Effects of the Sintering Process on Al₂O₃ Composite Ceramics Fabricated Using Material Extrusion and Photo-Polymerization Combined Process

Abstract

The sintering process can improve the microstructure of Al₂O₃ composite ceramics and enhance their comprehensive properties, but the effects of the sintering process on Al₂O₃ composite ceramics are still unclear. Herein, a novel Al₂O₃ composite ceramic was printed using the material extrusion and photo-polymerization combined process, and the final ceramic was obtained using one-step sintering (TS) and two-step sintering technology (TSS). Based on the testing results, such as the relative density (D_rel), average grain size (AGS), hardness, bending strength, and fracture toughness, TSS was suitable for the refinement of commercial Al₂O₃ ceramics. Moreover, the highest sintering temperature of the second step (T₂) was at 1550 °C, while that of the shortest holding time (t) was at 4 h (TSS₈), which was to ensure densification before rapid grain growth. The D_rel and AGS of the best ceramics obtained via TSS₈ were 97.65% and 1.52 µm, respectively. Their hardness, bending strength, and fracture toughness were also enhanced, and they were affected by T₂, t, and the interaction. In sum, the TSS obtained better fracture toughness and bending strength, which had great potential in the application of the additive manufacturing field.

1. Introduction

Al₂O₃ ceramics is one of the most common engineering materials, and its performance depends on the density and control of fine particles [1]. In the technology of obtaining dense and fine particle ceramics, fine particles in raw material will reduce the fluidity of the slurry, which has limited the form of the green body [2,3], and it will hinder the further application of the commercial Al₂O₃. Dense and fine-grained ceramics can be obtained via pulse plasma sintering, liquid-phase sintering, laser sintering, adding additives, etc., but the sintering process is complex, and the cost is high [4,5,6,7,8]. One-step sintering (TS) is common, economical, and easy to operate, which can not only improve compactness through higher sintering temperatures but also promote grain growth [9,10,11]. The slightly lower sintering temperature can uniformly refine the particles and reduce their compactness [12], which limits the application of ceramic structure densification and refinement.

As ceramic fabrication technology develops, two-step sintering (TSS) is adopted to control the sintering rate and achieve the density and refinement of ceramics [13,14,15] to improve their mechanical properties [16,17]. Generally, the first step of sintering to the maximum temperature is recorded as T₁. After a brief stay, it quickly cools to T₂ as the second step to sintering temperature and cools down to room temperature after holding temperature for t₂ [18]. The dense Al₂O₃ ceramics [19,20,21], BaTiO₃ ceramics [22], and other ceramics [23,24] are obtained using TSS to inhibit the growth of ceramic grains.

Z. Razavi et al. [25] prepared and characterized sub-micrometer Al₂O₃ ceramics (grain size, 150 nm) using different TSS and discussed the effects of T₁ and t₂ on the densification and grain of ceramics. Compared with TS, when t₂ = 60 h, T₁ goes from 1300 °C to 1150 °C, the grain size in the bulk was reduced from 1.2 µm to 0.85 µm. The grain size decreased to 0.5 μm when T₁ dropped to 1250 °C, and the relative density (D_rel) was less than 88% at T₂ = 1100 °C, indicating that temperature played a vital role for TSS, but the period of t₂ was longer, and the effects on average grain size (AGS) and D_rel has not been analyzed. N.J. Lóh et al. [26] used this technique to obtain the three commercial Al₂O₃ of different purity (92, 96, and 99 wt% of Al₂O₃), evaluate the effects of T₂ and t₂ on density, and conclude the maximum T₂ (1550 °C) and minimum t₂ (4 h). Moreover, the interaction of T₂ and t₂ significantly impacted the density of 99.7 wt% Al₂O₃ (particle size = 0.73 μm) [27]. However, the systematic evaluation of sintering parameters on compactness, AGS, and mechanical properties are still unclear, and the AGS is within the range of sub-micrometer (150–200 nm) and micrometer (0.73–2.16 µm), the application of TSS in multi-components of micro-nanometer particles composite ceramics has not been analyzed.

In this study, Al₂O₃, TiCN, and the other micro-nanometer particles were used as raw powder. The ceramic green body was fabricated via material extrusion and the photo-polymerization combined process (MEX-PPM) [28], and using two-step degreasing [29] and TSS, the final ceramic parts. The effect of the sintering process on Al₂O₃ composite ceramics fabricated using the MEX-PPM process was studied through Drel, AGS, mechanical properties, and microstructure measurements.

