ICMCTF2015 Session G1: Innovations in Surface Engineering

Cold forming is a manufacturing process which is of great importance due to the maximum degree of material utilization and the associated energy and resource efficiency in the production technology. Lubricants are applied to reduce friction between workpieces and forming tools in cold forming processes. There is a strong demand to avoid lubricants due to the economic and ecological aspects. (Cr_1-x,Al_x)N / MoS_y coatings promise a solution for replacement of lubricants on highly forced tools in dry cold forging. Wear resistant (Cr_1‑x,Al_x)N together with the self-lubricating MoS_y deposited using Physical Vapor Deposition (PVD) technology offers an extension of tool life without lubricants. This combination is insufficiently researched and its capabilities are unknown, yet.

In the present work (Cr_1-x,Al_x)N / MoS_y coatings were deposited on 1.3343 (S6-5-2) tool steel by means of PVD in an industrial scale unit. A special target which consists of a CrAl half and a MoS₂ half in form of triangles was used, which leads to a variation of chemical composition of the deposited coatings. The coating thickness and morphology are determined using SEM (Scanning Electron Microscopy). Glow Discharge Optical Emission Spectroscopy (GDOES) was used to analyze the composition. The surface roughness was measured by means of a confocal laserscanning microscope (CLSM). Phase composition was investigated by using XRD (X-ray Diffraction). Hardness and Young’s modulus were measured using nanoindentation. A pin-on-disk (PoD) tribometer was used for the simulation of tribological behavior during the dry cold forming. Decreasing the Cr and Al content from 22.88 and 20.21 at.‑% to 13.61 and 10.99 at.-% by increasing Mo and S content from 3.44 and 9.00 at.‑% to 9.75 and 20.24 at.-% leads to considerable influences on the properties. It was found that hardness and Young’s modulus fell from 11.33 to 4.88 GPa and from 206.99 to 96.16 GPa, respectively. However, an increase of the deposition rate from 2.46 to 6.68 µm/h was observed. A reduction of the friction coefficient with the increase of Mo and S contents was observed. The characterization of samples showed that observed trends were clear for all deposited samples. It could be observed that the microstructure changed slightly due to variation of the chemical composition.

High density oxygen plasma ashing method is proposed to remove the used diamond coating with the film thickness of 20 mμm. First, both the emissive light spectroscopy and the Langmuir probe method are employed to make quantitative diagnosis on the generated oxygen plasmas. The measured ion density becomes more than 1x10¹⁷ m^-3, higher than ICP in one order. Oxygen atoms are main population of generated species in this high density plasmas. This high density oxygen flux is responsible for complete removal of CVD diamond coating; e.g. the average ashing rate turns to be more than 10 mμm/H.

The end-milling tools are employed to describe the ashing behavior with time. Corresponding to the variation of CO-peak intensity in the measured spectra by on-line spectroscopy, the diamond film thickness reduces monotonically with time up to 3.0 to 3.8 ks, and, removal rate decreases with time after this period. SEM and Raman spectroscopy are utilized to describe the time variation of diamond films and to describe the damage of WC (Co) substrates. Fine tuning of oxygen plasma processing conditions is capable to reduce the damage depth of tool teeth tips less than 5 mμm.

Advanced belt finishing process is remarkably simple and inexpensive. Its principle of operation is well known: pressure-locked shoes platens come circumferentially press an abrasive coated belt on a rotated workpiece. Very used in automotive industry to superfinish journals crankshaft, this abrasive machining process allows to significantly reducing surface irregularities, improving geometrical quality and increasing wear resistance and fatigue life. However one of the major industrial issues about this manufacturing process is its efficiency and robustness. Nowadays passenger cars journals crankshaft superfinishing are generally obtained by processing three strokes of abrasive coated belt when decreasing successively their grits size, which involve substantial investment costs for the manufacturer.

One of the most promising ways to solve this issue is to control the distribution and morphology of the abrasive grits. Recently, new generation of abrasives belts, coated by structured and shaped agglomerate grits are commercially available. Those structured coated belts with mastered cutting edge orientations promise to be more efficient and would have a better wear resistance compared to the traditional coated abrasive belt. This work aims to discuss these assumptions and to establish the link between such structured coated belts, their surface state and the physical mechanisms which govern their wear performances when belt finishing journals crankshaft.

Although a multitude of such technologies is already available for these purposes, it is still a big issue in these markets to combine low production costs with an outstanding performance – and to have a high level of flexibility for individualization features simultaneously.

To enable these options, it was our approach to use the technology of Physical Vapor Deposition (“PVD”) for the deposition of thin films on a transparent foil material (e.g. PET), to place the coated films onto a product surface and to transfer these thin PVD coatings then from this carrier to the product surface with a laser beam.

In this presentation this laser-induced coating transfer is discussed and the parameters to influence the optical, mechanical and chemical properties of the transferred coatings are described. Examples are shown for laser transferred coatings on different products.

By using the optical spectroscopy, microscopy, REM, EDX and other methods for analyses it is proven, that not only optical features – like different color or color flops -, but also functionalities like excellent adhesion and enhanced functionalities like scratch or corrosion resistance can be applied on product surfaces by using this technology.

High density plasma nitriding below 693 K significantly promotes the high concentration of nitrogen solutes in the martensitic stainless steel type 420 (AISI420-SUS) to be higher than 5 mass% or 50 times higher content than the solubility limit in the phase diagram of Fe-N system. This extraordinary nitrogen solute content results in the solid-solution hardening of stainless steels without loss of their corrosion toughness and surface quality since the chromium content remains to be the same as the initial state because of no reactions between chromium or iron and nitrogen atoms. For an example, the surface hardness reaches to 1400 Hv only by the present high density plasma nitridng for 14.4 ks after the present high density plasma nitriding.

SEM-EDX, Auger-spectroscopy and XPS are utilized to describe the solid solution behavior of nitrogen into AISI420-SUS. With reference to the first principle calculation, the possible occupation process for nitrogen solute into Fe-Cr crystalline structure is considered to explain the high concentration of nitrogen solute in the martensitic crystalline structure around 40 at% in the far depth. XRD-analyses are used to describe the lattice expansion process with high concentration of nitrogen as a solute into crystalline. In particular, the induced phase transformation from martensitic state to austenitic one by lattice expansion is discussed by controlling the plasma nitriding conditions. The diffusion process of nitrogen solute in the direction of depth is analyzed by SEM-EDX with reference to the theoretical diffusion approach for inner nitriding. Anisotropy in this diffusion is discussed through precise investigation of nitrogen distribution both in the masked (or not nitrided) and the unmasked (or nitrided) regions.

Duplex and superduplex stainless steel are used in chemical and petrochemical industrial plants. These materials combine corrosion resistance with sound mechanical properties. In this project, the goal is to enhance the mechanical and tribological properties of both stainless steels by plasma immersion ion implantation (PI3). Both materials were PI3 processed in three different gas atmosphere: Pure nitrogen, 50% N₂/50%H₂ or 98%N₂/2% CH₄. The process was conducted in two different temperature: either 300^oC or 360^oC over 2 hour or 4 hour. After the processing, XRD, hardness and wear tests were done. The hardness was substantially increased in the samples processed by PI3, specially the superduplex stainless steel, which was treated in the atmosphere that consisted of 50%N₂/50%H₂ at 360^oC. The tribological behavior, consequently, was improved. The increment in the wear rate was as high one order of magnitude in comparison with the untreated samples.