The fatigue behavior of
cemented carbides mainly focuses on the determination of fatigue life and fatigue limit, as well as the evaluation of fatigue crack growth behavior based on fracture mechanics. This study conducts an in-depth analysis of short surface cracks in fine-grained WC-Co cemented carbides, explores their interaction with the microstructure, and evaluates crack growth characteristics under different stress ratios. The aim is to improve the service life of cemented carbides and reduce failures, highlighting the importance and necessity of this research in the performance optimization and application of cemented carbides.
Crack growth paths are classified into four types:
（1）Within WC grains
(2) Within the Co phase
(3) Along WC/WC grain boundaries
(4) Along WC/Co phase boundaries
The proportion of crack growth along WC/WC grain boundaries decreases from 50% to 30%, while the proportion of transgranular cracks within WC grains increases from 20% to 40%.
The short surface crack growth mechanism of WC-Co cemented carbides is a complex process influenced by multiple factors, involving the interaction of microstructure, stress state, and environmental conditions. Based on existing research, the mechanism can be summarized in the following aspects:
Competitive Mechanism between Intergranular and Transgranular Fracture
Intergranular fracture: In as-sintered WC-Co alloys, cracks tend to propagate along WC grain boundaries or WC/Co phase interfaces, showing brittle fracture characteristics. This mechanism is related to the low bonding strength of grain boundaries, and is more significant in alloys without heat treatment.Transgranular fracture: The introduction of plate-like WC grains or heat treatment can promote transgranular fracture. The anisotropy of plate-like WC grains induces cracks to pass through the interior of WC grains. At the same time, the Co phase bridging effect (plastic deformation of the Co phase behind the crack tip) can delay crack growth and significantly improve the flexural strength.
Plastic Bridging Effect of the Binder Phase (Co Phase)
The Co phase absorbs energy through plastic deformation during crack growth and forms a bridging region, thereby inhibiting crack propagation. Alloys with high Co content exhibit better fatigue life in the high-cycle fatigue region, which is partly attributed to this mechanism. Heat treatment can optimize the distribution and performance of the Co phase, improve its toughness, and further enhance the bridging effect, thus improving the fracture toughness.
Crack Deflection and Branching
The inhomogeneity of the microstructure (such as plate-like WC grains or ultrafine-grained structures) forces the crack path to deflect or branch, extending the growth path and consuming more energy. For example, in alloys containing plate-like WC grains, the contribution of crack deflection to strength improvement can reach a significant level. In ultrafine-grained WC-Co alloys (grain spacing of 169–179 nm), the fine-grained structure promotes crack branching by increasing the grain boundary density. At the same time, VC additives inhibit grain growth and further optimize performance.
Crack Growth Behavior under Fatigue Loading
Under high-cycle fatigue conditions, the crack growth rate is significantly affected by the stress amplitude. The high-stress region is dominated by strength, while the low-stress region relies more on the toughness mechanism of the microstructure (such as Co content). Fatigue crack growth is often accompanied by the fracture of WC particles and the extrusion of the binder phase, resulting in material loss in the wear mechanism.
Influence of Sintering Process on Microstructure
Microwave sintering: Refines the microstructure through volumetric heating and non-thermal effects, which may change the crack growth path.
Pressure-assisted sintering: Methods such as vacuum hot pressing can densify ultrafine-grained alloys, reduce defects, and inhibit crack initiation.
Oscillatory hot pressing sintering: Achieves densification at the solid-phase sintering temperature, retains the ultrafine-grained structure, and at the same time, the uniform distribution of the Co phase improves the fracture toughness.
There is a significant correlation between short surface cracks in cemented carbides (especially WC-Co series) and their fatigue life and impact life. The mechanism involves the complex interaction of crack initiation, growth resistance, and microstructure response. Combined with academic search results, the main correlation mechanisms can be summarized in the following four aspects:
Processing processes such as grinding and cutting form residual tensile stress (up to several hundred MPa) on the surface of cemented carbides, which becomes the core initiation source of short cracks. X-ray diffraction measurements show that the surface layer of WC-6wt.%Co alloy after grinding has significant Type I residual tensile stress, and the depth distribution of the stress layer is directly related to the processing parameters. This stress field significantly reduces the crack initiation energy barrier and accelerates early failure.
Defects such as surface micro-voids and WC/Co phase interface debonding (with a size of approximately 1–10 μm) tend to evolve into short cracks under cyclic loading. Impact fatigue experiments show that short cracks often initiate in WC grain aggregation areas or regions with uneven Co phase distribution, and the proportion of initiation life in the total life increases with the increase of impact energy (more significant at high temperatures).
Short cracks (length < 100 μm) are not limited by the traditional fracture mechanics threshold ΔKth in the early stage of growth, showing an accelerated growth phenomenon. Wang Zhen’s research points out that under low stress amplitude, the growth of short cracks in WC-Co alloys is delayed due to the plastic bridging of the Co phase; however, under high stress, the cracks quickly pass through the Co phase, leading to a sharp decrease in life.
WC Grain Size Effect
Coarse-grained WC (average grain size > 3 μm) extends the growth path by enhancing crack deflection and branching. Li Chenhui confirmed that when the Co content is fixed, the fracture toughness KIc increases with the increase of WC grain size, the short crack growth resistance increases, and the life is prolonged.
Gradient Structure Regulation
Huang Ziqian found that the Co-rich region on the surface of gradient cemented carbides has residual compressive stress, which can inhibit the growth of short cracks; while the core region with high WC content consumes energy through transgranular fracture. This dual effect improves the overall life.
Wang Zhen prepared a micro-pit array on the surface of WC-Co, and confirmed that it can store lubricants and reduce local stress concentration. Under MQL (Minimum Quantity Lubrication) conditions, the micro-pits reduce the wear rate of the WC/Ti6Al4V friction pair by more than 30% and delay the initiation of short cracks.
Wang Yong used boriding treatment on the smooth WC-Co substrate to form a boride layer before CVD (Chemical Vapor Deposition) diamond coating. This layer effectively inhibits the migration of Co to the surface, reduces interface microcracks caused by Co-induced graphitization, and increases the coating adhesion by 40%.
Aqueous-based ultrafine-grained WC-Co alloys (grain spacing of 169–179 nm) force short cracks to branch frequently through high-density grain boundaries, making the growth path tortuous. Huang Lin pointed out that this process can also reduce surface processing defects and lower the probability of short crack initiation.
Research trends indicate that short crack growth resistance (rather than the number of initiations) is the core indicator for life prediction. In the future, it is necessary to combine in-situ characterization technologies (such as high-cycle fatigue-micro-CT combination) to quantify the dynamic growth path of short cracks, so as to achieve the refinement of life models.
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