HVOF
Tungsten Carbide Cobalt
88Wc12CO - HRA - 86 HV - 1200
High Velocity Oxy-Fuel (HVOF) coating is a thermal spray process widely used to enhance surface properties such as wear and corrosion resistance. In this study, a tungsten carbide–cobalt (WC–Co) coating composed of 88% WC and 12% Co was applied using HVOF technology. Tungsten carbide provides exceptional hardness and abrasion resistance, while cobalt acts as a binder, improving toughness and adhesion. The HVOF process produces dense, well-bonded coatings with minimal porosity, ensuring superior performance in demanding environments. This combination of WC and Co delivers an optimal balance of hardness and durability, making it ideal for components exposed to severe mechanical and thermal stresses.
STEP 1: CUTTING THE WORKPEACE
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Cutting tungsten carbide–cobalt (WC–Co) coated samples requires special care due to their extreme hardness (≈70–75 HRC) and brittleness. Conventional abrasive wheels for steel are not suitable because they wear quickly and can cause edge chipping. Instead, use a diamond or cubic boron nitride (CBN) cutting wheel designed for hard materials.
RECOMMENDED METHOD
Machine: Precision cut-off saw with rigid clamping to prevent vibration.
Wheel: Resin-bonded diamond wheel (typically 200–300 mm diameter).
Coolant: Abundant water-based coolant to minimize heat and avoid thermal cracks.
Feed Rate: Slow, controlled feed to reduce mechanical stress.
Goal: Achieve a flat, undamaged surface with minimal microcracking for subsequent mounting
STEP 2: MOUNTING
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The samples for the lapping process were cold-mounted using an epoxy resin mixture with a 1:10 ratio (200 g resin and 20 g hardener), enough for two samples. After cleaning and drying, the samples were placed in molds and completely covered with the resin. The molds were first cured at 60 °C for 30 minutes, then the samples were removed and placed in an oven for a second curing at 80 °C for 8 hours to fully harden the resin. The resulting mounts were solid, uniform, and provided excellent support for subsequent lapping and polishing.
STEP-BY-STEP MOUNTING PROCEDURE FOR WC-Co SAMPLES
1 Clean the Sample: Remove oil, dust, and debris using ethanol or acetone.
2 Prepare Resin Mixture: Mix epoxy resin and hardener in the recommended ratio (e.g., 10:1 by weight).
3 Pour into Mold: Place the sample in a mold and cover completely with resin.
4 Initial Cure: Heat at 60 °C for 30 minutes to start polymerization.
5 Final Cure: Transfer to an oven at 80 °C for 8 hours for full hardening.
6 Demold and Inspect: Ensure the mount is solid, uniform, and free of bubbles.
7 Proceed to Grinding and Polishing: Start with coarse diamond discs and progress to fine polishing steps.
STEP 3: PLANAR GRINDING AND LAPPING
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If the sample surface is not sufficiently flat after cutting and mounting, planar grinding is performed to achieve uniformity. This step removes excess resin and levels the sample. If the starting surface is already flat, this step can be skipped.
Lapping refines the surface and prepares it for fine polishing. This produces a smooth, even surface with minimal deformation.
Figure 1 Lapping - Prolap 40um - 10x
STEP 4: PRE-POLISHING AND POLISHING
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The pre-polishing stage marks the beginning of the polishing process and serves as the transition from the more aggressive grinding steps to the finer finishing operations. At this point, the surface of the sample still shows scratches from the previous lapping stage, which need to be removed to achieve a smooth and uniform finish. In this step, a KGS PROLAP disc with 9 µm diamond abrasive is used together with water as a lubricant. The purpose of pre-polishing is to eliminate most of the deeper scratches and prepare the surface for the subsequent fine polishing stages. After this step, the sample should already exhibit a much more even appearance, with only very fine marks remaining.
Polishing continues with progressively finer abrasives, moving to the KGS PROLAP with 2µm diamond abrasive with water as lubricant. Each polishing step removes the scratches left by the previous one, resulting in a mirror-like surface free of deformation and suitable for microscopic examination.
Figure 2 Pre-polishing - Prolap 9um -10x
Figure 3 Polishing - Prolap 2um -10x
STEP 5: FINAL POLISH
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The final polishing step is crucial for achieving a flawless surface that is completely free of scratches and deformation. In this stage, the KGS Polishing Pad Viscose White disc is used to gently remove any remaining residues from the previous polishing steps. To enhance the effectiveness of this process, a 0.06 µm colloidal silica suspension is applied as the polishing medium. This extremely fine abrasive not only smooths the surface but also chemically assists in revealing microstructural details, such as grain boundaries and binder phases, which are essential for accurate materialographic analysis. After this step, the sample should exhibit a mirror-like finish suitable for high-resolution microscopic examination.
Figure 4 Final Polishing - Polishing Pad - 10x
Materialography
The term materialography used today is a factual extension of metallography, which include many other groups of materials, such as ceramics, plastics and composite materials that are examined in the same way. Materialography is the science of examining a material's microstructure, which is its internal composition at a microscopic level. By polishing and analyzing a material's surface, materialography helps engineers understand properties like strength, corrosion resistance, and potential failure points.