Effect of carbon resources on the synthesis of Ti3AlC2/Al2O3composite by pressureless sintering

Gengfu Liua,b, Yawei Lia,b

a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, Chinab National-Provincial Joint Engineering Research Center of High Temperature Materials and Lining Technology, Wuhan, 430081, China

IntroductionMAX phases are a family of ternary compounds with the general formula Mn+1AXn, where M represented an early transition metal, A represented an A group element, X represented C and/or N, and n=1–3. The MAX

phases are all layered hexagonal structures with space group P63/mmc (no. 194), consisting of alternate near-close-packed layers of M6X octahedra interleafed with layers of pure A atoms. Among the MAX phases reportedso far, Ti3AlC2 is the most light-weight and oxidation resistant. Due to the weak bonding between the Al atomic layer and Ti6C layer, Al atomic is preferred to migrate the sample surface for oxidation and forming a densealumina layer, which inhibits the further oxidation of the material. This oxidation behavior is also known as the self-healing of Ti3AlC2. Given the unique oxidation behavior and layered structure, Ti3AlC2 compared with Alwas introduced in Al2O3-C refractories in our previous work to overcome the evaporation of antioxidants related phases such as Al(g), Si(g), and SiO(g), which would deteriorate the properties of materials during high-temperature treatment/application. Consequently, the mechanical properties and oxidation resistance of Ti3AlC2 containing Al2O3-C refractories treated at 1600 ℃ were improved. Chen etl. also prepared Al2O3-Ti3AlC2-Crefractories by replacing graphite with Ti3AlC2 and found that the slag resistance of the materials was improved due to the formation of Ti-contained isolation layer. However, commercial Ti3AlC2 is expensive due to the highcost of elemental raw materials and assisted sintering method, which limit the large-scale application in refractories. Some studies have shown that Ti3AlC2-Al2O3 composites can be synthesized by the aluminothermicreaction of Al and TiO2. Besides, various carbon resources, such as graphite, active carbon, Al4C3 and TiC, have been reported different effect on the purity of final products. To compare the single element carbon resourcewith the multi element carbon resource, carbon black and TiC were selected as the carbon resource to synthesis Ti3AlC2-Al2O3 composite powders by aluminothermic reaction at atmospheric pressure.

ExperimentTwo synthesis routes were designed according to reaction (1) and (2) and named as CB (carbon black as carbon resource)

and TC (TiC as carbon resource) respectively. The raw materials were weighed according to the designed proportion, then wereball milled, dried and sieved. The mixed powder was pressed into Φ 20mm cylindrical pellets and fired in an argon atmospherefurnace at 1000 ~ 1400 ℃. According to DTA curves for raw material systems with different carbon sources (Fig. 1), there twomain endothermic peak and exothermal peak occurred during the reaction process. In order to study the effect of differentheating systems on the synthesis products, a steps heating system was applied, which the samples was dwelling at 700 ℃ and1000 ℃ for extra 1 hour before the final synthesis temperatures. The synthesized samples were named as CB-T and TC-Tcorrespondingly.

3𝑇𝑇𝑇𝑇𝑇𝑇2 𝑠𝑠 + 5𝐴𝐴𝐴𝐴(𝑠𝑠) + 2𝐶𝐶 𝑠𝑠 → 𝑇𝑇𝑇𝑇3𝐴𝐴𝐴𝐴𝐶𝐶2 𝑠𝑠 + 2𝐴𝐴𝐴𝐴2𝑇𝑇3 𝑠𝑠 (1)3𝑇𝑇𝑇𝑇𝑇𝑇2 𝑠𝑠 + 7𝐴𝐴𝐴𝐴(𝑠𝑠) + 6𝑇𝑇𝑇𝑇𝐶𝐶 𝑠𝑠 → 3𝑇𝑇𝑇𝑇3𝐴𝐴𝐴𝐴𝐶𝐶2 𝑠𝑠 + 2𝐴𝐴𝐴𝐴2𝑇𝑇3 𝑠𝑠 (2)

