Study on Densification of Gd₂Zr₂O₇-Based Ceramic Target for EB-PVD Application and Its Effect on Fracture Toughness

Abstract

In this study, the Gd₂Zr₂O₇-based ceramic target was densified via pressureless sintering which follows well with Kingery’s three-stage sintering theory. Sintering temperature is the key factor affecting the densification of targets. In the initial stage, when the sintering temperature is in the range of 1200–1450 °C, the porosity decreases with the density of targets slowly increasing to 64.71%. Grain boundary diffusion controls the densification process. In the middle stage, at 1450–1500 °C, the density ratio of the target rapidly rises to 77.6%. The competition between grain boundary migration rate and pore shrinkage rate leads to the maximum isolated porosity. In the final stage, when the sintering temperature is above 1500 °C, the density ratio of the target significantly increases to 97.28% at the temperature of 1600 °C. Even when the holding time is extended to 7 h at 1500 °C, the density ratio of the target only reaches 85.72%. With the increase in sintering temperature and prolongation of holding time, the fracture toughness of the ceramic targets exhibited a trend of initial increase followed by a decrease. Density ratio and grain size were identified as key factors influencing fracture toughness. When the density ratio reached approximately 80%, the fracture toughness achieved its maximum value of 2.245 MPa·m^0.5. When the sintering temperature exceeds 1450 °C, both the Young’s modulus and hardness of the targets increase rapidly, which significantly enhances their fracture toughness. However, with the increase in sintering temperature or holding time, the grain grows rapidly. This excessive grain growth reduces grain boundary.

1. Introduction

A total of 6%–8% yttria-stabilized zirconia (YSZ) is widely used as thermal barrier coatings [1]. However, when the service temperature is above 1250 °C, YSZ coatings undergo phase decomposition and accelerated sintering, reducing thermal insulation and strain tolerance which causes coating spallation [2]. A feasible solution is to add an outer layer of ultra-high-temperature thermal barrier coating to the YSZ layer, forming a dual-ceramic-layer structure [3]. Among these, gadolinium zirconate (Gd₂Zr₂O₇) has emerged as a candidate material for next-generation ultra-high-temperature thermal barrier coatings due to its relatively high thermal expansion coefficient (10.4 × 10⁻⁶ K⁻¹ in the temperature range of 30–1000 °C), low thermal conductivity (approximately 1.5 W·m⁻¹·K⁻¹ at 1000 °C), along with good high-temperature phase stability and CMAS corrosion resistance [4,5,6]. In particular, the Gd₂Zr₂O₇ with multivariate doping demonstrates comprehensive excellent properties that make it particularly suitable for this application.

Guo et al. [7] found that Yb-doped Gd₂Zr₂O₇, specifically with the formulation of (Gd_0.9Yb_0.1)₂Zr₂O₇, achieves ultra-low thermal conductivity. Through non-stoichiometric compound doping, Wu et al. [8] further revealed that (Gd_0.85Yb_0.15)_1.95Zr_2.05O_7.025 ceramics exhibit the lowest thermal conductivity, highlighting their potential as next-generation thermal barrier coating (TBC) materials. Furthermore, extensive research has focused on the structural design and performance optimization of gadolinium zirconate-based coatings, composition optimization and development of new materials, mechanisms for enhancing thermophysical and mechanical properties, as well as electron beam physical vapor deposition (EB-PVD) fabrication processes and microstructure regulation [9,10,11,12,13,14,15]. However, during the EB-PVD process, ceramic targets tend to be cracked, leading to instability in coating deposition [16,17,18]. The fracture toughness of ceramics is a critical factor affecting their fracture strength, yet studies on the fracture toughness of target materials have rarely been reported in the literature.

Fracture toughness (K_IC) is a key mechanical property indicating the crack resistance of ceramic. Based on Griffith’s theory [19], K_IC is closely related to the Young’s modulus (E) of materials. Arzt [20] found that E decreases with smaller grain sizes. Spriggs [21] proposed that for every 10% increase in porosity, E drops by 30%–50%. The above theoretical analysis demonstrates that controlling the grain refinement process and sintering densification can optimize the Young’s modulus of ceramics, thereby enhancing their fracture toughness.

