Active Screen Plasma Nitriding Process Research Articles

Influence of screen height and bias voltage on the active screen plasma nitriding of shot-peened Ti-6Al-4V titanium alloy

The effects of screen height and bias voltage on the active screen plasma nitriding (ASPN) of Ti-6Al-4V titanium alloy were investigated in this paper. Before nitriding, the Ti-6Al-4V alloy was pretreated by shot peening to allow the ASPN performed at a low temperature of 500 °C. The ASPN process variables concerned in this paper included screen height (20 and 100 mm) and bias voltage (0, 400 and 1100 V). The results show that the increase in active screen height effectively increased the yield of nitride particles transferred to the specimen surface by expanding the screen sputtering area and promoted the thickening of the nitrided layer. When the bias voltage increased from 0 V to 1100 V, the nitrided layer thickness and surface hardness of the nitrided Ti-6Al-4V showed a tendency of increasing first and then decreasing. Among the three selected bias voltages (0 V, 400 V, 1100 V), the thickest and hardest nitrided layer was obtained at 400 V, which balanced better between the deposition of nitride particles and the sputtering thinning of the nitrided layer. Meanwhile, the thickness and hardness of the nitrided layer on the shot-peened Ti-6Al-4V specimens were greatly improved compared to the original specimen. The thickest (5.30 μm) and hardest (1672 HV) nitrided layer was obtained on the shot-peened specimen at ASPN 500 °C–20 h with a bias voltage of 400 V and a screen height of 100 mm.

Spectroscopy study of composite coating created by a new method of active screen plasma nitriding on pure aluminum

In this study, the Active Screen Plasma Nitriding (ASPN) method was used to create a nitride layer on the aluminum Al1050. The goal of the study was to investigate the mechanism of formation of composite nitride coating (AlN/Fe2-3N) on an aluminum substrate and carry out a spectroscopic examination on this coating. For this purpose, the specimens of Al1050 were subjected to the ASPN process at temperatures of 450 and 500 °C for 5 and 10 h with constant conditions of 20%H2 + 80%N2 atmosphere, 80% duty cycle, and 10 kHz frequency. Coating characterization was performed by Grazing Incidence X-ray Diffraction (GIXRD) analysis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Glow-Discharge Optical Emission Spectroscopy (GDOES), and Field Emission Scanning Electron Microscopy (FE-SEM) with Energy Dispersive Spectroscopy (EDS) analysis. The results showed that the ASPN method can be used to form a composite nitride layer on an aluminum substrate. Based on the results, the paper also proposes a new mechanism for the formation of the composite layer, in which this layer is divided into two sections: a surface section made up of nitride particles and a sub-surface section consisting of diffusive layer.

Investigation of the ASPN process of low alloy steel by using Ni or Cr coated active screens

The mechanism of active screen plasma nitriding (ASPN) and the influence of the active screen's coating material on the surface properties of the nitrided layer was investigated on low alloyed steel samples, using chromium or nickel-coated active screens. ASPN experiments were performed at 510 °C, for 5 h in a 75% N2 + 25% H2 gas mixture using tempered 42CrMo4 type low alloy steel. The base material of the screens was unalloyed steel, but the screens were electroplated with chromium or nickel. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD), atomic force microscopy (AFM), and secondary neutral mass spectrometry (SNMS) were used for the characterization of surface properties of the samples and the screen. SNMS results show that the coating material of the active screen (chromium or nickel) was deposited on the surface of the samples in the form of compounds, with layer thicknesses between 160 and 200 nm (chromium) and 290–420 nm (nickel). Chemical analysis by X-ray photoelectron spectroscopy (XPS), performed at the interface of the steel sample and the deposited compounds reveals that iron-nitride was only formed in the case of samples that were treated with a Cr-coated active screen. Here the decomposition and oxidation of the unstable chromium-nitride provided the nitrogen atoms and the chemical drive for the formation of iron-nitride. In the case of the Ni-coated active screen, only molecular nitrogen was observed in the treated steel samples. The results prove that – contrary to widespread models on ASPN nitriding mechanisms – physically adsorbed nitrogen plays little role in the nitriding process of iron.

