Ultrahigh Hardness Research Articles

Super-hard (MoSiTiVZr)Nx high-entropy nitride coatings

High-entropy nitride coatings have become a promising alternative to traditional protective coatings due to their excellent mechanical properties, thermal stability, oxidation, and wear resistance. However, the design and fabrication of hard high-entropy nitride coatings remain a great challenge. In this study, (MoSiTiVZr)Nx high-entropy nitride coatings were deposited by reactive DC magnetron sputtering at different nitrogen flow. The chemical compositions, structures, hardness, damage-tolerance, friction and corrosion resistances of the coatings were investigated. With the increase of nitrogen content in the coatings, the phase structure transforms from an amorphous structure to a single face centered cubic structure, and all alloying elements form metal-nitrogen bonds. The coating with nitrogen content of 53.7 at% exhibits an ultrahigh hardness of 45.6 GPa and the best damage-tolerance and wear resistance. The coatings with low nitrogen contents show better corrosion resistance than 304 stainless steels, however, an increase in nitrogen content results in a slight decrease in corrosion resistance. The mechanism of the structures, mechanical properties and corrosion resistance of (MoSiTiVZr)Nx coatings are discussed in details.

One-step in-situ low damage etching of SiC/SiC composites by high-temperature chemical-assisted laser processing

SiC/SiC composites have a promising application prospect in the fields of aero-engine turbine blades and other hot-end components due to their excellent properties. It is difficult to machine SiC/SiC composites by traditional mechanical processing because of the ultra-high hardness and anisotropic character. The routinely laser direct processing in the air produce thermal effect damage, resulting in heat-affected zones and microcracks. Here high-temperature chemical-assisted nanosecond laser processing was investigated to realize one-step in-situ fast etching without causing damage to the structure of SiC/SiC composites. Firstly, the chemical wet etching only occurred in the laser-induced local high-temperature region. There was no additional etching damage to the substrate material in the region where the temperature was relatively low. Secondly, chemical liquid served as a transparent constraint layer, the ablation pressure on the material increased in a liquid confined environment. In comparison with laser direct etching in the air, the exposed fiber was not a needle-like structure and tubular structure but turned into a thin and flat tongue-like structure. The etching behavior of 0° fiber layer, 90° fiber layer, and crossing regions of fibers of different directions at low and high laser intensities were studied. Further, the hardness and friction coefficient of SiC/SiC composites after laser direct etching in the air and high-temperature chemical-assisted laser etching were compared. The chemical elementary composition before and after etching was measured to reveal the oxidized and carbonized characteristics. It was concluded that the action mechanism was the combined result of high-temperature decomposition, laser-induced ablation pressure in the liquid confined region, and local chemical wet etching. The results demonstrate that high-temperature chemical-assisted laser etching has some advantages over traditional laser beam machining and is a potential processing method for SiC/SiC composites.

Exploring the amorphous phase formation and properties of W-Ta-(Cr, Fe, Ni) high-entropy alloy gradient films via a high-throughput technique

A high-throughput technique can realize fast and efficient material screening, and greatly improve the discovery efficiency of advanced materials. In the present work, the gradient composition films of the W-Ta-CrFeNi high-entropy alloy were prepared by the three-target magnetron co-sputtering technique. The gradient alloy films exhibited a body-center-cubic (BCC) structure near the W or Ta element target, and an amorphous structure near the CrFeNi target region. Considering the effect of the undercooling degree, the amorphous formation region (using Ω ~ δ ) was larger than that of the bulk alloys. These alloy films displayed ultra-high nano-indentation hardness, with the maximum hardness reaching ~ 20.6 GPa. The elastic modulus in the high-entropy composition region was lower than that in the low-entropy composition region. There is a nonlinear relationship between the nano-indentation hardness and mix-entropy of the alloys. This study provides a reference and alternative material library for the development of high-performance advanced materials. • The rapid screening with properties and phase structure of the W-Ta-(Cr, Fe, Ni) films via the high-throughput technique. • The W 15.39 Ta 38.81 Cr 14.58 Fe1 5.45 Ni 15.77 high-entropy alloy film possesses ultra-high hardness of ~ 20.6 GPa. • The amorphous phase formation ruler of high-entropy alloy films was investigated. • There is a nonlinear relationship with the mix-entropy and properties in these high-entropy alloy films. • The high mix-entropy is a reason for low elastic modulus in these high-entropy alloy films.