Usually, the fracture toughness of ceramics can be measured by the single-edge notched beam (SENB) and indentation method (IM) [22,23]. Compared to SENB, the indentation method offers simpler operation, but unfortunately, the accuracy is significantly affected by the surface roughness, making it unsuitable for quantitative analysis of fracture toughness in low-density ceramics. Although SENB requires more complex specimen preparation, its test results are less influenced by external factors and can be reliably applied to quantitative fracture toughness analysis of diverse ceramic materials [24,25].

Hence, addressing the problem of poor fracture toughness of gadolinium zirconate-based ceramic target, firstly, the (Gd_0.85Yb_0.15)_1.95Zr_2.05O_7.025 target was prepared by cold isostatic pressing (CIP) and pressureless sintering densification using nano-powder as raw material. Then, the densification process including grain growth and pore evolution were investigated. Next, the fracture toughness of the target was measured by the single-edge-notched beam method, meanwhile Young’s modulus (E) and hardness (H_v) were measured by the nanoindentation method. Finally, the influence mechanism of fracture toughness is discussed according to Griffith’s fracture theory.

2. Experimental Design

2.1. Experimental Procedures

Commercial nano-sized (Gd_0.85Yb_0.15)_1.95Zr_2.05O_7.025 powder (purity > 99.9%, supplied by Grirem Advanced Materials Co., Ltd., Beijing, China) was used as the raw material. XRD testing was performed on the raw material powder, and the results are shown in Figure 1. As observed in Figure 1, the XRD characteristic peaks of the raw material powder exhibit an overall rightward shift compared to those of Gd₂Zr₂O₇. This phenomenon is attributed to the lanthanide contraction, which results in a smaller ionic radius of Yb³⁺ compared to Gd³⁺. According to Bragg’s law (nλ = 2dsin θ), a decrease in the interplanar spacing d leads to an increase in the diffraction angle θ, thereby causing the rightward shift of the characteristic peaks.

The raw powder was ball milled and dried. The green compact was prepared by cold isostatic pressing (CIP) under the pressure of 150 MPa and the holding time of 300 s. The gadolinium zirconate-based ceramic targets were pressureless sintered in a box-type high-temperature furnace.

2.2. Characterizations

The phase structure of raw powder and targets were analyzed by X-ray diffraction (XRD, Cu-Kα, λ = 1.5406 Å). The microstructure and grain size of the targets were characterized by scanning electron microscopy (SEM, JEOL JSM-7610F, Tokyo, Japan). The porosity and the density ratio were measured using mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics, Norcross, GA, USA). The fracture toughness was measured using the single-edge notched beam (SENB) method (MTS-C45.105, Shanghai, China). The hardness and Young’s modulus were measured via nanoindentation (Agilent-G200, Lexington, MA, USA).

3. Results and Discussion

3.1. Effect of Sintering Condidtions on the Phase Structure of the Ceramic Target

Figure 2a shows the phase structure of the targets sintered at different temperatures with a holding time of 3 h, while Figure 2b presents the phase structure of the targets sintered at 1500 °C with different holding time. As shown in Figure 2, consistent with the phase structure of powder, the phase structure of all targets is of a single fluorite phase (space group Fm3m, ICDD PDF# 00-024-1164). No phase transformations were observed during the experiments conducted at different sintering temperatures and holding times. And no peaks of the pyrochlore phase or residual oxide were detected.

3.2. Investigation of Sintering Densification Process of the Ceramic Target

Figure 3 shows the SEM micrographs of the targets sintered at different temperatures with a holding time of 3 h. As shown in Figure 3, with the increase in sintering temperature, the isolated particles gradually coalesced from a loose state. Sintering necks were observed in the target material sintered at 1350 °C, and the isolated particles nearly disappeared at 1600 °C, accompanied by gradual grain growth. The target material approached full densification under these conditions.

Figure 4 illustrates the effects of sintering temperature on the density ratio, average grain size, porosity, and isolated porosity of the ceramic targets.

