On the potential of polyetheretherketone matrix composites reinforced with ternary nanolaminates for tribological and biomedical applications

We report the synthesis and characterization of PEEK-MAX (Ti₃SiC₂, Ti₃AlC₂, and Cr₂AlC), and PEEK-MoAlB composites by hot-pressing. Detailed microstructure analysis by scanning electron microscopy showed that Ti₃SiC₂ particles are well dispersed in the PEEK matrix after the addition of 5 vol% Ti₃SiC₂ but at higher concentration (≥10 vol%), the Ti₃SiC₂ particles segregated at the phase boundaries and formed interpenetrating micro-networks. PEEK-Ti₃AlC₂ and PEEK-MoAlB composites also showed similar structuring at the microstructural level. PEEK-Cr₂AlC composites showed a different behavior where Cr₂AlC particles were well dispersed in the PEEK matrix. All the three PEEK-MAX composites have lower hardness than PEEK-MoAlB composites as MoAlB particulates are appreciably harder than MAX phases but were harder than PEEK. Due to heterogenous nucleation, the addition of MAX phases or MoAlB reduced the crystallization temperature (T_c) by a few ^oC. The formation of imperfect crystals also resulted in the lowering of melting point (T_m) of these composites. PEEK reinforced with 10 vol% Ti₃SiC₂, Ti₃AlC₂ and MoAlB showed plastic failure, and had higher strength than PEEK. Comparatively, PEEK reinforced with 10 vol% Cr₂AlC did not show any enhancement. All the PEEK-MAX and PEEK-MoAlB composites showed triboactive behavior and enhanced wear resistance.     

A High‐Temperature Neutron Diffraction Study of Nb2AlC and TiNbAlC

Herein, we report on the crystal structures of Nb₂AlC and TiNbAlC—actual composition (Ti_0.45,Nb_0.55)₂AlC—compounds determined from Rietveld analysis of neutron diffraction patterns in the 300–1173 K temperature range. The average linear thermal expansion coefficients of a Nb₂AlC sample in the a and c directions are, respectively, 7.9(5) × 10^?6 and 7.7(5) × 10^?6 K^?1 on one neutron diffractometer and 7.3(3) × 10^?6 and 7.0(2) × 10^?6 K^?1 on a second diffractometer. The respective values for the (Ti_0.45,Nb_0.55)₂AlC composition—only tested on one diffractometer—are 8.5(3) × 10^?6 and 7.5(5) × 10^?6 K^?1. These values are relatively low compared to other MAX phases. Like other MAX phases, however, the atomic displacement parameters (APDs) show that the Al atoms vibrate with higher amplitudes than the Ti and C atoms, and more along the basal planes than normal to them. When the predictions of the APDs obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Al atoms. In case of the Nb and C atoms, the agreement was more qualitative.     

Oxidation and hot corrosion behaviors of MAX-phase Ti3SiC2, Ti2AlC,Cr2AlC

Oxidation and hot corrosion behaviours of Ti₃SiC₂, Ti₂AlC and Cr₂AlC at 750 °C were investigated in this work. Ti₃SiC₂ and Ti₂AlC showed a linear increase in mass gain and a relatively poor oxidation resistance. This might be attributed to the porous TiO₂ scale. A dense α-Al₂O₃ layer was formed during the oxidation test. Cr₂AlC exhibited the best oxidation resistance. This dense oxide scale can effectively isolate the substrate from contact with oxygen leading to excellent oxidation resistance. In contrast to the oxidation test, Ti₃SiC₂ and Ti₂AlC showed relatively better resistance to hot corrosion, while Cr₂AlC showed inferior resistance to NaCl introduced hot corrosion. The hot corrosion mechanism of the MAX phases was analyzed. Due to the formation of Na₂TiO₃, Ti containing MAX phases showed a continuous increase in the mass gain. The corrosion products of Cr₂AlC were Al₂O₃, Cr₂O₃ and Na₂CrO₄. However, due to the volatilization of Na₂CrO₄, Cr₂AlC showed a mass loss during the hot corrosion test. The chemical reaction process of the MAX phase was also analyzed.     

