The mechanical and tribological characteristic of Aluminium-Titanium dioxide composites

Levent Ulvi Gezicia, Burak Güla, Uǧur Çavdarb,*

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:



The purpose of this work was to investigate the mechanical and tribological effects of Titanium dioxide (TiO2) reinforcement in Aluminium (Al). Aluminium composites consist of 99.8% pure aluminium reinforced with five different partions of TiO2. Aluminium powders were mixed with TiO2 by ball milling for 30 minutes in a planetary mixer. The powder mixture was compacted by the cold pressing technique at 250 MPa. Two different methods used for sintering. The green compact was sintered at 600 °C for 300 seconds in open atmosphere with an Ulta-High Frequency Induction System (UHFIS) and with furnace at 600 °C for 1800 seconds. The mechanical and microstructural properties of examples were compared for different amount of reinforcement. We have observed a maximum hardness for 5 wt.% TiO2 reinforced composites.



Características mecánicas y tribológicas de compuestos de dióxido de Titanio-Aluminio. El objetivo de este trabajo es investigar las propiedades mecánicas y tribológicas de refuerzos de dióxido de titanio (TiO2) en una matriz de aluminio (Al). Se utilizó aluminio de pureza 99,8% reforzado con TiO2 ensayándose cinco cantidades diferentes de dióxido de titanio. Se mezcló polvo de aluminio y TiO2 en un molino de bolas durante 30 min, utilizando un mezclador con eje descentrado. La mezcla se compactó mediante la técnica de prensado en frío a una presión de 250 MPa. Se utilizaron dos métodos diferentes para el sinterizado. El compactado en verde se sinterizó a 600 ºC durante 300 s en atmósfera ambiental con un sistema de inducción de ultra alta frecuencia (UHFIS) y con un horno convencional a 600 °C durante 1800 s. Las propiedades mecánicas y micro-estructurales de las muestras se compararon utilizando diferentes cantidades de refuerzo. La dureza máxima se observó para un refuerzo con 5% en peso de TiO2.


Submitted: 10 December 2016; Accepted: 4 December 2017; Available On-line: 29 May 2018

Citation/Citar como: Ulvi Gezici, L.; Gül, B.; Çavdar, U. (2018). “The mechanical and tribological characteristic of Aluminium-Titanium dioxide composites”. Rev. Metal. 54(2): e119.

KEYWORDS: Aluminium; TiO2; Induction sintering; Powder metallurgy

PALABRAS CLAVE: Aluminio; TiO2; Sinterización por inducción; Metalurgia de polvos

ORCID ID: Levent Ulvi Gezici (; Burak Gül (; Uǧur Çavdar (

Copyright: © 2018 CSIC. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) License.




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.


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 13). 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.

Table 1. By 250 MPa pressed and furnace sintered samples at 600 °C for 30 min
TiO2 wt.% Powder After Pressing After Sintering    
Weight (g) Weight (g) Volume (cm3) Weight (g) Volume (cm3) Density (g·cm−3) Hardness (HB)
1 2.0027 1.9999 0.7984 2.0033 0.7923 2.6907 48
3 2.0018 1.9951 0.7994 2.0004 0.7923 2.7249 50
5 2.0009 1.9917 0.7984 1.999 0.7920 2.7558 51
9 2.0108 2.0101 0.8004 2.0163 0.7923 2.7781 48
15 2.0509 2.0038 0.8004 2.0066 0.7943 2.7297 30
a(Error range [ER] is ± 2%).
Table 2. By 250 MPa pressed and induction sintered samples at 600 °C for 5 min
TiO2 wt.% Powder After Pressing After Sintering    
Weight (g) Weight (g) Volume (cm3) Weight (g) Volume (cm3) Density (g·cm−3) Hardness (HB)
1 2.0111 2.0067 0.8004 2.0074 0.7923 2.6879 49
3 2.0174 2.0131 0.8086 2.0156 0.7923 2.6996 53
5 2.0294 2.0166 0.8014 2.0172 0.7932 2.7258 54
9 2.0144 2.0131 0.7963 2.0128 0.7943 2.6986 49
15 2.001 1.9982 0.8004 2.0174 0.7973 2.7698 34
a(Error range [ER] is ± 2%).
Table 3. Surface roughness and weight change
Sintering Method TiO2 Wt.% Porosity (μm) Weight (g)  
Ra Rz Rq Before Wear Test After Wear Test Weight (g) Change
Furnace 3 2.88 16.78 3.49 2.004 2.001 −0.15
Furnace 9 2.58 20.62 3.32 2.016 2.012 −0.21
Furnace 15 2.91 19.47 3.58 2.007 1.994 −0.63
Induction 3 2.89 18.9 3.5 2.016 2.021 0.27
Induction 9 2.68 20.3 3.33 2.013 2.03 0.85
Induction 15 2.25 17.21 2.8 2.017 2.045 1.37
a(Error range [ER] is ± 2%).