Microstructure and mechanical property of novel nanoparticles strengthened AlCrCuFeNi dual-phase high entropy alloy

High entropy alloys are considered to be the best candidates for structural materials because of outstanding comprehensive properties resulted from their unique microstructure. Clarifying the microstructural evolution and controlling the content and morphology of the second phase are of great significance in the design of high-performance HEAs. Along this line of thinking, we report a research of adding little Mo(0–0.5 at%) to a novel Co-free AlCrCuFeNi alloy to tailor microstructure by introducing large lattice distortion and precipitation strengthening to achieve excellent comprehensive properties. The AlCrCuFeNi alloy exhibits BCC+FCC dual-phases, typical solid solution structures (B2 matrix, A2 flap-like and Cu-rich structure) and moderate mechanical properties. An ultrahigh hardness of 643 HV was achieved at the composition of Mo reaches 0.5 at%, which is approximately 1.5 times higher than AlCrCuFeNi alloy. A large amount of nano-sized σ precipitates with various sizes and morphologies were formed when the content of Mo was more than 0.3 at%. Most importantly, the Mo0.3 alloy exhibits an increase of ~18% in yield strength and an increase of ~30% in fracture strength accompanied by a reasonable plasticity which possesses potential application prospects in industrial areas. The solid solution strengthening and the formation of σ phase are the main factors contributing to the strengthening.

Effect of Ti5Si3 on the wear properties of Ti–3Si-1.5Fe–1Mo titanium alloy with ultrahigh hardness

In this work, Ti–3Si-1.5Fe–1Mo titanium alloy reinforced with in-situ synthesized Ti5Si3 was prepared, and ultrahigh hardness (55.6 HRC) was achieved by solution aging treatment. The effects of Ti5Si3 on the friction and wear behavior of the alloy were investigated. Results show that the wear rate of the alloy after hot treatment increases from 3.51 × 10−4 mm3 N−1 m−1 to 3.93 × 10−4 mm3 N−1 m−1, yet the ultrahigh hardness alloy exhibits poor wear resistance. A series of the wear tracks characterizations and thermodynamical calculations were performed to elucidate its wear mechanism: during the reciprocating sliding, the reduction of the Ti5Si3 content leads to the increase of the friction pair force on the matrix, TiO2 and SiO2 wear debris are generated then fall off, both the oxidative wear and adhesive wear aggravated, which seriously impair the wear resistance of the alloy.

Efficiently searching extreme mechanical properties via boundless objective-free exploration and minimal first-principles calculations

Despite the machine learning (ML) methods have been largely used recently, the predicted materials properties usually cannot exceed the range of original training data. We deployed a boundless objective-free exploration approach to combine traditional ML and density functional theory (DFT) in searching extreme material properties. This combination not only improves the efficiency for screening large-scale materials with minimal DFT inquiry, but also yields properties beyond original training range. We use Stein novelty to recommend outliers and then verify using DFT. Validated data are then added into the training dataset for next round iteration. We test the loop of training-recommendation-validation in mechanical property space. By screening 85,707 crystal structures, we identify 21 ultrahigh hardness structures and 11 negative Poisson’s ratio structures. The algorithm is very promising for future materials discovery that can push materials properties to the limit with minimal DFT calculations on only ~1% of the structures in the screening pool.