Results and discussionThe phase quantitative analysis results of CB and TC at 1400 ℃ with different heating systems were shown in Table 1. In the direct heating system, the relative content of Ti3AlC2 after alumina removal in sample TC was

much higher. However, in the steps heating system the relative content of Ti3AlC2 in sample CB was improved from to 78.6 wt%, even higher than that of TC as 76.4 wt%. From the XRD pattern of samples fired at differenttemperatures (Fig. 2), after extra dwelling at 700 ℃ and 1000 ℃, the formation of intermediate phases TiAl3 was promoted at 1000 ℃ and the content of Ti3AlC2 was increased significantly. The Ti3AlC2 morphologies ofsamples fired at 1400 ℃ was shown in Fig. 3 , well developed hexagonal lamellar Ti3AlC2 grains were found in sample BC while the lamellar Ti3AlC2 grains were observed grew insert from TiC particles in sample TC.

The SEM pictures, EDS element maps and EBSD phase maps of polished samples fired at 1400 ℃ were shown in Fig. 4, typical laminate Ti3AlC2 grains interlaced with residual TiC grain were found insample TC whileTi3AlC2 and residual TiC grains with smaller size and irregular sharp are observed in sample BC. According to the above results, the reaction pathway of Ti3AlC2-Al2O3 composites synthesized from TiO2-Al-C/TiC rawmaterial systems was shown in Fig. 5. Using nano carbon black as carbon source, due to the large content of TiO2, more Ti–Al melt is formed. As the Ti–Al melt spread out in the sample with increasing temperature, the nanocarbon black directly dissolved in Ti–Al melt due to its high reactivity. Then, the TiC began to precipitate when carbon is saturated in Ti–Al melt. With the temperature continue increasing, previously formed TiC crystallitesdissolved into the Ti–Al melt, and ternary phases Ti3AlC2/Ti2AlC began to precipitate from the melt when the temperature decreased. When TiC is used as carbon source, the continuous liquid phase cannot be formed in thesample with less Ti–Al melt formed in the system. However, the introduced TiC grains act as the crystal template for Ti3AlC2/Ti2AlC when contact with Ti–Al melt at even lower temperature (1000 ℃). With the increase oftemperature, more Ti3AlC2 grow and enlarged along the (111) plane of TiC through the diffusion of Al/Ti from Ti–Al melt, which was much slower than the liquid precipitation. The different reaction mechanisms alsoexplained the higher conversion efficiency of TiC to Ti3AlC2 in BC than TC.

ConclusionTi3AlC2/Al2O3 composite powders were synthesized by aluminothermic reaction with different carbon sources. The effects of carbon resources and heating systems on the Ti3AlC2 content of synthesized products were

investigated.(1) Compared to direct heating system, steps heating system promoted the formation of intermediate phases TiAl3 at 1000 ℃ and increased the content of Ti3AlC2 in final synthesis products at 1400 ℃.(2) Comparing the two raw materials systems with different carbon resources, Ti3AlC2 was formed at lower temperatures when using TiC as carbon source, due to the role of introduced TiC as crystal template for

Ti3AlC2/Ti2AlC. However, more Ti3AlC2 was formed with carbon black due to the larger content of Ti–Al melt and more efficient dissolution-precipitation mechanism at elevated temperatures.

0 200 400 600 800 1000 1200

CB TC

995 ℃

900 ℃

1017 ℃

657 ℃

907 ℃

0.1 uV/mg

Exo

Temperature/ ℃Fig. 1 DTA curves for raw material systems with different carbon sources from room temperature to 1200 ℃

SamplesPhase composition / wt%

Al2O3 Ti3AlC2 TiC

CB-1400 65.12 5.55(15.9) 29.34(84.1)

TC-1400 33.61 42.87(64.6) 23.52(35.4)

CB-T-1400 57.97 33.05(78.6) 8.98(21.4)

TC-T-1400 30.76 52.93(76.4) 16.31(23.6)

Table 1 Phase composite of samples fired at 1400 ℃

Fig. 2 XRD pattern of samples fired at different temperatures Fig. 3 SEM pictures of Ti3AlC2 morphologies of samples fired at 1400 ℃

Fig. 4 SEM pictures, EDS element maps and EBSD phase maps of polished samples fired at 1400 ℃ Fig. 5 Schematic diagram of aluminothermic reaction synthesis of Ti3AlC2