Oxidation resistance of Ti3AlC2 and Ti3Al0.8Sn0.2C2 MAX phases: A comparison

Ti₃AlC₂ and Ti₃Al_0.8Sn_0.2C₂ MAX phase powders are densified using Spark Plasma Sintering (SPS) technique to obtain dense bulk materials. Oxidation tests are then performed over the temperature range 800°C-1000°C under synthetic air on the two different materials in order to compare their oxidation resistance. It is demonstrated that, in the case of the Ti₃Al_0.8Sn_0.2C₂ solid solution, the oxide layers consist in TiO₂, Al₂O₃, and SnO₂. The presence of Sn atoms in the A planes of the solid solution leads to an easy diffusion of Sn out of the MAX phase which promote the formation of the nonprotective and fast growing SnO₂ oxide. Moreover, the small Al/Ti atom's ratio promotes the growth of a nonprotective rutile-TiO₂ scale as well. In the case of the Ti₃AlC₂ MAX phase, the oxide layer consists in a protective alumina scale; a few TiO₂ grains being observed on the top of the Al₂O₃ layer. The parabolic oxidation rate constants are about 3 orders of magnitude smaller for Ti₃AlC₂ compared to Ti₃Al_0.8Sn_0.2C₂.     

In situ synthesis,mechanical and cyclic oxidation properties of Ti3AlC2/Al2O3 composites

ABSTRACT

Ti₃AlC₂/Al₂O₃ composite materials were successfully fabricated from TiO₂/TiC/Ti/Al powders by the in situ reactive hot pressed technique. The microstructure, mechanical and oxidation properties of the composites were investigated in the paper. Vickers hardness increased with the Al₂O₃ content. The relative density of Ti₃AlC₂/Al₂O₃ composites exhibits a declining tendency with Al₂O₃ content especially exceeds 10 vol.?%. The Ti₃AlC₂/Al₂O₃ composites show excellent electrical conductivity. The flexural strength and fracture toughness of Ti₃AlC₂/10 vol. % Al₂O₃ are 461 ± 20?MPa and 6.2?±?0.2?MPa m^1/2, respectively. The cyclic oxidation behaviour of resistance of Ti₃AlC₂/10 vol. % Al₂O₃ composites at 800–1000°C generally obeys a parabolic law. The oxide scale of sample consists of a mass of α-Al₂O₃ and TiO₂, forming a dense and adhesive protect layer. The result indicates that the Al₂O₃ can greatly improve the oxidation resistance of Ti₃AlC₂.     

Effects of Al substitution with Si and Sn on tribological performance of Ti3AlC2

Since their discovery, the MAX phases have elicited engineering interest as potential choices for wear resistant parts. One such compound is Ti₃AlC₂ with nano-layered structure, low density (4.25 g/cm³), good oxidation resistant and self-lubrication properties. The purpose of this investigation was to evaluate the dominant effect of the A-site solid solution elements addition on dry sliding characteristics of Ti₃AlC₂ against 0.45% C steel (S45C) disk, a material which is widely used in wear-critical applications such as impeller, gear and axles. Dry sliding tribological behaviors of hot-pressed Ti₃Al_0.94C₂, Ti₃Al_0.78Sn_0.22C₂ and Ti₃Al_0.67Si_0.28C₂ solid solutions were conducted using a block-on-disk type tester at surface sliding speed range from 10 to 30 m/s and in the normal load range from 20 to 80 N. The results show that friction coefficient of Ti₃Al_0.94C₂ is higher than that of Ti₃Al_0.78Sn_0.22C₂, but lower than that of Ti₃Al_0.67Si_0.28C₂. However, the change in wear rate as a function of normal load for different sliding speed shows almost reversed trend. Difference in the phase composition of friction films were found to be responsible for observed change in tribological behaved after partial substitution of Al in Ti₃AlC₂ with Sn and Si. This study shows that friction coefficient of Ti₃AlC₂ can be adjusted from 0.2 to 0.38 by partially substituting Al with Si and/or Sn.     

