DOI:10.30919/esmm5f207

ES Materials & Manufacturing, 2019, 3, 57-65

Published online: 02 Feb 2019

Received 15 Nov 2018, Accepted 02 Feb 2019

 

Improved Bonding Strength Between Thermoplastic Resin and Ti Alloy with Surface Treatments by Multi-step Anodization and Single-step Micro-arcOxidation Method: a Comparative Study

Logesh Shanmugam, MohammaderfanKazemi and Jinglei Yang*

 

Department of Mechanical and Aerospace Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR

*E-mail: [email protected]

 

ABSTRACT:

Due to excellent specific mechanical and physical properties, titanium-based fibre metal laminates (FML) have attracted increasing attention in marine and defense for improved impact protection. Metal and composite interface (MCI) plays a pivotal role to determine the failure mode of FML. Surface treatment of metal is commonly carried out to improve the MCI properties. In this study, two different electrochemical surface treatments, anodization and micro-arc oxidation (MAO) were adopted. The tunable hierarchical structure observed after multiple physical and chemical surface treating processes, i.e., sandblasting, anodization, etching,and annealing on Ti6Al4V. In comparison single step MAO treatment shows more pores and craters on the surface with increasing anodic voltage. The results from lap shear experiment show that the adhesive strength between Elium® resin and titanium adherend by single-step MAO treatment has been improved compared to that by multi-step anodization. By increasing the anodic voltage of MAO treatment to 600V, the highest shear strength of 18.4MPa is achieved.

Table of Content

Multiple-step anodization can be replaced by single-step MAO for enhanced bonding strength between metal and composite interface.

 

 

 

Keywords: Bonding strength; Surface treatment; Anodization; Micro-arc oxidation


1. Introduction

Fibre metal laminate (FML) is a combination of metal,and composite sandwich layers which have the desirable impact damage resistance, superior strength, excellent fatigue resistance and decent corrosion resistance.1,2 The metal currently being used is either aluminium, titanium or magnesium alloy and the fibre-reinforced plastic layer is either kevlar-reinforced, carbon-reinforced, glass-reinforced composite. GLARE (GlAss Reinforced aluminium laminate) find its application in the upper fuselage of Airbus A380 aircraft.3  Apart from aerospace, FML used in marine4 and armor5 components due to their design resilience. Among metal, titanium alloyTi-6Al-4V possesses many fascinating properties such as high fatigue strength, corrosion resistance, merchantability and weldability.6 Titanium-based FMLs attracted among the researchers and engineers due to its unique feature which includes high durability, decisive anti-permeability and assuring structural applications.7-9To achieve the desired properties, excellent adhesion is a necessity between the titanium layer and fibre reinforced layer, which is very critical in determining the overall performance (fatigue, impact damage,and other mechanical properties) of titanium-based FML.

To improve the adhesion of Ti alloy to the polymer necessary surface treatment need to be carried out. Three major category of surface treatment are commonly followed they are (i) mechanical treatment - sandblasting 10-12 and shock peening,13 (ii) chemical treatment - anodizing,14,15 etching16,17 and micro arc oxidation (MAO),18, 19 and (iii) addition of interfacial layer - sol/gel methods,20,21 plasma-spray 22 and coupling agent.23, 24He25, 26 found higher shear strength by multi-step anodization .i.e., sandblasting for 20sec, anodization at 40°C for 15min, NaOH etching for 24hrs  and annealing for 5hrs at 600°C  leads to the formation of hierarchical macro to nanopores on Ti alloy surface. However, the improved shear strength was achieved after multiple steps. Thisinspires to develop a single-step Ti alloy surface treatment method to improve shear strength, work of fracture and wettability for polymer matrix. Another chemical treatment method, micro-arc oxidation (MAO) is a potential method to create a uniform ceramic-like TiO2 film on Ti alloy metal surface by discharging sparks relatively at a high voltage. This action creates more pores and craters and also forms thick oxide film compared to the conventional anodization.27 However, the ceramic-like oxide film, pore size depends on the parameters such as electrolytic solution, power supply (unipolar or bipolar), current density, the voltage of anode and cathode and the processing time.28 The oxide filmformed on the surface is derived from the electrolyte and the metal substrate during the MAO treatment.29The resultant oxide film comprised of amorphous-to-crystallinemixed phases.30

The oxide film formed on the MAO-treatedmetal surface comprisesof two layers with aloose, porous outer layer and a thick inner dense layer. The outer porous layer creates an interlocking with the resin which contributes to improving the bonding strength. This shows the porous outer layer has a significant effect on the improvement of bonding strength.31 The porous layer formed on the MAO-treated surface increases in size by increasing voltage, this also contributes higher oxide thickness which improves the mechanical interlocking with a polymer resin.32Hsieh33 found an improved adhesive strength of TiNi shape memory alloy treated by MAO in 20vol.% phosphoric acid electrolytic solution at pH of 4.1. Wu 27 demonstrated the pore size, the phase of the oxide layer depends on the applied voltage. Tang 34 highlighted pore size of the oxide surface film improves the bonding strength, increased pore size enhance mechanical interlocking and improves the stress distribution at the bonding interface. Gao35 achieved improved lap shear strength by MAO surface treatment by modifying the concentration of Na2SiO3 in the electrolytic solution.This investigation aims to identify the shear strength, work of fracture, wettability, and surface roughness on multi-step conventional anodization treatment and to compare with single-step micro-arc oxidation treatment by increasing anodic voltage. The comparison study on two extreme conditions of low voltage anodization and high voltage MAO chemical surface treatment is first of its kind.

