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Hydroxyapatite [Hap, Ca10(PO4)6(OH)2] is one of the most preferred bioceramic material for orthopedic implants and coatings due to its stoichiometric similarities with human hard tissues. However, foreign body implantation inside human body sometimes leads to bacterial film formation over the implant surface causing the implant failure, thereby needing a revision surgery. This study attempts to select the better dopant between zinc (Zn) and cobalt (Co) as per the antibacterial efficacy when doped in Hap. To prepare antibacterial transition‐metal‐doped Hap, Zn and Co are doped in Hap as per the chemical formula Ca10−x Mx(PO4)6(OH)2, (M = Zn or Co and x = 0.24) to improve antibacterial efficacy. Phase and microstructural characterization by Rietveld refinement, scanning electron microscopy (SEM), and Fourier transformation infrared spectroscopy (FT‐IR) confirms the doping. Evaluation of antibacterial activity against E coli reveals that both Zn‐ and Co‐doped Hap shows antibacterial property with the latter being more effective (zone of inhibition ~3 mm more) for the same level of doping. Inductively coupled plasma‐mass spectrometry confirms the presence of ~676 ppb Co+2 and ~303 ppb Zn+2 after leaching. In addition, cytotoxicity assay with NIH3T3 cell line reveals cytocompatibility of both the compositions with either dopant. The effect of spark plasma sintering on densification and mechanical properties of Co‐doped Hap is investigated for the first time and compared with Hap with the same level of Zn doping. It appears that Co‐doped Hap attains higher densification (~7% more) and fracture toughness (~2 times better) as compared to that of Zn‐doped counterpart (densification: 86% and fracture toughness: 0.75 ± 0.12 MPa √m). Thus, this study suggests that Co‐ and Zn‐doped Hap are promising candidates for bone tissue engineering with improved antibacterial properties and in addition, Co‐doped Hap can attain higher density and offer better fracture toughness than that of Hap doped with Zn. [ABSTRACT FROM AUTHOR]

The refractory diborides (HfB2 and ZrB2) are considered as promising ultra-high temperature ceramic (UHTCs) where low damage tolerance limits their application for the thermal protection system in re-entry vehicles. In this regard, SiC and CNT have been synergistically added as the sintering aids and toughening agents in the spark plasma sintered (SPS) HfB2-ZrB2 system. Herein, a novel equimolar composition of HfB2 and ZrB2 has shown to form a solid-solution which then allows compositional tailoring of mechanical properties (such as hardness, elastic modulus, and fracture toughness). The hardness of the processed composite is higher than the individual phase hardness up to 1.5 times, insinuating the synergy of SiC and CNT reinforcement in HfB2-ZrB2 composites. The enhanced fracture toughness of CNT reinforced composite (up to a 196% increment) surpassing that of the parent materials (ZrB2/HfB2-SiC) is attributed to the synergy of solid solution formation and enhanced densification (~99.5%). In addition, the reduction in the analytically quantified interfacial residual tensile stress with SiC and CNT reinforcements contribute to the enhancement in the fracture toughness of HfB2-ZrB2-SiC-CNT composites, mandatory for aerospace applications. [ABSTRACT FROM AUTHOR]

Tantalum carbide (TaC) is an ultrahigh temperature ceramic, where low damage tolerance limits its potential application in propulsion sector. In this respect, current work focuses on enhancing the toughness of TaC based composites via synergistic reinforcement of SiC and carbon nanotubes (CNTs). Spark plasma sintering of TaC, reinforced with 15 vol% SiC and 15 vol% CNT (processed at 1850 °C, 40 MPa, 5 min), has shown enhanced densification from ∼93% (for TaC) to ∼98%. Potential damage of the tubular CNTs to flaky graphite was revealed using transmission electron microscopy, and was supplemented via Raman spectroscopy. SiC addition has enhanced the hardness to ∼19.5 GPa while a decreases to 12.6 GPa was observed with CNT addition when compared to the hardness of TaC (∼15.5 GPa). The increase in the indentation fracture toughness (from 3.1 MPa m1/2 for TaC to 11.4 MPa m1/2) and fracture strength (from ∼23 MPa for TaC to ∼183 MPa) via synergetic reinforcement of SiC and CNT is mainly attributed to energy dissipating mechanisms such as crack branching, CNT bridging, and crack-deflection. In addition, the reduction of interfacial residual tensile-stresses with SiC- and CNT-reinforcement, resulting an overall increase in the fracture energy and toughening, is also established. [ABSTRACT FROM AUTHOR]

