Indentation Fracture Research Articles

Influence of sintering condition on tribological properties of (Hf-Ta-Zr-Nb-Ti)C carbides

The wear behavior of high-entropy (Hf-Ta-Zr-Nb-Ti)C ceramics (HECs) was investigated using the ball-on-flat linear reciprocating method/dry sliding in the air. The experimental materials were prepared by ball milling and spark plasma sintering at 2100 °C and 5, 10, or 20 min sintering time. The HECs achieved relative density from 99.5% to 99.9%, with homogeneous chemical composition and grain sizes of HEC from 8 μm to 11 μm. The sintering times showed no significant influence on the wear characteristics of the investigated specimens. However, very promising mechanical and tribological properties of HEC were obtained. The materials had Young's modulus from 452 GPa to 482 GPa, Vickers' hardness from 20.9 GPa to 22.0 GPa, and Indentation fracture resistance of ∼2.8 MPa·m1/2 - 2.9 MPa·m1/2. The measured values of specific wear rate varied from 1.18 × 10−6 mm3/N·m to 3.23 × 10−6 mm3/N·m and coefficient of friction under different loads were very similar, between 0.29 and 0.32. The dominant wear mechanisms in all cases were mild abrasion with limited oxidation-driven tribochemical reaction.

Influence of Different Shaping and Finishing Processes on the Surface Integrity of WC-Co Cemented Carbides

Investigation of four different surface-shaping and finishing sequences is carried out on the surface integrity of a WC-10Co hardmetal grade. The surface conditions include grinding, electrical discharge machining and grinding, followed by mechanical and dry-electrochemical polishing using the DryLyte® technology. The evaluation includes the measurement of roughness, residual stresses, the Vickers hardness, indentation fracture toughness determination and the damage induced by conical contact response. By scanning electron microscopy, a systematic and detailed examination of the residual imprints is carried out to determine the critical loads for damage initiation and development across the different surface conditions. The results indicate that the use of dry-electrochemical polishing enables the attainment of polished surfaces without any corrosive damage to the metallic binder. Moreover, it retains the mechanical attributes reminiscent of the core material, comprising 85% that were initially induced via grinding.

Effect of SiC on densification, microstructure and mechanical properties of high entropy diboride (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2

Highly dense high entropy diboride (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-SiC composites with different β-SiC content (5, 10, 15, 20 and 25 vol%) were prepared by Spark Plasma Sintering (SPS) at 1900 °C. High-purity high entropy diboride (HEB) powder was synthesized using boro/carbothermal reduction of oxide precursors at 1800 °C. X-ray diffraction patterns of the synthesized powders and sintered compacts of HEB showed a single solid solution with hexagonal AlB2 structure without any residual oxide impurities. Energy Dispersive X-ray Spectroscopy (EDS) mapping results indicated the uniform elemental distribution of transition metals in the HEB phase. The addition of SiC improved the densification of HEB-SiC composites. The composite with 20 vol% SiC exhibited a unique combination of mechanical properties, such as superior Vickers hardness (22.28 ± 0.25 GPa), flexural strength (750.21 ± 43.23 MPa), indentation fracture toughness (4.12 ± 0.2 MPa.m1/2), Young’s modulus (495.3 ± 0.3 GPa) and dynamic oxidation resistance (0.02 mg.cm−2.s−1).

Fracture resistance of binderless tungsten carbide consolidated by spark plasma sintering and flash sintering

Binderless tungsten carbide (WC) consolidated by spark plasma sintering (SPS) and electrical resistance flash sintering (ERFS) has emerged as a promising alternative to materials obtained by traditional sintering methods. In this study, we investigated the influence of SPS and ERFS techniques on the mechanical properties of binderless WC. Hardness, indentation fracture resistance and elastic modulus were compared and analysed with respect to the microstructure of the resulting material. The results show that SPS tungsten carbide is harder, stiffer and denser when compared to the material produced by ERFS. Nevertheless, the fracture resistance of SPS ceramics was limited due to the lack of macroscopic toughening mechanisms. Conversely, flash-sintered tungsten carbide, consolidated by ERFS, possesses unique biphasic WC/W2C microstructures that promote crack deflection and bridging toughening mechanisms. Additionally, flash-sintered materials exhibit a more ductile character and are characterised by a lower effect of the strain gradient on the material plasticity upon indentation. This study provides valuable insights into the mechanical properties of ultra-hard monolithic ceramics, such as WC, influenced by ultrafast/flash sintering techniques.