Effect of Al2O3 on the microstructure and mechanical properties of Ti3AlC2/Al2O3 in situ composites synthesized by reactive hot pressing

Ti₃AlC₂/Al₂O₃ in situ composites with different Al₂O₃ contents were successfully synthesized from the powder mixture of Ti, TiC, Al and TiO₂ by a reactive hot-pressing method at 1350 °C. The effect of Al₂O₃ on the microstructure and mechanical properties of the composites was investigated in detail. The results indicate that the as-fabricated products mainly consist of Ti₃AlC₂, Al₂O₃ and a small amount of TiC. With increasing the Al₂O₃ content, the flexural strength of Ti₃AlC₂/Al₂O₃ composites increase gradually, the fracture toughness reaches the peak value of 8.21 MPa m^1/2 as the Al₂O₃ content increasing to 9 wt%, the hardness attains the maximum value of 10.16 GPa for 12 wt% Al₂O_3. The strengthening mechanism of the composites was also discussed.     

Optimized mechanical properties and oxidation resistance of low carbon Al2O3-C refractories through Ti3AlC2 addition

The mechanical properties and oxidation resistance of the Al₂O₃-C refractories are of critical importance for iron and steel making processes. However, the evaporation of antioxidants related phases such as Al(g), Si(g), and SiO(g) would deteriorate these properties, especially during high-temperature treatment/application. Therefore, in the present work, a small amount of Ti₃AlC₂ compared with Al was introduced to overcome these problems. The phase compositions, microstructures, mechanical properties, and oxidation resistance of Ti₃AlC₂ containing refractories were investigated. The partial oxidation of Ti₃AlC₂ led to inherited lamellar structures such as Ti₃Al_1-xC₂, TiC, and granular Al₂TiO₅ phases. The controlled oxidation of Ti₃AlC₂ and its volume expansion contributed to the compact-structure, thereby limiting the escape of Si and SiO vapors at high temperatures. Consequently, the mechanical properties and oxidation resistance of Ti₃AlC₂ containing Al₂O₃-C refractories treated at 1600 ℃ were improved.     

Crack Healing in Ti2Al0.5Sn0.5C–Al2O3 Composites

Oxidation induced crack healing of Al₂O₃ composites loaded with a MAX phase based repair filler (Ti₂Al_0.5Sn_0.5C) was examined. The fracture strength of 20 vol% repair filler loaded composites containing artificial indent cracks recovered fully to the level of the virgin material upon isothermal annealing in air atmosphere after 48 h at 700°C and 0.5 h at 900°C. SEM‐EBSD analysis of crack microstructure indicates two different oxidation reaction regimes to govern the crack filling: near the surface SnO₂, TiO₂, and Al₂O₃ were formed whereas deeply inside the cracks Al₂O₃ and TiO₂ and metallic Sn were detected. The presence of elemental Sn was attributed to partial oxidation of aluminum and titanium which lowered the local oxygen concentration below a threshold value required for Sn oxidation to SnO₂. Thus, Ti₂Al_0.5Sn_0.5C may represent an efficient repair filler system to trigger oxidation induced crack healing in ceramic composites at temperatures below 1000°C.     

Investigation of preparation and properties of Ti3SiC2/Al2O3 multilayered composites

