Preparation Of Machinable Conductive Ceramics Ti₃AlC₂ And Cu/Ti₃AlC₂ Composites

Recently, Ti₃AlC₂ (titanium aluminum carbide) attracts increasing interest owing to its unique properties. It is a new structural/functional material, combining unusual properties of both metals and ceramics. Such as good electrical and thermal conductivity, high strength and modulus, high thermal shock, high-temperature oxidation resistance, low density, and being machainable with conventional high-speed tools without lubrication. In addition, in contrast to the normal brittle ceramics, Ti₃AlC₂ exhibits some abnormal room-temperature compressive plasticity. Due to above excellent properties, the applications of this material are very potential. For example, it can be used as a high-temperature structural material, and also to process abrasion-resistant components and rotating parts, and so on.However, Ti₃AlC₂ has a very narrow zone of stability in Ti-Al-C ternary phase diagram, therefore, preparation of high purity Ti₃AlC₂ has been the difficulties of its research. Since 1994, a variety of sintering methods were used to synthesize Ti₃AlC₂. Such as hot isostatic pressing (HIP), hot-pressing (HP), self-propagating high temperature synthesis (SHS), spark plasma sintering (SPS). Though high-purity, even single-phase Ti₃AlC₂ were synthesized, higher synthesis temperatures, longer holding time were required in these methods.In this paper, Ti, Al and C were selected as raw materials. Preparation of layered machinable ternary ceramics Ti₃AlC₂ by mechanical alloying is detailed for the first time; at the same time, the milled powders were heated treatment in order to obtain high-purity Ti₃AlC₂ powder. In addition, high-purity, even single-phase Ti₃AlC₂ bulks were synthesized by mechanical alloying and spark plasma sintering for the first time. The key scientific problems on preparation of Ti₃AlC₂ are solved, e.g. higher sintering temperature or longer holding time. High-purity Ti₃AlC₂ powder and Cu powder are used to synthesize Cu/ Ti₃AlC₂ metal-ceramic composites with excellent properties, and wear behavior is investigated. The major research efforts of the present study are as follows:(1) Research on preparation of Ti₃AlC₂ powder by mechanical alloyingEffects of mechanical alloying parameters, raw material ratio and Si addition on preparation of Ti₃AlC₂ by mechanical alloying were investigated systematically. XRD, SEM, EDX and TEM are used to identity phase and observed microstructure. The study results show that high content Ti₃AlC₂ was successfully obtained after ball milling of powder mixture at 600 rpm, charge ratio of 5:1 only for 3 h. The milled products consist of powder and a coarse granule with 8 mm in diameter, and both are mainly composed of Ti₃AlC₂ with TiC as impurity. It is believed that a mechanically induced self-propagating reaction (MSR) was triggered to form Ti₃AlC₂ and TiC during mechanical alloying process. Through adjusting mechanical alloying parameters and raw material ratio, the best process is as follows: the rotation speed is set as 600 rpm, and the weight ratio of balls to powders is 5:1, milling time is 3.5h. The obtained Ti₃AlC₂ content is 92.2 wt.%. Adding a small amount of Si in the starting material remarkably improves the content of Ti₃AlC₂ in the final products. High-content Ti₃AlC₂ powders with a phase purity of 94.2 wt.% and 95.1 wt.%could be fabricated by mechanical alloying of 3Ti/Al/0.1Si/2C and 3Ti/Al/0.1Si/1.8C, respectively.(2) Research on preparation of high-purity Ti₃AlC₂ powder by heating treatment the mechanically alloyed mixtureHigh-purity Ti₃AlC₂ powder (99 wt.%) is obtained by heating treatment the mechanically alloyed mixture. The best raw material ratio, heat treatment temperature and holding time is Ti:Al:C=3:1.1:1.8, 1000℃, 10 min, respectively. In addition, powder of lower content Ti₃AlC₂ was heated treatment in the same ball-milling process, high-purity Ti₃AlC₂ powder was also obtained. However, longer milling time doesn't obtained high-purity Ti₃AlC₂ powder. When the milled powders with appropriate Si addition are heated treatment at 1000℃, high-purity Ti3Al(Si)C2 solid solution powder is synthesized. And Si atoms substitute for Al atoms in solid solution distribute unevenly.