Preparation and Performance of Al₂O₃/Ti(C,N)-Added ZrO₂ Whisker and NanoCoated CaF₂@Al(OH)₃ Powder

Abstract

The Al₂O₃/Ti(C,N) ceramic material added micron ZrO₂ whisker and nano coated CaF₂@Al(OH)₃ powder was fabricated. The micron ZrO₂ whisker was for the toughening and reinforcing phase and the nano coated CaF₂@Al(OH)₃ powder was the lubricant. For obtaining a ceramic material with optimal comprehensive mechanical properties and friction properties, the influences of different compositions of the ZrO₂ whisker and nano coated CaF₂@Al(OH)₃ powder on the microstructure and mechanical properties were analyzed, respectively. The result demonstrated that as the addition of the ZrO₂ whisker was 6 vol% and the addition of the nano coated CaF₂@Al(OH)₃ powder was 10 vol%, the optimal self-lubricating ceramic material had optimal mechanical properties. The hardness of the ceramic material was 16.72 GPa, the flexural strength was 520 MPa and the fracture toughness reached 7.16 MPa·m^1/2. The formation of the intragranular structure, whisker toughening and the phase transition of ZrO₂ were the main mechanisms.

1. Introduction

The ceramic materials can be generally used in various fields because of excellent chemical and physical properties [1,2,3]. Aluminum oxide is an ideal material due to its inherent high hardness and excellent thermal stability [4,5,6]. Currently, aluminum oxide-based ceramic materials are broadly used in cutting tool molds, sealing rings and various high temperature engine parts [7,8,9]. However, the low-fracture toughness and the brittleness contribute to the aluminum oxide-based ceramic risk of being easily chipped in the course of processing, which seriously influences the cutting performance of the ceramic tool material [10,11,12]. The self-lubricating ceramic tool materials with excellent mechanical properties and friction properties have always been people’s pursuit goal.

To acquire the ceramic material with excellent mechanical properties, many scholars have studied gradient design and layered design [13,14,15]. Yang [16] prepared the gradient composite Al-7Si-5Cu/Al₂O₃. The flexural strength of the material increased significantly. Katsui [17] studied the deposition of SiC layers on SiO₂ and diamond powders. The result showed that the formation of the microstructure enhanced the mechanical properties. Dang [18] prepared mullite using Al₂O₃ powders and coated SiO₂@SiC powders. The mechanical properties were better than without whisker. The surface modification technology can effectively enhance the mechanical properties of the ceramic materials, and the preparation is becoming more and more mature, which has become the first choice of many scientists.

At present, strengthening the toughness of the ceramic tool material with fiber (including whisker) is an effective means [19,20,21,22]. Zhu [23] prepared an Al₂O₃ membrane by introducing ZrO₂ fiber. Compared with the sample of unadded ZrO₂ fiber, the addition of ZrO₂ fiber increased the flexural strength of the material. Zu [24] investigated multi-walked carbon nanotubes to optimize the carbon fiber-reinforced ZrB₂-based ceramic material. The fracture toughness was 7.0 ± 0.4 MPa·m^1/2, 1.6 times the value of the ceramic without the multi-walked carbon nanotubes. Zhai [25] prepared the Al₂O₃/MgAl₂O₄/ZrO₂ ternary eutectic ceramic. The ZrO₂ phase acted as a whisker in the ceramic. The fracture toughness reached 6.1 MPa·m^1/2, 1.7 times the value of the pre-sintered ceramic. The ZrO₂ has an extremely special phase transition toughening [26,27,28]. The Al₂O₃/ZrO₂/CeO₂ composites were processed in his research [29]. The transformation of m-ZrO₂ into t-ZrO₂ resulted in the toughening of the Al₂O₃ composite. Yu [30] prepared the Al₂O₃–ZrO₂ (Y₂O₃) powders with different Y₂O₃ addition. The additional Y₂O₃ phase contributed to the abundance of ZrO₂ polymorphs in the powders. The phase transition of ZrO₂ significantly enhances the mechanical properties of the ceramics.

