Synthesis of TiB₂/TiC/Al₂O₃ and ZrB₂/ZrC/Al₂O₃ Composites by Low-Exotherm Thermitic Combustion with PTFE Activation

Abstract

TiB₂–TiC–Al₂O₃ and ZrB₂–ZrC–Al₂O₃ composites were produced via PTFE (polytetrafluoroethene)-activated combustion synthesis involving low-exotherm thermites. The reactant stoichiometries were 3TiO₂ + 4Al + 0.5B₄C + (1 − x)C + xC_PTFE and 3ZrO₂ + 4Al + 0.5B₄C + (1 − y)C + yC_PTFE. PTFE played a dual role in promoting the reaction and carburizing reduced Ti and Zr. The threshold amount of PTFE for the TiO₂/Al-based reaction was 2 wt% (i.e., x = 0.15) and for the ZrO₂/Al-based reaction was 3 wt% (i.e., y = 0.25). The increase in PTFE increased the combustion front velocity and reaction temperature. The TiO₂/Al-based reaction was more exothermic than the ZrO₂/Al-based reaction and exhibited a faster combustion front and a lower activation energy. The TiB₂–TiC–Al₂O₃ composite was produced with the minimum amount of PTFE at x = 0.15. The formation of ZrB₂–ZrC–Al₂O₃ composites required more PTFE at y = 0.5 to improve the reduction of ZrO₂. Both triplex composites displayed mixed microstructures consisting of short-rod borides, fine spherical carbides, and Al₂O₃ agglomerates.

1. Introduction

The development of binary and ternary ceramic matrix composites has gained considerable attention because composite ceramics can overcome the intrinsic brittleness of monolithic ceramics. With a wide range of technological applications, TiB₂–TiC and ZrB₂–ZrC composites are the most studied boride–carbide composites of the transition metals [1,2,3,4,5]. They possess an exceptional combination of mechanical and physical properties, including high melting points, hardness, flexural strength, thermal shock resistance, chemical stability, and thermal and electrical conductivity, as well as good corrosion and oxidation resistance at high temperatures [4,5,6,7,8]. Additionally, their oxidation resistance, wear resistance, and fracture toughness can be reinforced by Al₂O₃ addition [9,10,11].

Conventional methods to fabricate multiphase ceramic composites include pressureless sintering, hot pressing, hot isostatic pressing, spark plasma sintering, and mechanochemical reaction [12,13,14,15,16,17]. As an alternative approach, combustion synthesis or self-propagating high-temperature synthesis (SHS) based on highly exothermic reactions has the advantages of high energy efficiency, a fast reaction rate, simplicity of operation, high-purity products, and in situ formation of composite components [18]. Once the thermite mixtures with strong exothermicity, for example Fe₂O₃/Al, MoO₃/Al, and WO₃/Al, are combined with the SHS reaction, thermitic combustion synthesis enhances the degree of reaction sustainability and provides an in situ fabrication route for various Al₂O₃-reinforced ceramics and intermetallics [19,20,21,22]. On the other hand, PTFE (polytetrafluoroethylene or Teflon) has often been employed as a reaction promoter for SHS reactions containing low-exotherm thermites such as Cr₂O₃/Al, SiO₂/Al, ZrO₂/Al, and ZrSiO₄/Al [23,24,25]. Abovyan et al. [24] indicated that the addition of PTFE led to the formation of gas-phase transport species in the SiO₂/Al/C combustion system and promoted the growth of SiC whiskers. The role of PTFE in transporting the reactant species by metal halides was verified in PTFE-activated combustion synthesis involving aluminothermic reduction of ZrSiO₄, SiO₂, and ZrO₂ [25].

