# The Influence of Sintering Pressure on the Preparation, Friction Properties, and Magnetic Properties of Ti2AlC-TiC and Ti3AlC2-TiC Composites Under High-Pressure and High-Temperature

Authors: Liang Li; Yuqi Chen
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Due to their high melting point, low density, and affordable price, TiC ceramics have emerged as one of the most promising ultrahigh temperature structural materials. However, its development and usage are severely constrained by its weak mechanical characteristics. The Ti2AlC-TiC and Ti3AlC2-TiC composites were synthesized by using the laminated metal-ceramic Ti2AlC/Ti3AlC2 as the binder, combined with high-temperature and high-pressure sintering technology (GPa order of magnitude confinement system). The thermal analysis shows that the exothermic heat of Ti/1.2Al/2TiC is less than that of 3Ti/1.2Al/2C, 2Ti/1.2Al/C, and 3Ti/1.5Al/C. The ratio of Ti/1.2Al/2TiC can suppress the thermal explosion phenomenon during the sintering process and improve the safety of the high-pressure sintering process. XRD results showed that the main composition of the sintered products was Ti2AlC-TiC at 2∼3.5 GPa and Ti3AlC2-TiC at 4∼5 GPa. As the sintering pressure increased from 2 GPa to 5 GPa, the bonding phase changed from Ti2AlC to Ti3AlC2. The thickness of the synthetic Ti3AlC2 layer is near 0.1 µm under 5 GPa. The average friction coefficient of Ti2AlC-TiC composites sintered 2 GPa with POM is as low as 0.1767, while the average friction coefficient of Ti3AlC2-TiC composites with POM increased to 0.3468∼0.3797. The wear width of Ti2AlC-TiC is lower than that of Ti3AlC2-TiC composites sintered under 4–4.5 GPa. The wear width of Ti3AlC2-TiC composites decreases following the rise of sintering pressure and is reduced to 0.3504 mm under 5 GPa. Ti2AlC-TiC and Ti3AlC2-TiC composites with PP ball counterparts have friction coefficients of 0.375 to 0.45 and worse wear resistance than that of POM balls. Ti3AlC2-TiC composites synthesized at 5 GPa with a Cu ball had the smallest friction coefficient (0.4558) and the smallest wear scar width (0.6289 mm). The friction coefficients and wear widths of synthetics with Al pairs ranged from 0.48–0.697 and 0.851–1.087 mm, respectively, and varied irregularly with increasing sintering pressure. Ti2AlC-TiC composites synthesized at 2 GPa with agate and glass pairs have coefficients of friction of 0.5 and 0.61, respectively. Abrasion widths of Ti2AlC-TiC composites synthesized at 2 GPa with agate and glass pairs were 1.3 mm and 0.72 mm, respectively. The friction coefficient of synthetics with agate and glass pairs increases with increasing sintering pressure. The magnetization and magnetization intensity of the Ti3AlC2-TiC composites are stronger than those of Ti3AlC2. Ti2AlC-TiC and Ti3AlC2-TiC composites held promise to serve in the field of ceramic grinding processing applications.


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## Body

## 1. Introduction

Titanium carbide (TiC) ceramics with covalent bonding possess a high melting point, high hardness, low density, good thermal conductivity, and noticeable wear and corrosion resistance [1]. However, due to its strong covalent bonding and relatively low diffusion coefficients, it is often difficult to manufacture dense TiC ceramics at low sintering temperatures of around 1800°C or tiny applied pressures of around 40 MPa [2]. Therefore, the application of monolithic TiC is greatly restricted by its intrinsic low-toughness.Ternary carbide Ti2AlC/Ti3AlC2 phases display a remarkable mix of the favorable qualities of ceramics and metals [3–5]. A planar Al slab is sandwiched between two Ti-C octahedral slabs, which make up its hexagonal crystal structure [6]. Ti2AlC/Ti3AlC2 has a low friction coefficient and great self-lubricating capacity due to the strong covalent Ti-C bond [6]. Meanwhile, Ti2AlC/Ti3AlC2 has excellent oxidation resistance by generating an adhesive continuous Al2O3 layer [7]. As a result, it is a prospective candidate as a ceramic binder in TiC-based ceramics [8].Ti2AlC-TiC and Ti3AlC2-TiC composite layers have been synthesized using a variety of techniques so far. The Ti2AlC-TiC composite layer was formed on the surface of die-casting aluminum alloys to improve surface hardness [9]. The thickness of the Ti2AlC-TiC coating was controllable, which may give effective protection to the carbon fiber under different conditions [10]. The shape, crystalline structure, and microstructure of Ti2AlC-TiC coatings were studied following the change of oxidation temperature and the adhesion between Ti2AlC-TiC coating and substrate is excellent [11].Dense Ti2AlC-TiC-Ti3Al compacts were prepared by spark plasma sintering (SPS) and the relationships of direction between the Ti2AlC and the TiC were confirmed [12]. The Ti2AlC-TiAl composites and TiC-TiAl composites with different consolidation paths were synthesized by the metallurgical method and tiny TiC particles are observed in Ti2AlC grains with uneven forms [13]. In the prior research, high-pressure during sintering was helpful to synthesize the dense MAX solids [14]. The highest pressure of the afore-mentioned methods is ∼200 MPa in the hot isostatic pressing (HIP, ∼200 MPa pressure). Ti3AlC2-TiC has been synthesized under the pressure of 0.5–2 GPa and a temperature of 800–1200°C [15]. Higher pressures of up to 2 GPa can promote the formation of Ti2AlC and increase Vickers microhardness.In the previous research on the synthesis of Ti3SiC2-cBN composites by high-temperature and high-pressure sintering, hot-pressure sintering, and atmospheric pressure sintering [16]. It was observed that the sintering pressure is an important factor affecting the composition, structure, and properties of the composites. In general, material synthesis is more often studied in terms of raw material ratios, temperature, and time. The effect of pressure on the properties of Ti2AlC-TiC and Ti3AlC2-TiC composites is less well reported. Therefore, the synthesis of TiC-based composites using high temperature and high-pressure technology is also meaningful for the innovative application.In this study, we investigated the fabrication of the Ti2AlC-TiC and Ti3AlC2-TiC composites at 950°C under 2∼5 GPa using high-pressure and high-temperature technology. The influence of sintering pressure on the composition of sintering products was studied. We also compared the stability of Ti2AlC-TiC and Ti3AlC2-TiC composites under high-pressure. The friction properties of high-pressure synthetic composites paired with POM, PP, Al, Cu, agate, and glass under normal temperature dry friction conditions were investigated. The magnetic properties of Ti2AlC-TiC and Ti3AlC2-TiC composites were characterized over the 4–300 K temperature range and magnetic fields up to 7 T.