The ceramic tool materials with lubricant have also been extensively studied. Wu [31] prepared Al₂O₃/TiC/CaF₂ self-lubricating ceramic material. The fracture toughness increased by 5.9%. Wu [32] prepared Al₂O₃/(W,Ti)C ceramic material added with h-BN@Ni powders. The friction coefficient and wear rate were both less than the Al₂O₃/(W,Ti)C/h-BN ceramic materials. Chen [33] prepared the self-lubricating tool with SiO₂-coated h-BN. The flexural strength and fracture toughness were obviously improved. The reasonable addition of lubricant can improve the mechanical properties and friction properties.

Herein, to acquire the ceramic tool material with excellent mechanical properties and friction properties, a kind of self-lubricating ceramic material added micron ZrO₂ whisker and nano coated CaF₂@Al(OH)₃ powder was introduced. The Al₂O₃ was the matrix material. Due to the high hardness, the flexural strength and the fracture toughness of Ti(C,N), the Ti(C,N) was the matrix material. The micron ZrO₂ whisker was the toughening and reinforcing phase. The nano coated CaF₂@Al(OH)₃ powder was the solid lubricant. The influences of the different compositions of the ZrO₂ whisker and nano coated CaF₂@Al(OH)₃ powder on the microstructure and mechanical properties were analyzed, respectively. The main mechanisms of the self-lubricating ceramic tool material were revealed.

2. Materials and Methods

2.1. Fabrication of Nano Coated CaF₂@Al(OH)₃ Powder

The preparation process of nano coated CaF₂@Al(OH)₃ powder was as follows: The ethanol, benzene and water were mixed according to the volume ratio of 6:2:1 as a solvent. The polyvinylpyrrolidone (PVP) was added to prepare the solution containing 0.5 mol/L. The nano CaF₂ (5~10 nm, self-made) was added to prepare the solution containing CaF₂ 0.1 mol/L, and afterward ultrasonic treatment was carried out for 40 min. The dilute ammonia water was mixed alcohol and ammonia water with a volume ratio of 5:1. The CaF₂ suspension was stirred with a magnetic stirrer (DF-101S) at 25 °C. The aluminum nitrate solution of 0.5 mol/L was slowly poured into the CaF₂ suspension, and the solution was continuously stirred for 20 min until the solution was uniform. Dropwise, diluted ammonia water was added at 2 mL/min to adjust the pH of the solution to 7. The Al(OH)₃ finally formed the heterogeneous nucleation coating on the surface of nano CaF₂. The nano coated CaF₂@Al₂O₃ powder can be obtained by centrifuging, washing and sintering the CaF₂@Al(OH)₃.

2.2. Fabrication of Al₂O₃/Ti(C,N)/ZrO₂/CaF₂@Al(OH)₃ Ceramic Material

Commercially available Al₂O₃ (200 nm, purity ≥ 99.9%, Shanghai Chaowei New Material Co., Ltd., Shanghai, China) and Ti(C,N) (80 nm, purity ≥ 99.9%, Hefei Yulong New Material Co., Ltd., Hefei, China) used here were raw materials. The ZrO₂ whisker (mean diameter and length were 1–3 μm and 10–20 μm, respectively) was raw material. MgO (1 μm, purity ≥ 99.9%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was used as a sintering aid. Polyethylene glycol (PEG 4000, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was the dispersant.

The Al₂O₃ and MgO powders, Ti(C,N) powders were added to the absolute ethyl alcohol with polyethylene glycol, respectively. Then, the dispersions were ultrasonically dispersed and mechanically stirred for 20–30 min. Mixing the Al₂O₃ dispersion, the Ti(C,N) dispersion and the dispersion containing the nano coated CaF₂@Al(OH)₃ powder, the mixed solution was ultrasonically dispersed and mechanically stirred for 10–30 min, thereafter the dispersion was poured into ball mill tank for ball milling. After 44 h of ball milling, the ZrO₂ whisker was added to the ball mill tank. The prepared multiphase suspension was dried in the vacuum drying oven (DZF-6050, Shanghai, China), and then the multiphase suspension was sieved to obtain composite powder. The composite powder was put into a graphite sleeve for cold-pressing. After the hot pressing in the vacuum hot-pressing sinter (ZR1050, Jinan, China), the Al₂O₃/Ti(C,N)/ZrO₂/CaF₂@Al(OH)₃ ceramic material was fabricated. The preparation process influenced the microstructure and further influenced properties of the material [34,35]. The parameters selected in this experiment were as follows: the sintering temperature was set to 1650 °C under 30 MPa, the soaking time was selected as 20 min and the heating rate of the preparation progress was set to 20 °C/min. Figure 1 shows the process flow chart of the composite ceramic material preparation.

