OPTICAL METHOD FOR EVALUATING THE SURFACE-HARDENING OF A METALLIC OR A METAL-CONTAINING MATERIAL

The invention relates to an optical method for evalu- ating the surface-hardening of a metallic or a metal- containing material.

The realization of materials that show characteristics of high hardness, and preferably of a considerable lightness, is of great interest for numerous technological applications. Among the techniques used for the hardening of materials that is more and more used, that provides the formation or induction of nanostructures in the material.

Those materials are considered to be nanostructured that present themselves as agglomerates of particles hav- ing size lesser than 100 ran. As the size of the agglomerates of particles or grains increases significantly the ratio increases significantly between the surface and the volume of the single nanoparticles . This characteristic affects the physico-chemical properties of nanostructured materials in a determinant way, and principally originates an increase of the hardness thereof. For instance, in the formation of steels, the physical and mechanical properties can be characterized by the crystalline structure. The crystalline structure depends on the physico-chemical composition, on the cooling process, on the melting tern-

peratures and on the cold treatment processes. For instance, martensite, a crystalline phase characteristic of high-hardness steels, shows a nanophase structure [E. Hoffmann et al., Microscopic martensitic transition in Fe- Ni, 1993, Physical Review B; M.Shimojo et al., Formation of nanosized martensite particles in stainless steels, 2001, Metallurgical and Materials Transactions A; Z. L., Wang, Characterization of nanophase material, 2000, Wiley eds . ] . Mertensite, customarily produced by fast cooling, can also be obtained by deformation, that is by impact [V.I. Levitas et al., Strain-induced material changes and chemical reactions, 1998, Acta materialia; K. Kadau et al., Microscopic view of ~ structural phase transition induced by shock waves, 2002, science]. The most widespread techniques of deformation by impact on the surface of steels consist in bombarding the surface itself with spherical projectiles. The quantity of martensite produced depends on the physico-chemical characteristics of the treated material and on a set of experimental process pa- rameters. The fundamental parameters are: mass; density; impact velocity; the impact surface, the number of impacts per second, the treatment time. Therefore, the longer the time length of the treatment, the greater the thickness of martensite produced. In the treatments of deformation by impact of steels usually one does not obtain martensite

thicknesses having a depth greater than a few hundred microns from the surface. Thus it is even more important to control or monitor the production of the layer of marten- site . The formation of these layers of martensite is usually observed, locally and in areas restricted to squares having sides of a few microns, by means of transmission electron microscopy (TEM) or X-diffraction [N. R. Tao et al., An investigation ^" of surface nanocrystallization mechanism in Fe induced by surface mechanical attrition treatment, 2002, Acta Materialia; H. W. Zhang et al., Formation of nanostructured surface layers on AISI 304 stainless steel by means of surface mechanical attrition treatment, 2003, Acta Materialia; M. Umemoto et al., For- mation of nanocrystalline structure in steels by air blast shot peening, 2003, Material Transactions; M. Umemoto et al., Nano-grained steels produced by various severe plastic deformation processes, 2004, in Ultrafine grained materials III. Zhu et al. eds . , The Minerals, Metals & Mate- rials Society Press] . The hardness characteristics can be measured with penetrators, such as Knoop' s diamond pyramid, pressed against the surface of a sample pf the material under test.

The aforecited techniques for studying the formation of nanostructures require the use of a costly and often

bulky instrumentation, are restricted to the execution of investigations per sample and, anyhow, require preparations of the samples which alter or damage the integrity thereof. _„ , Moreover, the techniques based on the utilization of the TEM are invasive techniques. The TEM, in particular, requires a long and costly preparation of the sample and of the experiment. This imposes, as a matter of fact, a method of quality control per sample. It is impossible to distinguish between hardening layers having different thicknesses, unless one executes cuts of the sample perpendicularly to the surface that destroy the integrity thereof.

The instrumentation for the , characterization of the sample requires the necessity of manipulating the sample during a measurement .

The hardness measurement techniques of the type of Knoob' s one are invasive techniques and of difficult application and interpretation in the case of hardening lay- ers of the depth of a few"microns from the surface.

On the other side, it is known that the presence of metallic nanoparticles in a material can be identified through changes induced in the spectrum of reflected light in the visible and near infrared region (VIS-NIR) . In fact, an increase is observed of the reflectivity to the

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greater wavelengths which is indicated as a reddening or increase of the spectral gradient.

Such a reddening has been historically discovered in the study of the rocks on lunar grounds and confirmed by experiments of addition of metallic nanoparticles on rocky samples. [S. K. Noble et al., The optical properties of nanophase iron: investigation of a space weathering analog, 2003, Lunar and Planet. Sci. 34 ^th ; S. K. Noble et al., Quantitative aspects of space weathering: implications for regolith breccia meteorites and asteroids, 2004, Lunar and Planet. Sci. 35 ^th ; CM. Pieters et al., Space Weathering on airless bodies: resolving a mistery with lunar samples, 2000, Met. Planet. Sci.].

