Strengthening and Toughening Mechanism and Impact Resistance of Graphene Nanosheet–Silicon Carbide Whisker Hybrid Reinforced PDC Composites

Abstract

The toughening mechanism and impact resistance of Graphene Nanosheets (GNs) and Silicon Carbide Whisker (SiCw) hybrid reinforced polycrystalline diamond compact (PDC) were systematically investigated. Pure PDC and reinforced PDC composites with different ratios were prepared by High-Temperature and High-Pressure (HPHT) synthesis (5.5 GPa, 1450 °C), and their microstructures, mechanical properties, impact resistance, and thermal properties were comprehensively characterized. The results showed that the hybrid introduction of GNs and SiCw produced an effective synergistic reinforcing effect. Compared with pure PDC, the optimal hybrid ratio group (PDC+0.5 wt % GNs+1.0 wt % SiCw) exhibited the most favorable comprehensive performance among the tested groups: Apparent density increased to 3.63 g/cm³, Vickers hardness reached 56.4 ± 1.1 GPa, flexural strength and fracture toughness increased to 682 and 7.8 MPa·m^1/2, while thermal conductivity stabilized at 550 W/(m·K) and Coefficient of Thermal Expansion (CTE) decreased to 2.5 × 10^–6 /K. Microstructure analysis showed that the multiscale hybrid reinforcing phase effectively suppressed abnormal grain growth and significantly reduced porosity to 0.4%. Based on two-dimensional images and fractal geometry analysis, it was confirmed that the hybrid network greatly increased the curvature of crack propagation (τ = 1.38) and the fractal dimension (D _f = 1.26), and the critical strain energy release rate (G _IC ) of the composite system was raised to 65.9 J/m². A multidimensional performance radar system was used to reveal the cross-scale multidimensional synergistic strengthening and toughening mechanism of GNs and SiCw in PDC composites. This study not only provides a solid theoretical basis for the toughening design of high-end superhard materials under complex conditions in deep strata, but also opens up new avenues for the development of multidimensional nanocomposites in extreme service environments.

1. Introduction

Owing to its superior hardness, excellent wear resistance, and superior intrinsic thermal conductivity, polycrystalline diamond compact (PDC) has become a core rock-breaking and cutting material for extreme service conditions such as deep formation oil and gas drilling, geothermal resource development, and precision machining. However, the underlying architecture of PDC materials is mainly composed of a network of highly rigid covalent bonds (D-D bonds) between diamond particles. This particular crystallographic feature, while giving it extremely high yield strength, also leads to its high intrinsic brittleness that cannot be ignored. In the face of deep complex formation drilling or strong discontinuous cutting conditions, high-frequency, high-strain rate dynamic microimpacts and alternating stresses triggered by the stick–slip vibration of the drill string can easily induce rapid nucleation and propagation of microcracks within the PDC, ultimately leading to macroscopic chipping, fragmentation or catastrophic failure. This contradictory feature of ″high hardness and low toughness″ and the low damage tolerance under high-frequency dynamic microimpact have become a technical bottleneck restricting the further large-scale application of high-end PDC cutting teeth in deeper and harder formations.

To break through this intrinsic performance bottleneck, researchers have extensively explored reinforcement and toughening strategies by introducing second-phase nanomaterials. In recent years, two-dimensional nanocarbon materials - particularly Graphene Nanosheets (GNs) - have been favored for their ultrahigh tensile strength of up to 130 GPa, excellent fracture toughness, and extremely high in-plane thermal conductivity. The academic community considers it an ideal toughening agent and thermal bridging phase for superhard ceramics and composites. , Studies have shown that an appropriate amount of graphene can effectively dissipate fracture energy through mechanisms such as crack deflection, crack bridging, and interlayer slip. However, in the actual High-temperature and High-pressure (HPHT) sintering process, the single introduction of GNs faces severe physical and chemical challenges: on the one hand, GNs with an extremely high specific surface area are prone to stacking and agglomeration in the composite matrix due to strong van der Waals forces, resulting in local stress concentration. On the other hand, the geometric properties of the two-dimensional plane limit the initial interfacial bonding force with the diamond/cobalt matrix, making it prone to debonding when subjected to high-frequency shear stress. This not only hinders the effective transfer of loads, but may even evolve into the source of microcracks locally, thereby weakening or even deteriorating the macroscopic properties of the material.

In response to the limitations of a single nanoscale reinforcement, constructing three-dimensional (3D) spatial hybrid networks has emerged as an effective strategy to balance mechanical and thermophysical properties. , To overcome the stacking effect of two-dimensional graphene nanosheets (GNs), introducing one-dimensional silicon carbide whiskers (SiCw) offers geometric complementarity. However, in the high-temperature and high-pressure (HPHT) synthesis of PDC, highly reactive liquid-phase cobalt is prone to intense interfacial reactions with SiCw to form brittle intermetallic compounds, significantly degrading the whiskers’ toughening efficacy. Recent studies suggest that synergistically combining 1D and 2D nanomaterials can establish a steric hindrance barrier to mitigate interfacial degradation. For instance, recent investigations on analogous transition-metal/ceramic systems (e.g., Ni-matrix cermets) have demonstrated that multilayer graphene coatings act as a physical shield, significantly inhibiting the high-temperature dissolution of SiCw and the subsequent formation of brittle silicides. Furthermore, while classical theories assume that interfacial defects strictly degrade thermal conductivity, advanced nanoscale thermal transport studies reveal that strategically engineered topological defects and vacancy sites can actually reduce interfacial thermal resistance (Kapitza resistance). This is achieved by mitigating directional phonon nonequilibrium, decreasing elastic modulus mismatch, and providing active sites for covalent bridging. − Despite these advances, the phase evolution, survivability, and defect-mediated synergistic mechanisms of the 1D SiCw and 2D GNs network within a highly rigid PDC matrix under the extreme HPHT conditions remain a significant knowledge gap.

To address this gap, this study pioneers the construction of a cross-scale 1D SiCw and 2D GNs spatial interlocking network within a PDC matrix via HPHT synthesis (5.5 GPa, 1450 °C). By integrating macroscopic mechanical evaluations with microscopic defect-mediated interface analyses, we systematically decouple the synergistic contributions of SiCw and GNs. Specifically, we investigate the multiscale toughening mechanismssuch as 3D crack deflection, whisker pull-out, and interlayer slipand quantitatively evaluate the defect-coupled thermal conduction behaviors. This work not only clarifies the physicochemical survival mechanisms of nanocarbons and whiskers under high-pressure liquid-metal erosion but also provides an evidence-based paradigm for optimizing the dynamic microimpact resistance and thermal stability of superhard composites in extreme service environments.

2. Materials and Methods

2.1. Main Materials and Instruments for the Experiment

The main materials used in the experiment included diamond micropowder, GNs, SiCw, cobalt powder, anhydrous ethanol and deionized water. The diamond micropowder, with a particle size of 0.5–2 μm and a purity higher than 99.9%, was provided by Henan Yellow River Cyclone Co., Ltd. as the main raw material for the PDC matrix. GNs with 3–5 layers and a specific surface area greater than 500 m²/g, purchased from Changzhou Sixth Element Materials Technology Co., Ltd., was used as a two-dimensional nanoreinforcing phase in composites. SiCw, with a diameter of 0.2–0.5 μm and an initial macroscopic length of 10–30 μm, was derived from Beijing Zhongke Silicon Crystal Materials Co., Ltd. and was used as a one-dimensional reinforcing phase to synergistically enhance the matrix performance with GNs. Cobalt powder with a purity of over 99.8% and a particle size range of 1–3 μm, provided by Jinchuan Group Co., Ltd., was used as a binder to promote densification and interfacial bonding during sintering. Anhydrous ethanol analytical grade, with a purity of no less than 99.7%, purchased from Sinopharm Group Chemical Reagent Co., Ltd. was mainly used for sample cleaning and as a dispersion medium. The ultrapure water had a resistivity of 18.2 MΩ·cm (at 25 °C) and was made in the laboratory. The main equipment used in the experiment is shown in Table .