2. Experimental

2.1. Materials

Ti alloy (90% Ti, 6% Al, and 4% V) of grade 5 with a thickness of 1.5 mm was used for surface treatment for both multi-step anodization and single-step MAO treatment. Elium®188 is a low viscous methyl methacrylate liquid thermoplastic resin which also has resin infusion capability was used as a polymer adhesive. Elium®188 resin undergo radical polymerization to complete the formation of tougheningthermoplastic matrix, in which benzoyl peroxide (BPO) of 2wt.% (weight ratio)is used to initiate the polymerization.

2.2. Surface treatment procedures

2.2.1. Anodization surface treatment process

The process of surface treatment started with grounding the Ti alloy by 2000 grid abrasive paper and cleaned with distilled water and ethanol separately in bath sonicator and dried after every hierarchical surface treatment process. Alumina powder of diameter 5–20µm was used to clean the surface in sandblasting procedure.The anodization electrolyte solution is a combination of EDTA - 0.1M (ethylene diamine tetra acetic acid) and Na-tartrate - 0.2M which is used as the impurity and Ti-complexing agent respectively, also 7.5M of NaOH (sodium hydroxide) added to the electrolytic solution. The Ti sample is anodized at 15V and 40°C for 15min in as prepared electrolytic solution. As treatedanodized sample is etched in 1M of NaOH for 24h at 60°C. The etched Ti samples were placed in the hot furnace at 600°C for 5h to complete the annealing process. The samplewere cut into a required dimension after each step of the surface treatment process for a single lap shear experiment.

2.2.2. MAO surface treatment process

The electrolytic solution is prepared by combining sodium pyrophosphate 5.0g/L and sodium silicate 10.0g/L in distilled water. After polishing and sandblasting of Ti alloy, the samples were cleaned with distilled water and ethanol separately before MAO process. MAO Ti alloy is processed at three constant voltage of 400V, 500V, and 600V for a constant time of 5 min each. One step voltage of 400V for 2min, 500V for 2min and 600V for 1minmakes a total treating time of 5min is also fabricated. This makes to compare with other MAO treated samples which are treated at constant voltage and constant time (5min). Anodic voltage was selected based on few trails, where no arc is observed on the Ti alloy at 300V and extensive arc observed at 600V. Also, while treating at 600V in Ti-MAO-Step sample, produces an intensified spark and prolonged activity (more than 2min) makes difficult to the operator during fabrication. This similar intensified spark was also observed while treating Ti sample at constant 600V for 5min.  This chooses to treat Ti-MAO-Step (5min) sample for 2min (400V), 2min (500V) and 1min (600V).

2.3. Single-lap-shear sample preparation

Ti sample after different surface treatments (multi-step anodization and single-step MAO), were cut into the dimension of 25.4 mm×100 mm ×1.5 mm to prepare single-lap-joint. Elium® resin wasprepared carefully and spread on the Ti sample without any air bubble and well aligned to fix properly to make an overlap of 12.5 ± 0.25 mm. Fig. 1 shows the sample dimension for a single lap shear bonding experiment. The samples were cured at room temperature for 2h and in the heating oven at 80°C for 4h. The thickness of the Elium® resin was maintained at 0.4mm for all different surface treated samples. In accordance with ASTM standard D1002, single lap shear experiment were carried out in the universal testing machine with a crosshead speed of 1.3mm/min,an average value with standard deviation reported.

Fig. 1 Ti-Elium®sample dimension geometries

 

Table 1 Surface treatment of samples prepared byanodization and micro-arc oxidation.

Sample code

Description

Time

 

Current (A)

Duty

(%)

Frequency

(Hz)

Start

End

Ti-P

Sandblasting only

20s

-

-

-

Ti-PA

Sandblasting + Anodization

20s + 15min

-

-

-

Ti-PAE

Sandblasting + Anodization + NaOH Etching

20s + 15min + 24h

-

-

-

Ti-PAEA

Sandblasting + Anodization + NaOH Etching + Annealing

20s + 15min + 24h + 5h

-

-

-

Ti-MAO-400

Micro-arc oxidation at 400V

5min

12.5

7.2

20

1000

Ti-MAO-500

Micro-arc oxidation at 500V

5min

18

13.9

20

1000

Ti-MAO-600

Micro-arc oxidation at 600V

5min

30.9

27.2

20

1000

Ti-MAO-Step

Micro-arc oxidation step 400V + 500V + 600V

2min +

2min +

1min

12.5

16.2

28.9

7.1

13.3

26.5

20

1000

 