## 2. Experimental Procedures

A V-type blinder was employed to mix the starting powders of Ti (325 mesh, 99.3 wt. % pure, Jinzhou Haotian Titanium Co., China), Al (200 mesh, 99.99 wt. % pure, Henan Yuanyang Aluminum Industry Co., China), and TiC (325 mesh, 99.3 wt. % pure, Aladdin Reagent Co., China) for 4 hours. The mixes were then cold pressed into a disc with a diameter of 10 mm and a height of 4 mm. At 950°C and 2–5 GPa, the samples were synthesized in a cubic anvil high-pressure device (SPD 6 1200). The increase in temperature and pressure took around 3 minutes. They were all kept for 60 minutes. After that, the heating power was turned off, and the pressure was removed after around three minutes.The mixed starting powders were analyzed by differential scanning calorimetry (DSC, Thermogravimetry and Differential Thermal Analyzer, Evolution24, Setaram, France). Scanning electron microscopy (SEM, JSM-6390LV, JEOL, Japan) was used to investigate the fracture surfaces of the samples obtained. The samples were finely powdered for X-ray diffraction (XRD; Bruker AXS Co., Germany). XRD patterns were obtained by using an aD8ADVANCE X-ray diffractometer based on Cu Kα radiation.The friction and wear tests of sintered synthetic bulks were performed on the pin-on-disk type of the CFT-I material surface performance comprehensive tester (Zhongke Kaihua Instrument Equipment Co., Ltd.). The surface of the synthetic sample was treated with sandpaper before the test, and the surface roughness of the treated sample was about 0.1 mm. The synthetic sample was fixed on hollow disc bolts in compression. The counter-abrasives were POM, PP, Al, Cu, agate, and glass balls of Φ 4 mm. The test balls and samples were ultrasonically cleaned with alcohol and then experimented with different conditions. The dry friction reciprocating sliding test was carried out at room temperature with a sliding distance of 5 mm, the drive motor speed of 300 rpm/min, the load of 12 N, and a test time of 30 min, and the dynamic real-time friction coefficient was automatically recorded by the computer during the test. A SQUID VSM (SQUID, Quantum Design) magnetometer was used to detect magnetization as a function of temperature (2–300 K) and magnetic field (between 0 and 7 T).


## 3. Results and Discussions

### 3.1. Thermodynamic Analysis

Figure1 shows the thermal analysis (DSC) of four kinds of starting materials. The ratio of 2Ti/1.2Al/C and 3Ti/1.5Al/C is employed commonly to fabricate single-phase Ti2AlC. As shown in Figure 1, the endothermic peak at ∼660°C was caused by the melt of Al. Compared with the mixture of 2Ti/1.2Al/C, the ratio of 3Ti/1.5Al/C had two strong exothermic peaks at about 945°C and 1100°C, which corresponded to the synthetic reactions of TiC and Ti2AlC, respectively. The value of 3Ti/1.2Al/2C is bigger than that of Ti/1.2Al/2TiC in all ranges of 400 ∼1300°C because both the formation of TiAlx [17–20]or TiC [21, 22] is an exothermic reaction. Gao et al. have systematically investigated the TiAlx generation process in the systems Ti-Al-C and Ti-Mn-Al-C and confirmed the composition of the phases at different temperature points and discuss the formation mechanism of TiAlx in Figure 5 of reference [20]. Professor Yanchun Zhou et al. showed that TiCx was formed at 950°C by DSC analysis (Figure 1 of the reference [22]) which can be seen from XRD shown in Figure 2(d) of reference [22]. Therefore, the ratio of Ti/1.2Al/2TiC is better for the fabrication of Ti2AlC/Ti3AlC2 to avoid thermal exploration during sintering.Figure 1
DTA curves of four kinds of starting materials.Figure 2
XRD patterns of synthetic samples synthesized under 2∼5 GPa and 950°C for 60 min.


