High modulus of about 1 TPa, high thermal conductivity of over 3000 W/mK, very low coefficient of thermal expansion (CTE), high electrical conductivity, self-lubricating characteristics and low density have made carbon nanotubes (CNTs) one of the best reinforcing materials of nano composites for advanced structural, industrial, high strength and wear-prone applications. This is so because it has the capacity of improving the mechanical, tribological, electrical, thermal and physical properties of nanocomposites.
Plots of COF Improvements | Remark | Ref. |
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5 vol.% of CNTs gave the optimal COF improvement. As the concentration increased, the COF depreciated in the Cu alloy. | [24,25,26,27][24][25][26][27] | |
At a very low concentration of CNTs, the composite had low COF improvement; at too excessive a volume fraction, the improvement was still low. It was an optimal concentration (4 vol.%) that gave the highest improvement in Al alloys. | [28,29,30,31,32][28][29][30][31][32] | |
The COF improvement in Al2O3 ceramic-CNTs composite absorbed higher volume fraction of CNTs than in metals and polymers. An optimum value of 10 wt.% gave the highest COF improvement of 67%. | [33,34,35,36,37][33][34][35][36][37] | |
Polymer matrices can wet out only minute fraction of CNTs. Optimum concentration of CNTs that gave the highest COF improvement was 0.7 wt.%. (PMMA, polymethyl methacrylate; EP, epoxy; PS, polystyrene; AMMA, polyacrylonotrile). | [38,[39,3840]][39][40] |
Production Technique Sketches | Description | Remarks | Ref. |
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(a) Powder metallurgy |
Powder metallurgy: This comprises spark plasma sintering, vacuum sintering, microwave sintering, hot pressing and conventional sintering. Prior to consolidation, matrix and reinforcement are blended via ball milling, tubular mixing and molecular mixing. Then, the application of heat and pressure is used to consolidate the composite. |
CNTs/Composites | Tribology Properties | Remarks | Ref. | ||||
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In SPS and other forms of advanced sintering, heat and pressure are applied concurrently, while in conventional sintering, heat is applied after green compaction. There is zero waste of materials. | |||||||
AlSi-10Mg-CNTs | Wear grove of composite = 150 μm2 Wear grove metal matrix = 224 μm2 Wear rate of composite is 33% lower | [ | 67] | ||||
The improvement in wear rate is attributed to the fact that CNTs increased the hardness of the MMCs and improved the microstructure. Selective laser melting (SLM) was the technique employed. This composite is useful in automotive brake pad and lining, pistons and engine sleeves. | [ | 70 | ] | (b) Additive process |
Additive manufacturing: This comprises laser cladding, sputtering, nanoscale dispersion, sandwich processing, plasma spraying and vapor deposition. | This technique reduces material wastes to the lowest level. | |
WC-20Co-6CNTs | Wear rate = 0.000307 ± 0.1 mm3/N.m COF = 0.08 ± 0.012 | [ | 68] | ||||
The introduction of CNTs on WC-20Co improved the wear loss, wear rate and coefficient of friction greatly through improving the mechanical strength. The processing technique was high-velocity oxy-fuel (HVOF) spraying. This ceramic composite is useful in cutting tools and grinding wheels. | [ | 57 | ] | (c) Electrochemical method |
Electrochemical technique: This comprises electro deposition via amperometry, potentiometry, conductometry, voltammetry or the galvanic cell technique. This involves the passing of electric current to initiate dissolution and deposition of materials. | The major advantages of this technique are its simplicity, low cost and speed. | [69] |
Al2O3-3%TiO2-3% CNT, | Wear resistance = 5800 Nm/mm3 Mass loss = 0.0035 g Average mass loss of unreinforced ceramics was 3.85 mg, while average mass loss of CNTs/ceramics was 3.45 mg (11.6% improvement). |
Uniform CNTs dispersion and good adhesion of coatings with the substrate invoked the enhancement of the wear properties. The method was Plasma Spraying of Al2O3-3TiO2-CNTs on AISI 1020 steel substrate. This ceramic composite is replacing steel pipes in oil and gas industrial pipes. | [71] | (d) Casting method |
Casting method: This comprises stir casting, ultrasonic cavitation, squeeze casting, liquid casting etc. Casting involves melting of base material, addition of reinforcing phase, stirring and solidification. | Virtually every component can be casted, using metal composites, ceramics or polymer composites. It is cheap and uses simple tooling. | [67] |
316 L Stainless steel-8CNT | Wear rate = 1.1 × 10−7 mm3/Nm (218% improvement) COF = 0.25 (60% improvement) The COF of unreinforced 316 L was 0.4, and the wear rate was 3.5 × 10−7 mm3/Nm. |
