Significant Hardness Enhancement Research Articles

High He-ion irradiation resistance of CrMnFeCoNi high-entropy alloy revealed by comparison study with Ni and 304SS

High entropy alloys (HEAs) have presented potential applications in nuclear power plants owing to their novel atomic structure based high irradiation resistance. However, understanding of He-ion irradiation of HEAs is still lacking. In this work, we reveal He-ion irradiation resistance of HEA CrMnFeCoNi by comparison study with a pure Ni and a 304 stainless steel (304SS). It is found that the damage structure in the three materials can be characterized with He bubbles and stacking faults/stacking fault tetrahedrons ((SFs/SFTs), which show a similar depth distribution after He-ion irradiation at both RT and 450 °C. Although the He bubbles have a similar size about 2 nm after irradiation at RT, the He bubble sizes of the HEA, 304SS, and Ni increase to 4.0 ± 0.9, 5.3 ± 1.0 and 6.7 ± 1.0 nm after irradiation at 450 °C, respectively. Moreover, the density of SFs/SFTs displays in an order of Ni < 304SS < HEA at both RT and 450 °C. The He-ion irradiation at RT causes significant hardness enhancement for the three materials, however, compared to RT, after irradiation at 450 °C, the Ni presents softening, while the 304SS, especially the HEA, shows further hardening. Thus, the HEA CrMnFeCoNi possesses the smallest He bubble size, the densest SFs/SFTs, and the highest hardening, indicating the best structural stability, as well as the best He-ion irradiation resistance, which can be attributed to its low mobility of He atoms and point defects.

Multilayer design of CrN/MoN protective coatings for enhanced hardness and toughness

We report on CrN/MoN multilayer coatings, their structure, elemental and phase composition, residual stresses, mechanical properties and their dependence on deposition conditions. The hardness and toughness were considered as main parameters for improvement of the protective properties of coatings. Multilayers with varying bilayer periods, ranging from 40 nm to 2.2 μm, were obtained by using cathodic arc physical vapour deposition (Arc-PVD) on stainless steel substrate. The elemental analysis was performed using wavelength-dispersive X-ray spectroscopy (WDS). The surface morphology and cross-sections were analysed with scanning electron microscopy (SEM). The X-ray diffraction (XRD) measurements, including grazing incidence X-ray diffraction (GIXRD), in-plane diffraction analysis and electron backscatter diffraction (EBSD), were used for microstructure characterisation. Mechanical properties of deposited films were studied by measuring hardness (H) and Young's modulus (E) with micro-indentation, H/E and H3/E2 ratios were calculated. The dependences of internal structure and, hence, mechanical properties, on layer thickness of films have been found. Significant enhancement of hardness and toughness was observed with decreasing individual layer thickness to 20 nm: H = 38–42 GPa, H/E = 0.11.

Effect of Deep Cryogenic Treatment on Tribological Behaviour of D2 Tool Steel - An Experimental Investigation

This paper deals with the experimental study of friction and wear behavior of D2 tool steel. In the present work, the role of multiple tempering before and after deep cryogenic treatment on tribological behavior like friction and wear of D2 tool steel is focused. The heat treatments like hardening (1020°C) for one hour, tempering (210°C) for two hours and deep cryogenic treatment (-185°C) for 36 hours were done on D2 tool steel. Wear tests were performed using pin-on-disc wear tester to which normal load of 50 N and velocity of 1.5 m/s is applied. The hardness of specimens was measured by using Rockwell Hardness tester. The execution of deep cryogenic treatment increases the hardness; reduces the wear volume, wear rate and coefficient of friction of D2 tool steel. In this experimentation, various combinations of heat treatment are used. The optimum combination of heat treatment obtained was Hardening, Cryotreatment and Single Tempering (HCT). In HCT specimens there was a large reduction in the wear volume, wear rate (WR) and coefficient of friction; significant enhancement in hardness of the D2 tool steel.

Tribological and corrosion behaviors of warm-and hot-rolled Ti-13Nb-13Zr alloys in simulated body fluid conditions.