Application and evaluation of nitriding treatment using active screen plasma

Plasma nitriding was performed on samples of JIS SKD61 die steel, and the microstructure near the surface was analyzed. The plasma nitriding was performed using active screen plasma (ASP). In the ASP nitriding process, a dependency on the bias current applied to the sample was found.Emission spectra showed the presence of atomic nitrogen ions (N+), molecular nitrogen ions (N2+), hydrogen radicals (H), nitrogen radicals (N), and radicals consisting of nitrogen and hydrogen (NiHi). The densities of nitrogen and hydrogen radicals observed by vacuum ultraviolet absorption spectroscopy were 1.5 × 1011 and 1.0 × 1011 cm−3, respectively. Once the device current reached 0.25 A or more, the surface became cloudy. The cause of this cloudiness was investigated by X-ray diffraction and cross-sectional bright-field scanning transmission electron microscopy and was found to be a deposition layer around 50 nm thick of a mixed crystal containing the ε phase Fe23N and γ' phase Fe4N and the formation of a structure with a period of around 300 nm at the surface. Although deposition of Fe23N and Fe4N also occurred at the surface in samples without cloudiness, the deposition layer was only around 10 nm thick, and no periodic structure was formed. A chromium oxide-rich layer was found directly under the Fe23N and Fe4N deposition layer by scanning transmission electron microscopy with electron energy-loss spectroscopy. This chromium oxide-rich layer likely occurred due to selective etching of iron oxide and chromium oxide. In ASP nitriding method, the sputtering action of charged particles of N2+ and N+ was experimentally shown to play an important role in nitriding and surface morphology. Furthermore, it was discovered that there was no compound layer several micrometers thick as would form in the conventional nitriding process.

Synthesis and in-vitro antibacterial properties of a functionally graded Ag impregnated composite surface

Silver is considered promising in medical devices to prevent infection due to its excellent properties of broad antibacterial spectrum and persistent antibacterial activity. Herein, silver impregnated functionally graded composite surfaces have been developed by a novel duplex plasma deposition technique, which combines the double glow sputtering process and active screen plasma nitriding process. The composite surfaces include a surface antibacterial layer and a bottom supporting layer, which are deposited simultaneously. The functionally graded structure endows the composite surfaces with antibacterial activity, combined with improved wear resistance. The multilayer structures were observed by scanning electron microscopy, and the graded distribution of silver and nitrogen was verified by glow discharge optical emission spectroscopy. X-ray diffraction and X-ray photoelectron spectroscopy were used to analyze the microstructures and chemical states of the components. Results from physical properties tests indicated that the composite surfaces have increased hardness, lower contact angles, excellent scratch resistance and wear resistance. The in-vitro antibacterial tests using the Gram-negative E. coli. NCTC 10418 also showed that over 99% of bacteria were killed after 5 h contacting with the composite surface.

Plasma nitriding of the inner surface of stainless steel tubes

Stainless steels, especially the austenitic grades, present a good corrosion resistance but poor mechanical properties. Nitriding can increase the surface hardness but typically at the expense of corrosion resistance. Besides, nitriding of inner surfaces is not straight forward, as control of the plasma there is challenging.This work has focused on the development of active screen plasma nitriding processes of inner geometries of the widely used AISI 316L austenitic stainless steel, with preservation of the corrosion resistance. The structure and composition of the nitrided surfaces were studied and corrosion resistance of the inner surfaces assessed.As a result of the study, a relation has been established between plasma treatment parameters, geometrical features and the corrosion resistance. Additionally, electro-tribological studies have been performed. It has been observed that wear resistance improved and that ECR (Electrical Contact Resistance) is a good indicator of the wear phenomena occurring along the test and potentially could be a useful and simple monitoring tool of the status of the surface and assist in defining safe and dependable operational lives. As part of the study, an equivalent electric circuit is proposed.In conclusion, treatment conditions for AISI316L inner tube surfaces have been defined, with corrosion resistance preservation and improved tribological performance, while ECR monitoring has been found insightful and informative of the evolution of the later.