1. Detailed Information on the Main Experimental Instruments.

Instrument Names	Model	Manufacturer	Main uses
High energy ball mill	QM-3SP2	Nanjing University Instrument Factory	For uniform mixing and dispersion of raw materials
Cubic press	SDP-16700	Xianyang Superhard Material Equipment Co., Ltd.	HPHT sintering for PDC composites
Scanning Electron Microscope (SEM)	SU8010	Hitachi, Japan	For microscopic morphology observation of materials
X-ray Diffraction (XRD) instrument	D8 Advance	Bruker, Germany	For phase analysis and crystal structure characterization
Universal material testing machine	Instron 5967	Instron, USA	For mechanical property testing
Impact testing machine	ZBC-4	Metz Industrial Systems, LTD	For impact resistance testing
Laser thermal conductivity meter	LFA 467	Netzsch, Germany	For thermal conductivity testing
Ultrasonic cleaner	KQ-500DE	Kunshan Ultrasonic Instrument Co., LTD	For sample dispersion and cleaning
Microscopic confocal Raman spectrometer	LabRAM HR Evolution	Horiba, Japan	For carbon bonding state and phase microarea analysis
Thermomechanical analyzer	TMA 402	Netzsch, Germany	For testing the Coefficient of Thermal Expansion (CTE) of materials

2.2. Experimental Methods

To systematically study the preparation process, microstructure, mechanical properties, and impact resistance of GNs and SiCw hybrid reinforced PDC, related experiments such as composite material ratio, material mixing and dispersion, HPHT synthesis parameter optimization, performance testing, and characterization were designed.

Design of composite material ratio: The specific reinforcement mass fractions were determined based on preliminary exploratory experiments and established dispersion thresholds for carbon and ceramic nanomaterials. In superhard composite systems, when the GNs content exceeds 0.5 wt %, strong van der Waals forces induce severe restacking and agglomeration, which act as structural defect sources. Similarly, increasing the SiCw content beyond 1.0 wt % significantly hinders the capillary infiltration of liquid-phase cobalt during HPHT sintering. Excessive whisker loading restricts particle rearrangement, leading to insufficient densification, increased porosity, and localized stress concentrations, which corresponds to the agglomeration-induced degradation phenomena recently reported in hybrid composites. ,,

Therefore, to balance structural densification and uniform reinforcement dispersion, 0.5 wt % GNs and 1.0 wt % SiCw were selected as the compositional limits. The experimental groups were designed as follows: pure PDC (reference control), PDC+0.5 wt % GNs, PDC+1.0 wt % SiCw, PDC+0.5 wt % GNs+0.5 wt % SiCw, and PDC+0.5 wt % GNs+1.0 wt % SiCw. In all groups, the initial addition of cobalt powder was fixed at 8.0 wt %, with diamond micropowder comprising the balance.

Test sample preparation: The well-mixed composite powder was first formed by uniaxial molding at room temperature to prepare the initial cold-pressed blank. Then the cold press was placed in a purified graphite cup, wrapped with a Pyrophyllite pressure transfer sealing medium, and assembled into a high-pressure composite block suitable for a six-sided top press. The synthetic block was placed in the six-sided top press chamber, first pressurized to 5.5 GPa, then heated by passing an electric current through the graphite tube to 1450 °C according to the preset procedure. Under these HPHT conditions, hold for 10 min to allow the cobalt binder to melt and penetrate, promoting the D-D bonding between diamond particles. After the synthesis was completed, the system was cooled down and depressurized. The pyrophyllite was then broken to take out the cylindrical PDC composite sample (dimensions: approximately Φ 16 mm × 12 mm).

Material synthesis and high-temperature high-pressure process: To ensure uniform dispersion of the reinforcing phase in the matrix, a high-energy ball mill was used for wet ball milling mixing. The ball-to-material ratio was 10:1, the milling medium was anhydrous ethanol, the milling time was 4 h, and the rotational speed was 300 rpm. It should be particularly noted that during this high-energy shear grinding process, although the high-aspect-ratio one-dimensional SiCw inevitably undergoes a certain degree of cleavage and shortening during this process, precise control of the ball milling parameters (4 h, 300 rpm) ensures uniform dispersion. Quantitative analysis indicates that while the whiskers are shortened, their retained aspect ratio remains well above the critical threshold required to activate the fiber pull-out toughening mechanism during subsequent fracture processes. Meanwhile, the two-dimensional GNs will also produce abundant edge topological defects due to mechanical impact. However, this artificially introduced high-density defect, along with the nascent active surface, provides an excellent thermodynamic driving force and suspension point for strengthening liquid-phase cobalt impregnation and constructing carbon–metal covalent bridges during subsequent HPHT sintering.

The mixed slurry was ultrasonically dispersed and vacuum-dried at 80 °C for 12 h, and then prepressed into shape at room temperature. The cold-pressed blanks were then loaded into high-purity graphite heating tubes, wrapped with Pyrophyllite as a pressure transfer and sealing medium, and assembled into high-pressure composite blocks. HPHT synthesis was performed using a six-sided top hydraulic press (SDP-16700). First, the hydraulic system was driven to increase the hydrostatic pressure in the chamber to 5.5 GPa, and then the heating was carried through a graphite tube at a rate of 100 °C/min to 1450 °C and held for 10 min. It is notable that under the extreme hydrodynamic hydrostatic pressure of 5.5 GPa, the molten liquid Cobalt (as a catalyst and binder) underwent a significant “Cobalt Sweep” effect, with some of the cobalt migrating toward the boundary of the pyroxylite medium with a lower pressure gradient. As a result, the final absolute cobalt content within the sintered body will be dynamically modulated by this thermodynamic and kinetic boundary. After the synthesis was completed, cylindrical PDC samples (approximately Φ 16 mm × 12 mm in size) were extracted following a controlled cooling and gradual depressurization procedure.

Microstructure and phase characterization: The prepared PDC samples were wire-cut, finely ground, polished and gold-sprayed and placed in the sample chamber. High-resolution micromorphology and backscattered electron images were obtained using SEM, and the average grain size and pore size of diamond were calculated using ImageJ image processing software according to the Linear Intercept Method (ASTM E112 standard). Furthermore, X-ray diffraction (XRD) was employed for phase analysis to evaluate the inhibitory effect of hybridization on grain boundary migration and grain growth. XRD systems were used for phase analysis of the sample. The surface of the polished sample was scanned with Cu Kα rays to qualitatively and quantitatively determine the phase composition (such as diamond, cobalt, graphite phase and silicon carbide) inside the composite material by analyzing the position and relative intensity of the diffraction peaks, and to monitor the phase transition mechanism during sintering.

Mechanical and impact resistance tests: Due to the size limitations of synthesizing superhard materials at HPHT, impact resistance tests were conducted using micro-Charpy V-notch impact tests. The synthesized specimens were processed into miniature impact specimens of size 3 mm × 4 mm × 15 mm (notched depth 1 mm) by precision slow wire cutting technology. The specimen was placed horizontally on the anvil of the impact testing machine, and the pendulum was released to fracture the sample. Dynamic impact toughness is calculated by measuring the transient work consumed to break the specimen and converting it based on the net cross-sectional area at the notches of the specimen. It should be objectively noted that due to the extremely high intrinsic stiffness of PDC composites, the energy consumed by Machine Compliance is inevitably coupled in the microsecond-level impact response. Therefore, the a _k extracted in this test is not the absolute intrinsic impact toughness, instead, it is defined as the ’relative apparent dynamic energy dissipation index’, which is only used for the lateral qualitative assessment of the impact buffering performance of different formulations under the same stiffness constraints.

The microhardness tester applied a constant load to the surface of the polished sample with a diamond indenter, held it for a period of time before unloading, measured the diagonal length of the indentation, and calculated the Vickers hardness to evaluate the material’s ability to resist local plastic deformation. All test equipment was connected to a computer data acquisition system to ensure that test signals were recorded in real time. A total of five independent samples (n = 5) were tested for each group, and the results were expressed as “mean ± standard deviation”. Statistical analysis was performed using one-way Analysis of Variance (ANOVA) followed by Tukey’s Honestly Significant Difference (HSD) posthoc test for multiple comparisons. Differences were considered statistically significant at an exact p-value <0.05. Detailed ANOVA parameters, including F-values and exact p-values, are incorporated into the text, and comprehensive ANOVA tables are provided in the Supporting Information (Tables S1–S3).