### 3.2. Composition and Microstructure Characterization of Synthetic Composites

XRD patterns of synthesized samples from Ti/1.2Al/2TiC synthesized under 2∼5 GPa and 950°C for 60 min are shown in Figure2. The composition of the synthetic bulk presents a symmetrical distribution. The inlayer is Ti2AlC, while the main phase translated from Ti2AlC to Ti3AlC2, and formed a TiC phase lastly on the surface of the specimen following the Al atom layer running off. From Figure 3, three samples have Ti2AlC and Ti3AlC2 peaks under 2∼3.5 GPa and 4∼5 GPa, respectively. This is true that the functional gradient material Ti2AlC/Ti3AlC2 can be synthesized at around 950°C. However, there are also strong TiC peaks in all synthetic compacts. The Ti2AlC phase changed into Ti3AlC2 with the increase of external pressure. It is similar to our previous results with pressure-less sintering [23].The influences of sintering pressure on the laminar structure of Ti2AlC/Ti3AlC2 are shown in Figure 3(a) via SEM observation. The typical layer structure of Ti2AlC with Ti/1.2Al/2TiC composition sintered at 2 GPa. With the increase of sintering pressure, the microscopic lamellar structure tends to be densified along with the change of chemical composition (Figure 2). Ti3AlC2 grains are lamellar structures in the vertical direction. Ti3AlC2 grains are tightly bonded to TiC under 4 GPa for 60 min (Figure 3(b)). Figure 3(c) shows the overall morphology of the fracture surface of compact sintered at 5 GPa for 60 min. From Figure 3(c), the average grain diameter is about 8 ∼ 10 μm. According to the XRD results (Figure 2), Ti3AlC2 surely exists. Figure 3(d) is an image with a high magnification of Ti3AlC2. The thickness of the Ti3AlC2 layer is near 0.1 um. Figure 3(e) is the formation process of Ti3AlC2 crystal nucleation, which will be discussed in detail about the formation mechanism of Ti3AlC2 in Figure 4.Figure 3
SEM micrographs of the layered structure of Ti2AlC/Ti3AlC2. (a) Synthetics prepared under 4 GPa (b) and 5 GPa (c) high magnification of Ti3AlC2 (d) interface of Ti3AlC2 and TiC prepared under 5 GPa (e).
(a)(b)(c)(d)(e)Figure 4
Stable area of Ti2AlC or Ti3AlC2 composites (a); possible mechanism of the enhancement of Ti2AlC or Ti3AlC2 stability (b)
(a)(b)