Investigation of nitrogen transport in active screen plasma nitriding processes – Uphill diffusion effect

The paper presents a structure of a nitrided layer formed with active screen plasma nitriding (ASPN) technique, which is a modification of plasma nitriding. The model investigated material was Fe Armco. The nitriding processes were carried out at 773 K for 6 h and 150 Pa. The main objective of this study was to confirm nitrogen migration effect and its influence on the nitride layer formation in different area of the layer interfaces (ε/ε+γ′/γ′). The results of the tests were evaluated using scanning electron microscopy (SEM, SEM/EBSD), transmission electron microscopy – electron energy loss spectroscopy (TEM-EFTEM), secondary ion mass spectroscopy (SIMS) and Wavelength Dispersive X-Ray Spectrometry (WDS). The analysis of the results suggests that the structures of the nitrided layers and nitrides morphology differ for various parameters and are dependent on the surface layer saturation mechanism for each of the temperatures and process parameters. New approaches in diffusion of nitrogen and carbon atoms and optimizing process were also analyzed. Nitrogen and also carbon transport in the sublayer was observed by several effects i.e. uphill diffusion effect which confirmed migration of the atoms in diffusive layer towards top surface (ε/ε+γ′ interface) and stress change effect in the nitrogen saturation area of the (Fe(C,N)+γ′) layer. Results showed in the paper might be used both for optimization of ASPN processes, modeling of nitrided layers formation mechanism and for controlling the nitrided layers morphology when nitriding different Fe based materials.

Scratch test of active screen low temperature plasma nitrided AISI 410 martensitic stainless steel

A nitrided case composed of expanded martensite and small quantities of hexagonal ε-Fe24N10 iron nitrides was formed in a martensitic stainless steel by means of active screen plasma nitriding process. Nanoindentation tests were carried out in order to assess the mechanical properties and to obtain an energy dissipation coefficient defined as the ratio of plastic to total deformation energy. Friction coefficient, mechanical failure mode and critical load for damaging the nitrided case were determined using linear scratch tests performed at both linearly-increased normal force and constant normal force according to ASTM C1624 standard. The scratch test results showed that the groove features and the friction coefficient could be well correlated to the energy dissipation coefficient. The expanded martensite strongly decreased the friction coefficient in comparison to the non-nitrided martensitic stainless steel. The critical load was 14N and tensile cracking was the mechanical failure mode of the nitrided case.

Influence of amorphous carbon layers on tribological properties of polyetheretherketone composite in contact with nitrided layer produced on Ti6Al4V titanium alloy

The key issue in bone implants biomechanics is – besides using biocompatible materials - the optimization of tribological properties of friction pairs occurring in joint implants e.g. knee and hip endoprothesis. Increasingly important role in these systems, alongside ceramics-on-ceramics and metal-on-metal, have metal-on-polymer friction pairs. Commonly used in orthopedics ultra-high molecular weight polyethylene (UHMWPE) does not meet the requirements of modern bone implants, mainly because of its harmful wear products which can cause inflammation and osteolysis of surrounding tissues after several years of using the endoprosthesis. Therefore, other polymer materials have been developed to replace UHMWPE, as well as several surface engineering methods are used for enhancing biocompatibility and tribological properties of applied materials. One of the materials to replace UHMWPE is increasingly used in medicine polyetheretherketone (PEEK) – a polymer material with a high biological indifference and mechanical properties.The article presents characteristics of TiN+Ti2N+αTi(N) nitrided layer produced on Ti6Al4V titanium alloy using glow discharge assisted nitriding process at the plasma potential, also known as the active screen plasma nitriding process and hydrogenated amorphous carbon doped with nitrogen layer a-C:N:H produced via RFCVD process on PEEK-based composite consisting of 10% graphite, 10% carbon fibers and 10% PTFE. Tribological properties of a-C:N:H – TiN+Ti2N+αTi(N) friction pair using “ball-on-disc” and “block-on-roll” tests were examined in correlation with microstructure (TEM, SEM, Raman spectroscopy) and surface morphology and topography (SEM, AFM, optical profilometer).The goal of this work is to present a new possibility of material solution for a ‘head-acetabulum’ friction pair in hip joint endoprosthesis using PEEK (PEEK T) as a replacement for commonly used UHMWPE and nitrided layer produced on Ti6Al4V titanium alloy using active screen plasma nitriding process.Amorphous carbon layer, 200nm thick, produced on PEEK composite significantly improved tribological properties of PEEK composite – TiN (nanocrystalline)+Ti2N+αTi(N) layer friction pair, decreasing the friction coefficient by 2 times and minimizing wear of both used materials in the point contact (“ball-on-disc” method) and in surface contact (“block-on-roll” method).