2.3. Experimental Comprehensive Equipment and Evaluation System

To systematically elucidate the microscopic strengthening and toughening mechanisms and macroscopic impact resistance of GNs and SiCw hybrid reinforced PDC composites, this study constructed a complete “synthesis - characterization - testing” closed-loop system based on the high-pressure physical synthesis and comprehensive material evaluation platform.

The core preparation unit of the system is the HPHT synthesis equipment. The study employed the domestic SDP-16700 six-sided top hydraulic press, which is equipped with six independently controlled tungsten carbide (WC) hard alloy top hammers, using Pyrophyllite as the pressure transmission and sealing medium and high-purity graphite tubes as the heating body, capable of creating a highly uniform hydrostatic and thermal field in three-dimensional space, Precisely provide and maintain the extreme synthesis conditions of 5.5 GPa and 1450 °C required for this study.

The microstructure characterization and phase analysis system is designed to reveal the multiscale distribution and interfacial behavior of the reinforcement in the diamond matrix, with the core relying on high-resolution SEM and XRD platforms for high-precision surface morphology observation and crystallographic structure analysis. In addition, macroscopic mechanics and dynamic impact performance assessment mainly rely on universal material testing machines and impact testing machines equipped with precision load–displacement sensors to achieve real-time acquisition and precise quantification of stress response and energy dissipation data of specimens during three-point bending, local indentation and dynamic impact fracture.

2.4. Experimental Analysis Methods

2.4.1. Microstructure Characterization

The microstructure, multiscale distribution of the reinforcing phase, interfacial bonding state and dynamic fracture morphology of the composites were observed using field emission SEM (SU8010, Hitachi). After wire cutting, precision polishing and ultrasonic cleaning, the samples were subjected to surface gold spraying to enhance conductivity. To accurately obtain diamond grain and pore parameters, high-magnification SEM images were processed using ImageJ image processing software and statistically analyzed strictly in accordance with the Linear Intercept Method (ASTM E112 standard). At least five fields of view were randomly selected for each sample, and no fewer than 200 diamond grains were counted to calculate the average grain size and evaluate the pinning effect of hybridized enhanced relative grain boundary migration.

2.4.2. Phase Analysis

Phase composition and crystal structure were identified using an XRD (D8 Advance, Bruker). Test conditions were strictly set as follows: Cu Kα radiation (λ = 0.15406 nm), acceleration voltage 40 kV, operating current 40 mA, scanning range 2θ = 10–80°, step size 0.02°, scanning speed 5°/min. The test results were compared with standard PDF cards using Jade software to precisely qualitatively analyze phases such as diamond, cobalt, graphite and silicon carbide in the system, and to determine the thermodynamic phase transition behavior and material purity during HPHT sintering.

In addition, to accurately identify the carbon bonding state (sp² to sp³ hybridization) and defect Evolution in the composites, microarea scanning of the sample fracture was performed using a confocal Raman spectrometer (Raman, LabRAM HR Evolution, Horiba). The test was conducted using a 532 nm solid-state laser as the excitation source, with the test range set at 600–3100 cm^–1 to target and capture the characteristic vibrational responses of the carbon nanophase at the diamond matrix and local grain boundaries.

2.4.3. Mechanical Property Tests

Due to the physical constraints of the cavity size of the HPHT synthesis equipment, the mechanical tests in this study were characterized using nonstandard microsamples. All mechanical samples were cut from synthetic PDC cylinders using precision slow wire cutting machines. The bending strength test was conducted using the microthree-point bending method. The test sample size was 2 mm × 2 mm × 12 mm, the test span was set at 10 mm, and the loading speed was 0.5 mm/min. The bending strength σ is calculated as shown in eq .

$$\sigma =\frac{3\mathit{FS}}{2b{h}^2}$$ 1

In eq , F represents the breaking load, S represents the span, and b and h respectively represent the width and height of the specimen.

The fracture toughness test was conducted using the Single-sided Notched Beam (SENB) method, with miniature specimens measuring 2 mm × 4 mm × 14 mm, a span of 10 mm, and notched depth a = 2 mm. In accordance with international fracture mechanics standards, the width of the specimen is denoted as B, the height (depth) in the direction of crack propagation is denoted as W, and the span is denoted as S. The fracture toughness K _IC is calculated as shown in eq .

$$K_{\mathit{IC}}=\frac{\mathit{FS}}{B{W}^{3/2}}f\left(\frac{a}{W}\right)$$ 2

In online Elastic Fracture Mechanics (LEFM), the dimensionless geometric flexibility function f(a/W) is highly dependent on the aspect ratio of the specimen. Due to the limitations of the HPHT synthesis cavity size in this study, the aspect ratio of the microspecimens is S/W = 2.5, which no longer meets the pure bending assumption of the international standard for long beams (S/W = 4). To eliminate the systematic error caused by the shear deformation of the Deep Beam, this study abandons the conventional high-order polynomial of the deep beam and strictly adopts the generalized deep beam fracture mechanics model with shear correction derived by Guinea et al. Since the crack depth-to-height ratio of the microspecimens in this experiment was constant (a/W = 0.5), this study directly substituted the specific dimensionless geometric flexibility constant f(0.5) = 2.557 under this boundary condition for precise calculation.

2.4.4. Impact Resistance Test

Since the traditional Charpy specimen size exceeded the limits of the synthetic cavity, the impact test was conducted using the micronotch impact test. Miniature Charpy V-notch specimens with dimensions of 3 mm × 4 mm × 15 mm and notch depth of 1 mm were tested using an instrumented impact testing machine (ZBC-4, Mette Industrial Systems Ltd.). The impact toughness is calculated as shown in eq .

$${\alpha }_k=\frac{A_k}{A_{\mathit{net}}}$$ 3

In eq , A _k represents the dynamic shock absorption energy consumed to break the specimen, and A _net represents the net cross-sectional area at the notch of the specimen.

In addition, to assess the damage tolerance of the material, the impact fracture specimen is subjected to wire-cut sectioning, resin mounting and precision polishing along the direction perpendicular to the fracture surface. The deep microcrack propagation morphology beneath the fracture was observed using a SEM. At least five fields of view were randomly selected from each sample, and the farthest vertical distance from the main fracture surface to the tip of the microcrack was measured to quantify and count the average Subsurface Damage Depth (Subsurface damage depth).

2.4.5. Thermal Performance Analysis

The thermal diffusivity of the material at room temperature was tested by laser flash (LFA 467, Netzsch, Germany), and its thermal conductivity was calculated in combination with the true density and specific heat capacity of the sample under test (determined by differential scanning calorimetry). The CTE was measured by a thermomechanical analyzer, and the test was conducted in accordance with ASTM E1461 and relevant standards for testing superhard materials to ensure the reliability of transient thermal conduction and thermal expansion response data.

3. Results and Discussion

3.1. Basic Physical Properties Test Results

The apparent density, porosity, relative density, and Vickers hardness of the samples were measured to systematically evaluate the effect of hybrid reinforcement on the underlying structure of the composites. The test results are shown in Figure . Figure a shows the evolution of apparent density and porosity. According to the mixing Rule (rule of Mixtures), the theoretical skeleton density of pure PDC (measured to contain approximately 7.2 wt % residual cobalt) was approximately 3.68 g/cm³. After the introduction of low-density GNs (2.20 g/cm³) and SiCw (3.21 g/cm³), the theoretical density of the optimal hybrid group (containing approximately 6.7 wt % residual cobalt) dropped to 3.645 g/cm³. However, the experimentally measured macroscopic apparent density showed the opposite slight upward trend (steadily increasing from 3.61 ± 0.02 g/cm³ of pure PDC to 3.63 ± 0.02 g/cm³ of PDC+0.5GNs+1.0SiCw). This physical phenomenon, which seemed to violate the rule of mixtures, was readily explained by porosity analysis: although the low-density reinforcement phase reduced the theoretical density baseline, its introduction caused porosity to decrease significantly from 1.8% for pure PDC to 0.4% for the optimal hybrid group (F­(4,20) = 45.6, p < 0.001). The greatly enhanced structural densification effect outweighed the mass pull-down effect of the low-density phase, resulting in a net increase in apparent density.