Development of submicrocrystalline structure in biomedical alloy such as Ti-13Nb-13Zr (in wt%) through warm-rolling process has been found to enhance mechanical properties compared to conventional thermomechanical processing routes including hot-rolling process. The present study investigated the tribological and corrosion behaviors of warm-rolled (WR) and hot-rolled Ti-13Nb-13Zr alloys which have not been studied to date. Both tribological and corrosion experiments were carried out in simulated body fluid conditions (Hank’s solution at 37°C) based on the fact that the investigated alloys would be used in a human body as orthopedic implants. The WR Ti-13Nb-13Zr demonstrated a submicrocrystalline structure that provided a significant enhancement in hardness, strength, and corrosion resistance. Meanwhile, there was no notable difference in wear resistance between the WR and hot-rolled samples despite the different microstructure and hardness. The present study confirmed the enormous potential of WR Ti-13Nb-13Zr with not only great mechanical properties but also high corrosion resistance in the simulated body fluid.

Bulk titanium nitride ceramics – Significant enhancement of hardness by silicon nitride addition, nanostructuring and high pressure sintering

Dense nanocomposites consisting of titanium nitride (TiN) and 1.7–8.1mol% silicon nitride (a-Si3N4) have been prepared by high-pressure sintering at 6.5GPa and at temperatures up to 1300°C. Microstructure, hardness, indentation fracture toughness, elastic modulus and the correlation of these parameters to the silicon nitride content of the obtained TiN–Si3N4 ceramics have been determined. The results demonstrate that even the addition of small amounts of Si3N4 leads to a considerable increase in Vickers hardness to HV500g∼25GPa and indentation fracture toughness up to 6MPam½. This significant enhancement of mechanical properties can be related to interfacial bonds between titanium nitride crystals strengthened by nano-size effects and amorphous silicon nitride forming an intergranular matrix. Realignment and rotation of such interlocked titanium nitride crystallites under deformational stress is suppressed and crack propagation reduced by a crack deflection mechanism.

Nanostructured Al–Zn–Mg–Cu–Zr alloy prepared by mechanical alloying followed by hot pressing

Nanostructured Al–7.8wt% Zn–2.6wt% Mg–2wt% Cu–0.1wt% Zr alloy was mechanically alloyed (MA) from elemental powders and consolidated by hot press technique. The effect of the milling time and hot pressing process on microstructure was investigated by means of X-ray diffraction measurements (XRD) and analytical and scanning electron microscopy (SEM). Furthermore mechanical properties of samples with different MA time as well as pure aluminum were investigated by microhardness and compression tests. The results show that an Al–Zn–Mg–Cu–Zr homogenous supersaturated solid solution with a crystallite size of 27nm was obtained after 40h of milling time. Microstructure refinement and morphological changes of powders from flake to spherical shape were observed by increasing milling time. Phase and microstructural characterization of high density bulk nanostructured samples revealed that increasing milling time up to 40h leads to formation of MgZn2 precipitation in the alloy matrix. With increasing milling time, density of the samples and crystalline size decrease. Significant enhancement of hardness and compressive strength is observed in the aluminum alloy by increasing milling time up to 40h which is much higher than pure aluminum. Crystallite size refinement in pure aluminum samples from micro- to nanoscales resulted in 107% and 100% improvement in compressive strength and hardness, respectively. Furthermore the compressive strength and hardness of Al–Zn–Mg–Cu–Zr alloy nanostructured samples increased to 179% and 172%, respectively, compared to nanostructured pure Al, which was produced as reference specimen. 40h of MA was the optimum case for preparing such an Al alloy and more milling up to 50h led to deterioration of mechanical properties.

