Effect of PyC coating on mechanical properties of C f /AZ91D composites

Abstract

The microstructure of high strength PAN-based T700 carbon fibre (C_f) with pyrolytic carbon (PyC) coating was characterised. Effects of PyC coating on the surface of carbon fibres, interface characteristics, and mechanical properties of C_f/AZ91D composites were evaluated. The results showed that the carbon fibres with PyC coating had higher surface roughness and higher graphitisation which were beneficial to ease hazardous interface reaction. C_f-PyC/AZ91D composites with an optimal PyC coating that is about 150 nm thick and has low texture exhibited ultimate tensile strength of 416 MPa, which demonstrated 35% improvement compared with the C_f/AZ91D composites. The increase of mechanical properties of C_f-PyC/AZ91D composites could be ascribed to a synergistic effect of the rough C_f surface interlocking, an optimum interfacial bonding between fibres and matrix, and protection of fibres from a corrosive attack of the aluminium element in the matrix.

Keywords

PyC coating microstructure of PyC mechanical properties

Introduction

Based on the concept of saving the limited resources and reducing environmental pollution, continuous carbon fibres reinforced Mg matrix composites have attracted huge interest for their high specific strength and modulus, high specific stiffness, low coefficient of thermal expansion [1 -3], as one of light and high-performance materials that have been widely used in automotive and aviation fields. However, carbon fibres have an excessive smoothness and fewer adsorption characteristics always lead to poor adhesion with magnesium matrix. Furthermore, the interface reaction between fibres and magnesium matrix can introduce defects that significantly decrease the fibre strength. The former one will degrade the load transfer from fibre to the matrix and the structural integrity of C_f/Mg composites [1 -4]. The latter one will influence the fully reinforce function of carbon fibres as the main reinforcers of composites, which leads to the decrease of the ultimate properties of composites, particularly for the fibre-dominated strength [4]. Thus, to maximise the mechanical properties of C_f/Mg composites, it is very necessary to inhibit these induced defects on carbon fibres during the fabrication of the composites and to improve the fibre/matrix interfacial bonding.

Many strategies have been applied to modify the surface of carbon fibres to optimise the interfacial adhesion between carbon fibres and Mg matrix [5]. Wang et al. [5] reported a self-regulating mechanism of interfacial reaction by introducing γ-Al₂O₃ and anatase-TiO₂ layers on carbon fibre surface and a huge enhancement in the tensile strength of C_f/Mg composites were observed. Wang et al. [6] also reported that the tensile strength of C_f/Mg composite with yttria stabilised zirconia (YSZ)-coated carbon fibres reached 1.08 GPa (90% of the theoretical prediction). They demonstrated that the tensile thermal residual stress triggered the ZrO₂ phase transformation from tetragonal to monoclinic, resulting in the alleviation of the tensile TRS and reduction of nanostructured defects in the interfacial layers. Reischer et al. [7] prepared nanostructured hexagonal boron nitride (BN) coating on carbon fibres and proved that interlayers with a definite texturing of the BN stacks can distinctly improve the bending strength by 42% to 1620 MPa compared to those with randomly oriented BN planes.

Another commonly used coating material adapted to improve the interface of composites is pyrocarbon (PyC) coating [8]. Dorner-Reisel et al. [9] prepared T800H/AZ91 composites by coating a PyC layer on carbon fibres, and the experimental results showed that the presence of PyC coating contributed to the formation of interfacial phases and enhanced the micromechanical response of the composites. Vidal-Sétif et al. [10] and Lancin and Marhic co-workers [11] interposed a PyC coating which acted as a mechanical fuse in M40J/C_pyr/A357 and attained higher mechanical properties in the as-cast composite (1360 MPa). Cao et al. [12] fabricated C/SiC-SiBC composites with optimal PyC interphase thickness exhibited flexural strength of 412 MPa and fracture toughness of 24 MPa m^1/2, and demonstrated that PyC interphase would accommodate the radial thermal residual stress, leading to a proper IBS (interface bonding strength) for the best load transfer. Hu et al. [8] used CNTs coated with PyC growing on carbon fibres to reinforce SiC composites. The PyC layer not only protected CNT but also weakened interfacial strength between CNTs/Matrix, leading to long pull-out of CNTs compared to brittle fracture of uncoated CNTs. Significant efforts have been done on optimising the interfaces of C_f/Al, C_f/ SiC, and CNTs/SiC composites by PyC coating to increase the composite properties. However, for carbon fibre-reinforced Mg matrix composites, rare evidence of previous works studied the effect of PyC coating microstructure on the mechanical properties of C_f/Mg composites.

