Fabrication And Properties Of Graphene Reinforced NC Ni Matrix Composites

Nanomaterials are materials that have one dimension in nanoscale. Due to the specialstructure, nanomaterials own unique properties comparing with conventional coarse-grainedmaterials. As a special sort of nanomaterials, nanocrystalline metals referring to metals withgrain size in nanoscale (10-9m) present outstanding properties, like excellent yieldingstrength, hardness, wear resistance and also superior corrosion resistance. On the other hand,nanocomposites have been the subject of widespread research in the past couple of decadesowing to their super overall performance. The introduction of nanoparticles like carbonnanotubes and ceramic nanoparticles etc. can bring the combination of the superiority ofmetal metrics and second phase particles, and thus enhance the properties of nanocompositesremarkably.Due to its special structure (ultra-high specific surface area~2600m2g-1and lowdensity~2.2gcm-3) and excellent mechanical properties (Young’s Modulus up to1TPa andtensile strength up to130GPa), graphene would be an ideal second phase material. Therehave been many studies on the use of graphene in composite material. Researches ongraphene-metal composites showed that the introduction of graphene can improve themechanical properties of the metal matrices. However, few works have been focusing on theconjunction of graphene and nanocrystalline metal metrics. So if thegraphene/nanocrystalline metal metrics composite coatings can be fabricated based on theavailable theory and techniques, it can be expected that these coatings would have ultra-highstrength and other superior properties.Based on the techniques to fabricate nanocrystalline metals, we proposed a method tointroduce graphene to nanocrystalline Ni matrices as ‘Second Phase’ through the addition ofRGO-Ni nanocomposites into Watts plating solution. Through controlling the concentrationof RGO-Ni nanocomposites and process parameters during electrodeposition, the optimalparameters were obtained. The X-ray diffraction (XRD), transmission electron microscopy(TEM) and scanning electron microscopy (SEM) were used to observe and analyze themicrostructure, surface morphology and ingredient of products at every stage during the fabrication. Then, electrochemical analyzer was used to analyze their corrosion resistanceand nanoindentation tests were performed to characterize their mechanical properties.The results are as follows:1. We fabricated GO through modified Hummer method. Then, RGO-Ninanocomposites were fabricated through hydrothermal reduction with different ratio of theweight of GO to the weight of NiSO4·6H2O. Through comparison, nanocompositesfabricated under the ratio of4:1with Ni nanoparticles having high concentration anddispersing well on graphene with lower aggregation are selected as the optimal secondphase.2. Based on the technique to synthesis NC metals through electrodeposition, RGO-Ninanocomposites are introduced into nanocrystalline Ni matrices through compositeelectrodeposition.3. According to the SEM images, with the concentration of RGO-Ni in plating solutionincreasing from0.1mg/mL to0.5mg/mL the surface morphology of the G-NC Ni compositecoatings become rougher obviously. Furthermore, the increase of the power of the ultrasonicwould increase the roughness of the surface, while the surface of samples fabricated underrotation is much smoother than the samples fabricated under ultrasonic.4. Through the analysis of polarization curves and electrochemical impedancespectroscopy, we found that the introduction of graphene has great impact on the corrisionbehavior of coatings and the concentration of it determines the defect density in coatings.Pure Ni coating can form effective passive film in NaCl and NaOH solutions, whichprevents the migration of Ni2+and electron to surface and subsequent chemical reaction.Meanwhile, the introduction of graphene increases the defect density in coatings and a largeamount of micro-batteries are formed. Therefore, the corrosion resistance of coatings woulddecrease with the introduction of graphene and would reduce further with increasing thedensity of RGO-Ni nanocomposites.5. Nanoindentation results showed that mechanical properties of pure Ni and compositecoatings exhibit little difference under rotation and untrasonic. The hardness of pure Ni is6.9~7.0GPa and elastic modulus is120~121GPa. The addition of GO into plating solutiondoes improve the mechanical properties of the composites with highest hardness of7.4GPaand highest elastic modulus of130GPa when the concentration of GO is0.5mg/mL. When the additive is RGO-Ni, the promotion in mechanical properties is more remarkable. Thehardness can reach8.0GPa and the elastic modulus can achieve177GPa when theconcentration of RGO-Ni is0.5mg/mL. Furthermore, the hardness and elastic modulus getreinforced with the increase of the concentration of RGO-Ni in plating solution.