### 3.3. Friction Properties of Ti2AlC-TiC and Ti3AlC2-TiC Composites

The effects of the sintering pressure of Ti2AlC-TiC and Ti3AlC2-TiC composites on the friction and wear properties of POM were investigated. The average friction coefficient of Ti2AlC-TiC composites sintered at 2 GPa is 0.1767, and the wear area of the POM ball is 0.46 mm2. Combined with XRD results in Figure 2, the main components are Ti2AlC and TiC. The average friction coefficient of Ti2AlC by hot pressing sintering is about 0.65 under a contact load of 5–15 N and a sliding velocity of 5–15 mm/s [24]. The friction coefficient of Ti2AlC/TiAl composites by hot pressing sintering is 0.45–0.58 [25]. It shows that the increase in sintering pressure can inhibit the grain growth, increase the number of grain boundaries, and reduce the friction coefficient and wear amount.When the sintering pressure is added to 4–4.5 GPa, the main component is transformed into Ti3AlC2 and TiC. The average friction coefficients are 0.3624 (4 GPa) and 0.3468 (4.5 GPa), respectively. The friction coefficient of Ti3AlC2-TiC composites changes less as the test time increases. The effect of Ti3AlC2 reduced the friction coefficient of the TiC/Ti3AlC2-Co cermet coatings by producing more TiC and Al2O3 hard phases [6]. However, the wear area is significantly increased compared to the Ti2AlC-TiC bulk sintered at 2 GPa. The average friction coefficient of Ti3AlC2-TiC composites sintered at 5 GPa is 0.3797, and the wear area of POM balls is 2.02 mm2. The average friction coefficient of Ti3AlC2-TiC composites is similar to that of POM sliding on carbon steel, at about 0.41 under a contact load of 98 N and a sliding velocity of 0.15 m/s [26]. With the increase of sintering pressure, the friction coefficient first increases and then decreases, and the wear loss increases first and then decreases.Figure5(b) presents the wear width of Ti2AlC-TiC and Ti3AlC2-TiC composites sliding on a POM ball. Ti2AlC-TiC composites exhibited a lower friction coefficient and average width than those of Ti3AlC2-TiC composites sintered under 4 GPa and 4.5 GPa. There are almost no evident changes in the friction coefficient of Ti3AlC2-TiC composites. The wear width of Ti3AlC2-TiC composites decreases following the rise of sintering pressure. The wear width of the POM ball first increases and then lessens with the rise of sintering pressure, and the highest wear width of the POM ball was obtained for the sintered under 4.5 GPa.Figure 5
Friction coefficient (a) and wear properties (b) of synthetics and POM pairs; friction coefficient (c) and wear properties (d) of synthetics and PP counterparts.
(a)(b)(c)(d)Figure5(c) depicts the friction and wear properties of Ti2AlC-TiC and Ti3AlC2-TiC composites and their PP ball counterparts. The friction coefficient first increases and then decreases with the increase of sintering pressure, and the friction coefficient reaches its maximum value at 4.5 GPa. Ti2AlC-TiC and Ti3AlC2-TiC composites have friction coefficients of 0.375 to 0.45. The effect of sintering pressure on the friction coefficient is not instantly obvious. The friction coefficient of 45 steel and PP friction pair is 0.467 under the contact load of load 200N and a speed of 200 r/min [27]. The wear width of synthetic and PP balls both increased with the increase of pressure as shown in Figure 5(d). There are minor scratches and adhesive wear on the wear-scarred surface of the PP balls, resulting in large differences in the amount of wear. Compared with the POM ball, the PP ball has poor wear resistance under the same working conditions due to its weak flexural and tensile strength, stiffness, and hardness. Ramesh et al. have tested the polyoxymethylene (POM) and polypropylene (PP) combinations on a pin-on-disk setup, alternating static and rotating elements at ambient temperature analyzed regarding the effects of load and velocity [28].Figure6(a) shows the friction coefficient between Ti2AlC-TiC and Ti3AlC2-TiC composites and their Cu counterparts. The friction coefficient of the synthetics increased sharply at the beginning of the experiment, indicating that a complete and stable lubricating film has not been formed between the friction pairs. The friction coefficient first increased and then decreased with the increase in sintering pressure. The samples synthesized under 2–4.5 GPa had a short running-in time, and the friction coefficient rose to roughly 0.7 before becoming steady. The friction coefficient of Cu–Ti3AlC2 composites decreases due to a lubricating film formed on the worn surface, and the wear rate initially decreases and then increases with increasing Ti3AlC2 content [29].Figure 6
Friction coefficient (a) and wear properties (b) of synthetics and Cu pairs; friction coefficient (c) and wear properties (d) of synthetics and Al counterparts.
(a)(b)(c)(d)The rising time of the Ti3AlC2-TiC composites sintered at 5 GPa was longer, and the friction coefficient and wear width decreased, which was presumed to be related to the change of the main component of the material and the growth of grains [14]. Abrasive wear, minor scratches, and tiny adhesive wear produced by wear and particle shedding are among the wear processes between Ti2AlC-TiC and Ti3AlC2-TiC composites and their Cu counterparts. The wear widths of the copper balls were similar, indicating that the sintering pressure and component fluctuations had little effect on the wear loss of the Cu ball, as shown in Figure 6(b).Figure6(c) shows the variation of the friction coefficient between synthetic and Al counterparts. With the increase of sintering pressure, the friction coefficient first decreased, then increased, and then decreased again. The reason for the friction coefficient decreasing finally is not fully understood but two major reasons might be attributed to the fluctuations in chemical composition and the influence of microstructural changes due to the elevated sintering pressure. The friction coefficient is between 0.5 and 0.7 (Figure 6(d)). Compared with the Cu counterpart, the friction coefficient is smaller, but there are partial adhesion marks on the surface of the Al counterpart. Ti3AlC2 displayed poor tribological properties when sliding against Al2O3 [30].Figure7(a) depicts the fluctuation in the friction coefficient of Ti2AlC-TiC and Ti3AlC2-TiC composites sliding with the agate ball. With the increase of sintering pressure, the friction coefficient of synthetics and their agate counterparts first increases and then decreases. The abrasion width is larger for both the specimen and the agate ball (Figure 7(b)), presumably due to the occurrence of abrasive wear between the hard ceramic particles of the agate ball and the high-pressure sintered Ti3AlC2 or TiC particles. Similar behavior was confirmed for Ti3AlC2 composites with Al2O3, Si3N4, and SiC balls tested in dry air in reciprocating mode [31].Figure 7
Friction coefficient (a) and wear properties (b) of synthetics and agate pairs; friction coefficient (c) and wear properties (d) of synthetics and glass counterparts.
(a)(b)(c)(d)Figure7(c) depicts the change in friction coefficient of the Ti2AlC-TiC and Ti3AlC2-TiC composites and the agate ball counterpart. With the increase of sintering pressure, the friction coefficient shows a gradually increasing variation trend. The friction coefficient of Ti3AlC2-TiC composites was similar to the friction coefficient of glass during the polishing process, in which the friction coefficient and load meet the positive ratio coefficient of 0.906 [32]. The friction coefficient of the test material of 2–4.5Gpa is relatively small, and the width of the test material abrasion marks is also much lower than that of the agate ball (Figure 7(d)).


