M2 Steel Substrates Research Articles

Effect of Temperature on the Structure and Tribological Properties of Ti, TiN and Ti/TiN Coatings Deposited by Cathodic Arc PVD

Monolayers of Ti and TiN coatings, as well as a Ti/TiN bilayer coating, were deposited on AISI M2 steel substrates using the PVD cathodic arc technique. The coatings had a thickness close to 5 μm and an average roughness between 98.6 and 110.1 μm due to the presence of microdroplets on the surface. The crystalline structure of the materials was analyzed using Grazing Incidence X-ray Diffraction (GIXRD) with an increase in temperature to study the dynamics of oxide formation. A phase composition study was conducted using the Rietveld refinement method. At the temperatures where critical growth of titanium oxides, both anatase and rutile, was observed, pin-on-disk tests were performed to study the tribological properties of the materials at high temperatures. It was determined that the oxidation temperature of Ti is around 450 °C, promoting the formation of a combination of anatase and rutile. However, the formation of rutile inhibits the formation of anatase, which is stable above 600 °C. In contrast, TiN showed an oxidation temperature of 550 °C, with an exclusive growth of the rutile phase. The Ti/TiN bilayer exhibited mixed behavior, with the initial growth of anatase promoted by Ti, followed by the formation of rutile. Oxidation and tribo-oxidation dominated the wear behavior of the surfaces, showing a transition from mechanisms related to abrasion at low and medium temperatures to a combination of abrasion and adhesion mechanisms at high temperatures (800 °C).

Application of honeycomb pattern to Ti2AlN MAX phase films by plasma etching

The honeycomb pattern possesses a distinctive hexagonal structure capable of seamless repetition on a flat surface, leaving no gaps. Moreover, all arm thicknesses and angles are equal to one another. This remarkable configuration is deemed biomimetic, with numerous examples found in nature. Notably, it exhibits remarkably low density and exceptional mechanical strength. MAX phase films have gathered significant attention due to their exceptional capacity to amalgamate the essential properties of both metals and ceramics. Additionally, they possess the unique ability to effectively mend surface cracks that may arise as a result of friction-wear, restoring the material to a certain degree of integrity. In this study, Ti2AlN MAX phase thin films were deposited on M2 steel substrates by a closed field unbalanced magnetron sputtering system (CFUBMS). 750 °C heat treatment was applied to obtain the produced films in crystalline form. In addition, plasma etching parameters suitable for the phase structure of the deposited film were determined. With the inductive coupling plasma etching (ICP) process, the honeycomb pattern was given to the Ti2AlN MAX phase films with a continuous and smooth geometry at a depth of 2 μm. This study gives ideas for the development of multi-coating systems in which patterns of different geometries are included in a single layer.

Pulsed-dc bias magnetron sputtered TiB2 ceramic coating

Titanium diboride, TiB2, is well known as a ceramic material with a hexagonal structure which presents various attractive properties, such as high hardness and excellent corrosion, thermal oxidation, and wear resistance. However, one drawback of TiB2 coatings is their poor adhesion to substrate materials due to high compressive residual stress after deposition. The present study aimed to assess whether a PVD-TiB2 coating with both high hardness and sufficiently good adhesion to the substrate and good wear properties can be developed by a closed-field unbalanced magnetron sputtering system using pulsed-dc biasing. The TiB2 coating deposited on AISI M2 steel substrates was characterized in terms of the structural, mechanical and tribological property. From the experimental results, it can be concluded that a closed-field unbalanced magnetron sputtering system using pulsed-dc biasing can be used to produce a TiB2 coating with sufficiently good adhesion (82 N), and high hardness (2300 HK0.01), and low friction (0.34) under given deposition conditions. The deposition conditions, particularly substrate biasing and rotation, which inhibited the formation of (001) orientation, played a role in the coating's lack of superhardness.

Tribological Evaluation of Boride Layers Formed on an AISI M2 Steel Substrate by the Powder Packing Method.