The aim of this study was to fabricate triplex ceramic composites of TiB₂–TiC–Al₂O₃ and ZrB₂–ZrC–Al₂O₃ via low-exotherm thermitic combustion synthesis with PTFE activation. Reactant mixtures were formulated using two thermite reagents of TiO₂/Al and ZrO₂/Al, as well as B₄C, carbon black, and PTFE powders. In this study, the effects of PTFE were explored on the initiation of the SHS reaction, variations in combustion velocity and temperature, and evolution of the product phases. The activation energy of the combustion synthesis reaction was deduced from combustion wave kinetics. The composition and microstructure of as-synthesized products were analyzed.

2. Materials and Methods

The starting materials included TiO₂ (Strem Chemicals, Newburyport, MA, USA, <45 μm, 99.5%), ZrO₂ (Alfa Aesar, Ward Hill, MA, USA, <45 μm, 99.5%), Al (Showa Chemical, <45 μm, 99.9%), B₄C (Showa Chemical, Tokyo, Japan, <10 μm, 99.5%), carbon black (Showa Chemical, 20–40 nm, 99%), and PTFE (–(C₂F₄)_n–, Alfa Aesar, 6–10 μm). Reactant mixtures were prepared based on the stoichiometries of Reactions (R1) and (R2) for the production of TiB₂–TiC–Al₂O₃ and ZrB₂–ZrC–Al₂O₃ composites, respectively.

$$3\mathrm{T}\mathrm{i}{\mathrm{O}}_2+4\mathrm{A}\mathrm{l}+0.5{\mathrm{B}}_4\mathrm{C}+(1.5-\mathrm{x})\mathrm{C}+\mathrm{x}{\mathrm{C}}_{\mathrm{P}\mathrm{T}\mathrm{F}\mathrm{E}}\stackrel{-{({\mathrm{C}}_2{\mathrm{F}}_4)}_{\mathrm{n}}-}{\to }\mathrm{T}\mathrm{i}{\mathrm{B}}_2+2\mathrm{T}\mathrm{i}\mathrm{C}+2\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$$

(R1)

$$3\mathrm{Z}\mathrm{r}{\mathrm{O}}_2+4\mathrm{A}\mathrm{l}+0.5{\mathrm{B}}_4\mathrm{C}+(1.5-\mathrm{y})\mathrm{C}+\mathrm{y}{\mathrm{C}}_{\mathrm{P}\mathrm{T}\mathrm{F}\mathrm{E}}\stackrel{-{({\mathrm{C}}_2{\mathrm{F}}_4)}_{\mathrm{n}}-}{\to }\mathrm{Z}\mathrm{r}{\mathrm{B}}_2+2\mathrm{Z}\mathrm{r}\mathrm{C}+2\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$$

(R2)

where C_PTFE signifies carbon supplied from the decomposition of PTFE and is considered as a part of the total carbon balance; that is, PTFE was adopted not only to act as the reaction promoter, but also to participate in carburizing reduced Ti and Zr. The coefficients, x and y, denote the mole content of C_PTFE. In R1 and R2, the sources of carbon for the formation of metal carbides include B₄C, carbon black, and PTFE. For the production of metal borides, B₄C is the only source of boron. According to experimental observations in this study, the threshold amount of PTFE for R1 to be initiated was about 2 wt%, which was equivalent to x = 0.15, and that for R2 was 3 wt% and y = 0.25. To evaluate the effect of the PTFE content on the combustion behavior and product evolution, this study performed x in the range from 0.15 to 0.35 for R1 and y from 0.25 to 0.5 for R2.

The reactant powders were well mixed in a ball mill and then compressed into cylindrical samples measuring 7 mm in diameter, 12 mm in length, and 55% of the relative density. The SHS experiments were conducted in a windowed stainless-steel chamber filled with Ar at 0.3 MPa to ensure an inert environment. The propagation velocity of the combustion wave (V_f) was determined from the time sequence of recorded images. The combustion temperature was measured using a fine-wire thermocouple with an alloy combination of Pt/Pt-13%Rh (Omega Engineering Inc.; Norwalk, CT, USA). The wire diameter was 62.5 μm and the bead size was 125 μm. Details of the experimental setup are reported elsewhere [26]. Constituent phases of the synthesized product were identified by X-ray diffraction (XRD) patterns using an X-ray diffractometer (Bruker D2 Phaser; Billerica, MA, USA) with CuK_α radiation. Scanning electron microscopy (SEM, Hitachi, Tokyo, Japan) analysis was performed with energy-dispersive spectroscopy (EDS, Hitachi, Tokyo, Japan) to examine the microstructure and elemental composition on the fracture surface of the final product.