The present research aims to investigate the effect of multi-walled carbon nanotube (MWNT) reinforcement on the mechanism of transformation toughening in zirconia matrix, and consequently, its fracture toughness. Monoclinic zirconia (un-doped ZrO), partially stabilized zirconia (3 mol pct yttria-stabilized zirconia (3 mol pct YSZ)), and fully stabilized cubic zirconia (8 mol pct YSZ) with and without 6 vol pct MWNT-reinforced nanocomposites were processed via multi-stage spark plasma sintering. Phase analysis of powders, and sintered and crushed pellets performed using X-ray diffraction reveals the absence of any phase transformation in monoclinic ZrO, 8 mol pct YSZ and their MWNT-reinforced nanocomposites upon application of stress by means of crushing. However, a significant decrease in the stress-induced phase transformation (81.1 pct metastable tetragonal phase retained with 6 vol pct MWNT reinforcement when compared to that of 68.4 pct tetragonal phase in 3 mol pct YSZ) is observed in the crushed pellet samples of partially stabilized zirconia upon 6 vol pct MWNT reinforcement. Transmission electron microscopy has been utilized for complementary phase analysis. Evaluation of mechanical properties indicates enhancement in fracture toughness (~23.6 to 26.4 pct) with the incorporation of 6 vol pct MWNT. Isolation of the net toughening contribution suggests that MWNT toughening mechanisms are ~3.8 times more effective than transformation toughening in enhancing the fracture toughness of YSZ/MWNT nanocomposites. [ABSTRACT FROM AUTHOR]

The current work focuses on enhancing the flexural strength and fracture toughness of zirconium diboride (ZrB 2 ) reinforced with silicon carbide (SiC) and carbon nanotubes (CNT). The flexural strength has shown to increase by ~ 1.2 times from 322.8 MPa (for ZrB 2 ) to 390.7 MPa and fracture toughness up to 3 times from 3.2 MPam 0.5 (for ZrB 2 ) to 9.5 MPam 0.5 with the synergistic addition of both SiC and CNT in ZrB 2 matrix through energy dissipating mechanisms such as deflection, branching and strong interfacial bonding evidenced from the transmission electron microscopy (TEM). A modified fractal model is used to evaluate the fracture toughness and delineate the contribution of residual stresses, and reinforcements (SiC and CNT) in enhancing the fracture toughness. Interfacial bonding, in terms of a debonding factor, was also evaluated by theoretically predicting the elastic modulus and then correlated with the microstructure along with other mechanical properties of ZrB 2 -SiC-CNT composites. [ABSTRACT FROM AUTHOR]

Current work pursues generating controlled bimodal microstructure by plasma spraying of micrometer-sized Al 2 O 3 and nanostructured spray-dried agglomerate with reinforcement of 20 wt% of 8 mol % yttria stabilized zirconia (8YSZ) and 4 wt% carbon nanotube (CNT) as potential thermal barrier coating (TBC) on the Inconel 718 substrate. Composite coatings exhibit bimodal microstructure of: (i) fully melted and resolidified microstructured region (MR), and (ii) partially melted and solid state sintered nanostructured regions (NR). Reinforcement with 8YSZ has led to an increase in hardness from ∼12.8 GPa (for μ-Al 2 O 3) to ∼13.9 GPa in MR of reinforced Al 2 O 3 -YSZ composite. Further, with the addition of CNT in Al 2 O 3 -8YSZ reinforced composite, hardness of MR has remained similar ∼13.9 GPa (8YSZ reinforced) and ∼13.5 GPa (8YSZ-CNT reinforced), which is attributed to acquiescent nature and non-metallurgical bonding of CNT with MR. Indentation fracture toughness increased from 3.4 MPam0.5 (for μ-Al 2 O 3) to a maximum of 5.4 MPam0.5 (8YSZ- CNT reinforced) showing ∼57.7% improvement, which is due to crack termination at NR, retention of t-ZrO 2 (∼3.3 vol%) crack bridging, and CNT pull-out toughening mechanisms. Modified fractal models affirmed that the introduction of bimodal microstructure (NR) i.e., nanometer-sized- Al 2 O 3 , nanostructured 8YSZ and CNTs in the μ-Al 2 O 3 (MR) contributes ∼44.6% and ∼72% towards fracture toughness enhancement for A8Y and A8YC coatings. An enhanced contribution of nanostructured phases in toughening microstructured Al 2 O 3 matrix (in plasma sprayed A8YC coating) is established via modified fractal model affirming crack deflection and termination for potential TBC applications. [ABSTRACT FROM AUTHOR]