Tribological Evaluation of Boride Layers Formed on an AISI M2 Steel Substrate by the Powder Packing Method.

Tribological aspects of the Diamond-like carbon film applied to different surfaces of AISI M2 steel

This study investigated the influence of the roughness of AISI M2 steel substrate on the mechanical properties and performance of the DLC film exposed to a tribosystems with a hypoeutectic aluminum-silicon alloy, evaluating the adhesion properties, resistance to microabrasive wear and friction coefficient in sliding test. The film was deposited in samples with three distinct roughnesses, depending on the finishing condition: casted, grinded and polished. Results showed that the substrate roughness does not influence the structure of the DLC, but its morphology and tribological behavior. A better adhesion of the DLC was observed in the casted and polished substrates. In the microabrasive test, less wear resistance was found for rougher substrates, the polished substrate showed lower wear rate and specific wear coefficient. In the sliding test, different film behaviors were observed in the running-in and steady state regimes, with the casted substrate responsible for the lowest friction coefficient.

Enhancing the hardness and damage-tolerance of CoCrNiAlTi coatings through dual-phase and nanotwinned structures

A series of CoCrNiAlTi coatings were deposited, via direct current (DC) magnetron sputtering, onto M2 steel substrates employing a range of substrate bias voltages (−20 V ~ -120 V). Dual phase (fcc and hcp) microstructures, with high compositional homogeneity, were observed for all as-deposited coatings. Highly refined columnar grains, with a high density of twin boundaries that were oriented perpendicular to the coating growth direction, were identified. As the bias voltage increased, elements in the high entropy alloy exhibited different sputtering yields that led to different degrees of re-sputtering in the deposited films. This, in turn, played a critical role in determining the coating composition and hence stacking fault energy. Accordingly, the ratio between the hcp and fcc phases varied, with a maximum fraction of the hcp phase observed at a bias voltage of −80 V. The fraction of the hcp phase then decreased as the voltage increased to −120 V. An exceptional hardness of value of ~9.5 GPa, along with appreciable damage-tolerance, was exhibited in the coating deposited at −80 V. It is suggested that the presence of nanotwins, as well as the dual-phase microstructure, contributed to this excellent strength-ductility unity.

The influence of substrate bias on the surface morphology, microstructure and mechanical behaviour of TiNiN coatings

The application of a negative substrate bias during physical vapor deposition can have important effects on the surface morphology, structure and properties of ceramic-based coatings. In this study, a number of TiNiN coatings were prepared onto AISI M2 steel substrates via cathodic arc deposition method by employing a range of different negative substrate bias voltages. The objective of this work is to study the influence of bias voltage on the surface morphology, microstructure and mechanical characteristics of these thin films. A change from a fine columnar grained structure to highly refined equiaxed grains was noted with increasing bias from 0 to −50 V, associated with the energetic ion bombardment at the higher substrate bias. A very high hardness value of ∼ 34.2 GPa was attained at a −100 V substrate bias, which was attributed to the synergistic effects of a number of factors such as the presence of compressive residual stresses, solid solution hardening effects along with grain refinement. In addition, significant reduction in the density of microparticle-related defects generated in these coatings was achieved at ≥ -100 V, which can presumably be ascribed to the higher repulsive force induced at higher substrate bias values.

High entropy alloy FeMnNiCoCr coatings: Enhanced hardness and damage-tolerance through a dual-phase structure and nanotwins

Three FeMnNiCoCr high entropy alloy (HEA) coatings were deposited onto M2 steel substrates using a direct current (DC) magnetron sputtering system under a range of substrate bias voltages (−20 V, −60 V and −120 V). The microstructure transformed from a fine elongated structure to coarse V-shaped columnar grains with increasing substrate bias voltage. A high density of nanotwins, together with the presence of a partial fcc-to-hcp transformation, was observed in the coating deposited at −120 V. This was attributed to the introduction of stacking faults whose presence was promoted by preferential re-sputtering effects during the deposition process. A high hardness value of ~9.1 GPa, accompanied by exceptional damage-tolerance, was achieved in the coating deposited at −120 V. Here, the formation of nanotwins and the dual-phase structure was found to contribute to this remarkable combination of hardness and resistance to plastic deformation.