As shown in Figure 4, when the sintering temperature is between 1200 °C and 1450 °C, the density of the targets increases slowly, from 52.79% to 64.71%, and the porosity decreases from 47.21% to 35.29%. At this temperature range, the densification process can be regarded as the initial sintering stage, which follows the particle rearrangement–viscous flow mechanism proposed by Kingery [26]. Driven by surface energy, mass transport occurs through diffusion and evaporation–condensation, leading to increased contact area between particles, while pores are deformed and progressively reduced [27]. Furthermore, the average grain size grows from 0.61 μm to 1.09 μm, and the isolated porosity increases from 0.57% to 6.82%, indicating that grain boundary diffusion acts as the rate-controlling mechanism.

When the temperature is between 1450 °C and 1550 °C, the densification process can be considered as the intermediate sintering stage. The density ratio of the targets increases from 64.71% to 77.6%, accompanied by continued grain growth (average grain size rising from 1.09 μm to 1.75 μm). The porosity decreases from 25.29% to 22.40%, while the isolated porosity further increases, reaching a maximum of 8.03% at 1500 °C. As shown in Figure 4, rapid densification of the ceramic target occurred between 1450 °C and 1550 °C. Notably, the grain growth rate exhibited an initial increase followed by a decrease around 1500 °C. This behavior aligns with Kingery’s intermediate sintering stage (Stage II), where competition between grain boundary migration rate and pore shrinkage rate governs densification. The isolated porosity reaches a maximum value when these two rates approach equilibrium, as observed at 1500 °C, where the near-balanced kinetics maximize pore entrapment within grains [27].

When the temperature is between 1550 °C and 1600 °C, the densification process can be regarded as the final sintering stage. The density ratio of the targets increases from 91.82% to 97.28%, the average grain size grows from 1.09 μm to 1.75 μm, and the grain growth rate increases. This stage was predominantly governed by pore shrinkage coupled with volume diffusion mechanisms, which collectively drove the densification process. Pores are progressively compressed and shrunk, resulting in a porosity reduction from 4.96% to 1.64% and isolated porosity decreasing from 3.22% to 1.08%. This behavior aligns with Kingery’s final sintering stage (Stage III) [27].

When the sintering temperature reaches 1500 °C, the isolated porosity attains its maximum value. However, isolated pores are a detrimental factor contributing to nodular defects during the coating deposition process [28]. Therefore, the effect of holding time on the densification process is investigated. Figure 4 illustrates the microstructural evolution of the targets sintered at 1500 °C under different holding times.

As shown in Figure 5 and Figure 6, similar to the effect of increasing sintering temperature, at 1500 °C, prolonged holding time leads to progressive coalescence of particles and growth of sintering necks, accompanied by subsequent grain growth. When the holding time reaches 7 h, the densification effect can be obviously observed. Figure 5 illustrates the effects of holding time on the density ratio, grain size, porosity, and isolated porosity of the targets.

As shown in Figure 6, when sintered at 1500 °C, increasing the holding time from 1 h to 6 h results in a gradual reduction in porosity, with the density ratio rising to 85.54%. Further increasing the holding time, the densification rate becomes slower, with the density ratio slightly increased, which shows there is still no deep densification. This indicates that with prolonging holding time at 1500 °C, the densification process remains in the intermediate sintering stage, suggesting that sintering temperature is a more critical factor than holding time in governing densification. Meanwhile, within the holding time range of 1–7 h, grain growth exhibits three distinct stages, which are identified as fast grain growth in 1–4 h, slowed growth rate in 4–6 h and accelerated growth rate in 6–7 h. This phenomenon may be attributed to the Zener pinning effect [29]. During 4–6 h, the segregation of solutes or secondary-phase particles at grain boundaries inhibits boundary migration, suppressing grain growth. As the holding time is more than 6 h, these particles coarsen and detach from grain boundaries, leading to pinning failure and renewed acceleration of grain growth. The isolated porosity first increases and then decreases. When the holding time is extended from 1 h to 4 h, rapid grain boundary migration entraps pores within grains, raising isolated porosity to a maximum of 8.15% at 4 h. When the holding time is more than 4 h, pore shrinkage dominates due to enhanced diffusion, causing isolated porosity to decline.

3.3. Effect of the Properties of Ceramic Targets on Fracture Toughness

The fracture toughness of the targets was measured using the single-edge notched beam (SENB) method. As shown in Figure 7, the relationship between sintering temperature and the density, grain size, and fracture toughness of the targets are illustrated.

