Effect of Silicon Addition on High-Temperature Solid Particle Erosion-Wear Behaviour of Mullite-SiC Composite Refractories Prepared by Nitriding Reactive

Solid particle erosion-wear experiments on as-prepared mullite-SiC composite refractories by nitriding reactive sintering were performed at elevated temperatures, using sharp black SiC abrasive particles at an impact speed of 50m/s and the impact angle of 90 in the air atmosphere. The effects of silicon powder addition and erosion temperature on the erosion-wear resistance of mullite-SiC composite refractories were studied. The test results reveal that Si powders caused nitriding reaction to form β-sialon whiskers in the matrix of mullite-SiC composite refractories.The erosion-wear resistance of mullite-SiC composite refractories was improved with the increase of silicon powder addition and erosion temperature, and the minimum volume erosion rate was under the condition of 12% silicon added and a temperature of 1400C. The major erosion-wear mechanisms of mullite-SiC composite refractories were brittle erosion at the erosion temperature from room temperature to 1000C and then plastic deformation from 1200C to 1400C.


Introduction
With the development of technology of circulating fluidized bed boiler, garbage incineration boiler, and coke dry quench there is a growing demand for high-temperature erosionwear-resistant materials with better performances whereas the conventional wear-resistant materials cannot fully meet these requirements anymore.For instance, the refractory linings materials for circulating fluidized bed boiler are commonly eroded between 850 ∘ C and 1100 ∘ C by the coal powders, high-melting mineral impurity particles, and limestone grits for desulfurizing; their services can be severely shortened by such high-temperature erosion-wear [1,2].At present, the commonly used wear-resistant refractories mainly include the traditional mullite-SiC [3], SiC bricks, and Si 3 N 4 /sialon bonded SiC [4].The traditional mullite-SiC bricks cannot meet the serve in high temperature due to their low erosionwear resistance.SiC bricks and Si 3 N 4 /sialon bonded SiC bricks cannot be used on a large scale because of their high cost.Therefore, it is essential to devise a new sort of promising candidate erosion resistant material with high performance, low cost, and long service life.
-sialon is a kind of solid solutions with the formula Si 6− Al  O  N 8− (0 <  ≤ 4.2) where Si 4+ and N 3− are partially replaced by Al 3+ and O 2− , respectively.Such replacements without changing the crystal structure only increase the crystal unit sizes [5,6].Sialon-based materials are regarded as a promising candidate for erosion resistant materials, because of their excellent properties such as high strength, high toughness, chemical inertness, good wear resistance, corrosion resistance, and outstanding thermal shock resistance.For example, these sialon-based materials are widely used in valve seats, cutting tools, engines, cylinder liners, and other mechanical systems, which all requiring wear resistance at elevated temperatures [7][8][9][10].Recently, the work on the wear properties of /-sialon composites ceramics revealed that under mild wear conditions, the single phase -sialon materials showed better resistant under mild wear conditions.However, under more severe conditions, composites with higher -sialon content showed better wear properties [11][12][13].Chen et al. [14][15][16][17] have proposed that sialon could be easily prepared by high-temperature combustion syntheses, which were suitable for use as reinforcing agent in composites.It was found that with increasing sialon content in the composites, there was a monotonic increase in both bending strength and fracture toughness.Also, the wear resistance and fracture toughness of the material increased with increasing the size of elongated sialon grains.Therefore, introducing sialon phase into the mullite-SiC composite matrixes would be a useful approach for enhancing their erosion resistant.Refractories consisting of aggregate and binder phases are regarded as representative brittle materials.The fracture toughness of material plays an important role in erosionwear resistance.We can predict the erosion-wear resistance to conduct the experiment on fracture toughness of materials.It is noted that a lot of research works have been done on the fracture toughness experiments for ceramics.In the present research work, fracture toughness was usually evaluated in three different stress states (which include pure mode I, pure mode II, and combined mode I and mode II) using the asymmetric in the four-point bending method by a centrally cracked Brazilian disk specimen for ceramics [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33].At present, we have not conducted the research on fracture toughness of materials in this paper, and we will emphasize and discuss the relationship between fracture toughness and the erosion-wear resistance in the next work.
In this paper, we employed silicon and Al 2 O 3 powders as raw materials to in-situ nitride to generate sialon, which surrounded mullite and SiC particles to form sialon bonded mullite-SiC composite refractories.Mullite-SiC composite refractories with high performance and low cost were prepared by nitriding reaction sintered.The work on their preparation and mechanical properties, heated at different temperatures by carbonization reaction sintering, has been reported recently [3].The as-prepared sialon bonded mullite-SiC composite refractories could be developed as the linings materials of circulating fluidized bed boiler, garbage incineration boiler, and coke dry quench, which are usually suffered solid particles erosion.Up to now, the study on the erosionwear resistance of mullite-SiC composites refractories has not been reported previously.Therefore, in this paper, we investigated the solid particles erosion-wear behavior of the as-prepared mullite-SiC composite refractories by nitriding reactive sintering at high temperature.This work may provide theoretical basis for the development of wear-resistant refractory materials.
The starting materials according to the mixture ratio designed in Table 1 were mixed in the mixer and the right water into the mixture during the mixing process was added.After the mixing uniformity, the materials mixture was pressed at 40 MPa for 30 s to get cylindrical samples of Φ 50 mm × 10 mm.Subsequently, the cylindrical samples were further compacted by cold isostatic pressing at 120 MPa for 90 s.Then these green bodies were dried at 110 ∘ C for 12 h.Finally, the green bodies were sintered at the temperature of 1300 ∘ C and 1400 ∘ C for 90 minutes, then rose to 1500 ∘ C for 3 h in a nitrogen atmosphere.When the sintering process was finished, the fired samples were furnace-cooled to room temperature.
The erosion-wear experiments were performed by self-designed high-temperature solid particle erosion-wear equipment, according to ASTM G76-04.The schematic diagram of erosion-wear experiment is presented in [2,34].Angular black SiC particles (the particles size of 325-830 m is more than 97%) were used as abrasive particles.The pressure of compressed air was 0.45 MPa.The flux rate of impact particles was 60 g/min.The impingement angle of impact particles stream on the target was 90 ∘ and impacted at different temperatures (25 ∘ C, 200 ∘ C, 400 ∘ C, 600 ∘ C, 800 ∘ C, 1000 ∘ C, 1200 ∘ C, and 1400 ∘ C) with 100 g of black SiC particles.
Intensity (a.u.) Solid particle erosion-wear resistance was characterized using volume erosion rate, which could be calculated with the following expression [2]: volume erosion rate (mm 3 /g) = average mass loss specimen density × the mass of impact particles . ( Archimedes water immersion method was used in the determination of bulk density and apparent porosity.According to the ASTM C1421-01b (R2007), the bending strength was determined via conventional three-point bending method with a support roller span of 20 mm and a crosshead speed of 0.05 mm/min at room temperature, and the bend samples (6 mm × 6 mm × 40 mm) were tested more than three times.The crystalline phase was monitored via Xray diffraction (XRD, XD-3, Cu K 1 radiation,  = 1.5406,Purkinje General Instrument Co., Ltd.).And the scanning electron microscopy (SEM, JEM-6460LV, Japan) was used to observe the microstructure of the samples eroded by SiC abrasive particles.Figure 2 shows SEM photographs and EDS spectrum of samples.There are large amounts of whiskers structure to  2. The properties of  sample M4 (12% silicon powder added) are better than other samples.