1.

Basic physical properties of PDC composites with different reinforcement phases: (a) density and porosity analysis, (b) Vickers hardness and relative density analysis.

Figure b further demonstrates the corresponding relationship between the relative density of the composite material and its Vickers hardness. It can be clearly seen that the introduction of the hybrid reinforcing phase greatly enhanced the relative density of the system (from 98.2% significantly to 99.6%). The multiscale nano/microphase acts as an effective “solid lubricant” in the early stage of HPHT sintering, promoting the rearrangement of diamond particles and the deep penetration of liquid-phase cobalt. The densification benefits brought by the pore-filling effect far exceed the mass pull effect caused by the introduction of low-density phases. From the thermodynamic perspective of the material, the large specific surface area of GNs and SiCw significantly changed the capillary infiltration angle of the liquid cobalt and broke the physical arch bridge between the diamond particles during the cold pressing stage (arching effect). This led to a more thorough particle rearrangement at the initial stage of the 5.5 GPa rheology. Under the combined effect of this densification and the aforementioned “Hall-Petch effect”, the Vickers hardness of the composite material increased significantly. The Vickers hardness of pure PDC was 50.2 ± 1.5 GPa, and that of the single reinforcing phase group increased to 51.5 ± 1.2 GPa (GNs) and 52.8 ± 1.4 GPa (SiCw), respectively. The hybrid reinforcement system showed the best mechanical resistance due to the strong hindering of dislocation slip and local plastic deformation by the three-dimensional network constructed by GNs and SiCw, with the PDC + 0.5GNs + 1.0SiCw group achieving the highest hardness of 56.4 ± 1.1 GPa (F(4,20) = 38.2, p < 0.001). The data fully confirmed the synergistic strengthening effect of the multiscale carbon-based whisker/lamellar structure in the superhard matrix.

3.2. Analysis of Mechanical Property Test Results

Based on the previous densification and inhibition of abnormal grain growth, the macroscopic mechanical properties of the composite materials were systematically evaluated by using microthree-point bending experiments, and the results are shown in Figure . Figure a shows the evolution trends of flexural strength and fracture toughness of the samples. The flexural strength of pure PDC was 525 ± 18 MPa. With the introduction of a single reinforcing phase, the load-bearing capacity of the material was significantly enhanced, reaching 584 ± 15 MPa and 608 ± 20 MPa for the PDC+0.5 wt % GNs group and the PDC + 1.0 wt % SiCw group, respectively. In the multiscale hybrid system, GNs and SiCw produced a significant synergistic reinforcing effect in resisting flexural tensile stress. The bending strength of the PDC+0.5GNs+1.0SiCw group reached a maximum of 682 ± 22 MPa (F(4,20) = 52.4, p < 0.001), which was approximately 29.9% higher than that of pure PDC.

2.

Mechanical properties from three-point bending tests: (a) flexural strength and fracture toughness, (b) elastic modulus and failure strain.

The trend of fracture toughness is consistent with flexural strength. Based on the deep beam shear correction model, the fracture toughness (K _IC ) of pure PDC was 5.4 ± 0.2 MPa·m^1/2, exhibiting intrinsic brittleness. Upon the introduction of the hybrid reinforcement network, the fracture toughness of the PDC + 0.5GNs + 1.0SiCw group increased to a maximum of 7.8 ± 0.3 MPa·m^1/2 (F(4,20) = 65.8, p < 0.001) (a 44.4% improvement). According to Linear Elastic Fracture Mechanics (LEFM), the essence of material toughening relies heavily on increasing fracture energy dissipation. This significant enhancement originates from the synergistic multistage fracture energy dissipation induced by the 1D+2D hybrid network. During crack propagation, the 1D SiCw resists crack opening via interfacial debonding and pull-out friction in the crack wake zone, consuming substantial strain energy. Simultaneously, the high-specific-surface-area 2D GNs induce microcrack branching and interlayer slip. This multidimensional spatial coupling effectively enlarges the damage tolerance zone and increases crack tortuosity, allowing the composite to dissipate significantly more fracture energy before catastrophic failure compared to the theoretical limit of single-phase reinforcements.

Figure b shows the test results of the elastic modulus and failure strain (bending fracture strain) of the composite. The elastic modulus of pure PDC was 845 ± 12 GPa. With the addition of the hybrid reinforcing phase, the elastic modulus steadily rose to 885 ± 15 GPa due to the increase in system density and the formation of a high stiffness reinforcing network. It is notable that the fracture failure strain of pure PDC was only 0.62%, showing typical catastrophic brittle fracture characteristics; The failure strain in the PDC + 0.5GNs + 1.0SiCw group increased to 0.76% (F(4,20) = 18.5, p = 0.002). The increase in failure strain was not due to the plastic yielding of the matrix; rather, the enhanced network activated abundant microdamage tolerance mechanisms (such as the branching of microcracks and the nonlinear slip at the interface). This “enhanced microdamage tolerance” feature indicated that the material could absorb and dissipate much greater strain energy before destabilizing fracture, fully verifying the effectiveness of the toughening design of superhard PDC materials.

It is imperative to acknowledge that, constrained by the dimensions of the HPHT synthesis cavity, miniature nonstandard specimens were utilized in this mechanical evaluation. According to the Weibull statistical size effect for brittle ceramics, miniature specimens contain a smaller stressed volume and statistically fewer critical volumetric flaws, which typically yields higher absolute flexural strength values compared to bulk materials. Furthermore, regarding fracture toughness, the reduced specimen thickness may not fully satisfy the stringent plane-strain constraint criteria defined in standard linear elastic fracture mechanics (LEFM). The potential relaxation of triaxial stress constraints at the crack tip could introduce a partial plane-stress state, marginally inflating the measured absolute K _IC values. Consequently, the mechanical properties reported herein are not intended as absolute design allowables for full-scale bulk PDC cutters. Rather, they serve as a highly reliable relative metric to elucidate the structural efficacy and synergistic toughening sequence among the different hybrid formulations under identical geometric constraints.

3.3. Dynamic Impact Resistance Test Analysis

Composite materials are often subjected to high-frequency dynamic impact loads in extreme service conditions such as drilling and cutting, so the assessment of their impact resistance is crucial. The impact energy dissipation behavior of each group of samples was tested using instrumental micro-Charpy impact tests, and the results are shown in Figure . Considering the specific cross-sectional dimensions of the microspecimens, Figure a comprehensively presents the evolution law of the relative dynamic dissipation index and dynamic fracture toughness (K _Id ) of the composites. The relative dynamic dissipation index of pure PDC was 21.6 ± 1.5 J/cm², and the dynamic fracture toughness was 4.9 ± 0.3 MPa·m^1/2, showing brittle characteristics that are difficult to effectively absorb energy at extremely high strain rates. After the introduction of a single reinforcing phase, the relative dissipation indices of the material increased to 25.7 ± 1.8 J/cm² (GNs) and 28.3 ± 1.6 J/cm² (SiCw), respectively. When constructing multiscale hybrid reinforcement systems, the composites showed highly significant synergistic toughening effects. The relative dynamic dissipation index of the PDC+0.5GNs+1.0SiCw group reached a maximum of 36.1 ± 2.0 J/cm² (F(4,20) = 42.1, p < 0.001), which was approximately 67.1% higher than that of pure PDC. At the same time, its dynamic fracture toughness increased to 7.1 ± 0.4 MPa·m^1/2. The hybrid network of GNs and SiCw effectively activated highly efficient instantaneous energy dissipation mechanisms such as fiber pull-out, interlayer slip and fracture of graphene during the very short response time of dynamic impact.