2.3. Performance Testing of the Ceramic Material

The ceramic material after hot pressing was cuboid with the size of 3 mm × 4 mm × 35 mm. The surface roughness Ra of the ceramic material was less than 0.1 μm. The instrument used for the hardness test of the ceramic tool material was the Hv-120 Vickers hardness tester (Hv-120, Jinan, China), which measured with the Vickers indentation method. The indentation load was set to 196 N for 15 s. The indentation of the ceramic material was observed and measured by optical microscope. Then, the length of the diagonal of the two indentations was recorded. The hardness of the ceramic material could be calculated by the function:

$${\mathrm{H}}_{\mathrm{V}}=\frac{1.8544\mathrm{P}}{{\left(2\mathrm{a}\right)}^2}$$

(1)

where ${\mathrm{H}}_{\mathrm{V}}$ is the hardness value (GPa); $\mathrm{P}$ is the indentation load (N); and $2\mathrm{a}$ is the arithmetic average value of the diagonal lengths of the two indentations produced.

The flexural strength of the ceramic material was measured by the three-point bending method. The span value was 20 mm and the displacement loading speed was 0.5 mm/min. The flexural strength of the ceramic material could be calculated by the function:

$${\mathsf{\sigma }}_{\mathrm{f}}=\frac{3\mathrm{PL}}{2{\mathrm{bh}}^2}$$

(2)

where ${\mathsf{\sigma }}_{\mathrm{f}}$ is the flexural strength (MPa); $\mathrm{P}$ is the maximum load (N) value loaded when the sample is broken; $\mathrm{L}$ is the distance (mm) between the two supports supporting the sample; and $\mathrm{b}$ and $\mathrm{h}$ are the width (mm) and height (mm) of the sample, respectively.

The fracture toughness was also measured by the indentation method. The instrument used for fracture toughness test of the ceramic material was the Hv-120 Vickers hardness tester (Hv-120, Jinan, China). The fracture toughness of the ceramic material could be calculated by the function:

$${\mathrm{K}}_{\mathrm{IC}}=0.203{\mathrm{H}}_{\mathrm{V}}{\mathrm{a}}^{\frac{1}{2}}{\left(\frac{\mathrm{c}}{\mathrm{a}}\right)}^{-\frac{2}{3}}$$

(3)

where ${\mathrm{K}}_{\mathrm{IC}}$ is the fracture toughness (MPa·m^1/2); ${\mathrm{H}}_{\mathrm{V}}$ is the hardness value (GPa) measured by the Vickers indentation method; $\mathrm{a}$ is the half length (mm) of the diagonal lengths; and $\mathrm{c}$ is the half length (mm) of the crack diagonal.

Density test of the ceramic material was performed by the drainage method. The dry weight ${\mathsf{{\rm M}}}_1$, submerged weight ${\mathsf{{\rm M}}}_2$ and wet weight ${\mathsf{{\rm M}}}_3$ of the sample were weighed by a precision electronic balance (ME105DU, Jinan, China). The density of the ceramic material could be calculated by the function:

$${\mathsf{\rho }}_{\mathrm{s}}=\frac{{\mathsf{{\rm M}}}_1{\mathsf{\rho }}_0}{{\mathsf{{\rm M}}}_3-{\mathsf{{\rm M}}}_2}$$

(4)

where ${\mathsf{\rho }}_{\mathrm{s}}$ is the density (g/cm³) of the sample; ${\mathsf{\rho }}_0$ is the density (g/cm³) of the distilled water; ${\mathsf{{\rm M}}}_1$ is the weight (g) when the sample is dried; ${\mathsf{{\rm M}}}_2$ is the submerged weight (g) in the liquid; and ${\mathsf{{\rm M}}}_3$ is the wet weight (g) measured in the air after the sample is sufficiently absorbed. The relative density of the ceramic material could be calculated by the function:

$$\mathsf{\rho }=\frac{{\mathsf{\rho }}_{\mathrm{s}}}{{\mathsf{\rho }}_{\mathrm{t}}}\times 100\%$$

(5)

where $\mathsf{\rho }$ is the relative density of the ceramic material; ${\mathsf{\rho }}_{\mathrm{s}}$ is the density (g/cm³) of the sample; and ${\mathsf{\rho }}_{\mathrm{t}}$ is the theoretical density (g/cm³).