The tribology improvement was made possible because the larger surface area of CNTs provided large surface roughing, thus having a better lubrication effect. Vacuum hot-press sintering was the technique applied. This steel composite is replacing conventional stainless steel in advanced applications. | (e) Pultrusion/Extrusion |
Low melting-point metals and polymer composite fabrication that comprises pultrusion and extrusion. This involves melting and pushing or pulling the melt through an orifice to assume specific shape. | This is used in the production of long components such as electric transmission conductors. It is very fast and economical. | [67] |
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AZ61Mg-0.5%CNTs | Volume loss = 0.81 mm3 (48% improvement) Mass loss = 0.008 g (38% improvement) COF = 0.3 (17% improvement) There was very low COF (0.22) at 1 wt%CNTs. The unrefined AZ61 had volume loss of 1.2 mm3, mass loss of 0.011 and COF of 0.35. |
The strengthening and bonding characteristics of CNTs led to a reduction in mass loss. Additionally, the CNTs provided lubrication effect which led to enhancement of the tribology. Stir casting with annealing was the method used. This MMC is useful in electrical and electronics packaging. | [72] |
Al3Ti-Cu-SiC-CNTs | Average COF = 0.46 Average COF of substrate = 0.57 This gives an improvement of 24% |
The COF was enhanced by the dispersion strengthening and fine grain strengthening of CNTs. The production technique was laser cladding. This is an advanced hybrid composite which can compete favorably with high entropy alloy in aerospace applications. | [73] |
Al2O3-3TiO2-6CNTs | Wear depth = 4 ± 0.8 μm Wear rate = 0.0003 ± 0.1 mm3/Nm COF = 0.11 ± 0.009 |
The improvement was attributed to bridging of CNTs in between the splats of the ceramics matrix. The technique used was Plasma Spraying of the composite on AISI 1020 mild steel. This is replacing steel alloy in bridges, rails and other structural applications. | [74] |
Polyimide-0.1CNTs | Wear rate = 0.0002 mm3/Nm COF = 0.26 |
The functionalization of CNTs contributed to the improvement of the wear resistance. This polymer composite is used as high temperature structural glues and laminating resin. It is used in wood work, structural and car body parts applications. | [38] |
Polyimide-0.7CNTs | Wear rate = 0.00065 mm3/Nm COF = 0.18 |
There was strong interfacial bonding between PI matrix and MWCNTs-COOH nanofillers, which enhanced the transfer of load effectively from the matrix to the functionalized CNTs. So, this improved its hardness, which in turn reduced the wear rate and COF. It is now prominent in automotive industries. | [38] |
Phosphate ceramic-0.75CNTs | Wear rate = 0.008 mm3/Nm COF = 0.39 |
It was observed that at temperatures below 500 °C, the lubrication effect of CNTs was intact. However, when this temperature was exceeded, the tribology properties diminished. | [75] |
Al2O3-2CNTs | Wear rate = 0.00000269 mm2/kg Wear rate of pristine Al2O3 = 0.00000294 mm2/kg |
The improvement in wear rate was attributed to the uniform dispersion and the reinforcement efficiency of CNTs. The composite was produced with cold spraying. This is a high-temperature structural ceramic composite that is replacing BN, WC etc. | [76] |
Al-CNTs | Specific wear rate of micro-sized channel reinforcement filling (MCRF) = 0.018 mm3/Nm Wear rate of pure Al = 0.022 mm3/Nm (20% improvement) |
The improvement in wear rate was attributed to the uniform dispersion of CNTs in MCRF-fabricated composite, which formed a solid lubricant layer. Friction stir processing was the method applied. This MMC is useful in electrical, electronic and structural applications which can replace conventional steel conductors. | [77] |
Al-Si-0.75CNTs | Wear rate = 0.00095 mm3/m Wear rate Al-Si = 0.0018 mm3/m (89% improvement) |
The improvement was attributed to the formation of a carbon layer which acted as a solid lubricant at a higher speed. Powder metallurgy was employed in the fabrication. | [78] |
Al-4CNTs | COF = 0.18 (52% improvement) Wear rate = 0.34 μm3/s (23% improvement) Wear volume = 20 μm3 (23% improvement) |
The improvement was attributed to strong densification and refined microstructure influenced by spark plasma sintering technique. Useful for high temperature transmission conductor. | [18] |
Al-8CNTs-8Nb | COF = 0.10 (79% improvement) Wear rate = 0.49 μm3/s (23% improvement) |
The improvement was attributed to the solid lubrication property of CNTs and formation of Nb2O5 that acted as a solid lubricant too. This can conveniently replace steel-reinforced aluminum conductors. | [79] |
Epoxy-10Carbon fibre-0.3CNTs | COF = 0.3 (97% improvement) Wear rate = 4 × 10−6 mm3/Nm (425% improvement) |
The improvement in the tribological properties was attributed to the lubricating effect as well as strengthening action of C-C bond between CNTs and short carbon fibre. This polymer composite is useful in high-temperature applications. | [80] |