It has been empirically recognized by Merzhanov [27] that the adiabatic temperature (T_ad) of a solid-state combustion reaction higher than 1800 K is a criterion for establishing a self-sustaining combustion process. T_ad was calculated for PTFE-free R1 and R2 reactions via an energy balance equation [26] with thermochemical data taken from [28].

3. Results and Discussion

3.1. Thermodynamic Analysis

The aluminothermic reduction of metal oxides (TiO₂ and ZrO₂) is considered as the initiating step in R1 and R2 but the solid-state combustion of R1 and R2 cannot start off without the activation of PTFE. This could be due to the weak reaction exothermicity or the high kinetic barrier of the initiation reaction. As shown in Figure 1, the calculated values of the reaction enthalpy (ΔH_r) and T_ad are relatively low for the aluminothermic reduction of TiO₂ and ZrO₂, especially in the case of 3ZrO₂ + 4Al. Unlike most of the thermites with T_ad exceeding 3500 K [29], TiO₂/Al and ZrO₂/Al mixtures are low-exotherm thermites. Figure 1 also shows significant increases in ΔH_r and T_ad for R1 and R2, since their products, namely transition metal boride (TiB₂ and ZrB₂) and carbide (TiC and ZrC), are high-exotherm compounds. Moreover, TiO₂/Al-based R1 is more exothermic than ZrO₂/Al-based R2. Even although R1 and R2 have T_ad values of 2665 K and 2080 K, respectively, it was found that their initiation was infeasible without PTFE addition. As a reaction promoter, PTFE is able to facilitate solid-phase diffusion by means of the gas-phase transport agent to overcome the kinetic limitation and assist the synthesis reaction to proceed [30].

3.2. Self-Propagating Combustion Kinetics

With the addition of PTFE at x = 0.15 in R1 and y = 0.35 in R2, Figure 2a,b represent typical SHS processes, which illustrate that upon ignition, self-sustaining combustion is achieved and a distinct combustion wave traverses the powder compact. Because of the generation of gaseous species associated with PTFE, gas discharged from the powder compacted during combustion wave progression, leading to the formation of pores and cracks in the burned sample and an elongated final product.

The effect of the PTFE content on the flame-front propagation velocity of R1 and R2 is presented in Figure 3a. The increase in PTFE from x = 0.15 to 0.35 was found to accelerate the combustion wave of R1 from V_f = 2.68 to 4.17 mm/s. A slower combustion wave was observed for R2, most likely due to its weaker reaction exothermicity, with the wave speeds ranging from 1.74 to 2.95 mm/s for the PTFE content from y = 0.35 to 0.5. It has been proposed that gas-phase transport agents in the form of metal fluorides can effectively improve the solid-phase diffusion, which is critical for solid-state combustion to proceed in a self-sustaining manner [30]. Regarding metal fluorides, TiF₃ could be produced in R1 from the interaction of C₂F₄ with TiO₂ and ZrF₄ in R2 from C₂F₄ reacting with ZrO₂ [25,31].

Figure 3b depicts typical combustion temperature profiles of R1 and R2. The contours feature an abrupt rise to their peak points, followed almost immediately by a substantial decline due to heat losses to the surroundings. The sharp surge signifies the fast arrival of the combustion wave and the peak value represents the combustion front temperature (T_c). An increase in T_c with PTFE content was noticed for both R1 and R2. As indicated in Figure 3b, T_c is about 1222 °C for R1 with x = 0.15 and reaches 1356 °C as the PTFE content increases to x = 0.35. Due to the weaker reaction exothermicity, lower T_c values of about 1082 and 1202 °C were measured for R2 with PTFE at y = 0.35 and 0.50, respectively. The increase in T_c with increasing PTFE content meant a thermal assistance from PTFE, because gas-phase metal fluorides are highly exothermic intermediates [32]. For example, the formation enthalpies of TiF₃ and ZrF₄ are –1188.7 and –1673.6 kJ/mol, respectively [28]. As stated by Merzhanov [27], even a small amount of gaseous transport species can make a significant impact on the SHS process, leading to pronounced changes in the combustion propagation speed, reaction temperature, elongation of the product, and phase transformation from reactants to products.