In the present work, carbon fibre modified by PyC-reinforced AZ91D matrix composites was prepared by a joint process of chemical vapour deposition (CVD) and liquid–solid extrusion following vacuum infiltration (LSEVI). CVD is a method for synthesising carbon coatings [13]. LSEVI was a special forming technique which integrated vacuum infiltration, squeezes casting and liquid–solid extrusion, and could form a composite tube, bar, and profile by one step [14,15], which is suitable for preparing C_f/Mg composite [16]. The utilisation of this process can not only reduce the cost of processing C_f/Mg composite but also realise near net forming of magnesium matrix composites and achieve mass production. The aim of this study was to improve the fibre/matrix interface through PyC coating for enhancing the mechanical properties of laminated C_f/AZ91D composites. Special attentions were paid on the microstructures of PyC coating, the interrelationship between coating microstructures and mechanical properties of C_f/AZ91D composite.

Experimental procedures

Fabrication

Preform and matrix preparation

AZ91D Magnesium alloy (Mg-9 wt-% Al-1 wt-% Zn-0.2 wt-% Mn) was used as the matrix (Magontec Xi'an Co., Ltd). PAN-based carbon fibre (bulk density: 1.8 g cm⁻³, fibres diameter: ∼7 µm, 12 000 single filaments per tow, manufactured by Toray Co., Japan) under the trade name of T-700 was selected as the reinforcement. T700 carbon fibre was cut, then laid with 0/90° with the orthogonal orientation layer by layer, finally fixed along Z-axis by needling carbon fibre string to obtain a preform with a size of 95 × 95 × 8 mm.

Coating

PyC coating was deposited on the surface of carbon fibres by the CVD method at 1000∼1150°C for 4 h, CH₄ were used as the precursor. Carbon fibres without and with PyC are noted as C_f and C_f-PyC, respectively.

C_f/AZ91D composite preparation

Approximately 45 vol.-% carbon fibre preform with and without PyC coating reinforced AZ91D magnesium matrix composite (∼45 vol.-% C_f/AZ91D and PyC-C_f/AZ91D composites) were fabricated by LSEVI. The previous work of our research group has already introduced the process [16 -18]. The whole process was considered as an infiltration process of magnesium alloy into a carbon fibre preform, and the schematic sketch was given in Figure 1. The specimens (75 × 10 × 2 mm) were cut from laminated composites for the strength test according to ASTM standard D 3552. Each test was repeated for five times to ensure the statistical accuracy of the results.

Characterisation and test

The morphology of T700 fibre, PyC coating, and fracture surface of all the specimens after the tensile testing was observed by SEM (FE-SEM, Supra-55). The microstructures of PyC coating were investigated by transmission electron microscopy (TEM, Tecnai F30G2). The crystalline structures of the coating and phase analysis were analysed by X-ray diffraction (XRD, X'Pert Pro, PANalytical, Almelo, The Netherlands). The graphitisation degrees of the carbon fibres before and after coating were investigated by Raman spectroscopy (Renishaw) using an inVia micro-Raman spectrometer with an Ar ion laser of 514.5 nm wavelength at room temperature.

Tensile tests of fibre bundles were carried out using a tensile stress testing system on a universal testing machine (CMT5304-30 kN). A fibre bundle (consist of 12 000 fibres) was glued at both ends onto two aluminium sheets (thickness less than 1 mm). The gauge length was 20 mm. A typical crosshead speed of 0.5 mm min⁻¹ was applied for the test. A minimum of 30 measurements were recorded for each fibre bundle sample. The averaged values of tensile strength were obtained.

Tensile tests of C_f/AZ91D composite were carried out by using a material testing equipment Zwick 1456 (model 1456, z010, Ulm Germany) at room temperature in accordance with ASTM standard D 3552. The size of specimens was 75 × 10 × 2 mm with a span of 20 mm.