To decrease the measurement error and ensure the accuracy of the measurement, every ceramic material was tested 5 times. The arithmetic average value was the measured value of the ceramic material.

3. Results and Discussion

3.1. XRD Phase Composition Diagram of the Ceramic Material

The XRD (XRD is the abbreviation of X-Ray Diffraction) diffraction analysis of the ceramic material with 6 vol% ZrO₂ whisker and 10 vol% nano coated CaF₂@Al(OH)₃ powder is shown in Figure 2. In the ceramic material, the phase analysis indicates that the predominant phases for the composites are Ti(C,N) and Al₂O₃. Before the experiment, the ZrO₂ whisker exists in monoclinic phase. From Figure 2, it can be seen that ZrO₂ exists mainly in the form of t-ZrO₂ in the ceramic material. This indicates that the phase transition took place during the sintering. The ZrO₂ whisker underwent a phase change. The introduction of nano coated powder does not have a significant influence on it, which provides the fundamental condition for the phase transition toughening. The presence of the characteristic peak of CaF₂ is clearly observed. In this paper, the addition of sintering aid MgO is 0.5 vol%. The characteristic peak of MgO is not observed, because the addition amount of MgO is too small. The chemical composition of the various components constituting the composite ceramic has a good chemical compatibility, and an obvious chemical reaction does not take place.

3.2. Influence of ZrO₂ Whisker Addition on Al₂O₃/Ti(C,N) Ceramic Material

Figure 3a–c represents the scanning electron microscope photographs of the fracture surfaces of the Al₂O₃/Ti(C,N) ceramic material with 3 vol%, 6 vol% and 9 vol% ZrO₂ whisker, respectively. It can be observed that the grains are coarser in Figure 3a or Figure 3c. However, the grains in Figure 3b are finer than those in Figure 3a and the uniformity of Figure 3b is improved. The addition of the ZrO₂ whisker has an influence on the grain refinement of the ceramic material, and the ZrO₂ whisker can suppress the abnormal growth of the crystal grain. When the addition of the ZrO₂ whisker is 9 vol%, more pores can be seen in the cross section. A possible reason for this is that under the process conditions selected by the experiment, the dispersion of the ceramic material with 9 vol% ZrO₂ whisker may not achieve the desired effect, contributing to the bridging or agglomeration of the whisker. The occurrence of pores leads to a decrease in the relative density. Due to the contrast, less pores can be seen in the cross section when the additive amount ZrO₂ whisker is low. The existence of pores influences the densification of the ceramic material and thus reduces the mechanical properties of the ceramic material. Whisker reunion can be found in Figure 3c mark 1, and the whisker section can also be observed by magnification, as showed in Figure 3d.

The consequences of adding 0, 3 vol%, 6 vol% and 9 vol% ZrO₂ whisker on the mechanical properties of the Al₂O₃/Ti(C,N) ceramic material are shown in Figure 4. The ceramic material without ZrO₂ whisker has a high hardness of 20.47 GPa. With the increasing addition of ZrO₂ whisker, the hardness is prone to decrease. When the addition of ZrO₂ whisker increases, the flexural strength of the ceramic material increases significantly from 555 to 584 MPa. Whereas when the additive amount of ZrO₂ is more than 6 vol%, the flexural strength of the ceramic material shows a decreasing trend. The ceramic material obtains maximum flexural strength when the ZrO₂ whisker addition is 6 vol%. With the increase in ZrO₂ whisker addition, the fracture toughness also shows an increasing trend. It reveals that the introduction of ZrO₂ whisker can enhance the fracture toughness of the material. The ceramic material without ZrO₂ whisker has the highest relative density. With the addition of ZrO₂ whisker increasing, the relative density decreases first and afterward increases. As can be seen from the analysis in SEM(Scanning electron microscope) morphology the presence of pores reduces the density of the material.