Effect of B 4 C addition on the lower damage tolerance of spark plasma sintered (Hf, Zr)B 2 based ceramics has been elucidated. Increased densification (from 89% to 95%) with B 4 C addition has elicited hardness and elastic modulus increase from 25 GPa to 38 GPa (52%) and 439 GPa–635 GPa (45%), respectively. Systematic 3–9 vol% B 4 C addition resulted in an increased fracture toughness (from 2.2 MPa m0.5– 4.4 MPa m0.5) and flexural strength (from 320 MPa to 357 MPa) compared to that of (Hf, Zr)B 2 due to synergistic effect of solid solutioning and compressive residual stresses generated around reinforcement. The decreased wear rate (from 0.163 mm3/N.m to 0.061 mm3/N.m) with optimal B 4 C addition, affirms enhanced damage tolerance of (Hf, Zr)B 2 composite. Enhanced cracking and fracture in (Hf, Zr)B 2 shifts to abrasion induced restricted micro-cracking as micro-scratch wear mechanism with B 4 C addition. Estimated elastic-modulus and extracted scratch-hardness are correlated with microstructural attributes of porosity, phase content and residual stress in (Hf, Zr)B 2 composites. • Spark plasma sintering of dense (up to 95%) HfB2-ZrB2-B4C composites. • Fracture toughness doubled to 4.4. MPa.m0.5 in HfB2-ZrB2-9 vol% B4C. • Modelling the effect of solid solutioning on estimated elastic modulus. • Estimating scratch toughness/hardness and correlating to damage mechanics. • 60% increased damage tolerance with B4C addition in HfB2-ZrB2 composites. [ABSTRACT FROM AUTHOR]

Abstract The fully dense HfB 2 -ZrB 2 -SiC composites were processed using spark plasma sintering (SPS) at 1850 °C. The effect of reinforcements (B 4 C and CNT) on the densification as well as mechanical properties were investigated and compared (with monolithic) in the present study. The study showed that the addition of B 4 C and CNT were not only beneficial for the densification but also towards enhancing the mechanical properties (hardness, elastic modulus, and fracture toughness) of HfB 2 -ZrB 2 -SiC composites. The augmentation in the mechanical properties establish the synergy between solid solution formation (with the equimolar composition of HfB 2 /ZrB 2) and the reinforcements (SiC, B 4 C, and CNT). The highest increase in the indentation fracture toughness with the reinforcements of B 4 C as well as CNT is >3 times (~13.8 MPam0.5 when it is 3–4 MPam0.5 for monolithic ZrB 2 /HfB 2) on HfB 2 -ZrB 2 -SiC composites, which is attributed to the crack deflection and pull-out mechanisms. An increase in the analytically quantified interfacial compressive residual stresses in the composites during SPS processing with the synergistic addition of reinforcements (SiC, B 4 C, and CNT) and its effect on the indentation fracture toughness has also been addressed. Graphical abstract Unlabelled Image Highlights • Processing of HfB2-ZrB2-SiC-based Ultra High Temperature Ceramics • Densification of HfB2-ZrB2-SiC with reinforcement of B4C and Carbon nanotube • Effect of solid solutioning on mechanical properties of HfB2-ZrB2-SiC composites • Estimating compressive residual stress generation in HfB2-ZrB2 system • Effect of residual stresses on fracture toughness of HfB2-ZrB2-SiC [ABSTRACT FROM AUTHOR]