As shown in Figure 7, when the sintering temperature is below 1450 °C, the density ratio and grain size of the targets increase gradually with rising temperature, while the fracture toughness exhibits an accelerated growth rate. When the temperature further increases to 1500 °C, the density ratio rises rapidly to 77.6%, accompanied by accelerated grain growth, and the fracture toughness reaches its maximum value of 2.245 MPa·m^0.5. However, as the sintering temperature approaches 1600 °C, the density ratio surges to 97.28% with rapid grain coarsening, yet the fracture toughness decreases sharply despite the increased density. According to Rosal et al. [30], this paradoxical behavior can be explained by crystal plasticity theory. Higher density in ceramic enhances their close-packed crystal structure, which strongly suppresses dislocation motion due to the lack of slip systems. Therefore, the suppression of crack tip blunting induced by plastic deformation exacerbates stress concentration at the crack tip, thereby reducing energy dissipation during crack propagation and ultimately leading to a decline in fracture toughness. Furthermore, the Young’s modulus and hardness of the targets at different sintering temperatures were measured via nanoindentation, as illustrated in Figure 8.

As shown in Figure 8, both the Young’s modulus and hardness increase within the temperature range of 1200–1600 °C. According to Griffith’s fracture theory, the fracture strength is proportional to E^0.5 (where E is Young’s modulus), and the enhanced hardness improves crack resistance by reducing indentation crack length. The synergistic effect of these factors leads to the maximum fracture toughness at 1500 °C. However, as the temperature further increases to more than 1500 °C, significant grain growth occurs. At this stage, grain boundary energy becomes a critical factor influencing fracture toughness [31]. The reduction in grain boundary energy caused by excessive grain growth weakens the crack deflection and bridging mechanisms, resulting in a sharp decline in fracture toughness at the temperature above 1500 °C.

Figure 9 illustrates the relationship between density, grain size, and fracture toughness of the ceramic targets under different holding times. Figure 10 illustrates the Young’s modulus and hardness of the targets measured via nanoindentation.

As shown in Figure 9, the fracture toughness of the targets reaches its maximum value at a holding time of 4 h and a density ratio of 80.7%, which mirrors the trend observed in Figure 7 (maximum toughness at 1500 °C with 77.6% density ratio). This similarity highlights that density ratio is a key factor governing fracture toughness. The influence of the density ratio reflects the combined effects of microstructural characteristics such as grain size and porosity on fracture behavior. However, when the holding time exceeded 4 h, grain growth persisted, reducing grain boundary energy and consequently degrading fracture toughness, as discussed in prior analyses [30]. From Figure 10, despite slight increases in Young’s modulus and hardness within the 1–7 h holding time range, their variations remain statistically insignificant. Therefore, density ratio and grain size are identified as the dominant factors controlling the fracture toughness of the targets.

4. Conclusions

This study investigated the sintering densification behavior of (Gd_0.85Yb_0.15)_1.95Zr_2.05O_7.025 ceramic targets for EB-PVD applications and its impact on the fracture toughness of the targets. The key conclusions are summarized as follows.

(Gd_0.85Yb_0.15)_1.95Zr_2.05O_7.025 ceramic targets were fabricated via pressureless sintering, and the sintering process aligns with Kingery’s three-stage sintering theory. The sintering temperature governs densification. At 1500 °C with a holding time of 3 h, the isolated porosity of the targets peaked at 8.03% (maximum value). When sintered at 1500 °C, prolonged holding time provided limited improvement in densification, which induced three-stage grain growth kinetics: initial rapid growth (1–4 h), followed by transitional slowdown (4–6 h), and resumed acceleration (6–7 h). This behavior is attributed to the grain boundary pinning effect [27], where solute segregation temporarily inhibits boundary migration, followed by particle coarsening and failure of the pinning effect.

Both increasing sintering temperature and prolonging holding time result in a nonlinear trend in the fracture toughness of the targets, characterized by an initial increase followed by a sharp decrease. Density ratio and grain size play a crucial role in determining fracture toughness. The peak value of the fracture toughness occurs at a density ratio of ~80%, where the synergistic enhancement of Young’s modulus contributes to improved toughness. However, excessive grain growth induced by prolonged sintering reduces grain boundary energy, which may significantly degrade fracture toughness due to weakened crack deflection and bridging mechanisms.