3.3. Influence of Nano Coated CaF₂@Al(OH)₃ Powder on Al₂O₃/Ti(C,N) Ceramic Material

The nano powder and nano coated powder have a crucial influence on the microstructure and mechanical properties of the ceramic material. The cross sections of the two ceramic materials were observed by scanning electron microscope (SEM). The results are displayed in Figure 5. Figure 5a is a SEM morphology photograph added with 10 vol% nano CaF₂ powder. It can be found that a few grains have an abnormal growth and the grain distribution is not uniform. Figure 5b is a SEM morphology paragraph added with 10 vol% nano coated CaF₂@Al(OH)₃ powder. Compared to the material added with the same components, the nano CaF₂ powder, the grain distribution is uniform and the density of the material is superior to the former. In Figure 5b, the material of the surface coating is nano coated CaF₂@Al(OH)₃ powder. Moreover, the nano coated CaF₂@Al(OH)₃ powder is well combined with the ceramic matrix. Compared with the added nano CaF₂ powder, there are many intragranular structures in the ceramic material. The intragranular structures have a good effect on enhancing the mechanical properties of ceramic material [36]. In the sintering process of the material, with the growth of crystal grains, the nano powders enter the crystal with the motion of boundaries of particles. The matrix grains merge and grow, forming the intragranular structures. The nano CaF₂ can be considered as completely entering the crystal. From Figure 5b, the fracture mode is mainly intergranular fracture, and partly transgranular fracture, which can also be found. The transgranular fracture consumes quantities of fracture energy, which is conducive to enhance mechanical properties. This is also one of the main reasons for the improvement of the mechanical properties of the prepared ceramic material. Nano coated powder contributes to the dispersion of CaF₂ in ceramic matrix material. As it can be found from Figure 5b, CaF₂ is more evenly distributed in the matrix material.

The results of adding 5 vol%, 10 vol% and 15 vol% nano coated CaF₂@Al(OH)₃ powder on the mechanical properties of the Al₂O₃/Ti(C,N) ceramic material are displayed in Figure 6. From Figure 6a, with the increasing addition of CaF₂@Al(OH)₃, the hardness is prone to decrease. The main reason is the low mechanical properties of the CaF₂. The increase in lubricant addition inevitably leads to a decrease in the hardness and other properties. The flexural strength of the ceramic material first increases and afterward decreases. When the addition of nano coated CaF₂@Al(OH)₃ powder is 10 vol%, the flexural strength is up to 471 MPa. From Figure 6b, with the increasing addition of nano coated CaF₂@Al(OH)₃ powder, the fracture toughness is increased from the initial 6.50 MPa·m^1/2 to 6.60 MPa·m^1/2. The main reason may be that the presence of massive nano coated CaF₂@Al(OH)₃ powder enhances the material’s fracture toughness. The relative density of the ceramic material shows a trend of increasing first and afterward decreasing. When the addition of the CaF₂@Al(OH)₃ powder is 10 vol%, excellent comprehensive mechanical properties are obtained.

3.4. Mechanism Analysis of Co-Modification of Micron ZrO₂ Whisker and Nano Coated CaF₂@Al(OH)₃ Powder

From the analysis above, the addition of micron ZrO₂ whisker and nano coated CaF₂@Al(OH)₃ powder have a significant influence on the ceramic material. The Al₂O₃/Ti(C,N) ceramic material added 6 vol% ZrO₂ whisker and 10 vol% nano coated CaF₂@Al(OH)₃ powder was fabricated. The performance of the ceramic material was measured. The hardness of the Al₂O₃/Ti(C,N)/ZrO₂/CaF₂@Al(OH)₃ ceramic material is 16.72 GPa, the flexural strength is 520 MPa and the fracture toughness is up to 7.16 MPa·m^1/2. For the convenience of analysis, the sintering diagram of the material is drawn in Figure 7, and the micro-morphology of the ceramic material after sintering is observed.

In the sintering course of the Al₂O₃/Ti(C,N)/ZrO₂/CaF₂@Al(OH)₃ ceramic material, with the development of temperature and soaking time, the crystal grains grow gradually. The matrix powders accommodated by ball milling will have the situation that small powders are gradually absorbed by large powders and the number of powders is continuously reduced. The growth of crystal grain rests with the motion of boundaries of particles. The boundaries in the matrix tend to migrate to the center, as indicated by mark 1 in Figure 7. Due to the shell material (Al₂O₃) of the lubricant being the same as the matrix powders (Al₂O₃) of the ceramic material, the nano powders are dispersed into the interior of the matrix material during the sintering of the ceramic material. With the intergranular nanostructure of the nano CaF₂ powders dispersed inside, the surface coating design avoids powder agglomeration growth.

