The mechanical and tribological characteristic of Aluminium-Titanium dioxide composites

Levent Ulvi Gezici^a, Burak Gül^a, Uǧur Çavdar^b,*

^aCelal Bayar University, Engineering Faculty. Department of Mechanical Engineering, 45140 Manisa, Turkey

^bİzmir Demokrasi University, Engineering Faculty, Mechanical Engineering Department, İDU Campus, 35140, İzmir/Turkey

*Corresponding Author: ugur.cavdar@idu.edu.tr

1. INTRODUCTIONTOP

Aluminium alloys used in many applications because of their high strength, low density and good formability. The limiting factor was the poor surface hardness and wear resistance (Xiang et al., 2015). Especially aluminium based metal matrix composites (MMCs) are used in manufacturing industry due to their low cost and weight (Singh et al., 2015). Aluminium based MMCs have improved wears resistance, toughness and superplastic forming which make Al more attractive than other metals (Pydi et al., 2013; Hassani et al., 2014; Singh et al., 2015). Aluminium matrix composites (AMCs) reinforced with ceramic particles are relatively easy to process which are applied in various industrial domains (Hassani et al., 2014). In the straight compaction and sintering of Al powder have some complexities due to the presence of an oxide layer covering the powder particles (Gökçe and Fındık, 2008). Typically the oxide layer on bulk aluminium at room temperature varies between 10 to 20 nm while the layer can vary between 50 and 150 nm on powder. This layer acts as a barier to solid state sintering (SSS). However additions of Mg have been shown to be effective in disrupting the oxide layer and faciliating the SSS of aluminium (Lumley, 2011).

Titanium dioxide (TiO₂) is a single crystalline system in the surface science of metal oxides. TiO₂ has three different phases in nature which are anatase, rutile and brookite. It is used as a heterogeneous catalyst and photocatalyst. It also finds applications in solar cells. TiO₂ used in gas sensors and in paint industries as white pigment (Chuang et al., 2011). The surface science of TiO₂ shows a rapid growth of interest among researchers and scientists. TiO₂ film coating is used for improving corrosion resistance, mechanical properties and surface energy (Leng et al., 2007; Aniolek et al., 2016). In addition TiO₂ coating is used on aircraft surface since it absorbs the radar signals (Raut and Choudhary, 2015).

TiO₂ has a high diffraction index and it requires a particle size of approximately half the wavelength of the light for strong light scattering capability. There are some difficulties by working with nanoparticles. TiO₂ particles tend to agglomerate due to their large-surface area and nano size effect. Large particle sizes improve dispersion but smale particle size leads to higher Van Der Waals Forces which decrease the dispersion of the particles. Only well dispersed TiO₂ nanoparticles can lead a unique properties of the composites (Pinto et al., 2015).

TiO₂ particle sintering does not happen up to 1000 °C and pure anatase transformation begins into a rutile at abaout 600 °C in air. The transformation also depends on the heating rate (Tubio et al., 2015). So the relative density and grain size of TiO₂ were usually investigated as a function of sintering time and heating rate. At higher heating rates amorphous TiO₂ transformed to rutile directly while at a lower heating rates the amorphous transformed to anastase at 500 °C and then to rutile at 800–900 °C (Li et al., 2010). The density does not change at temperatures between 650–800 °C. But at temperatures between 800–1000 °C, the grain boundary diffusion causes densification and grain growth (Mazaheri et al., 2009).

The aim of this study was to examine the effect of TiO₂ reinforcement in aluminium. For this purpose the density, porosity, hardness, wear resistance, and microstructure of Al-TiO₂ composites were investigated. The green compacts with the same percent of reinforcement were sintered with induction and furnace. The results of induction and furnace sintered examples have been compared with each other to see the mechanical and tribological effects of the sinter method.

