In this study, the effects of aluminium titanate and mullite added porcelain ceramic composites produced by the traditional ceramic production process on the wear behaviour of the applied load and time were investigated. Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃.2SiO₂) ceramics were synthesized by reaction sintering from Al₂O₃, SiO₂ and TiO₂ powders consisting of homogenization by wet ball milling followed by sintering Al₂TiO₅ (AT) and 3Al₂O₃.2SiO₂ (M) in atmosphere conditions at 1,550 ^oC for 2 h and 1,400 ^oC for 2 h, respectively. AT and M in amounts of 0 and 20 wt.% were mixed with Porcelain (P). AT and M reinforced porcelain ceramics were prepared by the powder metallurgy route. After drying, the powders were compressed to preforms of 56 ´ 12 ´ 10 mm by uniaxial pressing at 200 MPa. The green compacts were sintered at 1100-1200 ^oC for 1-5 h in air. Investigations were carried out, on the effect of addition of Al₂TiO₅ and 3Al₂O₃.2SiO₂ in terms of physical properties, microstructure, XRD phases, as well as wear and mechanical properties of 3Al₂O₃.2SiO₂ and Al₂TiO₅-reinforced porcelain ceramic composites. Phase and microstructural characterizations of the sintered materials were carried out by using X-ray diffraction technique (XRD) and scanning electron microscope (SEM). The Micro Vickers hardness testing was performed using the Shimadzu HMV-MIII hardness tester. PLINT brand abrasion tester was used for the abrasion tests of ceramics. As a result, as the load and time increased in all samples, the amount of wear increased. While the wear volume in the P sample under 70 N load for 5 min was 5.15 ´ 10^-2 mm³, it reached 33.33 ´ 10^-2 mm³ in 20 min under 120 N load. Similarly, these values were measured as 0,66 ´ 10^-2 mm³ and 5.48 ´ 10^-2 mm³ in the PAT sample, 1.09 ´ 10^-2 mm³ and 7.41 ´ 10^-2 mm³ in the PM sample and 1.75 ´ 10^-2 mm³ and 38.44 ´ 10^-2 mm³ in the PMAT sample. M and AT additives improve the wear properties of porcelain, while M and AT both have negative effects on wear

Keywords: Aluminium Titanate, Mullite, Porcelain, characterization, hardness, wear

Porcelain is a hard, fine-grained, nonporous, and usually translucent, highly vitrified materials. White ceramic ware consisting essentially of kaolin, quartz, and a feldspathic rock and is fired at a high temperature. Porcelains have a triaxial formulation comprising about 50% clay, 25% flux and 25% filler. It is a ceramic material that can be used for technical or artistic purposes, with or without glazing, as well as semi-light transmittance. Porcelain represents one of the most complex ceramics, formulated from a mix of clay, feldspar and quartz, sintered at temperatures between 1,200 ^oC and 1,400 ^oC to form a glass-ceramic composite. The clay is comprised of kaolinite conferring plasticity to green paste and is the precursor of mullite crystals. The fluxing agent is feldspar and the filler is quartz, which most likely leads to higher strengths of the unfired tiles. Firing bodies containing these three components exhibit a grain and bond microstructure, which consists of coarse quartz grains joined by a finer bond or matrix that contains mullite crystals and a glassy phase [1-7].
Aluminium Titanate (Al₂TiO₅) exhibits strength thermal shock resistance and low thermal conductivity, chemical resistance in molten metals. Aluminium Titanate (Al₂TiO₅) ceramics have low thermal expansion coefficient, low thermal conductivity and excellent thermal shock resistance enabling them for applications in glass, manufacturing industries. These properties and the thermal shock resistance make Al₂TiO₅ a promising candidate reinforcement in ceramic systems, whereas the expansion coefficient of Al₂TiO₅ is anisotropic and shows large mismatch with matrix alumina [2, 8-11].
Mullite is the unique stable intermediate crystalline phase of the binary system Al₂O₃ - SiO₂. Mullite (3Al₂O₃· 2SiO₂) is a good and low-cost refractory ceramic. Its applications have very large area in refractory field to technical applications. Mullite ceramics have many desirable properties and very good thermo-mechanical properties such as excellent high-temperature strength and creep resistance, good thermal and chemical stability. Its thermal expansion coefficient is relatively low leading to a good thermal shock resistance [12-19].
Ceramic materials can significantly improve the response of parts for applications involving contact loadings because of their high hardness, low friction, excellent corrosion resistance and ability to operate under high temperatures. Wear of ceramics are anisotropic and relate to crystal structure as with metals [18-25].
Consequently, the main objective of the present study is to investigate the influence of Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃·2SiO₂) phases added within porcelain on the wear behaviour of porcelain. On the other hand, physical (density, water absorption and porosity) and other mechanical (hardness, bending strength) properties of ceramic composite materials to be produced under different sintering conditions will also be examined.