Effect of strain rate on the hardness of relaxor ferroelectric PMN-0.32PT single crystals

The relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) has been considered to be a potential choice for high performance actuators and sensors owing to their ultrahigh electromechanical properties. Recent experiments have shown that the hardness and indentation fracture toughness of PMN-PT varies with load and loading direction. However, it is not clear from these studies that how the loading rates would affect the indentation response of these materials. Therefore, nanoindentation experiments at different strain rates are performed on [001] and [011] oriented poled PMN-0.32PT single crystals. Results show that the hardness, H decreases with increase in strain rate. This behavior is in contrast to the trend reported for PMN-PT single crystal in the recent past. The indentation size effect (ISE) is noticed in both the orientation for all the strain rates. In order to analyze the observed ISE, various mechanistic models are used such as Meyer's law, Hays-Kendall (H-K) model, proportional specimen resistance (PSR) model, and modified PSR (MPSR) model. The analysis of nanoindentation data using MPSR model satisfactorily describes the ISE in terms of resistance offered by the specimen to indenter penetration, the interface friction, and surface residual stress.

Microstructural evolution and mechanical properties of Ti(C,N)–FeCrMo-based green cermets

This work aims to develop Co and Ni-free Ti(C,N)–Fe-based cermets which are essential for developing cermets in the direction of green and low-cost materials. TiC and Ti(C,N) -based cermets prove to evince excellent mechanical properties at high temperatures as compared to conventional WC-based cermets. A binder system based on Cr, Mo, Fe, and their alloys may offer a viable solution to replace carcinogen elements such as Ni and Co from the binder. In the present work, high chromium Ti(C,N)-30 wt% FeCrMo cermets were fabricated by using Pressureless liquid-phase sintering. Samples at different sintering temperatures were prepared to investigate the influence of the temperature on microstructural formation and their mechanical characteristics. Scanning Electron Microscopy (SEM) is employed to observe the microstructural formation. A microstructural analysis of the consolidated cermets reveals that the addition of Mo to the binder system and an increase in sintering temperature affect the carbide grain size and densification, which ultimately affect their mechanical characteristics. The hardness and fracture toughness were determined by the Vickers hardness test and Indentation fracture toughness (IFT) method respectively. The maximum Hardness, Fracture toughness, and Transverse rapture strength achieved for the Ti(C,N)–FeCrMo-based green cermets are 665 ± 6.7 MPa, 1396 ± 26 HV30, and 9.23 ± 0.12 MPa m1/2 respectively.

An experimental study on 3D-printed continuous fiber-reinforced composite auxetic structures

Auxetic structures have negative Poisson’s ratios (NPR). Due to the unique deformation mechanism, auxetic structures possess extraordinary mechanical properties, such as indentation resistance, shear resistance, fracture toughness, and energy absorption capability. However, the stiffness and load-bearing capacity are the weak points for auxetic structures. 3D printing of continuous fiber-reinforced composite enables the fabrication of lightweight and highly stiff complex structures, providing a perfect manufacturing method to remedy the shortcomings of auxetic structures. This work investigated the mechanical properties of 3D-printed continuous fiber-reinforced composite auxetic structures. In this study, we utilized continuous fiber-reinforced composite 3D printing to fabricate two types of auxetic structures. The fiber path configurations were varied among the test specimens to explore the effect of fiber distribution on mechanical properties. A uniaxial tensile test was performed to evaluate the tensile properties and Poisson’s ratio of continuous fiber-reinforced composite auxetic structures. Results showed that the tensile modulus and strength have been dramatically improved with a minor mass increase. The auxetic behavior can be strengthened by properly allocating the reinforcing fibers. However, the addition of continuous fiber led to different performances on the selected auxetic structures. In summary, two out of the five specimens demonstrated simultaneous improvements in stiffness, strength, and auxeticity across the conducted tests.