After hot pressing, the nano powder exists inside the matrix crystal, and the matrix and nano powders form the intragranular structure. The related studies have shown that the intragranular structure can toughen the ceramic material [37,38]. The nano powder promotes the generation of more intragranular structures, which enhances the mechanical properties of the ceramic material. As shown in Figure 7, the nano powders form intragranular structures. The appearance of this intragranular structure is one of the main reasons for enhancing the mechanical properties of the ceramic material prepared in this paper. The ZrO₂ whiskers have a unique phase transition effect. In the sintering process, when the temperature reaches 1170 °C, the m-ZrO₂ is completely converted into the t-ZrO₂. In the cooling process, when the temperature is less than 950 °C, the matrix materials have the binding effect on the t-ZrO₂, which hinders the conversion of the t-ZrO₂ into the m-ZrO₂ and enables the t-ZrO₂ to be preserved at room temperature. The toughening effect of the phase transition generated by the ZrO₂ is mainly attributed to the crack growth being inhibited by the phase transition of t-ZrO₂. The existence of the t-ZrO₂ in the material is the necessary condition for phase transformation toughening. This makes up for the natural defects caused by the azimuth angle of the dispersion whisker reinforced the ceramic materials. This synergistic effect makes the ZrO₂ whisker-toughened ceramic material obviously different from the ceramic material toughened by adding the dispersion whisker alone. As indicated by the mark 2 partially ZrO₂ whiskers after hot pressing are distributed in intercrystalline, which provides the conditions for whisker toughening in the process of crack propagation [39]. The toughening mechanisms of the ZrO₂ whisker in the ceramic material are the whisker bridging and the crack deflection. During the whisker bridging and the crack deflection, the energy consumption occurs [40], which is conducive to enhancing the fracture toughness of the ceramic material.

4. Conclusions

The ZrO₂ whisker and nano coated CaF₂@Al(OH)₃ powder were added to the Al₂O₃/Ti(C,N) self-lubricating ceramic material simultaneously. The modification of the Al₂O₃/Ti(C,N)/ZrO₂/CaF₂@Al(OH)₃ ceramic tool material was completed by using two different toughening mechanisms.

(1).

The ZrO₂ whisker with 0, 3 vol%, 6 vol% and 9 vol% were added to the Al₂O₃/Ti(C,N) ceramic material. The result revealed that when the additive amount of the ZrO₂ whisker was 6 vol%, the hardness, the flexural strength and the fracture toughness values were 19.1 GPa, 584 MPa and 6.61 MPa·m^1/2, respectively. Whisker toughening and the phase transition toughening of the ZrO₂ enhanced the mechanical properties of the ceramic material.

(2).

The addition of nano coated CaF₂@Al(OH)₃ powder also had a significant influence on the mechanical properties of the ceramic material. The intragranular structures played a good role in enhancing the mechanical properties of the ceramic material. When the nano coated CaF₂@Al(OH)₃ powder addition was 10 vol%, the mechanical properties of the ceramic material was the best. The hardness, flexural strength and fracture toughness values of the prepared ceramic material were 18.58 GPa, 471 MPa and 6.50 MPa·m^1/2, respectively.

(3).

The ZrO₂ whisker of 6 vol% and nano coated CaF₂@Al(OH)₃ powder of 10 vol% were simultaneously added to the Al₂O₃/Ti(C,N) ceramic material. The hardness of the Al₂O₃/Ti(C,N)/ZrO₂/CaF₂@Al(OH)₃ ceramic material was 16.72 GPa, the flexural strength was 520 MPa, and the fracture toughness reached 7.16 MPa·m^1/2. The fracture toughness of the Al₂O₃/Ti(C,N)/ZrO₂/CaF₂@Al(OH)₃ self-lubricating ceramic tool material was better than the added ZrO₂ whisker or added nano coated CaF₂@Al(OH)₃ powder of the Al₂O₃/Ti(C,N) ceramic material, separately. The formation of intragranular structure, whisker toughening and phase transition of ZrO₂ were the main mechanisms.