2. MATERIALS AND METHODSTOP

By paper research we recognize that the TiO₂ reinforcement effect on mechanical properties does not change much after 10 wt.% reinforcement. And the maximum change is between 0–5 wt.% reinforcement. Therefore, we chose to study 1, 3, 5, 9 and 15 wt.% TiO₂ reinforcement rates. Aluminium based composities that consist of 99.8% pure Al was reinforced with 99.5% pure TiO₂. The size of particles was 50–70 micron for Al and 200–350 nm for TiO₂. The mixture was ball milled in a planetry ball mill with 30 steel balls at a speed of 56 rpm for 30 min for hindering agglomeration. The ball to powder mass ratio was 10:1, 15:1, 20:1. For each sample 2 gram powder was compacted at 250 MPa by cold isostatic pressing. Thirty green samples with 16 mm diameter and 4 mm thickness were pressed. Ten of them were sintered by a conventional sintering in an electric resistance Protherm furnace at 600 °C for 1800 seconds with a heating rate of 10 °C/second. Other ten samples were sintered with induction and with the same reinforcement portions at 600 °C for 300 seconds with a heating rate of 100 °C/second. The UHFIS using 2.8 kW and 900 kHz with a cylindrical coil with inner diameter of 20 mm. This system was called UHFIS by Çavdar and Güls¸ahin (2014) and Çavdar, P. and Çavdar, U. (2015), because of its very high frequency. Both sinter application was under an athmospheric environment.

The wear ressistance of sintered Al-TiO₂ composite samples was invastigated at room temperature using a CSM Tribometer with pin on disk test against a 5 mm diameter steel ball with 5 N load. The wear test performed approxiametly for 7950 laps, 100 m way and 1000 seconds. The weight of samples was recorded before and after testing (Tables 1–3). The Brinell hardness test of sintered samples was performed using a BMS 200-RB (Rockwell & Brinell) hardness measurement apparatus at a load of 60 N using a steel ball of 2.5 mm diameter. The Surface roughness test was made using a Mitutoyo Surf SJ-301 profilometer. To understand the relation between surface roughnesses and wear test results (friction coefficient, weight change) we chose the samples with 3, 9 and 15 wt.% TiO₂ reinforcement because they show the maximum effect (Table 3). After cutting the specimens they were hot mounted using a citopress mounting press. The surface of sample was grinded with 200, 400, 600, 800, 1000 and 1200 emery paper. After grinding the surface of sample was polished using a solution with 1 micron particle size of alumina suspansion. The images of the microstructure were taken with Meiji Techno Metallurgical Microscope.

3. RESULTSTOP

The influence of TiO₂ reinforcement on density and hardness values are given in Table 1 and 2 for furnace and induction respectively. The influence on wear resistance and porosity are given in Table 3.

It can be seen in Table 1 and 2 induction sintered samples are about 2% harder than the samples sintered in furnace. The hardness of the sample increases for wt. 3% and 5% TiO₂ reinforcement. By increasing the reinforcement it is obvious that the hardness decreases significantly.

Roughness and friction coefficient of porous titanium dioxide films increase with inrceasing TiO₂ concentration. Generally porosity causes higher roughness values and the coefficient of friction (COF) increases with increasing roughness according to the literature (Piwoński, 2007). Our observation shows that this is not the case. The roughness does not show the same correlation with COF depending on the amount of reinforcement.

During annealing the diffusion at the interface layer has an effect on mechanical properties of Al-TiO₂ composites. Since Gibbs free energy of formation for Al₂O₃ is lower than that of TiO₂, a short range thermal diffusion of constituent elements can °Cur at the interface. The annealing time stimulates the decomposition process and induces a large diffused area which results a mixture of Al₂O₃ and Al3Ti fragments in the aluminium matrix. The decomposition of the particles and their microstructural evolution depends on annealing time and the size of particles as mentioned in literature (Shin et al., 2013). Sintering methods cause difference in mechanical and tribological values, which can due to the different sintering time and heating rate. At higher heating rate amorphous TiO₂ transformed to rutile directly while at a lower heating rate the TiO₂ phase will be anastase at 500–800 °C. The Mohs scale for rutile is around 6.0–6.5 and for anatase 5.5–6.0. We observed that, induction sintered samples have higher hardness values then furnace sintered samples (Tables 1 and 2).

Increasing the weight percentage of TiO₂ as seen in microstructure images (Figs. 1–4) cause a increase in the grain size and grain boundary region. The porosity between grain boundaries increases with increasing reinforcement. For both sintering method above 5 wt.% reinforcement the hardness has a tendency to decrease (Figs. 8 and 9).