2.4. Surface characterization

SEM (scanning electron microscopy – JEOL-6390) was used to characterize the surface morphology after different surface treatment. SEI (secondary electron image) and BSE (backscattered electron) images operated at 20kV were used to find the oxide layer thickness by taking the cross-sectional images of all differently treated samples. Average and standard deviation of surface roughness were reported from four different readings measured by optical profilometry (Bruker NPFLEX) with an area of 480 µm X 640µm at a different location. The chemical composition of the oxide layer was detected by X-ray photoelectron spectroscopy (XPS, Axis-ultra, Kratos). An Al Kα, the x-raywas used at 15kV and 10mA, and the C1s peak is shifted to 285.0eV for energy calibration. The contact angle between resin and surface treated Ti was measured using contact angle measurement (Biolin Theta) with photo interval of 1s.

3. Results & Discussions

3.1. The microstructure of treated Ti alloys

 Fig. 2 and Fig. 3 shows SEM surface morphology of the metal after surface treatment of both the anodization and MAO process. In Fig. 2, for Ti-P, the surface shows microscopic and macroscopic bumps which disappear when the sample is anodized in Ti-PA. During the anodization process, the bumps were far more liable to be dissolved due to their contactwith the electrolyticsolution.36NaOH etching of Ti-PA sample has great influence on the surface morphology. After NaOH surface etching the surface becomes smoother than Ti-PA. But at higher magnification, the samples were shown with nano-structured bumps; this occurrence caused due to the chemical reaction in the corrosive NaOH solution at 60°C.The chemical reactionbetween titaniumand corrosive NaOH solution forms hydrated titanium oxide gel layer which contains Na+ ions on the surface of titanium.37This oxide gel layer contains an ample amount of water and hydrated ions,and it is mechanically unstable (Fig 2.c2). The heat treatment at 600°C for 5h after the process of NaOH etching, dehydrate and densify the mechanically unstable gel layer, forming a porous network structure whichmakes the oxide layer to firmly bonded to the metal substrate37 (Fig 2.d2).Annealing can influence the significance of the bonding strengthon Ti alloy.38The formation of nanopores on the titanium surface enhances the bonding strength and wettability. 25, 26

In Fig. 3, the SEM images depict the surface of MAO treated Ti alloy at different anodic voltages and can be seen that increasing the anodic voltage the outer porous layer formed on the surface also increases. By increasing the anodic voltage, the pore size increases from 2 μm to 10 μm which will improve the wettability and more prone to mechanical interlocking when the Elium® resin is placed on the treated surface. On increasing anodic voltage in MAO surface treatment, the current transmitsthrough the electrolyte provoke the spark initiationon the Ti alloy surfaceat high intensity. This intensified spark improves the Ti surfacemore rougher by creating regular growth mode of the volcano-shaped craters on the Ti surface.39From Table 1, the current requires for treating the samples was higher at initiation, and after 50S the current stabilizes. The higher current required at the spark initiation of all MAO sample is due to the more power required to create initial melting on the pristine Ti alloy. Once spark initiation starts, the current stabilizes showing lesser power is adequate enough to create a spark when being compared to spark initiation. However, for step voltage the current requires is slightly less from the constant voltage samples. This phenomenon is due to the fact when treating the Ti-MAO-Step sample at 400V it requires high current to create melt spark but on the consecutive voltage requires less current depicting the samples is already melted enough to form ceramic-like oxide layer.This shows that energy consumption is lower when treating the sample by a step voltage compared to constant voltage samples.

Cross-section of theoxide layer formed on the Ti surface after anodization and MAO treatment is shown in Fig. 4.The thickness of the oxide layer for all anodization treatment with further processing is relatively low when compared to the MAO samples which are treated at higher anodic voltage. It is found that the thickness of oxide layer was 1µm,1.7µm, 3.1 µm, for Ti-PA, Ti-PAE,and Ti-PAEA respectively. This shows anodization with further treatment of etching and annealing improves the oxide layer thickness.  The oxide layer thickness of MAO treated Ti alloy are 10.1µm, 20.6µm, 43.5µm, 37.5µm for Ti-MAO-400, Ti-MAO-500, Ti-MAO-600,and Ti-MAO-Step respectively. The result shows that increasing anodic voltage from 400V to 600Vin MAO treatment can increase the thickness of oxide layer which in turn improves the wettability of Elium® resin and improves the shear strength.

Fig. 5 depicts the surface roughness of Ti-PAEA has a higher surface roughness among anodization samples which is crucial in improving the bonding strength. On the other hand, Ti-PA and Ti-PAE show less roughness compared to Ti-P which is due to thedissolution of macroscopic and microscopic bumps in the electrolytic solution during the anodization and etching process respectively. However, the surface roughness of the MAO samples is much higher compared to the anodization process and has increased roughness on increasing the anodic voltage. The improved surface roughness is due to intensified spark on the surface at higher anodic voltage.