the assumption that the elastic modulus of austenite linearly changes with C content X-c (wt %) [17] E can be expressed by Eq. (8) being k = 5.5147 GPa / C wt%.
As reported by several works (e.g. see [18]), the kolsterized layer is not homogeneous since C concentration is maximum near the surface and decreases to reach a value close to that of not treated material at a depth of some tenths of microns. Fig. 4 displays a typical C content vs. depth profile measured by XPS after successive atomic layers removal by sputtering. The surface mainly consists of C (similar to 40 at%) and Fe (similar to 35 at%). Since the XPS depth of analysis is quite small, chemical composition profiles have been investigated also by multipoint AES on cross-sections. Fig. 5 shows the atomit ratios C/Fe, O/Fe, Cr/Fe recorded at different depth z on a cross-section. C/Fe ratio is 6 on the surface and steeply decreases with depth, whilst Cr/Fe ratio is nearly constant.
Since Fe concentration is approximate to 70 (wt%), that of C on the surface is 15% (wt%), i.e. a value much higher than the average one (2.2 %) of the S phase determined by XRD. The value determined by AES is also higher than that 5.5 %) found by Farrell et al. [18] through EDS. The binding energy of the photoemission peak C is (285.0 eV) suggests that C-C bonds are prevalent in the C layer. In literature, the D parameter is used to evaluate the nature of C [19-20]. This parameter is defined as the distance between the most positive Maximum and the most negative minimum of the first derivative of C KLL spectrum and is directly related to the moiety of 0 in sp(2) and sp(3) hybridization. In the case of kolsterized steel D ranges from 18.5 to 20.2 and corresponds to a 65-88% concentration of sp(2) C. This indicates that the surface layer is mainly made of C with graphitic nature with a minor part of sp3 C that can be attributed to DLC (Diamond-like carbon) or to taC (tetrahedral carbon).
The extremely high C concentration measured by AES and the elastic modulus value of similar to 400 GPa lead to consider the first surface layer as a mixed structure, DLC (Diamond-like carbon) - taC (tetrahedral carbon). In fact, a suitable mixture of the stiff taC (E= 900 GPa) and soft DLC (E= 62-213 GPa) [21-23] can give rise to values of elastic modulus like those determined by nanoindentation.