The correlation of the combustion wave velocity with the reaction temperature was applied to determine the activation energy (E_a) of the solid-state combustion reaction from the slope of a best-fitted linear line in the plot of ln(V_f/T_c)² versus 1/T_c [33]. As presented in Figure 4, the values of E_a equal to 102.3 and 123.9 kJ/mol were deduced for R1 and R2, respectively. A lower E_a suggests that R1 is more favorable in the kinetic aspect. Thermally, R1 is also more energetic. This explained why the threshold amount of PTFE required to trigger the SHS process was lesser for R1 than R2.

The role of PTFE in activating the reaction is two-fold: it first has to overcome the kinetic limitation due mostly to the insufficient reactivity caused by the presence of diffusion barriers, and secondly it has to enhance the degree of reaction exothermicity. Once gas-phase metal fluorides, TiF₃ and ZrF₄, were generated, they diffused to B₄C or carbon particles and brought about condensation of Ti and Zr on the solid particles [31,34]. The deposition of metallic compounds on B₄C and carbon facilitated their interactions to produce high-exotherm metal borides and carbides; that is, the reaction assisted by the gas-phase transport mechanism is benefited in both kinetical and thermal aspects. As soon as the initiation step was triggered, aluminothermic reduction of TiO₂ and ZrO₂ took place and then reduced Ti and Zr, which reacted with B₄C and carbon.

The discharge of volatile metal fluorides into the air might cause water and soil pollution. This results in a number of problems in the environment, including decreased plant growth and crop yield. Harmful effects of fluoride on the health problems of animals have also been reported [35]. In this study, the exhaust gas was treated through a filter containing absorbent particles of synthetic aluminum oxide, which acted as the fluoride removal media [36].

3.3. Phase Composition and Microstructure of Synthesized Products

The products of SHS were crushed and ground into powders for the XRD analysis by using a porcelain mortar and pestle. The XRD patterns of SHS-derived products from R1 with PTFE values of x = 0.15 and 0.35 are presented in Figure 5a,b, respectively. Both samples produced triplex composites composed of TiB₂, TiC, and Al₂O₃. The crystal structures and powder diffraction data for TiB₂ refer to JCPDS (Joint Committee on Powder Diffraction Standards) card number 85-2083, TiC to JCPDS 89-3828, and Al₂O₃ to JCPDS 88-0826 [37]. No unreacted reagents or impurities were detected, even for the sample with the threshold amount of PTFE of x = 0.15; that is, the complete conversion from the reactants to products was achieved by R1 under PTFE activation.

Figure 6a,b contains a representation of the XRD patterns, which were collected for the zirconium-containing samples. Three target phases, ZrB₂ (JCPDS 89-3930), ZrC (JCPDS 89-4054), and Al₂O₃, were identified along with ZrO₂. The existence of ZrO₂ indicates incomplete aluminothermic reduction. It was found that the increase in PTFE from y = 0.35 (the threshold amount) to y = 0.5 decreased ZrO₂ considerably. According to Lee et al. [38], PTFE is able to remove the oxide shell on the surface of Al particles and to enhance the reactivity of Al. As a consequence, the reduction of ZrO₂ by Al was improved and the amount of ZrO₂ left in the final product was trivial, as shown in Figure 6b.