FeMnNiCoCr-based high entropy alloy coatings: Effect of nitrogen additions on microstructural development, mechanical properties and tribological performance

(FeMnNiCoCr)Nx high entropy alloy (HEA) coatings were deposited onto M2 steel substrates via direct current (DC) magnetron sputtering at several nitrogen flow rates (4, 8, 15, 25 sccm). The microstructures exhibit a fine columnar pattern. Higher flow rates promoted higher nitrogen contents that led to a transformation from a fcc to bcc structure. Self-organising behaviours of metallic atoms, enhanced by nitrogen gas flow, led to the formation of the bcc structure, wherein metal atoms randomly occupy equilibrium lattice sites and nitrogen atoms segregate at grain boundaries. Coatings with low nitrogen content (~6 at. %) exhibit a fcc structure, offering the best scratch toughness and adhesion strength, but inferior hardness (~11 GPa) and wear resistance; coatings with medium nitrogen contents (~15–22 at. %) exhibit a bcc structure, which display improved hardness (~13–15 GPa) and wear resistance, but inferior scratch responses, that was attributed to the presence of the harder, but more brittle bcc phase. At high nitrogen content (~26 at. %) an impressive combination of superior hardness (~17 GPa), wear resistance and good scratch response were identified with a well-defined bcc phase. It is these bcc grains, together with nitrogen-reinforced grain boundaries that overcame the incompatibility between the strength and ductility.

Deciphering deformation mechanisms of hierarchical dual-phase CrCoNi coatings

Hierarchical CrCoNi medium entropy alloy (MEA) thin films with a dual-phase face-centred cubic (FCC) and hexagonal closed-packed (HCP) nanostructure were prepared on M2 steel substrates by closed field unbalanced magnetron sputtering. Nanoindentation tests show an ultra-high hardness of 9.5 GPa, attributable to large amounts of innate planar defects (i.e., growth twins and stacking faults) impeding dislocation motion in the coatings. A deep analysis of undeformed and post-mortem samples reveals grain refinement as the dominant deformation mechanism in FCC dominated regions, while phase transformation and shear banding played major roles in regions occupied by HCP phase. The grain refinement was facilitated by twin/matrix lamellae, with dislocations piling up and arranging into interconnecting grain boundaries. The shear banding was accelerated by innate planar defects in the HCP phase due to a lack of slip systems. Of particular interest is the observation of HCP → FCC phase transformation, which was catalysed by deformation-induced grain reorientation with innate stacking faults acting as embryos to grow the FCC phase. The results of this work suggest that multiple deformation pathways could be activated in CrCoNi coatings with assistance of growth defects, thereby imparting these technically important coatings appreciable ductility.

Synergistic effect of plasma nitriding and bias voltage on the adhesion of diamond-like carbon coatings on M2 steel by PECVD

Diamond-like carbon (DLC) coatings have excellent mechanical and tribological properties, as well as good wear and corrosion resistance. They are well established in medical, metalworking and automotive applications. However, further improvements are needed and substrate pre-treatment plays an important role in supporting and enforcing the adhesion and service performance of the DLC coatings. In this work, the synergetic effect of low-pressure arc plasma-assisted-nitriding (PAN) treatment of M2 steel and plasma-enhanced chemical vapor deposition (PECVD) on the DLC coatings adhesion was analyzed. Adhesion strength of a nitriding + DLC duplex coating system was compared to the performance of DLC-coatings applied on non-nitrided M2 steel substrates. The nitrided layer was analyzed by optical microscopy, electronic microscopy, X-ray microanalysis, X-ray diffraction, Vickers microhardness and atomic force microscopy. DLC coatings were analyzed using Raman spectroscopy revealing that DLC a-C:H type was obtained. The adhesion properties were analyzed by scratch testing supported by optical microscopy and scanning electron microscope, showing results with an improvement of the DLC's adhesion on the nitrided surfaces.