Nanoscale carbide precipitation in Co–28Cr–5Mo–0.3C implant alloy during martensite transformation

The precipitation of nanoscale carbides in Co–28Cr–5Mo–0.3C implant alloy during tungsten inert gas (TIG) welding has been systematically investigated. Based on the high resolution transmission electron microscopy (HRTEM) results, the nanoscale precipitates are Cr-rich M23C6-type carbides, with 10–100nm in size, and precipitation mostly occurred at hcp/fcc interfaces. In addition, X-ray diffraction analysis (XRD) showed that higher amount of athermal ε-martensitic (≈2-fold those found in the solution-treated sample) can be achieved after the welding process. Apparently, the higher content of athermal martensite after welding is responsible for the precipitation of M23C6 carbides in nanoscale and significant enhancement of hardness (850 HVN) is the concomitant result of that.

The mechanical properties of a nanocrystalline Al2O3/a-Al2O3 composite coating measured by nanoindentation and Brillouin spectroscopy

In this work, ellipsometry, Brillouin spectroscopy and nanoindentation are combined to assess the mechanical properties of a nanocrystalline Al2O3/a-Al2O3 composite coating with high accuracy and precision. The nanocomposite is grown by pulsed laser deposition at either room temperature or 600°C. The adhesive strength is evaluated by nanoscratch tests. In the room temperature process the coating attains an unusual combination of compactness, strong interfacial bonding, moderate stiffness (E=195±9GPa and ν=0.29±0.02) and significant hardness (H=10±1GPa), resulting in superior plastic behavior and a relatively high ratio of hardness to elastic modulus (H/E=0.049). These features are correlated to the nanostructure of the coating, which comprises a regular dispersion of ultrafine crystalline Al2O3 nanodomains (2–5nm) in a dense and amorphous alumina matrix, as revealed by transmission electron microscopy. For the coating grown at 600°C, strong adhesion is also observed, with an increase of stiffness and a significant enhancement of hardness (E=277±9GPa, ν=0.27±0.02 and H=25±1GPa), suggesting an outstanding resistance to wear (H/E=0.091).

Exploring the benefits of depositing hard TiN thin films by non-reactive magnetron sputtering

The aim of this paper is to compare the mechanical and tribological properties of TiN coatings prepared in a conventional magnetron sputtering chamber according to two different routes: the usual reactive sputtering of a Ti target in an Ar/N2 atmosphere vs. the comparatively more simple sputtering of a TiN target in a pure Ar atmosphere. Improved properties in term of hardness and wear rates were obtained for films prepared by non-reactive sputtering route, due to the lower presence of oxynitride species and larger crystalline domain size. Additionally, a significant hardness enhancement (up to 45GPa) is obtained when a −100V d.c. bias is applied during growth. This behaviour is explained by non-columnar growth and small grain size induced by effective ion bombarding. These results demonstrate that non-reactive sputtering of TiN target appears a simple and efficient method to prepare hard wear-resistant TiN films.

Microstructure and mechanical properties of nanocrystalline Al–Cr–B–N thin films

Al–Cr–N is a well-established hard film system with excellent mechanical and tribological properties. The effect of B addition on structure and mechanical properties of Al–Cr–N films grown by cathodic arc evaporation was systematically investigated. The B content in the targets was varied between 10 and 30at.% at a constant Al/Cr atomic ratio of 1.8. X-ray diffraction revealed that all coatings exhibit a face-centered cubic structure, with texture and grain size affected by B content and substrate bias voltage. X-ray photoelectron spectroscopy indicates formation of a nanocomposite structure consisting of a crystalline face-centered cubic Al–Cr–(B)–N solid solution and an amorphous BNx phase. Nanoindentation experiments revealed a significant hardness enhancement of Al–Cr–(B)–N with respect to B-free films, accompanied with a reduction of residual compressive stress.

Mineral concentration dependent modulation of mechanical properties of bone-inspired bionanocomposite scaffold

We demonstrate tunable mechanical properties of bone-inspired bionanocomposite scaffolds while maintaining the required viscoelasticity. Mechanical properties such as hardness and elastic modulus of the bionanocomposite scaffolds were controlled by varying mineral concentrations of the bioscaffold. In particular, higher calcium and oxygen contents in the bioscaffold resulted in a significant enhancement in hardness and modulus of the bionanocomposite. Moreover, the phosphorous content appeared to be a determining factor in the hardness and mechanical properties of the bionanocomposites. These results open up the possibility of designing new engineered biocompatible nanoscaffolds with desired and tunable biomimetic functions and biomechanical properties with significant potential for advanced bone tissue engineering platforms and bone substitutes.