Abstract: In order to address the problem of bacterial infections in bone-substitution surgery, it is essential that bone replacement biomaterials are equipped with bactericidal components. This research aims to optimize the content of silver (Ag), a well-known antibacterial metal, in a multiwalled carbon nanotube (CNT) reinforced hydroxyapatite (HA) composite, to yield a bioceramic which can be used as an antibacterial and tough surface of bone replacement prosthesis. The bactericidal properties evaluated using Escherichia coli and Staphylococcus epidermidis indicate that CNT reinforcement supports growth of Gram negative E. coli bacteria (~8.5% more adhesion than pure HA); but showed a strong decrease of Gram positive S. epidermidis bacteria (~diminished to 66%) compared to that of pure HA. Small amounts of silver (2–5wt.%) already show a severe bactericidal effect when compared to that of HA–CNT (by 30% and ~60% respectively). MTT assay confirmed enhanced biocompatibility of L929 cells on HA–4wt.% CNT (~121%), HA–4wt.% CNT–1wt.% Ag (~124%) sample and HA–4wt.% CNT–2wt.% Ag (~100%) when compared to that of pure HA. The samples with higher silver content showed decreased biocompatibility (77% for HA–4wt.% CNT–5wt.% Ag sample and 73% for HA–4wt.% CNT–10wt.% Ag). Though reinforcement of 4wt.% CNT has shown an increase of fracture toughness by ~62%, silver reinforcement has shown enhancement of up to 244% (i.e. 3.43 times). Accordingly, isolation of toughening contribution indicates that volumetric toughening by silver dominates over interfacial strengthening contributed by CNTs towards enhanced fracture toughness of potential HA–Ag–CNT biocomposites. [Copyright &y& Elsevier]

Abstract: Functionally Gradient Materials (FGM) are considered as a novel concept to implement graded functionality that otherwise cannot be achieved by conventional homogeneous materials. For biomedical applications, an ideal combination of bioactivity on the material surface as well as good physical property (strength/toughness/hardness) of the bulk is required in a designed FGM structure. In this perspective, the present work aims at providing a smooth gradation of functionality (enhanced toughening of the bulk, and retained biocompatibility of the surface) in a spark plasma processed hydroxyapatite-alumina-zirconia (HAp-Al2O3-YSZ) FGM bio-composite. In the current work HAp (fracture toughness ~1.5MPa.m1/2) and YSZ (fracture toughness ~6.2MPa.m1/2) are coupled with a transition layer of Al2O3 allowing minimum gradient of mechanical properties (especially the fracture toughness ~3.5MPa.m1/2). The in vitro cyto-compatibilty of HAp-Al2O3-YSZ FGM was evaluated using L929 fibroblast cells and Saos-2 Osteoblast cells for their adhesion and growth. From analysis of the cell viability data, it is evident that FGM supports good cell proliferation after 2, 3, 4days culture. The measured variation in hardness, fracture toughness and cellular adhesion across the cross section confirmed the smooth transition achieved for the FGM (HAp-Al2O3-YSZ) nanocomposite, i.e. enhanced bulk toughness combined with unrestricted surface bioactivity. Therefore, such designed biomaterials can serve as potential bone implants. [Copyright &y& Elsevier]

Abstract: In the present work, HA reinforced with Al2O3 and multiwalled carbon nanotubes (CNTs) is processed using spark plasma sintering (SPS). Vickers micro indentation and nanoindentation of the samples revealed contrary mechanical properties (hardness of 4.0, 6.1, and 4.4GPa of HA, HA–Al2O3 and HA–Al2O3–CNT samples at bulk scale, while that of 8.0, 9.0, and 7.0GPa respectively at nanoscale), owing to the difference in the interaction of the indenter with the material at two different length scales. The addition of Al2O3 reinforcement has been shown to enhance the indentation fracture toughness of HA matrix from 1.18MPa m1/2 to 2.07MPa m1/2. Further CNT reinforcement has increased the fracture toughness to 2.3 times (2.72MPa m1/2). In vitro biocompatibility of CNT reinforced HA–Al2O3 composite has been evaluated using MTT assay on mouse fibroblast L929 cell line. Cell adhesion and proliferation have been characterized using scanning electron microscopy (SEM), and have been quantified using UV spectrophotometer. The combination of cell viability data as well as microscopic observations of cultured surfaces suggests that SPS sintered HA–Al2O3–CNT composites exhibit the ability to promote cell adhesion and proliferation on their surface and prove to be promising new biocompatible materials. [Copyright &y& Elsevier]