B4C-SiC composites with tuneable mechanical properties: Role of Al2O3 - Y2O3 sintering additives

While outstanding properties of boron carbide (B4C)-silicon carbide (SiC) composites make them potential candidates for ballistic and wear resistance applications, the applicability is rather limited due to processing challenges to obtain high density and superior mechanical properties. In the present work, B4C-10 wt% SiC composites are spark plasma sintered without additives and with 6 wt% (Al2O3 -Y2O3) additives at 1600 °C, 1700 °C and 1800 °C. When sintered at 1600 °C, the relative density increased from 94% for the composites without additives to ∼99% with Al2O3-Y2O3 additives. The formation of liquid phase comprising Al-Y-Si-O improved density and mechanical properties of the composites. TEM analysis of the composite sintered with (2 wt% Al2O3 - 4 wt% Y2O3) additives at 1800 °C confirmed clean grain boundaries between B4C, SiC and liquid phase precipitates. When sintered at 1700° C, the indentation fracture toughness increased from 4.3 MPa.m0.5 for the composite without additive to 6.0 MPa.m0.5 for the composite with 6 wt% Al2O3 additive due to increased instances of crack splitting, bridging and deflection.

Mechanisms and mechanical properties of high-temperature high-pressure sintered vanadium carbide ceramics

Vanadium carbide (VC) is extensively employed as a reinforcing agent in coating materials and a grain refiner in hard alloys, owing to its exceptional mechanical properties. A series of bulk VC ceramics were fabricated via high-temperature high-pressure sintering, followed by a comprehensive and systematic investigation of various properties of the sintered samples. The results revealed significant influences of processing temperature on the properties of sintered VC at 5.0 GPa. However, the exact nature of temperature dependence varied depending on the specific property. It was found that VC ceramics sintered at 1100 °C and 5.0 GPa exhibited the best overall performance, providing a relatively optimal temperature condition for synthesis. The Vickers hardness of the material reached an astonishing 43.2 GPa under a 9.8 N load, placing it within the category of superhard materials. However, the sintered sample exhibited a gradually lower hardness value of 30.4 GPa under a higher load of 29.4 N. Highly dense with a relative density of 99.8%, exhibiting near full density, the vanadium carbide ceramics possess an impressive Young's modulus of 544 GPa, indentation fracture resistance of 5.4 MPa m1/2, and an oxidation initiation temperature of 758 °C. This study offers practical guidance for the design of novel transition metal carbide superhard materials and provides valuable insights for the exploration of visualizing directions in the next generation of superhard materials.

Microstructural and mechanical behaviours of Y-TZP prepared via slip-casting and fused deposition modelling (FDM)

This paper reports the microstructural characteristics and mechanical properties of yttria-stabilized zirconia prepared via fused deposition modelling and slip casting. X-Ray Diffraction peaks indicated that yttria-stabilized zirconia crystallized in tetragonal structure for both slip casted(SC) and fused deposition modelled(FDM) samples. Further, scanning electron microscopy of slip casted sample showcased closely packed structure with fine grains and an average grain size of ∼65 nm whilst fused deposition modelled samples showcased non-homogeneous pores with ∼20 nm grain size. Average relative density of slip casted samples was ∼99.4 % while that of fused deposition modelled sample exhibited ∼96.2 %. The Vickers Hardness of slip casted (∼15.26 ± 0.4 GPa) was ∼10 % higher than the fused deposition modelled samples (∼13.79 ± 0.3 GPa). Likewise, indentation fracture toughness of slip casted (5.78 ± 0.5 MPa m1/2) was 14 % higher than fused deposition modelled samples which could have been due to the change in grain size as well as porosity of the ceramics. Compressive strength of the fused deposition modelled samples was 32 % less than slip casted samples (∼510 ± 10 MPa) due to its non-homogenous pores which led to weakening van der Waals force of attraction.