2. Materials and Methods

2.1. Materials

Al₂O₃ (Tuopu Metal Materials Co., Ltd., Suzhou, China) was used as the matrix material, TiCN as the additive, MgO as the sintering additive, Ni and Mo as the metal binder (Tuopu Metal Materials Co., Ltd., Suzhou, China), 0.15 wt% oleic acid (OA) as the surfactant [28], and 1,6-hexanediol diacrylate (HDDA) (Changxing Chemical Co., Ltd., Shanghai, China) as the prepolymer solution. Diphenyl (2,4,6-trimethylbenzoyl)-phosphate oxide (TPO) (BASF GmbH, Shanghai, China) was used as a photoinitiator. The properties and content of raw materials are listed in

Table 1. Properties and content of raw materials [30].

Components	Grain Size (μm)	Theoretical Density (g/cm3)	Content (wt%)
Al2O3	1 μm	3.98	54
Al2O3	200 nm	3.98	10
TiCN	1 μm	5.08	30
Ni	1 μm	9.90	2
Mo	1 μm	10.20	2
MgO	1 μm	3.58	2

.

2.2. Preparation Process

The preparation of ceramic slurry for the MEX-PPM process can be divided into three steps: modifying ceramic powder, preparing prepolymer solution, and mixing slurry. Firstly, the ceramic powder was added to deionized water containing OA, mixed evenly, and dried to obtain the modified powder. Then, the modified ceramic powder was added into the HDDA prepolymer solution with TPO and milled for 4 h. Finally, the milled slurry was subjected to ultrasonic vibration to eliminate bubbles in the slurry, and the available ceramic slurry was obtained.

In the process of printing ceramic bulk using the MEX-PPM process, the ceramic slurry is extruded through the round nozzle and deposited on the workbench, which receives UV light radiation to maintain the shape and prevents the collapse and deformation of the deposited slurry caused by gravity [31]. The round nozzle and UV light source are fixed on the equipment. The 3D printing software slices the parts to generate a G-code, drives the workbench to move, and obtains the final ceramic bulk through the layer-by-layer deposition of slurry and UV light curing. The schematic diagram of the MEX-PPM process is shown in Figure 1.

The above MEX-PPM process was used to print the dense ceramic bulk at a printing speed of 5 mm/s and a radiation energy of 20 J/cm³ at room temperature and obtain the final ceramic parts through degreasing and sintering technology.

2.3. Degreasing and Sintering Process

2.3.1. Degreasing Process

This work adopted a two-step degreasing (TSD) with a controllable pyrolysis rate [29] to remove the organic binder HDDA in the ceramic bulk; the TSD process is shown in Figure 2.

Combined with the experimental conditions, the first step of degreasing in this study was carried out in a tubular furnace (GSL-1700X, Hefei Kejing Material Technology Co., Ltd., Hefei, China) at the rate of 1 °C/min. The temperature was hold for 30 min every 100 °C increased, hold for 180 minutes when it reaches 600 °C, and then the furnace was then cooled to room temperature. The second step of degreasing was carried out in an air furnace, which was heated to 200 °C, 600 °C (holding for 200 min), and 1000 °C (holding for 30 min) at 2 °C/min, 1 °C/min, and 4 °C/min, respectively, and then the furnace was cooled to room temperature to complete the whole degreasing process.

2.3.2. Sintering Process

After degreased Al₂O₃ composite ceramic adopts the traditional TS and the designed TSS to obtain the final ceramic parts, the process of TS and TSS is shown in Figure 3.

Figure 3a shows the changing curve of sample TS. Firstly, the temperature was raised to 1200 °C at a rate of 10 °C/min, then it was increased to T₁ at a rate of 3.75 °C/min. After that, the temperature was cooled to 1200 °C with a rate of 3.75 °C/min. Finally, the temperature was cooled down to room temperature. As shown in Figure 3b, the changing curve of TSS is similar to that of TS in the initial stage and then stays briefly after heating to the maximum temperature of T₁, and then it is cooled down to T₂ at a rate of 50 °C/min, and holding at t for a certain time. While the other temperature-controlling procedures were the same as that of TS, TSS sintering technology was carried out with two factors and three levels. The specific sintering parameters are shown in

Table 2. The specific parameters of TSS and TS sintering.

Sintering Process	T1/°C	T2/°C	t/h
TSS1	1500	1450	2
TSS2	1500	1450	4
TSS3	1500	1450	6
TSS4	1550	1500	2
TSS5	1550	1500	4
TSS6	1550	1500	6
TSS7	1600	1550	2
TSS8	1600	1550	4
TSS9	1600	1550	6
TS	1650	/	4

Notes: The sintering process is carried out in a high-temperature tube furnace with argon gas.