Results and discussion

Surface morphology of C_f and C_f-PyC

Acted as a barrier for reactions and diffusion, supported wetting and bonding, and prevent oxidation of the fibres [19], the PyC coating was coated on T700 carbon fibre successfully. The enlarged views of the square areas marked in (b) are shown in Figure 2(a,b) shows the surface of uncoated T700 carbon fibre. It can be seen that surface of C_f is smooth, without trenches and uneven fluff pulp. Figure 2(c) shows the cross-section of T700 carbon fibres. The surface of C_f has low energy and is chemically inert. Through coating, the surface of C_f becomes rougher, which could improve the chemical bonding and mechanical interlocking [20 -22].

Under the conditions described above, the PyC coating was prepared on the carbon fibre by the CVD method and examined by SEM. Figure 2(d–f) demonstrated that the uniform PyC coating was found around each C_f. After coating, the fibre has a rougher surface to allow a small degree of mechanical interlocking to take place between fibre and matrix, and interfacial roughness can provide crack shielding and plastic deformation at the interfacial region [23]. Compare with the relatively smooth surface of C_f in Figure 2(a,d) shows the coarse surface of C_f after the deposition of PyC at 1050°C for 4 h, which will obstruct the diffusion of Al or Mg element, and form a beneficial interface between magnesium matrix and C_f. The coating is very dense and without crinkle or crack. The fibre was coated uniformly by PyC coating with a thickness of about 150 nm (Figure 2(f)). These indicate that a PyC coating was formed on the C_f surface.

Figure 3 shows the three-dimensional (3D) surface profiles of the samples. The 3D measurement method was used to evaluate the characteristics of surface profiles comprehensively. It can be seen that surface roughness increased after deposition of the PyC coating and a coarse uniform surface was obtained, which will offer bigger sliding friction between fibre and matrix. The values of Sa and Sq of C_f-PyC were higher than the pristine C_f, as seen in Figure 3 and Table 1.

Table 1

The Sa and Sq of the sample surface.

	C_f	C_f-PyC
Sa (µm)	18.62	27.43
Sq (µm)	22.17	31.65

Microstructural comparison of C_f and C_f-PyC

In order to confirm the presence of PyC around the fibre and determine the thicknesses and textile structures, TEM observations were performed. TEM images clearly showed that the surface of the C_f was covered with a thin and uniform interfacial layer with a thickness of about ∼150 nm (Figure 4(a)). From the SAED pattern and based on the procedures preparation by our research group [24], orientation angle (OA) value of 88° could be obtained, which demonstrated that PyC in our samples would be considered low texture [25].

The composition and crystallinity of PyC coating were investigated by a combination of Raman spectroscopy and XRD analysis. The intensity ratio (R) of D and G bands (R = I_D/I_G) was regarded as a measure of the crystalline order of carbon materials. The lower R value corresponds to the higher quality of graphitisation within carbon materials [26,27]. Furthermore, it had been found empirically that the value of R was related to the microcrystalline planar size L_a calculated by the following formula [28]. (1) A typical comparative Raman spectra of pristine T700 C_f and C_f-PyC showed in Figure 5(a). The fitting Raman spectra of pristine C_f and C_f-PyC between 800 and 2000 cm⁻¹ were shown in Figure 4(b,c), respectively. Each Raman spectrum in Figure 4(a) exhibited strong well-defined D (disordered induced) and G (graphitic) bands that presented as a hump-like shape with peaks at ∼1360 and ∼1585 cm⁻¹ [25], and the C_f with PyC coating had second-order bands (2G), which meant that the C_f with PyC coating had higher graphitisation degree [29].

Additional information was obtained by peak fitting [26,30]. The C_f Raman spectrum was deconvoluted with four peaks located at ∼1200 cm⁻¹ (I band), ∼1362 cm⁻¹ (D band), ∼1542 cm⁻¹ (D′ band) and ∼1600 cm⁻¹ (G band), while C_f-PyC Raman spectrum was deconvoluted with five peaks located at ∼1180 cm⁻¹ (I band), ∼1353 cm⁻¹ (D band), ∼1463 cm⁻¹ (D″ band), ∼1587 cm⁻¹ (G band) and ∼1609 cm⁻¹ (D′ band). The D′ band was known to occur in defective graphitic systems and manifests as a shoulder on the G band. The I band had been linked with the disorder in the graphitic lattice, sp²–sp³ bonds or the presence of polyenes, whereas the D″ band was generally thought to stem from the presence of amorphous carbon [26]. Comparing the G band intensity and D band intensities of pristine and PyC-coated C_f, it can be observed that significantly increased in PyC-coated C_f, while the I_D/I_G relative band intensity ratio is 2.19 for pristine and 1.28 for PyC-coated C_f. This fitting result indicates that the graphitisation degree of C_f after the coating is higher than the pristine one, which is beneficial to the interface bonding of C_f/AZ91D, because the rate of carbide growth depends on the microstructure of the carbon fibres [31]. According to the calculation, L_a of pristine T700 C_f and C_f-PyC are 2.01 and 3.43 nm, respectively. This fact can prove further that applying PyC coating generated positive effects for interface bonding.