Influence of Al and Y content on the oxidation resistance of CrAlYN protective coatings for high temperature applications: New insights about the Y role

CrAlYN hard coatings with two different average Al contents: ∼16 at.% and ∼25 at.%, and Y concentration varying between 1.2 and 5.7 at.% were deposited by direct current reactive magnetron co-sputtering of mixed Cr-Al and Y targets on commercial M2 steel substrates. The samples were heated to 1000 °C in air during 2 h to study their oxidation resistance and thermal stability. The Y content is critical and the coatings present different behaviour depending on the Al content. The best oxidation resistance and thermal stability are obtained for the coating with ∼16 at.% Al and 3.4 at.% Y. The initial film microstructure and the cubic phase (fcc-CrAlN) were retained, and a thin (Cr,Al)2O3 oxide protective scale was formed. At lower Y content (1.2 at.%) iron, from the substrate crosses the coating, while a higher content (4.6 at.%) avoided the iron diffusion at the expense of a thicker oxide scale with new oxide phases. The coatings with higher Al content (∼25 at. %) were not thermally stable at 1000 °C. A good oxidation resistance was obtained for 2.6 at.% of Y although new phases (hcp-AlN and Cr-Fe) were formed. Higher amount of yttrium (∼5.7 at. %) led to the complete oxidation of the coating.

Microstructure and mechanical properties of a multilayered CoCrNi/Ti coating with varying crystal structure

Medium entropy alloys (MEAs), such as CoCrNi, have been demonstrated to combine high hardness and excellent ductility, thereby outperforming many high entropy alloys reported to date. In this study, a multilayered CoCrNi/Ti coating was deposited onto a M2 steel substrate using a DC magnetron sputtering system. Columnar grains can be observed in both the CoCrNi and Ti layers. A high density of periodic twin boundaries, aligned in a direction normal to the growth direction, was also observed within the columnar CoCrNi grains. Moreover, different crystal structures were identified for different CoCrNi layers. The outermost CoCrNi layer exhibited a FCC structure, whilst in contrast, both the middle and bottom CoCrNi layers exhibited a BCC structure. It was assumed that Shockley partial dislocations were responsible for the FCC to BCC transition occurring in both the bottom and middle CoCrNi layers. A high hardness of ~7.6 GPa and elastic modulus of ~233 GPa were determined for this multilayered coating by nanoindentation testing. Further, extraordinary damage tolerance was found in the multilayered CoCrNi/Ti coating under indentation loading. The steady shear banding behaviour during deformation may benefit energy dissipation and promote structural plasticity.

Microstructure, mechanical and tribological properties of TiBC coatings by DC magnetron sputtering onto AISI M2 steel using independent TiB2 and graphite targets

Despite TiB2 being a ceramic material widely known for its high hardness, chemical resistance and high thermal stability; its use in coatings for industrial conditions has been limited due to low adhesion, which is associated with high residual stresses. Recently, several investigations have shown that the incorporation of carbon within TiB2 coatings reduces residual stresses and increases adhesion, making Ti-B-C coatings excellent candidates for applications at high temperatures. In this research, DC UBMS Ti-B-C coatings were deposited onto AISI M2 steel substrates. The power density of TiB2 was fixed at 2.4W/cm2 and four different graphite power densities were made in order to obtain four different power ratios (WC/WTiB2) and carbon content in the Ti-B-C coatings. XRD and AFM results suggest the formation of nanocomposite coatings, with TiB2 nanocrystals embedded in an amorphous carbon matrix. SEM images revealed a glass-like structure for coatings with a low power ratio, which became a dense and featureless-like structure with a higher power ratio. A progressive decrease in hardness and Young's modulus was observed with the increase in carbon content. Hardness up to 27GPa was reached with lower carbon content and decreasing under 5GPa with the higher carbon content. The Young's modulus exhibited the same feature. Tribological properties of the coatings were investigated using a pin-on-disk tribometer. All coatings showed μ higher than the AISI M2 substrate, and only the coatings with a power ratio of 1.30 showed μ over 0.30. Regarding the wear rate, it remained almost constant for all the Ti-B-C coatings.