Coherent growth and superhardness effect in VC/Si3N4 nanomultilayers

A series of VC/Si3N4 nanomultilayers have been prepared by reactive magnetron sputtering in the mixture of Ar and C2H2. Microstructure evolution and mechanical properties of multilayers with various thicknesses of Si3N4 layer have been researched. The results show that the sputtered Si3N4 doesn't react with C2H2. In VC/Si3N4 multilayers, amorphous Si3N4 is crystallized at small layer thickness (≤0.71nm) due to the template effect of VC. Correspondingly, multilayers exhibit coherent growth, and obtain significant hardness enhancement with maximum hardness of 45.9GPa. Further increasing the thickness of Si3N4 layer (≥1.45nm), Si3N4 transforms to be amorphous and blocks the coherent growth in multilayer, which leads to the gradual decrease of hardness.

Understanding why the thinnestSiNxinterface in transition-metal nitrides is stronger than the ideal bulk crystal

One-monolayer-thick ${\text{SiN}}_{x}$ interfacial layer in superhard nanocomposites, consisting of 3--4 nm size TiN nanocrystals joined by that layer, is stronger than a bulk ${\text{SiN}}_{x}$ crystal due to valence charge transfer from the metallic TiN, thus providing the nanocomposites with significant hardness enhancement. However, this enhancement is lost when the thickness of the interfacial SiN increases to $\ensuremath{\ge}2$ monolayers and the hardness decreases. We show that the softening of the nanocomposites with thicker ${\text{SiN}}_{x}$ interface is caused by the weakening of the TiN bonds close to that interface, that increases with increasing of the ${\text{SiN}}_{x}$ thickness. Other possible mechanisms of the softening are briefly discussed and ruled out. This finding may open up possible way of preparing new, even stronger superhard nanocomposites.

Sliding wear characterization of microwave-glazed plasma-sprayed ceramic composites

The main objective of the present study is to present the wear characteristics of post-processed plasma deposits of alumina-titania (AT-13) ceramic composites under high-speed sliding. Spray deposits were post-processed (microwave glazed) by way of exposing them to microwave radiation at a frequency of 2.45 GHz in a multi-mode cavity by using the microwave hybrid heating principle. During ‘microwave glazing’, densification of the coating's structure takes place as a consequence of microwave-heating-induced phase transformation, leading to significant enhancement in hardness and reduction in porosity. Responses of the as-sprayed and microwave-glazed composites were monitored while they were subjected to sliding type of tribocontact at a speed of 5 m/s. Subsequent analyses are presented in terms of wear rate, pressure-velocity-time ( p- v- t) characteristics, fractography, elemental count, and acoustic emission (AE) emanated during the trials. The results show a steady sliding wear response after an initial higher rate. The p- v- t factor analysis confirms a better operating condition with glazed composites. Glazed composites exhibit higher stability towards elemental transfer between the wear surface and the counter surface. AEs emanating during trials indicate deformation and brittle fracture as the dominating wear mechanisms of the as-sprayed and microwave-glazed ceramic composites.

Superhard Nitride-Based Nanocomposites: Role of Interfaces and Effect of Impurities

Recently, a hardness similar to that of diamond has been reported for a quasiternary, nitride-based nanocomposite. The related, quasibinary nanocomposite "nc-TiN/a-Si3N4," which may be regarded as the prototype of the family of superhard nc-metal-N/a-Si3N4 systems, also exhibits a significant hardness enhancement. Extensive density-functional theory calculations indicate that the superhardness is related to the preferential formation of TiN(111) polar interfaces with a thin beta-Si3N4-derived layer. The strength of TiN in the 111 direction is similar to that of the weakest bonding direction in diamond. Oxygen impurities cause a significant reduction of the interface strength.