Ti₃SiC₂/Al₂O₃ multilayered composites were prepared by the combination of tape casting and hot pressing sintering. The slurry was produced by adjusting the amounts of each organic material, including triethyl phosphate (TEP) as a dispersion, polyvinyl butyrate (PVB) as a binder, dioctyl phthalate (DOP) as a plasticizer, and anhydrous ethanol as an organic solvent. When TEP content was 3 wt.%, PVB content was 4.5 wt.%, R-value (DOP/PVB) was 1.4, and solid content was 38 wt.%; the cast film with a smooth surface, good flexibility, and uniform thickness was obtained after defoaming, tape casting, and drying. Three samples were prepared, namely, S1–S3. The S1 was monolithic Ti₃SiC₂/Al₂O₃ (mass ratio is 1:1) composites. S2 and S3 were Ti₃SiC₂/Al₂O₃ multilayered composites, which matrix layers were Ti₃SiC₂/Al₂O₃ composites (mass ratio is 1:1) and Al₂O₃, respectively, and their interface layer was Ti₃SiC₂. S1–S3 were also sintered at 1550°C. The bending strength of multilayered materials were lower than that of monolithic material, but the fracture toughness of multilayered materials significantly increased. Due to the introduction of Ti₃SiC₂ interface layer, the friction coefficient and wear rate of Ti₃SiC₂/Al₂O₃ multilayered composites were reduced by 30.7% and 33.8%, respectively, compared with monolithic material.     

Influence of Ti3AlC2 content and load on the tribological behaviors of Ti3AlC2p/Al composites

In this study, Ti₃AlC₂ particles doped aluminum matrix composites were prepared by ultrasonic agitation casting method. Microstructure, mechanical properties, and tribological properties of pure aluminum and Ti₃AlC_2p/Al composites were characterized. Influence of different loads (10, 20, 30, and 40 N) and Ti₃AlC₂ contents (1.0, 2.0, 3.0, and 4.0 wt%) on the tribological behaviors of the composites were studied. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy dispersion spectroscopy (EDS), and 3D laser confocal were used to assist the analysis. The results indicated that fine and uniformly microstructure and the optimum comprehensive mechanical properties were exhibited on 2.0 wt%-Ti₃AlC_2p/Al composites. The abrasive grooves were widened and deepened with an increase in the load. The abrasion performance of composites improved distinctly with the addition of the Ti₃AlC₂ particles, which changed the wear mechanism from adhesive wear to abrasive wear. The 30 N load and the composites of 2.0 wt% Ti₃AlC₂ revealed the optimum tribological properties. The improvement of the tribological behavior of composites was attributed to the refinement of microstructure, the improvement mechanical properties and the three dimensional layered Ti₃AlC₂ phases with self-lubricating properties.     

Preparation of Ti3AlC2 bulk ceramic via aqueous gelcasting followed by Al-rich pressureless sintering

Ti₃AlC₂ bulk ceramics were prepared via aqueous gelcasting followed by C-rich and Al-rich pressureless sintering. The optimized pH value, zeta potential, dispersant content, and solid loading content were determined to be 10, 72.6 mV, 1.6 wt%, and 52 vol%, respectively. Impurities at ppm level containing in the flowing argon could cause severe decomposition of gelcasted bulk Ti₃AlC₂, forming whiskers of Al₂OC and Al₄O₄C and floccule of AlN. C-rich pressureless sintering resulted in the delamination of a duplex layer of Ti(CO) and Ti₃(AlO)C_x-Ti(O,C). The channels formed after debinding facilitated the outward diffusion of Al and the inward diffusion of O and C, and thereby promoting the decomposition of C-rich sintered Ti₃AlC₂. The combined effect of the unclosed channels and the porous reaction Ti₃(AlO)C_x-Ti(O,C) layer brought a catastrophic reduction in the density and mechanical properties of the C-rich sintered Ti₃AlC₂ ceramic. While the Al-rich pressureless sintering system isolated C, CO and N₂ and supplied a closed Al-rich atmosphere, thereby suppressing the decomposition reactions and promoting the sintering densification and ultimately leading to the superior in mechanical properties. The density, hardness, flexural strength and fracture toughness of the Al-rich sintered ceramic reaches 4.13 g/cm³, 4.36 GPa, 345 MPa, 4.79 MPa m^1/2, respectively.     