.

2.4. Characterizations of the Prepared Samples

To evaluate the properties of Al₂O₃ composite ceramic obtained by various sintering technologies, the ceramic sample was obtained by cutting, polishing with diamond particles, cleaning, drying, and gold spraying. The density was measured using the Archimedes drainage method; the microstructure was characterized using a scanning electron microscope (SEM, Evo18, Zeiss, Oberkochen, BW, Germany), and the grain size was measured using the line intercept method. The hardness, fracture, and toughness were measured using a Vickers hardness tester (HV-1000ZCM-XY, Anyi Instrument Co., Ltd., Shanghai, China), and the three-point bending test was carried out using an electronic universal testing machine (WDW-100KN, Instron Co., Boston, MA, USA). The fracture toughness and bending strength of ceramic specimens were obtained from Formulas (1) and (2), respectively.

$$KIC=0.203\times 1.8544 P/(4C^{3/2})$$

(1)

where P is the pressure load, and C is the average crack length.

$$\sigma =(3F\times L)/(2b\times h^2)$$

(2)

where F is the failure load, L is the span, and b and h are the width and thickness of the specimen, respectively.

In addition, the density, particle size, and mechanical properties of the ceramics were characterized using an average of seven tests per sample out of 20 samples from the same batch.

3. Results and Discussion

3.1. Comparison of Results from TSS and TS

The TS and TSS were applied to Al₂O₃ composite ceramics, and the D_rel and AGS of ceramic sintered bodies were obtained, as shown in Figure 4.

Figure 4 shows the D_rel (black mark) and AGS (blue mark) of Al₂O₃ composite ceramics under the action of each TSS and TS. The same holding time t = 4 h, the D_rel (TSS₈) (97.65%) obtained via TSS is slightly lower than that of TS (97.97%), but the AGS (TSS₈) (1.52 μm) is significantly lower than TS (2.56 μm). With the increase in t to 6 h, the D_rel (TSS₉) (97.91%) treated via TSS is close to TS, and the increased AGS (1.89 μm) is still lower than that of TS. Although the D_rel of other ceramics (93.61–97.18%) treated with TSS is lower than that of TS, their AGS (1.12–1.29 μm) is significantly lower than that of TS. TSS is more beneficial to Al₂O₃ composite ceramics from the compactness and grain refinement point of view.

In addition, the D_rel and AGS increase the amount of ceramic obtained via TSS with the T₂. When t = 2 h, T₂ from 1450 °C increases to 1550 °C, causing the D_rel from 93.61% to increase to 97.18% and the AGS from 1.12 μm to increase to 1.52 μm. When the t increases, the change in compactness is not always obvious, and the AGS still grows. At T₂ = 1550 °C, t from 2 h increased to 4 h, the D_rel from 97.18% (TSS₇) to 97.65% (TSS₈), and the AGS rapidly from 1.29 μm to 1.52 μm. The t increased to 6 h, the D_rel increased to 97.91%, and the AGS increased to 1.89 μm. Slightly lower T₂ ensures fine-grained ceramics; the changing compactness leads to different mechanical properties, as shown in Figure 5.

Figure 5 shows the mechanical properties of Al₂O₃ composite ceramics; the black, red, and green marks correspond to hardness, bending strength, and fracture toughness, respectively. Good ceramic hardness and fracture toughness can be obtained under the sintering conditions of T₂ = 1550 °C and t = 6 h (TSS₉), which are 17.01 GPa and 6.17 MPa∙m^0.5, respectively. The t was shortened to 4 h, and the ceramics obtained via TS (16.94 GPa, 6.11 MPa∙m^0.5) at 1650 °C were equivalent to TSS₈ (16.95 GPa, 6.09 MPa∙m^0.5) at 1550 °C. The bending strength obtained via TSS₇ (553.34 MPa) at 1550 °C for 2 h is higher than TS (541.23 MPa).

The mechanical properties of ceramic are improved with the T₂. When T₂ from 1450 °C to 1550 °C at t = 2 h, the hardness, bending strength, and fracture toughness increased from 15.74 GPa, 431.14 MPa, and 4.68 MPa∙m^0.5 to 16.67 GPa, 553.34 MPa, and 5.93 MPa∙m^0.5, respectively. But, the mechanical properties do not improve significantly with the increase in t. For example, the hardness at T₂ = 1550 °C, t = 4 h (TSS₈), and t = 6 h (TSS₉) are close, the fracture toughness increases slowly, and the bending strength decreases.