Microstructural analysis and ASTM B611-21 wear testing of novel binder hardmetals for future percussive drilling applications

Since their discovery exactly one hundred years ago, WC-Co hardmetals (cemented carbides) have been a well-established composite material. They are extensively used in multiple fields of industry such as cutting applications, drilling, mining, and milling, due to the combination of high hardness and toughness. In recent years the partial or complete substitution of Co as the metallic binder has been a central topic of hardmetal research for economical, legal, ethical, and health-related reasons. The presented work focuses on the complete substitution of Co as the binder element in hardmetals that in the future could be used in percussive drilling applications. Other suitable metallic elements have been used combination with Ni as the binder basis to strengthen the binder via the well-known principle of solid solution strengthening. Four alternative binder grades were selected, and various sintering experiments were conducted to determine the influence of maximum sintering temperature (Tmax), atmosphere, and annealing treatment. HV50, KIc via indentation fracture toughness, microstructure, magnetic saturation as well as differential thermal analysis (DTA) and dilatometry (DIL) were investigated. To simulate the high-stress and abrasive conditions in percussive drilling to a certain degree, wear tests in the form of a modified ASTM B611–21 test were conducted. The results were compared with industrially available reference grades and show that completely Co-free, Ni-based alloys can compete with Co when used as the binder matrix in hardmetals and upon further optimisation may very well be viable candidates for Co replacement.

High temperature corrosion behaviour of calcium magnesium aluminosilicate coated oxide-oxide ceramic matrix composites

Corrosion on turbine blades in calcium magnesium alumino-silicate (CMAS) environment is a crucial failure for turbine engines and its components. In this study, oxide-oxide (O-O) ceramic matrix composites (CMCs) (AS-N610), the potential materials for gas turbine components are examined for its corrosion behaviour at high temperatures at various intervals of time in presence of CMAS. The corrosion studies indicated that dip coated with CMAS revealed a weight gain of ∼3% owing to the formation of α-Al2O3 at 1400 °C. The SE images indicated cracks at the interface due to thermal mismatch between CMAS and O-O substrate. With increase in corrosion time, cracks at the interface propagated onto the matrix and fibres of O-O CMCs. This crack propagation is attributed to the diffusion of calcium aluminosilicate (CAS) with small traces of Mg which wicks the columns of O-O CMCs. Indentation fracture toughness of O-O CMCs degraded by ∼22% for 1400 °C in presence of CMAS compared to un-corroded sample.

A comprehensive review on in-plane and through-the-thickness auxeticity in composite laminates for structural applications

Auxetic laminates are part of a fascinating branch of materials denominated as auxetics, which display a NPR (Negative Poisson’s Ratio), an uncommon property in the engineering world. Auxeticity is at the root of enhancements in shear and indentation resistance, fracture toughness and energy absorption, making the design of NPR a desirable feat in structural engineering. In composite laminates made of conventional, i.e. positive Poissons ratio, materials, auxeticity is a result of the combination between particular angular configurations and anisotropic materials. Such laminates can be used in a wide array of engineering applications, especially those which require high energy absorption capacity, including aerospace, personal defense and sports industries. This review focuses on particular property enhancements reported in the scientific world brought about by the design and application of IP (In-plane) and TTT (Through-the-thickness) NPR fibre-reinforced polymer laminates under QSI (Quasi-static indentation), LVI (Low-velocity impact) and fatigue solicitations. Furthermore, some insight is given on some possible future paths for further investigation of this topic.