Ultrahard BCC-AlCoCrFeNi bulk nanocrystalline high-entropy alloy formed by nanoscale diffusion-induced phase transition

In the current work, the BCC-AlCoCrFeNi bulk nanocrystalline high-entropy alloy (nc-HEA) with ultra-high hardness was formed by nanoscale diffusion-induced phase transition in a nanocomposite. First, a dual-phase Al/CoCrFeNi nanocrystalline high-entropy alloy composite (nc-HEAC) was prepared by a laser source inert gas condensation equipment (laser-IGC). The as-prepared nc-HEAC is composed of well-mixed FCC-Al and FCC-CoCrFeNi nanocrystals. Then, the heat treatment was used to trigger the interdiffusion between Al and CoCrFeNi nanocrystals and form an FCC-AlCoCrFeNi phase. With the increase of the annealing temperature, element diffusion intensifies, and the AlCoCrFeNi phase undergoes a phase transition from FCC to BCC structure. Finally, the BCC-AlCoCrFeNi bulk nc-HEA with high Al content (up to 50 at.%) was obtained for the first time. Excitingly, the nc-HEAC (Al-40%) sample exhibits an unprecedented ultra-high hardness of 1124 HV after annealing at 500 °C for 1 h. We present a systematic investigation of the relationship between the microstructure evolution and mechanical properties during annealing, and the corresponding micro-mechanisms in different annealing stages are revealed. The enhanced nanoscale thermal diffusion-induced phase transition process dominates the mechanical performance evolution of the nc-HEACs, which opens a new pathway for the design of high-performance nanocrystalline alloy materials.

Ultra-high hardness induced by W precipitation within Ta-Hf-W-C ultra-high temperature ceramic coatings

Aging strengthening of traditional alloys is of great practical significance in industry, and the nano second phase precipitated from supersaturated solid solution alloys effectively prevent the movement of grain boundaries and dislocations, so as to improve the hardness of alloys. However, the concept is difficult to be realized in ultra-high temperature ceramics (UHTCs) by the existing sintering process, it is still a challenge to obtain large-scale high concentration supersaturated UHTCs. To achieve this, we prepared a supersaturated Ta-Hf-W-C solid solution ceramic coating by a step-by-step reactive plasma spraying, inducing a high-density precipitation of W by spark plasma sintering and finally obtained an ultra-high temperature Ta-Hf-W-C nano-composite ceramic coating with grid interlocking structure. The average hardness and elastic modulus of the Ta-Hf-W-C composite ceramic coating exceeded 47 GPa and 580 GPa due to fine grain strengthening, solid solution strengthening and ordered grain boundary strengthening.

Ultrahigh Hardness Coating with Excellent Crack Resistance Achieved by Ultrafine Eutectic

Ultrahigh Hardness Coating with Excellent Crack Resistance Achieved by Ultrafine Eutectic

Effect of C doping on structure and properties of TiAlCrN coatings by filter cathode vacuum arc deposition

Ultra-hard and wear-resistant (TiAlCrN)C coatings were deposited by filtered cathodic vacuum arc deposition method under the conditions of a fixed N2 flow rate (30 sccm) and an C2H2 flow rate of 0, 5, 10, 30 and 50 sccm. The effect of C2H2 gas flow on the phase structure, mechanical properties, friction properties, as well as corrosion resistance properties of the coating was studied systematically. The results show that C atoms exist in the form of replacing N atoms in the coating, making the coating structure more refined and transforming from a single face-centered cubic (fcc) to an amorphous state. A proper amount of carbon content aggravates the solid solution strengthening and lattice distortion effect of the coating, and increase the hardness of the coating. The coating (Fc = 10 sccm) presents ultra-high hardness (45.95 GPa), excellent wear resistance (1.1 × 10−6 mm3 N−1 m−1) and corrosion resistance properties (2.488 × 10−7 A·cm−2). When excessive carbon was incorporated into the coating (Fc ≥ 10 sccm), a large amount of N was supplanted by C, and the soft amorphous carbon increased in the coating, which reduced the hardness and H/E of the coating, resulting in an increase in the wear rate. The (TiAlCrN)C coating exhibits excellent wear resistance and corrosion resistance, and shows potential application value in surface protection in extreme environments.

Soft-Hard Segment Combined Carbonized Polymer Dots for Flexible Optical Film with Superhigh Surface Hardness.