### 3.4. Magnetic Properties of Ti3AlC2-TiC Composites

There is a controversy about the magnetic properties of Ti3AlC2, and to compare Ti3AlC2, the magnetic properties of the Ti3AlC2-TiC composites were studied. A SQUID VSM was used to measure the magnetic moment followed by the applied magnetic field and temperature in a 1T applied magnetic field to see how the amount of etching influenced the magnetic properties. Figure 8 illustrates the Ti3AlC2-TiC composites’ field-dependent magnetization at 2–300 K. The magnetization at 2 K and 7 T was 0.2017 emu/g for the Ti3AlC2-TiC composites but reduced to 0.1758 emu/g at 300 K. Although the magnetization curve did not saturate at 7T, the larger magnetization in Ti3AlC2-TiC composites proposed that it was ferromagnetic, which was similar to the reported Ti3AlC2 [33]. Moreover, the magnetization of Ti3AlC2-TiC composites sintered under high pressure is larger than that of commercial Ti3AlC2 powders (particle size >200 mesh) with a purity of 98%.There is a debate about whether Ti3AlC2 is ferromagnetic or paramagnetic. It has been reported that Ti3AlC2 is paramagnetic [34]. Scheibe et al. noted that EPR spectra of their Ti3AlC2 sample exhibit a weak peak at 0.3–0.35 T (Figure 6 in reference [34]). In our data, changes in magnetization intensity of Ti3AlC2-TiC composites were found at 2 K in the 0.5-1 T magnetic field, but no fluctuations were captured from 10–300 K. The low-field signal associated with Ti3+ ions is weaker, but still noticeable in Ti2AlC and Ti3AlC2 spectra [34].In accordance with the M vs. H data (Figure8), the M (T) behavior of Ti3AlC2-TiC composites at 1 T is shown in Figure 9, which reveals that it was essentially ferromagnetic. Ti3AlC2-TiC composites have a lower magnetic intensity than single-phase Ti3AlC2. Migration and escape of Al atomic layers in synthetic Ti3AlC2 by thermal diffusion during high-pressure sintering [35], which led to the vanishingly intense (104) Ti3AlC2 X-ray reflection at 39° in Figure 2. This process is similar to the preparation of Ti3C2 by HF etching of Ti3AlC2. The comparison of the magnetic intensity of Ti3AlC2 and Ti3C2 at 2–300 K suggested that the presence of the Al atom layer might impact magnetic behavior and/or a magnetic phase transition [34].Figure 8
Magnetization versus applied magnetic field for Ti3AlC2-TiC composites at 2–300 K (a); local enlargement of magnetization curve at 2 K (b).Figure 9
Temperature-dependent magnetization of Ti3AlC2-TiC composites.


### 3.5. Possible Mechanism of the Preparation of Ti2AlC or Ti3AlC2 Composites

Ti3AlC2 solids were synthesized under different sintering pressures. To manufacture solid Ti3AlC2, a pressure of 40 MPa is needed in the hot press [36]. SPS (50–8 MPa pressure) [37] and HIP [38] apply higher pressure to improve the properties of Ti3AlC2. The lattice parameters of Ti2AlC ( a-direction and c-direction) were studied under 50 GPa using a synchrotron radiation source, and they decreased with increasing applied pressure (Figure 2 in reference [39]).Ti3AlC2 was discovered in the Ti-Al-C ternary system [40]. Starostina et al. [15] had synthesized Ti2AlC-based composites from 3Ti/1.2Al/2C at 800–1200°C under 1–2 GPa. Earlier, the authors had successfully fabricated Ti3SiC2-cBN composites [16], and Ti3AlC2-cBN composites [14] under 4.5 GPa by HPHT. Li et al. fabricated Ti3AlC2-cBN [41] composites at 1350°C and Ti3AlC2-Al-cBN [42] composites at 1200–1500°C under 5.5 GPa. However, Qin et al. [35] reported that the raw materials of pure Ti2AlC decomposed at above 1300°C and 3 GPa. It needs to indicate that the second phase (such as diamond, cBN, and TiC) can improve the stability of Ti2AlC or Ti3AlC2 as shown in Figure 4(a).The possible mechanism of stability enhancement is shown in Figure4(b). In the second phase, TiC divides the binder Ti2AlC or Ti3AlC2 into individually sealed units. The thermal diffusion of the Al atomic layer of Ti2AlC or Ti3AlC2 is suppressed under high pressure. Thus, the stability of Ti2AlC or Ti3AlC2 in the Ti2AlC-based or Ti3AlC2-based composite is enhanced relative to that of pure Ti2AlC or Ti3AlC2.


## 3.1. Thermodynamic Analysis

Figure1 shows the thermal analysis (DSC) of four kinds of starting materials. The ratio of 2Ti/1.2Al/C and 3Ti/1.5Al/C is employed commonly to fabricate single-phase Ti2AlC. As shown in Figure 1, the endothermic peak at ∼660°C was caused by the melt of Al. Compared with the mixture of 2Ti/1.2Al/C, the ratio of 3Ti/1.5Al/C had two strong exothermic peaks at about 945°C and 1100°C, which corresponded to the synthetic reactions of TiC and Ti2AlC, respectively. The value of 3Ti/1.2Al/2C is bigger than that of Ti/1.2Al/2TiC in all ranges of 400 ∼1300°C because both the formation of TiAlx [17–20]or TiC [21, 22] is an exothermic reaction. Gao et al. have systematically investigated the TiAlx generation process in the systems Ti-Al-C and Ti-Mn-Al-C and confirmed the composition of the phases at different temperature points and discuss the formation mechanism of TiAlx in Figure 5 of reference [20]. Professor Yanchun Zhou et al. showed that TiCx was formed at 950°C by DSC analysis (Figure 1 of the reference [22]) which can be seen from XRD shown in Figure 2(d) of reference [22]. Therefore, the ratio of Ti/1.2Al/2TiC is better for the fabrication of Ti2AlC/Ti3AlC2 to avoid thermal exploration during sintering.Figure 1
DTA curves of four kinds of starting materials.Figure 2
XRD patterns of synthetic samples synthesized under 2∼5 GPa and 950°C for 60 min.