In this study, aluminium titanate and mullite added porcelain ceramic composites were produced by the traditional ceramic production process. The powders (Porcelain, Al₂O₃, SiO₂, and TiO₂) used in this study were obtained from the Eczacıbaşı (Eczacıbaşı Esan, Turkey). The mixtures were prepared by mechanical alloying method in acetone environment with alumina ball mill. The powders were dried in oven at 110 ºC for 24 h before mixing. Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃·2SiO₂) ceramic powders were synthesized by reaction sintering from Al₂O₃, SiO₂ and TiO₂ powders consisting of homogenization by wet ball milling. Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃·2SiO₂) ceramic powders were synthesized in air at 1,550 ^oC and 1,400 ^oC for 2 h, respectively. Then, the ceramic phases formed were made ready by crushing, grinding and screening processes. Then 0 and 20% by weight aluminium titanate (AT) and mullite (M) doped porcelain mixtures were prepared by powder metallurgy method (hereinafter these mixtures will be named P, PM, PAT and PMAT respectively). The prepared mixtures were wet milled with alumina ball mill for 3 h and sieved. After drying, the powders were compacted to preforms of 56x12x10 mm by uniaxial pressing at 200 MPa. The green compacts were sintered at 1,100-1,200 ^oC for 1-5 h in air conditions using a heating rate of 5 ^oC min^-1 in a high temperature furnace (Protherm Furnace).
The densities, porosity and water absorption of the sintered specimens were measured by the Archimedes method. The morphological parameters of the different phases were characterized by using a semiautomatic image analyser, EDX and the formed phases were analysed by X-ray powder diffractometer (Rigaku, Dmax, IIIC) using Cu Kα radiation. The microstructural characterization of the sintered samples was carried out using scanning electron microscopy (SEM, Tescan Mira). Micro hardness tests (Shimadzu, HMV-MIII) were measured on the polished surface of the samples at room temperature. At least six individual tests with a peak load of 1,000 g and a loading time of 20 s were performed for each set of composites. Plint brand abrasion tester was used for the abrasion tests of ceramics. Steel disc was used as wear disc. Wear tests were performed on each sample at 5, 10, 15 and 20 minute wear duration and 70, 90, 120 N force.