The rapid development of optical and electronic devices has driven up the demand of high performance optical protective films to avoid exterior influence and extend the service life. But it is not easy to obtain an ideal coating film with high transmittance, high hardness, and good flexibility. Herein, by taking advantage of the special core-shell structure of carbonized polymer dots (CPDs), we propose a strategy to build up a nanoscale soft-hard segment microstructure for optical protective coating materials. The CPDs with hard core and soft polymer chain shell are prepared from citric acid and (3-aminopropyl)triethoxysilane. The as-prepared CPDs can be converted directly to the coating film by the dehydration and cross-linking. In addition to the good optical transmittance, the final film exhibits simultaneously ultrahigh 9H pencil hardness to stand 4000 cycles of a steel-wool wear test, and excellent flexibility to stand bending and rolling-up.

Enhancement of energy storage and hardness of (Na0.5Bi0.5)0.7Sr0.3TiO3-based relaxor ferroelectrics via introducing Ba(Mg1/3Nb2/3)O3

• Simultaneously achieved high W rec of 5.50 J/cm 3 and η of 90.10% in this system. • Ultrahigh hardness ( H ) of 7.35 GPa is realized in this system. • The improved ESP are explained by first-principles calculation on basis of DFT. • The excellent stability in energy storage has been achieved in this system. High comprehensive performances with large energy storage density ( W rec ), high efficiency ( η ), good hardness ( H ), and large operating temperature range are the main challenge in applications of modern electronics and electrical power systems. Herein, excellent comprehensive energy storage performances [high W rec of 5.50 J/cm 3 , large η of 90.10%, and broad usage temperature range (20–200 °C)] and ultrahigh H of 7.35 GPa in lead-free (Na 0.5 Bi 0.5 ) 0.7 Sr 0.3 TiO 3 -based (BNST) ceramics are achieved synergistically. Improving dielectric breakdown strength ( E b ), mitigating early polarization saturation, large polarization difference, and decreasing grain size are beneficial to the enhancement of comprehensive performances. Further analysis of intrinsic electronic structure indicates that the introduction of Ba(Mg 1/3 Nb 2/3 )O 3 (BMN) is conducive to enhancing E b values of BNST via first-principle calculation upon density functional theory (DFT), which can also be verified by experiments. Significantly domain relaxor behavior, as evidenced by piezoresponse force microscopy (PFM) and Vogel-Fulcher (V-F) model, provides strong evidence for restraining early polarization saturation and large polarization difference. Additionally, for practical applications, the BNST-based ceramics exhibit a large power density (49.26 MW/cm 3 ) and fast discharge time (∼120.00 ns) over broad temperature range (20–140 °C). We believe that these findings in this study can provide an effective guideline approach to attain high-performance capacitors for application in pulsed power capacitors.

Formation of nanostructured surface layer, the white layer, through solid particles impingement during slurry erosion in a martensitic medium-carbon steel

The extremely altered topmost surface layer, known as the white layer, formed in a medium-carbon low-alloy steel as result of impacts by angular 10–12 mm granite particles during the slurry erosion process is comprehensively investigated. For this purpose, the characteristics, morphology, and formation mechanism of this white layer are described based on the microstructural observations using optical, scanning and transmission electron microscopies as well as nanoindentation hardness measurements and modelling of surface deformation. The white layer exhibits a nanocrystalline structure consisting of ultrafine grains with an average size of 200 nm. It has a nanohardness level of around 10.1 GPa, considerably higher than that of untempered martensitic bulk material (5.7 GPa) achieved by an induction hardening treatment. The results showed that during the high-speed slurry erosion process, solid particle impacts brought forth conditions of high strain, high strain rate, and multi-directional strain paths. This promoted formation of a cell-type structure at first and later, after increasing the number of impacts, development of subgrains following by subgrain rotation and eventually formation of a nanocrystalline structure with ultra-high hardness. The model confirmed that high strain conditions - much higher than required for the onset of plastic deformation - can be achieved on the surface resulting in severe microstructural and property changes during the slurry erosion test.