On the basis of the considerations above the present invention has been developed, which provides the utilization of VIS-NIR spectrum to evaluate the hardening of the surfaces of materials following the formation of metallic nanoparticles .

In particular, the present invention provides an op- tical method for evaluating the surface-hardening of a metallic or a metal-containing material, wherein the hardness depends on the formation of nanostructures in layers close to the surface. The method includes a first analysis of the spectrum in reflected light of the material, before it is subjected to hardening, in order to obtain a measure

of reflectivity as a function of the wavelength on a given surface of the sample; at least a second analysis of the sample in the spectrum in reflected light, in a desired subsequent moment during or at the end of the hardening treatment in order to obtain a measurement of the reflectivity as a function of the wavelength upon a given treated surface; a comparison between said at least second analysis and said first analysis; and the eventual ascertainment of an increase of the reflectivity that allows to evaluate the formation of metallic nanoparticles, and therefore an increase of hardness of the material, depending on the physico-chemical composition of the material and on the set of parameters of the hardening treatment which the sample has been subjected to. This approach allows ' to overcome the greater part of the shortcomings of prior techniques suggesting a reliable, non-invasive and inexpensive method for the diagnostics of the presence of nanoparticles associated with the hardening of the surfaces. The method is of particular efficaciousness when one wants to monitor the initial steps of the hardening process, that is the confinement thereof within the first microns from the surface.

Moreover, the present method allows to execute the monitoring of the hardening of the material also remotely,

that is with the sample at a distance and not under condi ^¬ tions of manipulation of the same.

The invention is now described for an indicative and not restricting purpose, referring to the enclosed draw- ing, wherein:

Figure 1 represents VIS-NIR spectra of an AISI 316 steel sample under various hardening conditions according to the method of the present invention, in a diagram where the wavelength is reported as the abscissae and the re- flectivity is reported as the ordinates. Figure 2 is a topographical image of AISI 316 steel sample of Figure 1, acquired with an atomic force microscope before a hardening treatment on the sample, with underlying magnification of the framed detail; and figure 3 is a topographical im- age of the same sample of preceding figures after the hardening treatment, to study the topography of the surface of the sample with--a high spatial resolution and to prove the results of Figure 1 obtained by the method of the present invention. In the following of the description, a scientific confirmation is first given of the validity of the method according to the invention and therefore some non restricting examples are given of industrial utilization of the method.

The method has been set up in the following way: an

AISI 316 steel sample was surface-peened with glass beads having a diameter of 100 microns and impact velocity of the order of a few meters per second for times variable up to 300 seconds.

Scattered light spectra have been acquired from 400 to 2400 mm of the surfaces before and after the treatment using a Perkin-Elmer 900 spectrophotometer with integrating sphere, keeping the geometry constant of the illumina- tion of the sample. The latter constitutes a fundamental requisite in the comparison between spectra in that a different angle between the source and the sensor can induce artefacts that cause modifications of the spectrum [B. Hapke Space weathering from Mercury to the asteroid belt, 2001, J. Geophys. Res.].

In Figure 1, which is a diagram with the wavelength as the abscissae and the reflectivity (normalized to 560 mm) as the ordinates, VIS-NIR spectra are reported relevant to surfaces of AISI 316 steel sample. By light lines the spectra are reported that refer to the not treated sample, whilst the bold lines are relevant to the spectra of the hardened sample.

As shown in Figure 1, the spectra acquired on the treated surfaces have shown a reddening with respect to those of all not treated surfaces. As an alternative to

spectrophotometer, instruments different therefrom can also be used for the recognition of the reflectivity of the material at different wavelengths in the spectral intervals of the visible and/or of the near infrared. Among the commercially available instruments filters, imaging and non-imaging sensors- "can be used, fit for realizing such a requisite. In any case the present invention is not to be considered to be restricted by the instruments employed. As a calibration example, following 180 seconds of peening on AISI 316 steel it has been observed that, normalizing the spectra in the visible around 560 nm, the average reflectivity in the NIR of the treated surfaces has turned out to be greater ^' than 5% with respect to that of not treated surfaces. Particularly it is to be remarked that the spectral gradient variations observed in different areas of the treated surfaces has not compromised the possibility of distinguishing not treated surfaces from treated ones. As a confirmation, at the same time of the spectral tests, the surfaces of AISI 316 steel sample have been studied utilizing an atomic force microscope (AFM) to survey the topography of the surfaces with a high spatial resolution. The results on the same sample of Figure 1 show, as turns out to be evident from Figures 2 and 3,

that a nanoparticle structure has been formed on all and only on the treated surfaces which on the contrary is absent on the not treated surfaces.