Effect of Silicon Addition and Erosion Temperature on the Volume Erosion Rates of Samples.
Figure 3 shows the effect of erosion temperature and silicon powder addition on the volume erosion rates of the samples.As Figure 3 reveals, the volume erosion rate tends to slightly change with increasing the erosion temperature from 25 ∘ C to 800 ∘ C, then sharply decreases from 800 ∘ C to 1400 ∘ C. The result reveals that mullite-SiC composites refractories had excellent high temperature erosion-wear resistance.The reasons can be explained as follows.The binder phase was softer and can show more plastic deformation at elevated temperatures.So it can consume and absorb more impact energy made by SiC erosion particles.Also at high temperature, the volume of aggregates would expand to a certain extent and make a higher bonding strength with the binder phase, which could improve the erosion-wear and spalling resistance of refractories.
As Figure 3 further reveals, the volume erosion rates of samples decreased sharply with the increasing of silicon powder addition at the erosion temperature from 25 ∘ C to 1000 ∘ C the reason could be due to the apparent porosity decrease and the mechanical properties improved with the increasing of silicon powder addition.Furthermore, the content of sialon increased with the increasing of silicon powder addition, which can improve the erosion-wear resistance at the experiment temperature from 25 ∘ C to 800 ∘ C. The refractory linings materials of circulating fluidized bed boiler, garbage incineration boiler, and coke dry quench are commonly used at the temperature between 850 ∘ C and 1100 ∘ C. The volume erosion rate of sample M4 (12% silicon added) tends to slightly change and reach the lowest value about 5 mm 3 /g under the use of temperature between 850 ∘ C and 1100 ∘ C. Therefore, sample M4 (12% silicon added) can meet the use as the lining wear-resisting refractories.