3.

Impact resistance properties from Charpy impact tests: (a) impact toughness and dynamic fracture toughness, (b) impact energy and subsurface damage depth.

Figure b shows the test results of the absolute value of dynamic shock absorption energy versus the depth of subsurface damage at the fracture. Under the impact load, not only did the high brittleness material suffer from backbone fracture, but the intense shock wave also induced deep microcrack networks (i.e., subsurface damage zones) in the matrix beneath the fracture surface. The apparent dissipation work of pure PDC was only 1.95 ± 0.15 J. Due to the lack of crack resistance mechanism, the main crack was highly unstable, and the residual impact kinetic energy caused subsurface damage to a depth of 185 ± 12 μm. This indicated that it was highly prone to severe deep structural degradation after being struck. With the addition of the hybrid enhancement phase, the total dissipated energy rose steadily, and the optimal mix group PDC+0.5GNs+1.0SiCw reached 3.25 ± 0.18 J. More importantly, due to the effective scattering of stress waves by the two-dimensional sheets of GNs and the strong pinning of microcracks by SiCw, the subsurface damage depth of the hybrid group was significantly reduced to 82 ± 8 μm (F(4,20) = 88.3, p < 0.001), a 55.7% reduction compared to pure PDC. This significant improvement in depth damage tolerance directly confirmed that the hybrid reinforced network could confine impact damage to extremely small surface areas, thereby greatly protecting the integrity of the underlying structure of the superhard composites.

3.4. Analysis of Thermal Conductivity and Thermal Expansion Properties

In extreme service conditions, such as deep rock drilling, the thermal diffusion and heat transfer capabilities of superhard materials are key to avoiding frictional heat accumulation and local thermal damage. The thermophysical responses of the composites at room temperature were measured using laser flash and a thermomechanical analyzer, and the consistency of the data was verified strictly based on physical equation k = α·ρ·C _p (where k is thermal conductivity, α is thermal diffusivity, ρ is apparent density, C _p is specific heat capacity), and the results are presented as a synthesis in Figure .

4.

Thermophysical properties of PDC composites with different reinforcement phases at room temperature: (a) thermal conductivity and thermal diffusivity, (b) thermal expansion coefficient and specific heat capacity.

Figure a illustrates the evolution of the system’s thermal conduction behavior. Pure PDC exhibits a thermal conductivity of 544 ± 8 W/(m·K) and a thermal diffusivity of 290 ± 5 mm²/s. The introduction of single reinforcing phases yielded distinct effects: adding 0.5 wt % GNs increased the thermal conductivity to 566 ± 10 W/(m·K) owing to the high in-plane intrinsic thermal conductivity of graphene. Conversely, the addition of 1.0 wt % SiCw introduced additional interfacial boundaries that act as phonon scattering cross sections, leading to a thermal conductivity drop to 529 ± 7 W/(m·K). In the multiscale hybrid system, GNs and SiCw demonstrate a synergistic thermal behavior. The 2D GNs construct a long-range thermal percolation network, acting as a “phonon highway” that bridges heat flow across the interfaces. , More importantly, as further revealed by structural analyses, the multidimensional interfacial coupling effectively mitigates the interfacial thermal resistance (Kapitza resistance) typically induced by isolated heterogeneous phases. , This synergistic thermal bridging compensates for the phonon scattering caused by SiCw. Consequently, the thermal diffusivity of the optimal hybrid composite (PDC+0.5GNs+1.0SiCw) was measured at 274 ± 6 mm²/s, corresponding to a rebounded thermal conductivity of 550 ± 9 W/(m·K). These results confirm that the multiscale hybrid network enhances the mechanical toughness of the composite while preserving its essential high heat dissipation capability.

Figure b further analysis of specific heat capacity and CTE. With the addition of the hybrid components, the specific heat capacity steadily increased from 0.52 ± 0.01 J/g·K of pure PDC to 0.55 ± 0.02 J/g·K, indicating that the hybrid composites could store more heat per unit mass, which is extremely beneficial for buffering transient thermal shock during drilling cutting. In addition, the CTE of pure PDC was 2.8 ± 0.1 × 10^–6 /K. The hybrid group PDC+0.5GNs+1.0SiCw had the lowest CTE, which decreased to 2.5 ± 0.1 × 10^–6 /K (F(4,20) = 15.2, p = 0.004). The hybrid phase with low thermal expansion properties formed a strong geometric constraint at the diamond grain boundaries, effectively limiting the volume expansion of the matrix under complex thermal fields. This excellent combination of high thermal conductivity and low CTE gives hybrid composites superior thermal shock resistance and high-temperature dimensional stability

3.5. Analysis of Microstructure and Grain Characteristics of Composites

To reveal in depth the influence of hybrid reinforcement relative to the material’s underlying architecture, the study used high-resolution SEM to characterize and analyze the micromorphologies of the samples of pure PDC, PDC+ 0.5 wt % GNs, PDC+ 1.0 wt % SiCw, PDC+ 0.5 wt % GNs+ 0.5 wt % SiCw, and PDC+ 0.5 wt % GNs+ 1.0 wt % SiCw, and the typical results are shown in Figures a–e. Microscopic morphology observations indicated that with the introduction of the hybrid enhancement phase, significant reconstruction and optimization of the internal structure of the material occurred. As shown in Figure a, the grain size distribution within the pure PDC sample is uneven, with significant Abnormal grain growth in some diamond grains. Meanwhile, distinct deep black microholes (mainly due to local undercompaction or grain pull-out) and some residual liquid cobalt pools with high brightness contrast can be clearly observed at the grain boundaries. After the introduction of a single reinforcing phase, in Figure b, very thin and conformed (Conformally attached) two-dimensional GNs sheets are seen covering or bridging the surface of the diamond grains and at the grain boundaries. The ultrathin two-dimensional nature of these sheets makes the underlying diamond morphology still clearly visible through electron beam penetration. Figure c clearly shows the high-strength SiCw interwoven in the matrix, accompanied by typical pull-out holes. To substantiate this observation quantitatively, statistical image analysis (n = 50) was performed on the whiskers before and after processing. The pristine SiCw exhibited an initial average length of 19.9 ± 4.1 μm, a diameter of 0.36 ± 0.04 μm, and an aspect ratio of 55.3 ± 11.1. After the high-energy ball milling and 5.5 GPa HPHT sintering, the embedded SiCw were shortened to an average length of 6.0 ± 1.6 μm with a diameter of 0.34 ± 0.05 μm, yielding a retained aspect ratio of 17.6 ± 4.7. Detailed statistical distributions are provided in Figure S1 of the Supporting Information. Based on the shear-lag model for composite toughening, an aspect ratio exceeding 10 is typically sufficient to ensure effective load transfer and prevent complete particle cleavage. Thus, this retained aspect ratio quantitatively confirms that the shortened SiCw successfully preserve their capacity to trigger critical toughening mechanisms such as whisker pull-out. In the multiscale hybrid reinforcement group (Figure d,e), one-dimensional SiCw and two-dimensional GNs are observed to interweave at the diamond grain boundaries, suggesting the formation of a spatially interlocking structural framework. This heterogeneous network effectively promotes the uniform penetration of liquid-phase cobalt, avoids agglomeration at the microscale, makes the interface bonding tight, and significantly reduces structural defects such as pores and microcracks.

5.

Characterization of GNs and SiCw hybrid reinforced PDC composites: (a) pure PDC, (b) PDC+ 0.5 wt % GNs, (c) PDC+ 1.0 wt % SiCw, (d) PDC+ 0.5 wt % GNs+ 0.5 wt % SiCw, and (e) PDC+ 0.5 wt % GNs+ 1.0 wt % SiCw.