Particularly, a topographical image is reported in Figure 2 of a surface of AISI 316 steel sample before the hardening treatment. It is put into evidence how the surface structure of the sample turns out to be devoid of nanoparticles . A topographical image is reported in Figure 3 of the same surface of sample of steel after the harden- ing treatment, characterized by the presence of nanoparticles having a size lesser than 100 run. Lower images of Figures 2 and 3 are magnified details of the frames indicated in the corresponding upper panels.

Therefore, one has had the confirmation that the peening, customarily utilized in hardening processes, has produced a nanoparticle" ^* structure the formation of which has been proven with the AFM and the presence of which has been detected with the reddening of VIS-NIR spectrum, making the efficaciousness apparent of the suggested method. The industrial application of the method according to the present invention is per se apparent. Naturally, as a method is dealt with for evaluating the hardening of a material, it needs, in the case of a quantitative investigation, for a reference ^"" scale dependent on the test mo- dalitities. Therefore, a use of the present method that

allows to substitute a scale of hardnesses and/or a measure of the hardening thickness obtained with a known method with an optical parameter scale, and to compare the results between different users, will have to provide a definition recognized internationally of common measurement procedures.

A possible operative utilization of the presented method is described in the following, for an indicative and not restricting purpose. As the formation of metallic nanoparticles depends on the physico-chemical composition of the material subjected to hardening process, the thickness of the hardened layer depends on the type of treated material. Then, for the purpose of going back up to the hardening characteristics through the analysis of the VIS-NIS spectrum, it is necessary to provide a calibration curve.

For each procedure for the hardening of a material, individuated by its set.,,.of treatment parameters, for instance in the case of peening, the diameter, the impact velocity, the density of the spheres and the time length of the treatment, some reference spectral parameters are to be measured. An example could be provided by the ratio between the intensity of reflected light, collected through suitable filters, to two reference wavelengths, in the band of the green and in the band of the near infra-

red. As in all calibrations, the hardness and the thick ^¬ ness of the hardened layer are to be associated, measured by conventional techniques. Once the transfer curve from the mechanical parameter to the optical one obtained, each measure of the abovementioned ratio provides an indirect measure of the hardness and/or hardness depth of the sur ^¬ face. At this point, an optical measurement and the use of the calibration curve allow one to monitor, in real time and/or remotely too, the course of the hardening process of a metal-containing material.

Again for an indicative and not restricting purpose, it is to be mentioned that the optical monitoring could be executed by a suitable instrument that would consist, in its simplest conception, of a sensor sensible to two suit- able wavelengths, for instance in the visible and in the near infrared, coupled with a light-focusing system, that utilizes lens, cameras, etc. The measure could be provided by an automatic analysis system that performs the ratio between the light signals through two filters, in the ex- ample, in the green or in the near infrared.

The technique is grounded on the detection, also as- certainable with the sample at a distance, of the formation of metallic nanoparticles upon the surface layer of a material subjected to a hardening treatment by plastic strain or fast cooling. The method is particularly inter-

esting for a characterization, depending on the optical depth of the material and technically very simple and inexpensive, of the happened structural transformation into metal-containing materials wherein one wants to limit the depth of the modified layer up to 5-10 microns. Moreover, the method can be implemented by techniques for the acqui- sition of two- or more-colour images that allow the recog ^¬ nition of regions of the surface of the material that have a different hardening. The invention turns out to be particularly useful for the development of metal-containing materials having characteristics of high hardness and limited weight.

The invention finds applications tied to the deveop- ment of materials exposed to bombardment of powders, to aggressive environments, in space, or that anyhow require particular requisites of strength to environmental stress and hardness.

It is considered that the invention can be employed in the field of the control of the processes of phase transformation that can be obtained, in metal-containing materials, through deformation, impact or fast-cooling processes. Particularly, the invention turns out to be of interest in the processes of artificially-produced, or natural, bombardment; for the remote control of the expo- sure to bombardment of powders or microparticles; for the

realization of hardening-s - with controlled penetration layers, or of the control of hardenings distributed in a different way on the surface of the materials.

With the invention a method is realized for the evaluation of the formation of nanoparticles at the surface of a sample containing metals through a nondestructive method which consists in the analysis of VIS- NIR spectrum. This approach allows one to monitor the hardening of the surface" of the material in an inexpensive way, which can also be executed with the sample at a distance and allowing the control of the process also for those applications in which it is required, or preferable, that the layer of penetration of the hardening is restricted to a few microns from the surface. The advantages with respect to traditional techniques are: a great inexpensiveness, a simplicity of the instrumentation required for the monitoring, the utilization of non-invasive techniques, the potentiality of executing the control of the hardening process also with the sample at a distance, that is without any contact with it.

The invention enjoys all the advantages of the utilization of an optical technique, such as, for instance, the possible wide field of view; the sensitiveness to variations of the hardening thickness within the first microns from the surface, making the method particularly desirable

for applications that require the realization of materials having small and controlled hardening thicknesses; the possibility of implementation by more-colour image acquisition systems that allow, in a non-invasive manner, the selective recognition of zones having different hardnesses by means of the observation of contrasts of reflectivity.