Repairing the cracks network of hard chromium electroplated layers using plasma nitriding technique

The use of hard chromium electroplated coatings for industrial and decorative applications is widespread. However, there seem to be some defects like the existence of an extensive micro-cracks network that restrict their applications. They affect corrosion resistance due to formation of direct routes to the substrate through which corrosive agents could reach the main metal. Plasma nitriding process is proved an effective technique to overcome such problems. The objective of this study is to investigate the effect of plasma nitriding parameters namely, time and temperature, on the closure of the cracks. Samples of a hard chromium electroplated hot work tool steel were heat-treated via conventional plasma nitriding (CPN) and active screen plasma nitriding processes (ASPN) at 500 and 550 °C for 5 and 10 h, and at 550 and 600 °C for 10 and 15 h, respectively. The morphology and microstructure of the samples were characterized by the scanning electron microscopy (SEM) imaging. The coating phases were determined by the X-ray diffraction (XRD) method. The corrosion properties of samples were evaluated using anodic polarization tests in a 3.5 wt% NaCl solution at 25 °C. The results showed that both techniques filled up the cracks. However, the closure mechanisms were different. In CPN method, the cracks were disappeared due to the transformation of chromium to its nitrides, which occur with volume expansion. It is worth noting that this phenomenon occurs at lower temperatures and shorter times compared to ASPN method. In ASPN method, the iron nitrides detached from active screen were deposited inside the cracks to fill them up. Moreover, it was found that the corrosion resistances of the samples modified by the CPN process were superior to the ASPN samples.

Effect of Surface Deposits on Nitriding Layer Formation of Active Screen Plasma Nitriding

Austenitic stainless steel SUS 316L was nitrided by active-screen plasma nitriding (ASPN) to investigate the effect of surface deposits from the screen on the nitriding layer formation. ASPN experiments were carried out using a DC plasma-nitriding unit. The sample was placed on the sample stage in a floating potential (bias-off) and a cathodic potential (bias-on). The screen, which was SUS 316L expanded metal with 38% open area ratio, was mounted on the cathodic stage around the sample stage. Nitriding was performed in a nitrogen-hydrogen atmosphere with 25% N2 + 75% H2 for 18–180 ks at 673 K under 200 Pa by the ASPN process. After nitriding, the nitrided samples were examined using scanning electron microscopy, X-ray diffraction, Vickers microhardness and glow discharge optical emission spectroscopy. From the surface observation of the nitrided sample, deposits were observed on the top surface of the sample nitrided with bias-off whereas deposits were not on that nitrided with bias-on. The nitrogen-expanded austenite (S phase) was formed on the surface of both samples. Layer thickness of the S phase increased with increasing the nitriding time. Additionally, the degree of an increase of the layer thickness of the S phase nitrided with bias-on was approximately 2.5 times greater than that nitrided with bias-off. This result suggests that the ASPN treatment with bias-on is effective for the increase of the nitriding layer thickness.

Spectroscopic diagnostics of active screen plasma nitriding processes: on the interplay of active screen and model probe plasmas

In a reactor used for active screen plasma nitriding (ASPN) the interplay of two plasma types, (i) the plasma of the cylindrical active screen driven in a pulsed dc mode (f = 1 kHz, 60% duty cycle) and (ii) the plasma at an internal model probe driven in a cw dc mode, ignited in a low pressure H2–N2 gas mixture (p = 3 mbar) containing small amounts of CH4 and CO2 have been studied by tunable diode laser infrared absorption (TDLAS) and optical emission spectroscopy (OES) techniques. Applying in situ TDLAS the evolution of the carbon containing precursors, CH4 and CO2, and of the reaction products, NH3, HCN, CO and H2O, has been monitored. The degree of dissociation of the carbon containing precursor molecules varied between 70% and 92%. The concentrations of the reaction products were found to be in the range 1012…1015 molecules cm−3. By analyzing the development of the molecular concentrations at changes of gas mixtures and plasma power values, it was found that (i) HCN and NH3 are the main products of plasma conversion in the case of methane admixture and (ii) CO, HCN and NH3 in the carbon dioxide case. The fragmentation efficiencies of methane and carbon dioxide (RF(CH4) ≈ 1…2 × 1015 molecules J−1, RF(CO2) ≈ 0.5…1.0 × 1016 molecules J−1) and the respective conversion efficiencies to the product molecules (RC(product) ≈ 1013–1015 molecules J−1) have been determined for different gas mixtures and plasma power values, while the influence of probe and screen plasmas, i.e. the phenomena caused by the interplay of both plasma sources, was analyzed. The additional usage of the plasma at the model probe has a sensitive influence on the generation of the reaction products, in particular that of NH3 and HCN. With the help of OES the rotational temperature of the screen plasma could be determined, which increases with power from 770 K to 950 K. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the -(0-0) band of the first negative system, increases strongly in relation to the intensity of the neutral component, represented by the N2-(0-0) band of the second positive system. In addition, the behavior of the emission of the plasma at the model probe has been studied during the off phase of the plasma of the active screen leading to a direct comparison of the emission characteristics of both plasma sources.