Preceramic paper-derived Ti3Al(Si)C2-based composites obtained by spark plasma sintering

The paper describes the structure and properties of preceramic paper-derived Ti₃Al(Si)C₂-based composites fabricated by spark plasma sintering. The effect of sintering temperature and pressure on microstructure and mechanical properties of the composites was studied. The microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. It was found that at 1150 °C the sintering of materials with the MAX-phase content above 84 vol% leads to nearly dense composites. The partial decomposition of the Ti₃Al(Si)C₂ phase becomes stronger with the temperature increase from 1150 to 1350 °C. In this case, composite materials with more than 20 vol% of TiC were obtained. The paper-derived Ti₃Al(Si)C₂-based composites with the flexural strength > 900 MPa and fracture toughness of >5 MPa m^1/2 were sintered at 1150 °C. The high values of flexural strength were attributed to fine microstructure and strengthening effect by secondary TiC and Al₂O₃ phases. The flexural strength and fracture toughness decrease with increase of the sintering temperature that is caused by phase composition and porosity of the composites. The hardness of composites increases from ~9.7 GPa (at 1150 °C) to ~11.2 GPa (at 1350 °C) due to higher content of TiC and Al₂O₃ phases.     

Effect of Ge on microstructure and mechanical properties of Ti3SiC2/Al2O3 composites

Owing to the good physicochemical compatibility and complementary mechanical properties of Ti₃SiC₂ and Al₂O₃, Ti₃SiC₂/Al₂O₃ composites are considered as ideal structural materials. However, TiC and TiSi₂ typically coexist during the synthesis of Ti₃SiC₂/Al₂O₃ composites through an in-situ reaction, which adversely affects the mechanical properties of the resulting composites. In this study, Ti₃SiC₂/Al₂O₃ composites were prepared via in-situ hot pressing sintering at 1450 °C. Ge, which was used as a sintering aid, improved the purity and mechanical properties of the Ti₃SiC₂/Al₂O₃ composites. This is because Ge replaced some of the Si atoms to compensate the evaporation loss of Si to form Ti₃(Si_1-xGe_x)C₂, which showed a crystal structure similar to that of Ti₃SiC₂. Furthermore, the molten Ge accelerated the diffusion reaction of the raw materials, increasing the overall density of the Ti₃SiC₂/Al₂O₃ composites. The optimum Ge amount for improving the mechanical properties of the composites was found to be 0.3 mol. The flexural strength, fracture toughness, and microhardness of the composite with the optimum Ge amount were 640.2 MPa, 6.57 MPa m^1/2, and 16.21 GPa, respectively. The formation of Ti₃(Si_1-xGe_x)C₂ was confirmed by carrying out X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy analyses. A model crystal structure of Ti₃(Si_1-xGe_x)C₂ doped with 0.3 mol Ge was established by calculating the solid solubility of Ge.     

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.     

Fabrication and electromagnetic interference shielding effectiveness of Ti3Si(Al)C2 modified Al2O3/SiC composites

A dense alumina fiber reinforced silicon carbide matrix composites (Al₂O₃/SiC) modified with Ti₃Si(Al)C₂ were prepared by a joint process of chemical vapor infiltration, slurry infiltration and reactive melt infiltration. The conductive Ti₃Si(Al)C₂ phase introduced into the matrix modified the microstructure of Al₂O₃/SiC. The refined microstructure was composed of conductive phase, semiconductive phase and insulating phase, which led to admirable electromagnetic shielding properties. Electromagnetic interference shielding effectiveness (EMI SE) of Al₂O₃/SiC and Ti₃Si(Al)C₂ modified Al₂O₃/SiC were investigated over the frequency range of 8.2–12.4 GHz. The EMI SE of Al₂O₃/SiC-Ti₃Si(Al)C₂ exhibited a significant increase from 27.6 to 42.1 dB compared with that of Al₂O₃/SiC. The reflection and absorption shielding effectiveness increased simultaneously with the increase of the electrical conductivity.     