The fine particles of dense ceramics can be obtained under the T₂ = 1550 °C, t = 4 h or 6 h (Figure 4), and the mechanical properties under the T₂ = 1550 °C, t = 4 h (Figure 5) are close to TS. So, the sintering parameter for obtaining the best Al₂O₃ composite ceramics is TSS₈ with T₂ = 1550 °C, t = 4 h. Using the TSS₈ process, the D_rel, AGS, hardness, bending strength, and fracture toughness were 97.65%, 1.52 μm, 16.95 GPa, 572.59 Mpa, and 6.09 MPa∙m^0.5. The above results preliminarily show that the T₂ and t in TSS have varying degrees of effect on the properties of Al₂O₃ composite ceramics. The impact and reliability of the T₂ and t on the properties of ceramics need to be further analyzed through data statistics.

3.2. Effects of T₂ and t on Al₂O₃ Composite Ceramics

The reliability of the impact of factors (T₂ and t) on variables (D_rel, AGS, and mechanical properties) was evaluated using SPSS software. In the statistical analysis, the adjusted R² is used to assess the fitting degree of the model, and the standard effect quantity (η²) and p-value are used to evaluate the degree and significance of the effects of T₂ and t on ceramic properties, respectively. It is assumed that when p-value < 0.05, the factors have significant effects on variables. On the contrary, the effect of factors on variables is not significant.

3.2.1. Effects of T₂ and t on D_rel and AGS

The statistical analysis results of T₂ and t for D_rel and AGS are shown in

Table 3. The effects of T₂ and t on D_rel.

Variable	Factor	η2	p-Value	The Adjusted R2
	T2	0.9861565	0.0016288
Drel	t	0.9639632	0.0029354	0.994
	T2 by t2	0.2768595	0.6149418

, which shows the effects of T₂, t, and their interaction (T₂ by t) on the D_rel of Al₂O₃ composite ceramics. The adjusted R² is 0.994, which indicates that the linear regression model has a high degree of fit. The p-value of T₂ (0.0016288) and t (0.0029354) is less than 0.05, and the p-value of interaction between both T₂ and t (0.6149418) is more than 0.05, which indicates that D_rel is only affected by individual T₂ and t. Moreover, T₂ (η² = 0.9861565) has a stronger impact on drel than t (η² = 0.9639632). Figure 6a shows the interaction between T₂ and t for D_rel. Figure 6a indicates that there is no interaction between T₂ and t because the three kinds of sintering temperatures at holding for 2, 4, and 6 h are approximately parallel in the t vs. D_rel curve.

Table 4. The effects of T₂ and t on AGS.

Variable	Factor	η2	p-Value	The Adjusted R2
	T2	0.8967313	0.0331859
AGS	t	0.9674137	0.0025214	0.986
	T2 by t	0.9755070	0.0038332

shows the effects of T₂, t, and their interaction on the AGS of Al₂O₃ composite ceramics. The adjusted R² = 0.986 indicates that the static models are valid. The η² of T₂, t, and their interaction gradually rose 0.8967313, 0.9674137, and 0.9755070, and their p-values were less than 0.5. The results show that the above factors had a significant impact on the AGS. However, the interaction of both T₂ and t is stronger, followed by T₂ and t. Figure 6b shows the interaction between the T₂ and t for AGS. Figure 6b indicates that there is an interaction between the T₂ and t because an intersection of the lines is observed.

3.2.2. Effects of T₂ and t on Mechanical Properties

The T₂ and t can be applied to the densification and refinement of Al₂O₃ composite ceramics to promote the mechanical properties. The statistical analysis of its effect on mechanical properties is shown in

Table 5. The effects of T₂ and t on hardness.

Variable	Factor	η2	p-Value	The Adjusted R2
	T2	0.8813812	0.0408536
Hardness	t	0.8513854	0.0254951	0.978
	T2 by t	0.7652519	0.1137373

,

Table 6. The effects of T₂ and t on fracture toughness.

Variable	Factor	η2	p-Value	The Corrected R2
Fracture toughness	T2	0.9608961	0.0077327
	t	0.9060330	0.0125882	0.995
	T2 by t	0.8801710	0.0414804

,

Table 7. The effects of T₂ and t on bending strength.