Medium entropy alloy CoCrNi coatings: Enhancing hardness and damage-tolerance through a nanotwinned structuring

Medium entropy alloys (MEA) are defined as alloys consisting of three equiatomic elements, such as CoCrNi. MEAs are reported to have superior mechanical properties and high thermodynamic stability, as well as excellent fracture toughness at cryogenic temperatures. Here, we investigate a series of equiatomic medium entropy alloy coatings, containing three elements, Co, Cr, Ni. These coatings were deposited onto M2 steel substrates with a range of coating thicknesses using a DC magnetron sputtering system using a CoCrNi alloy target (1:1:1at.%). The microstructure and mechanical properties were examined by a number of characterization techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. XRD analysis showed that the coatings were dominated by a fcc CoCrNi phase, with a smaller amount of hcp Co. TEM analysis demonstrated that the elongated grains contained a high density of {111} nanotwins. In addition, the residual stresses in the coatings were analysed using X-ray diffraction by adopting the conventional sin2ψ method. A high hardness value, ~10GPa, was determined by nanoindentation of these coatings. Exceptional damage-tolerance was also found in these coatings under contact loading. It is believed that the nanotwinned structure is responsible for the high hardness and damage tolerance observed in the new coatings.

Determining the critical loads of V and Nb doped ternary TiN-based coatings deposited using CFUBMS on steels

Transition metal nitride coatings are very attractive materials due to their hardness and toughness. On the other hand, adhesion properties of these coatings are generally low because of their brittleness. To overcome this disadvantage, these coatings are produced with different deposition techniques. In this work, V and Nb doped ternary TiN-based coatings were deposited on M2 and H13 steel substrates with closed field unbalanced magnetron sputtering (CFUBMS) system. The microstructure and thickness properties were determined using Scanning Electron Microscope (SEM). The chemical composition of the coatings was examined by Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD). The mechanical properties of the coatings were carried out by nanohardness test. The critical load values (Lc) of the coatings were pointed out using a scratch tester. The thickness values of the coatings were 400nm and 360nm for TiNbN and TiVN, respectively. The highest thickness, nanohardness and Lc values were obtained from TiNbN coating. The maximum nanohardness and Lc values were measured as 35±1.8GPa and 63N from TiNbN coating on M2 steel, respectively. On the other hand, TiVN coating on both steels showed lower nanohardness and adhesion behavior. It was clearly seen that different substrates had an important effect on microstructure and mechanical properties of coatings.

A comparative study of fatigue properties of TiVN and TiNbN thin films deposited on different substrates

Transition metal nitrides, especially ternary phase films attract attention due to their high mechanical and tribological features. Besides these, fatigue properties play a very important role on the performance in service life of these types of films. TiVN and TiNbN films were deposited on M2 and H13 steel substrates by reactive magnetron sputtering system. Fatigue properties of the films were characterized via multipass scratch tests. 100, 250 and 500cycles with bidirectional multipass scratch tests were conducted at room temperature by applying 20N constant load. Structural properties were determined with X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Mechanical features of the films were observed with nano hardness tests. Fatigue behaviors, deformation types, coefficient of frictions of the films and effect of different substrates were discussed comparatively. According to the results, the TiVN films showed better fatigue resistance than the TiNbN films although critical adhesion load value of the TiNbN films was higher than the TiVN films. The TiVN films generally showed ductile type cracks at the edge of the tracks and the TiNbN films showed more brittle type cracks. Additionally, the films deposited on M2 substrates exhibited better strength than those of H13 substrates.