Erosion-Wear Mechanisms Analysis.
Refractories are regarded as brittle materials, which consist of coarse aggregate and binder phase.So the material removal of refractories is the main brittle erosion mechanism.Figure 4 shows erosion morphologies of the binder phase in the samples.As Figure 4 reveals, the structure of sample M1 (no silicon added) was loosened (shown in Figure 4(a)), and the bonding strength was weak in both aggregate and binder phases.Therefore, the aggregate was easier to be dropped due to impact of the 36 SiC erosion particles, which caused the high volume of erosion rate.While the structure of sample M4 (12% silicon added) was denser, the bonding strength of the aggregate and binder phases of sample M4 was improved.So the erosion mechanism of binder phase was mainly started from the defects on the edge of pores (shown in Figures 4(b) and 4(c)) at low temperature in sample M4.Pores of the binder phase were enlarged gradually because of impact of the 36 SiC erosion particles, which caused the loss of binder phase.The erosion damage for the sample M1 is more serious than the sample M4, which shows that the overall strength and erosion resistance of samples were improved with the increasing of silicon addition.
Figure 5 shows the typical brittle erosion mechanism of aggregate in sample M4 (12% silicon added).As can be see from Figure 5, the cracks and brittle fractures have occurred in the aggregate during the impact of the SiC erosion particles.Then the material was removed as the brittle fracture of the aggregate, which causes the high erosion rates at low temperature.Given the above, we can know that the erosionwear resistance of samples was improved with the silicon addition.Also, the brittle erosion mechanism is a dominant model for the refractories removal at low temperature.
Figure 6 displays SEM images and EDS patterns of sample M4 (12% Silicon added) eroded at 1400 ∘ C. It can be seen from Figures 6(a) and 6(b) that -SiC on the surface of samples was oxidized at high temperature to form a thin dense SiO 2 glassy film which can blunt the surface cracks and improve the high-temperature erosion resistant materials [35].Figure 7 shows the representative plastic deformation mechanism of samples at high temperature.When erosion occurred at high temperature, the binder phase started to soften because of the oxidation and formed a layer of glass-like substance on the surface of samples, which can accommodate more plastic deformation.The plastic deformation can reduce sharply the volume erosion rate of material.However, there is still some brittle erosion at 1200 ∘ C in sample M1 (no silicon added), which caused the higher volume erosion rate than the sample M1.
We can conclude from the above study about phase, performance and microstructure of materials that the mechanical properties and erosion-wear resistance of samples increased with silicon powder addition.The results can be explained that the addition of silicon powder is helpful to form -sialon whiskers (depicted in Figure 2), which can fill the void and enhance the toughness of the material.Some studies indicate that in situ formed whiskers in the material are without pollution and spread evenly, and the formation of whiskers can prevent grain slip and improve the toughness of material through the bridge and pull-out mechanism [36,37].Furthermore, silicon powders had melted when 1500 ∘ C because of the low melting point 1410 ∘ C of silicon [3].The liquid silicon may fill the space between particles and make particles combine more closely by the surface tension, which can decrease the porosity and increase volume density [38].Therefore, the mechanical properties and erosion-wear resistance of samples were improved with silicon powder addition.

Conclusions
The main phase of mullite-SiC composite refractories by nitriding reaction sintering is mullite, -SiC and -Sialon, and in-situ form -Sialon whiskers in the matrix of mullite-SiC composite refractories can enhance the mechanical properties and erosion-wear resistance.The erosion resistance of mullite-SiC composite refractories increased with the increase of silicon powder addition and test temperature, and the minimum erosion rate was at 12% silicon added and 1400 ∘ C. The major erosion mechanism of mullite-SiC composite refractories is brittle erosion at the temperature from room temperature to 1000 ∘ C. The material removal is mainly due to the brittle fracture of aggregates and pores expansion of the binder phase causing the aggregates pullouts.From 1000 ∘ C to 1400 ∘ C, the major erosion mechanism of mullite-SiC composite refractories is plastic deformation, which mainly resulted from oxidation and softening of the material.

Figure 1 :
Figure 1: XRD patterns of samples with different silicon powder addition nitridation sintered at 1500 ∘ C for 3 h.

3. 1 .
Effect of Silicon Addition on Phase, Microstructure, and Performance of Samples.Mullite-SiC composite refractories were prepared using -SiC, mullite, Si and Al 2 O 3 as raw materials by nitriding reaction sintering at 1500 ∘ C for 3 h.

Figure 1
shows the phase compositions of samples.As this figure shows, the phase compositions consist of mullite, -SiC, and Si 3 Al 3 O 3 N 5 , which were formed in samples only when silicon powder has been added.The appearance of Si 3 Al 3 O 3 N 5 was due to the nitriding reaction of silicon powder and Al 2 O 3 .

Figure 2 :
Figure 2: SEM photographs and EDS spectrum of samples: (a) surface fracture microstructure of sample M4; (b) surface microstructure of sample M4; (c) EDS spectrum of point A in (b).

Figure 3 :
Figure 3: The volume erosion rates of the samples with respect to the test temperature and silicon powder addition.

Figure 6 :Figure 7 :
Figure 6: SEM images and EDS patterns of sample M4 (12% silicon added) eroded at 1200 ∘ C: (a) SEM micrographs of the surface of sample and (b) EDS result of area A.

Table 1 :
Compositions of the samples.

Table 2 :
Properties of mullite-SiC composite refractories prepared by nitriding reactive.