Insight into Superior Electrochemical Performance of 4.5 V High-Voltage LiCoO2 Using a Robust Polyacrylonitrile Binder

The polyvinylidene fluoride (PVdF) binder is the classic binder for cathodes because of its excellent electrochemical stability and superior adhesion property. However, PVdF is very expensive and sensitive to moisture. Therefore, pursuit of a cost-effective binder is highly desirable, especially for high-voltage cathode application. Polyacrylonitrile (PAN) possesses strong polar C≡N groups, endowing it with oxidation resistance and strong binding properties. Herein, the PAN binder was systematically investigated for the electrochemical performance of the 4.5 V high-voltage LiCoO2 (HV-LCO) electrode. The results showed that the HV-LCO-PAN electrode delivered superior rate capability and cycling stability to the corresponding HV-LCO-PVdF. The PAN binder was highly tolerant of moisture and was capable of forming uniform coating on the cathode materials. Moreover, the PAN binder had ultrahigh mechanical modulus (8.01 GPa) and hardness (381.45 MPa) in a liquid electrolyte, thereby improving cycling stability and rate capability. X-ray photoelectron spectroscopy analysis further demonstrated that PAN formed uniform coating on the surface of LiCoO2 and played a role of an artificial cathode–electrolyte interphase layer, suppressing decomposition of the liquid electrolyte under high cutoff voltage.

Application of femtosecond laser etching in the fabrication of bulk SiC accelerometer

Silicon carbide (SiC) is a promising material to fabricate MEMS accelerometers used in extreme environments for vibration monitoring. However, with ultra-high hardness and corrosion resistance, it is difficult to etch bulk SiC by traditional MEMS processing methods. Here, we presented a bulk SiC accelerometer structure and its femtosecond laser deep etching method. Combined with the sensor design and femtosecond laser etching method, the sensitivity of the sensor could be improved and the processing difficulty could be reduced. The sensitive beams and mass block of the designed accelerometer were fabricated by femtosecond laser and high size accuracy with the maximum size error of about 3.1% was achieved. A fillet transition on the sensitive beam can eliminate crack propagation during laser etching and improve the quality of the etched gap. The cross-section of the etched sensitive beam can be regarded as an isosceles trapezoid with a steepness of 84.76°. The laser etching sidewall surface of the beam is smooth with a surface roughness (Sa) of 0.255 μm. The piezoresistors on the beam of the accelerometer remain stable before and after laser etching. Generally, the results show that femtosecond laser etching is a feasible method to fabricate bulk SiC accelerometers.

Nanotwinned medium entropy alloy CoCrFeNi thin films with ultra-high hardness: Modifying residual stress without scarifying hardness through tuning substrate bias

High entropy alloy (HEA) and highly nanotwinned (NT) material with improved ductility and high strength demonstrate promising potential compared to nanocrystalline materials and traditional metallic alloys. In this study, CoCrFeNi-based medium entropy alloy (MEA) thin films with nanotwinned structure (NT) was fabricated by sputtering technology and their corresponding properties and microstructure were investigated. The NT-MEA thin films show simple face centered cubic (FCC) structure with nearly 100% (111) texture and twin thickness in 1.8–2.8 nm. The NT-MEA thin films exhibit both low electrical resistivity (100 ± 2 μΩ-cm) and high hardness (8.0–9.15 GPa) with low roughness (Rms < 1.3 nm). By controlling the substrate bias, the residual stress of the CoCrFeNi thin film can be manipulated from compressive (−0.89 GPa) to tensile (+0.6 GPa) without scarifying the hardness.

Grain boundary segregation in Si-doped B-based ceramics and its effect on grain boundary cohesion