Variable	Factor	η2	p-Value	The Corrected R2
Bending strength	T2	0.8996170	0.0318046
	t	0.2580958	0.2580958	0.918
	T2 by t	0.1742553	0.1742553

and

Table 8. The effects of T₂ and t on bending strength (Excluding T₂ = 1550 °C, t = 6 h).

Variable	Factor	η2	p-Value	The Corrected R2
Bending strength	T2	0.9829973	0.0170027
	t	0.9715431	0.0143312	0.995
	T2 by t	0.9105670	0.0894330

. The adjusted R² are 0.978, 0.995, and 0.995 (0.918), respectively, which indicates that the statistic models are suitable.

Table 5 presents the impact of T₂, t, and their interaction. Both T₂ (p-value = 0.0408536) and t (p-value = 0.0254951) significantly affect hardness. However, their interaction is not significant (p-value > 0.05). Moreover, T₂ has a stronger impact on hardness than t (0.8813812 against 0.8513854). Figure 7a shows the interaction between the T₂ and t for hardness. Figure 7a confirms that there is no impact on interaction between T₂ and t on hardness because the lines are approximately parallel, as shown in Table 5. In addition, the curve slope is approximately zero at T₂ = 1450 °C, which is caused by lower temperature.

Table 6 presents the impact of T₂ and t on fracture toughness. It is observed that T₂, t, and their interaction individually affect the fracture toughness and p-values of 0.0077327, 0.0125882, and 0.0414804, respectively. However, the T₂ has a more significant effect (η² = 0.9608961), followed by t and their interaction. Figure 7b shows the interaction between T₂ and t for fracture toughness.

Table 7 presents the p-value of T₂, which is less than 0.05. It is observed that T₂ individually affects bending strength, which obviously contradicts the previous research results (Figure 5). Table 8 shows the impact of T₂ and t on bending strength (excluding the data of 1550 °C for 6 h). It is observed that T₂ and t individually affect the bending strength and p-values of 0.0170027 and 0.0143312, respectively. Moreover, Figure 7c shows that the curves at T₂ = 1550 °C and t ≤ 4 h are almost parallel to each other, and then there is a cross trend at T₂ = 1550 °C, t = 6 h. Therefore, T₂ = 1550 °C and t = 6 h are unsuitable for sintering composite ceramics, which is consistent with the results shown in Figure 4 and Figure 5.

3.2.3. Microstructure of Sintered Ceramics

The performance of Al₂O₃ composite ceramics depends on the microstructure. This work verifies the advantages of TSS from the micro-level by comparing the microstructure of Al₂O₃ composite ceramics acted by TS and the TSS₈, as shown in Figure 8.

Figure 8a,b shows the microstructure of Al₂O₃ composite ceramics obtained via TSS₈ at T₂ = 1550 °C, t = 4 h, and TS at 1650 °C, t = 4 h, respectively. It is found that the ceramic grains obtained via TSS₈ are obviously smaller than those obtained via TS. However, there are small gaps between the grains, which results in lower compactness of ceramics obtained by TSS₈. The microstructure shown in Figure 8 is consistent with the results of D_rel and AGS shown in Figure 4. The above results show that the Al₂O₃ composite ceramics obtained via TSS₈ are significantly better than TS; although its compactness is slightly low, the fine-grained ceramics obtained at low cost can bring good comprehensive properties, especially the bending strength and fracture toughness that determine the engineering properties of ceramics.

4. Conclusions

In this work, an Al₂O₃ composite ceramic was prepared using the MEX-PPM combined process, and the final ceramic samples were obtained via TS and TSS. The effects of sintering processes on Al₂O₃ composite ceramics were studied using experiments, and we conclude the following:

(1)

Compared to TS, TSS effectively refined grain size and improved its comprehensive properties. TSS₈ can ensure the densification of ceramic before the rapid grain growth; its highest sintering temperature and shortest holding time were T₂ = 1550 °C and t = 4 h, respectively. Under this condition, the D_rel and AGS of the ceramics were 97.65% and 1.52 μm. Their hardness, bending strength, and fracture toughness were 16.95 GPa, 572.59 Mpa, and 6.09 MPa·m^0.5;

(2)

Both T₂ and t and their interactions individually affect the AGS, fracture toughness, and bending strength significantly, although T₂ has more impact. However, both T₂ and t affect the D_rel and hardness more significantly.

In addition, the microstructure of ceramics obtained via the TSS₈ and TS was compared, and the advantages of TSS8 from the microscopic point of view were verified. Commercial Al₂O₃ ceramics with good fracture toughness and flexural strength via TSS will have unique advantages in engineering applications.