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. 14) 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).

Figure 1. The micrographs of furnace sintered samples at 200X zoom: a) 3, b) 5, c) 9 and d) 15 wt. % TiO2.


Figure 2. The micrographs of induction sintered samples at 200X zoom: a) 3, b) 5, c) 9 and d) 15 wt.% TiO2.


Figure 3. The micrographs of furnace sintered samples at 500X zoom: a) 3, b) 5, c) 9 and d) 15% wt. TiO2.


Figure 4. The micrographs of induction sintered samples at 500X zoom: a) 3, b) 5, c) 9 and d) 15 %wt. TiO2.


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).

Figure 5. The absolute values of density change percent of samples with different reinforcement of TiO2.


Figure 6. Volume and density of induction sintered samples for various amount of reinforcement.


Figure 7. Volume and density change of furnace sintered samples for various amount of reinforcement.


Figure 8. Density and hardness of furnace sintered samples for various amount of reinforcement.


Figure 9. Density and hardness of induction sintered samples for various amount of reinforcement.


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 12 and shown in Figs. 89. 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.

Figure 10. Weight change percentage after wear test for various amount of reinforcement.


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.

Figure 11. Wear test results for induction sintered samples with various amount of reinforcement.


Figure 12. Wear test results for furnace sintered samples with various amount of reinforcement.


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.

Table 4. Green density of Al-TiO2 composites
Wt.% TiO2 Weight (g) Before Press Weight (g) After Press Density (g·cm−3)
1 2.0375 2.0334 2.6657
3 2.0085 2.0024 2.7086
5 2.0188 2.0157 2.6758
9 2.025 2.0178 2.6747
15 2.0192 2.0163 2.6395
a(Error range [ER] is ± 2%).

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. 1112).


The green density decrease after 9 wt.% of TiO2 (Table 4). The density of sintered samples are higher than green density. Also the density of samples with 15 wt.% TiO2 has a significant difference for different sintering methods.
The results Show that the mechanical and tribolgical values are different for induction and furnace sintering. This is because of the difference in the sintering time and the heating rate.
It can be concluded that the composities for low reinforcement show slight change in the wear resistance compared to pure aluminium. But reinforcement of wt. 15% TiO2 decreases the COF by both methods. Porosity has a important effect with increasing reinforcement on the sample.
The Brinnel hardness drops significantly to a minimum value for both sintering methods with wt. 15% TiO2. The hardness value has a maximum for value wt. 5% TiO2 for both sintering methods.
The microstructure images show that the particle bonds decreases and the porosity increases with increasing reinforcement. This has a great effect on hardness and wear resistance.
For the application Al-TiO2 composite in industries, furnace and induction sintering parameters is optimized. With a proper fabrication process, relatively high density and high dispersion degrees of dispersion of TiO2 nanoparticles are achieved. The strength of the composite increases with increasing volume contents up to 5 wt.% TiO2 nanoparticle reinforcement. It suggests that the 1–5 wt.% TiO2 reinforcement and induction sintering as fabrication processes with less time and less energy are suitable for successful composite preparation. These results may open new perspectives for the applications of such composites as structural components.


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



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