Boron carbide and boron suboxide are attractive for their low densities and ultrahigh hardness values, yet they exhibit poor fracture resistance. Strategies to improve mechanical performance aim to control bulk microstructures and properties by incorporating Si-based additives. A related approach to toughen boron carbide is tailoring the phase-like states of grain boundaries by applying the concept of grain boundary complexion engineering. This work directly investigates the potential of grain boundary complexion engineering to improve fracture resistance by systematically evaluating grain boundary segregation behavior of a polycrystalline silicon hexaboride / boron carbide diffusion couple using aberration-corrected scanning transmission electron microscopy and ζ-factor microanalysis. A main result was that all grain boundaries in the diffusion couple exhibited Si segregation that, depending on the bulk Si concentration, approached up to 3 monolayers of coverage, thereby resulting in nanolayer complexions (oftentimes known as intergranular films). Furthermore, upon analyzing 110 distinct grain boundaries throughout the diffusion zone and an impurity boron suboxide region, free energies of Si segregation in boron carbide and boron suboxide were experimentally determined using the Brunauer, Emmett and Teller (BET) multilayer grain boundary segregation theory. Subsequently, changes in grain boundary energy due to Si segregation were quantified and a maximum energy reduction of 51% and 81% were observed in boron carbide and boron suboxide, respectively, which led to decreases in works of adhesion of 18% and 17%, respectively. In summary, this work uncovers grain boundary processing-structure-property relationships that can be used to improve the fracture resistance in icosahedral boron-based ceramics.

Design of high strength and wear-resistance β-Ti alloy via oxygen-charging

Titanium (Ti) is a promising biomedical material due to its superior corrosion resistance, low elastic modulus and favorable biocompatibility. Nevertheless, Ti faces a dilemma because of its inferior abrasion performance and strength-ductility trade-off, which poses a limitation in application as biomedical implants. Here, we developed an oxygen-charging method to fabricate a β-Ti alloy with combination of ultrahigh surface hardness, strength, toughness and remarkable wear resistance. The superior mechanical performance of β-Ti alloy originates from a 200 μm-thick α+β phase hard shell, a 600 μm oxygen gradient region and an oxygen-free β-Ti core. The gradient phase and composition structures display different deformation mechanisms, transforming from simple but unusual basal slip in α phase to multiple-slip activities in β phase. The unique oxygen gradient distribution makes β-Ti alloy much stronger and tougher that can resist surface crack propagation and sample catastrophic failure. Oxygen charging is a novel technique to design high-performance Ti implants for biomedical applications.

Synthesis of High Refractive Index Silicone LED Encapsulation with Ultra-High Hardness

Synthesis of High Refractive Index Silicone LED Encapsulation with Ultra-High Hardness

Thermal simulation of the continuous pulse discharge for electro-spark deposition diamond wire saw

Due to their excellent physical and mechanical properties, third-generation super-hard semiconductor materials (such as SiC, GaN) are used widely in the field of microelectronics. However, due to its ultra-high hardness, the machining is very difficult, which has become the bottleneck of its development. Slicing is the first machining procedure that directly affects the subsequent process. Fixed abrasive diamond wire saw (DWS) has been widely used in cutting hard and brittle materials. However, the diamond abrasives are attached to a core wire by resin or electroplated, that slicing super hard crystals is very difficult and inefficient. In order to improve the slicing efficiency, it is necessary to improve the holding strength and wear resistance of the DWS. The electro-spark deposition (ESD) process can deposit electrode materials on the substrate under the condition of low heat input to achieve metallurgical bonding between metal materials. And it can improve the wear resistance, corrosion resistance, and repair the size of the workpiece. It has been widely used in the field of surface modification engineering. It can effectively improve the bonding strength of the abrasive grains, and the sawing ability of the wire saw to make the consolidated DWS by the ESD process. Due to its thin matrix and poor thermal properties, the saw wire is easy to burning or even breaking in the manufacturing process. At present, the selection of pulse interval time in the ESD process is generally determined by the duty factor. However, the pulse interval time selected according to duty factor is difficult to meet the heat dissipation requirements of electro-spark deposition DWS (ESDDWS). In this paper, two kinds of motion modes of ESDDWS manufacturing are put forward, according to the manufacturing characteristics of ESDDWS. The boundary conditions of the continuous pulse discharge of ESDDWS are established. The thermal simulations of continuous pulse discharge of ESDDWS under two motion modes are analyzed. According to the simulation results, the basis of the value of pulse interval in the ESDDWS process is put forward. The effect of pulse interval time on the mechanical performance of the wire saw is analyzed experimentally. The results show that selected the discharge interval time base on the simulation results can ensure the continuous production of the ESDDWS.