Microstructures and intrinsic lubricity of in situ Ti3SiC2–TiSi2–TiC MAX phase composite fabricated by reactive spark plasma sintering (SPS)
Introduction
Early transition-metal ternary metalloceramics composed of hexagonal nanolaminated layered structure with a chemistry M(n+1)AX(n) configuration have attracted a lot of attention [1,2]. Since first discovered in the late 1960s, MAX phases have been recently further explored to investigate their synthesis and structure-property relation owing to their unusual set of metal-like (machinability, stiffness, electrical and thermal conductivities) and ceramic-like (damage tolerance, thermal stability, oxidation resistance) properties [3,4]. The MAX phases are so-called because of their general formula where M are mainly group-4, group-5, and group-6 transition metals (mainly Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo), while A is mainly an A-group element from groups 9 (Ir), 10 (Pd), 11 (Cu, Au), 12 (Cd, Zn), 13 (Al, Ga, In, Tl), 14 (Si, Ge, Sn, Pb), 15 (P, As, Sb, Bi), X is either C or N and n = 1–3 and possibly higher [1,5,6]. Fig. 1 shows some of the elements in the periodic table that forms the Mn+1AXn phases.
These ternary phases (80 +) crystallize in a hexagonal structure (P63/mmc symmetry) with two formula units per unit cell, where Mn+1Xn layers are interleaved with pure A-group atoms thus resulting in a characteristic (Mn+1Xn)A(Mn+1Xn)A(Mn+1Xn) crystal structure [7]. The nature of their characteristic layered structure composed of stacking of n “ceramic” interposed with a “metallic” layer [8], coupled with the mixed covalent-metallic nature of the M–X bonds which are exceptionally strong and the relatively weak M–A bonds, endows the MAX phases with their signature mechanical, chemical, and electrical properties [4,9,10]. They represent the only class of ceramic-like material that deforms plastically via the nucleation and slip of basal dislocations (BDs) [11,12], incorporating a series of energy absorbing micro-scale events such as buckling of individual grains, diffuse micro-cracking, delamination of individual grains, kink and shear band formation followed by eventual grain push-outs and pull-outs [4,10,[13], [14], [15], [16], [17], [18], [19], [20]].
Ti3SiC2, a 312 compound, is the most studied representative member of the MAX phase family. It possesses unique metalloceramic properties such as low hardness, low density, high modulus, excellent thermal and electrical conductivity, high fracture toughness, damage tolerance and easy machinability [4,[21], [22], [23]]. Its hexagonal layered crystal structure similar to graphite and MoS2 [24] suggests it might be an excellent solid lubricant material with a low friction and wear properties suitable in a range of high-temperature structural engineering applications [25,26]. This is supported by the fact that Ti3SiC2 felt lubricious during machining as reported by Barsoum et al. [21]. Numerous research have since been conducted on the tribological behaviour of MAX phases for evidence of lubricity [27]. However, some researchers have reported that albeit Ti3SiC2 possessing layered hexagonal crystal structure similar to graphite, it is not intrinsically self-lubricating [28]. This was attributed to a three-body abrasive wear that stems from the fracture and pull-out of the Ti3SiC2 grains — representing the dominant wear mechanism at room temperature [27,29,30]. The ease of grain fracture and pull-outs has been linked to the weak grain boundary force of the Ti3SiC2 grains [31]. In addition, some other authors have highlighted the low hardness and oxidation resistance as the main factors deteriorating the friction and wear properties of monolithic Ti3SiC2 [32]. These observations have led to new studies focussing essentially on the incorporation of a second phase hard material in the soft matrix of Ti3SiC2 as an effective way to mitigate these weaknesses [32,33]. Some possible reinforcing materials are TiC and TiB2 as they possess high hardness, excellent oxidation resistance and close coefficient of thermal expansion (CTE) with Ti3SiC2 [32,34].
Nevertheless, little is known on the exact wear mechanism(s) as well as the intrinsic self-lubricating behaviour of monolithic Ti3SiC2 and Ti3SiC2-based material due to the lack of detailed investigation undertaken on the worn surface irrespective of the varying testing conditions reported to date. The scope of this work is to determine comprehensively the wear mechanism sequence of this solid and its associated composites during dry sliding friction at ambient conditions, in order to establish the existence of intrinsic self-lubricating behaviour as speculated [30,35] and to further elucidate the wear mechanism.
Section snippets
Powder preparation
Commercially available titanium powder (100 mesh, 99.7% purity, Aldrich), silicon powder (200 mesh, 99% purity, Acros organics) and graphite powder were used as starting elemental powders. 5.53 g of titanium powder and 0.98 g of graphite powder were dry-milled in a SPEX 8000 mill continuously for 2 h and subsequently mixed with 1.08 g of silicon powder according to a 3:1:2 stoichiometry.
Consolidation by spark plasma sintering (SPS)
The stoichiometric powder mixture was poured into an electrically and thermally conductive cylindrical
SPS sintering cycle
Fig. 3 shows the variation of temperature, applied force (pressure), punch displacement (piston movement) and sintering speed during the SPS cycle. As shown, the punch displacement has been divided into distinctive segments (I–IV) to represent the respective sintering stages and corresponding sintering events. In segment I, particle rearrangement initiated by the applied force resulted in positive punch displacement due to powder compression. A corresponding increase in sintering speed (~
Friction and wear
The evolution of friction coefficient as a function of time for the contact condition [0.5 N/50 rpm/60 min] and [0.5 N/100 rpm/30 min] are shown in Fig. 12. The noticeable features of these plots are the friction transition(s) and mild stick-slip phenomenon. The friction plots have been divided into three regimes (I, II, and III). In regime I, the friction was initially very low with no visible wear scar. This was then followed by a transition in friction to a high friction regime II, where the
Wear mechanisms
Following microstructural and chemical analyses before and after the wear tests, the wear mechanisms (Fig. 24) of this MAX phase composite system for the test conditions are designated oxidative–deformation–reoxidation as explained thus:
Conclusions
Dense polycrystalline MAX phase composite Ti3SiC2–TiSi2–TiC was successfully synthesized by spark plasma sintering via the elemental powder route. The following conclusions can be drawn upon exhaustive characterization of the deformation microstructure and tribological behaviour of this MAX phase composite system:
- 1.
Deformation microstructure revealed evidence of room temperature plasticity, toughening and anisotropy in mechanical response.
- 2.
Evidence of intrinsic solid lubrication was observed due
Declaration of competing interest
We have no conflict of interest to disclose.
Acknowledgement
We wish to acknowledge the Henry Royce Institute for Advanced Materials for equipment access at Royce@sheffield.
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