The XRD patterns of the pristine C_f and C_f-PyC were shown in Figure 6. It was obvious that only carbon diffraction peaks could be observed. The peaks at approximately 2θ of 26° and 43.6° were corresponding to (002) and (100) peaks of carbon, respectively. Compared with XRD patterns of pristine C_f, the relative intensity of characteristic peaks of C_f-PyC increased, the (002) peak position of C_f-PyC shifts rightwards and the peak becomes sharp and narrow, which indicated that part of carbon atoms in turbostratic structure were transferred and rearranged after PyC coating treatment, and the atomic vacancies on carbon atoms surface and defect twist at the carbon atom edges were reduced, finally the structure was more close to the ideal graphite structure [32]. The structural parameters calculated according to the diffraction data obtained from Figure 5 were listed in Table 1. The interlayer spacing (d₀₀₂) was calculated by Bragg equation (Equation (2)), and the microcrystalline height (L_c) was calculated by Scherrer equation (Equation (3)). The value of the graphitisation degree (g) of the composites was calculated by Maire and Mering equation (Equation (4)) [33]. (2) (3) (4) where d₀₀₂ was the interlayer spacing of adjacent carbon atom (nm), λ was the wavelength of Cu Kα radiation (0.154 nm), θ was Bragg diffraction angle (°), L_c was microcrystalline height of layer structure (nm), B was FWHM of (002) (radian), and K was empirical constant of 0.9.

From Table 2, L_c of C_f-PyC was 2.21 nm. Comparing with pristine of C_f, L_c of C_f-PyC increases by 28.4%, and the FWHM of the (002) peak decreased by 22.2%. The results demonstrated that the graphitisation degree of C_f after coating was higher than the pristine one, which was consistent with the Raman spectrum results above.

Table 2

XRD structural parameters of pristine C_f and C_f-PyC.

Sample	2 θ/(°)	FWHM/(°)	d₀₀₂/nm	L_c/nm	g/%
Pristine C_f	25.35	4.736	0.3516	1.72	…
C_f-PyC	26.17	3.684	0.3407	2.21	38.37

Effects of PyC coating on mechanical behaviour of C_f and C_f/AZ91D composites

Carbon fibres need to be coated before reinforced the magnesium matrix, and the inﬂuence of coating treatment on tensile properties of the modified carbon fibres need to be investigated. Figure 7 shows the tensile results of fibre bundles before and after coating PyC. After coating for 4 h, the fibre bundles lose approximately 31% of their pristine strength. The degradation is ascribed to two reasons. One is the surface damage and oxidation of the fibres by the high temperature. Another is the different thermal expansion coefficients between the carbon fibres and coating, which could cause residual stress at the interface between carbon fibre and coating. The strength loss of carbon fibre limited the reinforcing effect of carbon fibre to some extent.

In order to investigate the interfacial conditions, SEM micrographs of the fracture surface of C_f/AZ91D and C_f-PyC/AZ91D composites after the tensile tests were shown in Figure 7. The interfacial properties of the composites were corresponding to that of the mechanical properties and the fracture behaviour of the composites [23,31,34]. The initial damage of C_f/AZ91D composite occurred earlier than that of C_f-PyC/AZ91D composite under tensile strength test. C_f/AZ91D composite recovered with a narrower range after a sharp drop of loading until the fracture of the specimens. The ultimate tensile strength of C_f-PyC/AZ91D composites was 416 MPa (Figure 8), which improved by 35% compared with the C_f/AZ91D. It was believed that the increased strength was mainly attributed to the improved interfacial bonding strength, which led to the effective load transfer from the matrix to the fibre, and also the PyC interface had an ability to deform and debonding and thus to accommodate the thermal mismatch stresses between the fibre and the matrix [12]. Meanwhile, PyC coating also acted as a diffusion barrier to prevent degrading fibre–matrix reactions and protected the fibre from the loss of fibre strength [35].