Substituting Ce with La and Nd in Al11Ce3: Effect on microstructure and mechanical properties

Microstructure and mechanical properties are studied for five cast, coarse-grained intermetallic alloys: binary Al11Ce3 and Al11La3, ternary Al11(Ce0.75La0.25)3 and Al11(Ce0.5La0.5)3, and quaternary Al11(Ce0.54La0.27Nd0.19)3 with mischmetal composition. All compounds exhibit a single phase indicating a solid solution among the three rare-earth elements (RE=Ce, La, and Nd) on the Ce sublattice of Al11RE3, in disagreement with Thermo-Calc prediction of segregation of Ce and (La, Nd) into two insoluble compounds. The lattice parameters of this orthorhombic Immm-structured α-Al11RE3 phase increase as La concentration increases, while the α-β phase transformation temperature decreases. The five Al11RE3 compounds show very similar (i) propensity for twinning, (ii) high hardness (4.1–4.3 GPa), and (iii) low indentation fracture toughness (0.48–0.65 MPa m1/2). The Al-Al11RE3 interface lattice mismatches, as calculated for two stable interface orientations, have comparable values across all examined compounds. These similarities imply that these Al11RE3 compounds will exhibit comparable strengthening and coarsening resistance in Al-RE-based eutectic alloys, thus positioning them as economically- and environmentally-favorable alternatives to the well-developed binary Al11Ce3 compound formed in current eutectic Al-Ce alloys.

Interfacial Evolution of WC-Co/AISI 304L Diffusion Bonded Joint Obtained by Flash SPS Technique

Interfacial Evolution of WC-Co/AISI 304L Diffusion Bonded Joint Obtained by Flash SPS Technique

Investigation of Spark Plasma Sintering on Microstructure-Properties of Zirconia Reinforced Fluormica Glass for Dental Restorations.

Conventional sintering methods of dental ceramics have limitations of high temperature and slow cooling rates with requirements of additional heat treatment for crystallization. Spark plasma sintering (SPS) is an emerging technique that has the potential to process dental restorations with dense microstructures and tailor-made clinically relevant properties with optimized processing parameters. This study explored the potential of the SPS of zirconia-reinforced fluormica glass (FM) for dental restorative materials. FM glass frit was obtained through the melt-quench technique (44.5 SiO2-16.7 Al2O3-9.5 K2O-14.5 MgO-8.5 B2O3-6.3 F (wt.%)). The glass frit was ball-milled with 20 wt.% of 3 mol% yttria-stabilized zirconia (FMZ) for enhanced fracture toughness. The mixtures were SPS sintered at a pressure of 50 MPa and a heating rate of 100 °C/min for 5 min with an increase in temperature from 650-750 °C-850 °C-950 °C. Phase analysis was carried out using XRD and microstructural characterization with SEM. Micro-hardness, nano-indentation, porosity, density, indentation fracture toughness, and genotoxicity were assessed. The increase in the SPS temperature of FMZ influenced its microstructure and resulted in reduced porosity, improved density, and optimal mechanical properties with the absence of genotoxicity on human gingival fibroblast cells.

Effect of TiO2 reinforcement on low temperature (1300 °C) sinterability of zirconia toughened alumina ceramics by correlating its structural and physico-mechanical properties

The primary goal of this research is to investigate the sinterability of zirconia toughened alumina (ZTA) at low temperature (1300 °C) utilizing two different sintering techniques: hot press and cold press. The effect of titanium oxide (TiO2) content variation (0–5 wt%) in ZTA is compared for each sintering method. The solubility limits of TiO2 in ZTA are identified to be 2 wt% and 3 wt% for hot press and cold press sintering respectively which have served exceptional physical and mechanical properties. In the case of hot press, outstanding relative density (99%) is achieved due to excellent compaction, including high microhardness of ∼2092 HV0.5, nanohardness of ∼25.05 GPa, elastic modulus of ∼371.59 GPa and indentation fracture toughness of ∼5.43 MPa·√m for 2 wt% TiO2 inclusion. Whereas in the case of cold press, 3 wt% TiO2 addition has revealed a considerable relative density (96%), alongside microhardness of ∼1975 HV0.5, nanohardness of ∼20.92 GPa, elastic modulus of ∼280.39 GPa and an indentation fracture toughness of ∼4.84 MPa·√m. However, further addition of TiO2 leads to the formation of secondary phases i.e., tialite and zirconium titanate which deteriorate the mechanical and physical properties due to the abnormal grain growth. Therefore, it has been postulated that the small additions of TiO2 have enormous capability of lowering the sintering temperature of ZTA-MgO ceramic without compromising the physical and mechanical characteristics.