Progress in control of nitriding potential in ASPN process

AbstractA novel active screen plasma nitriding (ASPN) process provided excellent temperature homogeneity in the load and showed further progress in the control of nitriding potential. In addition to a variation of the nitrogen partial pressure in the process gas commonly used in the conventional plasma nitriding, the applied bias power strongly impacted the nitriding results. In the present work, an application of both methods for the control of nitriding potential in the ASPN process was systematically investigated for a wide range of process parameters to meet the treatment requirements for different types of engineering steel. A two-stage technique based on proper choice of process temperature and required nitriding potential in each stage has been applied in the ASPN process to avoid unnecessary compromises between sufficient thickness of the compound layer, the maximum case hardness and the acceptable nitriding hardness depth.

SELECTED PLASMA NITRIDING METHODS USABLE FOR THE THERMO-CHEMICAL TREATMENT OF AIRCRAFT PARTS

The aim of the work was to review selected methods of plasma nitriding, used in the thermo-chemical treatment of aircraft parts. The introduction explains the nitriding process and presents the requirements on the thermoDirect Current Plasma Nitriding (DCPN), Active Screen Plasma Nitriding (ASPN) and Low Pressure Nitriding in AEGD authors drew attention to known occur in this process such as edge effect or hollow cathode effect. The Keller’s model, the Marchand’s model and the Walkowicz’s model of nitriding literature, transportation of nitrogen during Active Screen Plasma Nitriding process and to show that in this nitriding method, defects typical for Direct Current Plasma Nitriding Low Pressure Nitriding in AEGD plasma, authors also presented the model of nitriding mechanism in this process, which shows four nitrogen diffusion paths (physisorption, chemisorption, adsorption of N2, ion implantation). nitriding a layer without film on the surface.

Optimization of the ASPN Process to Bright Nitriding of Woodworking Tools Using the Taguchi Approach

The subject of the research is optimization of the parameters of the Active Screen Plasma Nitriding (ASPN) process of high speed steel planing knives used in woodworking. The Taguchi approach was applied for development of the plan of experiments and elaboration of obtained experimental results. The optimized ASPN parameters were: process duration, composition and pressure of the gaseous atmosphere, the substrate BIAS voltage and the substrate temperature. The results of the optimization procedure were verified by the tools’ behavior in the sharpening operation performed in normal industrial conditions. The ASPN technology proved to be extremely suitable for nitriding the woodworking planing tools, which because of their specific geometry, in particular extremely sharp wedge angles, could not be successfully nitrided using conventional direct current plasma nitriding method. The carried out research proved that the values of fracture toughness coefficient K Ic are in correlation with maximum spalling depths of the cutting edge measured after sharpening, and therefore may be used as a measure of the nitrided planing knives quality. Based on this criterion the optimum parameters of the ASPN process for nitriding high speed planing knives were determined.