On the determination of the thermal shock parameter of MAX phases: A combined experimental-computational study

Thermal shock resistance is one of the performance-defining properties for applications where extreme temperature gradients are required. The thermal shock resistance of a material can be described by means of the thermal shock parameter R_T. Here, the thermo-mechanical properties required for the calculation of R_T are quantum-mechanically predicted, experimentally determined, and compared for Ti₃AlC₂ and Cr₂AlC MAX phases. The coatings are synthesized utilizing direct current magnetron sputtering without additional heating, followed by vacuum annealing. It is shown that the R_T of both Ti₃AlC₂ and Cr₂AlC obtained via simulations are in good agreement with the experimentally obtained ones. Comparing the MAX phase coatings, both experiments and simulations indicate superior thermal shock behavior of Ti₃AlC₂ compared to Cr₂AlC, attributed primarily to the larger linear coefficient of thermal expansion of Cr₂AlC. The results presented herein underline the potential of ab initio calculations for predicting the thermal shock behavior of ionically-covalently bonded materials.     

Nb doping in Ti3AlC2: Effects on phase stability,high-temperature compressive properties,and oxidation resistance

Nb-based ‘312’ MAX phase has not been recognized so far, raising a hypothesis that Nb doping would destabilize the isostructural Ti₃AlC₂. Here we report that (Ti_1−xNb_x)₃AlC₂ could persist with a doping limitation up to x = 0.15. As demonstrated by HAADF-STEM analysis, Nb dopants homogeneously distribute among polycrystalline grains at the microscale and randomly occupy the Ti sites at the atomic level. Beyond the limitation, Nb-doped ‘312’ phase Ti₃AlC₂ decomposes into (Ti,Nb)C, Nb-doped ‘211’ phase Ti₂AlC, and Nb-based ‘413’ phase. Compared to pristine Ti₃AlC₂, the compressive strength of (Ti_0.9Nb_0.1)₃AlC₂ at 1200 °C increases by 130%, whereas doping at this level impairs the oxidation resistance. Improving high-temperature strength without deteriorating oxidation resistance can be achieved by 5% Nb doping.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

Synthesis of the MAX Phases by Pulse Discharge Sintering

As an invited paper with the designated title to this focused session, this is to introduce the pulse discharge sintering process applied for the sintering and/or synthesis of a few representative MAX phases, such as Ti₃SiC₂, Ti₃AlC₂, Ti₂AlC, Ti₂AlN, Cr₂AlC, Cr₂GaC, Ti₂GaC, as well as the MAX-based solid solutions and some composites. The effects of processing parameters on the synthesis reactions, densification, microstructure, as well as the properties have been investigated. This paper not only briefly reviews our synthesis work on the MAX phases but also presents new results such as the comparison of PDS with conventional hot pressing, the newly obtained results on the synthesis of Cr₂GaC, Ti₂GaC, as well as some solid solutions and composites. With the results presented herein as summarized in tables and figures, none of which were published elsewhere, this article aims at a better understanding of the PDS process in the research and development of the MAX phases and related materials.     

Mechanical properties and toughening mechanisms of highly textured Ti3AlC2 composite material

Composite material consisting of Al₂O₃ and TiC in a matrix of highly textured Ti₃AlC₂ was fabricated in a two-step fabrication process. The Lotgering orientation factor for {00 l} planes of Ti₃AlC₂ in the textured top surface plane reached 0.71. Texture analysis showed an orientation relationship among Ti₃AlC₂, Al₂O₃ and TiC grains of [110] Ti₃AlC₂ // [110] TiC, (001) Ti₃AlC₂ // (111) TiC, and [110] Ti₃AlC₂ // [120] Al₂O₃, (001) Ti₃AlC₂ // (001) Al₂O₃. The texture grained material exhibited excellent mechanical properties, with compressive and flexural strengths of more than 2.5 times those of conventional coarse grained Ti₃AlC₂, and fracture toughness and hardness were 50% higher than those of conventional coarse grained Ti₃AlC₂. The microstructures of textured Ti₃AlC₂ and reported textured Ti₂AlC were investigated and compared to interpret the differences in mechanical behavior of the two textured MAX phases.     