To experimentally elucidate the suppression of deleterious interfacial reactions between reactive liquid-phase cobalt and SiCw under HPHT conditions, high-resolution transmission electron microscopy (HRTEM) coupled with EDS was employed (Figure ). As shown in the HAADF-STEM image (Figure a), a distinct phase boundary separates the SiCw and the Co/diamond matrix. Crucially, the HRTEM image (Figure b) reveals a highly preserved core–shell-like architecture, where approximately 4–5 layers of GNs (interlayer spacing d ≈ 0.34 nm) conformally wrap the crystalline SiCw (spacing of d ≈ 0.25 nm for the (111) plane). Furthermore, the EDS elemental mapping (Figure c) and the corresponding line scan profile (Figure d) demonstrate a sharp spatial decoupling between Co and Si. The Co signal drops sharply at the carbon-rich interface without exhibiting a broad mutual diffusion zone into the SiCw. This atomic-scale visual and compositional evidence directly corroborates that the flexible 2D GNs conformally wrap the SiCw, acting as a physical steric barrier. Significantly, these 1D-2D core–shell-like interfaces serve as the fundamental connective ’nodes’ that physically substantiate the formation of the spatially interlocking network distributed throughout the composite. This HRTEM/EDS methodology provides a direct and localized validation of the interfacial configuration. Drawing parallels to recent findings in graphene-wrapped SiCw/Ni systems, this structural configuration efficiently mitigates the direct permeation of liquid Co, thereby restricting the formation of brittle cobalt silicides to below macroscopically detectable levels.

6.

Interfacial microstructural characterization of the optimal PDC+0.5GNs+1.0SiCw composite. (a) HAADF-STEM image showing the overall morphology of the Co/Diamond matrix, GNs layer, and SiC whisker. (b) HRTEM image of the interface, highlighting the crystalline lattice fringes of SiC (111) and the conformal GNs layer. (c) Corresponding EDS elemental mapping of Co, Si, and C. (d) EDS line scan profile across the interface demonstrating the spatial decoupling of Co and Si elements.

Microstructural parameters were statistically analyzed using ImageJ software based on the linear intercept method, and the quantitative results are summarized in Table . The data indicated that the diamond grain size showed a significant suppressed trend after the introduction of the reinforcing phase. The average grain size of pure PDC was 1.45 ± 0.12 μm. The single enhancer phase can delay grain growth to some extent, while the inhibitory effect of the hybrid enhancer group is most prominent. The average grain size of the PDC+0.5 wt % GNs+1.0 wt % SiCw group was controlled at 0.86 ± 0.06 μm (F(4,20) = 31.7, p < 0.001), which was 40.7% smaller than that of pure PDC. This significant grain size anchoring effect was attributed to the synergistic ″Zener pinning effect″ of GNs and SiCw during HPHT sintering. They were abundantly distributed at the triple junctions of the grain boundaries, greatly increasing the resistance to grain boundary migration and effectively suppressing the recrystallization and secondary anomalous growth of diamond grains.

2. Microstructural Parameters of PDC Composites Determined by Quantitative Image Analysis.

group	average grain size (μm)	average pore size (μm)	relative density (%)
Pure PDC	1.45 ± 0.12	0.62 ± 0.08	98.2 ± 0.2
PDC+0.5 wt % GNs	1.28 ± 0.15	0.48 ± 0.06	98.6 ± 0.2
PDC+1.0 wt % SiCw	1.15 ± 0.10	0.35 ± 0.05	98.9 ± 0.1
PDC+0.5 wt % GNs + 0.5 wt % SiCw	1.02 ± 0.08	0.28 ± 0.04	99.3 ± 0.1
PDC+0.5 wt % GNs + 1.0 wt % SiCw	0.86 ± 0.06	0.21 ± 0.03	99.6 ± 0.1

^aNote: indicates a statistically significant difference compared with the Pure PDC group based on Tukey’s HSD posthoc test (p < 0.05). Exact F and p values are detailed in the text and Supporting Information.

In addition, the evolution of the micropore (and residual cobalt pool) size further confirmed the densification of the hybrid phase. The average pore size of pure PDC was 0.62 ± 0.08 μm, while the pore size of the optimal hybrid group PDC+0.5 wt % GNs+1.0 wt % SiCw decreased sharply to 0.21 ± 0.03 μm (p < 0.05). Correspondingly, the relative density of the samples steadily increased from 98.2 ± 0.2% of pure PDC to 99.6 ± 0.1%. This indicated that the multiscale hybrid reinforcing phase, by its complementary geometric dimensions, not only successfully filled the submicron gaps between the diamond skeletons, but also acted as a solid lubricant for particle rearrangement in the early stage of HPHT heating, promoting efficient migration of substances and interfacial fusion. This microstructure evolution based on suppressing grain coarsening and high densification laid a solid physical foundation for the subsequent leap in macroscopic strength and toughness of the composites.

3.6. Composite Phase Composition and Rietveld Quantitative Analysis

To precisely clarify the influence of hybrid reinforcement on the phase evolution of composites and to verify the structural stability of the reinforcement under extreme conditions of HPHT, systematic qualitative and Rietveld full-spectrum fitting fine-tuning quantitative analysis of XRD diffraction patterns of each group of samples was conducted, and the results are shown in Figures and .

7.

XRD patterns of PDC composites with different reinforcements, including a magnified view in the 22°-40° range.

8.

Rietveld quantitative phase analysis results of PDC composites.

Figure shows the overall XRD patterns and local high-resolution magnifications of each group of samples. All samples showed extremely sharp and intense diffraction peaks at 2θ = 43.9° and 75.3°, corresponding respectively to the face-centered cubic structure (111) and (220) crystal planes (PDF#06–0675) of diamond. Meanwhile, a faint (200) characteristic peak of face-centered cubic cobalt was detected at 51.5°, and the strongest (111) peak of cobalt (44.2°) overlapped implicitly with the main peak of diamond (111), resulting in an asymmetric broadening feature on the right side of the main peak of diamond. The above phenomenon indicates that the underlying phase architecture of PDC composites is highly pure.

Due to the extremely low addition of nanoreinforcing phases (GNs and SiCw) (≤1.0 wt %) and the extremely high crystallinity of the diamond matrix, the diffraction signal of the reinforcing phases is strongly suppressed by the matrix background in the global spectrum. However, the characteristic evolution of the hybrid phase can be clearly captured through local magnification observations in the range of 22 to 40° (Figure Inset). In the group with GNs added, a (002) broadened diffraction envelope peak (PDF#41–1487) representing the graphene layered structure emerged at 2θ ≈ 26.5°; In the group doped with SiCw, a sharp (111) diffraction peak (PDF#29–1129) of β-SiC was clearly detected at 35.6°. Within the detection limit of the XRD instrument, macroscopic formation of brittle Co–Si intermetallic compounds (e.g., CoSi, CoSi₂) or bulk amorphous carbon was not identified, which is consistent with the microstructural barrier effect observed in the HRTEM analysis.

The exact mass fraction of the phase was further combined with the Rietveld full spectrum estimate (Figure ). The quantitative results indicated that in the PDC+0.5GNs+1.0SiCw ultimate hybrid group, the detected β-SiC content was 1.0 wt %, the graphite-like phase rose precisely to 0.9 wt %, and the main diamond phase was adjusted accordingly to 91.4 wt %. It is notable that the residual face-centered cubic cobalt phase regularly decreased from 7.2 WWT % of pure PDC to 6.7 WWT %. This slight difference in cobalt loss is attributed to the nanoscale pore confinement effect caused by the multiscale enhanced network, which alters the capillary extrusion leakage dynamics of liquid-phase cobalt under extreme high pressure. The precise quantification of the nanocarbon phase/silicon carbide, which is highly consistent with the initial ratio, not only validates the high precision of the synthesis process based on the six-sided top press in controlling the stoichiometric ratio of the material, but also strongly suggests (strongly suggests) from the crystallographic statistical level: In the range of doping studied, the vast majority of GNs and SiCw did not undergo a complete consumable phase transition but coexisted in situ as independent second phases in the diamond matrix.