The physical properties of mullite and aluminium titanate-reinforced porcelain ceramics samples, sintered at 1,100-1,200 ^oC for 1-5 h are given in Table 1 [2, 3, 5, 7, 14].
As the temperature and sintering time increases, the density, relative density and strength are increasing, while the water absorption and porosity decreases. Relative density decreased with the addition of Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃·2SiO₂). Water absorption and porosity increased with the addition of Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃·2SiO₂). In the strength measurements, increased sintering temperature and time increased with the addition of AT added porcelain, while there was some decrease in the porcelain with M addition. The variation in bulk density with heating temperature is given porcelain compositions showed an increasing trend in their bulk density with temperature due to increased consolidation with temperature. Also, Firat at al. stated that the effect of sintering temperature on the apparent porosity that reduces with increasing sintering firing temperature in all porcelain compositions [28, 30]. Similarly, Koca et al. reported that densifications increase with the increase of sintering time in general [29, 30].
Figs. 1(a-d) are typical SEM micrographs taken from the surfaces of Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃·2SiO₂)-reinforced porcelain ceramic composite samples (P, PAT, PM and PMAT). It can be seen that P appears to be denser and to have fewer pores than the samples PAT and PM. Approximately 40% of the P samples are more intense in the vitreous phase. Aluminium titanate and mullite are uniformly dispersed in the porcelain matrix. The addition of aluminium titanate and mullite influences the grain structure, as can be seen in the microstructures of the composites. The grain size grows, as the dosage of aluminium titanate and mullite increases. Figs. 1(e) and 1(f) shows the microstructure of PMAT sample used for phase analysis with EDS. Analysis has revealed that the matrix consists of mullite, quartz and aluminium titanate phases, the A, B and C areas demonstrated porcelain, aluminium titanate and mullite, which consist of K, Al, Si, O and Ti [2, 3, 5, 7, 15].
Fig. 2 shows the representative XRD patterns of the ceramic composites (namely P, PAT, PM and PMAT) which were sintered at 1,200 ^oC for 5 h. XRD patterns of the resultant Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃·2SiO₂)-reinforced porcelain ceramic composites revealed that the main phases are Aluminium Titanate (Al₂TiO₅, PDF card No. 01-070-1434), Mullite (3Al₂O₃.2SiO₂, PDF card No. 00-015-0776) and quartz (SiO₂, PDF card No. 01-078-1252). In Fig. 2, the XRD pattern of the pure porcelain (P, PAT and PM) ceramic conform with that in the literature. The Al₂TiO₅ and mullite phases became present in the pattern of the PAT and PM sample first, while an addition of 20 wt. % Al₂TiO₅ and mullite completed [2, 3, 5, 7, 12-15].
Fig. 3 shows micro hardness plots for the P, PAT, PM and PMAT ceramic composites sintered at 1,100-1,200 ^oC for 1-5 h. Micro hardness (Shimadzu, HMV) was measured on the polished surface of the samples at room temperature. As the temperature and sintering time increased, the hardness of the all samples increased. In the hardness measurements, increased sintering temperature and time increased with the addition of AT added porcelain, while there was some decrease in the porcelain with M addition.
Plint brand abrasion tester was used for the abrasion tests of ceramics. Steel disc is used as wear disc. Wear tests were performed on each sample at 5, 15, 20 min wear duration and 70, 90, 120 N force (550 rpm constant speed). First, the specimen was measured with a precision scale of 0.0001 g, and the amount of wear was determined by measuring again after the specified wear time. The wear results are shown in Fig. 4, 5 and 6.
As the load and time increased in all samples, the amount of wear increased. Zhang et al. concluded that the specific wear rate of B₄C-SiC ceramics increased with the increase in load [31]. Similarly, Akkus et al. found that the wear rate increased with increasing wear load and load application time. [20]. The wear volume in the P specimen under 70 N load for 5 min was 5.15 x 10^-2 mm³ while it reached 33.33 x 10^-2 mm³ at 20 min under 120 N load. The wear volume in the PAT specimen under 70 N load for 5 min was 0.66 x 10^-2 mm³ while it reached 5.48 x 10^-2 mm³ at 20 min under 120 N load. The wear volume in the PM specimen under 70 N load for 5 min was 1.09 x 10^-2 mm³ while it reached 7,41 x 10^-2 mm³ at 20 minutes under 120 N load. The wear volume in the PMAT specimen under 70 N load for 5 min was 1,75 x 10^-2 mm³ while it reached 38,44 x 10^-2 mm³ at 20 min under 120 N load. While M and AT additive improved the wear characteristics of porcelain, P and AT had negative effects on two-in-one wear [20-22].

The effects of load and time on the wear properties of porcelain ceramic composites with aluminium titanate and mullite were studied. The following results were obtained:
1. Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃· 2SiO₂) ceramics were synthesized by reaction sintering from Al₂O₃, SiO₂ and TiO₂ powders which consisted of homogenization by wet ball milling followed by sintering Al₂TiO₅ (AT) and 3Al₂O₃·2SiO₂ (M) in air at 1,550 ^oC and 1,400 ^oC for 2 h, respectively.
2. Aluminium Titanate (Al₂TiO₅) and Mullite (3Al₂O₃· 2SiO₂) added porcelain ceramic composites have been prepared by powder metallurgy process.
3. Generally, As the sintering temperature and sintering time increases, the density, relative density and strength increased, while the water absorption and porosity decreased.
4. In the strength measurements, increased sintering temperature and time increased with the addition of AT added porcelain, while there was some decrease in the porcelain with M addition.
5. Aluminium titanate and mullite are evenly distributed in the porcelain matrix. Phase analysis has revealed that the matrix consists of mullite, quartz and aluminium titanate phases.
6. As the temperature and sintering time increased, the hardness of the all samples increased. In the hardness measurements, increased sintering temperature and time increased with the addition of AT added porcelain, while there was some decrease in the porcelain with M addition.
7. As the load and time increased in all samples, the amount of wear increased. The wear volume in the P specimen under 70 N load for 5 min was 5.15 x 10^-2 mm³ while it reached 33.33 x 10^-2 mm³ at 20 min under 120 N load. The wear volume in the PAT specimen under 70 N load for 5 min was 0,66 x 10^-2 mm³ while it reached 5,48 x 10^-2 mm³ at 20 minutes under 120 N load. The wear volume in the PM specimen under 70 N load for 5 min was 1.09 x 10^-2 mm³ while it reached 7.41 x 10^-2 mm³ at 20 min under 120 N load. The wear volume in the PMAT specimen under 70 N load for 5 min was 1.75 x 10^-2 mm³ while it reached 38.44 x 10^-2 mm³ at 20 min under 120 N load. While M and AT additive improved the wear characteristics of porcelain, M and AT had negative effects on two-in-one wear.