An experimental approach to delineate the reaction mechanism of Ti3AlC2 MAX-Phase formation

A comprehensive reaction mechanism of Ti₃AlC₂ MAX-phase formation from its elemental powders while spark plasma sintering has been proposed. Microstructural evaluation revealed that Al-rich TiAl₃ intermetallic forms at around 660 °C once Al melts. Gradual transition from TiAl₃ to Ti-rich TiAl and Ti₃Al intermetallic phases occurs between 700 °C and 1200 °C through formation of layered structure due to diffusion of Al from periphery toward the centre of Ti particles. Formation of TiC and Ti₃AlC transient carbide phases were observed to occur through two different reactions beyond 1000 °C. Initially, TiC forms due to interaction of Ti and C, which further reacts with TiAl and Ti and gives rise to Ti₃AlC. Later, Ti₃AlC also forms due to diffusion of C into Ti₃Al above 1200 °C. Above 1300 °C, Ti₃AlC phase decomposes into Ti₂AlC MAX-phase and TiC in presence of unreacted C. Finally, Ti₂AlC and TiC reacts together to from Ti₃AlC₂ MAX-phase above 1350 °C and completes at 1500 °C.     

Ice-templated assembly strategy to construct oriented porous Ti3SiC2 ceramics for thermal management and electromagnetic interference shielding in harsh thermal environments

Given the electromagnetic interference (EMI) and heat aggregation issue faced by electronic components, an urgent need exists to integrate EMI shielding and thermal conductivity in one material. Herein, a novel lightweight porous Ti₃SiC₂ ceramic with ordered structural arrangement was fabricated by using budget-friendly raw materials through ice template design and in-situ reaction synthesis. Leveraging the excellent conductivity and thermal conductivity of Ti₃SiC₂, a dual-functional advanced material with efficient EMI shielding and thermal management capabilities was obtained. At room temperature, porous Ti₃SiC₂ ceramics can achieve a shielding effectiveness of 35.44 dB and a thermal conductivity of 12.17 W/mK, with performance that can be tuned by porosity. In further, the porous Ti₃SiC₂ ceramic can work stably in thermal environments from room temperature to 700 °C or in corrosive environments rich in acid, alkali, and salts due to its excellent high temperature oxidation resistance and corrosion resistance. In view of the dual-functional characteristics and the stability of operation in harsh thermal environments, ordered porous Ti₃SiC₂ ceramics are promising for modern maritime and aerospace applications.     

HiPIMS induced high-purity Ti3AlC2 MAX phase coating at low-temperature of 700 °C

Ti₃AlC₂, one of Ti-Al-C MAX phases, has received extensive attention due to its unique nano-laminated structure and combined properties of metals and ceramics. However, ultra-high synthesis temperature exceeding 800 °C is a critical challenge for broad application of Ti₃AlC₂ coatings on temperature-sensitive substrates. In this study, Ti-Al-C coatings were deposited on Ti-6Al-4V substrates using high-power impulse magnetron sputtering (HiPIMS) and DC sputtering (DCMS) for comparison. Different from as-deposited amorphous Ti-Al-C coating by DCMS, nanocrystalline TiAl_x compound was achieved by HiPIMS deposition due to highly ionized plasma flux with high kinetic energy. Furthermore, HiPIMS promoted the generation of dense and smooth Ti₃AlC₂ phase coating after low-temperature annealing at 700 °C, while annealed DCMS coating only obtained Ti₂AlC. In-situ XRD demonstrated such Ti₃AlC₂ phase could be early involved in crystallization at 450 °C, lowest than synthesis temperature ever reported. The mechanical properties of Ti₃AlC₂ coating were also discussed in terms of structural evolution.     