The quantitative results indicated that the pure PDC system was highly pure, with diamond (92.4 wt %) as the main phase, accompanied by residual face-centered cubic cobalt phase (7.2 wt %) infiltrated as sintering aids and catalysts, and a very small amount of background graphite phase (0.4 wt %) remaining due to thermodynamic fluctuations. The phase composition of the material changed in a regular pattern with the introduction of reinforcing phases of different scales, and no significant secondary intermetallic phases were detected within the instrument’s resolution. In the group with 0.5 wt % GNs added, the content of the “Graphitic phase” in the system rose from 0.4% to 0.9% as the characteristic (002) diffraction peak of graphene overlaps with the graphite phase, according to semiquantitative estimates. Constrained by the physical limitations of XRD in quantifying low amounts of light elements (carbon phase), this value is mainly used to reflect the macroscopic trend of phase evolution, but its regular growth still highly suggests: The added two-dimensional GNs did not undergo a complete phase transition to diamond in the harsh HPHT environment of 5.5 GPa and 1450 °C, and still stably retained the core sp² hybrid crystal characteristics of their multilayer architecture, thus serving as an effective intrinsic two-dimensional toughening phase in the composite matrix.

Meanwhile, in the groups doped with SiCw (groups 3, 4, and 5), the silicon carbide (β-SiC) phase was accurately identified and detected. In particular, in the PDC+0.5GNs+1.0SiCw ultimate hybrid group, the detected SiC content was 1.0 wt %, the graphite-like phase was 0.9 wt %, the residual cobalt phase was stable at 7.0 wt %, and the main diamond phase was proportionally adjusted to 91.4 wt %. The quantitative results of the nanocarbon phase/silicon carbide showed excellent agreement with the initial designed ratio. Meanwhile, the extrusion leakage of liquid-phase cobalt under extreme high pressure remained within a reasonable range, with a weight loss of approximately 1.0 wt %. These observations not only confirm the high accuracy of the six-sided top press synthesis process in precisely controlling the stoichiometric ratio of the material. More importantly, from a crystallographic statistical perspective, the findings strongly suggest that within the studied doping range, the majority of GNs and SiCw did not undergo a complete consumable phase transition but instead coexisted in situ as independent second phases within the diamond matrix.

3.7. Raman Spectroscopy and Carbon Bond State Analysis

To further confirm the survival state and structural evolution of graphene and silicon carbide at the molecular bonding level (sp² and sp³ hybrid states) under HPHT conditions, microscopic confocal Raman Spectroscopy characterization was performed. As shown in the global spectrum in Figure , the Raman baselines of all samples presented characteristic nonlinear fluorescence backplanes due to the complex intrinsic residual stress and impurity levels within the HPHT synthetic diamond.

9.

Raman spectra of PDC composites with different reinforcements, including a deconvolution inset for the hybrid group in the 1200–1700 cm^–1 range.

From the responses of the characteristic peaks, the pure PDC group showed a sharp and intense Raman main peak at 1332 cm^–1, which is a hallmark of the typical diamond sp³ carbon–carbon single bond lattice stretching vibration, confirming the high crystalline quality of the matrix. After the introduction of SiCw (PDC+1.0 wt % SiCw group), a clear β-SiC Transverse Optical (TO) phonon vibration peak was detected at 796 cm^–1.

A significant structural evolution occurred in the GNS-doped group. In addition to the sp³ main peak of diamond, characteristic signals belonging to the graphene sp² carbon atom emerged clearly: the G peak at ∼1582 cm^–1 (in-plane carbon ring stretching vibration) and the 2D peak at ∼2690 cm^–1. Coupled with the HRTEM observations (Figure ), this characterization confirms that the two-dimensional carbon materials evade severe structural degradation or dissolution by liquid-phase cobalt at 1450 °C and 5.5 GPa. It further substantiates that the core sheet structure of GNs is stably retained at the local microarea interface of the composite system.

The overlapping Raman features of the hybrid composite (PDC+0.5GNs+1.0SiCw) between 1250 and 1650 cm^–1 were resolved using Voigt deconvolution (Figure , inset). The characteristic graphene D peak (∼1350 cm^–1) and G peak are distinct from the broadened 1332 cm^–1 diamond peak. It is noteworthy that the significant residual compressive stress generated during HPHT sintering leads to asymmetric broadening, and potentially Fano resonance, of the diamond peak. Consequently, the extracted D-band intensity may be coupled with substrate stress effects, leading to a higher apparent I _D /I _G ratio. Nevertheless, the copresence of G and 2D peaks confirms the structural retention of GNs at the matrix interface. The localized structural distortion and high-density edge defects resulted from the severe mechanical compression by diamond particles during the HPHT sintering process.

While these defects inevitably disrupt the sp ² conjugated networkslightly compromising the intrinsic in-plane thermal conductivity of individual GNsthey play a critical, positive role in the overall thermal and mechanical properties of the composite. In highly dense heterostructures, interfacial thermal resistance (Kapitza resistance) typically dictates heat dissipation. Recent nonequilibrium molecular dynamics (NEMD) and phonon transport studies demonstrate that an appropriate density of interfacial defects reduces the elastic modulus mismatch between 2D materials and the rigid matrix. These structural modifications alleviate mismatch-induced stress¹⁵ and induce a shift in the cross-plane phonon density of states (PDOS) toward lower frequencies, which significantly promotes low-frequency phonon coupling across the interfaces. Furthermore, localized defect scattering redirects oblique-propagating phonons, restoring directional equilibrium to enhance interfacial thermal conductance.

At the molecular level, the HPHT sintering conditions (5.5 GPa, 1450 °C) induce intense compressive stress, driving a localized sp2 to sp3 rehybridization at the defect-rich edges of the GNs. This rehybridization facilitates the formation of robust covalent bonds among the GNs, SiCw, and the diamond matrix. Such molecular-level covalent cross-linking plays a dual role: first, it creates a localized physical and thermodynamic barrier that restricts the complete dissolution and erosion of the reinforcements by liquid-phase cobalt during sintering; second, it establishes an ideal interface for efficient mechanical load transfer and phonon coupling. Therefore, this microscopic interfacial evolution effectively minimizes Kapitza resistance and enhances bonding strength, providing a solid physical foundation for the simultaneous retention of high macroscopic thermal conductivity and enhanced mechanical toughness.

3.8. Analysis of Sample Fracture Behavior and Fracture Energy Evolution

To eliminate the statistical interference of two-dimensional field-of-view dependence on microscopic measurement results, this study used ImageJ software to systematically process images of the Crack propagation profile induced by the indentation test and the three-dimensional Fracture surface. Crack tortuosity τ and the Fracture dimension D _f of the crack path were introduced to quantitatively evaluate the degree of crack deflection, and the roughness (R _a ) of the fracture surface was calculated, as shown in Table . In addition, based on the theory of linear elastic fracture mechanics under plane strain conditions (G _IC = K _IC (1 – ν²)/E), the critical strain energy release rates of each group of samples were calculated to reflect the true energy threshold required for crack propagation. (Note: Although the Poisson’s ratio of single crystal diamond is extremely small, considering that PDC is a cobalt-containing metal-ceramic composite system, this study follows the relevant literature and uses the macroscopic equivalent Poisson’s ratio ν ≈ 0.20 introduction formula G _IC = K _IC (1 – ν²)/E of polycrystalline diamond for precise calculation to ensure the rigor of the mechanical theory derivation).

3. Quantitative Evaluation of Crack Propagation Behaviors and Fracture Surface Characteristics.

Group	Crack tortuosity (τ)	Fractal dimension (D f )	Critical strain energy release rate (G IC ) J/m2	Fracture surface roughness (R a ) (μm)
Pure PDC	1.06 ± 0.03	1.08 ± 0.02	33.1 ± 2.4	0.8 ± 0.1
PDC+0.5 wt % GNs	1.15 ± 0.04	1.14 ± 0.02	43.2 ± 3.2	1.1 ± 0.1
PDC+1.0 wt % SiCw	1.18 ± 0.05	1.16 ± 0.03	47.1 ± 3.4	1.2 ± 0.1
PDC GNs + 0.5 + 0.5 SiCw	1.27 ± 0.06	1.22 ± 0.03	58.2 ± 4.0	1.4 ± 0.2
PDC GNs + 0.5 + 1.0 SiCw	1.38 ± 0.07	1.26 ± 0.04	65.9 ± 4.5	1.6 ± 0.2

^aNote: indicates a statistically significant difference compared with the Pure PDC group based on Tukey’s HSD posthoc test (p < 0.05). Exact F and p values are detailed in the text and Supporting Information. The critical strain energy release rate (G _IC ) was calculated based on the plane strain relation G _IC = K _IC (1 – ν²)/E (where an equivalent macroscopic Poisson’s ratio. ν ≈ 0.20. for PDC composites was applied). Crack parameters were quantitatively extracted using the ImageJ software.