## 3.2. Composition and Microstructure Characterization of Synthetic Composites

XRD patterns of synthesized samples from Ti/1.2Al/2TiC synthesized under 2∼5 GPa and 950°C for 60 min are shown in Figure2. The composition of the synthetic bulk presents a symmetrical distribution. The inlayer is Ti2AlC, while the main phase translated from Ti2AlC to Ti3AlC2, and formed a TiC phase lastly on the surface of the specimen following the Al atom layer running off. From Figure 3, three samples have Ti2AlC and Ti3AlC2 peaks under 2∼3.5 GPa and 4∼5 GPa, respectively. This is true that the functional gradient material Ti2AlC/Ti3AlC2 can be synthesized at around 950°C. However, there are also strong TiC peaks in all synthetic compacts. The Ti2AlC phase changed into Ti3AlC2 with the increase of external pressure. It is similar to our previous results with pressure-less sintering [23].The influences of sintering pressure on the laminar structure of Ti2AlC/Ti3AlC2 are shown in Figure 3(a) via SEM observation. The typical layer structure of Ti2AlC with Ti/1.2Al/2TiC composition sintered at 2 GPa. With the increase of sintering pressure, the microscopic lamellar structure tends to be densified along with the change of chemical composition (Figure 2). Ti3AlC2 grains are lamellar structures in the vertical direction. Ti3AlC2 grains are tightly bonded to TiC under 4 GPa for 60 min (Figure 3(b)). Figure 3(c) shows the overall morphology of the fracture surface of compact sintered at 5 GPa for 60 min. From Figure 3(c), the average grain diameter is about 8 ∼ 10 μm. According to the XRD results (Figure 2), Ti3AlC2 surely exists. Figure 3(d) is an image with a high magnification of Ti3AlC2. The thickness of the Ti3AlC2 layer is near 0.1 um. Figure 3(e) is the formation process of Ti3AlC2 crystal nucleation, which will be discussed in detail about the formation mechanism of Ti3AlC2 in Figure 4.Figure 3
SEM micrographs of the layered structure of Ti2AlC/Ti3AlC2. (a) Synthetics prepared under 4 GPa (b) and 5 GPa (c) high magnification of Ti3AlC2 (d) interface of Ti3AlC2 and TiC prepared under 5 GPa (e).
(a)(b)(c)(d)(e)Figure 4
Stable area of Ti2AlC or Ti3AlC2 composites (a); possible mechanism of the enhancement of Ti2AlC or Ti3AlC2 stability (b)
(a)(b)