Infiltration behavior of Cu and Ti fillers into Ti2AlC/Ti3AlC2 composites during tungsten inert gas (TIG)brazing

Herein we study the infiltration behavior of Ti and Cu fillers into a Ti₂AlC/Ti₃AlC₂MAX phase composites using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti₂AlC/Ti₃AlC₂ comes into contact with molten Ti, it starts decomposing into TiC_x, a Ti-richandTi₃AlC; when in contact with molten Cu, the resulting phases are Ti₂Al(Cu)C, Cu(Al), AlCu₂Ti and TiC. In the presence of Cu at approximately 1630 °C, a defective Ti₂Al(Cu)C phase was formed having a P63/mmc structure. Ti₃AlC₂ MAX phase was completely decomposed in presence of Cu or Ti filler-materials. The decomposition of Ti₂AlC to Ti₃AlC₂ was observed in the heat-affected zone of the composite. Notably, no cracks were observed during TIG-brazing of Ti₂AlC/Ti₃AlC₂ composite with Ti or Cu filler materials.     

Synthesis and enhanced mechanical properties of compositionally complex MAX phases

A novel strategy of fabricating compositionally complex MAX phases was successfully developed. Multicomponent 413 MAX phase solid solutions (Ti_0.36Nb_0.27Ta_0.37)₄AlC_2.8 and (Ti_0.28Nb_0.26Ta_0.28V_0.18)₄AlC_2.9 simultaneously containing 3 and 4 transition-metal elements at M site were experimentally synthesized via hot pressing equimolar mixture of 211 type MAX phase powders. By elemental analysis and structural characterization, it can be verified that those uniform compositionally complex solid solutions can be obtained only in the presence of Cr₂AlC in raw powders. Those compositionally complex MAX phase solid solutions exhibit typical layered structures with distinct elongated grains. This discovery further enriches the MAX phases family and provide a new avenue for tailoring the properties of these materials. MAX phase composites containing around 87.5 vol.% (Ti_0.36Nb_0.27Ta_0.37)₄AlC_2.8 exhibit a high flexural strength of 720 MPa and a high fracture toughness of 9.5 MPa m^0.5.     

Determination of interdiffusion coefficients of Si and Al in Ti3SiC2–Ti3AlC2 diffusion couple at 1373-1673 K

In the diffusion couple of Ti₃SiC₂ and Ti₃AlC₂, only interdiffusion of Si and Al occurred during diffusion treatment process. Based on the concentration profiles of Si and Al measured by electron probe microanalysis (EPMA), the interdiffusion coefficients of Si and Al at 1373-1673 K in Ti₃SiC₂–Ti₃AlC₂ diffusion couple were determined by both the Boltzmann-Matano (B-M) method and the Saucer-Freise (S-F) method. At the position of Matano plane with the composition of Ti₃Al_0.5Si_0.5C₂, the interdiffusion coefficient could be expressed as D_int (m²/s) = 5.6 × 10⁻⁴⋅exp [−246 ± 14 (kJ/mol)/RT]. Based on the two methods, the calculated interdiffusion coefficients increased with increasing temperature, and the magnitudes of their absolute values were on the order of 10^–13-10^–11 m²/s at 1373-1673 K. At 1373-1573 K, the calculated interdiffusion coefficients decreased monotonously with the increase of Si concentration, that is, x_Si/(x_Al + x_Si). But at 1673 K, the variation trend of interdiffusion coefficients with x_Si/(x_Al + x_Si) was no longer monotonous, probably due to the presence of Ti₅Si₃ phase and voids on Ti₃AlC₂ side.     