The analysis shows that the crack tortuosity τ of pure PDC is only 1.06 ± 0.03 and the fractal dimension D _f is as low as 1.08 ± 0.02, indicating that the crack propagation path presents typical straight cleavage characteristics, which is highly consistent with its intrinsic high brittleness. Its corresponding G _IC is only 33.1 ± 2.4 J/m². When a single reinforcing phase is introduced, the stress field at the crack tip is disturbed and forced to deflect and fork, resulting in τ and D _f steadily rising.

In multiscale hybrid reinforcement systems, the complexity of the fracture surface increases sharply. While two-dimensional (2D) cross-sectional images cannot fully capture the entire 3D spatial network, principles of quantitative fractography and stereology establish that the 2D profile fractal dimension (D _f ) is fundamentally correlated with the complexity of the 3D fracture surface. Consequently, the statistical evaluation of 2D profiles effectively mirrors the 3D spatial crack evolution without the need for resolution-limited 3D tomography. The PDC+0.5GNs+1.0SiCw hybrid group demonstrated substantially improved crack resistance, with the 2D profile crack tortuosity (τ) increasing to 1.38 ± 0.07 and the fractal dimension (D _f ) reaching 1.26 ± 0.04 (F(4,20) = 22.6, p = 0.001). This statistically significant increase in 2D fractal parameters serves as a quantitative indicator of a convoluted spatial crack evolution. This highly fractal and twisted crack morphology is driven by the spatially synergistic coupling effect of 1D SiCw and 2D GNs. High-strength SiCw forces large-angle crack deflection through interface dissociation and fiber pull-out, while GNs induce dense microcrack branching at grain boundaries. This continuous energy dissipation process, driven by whisker debonding and pull-out against interfacial friction, is a well-established toughening mechanism in multiphase networks. , Under the superposition of these mechanisms, the critical strain energy release rate (G _IC ) of the composites increased to 65.9 ± 4.5 J/m². Correspondingly, the roughness (R _a ) of the microfracture surface increased from 0.8 ± 0.1 μm in pure PDC to 1.6 ± 0.2 μm. The more tortuous path and rougher fracture surface directly correlate with the increased fracture dissipation energy, demonstrating the synergistic strengthening and toughening mechanism of the composite matrix.

3.9. Comprehensive Performance Evolution and Synergistic Effect Analysis

To fully assess the impact of the hybrid introduction of GNs and SiCw on the overall service potential of composites, this study abandoned the single-dimensional performance determination and extracted six core micromacro service indicators: density, Vickers hardness, flexural strength, fracture toughness, relative dynamic dissipation index, and thermal conductivity. By normalizing them to 100%, multidimensional Radar charts of the comprehensive performance of each group of samples were constructed, and the results are shown in Figure .

10.

Comprehensive performance evaluation radar chart of PDC composites with different reinforcements (normalized based on peak values).

The evolution of the composite’s comprehensive performance can be quantitatively observed from the performance envelope in Figure . Pure PDC exhibits a limited envelope area, primarily restricted by its intrinsic low toughness and dynamic dissipation capacity. Conversely, the PDC+0.5GNs+1.0SiCw hybrid group demonstrates an optimal multidimensional expansion across all six core indicators. At the macroscopic level, this comprehensive improvement is driven by a “spatial interlocking network”. The rigid 1D SiCw acts as structural “rebars” that carry primary loads and facilitate crack bridging and pull-out, while the flexible 2D GNs serve as “buffers” that wrap around grains, inducing large-area crack deflection and localized shielding. The expansion of the radar chart reflects this multiscale mechanism: the molecular-level covalent interfaces guarantee efficient interphase load transfer, while the 1D+2D spatial interlocking network maximizes fracture energy dissipation and constructs a continuous thermal percolation pathway.

It is worth noting that the area of the envelope polygon of the PDC+0.5GNs+1.0SiCw hybrid group (red solid line) has reached its maximum value, almost fully approaching the 100% contour line of the periphery. This optimal ratio eliminates the toughness shortcoming of pure PDC while effectively compensating for the thermal resistance effect of a single SiCw and the densification bottleneck of a single GNs. The simultaneous outward expansion of the six core dimension indicators is not a simple linear superposition, but achieves a high degree of thermal - force - structure through the inhibition of abnormal grain growth, multiscale interfacial thermal flow bridging, and three-dimensional tortuous deflection of microcracks as described earlier. The quantitative comparison of this radar map systematically reveals the synergistic toughening effect produced by the GNs and SiCw cross-scale hybrid network in the PDC matrix at the macroscopic physical characterization level.

4. Conclusions

This study systematically investigated the multidimensional strengthening, toughening, and thermal conduction mechanisms of GNs and SiC_w hybrid-reinforced PDC composites fabricated via HPHT synthesis. The main conclusions are summarized as follows:

• (1)Microstructural Optimization: The multiscale hybrid reinforcements effectively suppressed abnormal grain growth and promoted densification. For the optimal composition (PDC+0.5 wt % GNs+1.0 wt % SiC_w), the average pore size was reduced to 0.21 μm, the porosity decreased to 0.4%, and the relative density reached 99.6%. The 2D GNs network acted as an effective interfacial physical barrier that mitigated the liquid-cobalt ablation of SiCw. This structural preservation restricted the formation of brittle silicides to below macroscopically detectable levels, thereby maintaining the effective aspect ratio of the whiskers and ensuring their mechanical toughening capability.
• (2)Mechanical and Impact Resistance Enhancements: The synergistic network significantly improved the overall mechanical properties. The optimal hybrid group exhibited a Vickers hardness of 56.4 ± 1.1 GPa, a flexural strength of 682 ± 22 MPa, and a fracture toughness of 7.8 ± 0.3 MPa·m^1/2. Under dynamic impact, the dissipated energy increased, and the subsurface damage depth was constrained to 82 ± 8 μm, demonstrating improved damage tolerance.
• (3)The synergistic toughening mechanism was quantitatively characterized from the fracture mechanics and fractal geometry dimensions. The interlayer slip of GNs and the pull-out effect of SiCw induce highly convoluted spatial crack propagation, as evidenced stereologically by the significant increases in the 2D crack tortuosity (to 1.38) and fractal dimension (to 1.26).
• (4)Defect-Mediated Thermal Stability: Raman and thermal analyses indicated that the local sp²-to-sp³ rehybridization and edge defects formed on GNs during HPHT sintering acted as active bonding sites. This interfacial reconstruction helped reduce the elastic modulus mismatch between the reinforcements and the matrix, preserving a stable thermal conductivity (550 W/(m·K)) while enhancing the structural integrity.

Glossary

Abbreviations

CTE

coefficient of thermal expansion

GNs

graphene nanosheets

HPHT

high-temperature and high-pressure

LEFM

elastic fracture mechanics

PDC

polycrystalline diamond compact

SEM

scanning electron microscope

SiCw

silicon carbide whisker

TO

transverse optical

XRD

X-ray diffraction

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c08843.

• One-way ANOVA results for flexural strength, fracture toughness, and Vickers hardness (Tables S1–S3), and quantitative statistical distributions of SiCw aspect ratios (Figure S1) (PDF)

This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. L.M.: Conceptualization, methodology, investigation, data curation, and writingoriginal draft. Y.X.: Investigation, formal analysis, validation, writingreview and editing. C.G.: Conceptualization, resources, supervision, project administration, and writingreview and editing.

The authors declare no competing financial interest.