## 3.3. Friction Properties of Ti2AlC-TiC and Ti3AlC2-TiC Composites

The effects of the sintering pressure of Ti2AlC-TiC and Ti3AlC2-TiC composites on the friction and wear properties of POM were investigated. The average friction coefficient of Ti2AlC-TiC composites sintered at 2 GPa is 0.1767, and the wear area of the POM ball is 0.46 mm2. Combined with XRD results in Figure 2, the main components are Ti2AlC and TiC. The average friction coefficient of Ti2AlC by hot pressing sintering is about 0.65 under a contact load of 5–15 N and a sliding velocity of 5–15 mm/s [24]. The friction coefficient of Ti2AlC/TiAl composites by hot pressing sintering is 0.45–0.58 [25]. It shows that the increase in sintering pressure can inhibit the grain growth, increase the number of grain boundaries, and reduce the friction coefficient and wear amount.When the sintering pressure is added to 4–4.5 GPa, the main component is transformed into Ti3AlC2 and TiC. The average friction coefficients are 0.3624 (4 GPa) and 0.3468 (4.5 GPa), respectively. The friction coefficient of Ti3AlC2-TiC composites changes less as the test time increases. The effect of Ti3AlC2 reduced the friction coefficient of the TiC/Ti3AlC2-Co cermet coatings by producing more TiC and Al2O3 hard phases [6]. However, the wear area is significantly increased compared to the Ti2AlC-TiC bulk sintered at 2 GPa. The average friction coefficient of Ti3AlC2-TiC composites sintered at 5 GPa is 0.3797, and the wear area of POM balls is 2.02 mm2. The average friction coefficient of Ti3AlC2-TiC composites is similar to that of POM sliding on carbon steel, at about 0.41 under a contact load of 98 N and a sliding velocity of 0.15 m/s [26]. With the increase of sintering pressure, the friction coefficient first increases and then decreases, and the wear loss increases first and then decreases.Figure5(b) presents the wear width of Ti2AlC-TiC and Ti3AlC2-TiC composites sliding on a POM ball. Ti2AlC-TiC composites exhibited a lower friction coefficient and average width than those of Ti3AlC2-TiC composites sintered under 4 GPa and 4.5 GPa. There are almost no evident changes in the friction coefficient of Ti3AlC2-TiC composites. The wear width of Ti3AlC2-TiC composites decreases following the rise of sintering pressure. The wear width of the POM ball first increases and then lessens with the rise of sintering pressure, and the highest wear width of the POM ball was obtained for the sintered under 4.5 GPa.Figure 5
Friction coefficient (a) and wear properties (b) of synthetics and POM pairs; friction coefficient (c) and wear properties (d) of synthetics and PP counterparts.
(a)(b)(c)(d)Figure5(c) depicts the friction and wear properties of Ti2AlC-TiC and Ti3AlC2-TiC composites and their PP ball counterparts. The friction coefficient first increases and then decreases with the increase of sintering pressure, and the friction coefficient reaches its maximum value at 4.5 GPa. Ti2AlC-TiC and Ti3AlC2-TiC composites have friction coefficients of 0.375 to 0.45. The effect of sintering pressure on the friction coefficient is not instantly obvious. The friction coefficient of 45 steel and PP friction pair is 0.467 under the contact load of load 200N and a speed of 200 r/min [27]. The wear width of synthetic and PP balls both increased with the increase of pressure as shown in Figure 5(d). There are minor scratches and adhesive wear on the wear-scarred surface of the PP balls, resulting in large differences in the amount of wear. Compared with the POM ball, the PP ball has poor wear resistance under the same working conditions due to its weak flexural and tensile strength, stiffness, and hardness. Ramesh et al. have tested the polyoxymethylene (POM) and polypropylene (PP) combinations on a pin-on-disk setup, alternating static and rotating elements at ambient temperature analyzed regarding the effects of load and velocity [28].Figure6(a) shows the friction coefficient between Ti2AlC-TiC and Ti3AlC2-TiC composites and their Cu counterparts. The friction coefficient of the synthetics increased sharply at the beginning of the experiment, indicating that a complete and stable lubricating film has not been formed between the friction pairs. The friction coefficient first increased and then decreased with the increase in sintering pressure. The samples synthesized under 2–4.5 GPa had a short running-in time, and the friction coefficient rose to roughly 0.7 before becoming steady. The friction coefficient of Cu–Ti3AlC2 composites decreases due to a lubricating film formed on the worn surface, and the wear rate initially decreases and then increases with increasing Ti3AlC2 content [29].Figure 6
Friction coefficient (a) and wear properties (b) of synthetics and Cu pairs; friction coefficient (c) and wear properties (d) of synthetics and Al counterparts.
(a)(b)(c)(d)The rising time of the Ti3AlC2-TiC composites sintered at 5 GPa was longer, and the friction coefficient and wear width decreased, which was presumed to be related to the change of the main component of the material and the growth of grains [14]. Abrasive wear, minor scratches, and tiny adhesive wear produced by wear and particle shedding are among the wear processes between Ti2AlC-TiC and Ti3AlC2-TiC composites and their Cu counterparts. The wear widths of the copper balls were similar, indicating that the sintering pressure and component fluctuations had little effect on the wear loss of the Cu ball, as shown in Figure 6(b).Figure6(c) shows the variation of the friction coefficient between synthetic and Al counterparts. With the increase of sintering pressure, the friction coefficient first decreased, then increased, and then decreased again. The reason for the friction coefficient decreasing finally is not fully understood but two major reasons might be attributed to the fluctuations in chemical composition and the influence of microstructural changes due to the elevated sintering pressure. The friction coefficient is between 0.5 and 0.7 (Figure 6(d)). Compared with the Cu counterpart, the friction coefficient is smaller, but there are partial adhesion marks on the surface of the Al counterpart. Ti3AlC2 displayed poor tribological properties when sliding against Al2O3 [30].Figure7(a) depicts the fluctuation in the friction coefficient of Ti2AlC-TiC and Ti3AlC2-TiC composites sliding with the agate ball. With the increase of sintering pressure, the friction coefficient of synthetics and their agate counterparts first increases and then decreases. The abrasion width is larger for both the specimen and the agate ball (Figure 7(b)), presumably due to the occurrence of abrasive wear between the hard ceramic particles of the agate ball and the high-pressure sintered Ti3AlC2 or TiC particles. Similar behavior was confirmed for Ti3AlC2 composites with Al2O3, Si3N4, and SiC balls tested in dry air in reciprocating mode [31].Figure 7
Friction coefficient (a) and wear properties (b) of synthetics and agate pairs; friction coefficient (c) and wear properties (d) of synthetics and glass counterparts.
(a)(b)(c)(d)Figure7(c) depicts the change in friction coefficient of the Ti2AlC-TiC and Ti3AlC2-TiC composites and the agate ball counterpart. With the increase of sintering pressure, the friction coefficient shows a gradually increasing variation trend. The friction coefficient of Ti3AlC2-TiC composites was similar to the friction coefficient of glass during the polishing process, in which the friction coefficient and load meet the positive ratio coefficient of 0.906 [32]. The friction coefficient of the test material of 2–4.5Gpa is relatively small, and the width of the test material abrasion marks is also much lower than that of the agate ball (Figure 7(d)).