Synthesis and Oxidation Testing of MAX Phase Composites in the Cr–Ti–Al–C Quaternary System

According to the properties determined for the ternary end‐members, MAX phases in the quaternary Cr–Ti–Al–C system could be of interest as protective coatings for nuclear fuel cladding in the case of severe accident conditions. In this study, syntheses of 211 and 312 MAX phase compositions were attempted using pressureless reactions starting from Cr, TiH₂, Al, and C (graphite) powders. It was observed that both the Ti substitution by Cr in Ti₃AlC₂ and the mutual solubility of Ti₂AlC and Cr₂AlC are limited to a few atomic percent. In addition, the remarkable stability of the (Cr_2/3Ti_1/3)₃AlC₂ MAX phase composition was confirmed. Due to the low miscibility of MAX phases in the Cr–Ti–Al–C system, most samples contained substantial amounts of TiC_x and Al–Cr alloys as secondary phases, thus forming composite materials. After sintering, all samples were submitted to a single oxidation test (12 h at 1400°C in air) to identify compositions potentially offering high‐temperature oxidation resistance and so warranting further investigation. In addition to (Cr_0.95Ti_0.05)₂AlC, composite samples containing substantial quantities of Al₈Cr₅ and AlCr₂ formed a stable and passivating Al₂O₃ scale, whereas the other samples were fully oxidized.     

Phase formation of Nb2AlC investigated by combinatorial thin film synthesis and ab initio calculations

The phase formation of Nb₂AlC was studied by combinatorial thin film synthesis and ab initio calculations. Thin films with lateral chemical composition gradients were synthesized by DC magnetron sputtering at substrate temperatures of 710–870 °C. The lowest formation temperature for Nb₂AlC is between 710 and 750 °C. A predominantly single phase Nb₂AlC region where 99% of the X-ray diffraction intensity originate from Nb₂AlC was identified. Furthermore, selected area electron diffraction analysis reveals the local formation of single phase Nb₂AlC. The limited Al solubility in Nb₂AlC compared with Cr₂AlC can be understood by comparing the defect formation energy of Al substituting Nb and Cr in Nb₂AlC and Cr₂AlC, respectively. This methodology may serve as indicator for the magnitude of the A-element homogeneity range in M_n+1AX_n phases. The structural and elastic properties of Nb₂AlC determined experimentally are in very good agreement with the ab initio calculated data.     

Molten salt synthesis and formation mechanism of Ti3AlC2: A new path from Ti2AlC to Ti3AlC2

Fine, pure Ti₃AlC₂ powder is prepared in a very mild condition via Ti₃Al alloy and carbon black with the assistance of molten salts. X-ray diffraction, scanning electron microscopy, TG-DSC, and transmission electron microscopy (TEM) characterizations show that the high purity, nanosized Ti₃AlC₂ can be obtained at 900°C with the 1:1 salt-to-material ratio. The formation mechanism of Ti₃AlC₂ through this strategy of alloy raw material is fully studied under further TEM investigations, showing that the reaction process can basically be described as Ti₃Al and C → TiAl and TiC → Ti₂AlC and TiC → ψ and TiC → Ti₅Al₂C₃ and TiC → Ti₃AlC₂, where the key ψ, a modulated Ti₂AlC structure, is determined for the first time containing alternate-displacement Al layers along (0 0 0 2) of Ti₂AlC phase with a distinct selected area electron diffraction pattern. Such alternant displacement is considered a precondition of forming Ti₅Al₂C₃ through topotactic transition, followed by Ti₅Al₂C₃ converting into Ti₃AlC₂ by the diffusion of Ti, C atoms in the outside TiC. Several parallel orientations can be observed through the phase transition process: Ti₂AlC (0 0 0 2)//ψ (0 0 0 1), ψ (0 0 0 1)//Ti₅Al₂C₃ (0 0 0 3), Ti₅Al₂C₃ (0 0 0 3)//Ti₃AlC₂ (0 0 0 2). Such parallel orientations among these phases apply an ideal condition for the topotactic reaction. The distinct path of the phase transition brings a significant change of heat effect compared with the traditional method, leading to a fast reaction rate and a mild reaction condition.     