Dense nanocrystalline ReB2 bulk with ultrahigh hardness

Microcrystalline ReB2 with a high hardness of 48 GPa measured under a small load of 0.49 N has been reported as a superhard material. Nanocrystalline ReB2 is expected to achieve better mechanical properties than its microcrystalline counterparts. Here we report dense nanocrystalline ReB2 bulks synthesized by modified mechanical alloying and high-pressure sintering techniques. The modified mechanical alloying technique reduces the content of WC impurity from 7.75 wt% to 1.59 wt% in as-synthesized nanocrystalline ReB2 powders with an average grain size of 6.6 ± 2.0 nm. The extrinsic high-pressure (6 GPa) and the intrinsic nanocrystalline structure greatly promote the densification process of the ReB2 bulks during sintering. Our optimized nanocrystalline ReB2 bulk is ∼99% dense and with an average crystallite size of 40 nm, resulting in a high hardness of ∼33 GPa under a load of 9.8 N and indentation fracture toughness of ∼5 MPa m1/2. The high hardness can be ascribed to the synergistic effect of high density and nanocrystalline structure of ReB2 bulks. This work provides promising strategies to largely enhance the mechanical properties of different ceramics for practical applications.

WC-Ni cemented carbides prepared from Ni nano-dot coated powders

This study presents a novel approach for the synthesis of WC-Ni cemented carbides with enhanced mechanical properties. A low-cost solution-based route was used to coat WC powders with well-distributed metallic nickel dots measuring between 17 nm and 39 nm in diameter. Varying compositions with loadings of 2, 6, and 14 vol% Ni were consolidated using spark plasma sintering (SPS) at 1350 °C under 50 MPa of uniaxial pressure giving relative densities of 99 ± 1 %. The sintered WC-Ni cemented carbides had an even distribution of the Ni binder phase in all compositions, with retained ultrafine WC grain sizes of 0.5 ± 0.1 μm from the starting powder. The enhanced sinterability of the coated powders allowed for consolidation to near theoretical densities, with a binder content as low as 2 vol%. This is attributed to the uniform distribution of nickel and an extensive Ni-WC interface existing prior to sintering. The small size of the Ni dots likely also contributed to the solid-state sintering starting temperatures of as low as 800 °C. The mechanical performance of the resulting cemented carbides was evaluated by measuring the hardness at temperatures between 20 °C and 700 °C and estimating toughness at room temperature using Vickers indentations. These results showed that the mechanical properties of the WC-Ni cemented carbides synthesised by our method were comparable to conventionally prepared WC-Co cemented carbides with similar grain sizes and binder contents and superior to conventionally prepared WC-Ni cemented carbides. In particular, the 2 vol% Ni composition had excellent hardness at room temperature of up to 2210HV10, while still having an indentation fracture toughness of 7 MPa·m0.5. Therefore, WC-Ni cemented carbides processed by this novel approach are a promising alternative to conventional WC-Co cemented carbides for a wide range of applications.

Zirconia-Based Ceramics Reinforced by Carbon Nanotubes: A Review with Emphasis on Mechanical Properties

This review outlines the state of the art, processing techniques, and mechanical testing methods of zirconia (ZrO2)-based composites reinforced by carbon nanotubes (CNTs). The use of CNTs as a secondary phase in a zirconia matrix is motivated by their outstanding crack self-healing ability, the possibility to tailor the desired nano-structural properties, and their exceptional wear behavior. Therefore, a detailed investigation into CNT features has been provided. The debate of using the different Vickers indentation fracture toughness equations to estimate the resistance of crack propagation was critically reviewed according to crack characteristics. Finally, this review particularly highlights the exceptional role of ZrO2-based composites as a promising material owing to their outstanding tribo-mechanical properties.