Addition up to 1 wt.% TiO₂ in tin-led solders decreases the grain size and width of the grain boundary. The decrease resulted an increase in hardness. For reinforcement with 2 wt.% TiO₂ microporosity was observed at grain boundary region. Moreover, increased reinforcement porosity can form microscopic cracks which is detrimental to strength as reported by work (Lin et al., 2003). This is consistent with our observations and explains the sharp fall in hardness values (Figs. 8 and 9) for reinforcement above 9 wt.% TiO₂.

The density change absolute value of green and sintered composites for both sintering method are given in Fig. 5. Induction sintered samples have a little change in density up to 9 wt.% reinforcement. Furnace sintered speciements density increases linearly but drops after 9 wt.% reinforcement but at the same percent reinforcement the volume increases and hardness decreases sharply (Fig. 7). For both sinter method volume increases countinously after 5 wt.% reinforcement (Figs. 6 and 7). The hardness has a maximum value at 5 wt.% and a minimum value at 15 wt.% (Figs. 8 and 9).

At 900 °C sintered Cu-5 wt.% TiO₂ composites have a maximum hardness as mentioned in literature (Sorkhe et al., 2014). We have observed similar results. The hardness results are given in Tables 1–2 and shown in Figs. 8–9. An increase up to 5 wt.% reinforcement cause an increase in hardness. Above 9 wt. % reinforcement causes a sharp decrease in hardness.

According to literature (Khodabakhshi et al., 2014) annealing time and temperature have different effects on Al-Mg- 3, 5, 6 vol. % TiO₂ like microstructure and mechanical properties. Sintering time has an effect on the chemical reaction between Al and TiO₂. The heating rate and sintering time is quite different for furnace and induction sintering. This difference explains also the wear behavior of samples which are sintered with different methods. At higher heating rate like in induction, the samples are harder. An increase in hardness causes an increase in wear ressitance. The weight change (Fig. 10) after wear test (Tables 2 and 3) indicates that counterpart is worn up during wear test.

The wear test results for Al and Al-TiO₂ composites are shown in Figs. 11 and 12. The friction coefficient (COF) of furnace sintered pure Al and 3, 9 wt.% reinforced samples show an increase at the beginning of the first 10 meters of sliding distance and small fluctuations after 50 meters. It shows a negative drop in COF although reinforcement increased by 15 wt.% (Fig. 12). Negative friction force reported by Chen and Zhau (2003). The reason for this can be the high surface roughness (Table 3) which causes high vibration which can cause sample counterpart contact problems during wear test. In the same time the results show high wear loss (Fig. 10) and less COF (Fig. 12). As seen in Table 3 (Fig. 10) induction sintered samples have a higher wear resistance, higher hardness and a lower density than in furnace sintered samples. Comparing Figs. 3b, 3d and Figs. 4b, 4d it can be concluded that induction sintered samples have a beter compaction.

By comparition the friction coefficient of induction sintered samples (Fig. 11) with furnace sintered samples (Fig. 12) we see a good relation for 3 and 9 wt.% of TiO₂. But there is a significant change in COF for 15 wt.% of TiO₂. The weight change (Table 4) after wear test indicates wear loss in counterpart.

The wear test of induction sintered samples show a great fluctuation in COF compared to the samples sintered in furnace. It is obvious that the sintering method and reinforcement amount has a great effect on surface roughness, wear resistance and hardness. In furnace sintered samples have a greater wear loss during wear test (Fig. 10). The furnace sintered samples show a stable friction coefficient of 0.6 μ after 200 seconds of sliding time (Figs. 11 and 12).

Aniolek et al. (2016) reported for oxidised titanium surface at 600 °C in the initial stage of wear test a significant increase in the coefficient of friction up to 0.8–0.9 and stabilises at 0.5–0.65. Leng et al. (2007) reported in their work very stable friction coefficient about 0.5 after 500 cycles for titanium dioxide deposited Ti6Al4V substrate. The titanium alloy showed an unstable friction coefficient in the range of 0.5–0.8 after 2000 cycles. Also the mass loss was smaller for TiO₂ deposited substrate. Both are a good agreement with our results (Figs. 11–12).

4. CONCLUSIONSTOP

ACKNOWLEDGMENTTOP

This work was supported by Celal Bayar University Scientific and Research Commission under the Project number 2015-010.

REFERENCESTOP