(3) Preparation of Ti₃AlC₂ bulk by mechanically activated spark plasma sintering3Ti/xAl/2C(x=1, 1.1, 1.2, 1.3) raw material powders were milled at 400 rpm for 9 h and 9.5 h, respectively. For mixed powders of milling for 9 h, no new phase was formed, it was called no-phase transformation group. For mixed powders of milling for 9.5 h, Ti₃AlC₂ and TiC were formed, it was called phase transformation group. The milled powders of both groups were selected to spark plasma sintered. And dual activation mechanism of low temperature synthesis of high purity, even single-phase Ti₃AlC₂ is explored the mechanical alloying and spark plasma sintering (MA-SPS).For no-phase transformation group, high-purity and dense Ti₃AlC₂ was synthesized at lower sintering temperature by spark plasma sintering of mechanically milled elemental powders. It was found that elemental powders mechanically milled to superfine sizes and excess Al in the starting materials can obviously decrease sintering temperature for the synthesis of Ti₃AlC₂ during subsequent SPS. For mechanically milled mixtures of composition 3Ti/1.1Al/2C and 3Ti/1.2Al/2C in molar ratio, Ti₃AlC₂ with a purity of >99 wt.% and a relative density of >98% were obtained at 1100℃and 1050℃for 10 min, respectively. The reaction path for the formation of Ti₃AlC₂ is revealed as follows: Ti, Al and C mixtures induced chemical reactions to form TiC, Ti-Al intermetallics like Ti3Al and TiAl, and carbide Ti3AlC at low temperature of 600℃; the amorphous C further diffused in the Ti-Al intermetallics to form carbide Ti2AlC at elevated temperature; at last, the reactions between Ti2AlC and TiC yielded Ti₃AlC₂.For phase transformation group, fully dense bulks Ti₃AlC₂ with > 99 wt.% phase purity were obtained at the relatively low temperature and appropriate holding times by subsequent spark plasma sintering of mechanically alloyed powders from a starting mixtures of composition Ti:Al:C = 3:1.1:2 in molar ratio. But with the increasing holding time, the typical layered grain sizes of Ti₃AlC₂ increased and the Vickers hardness decreased. In addition, single-phase Ti₃AlC₂ and Ti3Al(Si)C2 solid solution were obtained by sintering the milled 3Ti/(1.1-y)Al/ySi/1.8C(y=0, 0.1, 0.2, 0.3) powder at 1000℃for 10 min. With Si content increasing, the Vickers hardness increased.(4) Preparation of Cu/Ti₃AlC₂ composites and wear behaviorHigh-purity Ti₃AlC₂ powder was selected to prepare Cu/Ti₃AlC₂ composites, and to study the wear behavior. Friction and wear properties and wear mechanism of Cu/Ti₃AlC₂ composites at different content Ti₃AlC₂ and test load. The results show that Ti₃AlC₂ and Cu occurred reaction at lower temperature. Moreover, this reaction became increasing serious with increasing sintering temperature. The reaction was mainly completed through diffusion of Al in Ti₃AlC₂ and Cu. As a result of the induced function of Cu, resulting in the loss of Al atoms in Ti₃AlC₂ decomposed into TiC. And Al diffused into Cu matrix to form Cu-Al alloy. During the wear process, Wear mechanism of copper is mainly plough wear and adhesive wear. With increasing Ti₃AlC₂ content, Cu/Ti₃AlC₂ composite wear rate and friction coefficient decreased, wear resistance was improved significantly. When volume fraction of Ti₃AlC₂ is lower (5-10vol.%), The wear mechanism is mainly adhesive wear, at the same time, accompanying by abrasive wear; With volume fraction of Ti₃AlC₂ further increasing (15-30vol.%), Furrows of the worn surface has become shallow and small, reducing the surface roughness. At the load of 100N, wear resistance of Cu/30vol.% Ti₃AlC₂ composites is the best. In a word, the studies of this paper provide a new technical path and the process parameters as reference for preparation of Ti₃AlC₂ powder and bulk material. The work in this paper is of rather available values in theory and practical application.