Microstructures and enhanced mechanical properties of (Al3Ti+Al2O3)/Al–Si composites with co-continuous network structure prepared by pressure infiltration

The (Al₃Ti + Al₂O₃)/Al–Si composites with three-dimensional co-continuous network structures are fabricated by a pore-forming agent and the pressure infiltration technique. The effect of the Al₃Ti content on the mechanical and wear properties of the developed composites is investigated. The Al₂O₃ (alumina) formation, fracture, and wear mechanisms of the composites are also analyzed. The results demonstrate that the granular Al₂O₃ particles scatter around Al₃Ti phases which are synthesized in-situ during the sintering process. The 20 vol.% (Al₃Ti + Al₂O₃)/Al–Si composites possess the optimal mechanical properties, i.e., compressive and flexural strength of 585 MPa and 489 MPa, respectively, which are 64.8% and 46.0% higher than those of the matrix. The specific wear rate of the composites (16.5 × 10^?14 m³/Nm) is 79% lower than that of the matrix. By further increasing the Al₃Ti content, the network structure is completed, the wear resistance properties are improved, while the mechanical properties are decreased. The enhanced mechanical properties can primarily be attributed to the three-dimensional co-continuous network structure of the Al₃Ti and Al₂O₃ phases, as well as the pinning effect of Al₂O₃ particles.     

Preparation and tribological behaviour of laminated Ti3AlC2 crystals as additive in base oil

Abstract

Laminated ternary compound Ti₃AlC₂ crystals were synthesised by pressureless sintering the mixture powders of 3Ti/1·1Al/1·8C, 3Ti/1Al/1·8C/0·2Sn, 1Ti/1·8TiC/1Al and 1Ti1·8TiC1Al0·1Sn at 1400°C with preliminary liquid magnetic stirring mixing. The X-ray diffraction results indicate that Ti₃AlC₂ prepared from 3Ti/1Al/1·8C/0·2Sn has the highest purity, and the addition of appropriate Sn favours the synthesis of high purity Ti₃AlC₂. Scanning electron microscopy images show that Ti₃AlC₂ samples exhibit lamellar-like microstructure with thickness of ～100 nm. The tribological properties of Ti₃AlC₂ as an additive in 100SN base oil were evaluated with a ball on disc tester. The results show that the Ti₃AlC₂ additives exhibited good friction reduction and wear resistance at 5 wt-% concentration. Under determinate conditions, the base oil containing 5 wt-% Ti₃AlC₂ samples presented good tribology performance under the load of 15 N. The improved tribological properties of the Ti₃AlC₂ samples could be attributed to the formation of tribofilm in friction process.     

Synthesis,microstructure and mechanical properties of Cr2AlC/Al2O3 in situ composites by reactive hot pressing

Al₂O₃ particle-reinforced Cr₂AlC in situ composites were successfully fabricated from powder mixtures of Cr₃C₂, Cr, Al, and Cr₂O₃ by a reactive hot-pressing method at 1400 °C. A possible synthesis mechanism was proposed to explain the formation of the composites in which Al₂O₃ was formed by the aluminothermic reaction between Al and Cr₂O₃, meanwhile, Cr₃C₂, Al, together with Cr reacted to form Cr₂AlC in a shortened reaction route. The effect of Al₂O₃ addition on the microstructure and mechanical properties of Cr₂AlC/Al₂O₃ composites was investigated. The results indicated that the as-sintered products consisted of Cr₂AlC matrix and Al₂O₃ reinforcement, and the in situ formed fine Al₂O₃ particles dispersed at the matrix grain boundaries. The flexural strength and Vickers hardness of the composites increased gradually with increasing Al₂O₃ content. But the fracture toughness peaked at 6.0 MPa m^1/2 when the Al₂O₃ content reached 11 vol.%. The strengthening and toughening mechanism was also discussed.     