Abstract: This study reports on the synthesis of novel bioceramic composite coating of hydroxyapatite (HA) reinforced with carbon nanotubes (CNTs) and aluminum oxide (Al2O3) using plasma spray technique. Fracture toughness of HA–20 wt.% Al2O3 improved by 158% as compared to HA coating whereas HA–18.4 wt.% Al2O3–1.6 wt.% CNT showed an improvement of 300%. Carbon nanotubes provided reinforcement via rebar mechanism. Human fiber osteoblast cell-growth studies showed that biocompatibility of the coating remained unaltered, as Al2O3 retained its bio-inertness and CNT, its bioactivity, within the composite coatings. Composite coating showed lower attachment, but higher proliferation rate, for the osteoblast cells, which has been attributed to the surface roughness. An optimized relation between coating composition, its biocompatibility and mechanical properties was established to predict the most suited coating material for orthopedic implants. HA–Al2O3–CNT composite coating displayed most improved mechanical properties while retaining its biocompatibility. [Copyright &y& Elsevier]

Tribaloy T400 (CoCrMoSi) caters to heavy-duty industrial tribological demands, but exhibits low fracture toughness with compromised resistance to crack propagation owing to the disparity in % Laves phases. In response to this limitation, hybrid suspension-powder plasma-sprayed novel Cr 3 C 2 (d 50 of 3.8 μm)/TiC (d 50 of 2.2 μm) reinforced T400 (average powder size of ~10–45 μm) coatings are deposited on grit-blasted SSAB Domex®350 LA steel. Continuous, adherent and co-existing lamellar T400-carbide coatings of ~100 μm thickness were revealed in microstructure analysis. Intermetallic CoMoSi/Co 3 Mo 2 Si Laves and eutectic Co 7 Mo 6 /Co 2 Mo 7 phases in T400 in addition to Cr 3 C 2 /TiC characteristic phases are confirmed via X-ray diffraction study. T400-Cr 3 C 2 and T400-TiC have exhibited enhancement in elastic modulus (E) by 39%, and 36%, Vickers hardness (H v) by 68%, and 82.5%; which consequently elucidates the augmentation in plasticity index (r e) by 15.7% and 26.7%, and the drop in maximum displacement amplitude (h max) by 14.9% and 19.8%, respectively, in T400-Cr 3 C 2 and T400-TiC with reference to T400 (E of 135.2 GPa, H v of 6.3 GPa, r e = 0.318, and h max = 1947 nm). A subsequent surmised fretting Hertzian contact diameter in T400-Cr 3 C 2 (~95.43 μm)/T400-TiC (~96.1 μm) evaluated against T400 (~106.9 μm) from optical profilometry indicates an improved damage tolerance. A contact area-based wear model, proposed herein to assess wear on a rough surface, further justifies the wear characteristics. Furthermore, synergistic L929 cell viability is recorded in T400-Cr 3 C 2 (by 46%) and T400-TiC (by 30%) when compared with the control (+ve) disk. To conclude, suspension-powder plasma-sprayed T400-Cr 3 C 2 /T400-TiC composite coatings allude potential application in wear-resistant articulating surfaces by eliciting significantly enhanced micro-hardness through refined microstructure retention, improved fretting wear resistance by forming protective tribofilm, and augmented cellular response. • Hybrid suspension/powder plasma spraying of Tribaloy T400 (CoCrMoSi alloy) • Tribaloy T400 reinforced with Cr 3 C 2 /TiC elicited enhanced hardness and elastic modulus. • Development of wear model accounting wear of rough surfaces during fretting • Lower Hertzian contact diameter (~95.4 μm) of T400-Cr 3 C 2 /T400-TiC than T400 (106.9 μm) • Cytocompatibility obtained for T400-Cr 3 C 2 /T400-TiC plasma-sprayed coatings [ABSTRACT FROM AUTHOR]