Investigations on the active screen plasma nitriding process

This study presents the mechanism of nitrogen transfer in a large scale active screen plasma nitriding (ASPN) unit. The active screen provides a homogeneous temperature distribution within the workload as well as an arcing tendency is reduced, since the plasma is moved from the treated work pieces to the active screen. In order to study the mechanisms in detail, plasma diagnostics is necessary. The applied diagnostic methods for analyses of the excited process gas synthesised by the active screen include mass spectrometry and infrared diode laser spectroscopy (TDLAS). Together with a metallurgical characterisation of nitrided steels a better understanding of the chemical phenomena in the N2-H2 plasmas is achieved. Various N2/H2 gas mixtures and variable bias activation powers at the pressure of 200 Pa were investigated. It was found that even during heating up in nitrogen free process gas exited nitrogen (atomic or molecular) releases from the active screen. Infrared diode laser spectroscopy has been used as a diagnostic methode to measure the concentrations of the stable NH3 molecules downstream the plasma source. The ammonia production turns out to depend on the mixtures of N2-H2 and the current input of the bias activation. The results are discussed with the plasma parameters measured by the electrostatic probe. Thus, a contribution to the interpretation of the mass transfer in ASPN process has been realised.

Surface hardening of duplex stainless steel by low temperature active screen plasma nitriding

The duplex stainless steel has high strength and excellent corrosion resistance. Since this steel has low hardness and wear resistance, the surface hardening treatment should be carried out under the wear environment. In this study, plasma nitriding has been applied for the improvement of wear resistance. However, traditional plasma nitriding at temperatures ranging from 723 to 823 K decreases the corrosion resistance of stainless steel. This is because the chromium in the steel reacts with nitrogen to form chromium nitride, and this decreases the concentration of chromium in the matrix, which is required for the formation of stable passive layers. Plasma nitriding carried out below 723 K forms a nitrided layer that is called the S phase. This phase improves the surface hardness without decreasing the corrosion resistance. Therefore, a low temperature plasma nitriding technique was used for treating stainless steel. Active screen plasma nitriding (ASPN) was carried out as the plasma nitriding process in addition to the conventional direct plasma nitriding (DCPN). ASPN has several advantages over the conventional DCPN, such as eliminating the edge effect, arcing, and carrying out hollow cathode discharge. The sample used was SUS 329J4L (X2CrNiMoCuN25-6-3). Nitriding was carried out at 673 and 723 K for 18 ks by DCPN and ASPN. The nitrided samples were then analyzed by various tests such as appearance observation, X-ray diffraction, SEM-EDX analysis, hardness testing, wear testing, and corrosion testing by measuring the pitting potential. After nitriding, the ASPN process improved the wear resistance and eliminated the edge effect when compared with the DCPN process. The S phase was observed on all the treated samples, and this phase was uniformly formed on both austenitic and ferritic phases of ASPN samples. Moreover, the corrosion tests showed that the pitting potential did not decrease with plasma nitriding.

Effect of Through Cage on Active Screen Plasma Nitriding Properties

Low carbon steel S15CK was nitrided by active screen plasma nitriding (ASPN) using various stainless steel cages to investigate the effect of the cage on the nitriding properties. Three types of austenitic stainless steel cages, such as pipe, foil, and wire mesh, were used. The sample was treated for 18 ks at 773 K under 630 Pa in 50% N2 + 50% H2 gases. The nitrided samples were characterized by surface roughness tests, optical microscopy, scanning electron microscopy, X-ray diffraction, and microhardness testing. In all samples nitrided by the ASPN process, the ‘edging effect’ was completely eliminated whereas hardness and thickness of the surface layer were comparable with those obtained from the DC plasma nitriding. Moreover, a comparison of the screens used in the ASPN process revealed that the screen hole size had a slight influence on surface properties such as microstructure and hardness. [doi:10.2320/matertrans.MRA2008431]

A fundamental contribution to a study of the active screen plasma nitriding process

Active Screen Plasma Nitriding (ASPN) is a novel plasma based nitriding process which challenges many of the existing hypotheses concerning mass transfer in plasma nitriding and, in turn, questions the influence of substrate bias during nitriding. In this study, a Field Emission Gun Scanning Electron Microscope analysis was used to investigate material transfer from the active screen. The results demonstrate that material transfer and deposition, in the form of nano-particles, take place from the active screen to parts that are in a direct line of sight of the active screen. The nano-particle distribution is significantly influenced by the type of active screen material used and the application of substrate bias. However, the suggestion that the mechanism of mass transfer in ASPN is directly related to sputtering of material from the active screen is questioned by the observation that AISI H13 steel samples treated in a non line of sight position in the ASPN chamber showed a significantly higher nitriding response than AISI H13 samples treated in direct line of sight of the active screen.