Figure 7 and Figure 8 present SEM images and EDS spectra of the fracture surfaces of the products synthesized from R1 with x = 0.25 and R2 with y = 0.5, respectively. As illustrated in Figure 7, the composite is composed of TiB₂, TiC, and Al₂O₃ with different morphologies, including short rods of TiB₂ (with length of 3–4 μm), small spheres of TiC (about 1–2 μm), and agglomerated grains of Al₂O₃ (about 5–7 μm). Because of the formation of gas-phase species during combustion, the discharge of gases created pores and cracks and caused delamination and elongation of the product. Therefore, the final products were porous, constituent grains were loosely bound, and composite powders were easy to obtain. The EDS spectrum of Figure 7 reveals the detection of all five elements from the SEM image. The quantitative elemental percentages are in reasonable agreement with the product composition of TiB₂ + 2TiC + 2Al₂O₃ in R1.

The SEM image in Figure 8 shows that the microstructure of the ZrB₂–ZrC–Al₂O₃ composite of R2 is similar to that of the TiB₂–TiC–Al₂O₃ composite of R1. Short-rod ZrB₂, fine spherical ZrC, and granular Al₂O₃ can be clearly observed in Figure 8. Additionally, the EDS analysis detected all five elements and provided an elemental ratio. The atomic percentages reported in Figure 8 are close to the composition of ZrB₂ + 2ZrC + 2Al₂O₃.

The grain sizes of SHS-derived products were reduced with different methods, such as by decreasing the combustion temperature; introducing modifiers; fast cooling of the product after the synthesis; and applying compaction, extrusion, shock waves, or ultrasonic waves in the process [39,40]. Hot pressing is typically applied for pressing and sintering of SHS-derived ceramic powders to obtain bulk ceramics in dense form and with a homogeneous microstructure [41]. Spark plasma sintering (SPS) is another method used to consolidate SHS-produced powders [42]. The HP approach is characterized by relatively long processing times (typically of the order of hours) because of the low heating rates. On the contrary, the SPS method involves a pulsed-current process in which the powder samples are loaded in an electrically conducting dye to accelerate the heating process and then sintered under uniaxial pressure.

4. Conclusions

Two triplex composites TiB₂–TiC–Al₂O₃ and ZrB₂–ZrC–Al₂O₃ were produced via PTFE-activated combustion synthesis involving the aluminothermic reduction of TiO₂ and ZrO₂. Reactant mixtures were prepared based on 3TiO₂ + 4Al + 0.5B₄C + (1 − x)C + xC_PTFE and 3ZrO₂ + 4Al + 0.5B₄C + (1 − y)C + yC_PTFE. Both reaction systems required the addition of PTFE to assist their initiation and the establishment of self-sustaining combustion. PTFE played a dual role in promoting the reaction and carburizing Ti and Zr. Experimental evidence indicated that the threshold amount of PTFE for the TiO₂/Al-based reaction was 2 wt% (i.e., x = 0.15) and for the ZrO₂/Al-based reaction was 3 wt% (i.e., y = 0.25). The increase in PTFE content accelerated the combustion wave and elevated the combustion temperature. The TiO₂/Al-based reaction was more exothermic than the ZrO₂/Al-based reaction and exhibited a faster combustion front. Based on the correlation of the combustion front velocity with temperature, E_a values of 102.3 and 123.9 kJ/mol were deduced for PTFE-activated TiO₂/Al- and ZrO₂/Al-based combustion reactions, respectively.

The XRD pattern confirmed that the TiB₂–TiC–Al₂O₃ composite was well produced from TiO₂/Al-based combustion with the minimum amount of PTFE added at x = 0.15. SEM graphs displayed a mixed microstructure consisting of TiB₂ in the form of short rods, TiC of micro-sized spheres, and Al₂O₃ agglomerates. However, the incomplete reduction of ZrO₂ was observed in the synthesis of ZrB₂–ZrC–Al₂O₃ composites. The reduction of ZrO₂ by Al was improved by increasing the PTFE, and the final product containing a negligible amount of ZrO₂ was obtained from the sample with PTFE of y = 0.5.