Coherent epitaxial growth and superhardness effects of c-TiN∕h-TiB2 nanomultilayers

TiN ∕ TiB 2 nanomultilayers with different TiB2 layer thicknesses were deposited by the multitarget magnetron sputtering method. Studies show that because of the template effects of the cubic TiN layer, the normally amorphous TiB2 layer crystallizes into a compact hexagonal structure when its thickness is less than 2.9 nm. As a result, the multilayers form a c-TiN∕h-TiB2 coherent epitaxial structure with the orientation relationship of {111}TiN∕∕{0001}TiB2,⟨110⟩TiN∕∕⟨112¯0⟩TiB2. Correspondingly, the multilayers show a significant hardness enhancement with a maximum hardness of 46.9 GPa. Further increase in TiB2 layer thickness leads to the formation of amorphous TiB2 that blocks the coherent growth of the films, and thus the hardness of the multilayers decreases gradually.

Template-induced crystallization of amorphous SiO2 and its effects on the mechanical properties of TiN∕SiO2 nanomultilayers

Ti N ∕ Si O 2 nanomultilayers with various thicknesses of the SiO2 layer have been prepared by multi-target magnetron sputtering. Studies show that amorphous SiO2, which is more favorable under sputtering condition, crystallizes at smaller layer thickness (0.45–0.9nm) due to the template effect of TiN layers. Correspondingly, multilayers exhibit coherent epitaxial growth with intensive (111) texture, and show significant hardness enhancement with maximum hardness of 44.5GPa. Further increase in the SiO2 layer thickness (≳1nm) leads to the formation of amorphous SiO2 which blocks the coherent growth of the films, and thus decreases the multilayer hardness gradually.

Investigation of the deformation behavior in nanoindented metal/nitride multilayers by coupling FIB-TEM and AFM observations

AbstractEpitaxial TiN/Cu bilayers and multilayers with periods L between 5 and 50 nm have been grown by ultrahigh vacuum ion beam sputtering on Si and MgO(001) substrates at room temperature. The deformation modes induced by a Berkovich nanoindent have been imaged using Focused Ion Beam – Transmission Electron Microscopy (FIB-TEM) and Atomic Force Microscopy (AFM). The observations suggest that the mechanical response of the multilayers is essentially governed by an extensive plastic flow inside the Cu layers, which is confined by a bending of the more rigid TiN layers. This specific deformation behavior, with no contribution of the interfaces as a barrier for dislocation motion could explain the absence of significant hardness enhancement in this system.

Nanoindentation Study of Amorphous Metal Multilayered Thin Films

AbstractNanoindentation studies were preformed on amorphous metal, multilayered thin films containing alternating layers of Fe50Ti50 and Cu35Nb65 in order to investigate the mechanism for plastic deformation in metallic glass. Films with a total thickness of 1μm and bilayer repeat lengths ranging from 2 to 50 nm were magnetron sputter-deposited onto sapphire substrates. In contrast to many crystalline multilayered systems, where large hardness enhancements have been observed when the bilayer repeat length is reduced below about 10 nm, no significant hardness enhancement as a function of bilayer repeat length was observed in the Fe50Ti50/ Cu35Nb65 amorphous metal system. This result suggests that a dislocation–like mechanism for plastic deformation may not be appropriate for these amorphous metals.

Roughness improvement and hardness enhancement in nanoscale Al/AlN multilayered thin films

Al/AlN multilayered thin films with periodic thickness λ less than 24 nm were developed by ion beam assisted deposition. A considerably small surface roughness comparable to that of the silicon substrate and much smaller than those of both monolithic Al and AlN films was obtained. Over the investigated range of λ, all the multilayers are harder than the homogeneous AlN film, and a significant hardness enhancement by a factor of ∼2 over that of the AlN film was observed in the multilayer with λ of 6 nm. Moreover, the hardness enhancement is not at the expense of the multilayer toughness, with the multilayer Al/AlN films showing improved plasticity as compared with the AlN film.