Molten salt dynamic sealing synthesis of MAX phases (Ti3AlC2, Ti3SiC2 et al.) powder in air

Since the synthesis of non-oxidized ceramic and alloy powders requires both high temperature and oxygen insulation conditions, here we demonstrate a cost-efficient molten salt sealing/shielded synthesis method with dynamic gas tightness. Compared to conventional synthesis method, it can prevent the loss of reaction materials at high temperature, cut off the connection between reacting material and outside air, and does not require long-time ball milling mixing treatment or provision of applied pressing before or during heating. Only low-cost salts (e.g., NaCl, KCl), a few minutes of raw material mixing, and regular heating molds are required to obtain high-purity (>96 wt%), micron-sized Ti₃AlC₂ and Ti₃SiC₂ powders with narrow size distribution, which significantly decreased the complexity and production costs in the synthesis process. The effect of temperature and raw material content on the products were investigated. The mechanism of diffusion reaction between reactants in molten salt environment was analyzed. The new method developed here was also applicable to Ti₂AlC, V₂AlC and Cr₂AlC MAX phases, as well as provided new ideas for the preparation of other MXenes precursors with certain stoichiometric ratios, air-sensitive materials and nanopowders.     

Point contact abrasive wear behavior of MAX phase materials

The room temperature abrasive wear behavior of three selected MAX phases, Ti₃SiC₂, solution strengthened Ti_2.7Zr_0.3SiC₂ and Cr₂AlC, is investigated by low velocity scratch testing using a diamond conical indentor with a final radius of 100 μm and a cone angle of 120° and applied loads of up to 20 N. All three materials showed a relatively low wear resistance in comparison to most engineering ceramics such as Al₂O_3, Si₃N₄ and SiC. For all three materials, the wear rate scaled more or less linearly with the applied load. The softer Ti₃SiC₂ with a hardness of 2.8 GPa showed the lowest wear resistance with extensive ploughing and grain breakout damage, both within and outside the direct wear track, in particular at the highest load. The hardest material, Ti_2.7Zr_0.3SiC₂, with a hardness of 7.3 GPa, showed a 5 times better wear resistance. The Cr₂AlC with a hardness of 4.8 GPa showed a wear resistance equal to or even better than that of the Ti_2.7Zr_0.3SiC₂. The wear mechanism depends on the applied load and the microstructure of the MAX phase materials tested. For the Ti₃SiC₂ sample, a quasi-plastic deformation behavior occurs below a point load of 10 N, resulting in grain bending, kink band formation and delamination, grain de-cohesion, as well as trans-and intra-granular fracture near the scratch groove. At this load, the Ti_2.7Zr_0.3SiC₂ and Cr₂AlC MAX samples display plastic ploughing, grain boundary cracks and material dislodgments.     

Anisotropic corrosion of Ti2AlC and Ti3AlC2 in supercritical water at 500 ºC

Textured and untextured M_n+1AX_n compounds, Ti₂AlC and Ti₃AlC₂, namely MAX phases have been synthesized and examined with respect to their corrosion resistance in static supercritical water at 500 °C. The textured ceramics were obtained by hot forging process at high temperatures. Both X-ray diffraction and SEM analysis revealed well alignment of c-plane of MAX phases parallel to the hot-forging surface. Better corrosion resistance on the surface perpendicular to the hot-forged direction was verified by SEM. On the other hand, the side surfaces of the samples showed thick oxidation layers and abundant cracks. The (00l) faces consist of strongly bonded Ti₃C₂ and Ti₂C layers in Ti₃AlC₂ and Ti₂AlC, respectively, hence exhibit higher resistance to water corrosion. On the contrary, the side surfaces where most of weakly bonded interlayers of these hexagonal phases were exposed tend to be easily corroded especially through Al-layers. The corrosion process involved a phase transition of oxidized product, i.e. TiO₂ from anatase to rutile phase, which gave rise to the formation of cracks due to accompanied volume changes.