## 3.4. Magnetic Properties of Ti3AlC2-TiC Composites

There is a controversy about the magnetic properties of Ti3AlC2, and to compare Ti3AlC2, the magnetic properties of the Ti3AlC2-TiC composites were studied. A SQUID VSM was used to measure the magnetic moment followed by the applied magnetic field and temperature in a 1T applied magnetic field to see how the amount of etching influenced the magnetic properties. Figure 8 illustrates the Ti3AlC2-TiC composites’ field-dependent magnetization at 2–300 K. The magnetization at 2 K and 7 T was 0.2017 emu/g for the Ti3AlC2-TiC composites but reduced to 0.1758 emu/g at 300 K. Although the magnetization curve did not saturate at 7T, the larger magnetization in Ti3AlC2-TiC composites proposed that it was ferromagnetic, which was similar to the reported Ti3AlC2 [33]. Moreover, the magnetization of Ti3AlC2-TiC composites sintered under high pressure is larger than that of commercial Ti3AlC2 powders (particle size >200 mesh) with a purity of 98%.There is a debate about whether Ti3AlC2 is ferromagnetic or paramagnetic. It has been reported that Ti3AlC2 is paramagnetic [34]. Scheibe et al. noted that EPR spectra of their Ti3AlC2 sample exhibit a weak peak at 0.3–0.35 T (Figure 6 in reference [34]). In our data, changes in magnetization intensity of Ti3AlC2-TiC composites were found at 2 K in the 0.5-1 T magnetic field, but no fluctuations were captured from 10–300 K. The low-field signal associated with Ti3+ ions is weaker, but still noticeable in Ti2AlC and Ti3AlC2 spectra [34].In accordance with the M vs. H data (Figure8), the M (T) behavior of Ti3AlC2-TiC composites at 1 T is shown in Figure 9, which reveals that it was essentially ferromagnetic. Ti3AlC2-TiC composites have a lower magnetic intensity than single-phase Ti3AlC2. Migration and escape of Al atomic layers in synthetic Ti3AlC2 by thermal diffusion during high-pressure sintering [35], which led to the vanishingly intense (104) Ti3AlC2 X-ray reflection at 39° in Figure 2. This process is similar to the preparation of Ti3C2 by HF etching of Ti3AlC2. The comparison of the magnetic intensity of Ti3AlC2 and Ti3C2 at 2–300 K suggested that the presence of the Al atom layer might impact magnetic behavior and/or a magnetic phase transition [34].Figure 8
Magnetization versus applied magnetic field for Ti3AlC2-TiC composites at 2–300 K (a); local enlargement of magnetization curve at 2 K (b).Figure 9
Temperature-dependent magnetization of Ti3AlC2-TiC composites.


## 3.5. Possible Mechanism of the Preparation of Ti2AlC or Ti3AlC2 Composites

Ti3AlC2 solids were synthesized under different sintering pressures. To manufacture solid Ti3AlC2, a pressure of 40 MPa is needed in the hot press [36]. SPS (50–8 MPa pressure) [37] and HIP [38] apply higher pressure to improve the properties of Ti3AlC2. The lattice parameters of Ti2AlC ( a-direction and c-direction) were studied under 50 GPa using a synchrotron radiation source, and they decreased with increasing applied pressure (Figure 2 in reference [39]).Ti3AlC2 was discovered in the Ti-Al-C ternary system [40]. Starostina et al. [15] had synthesized Ti2AlC-based composites from 3Ti/1.2Al/2C at 800–1200°C under 1–2 GPa. Earlier, the authors had successfully fabricated Ti3SiC2-cBN composites [16], and Ti3AlC2-cBN composites [14] under 4.5 GPa by HPHT. Li et al. fabricated Ti3AlC2-cBN [41] composites at 1350°C and Ti3AlC2-Al-cBN [42] composites at 1200–1500°C under 5.5 GPa. However, Qin et al. [35] reported that the raw materials of pure Ti2AlC decomposed at above 1300°C and 3 GPa. It needs to indicate that the second phase (such as diamond, cBN, and TiC) can improve the stability of Ti2AlC or Ti3AlC2 as shown in Figure 4(a).The possible mechanism of stability enhancement is shown in Figure4(b). In the second phase, TiC divides the binder Ti2AlC or Ti3AlC2 into individually sealed units. The thermal diffusion of the Al atomic layer of Ti2AlC or Ti3AlC2 is suppressed under high pressure. Thus, the stability of Ti2AlC or Ti3AlC2 in the Ti2AlC-based or Ti3AlC2-based composite is enhanced relative to that of pure Ti2AlC or Ti3AlC2.


## 4. Conclusions

Ti2AlC-TiC and Ti3AlC2-TiC composites were synthesized at 950°C and 2∼5 GPa from the mixture of Ti, Al, and TiC. XRD patterns show clear Ti2AlC/Ti3AlC2 peaks and the peaks of residual TiC. SEM micrographs suggested the layered structure of Ti3AlC2 is a clear existence. The friction coefficient of synthetics with POM and PP friction couples is between 0.17 and 0.45, and the main wear mechanism is abrasive wear. The friction coefficients of the sintered material with Cu and Al paired pairs are more affected by the composition, and the main wear mechanism is adhesive wear. The width of the abrasion marks of the sintered and agate paired specimens is much larger than that of POM, PP, Cu, Al, and glass. The average friction coefficient of the synthetic and glass pairs gradually increases with increasing sintering pressure. Both agate and glass have abrasive wear as the main wear mechanism. The magnetization rate and magnetization intensity of synthetic Ti2AlC-TiC and Ti3AlC2-TiC composites are greater than those of single-phase Ti2AlC or Ti3AlC2, which may be related to the migration of Al atomic layers in the Ti2AlC or Ti3AlC2 at high pressure. The stability of Ti2AlC or Ti3AlC2 increases as the TiC matrix hinders the migration of Al atoms.



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*Source: 9108736-2022-08-28.xml*