The fracture surface of C_f/AZ91D and C_f-PyC/AZ91D composites after the tensile test were shown in Figure 9. The mechanical behaviour of composites C_f/AZ91D was clearly different from C_f-PyC/AZ91D. It could be seen from Figure 9(a,b) that a large number of carbon fibre bundles were pulled-out, and most of the PyC protected the carbon fibres from corrosion during the C_f/AZ91D composite fabrication process had been left after the tensile test. Figure 9(d) showed the C_f/AZ91D composite with a brittle fracture behaviour without any fibre pullout, which indicated a strong interface bonding and a poor mechanical performance [9], such a result was in fact frequently observed in C-Mg composites and attributed to the presence of Al₄C₃ at the interface [29]. For the composites C_f-PyC/AZ91D, the PyC coating was about 150 nm, PyC coating residues were found adhered to the surface of the pull out fibres (Figure 9(b)), which indicated that rich crack deflection and crack branching occurred in interfacial zone and a large amount of energy had been consumed during the failure process, which is beneficial to the mechanical properties of C_f/AZ91D [36]. Energy disperses spectrum (EDS) (Figure 9(c)) was used to explore the composition of spot A, B, C in (b). It further proved that the residues adhered to the surface of the pull-out fibres were the PyC coating. The possible fracture mechanism can be explained as below: at first when the sample is subjected to the tensile test, a crack will be initiated at the AZ91D matrix. The crack then will propagate to the fibre and PyC coating interface. Because of the relatively weak interface between the fibre and PyC coating layer, the crack can grow in the interface between them and after crossing the fibre, a fibres pull out feature will appear [37,38].

Figure 10 showed the surface of PyC/C_f after corrosion using dilute nitric acid. The aim of corrosion is to remove magnesium alloys and to facilitate the observation of carbon fibres surface. Compared with the smooth surface of T700 carbon fibre in Figure 2(a) and the coarse surface of T700 carbon fibre in Figure 10(a), it could be concluded that part of carbon atoms from PyC coating take part in the reaction of carbon and aluminium, the left PyC coating can be observed in Figure 10. Some erosion pits resulted from the removal of the carbides was caused by interfacial chemical reactions [34]. EDS was used to probe the fibre surface after corrosion by nitric acid. Traces of carbon, oxygen, magnesium, and aluminium species are seen in Figure 10(b), there are carbon and oxygen in Spot A and C, but no magnesium and aluminium, which means that this position has distracted the carbon fibre from magnesium matrix completely. Oxygen comes from nitric acid and the process of composite making, only spot B has Mg and Al element, which stem from the AZ91D matrix.

Conclusions

Mg matrix composites reinforced by the carbon fibres were fabricated by the LSEVI process. Uniform low texture PyC coating with a thickness of 150 nm was successfully prepared on carbon fibre surface and offered higher graphitisation. The coating was acting as a diffusion barrier to inhibit strength-limiting interfacial reactions, and to protect the carbon fibre from Al or Mg atom attack during LSEVI process. The coarse surface of PyC coating offered bigger sliding friction and provided desired degree of bonding between fibres and matrix. The composites reinforced with PyC-coated carbon fibres had obviously improved mechanical properties compared to the composites reinforced by the uncoated fibres. These composites had ultimate tensile strength of 416 MPa, which resulted from the appropriate characteristics of the fiber/matrix interface.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

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Effect of PyC coating on mechanical properties of C f /AZ91D composites

Abstract

Keywords

Introduction

Experimental procedures

Fabrication

Preform and matrix preparation

Coating

Cf/AZ91D composite preparation

Characterisation and test

Results and discussion

Surface morphology of Cf and Cf-PyC

Microstructural comparison of Cf and Cf-PyC

Effects of PyC coating on mechanical behaviour of Cf and Cf/AZ91D composites

Conclusions

Footnotes

References

C_f/AZ91D composite preparation

Surface morphology of C_f and C_f-PyC

Microstructural comparison of C_f and C_f-PyC

Effects of PyC coating on mechanical behaviour of C_f and C_f/AZ91D composites