Development of diamond-like carbon (DLC) coatings with alternate soft and hard multilayer architecture for enhancing wear performance at high contact stress

Diamond-like carbon (DLC) coatings have attracted overwhelming interest due to a wide range of exceptional mechanical and tribological properties. Unfortunately, they have limited tribological applications where high contact stresses are required due to high residual stresses. We proposed that multilayer DLC coatings may have potential for tribological applications at high stresses. In this study, DLC coatings consisting of alternate soft-layer and hard-layer were deposited onto M2 steel substrates using unbalanced magnetron sputtering technique. The tribological performance at high contact stresses (2.5GPa, 3.2GPa, and 4GPa) was evaluated by a ball-on-disc tribometer. The residual stress, the hardness and reduced modulus are increased with percentage of hard-layer thickness. Multilayer coatings with the soft-top layer improve wear performance compared with single-layer DLC coatings. In contrast, multilayer coatings without the soft-top layer show poor wear performance under high stresses. The soft-top layer reduces the wear volume during run-in period by forming a transfer layer so that decreases the total wear. The 50% hard multilayer coating with the soft-top layer provides the best wear performance at an extreme stress of 4GPa among all coatings in this study because it combines the low magnitude of the residual stress with the acceptable hardness value. Such carbon/carbon multilayer structure offers an alternative to deposit low stress and high wear resistance DLC coatings without introducing other elements.

Effects of annealing temperatures on the morphological, mechanical, surface chemical bonding, and solar selectivity properties of sputtered TiAlSiN thin films

Quaternary sputtered TiAlSiN coatings were investigated for their high temperature structural stability, surface morphology, mechanical behaviors, surface chemical bonding states, solar absorptance and thermal emittance for possible solar selective surface applications. The TiAlSiN films were synthesized, via unbalanced magnetron sputtered technology, on AISI M2 steel substrate and annealed at 500 °C - 800 °C temperature range. SEM micrographs show nanocomposite-like structure with amorphous grain boundaries. Nanoindentation analyses indicate a decrease of hardness, plastic deformation and constant yield strength for the coatings. XPS analysis show mixed Ti, Al and Si nitride and oxide as main coating components but at 800 °C the top layer of the coatings is clearly composed of only Ti and Al oxides. Synchrotron radiation XRD (SR-XRD) results indicate various Ti, Al and Si nitride and oxide phases, for the above annealing temperature range with a phase change occurring with the Fe component of the substrate. UV–Vis spectroscopy, FTIR spectroscopy studies determined a high solar selectivity, s of 24.6 for the sample annealed at 600 °C. Overall results show good structural and morphological stability of these coatings at temperatures up to 800 °C with a very good solar selectivity for real world applications.

Role of Y in the oxidation resistance of CrAlYN coatings

CrAlYN coatings with different aluminum (4–12at.%) and yttrium (2–5at.%) contents are deposited by d.c. reactive magnetron sputtering on silicon and M2 steel substrates using metallic targets and Ar/N2 mixtures. The influence of the nanostructure and chemical elemental distribution on the oxidation resistance after heating in air at 1000°C is studied by means of cross-sectional scanning electron microscopy (X-SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and glow discharge optical emission spectroscopy (GD-OES). The sequential exposure to the metallic targets during the synthesis leads to a multilayer structure where concentration of metallic elements (Cr, Al and Y) is changing periodically. A good oxidation resistance is observed when Al- and Y-rich regions are separated by well-defined CrN layers, maintaining crystalline coherence along the columnar structure. This protective behavior is independent of the type of substrate and corresponds to the formation of a thin mixed (Al, Cr)-oxide scale that protects the film underneath. The GD-OES and XRD analysis have demonstrated that Y acts as a reactive element, blocking the Fe and C atoms diffusion from the steel and favoring higher Al/Cr ratio in the passivation layer after heating. The coating with Y content around 4at.% exhibited the best performance with a thinner oxide scale, a delay in the CrN decomposition and transformation to Cr2N, and a more effective Fe and C blocking.