High strengthening effects and excellent wear resistance of Ti3Al(Si)C2 solid solutions

The Ti₃Al_1.2−xSi_xC₂ (x = 0, 0.2, 0.4) powders were synthesized from Ti, Al, Si, and TiC powders, and nearly pure Ti₃Al_1.2−xSi_xC₂ bulks were fabricated by the means of two-time hot-pressing method. Significant strengthening effect in bulks was found after the addition of 0.2 Si and 0.4 Si to form Ti₃Al(Si)C₂ solid solutions. The flexural strengths of Ti₃AlSi_0.2C₂ and Ti₃Al_0.8Si_0.4C₂ were 485 and 554 MPa, 14% and 30% larger than the strength of Ti₃AlC₂, respectively. The Vickers hardness of these compounds were separately, 6.95 and 7.57 GPa, representing the enhancements of 37% and 49% over those of Ti₃AlC₂. The tribological behavior was studied by dry-sliding method with a S45C steel at the sliding speed of 30 m/s and the normal load of 20-80 N. The results showed that after incorporating different contents of Si, the friction coefficient was between 0.22 and 0.30, correspondingly lower wear rate was 3.19-2.61 × 10⁻⁶ mm³/Nm. These excellent tribological performances were attributed to the presence of continuous self-generated oxidized films during tribological examination. Finally, the phase compositions and microhardness of the oxidized films were analyzed and characterized.     

Synthesis and characterization of a new (Ti1-ε,Cuε)3(Al,Cu)C2 MAX phase solid solution

A new (Ti_1-εCu_ε)₃(Al,Cu)C₂ MAX phase solid solution has been synthesized by sintering at 760 °C compacted Ti₃AlC₂-40 vol.% Cu composite particles produced by mechanical milling. Using XRD and TEM-EDXS, it has been demonstrated that Cu can enter the crystallographic structure of the Ti₃AlC₂ MAX phase, whereas a Cu(Al,Ti) solid solution is also formed during thermal treatment. TEM-EELS analyses have demonstrated that Cu is mainly located on the A site of the MAX phase. The composition of the MAX phase solid solution, determined after selective chemical etching of the Cu(Al,Ti) matrix, by analyzing the filtrate and the solid phase using ICP-OES end EDXS methods respectively, is (Ti_0.93–0.97Cu_0.07–0.03)₃(Al_0.49–0.52Cu_0.51–0.48)C_2.     

Influence of Mo doping on the tribological behavior of Ti3AlC2 ceramic at different temperatures

(Ti_0·8Mo_0.2)₃AlC₂ solid solutions were successfully synthesized from Ti, Al, TiC, and Mo powders using the in situ hot-pressing sintering method. The tribological properties of (Ti_0·8Mo_0.2)₃AlC₂ and the reference Ti₃AlC₂ in the temperature range 25–800 °C were evaluated in ambient air with the counterpart of Al₂O₃ balls. The results show that (Ti_0·8Mo_0.2)₃AlC₂ has improved lubricating properties and wear resistance above 400 °C compared with Ti₃AlC₂. This can be contributed to the formation of tribo-oxidation films containing MoO₃ and MoO_3-x. Structural characterization of the tribo-oxidation films was conducted using SEM, EDS, Raman spectroscopy, and XPS to evaluate the effect of Mo doping on the wear mechanisms of Ti₃AlC₂ in detail.     

Pressureless Sintering and Properties of Ti3AlC2

Pressureless sintering of titanium aluminum carbide (Ti₃AlC₂) is difficult due to its easy decomposability at high temperatures, thus decomposition must be avoided during sintering. In this work, pressureless sintering was performed in different embedded powders and Al₄C₃ was found to be effective to inhibit Ti₃AlC₂ from decomposition due to the offering of Al rich ambience. High-density Ti₃AlC₂ was obtained by pressureless sintering in Al₄C₃ powder bed without additives. The good sinterability is due to the special crystal structure of Ti₃AlC₂ and the easy diffusion of Al atoms. The mechanical properties of pressureless sintered Ti₃AlC₂ are comparable to those of the hot-pressed ones.